McMurray formation geology

1

Field Excursion to the Outcrops and Mine Sitesof the Fort McMurray Area

Dr. Murray GingrasDept. of Earth and Atmospheric ScienceUniversity of AlbertaEdmonton, Alberta T6G 2E3 Canada

Copyright ©2001, 2003 Michael J. Ranger, Murray K. Gingras (except where noted)No part of this publication may be copied without permission.

Dr. Mike Ranger808 West Chestermere Drive,Chestermere. Alberta T1X 1B6 CanadaTel: 403 235-2712 Fax: 430 235-2723E-mail: [email protected]

Geology of theAthabasca Oil Sands

Field Guide & Overview

by

Michael J. RangerMurray K. Gingras

4th edition

2

TABLE OF CONTENTS

Introduction .................................................................................................................................... 3

Paleotopography of the McMurray Sub-Basin ............................................................................ 9

Sedimentology and Stratigraphy of the McMurray Formation in Core and Outcrop................... 14

Regional Interpretation of the McMurray Formation.................................................................... 26

Modern Physiographic Analogues for the McMurray Estuarine System .................................... 37

Structure of the North Athabasca Area ...................................................................................... 43

The Timing and Mechanism of Oil Migration and Trapping ...................................................... 48

Description of Selected Facies and Sedimentary Structures from the McMurray Formation ..... 71

Estuarine Ichnology of the Athabasca Deposit ............................................................................ 86

References .................................................................................................................................. 115

3

C A N A D A

A L A

S K

A

ALBERTA SASK.

0 200

km OIL SANDSHEAVY OIL

Athabasca

Cold Lake

Peace River

Lloydminster

CALGARY

Oil Sands and Heavy Oil Deposits of Western Canada

INTRODUCTION

The oil sands deposits of Alberta are collectively the greatest accumulation of bitumen inthe world. Total in-place reserves are estimated to be 267 billion m3 (1.68 trillion barrels). Theoil sands exist primarily in Cretaceous unconsolidated siliciclastic formations in three desig-nated Oil Sands areas: Peace River, Athabasca and Cold Lake. Crude bitumen has also beenidentified to exist in Paleozoic carbonates know as the "Carbonate Subcrop Trend" or simplythe "Carbonate Triangle". The Athabasca Oil Sands area (Fig. 1) contains by far the bulk of the

bitumen reserves of Alberta (148.5 billion m3 - 934.5 billion barrels). The Athabasca, PeaceRiver and Cold Lake Deposits are contained within the Lower Cretaceous Mannville Groupand its equivalents (Fig. 2), which consists dominantly of unlithified siliciclastic sediments of

mixed continental and marine origins.The reserves contained in Paleozoiccarbonates subcrop beneath the sub-Cretaceous Unconformity over a largearea of north central Alberta. The fullextent of the Subcrop Carbonate Trendis as yet poorly defined, and their dis-tribution is based on very limited andscattered borehole control. Recent es-timates of the in-place bitumen re-sources in the various deposits areshown in Table 1.

Figure 1

DEPOSIT m3 in place(billions)

bbls in place(billions)

Athabasca

Carbonate Subcrop

Cold Lake

Peace River

148.50

60.83

34.91

22.68

934.50

382.74

219.65

142.70

DEPOSIT m3 in place(billions)

bbls in place(billions)

Athabasca

Carbonate Subcrop

Cold Lake

Peace River

148.50

60.83

34.91

22.68

934.50

382.74

219.65

142.70

Table 1

4

AthabascaAn area equivalent to 7% of the Athabasca Oil Sands Deposit has been designated as

surface mineable, where overburden and top reject is less than 75m in thickness. The remain-der is overlain by increasing amounts of overburden towards the southwest. Overburdenthickness varies from 0m where it crops out along the Athabasca River, to about 500m at theextreme southwest of the deposit. Recently produced figures suggest that approximately 41billion barrels are recoverable by proven surface mining technologies. Athabasca is the onlydeposit that crops out at the surface as an oil sands reservoir and it is the only deposit withsurface mineable reserves. There are two commercial, surface mining, oil sands projects ac-tive at the present time: Suncor presently produces over 80,000 barrels per day, and Syncrudeproduces over 220,000 barrels per day of synthetic crude.

Most of the Athabasca reserves are contained within the Lower Mannville McMurrayFormation (Fig. 2), but there is some oil saturation in the overlying Clearwater Formationsands (Wabiskaw Member) in the western and the southern part of the deposit. The McMur-ray Formation averages 40 to 60m in thickness and consists of uncemented, very fine to me-dium-grained quartz sand, interbedded with shales in highly complex channel systems.Throughout much of the Athabasca Deposit the McMurray Formation is oil-bearing from thetop to the base, although there commonly is a discrete bitumen water contact, which is con-trolled by structure. Porosity in the clean sands generally varies between 25 to 35% and oilsaturations of 10 to 15 wt% are common.

Oil Sands Mining OperationsOf the 935 billion barrels of oil in place in the Athabasca Deposit, only about 92 billion

barrels lie in what is considered the surface mineable regions where there is less than 75m ofoverburden. It is in this surface mineable area that Suncor and Syncrude are located.

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DEVONIAN

Lower Cretaceous Stratigraphy of Northeast Alberta

MISSISSIPPIAN

"upper"

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Figure 2

5

The pioneering Suncor operation began operations in 1967 as Great Canadian Oil Sands(GCOS), a consortium of petroleum companies. GCOS was renamed Suncor Oil Sands Groupin 1979. It was the first commercially successful operation in the oil sands industry and wasbuilt at a cost of 235 million dollars, originally producing approximately 50,000 barrels ofsynthetic crude oil per day. In May 1995, Sun Oil Co. of Philadelphia sold its 55% share inSuncor for $1.2 billion. Current production rate (2002) at Suncor is over 225,000 barrels perday. The Suncor operation is located on the banks of the Athabasca River, about 40 km northof the town of Fort McMurray. Within the confines of the 4000 acre lease site, it is estimatedthat there are approximately one billion barrels of bitumen in place, of which 630 million aredeemed recoverable.

The Syncrude plant opened in 1978. It was originally about three times the size of Suncorand currently has a production rate of 250,000 to 260,000 barrels per day. Syncrude has 3,900permanent employees and 1000 contractors, and is run by a consortium that originally in-cluded various major companies and governments. The Syncrude operation is located imme-diately adjacent to the Suncor property. The total area of all the Syncrude leases is over 680km2 (25% of the world's countries are smaller than the total area of Syncrude's leases!) Themine itself covers an area of 28 km2 and is 60m deep.

Oil sands mines are amongst the largest earth moving operations in the world. A rule ofthumb is that it takes approximately two tons of oil sand to produce one barrel of syntheticcrude oil. In addition, the handling of tailings material and overburden increases the figure to5 tons per barrel produced. For a capacity of 200,000 barrels per day, Syncrude must dailyhandle in excess of 1,000,000 tons of earth material.

Excavation of the ore body at Syncrude was originally by drag lines, which casts the oilsand into "windrows" adjacent to the pit. From there it is reclaimed by bucketwheel excava-tors and transported to the extraction plant by conveyor belts. This operation is known as the"Base Mine" at Syncrude.

As of October 1993, Suncor has used truck and shovel equipment exclusively for oil sandmining. Four bucketwheel excavators, worth $50 million each, were retired upon being re-placed by the new trucks and shovels. Modern 320 ton heavy hauler trucks and large capacityhydraulic shovels are more efficient and flexible and are quickly making the draglines andbucketwheels obsolete. 400 ton trucks are currently in development. The largest shovels canload about 150,000 tons of oil sand per day, and can load a 320 ton truck in 2 minutes. Al-though Syncrude still utilizes draglines and bucketwheel reclaimers to mine the oil sand intheir Base Mine, the use of draglines and bucketwheels will be phased out when the BaseMine is depleted. The truck and shovel method is used exclusively in Syncrude's "NorthMine" (Fig. 3), as well as the new "Aurora Mine". All conveyor belts are also being eliminated.The bitumen will be partially extracted form the sand and then mixed with water before itleaves the mine and transported to the extraction plant as a slurry.

From the mine pit the oil sands are moved either by conveyor belts or pumped throughpipelines to the extraction plant in a continuous stream, where steam, hot water and causticare added. After passing through a screen to eliminate oversize material, the various slurrystreams pass into banks of separation cells where oil froths to the surface and is skimmed off.The sand sinks to the bottom and is pumped off to the tailings pond. Middlings in the separa-

6

tion cells are recycled and scav-enged. The bitumen may be sub-jected to centrifuging to remove anyremaining mineral matter. Overall,this type of commercial extractionprocess has proven to be on the or-der of 92% efficient. Only 8 % of theinput bitumen ends up in the tail-ings pond.

At Syncrude raw bitumen fromthe extraction plant is upgraded tosynthetic crude oil through a proc-ess of fluid coking. Various rela-tively pure liquid hydrocarbons aredrawn from the coker and thenblended together to form a synthetic crude product which can be pipelined to refineries inEdmonton. The principal by-products of the upgrading process are sulphur, of which theoriginal bitumen contains 5%, and coke.

Revegetation of tailings has proved a major difficulty. The sand is exceedingly sterile,having been subjected to boiling and caustic treatment. Nonetheless with the addition offertilizers and organic additives, ground cover foliage has been successfully established, andland reclamation is an ongoing commitment.

In June 1995, both Suncor and Syncrude announced plans for expansion. Suncor's newmine, the "Steepbank Mine" is located on the east side of the Athabasca River, whereas Syn-crude's new mine site, "Aurora" is about 35 km northeast of their present operations. Aurorahas been in production since 2000 and uses trucks and shovels exclusively. The oil sand ispipelined to the extraction plant as a slurry in a process termed "Hydrotransport". In thehydrotransport process, the oil sand is mixed with water and pumped through a pipelinerather than moved on an open conveyor belt. As it travels, the oil sand begins digesting andconditioning, eliminating the first step of the extraction process, and arrives at the extractionplant ready for separation. Aside from eliminating the need for tumblers at the processingplant, hydrotransport also uses less energy, is less expensive to build and operate, and ismore flexible than conveyor belts.

Together Syncrude and Suncor provide about 25% of Canada's petroleum requirements.The Alberta Energy and Utilities Board speculates that Alberta's oil sand reserves will be theprimary source for Canada's crude oil within a decade, offsetting rapid declines in conven-tional crude stocks. When Syncrude opened, it cost over CAD$24 to produce one barrel ofsynthetic crude. Today a cost of just over $12.50 per barrel has been achieved and is expectedto be $12 in the near future. Suncor has already achieves production costs of close to $12 perbarrel. A key advantage of oil sands bitumen operations is that the known location of its hugedeposits eliminates most exploration risk, a major cost for a conventional petroleum com-pany.

Figure 3 Syncrude North Mine

7

In Situ Bitumen RecoveryNinety percent of Alberta's oil sands lies deep below the surface and cannot be recovered

by surface mining. Most in situ techniques involve injecting steam through a series of wellsinto the oil sand. The pressure and high temperature cause the bitumen and water to separatefrom the sand particles, and lowers the viscosity of the bitumen. The hot liquid migratestowards producing wells, bringing it to the surface, while the sand is left in place. As a resultof extensive research, substantial improvements have been made in recovery of in situ bitu-men.

AOSTRA (Alberta Oil Sands Technology and Research Authority) is a test facility andresearch centre that has been operating since 1987, focusing on the deeply buried oil sandsreservoirs. Its Underground Test Facility (UTF) is located 70 km northwest of Fort McMurray.AOSTRA's research includes the development of new oil sands and heavy oil technology anddevelopment projects, ranging from small bench scale projects to major in situ pilot plants.The most successful process developed by AOSTRA has been SAGD (Steam Assisted GravityDrainage), which utilises vertical pairs of horizontal wells (Fig. 4). Each pair has a producercompleted 2-3m above the base of the bitumen, and an injector about 5m above the producer(Fig. 4). At the UTF, the hori-zontal wells were originallydrilled upward from shaftsthat were sunk into the un-derlying Devonian lime-stones. Subsequent projectsusing newer technologydemonstrated that the proc-ess can also be effective withhorizontal wells drilled fromthe surface. Using horizon-tal wells also obviates one ofthe major problems experi-enced with recoveryschemes using vertical wells:complex stratigraphy of theMcMurray Fm. makes lat-eral continuity betweenwells extremely unpredict-able.

There is presently much interest in oil sands development in Alberta. Many companiesare competing to establish a favourable land position and re-evaluating their existing landholdings. This appears to be due to several factors. First, in the mineable area, the realizationthat modern truck and shovel operations can be more economic than draglines and bucket-wheels makes smaller, lower capital mining projects feasible. Second, the potential of SAGDfor in situ recovery has been so promising that such techniques have made the transitionfrom experimental to commercial. Third, there are preliminary plans to expand the existingpipeline infrastructure to a maximum capacity of about 300,000 m3/d (1,900,000 bbl/d), givenfavourable market conditions. Pipeline capacity out of Athabasca, including the soon-to-be-

Figure 4 Principles of SAGD technology

8

completed Corridor Pipeline, is presently approximately 120,000 m3/d (750,000 bbl/d). Fi-nally, many long-term oil sands leases are coming up for renewal or relinquishment in thenext several years. This is the incentive for companies to re-evaluate their land holdings, aswell as the entire Athabasca Deposit, for potential sites where the most efficient recoverytechniques may be effectively applied.

Besides the UTF installation (now run commercially by Devon Energy, and known as the"Dover" project), several companies have commercial SAGD projects in development or inthe early stages of production, i.e. EnCana Foster Creek, EnCana Christina Lake, Suncor Fire-bag, and Petro-Canada MacKay River. Other potential projects in the planning stages includeDeer Creek Joslyn Project, Petro-Canada Meadow Creek, Japan Canada Hangingstone, OPTI-Nexen Long Lake, Conoco Surmont and CNRL (Rio Alto) Kirby.

9

PALEOTOPOGRAPHY OF THE McMURRAY SUBBASIN

In the area of the Athabasca Oil Sands, the McMurray Formation rests on truncated UpperDevonian strata, mainly limestone and calcareous shale of the Waterways Formation in theeast and somewhat younger carbonate rocks of the Woodbend Group in the west. In the mostgeneralised terms, the mainly marginal-marine sediments of the McMurray Formation canbe viewed as consequent valley fill of a broad, north-trending drainage system entrenched inthe exposed landscape of Devonian terrain. Deposition of the McMurray Formation ceased inthe middle Albian, when the Boreal Sea transgressed the entire region, ushering in marineconditions and giving rise to deposition of the mudstones of the Clearwater Formation.

During the Early Cretaceous (Neocomian/Aptian) in the Western Canada SedimentaryBasin, the unconformity terrain was an immature, continental, erosional landscape domi-nated by three major drainage systems. These drainage systems had developed their orienta-tions dominantly due to differential erosion of gently dipping strata. But tectonic and otherstructural elements certainly played a role. The subcropping strata dip to the southwest, andthus the erosional surface exposes older strata of the Middle to Upper Devonian BeaverhillLake Group in the northeast, and strata as young as Late Jurassic toward the southwest (Leckieand Smith, 1992). These drainage systems are separated from each other by major axial ridgesystems of resistant Devonian carbonates that constitute the drainage divides. Each of thesethree trunk drainage systems constitute what may be thought of as depositional subbasins.Certainly this is true as far as deposition of the Lower Mannville is concerned. During themajor sea-level transgressions of the Aptian and Albian, each valley system would have beenflooded and would have reacted independently depending on the topography and dynamicsof the sediment supply.

In the east of the basin is the axial ridge system of resistant Devonian carbonates known asthe Wainwright Ridge in central Alberta and the Grosmont High in northeastern Alberta (Fig.5). This axial ridge system forms the western boundary of a major drainage valley systeminformally referred to as the McMurray Valley System. It is in this valley system that theAthabasca and Cold Lake Oil Sands Deposits as well as the Lloydminster heavy oil fields arelocated. The valley system is confined to the east by the highlands of the Canadian Shield,and its axis follows a trend parallel to the strike of the outcrop of the Canadian Shield innortheast Alberta, through south-central Saskatchewan and Manitoba, and might be expectedto have its headwaters in the area of the Manitoba Escarpment or somewhat farther south.The paleotopographic low that forms the axis of the McMurray Valley System has been local-ised by the dissolution of evaporitic facies mainly of the Middle Devonian Prairie Evaporite,but also to some degree the Lower Devonian Cold Lake and Lotsberg Formations. This disso-lution was responsible for structural subsidence of the overlying basin before, during, andafter deposition of the Wabiskaw/McMurray sediments. The McMurray Valley System ofnortheast Alberta is eroded into Middle to Upper Devonian carbonates and shales of theBeaverhill Lake Group in the east and Upper Devonian carbonates of the Woodbend Groupin the west.

