- © The Society of Canadian Petroleum Geologists
Sea-level changes and synsedimentary tectonics affected the development of three potential hydrocarbon reservoir intervals in the Silurian–lowermost Devonian part of the Gaspé Belt on the margin of Laurentia. A relative sea-level curve is based on sequence analysis of facies and distribution of benthic faunas. The succession recorded two major, low-order sedimentary cycles, each composed of a regressive–transgressive couplet and represented by shallowing- and deepening-upward sequences. The first shallowing episode (Ss) covered the Rhuddanian–Sheinwoodian (Llandovery–early Wenlock) interval, whereas a rapid deepening episode (D1) followed during the Homerian (late Wenlock). A second shallowing episode (S2) started during the late Homerian, and ended in the late Ludfordian (late Ludlow) or earliest Pridolian, and was then followed by a latest Silurian–Early Devonian deepening episode (D2). Potential reservoir rock units developed mostly during sea-level lowstands or early transgressions in the Gaspé Belt. Comparison of the Gaspé relative sea-level curve with eustatic curves, proposed for the same time interval, indicates that the Gaspé curve was significantly influenced by local synsedimentary tectonics.
Salinic extensional tectonics, a predominantly Late Silurian–Early Devonian (Ludlovian–Pragian) event, resulted in block faulting and tilting along normal listric faults. Interaction between block faulting and eustatic sea-level fall at the end of the Ludlovian–early Pridolian exposed the highest parts of the blocks to subaerial erosion and diagenesis, and allowed reefs and reef complexes to settle at the margins of the blocks, or on erosional remnants. Salinic extensional tectonics may also have provided suitable pathways for hydrocarbon migration and charge at various times during the mid-Silurian to Early Devonian, particularly for hydrocarbons that may have been generated in the underlying Cambro–Ordovician shales. In the northeastern part of the Gaspé Belt, which is the most promising area for hydrocarbon reservoirs in the Gaspé Peninsula, block faulting and tilting have changed the fluid migration pattern from an updip-northeastward flow from the basin centre to basin margin during the Late Ordovician to early Late Silurian, to a potential updip-southwestward flow in each faulted block during the early Late Silurian. Basin tectonics may have played a significant role in driving hydrocarbon-rich fluids toward potential reservoirs, such as the Llandoverian Val-Brillant sandstone bodies, the Sayabec carbonate sands and knob reefs, or the West Point reefs.
Sea-level changes and synsedimentary tectonics are two major controls on terrigenous sandbody deposition and geometry, and on reef settlement and development. Other factors are: water energy, temperature, and chemistry; terrigenous and nutrient input; and community faunal dynamics. This paper investigates the effects and interplays of the first two parameters on the development of three potential hydrocarbon reservoir rock horizons in the Silurian–lowermost Devonian part of the Gaspé Belt: the Llandoverian sandstone bodies, the Wenlockian extensive reef-rimmed peritidal flats, and the Ludlovian–Lochkovian reef tract. All of these developed on the margin of Laurentia.
Deciphering the relative importance of eustasy and tectonism on sea-level changes is not easy, particularly for a basin that encountered significant synsedimentary tectonism during its evolution. The Gaspé Belt (sensu Bourque et al., 1995) developed and evolved between two major tectonic events that shaped the Appalachian mountains: 1) the Taconian Orogeny, which corresponds to the initial phase of closure in the Iapetus Ocean due to an oceanic arc-continent (Laurentia) collision the Late Ordovician; and 2) the Acadian Orogeny, which ended in a continent–continent collision when the western margin of Gondwana collided with the Laurentia margin in the Middle Devonian. The Upper Ordovician to Middle Devonian rock sequence that makes up the Gaspé Belt was deposited in a shelf-to-basin setting that was tectonically active from the Early Silurian to the end of the Acadian Orogeny in the Middle Devonian (Malo and Bourque, 1993; Bourque et al., 1995, 2000). Synsedimentary tectonic activity is recorded as extensional faulting and minor folding from the late Llandoverian to the Pragian, and is referred to as the Salinic extensional tectonic regime herein (the same as the Salinic disturbance of Boucot, 1962). This regime changed during the Pragian to a transpressional regime corresponding to the Acadian Orogeny in the Emsian-Eifelian (Bourque et al., 2001a, Fig. 4⇓, this issue).
This paper first presents and discusses the relative sea-level curve for the Silurian–earliest Devonian in the Gaspé Belt, based on sequence analysis, and compares that curve to proposed eustatic curves for the same time interval (e.g., several papers in Witzke et al., 1996; Landing and Johnson, 1998). It subsequently examines the relationships between sea-level fluctuations and the three potential rock reservoir units, emphasizing the critical eustatic versus tectonic controls on the genesis of these rock bodies. The effects of the Salinic disturbance on basin geometry and architecture are examined in detail, and a model with implications for oil exploration is proposed.
The geological map of the Gaspé–Témiscouata segment of the Gaspé Belt, as well as detailed stratigraphic nomenclature and correlations, are presented in Bourque et al. (2001a, Figs. 2⇓, 3⇓, this issue). This paper is concerned only with the Chaleurs Group, which ranges in age from the Early Silurian (Llandovery) to Early Devonian (Lochkovian). Detailed descriptions of each lithostratigraphic unit of the group are reported in Bourque et al. (1993) and Globensky (1993).
The Chaleurs Group contains more diversified sedimentary facies than other groups of the Gaspé Belt, which is helpful for sequence analysis. In order to summarize the sedimentary evolution of the Gaspé Belt for the Silurian–earliest Devonian, sequence analysis is presented for six representative rock successions of the Chaleurs Group (Figs. 1⇓, 2⇓). Five (Fig. 2, A, B, D, E, F⇓) are shelf successions (Fig. 1B⇓), in which relative sea-level changes are more likely to have influenced facies architecture and evolution, whereas succession C represents basin deposits where sea-level changes were less easily recorded. The curves in Figure 2⇓ show the positions of facies with respect to the shoreline and time, so they can be viewed as local paleobathymetric curves. Two major sedimentary cycles are recognized in the facies succession for the latest Ordovician (post-Taconian)–earliest Devonian. Each of these cycles is interpreted as a shallowing-deepening couplet, denoted S1–D1 or S2–D2.
