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| Chapitre 2. Taconian melanges in the parautochthonous zone of the Quebec Appalachians revisited: Implications for foreland basin and thrust belt evolution | ||
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Table des matières
In the Quebec Appalachians, disruption, imbrication and thrusting of the Taconian foreland basin sequence are responsible for the development of chaotic units within the turbiditic sequence of the Caradocian Sainte-Rosalie Group, the main lithologic assemblage of the parautochthonous zone. These chaotic units have been termed olistostromes or tectonosomes on the basis of field criteria and following Pini’s (1999) classification. Olistostromal units containing blocks of the middle mudstone (Utica Shale) and upper turbidite units (Sainte-Rosalie Group) of the foreland basin and spanning the Caradocian N. gracilis, C. americanus, O. ruedemanni and C. spiniferus graptolite zones were deposited and incorporated in the Sainte-Rosalie Group. Disruption of more competent beds of the flyschic sequence and fault stacking and slicing of older rock units occurred along major thrust faults, and now form structurally aligned corridors or tectonosomes. Graptolites and new chitinozoan data from both olistostromes and tectonosomes indicate older ages (early Late Ordovician) than the flysch units of Sainte-Rosalie Group (mid Late Ordovocian). Lithological, stratigraphic and structural criteria indicate that tectonosome slices are imbricated foreland basin rocks that are correlative to the Black River, Trenton, Utica, Sainte-Rosalie and Lorraine groups of the Laurentian platform. Thermal maturation data indicates that disruption of the autochthonous sequence, and folding and thrusting of the entire foreland basin sequence must have occurred shortly after their deposition. Contrary to what had been suggested, blocks in the olistostromes and tectonosomes were not derived from the allochthonous Chaudière thrust sheet, even though it presently marks the southern contact with parautochthonous zone. Imbrication of the foreland basin sequence must have occurred before emplacement of the Chaudière thrust sheet.
Dans les Appalaches du Québec, la rupture, l’imbrication et le chevauchement de la séquence de bassin d’avant-pays taconien ont provoqué la formation d’unités chaotiques au sein de l’assemblage lithologique typique de la zone parautochtone, soit la séquence turbiditique du Groupe de Sainte-Rosalie d'âge Caradocien. Ces unités chaotiques ont été nommées olistostromes ou tectonosomes sur la base de critères de terrain et selon la classification de Pini (1999). Les unités olistostromales, qui contiennent des blocs de l’unité médiane du bassin d’avant-pays dominée par des mudstones (Shale d'Utica) et de l’unité sommitale dominée par des turbidites (Sainte-Rosalie), ont été déposées et incorporées dans le Groupe de Sainte-Rosalie. Le boudinage des lits compétents de la séquence flyschique et l’empilement et l’écaillage par le jeu de failles sur les unités de roches plus anciennes se sont produits le long de chevauchements majeurs, et forment maintenant des corridors structuralement alignés et nommés tectonosomes. Les nouvelles données sur les chitinozoaires et les graptolites retrouvés au sein des olistostromes et des tectonosomes donnent des âges plus anciens (début de l’Ordovicien tardif) que l’unité flyschique du Groupe de Sainte-Rosalie (milieu de l’Ordovicien tardif). Les critères lithologiques, stratigraphiques et structuraux indiquent que les écailles de tectonosome sont des roches imbriquées du bassin d’avant-pays qui sont corrélées avec celles de la plateforme laurentienne, soit les groupes de Black River, Trenton, Utica, Sainte-Rosalie et Lorraine. Les données de maturation thermique indiquent que la fragmentation des roches autochtones ainsi que le plissement et le chevauchement de toute la séquence du bassin d’avant-pays, incluant les unités olistostromales doivent s'être produits peu de temps après le dépôt de la séquence flyschique. Contrairement à ce qui avait été précédemment suggéré, les blocs dans les olistostromes et les tectonosomes ne proviennent pas de la nappe de la Chaudière, quoiqu'elle marque actuellement le contact méridional avec la zone parautochthone. L’imbrication de la séquence du bassin d’avant-pays s'est produite avant l’emplacement de la nappe de la Chaudière.
During the Middle to Late Ordovician Taconian Orogeny in the northern Appalachians, carbonate sedimentation on the Laurentian passive continental margin was gradually replaced by flysch deposits in a foreland basin setting (St-Julien and Hubert 1975; Williams 1979). Convergence during the Taconian Orogeny resulted in growth of a tectonic wedge that migrated towards the continent. In the Quebec part of the orogen, Cambrian to Lower Ordovician slope and rise sediments accumulated on the southeast edge of the stable continental margin. In Middle Ordovician time, these deposits were detached and incorporated within the northwesterly transported thrust sheets and tectonically juxtaposed over the autochthonous platformal sequence (St-Julien and Hubert 1975). Deformation progressively migrated northwestward and eventually reached the more distal portion of the Laurentian platform/foreland basin, folding and imbricating these strata to form the Quebec Appalachians parautochthonous zone (Fig. 1). Thrust slices of the parautochthonous zone crop out within an orogen parallel corridor of a few kilometers wide that can be followed at depth in front of and beneath the allochthonous thrust sheets (Fig. 2b).
Figure 2.1 - General geology of the southern Quebec Appalachians. 1, St-Dominique slice; 2, Philipsburg slice; LSJO, Lac St-Jean outlier; Ch, Charlevoix area.
The foreland basin sequence is well preserved within the autochthonous zone of the Quebec Appalachians (Lavoie 1994). An important part of this basin can also be found within the outermost deformed zone of the Appalachian orogen in the Quebec parautochthonous zone. However, the presence of chaotic units within the parautochthonous sequence as well as the highly disrupted and imbricated nature of this zone at surface has complicated the stratigraphic correlations of these rocks with those of the autochthonous foreland basin. Within the parautochthonous zone, the observed facies differ slightly lithostratigraphically and chronostratigraphically from the autochthonous facies and reflect a more distal setting with respect to the autochthonous sequence. The first aim of this paper is to present a summary of foreland basin stratigraphy in the Quebec Appalachians and correlate strata of the parautochthonous zone with well known stratigraphic units of the autochthonous sequence.
The most remarkable unit within the parautochthonous sequence is the wildflysch exposed at Pointe Aubin (the « argiles-à-blocs » of St-Julien 1968) (Fig. 2a). The study of chaotic units, either of sedimentary or tectonic origin, is important in order to help reconstruct the depositional history and timing of deformation events within orogens. Chaotic units in the parautochthonous zone of Québec City were previously interpreted as gravity emplaced allochthonous slices produced during the Taconian Orogeny (St-Julien 1968; St-Julien and Hubert 1975; Beaulieu et al. 1980). Most of the units are highly chaotic and some are structurally ordered. The second objective of this study was to gather new field criteria to classify the units as olistostromes or tectonosomes following Pini's (1999) classification and to determine the nature of the chaotic units. Our work has established that the vast majority of chaotic units in the parautochthonous zone at Pointe Aubin consist of dismembered and boudinaged sandstone beds and thrust slices resulting entirely from tectonic processes such as folding and/or faulting. Better stratigraphic control, new paleontological and thermal maturation data, as well as structural relationships in the parautochthonous foreland sequence and its chaotic units in the Québec City area provide input on growth of the Taconian tectonic wedge, its migration towards the continent and the tectonic activity of the Quebec Laurentian margin during the Late Ordovician.
