CORRELATION OF MIDDLE PROTEROZOIC FLUVIAL DEPOSITS BETWEEN THE SOUTHERN BEAVERHEAD AND LEMHI MOUNTAIN RANGES, 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 the area suggested a Precambrian age with possible correlation to the Wilbert or Swauger Formations. One report refers to these 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 is unlikely, as these units are lithologically dissimilar to the section in Pierce Canyon. The Flathead Sandstone was 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 (altered to quartzite) in fining-upward successions with scattered thin lenses of mudstone (now slate). Differences between the localities are mainly in overall bed thickness and grain size and are interpreted to be the result of lateral facies variations. The results of this study are preliminary and further work is planned, including additional measured sections, petrographic analysis, and detrital zircon dating for more certain identification and correlation.