Geological History

The geological history of Sydney Olympic Park is presented here as a time line showing the main geological events that have shaped the area over the past several hundred million years. The formation of the island continent of Australia took millions of years and throughout the continental rifting and drifting, sedimentary rocks of the Sydney Basin were raised, tilted and then eroded by the action of the sun, wind and water.

In the vicinity of Sydney Olympic Park and to the south of the Parramatta River valley, The main rock types are sandstones and shales, known locally as the Hawkesbury Sandstone and Ashfield Shale.

250 to 210 million years ago
60 million years ago
20,000 years ago
6,000 years ago
20 years ago
Today

250 to 210 million years ago

At this time the Australian landmass formed part of the Gondwana supercontinent, which was located closer to the south pole, dinosaurs were yet to dominate the earth and the Sydney area was located within a depositional basin. The oldest rocks at Sydney Olympic Park can be viewed from the Brickpit Ringwalk. The rocks exposed in the Brickpit wall are sedimentary in origin, having been laid down by rivers that flowed across the area several hundreds of millions of years ago in the early to mid Triassic Period. There are Hawkesbury Sandstone outcrops out in the lower portion of the Brickpit. These record the deposition of massive sheets of sand and gravel from the large river systems which once flowed to the northeast across the Basin. The Ashfield Shale overlying the Sandstone records both a change in river flow direction and depositional style in the mid-Triassic Period, when southeast flowing rivers deposited fine grained sands and muds in a river delta which had formed along the shore of a shallow sea. The Ashfield Shale layer is twenty metres thick and comprised of the finely layered sediments with the occasional fossilised remains of leaves, fish, shells and insects typical of a deltaic environment. The shales form part of the Wianamatta Group of rocks, which occur throughout much of western Sydney.

60 million years ago

The break-up of Gondwana was not complete until some 40 to 60 million years ago, during the early Tertiary Period. Gondwana included Australia, Antarctica, India, Africa, South America and Arabia. The formation of the island continent of Australia took millions of years and throughout the continental rifting and drifting, sedimentary rocks of the Sydney Basin were raised, tilted and then eroded by the action of the sun, wind and water. The Tasman Sea flooded into the rift between Australia and New Zealand, defining the southeastern margin of the Australian continent. The sedimentary rocks of the Sydney Region were sculpted into a landscape characterised by bedrock valleys excavated into an elevated plateau. The largest rivers including the Hawkesbury, Parramatta, Georges and Hacking Rivers eroded the deepest valleys. In the vicinity of Homebush Bay and to the south of the Parramatta River valley, the Ashfield Shale weathered to produce a subdued landform characterised by a low undulating topography and moderately fertile soils. This area was in marked contrast to the spectacular landscape of plateaus, sheer cliffs and gorges formed on the sandstone areas elsewhere in the Sydney Region.

20,000 years ago

The Quaternary Period, and Pleistocene Epoch in particular (c.1.8 million to 10,000 years ago), was a time of frequent and rapid sea level change, caused by the melting and freezing of continental ice caps. Cool dry glacial periods with low sea levels alternated with relatively warm interglacial periods and associated higher sea levels throughout the Quaternary Period. During the most recent Ice Age, some 20,000 years ago, sea level in southeastern Australia is thought to have been around 120m lower than its present position. At this time, the present-day location of Sydney would have been some 20 kilometres inland from the coast and overlooking the Parramatta River valley. The area around Sydney Olympic Park would have been located near the headwaters of the Parramatta River and characterised by a series of small streams issuing from the Powells, Haslams and Duck Creek catchments, flowing north to meet the Parramatta River.

6,000 years ago

The warming of the earth and melting of the continental ice caps following the last Ice Age saw sea levels rise and stabilise around their present position some 6,000 years ago. In southeastern Australia the sea flooded across the continental shelf and into the ancient bedrock valleys eroded by the rivers over the tens of millions of years. In the largest river valleys, drowned river type estuaries were created, of which Sydney Harbour is a textbook example. What was once the Parramatta River valley was transformed into the estuary we now recognise as Sydney Harbour. River environments gave way to estuarine environments and sediment, which had once been transported to the coast, now became trapped within the estuary to form a variety of sub-tidal, intertidal and supratidal environments upon which a rich diversity of ecosystems developed: Seagrass beds, mangrove forests, saltwater-freshwater wetlands, casuarina woodlands. A shallow bay (Homebush Bay) formed within the catchments of Haslams and Powells Creeks. The tidal embayment was fringed by tidal mud flats built from the fine-grained sediments washed into the estuary from the local catchments. Over time, the mangrove forests would build into the deeper water of the bay, further trapping the fine grained sands and mud washed in from the local creeks or brought downstream by floods in the Parramatta River. Saltmarsh developed landward of the stands of mangroves and on the elevated ground fringing the tidal flats, casuarina forest grew.

20 years ago

Two hundred years of European occupation at Sydney Olympic Park effectively transformed the area from a wetland to a wasteland. By the late 1980's many of the low-lying areas near the Parramatta River estuary were either reclaimed with sediment from the bay bed or infilled with waste from the surrounding urban and industrial areas. Haslams Creek flowed through a concrete channel and the saline and freshwater wetlands were degraded by leachate draining from the waste. Excavations dug deep into the bedrock for building materials (The Brickpit and Clay Pit) lay abandoned or infilled with waste. Rejuvenation of the Sydney Olympic Park area commenced in the late 1980s with a remediation strategy that took over 10 years to implement. The clean-up consolidated the widely distributed waste into four main waste containment mounds and seven other consolidated waste areas, constructed soils were imported as capping material. Tidal flows to estuarine wetlands have been restored and the health of the surrounding ecosystems is on the mend.

Today

Remnant natural ecosystems, remediated landscapes and waste containment mounds have been integrated into the parklands of Sydney Olympic Park. The Parklands provide a unique opportunity to view the local geology, whether it be the ancient rocks of the Brickpit, the weathered shale soils of the Newington Nature Reserve or the organic-rich sands and muds underlying the mangrove forest and wetlands around the estuary foreshore.

This image was taken at Newington Armory. It shows a typical podzolic soil profile for the area identified as the "Blacktown Soil Landscape" on the 1:100,000 scale (1cm = 1Kilometre) map of Soil Landscapes of the Sydney Region. Red/Brown Podzolic soils are a common soil type developed on the Ashfield Shale in the Homebush Bay area. The soil profile shows three distinct horizons, or layers. The upper layer of the profile is comprised of a friable brownish black loam up to 30cm thick which overlies 10-20cm of hard setting brown clay loam (A horizon). The A horizon overlies up to 100cm of brown mottled light clay (B horizon) which is labelled B on the image. The soil profile then grades at depth to a plastic light-grey silty clay which occurs as a deep subsoil (C horizon) above the Ashfield Shale bedrock.

Note: Description drawn from field observations (photo) and notes to the Soil Landscapes of the Sydney 1:100,000 Sheet (1989).