Figure 2.2 - A) Geology of the Appalachian front in the Québec City area modified from Globensky (1987) and adapted from Castonguay et al. (2002). B) Cross section along 2001 seismic line adapted from St-Julien et al. (1983) and Castonguay et al. (2001). Location of the 2001 seismic line on figs. 1 and 2a. Abbreviations: Bts, Bacchus thrust sheet; CFS, Chambly-Fortierville Syncline; Cts, Chaudière thrust sheet; Lts, Lévis thrust sheet; PA, Pointe Aubin; PN, Plage Neuville; PQts, Promontoire de Québec thrust sheet; SFR, St-Flavien reservoir.
In the Québec City area, the parautochthonous zone is completely exposed at low tide between St-Nicolas and St-Antoine-de-Tilly on the south shore of the St-Lawrence river along a 15 km section (Fig. 2a). The southern contact between the parautochthonous and the allochthonous zones, i.e. Logan’s Line, occurs a few kilometers west of St-Nicolas and consists of a major fault zone, where rocks of the Upper Ordovician Sainte-Rosalie Group flyschoid sequence are overthrusted by red and green shales of the Cambrian Sillery Group outcropping within the allochthonous Chaudière thrust sheet (Fig. 2a). A few kilometers to the northwest, the limit between the autochthonous and the parautochthonous zone occurs along the Aston fault which juxtaposes rocks of the Les Fonds Formation (Sainte-Rosalie Group) to rocks of the Nicolet Formation (Lorraine Group) (Globensky 1987).
Structurally, the parautochthonous zone consists of a series of steep, southeast-dipping thrust faults that display imbricate thrust fan geometries (Fig. 2b) (St-Julien et al. 1983; Castonguay et al. 2001; Séjourné et al. 2003). Deformation within the zone features large-scale northeast-trending, slightly overturned folds. Smaller-scale mesoscopic folds are upright to slightly overturned, northwesterly-verging and plunge gently to the northeast or to the southwest. In cross-section, thrusts affect progressively older rocks as the main decollement surface cuts down to deeper stratigraphic levels (St-Julien et al. 1983). At outcrop, the rocks of the parautochthonous zone consist essentially of typical Upper Ordovician flysch of the foreland basin (Fig. 2a). However, thrust slices of Beekmantown and the lower argillaceous limestone unit of the foreland basin crop out within the imbricate sequence of parautochthonous zone within the St-Dominique slice (see Fig. 1) (Prichonnet and Raynal 1977; Chalaron and Malo 1998; Séjourné 2000) and are also present at depth at St-Flavien (Fig. 2b) (SOQUIP 1979; St-Julien et al. 1983; Castonguay et al. 2001; Bertrand et al. 2003).
In the Quebec Appalachians, the Upper Cambrian to Upper Ordovician sedimentary sequence of the autochthonous St. Lawrence Lowlands reaches a thickness varying between 1500 and 3000 m (Globensky 1987) and includes Upper Cambrian to Lower Ordovician sandstones of the Potsdam Group, Lower to lower Upper Ordovician carbonates and dolomites of the Beekmantown, Chazy, Black River and Trenton groups, Upper Ordovician flyschoid sediments of the Utica Shale, Sainte-Rosalie and Lorraine groups and uppermost Ordovician molassic red beds of the Queenston Group (Fig. 3). During Ordovician time, the Appalachian passive margin collided with a subduction complex at the leading edge of a magmatic arc, resulting in the Taconian Orogeny (St-Julien and Hubert 1975; Rowley and Kidd 1981; Bradley 1989). The first documented sign of tectonic activity along the Laurentian margin is the Middle Ordovician St. George Unconformity (Knight et al. 1991) that in the Quebec Appalachians records uplift and erosion of the platform between the Beekmantown and Chazy groups (Lavoie, 1994). The unconformity is somewhat diachronous throughout the northern Appalachians during the Middle Ordovician and is attributed to lithospheric flexure and the consequent passage of a peripheral forebulge (Jabobi 1981; Quinlan and Beaumont 1984; Bradley and Kidd 1991; Knight et al. 1991; Lavoie 1994). In the Quebec Reentrant, the erosion period was of short duration and was followed by the foundering, drowning, and burial of the platform (Lavoie 1994) and development of a Middle to Late Ordovician foreland basin (Rowley and Kidd 1981; Quinlan and Beaumont 1984).
Figure 2.3 - Stratigraphic framework for the Ordovician succession of the St. Lawrence Lowlands (modified from Lavoie 1994; time scale from Tucker and McKerrow 1995). Vertical line pattern indicates non-deposition and erosion period.
The Taconian foreland basin of Quebec can be viewed as a classical, under filled peripheral foreland basin and, following Sinclair’s (1997) nomenclature, can be divided into three diachronous lithostratigraphic units: I-a lower argillaceous limestone dominated unit, II-a middle mudstone dominated unit and III-an upper turbidite dominated unit (Fig. 3). Shallow-water carbonates of the Upper Ordovician Black River and Trenton groups are included in the lower argillaceous limestone unit (I of Fig. 3) that were deposited over the shallow-water, siliciclastic sands of the lower part of the Chazy Group (Globensky 1987) and recorded a progressive deepening upward cycle. Regional facies distribution, lithotectonic elements, and significant thickness variations indicate several depocenters and local growth faults dissecting the Trenton shelf (Lavoie 1994). Slump folds and limestone debris flows in the Caradocian Sainte-Irénée and Beaupré formations immediately overlying the Trenton Group northeast of Québec City also suggest that the shelf was indeed unstable.
The middle mudstone unit (II of Fig. 3) consists of up to 1300 m of deep water siliciclastic sediments and hemipelagic mud of the Utica Shale. They were deposited over the carbonate units due to rapid subsidence of the foreland basin (Globensky 1987). Black shale is characteristic of early flysch-phase fill along the distal flank of the Middle to Late Ordovician Taconian peripheral foreland basin (Bradley and Kidd 1991). The Utica Shale is a diachronous unit which is older when located closest to the Appalachian front, as in the Québec City vicinity ( Corynoides americanus-Orthogratus ruedemanni to the Climacograptus spiniferus Zones ) and younger to the southwest on the Laurentian platform, as in the Montreal region ( Climacograptus pygmaeus Zone ). Diachronous east to west progression of subsidence was coincident with the progressive westward change from carbonate-dominated to siliciclastic sedimentation within the foreland basin, also documented elsewhere in the Appalachian orogen (Ettensohn 1991; Lehmann et al. 1995). In the Québec City area, the Utica Shale is only 30 m thick and yields graptolites spanning the O. ruedemanni to the C. spiniferus Zones (Globensky 1987). This unit is distinguished from younger facies that contain more abundant clastic beds suggesting that it was laid down prior to overthrusting of the thrust sheets onto the continental margin.
The upper turbidite unit (III of Fig. 3) consists of synorogenic sediments accumulated during the Caradocian to early Asghillian stages of the Late Ordovician during and after the overthrusting of the external thrust sheets. It is dominated by thick successions of alternating sandstone and mudstone of the Sainte-Rosalie and Lorraine groups. The siliciclastic source was located to the south-east and debris were derived from the thrust sheets (Globensky 1987), representing a major reversal in the direction of sediment supply from the Laurentian shelf to more outboard elements of the tectonic wedge (Hiscott 1995). The sandstones that accumulate at the toe of, and on top of the thrust wedge are highly immature and rich in lithic fragments with rarer volcanic detritus derived from erosion of the thrust wedge (Beaulieu et al. 1980; Schwab 1986). Thrust-faulted highs generated during deposition of the middle unit lead to ponding of turbidite flows of the upper unit, generating thick sandstone beds overlain by thick mudstone drapes (Pickering and Hiscott 1985).
Flyschoid sediments of the upper turbidite unit are present both in the autochthonous and parautochthonous zones and assigned to the Sainte-Rosalie and Lorraine groups (Globensky 1987) (Fig. 4). The Sainte-Rosalie Group is a typical flysch sequence that consists essentially of a succession of siltstone, mudstone, silty shale and rare dolomitic units (Globensky et al. 1993). It is subdivided into the Lotbinière, Aubin, Les Fonds, Sainte-Sabine, and Iberville formations partly on the basis of the structural position within the orogen, i.e. autochthonous vs. parautochthonous zone. According to Globensky (1987), the Lotbinière Formation conformably overlies the Utica Shale on the south-east limb of the Chambly-Fortierville syncline in the autochthonous zone (Fig. 2a). On the north-west limb of the syncline, the Utica Shale is overlain by the Nicolet River Formation of the Lorraine Group, without the intervening Lotbinière. Thus the Lotbinière Formation is chronostratigraphically correlative to the upper part of the Utica Shale (Fig. 4). The Caradocian age of the Sainte-Rosalie Group is given by graptolitic faunas that indicate the C. americanus Zone to the C. pygmaeus Zone of Riva (1969). Regional correlatives of the Sainte-Rosalie Group are the Cloridorme Formation in the Gaspé Peninsula, the Iberville Formation of Vermont, the Snake Hill Formation of the Albany area of New York and the deformed Martinsburg Formation of New Jersey and Pennsylvania (Globensky and Riva 1982).
The Lorraine Group consists of interbedded grey shale, sandstone, siltstone and limestone and includes the Nicolet and Pontgravé formations (Fig. 4). It is the thickest and the most widespread group of the St. Lawrence Lowlands (Globensky 1987) and is restricted to the autochthonous zone. Its thickness reaches approximately 800 m along the Nicolet River (Clark et al. 1979). The middle and upper part of the Lorraine Group represent a shallowing upward environment. Pelecypods dominate the fauna which is extensive at the top (Clark et al. 1979). The Lorraine Group was interpreted by Belt et al. (1979) as a prodeltaic deposit environment, whereas Walters et al. (1982) suggest rather a deeper-water flysch deposit. The Lorraine Group is conformably capped by uppermost Ordovician red molasse deposits of the Queenston Group.
Figure 2.4 - Stratigraphy of the upper turbidite unit of the Taconian foreland basin. The Chambly-Fortierville syncline is shown on figure 2.
Siliciclastic rocks of the upper turbidite unit in the parautochthonous zone of the Québec City area were originally assigned to the Les Fonds Formation and the Breault and Chambly members of the Nicolet Formation, by Clark and Globensky (1973). Following detailed stratigraphic and sedimentologic work by Walters (1979), Walters et al. (1982) and Beaulieu et al. (1980), the parautochthonous flyschoid sediments were correlated to the Sainte-Perpétue and Sainte-Monique members of the redefined Rivière Nicolet Formation in the autochthonous zone (Fig. 4). Globensky and Riva (1982) reported graptolites of the C. americanus to C. pygmaeus zones in the parautochthonous flyschoid sediments indicating that they were in part older than rocks of the Nicolet River Formation (Lorraine Group), rather correlating them with the autochthonous Utica Shale. Finally, because of difficulties in following lithological contacts from the autochthonous to the parautochthonous zones due to the faulted contact between these zones, Globensky (1987) chose to follow Clark and Globensky (1973) and assigned the flyschoid sediments of the parautochthonous zone in the Québec City area to the Pointe Aubin Wildflysch, the Les Fonds and Lotbinière formations (Fig. 4), although the Les Fonds Formation is by far the most widespread. A few years later, Globensky et al. (1993) proposed to rename the Pointe Aubin Wildflysch the Aubin Formation. Total thickness of the Sainte-Rosalie Group is difficult to estimate due to faults, although 1000 m seems to be an educated minimum estimate. In the parautochthonous zone further southwest in the Iberville area (Fig. 1), lithostratigraphic and chronostratigraphic correlatives of the Les Fonds belong to the Sainte-Sabine Formation (Globensky 1987).
Stratigraphic repetitions and omissions by thrust faults make it difficult to correlate rocks of the parautochthonous zone with those of the autochthonous zone. For simplicity we propose that all outcropping rocks of the parautochthonous zone in the Québec City area be included within the Les Fonds Formation (Fig. 4). The Pointe- Formation is restricted to rocks that crop out at Pointe Aubin along the south shore of the St. Lawrence River near Québec City (Globensky et al. 1993). Detailed field observations, as well as stratigraphic and structural evidence presented below show that all the rocks assigned to the Aubin Formation are actually in fault contact with rocks of the Les Fonds Formation and occur within a 10-20 m wide fault zone made up of a series of folded and imbricate fault slices. Thus we propose to abandon the use of Aubin Formation all together (Fig. 4). Likewise, we propose to restrict the use of Lotbinière Formation to rocks of the Sainte-Rosalie Group flysch in the autochthonous zone.
The stratigraphic succession of the Les Fonds Formation can best be described in the Pointe Aubin area where a variety of lithological assemblages crop out in the core of a southeast-dipping and southwest-plunging syncline (Fig. 5). Thrust faults occur along both limbs of the syncline. In the axial zone of the syncline the stratigraphic succession of the Les Fonds Formation is rather well exposed and consists mainly of mudstone and fine- to coarse-grained sandstones with less abundant fine-grained limestones, conglomerate and chaotic units (lithologic assemblages A, B and C on Figs. 5 and 6). Chaotic units correspond to St-Julien's wildflysch or « argiles-à-blocs » (St-Julien 1968). St-Julien (1968) divided the wildflysch into two distinct chaotic units a polygenic and an oligomictic unit (Fig. 5b). Both units are interbedded in the same mudstone and sandstone matrix of lithologic assemblage C.
