The Late Quaternary geological history of the upper and central Sydney estuary (Australia)

Abstract

Sydney estuary, on the east coast of Australia, is a small, drowned river valley with a narrow, sinusoidal channel in the upper reaches; a wide, U-shaped valley with large embayments and tributaries in the central section; and an extensive, more marine lower waterway. The estuary has experienced repeated flooding episodes during Quaternary high sea-level stands, in the absence of neotectonism. The present work considered only the upper and central estuary and therefore, potentially records the highest Quaternary sea-level stands.Valuable information on the geology and sedimentology of Sydney estuary stored in poorly-accessible grey literature has been synthesised and summarised in the current work. The first seismic-stratigraphic model was correlated to borehole data to provide a new Late Quaternary geologic history for the upper and central estuary. Limited absolute geochronological data necessitated reliance on stratigraphic correlation and conformity (uniformitarianism).A basal fluvial unit was overlain by a thick, extensive estuarine sequence, which was followed by fluvial marsh and channel sands and muds with minor wood, gravel and peat. Holocene estuarine muds mantled the upper surface of the estuary. Seismic expression was generally consistent with this lithologic subdivision. The thick estuarine unit acted as an important chronological marker horizon and played a critical role in controlling seismic interpretation throughout this part of the estuary. The paleo-estuarine sequence onlaps the modern shoreline and was most likely deposited during the Last Interglacial highstand (MIS 5.5 at 130–115 kyBP) when relative sea-level in Sydney estuary was between ＋4.5 m to ＋5.5 m above the present surface. The channel and marsh sequences were probably backstepping, progradational units in the upper estuary and possibly bayhead deltaic deposits in the central section. An erosional upper surface, as well as the stratigraphic position of these sequences, suggested deposition during a subsequent Interstadial event, most likely the MIS 5.1 at 82–87 kyBP. The knowledge generated by this study provided useful information for greater understanding of Quaternary estuarine evolution on the Australian east coast and for current and future construction projects in and across the estuary.