Structure and stratigraphy of the Little Sheep Creek area, Beaverhead County, Montana

Graduation date: 1981The Middle and West Forks of Little Sheep Creek in the southern Tendoy Range have incised valleys across Cenozoic structural features exposing strata that range in age from the Mississippian to the Neoene. Paleozoic strata are 1,349 m thick and belong to the Mission Canyon Limestone, the Big Snowy, Amsden, Quadrant, and Priosphoria FormationS. The Scott Peak Formation of the White Knob Group of Idaho is allochtononous and forms the upper plate of the Medicine Lodge thrust. Paleozoic and Triassic strata of the thesis area represent sedimentation across a transition zone between a stable craton to the east and the Cordilleran miogeosyncline to the west. Regional unconformities are recognized locally at the top of the Mission Canyon Limestone and the Phosphoria and Thaynes Formations, but not at the top o the Big Snowy, Amsden, or Quadrant Formations, Mesozoic strata have a total thickness of 1,404 m and belong to the Dinwoody, Woodside, and Thaynes Formations, the newly recognized Gypsum Spring Tongue of the Twin Creek Formation, the Sawtooth and Rierdon Formations of the Ellis Group, the Morrison and Kootenai Formations, the Colorado Shale, and the Beaverhead Formation. Cenozoic strata are represented by the newly named Round Timber limestone (informal) of the Medicine Lodge beds (Miocene) and the Edie School rhyolite (Pliocene). Detailed stratigraphic and petrographic analyses were made of the Triassic Dinwoody, Woodside, and Thaynes Formations, the Jurassic Gypsum Spring Tongue, and the Niocene Round Timber limestone in order to determine environments of deposition. The limestones, calcareous siltstones, and silty limestones of the Dinwoody and Thaynes Formations were deposited in a shallow marine shelf environment as a result of two transgressive pulses separated by an Early Traissic regression. The Triassic seas had transgressed eastward onto the craton from the miogeosyncline. The Dinwoody arid Thaynes fauna indicate normal salinities and open marine conditions; the widespread regional distribution of the limestone-siltstone facies indicates broad equable conditions for sedimentation. Deposition was primarily a tractive process genarated by storm-driven, tidal, and long-shore currents within a maximum depth of approximately 50 m. The Early Triassic regressive phase is represented by the deposition of the variegated siltstone, sandstone, limestone, and dolomite of the Woodside Formation in a tidal flat environment. The Gypsum Spring Tongue consists of interbedded variegated siltstone, sandstone, limestone, dolomite, and limestone conglomerate that were deposited in a tidal flat and restricted marginal marine environment extending east from southwestern Montana and eastern Idaho across Wyoming arid southern Montana. The Middle Jurassic sea transgressed South across the North American continent. The Miocene Round Timber limestone was deposited in a fresh-water lake in which calcite was being deposited as encrustations on the green algae Chara and as a precipitate directly from solution. The folding and faulting within the thesis area are the result of cratonic and miogeosynclinal responses to Cretaceous-Early Tertiary orogenesis. A southwestward-plunging anticline has been refolded into northeastward-yielding, overturned, doubly-plunging folds oriented northwest-southeast. High angle reverse faults of minor displacement have occurred along the southeastern limb of Garfield anticline and within the axis of the Seybold syncline. Post-Laramide relaxation of compressional forces has caused north- and northwest-oriented normal faults that transect earlier structures. Sandstone and limestone of the Scott Peak Formation of the White Knob Group were implaced along the Medicine Lodge thrust