SYNOROGENIC SEDIMENTATION ALONG THE CORDILLERAN THRUST BELT FRONT, SOUTHERN NEVADA: ALLUVIAL SYSTEMS, SEDIMENT DISPERSAL, AND GROWTH STRATAL DEVELOPMENT

Upper Cretaceous conglomeratic strata (Baseline Formation) in the North Muddy Mountains of southern Nevada record the progressive eastward expansion of the wedge-top depozone of the Cordilleran foreland basin system into the adjacent foredeep depozone. Expansion of the orogenic wedge was facilitated by foreland-directed frontal thrusting (Willow Tank thrust) and associated development of fault-propagation anticlinal ridges in the older foredeep depozone. Growing anticlinal ridges associated with this frontal thrusting disrupted an Early Cretaceous alluvial plain comprising gravelly braided fluvial systems (Willow Tank Formation) delivering sediment from large, integrated drainage basins in the interior of the Cordilleran thrust belt to the west (Muddy Mountain-Keystone allochthon), and causing diversion of braided rivers around the growing frontal folds. Late Cretaceous Baseline Formation conglomerates record the kinematic development, topographic uplift, and erosional history of the thrust-cored frontal anticlinal ridge. Detailed lithofacies analysis of these cobble-boulder conglomerates and associated sandstones show synthetic sediment transport from and deposition proximal (< 1km) to the growing anticline in hybrid, sand-rich sheetflood and debris-flow alluvial fans sourced by small drainage basins that supplied a preponderance of sand from weathered Jurassic Aztec Sandstone exposures. More distally (>1 km), these fans were rimmed by sandsheet aprons that record deposition in shallow, ephemeral sand-bed streams. Abundant clast count data show an inverted clast population stratigraphy that represents the sequential unroofing of the frontal anticline. Conglomeratic strata in the lower portion of the Baseline Formation (White Member) record the kinematic growth of the frontal anticline in a composite syntectonic progressive unconformity composed of four cumulative wedge systems separated by three syntectonic intraformational angular unconformities. These growth strata preserve erosional truncation, onlap and offlap relationships that attest to the episodic uplift of the frontal anticline.