Date of Award:
Master of Science (MS)
Susanne U. Janecke
Grasshopper basin, located in southwest Montana, is an east-tilted graben bounded by the listric Muddy-Grasshopper fault and the Meriwether Lewis fault on the eastern and western margins of the basin, respectively. This basin contains a complex stratigraphy of intertonguing facies comprised of five unconformity-bounded sequences of Tertiary alluvial, flu vial, deltaic, and lacustrine sedimentary and volcanic rocks. Sequence 1 consists of the Challis volcanic Group (Middle Eocene). The sedimentary rocks of the Medicine Lodge beds (Late Eocene-Late Oligocene) represent sequence 2 and approximately 90% of the basin-fill within Grasshopper basin. Sequence 3 consists 11 of the Sedimentary Rocks of Everson Creek (Late Oligocene-Early Miocene), sequence 4 is represented by the Sedimentary Rocks of Bannack Pass (Middle-Late Miocene), and the Six Mile Creek Formation (Late Miocene?) corresponds to sequence 5.
Sequence 2 is the synrift deposit for the Muddy-Grasshopper fault, and was dominated by lakes that filled axially by rivers from the north. Transverse sediment influx was present on small fan-delta complexes shed into the lake from the eastern margin and periodic large fluvial-dominated alluvial fan and deltaic deposition from the western margin. Paleocurrent analyses are consistent with these observations and show predominantly south-southeastward axial paleoflow directions with west-directed and east-directed paleoflow on the eastern and western margins, respectively. Petrologic studies, including sandstone petrography and conglomerate petrology, reveal a mixed "basement uplift" and "recycled orogen" tectonic provenance. These findings support a model for Eocene-Oligocene rifting characterized by moderate to high relief superimposed on the Cretaceous-Early Tertiary Sevier fold-and-thrust belt.
Paleogeographic reconstructions of Grasshopper basin reveal the lack of a southern basin margin. A correlation of the basin-fill contained in the Medicine Lodge and Horse Prairie basins to the south with the Medicine Lodge beds (sequence 2) in Grasshopper basin suggests that each of these basins represents a third of a larger preexisting extensional basin that was partially dismembered by later phases of continued extension. Large extensional folds in Grasshopper basin had a small influence on facies architecture in the basin.
Matoush, Joseph P., "The Stratigraphic, Sedimentologic, and Paleogeographic Evolution of the Eocene- Oligocene Grasshopper Extensional Basin, Southwest Montana" (2002). All Graduate Theses and Dissertations. 6726.