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Montana State University, Bozeman, MT

Abstract: Fluvial Channel Systems of the Upper Cretaceous (Campanian) Two Medicine Formation, Willow Creek Anticline, Choteau, MT, USA

The Campanian Two Medicine Formation of western Montana represents alluvial sediments shed from the western Montana thrust belt during the Sevier orogeny. Two major sandstone bodies in the upper lithofacies of the Two Medicine Formation at the Willow Creek anticline are interpreted as fluvial channel deposits.

The two sandstone units (2-5m thick; >100-400 m in lateral extent) are separated by approximately 1 km and represent broadly coeval channel systems. The sandstone units have tabular geometry with planar lower bounding surfaces. Underlying overbank deposits grade upward from mudstone into the basal fine-grained sandstone of the channel bodies, suggesting widening of sandy stream beds through time. Upper bounding surfaces are generally abrupt and overlain by overbank mudstone.

Internal concave-up bounding surfaces separate cosets of trough cross-stratified sandstone, commonly truncating older cosets. Trough cross-stratification produced by migration of 2-D and/or 3-D dunes is ubiquitous and indicates deposition under lower flow regime conditions. Angular to subangular mud intraclasts in a silty, poorly sorted, fine-grained sandstone matrix are present at the base of most coset bounding surfaces suggesting reworking of floodplain fines and short intraclast transport distance during flooding events. Moderately to well-sorted sandstone exhibiting relict, faint lamination is interpreted as horizontally bedded sandstone formed during lower plane bed flow conditions and subsequently bioturbated. This interpretation is supported by the presence of rhizoliths and scattered intraclasts, suggesting deposition during channel avulsion events. The assemblage of sedimentary structures suggests a channel environment characterized by shallow, lower flow regime conditions in an anastamosing or sand-bed braided river system.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana