Australia: The Land Where Time Began

A biography of the Australian continent 

Situated 35 km north of Sydney Harbour, the geological history of this area has been studied in detail using seismic techniques that give a full picture of the drainage history of the area. This technique has enabled the discovery of the old bedrock channels that had been filled by sediment deposited in the valleys formed in the Pleistocene when the sea level was much lower than at the present, as well as the rock beneath the sandbars and tombolas (a deposition landform that connects an island to the mainland) that mask the palaeodrainage pattern beneath. The same history also applies to the formation of Sydney Harbour.

Broken Bay is the estuary of the Hawkesbury River, and a tidal waterway also opening into Broken Bay, Pittwater, lies between Barrenjoey and Commodore Heights, but has a different geological history. The Hawkesbury River system drains about 22,000 km² of the eastern highlands of New South Wales, and has essentially the same drainage pattern it had in the Early Tertiary, about 40 million years ago. During the Miocene the catchment was eroded down to flat plains, becoming a low-energy, sluggish river system, by about 14 million years ago, as the drop in elevation between the source and the plains declined. About 5 million years ago, in the Pliocene, the flat Miocene plain was uplifted during a phase of moderate tectonic activity. The newly rejuvenated rivers, having been increased in altitude had more energy to cut down through Triassic rocks as they deepened and widened the gorges. The process of rejuvenation was repeated at the start of the Quaternary.

The fluctuating sea level leading up to the Pleistocene ice age, and the fluctuations that occurred as the glacials alternated with the interglacials of the Pleistocene ice age, gave the coastal streams more power to erode down further into the bedrock of the river channels at times of low sea level. The old Pleistocene channel of the Hawkesbury River, between Barrenjoey and Box Head is about 125 m below the sea level of the present, the lowest point reached. In the middle of Broken Bay it is joined by tributaries from the mouth of Brisbane Water, that originated as an inland lake system before The Gap was breached in recent times, connecting it to the sea. Across The Gap is a bedrock bar that is now 4.5 m below sea level. It acted as a watershed during the Pleistocene.

The widest exposure of the continental shelf occurred at the same time as the lowest sea level and the deepest cutting of the bedrock. At about 14,000 years ago, contours show that there was a maximum exposure, at which time the margin of the land was about 20 km east of its present location,, the rivers crossing a wide coastal plain on their way to the sea. The Manly-Wariringah area at about 14,000 years ago would probably have been good hunting grounds for the Aboriginal People that were present at the time, with lagoons and estuaries on a wide plain, the rocky bluffs that form the present coastline were far inland. The shelf was gradually submerged as the sea rose until it stabilised about 6000 years ago. As the sea level rose the Hawkesbury was gradually silted up. Its large catchment allowed it to continue flowing across Broken Bay.

A surprise for the scientists carrying out the seismic survey of Pittwater came when their data indicated that it only joined the drainage channels of the Hawkesbury River system very recently. The bedrock under Pittwater showed that it was a steep-sided V-shaped valley running parallel to the coast that deepened towards the north. Its watershed in Church Point-Bayview-Mona Vale, the shallow valley is fed by 2 tributary valleys, 1 on the Bayview side and 1 from McCarrs Creek. The 2 valleys joined at a point in line with Towlers Bay, continuing on as a single river channel winding to the region of The Basin where it is joined by a tributary at a depth of 50 m. The channel veers eastward abruptly off Commodore Heights, draining to the sea across the bedrock between Barrenjoey and Palm Beach where the rock reaches a depth of 76 m.

A volcanic bar of rock, a Jurassic dyke about 170 million years old, connecting Barrenjoey to Commodore Heights, that is now 12 m below sea level, prevented the Pittwater River from merging with the drainage system of the Hawkesbury river. As the sea level rose after 14,000 years ago the Pittwater River slowly became Pittwater Swamp, not having an active enough flow to counteract the silting up of its channel. The bedrock joining Barrenjoey to Commodore Heights, and beyond towards the sea, formed a spur of rocky cliffs along the margin of Broken Bay up to 9,000 years ago, the drainage of the Pittwater being quite separate. The bar was breached and a new drainage pattern established as the sea level rose, allowing the tide to scour out the sediments that had silted up Pittwater Valley. The tombolo joining Barrenjoey to Palm Beach was built up on bedrock by sand carried by along-shore currents as the sea level rose. The old channel of Pittwater River to the sea was silted up until it was closed. The entire system had stabilised by 6000 years ago, Pittwater being the tidal system of the present, open to Broken Bay where it joins the Hawkesbury River and Brisbane Waters.

Between 12,000 and 6,000 years ago the sea is calculated to have risen at a rate of about 1 m per 100 years. The changes to the landscape seen in the study of Broken Bay as sea level rose can be a useful gauge to visualise the possible effects of a 1 m rise in 20 years that is the worst case scenario, or 1 m rise in 100 years rise if a slower rise occurs,  for present seal level rise.

Sources & Further reading

1. The Nature of Hidden Worlds, Mary E. White, Reed, 1993
2. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000