A PALEOENVIRONMENTAL REAPPRAISAL OF THE PENNSYLVANIAN (BASHKIRIAN-MOSCOVIAN) SOUTH BAR FORMATION AND LOWER SYDNEY MINES FORMATION OF CAPE BRETON, ATLANTIC CANADA

Previous work has established that the c. 1 km thick, Pennsylvanian South Bar Formation in the Sydney, Nova Scotia area was formed in a coastal alluvial plain setting under variable climatic conditions in the paleoequatorial realm of the late Paleozoic. Fluvial channel and overbank facies preserve a transition from deposits of perennial streams in a permanently waterlogged setting to those of more strongly seasonal fluvial environments that experienced periodic desiccation upward through the formation. The overlying lower Sydney Mines Formation preserves well-developed climatic cyclicity entailing alternations among deposits of humid climate settings to at times semi-arid to arid conditions. In this reappraisal we apply the concept of discharge variance to refine the sedimentological interpretation, finding that the formative rivers of the South Bar Formation were characterized by progressively higher discharge variance towards the top of the formation. This trend continues into the overlying Sydney Mines Formation, where differences in fluvial style correlate with interpreted paleoclimatic fluctuations. This long-term sedimentological transition coincides with an upward increase in Chemical Index of Alteration (CIA) values and associated Land Surface Temperatures (LST) at the top of the South Bar Formation that then become strongly cyclical into the overlying Sydney Mines Formation. These and other geochemical data (including stable isotopes and plant biomarkers) suggest that the paleoenvironment became progressively more seasonal and stressed upward through the Moscovian of Atlantic Canada, leading to a paleoenvironmental crisis near the Moscovian/Kasimovian boundary, a time of widespread extinction of tropical lowland plant taxa in the Euramerican paleotropics.