Sequence-stratigraphic concepts and nomenclature are predicated on the assumption that at any time, accommodation conditions are in phase across a depositional basin. In certain situations, however, such as in evolving retroarc foreland basins, the sense of accommodation may be spatially variable because of the growth of intrabasinal structures. Herein, we present evidence for the accumulation of a fluvially dominated deltaic sandstone under strong forcing from spatially and temporally variable low accommodation within the Cretaceous western Cordilleran foreland basin of North America. The Peay Sandstone Member is a coarsening-upward sandstone body (<60m [<197ft] thick) that is extensive across much of the eastern half of the present-day Bighorn Basin of Wyoming. It is remarkable for its elongate planform geometry, extending many tens of kilometers across the basin, and for distal thickening patterns that are, at face value, difficult to reconcile with conventional facies models for deltaic systems. The sandstone shows some of the characteristics of falling-stage and lowstand deltas (extending far into the basin from the contemporary shoreline to the west, lack of a preserved delta-plain topset) but is not incised into its substrate and does not show a descending regressive trajectory with respect to underlying strata. We submit that the Peay Sandstone Member was formed under a regime of both temporally and spatially variable accommodation forced by the heterogeneous growth of the forebulge within the western Cordilleran foreland basin and suggest that the origin of some other apparently isolated sandstone bodies in this and other basins might also be explained in a similar manner.

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