The main lithologic assemblage of the Les Fonds Formation (lithologic assemblage C) cropping out at Pointe Aubin consists of mudstone and very fine grained sandstone interbedded with thin- to thick-bedded sandstones and more rare chaotic units. The lithologic assemblage A consists of a 50 meters thick chaotic zone of bituminous mudstone blocks set in a shale matrix. Bituminous mudstone blocks vary in size from 1 m to more
Figure 2.5 - A) Geological map of the Pointe Aubin area and lithological assemblages of the Les Fonds Formation; B) Map of chaotic units by St-Julien (1968).
than 100 m length and from 1 m to about 10 m thick and are aligned on both limbs of the syncline. This horizon of meter-scale blocks is in fact interbedded within the flyschic sequence and is folded around the syncline. It is thus conformable to mudstone and sandstone beds meaning that it was deposited coevally within the stratigraphic sequence of the Les Fonds Formation. Two other very similar chaotic zones of 1-2 meters thick occur approximately 1 km to the north-east of Pointe Aubin and are interbedded conformably within mudstones and fine grained sandstones of Assemblage C (Fig. 7a). Texturally, they consist of a brecciated matrix with a chaotic distribution of clasts. The horizons contain small angular and rounded blocks (1-50 cm) of bituminous mudstone, green silicified siltstone and sandstone set in a sandy matrix. Bedding and pseudobedding orientations in the largest blocks do not have a similar trend.
The overlying lithologic assemblage B is composed of 75 meters of thick coarse sandstone beds (0.25 m to 1 m) alternating with shale (Fig. 6). Assemblage B corresponds to St-Julien's (1968) oligomictic chaotic unit (Fig. 5). This unit crops out mainly along the overturned, southern limb of the syncline. Although lithic sandstone and conglomeratic sandstone beds are quite continuous they can be boudinaged and sometimes strongly disrupted in which case they occur as relatively large blocks (1-50 m) (Fig. 5). This facies was recognized elsewhere in the parautochthonous zone just west of Pointe Aubin in the same stratigraphic position where it presents a thickness of 10 meters. Lithologic assemblage B is overlain by the turbiditic facies of assemblage C that fines upward into a 700 m thick succession of lithic sandstone beds (2 to 40 cm thick), shale and silty sandstone. The sandstone/shale ratio decreases upward, passing from 75% to 50%.
Figure 2.6 - Schematic stratigraphic succession of the Les Fonds Formation in the core of the syncline at Pointe Aubin.
The sandstone displays all the characteristics of a turbidite with graded beds, cross-laminations, flute casts and scour-marks. The detrital fraction of the sandstones is composed of 71% quartz, 9% feldspath, and 19% rock fragments, consisting of claystone, quartzo-feldspathic sandstone, argillaceous siltstone, various limestones and detrital fossil fragments (Beaulieu et al. 1980). Within the Les Fonds Formation, sandstone beds are more abundant in the lower half of the section (Beaulieu et al. 1980). Thickness of sandstone beds and size of the detrital fraction also diminishes towards the upper half of the section. This fining upward section suggests episodic tectonic activity in the basin at the time of deposition of the Les Fonds Formation (Fig. 6). Flute casts in the Les Fonds Formation indicate north westward paleocurrents, pointing to a source located south-southeast of the
Figure 7. a) Interbedded olistostrome zone within the flyschoid sequence of Les Fonds Formation; b) Close up view of the olistostrome unit. A perpendicular cleavage is recognized. The GPS receptor is used as scale; c) Aerial photography of the tectonosomes’ triaxial geometry at the Pointe Aubin. See fault location on figures 5 and 7; d) Folded tectonosome. The backpack and the geologist hammer are used as scale.
study area. Based on these observations, Beaulieu et al. (1980) proposed a sedimentary source to the east-southeast and paleotopography directly influenced by tectonic activity.
The northern limb of the Pointe Aubin syncline consists of a 10-20 m wide fault zone made up of a series of tectonically juxtaposed thrust slices, including distinct lithological facies and blocks of variable size and lithology, such as green silicified siltstone, bituminous mudstone, dolomitic limestone, red shale with calcarenite interbeds (Figs. 5 and 8). St-Julien (1968) included these fault slices in his polygenic chaotic unit and Globensky et al. (1993) later assigned them to the Pointe Aubin Formation. In outcrop, individual fault slices consist of tightly folded beds and sometimes highly fractured competent beds displaying internal continuity (Fig. 8). The fault slices are of variable size, from meters to hundred of meters, and are affected by faults and fractures fragmenting them into smaller blocks, with definite shapes, sharp outlines, and an evident triaxial geometry (Fig. 7c). All fault blocks show a common trend and a strong parallel preferred orientation of bedding plane. The finer-grained matrix surrounding the blocks is tectonized and displays a scaly texture occurring in small cm- to tens-cm thick corridors that are parallel to the preferred alignment of the blocks. Large blocks can be lozenge-shaped, highly fractured with numerous conjugate fault planes and disposed within 10-20 meters wide corridors parallel to thrust faults (Figs. 5 and 8). Thrust faults with down-dip striae can be recognized along the margins of the blocks. Strata within the blocks are occasionally tightly folded with axial traces (Fig. 7d) parallel to the regional cleavage.
Based on graptolite data (Riva 1972; Walters et al. 1982), the age of the stratigraphic units within the Les Fonds Formation is Late Ordovician. Most of the graptolites collected from the mudstones (assemblage C) of the Les Fonds Formation along the section belong to the Caradocian C. spiniferus Zone (Riva 1972; Walters 1979; Walter et al. 1982). However, graptolites collected in the chaotic units and thrust slices belong to the N. gracilis, C. americanus, O. ruedemanni and C. spiniferus Zones (St-Julien 1968; Riva 1972; Walters 1979; Walter et al. 1982).
Table 2.1. Chitinozoan identifications and their biostratigraphic equivalents in terms of eastern North American chitinozoan and graptolite zones.
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Our palynological investigations (Table 2.1 and Fig. 2.5 for sample location) confirm that the age of some units of the thrust slices is older than that of the enclosing turbidites of the Les Fonds Formation. Most of the chitinozoan collections from the Les Fonds turbidites have yielded species reported in the St. Lawrence Lowlands in biostratigraphic levels ranging from the K . multispinata - H . duplicitas to A . cancellata or S . gracqui to A . cancellata chitinozoan zonal levels (Achab 1989). These biostratigraphic levels are equivalent to C. americanus/O. ruedemanni to C. spiniferus graptolite zones (Table 2.1). Chitinozoan data from samples 30003 and 30004 suggest a correlation with levels in the Utica Shale belonging to part of the A . cancellata chitinozoan levels which are equivalent to the C. spiniferus graptolite Zone (Table 2.1). One sample (29998) from a tectonosome slice has yielded chitinozoan species characteristic (in Quebec) of the older C . hirsuta - Lagenochitina sp. a zonal part correlative to the N. gracilis graptolite Zone (Table 2.1). The calcareous mudstones of the Citadelle Formation forming the Quebec Promontory thrust sheet 25 km to the northeast (Fig. 2.2) and constituting the lowest structural thrust sheet of the Québec City area belong to the N. gracilis to D. multidens zonal interval (Riva 1985).