Chaleurs Bay Synclinorium (Succession A, Fig. 2⇑)
The Silurian succession of the Chaleurs Bay Synclinorium (Fig. 1⇑) is one of the most complete Silurian successions in North America. Following lime-turbidite sedimentation of the White Head Formation (Matapédia Group) in the earliest Silurian (Bourque et al., 1995), siliciclastic sedimentation predominated in the Chaleurs Bay Synclinorium. The Clemville–Weir–Anse Cascon–La Vieille succession resulted from a shallowing phase, S1. In detail, however, this shallowing-upward sequence resulted from a number of higher-order sea-level changes. A detailed paleobathymetric curve for this succession was established using the benthic assemblage scheme of Boucot (1975), mainly based on brachiopod paleo-communities and, to a lesser extent, on other benthic fauna (Bourque, 1981). The Clemville section north of Port-Daniel (Fig. 1A⇑) is representative of the benthic assemblage distribution of this succession (Fig. 3⇓). There, the Matapédia Group is absent, but the Clemville Formation and part of the Weir Formation are time-equivalent to the Silurian part of the Matapédia Group. The succession began with deposition of fine-grained siliciclastic tempestites of the Clemville Formation, containing an offshore benthic fauna. The depositional setting progressively shallowed to give way to deposition of the Weir sands and gravels characterized by an intertidal-type brachiopod association. Subsequently, and until the early Wenlockian, the depositional facies oscillated between shoreline sands (Anse Cascon Formation) and peritidal limestones fringed by coralgal knob reefs and shallow subtidal lime sands (La Vieille Formation) (Bourque et al., 1986; Desrochers and Bourque, 1989; Lavoie et al., 1992). One significant exception is a high-order deepening phase at the end of the Llandoverian, recorded in the offshore muds and muddy limestones of the Anse à Pierre-Loiselle Formation, which contains the brachiopod Costistricklandia, commonly indicative of below wave base marine conditions (Boucot, 1975). Trace fossil associations in the Anse Cascon and Anse à Pierre-Loiselle formations at the Anse à Pierre-Loiselle section, about 15 km east of the Clemville section, also reflect this environment (Pickerill et al., 1977). In addition, in the upper Anse Cascon and lower Anse à Pierre-Loiselle formations, a Chondrites trace fossil association (Chondrites, Planolites and minor Skolithos), indicative of sub-tidal and lagoonal, low-energy environment, is reported. The trace fossils are restricted to the top of the sandstone beds and developed during intervals of low deposition or even non-deposition. These beds are interbedded with strata deposited on a storm-influenced shoreface (Bourque, 1989a). Also, the presence of Diplocraterion indicates a very shallow subtidal or intertidal environment and occurs locally in the upper part of the Anse Cascon Formation.
The deepening phase, D1, began during the Wenlockian with deposition of outer shelf, nodular, lime-mudstones belonging to the upper member of the La Vieille Formation, and followed by offshore-type, fine-grained, siliciclastic sedimentation of the Zoophycos-rich Gascons Formation, deposited in a low-energy environment, below storm wave base. This interpretation is supported by the brachiopod faunal data, the trace fossil assemblages, and the lack of wave-formed sedimentary structures.
The progressive increase in stromatoporoid and coral benthic fauna in the upper part of the Gascons Formation, together with the transition from a Zoophycos to a Scalarituba–Cosmoraphe trace fossil association (Pickerill et al., 1977), indicates gradual shallowing, and the initiation of the S2 phase. Like the shallowing phase Sl, shallowing phase S2 contains a number of higher order sea-level changes. The West Point Formation above the Gascons Formation comprises at least twelve higher-order sequences (Fig. 4⇓). Together with the Scalarituba–Cosmoraphe-rich siliciclastics of the upper Gascons Formation, the stromatoporoid and coral-rich, mixed siliciclastics–limestones of the lower part of the lower mound and reef complex constitutes the first sequence. Its uppermost part is rich in colonial rugose thickets and small bioherms (Bourque et al., 1986) that more likely developed in the photic zone. They are overlain by a red stromatactis limestone (Gros Morbe Member) interpreted as representing sponge mounds (Bourque and Gignac, 1983; Bourque et al., 1986). The abrupt change from a coral-rich facies to sponge facies indicates a change from photic to aphotic conditions, related to deepening. The sponge community is viewed as representing the deepest benthic community that developed below the photic zone (Bourque et al., 1995; Bourque, 1997), and more probably into the oxygen minimum zone (Stanton et al., 2000).
The overlying approximately 200 m thick sponge mound and massive microbial reef limestone of the lower mound and reef complex constitutes the second higher order shallowing sequence of the West Point Formation (Fig. 4⇑). The sponge mounds existed below storm wave base and the photic zone, whereas the green alga-rich microbial reefs developed in shallower waters of the photic zone (Bourque et al., 1986; Bourque and Raymond, 1989). The top of this last sequence was subaerially exposed and is marked by a paleokarst surface (see Bourque et al., 2001b, this issue). The overlying 30 m thick middle crinoid bank complex comprises six shallowing-upward sequences. A typical one contains a lower crinoid-rich biostromal limestone that exhibits a progressive upward increase in the number of stromatoporoids and amount of siliciclastics. These are capped by nearshore siliciclastics and a subaerial erosion surface (Gosselin, 1981; Bourque et al., 1986; Bourque et al., 2001b, this issue). The three members of the 600 m thick upper West Point reef complex represent a stromatoporoid reef margin, back-reef to intertidal, and supertidal redbed facies (Fig. 4⇑). Each of these facies is repeated in the four shallowing-upward sequences of the West Point Formation (Morin, 1986; Bourque and Amyot, 1989).
Deepening phase D2 began in the late Pridolian with burial of the West Point buildup by Zoophycos-bearing fine-grained siliciclastics of the Indian Point Formation. No rocks younger than the latest Pridolian are known in the Chaleurs Group.
Ristigouche Syncline (Succession B, Fig. 2⇑)
The lithological and faunal assemblages of the Mann Formation overlying the lime-turbidites of the White Head Formation show a progressive shallowing-upward trend corresponding to phase S1, as recorded by the presence of the benthic fauna (Bourque and Lachambre, 1980). The Anse à Pierre-Loiselle nodular muds on top of the Mann Formation indicate a minor deepening phase, similar to the one recorded in the Chaleurs Bay Synclinorium succession. In the uppermost Anse à Pierre-Loiselle Formation, the occurrence of Pentamerus oblongus brachiopod banks indicates a high-energy subtidal environment in the photic zone (Boucot, 1975). The La Vieille Formation, which marks the end of the S1 phase, consists of the same peritidal limestones and coralgal knob reefs that characterize the formation in the Chaleurs Bay Synclinorium. The Salinic erosion (see below) removed all the Ludlovian-aged beds, except the Ristigouche volcanics where the D1 and S2 phases are not detected. The Pridolian New Mills conglomerates and redbeds above the Salinic unconformity mark the terrestrial to shallow-marine transition between S2 and D2. The D2 deepening phase is indicated by the deposition of fine-grained siliciclastics of the Lochkovian–lower Pragian Dalhousie Formation containing subtidal shelly fauna, and interbedded with mafic and minor felsic submarine lava flows.
Central Gaspe Peninsula (Succession C, Fig. 2⇑)
This composite succession is exposed in the Gastonguay Anticline, the western tip of the Saint-Jean River Anticline, the Mont Alexandre Syncline, and the Angers–Dugal Belt (Fig. 1B⇑). It is a basinal facies: lime-turbidites of the White Head Formation, nearly azoic claystone of the Burnt Jam Brook Formation, turbidites and debrites of the Laforce Formation (Lavoie, 1988), massive fine-grained siliciclastics with a few graptolites of the Saint-Léon Formation, and turbidites of the Fortin Group (Dalton, 1987; Hesse and Dalton, 1995). These deep-water facies did not contain the sedimentary cycles detected in the shelf successions.
Northeastern Gaspe Peninsula (Succession D, Fig. 2⇑)
In the Dartmouth River–Forillon area at the eastern end of the Northern Outcrop Belt (Bourque et al., 2001a, Fig. 1⇑, this issue), Upper Silurian (Salinic) erosion stripped off the entire Llandovery to Ludlow succession; therefore, phases S1, D1 and S2 are not observed. The Pridolian Griffon Cove River conglomerate and redbeds overlie the Salinic unconformity and mark the transition between S2 and D2 as in the Ristigouche Syncline. The D2 phase is well documented in this area (Fig. 5⇓) (Bourque et al., 1986; Bourque, 1989b). The West Point Formation above the Griffon Cove River Formation is composed of a supratidal–intertidal assemblage of mudcracked redbeds, microbial limestone, and stromatoporoid rubble, which correlate with the upper West Point reef complex in the Chaleurs Bay Synclinorium (Fig. 4⇑). Four shallowing-upward sequences are recognized in this intertidal–supratidal facies assemblage (Fig. 5⇓) including a lower, thick-bedded, structureless, argillaceous, and silty, fossiliferous lime mudstone of the Roncelles Formation, a middle turbiditic, fine-grained, siliciclastic succession assigned to the Indian Point Formation, and an upper, deep water lime mudstone of the Upper Gaspé Limestone Group (Lavoie, 1992a, 1992b).
