Andrew d. Miall Andbrian g. Jones 2003 Fluvial Architecture of the Hawkesbury Sandstone (Triassic),...

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 73, NO. 4, JULY, 2003, P. 531–545Copyright q 2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-531/$03.00

FLUVIAL ARCHITECTURE OF THE HAWKESBURY SANDSTONE (TRIASSIC), NEARSYDNEY, AUSTRALIA

ANDREW D. MIALL1 AND BRIAN G. JONES2

1 Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada2 School of Geosciences, University of Wollongong, Wollongong, New South Wales 2522, Australia

email: [email protected]

ABSTRACT: The Hawkesbury Sandstone has long been assumed to rep-resent the deposits of a large braided river system, comparable in styleand magnitude with the modern Brahmaputra River of Bangladesh.Such an interpretation is based mainly on the common occurrence ofvery large-scale crossbedding, but no architectural studies of the unithave hitherto been carried out. This paper represents a first attemptto estimate the magnitude of Hawkesbury channels and bars on thebasis of the preserved architectural evidence. Photomosaics were con-structed of two cliff sections south of Sydney, one 5.6 km in length. Onthe basis of these profiles we estimate that characteristic channel-scalearchitectural elements are at least 2.7 km wide, and individual macro-forms are 5–10 m high, indicating the constructional depth of typicalchannels. Hollow elements (scoop-shaped units interpreted to haveformed at channel confluences) are up to 20 m deep. These magnitudesare large, but measurably smaller that those of channels and bars inthe modern Brahmaputra River of Bangladesh.

INTRODUCTION

Facies models for fluvial deposits are based on data from modern riversand from ancient deposits, but it has been suggested that existing modelsare of little use because of a lack of primary three-dimensional data frommodern rivers, and because of the limited data base (typically vertical pro-file data or 2-D outcrop data) available for the ancient rock record (Bridge1993b). However, studies of the ancient record play an important role inthe development of facies models because they necessarily focus on whatis actually preserved, an emphasis that is only now becoming possible instudies of modern rivers with the use of high-resolution geophysical tech-niques, such as ground-penetrating radar. Documentation of the preservedarchitecture of fluvial sandbodies is essential for the improvement of ourunderstanding of reservoir heterogeneity (Miall 1988; Weber and VanGeuns 1990).

The Triassic Hawkesbury Sandstone of the Sydney Basin in New SouthWales, Australia, is an ancient fluvial unit that has been little studied buthas been cited as probably the deposit of a very large river system. It isparticularly well known because of the distinctive, very large-scale cross-bedding present in almost every outcrop, notably in cliffs and road cuts inand around Sydney. The sandstone is a craton-sourced unit that was de-posited within the foreland basin adjacent to the New England Fold Belt(Cowan 1993). Regional transport directions were towards the northeast;the unit extends for 225 km in that direction, and occupies a belt 75–100km wide, across depositional strike.

There has been some debate regarding interpretations of the depositionalenvironment of the Hawkesbury Sandstone, including arguments for shal-low-marine (Conolly 1969; Conolly and Ferm 1971) and eolian (Ashleyand Duncan 1977) environments. However, it is now universally agreedthat the Hawkesbury Sandstone is a fluvial deposit. A braided-fluvial en-vironment has been suggested, and analogies have been made with themodern giant braided river, the Brahmaputra (also locally called the Ja-muna), of India and Bangladesh (Conaghan and Jones 1975; Rust and Jones1987). These interpretations have been based, in part, on the very large-scale crossbedding, which has been interpreted as the deposits of largedunes, the scale of which is limited only by the sediment supply and the

size of the channel in which they form (Ashley 1990). A modern faciesanalysis of the Hawkesbury Sandstone in terms of a braided fluvial modelwas carried out by Rust and Jones (1987). In a separate study they analyzeda particularly characteristic facies of the unit, beds of massive sandstone,and developed an interpretation involving bank collapse and liquefactionof the sands (Jones and Rust 1983). Herbert (1997) assigned the Hawkes-bury Sandstone to the lowstand systems tract of a regionally extensivestratigraphic sequence within the Sydney Basin.

Given the long-standing assumption that the Hawkesbury Sandstone rep-resents the deposits of an unusually large river, there is a particularly ob-vious need to explore its architecture, and this paper is offered as a firststudy of the large-scale features of the Hawkesbury river system. The paperbuilds on the facies analysis of Rust and Jones (1987), and is based onstudies of the coastal cliff sections of the Kurnell Peninsula and the RoyalNational Park south of Sydney (Fig. 1), where the section is representativeof the central part of the Hawkesbury depositional system. The focus ofthis paper is on the large-scale architecture. Our data permit an order-of-magnitude comparison between the modern Brahmaputra River depositsand the Hawkesbury Sandstone, but the study also demonstrates that evenwhere very long outcrops, such as that at Kurnell Peninsula are available,making comparisons between large-scale depositional systems is fraughtwith uncertainty.

DATA AND METHODS

This paper is based primarily on the mapping of architectural featuresusing photographic mosaics of large outcrops as base maps. Photographsof the cliff sections of the Kurnell Peninsula (Fig. 2) were taken from achartered fishing boat. The peninsula is ringed by virtually continuous cliffsup to 40 m high showing nearly 100% exposure for a distance of at least6 km. We have constructed a profile more than 5.6 km long from ourphotographs. The scale of this profile is variable, because of the varyingdistance of the boat from the cliff, as it traversed past the various prom-ontories and bays in the cliff face. The cliffs are sheer, and accessibilityfor studies on the ground is poor except at the central part and northernend of this profile. For this reason, traditional field data, such as paleocur-rent readings, are sparse, but where it has been possible to obtain suchreadings they are internally consistent and help to clarify the architecturalinterpretations. Architectural features are defined mainly by photo inter-pretation, with some ground checking where the cliffs are accessible.

A second profile is based on photographs taken from the air and from aboat off the Royal National Park near Curracurrong (Fig. 3). Access to thebase of this profile, along the wave-cut platform at the base of the cliff, isgenerally good. Additional field data were obtained at The Cobblers andThe Waterrun, locations that were described in some detail by Rust andJones (1987). Cliff sections continue south to near Stanwell Park, but theyare largely inaccessible and have not been studied for this paper. Largeroad cuts flank the main highways inland from these coastal sections, andthese formed the source for some of the details described by Rust andJones (1987), but they are now somewhat weathered and have not beenrevisited for this study,

In places, especially near Potter Point, wave spray and wind have main-tained a strip of cliff top tens of meters wide completely bare of soil and

532 A.D. MIALL AND B.G. JONES

FIG. 1.—Project location map.

