2023-11-16 16:13:59
Inscribed in the rocks of the Grand Canyon of Colorado, the Great Unconformity is witness to a major event in the history of the Earth. Its end would also be marked by an impressive episode of flooding. A new study sheds light on the possible causes of this event during which North America and also other continents would have found themselves largely underwater.
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In geology, the study of the succession of different rock units which have been superimposed over time provides valuable information on the environmental conditions of the past. The analysis of successions of sedimentary profiles thus makes it possible to reconstruct variations in ocean levels for example, but also the oxygenation conditions of the aquatic environment or even temperature. However, it happens that the succession of strata is not continuous: instead of having a gradual evolution of ages, we find ourselves with two units certainly in contact, but whose ages impose a hiatus of time, as if a 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 which punctuate the history of the Earth. Understanding the origin of discrepancies 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”
Among the most emblematic unconformities, there is the one observed in the Grand Canyon of Colorado and which is aptly called “the Great unconformity”. Sedimentary rocks from the beginning of the Cambrian (around 525 million years old) thus rest on much older units. This temporal hiatus can reach in places up to more than 1.2 billion years! The event at the origin of this “erasure” of an entire section of the Earth’s geological history is therefore major, especially since this discontinuity is observed in numerous sites on the American continent.
Origin of the Great Unconformity
Even if the origin of this Great Unconformity is still subject to debate, it must be remembered that the end of the Proterozoic (around 700 million years ago) was marked by a rapid and intense cooling of the Earth’s climate (Earth ball episode). of snow). This massive glazing of the continents would thus 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 sometimes corresponding to more than a billion years of sedimentary deposit would have been abraded during this period.
North America under water
The multiple studies that have looked at the Great Unconformity show that although this event did not necessarily begin synchronously in the different American states where it is recorded, it nevertheless always ends with a major flooding event. 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, the cause of this unique episode of flooding in the geological history of the North American continent is linked to a global rise in ocean levels, linked to plate tectonics. The study, published in the magazine PNAS, explains that at this time, the production of oceanic crust is significant. However, a young crust is hot, and the ocean floor is therefore relatively “high” compared to the continental crust. Additionally, the researchers show that 510 million years ago, when the Great Unconformity ended, numerous subduction zones surrounded the newly formed supercontinent called Gondwana. By diving into the mantle, the oceanic crust tends to “suck in” the continental crust under which it sinks. However, the combined effect of the uplift of the ocean floor and the sinking of the Gondwana continental mass would have resulted in the global ocean level rising even further. Gondwana as well as the lands of North America which were then part of another supercontinent, Laurasia, then found themselves largely underwater! A situation that could have lasted a few million years.
A new scenario which finds resonance in the evolutionary history of terrestrial life, which is experiencing a major turning point at this same moment. The existence of numerous shallow seas could indeed have represented favorable conditions for the development of more complex organisms.
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