Tectonic Controls on Alluvial Systems in a Distal Foreland Basin: The Lakota and Cloverly Formations (Early Cretaceous) in Wyoming, Montana and South Dakota

Integrated surface and subsurface stratigraphic and sedimentologic analysis of the nonmarine Lower Cretaceous rocks along the eastern margin of the Rocky Mountain foreland demonstrates the critical role of intrabasinal and cratonic deformation in controlling alluvial architecture in a distal foreland basin setting Three coarse grained intervals within IheLakota Formation can be recognized and correlated throughout the Black Hills and into the subsurface of the Powder River Basin We informally designate these intervals from oldest to youngest as L1 L2 and L3 Detritus in the LI interval was deposited in northward flowing high sinuosity rivers The L2 interval lies atop of a regional intraformational angular unconformity which truncates rocks as old as the Middle Jurassic Sundance Formation L2 detritus was deposited by northeastward flowing braided rivers The coincidence of location of these rivers and lineaments reflecting recurrent movement of basement rooted structures is evidence of intrabasinal tectonic control A variety of evidence suggests that this movement was associated with transpression along steeply dipping basement rooted faults As much as 65 m of stratigraphic throw may have occurred on one of these faults in the study area Provenance analysis indicates that sediment deposited by L3 estuarine systems was derived from uplift of the Transcontinental Arch located within the craton approximately 600km southeast of the study area Preliminary chronostratigraphic correlation of Early Cretaceous alluvial deposits throughout Wyoming indicates that intrabasinal deformation may have started as early as 134 Main the western Wind River Basin and progressively advanced eastward through the foreland overapproximately the next 25 my culminating slightly less than I 10 Ma with theuplift of theTranscontinenIal Arch This uplift resulted in a 1200 change in direction of paleoslope in the distal Rocky Mountain foreland