FIG. 2.—Map of the Kurnell Peninsula area, showing the location of the cliffprofile.

vegetation, offering superb bedding-plane exposures of the HawkesburySandstone.

Classifications of facies and architectural features, including their lettercodes, are adapted, with minor additions, from those of Miall (1996, Chap.4). Paleocurrent readings have been precisely located on cliff photographs.

THE HIERARCHY OF DEPOSITIONAL UNITS AND BOUNDING SURFACES

The concept of a hierarchy of depositional scales and enclosing boundingsurfaces, and the relationship of this hierarchy to depositional processes onvarious time scales and physical scales, was first made explicit by Allen(1983). From his work evolved several approaches to the architectural sub-division of fluvial deposits. Two classifications are compared below. In thisstudy we employ the numerical ranking of Miall (1996). In this system,the rank of a unit is determined by the lowest rank of the surfaces thatenclose it. Bounding surfaces may be of higher rank, where a unit rests onor is cut into by an erosion surface of higher rank, for example a macroform(fourth-order unit) resting on or eroded into by a channel scour surface(fifth-order surface).

Bridge (1993a) Miall (1996)

microscale set (e.g., ripple)mesocsale set (e.g., dune)micro/mesoscale cosetmacroscale inclined stratummacroscale inclined strata setgroup of macroscale setsgroup of macroscale sets

1st-order unit (set)1st-order unit (set)2nd-order unit (coset)3rd-order unit (macroform increment)4th-order unit (macroform element)5th-order unit (channel element)6th-order unit (e.g., channel-belt; sequence)

In the outcrop profiles described in this paper the most prominent bound-ing surfaces, extending for hundreds of meters to a few kilometers alongthe outcrop face, are those that define the major sandbodies (the ‘‘macro-scale sets,’’ or ‘‘major stories’’ of other authors), and are classified as fifth-order in rank. They are here termed ‘‘channel elements.’’ Fifth-order sur-faces are typically visible in weathered cliff faces because of subtle weath-ering differences between coarser sandstone above the surface and finersandstone below. In some cases, fifth-order elements are capped by over-bank mudstones. These may be thin (a few centimeters, or less), but thecontrast in weathering characteristics between a well-cemented sandstoneand even a thin, underlying mudstone accounts for the prominence of somefifth-order surfaces in the cliff outcrops. In some instances a zone ofgroundwater seepage is indicated by a line of iron staining on the cliffsurface, indicating the presence of a porosity–permeability barrier in thesandstone, or the presence of a thin mudstone.

Macroforms constitute the major subdivisions of the fifth-order channelelements and are ranked as fourth-order architectural elements. The mostdistinctive macroforms are those that develop by horizontal accretion, asevidenced by the presence of accretionary cross-bedding extending fromthe top to the base of the unit. The fourth-order surfaces that define thetop surfaces of macroforms are commonly convex-up, curving in the di-rection of local flow downward to become surfaces dipping parallel to the

533FLUVIAL ARCHITECTURE OF HAWKESBURY SANDSTONE

FIG. 3.—Map of the Royal National Park area, showing the location of A) TheCobblers and The Waterrun, and B) the Curracurrong cliff profile.

TABLE 1.—Lithofacies types and lithofacies assemblages in the Hawkesbury Sand-stone near Sydney, Australia.

Stratified sandstone assemblageSs Pebbly sandstone, with siderite-cemented intraclasts. High-order erosion surface at base.St Trough crossbeds; sets are commonly 2–3 m thick (Fig. 4).Sp Large-scale planar crossbed sets; sets are commonly 2–3 m thick; sets up to 7 m have been recorded

(Fig. 5).Sl Low-angle crossbedding, dip at less than the angle of reposeScp Compound planar crossbed sets with rhythmically spaced reactivation surfaces. Dip of foresets is

commonly at less than the angle of repose. Set thickness 1–2 m.Sr Ripples and climbing-ripple cosets. Set thickness ,5 cm.

Massive sandstone assemblageSm Structureless to faintly laminated sandstone (Fig. 6).

Fine-grained assemblageFm Mudstone.Fl Interlaminated mudstone and laminated to rippled sandstone (Fig. 7).Sr Rippled sandstone.

FIG. 4.—Bedding-plane exposure of a large trough-crossbed set (St) forming partof a SD element, south of Cape Solander. Much of the cliff top near Cape Solanderand Potter Point is characterized by a complete lack of vegetation owing to wavespray and wind erosion.

internal accretionary bedding of the macroform. However, many macro-forms are truncated by within-channel erosion, and show flat fouth- or fifth-order upper bounding surfaces. The base of a macroform may be convex-up where it coincides with the top of an underlying macroform, but it istypically flat. Concave-up fourth-order surfaces occur at the bases of hollowelements and minor channels, in which case they define fourth-order units.Fourth-order surfaces normally do not have distinctive weathering char-

acteristics because they do not mark contrasts between distinctive lithofa-cies.

Surfaces of first- to third-order rank are shown but have not been dis-tinguished on the cliff profiles. During this study, only in rare cases couldarchitectural analysis be carried out at the scale small enough to permit thiskind of analysis.

LITHOFACIES

On the basis of their studies of road cuts and the cliffs near Bundeena,Rust and Jones (1987) subdivided the Hawkesbury Sandstone into threelithofacies assemblages. Individual lithofacies within these assemblages arelisted in Table 1. Most of these facies are common in fluvial deposits (andare therefore not described in detail here); the only unusual features are thelarge size of some of the crossbed sets and, unusually for a high-energyfluvial deposit, the lack of sandstone with parting lineation (Sh). The latterfacies is more common in fine- to very fine-grained sands, except underconditions of very high stream power (Allen 1984); its absence may simplyresult from the predominance of medium- to coarse-grained sand in theHawkesbury Sandstone. Some distinctive examples of the range of litho-facies present in the project area are illustrated in the accompanying pho-tographs (Figs. 4–7).

