Fluvial architectural style and stacking patterns in a high-accommodation coal-bearing succession: the upper Permian Newcastle Coal Measures, eastern Australia

Abstract

The upper Permian Newcastle Coal Measures (NCM) host world-class coal resources in coastal New South Wales, Australia. They formed close to the foredeep axis of a developing retroarc foreland basin associated with the Hunter-Bowen contractional event. In addition to the typical coal-bearing lithological suite of mudrocks, heteroliths, sandstones, and coals (with several beds of volcanic ash), the NCM preserve numerous, linear bodies of conglomerate up to 100 m thick and 20 km wide that are anomalous in the context of paralic coal-bearing successions worldwide. Four facies associations are recognized: A) linear bodies of conglomerate, gravelly sandstone and sandstone, interpreted as the deposits of major coastal plain channels, B) interbedded mudrocks and sandstones, interpreted as the product of coastal plain floodplains and floodbasins, C) coals and coaly mudrocks, interpreted as the product of coastal plain mires, and D) massive and bedded tuffs, interpreted as the product of pyroclastic falls and flows from nearby stratovolcanoes. Despite having formed on a coastal plain during a time of known sea-level changes, no depositional sequences can be recognized in the NCM, due to high rates of accommodation and sediment supply. The repeated co-occurrence of southward-trending major channel belts across the Newcastle coalfield is attributed to the area being located in the axial foredeep of the basin. A new palaeogeographic reconstruction is proposed in which the northern Sydney Basin, in the late Permian, extended further northwards across what is now the southern New England Orogen, and the north-south-elongate basin was drained by continental scale, basin-axial river systems that carried significant volumes of gravel.