What is it about?

Researchers found that across the scale of North America, about 3–5 km (2–3 miles) of erosion coincided with Snowball Earth glaciations at about 700–650 million years ago when the planet is believed to have been nearly covered in massive ice sheets. This suggests the Great Unconformity—which is an erosion surface that represents a mysterious period of “missing time” of up to a billion years or more (i.e., the rocks are gone!)—was primarily formed during the time of these glacial ice sheets.

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Why is it important?

The Great Unconformity involves a common gap of hundreds of millions to billions of years in the geologic record. The cause of this missing time has long eluded explanation, but recently two opposing hypotheses claim either a glacial or a plate tectonic origin in the Neoproterozoic. We provide thermochronologic evidence of rock cooling and multiple kilometers of exhumation in the Cryogenian Period in support of a glacial origin for erosion contributing to the composite basement nonconformity found across the North American interior. The broad synchronicity of this cooling signal at the continental scale can only be readily explained by glacial denudation.


The fact that so many places are missing the sedimentary rocks from this time period has been one of the most puzzling features of the rock record. Glaciation is the simplest explanation for erosion across a vast area during the Snowball Earth period since ice sheets were believed to cover most of North America at that time and can be efficient excavators of rock. The Great Unconformity sets the stage for the Cambrian explosion of life, which has always been puzzling since it is so abrupt in the fossil record—geological and evolutionary processes are usually gradual.

Kalin McDannell
Dartmouth College

Read the Original

This page is a summary of: Thermochronologic constraints on the origin of the Great Unconformity, Proceedings of the National Academy of Sciences, January 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2118682119.
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