Geologic Setting of the Utah FORGE Site, Based on New and Revised Geologic Mapping

摘要

As part of the Utah FORGE project, new geologic mapping of unconsolidated basin fill was combined with bedrock mapping of the central Mineral Mountains to create a 1:24,000 scale geologic map that covers ~ 700 km~2 adjoining the Utah FORGE site. The resulting geologic map provides basic constraints that include the Quaternary history of surface faulting and the distribution and character of bedrock lithologies and structural elements that are analogous to the FORGE reservoir rocks. Unconsolidated deposits in the map area consist primarily of west-sloping alluvial-fan deposits sourced from the Mineral Mountains and adjoining lacustrine deposits along the valley floor west of the FORGE site. A unique boulder-rich alluvial fan deposit is the oldest unconsolidated unit and likely represents debris shed from the Mineral Mountains in Miocene to Pliocene time. Relatively fine-grained middle Pleistocene- to recent-age alluvial-fan deposits underlie the FORGE site. South of the FORGE site, north-south trending fault scarps of the Mineral Mountain West fault cut upper Pleistocene-age alluvial fans. Based on the new mapping there is no evidence of Holocene offset along the Mineral Mountains West fault. Morphologically unique fault scarps associated with the Opal Mound fault and NM fault also lack evidence of Holocene faulting, and offset across these structures may be related to geothermal or volcanic processes. The bedrock of the central Mineral Mountains is dominated by an Oligocene- to Miocene-age batholith emplaced in Precambrian metamorphic rocks. Most exposed rocks consist of a variety of granitoid intrusives and fewer exposures of related diorites and adjoining gneiss, schist, and quartzite. These rocks are intruded by late-stage dikes that range from felsic to mafic in composition. Quaternary rhyolite and tuff, consisting of high-standing domes and drainage-filling flows, occur across the central Mineral Mountains.