LOW ACCOMMODATION, WAVE DOMINATED LOWSTAND DELTA DEPOSITS OF THE FRONTIER FORMATION IN THE NORTHEAST BIGHORN BASIN, WYOMING

A detailed sedimentologic and stratigraphic analysis of the 100-175 m thick Upper Cretaceous (Cenomanian-Turonian) Frontier Formation was conducted over ~ 30 km of depositional dip-oriented surface exposure in the northeast Bighorn Basin, Wyoming. These sediments were deposited in the Cretaceous Western Interior Seaway (KWIS). The study focused on the characterization of several laterally restricted, north-south-elongate, sandstone bodies of varying thickness, encased in open marine mudstones and siltstones within the Frontier Formation.

Observed sedimentologic and ichnologic characteristics were used to formulate an interpretation of the depositional environment. Sandstone bodies in places preserve gently southward dipping clinoform sets up to 25 m in height. Prominent internal sedimentary features include combined-flow wave ripples, hummocky cross stratification, interference ripples, laterally extensive pebble lags, low- angle to-flat stratification, synaeresis cracks, and trough cross-bedding. Low-diversity trace fossil assemblages are typical, with only up to four commonly recurring ichnotaxa. Paleocurrent data indicate uniformly southward sediment dispersal. The physical and biogenic sedimentary structures are interpreted to represent a brackish to-fresh-water- influenced, wave-dominated delta, with sediment reworking during intervening transgressions.

Several stratal cycles occur in the Frontier Formation, recording relative sea level fluctuations of <?50 m. Cycles are composed of basal prodelta sediments (claystones and siltstones) coarsening up to proximal delta front to river mouth sandstones and capped by pebble lags. Such bodies may be said to be top-truncated in that they preserve no delta platform facies. Sandstones of the basal Peay Member vary in thickness from < 20 m in the northeast portions of the outcrop belt to > 40 m to the southwest. These tabular and sharp-based sandstones do not thicken at the expense of underlying sediments. We argue that these characteristics represent lowstand deposition, possibly under the influence of synsedimentary fault offsets. The lower part of the Peay Member also preserves downdip-descending and offlapping sandstone lenses, potentially evidence for falling stage systems tract deposition beneath the main lowstand delta.