Western Australia’s changing coastline reveals a “drowning” continent

A study of two coastal landmarks in the southwest corner of Australia has traced their complex history over thousands of years and how they will change in the future.

Samples collected from locations along the coastline of Western Australia’s capital city Perth were analysed by geologists from Curtin University. Their findings are published in the journal Basin Research.

The researchers collected sedimentary rocks from the shore, waterways and the sea floor.

Among the sampled areas were tourist spots, the Swan River and Rottnest Island (Derbarl Yerrigan and Wadjemup respectively in the local indigenous Noongar language). Rottnest Island is known for its population of friendly and adorable marsupials called quokkas.

Mineral grains found at Rottnest Island date to 3.6 billion years old. The same grains were found in the Swan River estuary and Avon River tributary. These grains were deposited on the island thousands of years ago.

“Perth is sitting on a continental margin that is drowning,” says study lead Andreas Zametzer.

“Rottnest Island used to be the tip of a peninsula until about 6,500 years ago – similar to what we see in the Shark Bay region (about 650km north) – and because it’s still part of the drowned continental margin, the ocean between Perth and Rottnest is hardly deeper than 20m in most places.”

Mineral grains found at Rottnest Island date to 3.6 billion years old. The same grains were found in the Swan River estuary and Avon River tributary. These grains were deposited on the island thousands of years ago.

“We would have expected the grains to have eroded and washed away because they were in a very turbulent, high-energy environment of waves and tides,” says Zametzer

“But the distinctive mineral fingerprints of the river are still sitting there despite thousands of years of sea-level changes and shelf reworking.”

The find confirms that the mouth of Swan River used to extend more than 20km further west than it does today, just north of Rottnest Island.

“We have this river system imprint preserved offshore, confirming the coast’s history.”

Understanding the Perth coastline’s changes will help predict how shores will continue to evolve.

“It’s quite shocking to think about how fast some geological processes actually happen with sea levels continuing to rise quickly.”