DEPOSITIONAL FACIES, PALEOGEOGRAPHY AND PALEOCLIMATOLOGY OF THE LOWER JURASSIC MCCOY BROOK FORMATION, FUNDY RIFT BASIN, NOVA-SCOTIA

The McCoy Brook Formation is a 230+ m sequence of redbeds of Early Jurassic age that accumulated in the subtropical Fundy rift basin. The basin was asymmetrical, bordered on the northern side by the Cobequid fault system and adjacent highlands. The formation is a mosaic of terrestrial facies. (a) Multi- and single-storey fluvial sandstones were deposited by rivers that collected drainage from the northern highlands, and flowed southwest across the valley floor. (b) Graded-bed sequences of sandstone --> mudstone were deposited by flood events on the fluvial floodplains and the margins of playas. (c) Interbedded sandstones and finely laminated to disrupted red mudstones represent deposits of interfingering sandflats at the toes of alluvial fans and playas, respectively. The fabrics of both lithologies are commonly disrupted by desiccation and intrastratal precipitation of calcite and minor gypsum. (d) Plane-bedded conglomerate and pebbly sandstone record deposition in shallow channels that traversed alluvial fans. (e) Non-bedded breccias of basalt boulders up to 8 m in size in a sand to mud matrix formed as talus against palaeocliffs of the Lower Jurassic North Mountain Basalt. The palaeocliffs resulted from synsedimentary faulting within the basin. (f) Basalt conglomerate beds with matrix- to clast-supported fabrics comprise sequences deposited by debris-flows, probably generated by remobilization of talus. (g) A thin sequence of limestone, siltstone, and muddy sandstone containing fish scales and bones formed in a shallow lake on a basalt flow. (h) Sandstones containing m-scale crossbeds and a three-fold hierarchy of erosional surfaces accumulated as barchans and barchanoid transverse ridges that moved southwest in the valley. All of these facies occur within an area of 18 km2 located south of the fault-bounded highlands. Synsedimentary faulting within the basin during early deposition of the McCoy Brook Formation created palaeocliffs of North Mountain Basalt. Talus and debris-flow deposits accumulated adjacent to these cliffs, interfingering with eolian, fluvial, and lacustrine sediments. During later deposition of the McCoy Brook Formation, the early fault-controlled topography was buried as alluvial fans prograded southward from the highlands. Broad sandflats extended basinward from the toes of the fans, interfingering with playa mudflats on the valley floor. The palaeoclimate was semi-arid to arid as evidenced by gypsum-bearing playa redbeds, eolian sandstones, and caliche palaeosols.