Interim Geologic Map of the Browns Hole Quadrangle, Weber and Cache Counties, Utah

The Browns Hole quadrangle is in Weber and Cache Counties of northern Utah and covers the eastern part of Ogden Valley, a rapidly developing area of the Wasatch Range. The Middle and South Forks of the Ogden River bisect the quadrangle and are important watersheds and recreational areas to the communities of Ogden Valley and the Wasatch Front. The towns of Huntsville and Eden are just west of the quadrangle, unincorporated communities with year-round residents are present throughout the quadrangle, and numerous summer-cabin communities are present in the eastern part of the quadrangle. A portion of Powder Mountain ski resort, which draws year-round visitation and recreation, is present in the northwest corner of the quadrangle. The quadrangle contains the Willard thrust, a major thrust fault with approximately 30 mi (50 km) of eastward displacement that was active during the Cretaceous-Eocene Sevier orogeny (Yonkee and others, 2019). In the quadrangle, the Willard thrust places Neoproterozoic through Ordovician strata in the hanging wall over a fault-bounded lozenge of Cambrian strata and footwall Jurassic and Triassic strata (see cross section on Plate 2). Neoproterozoic strata comprise a succession of mostly clastic rocks deposited during rifting of western North America and breakup of the supercontinent Rodinia (Yonkee and others, 2014). These rocks include the Cryogenian-age Perry Canyon and Maple Canyon Formations, and the Ediacaran-age Kelley Canyon Formation, Papoose Creek Formation, Caddy Canyon Quartzite, Inkom Formation, Mutual Formation, and Browns Hole Formation. The Browns Hole Formation is a sequence of interbedded volcaniclastic rock and basalt lava flows that provides the only radiometric age control in the quadrangle. Provow and others (2021) reported a ~610 Ma detrital apatite U-Pb age from volcaniclastic sandstone at the base of the formation, Crittenden and Wallace (1973) reported a 580 ± 14 Ma K-Ar hornblende age for a volcanic clast, and Verdel (2009) reported a 609 ± 25 Ma U-Pb apatite age for a basalt flow near the top of the formation. Cambrian strata in the hanging wall include a thick basal clastic sequence (Geertsen Canyon Quartzite) overlain by a thick sequence of interbedded limestone, shale, and dolomite (Langston, Ute, and Blacksmith Formations). Hanging wall rocks are deformed by Willard thrust-related structures, including the Browns Hole anticline, Maple Canyon thrust, and numerous smaller folds and minor faults. Footwall rocks of the Willard thrust include highly deformed Cambrian strata within a fault-bounded lozenge exposed in the southern part of the quadrangle, and Jurassic and Triassic rocks exposed just south of the quadrangle. The Paleocene-Eocene Wasatch Formation unconformably overlies older rocks and was deposited over considerable paleotopography developed during late stages of the Sevier orogeny. The southwest part of the quadrangle is cut by a southwest-dipping normal fault system that bounds the east side of Ogden Valley. This fault is interpreted to have experienced an early phase of slip during local late Eocene to Oligocene collapse of the Sevier belt and deposition of volcanic and volcaniclastic rocks (Norwood Tuff) exposed west of the quadrangle (Sorensen and Crittenden, 1979), and a younger phase of slip during Neogene Basin and Range extension (Zoback, 1983). Lacustrine deposits and shorelines of Pleistocene-age Lake Bonneville are present in the southwest corner of the quadrangle near the mouth of the South Fork of the Ogden River and record the highstand of Lake Bonneville (Oviatt, 2015). Pleistocene glacial deposits, present in the northwest corner of the map, are likely related to the Pinedale glaciation, commonly expressed by two moraine building episodes in the Wasatch Range (Quirk and others, 2020). Numerous incised alluvial deposits and geomorphic surfaces are present along major drainages and record pre- and post-Lake Bonneville aggradational and degradational alluvial and colluvial sequences. Mass-movement deposits, including historically active landslides, are present throughout the quadrangle. Crittenden (1972) mapped the Browns Hole quadrangle at 1:24,000 scale, which provided an excellent foundation for the general stratigraphy and structure, but the 1972 map lacked important details of unconsolidated surficial units. As part of 1:62,500 scale mapping of the Ogden 30'x60' quadrangle, Coogan and King (2016) updated stratigraphic nomenclature, revised some contacts, and added more details for surficial units. For this map, we utilized new techniques for data acquisition and analysis to delineate surficial deposits, bedrock contacts, and faults more accurately and precisely. Mapping and field data collection were largely done in 2021–2022 using a combination of GPS-enabled tablets equipped with georectified aerial imagery (U.S. Department of Agriculture [USDA] National Agriculture Imagery Program [NAIP], 2009), orthoimagery (Utah Geospatial Resource Center [UGRC] State Geographic Information Database, 2018b, 2018c; 2021a, 2021b), and lidar data (UGRC State Geographic Information Database, 2006; 2011; 2013–2014; 2018a), previously published geologic maps, topographic maps, and applications for digital attitude collection. We also used hand-held GPS units, Brunton compasses, and field notebooks to collect geologic data. Field data were transferred to a Geographic Information System (GIS), where the map was compiled and completed.