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

This study reviewed the distribution of key stratigraphic units for major Lopingian formations, comprising the Peawaddy Formation, Black Alley Shale, and Bandanna Formation (in ascending order), and their equivalents in the Bowen Basin of eastern Australia. This review utilized geophysical logs from 700 drillholes across the basin, complemented by outcrop data. Spatial variation in thickness across different morphotectonic zones of the Bowen Basin (e.g., Nebo Synclinorium, Comet Ridge, Denison Trough, Taroom Trough, Roma Shelf) was analyzed. All three formations were deposited in a retroarc foreland basin setting that formed during the Hunter-Bowen contraction directed from the east-northeast. These formations are recognized over most of the Bowen Basin. However, the boreholes available for the southeastern part of the basin are not deep enough to penetrate the Peawaddy Formation.

The thickness of the Peawaddy Formation, Black Alley Shale, and Bandanna Formation varies across the basin, with a gradual thickening trend toward the eastern part of the basin that corresponds to the north-south-elongate depocenter (Taroom Trough). The Taroom Trough is the “interpreted” foredeep axis of the Bowen Basin. Isochore maps for the three formations highlight abrupt truncation at the eastern margin. Additionally, the thickness maxima do not coincide with the structural axis of the Taroom Trough, suggesting that the depocenter is only partially preserved in the Taroom Trough, while the initial foredeep was located further east. Hence, it is likely that during the late Permian, the eastern edge of the basin was positioned farther to the east than its currently preserved location, which may have been tectonically removed through faulting and/or erosion. We conclude that the Bowen Basin's original outline was significantly broader than its current preserved dimensions, with only about 50% of the basin's original area still intact.