MESOPROTEROZOIC LEMHI STRATA REPRESENT IMMENSE ALLUVIAL APRONS THAT PROGRADED NORTHWEST INTO THE BELT SEA, IDAHO AND MONTANA
Specific Lemhi-Belt correlations are less certain. New mapping of the Lemhi Group shows that the Inyo Creek and West Fork Fms are part of the Yellow Lake Fm, making the Big Creek Fm the lowest exposed Lemhi unit. Winston had correlated the Big Creek Fm with the Revett Fm of the Ravalli Group, and the overlying Yellow Lake Fm with the Piegan Group. In the Sapphire and northern Bitterroot Mountains of Montana, thick sandy units correlated with the Gunsight and Swauger Fms of the Lemhi strata directly overlie the Piegan Group, which is consistent with Winston’s correlations, leaving the Gunsight, Swauger, Lawson Creek, and Apple Creek Formations correlative with the Missoula Group. Alternatively, the Big Creek could be part of the sand sheet that fed the Wallace sea, so the Yellow Lake and Gunsight would be Snowslip, Shepard and Mt Shields Formations of the lower Missoula Group. The distribution and map patterns of the sandy Lemhi strata, although disrupted by Cretaceous-Tertiary tectonism, suggest that immense alluvial aprons or fans prograded north to northwest into the main Belt basin.