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Layer: Geologic Units (ID: 2407)

Parent Layer: Lake Mountain 7.5 MP-18-2DM

Name: Geologic Units

Display Field: UnitName

Type: Feature Layer

Geometry Type: esriGeometryPolygon

Description: The Lake Mountain quadrangle is about three miles north of the town of Tridell, Uintah County, Utah, along the south flank of the eastern Uinta Mountains. The area is popular for outdoor recreation, including fishing, hunting, four-wheeling, snowmobiling, hiking, and camping, mostly in Ashley National Forest in the northern part of the quadrangle.Bedrock units in the quadrangle are commonly covered by various surficial deposits, but in some areas good exposures of tilted bedrock can be found. The southern half of the map area is characterized by several sets of hogbacks and flatirons that consist of Mesozoic bedrock units separated by broad piedmont surfaces or stream valleys. The bedrock has been gently tilted towards the south and southwest about 15° to 25° because of the Laramide Uinta Mountains uplift (Hintze and Kowallis, 2009), although dips increase to 58° where Deep Creek crosses the Little Water Hills. The oldest bedrock unit in the quadrangle is the middle Neoproterozoic Red Pine Shale of the Uinta Mountain Group, which crops out in the Mosby Peak area in the northwestern corner of the quadrangle. The youngest bedrock unit mapped is the Oligocene Bishop Conglomerate, which is exposed throughout the northern half of the map area. The Lake Mountain quadrangle has a variety of unconsolidated surficial deposits of Quaternary age, dominated by landslide and slump deposits. Large landslides are located along the flanks of Lake Mountain and Mosby Mountain. Head scarps developed mostly in the Bishop Conglomerate, as the recessive and weak Mesozoic formations commonly undermine it. Other unconsolidated deposits include colluvium, stream alluvium, three levels of alluvium, three levels of large piedmont deposits, and glacial till and outwash. Each higher level of alluvium represents subsequently older stream valleys and each higher level of piedmont alluvium represents subsequently older broad alluviated surfaces. The numbers assigned to each level are applicable to the Lake Mountain quadrangle only and do not imply regional correlation of similarly numbered alluvial deposits mapped in other quadrangles. Two important landmarks within the quadrangle are Dry Fork Canyon, which cuts across the northeast corner of the map area and contains glacial deposits of Smiths Fork age, and Lake Mountain, the quadrangle’s namesake, located southwest of Dry Fork Canyon. The combined geology of the quadrangle makes for a majestic landscape that begins with low-lying hogbacks and broad valleys in the south and transitions to deep glacier-influenced canyons and broad mountain slopes in the north. The quadrangle’s structural history is challenging to interpret because fault systems along the south flank are commonly buried beneath Quaternary and Tertiary strata. A rare exception is the Deep Creek fault zone (DCFZ), a northwest-southeast-trending strain transfer zone of normal, reverse, strike-slip, and oblique faults (Haddox, 2005), which is exposed in several places within the quadrangle. This fault system consists of several graben and half-graben, which topographically forms an array of hogbacks and broad valleys that extends across the area north of Little Water Hills and cuts through the adjoining Ice Cave Peak quadrangle to the west as well as the Dry Fork, Steinaker Reservoir and Vernal NW quadrangles to the east. The DCFZ terminates just northeast of Asphalt Ridge (Hansen, 1986a, 1986b; Haddox and others, 2005; Sprinkel, 2006; Haddox and others, 2010a, 2010b; Schamel, 2013; Poduska and others, 2015). The DCFZ formed as a transfer zone between offsets in the frontal Uinta Basin-Mountain boundary fault zone (UB-MBFZ) (Haddox and others, 2005; Poduska and others, 2015), which had multiple periods of reactivation as smaller thrust wedges continued to shift independently during late-Laramide orogenic episodes (Ritzma, 1969, 1971; Hansen, 1986b; Stone, 1993). In our cross section of the Lake Mountain quadrangle (plate 2), we model the subsurface using multiple fault blocks to best account for the attitudes of bedrock exposed at the surface. This seems plausible because (1) smaller fault segments have been mapped along the UB-MBFZ, for example near Asphalt Ridge (Ritzma, 1971; Hansen, 1986a, 1986b; Sprinkel, 2007; Schamel, 2013), (2) previous interpretations of the subsurface geology of the surrounding area are similar to our model (Ritzma, 1974; Campbell and Ritzma, 1979; Hamilton, 1981; Covington and Young, 1985; Blackett, 1996), and (3) small thrusts in nearby areas have been proposed (Hansen, 1986b). Our model is also consistent with previous mapping of the UB-MBFZ (Johnson and Roberts, 2003; Sprinkel, 2015), though it assumes structural elements from Ritzma (1974) and Hansen (1986b). Having multiple faults also seems to fit with the Haddox (2005) interpretation of the DCFZ as small fault segments oriented orthogonally to each other with reverse, oblique, and strike-slip Laramide movement and later reactivated during extension as indicated by kinematic slip data preserved on some of the faults.

Copyright Text: Program Manager: Grant C. Willis (UGS) Project Manager: Douglas A. Sprinkel (UGS) GIS and Cartography: Kent D. Brown and Douglas A. Sprinkel (UGS) Geology review: Grant C. Willis, Stephanie M. Carney, and Michael D. Hylland (UGS) GIS and Cartographic review: Basia Matyjasik (UGS) Funding: U.S. Geological Survey, EDMAP award number 05HQAG0049 (2013).

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