ACCOMMODATION-LIMITED DELTA PROGRADATION RECORDED IN THE UPPER CRETACEOUS (CENOMANIAN-TURONIAN) FRONTIER FORMATION, NORTHEAST BIGHORN BASIN, WYOMING, USA

A detailed facies analysis of the Upper Cretaceous (Cenomanian-Turonian) Frontier Formation was conducted over a ~ 30 kilometer outcrop belt in the northeast Bighorn Basin, Wyoming, U.S.A. Nine recurring lithofacies are identified: A. dark-gray laminated mudstone with bentonite intervals (offshore marine), B. dark-gray laminated mudstone with thin sandstone intervals (prodelta), C. thinly interbedded siltstone and fine-grained sandstone (distal delta front), D. thickly bedded, sharp-based sandstones with thin siltstone partings (middle delta front), E. amalgamated, sharp-based sandstones (proximal delta front/river mouth), F. unbioturbated fine-grained sandstone lacking siltstone partings (upper shoreface), G. brown, fissile, laminated siltstone with abundant plant debris (coastal/alluvial floodplain), H. trough cross-bedded sandstone (coastal fluvial channel), I. erosionally-based, low-relief, laterally extensive pebble/cobble accumulations (transgressive lag). Correlation of key stratigraphic surfaces (pebble lags, bentonites) and individual sandstone beds, and analysis of vertical facies stacking patterns indicate the preservation of multiple coarsening-upward cycles. Such cycles consist of basal offshore marine/prodelta facies overlain by progressively more proximal deposits, containing sedimentary features consistent with the deposition and progradation of wave- (hummocky cross-stratification, symmetrical ripples) and tide- (bimodal cross-bedding) influenced, fluvially-dominated (syneresis cracks, current ripples and cross-bedding, impoverished trace fossil suites) deltas. Southward-directed cross-bedding (Peay and Torchlight Members) and gently-dipping clinoforms (Facies D, E; Peay Member) indicate the broadly southward progradation of delta fronts into shallow marine settings under accommodation-limited conditions. Cycles lacking significant sandstone accumulations (Facies D, E) likely represent distal expressions of individual progradational events. Occurrences of cycle-capping pebble lags (Facies I) suggest that transgressions led to the significant winnowing and top-truncation of accommodation-limited accumulations, generating isolated, mudstone-encased sandstone bodies observed throughout the Frontier Formation.