Quaternary Geology of the Utah FORGE Site and Vicinity, Millard and Beaver Counties, Utah

Abstract

We present new Quaternary geologic mapping, geomorphic analysis, and luminescence geochronology to better understand the Quaternary geologic history of the Utah FORGE site near Milford, Utah. Late Cenozoic unconsolidated deposits in the map area are predominantly coarse-grained alluvial-fan deposits sourced from the Mineral Mountains. Boulderrich basin-fill deposits are the oldest mapped unconsolidated unit (Taf) representing debris shed from the rapidly rising ancestral Mineral Mountains during Miocene to Pliocene time. Erosion of the Mineral Mountains continued into the early Pleistocene, forming the deeply embayed canyons and generally mature topography of the range today. In early middle Pleistocene time (~0.7–0.5 Ma), rhyolitic lava flows and domes partially filled pre-existing canyons. By the late middle Pleistocene (~0.5–0.1 Ma), alluvial fans began aggrading east, deep into the canyons and abutting against the rhyolitic flows. The distal ends of the alluvial fans were extensively reworked and etched by shorelines of Lake Bonneville, which occupied Milford Valley from about 20 to 18 ka. The recession of Lake Bonneville about 18 ka and resultant lowering of base level may have spurred the deep incision of Negro Mag Wash (NMW), Ranch Canyon, and Corral Canyon that has rendered older alluvial fans inactive. We mapped three fault zones near the Utah FORGE site that had apparent Quaternary displacement: the Negro Mag fault (NMF), the Opal Mound fault (OMF), and the Mineral Mountains West fault zone (MMWFZ). The steeply dipping NMF trends eastwest across much of the Mineral Mountains and bounds the northern margin of the Roosevelt Hot Springs geothermal area. We found no definitive evidence for displacement of Quaternary deposits along the inferred trace of the NMF and suspect the fault may be pre-Quaternary in age. The steeply east-dipping, north-trending OMF defines the western boundary of the Roosevelt Hot Springs geothermal system. The fault displaces late Pleistocene alluvial-fan deposits as much as 18 m. Hot springs emanating from the fault zone have deposited abundant siliceous sinter along the fault. Sinter deposition and faulting have been contemporaneous as evidenced by buried horizons of sinter detected in well logs, and by possible fault-rotated blocks of sinter mapped in the fault zone. However, Holocene sinter as young as 1.6 to 1.9 kyr covers the southern end of the fault and appears unbroken. Development of the OMF scarp has been greatly influenced by differential erosion of siliceous sinter exposed along the fault. Latest movement on the OMF was in the late Pleistocene (12–126 kyr), but its resultant scarp appears older than scarps formed by the nearby Lake Bonneville shoreline (>18 ka) and scarps formed by the MMWFZ. The north-south-trending MMWFZ extends for about 40 km from near the Utah FORGE site to south of Minersville. The fault zone has produced a series of eastand west-dipping scarps on late Pleistocene alluvial fans that are less than 5 m high. South of Corral Canyon, multiple strands of the fault zone converge into a single fault that has produced a scarp as high as about 12 m. MMWFZ scarps are deeply dissected and display similar scarp morphologies as the nearby Bonneville shoreline that formed about 18 ka. Unfaulted latest Pleistocene to Holocene alluvium covers parts of the fault zone, indicating that the most recent movement on the fault zone is late Pleistocene. INTRODUCTION The Utah FORGE site is in the eastern Basin and Range Province (Figure 1), a region of east-west extension that covers much of the southwestern United States. The province consists of north-south-striking, range-bounding normal faults that define a series of bedrock horst blocks and corresponding basins. Basin and Range extension began in the early Miocene (ca. 17.5 Ma) and continues today (Dickinson, 2006). Within the Basin and Range, most developed geothermal systems are fault controlled (Faulds and others, 2011). In the Sevier Desert of southwestern Utah, several geothermal systems may have magmatic sources (Faulds and others, 2011), and deep-seated faults allow for hydrothermal circulation. Some late Quaternary normal faulting in the Sevier Desert about 75 km north of the FORGE site may be related to rift-assisted magmatism (Stahl and Niemi, 2017; Figure 1). by Tyler Knudsen, Emily Kleber, Adam Hiscock, and Stefan M. Kirby Geothermal characteristics of the Roosevelt Hot Springs system and adjacent FORGE EGS site B2 The Utah FORGE site is on the western flank of the Mineral Mountains, a 45-km-long, north-south-trending mountain range consisting primarily of late Oligocene to late Miocene intrusive rocks (Nielson and others, 1986; Kirby, 2019). Late Cenozoic unconsolidated deposits in the map area consist of coarse-grained alluvial deposits of late Tertiary to Holocene age, and coarseand fine-grained lacustrine sediment deposited during at least two Quaternary lake cycles. The FORGE site is 2 km west of the Roosevelt Hot Springs (RHS) hydrothermal system that currently generates up to 35 MWe of gross power at the Blundell Power Plant (Simmons and others, 2016) (Figure 2). Quaternary hot spring deposits are common along the western margin of the RHS geothermal area. Thin eolian sand deposits are present throughout the study area, and form dunes and partially stabilized sand sheets and sand mounds. Late Tertiary and Quaternary rhyolitic lava flows and domes are present in the central Mineral Mountains. Three fault zones within the study area having suspected Quaternary movement are the north-south-trending Mineral Mountains West fault zone (MMWFZ), the Opal Mound fault (OMF), and the east-west-trending Negro Mag fault (NMF; Figure 2). Figure 1. (A) Regional overview of the study area relative to the Great Basin (GB), Basin and Range Province (B&RP), and the Wasatch fault zone (WFZ) in Utah. (B) Regional Quaternary faults near the Utah FORGE site include the southern terminus of the WFZ and faulting that may be driven by shallow magmatism in the Sevier Desert (Stahl and Niemi, 2017); colored fault lines indicate age of the most recent surface-rupturing movement. 0 40 20 km Li da r E xt en t Utah FORGE Site UTAH Wasatch Fault Zone Area of Southern Terminus of Wasatch Fault Zone, Fayette Segment