A mountainous mystery discovered in the pink sands of South Australia

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

The pink sand is composed of a mineral called garnet, and a research team from the University of Adelaide, led by Ph.D. candidate Sharmaine Verhaert and associate professor Stijn Glorie used a new, advanced method to demonstrate that the garnet grains are approximately 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 and 1.4 billion years ago. These ages are inconsistent with the garnet sands on the South Australian coast.

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

“Garnets require high temperatures to form and are usually associated with the formation of large mountain belts, and this was a time when the South Australian crust was relatively cool and non-mountainous.”

The researchers, who published their findings in the journal Communication Earth & Environmentdetermined that the garnet did not come from local source rocks, but they knew it had traveled from close range, as garnet is typically destroyed by long-term exposure to the marine environment.

They found that the glacial deposits of the Cape Jervis Formation, which stretch along the South Australian coastline, contain layers of sand with garnet that are also about 590 million years old.

Ice flow indicators in these glacial sedimentary deposits tell us that the garnet-rich glacial sand was brought to Australia by a northwest-moving ice sheet during the late Paleozoic glaciation, when Australia and Antarctica were connected in the supercontinent Gondwana.

Garnet from the same period has previously been found in an outcrop in the Transantarctic Mountains of East Antarctica, on the edge of a colossal area completely hidden by a thick ice sheet. Researchers believe this area harbors evidence of a 590-million-year-old mountain belt hiding beneath the Antarctic ice.

“Although it is not currently possible to sample directly beneath this ice sheet, it is conceivable that millions of years of ice transport have eroded the bedrock beneath it and transported this load of garnet northwestward into the Antarctic-Australian conjugate,” says Associate. Professor Glory.

“The garnet deposits were then stored locally in glacial 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 convergence.”

The new approach to lutetium-hafnium dating developed by the University of Adelaide, using a laser system coupled to a mass spectrometer, made this momentous discovery possible from a simple survey.

“This journey started with the question of why there was so much garnet on the beach at Petrel Cove,” said Dr. Jacob Mulder, who was also part of the research team.

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