ARCHITECTURAL ELEMENTS

Architectural-element analysis is most useful for identifying what Allen(1983) termed ‘‘packets of genetically related strata,’’ which define depo-


FIG. 5.—Large, simple planar-crossbed set (Sp) at center of view, overlain andunderlain by smaller sets, constituting altogether a simple dune architectural element(SD). South of Cape Solander.

FIG. 6.—Trough-shaped unit filled with massive sandstone (Sm) at center of fieldof view (base indicated with arrows), overlain and underlain by crossbedded sand-stone comprising SD elements. Near Potter Point.

FIG. 7.—Exposure of thin-bedded shale and fine-grained sandstone (Fl), formingpart of a FF(O) element, near Cape Solander. Top of element E, near 3000 m pointin Kurnell Profile.

sitional elements within fluvial systems larger than individual bedforms andsmaller than channels. The techniques are particularly valuable for mappingthe products of bar formation and channel-reach migration—‘‘macro-forms,’’ to use Jackson’s (1975) term. The depositional products of theseprocesses are termed architectural elements. In two-dimensional outcropsthat are largely inaccessible to ground inspection, element interpretationrelies on large-scale architectural features, primarily the configuration ofexternal bounding surfaces and internal stratification. Paleocurrent data canprovide some constraints on the third dimension of individual elements,where it is available. Architectural elements display one of two types ofinternal configuration. Bedding is either tabular, indicating growth by ver-tical aggradation, or characterized by a low-angle dip, indicating that itdeveloped by horizontal accretion. Theoretical or model cross-section di-agrams of braid bars, such as those provided by Willis (1989) and Bridge(1993b), provide a partial basis for element interpretations. A classificationof fluvial elements was provided by Miall (1996), based on earlier workby Allen (1983) and Miall (1985, 1988). Table 2 lists the architecturalelements at the ‘‘macroform’’ scale that have been identified in the Haw-kesbury Sandstone of the project area, and the following notes provideadditional details.

DA (Fig. 8) and LA elements are interpreted as downstream and laterallyaccreting compound bars, respectively, on the basis of the internal relativearrangement of accretion surfaces and Sp and St sets. These elements com-monly grade laterally into each other. Distinction between them is madeon the basis of the orientation of paleoflow directions relative to the ori-entation of accretion surfaces (Miall 1996, fig. 6.24).

Compound dune elements (element CD) are a distinctive component ofthe Hawkesbury Sandstone. Several were observed on the cliff top nearPotter Point, and another was recorded near The Waterrun. One of theseis illustrated in Figure 9. This element type is characterized by downward-climbing crossbed sets, resting on bounding surfaces that commonly aresignificant erosion surfaces. The structure of these elements is comparableto that of class IV tidal sand waves in the series of models developed byAllen (1980).

HO: The hollow element represents the rapid cut and fill of scour hollowsat channel confluences downstream from bars or junctions with tributaries.The distinctiveness of this architectural element was first documented inancient fluvial deposits by Cowan (1991). A facies model for this elementhas been provided by Bristow et al. (1993). The curved base, lacking a flatfloor, and a fill composed of a single coset of Sl dipping at an obliqueangle to the margin of the hollow (Fig. 10), are characteristics that help todistinguish these deposits from channel-fills.

TR(C): This element is not typical of the Hawkesbury Sandstone as a

whole. One example of it was observed, at the Cobblers (Rust and Jones1987) (Fig. 11), and a thin unit is present at the base of the Curracurrongsection (Fig. 12).

FF(C): This element is interpreted as a variant of the channel element(CH) in which the channel is of low energy, possibly undergoing aban-donment, and is filled by fine-grained deposits. Examples occur at TheCobblers (Fig. 11; see also fig. 15 in Rust and Jones 1987) and at Curra-currong (Fig. 12).

FF(O): Similar in composition to FF(C), but organized in extensive tab-ular sheets, representing overbank deposition. Most examples of this ele-ment are thin (a few centimeters), but an element 5 m thick and extendingalong the cliff for nearly 1 km was mapped near Cape Solander (Figs. 7,13, 14), and another element occurs between The Cobblers and The Wa-terrun.

TR(O): occurring as sandstone sheets in overbank settings, includingsingle dune sets, interpreted to represent individual overbank flood events(e.g., at The Cobblers; Fig. 11).

THE PROFILES

Kurnell Peninsula

A diagram showing the bedding traces interpreted from the outcrop pro-file is provided as Figure 13. Details of one of the more accessible portions


TABLE 2.—Architectural elements in the Hawkesbury Sandstone of the Sydney area, Australia.

Code Facies Composition Internal/External Architecture Interpretation

Channel element (fifth-order)CH Any assemblage of Sp, St, Sr. Ss typically at base of

unit. May include component SD, LA, DA units,etc.

Flat base with concave-up margins to element. Upper surface may begradational into overbank elements

Channel with preserved cutbanks

Within-channel elements (fourth-order)SD Single or stacked large-scale Sp sets, with or without

St topsets (Fig. 5)Tabular upper and lower bounding surfaces Field of large simple dunes

LA Stacked St cosets Tabular external form or with convex-up top. Contains accretion surfac-es with dip directions at high angle to orientation of St trough axes

Lateral-accretion deposit

DA Stacked Sp cosets. May be capped by St sets(Fig. 8).

Tabular external form or with convex-up top. Contains accretion surfac-es dipping in same direction as Sp crossbed dip

Downstream-accretion deposit

CD Stacked sets and cosets of Scp (Fig. 9). Tabular bounding surfaces. Rhythmically spaced reactivation surfaces. Compound dune elementHO Single Sl coset dipping obliquely across or down the

centre of the unit (Fig. 10)Scoop-shaped erosional base Formation and fill of erosional hollow formed

at channel confluenceSS Sm (Fig. 6) Irregular, concave-up erosional base Interdune or between-bar deposit, formed by

sediment gravity flowTR(C) Trough and ripple (St and Sr) assemblage (Fig. 12) Tabular sets and cosets. Field of small- to medium-scale bedformsFF(C) Interbedded Sr, Fm, commonly with intraclast mud-

stone breccias (Fig. 11)Channel form (concave-up erosional basal bounding surface) Fill or partial fill of abandoned channel

Overbank elements (fourth-order)FF(O) Mudstone blankets (Fm), with or without minor

sandstone lenses (Fl) and/or sandstone load struc-tures (Fig. 7).

