THE POISON CREEK FORMATION: A COARSE FAN-DELTA FACIES IN THE MIOCENE CHALK HILLS FORMATION OF THE WESTERN SNAKE RIVER PLAIN, IDAHO

This study documents sedimentological features of a fan delta within the upper Miocene Chalk Hills Formation. Cemented coarse, subangular sand and gravel, ranging in thickness from 2 plus meters to 40 meters, form mesas and cuestas along the Owyhee Mountain front for a distance of 4 kms northwest of Poison Creek, Sections 17 and 8, T.2 N., R.5 W. The coarse sediments are overlain and underlain by lacustrine mudstone. Thickness and grain size of coarse sediment diminishes and thins to the NW and SE, indicating the geometry of a fan delta. The center of the deposit is composed of coarse sandy gravel layers with basal scour surfaces. The layers range from 0.5 to 3 meters thick. Five stratigraphic columns on a NW-SE transect illustrate the lateral change in sediment structure, grain size, and thickness of the fan-delta. These coarse sand and gravel deposits are underlain by about 80 to 100 meters of mudstone, which rests upon the Jump Creek Rhyolite (10.9 Ma, Ekren et al., 1981). These sandstones and conglomerates were formerly called a type locality of the Poison Creek Formation, but are now regarded as a coarse facies associated with a mountain drainage basin that emptied into the lake during Chalk Hills Formation time.