Botany Bay Sands

Last week I again conducted my annual field trip for the Wentworth Group Master Class. I refer to this excursion as Cape to Cape from Cape Banks to Cape Solander. It encompasses the broad expanse of Botany Bay so named by Captain Cook and reflecting the incredible botanical finds of both Banks and Solander. But the Bay tells another story, one that is critical to the economy of Australia as well as to the lifestyle of many Sydney folk and the ecology of the Bay. This is the story of its sands.

Botany Bay lies along the axis of the Sydney Basin. While the two capes are composed of prominent sandstone headlands of the Hawkesbury Formation, most of the Bay is flanked with sand and mud deposits of Quaternary age. Many years ago Peter Roy was involved in writing a report on the depositional history of the Bay (Roy and Crawford, Records Geol. Survey NSW, Vol. 20, 1979). However, he focussed on the southern side including the Kurnell dune field, the source of sand for construction purposes for Sydney for many decades. This work suggested that sands blown in from Bate Bay to the south may have been one source for those that cover the bed of the Bay and to its north.

The western side of the Bay is less studied. A beach ridge complex covering the suburb of Ramsgate lies behind the massive foredune ridge described and illustrated by E.C. Andrews in his paper on Lady Robinson’s Beach (Proceedings Royal Society of NSW, 1916). Houses now cover these sands. However, it is reasonable to assume they are Holocene in age and relate to refracted waves that have entered the Bay forming a prograded beach-dune system.

To the north of the Bay lie an extensive dune and swamp complex extending all the way from Botany/Banksmeadow to Moore Park. Soils exposed in this area show signs of deep podsolization. To my mind, these dunes are likely to be Pleistocene in age and perhaps reflect several episodes of dune accumulation at times of lower sea level. This is in sharp contrast to the beach ridges of Ramsgate. The sands became an early source of bore water for the settlement of Sydney as the Tank Stream became more polluted. As industry expanded, ground waters not only were used they were also contaminated. Chemical pollutants started to creep as underground plumes towards the Bay eventually requiring government intervention and rehabilitation (see papers in the volume on Environmental Geology of the Botany Basin edited by McNally and Jankowski, Geol. Soc. Aust.1998; they dedicated the volume to the late Russ Griffin who they rightly say was the man who put the Botany Basin on the map).

It is the sands of the floor of the Bay that provide much support for transport functions that now dominate parts of Botany Bay. Beginning in the 1960s with the channel for oil tankers at Kurnell to the more recent expansion of Port Botany, the dredging of these sands has been critical to the transformation of the Bay. In places such as Silver Beach and Lady Robinsons Beach changes to the shape and depths of Bay floor has induced erosion of shores requiring protection works such as groynes. At Towra Point sand nourishment has been used to slow down shore recession. But it is the use of dredged sand to infill sections of the Bay for airport and shipping port expansion is where the value of this resource has proved important.

From drill records undertaken during investigations for the recent 60 hectare port expansion, it is apparent that two sand units cover the floor of the Bay. The total thickness is around 10-15m. The upper unit consists of loose quartz sand. It is presumably Holocene in age being swept into the Bay with the rising sea level. The lower unit is denser with discrete clay and organic lenses and appears to be linked to sands of the dunefield to the north and hence of Pleistocene age. This unit overlies a stiff clay and forms a similar stratigraphic sequence to that observed by Roy and myself in Port Stephens. Both sand units have proved ideal for reclamation fill. So from the development in the 70s of the main N-S runway, to the two phases of Port Botany establishment, and the construction of the second runway into the Bay, we have seen vast amounts of these sands extracted. All these major pieces of infrastructure have clearly benefitted from the readily accessible sands so ideal for reclamation.

Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 4th April 2017, for correspondence about this blog post please email admin@australiancoastalsociety.org

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