STRUCTURAL GEOLOGY AND SEDIMENT INFILL GEOMETRY OF A HIGH-ELEVATION GRABEN: THE FISH LAKE BASIN, CENTRAL UTAH

Fish Lake, Utah (elev. 2700 m) is a large (~10 km²) alpine lake in the transition zone between the Basin & Range and Colorado Plateau. The Fish Lake basin occupies a linear topographic depression in a NE-trending graben complex, and the lake itself fills a broad trough and has an average depth of 27 m. The graben was produced by displacement (up to ~600 m) on a suite of NE-striking normal faults that cut gently-dipping Oligocene-Miocene volcanic rocks. At the southwest end of Fish Lake, the graben is 2.5 km wide, relatively symmetric and bound by at least two antithetic faults. To the northeast, the graben widens becoming asymmetric as displacement is transferred to an array of listric faults. The Fish Lake graben cuts older NNW-trending grabens and truncates a pre-existing SE-trending drainage network on the Fish Lake Plateau. To the southwest, the Fish Lake graben is cut by a younger WNW normal fault. Faults on the NW-side of the graben are covered by 18-20 ka glacial moraines deposited by outlet glaciers sourced from an ice cap on the Fish Lake Hightop Plateau. Graben development likely produced an internally-drained basin that was later reintegrated into the Fremont/Colorado river system.

To better understand the structural geometry and sediment fill we conducted a gravity survey of the Fish Lake basin. We obtained gravity data from 101 stations, both around the lake and on the frozen lake surface, which was terrain corrected to eliminate anomalies caused by ~500 m of topographic relief within the study area. Gravity anomalies display a prominent NE-trend, which is parallel to the graben-bounding fault system. Maximum anomaly values in the lake range from -6.2 to -8.6 mGal. Sediment thickness, as determined with a 3-layer model (water, sediment, and volcanic bedrock), along 4 graben-normal cross sections had maxima that ranged from 90 to 230 m. The sedimentary fill increases towards the NE near a flooded glacial moraine presumed to be from the penultimate glaciation (Bull Lake, MIS 6, ~150 ka).