The Cretaceous Western Interior Basin, a marine retroarc foreland basin, has been filled from its western margin by a series of clastic wedges. The Coniacian-Campanian interval, exposed in central and western Wyoming, records sea-level fluctuations during a shift from thin-skinned (Sevier) to thick-skinned (Laramide) tectonics.
Both eustasy and tectonism have been cited as the prime force shaping this succession. Some constraints can be established by tracing a section from the passive-margin-like eastern basin through the central basin, and west toward the tectonic front. In the east, eustatic sea-level fluctuations combined with the autoretreat mechanism to build a well-developed shelf-slope-basin morphology. In the west, near the tectonic front, sequence stratigraphic principles are difficult to apply because the correlative shoreline deposits are not easily observed, and changes in base level at the shoreline are thought to be effective only a certain distance upstream. However, earlier studies have suggested that changes in accommodation/supply ratio are observable as shifts from single-story fluvial channel sands with floodplain shales (expanded section) to multistory valley fills (reduced section) and back again, and that these cycles reflect repeated uplift of the Laramide Wind River range to the north.
In comparison with the distal and proximal sections, the medial Coniacian-Campanian section in the Bighorn Basin contains both continental and marine deposits which sensitively record the interplay of tectonism and eustasy. The section is dominated by the look-alike Eagle and Judith River clastic wedges. Each starts with a forced-regressive succession of offlapping, erosionally based sandstone tongues capped by a regional unconformity, and ends with a transgressive “expanded section” of sandstones, shales, and coals whose only internal boundaries are flooding surfaces. A unique solution resolving the eustatic and tectonic signals cannot be extracted from the available evidence, but the “best fit” model suggests that the wedges reflect intensified subsidence during the change from Sevier to Laramide tectonics, coupled with an increased sediment supply from emerging Laramide uplifts. Eustatic modulation of sea level appears to have compartmentalized what would otherwise have been a single clastic wedge.
The succession is capped by the Teapot Member of the Judith River Formation. It is unique for its “condensed” shale-free sandstones, and the great time value of its lower bounding unconformity, which cuts across 17 ammonite zones. The Teapot-Ericson unconformity has been attributed to “relaxation” between the pulses of the Absaroka thrust. However, it is as likely due to a condition of supercriticality in the accretionary wedge induced by the onset of Laramide uplifts, and the restoration of critical taper by erosion.
The Wyoming section can be divided into five alloformations on the basis of six unconformable surfaces that can be traced from the eastern to the western margin of the basin. Four time-equivalent surfaces can be traced across the section in the Central Utah Embayment (Book Cliffs). Two of the other surfaces examined in Wyoming are perhaps among the multiple erosion surfaces identified within the Campanian Blackhawk Formation of Utah. While similarities occur, stratigraphic evolution within these two constraining surfaces followed other trajectories.
