Wianamatta Group Shale On Triassic Hawkesbury Sandstone

Title

Wianamatta Group Shale On Triassic Hawkesbury Sandstone

Artist

Miroslava Jurcik

Medium

Photograph - Photograph

Description

Areas of exposed rock are limited within Botany Basin and consist of Triassic Hawkesbury Sandstone but the overlying Wianamatta Group shale crops out on adjacent high ground to the west and southwest. These rocks have been shown to dip towards a meridional axis passing through the centre of the Bay. Unconsolidated sediment dominates in the Botany Basin, concealing the positions of the bedrock drainage channels. These channels extend for varying, but considerable, distances onto the shelf, and formed when the sea level was more than 110 m lower than at present (Albani, 1998).
The geological history of Botany Bay was strongly influenced by the climatic events of the Quaternary Period. During conditions of low sea level, the numerous coastal streams were rejuvenated and river valleys were entrenched and accentuated: a large volume of eroded material was deposited on the inner shelf. During high sea-level stands, some of the newly deposited sediment was remobilised by marine processes and transported back into the drowned river valleys causing partial siltation. The oscillating cycles of sea level of the Pleistocene are well recorded in the sediment cover of Botany Bay.
Bate Bay was once the main entrance to Botany Bay, and Kurnell was an island.
Since that time the gradual rise of sea level has led to progressive infilling of the bedrock channels with a tombolo forming between Kurnell and Cronulla. The present form of Botany Bay could not have existed until the water level had risen to about 30 m below present day sea level to cover the lowest part of Runway Ridge and inhabitants of Kurnell Island could have walked to Kyeemagh along this ridge until about 9000 years B.P.
ref. https://www.sutherlandshire.nsw.gov.au/files/assets/website/documents/waterways/waterways-study/vol_2_botany_bay_catchment_profile.pdf

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November 11th, 2019

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