Tabular unit Low-energy floodplain deposit

TR(O) Trough, ripple, and laminated assemblages (St, Sr,Fl)

Tabular unit Minor field of small- to medium-scale bed-forms

FIG. 8.—Panorama of a downstream-accretion element (DA), near Curracurrong. The element rests on a fifth-order bounding surface, and below this is a finer grainedsandstone bed, which accounts for the erosional overhang of the base of the DA unit. The element is capped by a fourth-order surface. Elements are labeled as in Figure17. The scale of this outcrop is indicated by the person, at center (arrow), but the photomosaic was constructed from four photographs all taken from the same point, sothe scale is reduced at either end of the view.

of the cliff section are shown in Figure 14. Structural dip is virtually zeroin the 0 to 5 km section of the profile, which exposes essentially the same40 m of strata. Northward from the 5.1 km point the profile bends north-westward and reveals a gentle northwestward apparent dip.

Interpretation of the rank of the bounding surfaces is dependent on theirshape, lateral extent, and associated lithofacies. A series of prominentbounding surfaces, interpreted to be of fifth-order rank, subdivides the out-crop into 16 major sand bodies. These are referred to here as ‘‘channelelements.’’ The 16 channel elements are lettered A to J and U to Z inFigure 13, from the base to the top of the profile. It is possible that someof these channel elements are laterally separated segments of a single el-ement. Thus the boundary between A and B, at the base of the cliff, cannotbe determined. Several major bounding surfaces plunge into the sea at the

base of the cliff, but not enough of each is exposed to indicate whetherthey are likely to be fifth-order surfaces. Similarly, the boundary betweenchannel elements F and J, at the top of the cliff, has been removed byerosion. D and E could be the same unit, separated into two by the deepincision of channel element J (although their paleocurrent directions aredifferent).

The order of lettering suggests an order of deposition, but in severalcases the suggested order cannot be confirmed by architectural relationshipsand is based only on relative vertical position. For example, the relativetiming of channel elements G and H cannot be known because they areseparated by more than 2 km. The same uncertainty as to the time sequenceis the reason for the separate lettering (U–Z) for the channel elements inthe 4700–5600 m interval of the profile. Channel elements A and U may


FIG. 9.—Panorama of a compound-dune element (CD), showing the rhythmically-spaced reactivation surfaces. On clifftop, near Cape Baily Lighthouse, Kurnell Peninsula.

FIG. 10.—Two examples of hollow elements (HO). In each case the base is shown by white arrows. A) Hollow approximately 30 m wide, with cosets of Sl dipping tothe left, oblique to the line–line of the hollow. Note the sharp erosional truncation of the beds below. Curracurrong. B) A smaller hollow, 18 m wide, filled with Sl setsshowing faint lamination, and dipping to the left. 5100 m interval in Kurnell Profile.

Tabbigai Gap

Yena Gap

Cape Solander

Cape Baily Blue Hole Cape Baily Gap

bend in profile

bend in profile

Fig. 14

Om 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 I600 1700 1800 1900IgOO 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 42y 4300 4400 4500 46,00 4800 4900 5100 5200 5300 5400 -55W 75600 -- ' I -

- 1st- to 3 d a d e r surfacas N

orientation of profile N

- 4thmder surfaces orientation orientation orientation of orientation of orientation of - 5th- and higher-oder surfaws of profile profile profile t of profile t 4 profile

Fig. 13

FIG. 13.-The Kurnell Peninsula profile. shown with X5 vertical exaggeration. A horizontal scale in 100 m increments is provided to facilitate reference to specific points along the profile (measured from an arbitrary zero point at the south end of the profile). The orientation of the profile varies somewhat along the cliff, as indicated by the orientation diagrams at the base of the profile and by the course of the boat shown in Figure 2. The top and bottom of the cliff are indicated by the dotted lines. Fifth-order architectural elements are labeled with capital letters in circles. Two- and three-letter codes refer to the architectural-element classification in Table 2. Paleocurrent arrows indicate local mean directions calculated from readings within individual elements. Orientations are plotted with respect to north directed vertically towards the top of the page, and are shown with number of readings.

Sutherland Point


FIG. 11.—Channel filled by fine-grained deposits of element FF(C), The Cobblers. The photographs from which this panorama was constructed were all taken from thesame location, and the scale diminishes to the right, as the cliff recedes into the distance. The cliff is curved, concave to the viewer, and the profile intersects the cut-bankof the channel on the left and a bank-attached bar at the right end of the field of view. Note the large gutter casts at the base of the overlying channel sandstone.

FIG. 12.—Oblique view along cliff showing sandstone bed consisting of troughcrossbed cosets (St), constituting element TR(C). The sandstone has partly slumpedinto and partly scoured into underlying fine-grained flood deposits (Fl) of an aban-doned channel-fill element FF(C).

be part of the same sand body, as could channel elements C and V. Giventhese caveats, the profile reveals much useful information about the con-struction of the Hawkesbury depositional system. The scales and, whereavailable, the paleocurrent data for each element are given in Table 3. Thelast column in this table indicates the interpreted orientation of the outcroprelative to depositional dip (down-flow) and strike (across-flow).

It is suggested that the 16 channel elements each correspond to the tem-porary position of a major channel within the Hawkesbury depositionalsystem. More than one of these channels may have been active at a giventime. None of the channel elements is completely defined within the avail-able outcrop, despite the unusual length of the profile. This attests to thesize of the Hawkesbury river system. Measured outcrop lengths range from500 m for channel element C to 3200 m for channel element B, but it isknown that element C continues for some distance farther to the northbeyond the end of the profile, and, as noted above, the bounds of elementB are difficult to determine, so these figures are not reliable as indicatorsof maximum and minimum widths. Channel element H probably terminatesa short distance to the south of the end of the profile (it thins to a fewmeters at point 0 m), and at 600 m is the smallest of the ten elements Ato J. Channel element J is bounded at both ends of the profile by relativelysteeply dipping surfaces that are interpreted as channel cutbanks. The topof this element is not defined but, given the typical thickness of the channelelements, at between 18 and 22 m, it seems likely that the 18 m of channelelement J represents most of it. Accordingly, the 1100 m width seems likelyto represent nearly all of this element. Channel element D is 2700 m wide.This element thins near the south end of the profile and may be close to

its termination there. At the north end it is cut out by channel element Jbut, as noted above, it could be a continuation of channel element E (oreven C), which would increase its maximum possible length to more than4 km.

