The Lower Silurian Sayabec Formation in northern Gasp6: carbonate ...

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.

R£StrM~ La Formation silurienne infrrieure de Sayabec reprrsente une rampe ~tcarbonates ~ 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 6galement 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 remplissage de pores et de fractures reprrsentant localement un mrlange de saumures de bassin et de fluides hydrothermaux. 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.

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THE LOWER SILURIAN SAYABEC FORMATION

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 reconstruction, 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. PALEOGEOGRAPHICAND ENVIRONMENTALSETTING 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 mudfiat, containing mostly microbial carbonates (laminites, stromatolites, 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

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Stratigraphy Carboniferous

Sample Locations RI = Ruisseau Isabelle LM = Lac Madeleine RM = Rivi~re Madeleine

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 accommodation space and a marine setting too deep to record the subsequent shallowing events (Lavoie et al., 1992). In detailed maps, Lachambre (1987) illustrated the smallscale 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 coarsegrained 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

285

THE LOWER SILURIAN SAYABEC FORMATION

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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 pseudosecondary origin: isolated, clustered, scattered, and randomly distributed in three dimensions.

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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 synchronous 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

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 replacement 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 testify 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 extrusive 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 kilometre 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.

D. LAVO1E and G. CH1

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 intersected the buried carbonate strata, the nature and reservoir potential of these strata are unknown. REGIONAL METEORIC DIAGENESISEFFECTS 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 stabilization 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

297

THE LOWER SILURIAN SA YABEC FORMATION

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. HYDROTHERMALDOLOMITERESERVOIRPOTENTIAL

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 replacement, 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

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D. LA VOIE and G. CHI

Shell C a n a d a for the study o f this L o w e r Silurian s u c c e s s i o n a n d for p e r m i s s i o n to p u b l i s h t h e s e results. T h e G e o l o g i c a l Survey

of

Canada

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Foreland

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Platform

N A T M A P p r o j e c t is also t h a n k e d for financial support. This is G e o l o g i c a l S u r v e y o f C a n a d a C o n t r i b u t i o n No. 200089. REFERENCES Bertrand, R. 1987. Maturation therrnique et potentiel prtrolig~ne des stiles post-taconiennes du Nord-Est de la Gasprsie et de l'lle d'Anticosti. Unpublished D.Sc. thesis, Nefichatel, Switzerland, 647 p. , Lavoie, D. and Goodarzi, F. 1992. Evolution de la maturation thermique dans les Calcaires Suprrieurs de Gasp6 entre la p6ninsule de Forillon et Murdochville, Qurbec. Current Research Forum of the Geological Survey of Canada, Program with abstracts, p. 9. Bourque, P.-A. 1975. Lithostratigraphic framework and unified nomenclature for Silurian and basal Devonian rocks in eastern Gasp6 Peninsula, Qurbec. Canadian Journal of Earth Sciences, v. 12, p. 858-872. 2001a. Sea level, synsedimentary tectonics, and reefs: implications for hydrocarbon exploration in the Silurian-lowermost Devonian Gasp6 Belt, Qurbec Appalachians. Bulletin of Canadian Petroleum Geology, this issue. and Lachambre, G. 1980. Stratigraphie du Silurien et du Dfvonien basal du sud de la Gasprsie, MinistSre de l'Energie et des Ressources du Qurbec, ES-30, 123 p. Amyot, G., Desrochers, A., Gignac, H., Gosselin, C., Lachambre, G. and Lalibertr, J.-Y. 1986. Silurian and Lower Devonian reefs and carbonate complexes of the Gasp6 basin, Qurbec - a summary. Bulletin of Canadian Petroleum Geology, v. 34, p. 452-489. , Bfisebois, D. and Malo, M. 1995. Gasp6 Belt. In: Geology of the Appalachian-Caledonian Orogen in Canada and Greenland. H. Williams (ed.). Geological Society of America, Geology of North America, v. F-I, p. 316-351. Malo, M. and Kirkwood, D. 2000. Paleogeography and tectonosedimentary history at the margin of Laurentia during Silurian-earliest Devonian time: the Gasp6 Belt, Qurbec. Geological Society of America Bulletin, v. 112, p. 4-20. and _ _ 2001a. Stratigraphy, tectono-sedimentary evolution and paleogeography of the post-Taconian-pre-Carboniferous Gasp6 Belt: an overview. Bulletin of Canadian Petroleum Geology, this issue. Savard, M.M., Dansereau, P. and Chi, G. 200lb. Diagenesis and porosity evolution of the Upper Silurian-lowermost Devonian West Point Reef Limestones, eastern Gasp6 Belt, Quebec Appalachians. Bulletin of Canadian Petroleum Geology, this issue. Choquette, P.W. and James, N.P. 1990. Limestones~the burial diagenetic environment. In: Diagenesis. I. Mcllreath and D.W. Morrow (eds.). Geoscience Canada, Reprint Series 4, p. 75-111. Desrochers, A. 1981. l~tude srdimentologique de la Formation de La Vieille dans la rrgion de Clemville-Port-Daniel, Baie des Chaleurs. Unpublished M.Sc. thesis, Universit6 Laval, Qurbec, 49 p. _

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