Misleading Basin Margins—Analysis of the Upper Permian Succession in the Retroarc Foreland Bowen Basin of Northeast Australia

The meridional, Permo–Triassic Bowen Basin of NE Australia became a retroarc foreland basin in the late Permian, with a mapped foredeep axis (Taroom Trough) running north–south adjacent to the eastern edge of the basin. The present outline of the basin is, nonetheless, shaped by significant structural deformation along its eastern margin, and stratigraphic pinch-outs and erosional truncation in the west. The plan form of the basin thus gives a potentially misleading representation of the basin's original shape and extent. By analysing wireline log data from over 1000 drillholes, we developed isochore, net sand thickness, and net-to-gross maps, which inform a new set of palaeogeographical maps for the three upper Permian formations of the Bowen Basin (Peawaddy Formation, Black Alley Shale, and Bandanna Formation, in ascending order). These maps were further refined using palaeocurrent data from outcrops and validated against logged vertical sections from various parts of the basin. Isochore plots for the three formations indicate abrupt truncation of contour lines along the structural eastern margin, in the Taroom Trough. This pattern is interpreted as evidence of significant erosion of stratigraphy along the eastern basin margin during contractional deformation. Based on these findings, we posit that the Taroom Trough synclinal axis does not represent the original foredeep axis, which we propose was located farther east and was erosionally excised. The configurations of sand-prone depositional systems, and the preservation of upper Permian strata outside the structural basin boundaries, support the hypothesis that the original eastern depositional edge of the basin extended farther east than its current structural boundary. Similarly, truncation and westward onlap of formations along the western margin suggest that the basin's original western boundary lay beyond its present margin. Based on the truncated isochores and other criteria, we estimate that the preserved part of the basin is ~50% of its original area.