REVISION OF LEMHI SUBBASIN STRATIGRAPHY TO ACCOMMODATE NEWLY RECOGNIZED MESOPROTEROZOIC STRATA OF THE BELT-PURCELL SUPERGROUP, IDAHO AND MONTANA

Cooperative field mapping by the Idaho Geological Survey and the Montana Bureau of Mines and Geology from 2007 through 2013, primarily in the Beaverhead Mountains along the Idaho/Montana border, shows that a very thick east-facing succession (> 10,000 m), mostly east of the Beaverhead Divide in Montana, is above the Gunsight Formation of the Lemhi Group. Reconnaissance in the Lemhi Range to correlate the Beaverhead succession with the Lemhi Group found that the overlying Swauger and Lawson Creek formations, the highest units previously described in the Lemhi subbasin, grade upward into a thick quartzite section that in turn grades up into the type Apple Creek Formation. This new insight led to the recognition that the base of the Beaverhead sequence includes the Swauger and Lawson Creek formations and that the overlying strata (quartzite of Janke Lake) are correlative with the quartzite below the type Apple Creek Formation in the Lemhi Range.

We propose a revised stratigraphic nomenclature that excludes the crinkle cracked carbonate-bearing unit below the Gunsight Formation from the Apple Creek, redefining it as the new Yellow Lake Formation. Redefinition also expands the type Apple Creek Formation downward, designating the thick quartzite package above the Lawson Creek Formation as the Lem Peak member of the Apple Creek, and includes all in the Lemhi Group. Together, these changes clarify the regional Mesoproterozoic stratigraphy by correcting previous miscorrelations.

The areal distribution of the upper quartzitic strata extends the Lemhi subbasin east and north into Montana and farther northwest in Idaho than previously recognized. Regional correlation of this thick and young quartzite sequence with the Bonner Formation and higher units of the main Belt basin requires that subsidence of the Lemhi subbasin was significantly faster than that of the main Belt basin during deposition of the upper Belt strata. Documenting how Mesoproterozoic strata of the Lemhi Range and Salmon River and Beaverhead mountains of east-central Idaho relate to better known Belt units to the north in both Idaho and western Montana continues to build upon the relentless efforts of Don Winston, whose pioneering stratigraphic work, encouragement, tutoring, and inspiration throughout the Belt basin make it all possible.