Abstract

Fault scarps up to 15 m high form a 5-km-long, northeast trending graben on Towanta Flat, an outwash plain on the southwest flank of the Uinta Mountains. Mapping, relative-age dating, and regional correlation of moraines and outwash terraces in the area indicate many of the faulted deposits on Towanta Flat are 250,000 to 500,000 years old. No scarps were mapped in outwash deposits of the “Bull Lake” glaciation that are interpreted to be 60,000 to 150,000 years old. Two backhoe trenches confirm that “Bull Lake” deposits are unfaulted. Colluvial stratigraphy and soil development in a third trench, across a 5-m-high graben scarp, suggest at least three “pre-Bull Lake” surface faulting events with stratigraphic displacements of 2 to 3 m and a recurrence of 50,000 to 100,000 years. Individual fault scarp slip rates derived from scarp and trench data are 0.02 to 0.06 mm/yr, but there is no net tectonic offset across the graben. Although the Towanta Flat scarps may be a response to regional Quaternary tectonic stresses, the scarps are of very limited extent and their orientation does not match that of planes defined by microearthquake studies in the area.