Intro to Geologic History of the Colorado Plateau
The geologic history of the Colorado Plateau can be subdivided into six broad periods ranging from Precambrian to Recent. Period 1 includes the whole of the Precambrian. Rocks of Period 1 are exposed only in the cores of some of the uplifts which flank the Colorado Plateau, in the Basin and Ranges to the south, or as isolated exposures along the deeply entrenched Grand Canyon of the Colorado River through the National Park. Two broad ages of Precambrian rocks are recognized in this area. An older highly metamorphized series of schist and gneiss is exposed in the cores of some of the marginal uplifts and along the deep V-shaped inner gorge of Grand Canyon. Younger Precambrian rocks blanket the older sequence and are only locally preserved, in areas that have not been subjected to significant uplift prior to deposition of Cambrian rocks. Upper Precambrian rocks are not preserved in uplifts where the young Precambrian and Paleozoic rocks were stripped away during late Paleozoic and Mesozoic erosion.
Period 2 extends from Cambrian through Mississippian time and is characterized over much of the Colorado Plateau by moderately thin sheetlike deposits of marine rocks. These thin rock units extend eastward from the miogeosyncline onto the western margin of the core of North America. Cambrian and Mississippian rocks occur extensively in the subsurface across the southern part of the Colorado Plateau, although they are only locally exposed where erosion has cut deeply through the Mesozoic cover. Cambrian and Mississippian rocks are well exposed in the Grand Canyon where the Redwall Limestone and the Tapeats Sandstone form prominent cliffs in the lower part of the canyon wall. Rocks of Period 2 are missing over many of the broad uplifts of the Colorado Plateau because these same uplifts functioned as sources for sediments during the later part of Paleozoic and early Mesozoic time.
Periods 2 and 3 are separated by the Colorado Mountain disturbance during late Paleozoic time. During Period 3 geology of the region was dominated by a series of northwest-trending basins and uplifts that affected much of the eastern rim and central part of the Colorado Plateau. For example, the broad Kaibab-Defiance-Zuni Uplift, in northeastern Arizona and west-central New Mexico, supplied erosional debris to the Paradox Basin that covered much of the immediate Four Corners area, northwestern New Mexico, and southwesternmost Colorado. The Uncompaghre Uplift, Emery Uplift, and Pedernal Uplift also shed debris into the sinking troughs that surrounded them. Much of the brightly colored shale, siltstone, and sandstone of the Colorado Plateaus is a result of accumulation of debris swept from the uplifts into the broad basins. These iron-bearing sediments generally are a bright red or green and have helped produce much of the distinctive scenery of the southern and eastern part of the Plateau. Initial sedimentary sequences are relatively coarse near the uplifts but the rocks become finer and finer grained in upper parts of the section where the ashy-appearing Chinle sequence buried or lapped high around the flanks of the old Colorado Mountain uplifts.
Period 4 extends from the Jurassic through the Cretaceous and was a time during which sediments were derived, in large part, first from the continent but then from major uplifts in the west. During Period 4 sediments were deposited in broad elongate marine troughs that extended roughly north-south across the present Colorado Plateau. This was also a period during which sand, from windblown deserts, blanketed much of the plateau province and resulted in accumulation of the several thick, cross-bedded, sandstones that are distinctive of the Colorado Plateau. Two major orogenies occurred west of the Colorado Plateau Province during Jurassic and Cretaceous time. The folds only indirectly affected the plateau area, however. The Nevadian orogeny, in the late Jurassic and earliest Cretaceous time, and the Sevier orogeny, in later Cretaceous time, supplied extensive quantities of elastic debris to the geosynclinal troughs. This resulted in accumulation of thick interfingering nonmarine and marine sequences over most of the plateau. The thick grey Mancos Shale, which is extensively exposed around the San Juan and Black Mesa Basins, is overlain by a series of eastward advancing sandy deltaic deposits. It is with these deltas that much of the coal of the plateau is associated.
The early Tertiary Laramide orogeny or disturbance separated Periods 4 and 5. It was during the Laramide event that many of the folds and minor faults of the Colorado Plateau were produced. Uplift of the Grand Canyon section, refolding of the Zuni Uplift, the Defiance Uplift, and the Monument Valley Upwarp produced high plateaus but also isolated broad basins, like the Black Mesa Basin and the San Juan Basin, which continued to subside and receive sediments. The sediments were derived, in large part, from erosion of the uplifted Laramide blocks. The striking monoclines of the district are associated with this particular period of folding, which has affected all but the youngest rocks of the plateau.
Period 5 extends from the middle Tertiary nearly until modern times and is dominated by volcanic activity around the periphery of the plateau. Volcanic activity of the San Francisco Peak field, Mt. Taylor and the Datil section, the San Juan Mountains, the Valles Caldera near Los Alamos, and the numerous small intrusions of the Navajo and Hopi Buttes fields are part of this phase of geologic history. Young lava flows in the Sunset Crater area and near Grants, New Mexico are typical of basaltic activity of the late phase of volcanic development of the region. Basin-and-Range-type block faulting has not materially affected the Colorado Plateau in most areas covered by the southern guidebook but such faulting has modified the eastern margin of the plateau across the guide route along the Rio Grande Depression. Here a major graben developed in late Tertiary and Quaternary time. The westernmost limit of the Grand Canyon section, beyond the guidebook routes, also has been affected by eastern block faults of the Basin and Range. The southern part of the plateau grades into the Sonoran section of Arizona and New Mexico where block faulting has also interrupted all but the youngest sediments and volcanic rocks in late Phase 5 development.
GEOLOGICAL TIME SCALE
Figure 1.5. Geologic lime scale, showing Periods 1 through 6 into which the geologic history of the southern Colorado Plateau can be conveniently subdivided. These six periods are informal subdivisions, whereas the named units are widely recognized formal subdivisions.
Broad uplift of the Colorado Plateau during the Laramide disturbance and later Tertiary regional uplift elevated rocks which were deposited in the sea or near sea level up to 10,000 to 15,000 feet above sea level. It was during and following these periods of uplift that major stream patterns became established and during which the scenery of the Colorado Plateau began to develop into what we know it now.
Phase 6 is the latest phase of history of the plateau and is characterized by erosion accompanying and postdating the prominent basin and range faulting and the uplift of the western part of North America. High peaks of the plateau were affected by Pleistocene glaciation but much of the lower part of the plateau has experienced only erosion following uplift of the region during middle and late Tertiary time.
Southern Colorado Plateau is an area rich in the early history of man in western North America and many ruins within the guidebook area have been brought under protection of the National Park Service. Mesa Verde National Park, Wupatki National Monument, the several associated ruins of Navajo National Monument, Canyon DeChelly and Chaco Canyon National Monument are but representatives of some of the archaeological riches of the plateau. Minor modifications of the countryside have been produced by man occupying the area. Mineral exploration, agricultural efforts and community development mark the final phase of modification of the plateau. Man’s activities appear relatively minor on the regional scale but on a local scale may have profound significance. Construction of dams along the Colorado River and other major streams of the province have modified the courses of the rivers. Extensive mining and attendant accumulation of waste, such as the coal mines of the Four Corners area or the waste piles resulting from uranium development near Grants, New Mexico, have also left an impression on the surface which locally may be significant. In general, however, the arid climate and the moderately high elevation has left much of the southern Colorado Plateau relatively untrampled by man.