CONTINENTAL SEDIMENTS OF THE COLORADO PLATEAU

Considering the geologic record as a whole, continental or non-marine sediments are far less common-than marine sediments. This probably does not mean that continental sediments have not always been abundantly produced, but rather, that they have not always been preserved. The critical prerequisite for continental rock is, of course, not the origin but rather the final site of deposition. Continental sediments reveal much regarding ancient land life, geography, and climate and have always attracted much attention wherever found. The sedimentary section of the Colorado Plateau as typified in the Four Corners area, is remarkable for the variety and volume of its continental rocks. The abundance of these sediments appears to be explainable on the basis of the coincidence of a number of geographic, climatological, and tectonic factors during a relatively long span of geologic time. Early Paleozoic: During the early Paleozoic, up to and including Early Mississippian time, the site of the present Colorado Plateau received only thin marine deposits from occasional temporary oceanic transgressions from the west. During periods of emergence it was subject to very little erosion probably because of the low gradients of the streams. During this time there were minor disturbances within the geosyncline to the west and even a major orogeni c movement in central Nevada. These events do not seem to have affected the Colorado Plateau. With passage of time a number of important structures came into' being in the interior, and the site of the Colorado Plateau became successively more isolated, more distant from the ocean and also subjected to greater climatic variations. The various tectonic features which had major influence upon sedimentation may be briefly noted as follows: The Transcontinental Arch: The broad, very long and poorly defined positive area which extends diagonally across the United States has been called the Transcontinental arch. Its history and sedimentary effects in the Southwest are not fully known, but it did act as an effective barrier between the western miogeosyncline and the interior seas during early Paleozoic time. The present Four-Corners lies near the axis of this uplift and its presence explains the thinness of Paleozoic rocks in the vicinity. During later time it was a low but effective barrier which prevented southward movement of sediment out of the Colorado Plateau. It also appears to have been near the southern edge of several Mesozoic marine invasions. Ancestral Rockies: During a relatively short interval of the late Pennsylvanian, a large scale disturbance generally known as the Ancestral Rockies orogeny created a great system of folds and basins in and adjacent to the Colorado Plateau. Most extensive of the uplifts were the Front Range element and the Uncompahgre element. These were elevated several thousand feet and broke directly across the trend of the Transcontinental arch where the previous sedimentary section was only a few hundred feet thick. This, in effect, created distinct drainage divides, a curving elevated barrier around the east and northeast side of the Colorado Plateau area and a source of abundant clastic detritus. This element was destined to affect profoundly all subsequent geologic development in the western interior. Mesocordilleran Geanticline: A very significant tectonic event was the creation of the Mesocordilleran geanticline during or near Middle Triassic time. Previous to this episode the western ocean had repeatedly invaded what is now the interior of the continent. After this time no further invasions of the sea from the west reached Utah. For a relatively short interval, between the time of the appearance of the Mesocordilleran geanticline and the rise of the Sierra Nevada, the western margin of the continent was in central Nevada. The Mesocordilleran geanticline extended northward into Canada and southward for an unknown distance in the Basin-Range province. It remained in existence and had profound effects from Late Triassic to early Tertiary time. The eastern margin of this important element evidently coincided with the present Wasatch line through northern and central Utah but curved southeastward and eastward through Arizona and New Mexico to coincide with the present elevated Mogollon Rim in that area. The complete collapse and breakdown of the Mesocordilleran geanticline makes its reconstruction difficult. Laramide Structures: With the beginning of Tertiary time, a very strong orogeny affected the zone of the Wasatch line and other margins of the Colorado Plateau. The Wasatch Mountains came into prominence and the Colorado Plateau and Great Basin became structurally distinct. The Uinta Range was uplifted to form the northern boundary of the Uinta Basin. These uplifts furnished sediments to the basin and impeded drainage to the ocean.