INTRODUCTION

1 Nova Scotia is home to several late Devonian—Carboniferous sedimentary basins, the genesis and development of which have been the subject of controversy. The Antigonish Basin, located in northeastern Nova Scotia (Fig. 1), is one of these basins and contains more than 6500 m of sedimentary rocks of Late Devonian to Late Carboniferous age (Boehner and Giles 1993). The basin fill unconformably overlies Neoproterozoic to Devonian basement lithologies of the Avalon terrane in the Antigonish Highlands to the west (e.g., Murphy et al. 1991), and is bounded onshore by the Morristown Fault to the north and by the Glenroy Fault to the south. To the northeast it is hidden beneath St. Georges Bay, on the north and eastern sides of which it is bordered by the Hollow Fault Zone and the Judique Fault, respectively.

2 The Antigonish Basin is located on the south-central margin of the larger Maritimes Basin (inset, Fig. 1), which consists of several depocentres that developed during the waning stages of the Acadian Orogeny (e.g., McCutcheon and Robinson 1987). Following Bradley (1982), the Antigonish Basin has traditionally been interpreted as a pull-apart structure that formed as a result of Alleghanian strike-slip faulting (Boehner and Giles 1982; Stockmal et al. 1986). On Cape Breton Island, however, sedimentological studies of broadly coeval basins suggest that they opened as a series of half-grabens (Hamblin and Rust 1989). More recently, Lynch and Tremblay (1994) have proposed that the basin evolved above a regional detachment fault (Margaree Shear Zone) that formed as a result of orogenic collapse following Acadian crustal thickening in the Devonian.

3 Following Fyson (1967), Lynch and Tremblay (1994) further proposed that extensional deformation within the Maritimes Basin is not limited to the Margaree Shear Zone, but reoccurred during the late Namurian—early Westphalian with the development of a regional flat-lying extensional fault (the Ainslie Detachment) that developed within the basin fill. Located near the base of an evaporitic sequence of Vis. age (Windsor Group) and distributed across 10,000 km2, the detachment surface is held to be responsible for as much as 2 km of stratigraphic omission and 10—25 km of westward displacement (Lynch and Giles 1995). The detachment is placed at the top of the basal limestone of the Macumber Formation, the rheological contrast between the limestone and overlying evaporitic sequence acting as a stratigraphically controlled zone of weakness.

4 In this study, we examine the deformation of the Macumber Formation in the Antigonish Basin in an attempt to determine its relationship to the Ainslie Detachment. The data suggest that deformation took place soon after deposition of the formation and occurred in response to repeated movement directed towards the centre of the basin located in St. Georges Bay. We therefore conclude that deformation of the Macumber Formation in the Antigonish Basin predated the Ainslie Detachment and was the result of local gravity sliding rather than major regional detachment faulting.

GEOLOGY OF THE ANTIGONISH BASIN

STRUCTURE OF THE MACUMBER FORMATION

15 To determine whether the deformation of the Macumber Formation reflects movement on the overlying Ainslie Detachment, detailed analyses of small-scale structures developed within the Macumber Formation were made at key sites identified from the existing literature. They included outcrops at MacIsaacs Point, Lakevale, Cape Jack, Aulds Cove, and along the western basin margin (Figs. 1 and 2).

STRUCTURAL FEATURES

16 A variety of structures within the Macumber Formation suggest that it records repeated episodes of deformation that began soon after its deposition. These structures include recumbent folds, breccia zones, extension veins, and normal faults, and occur at all sites exposing the Macumber Formation limestone.

17 MacIsaacs Point: At MacIsaacs Point on the west side of St. Georges Bay (Fig. 1), steep cliffs expose basement, Macumber Formation limestone, and Windsor Group evaporites. The limestone beds are highly deformed with inclined to recumbent buckle folds discordantly overlying an unfolded but finely laminated horizon at the base of the formation (Fig. 4a—c). The folds show broadly parallel geometry and have amplitudes that increase from 2—3 cm near the base of the deformed zone to 40—50 cm up section. The predominant sense of vergence throughout the deformed zone is eastward (seaward); all of the largest folds being consistent with a down-dip (due east) sense of movement (Fig. 5). However, westward-verging folds also occur; the consistent N-S orientation of their fold axes, coupled with the variable dip of the fold axial surfaces and the detached style of the folding, suggesting an origin through slumping. Such an origin is supported by the presence of previously folded fragments in overlying breccia units (see below), which requires at least some of the deformation to have been syndepositional with respect to the Windsor Group.

