Abstract
The Apple Creek Formation above the Swauger and Lawson Creek Formations comprises the upper part of the Lemhi subbasin strata of the Mesoproterozoic Belt Supergroup in the vicinity of Salmon, Idaho. Lithologic variation of sediment types and features within the Apple Creek Formation reflects the migration, change, or end of depositional environments. Some of this variation may record the effects of syndepositional tectonism, such as the initiation and activity of growth faults. A hindrance to environmental reconstructions is that the strata are exposed on several thrust plates whose original locations are poorly constrained. The Poison Creek thrust plate carries laterally varied Apple Creek units that are not known on the North Fork plate and are much different from those on the Jackson Plate. The Poison Creek fault is yet untraceable through the Mesoproterozoic igneous complex, which intruded rocks now on both the Poison Creek and North Fork plates. To the north-northwest, lack of the distinctive quartzite of the Swauger Formation and increased metamorphism and deformation preclude recognition of those plates. However, the North Fork Thrust may have continued northward to east of Elk City, where the Green Mountain Fault separates metamorphosed Lemhi Subbasin strata and Mesoproterozoic augen gneiss from quartzite and carbonate rocks likely part of the main Belt Basin. If so, strata northeast of the North Fork and Trail Gulch-Freeman Thrusts may correlate with strata of the main Belt Basin.