OCR Text |
Show River and its large tributaries, the Duchesne, Uinta, Yampa, and White Rivers. From the Wasatch Mountains the basin extends eastward for about 170 miles to the White River Plateau, as a border belt for the Uinta Range and the Axial Basin anticline. Southward from the base of Uinta uplift it extends to the crest of the Book and Roan Cliffs 40 to 120 miles distant. Within this large area a few remnants of an ancient erosion surface are exposed, but generally the older rocks lie beneath thick deposits of Tertiary age exposed to view only in the deep canyons. These Tertiary rocks, 500 to 12,000 feet thick, floor not only the Uinta Basin but also the lands east and north of the Uinta Mountains. ( Plate 13, Section 3, in pocket.) The Uinta Basin is both a topographic and structural depression. In generalized north- south profile across its central part, the surface descends from 7,000 feet at the base of the Uintas to about 5,000 feet ( along Green River, 4,000 feet), then ascends to altitudes of 8,000 to 9,000 feet. This topographic expression is largely in accord with the attitude of the underlying sedimentary rocks. In structure the basin is a long syncline; from its axis northward, the strata bend upward to the base of the Uintas and southward up the back slopes of the Tavaputs Plateaus. The Uinta Basin is a profitable agricultural section, a potential oil field, and the site of the largest and oldest Indian reservation in Utah. It is a famous source of fossils, and derives additional geologic interest from its scattered outcrop of solid hydrocarbons, which include rare types, some of them unique and little understood- uintaite, native asphalt, ozocerite ( mineral wax), wurtzilite, tabbyite, and bituminous sandstone. Especially peculiar are the deposits of commercially valuable gilsonite, which on both sides of the Green River fill deep vertical cracks 1 to 8 feet wide and 3 to 8 miles long, and appear at the surfaces as conspicuous black streaks. COLORADO PLATEAU PROVINCE Outline of geologic history.- The geologic history of the Colorado Plateau Province, now in vigorous process of destruction, is recorded in the composition, the sequence of deposition, fossil content, and geographic position of its constitutent rocks. In broad outline it is the story of enormous masses of sedimentary rock raised high above sea level and dissected by the Colorado River and its many tributaries. Thus, in strong contrast with adjacent regions, its history is fairly simple and easy to read. In it six major events are recorded. 1. During Paleozoic and Mesozoic times- periods aggregating as much as 490,000,000 years- sandstone, limestone, and shale were deposited alternately on the ocean bed and on land, and progressively sunk, thus leaving each set of beds periodically not far above sea level. 2. Near the close of the Cretaceous era, approximately 60,000,000 years ago, the entire plateau country was uplifted. For the region as a whole, the raising of this enormous mass was accomplished without greatly modifying the original, almost horizontal attitude of its constituent strata. In places, however, the beds were warped into broad syn- clines, anticlines, and domes, and locally were folded into long narrow monoclines, conspicuously represented by the San Rafael Swell, the Kaibab Plateau, the Waterpocket Fold, and the Comb monocline. 3. Following this general uplift both the flat- lying rocks and the folded rocks were so completely worn down by streams that the surface again became substantially level. 4. On the surface produced by post- Cretaceous erosion, great thicknesses of Tertiary rocks were deposited over most of the plateau province, and upon them a master stream and subordinate streams were developed- the ancestral drainage system of the Colorado River. 5. After a large part of the Tertiary sediments and some of the lavas had been laid down the plateau country was again uplifted- substantially to its present height- and in places broken by faults. In consequence of the uplift, the gradients of drainage channels were greatly steepened; the streams became powerful agents of erosion. 6. During the past 20,000,000 years, substantially the present plateau landscape of canyons, cliffs, plateaus, mesas, terraces, and the amazing variety of minor land forms has been modeled, chiefly by stream erosion. Regional features of northern portion.- The central part of the Colorado drainage basin is 28 |