Madeleine River Area (Succession E, Fig. 2⇑)
The succession in the Madeleine River area along the Northern Outcrop Belt (Fig. 1⇑) records the two major S1–D1 and S2–D2 sedimentary cycles. Its base lies unconformably on the Cambrian–Ordovician rocks of the Humber Zone deformed during the Taconian Orogeny. Unlike the succession to the east, the Salinic erosion of Silurian units was minor. The transition from deep-water clays and carbonate muds of the Awantjish–Sources Formation to coastal, clean quartz sands of the Val-Brillant Formation (Lajoie, 1968), and finally the peritidal limestones of the Sayabec Formation (Lavoie, 1988; Lavoie et al., 1992), represents the S1 phase. The following D1 phase started in the Wenlockian with deposition of the nodular limestones of the upper part of the Sayabec Formation and culminated in deposition of the offshore, fine-grained siliciclastics of the Gascons Formation. The ensuing S2 phase culminated in Salinic uplifting that caused partial and local erosion of Silurian deposits. Pinnacle reefs of the West Point Formation settled on topographic highs that resulted from differential erosion (Bourque et al., 2000, Fig. 6⇓) during early stages of phase D2 and were buried under fine-grained siliciclastics of the Indian Point Formation.
Temiscouata Region (Succession F, Fig. 2⇑)
As in the Chaleurs Bay Synclinorium, the two major sedimentary cycles are well developed in the Témiscouata region (Fig. 1⇑). At the base of the succession, the deep-water turbiditic Cabano Group passes upsection to volcaniclastic gravels and shallow-water lava flows of the Pointe-aux-Trembles Formation. In the uppermost part of the latter, the facies is terrestrial (David et al., 1985) and grades into the overlying redbeds of the Robitaille Formation. On top of this shallowing-upward sequence (phase S1), the Sayabec Formation contains redbeds and peritidal limestones, that were deposited as a result of minor, high-order, sea-level fluctuations. The following D1 phase started in the Wenlockian with the deposition of outer shelf, nodular limestones of the upper member of the Sayabec Formation (Lavoie et al., 1992) and was completed with the deposition of the offshore-type, fine-grained, siliciclastics of the Saint-Léon Formation. During the following S2 phase, lower reefal limestones of the Lac Croche Formation were deposited. Recurrence of the fine-grained siliciclastic strata of the Saint-Léon Formation above the reef limestones, followed by deep-water limestones of the Cap Bon Ami Formation, represents phase D2.
Relative Sea-Level Curve of the Gasple Belt, and Comparison with Silurian Ecstatic Curves
A relative sea-level curve for the Silurian–lowermost Devonian shelf at the margin of the Québec Re-entrant–St. Lawrence Promontory is based on data from the Chaleurs Bay Synclinorium (Figs. 3⇑, 4⇑) and the eastern part of the Northern Outcrop Belt (Fig. 5⇑), and is compared to Silurian eustatic curves reported elsewhere (Fig. 6⇑).
We built the Llandovery part of the Gaspé Belt sea-level curve, using Boucot’s (1975) benthic assemblages as bathymetric indicators (Fig. 3⇑), as did Johnson et al. (1998) in their study of Silurian cratonic strata. Johnson et al. (1998) interpreted two deepening events within the Rhuddanian–Aeronian interval, while Ross and Ross (1996) interpreted general deepening. However, we infer general shallowing followed by lowstand in the Gaspé Belt. The Weir Formation arkosic conglomerate and sandstone that abruptly overly the fine-grained siliciclastics of the Clemville Formation in the Chaleurs Bay Synclinorium (Fig. 2A⇑) indicate rejuvenation of the source area. Such a rejuvenation may have occurred either because of local uplift of the adjacent Maquereau-Mictaw source area, or eustatic sea-level fall, or a combination of both. In the nearby Ristigouche Syncline area to the west (column B, Fig. 2⇑), the coeval White Head–Mann sequence records the same shallowing trend, a conclusion based on the benthic assemblage succession (A.J. Boucot, written comm. to P.-A. Bourque). The same general shallowing trend is recorded in the coeval Cabano–Pointe-aux-Trembles sequence in the Témiscouata region. Therefore, in contrast to the sea-level curves of Ross and Ross (1996) and Johnson et al. (1998), the Rhuddanian–Aeronian shallowing episode of the Gaspé Basin is probably related to basin infill and local tectonism following the Taconian Orogeny, and not to eustacy.
The two Telychian sea-level highstands shown by the Gaspé Belt curve (earliest and latest Telychian) are recorded in the Chaleurs Bay Synclinorium, the Ristigouche Syncline, and the Northern Outcrop Belt between Madeleine River and Lake Matapédia (Lavoie et al., 1992). They are viewed as basin-wide events, equivalent to highstands 3 and 4 of Johnson et al. (1998). The intervening sea-level lowstand corresponds to the deposition of extensive sandbodies in the Gaspé Belt (Anse Cascon, Mann, Val-Brillant, and Robitaille formations). This lowstand may correspond to the Telychian lowstand of Ross and Ross (1996), and therefore be eustatic in origin, just as the two Telychian sea-level highstands.
The Wenlockian Sayabec–La Vieille platformal limestones are the oldest shallow-water carbonates in the basin. The Sayabec Formation of the Temiscouata area (Lavoie et al., 1992), as well as the La Vieille Formation in the Chaleurs Bay Synclinorium (Black Cape section; Bourque, 1981), record two shallowing episodes, each culminating in peritidal algal–microbial facies (equivalent to benthic assemblage 1). It is tentatively suggested that these two lowstands are eustatic in origin. The first one may correspond to the early Sheinwoodian lowstand reported by Ross and Ross (1996) and Johnson et al. (1998), and also McKerrow (1979) for the continental mass west of the Iapetus Ocean. The second lowstand possibly corresponds to the Sheinwoodian–Homerian boundary lowstand of Ross and Ross (1996), but it was not reported by Johnson et al. (1998). However, it should be stressed that the age of these two low-stands in the Gaspé Belt is poorly constrained: the first is post-Llandoverian C6, while the second is in the pre-Monograptus ludensis graptolite Zone (pre-late Wenlockian).
The age of the Gaspé Belt D1 deepening phase is poorly constrained, but the relative sea-level rise was demonstrably near, or at, its maximum by the late Wenlockian (M. ludensis Zone). Fine-grained siliciclastics of the Gascons Formation in the Chaleurs Bay Synclinorium locally contain deep marine shelly fauna (benthic assemblage 4 or 5), or elsewhere are devoid of shelly fauna but contain graptolites (Bourque and Lachambre, 1980). For the same time interval, overall shallowing (Johnson et al., 1998) and highstand (Ross and Ross, 1996) are reported.
