STATEMAP RESULTS FROM THE IDAHO-MONTANA LINE: PROTEROZOIC STRATA ARE THICKER AND YOUNGER THAN PREVIOUSLY THOUGHT

The relationship between Mesoproterozoic strata of the Lemhi subbasin of the Belt Supergroup in southwest Montana and eastern Idaho and strata typical of the Belt Supergroup has been debated for decades. One contribution to disparate correlations is that not all of the strata in the Lemhi subbasin have been described. Our STATEMAP work from 2007 to 2013, primarily in the Beaverhead Mountains, has identified a very thick succession (>10,000 m) of east-facing strata in the range. Included in this is more than 5,000 m of poorly sorted, typically medium-grained quartzite that we identify as Swauger Formation because of its similarity to the Swauger Formation in the Lemhi Range and because of its stratigraphic position gradationally above fine-grained quartzite similar to the Gunsight Formation, which lies below the Swauger in the Lemhi Range. In the Lemhi Range, the Swauger grades up into argillite-bearing strata of the Lawson Creek Formation. A similar succession upward into argillite-bearing strata is present in the Beaverhead Mountains. There the argillite-rich strata grade upward into a thick succession of very fine to fine-grained quartzite. A simple explanation is that highest strata in the Beaverhead Mountains comprise Gunsight grading up through the Swauger and Lawson Creek formations into more than 4,300 m of stratigraphically higher rocks that have not been described as such. We speculate that the same upper part of the section exists west of Lost Trail Pass, as well as in the Lemhi Range, where we reinterpret the Lem Peak fault as a conformable contact. It seems likely that the Swauger and overlying units correlate with the Bonner, McNamara, and Garnet Range formations of the Belt-Purcell Supergroup, but their greater thickness supports deposition in a more rapidly subsiding subbasin. In addition, tentative correlation of strata west of the Beaverhead Divide with lower Lemhi Group units may add about 2400 m to the bottom of the Lemhi Group, whose base is still not known. These stratigraphic discoveries have aided in identification of two regionally extensive but previously unmapped thrust faults.