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Great Unconformity

Geological history: news of the ‘Big Gap’ – Rodinia was responsible for the disappearance of deposits from a billion years of geological history

Disappearing Layers: Geologists have discovered more about the cause of the mysterious “Great Misalignment” – a gap of about a billion years in rock layers in the Grand Canyon and other formations around the world. Accordingly, the minerals on both sides of this gap show that the lost layers did not disappear at once. Instead, various regional operations were responsible for the removal. This strengthens the hypothesis, which states that the fracture of the Orcontinental Rodinia was the impetus for the Great Void.

Charles Darwin wondered why there was such a large gap in chronologically successive rock layers in some places: a lower piece of sediment about 540 million years old from the Cambrian period, there is a sudden change in rock layers over a billion years old. This “great mismatch” is especially evident in the Grand Canyon.

The Grand Canyon’s Great Misalignment Model: the strata sloping from the end of the Precambrian abruptly and covered by a much smaller upright stratum. © Miguel Hermoso Cuesta / CC-by-sa 3.0.0 Update

From analyzes of the rock layers on either side of this large gap, geologists have long concluded that the sediments in between were not initially lost, but were later removed by an extensive erosion phase. However, it has remained controversial as to whether this erosion was caused by tectonic processes such as the formation or break-up of the supercontinent Rodina, or whether the global glaciation was roughly during “earth snowball“- A good 700 million years ago, the rock layers were eroded.

Find guides in the Grand Canyon

Analyzes of the Grand Canyon’s “great mismatch” now provide new indicators. For their study, Barra Beck of the University of Colorado at Boulder and her colleagues used thermochronology to examine rock samples from gap edges from different sections of the Grand Canyon. In this case, the helium content in the zirconium crystals reveals whether the metal was heated more strongly at any point in its history.

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“At temperatures above 220 degrees, helium exits the zirconium crystal and completely diffuses,” Buck and her team explain. “At lower temperatures, on the other hand, the helium formed from the decomposition of uranium remains trapped in the crystal.” In principle, this method can be used to determine when and whether a mineral has reached the surface far enough to remain cold.

There is no standardized process

The analyzes revealed something surprising: Contrary to a previous assumption, the Grand Canyon’s large gap is not due to a uniform event or phase. Instead, the layers in the western and eastern fringes have been removed by various processes, although they are only about 10 kilometers apart. “Basic rocks are not a uniform mass with the same temperature history,” Beck says.

Instead, the rock was under the western part of the Great Gap on the Earth’s surface 700 million years ago – apparently at the time erosion was at work there. The layer of the same age at the eastern end of the Grand Canyon was at this point under a few kilometers of sediment. There, the erosion of the lost sediment must have occurred at a different point in time.

Evidence for the Rodinia hypothesis

“Our results indicate a multi-stage fault and erosion in the Grand Canyon, which over a long period of time contributed to the development of the Great Nonconformity,” Beck and her team report. In their opinion, there were many erosion events caused by small-scale tectonic processes. This pattern is likely due to tectonic causes and regional clarity different from the global primordial ice age.

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This confirms the new analysis ConclusionsThe team pulled it off in 2019 after investigating the Great Nonconformity in the Colorado Front Range east of the Rocky Mountains. Because there, too, the gap dates were more in line with the decay of Rodinia than with the glacial phase of Snowball Land.

The new findings now confirm suspicions expressed at the time that the ‘large hiatus’, despite its wide geographic distribution, is likely to be due to numerous turbulences and limited regional elevations. True, they were all most likely related to the disintegration of the ancient continent of Rodinia, and therefore moved roughly in one region in time. However, it is assumed that the catalysts and tectonic processes were locally different. (Geology, 2021; doi: 10.1130/G49116.1)

Coyle: University of Colorado at Boulder