Sedimentology and Provenance of an Alluvial Fan Progradation, Middle Thumb Member, Horse Spring Formation, Western Lake Mead, Nevada

Abstract

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 in the western Lake Mead area.

The Lake Mead Domain is located in the Central Basin and Range Province (CBRP). The CBRP is dominated by extensional and transtensional tectonism. Dextral Las Vegas Valley Shear Zone, sinistral Lake Mead Fault System, and north-south trending normal faults come together in the Lake Mead area and they create a structurally complex region. Therefore, sedimentary basins in the Lake Mead domain were affected by this structural complexity. However, the interaction between extensional/transtensional tectonism and sedimentary basins in the Lake Mead area helps us to understand the timing of the extension in the CBRP. The beginning of the extension in the CBRP was recorded by the Horse Spring Formation. This study focuses on the extensive conglomerate deposits in the middle Thumb Member of the Horse Spring Formation at ca. 15.5 Ma. The extensive nature and sudden retreat of the progradation makes this event unusual. The purpose of this study is to understand the nature (exact extent, timing of the beginning and the end, gradual or abrupt), and cause of a major progradation on the unfaulted side of the Thumb basin during the peak of detachment faulting. Moreover, this research aims to reveal the effect of this major progradation on the Thumb basin. These research questions will help to provide higher resolution paleogeographic reconstructions for the Thumb basin between 15.7-15.35 Ma; and will aid to better understand the relationship between tectonism, climate, and sedimentation. In hydrocarbon exploration point of view, this region is a natural laboratory to understand the effects of faulting on sedimentary basins (stratigraphic and structural traps). In order to answer these questions, U-Pb detrital zircon geochronology, traditional provenance methods (point counting and lithological analysis), detailed structural and lithofacies mapping, and high resolution measured sections will be used.

To sum up, this study will extend our knowledge about the paleogeography of the Thumb basin and the relationship between tectonism, climate, and sedimentation in an extensional/transtensional basin setting.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018