STRATIGRAPHIC ARCHITECTURE OF THE LOPINGIAN SUCCESSION IN THE BOWEN BASIN OF EASTERN AUSTRALIA

This study reviewed the distribution of key stratigraphic units and depositional environments of the Lopingian succession in the Bowen Basin of eastern Australia using geophysical logs from 700 drillholes across the basin, complemented by outcrop data. Spatial variation in thickness across different morphotectonic zones of the Bowen Basin are presented as a series of cross sections and maps that illustrate the regional-scale distribution of major Lopingian units comprising the Peawaddy Formation, Black Alley Shale, and Bandanna Formation (in ascending order), and their equivalents. All three formations were deposited in a retroarc foreland basin setting that formed during the Hunter-Bowen contraction directed from the east-northeast. This gave rise to a north-south-elongate depocenter (Taroom Trough) close to the eastern basin margin. Isochore maps for the three formations highlight variations in thickness across the basin, with a gradual thickening trend toward the Taroom Trough. The abrupt truncation of the contour lines in the isochore maps suggests that the depocenter is only partially preserved in the Taroom Trough, while the initial foredeep extended further to the east, and it has been tectonically removed by faulting and/or erosion.

The Peawaddy Formation (up to 758m) displays a series of coarsening-upward, progradational cycles across much of the basin. The Black Alley Shale (up to 915m), in the central and southern regions of the basin, exhibits marine mudstone facies, whereas in the northern part, it becomes increasingly sandy and coal-bearing. During the deposition of both the Peawaddy Formation and overlying Black Alley Shale, the dominant depositional environments in the Bowen Basin were deltaic, as evident from successive coarsening-up sequences recorded in gamma logs. Notably, arc-related volcanic activity was most prominent during this period, reflected in the wireline logs by intervals of significantly high gamma ray values (>175 API). In contrast, during the deposition of the uppermost Bandanna Formation (up to 635m), wireline logs show a sharp decrease in tuffaceous material. This formation consists of alternating layers of sandstones and mudrock, containing significant coal beds and preserved fossilized plant roots, formed in a vast alluvial plain depositional environment.