TECTONIC CONTROL ON SEDIMENT DELIVERY TO THE CORDILLERAN MARINE FOREDEEP, EARLY CRETACEOUS ALBIAN-CENOMANIAN, SOUTHWEST MONTANA

Deposition of the Early Cretaceous (Albian-Cenomanian) Blackleaf Formation and Thermopolis Shale in southwest Montana was strongly influenced by regional tectonism in the Cordilleran fold-thrust belt. Detailed sedimentological and stratigraphic field investigations provide evidence for thirteen lithofacies, which predictably recur as six lithofacies assemblages, representing distinct depositional episodes within this portion of 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 these 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.

Offshore, mudrock-dominated deposits of the Blackleaf Formation and Thermopolis Shale 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, while 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.