Programmed pyrolysis (Rock Eval) and reflectance data on dispersed organic matter were collected from sandstones and mudstones of the Les Fonds Formation, as well as in both the olistostromes and tectonosomes (Tables 2, 3). Production index (PI) and temperature of maximum pyrolysis ( T max) indicators from Table 2 are reliable thermal maturation evaluation for sedimentary successions (Hunt 1995). Values of reflectance equivalent to that of vitrinite in the Pointe Aubin syncline are below 1.35% and indicate that rocks of the Les Fonds Formation are in the lower half of the mature zone (Figs. 9, 10). Such a low degree of maturation is very rarely observed in the St. Lawrence Lowlands and in the foreland basin sequence. Analyses in the Les Fonds Formation 7 km west of the Pointe Aubin section in the parautochthonous zone along the St. Lawrence shore, display reflectance values 0.2% to 0.3% higher (Héroux and Bertrand 1991). This lower level of thermal maturation indicates that the Pointe Aubin section is near the top of the parautochthonous structural pile.
Table 2.2. Total organic carbon (TOC) and Rock Eval analysis of selected Pointe Aubin samples. GP = genetic potential (kg hydrocarbon (HC)/ton of rock, PI = production index (ratio of in situ HC on total in situ and kerogen hydrocarbon), Tmax = temperature °C of maximum pyrolysis of kerogen, HI = hydrogen index (mg HC/ g of TOC), OI = oxygen index (mg CO2/ g of TOC). Detailed explanations about the purpose of those thermal maturation and source rock paramaters are found in Espitalié et al. (1977) and Hunt (1995).
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Figure 2.9 - Modified Espitalié (Espitalié et al. 1977, 1984; Hunt, 1995) with the addition of HC yield that is determined from HI (left vertical scale) and TOC (upper-right horizontal scale) (modified from Bertrand and Malo, 2001). Data of black shale of Tectonostome unit (●), Les Fonds Formation (■) (this paper) and Utica Shale from Montmorency River (○) and Billings Formation (□) (Bertrand 1991).
Table 2.3. Reflectance of dispersed organic matter (DOM) in Pointe Aubin samples. Ch = chitinozoans, Gr = graptolites, Sc = scolecodonts, pro = protobitumen-bituminite, Dpt = migrabitumen droplet, Mig = migrabitumen Cok = natural coke, Est. Ro Vitrinite = estimated vitrinite reflectance from migrabitumen or zooclasts (combination of Ch, Gr and Sc). Organic matter concentrates were obtained according the method of Bertrand and Héroux (1987). Transparent strew mount used for OM identification and reflectance analysis were prepared following the method of Bertrand et al. (1985). Method used for correlated DOM reflectance to vitrinite reflectance is from Bertrand and Malo (2001).
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Spatial variation of reflectance values in the Pointe Aubin successions shows a rather clear relationship with the syncline. Low values of reflectance are observed in the vicinity of the
axis, whereas highest reflectance values are found on both limbs of the syncline, at the base of the structural succession (Fig. 10). This relationship indicates that the folding of the synclinal structure postdate the thermal maturation of those successions (pre-tectonic thermal maturation of Taylor et al. 1998). Maximum thermal maturation of the parautochthonous zone is thus presumably due to sedimentary and (or) tectonic burial. Tectonosome units outcropping on both limbs of the synclinal structure also show similar reflectance values.
The chaotic units at Pointe Aubin were interpreted as olistostromes genetically related to gravity sliding and emplacement of thrust sheets during the Middle Ordovician Taconian Orogeny (St-Julien 1968; St-Julien and Hubert 1975; Beaulieu et al. 1980). According to Flores (1955) who first defined olistostromes, they consist of sedimentary bodies interbedded within sequences of normal marine strata. However, the term olistostrome has been widely used over the years to designate any structural unit, tectonostratigraphic unit or sedimentary body characterized by a chaotic appearance and a block-in-matrix fabric in outcrop. Disrupted allochthonous bodies and mélange-type rocks characterized by a scaly fabric (i.e., argile scagliose) and derived from distant paleogeographic settings have often been confused with olistostromes (Pini, 1999). Tectonosomes basically consist of strongly deformed and tectonized formations, whereas olistostromes are sedimentary bodies emplaced by debris flows or avalanches.
Chaotic units in the parautochthonous zone of the Québec City area described at Pointe Aubin by St-Julien (1968) have, at first observation, common characteristics, such as a block-in-matrix fabric and a seemingly chaotic disposition of blocks. However, using basic criteria for distinguishing tectonosomes and olistostromes (Pini 1999) we were able to separate the chaotic units and to deduce their sedimentary and/or tectonic origin.
Typical olistostromes tend to display a bimodal distribution of block size and can be grouped into two types: Type A olistostromes, that contain small cm-sized fragments and m-sized blocks of single beds floating in a matrix, and Type B olistostromes, that are made up of large blocks including decameter slabs of single beds to packages of beds that are either clast or matrix supported (Pini 1999).
Chaotic units of lithological assemblage A are aligned on both limbs of the Pointe Aubin syncline (Fig. 5). This horizon of meter-scale bituminous mudstone blocks is in fact interbedded in the flyschic sequence and is also folded around the syncline. We have designated most of lithological assemblage A as Type B olistostromes of Pini (1999) due to the size of these blocks within this olistostromal unit. Graptolites which are from the N. gracilis Zone were identified in the bituminous mudstone blocks by Riva (1972) and reconfirmed by Riva (personal communication, 2000). Since these blocks are much older than the surrounding shaly sandstone ( C. spiniferus Zone ), we believe that these bituminous mudstones are olistoliths which have been deposited by debris flows, i.e. “ sedimentary olistostromes ” within the flyschic sequence of the Les Fonds Formation.
Type-A olistostrome zones of 1-2 meter thick also occur sporadically throughout the Les Fonds Formation in the parautochthonous zone and at least two of these can be observed along the section at Pointe Aubin (Fig. 7a). In outcrop, olistostromes are clearly affected by major northeast-directed folds (Fig. 7a). In the hinge of major folds, a perpendicular to bedding cleavage, coaxial to folds and parallel to the regional cleavage affects matrix, clasts and small blocks within the chaotic units (Fig. 7b). The fact that cleavage cuts through the chaotic horizons and across clasts is evidence that the deposition of chaotic units predates cleavage development and cannot be related to tectonic processes such as folding and thrusting. These relationships argue in favor of a sedimentary or pre-tectonic origin and we propose that the chaotic horizons are true olistostromes deposited by debris flow. Mapping relationships in the syncline at Pointe Aubin indicate that these debris flows seem to be restricted to the lower part of the section and seem to be roughly contemporaneous with the thick sandstone and conglomerate beds (Fig. 5).
