Desert dunes and estuarine fill in the Fitzroy estuary (North-Western Australia)

Abstract

Summary In the R. Fitzroy estuary, Western Australia, there is a remarkable interfingering of linear desert dunes and tidal flats. The dunes, fixed by Acacia shrub and at right angles to the resultant of present sand-shifting winds, belong to a former more arid climate. The extensive tidal flats form part of considerable Holocene estuarine sedimentation. The age of the dunes and associated aridity was the prime object of this investigation. Despite difficulties with insensitive sea level indicators and large tidal ranges, it is evident that sea level reached much closer to its present position by 7400–6000 BP than in Europe or N. America. This agrees with recent work elsewhere in Australia showing that it behaved differently in this respect. At the maximum of the Holocene transgression tidal action cut back the flanks and tips of the desert dunes and truncated others completely, leaving only concealed plinths. Sediment mixing through this erosion produced a shore facies, reaching to 1–2m above HW springs and subjected to pronounced pedogenesis. Whether there was a Mid-Holocene sea level of + 1–2m remains uncertain but various geomorphic traits fit with this hypothesis better than with self-exclusion of the tide by estuarine sedimentation. Differences between air photography in 1949 and 1967 witness rapid erosional loss and constructional gain of mangrove swamp and salt marsh, with a marked net loss. Much of the wide high-tidal land lacks vegetation because of dry season hypersalinity; other parts form shallow intertidal lagoons behind low estuarine levees. Low mangrove forest and mangrove scrub occupy restricted mid-tidal zones. Greater extent and vigour of mangrove growth, including tall mangrove forest, in the period 7400–6000 BP require a longer and rainier wet season than is found with the present tropical monsoonal semi-arid climate. The desert dunes are overlain by the sediments of the Holocene transgression, including mangrove clays dated at 6000 BP. Dune sands were proved to nearly 15m below HW springs, taking their minimum age back to about 8000 BP. Well preserved dune shape implies a maximum age within the Upper Pleistocene. Related evidence from the Timor Shelf points to a last pleniglacial age for the Fitzroy dunes, whose direction is best interpreted on the basis of the likely wind regime of a full glacial paleoclimate with more prolonged Trade Winds and reduced rainbearing monsoonal winds.