The erosional relief on the fifth-order surfaces provides an indication ofthe scour depth of the Hawkesbury channels, while the vertical height ofidentifiable macroforms, such as DA and LA units, provides indications ofthe height of bars and indicates the minimum depth of channels undergoingdepositional accretion. Deep scours commonly reflect enhanced scour atchannel confluences (Best 1987; Best and Ashworth 1997) and upstreamfrom large bars (Cant 1976). Scour depths at such locations may be threeto six times mean channel depth (Mosley 1976; Cant 1976; Best and Ash-worth 1997). Cut-and-fill erosional relief on the fifth-order surfaces in theKurnell Peninsula profile is commonly as much as 10 m over lateral dis-tances of a few hundred meters or less. The base of surface J rises 20 mbetween the 2820 and 3140 m points of the profile, and slightly morebetween 2820 and 2030. Both margins of this element are relatively steep(not forgetting the vertical exaggeration of Fig. 13), and may correspondto the cutbanks of the original channel. Even steeper cutbanks are presentat the base of channel element G at 3400 and 3580 m. They indicate anerosional relief of up to 12 m on each margin of what appears to be achannel, some 180 m wide, cutting down into a thick overbank unit. Thisprobably represents a channel in the process of avulsion across the flood-plain (the FF(O) element at the top of channel element E) and is orientedat a high angle to the interpreted paleoflow direction of element E. Theselines of evidence suggest that the channel at the base of element G can beinterpreted as a crevasse channel.

Hollow elements (HO) are present in several places along the profile.Two have been identified in the 2100–2200 m interval. One, 10 m deep,appears to form an integral part of the base of channel element D, and theother, a short distance to the north, is truncated by the major surface at thebase of channel element D and clearly predated the main constructionalphase of that element. Two more hollow elements are present at 5100 (Fig.10B).

Where accretionary architectural elements (LA and DA) can be identifiedby stacking of third-order units or by the tracing of fourth-order surfacesthey average about 10 m in thickness. One such element, at the 2330-mpoint in channel element D, is cut by a scour hollow about 4 m deep atits south end.

In summary, the thicknesses of accretionary architectural elements (LAand DA units) indicate that channel bars ranged up to about 10 m in height.This height indicates the minimum constructional depth of the channels,although bankfull depth was undoubtedly somewhat greater. Scour depthsranged from 4 to 20 m.

The relative arrangement of the major channel elements provides someinsights into the larger-scale geomorphic history of the Hawkesbury riversystem. The order in which the channel elements were deposited indicates


FIG. 14.—Example of detailed interpretation of the Kurnell Peninsula profile. Fifth-order elements are indicated by circled letters. Individual paleocurrent readings areshown, with the points of the arrows located at the point of measurement. Orientations are shown with respect to north towards the top of the page.

TABLE 3.—Size and orientation of the major (fifth-order) elements in the KurnellPeninsula profile.

Element Width (m)Max.

Thickness (m)

Paleocurrent

Mean n

InterpretedOutcrop

Orientation

ABCDE

.600?.3200?.500

;2700.900

.8.20.82022

257

297082360

10

539

strike?strikestrikedip

FGHIJ

.1600.800;600

.1300

.1100

.20

.1811

.20

.18

097

112

12

14

?strike??strike

UVWXY

?.1200.600.500.300

.1010131511

135342101095

615

75

?dipdipobliqueoblique

major lateral shifting in position of channels, although as noted above, theorder in which the channel elements have been lettered on the profile maynot correspond to the precise succession of events and, indeed, some ofthe channel elements may represent entirely different portions of the chan-nel system, as the rivers underwent avulsion or river capture. The limitedpaleocurrent data available from the north end of the profile may reflectchannel sinuosity, or it may indicate that the channels underwent majorshifts in transport direction as well as changes in position. For example,the vertical succession of channel elements A, C, E, and G in the 3500–4200 m portions of the profile is accompanied by shifts in mean channeldirection of 408, 638, and 978 from one element to the next. A schematicinterpretation of this succession of channel elements is shown in Figure 15.At the northwest end of the profile the succession of channel elements V,W, X, and Y is accompanied by shifts in mean channel direction of 2068,1208, and 68, respectively.

According to Cowan (1993) the Hawkesbury rivers were, in general,flowing northeastward, from the craton across the Sydney Basin oblique tostructural trends. In detail, however, the data from the Kurnell Peninsula

indicates that mean channel orientations ranged from west-southwest (chan-nel element A) to east-southeast (channel elements J, V). The fifth-orderbounding surface at the base of each element represents channel scour, asa result of lateral channel shifting or avulsion. Both processes are autogenicand are interpreted to have occurred because of local slope advantages onthe alluvial plain. Deposition within a given position of the channel resultedin accretion and aggradation up to a certain critical thickness, beyond whichcrevassing following a flood event or lateral channel-bank erosion led toflow diversion and the initiation of a new channel or channel belt (Jonesand Schumm 1999). The fact that the fifth-order channel elements all havemaximum thicknesses within the 18–22 m range suggests that there was alimit on the Hawkesbury alluvial plain beyond which one or more geo-morphic thresholds was reached, one of which triggered the avulsion.

The steep cutbanks at the base of channel element G (Figs. 13, 15), andthe presence of the thick floodplain unit into which this element has incised,are both unusual features in the Hawkesbury Sandstone. Floodplain depos-its tend to be minor components of braided fluvial systems, but are notunknown (e.g., Reinfelds and Nanson 1993). Channel element CH (Fig.13) is interpreted as a crevasse channel.

Curracurrong

The profile at Curracurrong is illustrated in Figure 16. An example ofthe photomosaic interpretation is shown in Figure 17. The profile is 770m long and up to 35 m high.

