Geometrical Facies Analysis of a Mixed Influence Deltaic System: The Late Permian German Creek Formation, Bowen Basin, Australia

The Late Permian German Creek Formation outcrops along the western margin of the foreland Bowen Basin in central Queensland, Australia. The unit comprises a coarse clastic wedge enclosed below, above and to the south and east by rocks of interpreted marine shelf origin. This coarse clastic interval is interpreted to be of deltaic origin, sourced from the cratonic Australian landmass to the west, and prograding eastwards (transversely) into a marine basin. The upper part of the formation, which is extensively mined for coal, is the focus of this study. Six facies have been identified: Facies A, thick and laterally extensive, channelized sandstone bodies, interpreted as deltaic distributary channel fills; Facies B, mostly siltstone with thin sandstone interbeds interpreted as interdistributary bay fills and tidal flat deposits; Facies C, sheet-like, mostly horizontally laminated, well sorted sandstones interpreted as foreshore deposits; Facies D, laterally extensive, tabular sandstone bodies internally dominated by hummocky cross-stratification interpreted as proximal mouth bar deposits; Facies E, heavily bioturbated, interbedded siltstone and sandstone with fining-upward graded units and occasional hummocky cross-stratification interpreted as distal mouth bar deposits; and Facies F, coals, interpreted as in situ mire deposits which accumulated on an extensive coastal plain. Palaeocurrent data from Facies A–C indicate bipolar east–west directed currents. The facies assemblage is interpreted as the deposits of lower delta plain and delta front environments within a mixed influence (i.e. wave-tide-fluvial influenced) deltaic complex. The great lateral extent of the major coal seams within the German Creek Formation is difficult to explain in terms of purely sedimentary processes in the context of a lower delta plain, and may imply the role of external base level changes.