Sediment Provenance and Dispersal in the Late Cretaceous Cordilleran Foreland Basin of Montana

Schmitt, James G. ¹ (1) Montana State University, Bozeman, MT

Sediment delivered to the Cordilleran foreland basin of Montana during Late Cretaceous time was derived from several tectono-magmatic elements of the adjacent orogenic wedge. The Helena salient of the Cordilleran thrust belt in west-central Montana served as the major source of siliciclastic (pre-80 Ma) and volcaniclastic (post- 80Ma) sediment for fluvial systems draining the orogenic wedge during Late Cretaceous time. Eruption of the Elkhorn Mountains volcanic field (80-70 Ma) within the Helena salient produced abundant pyroclastic (Campanian Two Medicine-Judith River clastic wedge) and epiclastic (Maestrichthian St. Mary River/ Hell Creek formations) sediment that overwhelmed fluvial dispersal systems well into Paleogene time.

In the southwest Montana re-entrant, the Snake River conglomerate complex, comprising strata of the Upper Cretaceous-Paleocene Beaverhead Group and adjacent Harebell (Upper Cretaceous) and Pinyon (Upper Cretaceous-Paleocene) formations of northwest Wyoming, was deposited by a highly-integrated drainage system occupying a major long-lived transverse structural zone that delivered an abundance of quartzite detritus from the interior of the orogenic wedge. Thrust-cored anticlinal ridges bounding piggyback basins adjacent to this fluvial megafan system sourced alluvial fans comprising locally-derived sedimentary lithic detritus with a recycled orogen provenance.

In northwest Montana, the proximal record of the sediment dispersal systems is missing due to erosion and structural burial beneath thrust allochthons. Ash-fall processes driven by explosive activity in the nearby volcanic arc delivered a major component of fine-grained sediment to the Western Interior seaway. During latest Cretaceous time, Laramide-style uplifts became locally important sources of sediment within the Cordilleran foreland basin.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana