Off the coast of Australia, scientists have found, 60 meters below the surface, an impressive structure similar to an underwater dune. Extraordinarily well preserved, these mounds of sand have benefited from a particular composition that allowed them to solidify.
At first glance, the Ice Age is more likely to be associated with vast frozen areas than with waves washing up peacefully on the southern coast… And yet, it is well into the ice age that would have formed — off the cost of Australia — amazing sand dunes, geological structures now swamped by the waters, which are now being studied by researchers, as revealed by their forthcoming study in the Australian Journal of Earth Sciences.
The discovery of the structure is a happy coincidence. The scientists were originally probing Australian waters as part of a mission to explore the Great Barrier Reef, when their sonar revealed underwater structures to say the least enigmatic. Other works then led to revealing their origins and precise nature.
Located 60 meters below the surface, and almost 15 meters high, these strange structures are indeed dunes stretching for nearly 70 kilometers; from Fraser Island — near the east coast of Queensland — to Moreton, located in Moreton Bay, off Brisbane. But besides their importance, it is above all the mineralogical characteristics of these submarine dunes that aroused the most the researchers’ interest. “I think the most curious aspect is the mechanism that allowed them to be so well preserved,” says one of the members of the research team, Tiago Passos, of researcher at the University of Sydney.
But the key to the mystery lies in their very particular mineralogical composition. Instead of the quartz sand dunes usually contain, they contain calcium carbonate. A mineral that is at the origin of their extraordinary state of conservation.
The calcareous substance has indeed allowed the dunes to literally freeze under the effect of a process called diagenesis, a geological phenomenon which designates the solidification of a sedimentary deposit. A typical example of comparable formation is that of sandstone, a sedimentary rock which is none other than indurated sand. And this hardening results from a well-known reaction known as cementation.
Like those of a sandstone, the sand grains of the Australian dunes would thus have joined one another after the famous calcium carbonate formed a real cement in the interstices that separate them. A true wall at the microscopic scale, in which the bricks would be replaced by grains of sand, and the mortar joints by calcium carbonate cement.
This extraordinary geological treasure discovered could thus shed new light on the way in which such structures form in the open air.