- © The Society of Canadian Petroleum Geologists
This contribution provides an overview of the geological setting, stratigraphy, tectono-sedimentary evolution, and paleogeography of the post-Taconian–pre-Carboniferous sequence of the Gaspé Belt. The sequence has been shaped by two tectonic pulses: the Salinic disturbance that began during the Early Silurian (Telychian) and persisted until the Early Devonian (Pragian), and the ensuing Acadian Orogeny in the Early to Mid-Devonian (Emsian-Eifelian).
The shelf and shelf-edge history at the Laurentian margin along the Québec Re-entrant–St. Lawrence Promontory can be summarized in five broad phases that resulted from the interplay of tectonics and sea-level change. Phase 1 is a Llandoverian–Wenlockian regressive phase (R1) related to post-Taconian successor basin infilling, which culminated in extensive carbonate platform development. Phase 2 is a late Wenlockian–Ludlovian transgressive phase (T1). Phase 3 corresponds to a later Ludlovian-Pridolian second regressive phase (R2). Phases 2 and 3 were accompanied by extension faulting, block tilting, and the development of reefs, reef complexes and reef tracts along the Gaspé–Témiscouata shelf. Phase 4 is an Early Devonian phase of accelerated subsidence (transgression T2) affecting the northwestern part (Québec Re-entrant area), while the southeastern part (St-Lawrence Promontory area) was previously uplifted as a result of Laurentia colliding with the western margin of Gondwana-related terranes to the south. Phase 5 is the final regression (R3) related to the Acadian Orogeny.
The purpose of this contribution is to provide an overview of the geological setting, stratigraphy, tectono-sedimentary evolution, and paleogeography of the post-Taconian–pre-Carboniferous Gaspé Belt. It also provides a framework for the contributions of this special issue of the Bulletin of Canadian Petroleum Geology. It summarizes and updates the synthesis of Bourque et al., published in 1995, but written in the early 90s, and the subsequent work by Lavoie (1992a, 1992b) on the Upper Gaspé Limestone Group; Bourque et al. (1993) on stratigraphy and paleogeography of the Silurian–earliest Devonian interval; Malo and Bourque (1993) on timing of the deformation events in the Gaspé Peninsula; Hesse and Dalton (1995) on the Fortin Group and Témiscouata Formation in the southern Matapédia River Valley and Lake Témiscouata area; Kirkwood (1999) on pre-Acadian palinspastic restoration of the Gaspé Belt; and Bourque et al. (2000) on the paleogeography and tectono-sedimentary history at the margin of Laurentia during the Silurian to earliest Devonian. It presents only key information so the reader can gain a broad picture of belt composition and evolution. Detailed accounts and supporting evidence on lithology, age, correlation, sequence analysis, paleogeography and tectono-sedimentary evolution of the belt can be found in the published works previously cited.
Rocks of the Gaspé Belt range in age from Late Ordovician (Caradocian) to Late Devonian (Frasnian). They belong to three major structural units, from north to south: the Connecticut Valley–Gaspé Synclinorium, the Aroostook–Percé Anticlinorium and the Chaleurs Bay Synclinorium (Fig. 1⇓).
The stratigraphy of the Québec segment can be summarized in four broad temporal and lithological packages (Figs. 2⇓, 3⇓): 1) Upper Ordovician–Lowest Silurian (Caradocian to Llandoverian) deep-water, fine-grained siliciclastic and carbonate facies of the Honorat and Matapédia groups, which occur chiefly in the Aroostook–Percé Anticlinorium, and the Cabano Group of the Témiscouata region, occurring in the Connecticut Valley-Gaspé Synclinorium; 2) Silurian–lowest Devonian (Llandoverian to Lochkovian) shallow to deep shelf facies of the Chaleurs Group occurring in the Connecticut Valley–Gaspé and the Chaleurs Bay synclinoria; 3) Lower Devonian (Pragian–Emsian) carbonate and siliciclastic outer shelf to basin facies, including the Upper Gaspé Limestone and Fortin groups, and the Témiscouata Formation, occurring in the Connecticut Valley–Gaspé Synclinorium; 4) Upper Lower to Upper Devonian (Emsian to Frasnian) nearshore to terrestrial coarse-grained facies comprising the Gaspé Sandstone Group (Emsian–Eifelian) of the Connecticut Valley–Gaspé Synclinorium, and related units (Emsian–Frasnian) of the Ristigouche Syncline in the Chaleurs Bay Synclinorium.
