Tectonic Control on Sediment Delivery to the Cordilleran Marine Foredeep, Early Cretaceous (Albian-Cenomanian), Southwest Montana
Department of Earth Sciences, Montana State University, Bozeman, MT
Regional tectonism in the Cordilleran fold-thrust belt strongly influenced deposition of the Early Cretaceous (Albian-Cenomanian) Blackleaf Formation and Thermopolis Shale in southwest Montana. Detailed sedimentological and stratigraphic field investigations provide evidence for thirteen lithofacies, which predictably recur as six lithofacies assemblages, representing distinct depositional episodes within the Cordilleran foredeep. These lithofacies assemblages are interpreted as: 1) fluvial/alluvial plain; 2) middle- to lower-shoreface; 3) offshore-transition; 4) offshore; 5) offshore with rare storm beds; and 6) offshore with abundant hyperpycnites.Alluvial strata of the Blackleaf Formation are conformable with underlying Kootenai Formation deposits. A transgressive unconformity, representing a sequence boundary, separates alluvial sediments from overlying shoreface intervals that grade upward into offshore-transition, mud-rich offshore deposits, and sand-rich storm- and hyperpycnal flow-influenced offshore deposits. Subsequent shoreface deposits grade upward into nonmarine strata, recording basin shallowing.Mudrock-dominated offshore deposits of these strata indicate increased accommodation and starved basin conditions, and may be interpreted as the result of two end-member processes: thrust emplacement causing rapid foredeep subsidence or eustatic sea level rise. Sand-rich hyperpycnites, present stratigraphically above offshore mudrock, record a fundamental hydrologic reorganization of the sediment delivery system and provide evidence for a tectonically-dominated basin. Hyperpycnal flows typically develop where small drainages deliver abundant sediment, such that resultant high-density flows evolve into negatively buoyant underflows upon debouching into a basin; such conditions were achieved in nearby mountainous drainages of the tectonically active Cordilleran orogen. Thrust emplacement provided accommodation and partitioned hinterland drainages; erosional denudation of the highlands, as recorded by hyperpycnal flow deposits, subsequently reduced the tectonic load, and hence rate of basin flexure, providing a mechanism for regional shallowing independent of eustasy.
AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid