A mountainous secret revealed in the pink sands of South Australia

Deposits of deep pink sand washed up on South Australian shores have shed new light on when the Australian tectonic plate began to subduct beneath the Pacific plate, as well as the presence of previously unknown ancient Antarctic mountains.

The pink sand is made up of a mineral called garnet, and the University of Adelaide research team, led by Ph.D. candidate Sharmaine Verhaert and Associate Professor Stijn Glorie used a new, cutting-edge method to show that garnet grains are about 590 million years old.

Garnet is known to have formed locally during the Delamerian Orogeny, an event that created the Adelaide Fold Belt about 514–490 million years ago, and during the formation of the Gawler Craton in western South Australia between 3.3–1.4 billion years ago. These ages do not match the garnet sand on the South Australian shores.

“The garnet is too young to be from the Gawler Craton and too old to be from the eroding Adelaide Fold Belt,” says Verhaert.

“Garnet requires high temperatures to form and is usually associated with the formation of large mountain ranges, and this was a time when the southern Australian crust was relatively cool and non-mountainous.”

Scientists who published their findings in a journal Earth and environment communicationthey found that the garnet did not come from the local source rocks, but they knew that it came from nearby because garnet is usually destroyed by long-term exposure to the marine environment.

They found that the glacial sedimentary deposits of the Cape Jervis Formation, which crops out along the coast of South Australia, contain layers of sand with garnet that are also about 590 million years old.

Indicators of ice flow in these glacial sedimentary deposits tell us that garnet-rich glacial sands were brought to Australia by a northwest-moving ice sheet during the late Ice Age, when Australia and Antarctica were joined into the supercontinent Gondwana.

A garnet dated to the same period was previously found in an outcrop in the Transantarctic Mountains in East Antarctica, on the edge of a colossal region that is completely hidden by a thick ice sheet. Scientists believe the area hosts evidence of a 590-million-year-old mountain belt hidden beneath the Antarctic ice.

“Although it is not currently possible to sample directly beneath this ice sheet, it is possible that millions of years of ice transport eroded the underlying bedrock and transported this garnet load northwest, towards the conjugate Antarctic-Australian margin,” says Associate. Professor Gloria.

“The garnet deposits were then locally deposited in glacial sedimentary deposits along the southern Australian margin until erosion freed them and waves and tides concentrated them on South Australian beaches.

“We have effectively uncovered a major mountain building event that redefines the timing of the onset of Pacific Ocean convergence.”

A new approach to lutetium-hafnium dating developed by the University of Adelaide, which uses a laser system attached to a mass spectrometer, made this important discovery possible with a simple interrogation.

“This journey started with the question of why there are so many shells on the beach at Petrel Cove,” says Dr. Jacob Mulder, who was also on the research team.

“It’s fascinating to think that we were able to trace tiny grains of sand on a beach in Australia to a previously undiscovered mountain belt beneath the Antarctic ice.”