Cavern Collapse Begets Beautiful Breccia
Combination of fractures and stratigraphy define the large-scale architecture of most caverns
This beautiful mosaic in stone decorates a shady wall of Paleozoic rock in Fall Canyon, Death Valley National Park.This is not a type of fracture, but rather a breccia, which is a textural term.A breccia is a rock composed of coarse, angular, rock fragments.That certainly describes this rock.Breccia forms in a variety of settings in consequence of distinctly different geologic processes, all of which involve breaking pieces from an antecedent rock to form the breccia clasts. Moreover, in the case of this photo a fracture system likely forms the precursor for the development of this deposit.
The breccia in this photograph is due to roof collapse in a limestone cavern, which was subsequently infilled and its clasts were cemented with calcite.It is a solution-collapse breccia.The clasts are monomictic, which is typical of cave breccia. The strata within this paleocavern dip to the left (west) along with the bedding of the host strata, therefore the cave formed and its roof collapsed prior to the tilting of the rock, which probably dates to the Cretaceous.Therefore this is a paleocavern system.The vertical, brown streaks that cross the outcrop are recent.
Fractures play a larger role in the development of this breccia.The large-scale geometry of most cavern systems is determined by dissolution along fractures in susceptible stratigraphic layers.Cave systems are often mapped with great care by spelunkers, and are generally the closest we have to true 3D fracture system models.The fracture system that acted as the nucleus of the breccia-filled paleocavern in the photo is not evident, but considering the presence of breccia here and in other nearby canyons the cavern system in this area was likely extensive.
Breccia forms in other settings.Tectonic breccia forms adjacent to some faults, wherein the clasts may be polymictic and may show structures useful in the interpretation of the fault kinematics.Depositional breccia often forms proximal to collapsing slopes that are oversteepened due to erosional, biogenic, or tectonic activity.Breccia also forms in volcanic settings due to explosive activity or the collapse of internal or external interfaces.