The cliff exposes a series of major channel elements bounded by fifth-order surfaces. Ten of these have been defined and identified by the lettersA to J. The large HO elements at the north end of the profile have notbeen given separate letters. Only one of the labeled channel elements, F,is completely exposed in cross section by the cliff. It is 450 m wide andup to 4 m thick. All the other elements are . 770 m wide. Most of theelements are up to 10 m thick. Many consist of two or three macroformssuperimposed vertically or laterally, as indicated by the fourth-order sur-faces in Figure 16. Several of these surfaces are in part convex-up, indi-cating that they are preserved accretionary surfaces little modified by sub-sequent erosion. Ground observations along the base of the cliff indicate


FIG. 15.—Schematic interpretation of the3500–4000 m interval in the Kurnell Peninsulaprofile, showing schematic channel and barreconstructions for the four successive elementsat this location. The front face of each blockdiagram represents the exposure seen in theprofile. Large arrows indicate paleoflowdirection.

that downstream accretion was an important process in the generation ofthese deposits. One of these is illustrated in Figure 8. In this case, individualcrossbed sets indicate growth toward the north (right), in the same directionas the dip of the accretionary surfaces.

Hollow elements (HO) are common in this profile. There is, in particular,a cluster of these elements at the north end of the profile. They are up to10 m thick and 60 m wide, and they are superimposed on each other,indicating repeated scour at bar heads or channel confluences.

In some cases fifth-order channel elements are interpreted as being sep-arated by thin mudstone units, as suggested by the prominence of somesurfaces in the cliff section—they are indicated by the shadow of an over-hang, where a prominent, resistant, basal sandstone unit overlies a recessivemudstone. Elsewhere, sandstone grain-size contrasts across the fifth-ordersurface probably account for the weathering out of the surface. In a fewcases these mudstones or grain-size contrasts are not present, and fifth-order surfaces are then very difficult to trace. In a few cases, as indicatedby the notation FF(O) in the upper parts of channel elements E and G inFigure 16, thicker floodplain units are indicated by darker, recessive inter-vals along the cliff.

An example of the photomosaic from which Figure 16 was drawn isillustrated in Figure 17. Elements are labeled as in Figure 16, and the twolowest channel elements are subdivided into sub-elements based on theirinternal architectural characteristics. Channel element A rests on an exten-sive, recessive fine-grained unit interpreted as the fill of an abandoned

channel (FF(C)). The base of the channel element, sub-element A9, consistsof a relatively massive sandstone capped by a convex-upward surface, in-terpreted as a fourth-order surface capping a macroform unit, probably thedeposit of a mid-channel bar form. Sub-element A0 onlaps the fourth-ordersurface and consists of stratification with a concave-up geometry. It is in-terpreted as an oblique cross section through a channel-fill succession, pos-sibly developed as a result of convergent accretion of bars from oppositebanks of the channel. Sub-element A- is interpreted as a downstream-accretion unit.

Channel element B has been subdivided into four sub-elements. B9 ischaracterized by accretionary stratification, but the details of the bar typecannot be determined. Sub-element B0 is a small unit about 30 m long,with a concave-up base. It is probably a cross section through a chutechannel that cut across the top of sub-element B9. B- is a large down-stream-accretion unit, at least 330 m long. It extends across nearly half ofthe Curracurrong profile, from near the left end of Figure 17 to where itis cut out by the cluster of HO elements near the north end of the profile.Part of this DA unit is illustrated in Figure 8. Sub-element B, is boundedby a concave-up surface, and is interpreted as a hollow element formed ata site of secondary channel confluence.

AN EVALUATION OF THE BRAHMAPUTRA RIVER COMPARISON

Although the Hawkesbury Sandstone has long been interpreted as thedeposit of a large braided river of the scale of the modern Brahmaputra,


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FIG. 17.—Example of detailed photomosaic interpretation of part of the Curracurrong cliff profile. Fifth-order elements are labeled as in Figure 16. Sub-elements areshown with tick marks to facilitate discussion in the text. The DA element illustrated in Figure 8 is a part of element B-, off this photograph to the north (right).

the basis for such an interpretation has always been limited, consistingmainly of remarks about the large scale of the crossbedding (Figs. 4, 5;see Conaghan and Jones 1975; Rust and Jones 1987). Early modern workon the bedforms, bars, and channels of the Brahmaputra River by Coleman(1969) provided some justification for this comparison. From echo-sound-ing profiles, Coleman (1969) identified bedforms he termed ‘‘dunes’’ upto 7.6 m high, and ‘‘sand waves’’ up to 15 m high, although the latter areprobably not simple trains of angle-of-repose bedforms (mesoforms) butthe accretionary tops of bars (macroforms) which may be composed ofstacked mesoforms. Ashworth et al. (2000), recorded dunes up to 3.5 m inheight and they report records of other workers in the river that indicatedunes reaching 6 m in height. A comparison of the facies of the Hawkes-bury Sandstone (Rust and Jones 1987) with those of the modern Brah-maputra river (e.g., Bristow 1987, 1993) reveals many similarities, but sucha comparison is not definitive because the facies of most fluvial systemsare rather universal, reflecting hydraulic conditions at and near the turbulentboundary layer that are not unique to any particular river planform or chan-nel scale. More recent work on the morphology and dynamics of channelsand large bar forms in the modern Brahmaputra River (e.g., Thorne et al.1993; McLelland et al. 1999; Ashworth et al. 2000; Best et al. 2003) formsa better basis for the comparison, because these studies provide more in-formation about the scale and dynamics of the large-scale features of themodern river, the large channels and giant bar forms. It was data of thiskind for the Hawkesbury Sandstone that our research was primarily de-signed to provide. Nevertheless, the discussion presented here points upthe limitations in the respective ancient and modern databases upon whicha comparison between them must be based.

Channels and bars in the Brahmaputra River (or Jamuna River, as it islocally called in Bangladesh) can be classified using a three-fold hierarchy(Bristow 1987). Bars of a given order are scaled to the channel of the sameorder. North of Dhaka the main channel belt (first-order river) is up to 20

km wide, consisting of braided and anastomosing second-order channelstypically 0.5–2 km wide (Fig. 18). Semipermanent islands (first-order bars)up to about 8 km wide and 20 km long occur regularly along the rivercourse. These are composed of amalgamated braid bars of second-orderrank.