Several studies over the last few years have shown how the underlying basin topographyhas profoundly influenced the distribution of facies and, in particular, reservoir facies in theMannville Group. Zaitlin and Schultz (1984) demonstrated that the geomorphology of anUpper Mannville Lloydminster estuarine system in the Senlac area was probably inheritedfrom differential compaction over a buried valley on the sub-Cretaceous unconformity. In theWabasca area, the Wabiskaw Member reservoir sands of the Athabasca Oil Sands are also

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partly controlled by a buried valley system eroded into Devonian carbonates that subcrop atthe unconformity (Ranger et al., 1988). These Wabiskaw sand bodies evidently form severaldiscrete, broad, shoreface aprons that onlap the buried Paleozoic highlands of the Red EarthRidge (Ranger, 1994). The main reservoir of the Athabasca Oil Sands, the McMurray Forma-tion, has been shown in several studies to have been profoundly controlled by the valleysystems of the underlying sub-Cretaceous unconformity that constitute the McMurray sub-basin described above (Stewart, 1963, 1981; Stewart and McCallum, 1978; Flach, 1984; Keithet al., 1988; Ranger and Pemberton, 1988; MacGillivray et al., 1989; Ranger, 1994).

The unconformity is a hard, indurated surface. It can be considered as the basement forthe Lower Cretaceous succession and no doubt had a profound effect on the distribution offacies in the McMurray Formation. The topography on this erosional surface is therefore ofvital importance, because it is on this surface that the reservoir rocks of the Athabasca OilSands Deposit were deposited.

The sub-Cretaceous unconformity surface can be modelled either by mapping its struc-tural elevation or, probably more accurately, by mapping the thickness of a suitable intervalwhose base lies directly on the unconformity surface. If it is assumed that some overlyingstratigraphic marker approximated a regionally “flat” surface (relative to paleo-sea level),then an isopach map of the interval between the upper marker and the unconformity forms amould of the unconformity surface, where the thins represent the highs on the unconformityand the thicks define the lows.

An ideal datum for this technique is the top of the Mannville Group, but in the regionalAthabasca area the top of the Upper Mannville Group is unsuitable as a datum due to erosionin the northeast. In this study the isopach of the McMurray Formation itself is used as amodel of the unconformity paleotopography (Figure 5).

The regional map of the sub-Cretaceous unconformity topography underlying the Atha-basca Deposit reveals a northerly-trending, axial ridge that effectively divides the area intotwo subbasins, informally termed the "McMurray Subbasin" in the east and the "WabascaSubbasin" in the west. This ridge is informally known as the Grosmont High because it ap-parently results from resistant carbonates of the Grosmont Formation. In the bigger picture,the Grosmont High is a north-trending spur of the Wainwright Ridge.

The McMurray formation is missing, and apparently was not deposited on the crest of theGrosmont High. These areas are shown in a brickwork pattern, and would have been high-land areas and then islands during marine transgressions. The Wainwright Ridge - GrosmontHigh complex has numerous secondary spurs branching obliquely away from it on both theeast and west sides. On the east side these spurs trend in a northeast direction, and the valleysbetween them form major northeast flowing tributaries that can be mapped into the centralvalley of the McMurray subbasin. In the south-central portion of the study area is a largeridge that extends along ranges 9 and 10 from township 77 down to at least township 70. Thisis the extension of a major spur from the Wainwright Ridge south of the Athabasca area. Tothe north another major spur extends at an oblique angle to the main ridge. The interveningvalley forms another major tributary of the McMurray system, but one that flows dominantlynorth to approximately township 94 where it abruptly turns to the east and enters the trunksystem in the area of the Bitumount subbasin.

Over the Bitumount Basin area, there is no suitable stratigraphic datum to use in an iso-pach map due to widespread Pleistocene erosion. Here only the structural elevation of theSub-Cretaceous Unconformity can be used as a model of its paleotopography (Figure 6). Onemust be aware, however, that the structure map reflects not only erosional paleotopography,

13

but also any structural effects that have occurred in the basin up to the present day. And thereappear to have been some profound structural effects in some areas. In particular there hasbeen post-Mannville structural collapse of 50 metres or more over part of the Bitumountbasin.

Despite the uncertainties of using a structure map to reconstruct the erosional paleotopog-raphy of the Unconformity, general observations can be made regarding the regional settingof the North Athabasca area (Fig. 6). The structural low known as the Bitumount Basin appar-ently lies at the northern reaches of the McMurray Channel Valley system, near the mouth ofwhat appears to be a major secondary valley system. The main trunk valley system of theMcMurray lies to the south, and appears to bifurcate. Koch Fort Hills, Aurora and OSLO liewithin the eastern valley, while the Syncrude base mine complex and Suncor lies within thewestern valley. In the Bitumount Basin, subsidence is apparent in the Mannville sediments,and furthermore, regional isopach maps show a dramatic thickening of the lower Mannvilleinterval within this area, even taking into account the eroded nature of its top. This indicatesthat the Bitumount Basin was a topographically low area during McMurray deposition, andthat structural subsidence continued even after the end of McMurray time.

The Bitumount Basin appears to have be a local catch basin. Both arms of the regionalbifurcation of the trunk valley system to the south appear to drain into the Bitumount Basin.As well there is another secondary valley system draining the Grosmont High far to west,which ultimately joins the McMurray trunk valley system in the Bitumount Basin area. Fur-ther north from the Bitumount Basin the trunk valley system appears to continue northwardup range 10 to approximately township 99 where the entire interval has been removed byerosion. It is these regional valley systems that provided the topographic conditions for thedevelopment of widespread estuarine complexes during Lower Cretaceous sea level rise.The Lower Mannville sediments therefore are the consequence of the filling of a drownedmajor river valley.

14

SEDIMENTOLOGY AND STRATIGRAPHYOF THE McMURRAY FORMATION IN CORE AND OUTCROP

Carrigy (1959) established the informal threefold stratigraphy of the McMurray Forma-tion consisting of lower, middle and upper units. This basic stratigraphy has not evolvedmuch since then and remains informal, although the units are often referred to as members.(“upper”, “middle” and “lower” are not capitalised in this guide in keeping with the infor-mal nature of the subdivision and the rules of stratigraphic nomenclature.) Many of the Mc-Murray Formation cores and outcrops in the Athabasca region appear to exhibit this three-fold facies. These vary in thickness and expression from place to place. The lower member,where present, typically consists of thick-bedded to massive sands, commonly medium tocoarse grained, characterised throughout by current cross-stratification.

The middle member is a complex set of facies associations, but the best reservoir sandsare thick bedsets of clean sand dominated by planar tabular to sigmoidal megarippled bed-ding. The are also good reservoir sands contained in very large scale sets of inclined strata,up to 25 m in thickness. In all essential regards these conform to Allen’s definition of epsiloncross-strata (Allen, 1963), or in the more modern context, Inclined Heterolithic Stratification(IHS) (Thomas et al., 1987).

The upper member typically consists of horizontally-bedded argillaceous sands and silts,often coarsening and becoming sandier upwards. Overlying the McMurray, apparently un-conformably, are the muds and glauconitic sands of the Clearwater Formation and its Wabis-kaw Member.

In the north Athabasca area, this tripartite stratigraphic subdivision generally manifestsitself strongly. Where preserved, the upper McMurray is generally a coarsening and sandierupwards unit, 10 to 15 metres in thickness. Physically the basal contact of the upper McMur-ray is most dramatic in outcrop where it typically overlies estuarine IHS beds of the middleMcMurray (Fig. 7). Here, the locally flat-lying stratigraphy of the upper McMurray is in sharpcontrast to the IHS beds which display a dip of up to 12 to 15 degrees. In core the contact maybe more subtle, and therefore not easily determined.

Many workers have fit their studies into this threefold subdivision (James, 1977; Nelsonand Glaister, 1978; Stewart and MacCallum, 1978; Flach, 1984). Yet no one has yet been able toreconcile and correlate the stratigraphy observed in these various studies. Given the acknowl-edged difficulty in correlating beyond a limited area (Mossop, 1980a; Flach, 1984), it seemsthat most workers are reconciled to let McMurray stratigraphy remain on an informal basis.However one study (Nelson and Glaister 1978) stands out for recognising widespread, corre-latable, radioactive (gamma ray) signatures from wells in a local subsurface study in thecentral Athabasca Deposit. Nelson and Glaister pointed out that within the McMurray For-mation there exists at least two correlatable shales, which they believed to be time strati-graphic markers. They used these markers to subdivide the McMurray Formation into threeunits, each of which they mapped as a discrete depositional system.

Carrigy (1971) observed large inclined bedsets exposed at the Steepbank River interpret-ing them as delta foresets. These well-known outcrop exposures are now believed to be in-clined heterolithic stratification (IHS) of point bars in a deep incised channel complex (Flachand Mossop, 1978). Carrigy (1971) went on to interpret much of the McMurray Formation inthe northern part of the deposit as fluvial-dominated deltaic and related deposits. His con-clusions were based partly on the interpretation that the McMurray Formation was primarily

15

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of freshwater origin, except for a marine wedge at the top that thickens towards the north andwest. A deltaic model has been proposed in several other studies, the most detailed being thatof Nelson and Glaister (1978).

Seminal work on the outcrop exposures around Fort McMurray has contributed greatly toa basic understanding of the sedimentology of the reservoir facies (Mossop, 1980a; Mossopand Flach, 1983; Flach, 1984; Flach and Mossop, 1985). Flach and Mossop have demonstratedthat some of the best reservoirs of the Athabasca Deposit are deep, sand-filled, incised chan-nel complexes. This observation is of prime economic importance. However, these channels,or at least their sandy facies, appear to be of relatively limited extent and, while common inoutcrop, there has been only limited success in extrapolating the outcrop observations intothe subsurface (Mossop, 1980a; Flach and Mossop, 1985). Moreover, it appears that sandyfacies of the McMurray Formation are preferentially preserved in outcrop, therefore giving abiased, but highly visible and influential sample of the reservoir architecture.

The suggestion that much of the McMurray Formation may have been deposited underestuarine conditions was first proposed by Stewart and MacCallum after many years of sub-surface and outcrop study (Stewart, 1963, 1981; Stewart and MacCallum, 1978). They putforth the commonly held interpretation that the McMurray Formation consists of a lowerfluvial unit, a thick middle estuarine unit and an upper marine unit, and they mapped thesefacies over much of the northern part of the deposit. Much of their detailed work has sur-vived the test of time, and the basic threefold subdivision is still generally accepted. In manystudies the threefold facies model is equated to the informal threefold stratigraphic frame-work of Carrigy (1959).

Lower McMurrayThe lower McMurray is distinctive for its generally coarser grain sizes, massive sand units

and rare to no bioturbation. The lower McMurray can be somewhat elusive, being limited tostructural lows on the sub-Cretaceous unconformity, confined by Devonian carbonate high-lands. The typical lower McMurray succession is an ideal genetic unit whose facies weredeposited in a recurring association, characterized by sand units that fine upwards from me-dium- or coarse-grained, large-scale, cross-stratified sand to fine-grained, small-scale, cross-stratified sand and then is abruptly overlain by a muddy facies. The large-scale cross-strati-fied sand lies on the angular Sub-Cretaceous Unconformity or a thin paleosol immediatelyabove the unconformity. Intraclasts of underlying calcareous mud may be present in the sandjust above the contact, and the basal sands are generally pebbly or very coarse-grained. Somewells also contain a thin, "hot", felspathic sand near the contact that produces a radioactivespike on gamma ray logs.

The large-scale cross-stratified sand grades upward into small-scale cross-stratified or mas-sive sand, and more rarely into interbedded sand/mud beds interpreted as sand-dominatedIHS. The muddy facies is typically a grey mud, which may contain rooted horizons. In thenorthern Athabasca area, the lower McMurray is capped by a unit consisting of coal, organicshale and/or rooted, light grey shale (Fig 10). This unit varies in thickness from nil to tens ofmetres in thickness and probably represents a late stage aggradational marsh/paleosol envi-ronment when sea level rise gradually began to outpaced sediment supply. The light col-oured shales probably result from pedogenesis within a humid oxidising environment. Thedark grey carbonaceous muds and coal indicate increasing organic content of facies undermore reducing, probably shallow subaqueous, marsh conditions (Fig. 10). Bioturbation is

19

rare to absent within most of the lower McMurray, although it maybe present in many wells in the upper units and in sand facieswithin the marsh paleosol.

The genetic units of the lower McMurray indicate depositionwithin a fluvial environment. The overall succession suggests pres-ervation of a fluvial meandering channel environment. The fin-ing-upward successions, paucity of burrowing, and presence ofrooted and coal horizons are all typical of a fluvial origin. Palyno-logical evidence has indicated generally fresh water conditions withrare brackish water influences for the lower McMurray (Flach,1984).

The sharp lower contact results from channel erosion and inci-sion. Where it lies directly on the sub-Cretaceous unconformity,erosion has further entrenched the basement carbonates. The large-scale cross-stratified sand units result from high flow regime duneswithin the basal channel bed. Small-scale cross-stratified sand re-sults from waning flow conditions on larger channel bedforms andon point bar surfaces. The occurrences of sand-dominated IHS arepoint bar lateral accretion deposits, and suggest a tidal influence.Grey rooted mud is the result of floodplain and overbank depos-its. Some small-scale, cross-stratified sands grade up into chaoticcarbonaceous sand with abundant wood fragments and carbona-ceous debris. These units probably represent crevasse splays. Oc-casional mud intraclast breccias result from erosion of overbankdeposits. These typically occur at the bottom of a channel succes-sion as a channel lag and probably survive transport over only ashort distance within the channel.

The rare presence of bioturbation (Fig. 10) indicates that thefluvial system may have been the upper reaches of a greater estua-rine system, whose marine influence began to encroach up the flu-vial valleys during the later stages of the aggradation of the lowerMcMurray. Taken in the transgressive context of the entire McMur-ray succession, a basinward estuarine component to the lower Mc-Murray systems tract in not unreasonable.

Previous studies have invariably assigned a fluvial interpretation to the lower McMurray(Carrigy, 1971; Stewart and MacCallum, 1978; Flach, 1984; Flach and Mossop, 1985; Rennie,1987; Fox, 1988).

Middle McMurrayTypical middle McMurray genetic units were deposited in recurring facies associations.

Large-scale, cross-stratified sand developed initially on a sharp erosional contact with lowerMcMurray, or lies directly on the Sub-Cretaceous Unconformity where the lower McMurrayis not present. We term this facies association FA1.

The dominant facies of FA1 consists of bedsets up to 0.5 m or more in thickness. Planar-laminated cross-stratification with toeset development indicates that these are high flow re-gime, sigmoidal, megarippled dunes (Fig 11). Topset laminae are rarely preserved. Thesebedsets commonly contain tidal indicators, such as reverse flow ripples, reactivation sur-

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faces, herringbone beddingand other evidence of localflow reversals as well asrhythmic grain size couplets(often in recurring series of 7,14 or 28). Within bedsets, bio-turbation is absent. But thetruncated upper surface of abedset may be capped by athin shale lamina, and/or abioturbated horizon suggest-ing a period of quiescence orabandonment. Trace fossilsare rare but robust, and theassemblage has a very low di-versity, typically consistingonly of Cylindrichnus and Sko-lithos. At several locationsrare Conichnus have been observed. Conichnus is believed to be the resting trace of a seaanemone, a marine organism intolerant of brackish or fresh water conditions.

Although FA1 is perhaps easily mistaken at first glance (especially in core) for high flowregime fluvial channel deposits, the presence of marine trace fossil indicators suggests atleast periodic incursion of marine conditions. But combined with the tidal structures, a strongmarine influence is indicated. We interpret this facies association to have originated in thelower (outer) estuary, proximal to an estuary mouth, with flow velocities magnified by tidaleffects.

Sand-dominated and/or mud-dominated IHS (In-clined Heterolithic Stratification) typically overlies FA1with an erosional contact. The IHS bedsets are a majorcomponent of a facies association we term 'FA2'. TheIHS sand/mud couplets vary greatly in thickness fromapproximately 7 cm to 50 cm, generally either mud- orsand-dominated. Bedding is inclined anywhere fromnear horizontal up to a maximum apparent angle in coreof 15° (Fig. 12). Each sand/mud couplet consists of afine- to very fine-grained sand bed whose base may beerosional, followed by a sharp transition up into a siltymud bed. The contact is generally bioturbated with mudfilled burrows penetrating down into the sand (Fig. 13).The sands are cross-stratified on a small-scale and prob-ably result from waning flow conditions on point barsurfaces. Individual sand and mud beds are known fromoutcrop to be laterally continuous from the top of a faciesunit to near the base where the mud beds graduallypinch out into cross-stratified sands, where they wereprobably scoured away and therefore not preserved.

Figure 11 Cross-stratified sigmoidal bedsets of FA1. Lenscover is approximately 6cm in diameter

Figure 12 Sand-dominated IHS in thesubsurface middle McMurray

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Mud intraclastbreccia is commonlyfound in associationwith the cross-strati-fied sands, and occa-sionally with theIHS beds. Grey mudor interlaminatedsilt and mud typi-cally caps the suc-cession. Bioturba-tion is commonthroughout the mid-dle McMurrayfacies, except for thecross-stratified sandwhere it is rare. Thisideal association offacies is often re-peated within themiddle McMurrayalthough complete successions are rare. Typically, multiple partial successions are complexlystacked such that individual genetic units are difficult to differentiate in core alone.

We suggest that this facies association, FA2, supports deposition within a channelisedcentral estuarine environment. The fining-upward successions, bioturbation overprint indica-tive of brackish water conditions, and overwhelming abundance of IHS cycles suggest thatthe estuarine channel point bars have been preferentially preserved. The basal sands of anIHS set are the expression of subaqueous dunes within the thalweg of the estuary channel.The erosional lower contact of FA2 results from channel meander erosion and incision. Paleo-current directions are locally unidirectional within IHS sets, although may vary regionally byup to 90 degrees.

