The Lower Silurian Sayabec Formation in northern Gasp6 ...uregina.ca/~chiguox/s/2001 Lavoie and Chi...

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 49, NO. 2 (JUNE, 2001), R 282-298

The Lower Silurian Sayabec Formation in northern Gasp6: carbonate diagenesis and reservoir potential

DENIS LAVOIE AND GUOXIANG CHI

Geological Survey of Canada (Quebec) Centre Gdoscientifique de Qudbec 880 Chemin Sainte-Foy, C.P. 7500

Qudbec, QC G1V 4C7

ABSTRACT

The Lower Silurian Sayabec Formation represents a peritidal-dominated carbonate ramp that developed at the northern edge of the post-Taconian Gasp6 successor basin. In the Late Silurian, during the Salinic disturbance, the Sayabec ramp was subaefially exposed locally. This could have lead to the formation of economically significant secondary dissolution porosity. A detailed diagenetic study of the Sayabec Formation was carried out at selected localities along the Northern Outcrop Belt in the Gasp6 Peninsula, where the Salinic unconformity and hydrothermal alteration of the carbonate facies have been documented.

The diagenetic history consists of initial minor marine diagenesis (marine cements in boundstones and neptunian dykes) followed by pervasive burial diagenesis that resulted in the emplacement of various pore- and fracture-filling calcite cements, due to the mixing of basinal brines and hydrothermal fluids. Late Silurian tectonic exhumation of the lithifled carbonate ramp is recorded locally in meteoric-cement-filled fractures that were dissolution-enhanced after early burial. The significance of this event in generating porosity was relatively minor.

Preserved porosity is observed where limestone facies and calcite cements were completely replaced by hydrothermal saddle dolomite. However, the porous dolostone is of geographically limited extent. The hydrothermal event is mostly recorded in high-temperature calcite cements that occlude burial fractures.

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La Formation silurienne infrrieure de Sayabec reprrsente une rampe ~t carbonates ~ dominance prfitidale qui bordait la limite nord du bassin successeur post-taconien de la Gasprsie. Cette rampe fut, au Silufien tardif, localement exposre ~t des conditions sub-arfiennes (Discordance Salinique) menant h la formation d'une porosit6 secondaire de dissolution possiblement 6conomiquement importante. Une 6tude diagrnrtique drtaillre du Sayabec fut menre pour des sites reprrsentatifs le long de la Bande du Nord en Gasprsie o0 la discordance Salinique est bien documentre et o0 6gale- ment une altrration hydrothermale des facibs h carbonates est connue.

L'histoire diagrnrtique consiste en une diagen~se marine rnineure (ciments marins dans des bioconstructions et dykes neptuniens), suivie d'un syst~me diagrnrtique d'enfouissement enregistr6 dans les grnrrations de calcite de rem- plissage de pores et de fractures reprrsentant localement un mrlange de saumures de bassin et de fluides hydrother- maux. L'exhumation tectonique au Silurien tardif de la rampe h carbonates drjh lithifire fut localement enregistrre grace ~ la prrsence d'un 6pisode de fracturation/dissolution ciment6 par des calcites mrtroriques qui a suivi un enfouissement initial. L'importance de cet 6vfnement pour le drveloppement de porosit6 fut mineure. Une porosit6 actuelle s'observe localement due ~ un remplacement total des faci~scalcaires et des ciments calcitiques par une dolomite baroque d'origine hydrothermale. Cette derni~re poss~de cependant une extension grographique limitre. L'rvrnement hydrothermal fut principalement enregistr6 dans des ciments de calcite de hautes temprratures colmatant les fractures d'enfouissement.

Traduit par les auteurs.

282

THE LOWER SILURIAN SAYABEC FORMATION 283

INTRODUCTION

The Paleozoic sequence in the Acadian Gasp6 Belt (Bourque et al., 1995) of the northern Appalachian Orogen (Bourque et al., 2001a, Figs. l, 2, this issue) contains few shallow-water carbonates. The notable exception to this is the occurrence of reef and carbonate complexes that developed during the Late Llandoverian-Wenlockian and the Ludlovian-Lochkovian. Limestone facies of the former, including microbial laminites, stromatolitic limestones, biohermal and biostromal boundstones, well-bedded packstones and grainstones, and muddy nodular limestones, have been described from various parts of the Gasp6 Peninsula and northern New Brunswick (Fig. 1) (Hrroux, 1975; Bourque, 1975; Noble, 1976; Lee and Noble, 1977; Bourque and Lachambre, 1980; Desrochers, 1981; Lachambre, 1987; Lavoie, 1988; Lavoie et al., 1992). All these occurrences are within two formations, the Sayabec Formation in northern Gasp6 Peninsula and Mataprdia Valley, and the La Vieille Formation in southern Gasp6 Peninsula and northern New Brunswick. The Lower Silurian Sayabec and La Vieille formations represent the first occurrence of shallow-water limestones in the Paleozoic succession of the northern segment of the Appalachian Orogen (Bourque et al., 2001a, this issue). They have been interpreted, on the basis of palinspastic recon- struction, as part of the same carbonate ramp that developed along the margin of the Qurbec Re-entrant and St. Lawrence Promontory (Lavoie et al., 1992; Bourque et al., 1995; Bourque et al., 2001a, Fig. 5c, this issue). Various aspects of the sedimentology, paleobiological communities, and paleogeography of peritidal-dominated Sayabec/La Vieille ramps have been published elsewhere (Bourque et al., 1986; Lavoie, 1988; Desrochers and Bourque, 1989; Lavoie et al., 1992; Bourque et al., 1995). Both units have similar facies and overall sequence evolution (Lavoie, 1988; Lavoie et al., 1992).

The general diagenetic evolution of the Sayabec and La Vieille formations is characterized by little marine cementation followed by burial diagenesis (Lavoie, 1988; Lavoie and Bourque, 1993). A significant Upper Silurian (Pfidolian) meteoric diagenetic event has been recognized in the La Vieille Formation in southern Gasp6 Peninsula (Lavoie and Bourque, 1993). This event, related to the combined effects of tectonic uplift, relative sea-level lowstand, and erosion (Salinic unconformity), led to the generation of significant secondary porosity that was filled ablruptly by meteoric calcite cement (Lavoie and Bourque, 1993). This event also affected the lower part of the Ludlovian-Pridolian reef complex in southern Gasp6 (Bourque et al., 2001b, this issue). Although the Salinic unconformity truncates the Sayabec Formation in northern Gasp6 Peninsula (Lachambre, 1987; Lavoie et al., 1992), its significance in porosity generation has not been documented previously. The limited diagenetic information of previous studies of this segment of the Gasp6 Peninsula has focused only on the burial diagenetic trend.

This paper aims to improve our understanding of the diagenetic evolution of the Sayabec Formation in the Silurian-Devonian Northern Outcrop Belt (sensu Bourque et al., 1995) of the Gasp6 Peninsula (Fig. 1). In particular, and as observed for the La Vieille Formation in southern Gasp6 (Lavoie and Bourque, 1993), the possible diagenetic effects of the Salinic disturbance on porosity will be addressed. As a whole, this paper aims to document the diagenetic history and fluid evolution recorded in cement in order to evaluate the hydrocarbon reservoir potential of the northern segment of the Gasp6 Peninsula.

PALEOGEOGRAPHIC AND ENVIRONMENTAL SETTING

In the Gasp6 Belt, the Sayabec and La Vieille carbonate ramps encompass approximately 10,000 and 20,000 km 2, respectively. These ramps developed at the margin of the Qu6bec Re-entrant and St. Lawrence Promontory (Bourque et al., 2001a, Fig. 5c, this issue). The Sayabec and La Vieille formations constitute a thin limestone unit in an otherwise thick Silurian siliciclastic succession (Bourque et al., 2001a, Fig. 2, this issue). The Sayabec attains a thickness of nearly 320 m, whereas the La Vieille at its thickest comprises only 180 m of the 2500 to 4000 m thick Silurian succession of the northern and southern parts of the Gasp6 Belt. Carbonate ramp development coincided with the peak of the first shallowing phase in the Gasp~ Belt following the Taconian Orogeny (late Middle Ordovician) (Bourque et al., 1995; Bourque et al., 2001a, this issue; Bourque, 2001, this issue). Carbonate ramps were deposited during two transgressive-regressive cycles of earliest to late Wenlockian age (Lavoie et al., 1992; Bourque, 2001, Fig. 6, this issue). Both ramps were gently south-dipping and exhibited the same lateral facies zonation (Lavoie et al., 1992). Four parallel depositional belts are recognized: from nearshore to offshore (Fig. 2, Table 1) they are (1) a wide peritidal mud- fiat, containing mostly microbial carbonates (laminites, stroma- tolites, thrombolites, oncolites) and local elastics; (2) a narrow knob reef rim built by a consortium of skeletal metazoans (corals, bryozoans, stromatoporoids), skeletal calcareous algae, and microbial communities; (3) a well-sorted lime sand belt and; (4) a deeper water nodular mud belt. The ramps were buried by an influx of deep-water siliciclastic sediments during a late Wenlockian transgression (Lavoie et al., 1992; Bourque, 2001, Fig. 6, this issue), but part of the carbonate ramps was uplifted and eroded in earliest Pridolian time (Bourque et al., 1986; Lavoie and Bourque, 1993).

