QUATERNARY FAULTING IN THE LITTLE GRAND WASH FAULT, EMERY COUNTY UTAH

Located at the Northern end of the Paradox basin in Emery County, Utah, the Little Grand Wash Fault is an east-west trending south-dipping normal fault. Offset on the fault juxtaposes the Jurassic Morrison Formation against Cretaceous Mancos Shale. This anastomosing fault contains multiple splays and interaction zones and records a complex history of fault movement and multiple fluid flow events. We identify isolated tectonic clasts of fractured pebble conglomerate in the hanging wall damage zone. These tectonic clasts are isolated blocks and surrounded by fault rock thought to be sourced from Quaternary fluvial terrace deposit.

Using detailed clast counts on adjacent Quaternary terrace deposits and those of outwash on the valley floor we verify that fractures and offset observed in the tectonic clasts are associated with faulting. Clasts were grouped into three categories, rounded, fractured, rounded and fractured, and the rock type of each clast was noted. We observed significant differences in the amount of clasts that were rounded (10-75%) and the fractured and rounded (14-72%) at the different localities. Of the rounded and fractured clasts up to 5 mm of shear offset is noted in the field and an average of 0.39 mm fracture aperture in thin section. Results from these counts link the source of the tectonic clasts to the Quaternary fluvial terrace deposits, evaluates the potential for Quaternary fault motion and earthquake strength using published rock strength values.