Tectonosomes are defined as block in matrix rocks derived from an original in situ deformation of stratigraphic units (Pini 1999). The term tectonosomes is used here instead of terms such as broken formation, disrupted formation, or dismembered sequence (Raymond 1984), to emphasize that tectonosomes consist of either boudinaged or completely disrupted parts of a single lithostratigraphic unit and that the high structural order evident at the outcrop to map scale coincides with a structural fabric.
In the study area, two tectonosome types occur along both deformed limbs of the Pointe Aubin syncline parallel to thrust faults (Figs. 5, 7 and 8). First, blocks of thick and coarse sandstones that we have assigned to the lithologic assemblage B of the Les Fonds Formation occur along the southern limb of the syncline. St-Julien included these sandstone blocks in his oligomictic unit (St-Julien 1968). The blocks are structurally ordered and display a common trend and a strong preferred orientation in line with and parallel to undisrupted sandstone beds further to the northeast (Fig. 5). Mesoscopically, they consist of deformed competent sandstone beds that are boudinaged and completely fragmented into sharp angular blocks and dismembered sandstone bed packages.A pervasive scaly fabric is clearly observed within the matrix between the blocks. Nonetheless, the continuity between competent fragmented beds is still recognizable (Fig. 5), where some isolated blocks represent either relicts of thicker channels or fold hinges. The structural order is defined by fragments or boudins that have long axes that coincide with the orientation of the bedding plane along the southern limb of the syncline.
The other tectosome type occurs along the northern limb of the syncline and is made up of tectonically juxtaposed fault slices described above (Fig. 8). Well documented criteria such as internal continuity, highly fractured competent beds and numerous faults within individual fault slices, triaxial geometry and common trend of slices, and a tectonized scaly-matrix are all typical of tectonosomes (Pini, 1999). Also, contacts between individual slices are defined by thrust faults. The rather consistent orientation of fold axes within the individual slices (Fig. 5) refutes the slumping hypothesis for the origin of folds in the blocks proposed by St-Julien (1968). Rather, we believe that stratal disruption resulted from prevalent brittle, simple-shear deformation, related to boudinage, transposition of bedding, stacking of blocks, and thrusting.
Distinguishing olistostromes from tectonosomes within the foreland sequence of the parautochthonous zone has improved our understanding of the tectonosedimentary evolution in this part of the transitional parautochthonous zone between the St. Lawrence Lowlands and the allochthons. Based on sound field observations and litho- and chronostratigraphic correlations with foreland sequences, we recognize that chaotic units at Pointe Aubin are the result of two distinct mechanisms. The olistostromes are sedimentary bodies interbedded within the flyschic sequence of the foreland basin and contain randomly distributed blocks of foreland lithotypes mainly from the middle mudstone unit of the foreland basin sequence in a mudstone to sandstone matrix typical of the turbiditic facies of the Les Fonds Formation. Contrary to previous interpretations that proposed a sedimentary origin for all chaotic units at Pointe Aubin (St-Julien 1968; Riva 1972; Globensky 1987), our study shows that true olistostromal units represent only a minor proportion of the lithological units. Rather, a large proportion of the chaotic units at Pointe Aubin have all the main characteristics of tectonosomes and units are found within two structurally ordered, map scale corridors along both limbs of the syncline. Tectonic mechanisms such as folding and boudinage were entirely responsible for the development of the oligomictic tectonosome made up of moderate to strongly deformed sandstone beds of the Les Fonds Formation along the southern limb of the syncline.
Correlating rocks within the tectonosome slices on the northern limb of the Pointe Aubin syncline with other known lithostratigraphic units of the autochthonous foreland basin has always been hazardous which is why Globensky et al. (1993) assigned these rocks to the Pointe Aubin Formation. Lithologically, each tectonosome slice contains distinct lithological assemblages, either 30-100 cm thick sandstone beds, dolomitic limestone, brown calcareous mudstone, red shale with interbedded calcarenite or slumped limestone with shale. These lithologies are all representative of typical foreland basin lithotypes from the lower argillaceous limestone unit as well as from the middle mudstone, or the upper turbidite units of the Quebec foreland basin described earlier. TOC values and correlative organic petrographic compositions determined on samples fromthe tectonosome black shales are similar to those of the Utica Shale of the autochthonous platform (St. Lawrence Lowland sensus stricto ) outcropping at Montmorency Fall, 25 km northeast of Québec City (see Fig. 2a). This indicates that the black shales in the tectonosome units at Pointe Aubin are most likely blocks of Utica Shale from the underlying autochthonous platform. Walters (1979) identified graptolites within fault blocks ranging from the C. americanus to the O. ruedemanni Zone , meaning that they are in fact chronostratigraphic equivalents to the Upper Ordovician sequences of the foreland basin. New palynological data presented here also suggest that the tectonosome slices are imbricated foreland basin rocks that contain chitinozoan species reported units of the Black River, Trenton, Utica and Sainte-Rosalie groups of the Laurentian platform.
A large slice of peculiar green and black siliceous argillites and radiolarian cherts generally assigned to the Middle Ordovician is caught up in the fault zone along the northern limb of the Pointe Aubin syncline (Fig. 5a). This lithotype has been previously ascribed to the Sillery Group outcropping just a few kilometers to the south in the Chaudière thrust sheet (St-Julien 1968). However, a recent petrological and geochemical study of cherts in the Quebec Appalachians area concluded that the chert at Pointe Aubin is of sedimentary origin and was deposited in the foreland basin with alternating beds of black shale and limey shale during the Middle Ordovician (Morin 1997). Based on these petrological, geochemical and lithostratigraphic considerations, we assign these siliceous argillites to the middle mudstone unit of the foreland basin, and suggest that siliceous argillites and chert were faulted to surface from similar units from the underlying autochthonous foreland basin.
Different lines of evidence presented above prompt us to reevaluate St-Julien’s (1968) interpretation of the chaotic units at Pointe Aubin with respect to the formational mechanisms and the origin of olistostromes and tectonosomes. Olistostrome fragments, clasts, blocks, originated from the foreland basin sequence. Likewise olistoliths and slivers within the tectonosome bodies were the result of tectonic imbrication of the foreland basin sequence with very little if no contribution from the far travelled allochthons. St-Julien (1968) hypothesized that blocks, olistoliths and tectonic slices were derived from the Chaudière thrust sheet comprising Cambrian to Lower Ordovician rocks of the Sillery Group (St-Julien 1995). He proposed a two-stage scenario for the formation of the chaotic units at Pointe Aubin: (1) oligomictic units resulting from submarine debris flows produced by ploughing and scraping sediments in front of the advancing Chaudière thrust sheet, and (2) the polymictic chaotic units that developed soon afterwards contain blocks derived from the base of the Chaudière thrust sheet. Alternatively our model proposes: (1) tectonically controlled deposition of turbidites of the Les Fonds Formation in the southeastern extension of the Quebec Taconian foreland basin, (2) imbrication of the foreland basin sequence and (3) folding, faulting and development of the parautochtonous zone.
