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The structure of the northern Cottonwood Mountains, located in the Death Valley region of southeastern California, is dominated by a faulted, east-facing, monoclinal flexure developed in Paleozoic strata of the Cordilleran miogeocline. We interpret this flexure as a rollover or fault-bend fold above a listric normal fault, probably the north-ward continuation of the Tucki Mountain normal fault system exposed about 45 km south of the study area. Synorogenic Tertiary sediments in the Ubehebe basin, overlying the rollover at the northern end of the Cottonwood Mountains, record the inception of normal faulting and eastward tilting around a horizontal axis trending about N25°W. Paleozoic strata in the Dry Mountain block to the west are subhorizontal. Racetrack Valley, a north-trending topographic depression between the Dry Mountain block and the northern Cottonwood Mountains, is interpreted as a major graben that accommodates differential stratal rotations within the rollover. The juxtaposition of Mesozoic thrust plates by Tertiary normal faults has obscured the structural simplicity of the rollover. Structural and stratigraphic correlations indicate that strata exposed in the Dry Mountain block and northern Cottonwood Mountains are parts of a single Mesozoic thrust plate. This new evidence suggests that the Racetrack thrust of McAllister (1952) does not root in northern Racetrack Valley, contrary to previous interpretations. A previously unrecognized klippe and other parts of the Ubehebe thrust plate lie structurally above the northern Cottonwood Mountains. We suggest that normal faulting has downdropped a portion of the Ubehebe thrust plate into Racetrack Valley between strata of the Dry Mountain block and northern Cottonwood Mountains. Thus, the development of a major graben within the rollover has produced an apparent thrust relation along both sides of the Racetrack Valley block. We correlate the Racetrack Valley block and Ubehebe thrust plate with the Last Chance thrust plate of Stewart and others (1966). Sequentially deformed, palinspastic reconstructions of the rollover indicate that conjugate faults formed symmetrical grabens during development of the rollover. The grabens accommodated bending within the rollover and probably nucleated above areas of maximum curvature on the basal listric fault. Thus, the nucleation of progressively younger grabens during extension apparently migrated toward the breakaway within the hanging-wall block while remaining fixed relative to the bend in the basal fault plane. The pre-middle Pliocene(?) age and structural position of strata in the Ubehebe basin suggest that they are partially correlative with upper Miocene to lower Pliocene strata in the Nova basin, also located on the eastern margin of the Cottonwood Mountains structural block but adjacent to Tucki Mountain. The location of these basins in the hanging wall of the Tucki Mountain normal fault system suggests that they may be allochthonous parts of an earlier basin. Distinctive cobbles with no known local source are abundant in a relatively thin stratigraphic zone within the Ubehebe basin. We interpret these cobbles as reworked clasts derived from the Oligocene Titus Canyon Formation. This is consistent with structural and stratigraphic correlations that indicate about 70 km northwest-directed transport of the Cottonwood Mountains block from a position adjacent to the Funeral Mountains (Snow and Wernicke, 1988,1989; Snow, 1989; Snow and others, 1989).

Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach of the river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to the north and Mohave Valley to the south. Lacustrine marls interfingered with and conformably overlying a sequence of post–5.6 Ma fine-grained valley-fill deposits record an early phase of intermittent lacustrine inundation restricted to Cottonwood Valley. Limestone, mud, sand, and minor gravel of the Bouse Formation were subsequently deposited above an unconformity. At the north end of Mohave Valley, a coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan deposits from subaqueous deposits of the Bouse Formation. We interpret this key unit as evidence for overtopping and catastrophic breaching of the paleodivide immediately before deep lacustrine inundation of both valleys. Exposures in both valleys reveal a substantial erosional unconformity that records drainage of the lake and predates the arrival of sediment of the through-going Colorado River. Subsequent river aggradation culminated in the Pliocene between 4.1 and 3.3 Ma. The stratigraphic associations and timing of this drainage transition are consistent with geochemical evidence linking lacustrine conditions to the early Colorado River, the timings of drainage integration and canyon incision on the Colorado Plateau, the arrival of Colorado River sand at its terminus in the Salton Trough, and a downstream-directed mode of river integration common in areas of crustal extension.

Abstract A detailed record of the late Cenozoic history of the lower Colorado River can be inferred from alluvial and (likely) lacustrine stratigraphy exposed in dissected alluvial basins below the mouth of the Grand Canyon. Numerous sites in Mohave, Cottonwood, and Detrital valleys contain stratigraphic records that directly bear on the mode, timing, and consequences of the river’s inception and integration in the latest Miocene–early Pliocene and its subsequent evolution through the Pleistocene. This field trip guide describes and illustrates many of these key stratigraphic relationships and, in particular, highlights evidence that supports the hypothesis of cascading lake-overflow as the principal formative mechanism of the river’s course downstream from the Grand Canyon.