The Great Unconformity, carved into the rocks of Colorado’s Grand Canyon, bears witness to a major event in Earth’s history. Its end will also be marked by an impressive flood event. A new study sheds light on the possible causes of this event during which North America and other continents may have found themselves largely underwater.
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In geology, the study of the sequence of different rock units superimposed over time provides valuable information on past environmental conditions. Thus the analysis of sequences of sedimentary profiles makes it possible to reconstruct for example variations in sea level, but also the oxygenation conditions of the aquatic environment or even the temperature. However, it happens that the sequence of layers is not continuous: instead of having a sequential evolution of ages, we find ourselves with two units that are certainly in contact, but whose ages impose a time interval, such as One or several units in between. These types of structures are called “discontinuities” in geology. They represent a break in the sedimentary record and mark the occurrence of particular and sometimes major events that punctuate Earth’s history. Understanding the origins of anomalies is therefore particularly important to understand the evolution of a place or even a continent.
More than a billion years of Earth’s geological history literally “erased”
One of the most emblematic unconformities is observed in the Grand Canyon of Colorado and is aptly called the “Great Unconformity”. Sedimentary rocks of the beginning of the Cambrian (about 525 million years old) thus rest on much older units. This time interval can reach more than 1.2 billion years! The event at the origin of this “erasing” of an entire segment of the Earth’s geological history is therefore prominent, especially since this imbalance has been observed in many places on the American continent.
Origin of the Great Discontinuity
Even though the origin of this great discontinuity is still a matter of debate, it should be remembered that the end of the Proterozoic (about 700 million years ago) was marked by a rapid and intense cooling of the Earth’s climate (the Earth Ball episode). Ice). This large-scale glazing of the continents would have lowered the global level of the oceans, leading to increased erosion of continental surfaces through the formation and movement of glaciers. A recent study suggests that several kilometers of rocks may have been destroyed sometime during this period, corresponding to more than a billion years of sedimentary deposition.
North America underwater
Several studies looking at the Great Unconformity show that although the event does not necessarily begin simultaneously in the different US states where it has been recorded, it always ends with a major flood event. Is. Even in the middle of the American continent, the end of the Great Unconformity is marked by the deposition of marine sediments. For some researchers, this unique flood event in the geological history of the North American continent is linked to a global rise in sea levels associated with plate tectonics. Study, published in journal PNAS, suggests that oceanic crust production is important at this time. However, a new layer is warmer, and so the ocean floor is relatively “higher” than the continental crust. Additionally, the researchers point out that 510 million years ago, when the Great Unconformity ended, several subduction zones surrounded the newly formed supercontinent called Gondwana. By diving into the mantle, oceanic crust begins to “suck” the continental crust beneath which it sinks. However, the combined effect of sea level rise and sinking of the Gondwana continental mass would have resulted in global ocean levels rising even further. The lands of Gondwana, as well as North America, which at that time were part of another continent, Laurasia, found themselves largely under water! A situation that could have lasted for a few million years.
A new scenario that echoes the evolutionary history of terrestrial life, which is experiencing a major turning point at this very moment. The existence of numerous shallow seas may actually represent favorable conditions for the evolution of more complex organisms.