The sand- and mud-dominatedIHS result from lateral accretion depo-sition on tidally-influenced point bars(Fig. 14). Relative proportions of sandand mud in IHS point bar depositsmay reflect normal brackish condi-tions relative to the position of a tur-bidity maximum in the estuary. Mud-dominated point bars slowly accumu-late mud drapes over a relatively longperiod due to the position within theturbidity maximum, seaward of thehead of the salt wedge; sand-domi-nated point bars, beyond the influenceof the turbidity maximum, accretecoarser sediment. During periods ofhigh fluvial discharge, which may be

Figure 13 IHS beds in outcrop, Ichnofossil assemblage is monospecific, consist-ing solely of Cylindrichnus

Figure 14 Point bar model for IHS

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a seasonal flooding phenomenon or a random, intense storm event, the salt wedge becomesvertically homogenous and is displaced downstream, along with the turbidity maximum.Entrained in the flood waters is an influx of sand, which accumulates rapidly over only a fewdays. The surface of a new sandy substrate covering a previously mud-dominated point barcan then become the site of an "opportunistic" colonisation of fauna. In the middle McMurrayFA2 units, such fauna seems to consist largely of the Gyrolithes and Cylindrichnus trace-mak-ers. In contrast, the displaced turbidity maximum may deposit mud over a previously sand-dominated point bar, also disrupting the normal faunal populations. Opportunistic colonisa-tion would necessarily be short-lived, because once the turbidity maximum returns to itsnormal physiographic position and sediments once more begin to accumulate under "fair-weather" conditions, the opportunistic fauna are smothered or otherwise displaced. There-fore whether the IHS point bars are sand- or mud-dominated probably depends mostly ontheir physiographic position relative to the turbidity maximum in the estuary, although otherfactors may certainly play a role, such as the morphology of the point bar, and relative strengthof currents.

The mud intraclast breccias result from erosion of older muddy point bars and overbankcollapse deposits. These typically accumulate at the bottom of a channel succession as a chan-nel lag and probably survive transport over only a short distance within the channel.

The grey mud and thinly interbedded silt/mud deposits are typically highly bioturbatedand represent tidal flats flanking the estuarine channel/point bar complex. The tidal flat en-vironment is topographically higher and accumulates over the point bar as vertical accretiondeposits. This can be considered the estuarine channel "overbank" environment, and is heav-ily influenced by tides in contrast to purely fluvial overbank deposits. Within modern estua-rine complex environments, tidal flats are typically the most heavily populated and biologi-cally complex environments.

Mud intervals several metres, or tens of metres, thick that are barren of ichnofossils areinterpreted as fill in abandoned estuarine tributary channels, which were periodically floodedwith turbid fresh water due to overbank flooding during periods of high fluvial discharge.This facies is seldom seen in outcrop, because it has almost no resistance to erosion, but isrelatively common in core.

Flach (1984) recognised that the ideal middle McMurray succession displays many of thecharacteristics of a channelised system: a scoured base; fining-upward sand grain size; anupward increase in the number and thickness of mud beds; and the paleocurrent indicatorsare unidirectional. Although Flach downplayed the estuarine interpretation of the channelsystems, the evidence of marine brackish influence is unequivocal. Trace fossils are rare toabsent in freshwater fluvial systems. The trace fossils found within the middle McMurray arenoteworthy for their small size, generally simple morphology, low diversity and high abun-dance, and they represent a combination of traces from both the Skolithos and Cruziana ichno-facies, all typical of a brackish water environment (Pemberton et al., 1982; Beynon andPemberton, 1992; Ranger and Pemberton, 1992).

The cyclicity of the interbedded units on all scales is also compelling evidence for marinetidal influence. The characteristic sand-mud couplets of the IHS units can only result fromcyclic energy levels. Within the upper mud flats, rhythmic wavy and lenticular bedding areindicative of flow-reversals. Alternating burrowed and unburrowed zones within the samefacies indicate variations in the physical regime of the environment.

Mud is a common constituent of estuarine channel point bars, but is rare in fluvial pointbars, except were tidally influenced. Allen (1991) recognised the increasing mud content of

23

estuarine point bars downstream from the fluvial-tidal transition in the Gironde estuary. Theestuarine point bars contain ripple cross-bedded sand with abundant mud laminae and fla-sers, resulting from the proximity of the turbidity maximum. In outcrop exposures of themiddle McMurray, mud layers drape the entire point bar surfaces from the top almost to thebase (Flach, 1984; Flach and Mossop, 1985).

Flach suggested that there is apparent continuity of bedding from cross-bedded sands(FA1) up into the IHS units (FA2), indicating that they are two facies of the same genetic unit.Close examination of the Steepbank River outcrops as well as many other outcrops visited onthis field trip indicate that this is not so. FA1 and FA2 are almost invariably in erosional con-tact, an important, highly relevant, point when we talk about the stratigraphic developmentof the McMurray Formation in subsequent chapters.

Upper McMurrayThe upper McMurray in outcrop is distinctive for its dark grey mud, coarsening-upward

nature, and horizontal strata often in sharp contrast to IHS beds ofthe middle McMurray. It has a sharp, erosive lower contact with theunderlying middle McMurray. There may be a veneer of poorly sortedfine to coarse sand immediately above the basal contact. This is fol-lowed by interbedded sand and dark grey mud grading upward intofine-grained sand with dark grey mud interlaminae, reportedly withthe presence of glauconite. At least two of these coarsening-upwardunits are present, and each may be capped by a rooted horizon. Theupper contact with the Wabiskaw Mbr. is also sharp. Flach and Mossop(1985) used sedimentology, palynology, and the presence of glauco-nite to suggest open marine conditions for their upper McMurray “ma-rine unit”. The unit was interpreted to be an offshore marine bar (Flach,1984; Flach and Mossop, 1985).

In core, at least two coarsening-up units may be preserved in theupper part of the McMurray Fm. (Fig. 16), and are probably correla-tive to those seen in outcrop. They are bounded by regional marinemuds that accumulated on flooding surfaces. These units are referredto as the Upper C/U (Coarsening-Upward) or “Red” cycle and theLower C/U or “Blue” cycle. These two cycles are separated from eachother by a thin unit referred to as the “Green” unit.

Each cycle exhibits a coarsening- and sandier-upward trend,wherein the occurrence and thickness of sandstone beds increases up-wards accompanied by a slight increase in sand grain size. Fully de-veloped cycles are composed of three lithofacies, the 'A', 'B' and 'C'and may be capped by a rooted organic shale and/or thin coal. Litho-facies A to C successively exhibit greater proportions of sand upward.Cycles commence with bioturbated regional marine muds of Litho-facies A, grading upward into interbedded, oscillation rippled, sandsand muds of Lithofacies B, (Fig. 15), which finally passes up into swa-ley, oscillation rippled sands of Lithofacies C. Lithofacies B typicallycontains indications of waning flow conditions such as oscillation andcombined flow ripples, and graded beds.

The base of a cycle (base of lithofacies 'A') is always sharp and

Figure 15 upper Mc-Murray oscillation-rippledshoreface

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constitutes a floodingsurface. The floodingsurface may be associ-ated with a transgres-sive surface of erosion,demarcated by thinbeds of grit. In someinstances, a Glossifun-gites surface (bur-rowed exhumed firm-ground) has been iden-tified at the base of acycle where heavilybioturbated silty-shales abruptly overlielaminated estuarinedeposits.

Facies A (the re-gional marine mud) isalmost always thor-oughly bioturbatedwith a diverse assem-blage of trace fossilsincluding Teichichnusand Helminthopsis.Stratification is typi-cally totally obliter-ated. Facies B and C ofthe C/U cycles are dis-tinctly different in ap-pearance. The ichno-fossil suites have ex-tremely low diversity,typically monospecific, and constitute very simple structures. Numbers are extremely low,often barren. These characteristics are at the opposite pole from any fully marine ichnofacies,and indicative of some extreme stress in the environment. This stress may be anticipated to bevery low salinities, rapid deposition and/or extreme turbidity (possibly associated with rapiddeposition) However, the presence of abundant synaeresis cracks suggests episodic salinityfluctuation. The synaeresis cracks are almost invariably present in clayshales that are com-pletely barren of ichnofossils.

The vertical arrangement of lithofacies A through C represents the superposition of proxi-mal through distal shoreface environments. The general upward increase in grain-size, de-crease in mud content, observed ichnofaunal suites and progressive change in style of sedi-mentary structures suggests the aggradation/progradation of a shoreface whose distal envi-ronment is clearly marine, but which exhibits increasing stress due to low salinity in the shore-ward direction. The bioturbated silty-muds at the base of each cycle (Lithofacies A), weredeposited below fairweather wave base and lie on a marine flooding surface/transgressive

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REMARKS

Figure 16 Coarsening- and sandier-upward parasequences in core fromthe upper McMurray.

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Plan View Cross Section

Facies Modelupper McMurray Coarsening-Upward Cycles

A A’

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© 2002 Mike Ranger and Mark Caplin

Fluvial/Wave-Dominated Delta

surface of erosion. The upwards increase in occurrence and thickness of sharp-based, paral-lel-laminated sand beds reflects the progressive increase in intensity of apparently freshwa-ter, flood-derived, currents over time. These currents are thought to have been dominantlyfreshwater in composition as they would make environmental conditions very difficult, if notintolerable, even for the most opportunistic marine organisms. The presence of wave-inducedoscillation ripples and swash cross-stratified sands within lithofacies B and C indicates aninfluence from background wave-dominated processes. The cycles are herein tentatively in-terpreted as shoreface parasequences (Fig. 17) bounded by flooding surfaces, which consistof mixed fluvial-wave-dominated deltas that prograded into a highstand brackish to marinebay (bayhead delta?). The synaeresis-cracked clayshales may represent the shoreface abovethe salt wedge. The delta would only be exposed to marine water during periods of lowfluvial flow, during storm surges, periods of extreme high spring tides or perhaps duringpost-storm oscillation of the salt wedge (Caplin and Ranger, 2001).

The trace fossil assemblage of the upper McMurray is distinct from underlying estuarinedeposits of the middle McMurray, and yet is somewhat inconsistent with what would beexpected from a fully marine shoreface. The trace fossil forms are larger but less abundantthan the estuarine examples, and they appear to have a generally low diversity of forms. Thelow diversity and low abundances of trace fossils within these shoreface deposits may resultfrom a combination of energy and salinity stresses. Fluctuating conditions provide insuffi-cient time for colonization of a substrate resulting in a predominance of physical structuresover biogenic. Furthermore, the basin may never have reached fully marine salinity condi-tions. This may be due to the restricted nature of the northern part of the basin caused by theconvergence of the Grosmont High in the west and the Canadian Shield in the east. Thecontinued influx of fresh water from the McMurray Valley System even during highstand sealevel maintained brackish conditions to some degree, and the constriction in the basin pre-vented rapid dispersion of brackish water into the boreal sea to the north. Therefore the shore-face/delta was deposited within a broad embayment rather than an open coastline.

Overall, the upper McMurray should be interpreted in the context of the transgressivehistory of the basin due to rising sea levels. The progradation of a shoreface indicates relativeslowing or a tempo-rary halt to the trans-gression, probably re-flecting an increasedsediment supply andless subsidence in re-lation to eustatic sealevel, i.e. a stillstandevent. This was ap-parently short-livedbecause the immedi-ately succeeding Wa-biskaw is indicativeof a basin suddenlyengulfed in fully ma-rine offshore condi-tions. Figure 17

26

REGIONAL INTERPRETATION OF THE McMURRAY FORMATION

The previous chapter has outlined the general sedimentology of the three informal unitsof the McMurray Fm., the lower, middle and upper members. The boundaries between thethree have always seemed rather fuzzy (hence the lack of a formal stratigraphic subdivision),and possibly for this reason, it was easy to suggest that the entire McMurray interval repre-sents the development of a single systems tract. The base appearing fluvial, the middle, es-tuarine, and with evidence of more marine influence towards the top, an overall transgres-sive system seemed most likely. This simplistic interpretation had become the working modelfor the interpretation of the McMurray Fm. and the complexity of facies seemed to precludeanything more than such a broad-brush approach. The McMurray Fm. is also blessed (somewould say cursed) with an abundance of data over an area of at least 50,000 km2. Tens ofthousands of exploratory wells have been drilled into the McMurray Fm, many with corerecovery, and that does not include thousands of additional closely spaced wells that havebeen drilled for commercial recovery of the resource. Most studies have been sponsored bycommercial interests, and therefore focus on relatively small areas, perhaps 1 or 2 townshipsat the most, on and around a private lease. The amount of data available, (not to mention theexpense of gaining access to it) made more regional studies prohibitive in time and money.Most researchers therefore have a detailed knowledge over a small area, but lack a regionalperspective.

In the early 1990's, a thesis study based on digitised data from over 1600 wells in SouthAthabasca demonstrated that in the upper part of the McMurray Fm., there exist a series ofregionally correlatable thin parasequences (Fig. 18), 8 to 12 metres in thickness, consisting ofwhat appeared to be prograding shoreface deposits (Ranger, 1994; Ranger and Pemberton,1997, Caplin and Ranger, 2001). The shoreface parasequences are bounded by regional ma-rine shale units that were interpreted to represent transgressive marine muds lying on a flood-ing surface/ transgressive surface of erosion. The three upper stratigraphic units are espe-cially obvious, and were informally termed the “red”, “green” and “blue” intervals, and theirlog signature can be recognised sporadically over much of the south Athabasca area (Fig. 18).The distinct signatures of the three units is not ubiquitous however. It cannot be recognisedin all wells. There are many areas where the signature is anomalous, suggesting that theshoreface unit has been eroded. A map of the distribution and thickness of one of the shore-face intervals demonstrates this (Fig. 19). The areas in black are areas where the “blue” signa-ture is anomalous. These anomalous signatures indicate a wide variety of sandy, shaly orheterogeneous fills which were interpreted broadly as brackish estuarine channel fills (Ranger1994), although these units have undergone little detailed study.

Incised Valley Fill Systems as a Working Model for the upper McMurrayThese observations intuitively seemed to fit well with what is known about incised valley

systems, (Fig. 20), wherein the Athabasca stacked parasequences sets represent progradingbasin fill during highstand conditions, bounded by flooding surfaces. These provide acorrelateable "background" stratigraphy with predictable log signature and facies associa-tion. Subsequent sea level drop results in an erosional unconformity and incision into theparasequence "stack", -i.e. an incised valley, and sequence boundary.

The presence of incised valley fills in South Athabasca is interpreted more by the omis-sion of evidence than by direct observation, however. That is, where the "background' stra-

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miles

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Isopach of the "Blue" Parasequence,McMurray Formation

Contour Interval: 2m

0 10 20 30 40 50

> 10m8 - 10m6 - 8m4 - 6m2 - 4m0 - 2m

© 1993 Mike Ranger

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 R4w4

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Paleozoic subcrop

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29

tigraphy is missing, the assump-tion is that these anomalies con-stitute the incised valley fill. Butto reiterate, whereas the para-sequence sets have been well-studied, at least in core, the sup-posed incised valley fills havenot. To date the relationship be-tween the supposed incised val-ley fills and the backgroundshoreface parasequences has notbeen worked out. It has provendifficult to recognise unconformi-ties or to relate incision events toa particular stratigraphic horizon.It is possible that both the 'back-ground' parasequences and theanomalous intervals are simplysub-environments of the samesystems tract; for example, dis-tinctive deltaic shoreface pulsesand more heterogeneous inter-lobe bay fill.

While recognising the uncer-tainties, a highstand para-sequence/lowstand to transgres-sive incised valley fill systemseems a reasonable model for theupper part of the McMurray. What about the rest of the McMurray Fm.? Wireline log signa-tures do seem to suggest that there may be additional highstand parasequences preservedbelow the "Blue Parasequence", at least in south Athabasca (Fig. 18). None have been recog-nised from the middle McMurray outcrops in north Athabasca. The middle McMurray isknown for its thick estuarine channel systems, easily recognised by the almost ubiquitouspresence of sets of IHS point bars, whose architecture makes them easily distinguished inboth core and outcrop. When Flach and Mossop first recognised these for what they are (Flachand Mossop, 1978; Mossop and Flach, 1983) they raised the question of what the channelsystems may have been incising into. With this in mind, Ranger (1994) speculated that thedevelopment of stacked, prograding shoreface parasequences and their subsequent incisionand destruction may have been the theme of the development history during all of McMur-ray time. If they are not present now in the middle and lower part of the McMurray, perhapsthat is simply because the highstand parasequences had very poor preservation potentialearly in the development of the McMurray Fm. During early McMurray time, the influence ofthe sub-Cretaceous unconformity was substantial, and extensive exposed carbonate ridgesseparated the McMurray subbasin into long narrow valleys. Channels would therefore havebeen more confined early in the depositional history of the McMurray Formation and it islikely that the channels would destroy all or most of any existing highstand shoreface bymigrating from valley wall to valley wall.

Figure 20 Standard model for the development ofan incised valley.