According to McKerrow (1979) and Ross and Ross (1996), the latest Ludlovian-earliest Pridolian was characterized by a global sea-level fall, although, according to Johnson et al. (1998) a sea-level rise (their highstand 8) occurred in the early Pridolian after a latest Ludlovian sea-level fall. In the Chaleurs Bay Synclinorium, the shallowing trend (S2) started during the deposition of the upper part of the Gascons Formation, which has not been dated more precisely than Ludlovian, and culminated with the paleokarst surface on top of the lower mound and reef complex of the West Point Formation (Fig. 4⇑). Thereafter, the West Point platform was influenced by minor oscillations in sea-level, that resulted in an interbedded succession of supratidal and shallow subtidal facies (middle bank and upper reef complexes). This was formed by D2 deepening. The lowstand between the culmination of the S2 phase and beginning of the D2 phase is not precisely age constrained, because the age of the West Point Formation is uncertain. The following are known: 1) the base of the West Point Formation is dated no more precisely than Ludlovian; 2) the fine-grained siliciclastics of the Indian Point Formation, overlying the upper West Point reef complex, are late Pridolian, based on conodonts from the Chaleurs Bay Synclinorium (Bourque and Lachambre, 1980); 3) the entire West Point upper reef complex is Pridolian, based on its occurrence above the Pridolian Griffon Cove River conglomerate in northeastern Gaspé (Bourque, 1977; Bourque and Lachambre, 1980); and 4) the Salinic unconformity occurs below several Pridolian conglomerate units that were derived from uplifted fault blocks. The sea-level lowstand associated with this tectonic event is likely the same age as the paleokarst surface on top of the lower mound and reef complex of the West Point Formation in the Chaleurs Bay Synclinorium (see Bourque et al., 2001b, this issue). Based on these, the culmination of the S2 phase is thought to have occurred during the latest Ludlovian–earliest Pridolian, whereas the beginning of the D2 phase more likely occurred during the late Pridolian (Fig. 6⇑). However, the possibility that the culmination of the S2 phase may be slightly older or younger cannot be ruled out.
The S2 flooding event that corresponds to the change from colonial rugose coral facies to sponge mound facies at the base of the sponge mounds of the West Point mound and reef complex in the Chaleurs Bay Synclinorium is equated with highstand 7 of Johnson et al. (1998) (Fig. 6⇑). However, age data in support of this correlation are unavailable, except that the flooding event occurred before the culmination of the S2 phase. It may correspond to highstand 6 of Johnson et al. (1998) or reflect an independent event.
The late Ludlovian–Pridolian lowstand of the Gaspé Belt relative sea-level curve possibly correlates with the end of the Ludlovian–Pridolian worldwide eustatic lowstand, although in the Gaspé Belt it is difficult to distinguish the role of local tectonism and eustasy. On the one hand, it is hard to envisage that this lowstand and associated significant erosion were the result of general uplift of the shelf area due to the Salinic disturbance, because the overall tectonic regime was extensional, and prone to subsidence rather than uplift. On the other hand, the presence of redbeds in the shelf area of the Gaspé Belt (Bourque et al., 2000, Fig. 13E) and in western Newfoundland at the St. Lawrence Promontory (Clam Bank Formation, Port-au-Port Peninsula), clearly indicates the development of an extensive coastal plain during the Pridolian, suggesting eustatic sea-level fall. There is no evidence for a Ludlovian-Pridolian sea-level fall in the basinfill (Fig. 2C⇑), because there was no such event; subsidence was faster than sea-level fall, or the sea floor was too deep to record eustacy.
The D2 deepening phase likely began during the late Pridolian in the Chaleurs Bay Synclinorium and in the eastern part of the Northern Outcrop Belt. In the early stage of transgression, pinnacle reefs of the West Point Formation developed along the Northern Outcrop Belt (Bourque et al., 2000), but were quickly buried beneath prograding siliciclastics. This second deepening phase was likely local, because a worldwide sea-level rise of this age is unknown (Ross and Ross, 1988). It could reflect increased foreland subsidence related to overthrusting during the Acadian Orogeny farther southeast (Bourque et al., 2001a, this issue). In New Brunswick, Nova Scotia, and Newfoundland, early deformation related to the Acadian Orogeny occurred during the Late Silurian (Van Staal and De Roo, 1996; Lin et al., 1994) and affected the Cambro–Ordovician platform succession, the Taconian foreland clastic succession, and the Humber Arm Allochthon in Newfoundland (Waldron and Stockmal, 1991).
Salinic Disturbance and Synsedimentary Tectonics
Although the Québec Appalachians were shaped mainly by two major orogenies, the Taconian and the Acadian, Silurian and Lower Devonian facies and distribution were significantly influenced by synsedimentary extensional tectonism related to the Salinic disturbance (Bourque et al., 2001a, Fig. 4⇑, this issue). This orogenic event was first recognized by Boucot (1962), based on a Late Silurian unconformity in the Gaspé. The Salinic disturbance is now recognized as a distinct, regional tectonic event which, in the Gaspé Belt, preceded the transpressional tectonic regime of the Acadian Orogeny. It began in the late Llandoverian (Telychian) and lasted until the Early Devonian (Pragian) when the Acadian tectonic regime predominated (Bourque et al., 1993; Malo and Bourque, 1993; Malo and Kirkwood, 1995; Bourque et al., 2000; Bourque et al., 2001a, this issue). Salinic extensional tectonics developed in response to the oblique collision of Laurentia and the western margin of Gondwana-related terranes to the south.
The effects of the Salinic disturbance on facies composition and distribution are particularly well exposed in the northeastern part of the Gaspé Peninsula (Fig. 7⇓). This part of the Gaspé Belt is characterized by NW-trending faults. The most significant are the Bras Nord-Ouest, the Troisième Lac, and the Gastonguay faults. The latter fault is a northwestern extension of the E-trending Grande Rivière Fault. These faults have been used to delimit three tectonic blocks (Roksandic and Granger, 1981; Héroux et al., 1983; Amyot, 1984; Bertrand, 1987; St-Julien and Bourque, 1990; Lavoie, 1992b; Berger and Ramsay, 1993): a NE block, north of the Bras Nord-Ouest Fault; a central block, between the Bras Nord-Ouest and the Troisième Lac faults; and a SW block, between the Troisième Lac Fault and the Grande Rivière–Gastonguay Fault.
The east-trending Grande Rivière Fault is part of the Acadian dextral strike-slip fault system of the southern part of the Gaspé Belt (Malo and Béland, 1989). Detailed structural analysis along the Bras Nord-Ouest and Troisième Lac faults (Béland, 1980; Berger and Ramsay, 1993) showed that these are Acadian dextral strike-slip faults with a vertical component of movement. The dextral strike-slip movement probably corresponds to the final Acadian movement in the northeastern part of the Gaspé Belt. However, these NW-trending faults are probably older than Acadian, and are thought to have been active during the Silurian and Lower Devonian in the Gaspé Belt. Sedimentological analysis of the Gaspé Sandstones (Rust, 1981; Amyot, 1984), the Upper Gaspé Limestones (Amyot, 1984; Lavoie, 1992b), and the Chaleurs Group (Bourque et al., 1993, 1995, 2000), together with seismic reflection data (Roksandic and Granger, 1981), suggest synsedimentary fault movement (see below). These movements were initiated in the late Llandoverian but terminated in the Pragian. Extension along the Gastonguay and Grande Rivière faults began during the sedimentation of the Burnt Jam Brook Formation (Bourque et al., 2000), whereas the Bras Nord-Ouest Fault was still active during deposition of the Gaspé Sandstones (Rust, 1981; Amyot, 1984).
Besides extensive evidence for the Salinic erosional unconformity, drastic changes in unit thicknesses on both sides of any given fault argue for synsedimentary normal fault movement. For example, significant thickening of the Saint-Léon Formation southwest of the Grande Rivière Fault (Bourque et al., 2000, Figs. 7⇑, 8⇓) suggests increased subsidence in the basin related to motion along an active fault ancestral to the Grande Rivière Fault. Erosion of several hundreds of metres of strata (up to 500 m in the Saint-Jean River Anticline, and 400 m in the Northern Outcrop Belt) cannot be explained solely by absolute sea-level fall. A significant component of this erosion must have been related to local fault-controlled block uplift.
