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. . Chapter 2. Geology of the Colorado Plateau Introduction The Colorado River drainage is a relatively recent feature etched into the Colorado Plateau, and lays bare more than 1.7 billion years of the earth's history (Table 3). As part of the Rocky Mountain Geologic Province, the Colorado Plateau has remained relatively stable while the rest of the Rocky Mountain province and the adjacent basin and range province have been rendered by tectonic activity from pre-Cambrian times to the present. The plateau is underlain by the oldest stratum west of the Rocky Mountains, a 1.8 billion year (b.y.)-old crystalline basement complex. Strata overlying that metamorphic sequence provide a record of all subsequent geologic periods except the Ordovician and Silurian, with pre-Cambrian and Paleozoic exposures in the Grand Canyon, and Mesozoic and Cenozoic strata exposed progressively farther north in a "Grand Staircase" (Fig. 5). Cenozoic strata in the headwaters of major tributaries, in the upper Colorado River basin, and in the Grand Canyon region reveal some details of how the Colorado River assumed its present course. Thus, although depositional events account for much less than half of the geologic time that has elapsed there, the Colorado River and its tributaries expose the most complete record of geologic events on earth. . . . . Historical Geology Precambrian Events (Prior to 550 Million Years Ago) Thousands of meters of sedimentary and some igneous deposits accumulated on the floor of a pre-Cambrian sea prior to 1.8 billion years (b.y.) ago, covering what is now the Colorado Plateau. Tectonic activity between 1.8 and 1.7 b.y. ago resulted in deep burial, high grade metamorphism, igneous intrusion, and produced the Vishnu schist and Zoroaster granite exposed in the Granite Gorges of the Colorado River in the Grand Canyon (Fi~. 6). Crystallization of the schist was followed by about 400 milllon years (m.y.) of erosion, creating a low, rolling peneplain. Shallow marine sediments began to accumulate on top of the peneplain about 1.2 b.y. ago. Algal stromatolite reefs in the bass limestone are the Plateau's oldest fossils. The pre-Cambrian sedimentary and igneous deposits that comprise the Grand Canyon supergroup accumulated for about 400 m.y., attaining a depth of 4,300 m. These strata were rather quickly eroded during the "Grand Canyon disturbance", a period of geologic upheaval between 820 m.y. and 700 m.y. (Elston 1979). The only remnants of this pre-Cambrian supergroup were preserved in down-dropped fault wedges found along the Colorado River corridor in the Grand Canyon. . . . . 3 .