Late Quaternary geological history of the Sydney estuary, Australia

Abstract The present study provides valuable new information on the evolution of Sydney estuary by tracing the development of the complete marine–estuarine–fluvial system through a full glacial cycle (Last Interglacial, LIG, to the present Interglacial). Extensive seismic (361.3-line km) and sedimentological studies provided a sound foundation for production of a detailed litho- and seismic-stratigraphic record for the estuary. In the absence of reliable age data, a relative chronology was constructed based on Quaternary flooding surface elevations constrained by a recent local relative sea-level record supported by other global studies. A thick, ubiquitous estuarine unit deposited during the LIG period (MIS 5.5; 130–115 ka BP) was an important chronological marker horizon and played a critical role in controlling seismic interpretation and correlation throughout the estuary. Deposition during the MIS 5.1/5.3 interstadial period (100–80 ka BP) resulted in deposition of fine-grained, estuarine sediments in the lower estuary and time-equivalent, fluvial-sourced estuarine and channel sediments, and marsh sediments in the upper and central estuary, respectively. The MIS 3 interstadial event did not play a significant role in sedimentation in Sydney estuary. An eolian dune field formed adjacent to the southern shores of the estuary during the last glacial (31–24 ka BP) when most of the sediment in the lower estuary had been removed by fluvial erosion. Transgressive marine sand, which deposited in the lower paleovalley after the ocean re-entered the estuary, experienced repeated erosion and infilling by laterally migrating paleoriver channels. A marine flood-tide delta now occupies the estuary mouth, and the lower and upper/central estuary are mantled in a veneer (mean 7 m) of Holocene sand and mud, respectively. KEY POINTS A relative chronology was based on Quaternary flooding surface elevations constrained by relative sea-level. First geological history of the Sydney estuary with a complete marine–estuarine–fluvial system. A late Quaternary estuary evolution through a full glacial cycle. Geological history includes an interstadial (MIS 5.3/5.1) estuarine sequence.