SURFICIAL GEOLOGY OF THE UM CREEK DRAINAGE BASIN, FISH LAKE PLATEAU, UTAH: GLACIATION, TECTONICS, AND MASS WASTING

The Fish Lake Plateau of south-central Utah is underlain by a thick sequence of Tertiary volcanics and sedimentary rocks. The Fish Lake Plateau was glaciated at least twice during the Pleistocene. The UM Creek drainage basin (~110 km²) is located in the northeastern part of the Fish Lake Plateau with elevations between 3540 – 2440 m. UM Creek flows south through the elongate N-S trending basin (~110km²) that empties into the Fremont River. The structural geometry of the UM Creek basin is dominated by NNW trending grabens and their bounding normal faults. Geologic mapping in the northeastern Fish Lake Plateau reveals abundant surficial deposits overlying Tertiary bedrock units. Many of these Quaternary deposits formed due to landslide and mass wasting processes associated with the failure of weak silty limestones in the Flagstaff Formation. The upper UM Creek basin is extensively mantled by surficial deposits, whereas in the south, lower UM Creek flows over a predominately bedrock channel. Geologic mapping and kinematic GPS data suggest that Water Flat, a ~1 km² low-relief flat underlain by shallow lacustrine deposits, was dammed by a large debris flow event that covers ~3 km². In addition, another large mass movement debris flow was mapped that covers ~3.4km². Danish Meadows, an elongate ~2 km² flat, formed by Quaternary slip on a cross fault that temporarily truncated the UM Creek drainage. In the upper UM Creek basin the Potholes, a topographically hummocky south-facing feature (~2800 m), forms a prominent terminal moraine complex. Three trachyandesite boulders were sampled along the crest of the Potholes moraine for ³He exposure age dating using pyroxenes. Pending results from mass spectrometry analysis, exposure ages will likely correlate with the Last Glacial Maximum (ca.18-20 ka) ages obtained from similar moraines on the Fish Lake Plateau.