30

Later, as the valleys in the basin became filled and the carbonate ridges were buried, shore-face deposits would have had much wider areal development, and thus more potential forpreservation from destruction due to lowstand erosion and incision. This notion is summa-rised in Figure 21. Could it be that facies associations of the middle McMurray are the expres-sion of Incised Valley fill systems, and there really is a developmental continuum through theentire McMurray Fm., modified only by preservational potential? It can be observed on thedistribution map of the “blue” parasequence (Fig. 19), for example, that preservation of theupper McMurray shoreface parasequences is significantly poorer in the north than in thesouth. Perhaps they have been totally eradicated from the record by compound incised valleyfills that dominate the system increasingly towards the north, and therefore in the north Atha-basca area in general one is left only with the complex of channel systems with no back-ground stratigraphic framework.

Middle McMurray: Incised Valley Fill or Progading Tide-Dominated Delta?The middle McMurray has been the unit most studied in outcrop, mainly because it is the

best exposed, has relatively easy accessibility, and it contains the best reservoir facies. In the

Highstand Shoreface ComplexIncised ValleySystem

Stacked, prograding,brackish shoreface

(Parasequences)

Fluvial - estuarine channeland channel fill complex

Highstand Systems Lowstand & Transgressive Systems

Carbonate "Basement"

Schematic Depositional Model for theLower Cretaceous McMurray Formation

Basinward Configuration

Strike Configuration

© 2000 Michael J. Ranger

Figure 21 Incised Valley Fill model for the upper McMurray. Can it be extrapolated to explainthe entire McMurray succession as shown here? Narrow valleys may explain why no highstandshoreface parasequences are preserved.

31

previous chapter, we subdivided the middle McMurray into two dominant facies associa-tions: FA1, thick bedsets of sigmoidal, megarippled sand with rare but unequivocal marinetrace fossil suites and abundant tidal indicators; and FA2, muddy to sandy channel deposits,dominated by IHS point bar architecture, and generally fingerprinted with a brackish tracefossil suite. In previous studies (Mossop and Flach, 1983) including more recent work(Langenberg et al., 2002) these two facies associations were believed to be gradational intoone another such that the megarippled sands of FA1 (trough cross-bedded facies of otherstudies) represent large dune bed forms in the deepest part of a channel, and FA2 (IHS beds)represent lateral accretion of point bars into the same channel. Therefore FA1 and FA2 werethought to be elements of the same genetic unit.

It is true that these two facies association are always closely related, and where present,FA2 always overlies FA1. Yet they are not gradational into one another. Where exposed inoutcrop, the contact be-tween these two facies as-sociations is almost al-ways seen to be separatedby a sharp, often erosional,discontinuity (Fig. 22). Wehave already pointed outthat FA1 contains strongmarine indicators and isnot likely part of a brack-ish estuarine channel. IfFA1 can be interpreted tobe a high tidal energy,outer estuary, and FA2 isa middle estuary channelsystem, then the relation-ship between the twowould seem to be progra-dational and therefore re-gressive. The erosionalcontact between the twois therefore probably aRegressive Surface of Erosion. An unconformable contact is unlikely, since in at least onelocation, slump blocks of FA2 are seen to be entrained in FA1 directly at the contact, a featuredifficult to explain by a process of exhumation.

Such a progradational succession is in conflict with what we know about the internalarchitecture of modern (and other ancient) estuary/incised valley fills. In such systems anincised valley fill is seen to be a Transgressive Systems Tract. i.e. progressively more basin-ward facies, punctuated by ravinement surfaces, fill the valley vertically from the outer throughmiddle part of the valley system, up to some maximum flooding surface (Fig. 23). This is indirect contrast to what we observe in the McMurray Fm. Our observations show that theMcMurray Fm. was a strongly prograding body in an open valley system. FA1 is alwayserosionally or abruptly overlain by FA2. This suggests that FA2 is strongly progradationaland developed in a low accommodation-space basin. Furthermore, the ubiquitous presenceof FA2 over FA1 suggests that together they represent a sedimentological succession, not

Figure 22 FA2 (middle estuary IHS point bar) in erosional contact withFA1 (outer estuary tidal bar complex). FA1 is not simply the basal channelfacies of the IHS channel, but is separated from it by a Regressive Surfaceof Erosion.

32

genetically distinct channel-fill complexes. Furthermore, the erosional contact between them,as well as the ichnofossil evidence would indicate that they are not elements of a single chan-nel. The architectural relationship between FA1 and FA2 is explained by linking them as depo-sitional elements of a tide-dominated delta that originated in the valley low and progradedbasinward. Thus FA1 represents strongly tidal-influenced outer estuarine sediments, and FA2middle estuarine distributary channels.

This may indeed be an unusual example of a regressive rather than transgressive incisedvalley fill given a low accommodation, high sedimentation system. But the classification may

Marine

Shelf Ramp

Segment 1

Outer IVS

Segment 2

Middle IVS

Segment 3

Inner IVS

Non-Incised

Fluvial

Central Basin Bayhead DeltaHST Fluvial

TST Fluvial

Transgressive Surface

Sequence BoundaryLST FluvialTidal Ravinement

SurfacesBarrier/Inlet

LST Shoreline

Transgressive Surface

HST Shoreline

Wave Ravinement Surface

Tidal Ravinement Surfaces

Bayhead Diastem

Incised Valley System

SystemsTract

Ravinement Surface

Sequence BoundaryLowstand

Transgressiveto

Highstand

Tide-Dominated EstuaryTide-Dominated Estuary

Tidal sandbars

Inner straighttidal-fluvial

Alluvialdeposits

Tidalmeanders

UFRsandflats

Transgressiveshelf deposits

Flooding Surface

Figure 23 Established models for Incised Valley Systems. IVS systems tracts are known to betransgressive in nature up to a maximum flooding surface. Middle McMurray facies associations donot appear to be part of a transgressive system, but rather regressive and progradational.

Zaitlin et al. 1994

Dalrymple et al. 1992

33

then be one purely of semantics, since almost by definition such a prograding system wouldbe a delta (Fig. 24).

Fluvial - Inner EstuaryIf FA1 and FA2 represent the outer and middle segment of an estuarine system, it is fair to

ask if an inner estuary/fluvial environment is exposed in the McMurray outcrops. A coarse-grained, apparently fluvial channel crops out along the east bank of the Athabasca River nearthe boundary between Twp 95 & 96 (Fig. 25) The channel incises dramatically into an IHSpoint bar system that is capped with a marsh-paleosol unit. The channel is approximately 170m in apparent width, both sides being well exposed on the banks of the Athabasca River.Paleocurrent measurements indicate flow towards the north-northeast, with point bars dip-

Tidal limit

Fluvial point bars(inner estuary)

Estuarine point bars(middle estuary)

Tidal bars/distributaries(outer estuary)

Delta front

Marsh/Tidal flat

Figure 24 Depositional model for the middle McMurray Fm. in North Athabasca. Progradingtide-dominated deltas developed in valleys entrenched in the sub-Cretaceous unconformity. Thedeltaic deposits are mainly estuarine in nature.

34NO

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35

ping approximately towards the east. (If flow was towards the NNE, the outcrop is not paral-lel to the channel cross-section, and the actual channel width is calculated to be about half theapparent width, i.e. about 85 m). Other similar coarse-grained channel deposits crop out inthe nearby area along the banks of the Athabasca River. In some places these channel depositscan be seen to interfinger with the marsh-paleosol unit that is incised by the main channel.Therefore these units appear to be somewhat contemporaneous (or at least overlap in time)and, we suggest, indicates progradation of an inner estuary fluvial environment into the mid-dle estuary.

WabiskawThe contact of the upper McMurray with the overlying Wabiskaw Mbr of the Clearwater

Formation is a low-relief, erosive contact probably representing a transgressive wave ravine-ment erosional surface. Where preserved, the Wabiskaw sand/silt is a thin, glauconitic, fin-ing-upward unit. It contains an abundant and diverse suite of large trace fossils, indicative offully marine offshore conditions (Fig. 26). TheWabiskaw in Northern Athabasca is probably athin reworked transgressive deposit. Overlyinginterbedded silt and shale of the lower Clear-water Formation shows the influence of storms,and was probably deposited within the middleto lower offshore. The Wabiskaw has a sharp,apparently erosive lower contact with the un-derlying upper McMurray and in some areas isassociated with a Glossifungites ichnofacies(Bechtel, 1996).

Sequence Stratigraphic ModelThe sequence stratigraphic model proposed for the North Athabasca area is summarised

in a series of time slices shown in Fig. 27 on the following page.

Figure 26 Typical examples of bioturbation from the Wabiskaw Mbr. (Clearwater Fm.), which overlies theMcMurray Fm. (A: outcrop; B: core) Trace fossils are large, robust, numerous and diverse, in stark contrastto the typically impoverished assemblages observed in the McMurray Fm.

BA

36

prograding shorefaceflooding surfaces

RSE

RSE

RSETSE

TSE

Tidal Bar Complex

RSE

SB

SB

SB

SB

RSE

RSETSE

SB

RSE

RSETSE

SB

RSE

RSETSE

SB

SB

SB

SOUTH NORTH

© 2003 Michael J. Ranger, Murray K. Gingras

TSE: Transgressive Surface of Erosion RSE: RegressiveSurface of Erosion SB: Sequence Boundary

F. Sea Level Rise - transgression.Development of coarsening/sandier upwardshoreface parasequences, capped by rootedpaleosol horizons. Apparent evolution of basinto wave/storm-dominated deltaic systems.upper McMurray Fm.

G. Sea Level Drop & Sequence Boundary(?).Development of prograding shoreface.Distinctive dark shales suggest basin becomessomewhat anoxic.

H. Sequence Boundary.Unconformity at top of McMurray Fm.Deposition of storm-dominated distalshoreface/proximal offshore. (Wabiskaw Mbr.)

A. Aggradational Parasequence Set.Early fluvial of lower McMurray

C. Further development of tide-dominateddelta. Extensive progradation of middle estuarychannels which truncate tidal bar complex.(Regressive Surface of Erosion)

E. Further development of tide-dominateddelta. Extensive progradation of middleestuary channels.

D. Sea Level Rise - transgression.Subsequent progradational development ofsecond parasequence. Incipient tidal bar complex of tide-dominated delta.

B. Early Transgressive Systems Tract.Development of prograding parasequence.Tidal Bar Complex of tide-dominated delta(incipient estuary system ofmiddle McMurray)

TSE

TSE

TSE

TSE

Figure 27

37

MODERN PHYSIOGRAPHIC ANALOGUES FOR THEMcMURRAY ESTUARINE SYSTEM

Chesapeake BayChesapeake Bay may be a reasonable modern analogue for the McMurray estuarine sys-

tem (at least the upper McMurray), primarily because of its size and geomorphology. Al-though the mouth of Chesapeake Bay is open to the high wave energy marine environment ofthe North Atlantic Ocean, the morphology of the Bay is such that directly landward of itsmouth, the main channel turns abruptly north, so it is sheltered from much of the wave en-ergy of the open Atlantic (Fig. 28).

The entire Chesapeake Bay estuarine system is a drowned system of stream valleys whichevolved during the last glacial stage of the Pleistocene. Similar to the McMurray system, thebay proper follows a master valley (Fig. 29), which developed from both consequent andsubsequent valleys. Modern bay sediments partially fill these valleys so that the bottom to-pography of Chesapeake Bay is simply a modification of the ancient valley topography. Theaccumulation of muds has been primarily in the central channel.

Siltation of the minor tributaries and reentrants is probably related to the tidal movement.


miles

kilometres0 20 40 60 80

0 10 20 30 40 50

DISMAL SWAMP

James

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Eastern M

argin Eroded

Comparison of the McMurray estuarine system with modern Chesapeake Bay to common scale demonstrating size and morphology

Figure 28

38

Suspended silt and clay are driven into these areas during high tide. Because these are areasof relatively stagnant water, deposition takes place and mud accumulates.

Since the close of the Pleistocene epoch, the rate of sedimentation has not been uniformthroughout the bay. The Southern Bay valley has been about 90 per cent filled, the Mid-Bayvalley has been about 50 per cent filled, and the Northern Bay valley has been about 80 per

cent filled.The main source of sediments in the northern

bay area is the Susquehanna River. In the past, anenormous volume of shore-eroded material hasbeen removed from the western shore of the mid-bay area and probably makes up a large portionof mid-bay bottom sediments. The Atlantic Oceanand the large rivers discharging into the south-ern bay area are the main sources of southern baysediments. The largest quantities are probablyfrom the Atlantic Ocean and the Potomac River.

The total volume of sediment that has beendeposited in Chesapeake Bay since the close ofthe Pleistocene is about 61,150,000,000 cubicyards. Assuming that the last glaciers began theirretreat about 10,000 years ago, an average of6,115,000 cubic yards of sediment has been de-posited in the bay each year (Folger, 1972).

Sediment distribution maps (Fig. 30) show thatat the present day the finest sediments are depos-ited in centre of the channel valleys, becomingcoarser towards the banks. This indicates that atpresent, the overall depositional system is coars-ening upwards and progradational. At present sealevel is increasing and approaching a high stand.In relation to the McMurray estuarine system thiswould be equivalent to the development of pro-gradational shoreface parasequences, as recog-nised in South Athabasca, and perhaps analogousto the upper McMurray in northern Athabasca.Subsurface studies of Chesapeake Bay haveshown that there are at least three Quaternary in-cised valley paleochannel systems (Fig. 31, 32) be-neath the present bay (Colman et al., 1990), analo-gous to the incised valley systems currently rec-ognised at Aurora (middle McMurray, "MarineChannel" and "Marine Transition").

Chesapeake Bay has also been studied withrespect to the effects of major storms, in particu-lar that of tropical storm Agnes in 1972 (Chesa-peake Research Consortium Inc., 1976). TheChesapeake Bay watershed received rainfall in ex-

Figure 29

39

Figure 30 Figure 31

40

Figure 32

cess of five inches with many areas receiving more than twelve inches of water over a threeday period of June, 1972. This resulted in immediate flooding of the major tributaries of Chesa-peake Bay, the Susquehanna, Potomac, Rappahannock, York and James Rivers. Most riverscrested at levels higher than previously ever recorded. The Susquehanna River had flowsaveraging 15.5 times greater than normal.

The immediate effect on the estuary was that of the salinities (Fig. 33). Initially, floodwaters displaced surface salinities downstream several miles while bottom salinities remainedsomewhat constant, producing highly stratified estuaries. After a time lag of a few days bot-tom salinities shifted downstream as well, resulting in vertically homogeneous estuaries ofvery low salinity.

Recovery due to a gravitational density reaction followed, causing net transport of saltwater up the estuaries. This recovery started as a basal wedge, but eventually included sur-face water and moved salt water upstream substantially beyond the pre-storm position. Tribu-taries to the bay were subjected to internal oscillations in salinity, dissolved oxygen and sus-pended sediment over periods of four to fifteen days. The entire biological community wasdisrupted to some degree. Sessile bottom dwelling biota experienced severe mortalities. Even-tually vertical mixing between surface and bottom water resulted in a salinity profile throughChesapeake Bay similar to normal conditions. But total recovery took over 100 days (Chesa-peake Research Consortium Inc., 1976).

41

Figure 33

Gironde EstuaryAnother modern analogue that

has been relatively better studiedthan Chesapeake Bay (perhaps be-cause it is much smaller and physi-ographically less complex) is theGironde estuary.

The Gironde estuary has alsoformed in a drowned Pleistocene in-cised fluvial valley. It is presently be-ing filled mostly by fluvial sedi-ment, in a mixed tide/wave domi-nant estuarine environment. TheGironde estuary has the typicalthreefold geomorphology of: 1) amixed fluvial/tidal upper estuarywith meandering channels and tidalpoint bars; 2) a tide-dominated, fun-nel-shaped middle to lower estuarywith non-erosional muddy channelsand elongate sandy tidal bars; and3) a wave and tide-influenced inletwith sandy coastal barriers and tidaldelta shoals. (Allen, 1991)

Tidal effects can occur as far as130 km landward from the estuarymouth. The fluvial sediment load isprimarily deposited in the upper es-tuary channels. A turbidity maxi-mum exists in which suspendedsediment is trapped. The turbidity

maximum migrates longitudinally within the estuary depending on river flow. Only fine-grained sand enters the estuary from the river system. Coarser sands and gravels are blockedby tidal currents.

Facies distribution in the Gironde depends on the morphology of the estuary (Fig. 34).The upper estuary comprises sandy and muddy estuarine point bars. Here, there are no allu-vial levee and crevasse splay deposits, because tidal flow in the estuary attenuates fluvialfloods. The lower estuary is marked by a transition from point bars to tidal bars that progradeseaward over estuarine mud. The estuary inlet contains coarse sand with tide and wave struc-tures. These sands are of marine origin and are introduced into the estuary from the coast bylongshore drift and flood tidal currents. They form the transgressive Holocene substratumover which accumulates the present regressive estuarine wedge.

These facies patterns are easily recognizable in cores and outcrops and may provide ana-logue criteria to help delimit paleogeographic boundaries in cores from the McMurray ofNorth Athabasca. They may also provide stratigraphic markers to identify and correlate thedifferent phases of an estuarine valley fill.

42

Figure 34

43

STRUCTURE OF THE NORTH ATHABASCA AREA

There are several elements that contribute to what appears to be a complex structuralhistory of the area (see Fig. 6). In general, the entire North Athabasca region is structurallyelevated. This is partially due to its position near the edge of the basin, sitting on the rela-tively stable craton of the Canadian Shield relative to the homoclinal Laramide subsidence ofthe Late Cretaceous/Early Tertiary. But it may also have experienced some uplift due to ac-tivity along the Peace River Arch. North Athabasca is directly on trend with the axis of thestructural elements of the Peace River Arch.

