Stratigraphic Response to Tectonic Forcing in the Cretaceous Western Interior Seaway,

Lewis Shale and Fox Hills Sandstone, Wyoming

By

David R. Pyles

University of Colorado, Boulder, Boulder, CO

Roger M. Slatt

University of Oklahoma, Norman, OK

The lower Maastrichtian Lewis Shale and Fox Hills Sandstone of south-central Wyoming were deposited in a tectonically active embayment during the final major transgression and regression of the Cretaceous Western Interior seaway. Localized uplifts along the margins of the embayment, coupled with changes in sediment supply into the embayment, had a profound effect on basin physiography, water depths, stacking patterns, facies and sediment-accumulation rates. Four uplift events impacted Lewis/Fox Hills deposition. The first three occurred at the ancestral Lost Soldier anticline area, creating a shelf-slope-basin physiography, which would remain through Lewis time. Water depths during this time were up to 500 meters (without considering sediment decompaction). The fourth uplift occurred at the Sierra Madre uplift. Sediment supply from the north increased during late Lewis and Fox Hills time. The combination of localized uplifts at the ancestral Lost Soldier anticline and increasing sediment supply into the embayed depositional basin resulted in aggradational/progradational stacking patterns. Thick-bedded turbidites and debris-flow deposits, which punctuate the pelagic and hemipelagic mudstones in the slope and basin facies tracts, began with the development of the shelf-slope-basin physiography. Compacted sediment accumulation rates in the depositional basin were 30 cm/ky during tectonically inactive times and up to 430 cm/ky during tectonically active times. The shelf-slope-basin physiography and thick-bedded turbidites and debris-flow deposits are anomalous to all other Cretaceous Western Interior seaway deposits.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming