The 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).