Great Unconformity: How Earth Lost a Billion Years
Researchers used models of ancient supercontinent formation and breakup to explain how the Great Unconformity formed. The geological gap, found around the world, separates Cambrian sandstone from much older basement rock and may reflect land uplifted by mantle movement before erosion wore it down.
Researchers Santosh Dhungana and Nicolas Flament say the Great Unconformity formed as ancient supercontinents broke apart, offering an explanation for why earth lost a billion years or more of its geological record. Their models point to mantle movement that pushed land upward before erosion wore it down.
The Great Unconformity is a vast gap in Earth’s rock record found around the world. Cambrian sandstone dating to about 538 million years ago can lie directly on basement rock between 1.2 billion and 1.8 billion years old.
The missing strata suggest that a major process erased rock layers across broad areas, though scientists have debated the cause. Dhungana and Flament, researchers at the University of Wollongong, said several explanations have been proposed.
“The cause or causes of the Great Unconformity are unknown,” they said. Proposed mechanisms include glacial erosion during Snowball Earth, rifting, plume activity, falling sea levels and mantle-driven changes in land elevation.
The researchers tested the idea by modeling the formation and breakup of ancient supercontinents in the eons before the Cambrian period. Their models showed that mantle motion pushed massive landmasses upward.
Once exposed, the elevated land faced weathering and erosion from glaciers and, later, oceans. Over time, those forces stripped away rock and created the gap now visible in the geological record.
The team said its models also explain why the Great Unconformity is best preserved in North America. They described it as a surface sign of deep processes that reshaped landscapes and the movement of sediment around the globe.
“Our models explain why the Great Unconformity is best preserved in North America and show that it is a first-order surface expression of deep geodynamic processes that transformed global landscapes and sediment fluxes,” Dhungana and Flament said.
The researchers said earlier proposals had not been dynamically tested against global models and datasets to explain where the unconformity appears. Their work connects mantle processes to the distribution of the geological gap, while acknowledging that scientists continue to debate its cause.
The Great Unconformity formed before the Cambrian, when complex life rapidly proliferated in Earth’s seas. The researchers’ account links the processes that erased the rock layers to changes in the planet’s landscapes and sediment flows.