Dissolution of underlying Devonian evaporites (mainly salt) is a well documented fea-ture along the eastern edge of the Western Canada Basin. Effects of this process can be ob-served to have occurred more or less continuously in time along the eastern edge of the basinas a whole, since at least early Cretaceous. Regionally, however, not all areas have experi-enced the effects continuously or contemporaneously. It appears that salt dissolution in northAthabasca was more or less complete by Lower Mannville time. This can be demonstrated bythe fact that structural rollover due to salt dissolution plays the major role in the trappingmechanism along the eastern edge of Athabasca Deposit in South Athabasca, whereas in thenorth, bitumen has accumulated far to the east of the present day edge of salt solution col-lapse (Fig. 35A). North of about township 90, almost all of the salt dissolution and conse-quent structural collapse apparently occurred before Mannville time. Therefore any preexist-ing structural collapse would probably have simply been expressed as topographic lows pro-viding accommodation space during accumulation of the McMurray. The overlying Wabis-kaw shale which forms the seal is relatively flat and did not experience the dip reversal seenin the south (Fig. 35B). Furthermore, no deep wells in North Athabasca have penetrated asignificant accumulation of evaporite in the Devonian. (There are however saline springscoming to surface in at least one location -on the east shore of Saline Lake just east of theAthabasca River, and immediately north of the mouth of the Steepbank River.)

At the local scale the major structural element affecting the elevation of the sub-Creta-ceous Unconformity is the erosional topography that resulted from the drainage systems thatincised the Devonian bedrock up until the continent-wide increases in sea level of the LowerCretaceous. The trunk valleys of these drainage systems were evidently themselves control-led in large part by the trend of structural collapse due to salt dissolution. This erosionaltopography obviously played the dominant role in the thickness and facies distribution ofthe lower McMurray, which sits directly on the unconformity. One may also surmise that italso played a role in the distribution of facies in the younger middle and upper McMurrayunits due to differential compaction of the underlying valley fill. No doubt the effects of thesub-Cretaceous Unconformity diminish as younger units accumulated, but its effects are allalways a debatable point, and difficult to quantify.

Because of the overwhelming influence of the erosional topography on the structure ofthe sub-Cretaceous Unconformity, other effects are difficult to differentiate from this struc-ture map alone (Fig. 36). There does appear to be a large, anomalous, structural low (Bitu-mount Basin) centred around Twp 96 Rng 11W4. The structural low is reflected in the Wabis-kaw surface as well. Of course Wabiskaw structure only reflects structural history of thebasin subsequent to Wabiskaw deposition. Earlier structural history can be implied by thefact that evaporite dissolution appears to be complete before Mannville time, and the LowerMannville appears to be overthickened in the Bitumount Basin. The Wabiskaw structure in-

44

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dicates that it underwent at least 50m+ of structural collapse in the Bitumount Basin. Thisstructural collapse can also be seen in outcrop. Along the Athabasca River, just north of themouth of the Ells River immediately East of MicMac, the Wabiskaw Member locally cropsout at river bank level (Fig. 37). This is considerably lower structurally than in any otherknown outcrop along the Athabasca River.

The structural elevation of the basal bitumen-water contact is also useful as an aid instudying the structural history of the region (Fig. 38). For this one must rely on the reasonableimplication that the bitumen-water contact was originally a flat-lying, conventional oil-wa-ter contact that has been frozen in place due to degradation and conversion of the oil intoheavy, viscous bitumen. After the bitumen has become fixed in stratigraphic position, anystructural movement in the basin is reflected in the bitumen-water contact by displacementfrom horizontal. A map of the structure of the bitumen-water contact, not surprisingly, shows

many structural irregularities. Over much of North Athabasca, the bitumen-water contactlies at an elevation of approximately 210 to 220 metres above sea level, but trending some-what higher towards the east (Fig. 38). Over the Bitumount Basin in the northwest, the con-tact drops to generally around 190 m, but as low as 121 m at the well AB01249611. Similarly ineast Aurora there exists an anomalous structural low in section 5 and 8, T96, R11W4. Herealso the bitumen-water contact drops to just below 190 m (apparently corresponding to asubtle low on the Sub-Cretaceous Unconformity). In these areas the timing of the structural

Figure 36


46

collapse is further restricted in time to no earlier than the time of degradation of the bitumen.This was probably at approximately the end of Upper Cretaceous time, very early in thehistory of the Laramide Orogeny (Ranger, 1994).

Figure 37 Outcrop of the Wabiskaw Member on the Athabasca RiverGlauconitic marine siltstone of the Wabiskaw Member is exposed along the shoreline of the Atha-basca River near the eastern border of the MicMac lease site. The Wabiskaw is exposed here atriver level, and marks the southern extent of the "Bitumount Basin".

One question presents itself: what has caused the structural collapse in the North Atha-basca area? Generally this has been relegated to dissolution of underlying Devonian evapor-ites. However, no evaporites have been found in deep wells in North Athabasca and, as dis-cussed previously, it appears that dissolution under the Aurora area (and probably much ofnorth Athabasca) was complete no later than the time of oil migration, trapping and degrada-tion. Yet structural collapse has affected the bitumen-water interface. Furthermore the struc-tural collapse observed is quite localised, which is typical of what would be expected early inthe dissolution history of an area. In later or final stages one might expect isolated pinnaclesrather than isolated lows. Differential thickness of evaporite causing differential structuralcollapse has also been proposed. The most likely occurrence of this would probably involvereefs and bioherms, but here once again, one might expect residual structurally high pinna-cles, rather than the observed residual lows. Two possibilities may be periodic Karsting ofunderlying Devonian carbonate surfaces or localised basement faulting such as small grabens.

47

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ure

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48

THE TIMING AND MECHANISM OFOIL MIGRATION AND TRAPPING

IN THE ATHABASCA OIL SANDS DEPOSIT

IntroductionConsidering that the Athabasca Oil Sands Deposit constitutes what is probably the largest

single hydrocarbon accumulation on earth, there is surprisingly little agreement on its migra-tion and trapping mechanisms or the timing of these events. The aspect that has received themost attention in recent years is the source of the hydrocarbons (du Rouchet, 1985; Moshierand Waples, 1985; Brooks et al., 1988; Brooks et al., 1989; Creaney and Allan, 1990; Allan andCreaney, 1991; Creaney and Allan, 1992). Studies of biomarkers appear to pin down the sourcebeds as dominantly Jurassic in age, with possibly some contribution from source rocks ofTriassic and Mississippian age (Allan and Creaney, 1991; Creaney and Allan, 1992). This im-plies that long distance migration has occurred from a source in the west to a trap in the east,since it is only in the foreland basin trough that potential source beds have been buried deeplyenough (Fig. 39). This theory for the origin for the oil sands is not unanimously accepted, andhistorically in fact has been one of the least acceptable source theories (Corbett, 1955a, 1955b).Various other theories include in-situ formation of bitumen (Ball, 1935; Hume, 1951; Corbett,1955a, 1955b), leaking from underlying Paleozoic reef reservoirs (Sproule, 1938, 1955) andmechanical deposition of the tars eroded from underlying Devonian bitumen (Link, 1951a).These various theories are briefly discussed here.

Previous theories

In Situ Source TheoryAt one time it was suggested that the bitumen is an immature oil that was sourced from

organic matter contained in the enclosing reservoir rocks (Ball, 1935; Hume, 1951; Corbett,1955a, 1955b). A possible alternative chemical source was thought to be humic acids in riversthat existed contemporaneously with deposition of the reservoir rocks (Corbett, 1955a, 1955b).This theory is all but discounted today based on modern studies of petroleum geochemistryand organic maturation. The Athabasca bitumens are not "immature" oils, but degraded con-ventional oils (Deroo et al., 1977), and the reservoir rocks and surrounding possible sourceshales have not experienced conditions necessary for formation of oil (HacqueBard, 1977). Itis certainly possible however, that some of the associated gas is biogenic in origin, obviatingthe need for deeply buried source.

Leaky Reef TheoryWith the discovery in the late 1940's of major petroleum reservoirs in Paleozoic reefs un-

derlying and downdip from the Athabasca Deposit, it was suggested that the Athabasca bitu-mens are the result of oil breaching these reef reservoirs and accumulating in the overlyingLower Cretaceous sands (Sproule, 1938, 1955). The problem with this theory is that massvolumes are totally inadequate. However it is possible that reefs near the sub-Cretaceousunconformity constitute part of the migration path (du Rouchet, 1985). There is commonly noseal between porous carbonates that subcrop at the unconformity and overlying sand reser-voirs, and many of these porous carbonates are themselves bitumen reservoirs.

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Eroded Devonian Tar TheoryBitumen deposits hosted by Devonian carbonates subcrop at the sub-Cretaceous uncon-

formity. Some researchers have suggested that these deposits were eroded during Lower Cre-taceous exposure, and subsequently deposited along with regular sediments (Link, 1951a,1951b, 1954). This theory is untenable because the bitumen, although presently having a den-sity equal to or slightly greater than water, displays a conventional reservoir relationship,that is, gas overlies bitumen which overlies water. The few rare exceptions to this relationshipcan easily be explained by the fact that the bitumen is presently immobile at reservoir condi-tions, and an overlying "paleo-gas cap" has breached its seal and been displaced by water.Furthermore, maturation studies have suggested that by far the dominant period of oil for-mation in the Western Canada Basin is during major subsidence during the Late Cretaceous(Deroo et al., 1977), long after exposure of the Devonian reservoirs at the unconformity. Thebitumen therefore must have accumulated as a conventional oil with density less than water,not as blebs of eroded tar.

Long Distance Migration TheoryIt is generally accepted today that the bitumen in the Athabasca Deposit originated from

source beds downdip in the western foreland basin as these beds subsided through oil win-dow conditions (Deroo et al., 1977; Creaney and Allan, 1990; Creaney and Allan, 1992). Theythen underwent long distance migration to the east, there to be trapped as conventional oils.These conventional oils were then degraded to higher density, high viscosity bitumen throughwater-washing and/or bacterial activity (Deroo et al., 1977; Brooks et al., 1988) Thus, despiteits size, no extraordinary mechanism or chemistry need be invoked to account for the accu-mulation. This theory is not a modern development however. Gussow (1955) came to essen-tially the same conclusions early in the debate on bitumen origin. Much of the modern evi-dence for the source of the bitumen confirms his earlier conclusions.

This short, historical review of the source theories does not do justice to the reasoning,discussion and evidence presented at the time of espousal of the various theories. For de-tailed reviews of the history of ideas on the origin of the Athabasca Deposit see Coneybear(1966), Vigrass (1968) and DeMaison (1977).

Western Source BedsBased purely on structural relationships Gussow (1955) came to the conclusion that the

source beds of the oil sands were primarily Jurassic in age, with possible contributions fromshales of Triassic, Permo-Pennsylvanian and Mississippian age. A primarily Triassic source,with some possible contribution from Jurassic shales, has been suggested for the Peace RiverOil Sands Deposit and, by extension, for all of the Cretaceous oil sands deposits (du Rouchet,1985). The Lower Cretaceous shales have been suggested as possible source beds for the oilsands (Deroo et al., 1973; Deroo et al., 1977; Hacquebard, 1977), given that they are the sameage as the reservoir rocks and are thus stratigraphically directly downdip from the reservoir.Allan and Creaney (1991) have determined that the Lower Cretaceous Joli Fou Shale (Fig. 2)is a regional seal isolating reservoir systems above and below it from each other. Thereforethe only possible Lower Cretaceous source beds must be confined to the Mannville Groupdirectly underlying the Joli Fou shale. However Moshier and Waples (1985) showed that evenusing very optimistic assumptions, the Mannville Group could not have generated the vol-ume of hydrocarbons known to exist in the oil sands deposits of Alberta.

The geochemical classification of oils by Deroo et al. (1977), and more recent work by

51

Allan and Creaney (1991), who used biomarkers to fingerprint oils and source beds through-out the Western Canada Basin, have divided the oils of the basin into distinct families. Corre-lation of these geochemical families to source rock geochemistry, and recognition of discretereservoir systems constitute persuasive evidence that the source of the Lower Cretaceous oilsands and heavy oils is primarily the Jurassic Nordegg Member, and the Mississippian Ex-shaw shale.

Migration MechanismThe driving mechanism of fluid migration for the precursor oils of the oil sands has been

a controversial issue. There are two competing theories for the migration mechanism: com-paction causing expulsion of water and oil either as separate phase or in miscible solution(Gussow, 1955; Vigrass, 1968; DeMaison, 1977) and topographically driven hydrodynamicflow (DeMaison, 1977; Hitchon, 1984; Garven, 1989). The hydrodynamic model depends onthe establishment of thrusting and basin uplift (Laramide orogeny) in the west, which estab-lished the Rocky Mountains and formed a hydraulic head for the major recharge area in theforeland basin.

TimingThe timing of hydrocarbon generation and migration in the Athabasca Deposit must be

discussed in relation to the timing of the Late Cretaceous-Early Tertiary Laramide orogeny,because it is the initiation of the Laramide that produced the deep burial of potential sourcebeds and initiated compaction water flow. Its culmination in the Paleocene caused the upliftin the west to produce the topography that would have made possible basin wide hydrody-namic flow. It is probable that little hydrocarbon generation took place much before Lara-mide, because potential source beds would not have been buried deeply enough to be ex-posed to temperatures that would initiate organic diagenesis (Deroo et al., 1977).

Gussow (1955) deduced that migration and trapping in the Lloydminster heavy oil fields,and by extension the oil sands deposits, took place no later than Lea Park (Campanian) timebased on an analysis of solution gas saturation pressures. Secondary migration would havebeen driven by compaction dewatering and buoyancy effects, because little or no Laramideuplift existed at this time to provide a hydraulic head to drive a hydrodynamic basin system.The hydrodynamic migration model of Garven (1989) would require secondary migrationand trapping to have commenced no earlier than Paleocene when Laramide uplift providedthe topography to drive a basin wide hydrodynamic system. Once established, however hy-drodynamic drive for petroleum migration under this model could have continued throughlate Tertiary until Pliocene time when the regional flow system was disrupted by erosion.

Trapping MechanismThe trapping mechanism for the bitumen in the Athabasca Deposit has also been a contro-

versial topic for many years. The vast size of the deposit did not allow the recognition of atrapping mechanism until many wells had been drilled into the deposit providing a struc-tural data base. In fact Corbett (1955b) used the apparent lack of recognition of a trap asevidence for the in-situ theory of bitumen accumulation as immature oil. Gussow (1955) re-fused to accept the theory, preferring long distance migration and therefore suggested theexistence of a stratigraphic trap. Vigrass (1968) recognised the existence of an updip rolloverand reversal of dip from the regional southwest dipping homoclinal structure of the top ofthe reservoir. This rollover is now a well known feature of the structural setting and is attrib-

52

uted to dissolution of underlying salt from the Middle Devonian Prairie Evaporite. Its exist-ence and proximity to the eastern limit of the bitumen suggests that anticlinal structural clo-sure provided at least part of the trapping mechanism.

The problem with invoking a purely structural trapping mechanism is that bitumen isknown to be trapped downdip on the southwestern flank of the anticline at least 500 metresbelow the level of a distinct bitumen/water contact that exists on the opposite flank of theanticline. It is this that has led to the common belief that structural trapping is not sufficientand that there must be a significant stratigraphic component to the mechanism (DeMaison,1977). As an alternative, Mossop (1980b) suggested that oil initially migrating updip wouldhave undergone biodegradation very early in its migration history, forming a bitumen plugthat prevented further fluid displacement, and creating an updip seal to the trap. Masters(1984) suggested that the trap was entirely structural, and that the anomalies in the distribu-tion of the bitumen are due to structural deformation of the trap after the oil was degradedand "frozen" in place.

Latest Theory

The Nature of the Bitumen/Water ContactRoutine regional mapping of the bitumen/water contact at the base of the Athabasca De-

posit led to new evidence relevant to all three topics discussed above, that is the migration,timing and trapping of the bitumen deposits in the Athabasca Deposit (Ranger 1994). Thereexists a discrete bitumen/water contact under much of the eastern and southwestern portionof the Athabasca Deposit. This contact appears to be a single continuous horizon with only afew rare exceptions where some wells appear to indicate local multiple stacked reservoirswith two or more bitumen/water contacts. The bitumen/water contact is easily recognisedon geophysical logs, especially on the deep resistivity response where it is indicated by eithera sharp or gradual drop in resistivity over a porous reservoir sand. The SP log is also useful,indicating the contact by an increase in potential over water saturated porous sands. In wellswhere the contact horizon intersects a shale facies, the exact depth of the contact cannot easilybe determined, but its existence is evident by bitumen saturated sands above the shale andwater saturated sands below.

The structural elevation of the bitumen/water contact is not horizontal on a regional scale(Fig. 40). A first order observation of its attitude indicates a distinct southwesterly dip insouthwest Athabasca, rising gradually to the north to arch over the Ft. McMurray Area (Town-ship 90). The contact intersects the unconformity over much of the northwest and west-cen-tral part of the basin, which is shown on the map as a brickwork pattern. Over these areasthere is no basal water zone in the Mannville reservoirs. However it is possible that the waterleg may continue through subcropping porous Devonian carbonates. Note also that the con-tact horizon is not perfectly planar, but has many perturbations over its surface (Fig. 40).

