BIOGEOGRAPHIC, ECOLOGICAL AND STRATIGRAPHIC RELATIONSHIPS OF THE MIOCENE BROWN-COAL FLORAS, LATROBE VALLEY, VICTORIA, AUSTRALIA

Palaeobotanical studies of the brown coal deposits of the Latrobe Vall ey have contributed significantly towards an understanding of the age of the deposits, existing climatic conditions and detailed depositiona l environments, This paper re-assesses some past reconstructions fbr t he Early to Mid Miocene coals using recent information on plant distri butions and their bioclimatic significance and on a marine incursion m odel for coal lithotype formation. The brown coal flora is composed ov erwhelmingly of rainforest taxa that presently cover a range of differ ent environments within the Australasian region. The application of a bioclimatic prediction model to these taxa allows the construction of consistent and quantitative estimates of climates during coal-forming phases. It is considered that rainfall in the area was more than twice the 850 mm received today, with significant seasonal variation. The m ean annual temperature estimate of about 19 degrees C, indicating a me sothermal or subtropical environment, is some 4-5 degrees C higher tha n present and higher than previous estimates. It does, however, corres pond with sea-surface temperature estimates for the Southern Ocean at this time. The original model of lithotype formation, which suggests t hat lithotypes conform to a successional sequence from open water in t he lightest coloured lithotypes to raised bog in the darkest lithotype s, is considered to be inconsistent with the evidence for lightening u pwards sequences within the coals, a central feature of the proposed m arine incursion model of coal formation, The two models are reconciled to some degree by a reinterpretation of the palaeobotanical data and by the postulation of climatic rather than autogenic successional cont rol over lithotype formation.