The influence of growing structures on Cretaceous alluvial systems along the Cordilleran thrust belt front, Valley of Fire, Southern Nevada

Abstract

Outstanding preservation and exposure of Cretaceous tectogenic deposits in the North Muddy Mountains of southern Nevada provide a rare opportunity to examine the influence of frontal structures on provenance and sediment dispersal to the Cordilleran foreland basin. Eastward migration of the Sevier wedge-top depozone into the contiguous foredeep depozone was facilitated by development of the Willow Tank thrust-cored fault-propagation fold. The resulting thrust-cored frontal ridge diverted pre-existing fluvial systems of the Willow Tank Formation and promoted proximal alluvial fan deposition of the Baseline Formation. The Albian Willow Tank Formation represents the earliest foreland basin sediments derived from large, integrated drainage basins of the thrust belt interior. Sandstone point counts reveal an up-section increase in quartz locally derived from erosion from the Jurassic Aztec Sandstone along the frontal anticlinal ridge in the proximal Willow Tank thrust hanging wall. Continued fold growth is documented by growth strata development in and provenance of the overlying White Member of the Baseline Formation. Conglomeratic clast counts and detailed lithofacies analyses in the Cenomanian Red Member of the Baseline Formation record sequential unroofing of the frontal anticline and interaction of adjacent alluvial fan and fluvial depositional environments, respectively. Clast counts of poorly-sorted, massive ungraded conglomerates indicate a reverse clast stratigraphy based on progressively increasing amounts of carbonate framework clasts up-section. This trend is attributed to exposure and erosion of Upper Paleozoic strata in the Willow Tank hanging-wall anticline. Evidence of interfingering depositional environments is shown by up-section trends including: 1) replacement of poorly-sorted and organized coarse-grained conglomerates by well-sorted stratified to normally-graded conglomerates, 2) an overall decrease in grain size, 3) a decrease in soft-sediment deformation and increase in bioturbation, and 4) an increase in lateral bed continuity. This upsection transformation represents a spatial and temporal transition from foldproximal debris flow-dominated fans to an integrated braided stream system that transported detritus from the same source. Facies examination, clast composition and intertonguing relationships between the Red and Overton Conglomerate Members suggest contiguous braided stream networks that tapped separate, distinct source areas. These observations, as well as structural relationships, imply out-of-sequence movement on the Muddy Mountain thrust.