CLIMATIC EVOLUTION AND CONTROL ON CARBONATE DEPOSITION IN NORTHEAST AUSTRALIA

The characteristics of carbonate facies deposited along continental margins are directly controlled by seawater temperature. The oxygen isotopic composition of foraminifera tests reflect seawater temperature, and accordingly isotopic and age data may be combined to derive a paleotemperature record. Paleotemperature data may be used both to account for the known distribution of carbonate facies, and also to predict facies characteristics in poorly known areas. Oxygen isotope data from Deep Sea Drilling Project holes throughout the southwest Pacific have been used to compile a paleotemperature curve for offshore northeast Australia. The accuracy of paleotemperature estimates used in this compliation is dependent on the precise estimation of global ice volumes; on the estimation of surface water isotopic ratios from near-surface planktonic foraminifera; on the accuracy of biological disequilibrium isotopic fractionation constants for benthonic foraminifera; and on the identification of recrystallization, encrustation, and selective dissolution of samples. Decreasing temperatures during much of the early Cenozoic portion of the northeast Australia paleotemperature curve reflect the global high-latitude cooling trend which persisted throughout the Tertiary following the earliest Eocene temperature maximum. Warming during the middle Oligocene to Recent part of the curve reflects northeast Australia's transition from a mid-latitude situation in a world with little climatic zonation, to a low latitude situation in a world with pronounced latitudinal temperature gradients. The carbonate buildups of northeast Australia directly reflect this climatic variation. Restricted warm temperate or subtropical buildups developed during the Eocene; carbonate buildups did not develop at all during the cool Oligocene; and subtropical buildups succeeded by tropical coral reefs first developed during the latest Oligocene or Early Miocene in the northenmost part of area, and later developed further south with continued warming in the Late Miocene and Pliocene.