Dinoflagellate cysts and environmental evolution of the Oligocene to Lower Miocene at site 1148, Odp Leg 184, South China Sea

Abstract

The Oligocene to Lower Miocene of Site 1148, Ocean Drilling Program (ODP) Leg 184 was investigated palynologically to explore environmental change within the newly formed rifted South China Sea. The basin first developed 32.8 Ma ago during an initial rifting phase, and before sea floor spreading. Palynomorph Assemblage A contains abundant coastal and neritic dinoflagellate cysts (for example, Lingulodinium and Spiniferites) and a small number of oceanic Impagidinium species, together with abundant pollen, spores, and terrigenous phytoplankton. Offshore transportation induced by basement subsidence played an important role in the makeup of this assemblage. Paleoenvironments during the earliest Oligocene include shallow shelf, shelf/slope boundary, and mid slope regimes. The latter is indicated by the intermittent and rare occurrences of Impagidinium. Later, in the Early Oligocene to earliest Late Oligocene, there was a deepening of the basin with increasing influence of lower slope environments, indicated by increasing abundances of Impagidinium. A barren zone corresponding to a period of sea floor spreading during the latest Oligocene to the earliest Miocene effectively separates assemblages A and B. The Early Miocene environment deepened to a lower slope (>1500 m) regime, indicated by Assemblage B with consistent Impagidinium. This regime was relatively stable with much less terrigenous input, indicated by the rare occurrence of pollen and spores, and the absence of terrigenous phytoplankton.