Fluviatile evidence for a period of late-quaternary pluvial climate in coastal southeastern Australia

Abstract

The Cranebrook Terrace near Sydney, New South Wales, yields sound evidence of a period of exceptional fluvial activity in lowland basins of southeastern coastal Australia well prior to the last glacial maximum. This episode, titled here the “Cranebrook Pluvial”, is dated from radiocarbon and thermoluminescence age determinations as ending about 40,000–45,000 yr B.P. It was characterised along the lower Nepean River by abundant gravel transported over a braid plain proximal to confining sandstone gorges. Thick basal gravels were deposited almost contemporaneously with a sandy-clay overburden until this time when the river quite quickly became confined to two laterally stable channels. Additional overbank deposition probably accompanied this stable phase. The easternmost channel was abandoned about 34,000–37,000 yr B.P. and the Nepean River appears to have occupied only the western channel since then. Isolated from eustatic changes occurring downstream, the Nepean River has left no significant alluvial deposits from the last glacial maximum or from the Holocene, periods of time which, in this fluvial environment, appear quiescent by comparison with preglacial pluvial. However, stripping and replacement of the fine overburden appears to have occurred over the western part of the terrace about 14,000 yr B.P., or somewhat younger from 14C determinations. These conclusions are at odds with the proposal that the main period of alluvial deposition in coastal valleys of southeastern Australia was triggered by widespread slope instability during the last glacial maximum. Alluvial and pollen data suggest the glacial was a cool dry period of relatively little fluvial activity in basins not affected by glaciation. The Cranebrook Pluvial was probably the most important erosional-depositional episode in the non-glacially effected areas of southeastern Australia in the last 50,000 years.