THE SAYABEC FORMATION

In the Northern Outcrop Belt (NOB; Fig. 1), the Sayabec Formation has been divided into four informal members (A to D, Table 1, Lavoie et al., 1992). These members correspond broadly to a major initial transgressive-regressive cycle followed by a transgressive event (Lavoie et al., 1992). In the central segment of the NOB, the Sayabec Formation contains only

284 D. LAVOIE and G. CHI

w

St. Lawrence River

Taconian AIIochthons

! 8S o

I 2 5 k m i

iiiiiiiiiiiiYl iiiiiiiiiiii!iiiiG iliiiiiiiiii!iiiiiii i

Sample Locations RI = Ruisseau Isabelle

LM = Lac Madeleine

RM = Rivi~re Madeleine

Stratigraphy Carboniferous Gaspd Sandstone Group

[ ~ ] Upper Gasp~ Limestone/ Fortin/T6miscouata groups Chaleurs Group Matap6dia and Honorat/Cabano groups Quebec Supergroup

Fig. 1. Simplified geological map of eastern Gaspe showing the position of studied stratigraphic sections (RI, LM, RM) of the Sayabec Formation along the Northern Outcrop Belt. CVGS = Connecticut Valley-Gasp~ Synclinorium, BNO = bassin Nord-Ouest, TL = Troisieme Lac and GR = Grande Rivi~re, NOB = Northern Outcrop Belt. Modified from Bourque et al. (1995).

the lower two members (A and B), reflecting increased accom- modation space and a marine setting too deep to record the sub- sequent shallowing events (Lavoie et al., 1992).

In detailed maps, Lachambre (1987) illustrated the small- scale facies architecture of the Sayabec Formation in the NOB and stratal truncation that can be attributed to the Salinic disturbance. The Sayabec Formation has been partly eroded in the western segment of the NOB, but east of the Rivi~re Madeleine area, the entire formation has been removed (Bourque et al., 1995) (Fig. 1). In the Rivibre Madeleine area (Fig. 1), we document for the first time the occurrence of stromatoporoid-algal boundstone. This is the only known occurrence of this facies outside the type-area (Lac Matapddia Syncline).

The diagenetic evolution of the Sayabec Formation has been mostly evaluated in the Lac Matap6dia Syncline (Lavoie, 1988; Lavoie and Bourque, 1993). For the most part, the Sayabec Formation of the NOB has not been sampled for diagenetic

study, in particular for meteoric diagenesis associated with the Salinic unconformity. Lachambre (1987) documented massive dolostone in the central segment of the NOB (Ruisseau Isabelle; Fig. 1). The origin of this unit is investigated in this study for the first time.

METHODS

During the summer of 1996, field sections of the Sayabec Formation were examined and sampled along the NOB. Outcrops featuring exposures of massive dolostone (Ruisseau Isabelle) (Fig. 1), or the Sayabec Formation partly removed by the Salinic unconformity, were a focus for this study (in the Rivi~re Madeleine and Lac Madeleine areas; Lachambre, 1987)

Carbonates are massive dolostone, wackestone, or coarse- grained packstone and grainstone interpreted as being of peritidal and shallow subtidal origin (Lavoie et al., 1992). Fifty-five thin sections were examined under the light microscope and 33


S T - L I ~ O N F O R M A T I O f "

Z O

¢1¢ C) L I -

O

V A L - BRILLAL FORMATION (A)

S h o r e l i n e

Nodular

I ( ~ Bi0herm

12 ,~, Coquina I~ _a..._ Siltstone I t C p, loa, I) Um0,tone ~ . . ~ Sandstone

T r

_ mht

pedtidal belt-....-)m - ~ ~ ~ - ~ ~ " ' ~ -'~,.- < - - mlt

. . . . . . . nodular knob reef bel{ . . . . ~ ,,,~2~mUul~r ~lp.

MEMBERS A and C - , . .1 ~ A T

MEMBERS B and D

LOW ,, LOW Energy level

(B)

Fig, 2. (A) Schematic stratigraphic section of the Sayabec Formation illustrating the four members (A to D) and relative sea-level curve. Tr = transgressive phases, Re = regressive phases. Modified from Lavoie et ai. (1992). (B) Carbonate ramp model for the Sayabec Formation showing major depositional belts and their relationships with members A to D as well as the energy level across the depositional ramp. WB = wave base, mlt = mean low tide, mht = mean high tide. Modified from Bourque et al. (1986).

under a cathodoluminoscope (CL). Distinct generations of cement were recognized and microsampled for 40 carbon and oxygen isotope analyses at the GSC-Quebec Delta Laboratory. Samples were microdrilled from rock slabs, and the integrity of cement samples was assured through CL examination of milled areas. The carbonate powders were then treated and analyzed at the GSC-Quebec Delta Laboratory. Data are reported in the usual 'per mil' (%0) notation relative to the standard VPDB for carbon and oxygen. Precision of the data is always better than >0.1%o for both 5180 and ~il3C. Finally, l0 double-polished thin sections were examined for fluid inclusions. Fluid inclusions showing clear relationships with crystal growth zones were not observed in every sample. We considered the following occurrences of fluid inclusions as being of primary or pseudosec- ondary origin: isolated, clustered, scattered, and randomly dis- tributed in three dimensions.

PETROGRAPHY OF CEMENTS AND OTHER DIAGENETIC

ELEMENTS

Pore- and fracture-filling cements and other rock textures, such as sediment infills, stylolites, dolomite, and crosscutting relationships were carefully documente d in order to reconstruct the syn- to post-depositional evolution of the strata.

Figure 3 (A-C) is a schematic summary of the observations presented below. The colours of illustrated pore-filling and fracture-filling calcite cements are related directly to CL observations. The Rivi6re Madeleine succession is represented in Figure 3A, B, and the Lac Madeleine succession is illustrated in Figure 3C.

Table 1. Facies, depositional interpretation and members of the Sayabec Formation.

Lithology

Stromatolite/thrombolite Cryptalgal laminites Fenestral lime mudstone Oncolite lime mudstone Peloid packstone/grainstone Oolite grainstone

Intraclast and peloid l imestone

!Algal-metazoan boundstone Limestone conglomerate

Bioturbated nodular wackestone and mudstone Pentamerid-rich coquinas Intraformational conglomerate

Relative Depositional environment abundance

<10% 25% 35% 15% 15% <1%

99% 1%

90% 10% <1%

Peritidal flat Intertidal to shal low subtidal

Members A and C

Shallow subtidal sands Members A and C

Shallow subtidal knob reefs Member C

Offshore-type muds Photic zone Members B and D

286 D. LAVO1E and G. CH1

PORE-FILLING CALCITES Pore-filling calcite cements were recognized in nearly all

limestone samples, and occurred in primary and secondary )ores.

A) L

a)

c)

Primary Porosity

The volume of depositional and constructional primary porosity varies from a maximum of 30% in the boundstone, to 25% in the well-sorted grainstone, and 10% in the wackestone. All pores are filled by calcite cement.

Fig. 3. Summary of petrographic observations (light and cathodolu- minoscopy) of various diagenetic-tectonic elements recognized in the Sayabec Formation. The colour of illustrated cements (pore-filling: LF1 to LF5, fracture-filling: LF5, DF, EBF, BF) relates to the cathodoluminescence characteristics of these cements described in the text. (A) Algal- metazoan boundstones of the Sayabec Formation, Riviere Madeleine outcrop area. (B) Burial succession for the Sayabec Formation in the Riviere Madeleine outcrop area. (C) Burial-meteoric succession of the Sayabec Formation in the Lac Madeleine outcrop area. See text for explanation. No scale implied.

Fig. 4. (A) Plane-polarized photomicrograph of a growth cavity floor showing an isopachous crust of recrystallized, cloudy calcite marine cement. The floor of the cavity is lined with geopetal lime silt. Top is upward. (B) Same as (A) under CL, showing an early lamellar, non-luminescent cement phase with blotchy luminescence, Riviere Madeleine area. Scale bars are 0.1 ram.


The earliest calcite cement consists of thin (less than 5 mm) isopachous crusts of radiaxial calcite that coats boundstone growth pores (Fig. 4A). This cement contains abundant tiny inclusions, giving them a cloudy aspect. The cement crusts are locally interlayered with geopetal lime silt. Under CL, the cement consists of non-luminescent, lamellar calcite with a sig-

nificant number of blotchy zones (Fig. 4B). Following Lavoie and Bourque (1993), this cement is designated LF1 (Luminescence Facies 1), and can occlude up to 50% of the primary porosity. In the remaining primary porosity of the boundstone, small idiomorphic to large blocky calcite cements are present. Under CL, these cements consist of (1) non-luminescent pyramidal spar with thin, bright luminescent laminae, followed by (2) a zone of bright luminescent pyramidal spar capped by (3) a xenomorphic, dull-luminescent calcite cement (Fig. 5B). The cement succession is similar to the Sayabec LF2-LF3-LF4 succession documented previously by Lavoie and Bourque (1993). The complete cement succession occurs only in centimetre-sized pores (Fig. 5A); smaller pores were occluded before precipitation of the dull-luminescent calcite (Fig. 5B).

Secondary Porosity

Dissolution-enhanced, millimetre- to centimetre-sized secondary pores are observed in the Lac Madeleine area and are always connected by fractures. The irregular walls of these voids are seen cutting through various allochems or calcite cements. The voids are now occluded by xenomorphic calcite cements consisting of bright-zoned pyramidal crystals and late dull xenomorphic calcite (Fig. 6). The bright-zoned pyramidal crystals outline gravitational fabrics. In large cavities, this calcite cement is confined usually to the roof of the voids (Fig. 6). These cements are similar to the LF5 and LF6 phases of the La Vieille Formation of southern Gasp6 (Lavoie and Bourque, 1993).

Fig. 5. (A) Photomicrograph under CL of a primary pore space filled by an early scalenohedral, non-luminescent cement (LF2), overlain by a bright luminescent zone (LF3), then by large, xenomorphic, dull-luminescent spar (LF4). The cemented pore is cut by a late fracture that has been filled by a dark, dull-luminescent calcite. Riviere Madeleine area. (B) Photomicrograph under CL showing pore space filled by LF2 and LF3 cements. Lac Madeleine area. Scale bars are 0.1 mm.

Fig. 6. Photomicrograph under CL showing secondary (dissolution) pore space filled by strongly zoned, luminescent, pyramidal spar (LF5) on the roofs of large cavities. Note the gravitational fabric. The remaining pore space is filled by a dull-luminescent calcite (LF6). Lac Madeleine area. Scale bar is 0.1 mm.

288 D. LA VOlE and G. CH1

Fig. 7. (A) Photomicrograph under CL showing an early fracture filled by scalenohedral non-luminescent cement. This fracture is floored by marine lime silt and is interpreted as a neptunian dyke (see text). Rivi6re Madeleine area. (B) Photomicrograph under CL of a dissolution- enhanced fracture (DF). The fracture is filled by strongly zoned, bright luminescent calcite cement that was precipitated over an accumulation of crinoid debris (arrows) on the floor of the cavity. Lac Madeleine area. (C) CL photomicrograph showing primary pores in oolitic grainstone, filled by dull-luminescent LF4 cement. This cement is cut by a dissolution-enhanced fracture (DF), filled by zoned bright luminescent calcite. This relationship suggests that dissolution-enhanced fractures were part of a late diagenetic phase. Lac Madeleine area. Scale bars are 0.1 mm.

FRACTURE-FILLING CALCITES

Particular attention has been given to, and significant new observations made on, the fracture-filling calcite cements. These were studied in earlier investigations (Lavoie, 1988). Five episodes of fracturing are recorded and distinguished on the basis of crosscutting relationships among fractures, pore- filling cements, and stylolites.

Early Fractures

Early fractures are rare in the Rivi~re Madeleine area (Fig. 7A). These centimetre-wide fractures have straight to irregular margins. Their early origin is supported by the presence of lime silt resting on irregular fracture floors, which are succeeded by LF2 non-luminescent cement.

Dissolution-Enhanced Fractures

The dissolution-enhanced fractures (DF) occur in four samples from the Lac Madeleine area. The DF have irregular walls with evidence of grain dissolution and corrosion in fracture walls. These fractures are a few millimetres to, rarely, 1 to 2 cm wide. Some fractures locally merge into larger (millimetre- to centimetre-sized) cavities with various elements (fragments of crinoids and syntaxial finds) on the floor (Fig. 7B). The DF are either pre- or post-stylolites and pre-date most other fractures. Moreover, in one sample, crosscutting relationships with pore- filling calcite indicate that DF followed precipitation of the dull-luminescent LF4 cement (Fig. 7C). This fracture-dissolution event followed some initial burial and was likely succeeded by burial diagenesis.

The DF are filled by zoned, bright luminescent, small-sized pyramidal crystals (Fig. 7B, C), although in a few places, the cement displays a zoned, orange luminescence. This fracture set accounts for about 2% of the secondary void space, but has been occluded by calcite cements.

Early Burial Fractures

Early burial fractures (EBF) are common in samples from the Rivi~re Madeleine and Lac Madeleine areas. These EBF have straight margins and are typically 0.5 to 1 cm wide. The EBF are either pre- or post-stylolites and are locally coeval with pore-filling LF4 cement (Fig. 8A, C). The EBF are filled with orange, dull-luminescent xenomorphic crystals that form a 0.1 to 1 cm thick cement layer. These fractures generated 5% of the secondary porosity, but they have been occluded by younger calcite cements.

Late Burial Fractures

Two sets of late burial fractures (BF1 and BF2) are recognized. These are abundant in the Rivi~re Madeleine and Lac Madeleine areas. The BF1 consist of centimetre-wide fractures that post-date EBF and stylolites. BF1 are filled by very dull- luminescent xenomorphic crystals with local, internal, non- luminescent, 0.1 and 1.5 cm wide growth bands (Fig. 8B, C). BF2 post-date BF1 and consist of small (<0.5 ram), regular fractures filled with small, uniformly bright, luminescent calcite crystals (Fig. 8A). These two late stage fracture sets


created about 5% of the secondary porosity, but they are also occluded by calcite cements. In the Lac Madeleine area, fractures are locally pervasive and breccia beds are known to occur.

SEDIMENT INFILLS

Void-filling rock textures occur in the Sayabec Formation. The first type consists of lime silt embedded in isopachous, radiaxial calcite cement crusts that coat pores of the metazoan boundstone facies. The second type consists of lime silt deposited on irregular floors of pores and early fractures, and LF2 (non-luminescent) cements overlie the lime silt. The third type consists of coarse-grained, bioclastic-intraclastic sediment (fragments of crinoids and corroded syntaxial rinds) deposited in large cavities associated with secondary pores and dissolution-enhanced fractures (DF).

STYLOLITES

Stylolites are common in all facies of the Sayabec Formation. More than 95% of the stylolites are roughly parallel to bedding, and the remainder consist of very late stage, small stylolites oriented at a high angle to the bedding. Stylolites occur as small, wispy, sutured and non-sutured seams or as sin- uous, high amplitude structures. However, a temporal relationship between the two types cannot be interpreted, based on the present data. Stylolites occur as early as LF4 cements and EBF, although most of them post-date EBF but pre-date BF.

DOLOMITIZATION

Dolomite is rare in the Sayabec Formation. Previous studies (Lavoie, 1988) describe two types of dolomite. The first one is a facies-specific dolomite (intertidal flat). The second type is a massive dolomite that has locally replaced limestone (Ruisseau Isabelle area). This dolomite has been interpreted as being of hydrothermal origin (Lachambre, 1987), based on the presence of high-temperature sulphides in pores, the lateral transition from limestone to dolostone, and the spatial association of dolomite and faults. In addition, dolomite rhombs occur with stylolites.

Facies-Specific Dolomite

The facies-specific dolomite occurs in intertidal lithofacies (microbial laminites, massive micrite), although adjacent beds can be dolomite-free. Under CL, this dolomite consists of well- zoned, small (from 0.01 to 0.08 mm) rhombs showing bright yellow and red luminescent growth bands (Fig. 9A). This dolomitization is early and occurred before stylolitization and precipitation of pore-filling calcite.

Massive Dolomite

Massive dolomite predominates in the Ruisseau Isabelle area (Fig. 1) and extends eastward for a few kilometres (Lachambre, 1987). The dolomite consists of centimetre-sized crystals lining pores (Fig. 9B) or 0.5 to 1 millimetre-wide rhombs in a ground- mass-like texture throughout the rock or within crosscutting fractures. Microscope examination indicates that the dolomite is an inclusion-rich saddle dolomite that, under CL, exhibits a

Fig. 8. (A) CL photomicrograph showing a fracture filled by orange, dull-luminescent calcite cement (EBF). This is cut by a late, very thin fracture, filled by bright yellow luminescent calcite. Lac Madeleine area. (B) CL photomicrograph showing a fracture filled by dull-luminescent calcite cement (BF). Similar cements occurred as a final cement phase (LF4) in the surrounding primary pore space. Lac Madeleine area. (C) Photomicrograph under CL showing a large (total width of photomicrograph) fracture filled by orange, dull-luminescent calcite cement (EBF) with a zoned, very dull to non-luminescent late cement (BF). Lac Madeleine area. Scale bars are 0.1 mm.

290 D. LAVOIE and G. CHI

faint red luminescence or non-luminescence. Locally, porosity is preserved, and sulphide mineralization is present (malachite and chalcosine).

Stylolite-Associated Dolomite

The stylolite-associated dolomite consists of small (0.05-0.1 ram) rhombs that form millimetre-wide bands that follow the surface trace of the stylolites. The dolomite consists of inclusion-free crystals that are red luminescent under CL.

STABLE ISOTOPES

Oxygen and carbon stable isotope ratios were measured on calcite and dolomite samples from the Sayabec Formation. The selected samples are considered representative of the discrete cement generations. Samples contain few pore-filling calcite cements (LF1) and pore sediment infills. Calcite forms the bulk of the analyzed material of fracture fills. Table 2 presents the isotopic results for sampled areas, and the nature of the analyzed material, Results from the Rivi~re Madeleine area are shown in Figure 10 and those of the Lac Madeleine and Ruisseau Isabelle areas in Figure 11. The results from the Rivi~re Madeleine and Lac Madeleine areas are different, with the latter having lower 8180 values for petrographically similar cements.

SEDIMENT INFILLS

Two analyses of sediment infills were made in order to assess the origin of sediment infills and to understand the nature of associated cements. One of these analyses is of an infill associated with the LF1 cement that yielded a 8180 value of -5.6%0 and a 813C value of +1.9%o. The other analysis is of a lime-silt infill found in very early fractures; it yielded a 8~80 value of -4.9%0 and a 813C value of +2.2%o.

PORE-FILLING CALCITES

Pore-filling calcites consist of the LF1 cement phase only, namely the isopachous, lamellar, non-luminescent to blotchy luminescent calcite. This cement phase is present only in the Rivi~re Madeleine samples. Three analyses yielded 8180 values of -5.1, -5.3 and -5.5%0, and 813C values of +1.8, +1.5 and +1.5%o, close to those of the interlayered lime silt.

FRACTURE-FILLING CALCITES

A large number of fracture-filling calcite samples were analyzed. Results obtained from the Rivi~re Madeleine and Lac Madeleine areas are discussed separately.

Rivibre Madeleine: Early Fracture

Two analyses of non-luminescent calcite cement were made in order to constrain the origin of this phase in an early fracture floored by lime silt. They yielded 8180 values of -4.8 and -6.5%o, and 813C values of +2.1 and +1.6%0.

Fig. 9. (A) Photomicrograph under CL showing red-to-bright orange, zoned, small dolomite rhombs that form the matrix of a dolomitized peritidal mud. Lac Madeleine area. Scale bar is 0.1 mm. (B) Plane-bedded outcrop view of large voids in massive dolostone largely composed of saddle dolomite. Large do lomi te crystals line the voids. Ruisseau Isabelle area. Lens cap for scale = 6 cm diameter.

Rivibre Madeleine: Early Burial Fracture

Two analyses of the early burial (EBF), orange, dull-luminescent, fracture-filling calcite cement yielded 81so values of -6.3 and -7.1%o and 813C values of +1.8 and +1.6%o.


I I I I I -10 ~ -8 -6 -4 -2

• LF1 cement [] Sediment infill X Non-luminescent cement, early fractures 0 EBF = early burial fracture cement ~ B F = burial fracture cement

-3

-2

--1

-2

V P D B

j ~ 1 8 0

2 V P D B

Fig. 10. Crossplot of 5180 and ,51aC values for pore- and fracture-filling calcite cements and sediment infills from the Sayabec Formation in the Rivi~re Madeleine area. See Table 2 for data. Vertical box is the assumed Early Silurian isotopic marine signature of the Gaspe Basin from Lavoie and Bourque (1993).

J0

d d

Marine end-member(] O

Mixing line I for EBF d

I

/ D . I nml '~s • I%~" I I I I .,8 , .re [t4 .~= .10 .8 -6 -4 .2

~'i~" ~ ~2' • OF = dissolution fracture cement O EBF = early burial frosture cement

D 0 I 0 ~ BF = burial fracture cement

;~, I d Facies-specific dolomite ~'i" ~' I D Saddle dolomite

~: 0 I "~ ] RM = Field for Rlvl6re I J Madeleine results

[ (see Fig. 10) I o

Hydrothermal end-member

,4 V P D B

3

2

-1

~ 1 8 0

V P D B --1

--2

--3

-4

Fig. 11. Crossplot of 5180 and ~513C values for pore- and fracture-filling calcite cements and dolomite samples from the Sayabec Formation at the Lac Madeleine and Ruisseau Isabelle outcrop areas. Mixing line (dashed) represents parent waters for EBF orange-luminescent calcite cements precipitated from a mixture of marine-derived and hydrothermal fluids. See Table 2 for data. Circle represents the results for the calcite cements in the Riviere Madeleine (RM) area (see Fig. 10). Vertical box is the assumed Early Silurian isotopic marine signature of the Gasp6 Basin from Lavoie and Bourque (1993).

Rivibre Madeleine: Late Burial Fracture

Three analyses of the late burial (BF), dull to very dull-luminescent, fracture-filling calcite cement gave 8180 (-6.0, -6.4 and -7.4%o) and 813C (+2.6, +1.9 and +1.8%o) values, fairly close to the interpreted early burial cement.

Lac Madeleine: Dissolution.Enhanced Fractures Three analyses of the bright luminescent calcites in dissolu-

tion-enhanced fractures (DF) yielded the least ]80-depleted results for this area, with 8180 values of-9.1, -9.8 and -10.3%o, and 813C ratios of +0.5, +0.3 and 0.3%0.

Table 2. 8180 and ~130 results for sediment infills, calcite cements and dolomites for the Sayabec Formation.

SECTION: RIVIERE MADELEINE t3 Sediment inflll 5 C ~180VPOS FF-sediment 2,2 -4,9 PF-sediment 1.9 -5,6 Pore-filling cement (LFI) PF-MAR 1,8 -5.1 PF-MAR 1,5 -5,5 PF-MAR 1.5 -5,3 Very early fracture-fill calcite FF-NL 2,1 -4.8 FF-NL 1.6 -6.5 Early burial fracture-fill calcite EBF-orange 1.6 -7.1 EBF-orange 1.8 -6.3 Burial fracture-fill calcite BF-dull 1.9 -6.4 BF-dull 1.8 -7.4 BF-dull 2.6 -6 SECTION: LAC MADELEINE Dissolution-enhanced fracture-fill calcite DF-bright 0.5 -9.1 DF-bright 0.3 -9.8 DF-bright -0.3 -10.3 Early burial fracture-fill calcite EBF-orange -1.2 -14.4 EBF-orange -1.4 -16 EBF-orange -2.5 -16.9 EBF-orange -3.3 -15.3 EBF-orange 2.7 -12.4 EBF-orange 2.1 -13.4

Average EBF I -0.6 I -t4.7 Burial fracture-fill calcite BF-dull -2,5 -17.5 BF-dull -2 -17.6 BF-dull -2,2 -16.7 BF-dull 0,2 -9.1 BF-dull 0,3 -13.6 BF-dull -0.9 -15.8 BF-dull -1,6 -20.4 BF-dull -2.7 -14.9 BF-dull -0,9 -15,8 BF-dull -0.7 -16.6 BF-dull -2.1 -13.4 BF-dull -0.8 -14.4

Average BF I -1.6 I -16.1 Late burial fracture-fill calcite LBF-bright 0 -15.8 Facies-specific dolomite Dolomite 3.9 -7.4 Dolomite 3.7 -6.2 Dolomite 1.2 -10.7 SECTION RUISSEAU ISABELLE Saddle dolomite Saddle dolomite -1.6 -17.4 Saddle dolomite -0.2 -16.9 Saddle dolomite -0.3 -17.9 FF = fracture filling, PF = pore filling. NL = non-luminescent, EBF = early burial fractures. LBF = late burial fractures, DF = dissolution-enhanced fractures

292 D. LA VOIE and G. CHI

Table 3. Fluid inclusion microthermometric data.

Sample Host Occurrence Size Tm. ice(°C) Salinity (wt % NaCI equiv.) m h (°C) mineral (mm) Range Mean (n) Range Mean (n) Range Mean (n)

S-7 EBF Cluster 3 ~ 10 -11.2 -11.2 (1) 15.2 15.2 (1) 74.6 ~ 90.1 82.4 (2) Cluster 7 ~ 9 70.3 ~ 81.5 75.9 (2) Isolated 11 -11.8 -11.8 (1) 15.8 15.8 (1) 93.4 93.4 (1) Isolated 5 91.2 91.2 (1) Isolated 9 -11.3 -11.3 (1) 15.3 15.3 (1) 103.3 103.3 (1) Isolated t2 -6.9 -6.9 (1) 10.4 10.4 (1) 111.2 111.2 (1) Isolated 11 -7.0 -7.0 (1) 10.5 10,5 (1) 127.0 127.0 (1)

S-36 PF-LF2 Isolated 6 -7.6 -7.6 (1) 11.3 11.3 (1) 85.2 85.2 ( l i Isolated 9 73.9 73.9 (1) Cluster 5 -11.8 -11.8 (1) 15.8 15.8 (1) all liquid all liquid (1)

S-30 DF Growth zone 5 -1.3 -1.3 (1) 2,2 2.2 (1) all liquid all liquid (1) Growth zone 11 -2,8 -2.8 (1) 4.6 4.6 (1) all liquid all liquid (1) Growth zone 6 54.6 54.6 (1)

Fracture 6 ~ 8 -8.9 -8.9 (1) 12.8 12.8 (1) 125.4 ~ 127.7 126.6 (2) S-33 DF Isolated 8 -0.2 -0,2 (1) 0.4 0.4 (1) all liquid all liquid (1) S-16 EBF Random 4 ~ 10 -6.0 -6.0 (1) 9.2 9.2 (1) 42.6 ~ 84.9 67.5 (3) S-39 EBF Random 4 ~ 7 -3.8 -3.8 (1) 6.2 6.2 (1) 37.4 ~ 75.6 62.0 (4) S-26 EBF Scattered 7 ~ 8 -12.8 -12.8 (1) 16.7 16.7 (1) 80.9 ~ 82.7 81.8 (2)

Isolated 12 -6.3 -6.3 (1) 9.6 9.6 (1) 106.7 106.7 (1) S-20 BF Random 8 ~ 9 -13.7 -13.7 (1) 17.6 17.6 (1) 148.9 ~ 157.3 153.1 (2)

Random 5 ~ 7 -15.5 ~ -17.5 -16.3 (3) 19.0 ~ 20.6 19.6 (3) 165.3 ~ 186.0 176.2 (4) S-25 BF Isolated 12 -10.8 -10.8 (1) 14.8 14.8 (1) 216.4 216,4 (1)

Isolated 13 226.2 226.2 (1) Isolated 7 -10.1 -10.1 (1) 14.1 14,1 (1) 218.1 218.1 (1) Isolated 8 208.8 208.8 (1) Isolated 5 196.5 196.5 (1) Cluster 5 ~ 12 -10.5 -10.5 (1) 14.5 14.5 (1) 180.0 ~ 185.6 182.6 (3)

S-51 SD-core Growth zone 7 ~ 10 -20.1 ~ -24.2 -22.2 (2) 22.4 ~ 23.9 23.2 (2) 133.7 ~ 142.0 136.6 (4) SD-core Random 6 ~ 9 -24.2 -24.2 (1) 23.9 23.9 (1) 117.7 ~ 145.3 129.5 (4) SD-core Random 9 ~ 11 -24.0 -24.0 (1) 23.8 23.8 (1) 150.9 ~ 161.3 156.1 (3) SD-core Random 6 ~ 13 -23.7 -23.7 (1) 23.7 23.7 (1) 150.2 ~ 151.7 150.7 (3) SD-core Random 7 -25.8 -25.8 (1) 24.3 24.3 (1) 162.6 ~ 173.1 167.9 (2) SD-core Random 6 ~ 7 159.6 ~ 162.2 160.9 (2) SD-core Random 6 ~ 8 -24.2 -24.2 (1) 23.9 23.9 (1) 111.1 ~ 117.3 114.1 (3) SD-core Random 5 ~ 6 177.6 ~ 187.5 182.7 (2) SD-rim Isolated 12 -24.8 -24.8 (1) 24.0 24.0 (1) 161.9 161.9 (1) SD-dm Isolated 7 161.0 161.0 (1) SD-rim Isolated 12 -24.4 -24.4 (1) 23.9 23.9 (1) 159.2 159.2 (1) SD-rim Isolated 15 -23.7 -23.7 (1) 23.7 23.7 (1) 162.1 162.1 (1) SD-rim Cluster 9 ~ 12 -18.1 ~ -18.4 -18.2 (4) 21.0 ~ 21.2 21.1 (4) 172.7 ~ 175.1 173.7 (4) SD-rim Cluster 7 ~ 15 -23.4 ~ -24.2 -23.8 (2) 23.7 ~ 23.9 23.8 (2) 153.5 ~ 161.9 157.9 (4) SD-rim Isolated 28 -22.7 -22.7 (1) 23.5 23.5 (1) 166.1 166.1 (1) SD-rim Isolated 25 194.0 194.0 (1) SD-rim Cluster 5 ~ 14 152.8 ~ 156,0 154.4 (2) SD-rim Isolated 8 187.9 187.9 (1) SD-rim Isolated 12 -39.1 -39.1 (1) 28.0 28,0 (1) 183.7 183.7 (1) SD-rim Cluster 10 ~ 16 -23.7 ~ -23.9 -23.8 (2) 23.7 ~ 23.8 23.8 (2) 148.8 ~ 163.1 157,5 (3) SD-rim Isolated 17 -23.9 -23.9 (1) 23.8 23.8 (1) 152.6 152.6 (1)

EBF = early burial fractures, PF = pore-filling, LF2 = luminescence facies 2, DF = dissolution-enhanced fractures, BF = burial fractures, SD = saddle dolomite


Lac Madeleine: Early Burial Fractures Six analyses of the early burial (EBF), orange, dull-lumines-

cent fracture-filling calcite were made that yielded 5~80 values ranging from-12.4 to -16.9%~ (average, -14.7%o) and ~13C values from 3.3 to +2.7%o (average, 0.6%o).

Lac Madeleine: Late Burial Fractures

Twelve analyses of the predominant late burial (BF), dull- to very-dull-luminescent, fracture-filling calcite yielded 8180 values o f -9 .1 to -20.4%0 (average, -16.1%0 and 813C values of -2.7 to +0.3%0 (average, -1.3%o). Finally, one analysis of the latest bright luminescent fracture-filling calcite cement gave a 5180 value of-15.8%~ and a 513C ratio of 0%0.

DOLOMITE

Three analyses of the facies-specific dolomite at the Lac Madeleine area and three analyses of the saddle dolomite at the Ruisseau Isabelle area were obtained. The facies-specific dolomite yielded ~180 values o f -6 .2 , -7.4 and -10.7%o and ~13C values of +3.7, +3.9 and +1.2%~. The saddle dolomite samples gave 6180 values of -16 .9 , -17.4 and -17.9%e and ~13C values of-0.2, -1.6 and -0.3%~.

30.

cr 25.

20.

z 15.

~ 10.

c

(A) A

6 b o o • i

O , •o o

0

0 ~ O i i i

z..__ All-liquid fluid inclusions

Homogenization Temperatures (°C)

R i v i ~ r e M a d e l e i n e

¢. E B F

L a c M a d e l e i n e

• PF-LF2 o DF • E B F

• B F

R u i s s e a u I s a b e l l e • SD.core

A SD.rim

25O

$

E

z

~) R i v i e r a M a d e l e i n e

. . . . . . . . i • ' i . . . . i '"'"'' • 0 5'0 100 t50 200 250

47 Lac Madeleine - PF- F2 -

2 "l aft-liquid FIs ~J m . . . . . I ~ DF 0 ~ _ ~ . . ~ ~ f ' l ~ r ] r ] ~l I i l l i l l 1

0 50 . . . . 100 . . . . 150 . . . . 200 . . . . 250

0 i . . . . o

1 R u i s s e a u I s a b e l l e

• SD-core o S D - r i m

l i b FI FI, i . . . . i . . . . i . . . . . . . . i

50 100 150 200 250

Homogenization Temperature (°C)

Fig. 12. (A) T h - salinity crossplot for fluid inclusions from the Sayabec Formation. A T h value of 40EC is arbitrarily assigned to liquid-only inclusions in order to include the salinity data of these inclusions in the dia- gram. (B) T h histogram of fluid inclusions in the Sayabec Formation. See Table 3 for data.

FLUID INCLUSIONS

Fluid inclusions were studied in calcite cements from the RiviSre Madeleine (EBF) and Lac Madeleine (LF2, DF, EBF and BF) areas and in saddle dolomite from the Ruisseau Isabelle area. The results are shown in Table 3 and Figure 12. Secondary fluid inclusions in microfractures that cross- cut crystal boundaries were avoided, except in one sample (S-30), where fluid inclusions cutting growth zones were studied for comparison with primary fluid inclusions.

The homogenization temperatures (Th) and ice-melting temperatures are largely convergent within individual fluid inclusion assemblages (Table 3). The overall range of T h is <50°C (all-liquid fluid inclusions) to 226°C. Salinity ranges from 0.4 to 28 wt % NaCl-equivalent. This range is likely related to the variable conditions under which the diagenetic minerals were precipitated.

RIVIERE MADELEINE AREA

Only one sample was studied from the RiviSre Madeleine area. Fluid inclusions were studied in the early burial, orange, dull-luminescent, fracture-filling calcite (EBF). Values for T h and salinity vary from 75.9 to 127°C, and 10.4 to 15.8 wt % NaCl-equivalent, respectively.

LAC MADELEINE AREA

Eight fluid inclusion samples were obtained from the Lac Madeleine area. Analyses were performed on a single pore- filling calcite (PF-LF2) and from almost all types of fracture-filling calcite that are discussed in the petrography section.

Fluid inclusions from the pore-filling, non-luminescent calcite cement (PF-LF2) yielded T h values from 73.9 to 85.2°C. Several all-liquid fluid inclusions were observed. Salinities ranged from 11.3 to 15.8 wt % NaCl-equivalent. Fluid inclusions from the calcite cement in dissolution- enhanced fractures (DF) are mainly all-liquid inclusions and show relatively low salinity (0.4 to 4.6 wt % NaCl-equivalent). These primary fluid inclusions are post-dated by secondary fluid inclusions showing relatively high salinity (12.8 wt % NaCl-equivalent) and high T h (126.6°C). Fluid inclusions from the orange, dull-luminescent calcite cement associated with early burial fractures (EBF) show moderate T h (37.4 to 106.7°C) and salinity (6.2 to 16.7 wt % NaC1- equivalent). The dull to very dull calcite cement associated with the late burial fracture (BF) is characterized by relatively high salinity (14.1 to 19.6 wt % NaCl-equivalent) and high T h (153.1 to 226.2°C).

RUISSEAU ISABELLE AREA

A single fluid inclusion sample was obtained from the Ruisseau Isabelle (S-51, Table 3), from the saddle dolomite (SD). Results are reported separately for the cloudy saddle dolomite core (SD-core) and the relatively clean saddle dolomite rim (SD-rim). Values for T h are slightly higher in the crystal rim (152.6 to 194°C) than in the cloudy core


SAYABEC / RIVIERE MADELEINE TIME

EVENTS

Sedimentation Cementation Compaction

Pressure solution

Early fractures

Late fractures Uplift

Primary Po

Secondary Po

i BURIAL Shallow ] Interm. (<l=rc) I Deep

LF1 LF2-LF3 LF4

m NO EBF

,BF

F ~ -

POST-BURIAL

Fig. 13. Diagenetic history of the Sayabec Formation in the Rivi~re Madeleine area. Cementation started early in the marine realm with the occlusion of primary pores in the shallow burial environment. Early- and late-burial fractures generated some secondary porosity rapidly (?) filled by fracture-filling calcite (EBF and BF).

(114.1 to 182.7°C). Salinity ranges from 23.2 to 24.3 and from 21.1 to 28.0 wt % NaCl-equivalent for SD-core and SD-rim, respectively.

INTERPRETATION OF ISOTOPE GEOCHEMISTRY AND FLUID

INCLUSION RESULTS

Based on an integration of the diagenetic tracers (C and O stable isotopes and fluid inclusions) and petrographic observations, we have interpreted the fluid evolution history of the Sayabec Formation. An emphasis is placed on the evolution of porosity, because its development and/or destruction are critical for the occurrence of hydrocarbon reservoirs. Because the isotopic and fluid inclusion results indicate that differences exist between the Rivibre Madeleine and Lac Madeleine areas, the two sectors are discussed separately.

RIVI~RE MADELEINE AREA (FIG. 13)

Marine Diagenesis

Primary porosity of the Sayabec limestone at the Rivibre Madeleine area was occluded rapidly. A marine origin for LF1 cement is suggested by its 8180 signature (-5.1 to -5.5%o). These ratios are similar to those obtained from marine cements in the Sayabec organic buildups in the Lac Matap6dia Syncline (8180 = -5.3 to -5.5%o; Lavoie and Bourque, 1993). The marine interpretation is also supported by the presence of interbedded lime silt, which yielded a similar 8~80 value (-5.6%o). Marine cementation occluded about 50% of the growth porosity in the metazoan boundstones.

The early fractures are interpreted as neptunian dykes. This interpretation is supported by the occurrence of sediment infill that is overlain by LF2 non-luminescent calcite cement (interpreted as early burial; see below). The marine origin is supported by the presence of 5180 values of the infills (-4.9%o) and the calcite-filling cement (--4.8%o and -6.5%o) and by marine- like 813C values.

Burial Diagenesis The overlying pore-filling cement succession LF2, LF3 and

LF4 were not sampled for stable isotopes nor for fluid inclusions. However, as reported above, LF2-1ike cements in neptunian dykes yielded 5180 and 813C values that suggest precipitation from slightly modified marine fluids. Cements precipitated under shallow to moderate burial depths (Fig. 13) occluded the remaining constructional and depositional porosity. It is noteworthy that, besides fracturing, no secondary porosity was observed in the Sayabec Formation in the Rivi~re Madeleine area, even though the Salinic unconformity is documented there (Lachambre, 1987; Bourque et al., 2000, Fig. 6).

Deep-seated burial fracturing occurred repeatedly in the Rivibre Madeleine area. Fractures are placed in chronological order, based on a crosscutting relationship with stylolites. Early burial fractures likely formed early, perhaps during the precipitation of pore-filling LF4 cement. From the limited data (5 analyses), fracture-filling calcite cements of the early (EBF) and late (BF) fractures yielded similar 8180 and 813C values (Table 2, and Fig. 10) that are only slightly lower than marine calcite values (LF1 and marine lime silts; Table 2 and Fig. 10). Fluid inclusion data from one sample (early burial calcite cement) suggests precipitation from saline fluids of moderate temperature (T h = 75.9-127°C). If this sample is representative of the fracture-filling cement succession, it indicates that some influx of 180-enriched brines occurred during the diagenetic burial stages of the Sayabec Formation in the Rivibre Madeleine area.

LAC MADELEINE AREA (FIG. 14)

Organic buildups were not observed in the Sayabec Formation in the Lac Matap6dia area. The succession contains predominantly peritidal and shallow, open-marine, subtidal facies, and evidence for marine diagenesis is lacking. The overall diagenetic evolution of the Sayabec Formation at Lac Madeleine consists of an initial shallow burial, pore filling by calcite cements, followed by a fracture-dissolution phase. Diagenesis of these strata terminated with fracturing during deep-seated burial.

Initial Shallow Burial Diagenesis

Primary porosity of these strata was initially rather low (10-25%) and was occluded by the LF2-LF3-LF4 cement succession, which is also recognized farther west (Lavoie and Bourque, 1993). Based on data provided by Lavoie and Bourque (1993) and on fluid inclusion measurements of the LF2 pore-filling calcite cements herein (Table 3; Fig. 12), it was interpreted that saline fluids were responsible for primary pore occlusion in a relatively shallow burial environment (Fig. 14).

Post-Shallow-Burial Meteoric Diagenesis

Petrographic evidence suggests that initial shallow burial diagenesis was followed by local subaerial exposure, related to the Salinic unconformity (sensu Bourque et al., 2000, Fig. 6; Bourque, 2001, this issue). Fresh water diagenesis occurred


during exposure. This interpretation is supported by the presence of dissolution-enhanced fractures and the isotopic signature of related cements (see below). The presence of gravitational cement fabrics (Fig. 5) also indicates diagenesis related to exposure. Elsewhere, gravitational cement fabrics are representative of vadose diagenesis (James and Choquette, 1990). The meteoric signature of cements is indicated by the 8180 (-9.1, -9.8 and -10.3%o) and 813C (+0.5, +0.3 and 0.3%0) values, which are significantly lower than the marine and burial cement results obtained in the Rivibre Madeleine area (see above). Based on paleolatitudinal and paleogeographic position and on assumed Silurian seawater isotopic composition, Lavoie and Bourque (1993) proposed that the Silurian Gasp6 Basin meteoric waters should have a 8]sOsMow composition of-7.5 to -8.5%0. Based on these data and on the Friedman and O'Neil (1977) equation, the temperature of precipitation for the Sayabec meteoric cements was about 20-30°C. The meteoric interpretation is supported by low salinity values (below that of seawater) and by the low T h of fluid inclusions (<50°C).

This is the first petrographic mad geochemical report of post- burial subaerial exposure of the Sayabec Formation in the Northern Outcrop Belt. The same suite of events (marine diagenesis, initial burial, local uplift, and meteoric diagenesis) was documented for the coeval La Vieille Formation in southern Gasp6 (Lavoie and Bourque, 1993), where secondary porosity and pore-filling calcites were described with nearly identical 8]80 values (-9.4 to -11.8%o).

F i n a l B u r i a l Diagenesis Following sub-aerial exposure (Salinic unconformity, sensu

Bourque, 2001, this issue), the Sayabec platform was flooded by marine water (transgressive phase T2; Bourque et al., 2001, this issue) and the Sayabec platform was again subject to burial diagenesis. The Salinic tectonic event caused uplift of the platform (Bourque, 2001, this issue) and fracturing. Fractures were later filled with cements precipitated from burial fluids (Fig. 14). The first burial fractures were largely coeval with the final stages of calcite cementation during primary porosity. Thus, some burial fractures may have been generated before Salinic tectonism, although most cementation is assumed to have occurred during uplift.

The fracture-filling, orange, dull-luminescent calcite cement (EBF) is moderately depleted in 180 (8180 = -12.4 to -16.9%e); it contains a wide range of 813C values (-3.3 to +2.7%o), moderate salinity (6.2 to 16.7 wt % NaCl-equivalent), and fluid inclusion T h values up to 106.7°C. The late burial fracture-filling, dull-luminescent calcite (BF) has more 180 depleted values (8180 = -9.1 to -20.4%o), a wide range of 813C values (-2.7 to +0.3%~), high salinity (14.1 to 19.6 wt % NaCl-equivalent), and very high fluid inclusion T h values of up to 226°C.

It is noteworthy that the 8180 values of the Lac Madeleine area are significantly more depleted in 180 compared with petrographically similar cements of the Rivibre Madeleine area (818ORiv. Madeleine-- 8180 Lac Madeleine = 5 to 13%o). In addition, T h

EVENTS

Sedimentation Cementation Compaction

Pressure solution

Salinlan uplift Fractures .i. meteoric ! dissolution

Meteoric cementatior

Burial fractures and cements

Hydrothermal events Uplift

Primary Po

Secondary Po

SAYABEC I LAC MADELEINE T I M E °'"l I"°°r'l

Shal low In term. (us-c) I

UoZ-LFS

LFS - LFS

EBF EBF-BF

Meteoric dissolution turlal fracture[,::>,..

Fig. 14. Diagenetic history of the Sayabec Formation in the Lac Madeleine area. Occlusion of primary pore space occurred in the shallow burial environment (LF2-LF3-LF4) with the final occlusion almost coeval with initial burial fractures (EBF) in the intermediate burial realm. This burial trend was stopped by sub-aerial meteoric diagenesis and secondary porosity generation (fractures + dissolution) rapidly closed by meteoric cements (LF5 and LF6). Renewed burial resulted in new fractures and fracture-filling calcite cementation (EBF-BF) almost synchro- nous with local hydrothermal events.

values of the fracture-filling calcite cements of the Lac Madeleine area are also significantly higher. The Lac Madeleine 5]80 values of the EBF and BF are too depleted in 180 to be related to meteoric fluids, an interpretation also supported by the high salinity and the T h values in associated fluid inclusions (see also the above discussion). Geochemical data from the Lac Madeleine fracture-filling calcites suggests that they are the result of precipitation at temperatures significantly higher than similar cements at the Rivibre Madeleine section. There are three possible explanations for this: (1) the cement precipitated from marine-derived brine in an area where the geothermal gradient was significantly higher; (2) precipitation occurred from a hydrothermal-type fluid; or (3) cement precipitation occurred in a mixture of these two end-member fluids.

The wide range of 813C values suggests that mixing of different carbon isotopic reservoirs occurred. A 5180-813C crossplot of the Lac Madeleine EBF calcite cements (Fig. 11) suggests that burial fluid composition was the result of a mixed, high-temperature system, containing an approximately equal measure of ]80- and 13C-depleted hydrothermal-like and marine-derived fluids (less depleted in 180 and normal marine 813C; e.g., values typical of the Rivi~re Madeleine area, see Fig. 10).

From a regional perspective, the Rivibre Madeleine and Lac Madeleine areas are 30 km apart and are characterized by a similar stratigraphic succession overlying the Sayabec Formation, without any significant variations in assumed burial depths (Bourque et al., 2001a, this issue). The post-Sayabec Formation succession is similar lithologically through northern Gasp6

296 D. LAVO1E and G. CH1

Peninsula, and is overlain by the upper part of the Chaleurs Group ( 2 kin), the Upper Gasp6 Limestones ( 1.5 kin), and the Gasp6 Sandstones (>2 kin) (Bourque et al., 2001a, this issue).

There is only one known in situ organic matter reflectance value for the entire Sayabec Formation, an R ° of 1.6% derived from the Lac Mataprdia Syncline at the western end of Gasp6 Peninsula (R. Bertrand, 2000, pers. comm.). This value suggests a hydrocarbon condensate zone and burial temperature of about 150°C, which is consistent with burial under roughly 5 km of strata (assumed geothermal gradient of 30°C/km, Bertrand, 1987). Such burial is consistent with the available stratigraphic information (Bourque et al., 2001a, this issue).

Magmatic activity occurred west of the Lac Madeleine area. Lachambre (1987) interpreted major hydrothermal alteration of the Sayabec Formation as being the result of massive replace- ment dolomitization in the Ruisseau Isabelle area (Fig. 1). Dolomitization has been shown to be associated spatially with a dense network of faults and fractures, and Upper Silurian to Lower Devonian felsic dykes and sills, and abundant hornfels and skarns locally intrude the Chaleurs Group. These strata tes- tify to intense high-temperature alteration of the sedimentary succession (Lachambre, 1987). This area, known as the "Drme de Lemieux", also contains many coeval mafic and felsic extru- sive flow deposits (Lachambre, 1987).

The dolomite at Ruisseau Isabelle is a massive saddle dolomite, which locally has significant porosity (Fig. 9B) filled with copper mineralization. Saddle dolomite is usually considered to precipitate from saline brines at relatively high temperature (>60°C; Radke and Mathis, 1980). Our stable isotope results (5180 = -16.9, -17.4 and -17.9%o; ~13C = -0.2, -1.6 and -0.3%0) are consistent with high-temperature precipitation from a 13C-depleted fluid. Moreover, the fluid inclusion data support precipitation of the dolomite from a highly saline (21.1 to 28.0 wt % NaCl-equivalent) and high temperature (up to 194.0°C) fluid.

In the area immediately west of the Lac Madeleine area, Lachambre (1987) mapped significant hydrothermal alteration zones (skarns, hornfels and dolomite) in the Sayabec Formation and in the Upper Silurian to Lower Devonian reefal West Point Formation. Base metal deposits (Sullipek and Barter deposits) occur in skarns and dykes. This high-temperature event was also recorded in the West Point Formation, based on a detailed diagenetic study by S avard et al. (1997). The eastern limit of the mapped alteration zone (Lachambre, 1987) is less than a kilo- metre away from studied sections of the Sayabec Formation in the Lac Madeleine area. The area immediately west of the Lac Madeleine area is interpreted as having been influenced by a major synsedimentary fault (the ancestral Grande Rivibre Fault; Bourque et al., 2000). The distribution of some Upper Silurian volcanics in central Gasp6 (Lac Mckay Volcanics, Bourque et al., 2000) is associated spatially with this major fault. Our cement geochemical data suggests that the hydrothermal event also affected the Sayabec Formation in the Lac Madeleine area, although dolomitization is not evident in outcrop.

In the Lac Madeleine area, well-cemented limestone ramp strata of the Sayabec Formation were exposed and altered in a subaerial environment during development of the Salinic unconformity. Following this, fractures in these carbonates were flow pathways for hydrothermal fluids and basinal brines, which together resulted in fracture occlusion by calcite cements (Fig. 14).

DISCUSSION

The data presented here are considered representative of only the exposed Lower Silurian carbonate platform in the Gasp6; a large part of the succession is buried beneath Silurian-Devonian strata. Because boreholes have not inter- sected the buried carbonate strata, the nature and reservoir potential of these strata are unknown.

REGIONAL METEORIC DIAGENESIS EFFECTS ON RESERVOIR POTENTIAL

The main objective of this research was to address the origin and distribution of dissolution porosity in the Sayabec Formation in areas where the formation had been partly eroded beneath the Salinic unconformity. Limited petrographic and geochemical evidence of subaerial diagenesis of the Sayabec Formation occurs in samples from the Lac Madeleine area. Although dissolution porosity did develop in the limestone, meteoric-related cements subsequently filled it. Interestingly, similar features are reported for the La Vieille Formation in southern Gasp6 (Lavoie and Bourque, 1993).

The limited evidence for meteoric diagenesis in the Sayabec limestones is problematic, because field relationships and regional distribution of unconformity-related conglomerates suggest that the Salinic unconformity was a significant regional event, particularly in our study area. A possible explanation for the paucity of meteoric diagenesis is suggested in the following. Late Silurian subaerial exposure of the Lower Silurian Sayabec Formation occurred following a previous diagenetic burial event, which most likely led to the mineralogical stabi- lization of metastable primary mineralogical components (aragonite and high-magnesium calcite to diagenetic low-magnesium calcite) (Choquette and James, 1990). It is well known that, over a given time, percolation of meteoric waters through a low-magnesium, calcite-dominated succession will result in less dissolution compared with a succession containing predominantly aragonite or high-magnesium calcite (James and Choquette, 1990). Nonetheless, given the relatively significant time of subaefial exposure suggested by the thickness and nature of unconformity-related conglomerates in eastern Gasp6 (Griffon Cove and Owl Cape conglomerates; Bourque et al., 1995), some evidence for dissolution should be expected.

However, limited evidence for subaerial porosity development is present in the Lac Madeleine area. There, porosity appears to have developed due to the movement of hydrothermal fluids, augmented by faults and intrusives (Lachambre, 1987). The diagenesis that resulted from these has not been recognized previously in the Sayabec Formation at this locality. It

THE LOWER SILURIAN SA YABEC FORMATION 297

is possible that hydrothermal alteration, which resulted in significant fracturing, obscured all evidence for subaerial exposure in the Sayabec Formation.

The eastward extension of hydrothermal alteration in the Sayabec Formation is unknown. The succession at Rivi6re Madeleine (30 km east of the Lac Madeleine area; Fig. 1) does not appear to have been altered by hydrothermal fluids, and no evidence for meteoric diagenesis is observed.

HYDROTHERMAL DOLOMITE RESERVOIR POTENTIAL

Another point addressed herein is the reservoir potential of the local massive dolostone of the Sayabec Formation. The study of the dolostone at Ruisseau Isabelle has shown that it comprises high-temperature saddle dolomite. Locally, some large vugs are visible and partly filled by base metals (Lachambre, 1987).

Fractures generated during Late Silurian tectonism acted as conduits for high-temperature, highly saline fluids. These fluids resulted in dolomitization of the limestone facies, and dolomite was locally precipitated in the newly formed secondary porosity. The reservoir potential of this porosity network in a system that contains evidence of late stage hydrocarbon migration is currently unknown. A late hydrocarbon migration event is suggested by the presence of highly fluorescent and likely low maturity hydrocarbon fluid inclusions in cements in underlying Val-Brillant Sandstones (Lavoie and Chi, 1997) and overlying West Point Formation reefal limestones and Upper Gasp6 Limestones (Lavoie et al., 1997; Savard et al., 1997; Bourque et al., 2001b, this issue; Lavoie et al., 2001, this issue). However, it is clear that more detailed studies on the hydrothermal dolomite of the Ruisseau Isabelle area and timing of hydrocarbon charge are needed in order to seriously evaluate the reservoir potential of this unit.

CONCLUSIONS

This is the first diagenetic study of the Lower Silurian Sayabec Formation in the Northern Outcrop Belt of northern Gasp& Three areas were investigated in detail. The first two areas, the Rivi~re Madeleine and Lac Madeleine areas, were selected because the strata contains evidence of a subaerial exposure event that affected part of the limestone platform. The third locality (Ruisseau Isabelle) was selected because of the occurrence of massive dolostone. Several diagenetic analyses were performed on the carbonate platform facies, including conventional and cathodoluminescence petrography, fluid inclusion microthermometry, and C and O stable isotope geochemistry. We have extended the distribution of the carbonate platform facies that were known previously to occur only in the type-area (Matap6dia Syncline, western Gasp6; Lavoie, 1988; Lavoie et al., 1992), and recognize these strata throughout northern Gasp&

The overall diagenetic evolution of the Sayabec Formation consists of (1) local (Rivi~re Madeleine) marine calcite cementation, synsedimentary neptunian dykes, and minor dolomitization; (2) burial calcite precipitation in primary pore spaces and

stylolites; (3) fracture-dissolution and meteoric calcite cementation (Lac Madeleine) due to the combined effects of tectonism and relative sea-level fall (Salinic unconformity); and (4) multiple episodes of burial fracturing, calcite cementation, and local hydrothermal dolomitization (Ruisseau Isabelle). Besides the occurrence of large, open vugs associated with local, late hydrothermal dolomitization, no visible porosity was observed. Interestingly, the area between Ruisseau Isabelle and Lac Madeleine contains evidence of a regional hydrothermal event that followed Late Silurian tectonism (Salinic disturbance), based on stable isotopes and fluid inclusion microthermometry.

The overall diagenetic sequence of the Sayabec Formation in the Northeru Outcrop Belt differs from that of the type-area (Lac Matap6dia Syncline; Lavoie, 1988; Lavoie and Bourque, 1993) because it contains evidence of subaerial exposure and a late, high-temperature alteration event related to Late Silurian tectonism. Conversely, diagenesis of the Sayabec Formation in northern Gasp6 compares well with that of the coeval La Vieille Formation of southern Gasp6, where marine diagenesis was followed by burial diagenesis. This normal diagenetic succession was interrupted locally by subaerial exposure and meteoric diagenesis related to the Salinic unconformity (Lavoie, 1988; Lavoie and Bourque, 1993). As stated above, the absence of the late, high-temperature alteration event is the only significant difference between the two areas.

From a reservoir perspective, primary porosity in the Sayabec Formation was eliminated abruptly during initial shallow burial, most likely before hydrocarbon migration. The Late Silurian (post-initial burial) Salinic unconformity locally generated some secondary (fracture and dissolution) porosity. However, the pore volume was small and subsequently filled by meteoric-derived calcite cements.

Multiple-episode fracturing occurred during burial but these fractures were also filled with calcite cements, with no evidence of hydrocarbon migration during cementation. A significant hydrothermal event is recorded in the western part of the study area. Finally, massive dolostone occurs locally as a replace- ment, and fracture-fill saddle dolomite occurs in the Ruisseau Isabelle area. There, significant secondary porosity is developed, although its timing and potential for hydrocarbon charge is not presently known.

ACKNOWLEDGMENTS

Discussions with all other researchers involved in the Gasp6 Project were certainly helpful in understanding the tectono-diagenetic evolution of the Silurian-Devonian Gasp6 Belt. I am particularly indebted to Pierre-AndrE Bourque who, as a thesis supervisor, introduced me to the superb geology of the Gasp6 Belt. I am grateful for his constructive comments on this paper. An early draft of this manuscript benefited from an in-depth review by Martine Savard, and CSPG reviewers George Dix and Andre Desrochers provided helpful and welcome comments. We would like to acknowledge the financial support of


Shell Canada for the study o f this L o w e r Silurian success ion

and for pe rmiss ion to publ ish these results. The Geologica l

Su rvey o f C a n a d a A p p a l a c h i a n F o r e l a n d and P l a t f o r m

N A T M A P projec t is also thanked for f inancial support . This is

Geologica l Survey o f Canada Cont r ibut ion No. 200089.

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