18 In outcrop, the finely laminated horizon discordantly underlying the folded zone (Fig. 4c) resembles the calc-mylonite described by Lynch and Giles (1995). To test this interpretation, several oriented samples of the horizon were examined in thin section. In the absence of a stretching lineation, the sections were cut north-south, parallel to the direction of movement proposed for the Ainslie Detachment by Lynch and Giles (1995). However, no kinematic indicators were observed and the fabrics were found to be those typical of undeformed algal limestone rather than calc-mylonite.

19 Directly above and locally truncating the folded interval erosionally, the Macumber Formation contains horizons of intraformational limestone breccia several metres thick that are separated by undisturbed strata. Crudely bedded and almost entirely comprised of reworked and corroded (karstified) fragments of Macumber Formation limestone, some of which contain calcite veins (e.g., Keppie et al. 1978), these breccias have been variously attributed to slumping (Lavoie and Sangster 1995), to post-Windsor dissolution (Clifton 1967; Geldsetzer 1978), to syndepositional (Schenk 1984) or post-depositional (Boehner 1980) karstic processes, and to later hydraulic fracturing by hydrothermal fluids (Ravenhurst and Zentilli 1987; Lavoie and Sangster 1995). Their association with zones of folding and their proximity to the carbonate-evaporite interface also suggests a genetic linkage to the Ainslie Detachment (Lynch et al. 1998). Regardless of their origin, however, the breccias exposed at MacIsaacs Point are clearly of depositional origin and, hence, part of the stratigraphy, the repetition of horizons implying that the Macumber Formation limestone experienced repeated episodes of karstification and brecciation prior to the deposition of the overlying evaporitic sequence of the Windsor Group.

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20 Like the underlying Macumber Formation limestone, the breccia horizons are folded (Fig. 6a), and the folds predominantly verge eastward with amplitudes of 40—50 cm. In addition, the layering within individual corroded breccia fragments is buckle folded (Fig. 6b), indicating that some folding pre-dated karstification and, while clearly post-dating lithification, must have been initiated soon after limestone deposition. Folding of the breccia horizons requires a repeated episode of deformation following their deposition. Hence, the deformation, karstification, and brecciation are considered penecontemporaneous processes that recurred syndepositionally with respect to the Windsor Group.

21 Lakevale: North of MacIsaacs Point, the Macumber Formation limestone is again exposed in sea cliffs at Lakevale (Fig. 1). Here, thinly laminated limestone beds developed near the top of the formation contain small calcite crystals and are crenulated about fold axes that plunge gently northeast. These horizons, like those at MacIsaacs Point, closely resemble those interpreted by Lynch and Giles (1995) as calc-mylonite produced during movement of the Ainslie Detachment. In thin sections cut perpendicular to the fold hinges, the calcite crystals are indeed enveloped by the laminations in a manner resembling sigma structures in ductile shear zones (Fig. 7a). However, the crystals are highly angular and do not resemble porphyroclasts, and the apparent sigma structures show no preferred sense of shear. Instead, the subhedral shape of the calcite crystals suggests that they are pseudomorphs after gypsum and that the crystal-laminated interval exhibits a primary fabric that records little post-depositional deformation other than compaction. Similar fabrics enveloping pseudomorphs after gypsum have been described from undeformed limestone of the Macumber Formation in Cape Breton Island by Savard et al. (1996).

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22 However, deformation is evident towards the base of the Macumber Formation where a series of large inclined to recum-bent folds are developed that resemble the slump structures at MacIsaacs Point. Fold axes plunge generally north-northeast and, where asymmetric, the predominant sense of vergence is again broadly eastward (Fig. 7b). Deformation also occurs in thin limestone beds immediately above the crystal-laminated layer, where it takes the form of boudinage and small-scale normal faults. Although the age of these structures relative to the folds is unknown, their orientations are consistent with broadly NE-SW extension (Fig. 7c).

23 Cape Jack, Aulds Cove, and Western Basin Margin: Along the coastline at Cape Jack (Fig. 1.), outcrops of the Macumber Formation limestone dip gently westward and contain calcitefilled fractures or veins that dip steeply north-northeast (Fig. 8a), indicating extension in this orientation. As clasts containing similar veins occur in the breccia horizons at MacIsaacs Point, their formation is considered to have been penecontemporaneous with the folding of the Macumber Formation limestone. At Cape Jack, minor symmetrical folds in the bedding plunge gently west-southwest.

24 Similar calcite veins and folds occur in gently NW-dipping beds of the Macumber Formation at Aulds Cove, where the limestone is overlain by siltstone and shale of the Mabou Group. The unex-posed contact is, therefore, the Ainslie Detachment of Lynch and Giles (1995), who attributed the absence of the evaporitic Windsor Group to movement on the detachment surface. The isoclinally folded calc-mylonite that they describe from the contact suggests broadly NW-directed movement. In the Macumber Formation limestone, the calcite veins dip steeply north and west while the fold axes are subhorizontal and trend NE-SW (Fig. 8b), consistent with broadly NW-SE extension.

25 Southwest of Antigonish, where the Macumber Formation occurs along the western basin margin, calcite veins similar to those at Cape Jack and Aulds Cove occur in outcrops beside the Canadian National railroad (site M1, Fig. 9). Here the limestone dips moderately south and the veins dip steeply north-northeast (Fig. 8c).

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STRUCTURAL INTERPRETATION

26 Figure 9 summarizes directional evidence for movement and/ or extension at localities exposing limestone of the Macumber Formation. At MacIsaacs Point and Lakevale, large-scale folds interpreted as being post-depositional slump structures verge eastward into St. Georges Bay, whereas the orientation of small-scale normal faults and boudinaged beds at Lakevale is consistent with NE-SW extension. A similar extension direction is recorded in the orientation of calcite fractures at the western basin margin, whereas those exposed at Cape Jack and Aulds Cove record N-S to NW-SE extension. In all cases, however, the structures imply a component of extension towards the centre of the basin beneath St. Georges Bay.

27 The presence of calcite veins and previously folded layering in the corroded limestone clasts of the Macumber Formation breccia further suggests that, although clearly post-depositional, many of the small-scale structures in the Macumber Formation limestone were produced prior to the deposition of the overlying evaporitic sequence of the Windsor Group and, hence, are Vis. in age.

DISCUSSION

28 Deformation of the Macumber Formation limestone throughout the Antigonish Basin is consistent with low-angle gravitational movement towards the basin centre. The nature and location of this deformation clearly invites its association with the Ainslie Detachment, which Lynch and Giles (1995) placed along the immediately overlying carbonate-evaporite interface. The kinematics and timing of the deformation, however, argue against such a correlation.

29 According to Lynch and Giles (1995), the Ainslie Detachment is a regional, broadly west-vergent structure that can be traced across much of northern Nova Scotia and western Cape Breton Island. It is held responsible for the local stratigraphic omission of the Windsor Group and is considered to have developed in the late Namurian to early Westphalian. The structures in the Macumber Formation, on the other hand, appear to record within-basin deformation in which extension was directed toward the basin centre. Structures developed along the western margin of the basin further suggest that deformation occurred, not as a result of west-vergent Namurian—Westphalian detachment, but in response to east-vergent gravitational sliding during the mid-Vis.. Although neither precluding the existence of the Ainslie Detachment nor negating the timing of its movement, these relations rule out movement on the detachment surface as the cause of deformation and raise the possibility that similar structures developed in the Macumber Formation in other Carboniferous basins may likewise be unrelated to movement on the Ainslie Detachment.

30 In the case of the Antigonish Basin, slumping of the Macumber Formation toward the basin centre is most easily attributed to movement on the basin-bounding faults (Fig. 1). Following the deposition of the Macumber Formation, significant movement on these faults must have occurred in order to accommodate the subsequent increase in thickness of the basin fill. During basin-wide deposition of the Macumber Formation, the decametre thickness of which records very shallow water deposition, the floor of the Antigonish Basin can have had little or no topography. Yet the remaining formations of the Windsor Group exceed 1.5 km in thickness. Movement compatible with such subsidence can be seen along the Glenroy Fault, which juxtaposes the Macumber Formation limestone to the south against the uppermost (Hood Island) formation of the Windsor Group to the north, a throw in excess of 1 km. Such movement is likely to have caused local instability of the basin floor and, coupled with similar movement on other basin-bounding faults, may have provided the impetus for repeated post-depositional slumping of the Macumber Formation down-slope toward the basin centre.

CONCLUSIONS

31 A variety of structures occurs in the Vis. Macumber Formation of the Antigonish Basin immediately beneath the basal carbonate-evaporite contact of the Windsor Group, an interface along which significant low-angle west-directed extensional faulting (Ainslie Detachment) has been proposed for the late Namurian—early Westphalian (Lynch and Giles 1995). The structures include recumbent folds, boudins, small-scale normal faults, and calcite veins, from which the timing of deformation and the direction of extension can be determined. The occurrence of veins and folds in clasts within folded zones of depositional breccia show that deformation was repeated and closely followed deposition of the Macumber Formation in the Vis.. Locally east-vergent fold asymmetries and the orientation of calcite veins and small-scale normal faults further require extension to have converged on the centre of the basin located in St. Georges Bay. These results argue against post-Windsor movement on the Ainslie Detachment as the cause of deformation and suggest, instead, an origin through earlier within-basin slumping. Renewed movement on basin-bounding faults required to accommodate the increase in basin fill thickness following deposition of the Macumber Formation may have provided the necessary instability.