An assumption is made here that is critical to the arguments developed from the observa-tions of the bitumen/water contact. That is that the oil was originally trapped in a conven-tional density relationship with gas and water i.e. gas overlying oil overlying water, and thatthe fluid contacts were originally horizontal. These contacts are now frozen in place becauseof the biodegradation of the oil and high viscosity of the bitumen end product.

In almost all instances where gas and or water is present with the bitumen this conven-tional density relationship is observed even though much of the bitumen is presently denserthan water, and therefore the precursor oil originally had a lower density and lower viscosity.

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In some instances a water leg is seen to directly overlie the bitumen. The existence of theseoverlying water zones have occasionally been used as evidence of bitumen with a densitygreater than water migrating into lows in the reservoir (Kidd, 1951). However, it is not un-common to find a water leg below the bitumen as well as above, with no seal separating anyof the fluids (Fig. 41). Thus the bitumen is truly immobile in its current state at reservoirconditions, and it became immobile before its density was reduced below that of the underly-ing formation water. Therefore these overlying water zones cannot be examples of densitysettling of the bitumen. There can be little doubt that these zones represent "paleo-gas caps"that were breached or depressurised after the oil became immobile, allowing water to dis-place the gas. In some cases a remnant gas cap remains above the water (Fig. 41).

The conclusion can be made that the immobilised fluid contacts are in fact paleo-horizonsthat were parallel to paleo-sea level at the time of degradation. The only other explanation is

Figure 41 East-west cross-section through the Athabasca Oil Sands Deposit in township 74, range 5W4.This reservoir shows a normal oil field density relationship of water, oil and gas, except for an additional waterleg between the oil (bitumen) and the gas. Note the presence of porous sand both above and below the bitumen,yet the bitumen has not migrated either upward or downward to displace the water. The water leg above thebitumen was presumably once part of the gas cap, but some of the gas has leaked off, and been displaced bywater. Immobility of the bitumen is a result of two factors: it has a very high viscosity, and the density of thebitumen is very close to that of the formation water.

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that the tilted fluid contacts preserve a hydrodynamic tilt. However the only possible sourcebeds are deeper in the basin towards the southwest, and therefore large scale fluid migrationmust have occurred towards the northeast. But such a hydrodynamic system would haveproduced fluid contacts dipping towards the northeast, opposite to the southwestern dipobserved today. Therefore any present-day deviation from the horizontal represents struc-tural deformation that has affected the reservoir since the time of oil degradation and subse-quent immobility. There is an important implication to this deduction: by correcting the struc-ture of the top of the reservoir so that the bitumen/water contact is flattened, the geometry ofthe trap during accumulation and degradation can be reconstructed.

Mapping TechniquesThe high frequency perturbations on the structure elevation map of the bitumen/water

contact (Fig. 40) are due to a number of different causes. Because the contact horizon is immo-bilised in relation to the enclosing reservoir rocks, any structural dislocation affecting thereservoir will also affect the contact as long as the dislocation occurred after biodegradationof the bitumen. The most common structural anomalies are probably minor faulting and iso-lated collapse due to underlying salt dissolution or karst structure in the underlying Devo-nian carbonates. Also, the bitumen/water contact is not always sharp on the geophysicallogs. In places it may be transitional over several metres, or there may be a stained zone at thebase of the bitumen, masking the precise contact. Transitional or stained zones may resultfrom the loss of volume due to biodegradation or water-washing of the precursor oils. Anoma-lous contact elevations may also simply be error, either in the surveyed ground elevations, orthe logging operation.

Trend analysis of the bitumen/water contact eliminates all of this high frequency noiseand demonstrates the regional trends. One advantage of trend surface analysis is that wherethe bitumen/water contact is truncated by Devonian highs on the sub-Cretaceous uncon-formity, it is possible to extrapolate the trend of the contact through these zones. A simplethird order trend has been used to estimate the surface (Fig. 42). (Trend surface analysis is aspecial case of multiple regression wherein a plane is fitted by least squares to a set of datapoints in 3 dimensional space. A third order trend fits a curved plane that is constrained to nomore than 2 inflections). This trend then serves as a datum from which the regional structureon the top of the reservoir can be corrected.

The regional structure of the reservoir can be estimated by mapping the top of the Wabis-kaw Member (Fig. 43), which is the uppermost of the reservoir units. The top of this unit is amarked by a persistent and readily recognised stratigraphic horizon that has a distinct signa-ture on resistivity logs. This horizon is a widespread shale, which is the seal for the Athabascareservoirs, and thus represents the configuration of the top of the trap. Trend analysis alsofilters out high frequency "noise" on this surface.

Given these topological data, a reconstruction of the reservoir surface can be approxi-mated by simply subtracting the actual elevation of the bitumen/water contact from the el-evation of the Wabiskaw marker. In effect this treats the bitumen/water contact as a datumand thus corrects the reservoir structure so that the datum appears flattened. This provides adetailed representation of the original configuration of the trap. Furthermore some of thepossible errors, such as ground elevation survey errors and any post-degradation faultingand collapse are cancelled out by the subtraction of surfaces. One drawback with this methodis that over much of the western part of the Athabasca Deposit, the bitumen/water contactdoes not exist since it is truncated by highs on the sub-Cretaceous unconformity. Over these

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areas the extrapolated trend of the bitumen/water contact can be substituted for the actualhorizon.

DiscussionThere are two major components to the structure of the top of the Wabiskaw, the primary

being the homoclinal Laramide subsidence to the southwest into the foreland basin. No lesssignificant from an economic point of view is the reversal in structural dip to the east (Fig. 43).This is caused by dissolution of underlying Middle Devonian salt accompanied by structuralcollapse and differential compaction of overlying strata.

TrapThe structure of the Wabiskaw marker corrected by flattening on the bitumen/water con-

tact is shown in Fig. 44. This surface represents the trap structure restored to its configurationat the time the oil was degraded to bitumen and immobilised. The trend of this surface (Fig.45)emphasises the broad regional attitude of the original trap structure.

Since the general southwest dip of the bitumen/water contact is only slightly less thanregional structural dip of the top of the Wabiskaw, the restored trap structure has a muchgentler southwesterly dip and provides a tremendously larger area of structural closure thanexists today (Fig. 46). The configuration of the trap is that of a very broad, very low amplitudeanticline with axis along the northeastern edge of the basin, confirming the existence of whathas been called the "Athabasca Anticline" (Masters, 1984). The maximum east-west closureacross the width of the anticlinal trap extended for over 150 km, but maximum vertical clo-sure was little more than 60 m (Fig. 46b).

The distribution of bitumen in the restored trap indicates that the anticlinal structure is byitself a sufficient trapping mechanism for the southern and central part of the deposit (Fig.48). No stratigraphic component or updip bitumen plug is required at least in the southeast.The northern edge of the anticline is eroded however, and its configuration will never beknown. Although the restored anticlinal structure plunges towards the south-southeast, thenorthern apex shows no reversal of plunge up to the erosional edge. It is generally acceptedthat the McMurray Formation prograded northward into the boreal sea, and it may be sur-mised that it shaled out to the north beyond the present day erosional edge, suggesting theexistence of a northern stratigraphic component to the trap.

One problematic area of the trapping mechanism remains. The northeastern limit of thedeposit is not explained by structural trapping due to salt solution dip reversal. Bitumen isfound far to the east of the present day edge of salt solution collapse (Fig. 47). North of town-ship 90, almost all of the salt dissolution and consequent structural collapse probably oc-curred before Mannville time. Therefore the Wabiskaw shale that forms the seal is relativelyflat and did not experience the salt dissolution dip reversal seen in the south.

How then was the bitumen trapped in the northeast? There is no evidence that the Mc-Murray Formation shales out towards the east. Furthermore, there appears to be an updipgas leg in the area. In crops out along the Christina River, and in nearby wells there are barrensands above the bitumen saturated sands. As discussed above, these can only be evidence ofa paleo-gas leg that has leaked off after degradation of the oil and been displaced by water.Additional, circumstantial evidence of the former existence of gas in these barren zones is thepresence of sulphur precipitate associated with carbonaceous matter in outcrops of the over-lying barren zone on the High Hill River. This infers that hydrogen sulphide (a common

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Figure 38. Trend surface cross-sections through the Athabasca reservoir, approximately perpendicular to the strike of the Laramide subsidence.

A. Present day configuration of the Athabasca reservoir. Note that the bitumen-water contact dips slightly less than the stratigraphic dip of the host reservoir.

B. The structure of Athabasca reservoir, restored to its configuration at the time of trapping of the oil by flattening the bitumen-water contact. The salt dissolution roll-over formed before migration of the bitumen, thus forming the trap.

Figure 46

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component of natural gas) was present in the porous sand at some time in its diagenetichistory. This overlying paleo-gas leg means that the bitumen plug trapping mechanism(Mossop, 1980b) can be discounted since the gas was updip from the bitumen, shared thereservoir, and would have required a trap of its own. Also the bitumen saturated zone doesnot end abruptly by laterally interfingering with unsaturated sand as depicted in diagrams ofthe bitumen plug theory (Mossop, 1980b), but it has what would have been a normal densityrelationship. The barren zone (originally gas) vertically displaces the bitumen as the struc-tural elevation of the reservoir rises.

A simple stratigraphic mechanism can easily explain the trap. During transgression of theClearwater sea, shales overstepped the reservoir sands sealing them by onlap against thePrecambrian Shield (Fig. 49). Outcrop evidence for this no longer exists because erosion at theedge of a tilted basin typically exposes older beds towards the edge and their original con-figuration is destroyed. If this trapping mechanism was indeed the operative one, one wouldperhaps not expect a perfect seal at the edge for a number of reasons. First, as the ClearwaterFormation transgressed and onlapped the Precambrian Shield it is likely to have left a coarsedetrital lag at its base, the thickness of which would depend on the rate of sea level rise, theangle of incline of the shoreface and the amount of detritus available. Second, any porosity inthe Precambrian regolith such as a fracture network or porous clastics could also provideminor conduits through the seal. Either or both of these conditions could have provided apathway for oil or gas seeps along the edge of the basin, when gas still existed and when theoil was still mobile. It is noteworthy that bitumen is very common in fractures and porousclastics of the Precambrian Athabasca Group and in fractures of older basement. These occurboth in outcrop and in boreholes as far away as the eastern shore of Lake Athabasca approxi-mately 150 kilometres northwest of the edge of the Athabasca Oil Sand Deposit (Wilson, 1985).Furthermore, analysis of a single sample of this Precambrian bitumen indicates a composi-tion that compares closely with bitumen from the McMurray Formation (Wilson, 1985, ap-pendix G) but is heavier and presumably more degraded. The amount of bitumen trapped inPrecambrian rocks has never been determined, and its extraction potential, if any, is unknown.However its occurrence can be explained as minor seeps through the stratigraphic pinch-outtrap of the McMurray Formation against the Precambrian Shield (Fig. 50). One can surmisethat much gas also escaped by this route even before the McMurray Formation reservoir wasbreached by erosion.

South of township 90, where the trapping mechanism is structural due to post deposi-tional rollover from salt dissolution collapse, it can be conjectured that the existence of thisstructure was probably not crucial to the trapping mechanism. The same kind of stratigraphicpinch-out of the reservoir, sealed by onlapping shales can be expected further to the east. Ifanything, the salt solution collapse structure probably restricted the areal size of the trap bymany thousands of square kilometres.

TimingThe southwestern dip of the bitumen/water contact is only slightly less than regional

structural dip of the Lower Cretaceous strata (Fig. 46). This dip is due to subsidence of theforeland basin that was caused by Laramide uplift and subsequent tectonic and sedimentloading. Since the immobilised bitumen/water contact has experienced all but a small part ofthis subsidence, it can be concluded that migration, trapping and degradation all were com-pleted very early in Laramide time. Tectonic studies of the western part of the basin suggestthat the earliest effects of the Laramide orogeny were felt in Late Cretaceous time (Porter et

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al., 1982). It is difficult to estimate a more exact upper limit for the time of final immobility ofthe oil-water contact.

The Upper Cretaceous limit on the formation of the oil may also explain the apparentpresent day volume deficit of source rocks for the oil sands of western Canada. Pre-Laramidepalinspastic reconstruction of the western Canada basin (McMechan and Thompson, 1992;Gabrielse and Yorath, 1992) provides evidence of a much larger potential volume of sourcerocks than would have been available later in the history of the Laramide orogeny, whenthrust and translational tectonics shortened the foreland belt by up to 200 km (Gabrielse andYorath, 1992), removing much potential source from the hydrocarbon "kitchen".

It seems apparent that the formation, migration, trapping and degradation of the Atha-basca bitumens must have been all but completed early in the development of the Laramideorogeny, by Late Cretaceous time. However, the onset of oil formation could not have begunmuch earlier than this time, because the burial history of the basin indicates that conditionswere inadequate (Deroo et al., 1977). It follows therefore that the vast volumes of oil requiredto produce the Athabasca bitumen (not to mention the other large oil sands and heavy oildeposits) appear to have been generated and to have migrated in a relatively short period oftime. An explanation for the rapid migration over many hundreds of kilometres of the hugevolumes of hydrocarbon required is beyond the scope of this paper, however it should benoted that time may not be a critical factor for the maturation of organic matter into hydrocar-bon. Price (1983) has concluded that beyond about one million years, time is not a factor inorganic metamorphism. That is, once temperature and pressure conditions reach critical 1evels,organic metamorphism is relatively instantaneous (on the scale of geologic time).

Restrictions on Migration Timing and MechanismVery early in the Laramide orogeny, significant Cordilleran hydraulic head could not have

been present at the time of migration and therefore the large scale, basin ground-water flowof Garven (1989) and Hitchon (1984) could not have been significant at this time. Thus migra-tion was probably driven by compaction and buoyancy. Furthermore, the time window formigration did not extend into the Tertiary as proposed by Garven (1989), but was finished byLate Cretaceous time.

These conclusions have implications and raise more questions regarding the method ofoil migration. Specifically, if the oil migrated by buoyancy, then migration as a separate phaseis probably indicated rather than as a miscible phase where migration is more easily accom-plished by hydrodynamic flow. Here as well, further discussion of this topic is beyond thescope of this paper, but any discussion of migration methods and phases must take into ac-count the conclusion borne out here regarding timing of migration and structural attitude ofthe basin.

GasLarge and small gas accumulations that share the bitumen reservoirs are common in the

Athabasca Deposit. The largest fields are concentrated along the southeastern edge of thedeposit, extending beyond the eastern limit of the bitumen.

Some of the gas was possibly formed biogenically in-situ at an early stage (Deroo et al.,1977), and filled small structural anomalies in the Athabasca reservoir. However, potentialsource rocks would not have been entirely horizontal during the early Laramide, and theyprobably dipped down through immature to mature to overmature windows of organic meta-

68

morphism. Therefore gas could have formed downdip from the zone of contemporaneous oilformation.

The existence of large gas accumulations beyond the eastern limit of the bitumen (Fig. 51)may be evidence for a phase of late gas generation. This area would have been downdip onthe structural trap at the time of bitumen accumulation. However subsequent Laramide tilt-ing of the structure has created new structural traps into which the degraded bitumen couldnot migrate. Any late generation of gas could then accumulate in this updip position of thetilted anticlinal structure.

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DESCRIPTION OF SELECTED FACIES ANDSEDIMENTARY STRUCTURES

FROM THE McMURRAY FORMATION

Common Reservoir and Non-reservoir FaciesThe two dominant facies associations that constitute potential reservoirs are massive-ap-

pearing clean sand, and interbedded sand and mud. The interbedded facies association maybe sand-dominated or mud-dominated. Interbedded facies that contain anything less than 5-10% mud may be considered good reservoir. Bitumen-saturated sands may appear massivein fresh core, but heavy bitumen saturation typically masks all sedimentary structures. Inoutcrop, weathering exposes and accentuates the bedding allowing an interpretation of hy-drodynamic conditions.

Clean Sand FaciesThese make up the most desirable reservoir facies, and may be bedded or massive. The

thickest bedded facies constitute bedsets of half a metre or more in thickness, exhibiting pla-nar-laminated cross-stratification with toeset development indicating that these are high flowregime, sigmoidal, megaripple dunes (Fig. 53, 54). The topset laminae are rarely preserveddue to truncation by succeeding events. These bedsets contain numerous examples of tidalindicators, such as reverse flow ripples, reactivation surfaces and rhythmic grain size cou-plets. Within bedsets, bioturbation is absent. But the truncated, scoured, upper surface ofeach bedset may be capped by a thin shale, a centimetre or less in thickness, and/or a biotur-bated horizon indicating a period of quiescence or abandonment. The ichnofacies suite is lowdiversity, typically consisting of only Cylindrichnus and Skolithos. However, at various loca-tions rare Conichnus have been noted. Conichnus is believed to be the resting trace of a seaanemone, a marine organism intolerant of brackish or fresh water conditions.

