Late Eocene-Oligocene nonmarine mollusks of the northern Kishenehn Basin, Montana and British Columbia

Abstract

The Kishenehn Basin is a narrow, asymmetric graben, developed during an episode of crustal extension occurring from middle Eocene to early Miocene time . The Flathead listric normal fault system defines the northeast margin of the basin. The southwest margin is either bounded by antithetic normal faults or is onlapped by Kishenehn Basin strata. High rates of basin subsidence and synextensional sedimentation resulted in an immense thickness of middle Eocene-early Miocene strata (stratigraphic thickness, -5000 m) and a relatively uninterupted depositional record. Provenance and paleocurrent studies indicate that the ancestral Livingston, Lewis, and Clark ranges to the east contributed not only vast amounts of detritus to the Basin, but also ample runoff that included seasonal snow-melt. Sediment ages of outcrops within the basin are linearly banded, with the course of the Flathead River exposing late Eocene-early Oligocene sediments (Chadronian-Orellan NALMA's), and with the oldest sediments on the southwest and youngest on the east margins of the basin, respectively. An unnamed tephra at the north end of the basin provides a date of 33.01.0 Ma (early Oligocene, Orellan NALMA).