PRELIMINARY GEOLOGY OF THE PROFFIT MOUNTAIN FLOOD SCOUR, REYNOLDS COUNTY, MISSOURI

The 2.4 km-long Proffit Mountain flood scour formed when the upper reservoir of the Taum Sauk Power Plant, a reversible pumped electric storage facility, failed on December 14, 2005. Approximately 1.3 billion gallons of water drained from the reservoir in roughly 12 minutes, scouring a small tributary on the west flank of Proffit Mountain to bedrock and depositing debris in the valley floor and in the valley of the East Fork of the Black River. The stratigraphic succession exposed includes Mesoproterozoic Taum Sauk Rhyolite and Munger Granite, Cambrian conglomerate and dolomite and flood deposits. The site provides a unique opportunity to study a landscape-scale outcrop. Taum Sauk Rhyolite is exposed in the upper part of the scour. The dark red to purplish rhyolite has alkali feldspar and quartz phenocrysts that vary from a few percent to approximately 30 percent of the rock. Exposures exhibit flow bands, pumice fragments and xenoliths or dikes of felsic igneous rock. Several faults in the rhyolite trend nearly east-west and are expressed as linear depressions related to associated shearing.

The Munger Granite is a ring dike composed of red to red-gray, medium- to coarse-grained granite with orthoclase, biotite and hornblende. Low angle exposures display gouging by flood-transported material. The lower part of the granite exposure has extensive post-flood weathering that suggest that this area is a grus.

The Cambrian basal conglomerate onlaps the western part of the Munger Granite exposure. The conglomerate has granite, rhyolite and minor mafic rock clasts cemented by sandstone and minor siltstone. It represents shoreline and near shore facies or a possible alluvial fan margin. Cambrian dolomite is medium-bedded, medium-crystalline and suggestive of a planar stromatolitic origin. It is marked by numerous solution and karst-related features, including a small cave. Dissolution commonly follows joint trends with variable fissure size and depth.

Flood debris deposition is minimal until the slope break on the flank of Proffit Mountain. The material varies from poorly to well sorted; variability may represent different stages in the flood or sediment sieving. Later channels cut early deposits. Several channel margin deposits are suggestive of point bar sequences. Natural levees also developed during flood deposition.