American West
the Colorado Plateau,
a little of its genesis
The Colorado Plateau covers a lot more than the park circuit that we have used (especially towards the southeast).
In fact, this immense base without much relief (in any case, of an order of magnitude that is not that of the alpine elevations or the neighboring Rocky Mountains) extends over 300 000 km², at an altitude of between 1500 m (towards the Grand Canyon) and 3000 m towards Bryce Canyon for example.
It is formed by a stack of tabular sedimentary rocks ranging from the Cambrian (from -545 My pendant 65 Ma) in the Eocene (from -66 My pendant 22 Ma), and which are either of marine origin or of continental origin (by rivers and wind).
the following table shows how are these cumulative stacks distributed according to the major sites of the Plateau, the oldest concerning the Grand Canyon in particular, and the most recent the Bryce Canyon region.
Without great tectonic deformation since 500 Ma, this plateau has been raised en bloc (the geomorphological term is " uplift ») From about 2000 to 3000 m, from about -30 Ma, with a maximum thrust between -20 and -6 Ma, which still seems to continue.
Before the uplift, we had a low plain crossed by many rivers, including the current Colorado and its tributaries.
With uplift, the sedimentary plain gradually becomes a plateau, which is then exposed to erosion and subjected to its effects.
The rivers dig their beds and form the canyons that we know.
The alternation of layers of hard and soft rocks leaves the cuestas, witness mounds, mesas,… under the work of erosion.
Grand Canyon Stratigraphy Details
Stratigraphy of Zion Park
In Canyonlands in particular, the outermost (outer) sandstone layer is harder and more protective than the lower clay-sandstone layers, which are softer and therefore easier to erode and undermine.
On the edge of the canyons, the layer of sandstone on the surface ends up overhanging the canyon, and in turn crumbles, so that the cliff remains vertical.
The phenomenon is more sensitive at the bottom of the "reculée" (geological term for this type of morphology) than on its flanks, so that the retreat tends to lengthen without widening.
