MESOPROTEROZOIC LEMHI STRATA REPRESENT IMMENSE ALLUVIAL APRONS THAT PROGRADED NORTHWEST INTO THE BELT SEA, IDAHO AND MONTANA

The Lemhi Group and related strata of the Lemhi subbasin in Idaho and Montana comprise a very thick (>14,000 m) succession of mainly feldspathic quartzite deposited in a southwestern arm of the Mesoproterozoic Belt basin. New mapping and published detrital zircon data support correlation of most of the Lemhi subbasin strata with the much thinner (<3,500 m) and finer-grained Missoula Group of the Belt Supergroup. Stratigraphic sections show that Lemhi subbasin strata interfinger with and grade laterally northward into the Missoula Group units as sand and gravel tongues that pinch out basinward. Lemhi subbasin strata show similar trends of fining and thinning westward into the Salmon River Mountains, where the Gunsight and Swauger Fms may grade laterally into the Yellowjacket and Hoodoo Fms, respectively, and the overlying Apple Creek Fm changes facies, with sedimentary structures there indicative of more sub-aqueous deposition. Don Winston postulated that the Missoula Group represents a series of sandy alluvial aprons that prograded into the Belt basin, and this study suggests that most of the Lemhi subbasin strata are relicts of the southeastern, upstream ends of these alluvial wedges.

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.