Eocene-Oligocene coals of the Gippsland and Australo-Antarctic basins – Paleoclimatic and paleogeographic context and implications for the earliest Cenozoic glaciations

Abstract

Australia's Gippsland Basin contains a semi-continuous Eocene-Oligocene (41.5–28.4Ma) near-coastal coal record that formed adjacent to Pacific Ocean. Traralgon and Morwell Formation brown coals include 4 main seams (T2, T1, T0, M2). Coal seam palynology records show late Middle Eocene (T2) coals formed under megathermic conditions characterized by high-gymnosperm contents, Late Eocene (T1) coals formed under mesothermic conditions characterized by reduced-gymnosperm contents and earliest indications of palaeoclimate cooling. Earliest Oligocene T0 coal record (33.9–31.5Ma) contains high-gymnosperm palynology profile, very similar to the T2 coals. The earliest indication of cooler climes only begins after this coal formed as indicated by low-gymnosperm high-Nothofagus (southern beech) pollen proportions. We suggest in Gippsland the earliest evidence for major glacial cooling (by inference the Oi1 event) be placed immediately above the T0 coal seam where Early to Late Oligocene Morwell Formation sands, clays and coals contain low counts of gymnosperms (<10%) but high average proportions of Nothofagus (50%). This is the main definitive indicator that palaeoclimates had cooled between the Eocene and Oligocene. This agrees with the current ocean drilling position of the earliest (Oi1) glacial event shortly above the Eocene-Oligocene boundary.A number of contemporaneous Middle to Late Eocene brown coals occurred in near-coastal settings across 1200km of southern Australia. Palaeogeographically, all these coal basins faced the Australo-Antarctic Gulf and have a much lower gymnosperm proportion (<10%), low Nothofagus proportion (<10%), but very high (non-Nothofagus) angiosperms proportion. This suggests a different climatic regime separated a cooler and wetter Gippsland Basin flora that responded to the cooler Proto-Ross Sea Gyre rotating around a wide Pacific Ocean, and a warm-wet climate associated with a warm proto-Leeuwin current of the Australo-Antarctic Gulf.