Although often interpreted as high energy fluvial channel deposits, the presence of brack-ish as well as rare, fully-marine, trace fossil indicators suggest at least periodic incursion ofmarine conditions. The tidal structures further indicate a strong marine influence. Thereforewe tend to place this facies association in the lower (outer) estuary proximal to the estuarymouth, possibly with flow velocities magnified periodically by ebb tidal discharge.

Mud Breccia FaciesA related facies frequently observed within the clean sand facies association constitute

intervals of massive sand containing brecciated shale clasts varying in size from decimetre ormore down to centimetres or less. Although by itself this would not normally be consideredas potential reservoir, it is most likely indicative of being in close proximity to reservoir facies.These units are believed to be the result of overbank collapse, and probably constitute a de-bris flow of limited areal distribution (Fig. 55). As such they probably reside in the thalweg ofa channel, associated with the sandy, basal channel traction load. The mud clasts are uncon-solidated and once entrained in the traction load, they would quickly become disaggregated,presumably within several tens to hundreds of metres. Therefore they would not be expectedto have much lateral extent in the downstream direction, and even less in the cross-channeldirection.

Surrounding, and/or interbedded with the breccias, one frequently observes intervals ofmassive, structureless sand. These can properly be considered as part of the same debris flowor associated grain flow that entrains the breccias. These massive, clean sands are obviously

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Figure 53. Sigmoidal megaripple bedding (Chris-tina River outcrop). Bedsets are 70cm to 1m in thick-ness even without the topsets preserved. Note back-flow climbing ripples at toesets (A). Topsets are al-ways scoured before deposition of a succeeding bed-set (B). The upper scoured surface of a bedset com-monly displays evidence of a quiescent period, ac-companied by opportunistic colonisation by brackish-tolerant organisms, commonly Cylindrichnus.

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excellent reservoirs, but, as with the breccias, would not be expected to have much lateralextent. Nonetheless, because of their interpreted position within a channel environment, boththe breccias and the massive sands are normally included as indicative of potential reservoirwhen evaluating reserves and planning a recovery scheme.

Inclined Heterolithic Stratification (IHS)Parallel to sub-parallel inclined strata previously known as “epsilon cross-stratification”

(Allen, 1963) or “longitudinal cross-bedding” has been redefined and classified by Thomas etal., (1987) as “Inclined Heterolithic Stratification” (IHS). Thick sets of IHS have been recog-nised in many examples of modern and ancient strata (de Mowbray, 1983; Flach and Mossop1985; MacEachern, 1989; as well as numerous others). IHS develops as lateral accretion de-posits, and is generally interpreted as a migrating point bar (Thomas et al., 1987; Rahmani,1988; Wood, 1989). Many of the modern and ancient examples of IHS invoke a tidal influenceon a fluvial or estuarine system to provide the fluctuating energy regime required to producethe heterogeneous bedding, which typically consists of repetitive sets of mud-sand couplets

Transport downstream (10's to 100's (?) of metres)

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on a scale of centimetres to decimetres. There are examples of purely fluvial settings thatproduce IHS (Jackson, 1978, 1981; Thomas et al., 1987). These systems have seasonal floodcycles producing fluctuating discharge that is probably a factor in the development of themud drapes, although the full mechanism that produces IHS in a fluvial environment is notyet completely understood.

By far the large majority of studied examples of IHS are observed in what is interpreted asmigrating point bars under the influence of tidal effects, whether in a brackish lower estuary,or in the fluvial fresh water reaches of the upper estuary where tides still have an effect on theflow regime (Thomas et al., 1987; Smith, 1988).

IHS is an extremely common bedding configuration in the McMurray Formation, as onecan observe in the outcrops (Fig. 57). In core, “interbedded sand and mud” is one of the mostfrequently observed facies associations, much of which can be recognised as IHS, even in thelimited “window” that core affords (Fig. 56). The classic outcrop example of IHS, and the site

where it was first recognised as an important component of the McMurray Fm., is at theSteepbank River outcrops (see Fig. 7). Here, as well as at several nearby mine face exposures,the channel fills are dominated by IHS. These consists of repetitive sets of decimetre to metrethick couplets of sand and mud, inclined at angles of 8° to 12°, grading into a coarse-grainedtrough cross-bedded sand facies towards the base (Mossop and Flach, 1983). Overlying theIHS beds at Steepbank are horizontally bedded, silty, argillaceous sands and muds that areextensively bioturbated.

Originally interpreted as delta foresets (Carrigy, 1971), it is generally accepted that thissuccession represents lateral accretion of point bars in a channel (Mossop and Flach, 1983;Smith, 1987, 1988). Although Mossop and Flach originally believed the Steepbank IHS to

Figure 56. Examples of Inclined Heterolithic Stratification in core. A: sand-dominated. B: mud-dominated.(Sand beds are bitumen-saturated and black) Note the generally inclined natured of the bedding. True dip istypically a maximum of 10 to 12 degrees.

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represent a generally fining upwards fluvial channel system, subsequent ichnological andsedimentological studies (Pemberton et al., 1982; Smith, 1987) indicated a strong marine in-fluence, suggesting that the channels were indeed brackish estuarine in nature, an environ-ment originally proposed by Stewart and MacCallum (1978). The overlying horizontal andextensively bioturbated argillaceous sand unit is now interpreted as a tidal flat, the “over-bank” expression of an estuarine channel system. The estuarine channel system at the Steep-bank outcrop is 30 to 40 metres in thickness. However the system consists of amalgamatedstacked channel fills that are typically 5 to 8 metres in individual thickness, and which erodeand interpenetrate each other laterally as well as vertically (Fig. 57).

The effects of periodic events that are responsible for high rates of deposition, as well assalinity and turbidity stress are clearly evident in the ichnofaunal assemblages in the IHSbeds. These consist of intervals that are barren of ichnofossils capped by bedding interfacesthat supported an opportunistic population. On the other hand, the ichnofaunal suites thatrepresent the stable, equilibrium population display many of the characteristics of brackishwater assemblages. (See chapter on “Estuarine Ichnology of the Athabasca Deposit.)

Whether the point bar IHS intervals are sand- or mud-dominated probably depends ontheir physiographic position relative to the turbidity maximum in the estuary. However otherfactors may play a role, such as the local sediment source, morphology of the point bar, andrelative strength of currents. From a reservoir point of view, the sand-dominated point barsare clearly to be preferred. But even within the sand-dominated point bars, the presence ofcyclic mud beds may lower the quality of the reservoir. Based on observations of outcrop, the

Figure 57. Inclined Heterolithic Stratification (IHS) at an outcrop on the Steepbank River. IHS constitutesthe point bar bedding of several genetic units interpreted as estuarine channels. Stacked channels resultfrom incision, both vertically and laterally, of earlier generation channels. Note trees at upper right forapproximate scale.

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mud beds can be expected to be relatively continuous from the top of the point bar almostdown to the toe, thus affecting both lateral and vertical permeability. However, the point barstypically do become coarser and/or sandier downwards reflecting increasing energy levelstowards the thalweg of a channel.

Barren “Mud Plug” FaciesMud intervals several metres or tens of metres thick that are barren of ichnofossils are

interpreted as the fill in abandoned estuarine channels, which were periodically flooded withturbid fresh water due to overbank flooding during periods of high fluvial discharge. Suchintervals are relatively common in core, but are not know to exist in natural outcrop, probablybecause of their poor preservation potential once exposed.

Coal/Oxidised Mud FaciesIn the subsurface, coal and organic shale associated with rooted horizons and oxidised

paleosols are frequently observed in core. These are potentially critical horizons in the emerg-ing sequence stratigraphic history of the McMurray Formation. These marsh-paleosol unitscan be observed in outcrop at several locations (Fig. 58), and may be associated with whatappear to be fossilised bog iron horizons.

Figure 58. "Marsh/Paleosol" unit lying on the sub-Cretaceous Unconformity (not visible) 75cm to 1m of verylight oxidised mud is overlain by dark organic shale and thin coal. (Unit closer to the foreground is slumped fromabove.)

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Figure 59. Some of thepotential variations thatmight be observed with tid-ally-generated pinstripes (A)and couplets (B through E).The inset is a subsurface ex-ample that corresponds toB.

Brackish and Tidal Physical Structures

Couplets and PinstripingAmong the most common sedimentary structures associated with tidally-influenced sedi-

mentation are the various expressions of sediment couplets and pinstripe lamination (Fig. 59,60, 61). Although they only provide equivocal evidence of semidiurnal processes, where cou-plets and pinstripes are repeatedly observed they are taken to provide reasonable evidence oftidal influence. Both of these sedimentary structures result from rhythmic fluctuations in cur-rent energy, thereby inducing textural or mineralogical heterogeneity. If attributed to tidalprocesses, the coarser-grained fraction is generally deposited during the flood or ebb of thetidal waters, whereas the finer-grained fraction accumulates during the slack period (the switchfrom tide-flood to -ebb, or vice versa).

Because most tidal channels are either flood- or ebb-dominated, the laminae are com-monly asymmetrical and occur in rhythmic groups of four laminae; these are characteristic oftidal couplets. Although couplets are commonly composed of two sand and two mud lami-nae, they might also comprise sand and organic detritus or quartz sand and heavy-mineralsand.

In pinstripe lamination, the sedimentary laminae are evenly-spaced and laterally continu-ous. In such cases flood and ebb currents are considered to be almost equal. It is worth notingthat although pinstripe lamination is commonly associated with tidal deposition, many othersedimentary processes might be invoked that could explain the presence of similar lamina-tion.

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Figure 60 A. Couplets defined by sand and silty mud. In this example, organic detritus drapes some of thesand beds forming tidal ‘triplets’. The uneven, but rhythmic distribution of the laminae is typical of tidalbundles. Photograph courtesy of Jason Lavigne.

Figure 60 B. Variations in tide height are rarelypreserved in the rock record. For instance, the rhyth-mical thinning and thickening of several tidal bun-dles probably represents tidal fluctuations that areassociated with the phase of the moon (neap/springbundles). New moons and full moons are linked tospring tides, which generate the highest tide heights.Half moons are related to the lowest energy neaptides. Correspondingly, more sediment can be trans-ported during spring tides resulting in a thickening ofthose laminae. Phases of the moon follow a 28 daycycle and the time between spring tides is 14 days.The number of sand laminae in a neap/spring shouldadd up to 28 in a 14 day cycle.

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A B

Figure 61 A and B. Examples of couplets from core of the McMurray Formation.

A. Rythmic lamination produced from semidiurnal (?) fluctuations in depositional current. The apparent bifurca-tion of laminae is due to draping of starved ripples. Note the pinstripe laminae near the top of the sample.

B. Rythmic couplets that show systematic thinning of the couplets forming apparent neap/spring bundles.

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Figure 62. The depositional condi-tions that lead to the development of re-activation surfaces. The important pointsto note are: 1) the necessity of revers-ing depositional currents to modify thebedform following its initiation; 2) the im-plication that one of the currents is domi-nant over the other; and, 3) the local de-velopment of sigmoidal bedding in as-sociation with reactivation surfaces.

Reactivation SurfacesReactivation surfaces and sigmoidal bedding are commonly invoked as evidence for regu-

larly reversing depositional currents. In the case of reactivation surfaces (Fig. 62, 63, 64), bed-forms are first deposited under one current direction and then subsequently modified by asubordinate current that has the opposite flow direction. This process is iterative and resultsin cross-bedded sands with several internal erosional surfaces that have a slightly lower dipthan the sandwave foresets. Sigmoidal beds are commonly associated with these processesand are illustrated in some of the following photographs.

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Figure 63. Reactivation surfaces in outcrop. The dominant current direction is left to right and the subordinatecurrent modified the bedform from the right to left. Note how difficult it might be to identify reactivation surfaces incore.

Figure 64. Reactivation surfaces in outcrop. These are larger than those shown in the previous figure.Couplets have also developed in this bedform. Close inspection of the reactivation surfaces reveals rythmicspacing. It is worth knowing that such spacing can sometimes be related to neap/spring bundles.

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Figure 65. An excellent example of herringbone cross-lamination from an outcrop of the McMurray Formationon the Christina River. The lower set indicates sediment transport to the left and the upper set, which sharplytruncates the lower set, indicates sediment transport to the right.

Herringbone Cross-stratificationHerringbone cross-lamination (Fig. 65) and cross-stratification are probably the most glo-

bally recognized tidal sedimentary structure. As a first principle, this is based on the fact thatthe dip direction of a ripple‘s or subaqueous dune’s foresets indicates the direction of sedi-ment transport. Where foresets truncate each other and indicate opposite flow directions (forexample: one set dips left and the next set dips right), it is commonly inferred that tidal cur-rents were the depositional mechanism at work. The logic is sound but great care must betaken to ensure that the foresets truly dip in opposite directions: this is difficult in two dimen-sional exposures.

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Figure 66. Reverse-flow ripples from an outcrop of the McMurray Formation on the Christina River. The knifeindicates current ripples that are climbing up the foresets of a larger megaripple. The ripples migrated to the rightduring the subordinate tidal flow and the megaripple migrated to the left in response to the dominant tidal current.

Reverse-flow RipplesOther types of tidal sedimentary structure include the various reverse-flow ripples (Fig.

66). These are conceptually like herringbone cross-stratification in that they indicate oppositeflow directions. Reverse-flow ripples consist of small ripples that are climbing back up thedipping foresets of larger bedforms. They are thought to form during the subordinate tidalflow on the down current side of larger bedforms that are shaped by the dominant tidal flow.One should be careful not to confuse these with backflow climbing ripples formed by vorti-ces in the troughs on the lee side of megaripples. These normally form only at the base of, oras an integral part of, the toeset of a megaripple.

85

Figure 67. Synaeresis cracks from a core in the McMur-ray Formation. The cracks shown here are filled with sand.Notably, the extremely impoverished trace-fossil suite at-tests to the brackish nature of synaeresis cracks.

Synaeresis CracksSynaeresis cracks (Fig. 67) are one of the very useful indicators of salinity fluctuation. In

Cretaceous strata of the Western Canada Sedimentary Basin, they have been consistently as-sociated with deposits of brackish affinity. Units that have abundant synaeresis cracks, which

are linked to brackish-water deposits, in-clude the McMurray, Bluesky, Viking, BellyRiver, and the Dunvegan Formations. Theinterpretation that synaeresis cracks arelinked to salinity fluctuations is based onclay volume shrinkage resulting fromchanges in the ionic strength of the depo-sitional waters. The cracks are initiatedwhen salt water comes into contact withfine sediments that accumulated in freshwater. Once they have formed, synaeresiscracks are filled with sediment from above,commonly sand or silt. These sedimentarystructures are similar to desiccation cracks.However, because they form in wet, un-consolidated sediments, as opposed to dry-ing sediments, synaeresis cracks are com-monly compacted after forming and look‘crenulated’; on bedding planes they arerandomly oriented and commonly have agash-like appearance.

There is some debate as to the true na-ture of synaeresis cracks. Some have re-cently been interpreted to be related to tec-tonic events and may be controlled by theinitial substrate consistency. However, theclose association of synaeresis cracks andfluctuating water salinity is well estab-lished.

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ESTUARINE ICHNOLOGY OF THE ATHABASCA DEPOSIT

Several different assemblages of ichnofauna are observed in Athabasca IHS point bars.Typically the sands lack internal burrowing. However the upper surface of the sand bed ofeach sand-mud couplet often contains abundant specimens of the ichnogenera Gyrolithes, anichnofossil with a spiral burrow morphology or Cylindrichnus. These simple, monospecificichnofossil suites represent the dwelling burrows of organisms that colonised the sandy sub-strate of the point bar following deposition of the sand component of the couplet. Both ichno-fossil forms are invariably filled with mud from the overlying mud bed. This assemblage isobserved at the interface between the sand and mud beds, and evidently the environmentalconditions immediately following deposition of the sand were conducive then, and only then,for the colonization of the substrate by fauna exhibiting such burrowing behaviour.

The silty mud beds of the IHS couplets directly overlying the sand are extensively bur-rowed. Individual traces may be difficult to discern, and the bioturbation generally takes theform of disruption of primary bedding in the mud. In other studies of IHS, silty and sandylaminae have been observed in the mud units and interpreted as the diurnal tidal signature ofthe strata (Thomas et al., 1987). In the McMurray mud beds typically no internal beddingremains, although reworked ‘pods’ of silt and fine sand may represent the remnants of suchlaminae. Individual ichnofossils that can be recognized in the mud include the ichnogeneraPlanolites, and Teichichnus, representing the behaviour of deposit feeders.

The cyclic nature of these sand-mud units and their characteristic ichnofossil assemblagesconform well with what is known about the physical characteristics of an estuary. The mudbeds reflect the normal brackish conditions in the estuary; the point bars slowly accumulatemud drapes over a relatively long period due its position within the turbidity maximum,seaward of the head of the salt wedge. The muddy substrate and low energy conditions sup-port elements of the Cruziana ichnofacies, influenced by the variable salinity conditions im-posed by the spring-neap tidal cycles. During periods of high fluvial discharge, which maybe a seasonal flooding phenomenon or a random, intense, storm event , the salt wedge be-comes stratified and is displaced downstream, freshening the subaqueous environment inthe vicinity of the point bar. Accompanying this is an influx of sand entrained in the floodwaters, which accumulates rapidly over only a few days. The rapid deposition and the pres-ence of turbid fresh water precludes the colonisation of the new coarser substrate until netvelocities and sediment transport return to normal rates, which may take several weeks. Thesurface of the sandy substrate can then become the site of an opportunistic colonisation chieflyby the Gyrolithes or Cylindrichnus trace-making organisms. This colonisation would necessar-ily be short-lived because once the turbidity maximum returns to its normal physiographicposition and mud once more begins to accumulate, the substrate is no longer conducive tothe type of behaviour exhibited by the opportunistic fauna; they are literally ‘smothered’, andtheir burrows filled with mud. The faunal population then returns to its equilibrium state:deposit feeders and foragers. In many ways, the equilibrium ichnofauna may also be consid-ered "opportunistic", especially if fluvial flood events are regular seasonal occurrences. Sur-vival in such conditions would require rapid colonisation, rapid reproduction and rapid growover a period of less than one year.

