Geology of the Great Basin

11. The Mesozoic

11 the mesozoic the rock buster: the cordilleran orogeny Throughout the 4.5 billion–year history of the Great Basin there have been many significant geologic events. None, however, was more consequential than the Cordilleran orogeny. This mountain-building episode radically transformed the western one-third of the continent. Great tectonic movements swept in waves across our region from Late Jurassic to the Eocene. So immense were these tectonic movements that geologists originally described them as being a series of orogenies. Current plate tectonic concepts more logically relate the mountain building to one almost continuous period of tectonism. Diastrophism on this scale could only result from the motions of major plates along and under the western margin of North America. It is convenient for clarity of description, however, to divide the tectonism into three distinct phases. The earliest phase, the Nevadan, describes the Middle to Late Jurassic movements. This rock deformation and time of plutonic emplacement largely involved the western Great Basin. By Late Cretaceous time, the Sevier phase was thrusting rocks along the eastern margin of the Great Basin. The orogeny continued through the Cenozoic Eocene, when the Laramide phase was deforming the Rocky Mountain region. North America, from California to Colorado, from Canada to Mexico, was reshaped by this great rock buster—the Cordilleran orogeny. 116 The Mesozoic the quiet before the storm: the triassic The early Paleozoic history of the Great Basin is one of gentle subsidence. In the Devonian, the Antler Mountains were uplifted and soon eroded. Again, 251 million years ago, near the close of the Paleozoic, orogeny uplifted the central Great Basin. This time the uplift was the result of the collision of the microplate Sonomia with the continental margin. The late Paleozoic arc welded firmly to the continent. As Sonomia and the continent ground together, the intervening oceanic floor was completely consumed by subduction . A new subduction zone began, far to the west on the western borderland of Sonomia. This time, easterly subduction slid the ocean floor under the sutured Sonomia. The collision of Sonomia uplifted the continental margin , and it remained a highland in the central Great Basin throughout most or all of Triassic time. Basins formed on either side of the uplift. The history of each of these basins is radically different through early and middle Mesozoic time. The Eastern Basin The eastern side of the upland was flooded by a sea. Fine clastics and limestones were deposited in the warm, shallow waters. Rivers carried muds and other clastics off the continent to the east. Some fine clastics washed down the flank of the western highlands and built broad deltas. Floods washed trees and plant debris into the basin, and their fragments remain today, imprinted in the rocks. In the Late Triassic, the region to the east of the uplift slowly emerged from the sea. Sandstone, conglomerate, and claystones, all containing abundant volcanic detritus, were deposited on the floodplains of this continental area. The windblown volcanics settled to the ground and were transported northerly by streams. The major source of clastics for the eastern basin was the highlands to the east and south in Colorado and New Mexico. These latest Triassic rocks are dominated by red beds. They add an exciting splash of color to the mountains of the southern and eastern Great Basin today. The Western Basin The history of the western basin is strikingly different. The Sonomia microplate had been heated while en route to its continental collision by the upward intrusion of subduction-melted magmas. Volcanoes crowned the higher parts of the microplate. After collision 235 million years ago, the The Mesozoic 117 subduction zone migrated farther to the west, and Sonomia igneous activity quieted down and eventually ceased. As the sutured plate cooled, it contracted and began to subside, forming a deep basin to the west of the central Great Basin highlands. The eastern and southern hinge lines of the sinking region are approximately coincident with the suture zone between the continent and Sonomia. When the microplate was in the early phase of subsidence, 251 to 215 million years ago, the deep-water offshore basin was the site of the fine carbonate and siliceous mud deposition over the late Paleozoic basement. Pebbles of Paleozoic debris were carried down the flanks of the uplift by turbid clouds of sediment. They settled into the depths. To the southwest of the now firmly welded, defunct, and subsided Sonomia...