Question

List from oldest to youngest the six orogenic events that occurred during the development of the Cordillera in Western North America (list oldest to youngest). Provide the name, age, location, important characteristics of each orogeny, and any other details that distinguish the events. See Figure 18-25 in your textbook, which only shows five; add the Ancestral Rocky Mountains in Colorado as the 6th (and put in in the correct order). What caused the "eastward shift" of mountain building in the western US and why is it important?(20 pts) 2)

Answer 1

Answer:

The orogenic events that occurred during the development of Cordillera in Western North America (from oldest to youngest) are Antler Orogeny, formation of Ancestral Rocky Mountains in Colorado associated with Ouachita-Marathon Orogeny, Sonoma Orogeny, Nevadan Orogeny, Sevier Orogeny and Laramide Orogeny.

Antler Orogeny: Antler orogeny began in late Devonian (419 Ma) and spread into Early Pennsylvanian (Upper part of Carboniferous). Great volume of conglomerate beds got deposited during this orogeny. But the most important thing about this orogeny is that, typical orogenic features like metamorphism or granitic intrusion did not occur here. Most of the features are seen in Nevada. In the eastern part, fossiliferous carbonate rocks are dominant which are Ordovician to Devonian age, which indicates shelf domain or basin domain. On the other hand, in the western part, carbonate rocks and fossils are almost absent, indicating deep-water depositional environment.

Formation of Ancestral Rocky Mountains: Ancestral Rocky Mountains were intracratonic block that were uplifted during Pennsylvanian (332 to 298 Ma). This is found in Colorado. The development is associated with collision of North America and South America-Africa, which caused Ouachita-Marathon orogeny.

Sonoma Orogeny: Despite controversy regarding age of Sonoma orogeny, it is believed to occur during Permian-Triassic transition i.e. most commonly known as K-T boundary (250 Ma). It was thought to have caused by closure of the basin between island arc of Sonomia and North American continent. The features are very evidently found in northwest Nevada. Most important sequence of this orogeny is Havallah formation where the dominant rocks are basalt, bedded chert, limestone, conglomerate, argillite. Havallah is underlain by Golconda thrust.

Nevadan Orogeny: Nevadan orogeny occurred during Middle Jurassic to Early Cretaceous (155 to 145 Ma). Subduction of Farallon oceanic plate beneath North American continental plate gave rise to Andean type magmatic arc in early stages of this orogeny. In the latter stages, number of oceanic arcs accreted into cordilleran type accretionary orogen. In the Klamath mountain range and Sierra Nevada, this deformation related to these accretionary arcs is found. The most interesting feature of Nevadan orogeny is that unlike other orogneies, which lasted for around 100 Ma, Nevadan orogeny lasted for only 10 Ma.

Sevier Orogeny: Sevier orogeny was the result of convergence between Farallon oceanic plate and North American continental plate in the time period between 140 Ma and 50 Ma, i.e. starting in Early Cretaceous and ending in Middle Paleogene. Sevier river region of Central Utah is the area from which the name is derived. Mountain building was caused due to crustal thickening, which was the result of combination of compressional forces and conductive heating. Eastern edge of Sevier is recognized by presence of conglomerates. The Sevier thrust belt is seen in north and south of Salt Lake City. The northern part has oil and gas formations. In Utah and Wyoming, recesses and salients are seen which are produced by Sevier orogeny. Transverse zones and sandstone arches are also found.

Laramide Orogeny: Laramide orogeny was the last orogenic event which began in late Cretaceous (70 to 80 Ma) and ended in late Eocene (35 Ma). The distinctive feature was deep-seated and thick-skinned deformation. Evidences are found from diverse range of places, from Canada to northern Mexico, extending up to Black Hills of South Dakota. Intermontane structural basins and adjacent mountain blocks were formed as a result of deformation. The core of Rocky mountains was formed during Laramide orogeny.

Volcanism in the Colorado was thought to be the eastward shift of volcanism and mountains prior to 75 Ma.