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 Stratigraphic relationships and patterns of Quaternary tectonic-geomorphic development together record a complex history of late Cenozoic deformation and crustal shortening at the northeastern Diablo Range-northwestern San Joaquin Valley mountain front. Neogene uplift of the northern Diablo Range is marked by a regional reversal in sediment transport direction in the ancestral northwestern San Joaquin Valley and deposition of a late Miocene-Pliocene fanglomerate. The fanglomerate and older Neogene strata were subsequently uplifted and tilted homoclinally eastward as mountain-front deformation migrated eastward into the ancestral San Joaquin Valley. Drill-hole data indicate that Neogene and underlying units are essentially flat-lying in the central San Joaquin Valley, so the east-tilted strata must flatten across a synformal flexure located at or directly east of the present mountain front. Pediments were beveled across the east-tilted strata in middle Pleistocene time. Mapping and reconstruction of the pediments reveal that the surfaces have been uplifted, tilted eastward, gently folded and locally faulted. East-draining streams have nested, inset sequences of middle to late Pleistocene terraces and alluvial fans that also have been variously uplifted, folded, tilted or offset. Similar patterns of late Cenozoic tectonism are observed in the northern Coast Ranges and western Sacramento Valley, and are consistent with surface deformation related to blind thrust faulting within an eastward-tapering tectonic wedge or triangle zone. Propagation of an underthrust wedge can account for uplift and homoclinal tilting along the northeastern Diablo Range piedmont. The regional synformal flexure at the base of the homocline lies approximately above the wedge tip and is analogous to similar structures at the leading edges of many fold and thrust belts. Structural and stratigraphic relationships indicate that tectonic wedges were emplaced beneath the western Great Valley in early Tertiary time or earlier. Pleistocene uplift and tilting west of the wedge tip may be related to movement on reactivated west-vergent thrust faults splaying upward from the roof thrust of the wedge, or from the development of second-order wedge-style pop-up blocks. Reverse faults displace Pleistocene deposits above the wedge tip and locally accentuate the steepness of the mountain front. Instrumental microseismicity and historic large magnitude earthquakes elsewhere along the western margin of the Great Valley suggest that the underlying blind thrust faults may be seismically active and thus may pose a heretofore unrecognized seismic hazard to communities in the northwestern San Joaquin Valley.