AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Isolated, Top-Truncated, Wave-Dominated Lowstand Delta Deposits and the Potential for Stratigraphic Entrapment Within the Frontier Formation, Northeast Bighorn Basin, Wyoming
(1) Earth & Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE.
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. The study focused on the characterization of several laterally restricted, north-south-elongate, sandstone bodies of varying thickness enclosed by open marine mudstones, deposited within the Cretaceous Western Interior Seaway (KWIS). Observed sedimentologic and ichnologic characteristics form the basis for an interpretation of formative depositional environment. Hummocky cross stratification, laterally extensive pebble lags, trough cross-bedding, low-angle to-flat stratification, and synaeresis cracks are common internal sedimentary features. Low-diversity trace fossil assemblages are typical, with only four frequently recurring ichnotaxa. Paleocurrent data, as well as gently dipping clinoform sets indicate southward sediment dispersal. The physical and biogenic sedimentary structures are interpreted to represent brackish to-fresh-water-influenced, wave-dominated deltas, with sediment reworking during intervening transgressions. Several stratal cycles occur in the Frontier Formation, consisting of basal prodelta sediments (claystones and siltstones) coarsening up to pebble lag-capped proximal delta front to river mouth sandstones. A lack of preserved delta platform facies, along with occurrences of pebble lags, may indicate top-truncation of sandstones. Correlation of measured sections within the Peay Member demonstrates a northwest to southeast bed thickness increase from < 20 m to > 40 m, and thinning from > 40 m to < 10 m west to east. This thickening and thinning behavior does not occur at the expense of underlying sediments. As such, these characteristics are interpreted to represent possible lowstand deposition, influenced by synsedimentary or immediate post-sedimentary fault offsets. Downdip-descending and offlapping sandstone lenses also appear in the lower portions of the Peay Member, potential evidence for falling stage systems tract deposition beneath the main lowstand delta. These laterally restricted, top-truncated, low accommodation sandstones encased in low-permeability marine sediments form probable stratigraphic traps, increasing the potential for stratigraphic entrapment within the Frontier Formation.