Deltaic and Shallow Marine Sediment Accumulation under Spatially and Temporally Variable Accommodation Associated With Structural Growth: Data from the Cretaceous Frontier Formation of the Bighorn Basin, Wyoming, USA

Fielding, Christopher; Hutsky, Andrew J.; Hurd, Trevor J.

Enduring controversy surrounds the origin and stratigraphic context of isolated shallow marine sandstone bodies preserved in the Cretaceous Western Interior Seaway Basin of North America. In this paper, we provide a new interpretation of one such sandstone body that may shed light on this long-standing issue. We mapped the Peay Sandstone Member (PSM) of the Cenomanian Frontier Formation at surface and in the subsurface across the northern Bighorn Basin of Wyoming. An isolith map of the PSM shows that it has a digitate planform, extending initially east, turning southeast and terminating distally. Facies analysis indicates that the PSM records a full cycle of progradation of a fluvially-dominated delta, but no delta top deposits are preserved. The PSM thickens southeastward from 10-20 to 60 m near the town of Greybull, before thinning abruptly further southeast. Outcrop sections and wireline log data indicate that the PSM is coarsening-upward over its entire length, nowhere displaying an erosional base. In many places, the underlying mudrock interval is unusually thick beneath the deltaic axis rather than thinned as would be the case if the delta were incised. Thickness variations within the Frontier Formation define synsedimentary growth folds with 10s of km wavelength across the study area.

Thickness of the top Mowry Shale to top PSM interval varies nonlinearly, suggesting that accommodation during that interval was spatially variable, and at times locally negative. The considerable extent of the PSM across the basin and the lack of a topset are suggestive of a falling stage or lowstand context, yet the body is neither incised into its substrate nor exhibits a descending regressive trajectory. Rather, the characteristics of the PSM are best explained by formation under a regime of both spatially and temporally variable, but limited accommodation forced by synsedimentary structural growth. Sediment was trapped and preferentially accumulated in zones of more rapid accommodation while bypassing or spilling into other, lower accommodation compartments of the basin. Patterns of thickness variation suggest a rumpling of the depositional surface rather than growth of a continuous barrier such as a forebulge ridge. The orientation and location of the PSM coincide closely with those of the Laramide age (early Cenozoic) Rio Thrust and Sheep Mountain Anticline, suggesting that precursors of these structures were already active during the Cretaceous Sevier Orogeny.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013