Australia's mysterious pink sand originated in the mountains of Antarctica: new research

Australia's mysterious pink sand originated in the mountains of Antarctica: new research

In parts of South Australia, long stretches of beach are often covered with large pieces of pink sand. Strong waves can wash grains of red garnet onto the shore – but the origin of these colourful crystals has remained a mystery until now.

Garnet is rare in beach sand, because it is eroded by long-term exposure to ocean waves and currents. If we find large quantities of garnet in beach sand, it means there must be a local source of garnet-containing rock. But where is this rock?

The search for the source of South Australia's pink sand took us thousands of kilometres and half a billion years back, to a previously undiscovered mountain range that we believe is now buried deep beneath the Antarctic ice sheet. New Study has been published in communication earth and environment,

Any local sources?

Geologists get excited when we find garnets in beach sand or other sediments, because these minerals develop deep below the Earth's surface, under the same conditions in which diamonds form.

One way for diamonds or garnets to reach the surface is through carrot-shaped volcanic structures called kimberlite pipes. Kimberlite (and diamonds) can be found In South Australia – For example, in Eurelia. However, these deposits are far from the coast, not very abundant, and are only 170-190 million years old – so they are unlikely to be the source of our beach garnets.

Another way for garnets to reach the surface is through long-term erosion.

Garnets usually form in greater quantities in places where the crust is thicker, such as beneath mountains. As mountains erode, garnets can be exposed as a record of former mountain belts.

So another possible origin of the garnets found on the beach is the erosion of the Adelaide Fold Belt. This mountain belt, which extends northward for hundreds of kilometres from Adelaide, developed between 514 million and 490 million years ago.

The third possible source is the Gawler Craton, a huge slab of ancient rock beneath South Australia, which has outcrops in the Adelaide Fold Belt. The Gawler Craton contains a lot of garnet, formed in multiple events between 3.3 billion and 1.4 billion years ago.

To find the source of our beach sand garnets, we started by finding out their age. Very old garnets may be from the Gawler Craton, while younger garnets are more likely to be from the Adelaide Fold Belt.

Time mismatch

We analyzed several hundred grains of coastal garnet and found that most of them were formed about 590 million years ago. Instead of answering our questions, this result raised even more questions.

The sand garnets found on the beach were too young to have come from the Gawler Craton, but too old to have come from the eroding Adelaide Fold Belt. In fact, about 590 million years ago is considered a tectonically quiet period in the region, where we would not expect garnets to grow.

Our dating results completely ruled out a local source of the garnet. So what's left?

Long-distance travellers

If the garnets did not come from a local source, we can say two things about them. First, they must have traveled in a way that would not have shattered them. Second, they must have been stored in a locally protected environment before arriving on the beaches.

A potential solution that meets both of these criteria can be found at Hallett Cove Conservation Park, located on the South Australian coast about 20 kilometres south of Adelaide.

Here we find sedimentary rocks that were formed about 280 million years ago, during a very icy phase of Earth's history. Ice is important, because glaciers and icebergs can transport large quantities of rocks over long distances without damaging the rocks' internal structure.

In addition, garnets found in glacial sediments on Kangaroo Island, which were deposited at about the same time as the Hallett Cove sediments, have also been dated to about 590 million years. The garnets were not born in these deposits, but were carried into them by ice flows.

A former land bridge

So, if beach garnets were deposited in sedimentary glacial deposits along the South Australian coast, dating from the end of the Palaeozoic Ice Age, before washing ashore, where did they originally come from?

About 280 million years ago, during the Late Paleozoic Ice Age, Australia was connected to Antarctica by a huge landmass called Gondwana, which was covered by a vast ice sheet.

Reconstructions of ice flow at this time suggest that glaciers may have brought ice northwest from the present-day Transantarctic Mountains in East Antarctica.

The Transantarctic Mountains are the expression of an older mountain belt, the Ross Orogen, which began to develop about 550 million years ago but was not experiencing any peak garnet-forming conditions until about 520 million years ago – 60 million years after the garnets in the pink sands. So we are warming, but the Transantarctic Mountains are not a suitable source either.

A hidden treasure

There is a bulge of rock in eastern Antarctica where garnets of the right age have been found, Skelton Glacier in southern Victoria LandHowever, such a small rock could not have produced such large quantities of garnet as we see on the Australian coasts.

This rock is located on the edge of a vast area of ​​about 2 million square kilometers that is buried under a thick ice sheet. We estimate that this area contains abundant garnet that originated in an unknown mountain region about 590 million years ago.

It is not currently possible to sample the rock beneath this ice sheet to confirm our theory. But it is conceivable that millions of years of ice transport eroded the bedrock underneath, and carried the rock above ground – including the garnet – northeastwards towards the region that is now divided by the coastlines of Antarctica and Australia.

The transported rock was brought to the South Australian coast about 280 million years ago and stored in sedimentary deposits such as Hallett Cove. Here it lay undisturbed until erosion caused the garnets to fall into the sea – and then, eventually, onto the beaches of South Australia.

This article is republished from The Conversation under a Creative Commons license. Original article,

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