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Tasmania’s Oldest Rocks Reveal North American Link

Ancient minerals recovered from Tasmania have revealed that western North America was once our close geographical neighbour.

In research published in Precambrian Research, Dr Jacqueline Halpin and Dr Peter McGoldrick of The University of Tasmania found that tiny monazite and zircon minerals in sedimentary rocks in north-west Tasmania were deposited in an ancient ocean between 1.45 and 1.33 billion years ago, making them the oldest rocks in Tasmania.

Halpin said the patterns of ages in the Rocky Cape Group strongly resemble the “Belt-Purcell Supergroup” rocks in Idaho and southern British Columbia, providing strong evidence that the Rocky Cape and the Belt-Purcell rocks were geographically close 1.4 billion years ago.

“At this time, both Tasmania and North America were part of a supercontinent called Nuna,” Halpin said. “As plate tectonics and the supercontinent cycle started to rift Nuna apart, a large sedimentary basin formed that included the Rocky Cape Group and Belt-Purcell Supergroup rocks.”

The continued breakup of Nuna eventually dispersed parts of this ancient sedimentary basin to opposite sides of the Earth.

McGoldrick said the new mineral dates also provide an age constraint for the Horodyskia (“string of beads”) fossils recently discovered in the Rocky Cape Group. These fossils have also previously been found in the Belt-Purcell rocks.

“Fossils visible to the naked eye are exceedingly rare from rocks older than 635 million years,” McGoldrick said. “Horodyskia from the Rocky Cape Group and the Belt-Purcell Basin are nearly twice this age.”

From an evolutionary viewpoint, Horodyskia are exceedingly important. “Unlike stromatolites, which are formed by communities of simple, single-celled organisms, Horodyskia may represent the oldest known ‘tissue-grade’ multicellular organism,” he said.