The stratigraphic framework of the post-Taconian–pre-Carboniferous sequence of the Gaspé Belt is shown in Figure 3⇑. The following is a brief description of the main lithology of these four packages.
Honorat, Matapedia, and Cabano Groups
The Honorat–Matapédia–Cabano rock package corresponds to the deep water, turbiditic, terrigenous to carbonate sediments that filled the post-Taconian successor basin (Ducharme, 1979; Malo, 1988; Malo and Bourque, 1993). The Honorat is a terrigenous unit, up to 1200 m thick, consisting of claystone, mudstone, siltstone, quartz and lithic wackes, conglomerate, and silty limestone. The overlying Matapédia Group is carbonate-dominated, consisting of a lower calcareous mudstone to argillaceous limestone (Pabos), and an upper, thin-bedded calcilutite with shale partings and a few thin beds and lenses of calcarenite (White Head) (Malo, 1988).
The Honorat and Matapédia occur only in the Gaspé Peninsula. In Témiscouata, both groups are replaced laterally by the Cabano Group, which is composed of dark grey, turbiditic, lithic wacke and conglomerate interbedded with silt-stone and mudstone, lithologically similar to the Honorat (Lespérance and Greiner, 1969; David et al., 1985).
The Chaleurs Group is a heterogeneous rock package that contains three assemblages (Bourque et al., 1993): a lower terrigenous assemblage, a middle carbonate assemblage, and an upper terrigenous assemblage with local reefs and volcanic bodies.
Lower Terrigenous Assemblage
In the southern and northern parts of the Gaspé Belt, the lower assemblage is generally a coarsening-upward sequence of mudstone or claystone and sandstone with variable amounts of conglomerate, whereas in the central belt, the sequence is composed of deep-water, graptolitic claystone with minor fine-grained sandstone (Burnt Jam Brook). The upward-coarsening terrigenous sequence is best exemplified in the southern part of the Gaspé Belt, where facies range from fine-grained siliciclastic tempestites (Clemville) with offshore-type benthic fauna, to sands and gravels (Weir) with intertidal brachiopods, and shoreline sands (Anse Cascon) (Bourque, 1981). In northern Gaspé, the sequence is made up of a lower deep-water clay-stone (Awantjish) or mudstone (Lac Raymond), overlain by a distinctive nearshore and terrestrial quartz sandstone (Val-Brillant and Robitaille, respectively).
Middle Limestone Assemblage
A distinctive limestone interval consisting of platformal and deeper water limestones is mapped across most of the Gaspé Belt. The platformal limestones (Sayabec in the northern part of the Gaspé Belt, and La Vieille in the southern part of the belt) are composed of peritidal, reefal, and various subtidal facies, and constitute a good chronological marker (Llandoverian C6–Wenlockian) (Bourque et al., 1986; Lavoie et al., 1992). The lateral equivalent but slightly younger (Wenlockian–lower Ludlovian) deeper water limestone (Laforce, in the East-Central Outcrop Belt, Saint-Jean River Anticline, and Mount Alexandre Syncline; Fig. 2⇑) is composed of sandy lithoclastic calcarenites and calcirudites. The Laforce material appears to be derived from at least part of the Sayabec–La Vieille platform (Lavoie, 1988). Age relationships, together with the lithoclastic nature of the limestones, where a variety of carbonate platform clasts are recognized, suggest this. An alternative derivation is from the coeval, nearby Anticosti shelf.
Upper Terrigenous Assemblage
The upper terrigenous assemblage is typically fine-grained (Saint-Léon, Gascons and Indian Point) and contains three distinctive lithotypes: conglomerates, volcanic rocks, and reef limestones. It also contains a striking feature, the Salinic erosional unconformity.
This unconformity resulted from block faulting and a Late Silurian eustatic sea-level fall. The topographically highest parts of the basin were eroded during this time (Bourque, 1997). The unconformity affected Ludlovian–Pridolian strata and is exposed sporadically in the Gaspé Peninsula.
