The Late Quaternary geological history of the lower Sydney estuary (Australia)

Abstract

Sydney estuary is a dendritic, drowned river valley on the mid-east coast of Australia and comprises an upper and central section of narrow, sinusoidal channels and embayments and a wide, more marine lower estuary. The current work considered only the lower estuary.Minimal research has been undertaken into the structure of the lower Sydney estuary and previous attempts to integrate geologic and geophysical information are limited. The current study combines previously uninterpreted seismic records and poorly-accessible borehole data into a Late Quaternary history of the lower estuary.High-resolution, continuous reflection seismic (212-line km) interpretation matched borehole-based stratigraphy reasonably well. A bedrock paleo-valley deepened from approximately 40 m below present sea level (bpsl) at the Sydney Harbour Bridge (SHB) to > 90 m bpsl at the estuary mouth. Stratigraphy was dominated by two estuarine sequences, which, in the lower part of the estuary comprised mainly fine-grained, organic-rich sediments with peat, that manifest as multiple, strong dark and light-reflectors on seismic. At the SHB, the upper estuarine sequence developed into a transgressive marine to tidal delta deposit. A thin veneer (up to 10 m) of Modern estuarine sands mantled most of the lower estuary. The elevation of the estuarine sediments at approximately present-day levels and at 20 to 30 m bpsl respectively, strongly indicated deposition during periods of high-level marine transgression, i.e. the Last Interglacial (Marine Isotope Stage, MIS 5.5) event at 130 kyPB to 115 kyBP and the Penultimate Interstadial event (MIS 5.1 at 110–80 kyBP), respectively.Late Pleistocene aeolian sands contributed sediment in the south of the estuary, and remobilisation may have dispersed these sediments more widely. Present-day bathymetric features frequently reflected subsurface erosional structures due to starvation of bedload sediment.