GEOLOGIC MAP OF THE FISH LAKE 7.5 MINUTE QUADRANGLE, CENTRAL UTAH: INSIGHTS INTO THE VOLCANIC, TECTONIC, GEOMORPHIC, AND LACUSTRINE HISTORY OF THE FISH LAKE PLATEAU

The Fish Lake quadrangle is located in central Utah and is part of the Fish Lake Plateau. The bedrock geology is dominated by thick, densely welded ash flow sheets that are late Oligocene to early Miocene in age. All of these ash flow sheets are from the Marysvale Volcanic Field located ~70 km to the SW of the quadrangle. Volcanic units include (from base to top) the Johnson Valley trachyandesite, the Lake Creek trachyite (whole rock ⁴⁰Ar/³⁹Ar age: 25.15 ± 0.14 Ma), and the Osiris trachyite (mean ⁴⁰Ar/³⁹Ar total fusion ages of sanidines: 23.03 ± 0.08 Ma). The structural geometry of the quad is characterized by a suite of grabens of varying size, orientation, and age. The Fish Lake basin, where the ~12 km² Fish Lake is located, is a 4 km-wide asymmetric graben formed by a suite of NE-SW striking faults. Volcanic rocks are displaced by ~300 m along a major NW-dipping fault and flanked by related synthetic and antithetic faults. The Crater Lakes occupy an internally drained graben ~2-5 km NE of Fish Lake. Displacement along these faults ranges from 60 to 140 m. Transverse faults bound the N and S ends of the Crater Lakes graben and a horst occurs at the N end.

Surficial deposits in the quad include alluvium, colluvium, and glacial deposits. The glacial deposits are composed of thick till units of two different ages with the younger unit having distinct morainal morphology. These deposits are from glaciers originating on the Fish Lake Plateau Hightop and from cirque glaciers on the E flank of the Hightop. Exposure ages of boulders on moraine crests indicate deposition of the younger moraines occurred at ~20 ± 2 ka with some recessional moraines yielding ages of ~16 ± 2 ka. Fish Lake is the largest alpine lake in Utah and is 7 km long by ~1.7 km wide. The NE end of the lake is partly occupied by glacial deposits from Pelican Canyon that have created a shallow lagoon filled with marsh. Fetch parallel to the long axis of the lake has created spits and bermed shorelines at the NE and SW lake margins. The maximum depth of the lake is 37 m and the average depth is 27 m. The gently sloped lake bottom is composed of organic-rich muds except over a prominent ridge where boulders dominate. This ridge is likely an older glacial moraine that is now submerged. Gravity data suggests sediment thickness in Fish Lake exceeds 100 m.