SEDIMENTARY AND TECTONIC ANALYSIS OF THE KISHENEHN BASIN, NORTHWEST MONTANA, AS AN ANALOG FOR TERTIARY EXTENSIONAL BASINS OF THE WESTERN UNITED STATES

The Kishenehn basin in NW Montana is unique among sedimentary basins of the western U.S. in that it contains a nearly complete paleontological and sedimentological record across the poorly constrained Eocene-Oligocene boundary. Using lithofacies analysis to recognize the modifications in paleoenvironmental conditions within the basin, we gain a more detailed understanding of sedimentary processes that are synchronous with the earliest phase of post-Laramide extension. A plethora of pollen and molluscan age data from the Kishenehn Formation has been used to postulate a fundamental tectonic change in the Rocky Mountains from crustal shortening to extension. During an episode of post-Laramide crustal extension from the mid-Eocene to the early Miocene (49-20 Ma), extension of the Cordilleran fold-thrust belt resulted in evolution of the Kishenehn basin as a depositional zone. Kishenehn growth strata indicate continuous syntectonic accumulation of approximately 3 km of nonmarine sediments within the basin while subsidence occurred along the Flathead listric normal fault at the eastern boundary of the basin. A portion of this fault is recognized as an extensional reactivation of a footwall ramp of the Lewis thrust, a Laramide structure exposed in Glacier National Park. Kishenehn sedimentation occurred under paludal, lacustrine, and other transient paleoenvironmental conditions presently recognizable as individual sedimentary lithofacies. Using preexisting paleontological age constraints from throughout the Kishenehn basin, a detailed sedimentological study along the middle fork of the Flathead River in the southernmost Kishenehn basin suggests a progressive change in the nature of deposition and depositional environment that is evident on a detailed scale. This evolutionary interpretation may be used as an analog for other Tertiary extensional basins of the western United States.