STRATIGRAPHIC ARCHITECTURE AND FACIES CHARACTERIZATION OF THE LATE DEVONIAN/EARLY MISSISSIPPIAN SAPPINGTON FORMATION, BRIDGER RANGE, MONTANA

Detailed facies analysis of the Late Devonian/Early Mississippian Sappington Formation offers insight into a period of complex paleogeography in Montana, characterized by deposition in sub-basins and structural highs, and eustatically- and tectonically-driven transgressive-regressive cycles. The study area is located in the Bridger Range in southwestern Montana and contains some of the most laterally continuous exposure of the Sappington Formation.

The Sappington Fm. is a marine unit comprised of lower and upper organic-rich shale members and a middle calcareous, siltstone to very fine-grained sandstone member. A variety of facies are observed, including organic-rich mudstone, muddy siltstone, cross-stratified and rippled sandstones, and bioturbated sandstones and siltstones. Middle Sappington member ichnofacies include normal-marine, shoreface assemblages of Palaeophycus, Diplocraterion, Planolites, Teichichnus, and Thallasinoides. Sappington Fm. depositional environments therefore record a transition from dysoxic (anoxic?) conditions below wave base for the lower shale, to normal-marine shoreface deposition for the middle siltstones and sandstones.

Depositional elements of the Sappington Fm. change spatially through time. The heterogeneity and geometries of depositional elements preserved in the middle Sappington member are linked to sediment dispersal along a shoreface system of a nearby source area. In the Bridger Range, the southern Sappington Fm. outcrops contain planar and high angle cross stratification and the highest abundance of the coarsest grained sand (upper very fine-grained) in the system. The northern outcrops are more heavily bioturbated, silitier, and contain more bidirectional and unidirectional ripple lamination. Laterally continuous outcrops of Sappington Fm. across the Bridger Range therefore record time-equivalent proximal to distal basin marginal marine facies from south to north. The closest, potential sediment source area for the middle Sappington member is the Beartooth Uplift southeast of the study area. Study results will provide a new framework of the facies heterogeneity observed in the Sappington Fm. in southwestern Montana.