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

The major stratigraphic units of the Lopingian succession in the meridional Bowen Basin of eastern Australia have been analyzed using wireline logs from c. 700 boreholes. The Lopingian succession in this study comprises 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 due to crustal contraction directed from ENE to WSW, associated with the Hunter-Bowen orogeny. This gave rise to a north-south-elongate depocenter (Taroom Trough) close to the eastern basin margin. Spatial variation in thickness across different morphotectonic zones of the Bowen Basin has been analyzed. Isochore maps for the three formations highlight variations in thickness across the basin with a gradual thickening trend toward the Taroom Trough. The asymmetric subsidence pattern facilitated accumulation of a thick Lopingian succession in the Taroom Trough, derived from the orogen to the east. The formations thin from east to west as they laterally pass away from the contractional deformation zone. The Peawaddy Formation displays a series of coarsening-upward, progradational cycles across much of the basin. In the central and southern region of the basin, the Black Alley Shale 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, which is evident from successive coarsening-up sequences recorded in gamma logs. Notably, arc-related volcanic activity was most prominent during this period, reflecting in the wireline logs by intervals of significantly high gamma ray values (>175 API). In contrast, during the deposition of the uppermost Bandanna Formation, wireline logs show a sharp decrease in tuffaceous material. This formation consists of alternating layers of sandstones and mudrocks, containing significant coal beds and preserved fossilized plant roots, formed on a vast alluvial plain depositional environment.