Was the Oligocene–Miocene a time of fire and rain? Insights from brown coals of the southeastern Australia Gippsland Basin

Abstract

Lithotype cycles (ranging from 10 to 30m thick) in the brown coals of the Latrobe Valley, Gippsland Basin, Australia, display well-developed lightening-upward trends. Cycle tops are characterized by abrupt and unconformable boundaries with the overlying cycle. Geological, geochemical, palynological and macrofossil evidence is consistent with a relative drying (terrestrialization) upward depositional model for the cycles.The abundance of charcoal in dark lithotypes near the cycle bases is explained by the fire-prone and highly flammable nature of the herbaceous/reed wetlands, in common with similar modern wetlands in modern Australasia, in which the dark lithotypes are suggested to have formed. This, together with the greater preservation potential of charcoal in subaqueous environments, results in the wettest facies of the Latrobe Valley coals having the highest charcoal contents. Despite prevailing warm, wet climate conditions and the predominance of rainforests that are suggested to have characterized the Cenozoic of southern Australia, some swamp taxa were clearly already pre-adapted to tolerate fire and are likely to have been the ancestors of the fire-adapted floral communities of modern arid Australia.