Miocene to Pleistocene foraminiferal biostratigraphy of dredge samples from the Marion Plateau, offshore Queensland, Australia

Planktic and larger benthic foraminiferids have been studied in thin section from a suite of dredge samples of two sites on the Marion Plateau, offshore Queensland. The two sites gave different results. (1) The northern site sampled an Early to Middle Miocene carbonate platform, with shallow-water foraminiferal assemblages. This limestone has solution cavities (developed during periods of subaerial exposure) infilled with Late Miocene shallow-water planktic foraminiferid-bearing sediments. The rock is cross-cut by several generations of mud-filled borings. Some of the fill in these borings could be biostratigraphically dated, with ages ranging from Pliocene to Pleistocene. (2) At the southern site, a Late Miocene carbonate platform was sampled; the only sample recovered was a limestone slab consisting of a late Miocene rhodolith-bearing floatstone with a marine hardground overlain by a condensed section of phosphatic wackestone of Pliocene age. A series of mounds is present down the slope from the Late Miocene platform sampling site; these also were sampled and were found to be of probable Pliocene and probable Pleistocene age (Zones N.18 and N.22). Water depths for the deposition of the platform phase were shallow, probably less than 50 m, as testified by the presence at the northern site of large benthic foraminiferids, Halimeda , hermatypic corals, and nodular coralline algae. The cavity-fill sediments, probably typical of the overlying sediments, have good planktic foraminiferal faunas, but also include some shallow-water faunal components (Amphistegina) and reworked older faunas (Lepidocyclina). The infilled borings tend to be almost totally composed of planktic foraminiferids, as do some discrete samples. This faunal evidence indicates that for the northern platform no deposition took place during much of the Middle Miocene and early Late Miocene, probably due to a long period of subaerial exposure. During the Late Miocene, the northern platform slope was flooded and subsided to its present depth by the Pleistocene. The southern platform was built during the Late Miocene and subsided to its present depth in the Pliocene. Samples from the mounds contain shallow-water assemblages of two ages: one may be of similar Pliocene age as found on the platform sites, and another of probable Pleistocene age. The presence of hermatypic corals, Halimeda, and larger benthic foraminiferids suggest warm surface water temperatures since the Early Miocene.