NEW STRATIGRAPHIC EVIDENCE FOR THE LATE CENOZOIC INCEPTION AND SUBSEQUENT ALLUVIAL HISTORY OF THE COLORADO RIVER FROM NEAR LAUGHLIN, NEVADA

Detailed geologic mapping of alluvial deposits near Laughlin, Nevada, offers important new insights into the late Cenozoic history of the Colorado River. Foremost are two particularly significant findings: 1. Stratigraphic evidence for a link between local catastrophic flooding and the early Pliocene Bouse Fm; and 2. Unequivocal evidence for major unconformities in the Pleistocene Chemehuevi Fm. Finding 1 concerns the mechanism of integration of the lower Colorado River. Lowest exposures along the river south of Laughlin contain a sequence of locally derived, late Tertiary alluvial fan deposits. These are disconformably overlain by a distinctive, monolithologic deposit of coarse, fluvial gravel derived almost entirely from a limited area upstream in lower Pyramid Canyon. The gravel is eroded into underlying fan deposits, has sedimentary structures consistent with high-energy (flood) deposition, and occupies paleochannels that follow the trend of the valley axis. The gravels are overlain by the basal marl of the Bouse Fm. The marl and the gravel are disconformably overlain by a thick sequence of sand, gravel, and mud unequivocally related to the early Colorado River. This sequence is consistent with catastrophic flooding into a local basin from an upstream breach in a bedrock divide (flood gravel over fan deposits), formation of a temporary lake (Bouse over flood gravel), and subsequent development of a through-going river (sand, gravel, and mud over Bouse). Finding 2 partly demystifies the enigma of the Chemehuevi Fm, a distinctive and reputedly unique stack of fluvial mud and sand occupying protected niches along the lower river. An excellent set of exposures SW of Davis Dam reveals at least one major disconformity within what is typically called the Chemehuevi Fm. This disconformity coincides with the point of contact between two major fluvial terraces. Other exposures in the area indicate at least three predominantly fine-grained fluvial sequences separated by disconformities. Thus, it appears that the Chemehuevi Fm contains numerous, very similar fluvial sequences comprised of vertical and lateral facies assemblages typical of a large river and its floodplain. The juxtaposition of these similar sequences has resulted from multiple episodes of deep incision followed by channel and floodplain aggradation.