Abstract

A detailed facies analysis of the Upper Cretaceous (Cenomanian) Frontier Formation was conducted over a ~ 35 kilometer outcrop belt in the northeast Bighorn Basin, Wyoming, U.S.A. Nine recurring lithofacies are identified: 1. dark-gray, laminated mudstone with bentonite intervals (offshore marine), 2. dark-gray, laminated mudstone with thin sandstone intervals (prodelta), 3. thinly interbedded siltstone and fine-grained sandstone (distal delta front), 4. thickly bedded, sharp-based sandstones with thin siltstone partings (middle delta front), 5. amalgamated, sharp-based sandstones (proximal delta front/river mouth), 6. non-bioturbated, fine-grained sandstone lacking siltstone partings (upper shoreface), 7. brown, fissile, laminated siltstone with abundant plant debris (coastal/alluvial floodplain), 8. trough cross-bedded sandstone (coastal fluvial channel), and 9. erosional 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 4, 5; Peay Sandstone Member) indicate the broadly southward progradation of digitate delta fronts into shallow-marine settings under accommodation-limited conditions. Cycles lacking significant sandstone accumulations (Facies 4, 5) likely represent distal expressions of individual progradational events. Occurrences of cycle-capping pebble lags (Facies 9) 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.