The conglomerate bodies of the upper terrigenous assemblage are unequivocally associated with the Salinic unconformity. The main bodies are 1) the Griffon Cove River conglomerate (Northern Outcrop Belt), composed of pebbles and cobbles of quartz, chert, mafic and felsic volcanic rocks, various sedimentary rocks and, in places, stromatoporoid clasts; 2) the New Mills conglomerate (Ristigouche Syncline), composed of pebbles and boulders of the underlying lithotypes (mafic volcanic rocks, calcilutite); and 3) the Owl Capes conglomerate (Saint-Jean River Anticline), composed of quartz pebbles and fragments of various limestones, corals, stromatoporoids, feldspathic wacke and mafic volcanic rocks. The conglomerates are commonly associated with nearshore to terrestrial facies, such as peritidal limestones and siliciclastic redbeds.
Volcanic Rock Bodies
Volcanic rock bodies form two packages in the upper terrigenous assemblage: the Silurian Lac McKay and Ristigouche members, and the uppermost Silurian–Lower Devonian Baldwin, Black Cape and Dalhousie members (Laurent and Bélanger, 1984; Bédard, 1986; Bourque et al., 1993; Doyon and Dalpé, 1993; Dostal et al., 1993). The Ristigouche and Lac McKay members are basaltic lava flows and volcaniclastics, with a few felsic lavas, whereas the Baldwin, Black Cape and Dalhousie members include a significant portion of intermediate rocks and minor rhyolites, pyroclastics and hyaloclastics, in addition to the tholeiitic basalts. All these volcanic bodies are related to intraplate volcanism in an extensional and/or transtensional tectonic regime (Doyon and Dalpé, 1993; Dostal et al., 1993), and provide important clues about the Upper Silurian geotectonic setting.
Reef Limestone Bodies
The reef bodies are best developed in the Chaleurs Bay Synclinorium (West Point) and are up to 800 m thick (Bourque et al., 1986). Reef limestone bodies contain sponge and microbial mounds, crinoidal banks, stromatoporoid reefs and rubble, microbial laminites, and mudcracked redbeds. The reef limestone facies are very similar regionally. Some constitute the clasts of the Neigette breccia in the northern Témiscouata–Matapédia River Valley area (Dansereau and Bourque, 2001, this issue), indicating that reef complexes existed north of the Neigette Fault (Fig. 2⇑). Some typical stromatoporoid biostromes of the Port Daniel area also occur 300 km to the west in the Témiscouata region (Lac Croche). Reef construction ended in Late Pridolian time in the Chaleurs Bay Synclinorium, but persisted into the Lochkovian in the East-Central and Northern outcrop belts (Bourque, 2001; Bourque et al., 2001, this issue).
Upper Gaspe Limestone and Fortin Groups, and Temiscouata Formation
This rock package is known only in the Connecticut Valley–Gaspé Synclinorium. It is made up of two distinct, laterally equivalent assemblages: the Upper Gaspé Limestones, developed along the northern rim of the synclinorium, and the Fortin Group and Témiscouata Formation (referred to as the Fortin/Témiscouata here), which outcrop in the central and southern parts of the synclinorium.
The Upper Gaspé Limestone Group was originally described in the northeastern part of the Gaspé Peninsula (Lespérance, 1980). In northern Gaspé Peninsula, the unit is up to 500 m thick (Lavoie, 1992b) and contains a lower, homogeneous succession of shaly dolomitic and siliceous calcilutite or limy mudstone (Forillon). The overlying middle part of the group is more heterogeneous and has a higher siliciclastic content (Shiphead), consisting of thinly to very thickly bedded siliceous and dolomitic limestone and mudstone, with minor calcarenite, sandstone and bentonite beds. The upper part of the group is a homogeneous unit of thin-to medium-bedded cherty to siliceous or silty calcilutite (Indian Cove).
The Fortin/Témiscouata form thick, monotonous, largely unfossiliferous, well-bedded sequences of dark siltstone and shale with intercalated sandstone-rich intervals and minor volcanics (Lespérance and Greiner, 1969; Kirkwood and St. Julien, 1987; Hesse and Dalton, 1989, 1995). Although the rocks below the Fortin/Témiscouata are unknown, it is more likely that the group lies conformably on the fine-grained siliciclastics of the Chaleurs Group (Bourque et al., 1995). Hesse and Dalton (1989, 1995) recognized seven turbiditic lithofacies in the Fortin/Témiscouata of the southern Matapédia River Valley and the Lake Témiscouata areas, which they interpreted as sub-marine overbank and channel-fill deposits from base-of-the- slope to deep-basin settings.
Relationships between both assemblages are known only in the eastern part of the Gaspé Belt. Elsewhere, they are in fault contact (Gastonguay, Ste-Florence, or Témiscouata faults, Fig. 2⇑). Along a north-south transect from the Forillon Peninsula area to the Grande Rivière Fault in the eastern part of the Gaspé Belt, Lavoie (1992a and 1992b) showed that the lateral transition of the threefold Upper Gaspé Limestones to the Fortin represent a transition from a storm-influenced carbonate shelf in the north, to a distal outer shelf with significant thickening of the limestone sequence, homogenization of the lithofacies, and disappearence of the storm layers, and finally to a tectonically active distal slope, or toe of a slope, where the Upper Gaspé Limestones interfinger with the Fortin siliciclastic facies in the south.
Volcanic rocks are present within the Upper Gaspé Limestones in west-central Gaspé (Lavoie et al., 1991; Lavoie, 1995). Rhyolitic flows and felsic volcaniclastics occur within the Shiphead Formation, whereas mafic tufts, flow breccia, hyaloelastics, and massive to pillowed basaltic flows are present in the upper part of the Indian Cove Formation.
Gaspe Sandstone Group and Related Units
The Gaspé Sandstone Group constitutes the youngest (Emsian–Eifelian) rock package in the Connecticut Valley–Gaspé Synclinorium, whereas related units (Lagarde, Pirate Cove, Fleurant, and Escuminac formations), unassigned to any group and ranging in age from Emsian to Frasnian, occur in the Ristigouche Syncline, in western Chaleurs Bay.
The Gaspé Sandstone Group in the Connecticut Valley–Gaspé Synclinorium
The Gaspé Sandstone Group occupies roughly the northern half of the Gaspé Belt (Fig. 2⇑). To the south, it is replaced by the Fortin/Témiscouata succession. The group is composed of four rock assemblages: 1) a lower transitional unit (York Lake) composed of alternating siliceous calcilutites with minor quartz arenites and wackes, like those of the underlying Upper Gaspé Limestone Group, and greenish grey, medium-grained, feldspathic wackes, similar to those of the overlying sandstones (York River); 2) an overlying sequence (York River) that coarsens upwards from a mudstone–siltstone–sandstone assemblage, to an upper, thick-bedded sandstone with large-scale crossbedding and minor mudstone; 3) a sequence (Battery Point) of conglomeratic sandstone, medium to coarse-grained sandstone, and minor siltstone and mudstone overlain by a redbed unit of sandstone, siltstone and mudstone (Rust, 1981; Walker and Cant, 1979; Cant and Walker, 1976); 4) an upper sequence (Malbaie) of thick-bedded conglomerate composed of pebbles and cobbles of limestone, siliciclastic and volcanic fragments derived from the older Matapédia and Chaleurs groups, and interbedded with medium to coarse-grained red sandstone (Rust, 1976, 1981). The Malbaie is unconformably overlain by Carboniferous rocks.
In the Forillon–Percé area, the transitional York Lake is absent. In the Big Berry Mountains Syncline area (Fig. 2⇑), the Gaspé Sandstone Group shows roughly the same stratigraphic succession as in northeastern Gaspé Peninsula area, except for the following: the occurrence of a thick unit of unfossiliferous red to brownish red shale with siltstone and fine-grained sand-stone, and sparse mudcracks and ripple marks (Lake Branch) between the York River and the Battery Point formations, and the absence of the Malbaie conglomerate on top of the sequence. The York Lake and York River formations are nearly the same as in northeastern Gaspé Peninsula, except that they contain thick, bimodal volcanic sequences, each consisting of basal basaltic flows and mafic pyroclastics, overlain by rhyolitic flows and felsic pyroclastics, and capped by rhyolitic flows (Doyon and Valiquette, 1991; Doyon, 1988).
The Gaspé Sandstone Group corresponds to an abrupt shoaling event, from shallow marine facies to terrestrial facies. The transition from the deep water facies of the Upper Gaspé Limestones to the shallow water sands of the Gaspé Sandstones is gradational (York Lake), except in the Forillon–Percé area, where it is abrupt (York River). The York Lake and York River formations contain marine fauna, and a deltaic or estuarine environment was postulated for the York River Formation (Mason, 1971; Sikander, 1975; Lawrence, 1986; Desbiens, 1992).
The overlying Battery Point Formation contains a lower, distal, braided stream deposit and interbedded marine deposits (Cant and Walker, 1976), and an overlying meandering river system associated with playa or ephemeral lake deposits (Rust, 1981). The overlying Malbaie is interpreted as a proximal braided plain deposit (Rust, 1981).
Units Related to the Gaspé Sandstone Group in the Ristigouche Syncline
The core of the Ristigouche Syncline in western Chaleurs Bay is occupied by a sequence of Emsian to Frasnian terrestrial clastic rocks, overlying the mafic to intermediate volcanic rocks of the Lower Devonian Dalhousie Formation. The sequence is composed of four conglomerate units (Alcock, 1935; Williams and Dinely, 1966; Zaitlin and Rust, 1983): 1) the Lagarde conglomerate, composed of thick-bedded, well-rounded pebble and cobble conglomerate interlayered with grey or greenish grey, coarse to fine-grained, crossbedded sandstone and lesser mudstone; 2) the Pirate Cove conglomerate, consisting predominantly of red and grey sandy siltstone and mudstone with lenses of channelled sandstone, and subsidiary limestone conglomerate with clasts derived predominantly from the Matapédia Group exposed to the north; 3) the Fleurant conglomerate, a grey, well-rounded pebble and cobble sandy conglomerate, whose clasts are dominantly limestone, with lesser amounts of volcanics and sandstone, and minor plutonic rocks; and 4) the Escuminac conglomerate, composed of greenish grey, thin to thick-bedded sandstone, siltstone and varve-like mudstone, with abundant sole marks, parallel lamination, current ripples, and fossil fish and plants (e.g., Miguasha fossil locality).
Several authors have suggested a correlation between the Devonian Gaspé sandstones of eastern Gaspé and those in the Ristigouche Syncline. The Battery Point and the Lagarde formations appear to be of similar age, and of approximately similar lithology. The Lagarde is predominantly an alluvial deposit, hence is broadly similar to the Battery Point. The Malbaie Formation and Pirate Cove conglomerate are also similar in age and superficially similar in lithology, both containing limestone pebble conglomerate. However, mudrocks are essentially absent from the Malbaie Formation, but abundant in the Pirate Cove Formation. The locally variable lithology in the Pirate Cove conglomerate suggests alluvial fan deposition, whereas the internal consistency of the Malbaie Formation indicates deposition on a broad, uniform braidplain (Rust, 1989).
No equivalents of the Givetian–Frasnian Fleurant and Escuminac formations exist in eastern Gaspé However, the age of the Fleurant Formation is not well established. The unit is homogeneous lithologically, which, together with the abundant horizontally stratified conglomerate, high angle imbrication and the absence of muddy matrix-supported conglomerate, indicates deposition on a proximal gravelly braidplain (Zaitlin, 1981). In terms of facies abundance, it is more like the Malbaie Formation rather than the Pirate Cove Formation (Rust, 1989). The clast composition of the Fleurant is also like that of the Malbaie Formation although limestone lithology is less abundant.
According to Hesse and Sawh (1982, 1989) the sedimentary structures of the Escuminac sandstone suggest deposition by sediment gravity flows. Flute casts, groove casts, brush and prod casts, rill-casts that bifurcate downstream, and squamiform load casts are abundant. Paleocurrent directions indicate west-southwest flow. Most authors have interpreted the Escuminac Formation as a lacustrine deposit (Dineley and Williams, 1968a, b; Carroll et al., 1972; Hesse and Sawh, 1982), based on its close association with other continental deposits and on its fauna. However, the fauna contains elements that have also been described from brackish or marine environments (Schultze, in Carroll et al., 1972). More recently, Chidiac (1996) suggested deposition in a setting that had a salinity transitional between lacustrine and truely marine conditions, based on isotope geochemistry.
Depositional Environments, Cyclic Sedimentation and Paleogeography
The Upper Ordovician to Upper Devonian succession of the Gaspé Belt developed at the margin of the Québec Reentrant and the St. Lawrence Promontory. During the Late Ordovician to Early Silurian, deposition occurred in a basinal setting. Later, in an unstable shelf to basin environment, cyclic sedimentation occurred, in part related to tectonism. The main sedimentary and tectonic events are summarized in Figure 4⇓. Paleogeographic evolution of the Gaspé Belt is summarized in Figures 5⇓ to 8⇓. Maps for the latest Ordovician to earliest Devonian are reproduced from Bourque et al. (2000), whereas maps for Pragian and Emsian times are updated from Bourque et al. (1995). Facies distribution has been plotted on palinspastic maps after Kirkwood (1993, 1999).
Regressive Phase R1
The first regressive phase corresponds with the deposition of the Honorat–Matapédia–Cabano deep-water strata, followed by the shallow-water, lower terrigenous and middle limestone assemblages of the Chaleurs Group. After the Late Ordovician Taconian Orogeny, the successor basin was filled by the thick strata of the Honorat, Matapédia and Cabano groups. The sandstones of the Honorat and Cabano, the siliciclastics of the lower part of the Matapédia Group, and the silty limestones and calcarenites of the upper part of the Matapédia Group, commonly possess internal sedimentary structures typical of turbidites. These turbidites occupied a relatively deep-water marine basin (Figs. 5A, B⇑). The source area for the Honorat was to the south-east (Ducharme, 1979; Malo, 1988), whereas, for the Cabano, it was to the northeast (Lajoie et al., 1968; David et al., 1985). The coeval Anticosti carbonate shelf more likely acted as the source area for the Matapedia limestones (Ducharme, 1979; Malo, 1988). The progressive change from deeper terrigenous (Honorat) to shallower limestone (Matapédia) facies reflects basin infill.
After deposition of the Matapédia Group lime turbidites, deposition was dominantly siliciclastic, but ended with lime-stone. This phase is exemplified by the superposition of the lower siliciclastic and the middle limestone assemblages of the Chaleurs Group. The facies evolved from terrigenous offshore muds with sandy storm layers, to lower shoreface sands (Bourque, 1981), and finally to two extensive peritidal carbonate platforms (Sayabec to the north, and La Vieille to the south; Fig. 5C⇑) with local subaerial facies (Bourque et al., 1986; Lavoie, 1988). The Sayabec–La Vieille carbonate unconformity may be, at least in part, related to the Wenlockian eustatic sea-level fall (Bourque, 2001, this issue). It is not clear whether these two very similar platforms were connected (Lavoie et al., 1992). Grain composition of the carbonate sand and gravel bodies (Laforce) in a central clay basin (Burnt Jam Brook) suggests erosional dismemberment of the shallow water carbonate platform. This material was derived either from erosion of the contiguous Anticosti platform, (Sami and Desrochers, 1992), or from erosion of the coeval Sayabec–La Vieille platform, in which case the Sayabec–La Vieille formed a single, continuous carbonate belt.
Transgressive Phase T1
Following regressive phase R1, deep shelf siliciclastic muds of the upper terrigenous assemblage of the Chaleurs Group were deposited during a transgressive episode (T1). This deepening phase might correspond to the late Wenlockian–early Ludlovian general eustatic sea-level rise (Bourque, 2001, this issue), combined with inception of significant basin subsidence related to Upper Silurian extensional tectonism.
Regressive Phase R2
In the middle part of the upper terrigenous Chaleurs Group, terrestrial redbeds, conglomerate, and reef limestone bodies occur (Figs. 6A, B⇓). This indicates a second regressive phase (R2), which resulted from the combined effects of the Late Silurian eustatic sea-level lowstand and Salinic block faulting during Pridolian time. Terrestrial redbeds and conglomerate are associated with uplifted areas. Field data indicate several hundreds of metres of erosion in the Ristigouche Syncline, the Saint-Jean River Anticline, and in the Northern Outcrop Belt. A significant part of this erosion is probably related to faulting and block tilting. Seismic profiles in northeastern Gaspé Peninsula (Roksandic and Granger, 1981; St-Julien and Bourque, 1990; Bourque, 2001, this issue) support the block-faulting interpretation. Reef complexes (West Point) at the margin of the uplifted blocks formed a reef tract all along the northern margin of the Gaspé–Témiscouata Basin (Bourque, 1979; Bourque et al., 1986; Bourque, 2001, this issue). Several reef facies indicate tectonic instability.
Significant thickening of the siliciclastic mudstone in the Saint-Léon Basin, suggests growth faulting (Figs. 6A, B⇑).
Transgressive Phase T2
Following Pridolian shallowing and erosion, a second transgressive phase (T2) resulted in a deepening upward sequence represented by the fine-grained sediments and turbidites of the upper portion of the upper terrigenous assemblage of the Chaleurs Group (Bourque, 1989), and the fine-grained deepwater limestones of the Upper Gaspé Limestone Group (Achab et al., 1997; Mussard and Lavoie, 1997) or the deeper water siliciclastics of the Fortin/Témiscouata. During mid-Lochkovian time, accelerated subsidence in the Saint-Léon Basin, in response to uplift in the south (Chaleurs Bay Synclinorium) and docking of the western margin of Gondwana-related terranes farther south, compartmentalized the Pragian–Emsian Fortin Trough (Fig. 7⇓). The turning point of the extensional (Salinic) and the transpressional (Acadian) tectonic regimes occurred during the Pragian (Fig. 4⇑), but not the earliest Pragian, as listric faults were still active (Bourque, 1990, 2001, this issue; Lavoie, 1992b). Two deformation styles of the Acadian Orogeny occurred during this time: 1) a regional, northwest-directed shortening accomodated by northeast-trending folds, cleavage, and reverse faults, and 2) a tightening of folds as a result of enhanced cleavage development and initiation of strike-slip faulting, with about 77% shortening in the southern part of the Connecticut Valley–Gaspé Synclinorium and 65% in the Aroostook–Percé Anticlinorium (Kirkwood, 1999). Basin shortening most likely began during the Pragian (Fig. 7B⇓). The ubiquitous slumps in the southernmost exposures of the Upper Gaspé Limestone (Lavoie, 1992b) and the Fortin/Témiscouata rocks may be related to synsedimentary tectonics. However, Acadian folding and penetrative cleavage developed during the Late Emsian, because the youngest age of the deformed Fortin/Témiscouata is Middle Emsian, and the Eifelian Malbaie, Lagarde and Touladi formations are not deformed.
Two facies belts characterize deposition during the Pragian (Fig. 7B⇑): a northern, outer shelf dominated by fine-grained carbonates (Lavoie et al., 1991; Lavoie, 1992b) with local lava flows, and a southern deep basin into which southeast-directed siliciclastic turbidite flows were deposited parallel to the north-western margin of the trough (Hesse and Dalton, 1995).
Regressive Phase R 3
The third regressive phase (R3) is recorded in the shallow marine to terrestrial facies of the Gaspé Sandstone Group of the northern half of the Connecticut Valley–Gaspé Synclinorium. The main facies are deltaic or estuarine (Mason, 1971; Sikander, 1975; Desbiens, 1992), distal braided stream deposits (Cant and Walker, 1976) probably periodically subjected to marine influx, meandering river deposits (Rust, 1981), and proximal braidplain deposits (Rust, 1981). These facies developed in response to the Acadian mountain building to the south.
South of the Gaspé Sandstone belt, the Fortin Trough was still receiving siliciclastic turbidites during the Early and Middle Emsian. It is likely that by then, some basin shortening had already occurred. We arbitrarily plotted our paleogeographical map for Emsian time (Fig. 8A⇓) with 50% shortening. Farther south, movement associated with the Grand Pabos Fault resulted in the area south of the fault being uplifted. Movement was predominantly dextral strike slip, probably with a reverse component. Lateral displacement along the fault and its associated ruptures has been evaluated as 85, 22, and 10 km for the Grand Pabos, Grande Rivière, and Rivière Garin faults, respectively (Malo et al., 1992). If we consider an interval of 15 My for the Emsian, and that most of the movement along the Grand Pabos Fault likely occurred during the last third of the Emsian, the displacement rate would be in the order of 2.5 cm per year.