Deposition of the flyschoid sediments of the upper turbidite unit mostly occurred as turbidity currents in the subsiding Quebec foreland basin in front of the advancing tectonic wedge (Beaulieu et al. 1980). Submarine-fan deposits of the Sainte-Rosalie and Lorraine groups, and the Les Fonds and Lotbinière formations composed of graywackes and turbiditic shales were derived from orogenic sources to the east. The proximal fill consists of coarse sandstones and shales of the Les Fonds Formation deposited into an elongate northwest-southeast trending trough. Spatial variation of reflectance values in the Pointe Aubin successions indicate that maximum thermal maturation of the parautochthonous zone is due to sedimentary and/or tectonic burial.
The advancing tectonic wedge imbricated the eastern part of the foreland basin where sediments of the middle mudstone, and upper turbidite units were, in turn, deformed and incorporated as blocks in olistostromal units deposited contemporaneously with the Les Fonds sandstones and shales during the Late Ordovician Caradocian ( C. spiniferus graptolite zone) (Fig. 11a). Relief along fault scarps in the overthrust wedge allowed erosion and incorporation of olistoliths, blocks and fragments within olistostromes originating in part from older rocks of the foreland basin such as bituminous mudstone ( N. gracilis Zone) and possibly also siliceous argillites and radiolarian cherts that we assign to the middle mudstone unit. Reflectance values suggest that folding of the entire foreland basin sequence including the olistostromal units occurred soon after burial (Fig. 11b). Similar relationships have been documented in the Chambly-Fortierville Syncline that outcrops in the same rock section along the St. Lawrence shore, 15 km west from the Pointe Aubin section (Héroux and Bertrand 1991).
Thrust faults of the tectonic wedge evolved into major detachment zones imbricating the entire eastern portion of the foreland basin to form what is now defined as the parautohthonous zone (Fig. 11c). Disruption of sandstone beds like those located along the southern limb of the Pointe Aubin syncline and fault stacking and slicing of older rocks buried beneath the flyschic sequence occurred along the major faults and now form structurally aligned corridors that correspond to tectonosomes along the Pointe Aubin section.
Precise timing of the structural development of the parautochthonous zone is uncertain although thermal maturation data presented above do give some indications. According to reflectance values and Héroux and Bertrand’s gradient (1991) deduced for the parautochthonous domain, maximum burial of Les Fonds Formation successions ranges between 3.9 and 5.1 km. Maximum burial of blocks from the tectonosome slices are also within this range, from 4.5 to 4.9 km. In order to show similar reflectance and maximum burial, the thrusted rocks in the tectonosome slices could not have been buried very deep, or during a very long time span, before being maturated coevally with the Les Fonds Formation successions. In other words, disruption of the autochthonous rocks and thrusting of the blocks must have occurred shortly after their deposition. Also, black shales in the tectonosomes are less mature (Ro valuesbelow 1%, Fig. 10) than the correlative Utica Shale in the autochthonous platform (revised Ro values between 1.07 and 1.19%) outcropping on the opposite shore of St. Lawrence River, just across the river from the Pointe Aubin section (Plage de Neuville in Bertrand 1991) (see Fig. 2a). Since Ro values of blocks of black shale in tectonosomes are lower than the immediately surrounding autochthonous black shales, the faulted blocks of the parautochthonous zone most probably reached their actual structural position before complete burial and the thermal maturation of the autochthonous successions, which presumably occurred during deposition of the Lorraine and Queenstone groups in early Ashgillian time.
Lithotypes within tectonosome units at Pointe Aubin are of two types: imbricated foreland basin turbidites and fault slices of older strata belonging to the lower argillaceous limestone and the middle mudstone units of the foreland basin sequence. Indeed, the thrust that cuts through the northern limb of the syncline at Pointe Aubin and highlighted by a 10 to 20 m thick zone of fault slices represents a major structure bringing to outcrop slices of the lower argillaceous limestone unit ( C. americanus to O. ruedemanni time). As pointed out by our palynological data, the Pointe Aubin fault juxtaposes slices of rocks that are correlative to the Black River and Trenton limestones, as well as the Utica Shale. This suggests that deformation of the foreland sequence of the parautochthonous zone occurred during two distinct stages: a first stage related to deposition of olistostromes and imbrication of the flyschoid strata of the foreland basin sequence (Figs. 11a and 11b) followed by late-stage emplacement of thrust slices, i.e. tectonosomes, (Fig. 11c) and out-of-sequence thrusting. A spread in orientations of folds and thrust faults in the parautochthonous zone also support two stages of faulting.
Rocks of the Les Fonds Formation at Pointe Aubin display low values of thermal maturation (Fig. 9) even though the section is located only 3 km to the north of Logan’s Line, the northwestern limit of the taconian allochthons, and structurally underlies the Sillery Group rocks of the Chaudière thrust sheet (Fig. 2). Ogunyomi et al. (1980) estimated reflectance values of between 1.71 and 2.30% for the Sillery Group deep-water clastic rocks of the Chaudière thrust sheet and concluded that thermal maturation was not caused by thrusting (Ogunyomi et al. 1980). These reflectance data indicate maximum burial depths of between 7 and 8 km for the Sillery Group compared to less than 5 km for blocks within the olistostromes and tectonosomes of the parautochthonous zone in the Québec City. Obviously, thermal maturation of the Les Fonds Formation succession including olistostromes and tectonosomes, was not caused by overthrusting of the Chaudière thrust sheet even though the Chaudière thrust sheet presently marks the southern contact with the parautochthonous zone. Likewise, olistostrome blocks and tectonomes simply can not be derived from the allochthonous Chaudière thrust sheet. For all these reasons, we propose that imbrication of the foreland basin sequence occurred before emplacement of the Chaudière thrust sheet to its actual position. An anomalous reflectance value of 1,58% was however determined in the Les Fonds in the footwall of the Chaudière thrust fault, indicating some local heating due to emplacement of the thrust sheet (Bertrand 2003, personal communication). Emplacement of the Chaudière thrust sheet has long been interpreted as out-of-sequence, since it is thrust over and covers the parautochthonous zone as well as the Promontoire de Québec, Lévis and Bacchus thrust sheets (Fig. 2a) (St-Julien 1995).
In the parautochtonous zone of the Québec City area, fault slices made up of strata from the lower units of the foreland basin (lower argillaceous and middle mudstone unit) are unusual and only occur at surface at Pointe Aubin. We believe that this major fault zone is similar to other important thrust faults that define the soles of three tectonic slices, the St-Flavien, Saint-Dominique and Philipsburg slices. Evolution of the foreland basin, structural relationships and timing of tectonic events presented here for the Pointe Aubin parautochthonous zone are thus relevant to better understand the emplacement of thrust slices at a more regional scale. A complex structural history involving two-stages of thrusting has also been proposed for the parautochthonous zone based on recent re-evaluation of industry seismic lines located between Drumondville and Iberville (Fig. 1). Post-stack reprocessing imaged long décollement planes and tectonic slices on the seismic lines that define hinterland-dipping duplexes and dominate the structural style of the parautocthonous zone (Séjourné et al. 2003). Small-scale out-of-sequence thrusts cut across the slices at steep angles.
