THE MESOPROTEROZOIC BELT–LEMHI CONNECTION, WESTERN MONTANA AND EAST-CENTRAL IDAHO

The Mesoproterozoic Belt-Purcell Supergroup’s immense thickness and extent, its repetitious and monotonous lithologies, its dismemberment by multiple tectonic events, and its obscuration by magmatism and metamorphism, have confounded correlations with the Mesoproterozoic Lemhi strata of east-central Idaho for decades. New and old field work combined with detrital zircon data support the correlation of the entire Lemhi subbasin section with the Missoula Group of the upper Belt Supergroup in western Montana. The Lemhi subbasin strata are much sandier and thicker than the equivalent Missoula Group strata and represent the upstream ends of immense alluvial aprons that originated to the south and southeast. These sand tongues alternately advanced into and retreated from the Belt Sea during Missoula Group time. Similarly, the coarser Lemhi strata also grade laterally westward into the finer grained Lemhi rocks of the Salmon River Mountains. Although the Bonner and McNamara Formations of the Belt are tentatively correlated with the Swauger and Lawson Creek Formations of the Lemhi subbasin, other specific formation correlations remain uncertain. During Missoula Group time, subsidence in the Lemhi subbasin must have been much faster than in the main Belt basin in order to accommodate the much thicker Lemhi section. No strata older than the Missoula Group are known in the Lemhi subbasin, and it is uncertain whether the Belt basin extended this far south in pre-Missoula Group time. It is also uncertain whether the central Lemhi subbasin of the Salmon River Mountains was connected to the main Belt basin, but the two are time correlative.