Collapsed Continental Margins, Bass Lake, and Submarine Canyons off Southeast Australia
10 April 2001
The CSIRO-run research vessel Franklin returned to Hobart today after a 21-day geoscience research expedition off Tasmania and the Otway coast of Victoria.
Chief Scientist, Dr Neville Exon of Geoscience Australia, said "We carried out seismic profiling east of Tasmania and sea bed sampling in three different areas, each addressing different research questions. The seismic profiles revealed a cross-section through the Earth's crust several kilometres thick, and the cores and dredges gave information about various aspects of the geological history of offshore Australia as far back as the Age of Dinosaurs, seventy million years ago."
The cruise brought together scientists from Geoscience Australia, CSIRO and the Universities of Sydney and Melbourne, and was the third cruise of the Franklin in 2001, and the first to depart from Hobart, its home port, for several years. Among the data brought back by the expedition for further study are 1300 kilometres of seismic profiles, 30 samples of surface samples, 25 cores of the strata just below the sea bed, and 10 dredges of ancient rocks. The major results came from the continental slope east of Tasmania, from Bass Strait, and from the Otway continental slope.
Of the eastern offshore margin of Tasmania Dr Exon said "This poorly known part of Australia extends 100 kilometres offshore with its outer limit 4500 metres deep, and is one-third the size of Tasmania. The margin supports a fishing industry and has some potential for petroleum, but our main aim was to better define its nature and its history since this part of Gondwana broke up 85 million years ago.
"The Lord Howe Rise, now a sunken ridge extending from Lord Howe Island to New Zealand, was dry land east of Australia including Tasmania until it was rifted off. The cruise results show that, although the margin subsided steadily after its separation from the Lord Howe Rise, it was not covered by the thick blanket of younger sediment present on most of the world's margins.
"In fact, the submarine landscape sloping down to the abyssal plain would not be very different to that on dry land in Gondwana when the dinosaurs roamed the land," said Dr Exon. "The granites, basalts, metamorphic rocks and sandstones familiar to many east coast Tasmanians are right there at the sea bed. Much of the sediment coming from the land has cut underwater canyons on its path to the abyssal plain, but some has been trapped in sedimentary basins that could have produced hydrocarbon deposits."
In Bass Strait, Professor Jock Keene pursued the secrets of the ancient Bass Lake that existed in Bass Strait following the last ice age. "This expedition was able to collect a core of the sea floor over 8 metres long, the longest ever taken from the Franklin. It will enable us to study past climates as the lake came and went over thousands of years," said Professor Keene, from the University of Sydney. "This all occurred when Aborigines were living and walking across Bass Strait."
Using shorter cores, Professor Keene and his research students had already determined that Bass Strait flooded 9,000 years ago as the sea level rose from the melting of the ice sheets. This new and longer core will go back to older ice-ages and allow studies of the Tasmanian and Victorian climates during this time. As the snow and ice melted on the Tasmanian Highlands 18,000 years ago, rivers drained the melt water into a lake in the centre of Bass Strait. This lake may have dried out as the Australian continent got drier. "This will all be recorded in the layers of mud, fossil shells and sand from the desert," said Professor Keene. "As the sea level rose further, Bass Strait became a large embayment open to the west, and then was flooded to form the strait we know today."
On the Otway margin off Port Campbell and Portland, studies of seismic profiles by Andrea Leach, a Melbourne University doctoral student, had shown that canyons first formed there about 30 million years ago. This was soon after the margin had collapsed, as did that off eastern Tasmania. Some had filled in and some new ones had formed.
"My aim on this expedition was to sample the sediments moving down the canyons to help understand the processes going on today and in the past. We got a number of cores and grab samples showing that mud is slumping down the canyons and from their walls. My laboratory studies of the cores may indicate whether the canyons are at present being filled or are cutting deeper," said Ms Leach.
Of the cruise as a whole, CSIRO's on-board cruise manager Ron Plaschke said "This was my first geoscience research cruise and I was very pleased that we managed to get so much good work done. For me a highlight was the deployment of a one-tonne piston corer with a ten metre long sampling pipe from Franklin. I'll long remember the delighted smiles on the faces of the scientists when eight metres of Bass Lake sediments were cut up on deck. I understand that many thousands of years of climate change will be revealed by the core."
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