Underwater landslip on the downward slope of the continental shelf off Byron Bay. New research shows a large number of such slips, many of which have occurred in the last 25,000 years. These were within the period of Aboriginal occupation of New England, a time of great changes in sea levels.
Interesting article in The Conversation (11 December 2017) by Samantha Clarke, Hannah Power, Kaya Wilson and Tom Hubble, Scars left by Australia’s undersea landslides reveal future tsunami potential on evidence for sometimes large underwater landslides on the downward slope where the continental shelf falls away to deeper water. This diagram from the article illustrates the process.
The focus in the article is on tsunami risk from future landslips. However, I was interested both in the undersea pictures and the reference to frequency over the last 25,000 year for this was a period of substantial sea level change.
Pattern of Sea Level Change
Without bogging down in dates, the last ice age began about 110,000 years ago. Sea levels fluctuated, falling during glacial periods, rising during warmer interglacial periods. Around 40,000 years ago, sea levels were perhaps 50 metres below present levels. From around 36,000 years ago, the climate became cooler and drier. Then from around 25,000 years ago the climate deteriorated very significantly with spreading ice sheets globally. This climatic regime peaked during what is called the Last Glacial Maximum (LGM), 21,000 to 15,000 years ago.
As more water turned to ice, sea levels fell significantly, bottoming around 18,000 years ago at perhaps 120-130 metres below present levels. At this height, parts of the continental shelf slope break line could have been 20-30 metres above water.
The LGM began to ease around 15,000 years ago. The North American ice sheets melted. Around 12,000 years ago, the Antarctic ice sheets began to shrink. The Holocene with its higher rainfall and warmer temperatures had begun. The seas rose and rose again, reaching present levels around 6,000 years ago, a total sea level rise of 120-130 metres over 9,000 years. .
The erosive effects as the sea rose onto and then submerged the continental shelf must have been very considerable. I wondered what the connection might be between the number of identified slips and this sea level rise. Whether triggered by earth movements or erosion, the bigger slips could have had significant localised impacts.
The Clarke et al article provides another little building block. In checking the latest material, I also found a 2010 paper that I had not seen before: Alan Jordan, Peter Davies, Tim Ingleton, Edwina Foulsham, Joe Neilson and Tim Pritchard, Seabed habitat mapping of the continental shelf of NSW, Department of Environment, Climate Change and Water NSW, Sydney 2010. This focuses on the current seabed habitat along the NSW, but it contains some of the best local descriptions that I have seen of the shape of the continental shelf.
I have not had time to absorb this properly. I am referencing it here as a record for later study.