Second-order channels are typically 10–12 m deep, but scour depths ofup to 50 m have been recorded (Best and Ashworth 1997). Maximum(bankfull) depths are reached for short periods during each monsoon flood.Bars typically range from half to slightly less than bankfull depth; therefore,in a channel 12 m deep, bars would typically be about 7 m high (C. Bris-tow, personal communication 2001). Second-order bars are 1–2 km wideand 3–6 km long. Ashworth et al. (2000) and Best et al. (2003) describedthe evolution of a second-order bar 4 km long, 1.5 km wide, and 12 mhigh. Width-to-depth ratios at bankfull flow approach 700 (Thorne et al.1993). Observations by Bristow (1987, p. 68) suggest that it is the second-order channels that control the internal stratigraphy of the resulting depos-its. The major sand bodies that result would be fifth-order units accordingto the architectural classification used here, and it is with these that theHawkesbury Sandstone bodies should be compared.

According to Bristow (1987) third-order channels are the product of flowacross second-order bars at falling or low stage. Bristow’s (1993) faciesmodel for bar-top sedimentation in the Brahmaputra River shows channelsof this type approximately 100 m wide and up to 2 m deep. Sub-elementsA9 and B0 in the Curracurrong profile (Fig. 17) may represent channels ofthis kind. The strongly seasonal discharge that characterizes the Brahma-putra River might be represented in the Hawkesbury Sandstone by the unitsof Sm, interpreted by Jones and Rust (1983) as the product of bank collapsefollowing falls in river stage.

Very little information on the architecture of preserved sand bodies inthe Brahmaputra channel belt is available. A first attempt to document thesubsurface of a modern bar complex based on ground-penetrating radar


FIG. 18.—Comparison of the architecture of the Hawkesbury Sandstone at Kurnell Peninsula with the dimensions of channels and bars of the modern Brahmaputra River.At top, a typical reach of the Brahmaputra (Jamuna) River of northern Bangladesh is shown next to a summary of the Kurnell Peninsula outcrop profile reduced to thesame horizontal scale. Numbers 1 and 2 in circles indicate the first- and second-order channels and within-channel bar complexes of the river, as classified by Bristow(1987). Note the scale of the interpreted crevasse channel in the Kurnell Peninsula profile. At bottom, the Kurnell Peninsula profile is shown at the same scale as alongitudinal cross-section through a modern bar in the Brahmaputra River, as reconstructed from GPR data.

data (GPR) was offered by Best et al. (2003), and the discussion providedhere draws on this study and on others that have been carried out on thesurface bed- and bar-forms. The following considerations provide the basisfor an estimate of sand-body dimensions. In its simplest condition the evo-lution of a braided channel can be considered as the development of op-posite-facing low-sinuosity meanders migrating away from a central (mid-channel) bar (Bridge 1993b). The work of Ashworth et al. (2000) explicitlyruled out this mode of evolution in the case of the bar they studied, al-though they made a comparison with the small bar in the Calamus River,Nebraska, analyzed by Bridge et al. (1998), which the latter demonstratedto have grown by a comparable pattern of lateral and downstream accretionfrom an upstream nucleus. Where bar migration is symmetrical, as pro-posed by Bridge (1993b), channel scour would be expected to sweep outan erosional channel form approximating the width of two channels plus

the intervening bar. Assuming two channels of second-order Brahmaputrascale, each 2 km wide, and a mid-channel bar also 2 km wide, if bothchannels were filled prior to abandonment this theoretically could generatea second-order sand body bounded by a fifth-order surface on the order of6 km wide. With an average depth of 12 m such a sand body would havea width-to-depth ratio of 500. If a channel remained in approximately thesame position, undergoing aggradation until it switched elsewhere as aresult of some geomorphic event, the resulting sand body would be evendeeper and wider. However, this scenario is quite speculative. Severalgroups of researchers have demonstrated patterns of active anabranch mi-gration and bar growth and erosion in the Brahmaputra/Jamuna River(Thorne et al. 1993; Ashworth et al. 2000) which indicate that sand bodiesof the full theoretical width estimated here may never develop. Sand bodiesbounded by surfaces of fifth-order rank are likely to be substantially less


FIG. 19.—Comparison of sandbody scalesbetween the modern Brahmaputra River and theHawkesbury Sandstone at the Kurnell Peninsula.The Brahmaputra architecture reconstruction isspeculative, based on data provided by Bristow(1987), Thorne et al. (1993), Ashworth et al.(2000) and Best et al. (2003).

than 6 km wide. The final preserved architecture of sand bodies of the typedescribed by Ashworth et al. (2000) would depend on the balance between(1) lateral growth of the bar under conditions of anabranch migration, and(2A) erosional incision brought about by events of avulsive anabranchswitching or (2B) migration and lateral erosion of an anabranch from an-other location within the channel belt. Final preserved sand body widthsare presumably somewhere between the hypothetical maximum of 6 kmand the width of individual bars—a minimum of 1 km. The bar documentedby Best et al. (2003) is approximately 3 km long and 1 km wide, and itcontains a complex pattern of internal third-order upstream, downstream,and lateral-accretion surfaces (a simplified rendering of this architecture,adapted from Best et al. 2003, is included in Fig. 18).

Even less information is available regarding the lengths of preservedsand bodies in the modern Brahmaputra floodplain. Channel reaches mi-grate and avulse laterally, and given the moderate sinuosity of the modernriver, with channel reaches locally oriented at a high angle to the averagedownstream direction, this means that at least some of the erosional effectsof channel migration and avulsion result in the truncation of sand bodiesalong the downstream trend. Intuitively we might expect sand-body lengthsto be greater than widths, but there is no reason to predict that the down-stream lengths of second-order sand bodies in the modern Brahmaputrafloodplain would have dimensions significantly different than the widths ofthe same sand bodies. Given the dynamic nature of this river, the 3 km 31 km bar mapped by Best et al. (2003) will probably not survive intactinto the geological record but will undergo both further depositional ex-tension and subsequent fragmentation by channel migration and avulsion.

Turning to the Hawkesbury Sandstone: despite the length of the twostudied profiles, data regarding sand body dimensions is very sparse. Theexposures of the sand bodies are at varying orientations with respect topaleoflow, and in most cases this orientation is not known. The outcroplengths of the sand bodies therefore represent lengths, widths, or obliquecross sections (relative to paleoflow) through each channel and bar com-plex.

