IDENTIFICATION AND CORRELATION OF PRECAMBRIAN FLUVIAL UNITS IN THE SOUTHERN BEAVERHEAD MOUNTAIN RANGE, IDAHO

Geologic mapping in the southernmost Beaverhead Mountain Range of central Idaho has revealed a previously undescribed unit. Based on stratigraphic position, earlier reconnaissance mapping in nearby areas has suggested a Precambrian age with correlation to either the Wilbert or the Swauger Formation. Another report refers to similar exposures as “unnamed Cambrian.” As part of this study, a reference section was measured in Pierce Canyon along the western flank of the southern Beaverhead Range and visually compared to type localities for the Wilbert, Swauger and other formations located in the Lemhi Mountain Range to the west. Comparison was also made to the Cambrian Flathead Sandstone found in the Teton Range to the east. Correlation to the Wilbert and Swauger Formations was dismissed, as these units are lithologically dissimilar to the section in Pierce Canyon. The Flathead Sandstone was also ruled out, as no other known Cambrian outcrops are found in the Beaverhead or adjacent mountain ranges and the unit in question appears to have been subjected to significantly deeper burial.

A better match for the Pierce Canyon exposures appears to be the Gunsight Formation, found on the western flank of the southern and central Lemhi Range. Both units consist primarily of reddish, cross-bedded sandstone (now more of a quartzite) in fining-upward successions with scattered thin lenses of mudstone (now slate). Differences in overall bed thickness and grain size may be lateral facies variations. Additional sections will be measured in both localities and combined with a petrographic analysis to determine potential correlation.