Some of the SOQUIP seismic profiles in northeastern Gaspé Peninsula (Roksandic and Granger, 1981; St-Julien and Bourque, 1990), a sector considered to be the best area for oil exploration, support a block-faulting model and provide clues to the local style of the Salinic extensional tectonics. A cross-section compiled from seismic profiles (Fig. 7⇑; SOQUIP lines P-19, P-20 and P-31), shows that a northeasterly thickening of the Chaleurs and the Upper Gaspé Limestone groups occurs in both the central and the northeast blocks, strongly suggesting that significant tilting of the faulted blocks occurred during Salinic tectonic activity.
Controls on Reef Settlement and Development
The La Vieille–Sayabec Platform
The depositional model proposed for the La Vieille carbonate platform in the Chaleurs Bay Synclinorium (Bourque et al., 1986) is applicable to the Sayabec platform in the northern part of the Gaspé Belt (Lavoie et al., 1992). The reefs on this platform are low relief, small algal–coral–bryozoan knobs that developed on the margin of an extensive peritidal flat belt, in a very shallow low-energy environment (see Bourque et al., 2001a, Fig. 5C⇑ this issue, for paleogeographical extension of this platform at the margin of Laurentia). These reefs have little primary porosity, because the metazoan framework pore space was occluded by stromatolites or filled by lime mud (Lavoie and Bourque, 1993; Lavoie and Chi, 2001, this issue). However, a belt of clean carbonate sands, with higher primary porosity, lies seaward of the reef belt (Lavoie and Bourque, 1993).
The La Vieille–Sayabec platform likely developed during relatively stable tectonic conditions. The Salinic disturbance was initiated during the late Llandoverian, but later extension was confined to the Grande Rivière-Gastonguay Fault in the deeper basin, and did not affect the platformal facies until the Ludlovian to Pridolian. The shallow marine character and distribution of the facies were, therefore, likely controlled by eustacy (Fig. 6⇑, Sheinwoodian sea-level fall).
The West Point Reef Platform and Pinnacle Reefs
Reef complexes and isolated pinnacles formed a long, almost continuous, shelf-situated reef tract at the margin of Laurentia along the St-Lawrence Promontory–Québec Re-entrant during the Late Silurian–Early Devonian (Bourque et al., 2001a, Fig. 6⇑, this issue). Reef settlement and development were controlled by a complex interplay of synsedimentary tectonics and eustacy.
The West Point Formation of the Gaspé Belt is roughly divided into two reefal limestone packages (Bourque et al., 2001a, Figs. 3⇑, 4⇑, this issue): 1) Upper Silurian reef and bank complexes, well developed in the southern part of the Gaspé Belt (Chaleurs Bay Synclinorium) and locally present in the northeastern part of the Gaspé Belt (Fig. 5⇑); and 2) Lower Devonian (Lochkovian) pinnacle reefs, mostly known in the Northern Outcrop Belt and the East-Central Outcrop Belt. In this discussion, I recognize two informal subdivisions of the West Point Formation: the Silurian West Point and the Devonian West Point.
Silurian West Point
Of the three complexes that make up the Silurian West Point (Fig. 4⇑), the upper reef complex is the best developed, and represents a laterally well-zoned reefal platform composed of facies ranging from supratidal to reef margin. Although it is better developed in the southern part of the Gaspé Belt (Chaleurs Bay Synclinorium), the Silurian West Point is known also in the northern part of the Gaspé Belt. The landward part of the upper reef complex (Sandy Cove and Plage Woodmans members; Bourque et al., 1986), composed of distinctive stromatoporoid rubble, microbial laminites and mudcracked redbeds, occurs above the Griffon Cove River conglomerate in the easternmost part of the Northern Outcrop Belt. This exposure of the Silurian West Point Formation in the Northern Outcrop Belt presents the same four sedimentary cycles as those recorded in the Chaleurs Bay sections (Fig. 5⇑). The presence of stromatoporoid rubble units, interpreted in the Chaleurs Bay area as storm rubble derived from the reef margin (Bourque et al., 1986), indicates the occurrence of a reef margin south of the Northern Outcrop Belt. Farther west, the occurrence of large clasts comprising all facies of the three Silurian West Point complexes in a fault-controlled breccia (Neigette breccia; Dansereau and Bourque, 2001, this issue) south of Rimouski (Fig. 1⇑), supports the idea that the Silurian West Point also developed along the northern margin of the Gaspé Basin.
Devonian West Point
The Devonian West Point differs from the Silurian West Point. It is composed of two main facies: a shallow water, well-bedded, crinoidal limestone facies, and a massive stromatoporoid limestone containing pinnacle reefs. The crinoidal limestone is a thin, continuous sheet (a few tens of metres thick at maximum) in the Northern and East-Central Outcrop belts. A number of pinnacle reefs attaining thicknesses of up to 300 m and diameters of 1 to 2 km are known along the Northern Outcrop Belt, between Madeleine River and Madeleine Lake (mapping by Lachambre, 1987) (Fig. 9⇓). One of these pinnacles, the Madeleine River buildup, has been the subject of detailed studies (Bourque, 1972, 1977; Bourque et al., 1986), and is used herein as a representative reef type.
The reefs show vertical zonation from a well-bedded, crinoid-dominated, grainstone-packstone and rudstone base to a massive, stromatoporoid framestone and rudstone top. The limestones in the lower half of the reef bodies contain a significant proportion of well-rounded quartz grains similar to those of the older Val-Brillant Formation, which locally unconformably (Salinic erosion) overlies the West Point Formation limestones (Bourque et al., 1986). In the Madeleine River buildup, the basal stromatactis lime mudstone is similar to the Silurian Gros Morbe Member of the West Point Formation at Chaleurs Bay. However, no age dating is available for stromatactis lime mudstone at Madeleine River. The Devonian age assigned to the base of the Madeleine River buildup (Bourque, 1977) is based on brachiopod collections obtained stratigraphically immediately above the Gros Morbe-like facies. Another buildup along the Northern Outcrop Belt (Madeleine Lake buildup) represents limestone facies similar to the basal Silurian West Point. Therefore, the possibility that the Silurian West Point occurs at the base of the pinnacle reefs along the Northern Outcrop Belt cannot be ruled out.
Reef Settlement and Development in the Southern Part of the Gaspe Belt (Areas A and B, Fig. 1⇑)
The Laurentian shelf in the southern part of the Gaspé Belt was deposited at the tip of the St-Lawrence Promontory during the Late Silurian–earliest Devonian. Only Silurian reefs occur in that part of the Gaspé Belt, and they are among the best developed of the entire reef tract.
The Salinic unconformity is angular in the Ristigouche Syncline (area B, Fig. 1⇑), where pre-erosion units are mildly folded, (Bourque and Lachambre, 1980, Map 1958; Bourque et al., 1995, Fig. 4.8) (Fig. 10⇓). This unconformity is overlain by conglomerate and redbeds (New Mills) that are coeval with those of the West Point Formation upper reef complex (Fig. 4⇑). To the west, in the Chaleurs Bay Synclinorium (area A, Fig. 1⇑), the Salinic unconformity is not clearly defined. As discussed above, Salinic erosion occurred sometime between deposition of the uppermost beds of the lower mound and reef complex and the first beds of the upper reef complex of the West Point Formation in the Port-Daniel area (eastern Chaleurs Bay Synclinorium). At the Black Cape section (a few kilometres southeast of New Richmond in the western part of the Chaleurs Bay Synclinorium, Fig. 1⇑), there is an abrupt sedimentary transition between deep-water fore-mound siliciclastics of the West Point Formation lower complex and the subaerial redbeds of the upper reef complex, the middle bank complex being absent there. North of the exposed West Point Formation, toward the synclinorium axis, only Ludlovian or older rocks are present, so the unconformity cannot be mapped.
It is postulated that the West Point reef and bank complexes of the southern part of the Gaspé Belt developed over, or at the margin of, an uplifted area during Late Silurian sea-level fall (Fig. 10⇑). North of the reef tract, redbeds of the upper reef complex overlie the Salinic unconformity, whereas south of it, in New Brunswick, allochthonous reef blocks of the West Point upper reef complex occur within deep-water siliciclastic facies (Noble, 1985).
The deep-water carbonate mounds and overlying shallower microbial reefs of the West Point Formation lower complex were deposited during the first Upper Silurian S2 shallowing phase (beginning of the regressive phase R2 of Bourque et al., 2001a, this issue) (Fig. 4⇑), before development of the Salinic unconformity. In contrast, subsequent high-order shallowing-deepening cycles of the middle and upper complexes likely occurred in response to tectonic instability at the margin of an uplifted area (Fig. 10⇑) during Late Silurian eustatic sea-level lowstand.
Reef Settlement and Development in the Northeastern Part of the Gaspe Belt (Area D, Fig. 1⇑)
The Salinic disturbance was associated with extensional tectonics in the northeastern part of the Gaspé Peninsula. A north-easterly thickening of the Chaleurs and Upper Gaspé Limestone groups in both the central and the northeast blocks indicates tilting of fault blocks along listric faults (Roksandic and Granger, 1981; St-Julien and Bourque, 1990) (Fig. 8⇑).
The following scenario is proposed to illustrate the tectono-sedimentary evolution of the northeastern part of the Gaspé Belt during the Silurian–Early Devonian, between the Taconian and Acadian orogenies (Fig. 11⇓).
Stage 1. Late Ordovician to Late Llandoverian
Sedimentation of the Matapédia Group, Burnt Jam Brook Formation, Laforce Formation and base of the Saint-Léon Formation in a post-Taconian successor basin; deposition of the nearshore Val-Brillant sands and overlying Sayabec carbonate platform in shallow marine areas; end of shallowing phase S1; movement of the Grande Rivière-Gastonguay Fault affects sedimentation of the Burnt Jam Brook Formation and forms a hinge line separating the Gaspé Belt into a northeastern shelf (northeastern Gaspé Peninsula) and southwestern basin; progressive burial of the Matapédia Group and the Burnt Jam Brook Formation.
Stage 2. Late Wenlockian to Early Ludlovian
Deposition of fine-grained, deep-water siliclastics of the Saint-Léon and Gascons formations during the D1 deepening phase (transgressive phase T1 of Bourque et al., 2001a, this issue), probably related to general subsidence in response to Salinic extensional tectonics and late Wenlockian eustatic sea-level rise.
Stage 3. Late Ludlovian–Pridolian
Generalized S2 shallowing (regressive phase R2 of Bourque et al., 2001a, this issue), block faulting, and tilting of the shelf area due to movement of the ancestral Bras Nord-Ouest and Troisième Lac faults and three associated fault blocks: the SW, central, and NE blocks; coeval eustatic sea-level fall leading to exposure, subaerial erosion, and diagenesis of elevated tilted blocks; significant erosion of most of the Silurian succession in the eastern portion of the NE block, which resulted in extensive development of the Salinic unconformity; erosion and subaerial diagenesis of emerged parts of the central and SW blocks produced local Salinic unconformities; development of Silurian West Point reef complexes at the margin of fault blocks; wedge-shape geometry of the Chaleurs Group, suggesting tilting of blocks bounded by listric faults.
Stage 4. Early Lochkovian
Beginning of deepening phase D2 (transgression T2 of Bourque et al., 2001a, this issue); settlement of the Devonian West Point pinnacle reefs (Fig. 11⇑) on shoals formed by older Silurian reefs and/or paleotopographic highs left after Salinic erosion (Bourque et al., 2000, Fig. 6⇑).
Stage 5. Late Lochkovian–Early Pragian
Late stage of D2 deepening phase; burial of the pinnacles by fine-grained siliciclastic sediments; continued block-faulting and tilting as expressed by the sedimentary wedge in the upper part of the Chaleurs and the Upper Gaspé Limestone groups.
Stage 6. Late Pragian–Emsian
A third shallowing phase (regressive phase R3 of Bourque et al., 2001a, this issue; not illustrated in Fig. 10⇑) related to the transpressional tectonic regime of the Acadian Orogeny, resulted in sedimentation of the Gaspé Sandstone Group in nearshore, coastal and terrestrial environments; significant shortening by folding and reversal of the movement along the major faults (from normal to transpressional). The present-day structural geology of the study area was not modified significantly by the younger Alleghanian Orogeny (Permian).
Fault-Related Reef Complexes in the Northwestern Part of the Gaspe Belt (Area F, Fig. 1⇑)
In the northwestern part of the Gaspé Belt (Fig. 1⇑), the Neigette Fault of the Témiscouata–Lake Matapédia area is shown to have acted as a late Pridolian synsedimentary normal fault (Dansereau, 1989; Dansereau and Bourque, 2001, this issue). Huge clasts in calci-debrites, forming the Neigette breccia, are composed of mound and reef limestones, similar to those of the Silurian West Point reef complexes of the Chaleurs Bay Synclinorium. This suggests the occurrence of Silurian reef complexes north of the ancestral Neigette Fault (Bourque et al., 2001a, Fig. 6⇑, this issue), and their exhumation and dismembering during the latest Silurian. It is likely, therefore, that reef settlement and development were related to the Neigette Fault with overall controls similar to those proposed herein for the Silurian reefs in the northeastern part of the Gaspé Belt.
Farther west, the platformal and reefal limestones of the Lac Croche Formation show some facies similarities with the upper reef complex of the Silurian West Point Formation in the Chaleurs Bay Synclinorium. However, this formation has not been studied in detail, and evolution of its facies cannot be ascertained with certainty.
Implications for Hydrocarbon Exploration in the Gaspe Belt
Silurian–lowermost Devonian potential reservoir units in the Gaspé Belt were deposited in the late stages of shallowing phases Sl and S2 (regressive phases R1 and R2 of Bourque et al., 2001a, this issue). These are the Llandoverian sandstone bodies of the Anse Cascon–Weir and Mann formations in the southern part of the Gaspé Belt (Chaleurs Bay Synclinorium and Ristigouche Syncline, respectively), and the Val-Brillant and Robitaille formations in the northern (Madeleine River and Matapédia River Valley areas) and western (Témiscouata) parts of the Gaspé Belt; the Wenlockian well-bedded sands and coralgal reef knobs rimming extensive peritidal flats of the shelf area (Sayabec and La Vieille formations); and the reef bodies of the Ludlovian–Lochkovian West Point reef tract at the margin of the Laurentian shelf. Reservoir potential in terms of porosity development for these units is discussed by Lavoie and Chi (1997) for the Val-Brillant Sandstone, Lavoie and Bourque (1993), and Lavoie and Chi (2001, this issue) for the Sayabec–La Vieille limestones, and Savard and Bourque (1989), and Bourque et al. (2001b, this issue) for the West Point reefs.