Eifelian paleogeography is not well understood, because of the lack of representative outcrops. Basin shortening and dextral translation along the Grand Pabos and associated faults had been completed (Fig. 8B⇑). In the north, a terrestrial sand-and-gravel belt, with poorly constrained extension, reflects erosion of the Acadian mountain belt to the south. Little is known about the facies that succeeded the Fortin/Temiscouata deep water facies in the previous Fortin Trough. Only the narrow outcrop belt of the Eifelian Touladi Formation (Fig. 2⇑) east of Lake Témiscouata indicates the occurrence of a marine embayment between the northern sand-and-gravel belt, and the southern uplifted area. This formation is composed predominantly of thin to thick-bedded, dark grey, shaly to sandy, fossiliferous limestone containing shallow marine brachiopods, with local sandstone and conglomerate (Lespérance and Greiner, 1969). Eastern extension of the embayment is not known, but its southwestern extension is represented by the coeval Famine and Mountain House Wharf formations of the Québec eastern townships (Bourque et al., 1995).
In the Chaleurs Bay Synclinorium, the only record of regressive phase R3 is the sands and gravels of the Lagarde, Pirate Cove, Fleurant and Escuminac formations. These represent various types of terrestrial sedimentation: alluvial deposits (Lagarde), alluvial fan to flood plain deposits (Pirate Cove), gravelly flood plain deposits (Fleurant), and lake or estuarine turbidites (Escuminac).
Summary and Conclusions
The Middle Ordovician to Upper Devonian stratigraphy of the Gaspé Belt is subdivided into four rock packages: 1) Upper Ordovician–lowest Silurian (Caradocian to Llandoverian) deep water siliciclastic and carbonate facies comprising the Honorat, Cabano and Matapédia groups; 2) Silurian–lowest Devonian (Llandoverian to Lochkovian) shallow to deep-water shelf facies of the Chaleurs Group; 3) Lower Devonian (Pragian–Emsian) carbonate and siliciclastic outer shelf to basin facies of the Upper Gaspé Limestone and Fortin groups, and the Témiscouata Formation; 4) Upper Lower to Upper Devonian (Emsian to Frasnian) nearshore to terrestrial coarse-grained facies consisting of the Gaspé Sandstone Group.
Three distinctive unconformities (Fig. 4⇑) occur in the Middle Ordovician to Upper Devonian succession of the Gaspé Belt. The oldest is the Taconian unconformity that separates the pre-Taconian rocks from the herein discussed Gaspé Belt succession. The second unconformity is located within the Chaleurs Group. It is dated as late Ludlovian–early Pridolian, and corresponds to the Salinic disturbance (Boucot, 1962). It is either an angular unconformity (Ristigouche Syncline), or an erosional surface that in places cuts deeply into the underlying, older Silurian and Ordovician rocks (Northern Outcrop Belt in the Connecticut Valley–Gaspé Synclinorium). The third unconformity is angular and occurs between Middle or Upper Devonian and Carboniferous rocks. It is related to the Acadian Orogeny.
The Middle Ordovician to Upper Devonian facies succession of the Gaspé Belt results from two broad sedimentary cycles, each made up of a regression–transgression couplet (R1−T1, and R2−T2), and a final regressive phase (R3) (Fig. 4⇑). Cyclic sedimentation is related to the interaction of basin tectonism and eustasy. Regressive phase R1 corresponds to successor basin infilling occurring after the Taconian Orogeny, and culminating in the middle Wenlockian eustatic sea-level low-stand. Transgressive phase T1 is likely linked to a combination of basin subsidence related to the Salinic extensional tectonic regime, and late Wenlockian–early Ludlovian eustatic sea-level rise. Regressive phase R2 resulted from a combination of the Late Silurian eustatic sea-level fall, and the Salinic-related block faulting, which led to the Salinic unconformity (see Bourque, 2001, this issue). Transgressive phase T2, which affected the central and northern parts of the Gaspé Belt, developed as basin subsidence in a transpressional tectonic regime in response to the Acadian Orogeny. Finally, regressive phase R3 is a response to the general basin uplift that accompanied the Acadian Orogeny.
↵1 GIRGAB. Groupe interuniversitaire de Recherches en Géodynamique et Analyse de Bassins