Depositional Environments and Sediment Dispersal Patterns of the Jurassic Walloon Subgroup in Eastern Australia

Abstract

This paper summarises results from a regional evaluation of depositional environments of the Middle Jurassic Walloon Subgroup in eastern Australia. The Walloon Subgroup (WSG), which extends from the Clarence-Moreton and other basins of coastal SE Queensland, westward into the Surat and Eromanga Basins, is currently the focus of extensive resource evaluation for coal seam gas. The study entailed analysis of a large array of natural surface exposures, opencut mine highwalls and other artificial exposures, and drillcores. The WSG comprises complexly interbedded sandstones, mudrocks, coals and coaly shales, and bentonites, with a characteristic volcanic lithic mineralogy. Facies and architectural element analysis has allowed formulation of a depositional model for the WSG. The environment of deposition was a broad, low-gradient alluvial plain, crossed by mainly sinuous streams of varying (but generally modest) dimensions that were flanked by extensive shallow lakes, ponds and mires, and periodically showered by volcanic fallout. Mine exposures indicate a motif of complex lateral facies variation, leading to a highly heterogeneous three-dimensional facies mosaic. Alluvial channel bodies vary from simple, single channel fills to complex, multilateral and/or multistory lithosomes with a variety of architectural elements preserved. Highly dispersed palaeocurrent distributions from single exposures attest to the highly sinuous character of formative streams. The voluminous volcanic fallout component to the WCM suggests that alluvial systems were at times choked by volcanic sediment, perhaps causing frequent avulsions and contributing to the complex internal architecture of many channel bodies. Overbank environments include levee and shallow floodbasin deposits including lacustrine delta deposits. Mire deposits (coals and coaly shales) are also highly laterally variable, with thick pods of coal restricted to discrete areas isolated within much broader zones of thinner and/or more clastic-rich mire facies. Overall sediment dispersal was westward from what is now coastal eastern Australia into the interior of the likely endorheic Great Artesian Basin. It appears that channels became gradually smaller down-palaeoslope, and may have exhibited either a distributive or avulsive pattern. Palaeocurrent data, however, indicate a centripetal, rather than a radial, pattern, suggesting that the WCM were not formed as part of a large fluvial fan.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015