Ancient deposits of river, wave- and tide-influenced deltas are described and interpreted in the context of new allostratigraphic subdivisions of the Cenomanian age lower Belle Fourche Member of the Frontier Formation in outcrops and subsurface of the Powder River Basin. These deltas formed near the center of the Cretaceous Western Interior Seaway during a lowstand of sea level and were deposited over the slowly subsiding, gently sloping floor of the Sevier foreland basin. These sandstones have been variably interpreted in the past as offshore shelf ridges, prodelta shelf plumes, deltas, and incised valleys.

Four allomembers, bounded by erosional transgressive discontinuities, are named, from oldest to youngest; the Harlan, Willow, Frewens, and Posey. Each allomember grades upwards from prodelta and shelf mudstones into top-truncated sandy shoreface and delta front facies with basinward inclined internal bedding. Lobate sandstones within the allomembers thicken and narrow to the northwest, pinchout into shales to the southwest, south, and southeast, and show south to southeast paleocurrents, suggesting a deltaic origin supplied by rivers to the northwest. The thickest parts of successive sandstones are offset along depositional strike, suggesting differential compaction encouraged successive delta lobes to avoid one another. Successive sandstones within the Harlan, Willow and Frewens allomembers progressively backstep. Deltas that extended farther into the basin left broader, wave-dominated, and extensively burrowed lobate sandstones that were more deeply ravined during subsequent transgression. The Frewens sandstone, positioned farthest landward, is more elongate, more heterolithic, more tide-influenced and sparsely bioturbated.

Subtle basin uplift between episodes of allomember deposition formed local high areas that were stripped of accumulated sediment by the action of waves, storms, and tides leaving pebbly lags in place of originally thicker sandstones. The highly elongate Frewens Allomember prograded into an embayment between the older Willow delta to the south and one such subtle structural high to the north. In the low area, tidal energy is enhanced and wave energy diminished. The Posey sandstone indicates a major southeasterly shift in deposition during the opening of the seaway to the south. This may indicate structural rejuvenation of the orogenic belt.

The position of these sandstones in a distal basin setting suggests that they are lowstand deltas because no significant sandy delta or shoreline deposits have been mapped farther seaward. The lack of preserved delta plain or paralic to non-marine facies reflects a low accommodation setting, probably resulting from deposition of deltaic sandstone during falling sea level and later removal and reworking during marine transgression, with erosion enhanced by uplift. Despite deposition during falling relative sea level, the basal contact is gradational and conformable because falling shoreline trajectory is greater than the sea floor slope. Neither regressive surface of marine erosion, basal unconformity nor “sequence boundary” could be reliably observed or picked below the sandy portion of the allomembers. The major bounding discontinuity used to define the allomembers is thus the transgressive disconformity on top of the sandstones, versus the correlative conformity at base. The lateral stacking of sandstones, common in low accommodation settings, makes interpretation of systems tracts nearly impossible from isolated vertical successions or cross sections; stacking patterns and systems tracts are best seen in plan view based on mapping position of sandstones within the basin.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah