Facies Architecture and Depositional Dynamics of the Upper Permian Rangal Coal Measures, Bowen Basin, Australia

Abstract

The Upper Permian Rangal Coal Measures in the northern Bowen Basin, Australia, accumulated during rapid basinal aggradation in a retroarc foreland basin setting. They reflect peatland deposition towards the close of a long-lived climatic regime that came to an abrupt termination at the Permian-Triassic boundary, which is well developed in the Newlands mine highwall and adjoining borefield. Six lithofacies are represented in the Newlands district: (1) erosionally based, vertically stacked, ribbon and sheet-like sandstone bodies (up to 30 m thick and 1000 m wide); (2) heterolithic lobes and wedges; (3) interbedded siltstone and sandstone sheets; (4) interlaminated claystone and coal stringers; (5) laterally extensive bituminous coal sheets; and (6) laterally extensive laminated carbonaceous siltstone sheets. This facies assemblage is interpreted to correspond to six alluvial, cold-climate depositional environments: (1) trunk river channels and crevasse feeder channels; (2) levee bank-proximal crevasse splay; (3) distal splay-overbank; (4) marsh; (5) peat mire and (6) floodbasin lake, respectively. The sequence architecture at Newlands reveals that a prolonged period of peat deposition was progressively terminated by northward-stepping lobes of fluvial sediment in the south and by lacustrine drowning in the north. Exceptionally well developed, composite, elongate crevasse splay microdeltas, up to 26 m thick and >3000 m long, indicate that crevassing was responsible for the localized emplacement of thick sediment packages on the proximal floodplain. In the absence of active siliciclastic sedimentation, peat mire environments expanded from the distal floodplain, extending across abandoned depositional lobes. The geometry of these deposits reflects in large part accommodation space developed by the compaction of the peat precursor of the thick and laterally extensive Upper Newlands Seam. Sequential compaction of peat by clastic overburden controlled the nucleation and location of succeeding clastic packages. The behavior of the peat under compactional load largely controlled the lateral facies mosaic and stacking pattern of depositional units on a district scale.