The St-Flavien slice (Fig. 2b) is located at depth beneath the Chaudière thrust sheet, 15 km southeast of Pointe Aubin and hosts a natural gas reservoir that produced 5.7 Bcf and currently serves as a gas storage facility. Like the Pointe Aubin fault, the St-Flavien fault imbricates strata from the lowermost units of autochthonous platform in the subsurface, and belonging to the Beekmantown dolomites, Chazy-Trenton limestones and Utica Shale (Fig. 2b). Other similar structural settings have been described in southwestern Quebec within the Saint-Dominique and Philipsburg slices, two structural units that were emplaced between the parautochthonous zone and the allochthons (Fig. 1). In the Saint-Dominique slice, Beekmantown and Trenton group strata from the autochthonous platform sequence were deformed during repeated thrusting events (Séjourné 2000). In the Philipsburg slice located 30 km east of Iberville (Fig. 1), the deformation history involved regional folding during overthrusting by the Stanbridge thrust sheet and reactivation of pre-existing faults during thrusting of the slice over the St. Lawrence platform (Séjourné and Malo 2001).
Olistostromes such as those found in the Taconian parautochthonous zone of Quebec and derived from the advancing tectonic wedge also occur throughout the foreland basin sequence in other parts of the northern Appalachians. In the Gaspé Peninsula, the Cap Chat Mélange crops out discontinuously in the Humber Zone for over more than 200 km and is composed of dismembered rocks of adjacent formations including rocks of the Middle Ordovician Tourelle Formation, the first orogenic flysch in this part of the Taconian foreland basin. The Cap Chat Mélange is interpreted as the result of tectonic processes, involving both extensional and contractional faulting making it difficult to classify as either a purely sedimentary or a purely tectonic mélange (Cousineau 1998). Although the exact age of the Cap Chat Mélange is uncertain, a Lower to Middle Ordovician age (Arenigian to Llandelian) for the blocks in the melange (Cousineau 1998) suggests that it developed before the Pointe Aubin olistostromes and tectonosomes. In the eastern New-York external zone, Bosworth and Vollmer (1981) and Vollmer and Bosworth (1984) demonstrated that the dominant mechanism for mélange formation appears to have been the tectonic disruption of beds of the synorogenic flysch sequence (Middle Ordovician). Other Taconian chaotic units occur along the Appalachian orogen, for example the Caradocian melanges of the Citadelle Formation outcropping within the Promontoire de Quebec thrust sheet (Fig. 2) in the Québec City area (St-Julien 1995; Kirkwood et al. 2000; Gayot 2002), and the Mid-Ordovician (Llanvirnian) Cape Cormorant Formation (Table Head Group) (Stenzel et al. 1990; Waldron et al. 1993). These melange units differ from the Les Fonds olistostromes since they contain limestone blocks derived from the stable platform to the north and are interpreted as debris flows deposited at the foot of syn-depositional normal faults. Bradley and Kidd (1991) invoked bending of the lithosphere and resulting flexure-induced normal faults in the Taconian collisional foredeep as the preferred model for deposition of Caradocian limestone conglomerates in the carbonate sequence of the subsiding foreland basin.
Detailed description of chaotic units within the parautochthonous zone of the Quebec Appalachians and the use of Pini’s (1999) classification scheme helped distinguish olistostromes from tectonosomes. The importance of clearly identifying processes involved as well as determining the origin of blocks, i.e., platform, foreland basin sequence or allochthons, was essential in unraveling timing of tectonic events and migration of the taconian tectonic wedge onto the continental margin.
Disruption, imbrication and thrusting of the foreland basin sequence during the Caradocian ( C. spiniferus graptolite zone) are responsible for the development of chaotic units within the flysch sequence of the Les Fonds Formation. We have distinguished the chaotic units on the basis of processes involved during their development: (1) olistostromes are sedimentary bodies emplaced by debris flows or avalanches, composed of variably sized blocks and exclusively derived from the foreland basin; (2) tectonosomes that are strongly deformed and tectonized foreland basin units in fault zones or along fold limbs. Structural and lithological evidence suggests that a large proportion of the chaotic units found at Pointe Aubin are tectonosomes that developed during the tectonic folding and fault imbrication.
Sandstones and shales of the Les Fonds Formation of the parautochthonous zone were deposited as turbiditic proximal fill in the Taconian foreland basin in Late Ordovician times. Olistostromal units containing early Late Ordovician blocks of the middle mudstone, and upper turbidite units of the foreland basin and spanning the Caradocian N. gracilis, C. americanus, O. ruedemanni and C. spiniferus graptolite zones were deposited and incorporated in the Les Fonds. Disruption of competent sandstone beds of the flyschic sequence and fault stacking and slicing of older rock units occurred along major thrust faults, and now form structurally aligned corridors of boudinaged and disrupted strata and/or thrust slices defined as tectonosomes. Paleontological and thermal maturation data help conclude that the tectonosome slices are imbricated foreland basin rocks that are correlative to the Black River, Trenton, Utica and Sainte-Rosalie groups of the Laurentian platform.
Deformation of the foreland sequence of the parautochthonous zone involved imbrication of the flyschoid strata of the foreland basin sequence (Figs. 11a and 11b) followed by late-stage emplacement of thrust slices, i.e. tectonosomes, (Fig. 11c) and out-of-sequence thrusting. Thermal maturation data indicates that disruption of the autochthonous rocks and thrusting of the blocks must have occurred shortly after their deposition. Contrary to what had been previously suggested by St-Julien (1968) and Beaulieu et al. (1980), we found no lithological, stratigraphic or structural evidence for the derivation of olistoliths from Cambrian strata of the Chaudière thrust sheet, presently exposed immediately southeast of the parautochthonous zone. New thermal maturation and palynological data confirm that the blocks in the olistostromes and tectonosomes cannot be derived from the allochthonous Chaudière thrust sheet, even though the Chaudière thrust sheet presently marks the southern contact with parautochthonous zone. We believe that the major fault that cuts through the northern limb of the syncline at Pointe Aubin is similar to other important thrust faults that define the soles of three tectonic slices, the St-Flavien, Saint-Dominique and Philipsburg slices.
The study was financially supported by a Natural Sciences and Engineering Research Council grant to Donna Kirkwood and by the Geological Survey of Canada through the Appalachian Foreland and St. Lawrence Platform NATMAP project. The authors would like to express their gratitude to the late Gifford Kessler of Marathon Oil for discussions on various aspects of the Pointe Aubin melange. We also benefited from discussions with John Riva on the significance of graptolite data and the geology of the Québec City vicinity. Finally, we acknowledge journal referees S. Castonguay and J. Beaulieu as well as associate editor D. Lavoie for their useful comments. This work is part of the MSc. studies of the first author, who would like to personally thank M. Lavoie and V. Lavoie for their contribution to the 2000-2001 summer field surveys and P. Therrien for technical help in positioning and mapping.
© Félix-Antoine Comeau, 2006
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| Introduction |
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Chapitre 3. Analyse structurale de la zone de failles imbriquées de la région de Québec |