Seaward of the turbidity maximum, the point bar is subject to little mud deposition exceptduring periods of high fluvial discharge, which would displace the turbidity maximum down-stream. Most of the mud beds will be poorly preserved, totally eroded by subsequent events.Where preserved, the muds appear to have supported an abundant, stable, faunal popula-

87

tion. The ichnogenera Asterosoma, Arenicolites, Palaeophycus, Planolites and Skolithos are com-mon, representing both deposit-feeding and dwelling structures. The sands may be locallysourced from erosion of the cutbank, or brought in from seaward direction during majorstorm surges. Therefore, the sand beds may also be the result of episodic events, the equilib-rium state on the point bar being reworking of sand by diurnal tidal currents. The sandscommonly contain an internal monospecific ichnofauna typically consisting of rare, small,Cylindrichnus shafts. This suggests that the sand accumulated relatively slowly, under brack-ish conditions.

Thick, laminated, but otherwise structureless, mud that is almost totally barren of ichno-fossils is interpreted to represent an estuarine channel abandonment mud plug. The onlybioturbation observed in these intervals is associated with a few minor laminae of silt orsand, and consists of almost entirely of Planolites. Following avulsion, an abandoned estua-rine channel meander would slowly become plugged in a similar fashion to an abandonedfluvial meander. The only sediment influx into the resulting pond would be due to turbid,overbank, freshwater flow during flood events. As discussed above, flood events in an estu-ary result from large fluvial discharge, which is accompanied by freshening of the estuary,especially near the surface. The abandoned channel thus periodically becomes flooded withfresh water and suspended sediment, which slowly accumulates over time creating the pre-served plug. The rare bioturbation that is observed may be due to organisms washed in alongwith suspended silt and very fine sand during extreme spring tidal surges, which overwhelmthe fresh water lens in the estuary.

Mud-dominated, estuarine point bar deposits, composed of IHS, are interpreted to haveaccumulated largely within the turbidity maximum of an estuarine system. The mud bedsmay or may not be bioturbated. The bioturbated muds represent the equilibrium state on thepoint bar, which aggraded predominantly by settling of fines out of suspension aided byflocculation of clays. These muds are commonly dominated by burrows of the ichnogeneraCylindrichnus, Planolites and Skolithos. These forms are considered the resident equilibriumpopulation, and represent a mixed Skolithos - Cruziana ichnofacies. The ichnofaunal popula-tion may be abundant yet low in diversity, representing an impoverished marine assemblage.All forms are generally small in size. These features are all characteristics of brackish condi-tions. Barren muds probably represent periods of rapid deposition of flocculated clays set-tling from suspension accompanied by an influx of fresh water, conditions indicative of highfluvial discharge associated with flood conditions.

Examples of Estuarine-Associated Trace Fossils from the Athabasca Oil Sands DepositTrace fossils are both sedimentological and paleontological entities. They represent a unique

blending of potential environmental indicators in the stratigraphic record. Like physical sedi-mentary structures, trace fossils reflect many of the effects of prevalent environmental pa-rameters. To a greater extent than body fossils, trace fossils are a record of the behaviour ofactive, in-situ organisms. The behavioural record of benthic organisms, as dictated or modi-fied by environmental constraints, is thus the mainstay of ichnology.

Biogenic structures may be preserved in many forms, but in core one is concerned mainlywith tracks, trails, burrows, and borings. The objective in this short overview is to show howto recognize estuarine trace fossils that are commonly encountered in Athabasca cores, and topoint out the facies implications of these various structures.

Recent summaries dealing with the recognition of trace fossils in core can be found inChamberlain (1975,1978), Ekdale (1977, 1978, 1980), Ekdale et al. (1984), Bromley (1990), and

88

the papers in Pemberton (1992). Although it is important to be able to identify specific ichno-fossils, it is equally important to be able to differentiate between ichnofossil associations thatmay represent behavioural or “ethological” groups. This distinction has considerable ramifi-cations in delineating estuarine ichnofacies.

Recognition of trace fossils in cores is based mainly on their appearance in vertical cross-section, either on the curved outer surface of an unsplit core or the flat face of a slabbed core.Simple vertical burrows (e.g., Skolithos) will be under-represented in core descriptions, be-cause the probability of a core intersecting a burrow with a long axis parallel to the core is lessthan the probability of the core cutting across a horizontal burrow (e.g., Planolites) that isoriented perpendicular to the core.

Recognition of portions of particular ichnogenera cut at various angles is a practiced skill.Typically, more than one view is essential to assure an accurate identification. For example,an oval in two dimensions could represent the true cross-section of a compacted horizontalburrow, an oblique cut through an uncompacted burrow that actually is circular in cross-section, or just an ovoid clast.

This overview contains schematic drawings and photographs to illustrate the appearanceof several common Athabasca trace fossils in cross-section and at various angles in core. Onemust keep in mind that large traces have to be recognized in parts, and it may be small ordelicate features (e.g. pellets, spreite structure, etc.), that allow for a proper identification.

Key To Abbreviations

Ar Arenicolites

As Asterosoma

Be Bergaueria

Ch Chondrites

Co Conichnus

Cy Cylindrichnus

D Diplocraterion

G Gyrolithes

Pa Palaeophycus

P Planolites

Sk Skolithos

Te Teichichnus

Th Thalassinoides

Esc escape traces

89

ARENICOLITES

Description: Simple, vertical, U-shaped tube with no spreiten between the limbs. Exterior walls

generally smooth with no ornamentation; apertures of one or both tubes may flare. Generally pre-

served in vertical relief, but may be recognized in plan view by paired openings.

Interpretation: Arenicolites is interpreted as the dwelling burrow of an annelid worm or a small

crustacean. Probable originators include the common polychaete Arenicola. A predominately suspen-

sion feeding behaviour has been postulated for the organisms occupying such structures.

Environmental Considerations: Arenicolites is generally associated with sandy substrates in low

energy shoreface or sandy tidal flats. It is a common element of the Skolithos ichnofacies. When found

in great numbers it can be indicative of mixed tidal flats.

Figure 68

90

Figure 69

91

ASTEROSOMA

Description: Star-shaped burrow system consisting of radial, bulbous arms tapering inward to-

wards an elevated centre. The arms tend to be circular in cross-section and consist of concentric lami-

nation of sand and clay, packed around a central tube; the exterior is generally smooth, but may exhibit

longitudinal striae or wrinkles.

Interpretation: Based on the tubular construction of galleries and the details of sediment rework-

ing, Asterosoma has been interpreted as the feeding burrow of a worm. The organism seems to have

probed repeatedly into the sediment to enlarge the gallery and to work more sediment both vertically

and laterally. Exact details of this process remain conjectural. The sediment fill may be related to

feeding/waste disposal functions.

Environmental Considerations: Asterosoma represents a specialized feeding structure and is there-

fore more commonly associated with fully marine conditions. Generally found in the upper part of the

lower shoreface in association with Rosselia. It is a common form of the Cruziana ichnofacies.

Figure 70

92

Figure 71

93

BERGAUERIA

Description: Cylindrical to hemispherical, plug-shaped, vertical burrows with smooth, unorna-

mented walls; circular to elliptical in cross-section; infillings essentially structureless; rounded base

with or without shallow central depression and radial ridges. The length-diameter ratio generally var-

ies from 2:2 to 2:8.

Interpretation: Bergaueria represents the activities of marine anemones. Behaviourally, two inter-

pretations have been postulated representing either a resting trace or dwelling burrow. Both are prob-

ably correct, with lined specimens representing dwellings and unlined specimens (in most cases) rep-

resenting resting traces.

Environmental Considerations: Generally Bergaueria is indicative of normal marine conditions

on a wave- or tide-dominated shoreface. It is a common element in the Skolithos ichnofacies and can

be found in brackish-water estuarine environments but greatly reduced in size.

Figure 72

94

Figure 73

95

CHONDRITES

Description: Chondrites is a complex rootlike burrow system of regularly branching feeding tun-

nels of uniform diameter which never anastamose, interpenetrate, or cut across one another. Branching

typically is in the form of side branches (up to five or six orders), which angle off of a higher order or

main tunnel at 30 40 degrees, rather than bifurcating at Y-shaped junctions.

In core, Chondrites commonly appears as an array of tiny elliptical dots where the vertical face of

the core truncates the numerous branching tunnels. In some instances, longitudinal sections through

individual tunnels and broken portions of branches are exposed, and are diagnostic.

Interpretation: Chondrites is probably produced by a vermiform animal dwelling within the struc-

ture and moving bodily through the sediment.

Environmental Considerations: Well-known to cut across facies. Chondrites is also a common

element of the Cruziana ichnofacies. It represents a complex feeding behaviour and is therefore more

commonly associated with more fully marine conditions. A monospecific association of Chondrites

may be indicative of low oxygen zones.

Figure 74

96

Figure 75

97

CONICHNUS

Description: Conical, amphora-like, or subcylindrical structures oriented perpendicular to bed-

ding; base may be rounded or may exhibit distinct protuberances. Fillings may reveal patterned inter-

nal structures such as chevron laminae but not radial symmetry. The lining, although very thin, consti-

tutes a distinct discontinuity between the infill and the adjacent matrix and is often subject to diage-

netic alteration.

Interpretation: Conichnus has been interpreted as representing dwelling and resting structures of

anemone-like organisms.

Environmental Considerations: Conichnus is generally associated with higher energy, sandy, mid-

dle shoreface environments deposited under more normal marine conditions and is commonly associ-

ated with the Skolithos ichnofacies.

Figure 76

98

Figure 77

99

CYLINDRICHNUS

Description: Cylindrical to conical burrows, straight to gently curved, having multiple concentri-

cally layered walls. Orientations range from vertical to horizontal, but never branched.

Interpretation: The most diagnostic feature of this form is its multiple lining which has been attrib-

uted to the activity of the organism in its burrow, responding to a slow, continuous sedimentation rate.

As sediment entered the burrow, it was pressed against the burrow wall. Multilined burrow walls have

been recognized in structures produced by polychaete worms and crustaceans.

Environmental Considerations: Cylindrichnus is a common element of the Skolithos ichnofacies

and the proximal end of the Cruziana ichnofacies, frequently associated with sandy tidal flats and is a

typical element, with Skolithos, in lateral accretion deposits within estuarine channels.

Figure 78

100

Figure 79

101

DIPLOCRATERION

Description: Vertical, U-shaped spreiten burrows. Apertures of the tubes may be cylindrical or

funnel-shaped; limbs of the U may be parallel or divergent. In some instances they may appear in core

as dumbbell-shaped burrows in plan view. The paired circular openings are joined by a horizontal band

of reworked sediment corresponding to the spreite.

Interpretation: Diplocraterion is the dwelling burrow of a suspension-feeding organism. Probable

originators include polychaete worms, and crustaceans (amphipods).

Environmental Considerations: Diplocraterion is a common element in the distal end of the Sko-

lithos ichnofacies in middle shoreface settings. It is also common on sandy tidal flats and in estuarine

channel deposits.

Figure 80

102

GYROLITHES

Description: Dextrally or sinistrally coiled burrows up to several centimetres high. Whorls are

typically several millimetres in diameter, but commonly less than a millimetre in estuarine environ-

ments. In core, Gyrolithes appears as layers of paired tunnels converging upwards or downwards.

Environmental Considerations: Gyrolithes occurs in wave-dominated bays and splay sands. It has

a tolerance of very low salinity environments and is commonly found in a high density monospecific

assemblage. In brackish environments they may appear as masses of needle-like burrows, whose coiled

nature is only evident under a hand lens. Gyrolithes is commonly found in lateral accretion deposits

within estuarine channels.

Figure 81

103Figure 82

104

PALAEOPHYCUS

Description: Infrequently branched, distinctly lined, cylindrical, horizontal to inclined burrows in

which the sediment fill typically is of the same lithology and texture as the host stratum. Wall linings

may be smooth or longitudinally striated

Interpretation: Palaeophycus is distinguished from the morphologically similar ichnogenus Plan-

olites primarily by wall linings and the character of burrow fills. Fills of Palaeophycus represent pas-

sive, gravity-induced sedimentation within open, lined burrows; the fillings therefore tend to be of the

same composition as the surrounding matrix. Passively filled, lined burrows are typically interpreted

as dwelling structures.

Environmental Considerations: Palaeophycus is associated with the Skolithos ichnofacies in both

high energy and low energy shoreface environments and is commonly found with Planolites or Maca-

ronichnus. It can also be found in episodic storm sands and brackish-water assemblages.

Figure 83

105

Figure 84

106

PLANOLITES

Description: Unlined, rarely branched, straight to tortuous, smooth to irregularly walled or annu-

lated burrows, circular to elliptical in cross-section, of variable dimensions and configurations; fillings

essentially structureless, differing in lithology from the host rock.

Interpretation: Planolites is distinguished from Palaeophycus primarily by having unlined walls

and burrow fills differing in texture from that of the adjacent rock. Fills may differ in fabric, composi-

tion, as well as colour. Fills of Planolites represent sediment processed by the tracemaker, especially

through deposit-feeding activities of bottom-feeders and wormlike organisms.

Environmental Considerations: Non-diagnostic. Found in virtually all environments from fresh-

water to deep marine.

Figure 85

107

Figure 86

108

SKOLITHOS

Description: Single entrance, cylindrical to subcylindrical, straight to curved, vertical to subverti-

cal, unbranched burrows that do not cross over or interpenetrate. The shafts are either lined or unlined

with generally smooth walls. The infill is typically structureless.

Interpretation: In behaviour, Skolithos represents the dwelling burrows of suspension-feeding or-

ganisms or passive carnivores. A multitude of probable originators have been postulated, including

worms and insect larvae.

Environmental Considerations: Lined specimens of Skolithos are generally associated with ma-

rine or brackish environments. It is an element of the Skolithos ichnofacies, but because Skolithos can

be constructed by many different kinds of organisms it is found in virtually every type of environment

from marine to non-marine.

Figure 87

109

Figure 88

110

TEICHICHNUS

Description: Teichichnus appears in slabbed core sections as a vertical series of tightly packed

concave-up or (more rarely) concave-down, laminae. Longitudinal sections show wavy, long laminae

that usually merge upwards at the ends. It is formed by the upward migration of a horizontal to sub-

horizontal tunnel produced by an organism moving back and forth in the same vertical plane, probing

the sediment for food.

Interpretation: The Teichichnus-producing animal appears to be a deposit-feeding, wormlike or-

ganism, which migrated upward in its burrow to keep pace with sedimentation.

Environmental Consideration: Teichichnus is commonly found in lower shoreface to offshore

environments associated with the Cruziana ichnofacies and is prevalent in low energy lagoon/bay

settings characterized by brackish-water conditions. It is never associated with freshwater.

Figure 89

111Figure 90

112

THALASSINOIDES

Description: Relatively large burrow systems consisting of smooth-walled, cylindrical compo-

nents. Branches are Y- or T-shaped and are typically enlarged at points of bifurcation. Burrow dimen-

sions may vary within a given system and cross-sections range from cylindrical, half-moon shaped, to

elliptical. Most systems are essentially horizontal with some irregularly inclined.

Interpretation: Very thinly-lined to essentially unlined burrow systems are characteristic of fine-

grained coherent substrates, in which wall reinforcement is unnecessary. Structureless to parallel-lami-

nated or graded burrow fills represent passive (gravity-induced) sedimentation, whereas meniscate or

chevron-laminated sediments represent active backfilling by the tracemaker. Thalassinoides is gener-

ally regarded as a dwelling and/or feeding burrow of a decapod crustacean (shrimp). Enlarged junction

points are often used as turning points for the organism, or as breeding chambers.

Environmental Consideration: Thalassinoides is associated with the Cruziana ichnofacies in lower

shoreface to offshore environments and may also be found in low diversity, brackish-water associa-

tions.

Figure 91

113

Figure 92

114

ESCAPE TRACES

Sedimentation is the most frequent cause for escape movements. Each new layer of sediment

separates the marine organism living on the sea floor or within the sediment from the oxygen-rich

water above the sea floor. After the animal is covered by sediment it can only survive if it succeeds in

escaping upward and reaching the new surface. Many fossils are perfectly preserved for the very

reason they failed in this attempt.

Escaping locomotion from sedimentation usually causes perturbed burrowing textures. Escape

structures are not consolidated, although gastropods do secrete a mucus layer to crawl on, leaving

behind a near-vertical digging trail. Escape trails may rarely spiral upward. Complete destruction of a

bed may occur when the shells of gastropods cut across bedding layers. In escape traces laminae are

deformed downward compared to water escape structures where laminae may also be convoluted but

are deformed upward.

Figure 93

115

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