Northeastern Part of the Gaspe Belt
The northeastern part of the Gaspé Belt (area D, Fig. 1⇑) has been viewed traditionally as the most promising area for the occurrence of hydrocarbon reservoirs in the Gaspé Peninsula. Exploration strategies have been enhanced by a further understanding of Salinic tectonics (Malo and Bourque, 1993; Bourque et al., 2000). Key points in evaluating the hydrocarbon potential with respect to the Salinic disturbance and unconformity in the northeastern part of the Gaspé Belt are as follows.
1) The extensional tectonic regime of the Salinic disturbance may have provided suitable conditions for hydrocarbon migration at various times during the mid-Silurian to Early Devonian, particularly for hydrocarbons that might have been generated in the underlying Cambro–Ordovician shales. Block faulting and tilting along normal listric faults (Fig. 11⇑) may have modified fluid migration directions, from an updip, northeastward migration from the centre of the basin to its margins during the Late Ordovician to early Late Silurian, to a potential updip south-westward migration in each fault block (Fig. 12⇓). This may have played a significant role in driving hydrocarbon-rich fluids toward potential reservoirs, such as the Llandoverian Val-Brillant sandstone bodies, the Sayabec carbonate sands and knob reefs, or the West Point reefs.
2) As noted previously, Silurian West Point reef complexes likely developed south of the Northern Outcrop Belt, possibly on the margin of fault blocks (Fig. 11⇑). Therefore, based on the model of reef settlement proposed here, exploration for them should be concentrated in the subsurface southeast and south of the Northern Outcrop Belt between the Madeleine Lake and Salmon Hole buildups (Fig. 9⇑). Of particular interest are the E–W-trending portion of the Bras Nord-Ouest Fault, and the northeast areas of both the Bras Nord-Ouest and the Troisième Lac faults (Figs. 7⇑, 11⇑). For the same areas, the Devonian West Point pinnacle reefs should also occur in the subsurface, above the Silurian West Point reefs.
3) The Devonian West Point crinoidal limestone sheet occurring in the Northern and East-Central Outcrop belts (Bourque et al., 2001a, Fig. 2⇑, this issue, for location) has significant lateral extension. Besides its surface expression, St-Julien and Bourque (1990) traced on SOQUIP seismic profiles (lines P-11, P-13, P-15, P-16, P-17, and P-32) a regional reflector within the Chaleurs Group in a stratigraphic position compatible with this limestone sheet. The extent of the sheet seems to be particularly important within the NE and central blocks.
4) Salinic extensional tectonics may have created conduits for hydrothermal fluids responsible for potential limestone dolomitization. Pervasive dolomitization of the Sayabec Formation is associated locally with faults in the Northern Outcrop Belt, and has been interpreted as hydrothermal in origin (Lachambre, 1987). The dolomite is highly porous. Dolomite is uncommon in the West Point Formation in the Northern Outcrop Belt but pervasive in the East-Central Outcrop Belt. Although not studied in detail (Bourque et al., 1996; Savard et al., 1997), dolomitization took place in a burial setting. No evidence of hydrothermal dolomitization has been observed.
5) Salinic extensional tectonics may have fractured buried limestone units, such as the Matapédia Group, the Sayabec Formation, and the Laforce Formation, and created subsurface fracture porosity.
6) Salinic erosion led to local subaerial exposure and probable dissolution of pre-Salinic limestone units. As shown by the mapping of Lachambre (1987), an irregular erosional surface can be traced in the Silurian succession between the Madeleine Lake and Madeleine River sections (Fig. 9⇑).
Southern Part of the Gaspe Belt
The southern part of the Gaspé Belt (areas A and B, Fig. 1⇑; Chaleurs Bay Synclinorium and Ristigouche Syncline, respectively) has never attracted hydrocarbon exploration. Except for the Lower Devonian Black-Cape volcanics and the patchy Carboniferous cover, the youngest rocks in the Chaleurs Bay Synclinorium are Late Silurian in age (Bourque et al., 2001a, Figs. 2⇑, 3⇑, this issue). The Anse Cascon–Weir sandstones, the La Vieille carbonate sands, and coralgal knob reefs likely occur in the subsurface, but the structural geology of the synclinorium (Bourque and Lachambre, 1980) seems unfavourable for structural traps. Silurian West Point reefs and banks form a buildup up to 800 m thick that is only present in the southeastern portion of the synclinorium (Port-Daniel–Gascons area). North and west of these occurrences, the West Point Formation comprises terrestrial redbeds (Bourque et al., 2001a, Fig. 6B⇑, this issue), with no reservoir potential.
In the Ristigouche Syncline, the Mann sandstone and the La Vieille limestone likely occur in the subsurface core of the syncline. The Salinic unconformity, overlain by the New Mills conglomerate and redbeds, is mapped in the northern limb of the syncline, and also in the southern limb of the syncline in New Brunswick, although the situation is not as clear as in Gaspé (Greiner, 1973; Bourque et al., 1995). Therefore, it is likely that the unconformity occurs throughout most of the syncline and that Silurian West Point reefs, if they ever existed, would have developed south or west of the area covered by the Ristigouche Syncline (Fig. 10⇑). Moreover, the Salinic unconformity may have acted as a seal above potential reservoirs in the Mann or La Vieille formations.
Of particular interest for reservoir potential is the overall setting of the Sellardsville tectonic slice at the western end of the Ristigouche Syncline in Québec (Bourque and Lachambre, 1980; Bourque et al., 2001a, Fig. 3⇑, this issue). The tectonic slice is limited by two northeast-trending, northwest-dipping reverse faults. The eastern fault cuts the northwestern flank of the Ristigouche Syncline. The slice contains a very thick Ludlovian–Pridolian succession, estimated to be up to 4000 m thick (Bourque and Lachambre, 1980), but the thickness is possibly overestimated because of internal folding. The succession consists of mudcracked redbeds and green, fine-grained siliciclastics, and minor limestone conglomerates with marine fossils that are overlain by small stromatoporoid bioherms and biostromes. The redbeds, as well as the bioherms and biostromes, correlate with the upper part of the West Point Formation of the Chaleurs Bay Synclinorium. Above the Silurian West Point redbeds and stromatoporoid limestones, 500 m of Pridolian–Lochkovian greenish, calcareous, fine-grained siliciclastics are correlative with the Indian Point Formation. The thick West Point and Indian Point succession of the Sellarsville slice is coeval with the Salinic unconformity and the New Mills conglomerate and redbeds of the nearby Ristigouche Syncline, indicating accelerated subsidence west of the emerged Ristigouche Syncline (Fig. 10⇑). The faults limiting the Sellardsville slice acted as reverse faults during the transpressional Acadian Orogeny, mostly to accommodate translation along the Grand Pabos strike-slip fault, although they could also have been ancestral normal listric faults during the Salinic extensional tectonics.
