The stratigraphic, sedimentologic, and paleogeographic evolution of the Eocene-Oligocene Grasshopper extensional basin, southwest Montana

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, fluvial, deltaic, and lacustrine sedimentary and volcanic rocks.; Sequence 2, representing 90% of the total basin-fill, is the synrift deposit for the Muddy-Grasshopper fault. This sequence was dominated by lakes that filled axially by rivers from the north and transverse sediment influx along the eastern and western margins. Paleogeographic reconstructions reveal the lack of a southern basin margin. Correlating the basin-fills contained in the Medicine Lodge and Horse Prairie basins to the south with sequence 2 in Grasshopper basin suggests that each basin represents a third of a larger preexisting extensional basin that was partially dismembered by later phases of extension.