The age and origin of Sydney Harbour and the Parramatta River: the Cenozoic history of the coastal rivers of central New South Wales

Abstract

Neither the age nor the origin of the Parramatta River of coastal central New South Wales, Australia is well understood. Despite previous assertions, there is no evidence that the river is underfit or that its drainage pattern has experienced radical rearrangement. Yet the river ignores the obvious route to the sea at Botany Bay, perhaps because its course was set by the Oligo-Miocene when local drainage was realigned in response to the subsidence of the Botany Basin. Similarly anomalous is the river’s route through the Hornsby Plateau to the ocean at Port Jackson. This is the result of antecedence, with the river pre-dating the upwarping of the plateau and maintaining its course across an uplifting surface. Two topographically distinct suites of drainage channels are preserved offshore. The earlier is traceable to depths of 260 m and is continuous with modern terrestrial drainage along the Hawkesbury River—the preservation of these channels may be a result of the subsidence of the east Australian margin since the Eocene. The more recent channels extend to depths of 120 m. These may post-date the earliest Oligocene when global sea levels fell by ≫50 m, resulting in intermittent exposure of the continental margin arising from glacio-eustatic oscillations in sea level. Deeper and older paleodrainage is absent offshore of Port Jackson and Botany Bay. Drainage along the proto-Parramatta River thus cannot pre-date the earliest Oligocene and it was only with the subsidence of the Botany Basin in the Oligo-Miocene that terrestrial drainage was realigned to the coast and channels were entrenched across the inner shelf. Sediments representative of a major flow-line sub-parallel to the path of the modern Hawkesbury–Nepean pre-date the uplift of the Blue Mountains Plateau and the upwarp of the Hornsby Plateau. These confirm the long-term role of the Hawkesbury–Nepean–Warragamba as the regionally dominant drainage line. KEY POINTS There is no evidence that the Parramatta River is underfit or that it has experienced radical rearrangement. The lower part of the Parramatta River is antecedent, existing prior to the upwarping of the Hornsby Plateau. An older suite of submarine channels may preserve evidence of terrestrial drainage along the Hawkesbury River since the Eocene; a younger suite may post-date the earliest Oligocene when falling global sea levels resulted in the intermittent exposure of the continental margin. The Parramatta River is likely to have originated in the Oligocene.