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Abstract
New research from the Shoalhaven deltaic plain prompts revision of ideas on fluvial deposition in coastal New South Wales. Extensive remnants of Pleistocene alluvium occur on the surface and beneath Holocene sediments. Thermoluminescence (TL) analysis indicates that there were at least two and probably four phases of Pleistocene deposition spanning much of Last Interglacial and Glacial times. Deposition continued even when sea levels were low. Holocene infilling was faster than previously thought, for the Shoalhaven was feeding fluvial sand onto the shore and the adjacent barrier dune system by 6 ka. Away from the main channel, deposition occurred mainly as levee building along tributary and distributary channels. Claims for an initial southerly course of the Shoalhaven across the plain followed by avulsion northward to the present mouth are discounted. Channels flowing southward are too small to be former courses of the Shoalhaven, and are better explained as distributaries caused by crevassing of levees. A terrace sequence includes bedrock straths of possible Tertiary age, eroded Pleistocene remnants, and three main Holocene levels apparently linked to falling sea levels. Alluvium was stripped and fluvial channels truncated by tsunami surges up estuaries. Tsunami impact is also indicated by relic boulder and sand deposits. C14 evidence indicates that the impact occurred at c.0.52 ka.
