SEDIMENTOLOGY AND PROVENANCE OF AN ALLUVIAL FAN PROGRADATION, MIDDLE THUMB MEMBER, HORSE SPRING FORMATION, WESTERN LAKE MEAD, NEVADA

The purpose of this study is to understand the nature and cause of an extensive alluvial fan progradation and its related depositional environments between 15.70-15.35 Ma in the middle Thumb Member of the Horse Spring Formation located in the western Lake Mead domain. The Thumb Basin records extension and the initiation of breakup of the basin into multiple subbasins. Therefore, soft sediment deformation and growth strata are common in the middle Thumb Member. The middle Thumb Member corresponds to the start of the peak of extension on the South Virgin-White Hill detachment fault to the east at 15.5–14.5 Ma before faulting moved to the west. This study focuses on the extensive conglomerate deposits in the middle Thumb Member at 15.6 to 15.4 Ma, which are unusual for the Lake Mead domain. The sources of the alluvial fan progradation have been postulated to be intra-basinal or from the margins of the basin to the west (Spring Mountains), northwest, and north (Muddy Mountains). We are using U-Pb detrital zircon geochronology, traditional provenance methods, detailed (1:5,000) structural and lithofacies mapping, and high resolution measured sections to build on previous studies and to determine the location and direction of the source areas for the middle Thumb conglomerate. Another important aspect of this study is to determine the cause of the alluvial fan progradation and retreat. Faulting within the Thumb Basin connected to the peak of extension on the South Virgin-White Hills detachment fault might be responsible for the progradation at ca. 15.5 Ma; the projection of the Las Vegas Valley shear zone into the basin from the west may be another cause. Soft sediment deformation, intra-basinal growth strata and their relation to facies changes are being used to investigate the local faulting. The retreat of the alluvial fans is correlated to a transition to higher sedimentation rates in the basin and therefore tentatively tied to higher faulting rates. The middle Miocene climatic optimum could be another factor affecting the progradation. Climate driven changes in the basin from a large evaporitic lake at ca. 16 Ma to increasing carbonate lakes from springs at 15-14 Ma support an overall trend to wetter conditions. The initiation of the progradation at 15.7 - 15.6 Ma, soon after the end of the evaporitic lake suggests a climatic control.