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

The c. 1 km thick, Pennsylvanian South Bar Formation (SBF) in the Sydney, Nova Scotia area was formed in a paleoequatorial, coastal alluvial plain setting. 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. Paleosols are representative of gleyed conditions until the top 100 m of the formation, when primary reddening indicates more freely or at least seasonally drained conditions. The overlying lower Sydney Mines Formation (SMF) preserves a more diverse array of facies, recording fluvial channel, overbank and lacustrine/paralic conditions on a coastal plain. The SMF preserves well-developed cyclicity entailing alternations among deposits of humid to semi-arid/arid conditions. Paleosols range from paleo-gleysols, through vertisols and oxisols to calcisols. In this reappraisal, we find that the formative rivers of the SBF were characterized by progressively higher discharge variance towards the top of the formation, continuing into the overlying SMF, where differences in fluvial style (low vs. high discharge variance) correlate with interpreted paleoclimatic fluctuations. These styles covary with other indices of cyclical paleoclimatic change, including upward fluctuations in Chemical Index of Alteration (CIA) values at the top of the SBF and into the overlying SMF. Geochemical data suggest that the paleoenvironment became progressively more seasonal and stressed upward through the Moscovian of Atlantic Canada, leading to a crisis near the Moscovian/Kasimovian boundary, a time of widespread turnover in paleoflora.