Jurassic Onset of Foreland Basin Deposition in Northwestern Montana, USA: Implications for Synchronous Cordilleran Orogenic Activity
University of Arizona, Department of Geosciences Tucson, Arizona; [email protected]
Stratigraphic, provenance and subsidence analysis suggest that by the Middle to Late Jurassic a foreland basin system was active in northwestern Montana. U-Pb ages of detrital zircons and detrital modes of sandstones indicate provenance from accretioned terranes to the west and from deformed miogeoclinal rocks. Sediment-accumulation history shows onset of subsidence at ~170 Ma and a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits of the Ellis Group and Morrison Formation probably accumulated in a backbulge depozone. A regional unconformity/paleosol condensation zone separates these deposits from unambiguous Early Cretaceous foredeep deposits of the Kootenai Formation. This unconformity possibly resulted from forebulge migration and was accentuated by a global sea level fall. This model is consistent with regional deformation events registered in hinterland regions, and challenges previous interpretations of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern Montana to Alberta, Canada.
AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid