Print this pageStratigraphic and Sedimentologic Analysis of Nonvolcanic Strata in the Late Cretaceous Golden Spike Formation, Western Montana
WADDELL, AMY M., and BENJAMIN M. WEBB
During Campanian time in southwestern Montana, propagating thrust sheets of the western Sevier orogen shed sediment into an approximate 9-mile long E-W oriented trough. Concurrently, the Elkhorn Mountains to the East supplied this basin with andesitic lava flows and volcaniclastic debris. This amalgamation of interfingered nonvolcanic and volcanic strata forms the Golden Spike formation.
Recent stratigraphic and sedimentologic research of nonvolcanic units in the Garrison, MT area provides a more detailed interpretation of the facies and deposystems associated with the Golden Spike, and provides first-time analyses of provenance information from this synorogenic unit. Crossbedded fluvial channel deposits, composed of sandstone and pebbly conglomerate, interfinger laterally and vertically with floodplain shales, paleosols, and thin crevasse splay deposits. Locally, river channel deposits interfinger with cobble- to pebble- bearing mudstones which we tentatively interpret as the distal deposits of debris flows. These relations imply a pediment topography dissected by meandering rivers early in the Golden Spike's deposition. Upsection, the Golden Spike comprises a poorly indurated megaconglomerate ("Chaos beds" of Gwinn and Mutch, 1965) that we interpret as proximal debris flow deposits likely related to the eastward propagation of thrust sheets. Analysis of the Chaos beds includes photo mosaic delineation of sedimentary and structural relations and cobble count provenance interpretation. While pebble to boulder sized nonvolcanic and volcaniclastic clasts both compose the Chaos beds, nonvolcanic debris constitutes the majority. Provenance observations suggest that clast compositions change upsection within the megaconglomerate with clasts of gray biomicrite (Madison Group) supplanting red quartzitic (Belt Supergroup) and black chert (Madison and/or Phosphoria?) lithologies.