Half‐graben lacustrine sedimentary rocks of the lower Carboniferous Strathlorne Formation, Horton Group, Cape Breton Island, Nova Scotia, Canada

Abstract

ABSTRACTThe Strathlorne Formation is the middle formation of a three‐part Horton Group stratigraphy present throughout the post‐Acadian Orogeny Maritimes Basin in Atlantic Canada. It is up to 600 m in thickness and is of Tournaisian age. The formation was deposited in a complex lacustrine system during the period of maximum fault‐bounded extensional subsidence within two asymmetric half‐graben sub‐basin segments of a large rift. This rift was located at a palaeolatitude of 10–15°S. Four facies assemblages are identified and interpreted: (1) dark grey mudstone (open lacustrine), (2) grey, very fine to fine‐grained sandstone (nearshore/shoreline), (3) grey, medium‐grained sandstone to conglomerate (fan delta) and (4) red siltstone to fine‐grained sandstone (interdeltaic mudflat). Interpreted structural asymmetry of the fault‐bounded sub‐basins is evidenced by asymmetry of sediment input, facies distribution and palaeoflow in the lacustrine sedimentary fill. These indicators suggest that the sub‐basins, which were linked end‐to‐end, had opposed polarity of structural asymmetry during deposition of the Strathlorne Formation. Open lacustrine sediments are typified by stacked shallowing‐upward sequences, each representing deepening due to sub‐basin‐wide subsidence events followed by gradual infilling to shallow water depths. Sub‐basin asymmetry is also reflected in the contrast of thick sequences and grouped thinner sequences at marginal and axial positions, respectively. The lakes which occupied the sub‐basins were large (up to 100 × 50 km), tens to hundreds of metres deep and periodically stratified (presence of an anoxic hypolimnion, at least near sub‐basin axes).