The tectonic setting of the Silurian–Lower Devonian shelf area in the Chaleurs Bay Synclinorium and the Ristigouche Syncline at the margin of Laurentia (Fig. 1⇑) is similar to that of the NE block of the northeastern part of the Gaspé Belt (Fig. 12⇑). From this we can speculate that, as in northern Gaspé where a number of faulted blocks are known, similar fault-bounded blocks with associated reefs were probably present south of the Chaleurs Bay Synclinorium–Ristigouche Syncline area, i.e., in the Chaleurs Bay and northern New Brunswick. The West Point allochthonous reef blocks within siliciclastic deep water facies in northern New Brunswick (Noble, 1985) may originate from one of these hypothetical blocks. However, a reservoir seal has not been identified in the southern part of the Gaspé Belt. South of the Chaleurs Bay Synclinorium, it is likely that the fine-grained siliciclastic Indian Point Formation or equivalent units, overlying the West Point reefs, are rather thin and that no other sediments were deposited before the post-orogenic Carboniferous rocks. Bourque et al. (2000) showed that this area was uplifted during the middle Lochkovian (Bourque et al., 2001a, this issue), which prevented deposition of the Upper Gaspé Limestones and Sandstones, and the Fortin Formation. In the Chaleurs Bay Synclinorium, Savard and Bourque (1989) estimated a burial depth for the base of the upper West Point reef complex to be less than 1 kin. In the Ristigouche Syncline, however, up to 1500 m of Lower Devonian Dalhousie Formation fine-grained siliciclastic rocks are covered by, and/or interbedded with, up to 5000 m of volcanic rocks, and these overlie the Silurian West Point Formation or equivalent units. Based on the model proposed for the northeastern part of the Gaspé Belt (Fig. 11⇑), it is also possible that, in the Ristigouche Syncline, pinnacle reefs appeared locally during the Early Devonian deepening phase D2, on highs that had developed during the Late Silurian.
Western Part of the Gaspe Belt
In the Témiscouata–Lake Matapédia area (area F, Fig. 1⇑), the Robitaille and Val-Brillant sandstones occur in the subsurface (see Bourque et al., 2000, Fig. 10), and may be involved in local structural or stratigraphic traps. The poorly known Lac Croche reefal limestone, correlative with the Silurian West Point Formation, apparently formed, together with the Neigette reef complexes to the northeast, a carbonate platform during the Pridolian (Bourque et al., 2001a, this issue). However, it is not clear how the Silurian tectonics influenced these strata. It is likely that the Neigette reefs developed on the margin of the active Neigette Fault (Dansereau and Bourque, 2001, this issue). The right-angle offset of this platform margin with respect to the Neigette Fault (Bourque et al., 2001a, Fig. 6B⇑, this issue) suggests synsedimentary faulting, but no other evidence supports this hypothesis. Moreover, as for the Upper Silurian reef limestones in this area, a suitable seal rock has not been identified. However, oil shows were discovered recently in the Upper Gaspé Limestones during a gold exploration drilling program in the nearby Lake Matapédia area. These shows indicate an increased potential for hydrocarbon pool occurrence in the study area.
Summary and Conclusions
The interplay of extensional tectonics and sea-level fluctuations during the Silurian–earliest Devonian in the Gaspé Peninsula–Témiscouata segment of the Gaspé Belt in the Québec Appalachians created conditions favourable for hydrocarbon reservoir development.
1) The succession recorded two major, low-order sedimentary cycles, each composed of a regressive-transgressive couplet and represented by shallowing- to deepening-upward sequences S1–Dl and S2–D2. The first shallowing episode (S1) spanned the Rhuddanian–Sheinwoodian (Llandovery–early Wenlock), whereas the following abrupt deepening episode (D1) occurred during the Homerian (late Wenlock). The second shallowing episode (S2) began in the late Homerian and culminated during the late Ludfordian (late Ludlow) or earliest Pridolian, and was followed by a latest Silurian–Early Devonian deepening episode (D2).
2) When compared to proposed eustatic sea-level curves for the Silurian, the relative sea-level curve for the Silurian–earliest Devonian succession of the Gaspé Belt, based on Boucot’s (1975) benthic assemblage scheme for its lower part, and the carbonate facies sequence analysis of Bourque et al. (1986) for its upper part, shows similarities and differences (Fig. 6⇑). The curve of Ross and Ross (1996) indicates a general sea-level rise for the entire Rhuddanian to Telychian (Llandovery), whereas the Gaspé curve shows a general sea-level fall, at least for the Rhuddanian–Aeronian (lower two thirds of the Llandovery). However, the Gaspé curve parallels that of Johnson et al. (1998) for the Telychian–Sheinwoodian (late Llandovery–early Wenlock) interval. During the Homerian–Ludfordian (late Wenlock–Ludlow), the Gaspé Belt sea floor was likely at too great a depth to record the minor sea-level fluctuations suggested by Ross and Ross (1996) and Johnson et al. (1998). The major late Ludfordian (latest Ludlow) sea-level lowstand suggested by Ross and Ross (1996) is recorded in the Gaspé succession, but the early Pridolian highstand of Johnson et al. (1998) is not. Overall, these observations suggest that the relative sea-level curve for the Gaspé Belt was partly influenced by local tectonics.
3) Potential reservoir rock units developed mostly during sea-level lowstands in the Gaspé Belt. Reservoir units include the extensive sandbodies of the Val-Brillant, Robitaille, Anse Cascon, and Mann formations of the Telychian; the carbonate sands and/or knob reefs rimming the extensive peritidal flat of the two phases of the Sayabec–La Vieille Formation of the early Sheinwoodian, to late Sheinwoodian–early Homerian, respectively; and the barrier reef of the West Point Formation of the late Ludlovian to Pridolian (Fig. 6⇑). However, the early Lochkovian pinnacle reefs of the West Point Formation developed at the beginning of sea-level rise following the Ludlovian–Pridolian eustatic lowstand.
4) Salinic extensional tectonics, a predominantly Late Silurian–Early Devonian (Ludlovian–Pragian) event, resulted in listric block faulting and tilting. Block faulting and eustatic sea-level fall at the end of the Ludlovian–early Pridolian exposed the highest parts of the blocks (including early Silurian platformal limestones) to subaerial erosion and diagenesis (Salinic erosion) and allowed subsequent reefs and reef complexes to settle at margins of the blocks, or on erosional remnants (Figs. 11⇑, 12⇑).
5) The most promising area of the Gaspé Belt in terms of hydrocarbon reservoir potential is the northeastern segment. Salinic extensional tectonics may have provided suitable conditions for hydrocarbon migration and charge at various times during the mid-Silurian to Early Devonian, particularly for hydrocarbons that may have been generated in the underlying Cambro–Ordovician shales. Block faulting and tilting changed fluid migration pathways, from an updip northeastward migration from the centre of the basin to the margins during the Llandoverian to Ludlovian, to an updip, southwestward migration on each faulted block (Fig. 12⇑). This may have driven hydrocarbon-rich fluids toward potential reservoirs, such as the Llandoverian Val-Brillant sandstone bodies, the Sayabec carbonate sands and knob reefs, or the West Point reef bodies. Porosity development of the Sayabec and the West Point formations, and ensuing reservoir potential of these units, are discussed further by Lavoie and Chi, 2001, this issue, and Bourque et al., 2001b, this issue.
I am much indebted to my colleague Denis Lavoie and to Bulletin reviewers Paul Copper and Markes E. Johnson, whose constructive comments on the manuscript significantly improved this paper. Discussions on the Gaspé tectonics with my colleagues Donna Kirkwood, Michel Malo, and the late Pierre St-Julien were very fruitful. Most of the field expenses for the research in the early phases of my work on the Gaspé Peninsula, particularly during the 1980s, were defrayed by the Ministère des Ressources naturelles du Québec, A more recent research contract with Shell Canada encouraged me to revisit older work. I am particularly indebted to Peter Immertz and Erdem Idiz, both from Shell Canada, for fruitful discussions. I benefited from an individual research grant from the Natural Sciences and Engineering Research Council of Canada. To all these persons and organizations, I express my deepest thanks for their support.
↵1 GIRGAB, Groupe interuniversitaire de Recherches en Géodynamique et Analyse de Bassins