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<br />. <br /> <br />. <br /> <br />The Western Colorado River Basin <br /> <br />In the western Grand Canyon, Young (1982) found no evidence <br />of a Colorado River drainage at the beginning of basin and range <br />tectonism 18 m.y. ago on the Hualapai Plateau. He suggested that <br />the Colorado River flowed southwest of its present course at that <br />time, emerging through the Colorado Plateau at the southern end of <br />the Hurricane Fault. Luccitta (1988) concluded that the Colorado <br />River existed since Miocene or even Oligocene time in northeastern <br />Arizona, Utah, and Colorado, flowing through a broad, mature <br />valley across what is now the Kaibab Plateau, with exterior <br />drainage to the northwest. Beginning about 18 m.y. ago, basin and <br />range rifting west and south of the Colorado Plateau disrupted <br />existing drainages. Endorheic drainage developed in basin and <br />range grabens, with erosional infilling of the basins. These <br />deposits show that the lower Colorado River did not run along its <br />present course until about 5.5 m.y. ago. <br /> <br />The Gulf of California began opening in late Miocene time and <br />a drainage system grew northward into the area of the western <br />Grand Canyon, where it apparently captured the ancestral Colorado. <br />The western Grand Canyon was apparently cut in approximately <br />4 m.y. between 5.5 m.y. and about 1.1 m.y. ago, when it reached <br />its present relative elevation in the western Grand Canyon <br />(Hamblin 1989). This process was driven by 1 km of regional <br />uplift of the southern margin of the Colorado Plateau during the <br />last 5 m.y. (Lucchitta 1979). Thus, it appears likely that the <br />Colorado River drainage became fully integrated only in the last <br />5.5 m.y., with rapid erosion driven by uplift causing the canyon <br />country to encroach on the rolling plateau terrain. <br /> <br />Evolution of Wetland Vegetation <br /> <br />Wetland vegetation, in the form of coastal, swamp or deltaic, <br />and riparian or lacustrine plant life has been continuously <br />present in the upper Colorado River Basin for at least 65 m.y., <br />and at least sporadically since the Cambrian period, 590 m.y. ago <br />(Tidwell et al. 1972). Although riparian vegetation has been <br />continuously present and therefore available as food and substrate <br />for microbial, invertebrate, and animal populations, the species <br />composition of that riparian vegetation has changed tremendously. <br /> <br />The pre-Cenozoic fossil flora record is poorly preserved, <br />Pennsylvanian root casts of terrestrial plants are common in the <br />Supai group. Numerous fern fossils in the Permian Hermit shale in <br />the Grand Canyon are suggestive of a profuse terrestrial flora in <br />a dry region. <br /> <br />Mesozoic flora are better preserved and were generally <br />dominated by conifer-related taxa, such as Araucarioxylon <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />9 <br /> <br />. <br />