A SEDIMENTOLOGICAL EVALUATION OF THE PERMO-TRIASSIC BOUNDARY INTERVAL IN THE NORTHEAST SYDNEY BASIN, AUSTRALIA

The end Permian was characterized by major climatic and environmental disturbances on land, including increased seasonality, wildfires and increased concentrations of atmospheric CO₂, which ultimately led to the most severe biotic turnover of the rock record. In this study, we evaluate a continental margin record of the end Permian Mass Extinction (EPME) and the Permo-Triassic Boundary (PTB) in a high-paleolatitude location. We use measured sections and stratigraphic relationships to evaluate the expression of the EPME and PTB along laterally continuous coastal exposures, open pits, and drill cores from the northeast Sydney Basin of New South Wales, Australia. The succession is predominantly fluvial, consisting of thick sandstone and conglomerate channel bodies, separated by mudrocks and heterolithic deposits of floodplain, mire and volcanic fallout origin. In the uppermost Permian succession, thick lensoid conglomerate bodies reflect west-draining, deep channels that interfinger with more southerly draining sandstone-filled channels. Interbedded with these bodies are thick coal seams and drab grey, root-penetrated mudrocks that indicate waterlogged conditions. Strong seasonality is indicated by abundance of in-situ tree stumps preserved in channel floor deposits. Tuff deposits up to 13 m thick occur locally. Above the putative PTB, coals and conglomerate filled channels are no longer present, tuffs become less common, and mudrocks become progressively more colored upward. Coaly plant debris persists into the basal Triassic succession. The interval immediately succeeding the uppermost coal is highly variable as some sections preserve up to 18 m of mudrock, whereas others are erosionally truncated by sharply-based, pebbly sandstone bodies. The PTB is believed to lie within this interval but is only preserved in sections unaffected by fluvial erosion. Our data indicate that there is insubstantial change in fluvial style, internal sedimentary structure or sediment dispersal direction across the PTB/EPME. Paleoenvironmental perturbations across the boundary occurred in a depositional environment that did not significantly change.