These arguments about the scale of the modern Brahmaputra sand bodiesand the significance of outcrop lengths of the Hawkesbury Sandstone sandbodies permit only an order-of-magnitude comparison between them. Acomparison with the exposure lengths of the sand bodies in the HawkesburySandstone suggests that the Hawkesbury Sandstone units are comparable,but perhaps smaller (Figs. 18, 19). The largest sand body for which thewidth is reasonably certain is channel element D, at ; 2.7 km wide, in

the Kurnell profile. The extremely limited paleocurrent data available fromthe crossbedding in this body suggests that the Kurnell Profile is orientedapproximately perpendicular to the flow direction within this channel, sothat the 2.7 km figure is an approximate measure of sand body width. Thisis in the middle of the range of possible widths of a typical second-orderBrahmaputra sand body (Fig. 19). Other Hawkesbury Sandstone bodiesappear to be somewhat smaller, although most are incomplete (Table 3).

Fifth-order channel elements in the Hawkesbury Sandstone are typically18–20 m thick, which is considerably thicker than the average channeldepth of 12 m for the Brahmaputra, but most of the Hawkesbury Sandstonechannel elements consist of two or three superimposed macroforms, each5–10 m thick, which again suggests a scale for constructional channel depthcomparable but perhaps slightly smaller than that of the Brahmaputra. Bris-tow (1987) noted that macroforms in the Brahmaputra River typically rangefrom ‘‘between one-half and just less than total bankfull depth.’’ Scourdepths in the Hawkesbury Sandstone, as indicated by the height of HOelements, range up to about 20 m, less than half the maximum 50 m scourdepths recorded in the Brahmaputra. However, some of the HO elementsmay be top-truncated, and some may represent scour at anabranch conflu-ences rather than the junction between major tributaries. Note that the in-terpreted crevasse channel in the Kurnell Peninsula profile (channel elementG in Fig. 13) is comparable in scale to the smallest of the channels shownin the map of the modern Brahmaputra River (Fig. 18).

The conclusion is that earlier interpretations of the Hawkesbury Sand-stone as the deposit of a very large river system are correct. The majorparameters that can be estimated for the Hawkesbury Sandstone rivers—channel width, constructional channel depth, and scour depth—show thatmaximum values are within the mid range of those of the modern Brah-maputra/Jamuna, as estimated from modern channel and bar patterns innorthern Bangladesh, suggesting that the Hawkesbury river system waslarge but measurably smaller than the modern Brahmaputra. However, eventhe availability of the exceptionally long Kurnell Peninsula profile is ade-quate only as a basis for tentative interpretations of scale, given the largesize of the ancient rivers that deposited the Hawkesbury Sandstone.

The 75–100 km preserved width of the Hawkesbury Sandstone deposi-tional system indicates that the river or rivers responsible for its depositionwere free to comb across a wide, flat, alluvial plain. The marked changesin paleocurrent direction recorded in successive channel-fill deposits in theCurracurrong and Kurnell sections attest to the lateral mobility of the riversand the very low depositional gradient in this part of the Sydney Basin.


Farther north, marine incursions have been recorded within the HawkesburySandstone (e.g., Herbert 1997), but they are not known to extend into thecentral part of the basin. Judging by the occurrence of compound dune(CD) elements, it seems likely that the Hawkesbury rivers were in parttidally influenced within our project area, but we have not yet been ableto discern any stratigraphically consistent patterns of occurrence of thiselement in the study area that would permit an interpretation in terms ofchanging accommodation and transgression–regression of the kind thatShanley and McCabe (1994) used in their sequence model for non-marinedeposits.

In conclusion, this preliminary architectural study of the HawkesburySandstone has demonstrated that in the project area south of Sydney, chan-nel fills and their component bars were large but measurably smaller inwidth and depth than those in the modern Brahmaputra River. Further studyis required to explore the regional consistency of these estimates.

FURTHER QUESTIONS

One of the outcomes of this study has been a realization of the limitationsof the databases upon which modern-to-ancient comparisons are based inthe fluvial realm. While individual lithofacies and vertical profiles throughbar and channel deposits may readily be compared—and have been doneso repeatedly since the development of the facies-model concept in the1960s—such comparisons are limited in their value by the nearly universalnature of such deposits. Bedform character reflects the common physics ofturbulent flow (Ashley 1990), and bar development is increasingly becom-ing recognized as a largely scale-independent process. However, the data-base on large-scale fluvial architecture, especially sandbody width andlength, remains extremely small, and more studies of this type (e.g., seeRobinson and McCabe 1997) need to be carried out.

While the Hawkesbury Sandstone remains a focus of interest because ofits outstanding outcrop character as the deposit of a large river, the natureof its facies and architectural assemblage raises questions about how typicalthis unit is as the deposit of a large braided fluvial system. Sandstone withprimary current lineation (lithofacies Sh) is virtually absent, possibly re-flecting a particular combination of high stream power and coarse grainsize, whereas the hollow element (element HO), a product of scour atchannel confluences, might have been expected to be much more abundantin a river system as vigorous as the one that deposited the HawkesburySandstone is interpreted to have been. We can only ask why, while notingthat the recognition of scour-fill deposits (the HO architectural element) isa relatively recent addition to our standard suite of fluvial units.

In addition to a need for further bar- and channel-scale architecturalanalysis it would be instructive to investigate the details of the internalsequence stratigraphy of the unit, to determine whether the occurrence oftidal influences reveals any systematic regional or vertical (time-related)variations in accommodation during Hawkesbury sedimentation. Such re-gional studies are also required to evaluate the significance of the fine-grained channel fill and thick ripple-cross-stratified units exposed at TheCobblers. These are the only locations in the project area where such faciesare exposed, and they may also provide important information on accom-modation changes if they can be situated within a regional stratigraphiccontext.

ACKNOWLEDGMENTS

This research was supported by the Natural Science and Engineering ResearchCouncil, Canada (ADM) and the Sustainable Earth Research Centre at the Universityof Wollongong (BGJ). The comments and suggestions on various versions of thispaper by Jim Best, Charlie Bristow, Brian Willis, Colin North, Peter Friend, andPhil Ashworth have led to many useful refinements and, we hope, improvements inthe final product. Our thanks to all of these individuals. Thanks are due to CharleneMiall for assistance in the field.

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Received 17 June 2002; accepted 15 November 2002.

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