Holocene development of the lower Mary River plains, Northern Territory, Australia

Abstract

Extensive coastal and estuarine plains, associated with several large seasonal rivers, characterize the southern shore of van Diemen Gulf, Northern Territory, Australia. The Mary River is unique among these rivers in that, despite a catchment of 7700 km2, it fails to reach the sea as a discrete channel but breaks up into a series of terminating, multiple channels, leaving much of the freshwater discharge to evaporate from the plains. The extent and morphology of these plains have changed markedly during the Holocene. Drilling, radiocarbon dating and pollen analysis indicate a transgressive phase that began about 7000 years ago as the rising seas flooded the prior valley. This was followed by a big swamp phase as sea level stabilized around 6000 years ago, during which widespread mangrove forest development occurred. Rapid progradation of the coast occurred between 6000 and 4000 years BP. The shoreline has prograded little since that time, but has been modified by episodic chenier ridge deposition and palaeochannel abandonment, infill and reoccupation. The lower Mary River differs from adjacent systems in that several former estuarine channels across the plains appear to have become completely blocked with sediment, leaving low-lying billabongs and palaeochannels which have been preferentially reinvaded by later channels. Several alternative explanations are examined, but it remains unclear what has driven major variations in palaeochannel activity and sedimentation during the mid-Holocene, and why these have decreased so markedly in the last 2000 years.