The Unconformity That Isn’t: A Sequence-Stratigraphic Reinterpretation of the J-5 Unconformity and the Redwater–Windy Hill–Morrison Transition in Wyoming, USA

Abstract

The J-5 unconformity was originally described as occurring in Wyoming at the base of the Upper Jurassic Windy Hill Sandstone, where it separates the underlying Redwater Shale from the overlying Morrison Formation. The existence of this unconformity has been questioned, as has the closely coupled issue of the depositional environment of the Windy Hill Sandstone. In this article, we document the facies and architecture of the Redwater–Windy Hill–Morrison transition. Facies analysis indicates that the Windy Hill Sandstone represents a tide-dominated deltaic system that prograded northward across the shelf mudstones of the Redwater Shale, followed by the coastal plain of the Morrison Formation. The modern Han River Delta of Korea is a close analog to the Windy Hill Sandstone. The base of the Windy Hill Sandstone is not an unconformity; instead, it is conformable and time transgressive, becoming younger northward along depositional dip, implying that portions of the Redwater Shale and Morrison Formation are coeval. This reinterpretation of the J-5 unconformity in Wyoming, along with a greater appreciation of how abrupt stratigraphic contacts are formed stratigraphically, how they are recognized, and how unconformities are generated eustatically and tectonically, all suggest that a broader reappraisal of the Pipiringos and O’Sullivan unconformities is warranted. In addition, the recognition of a prograding tide-dominated delta in the highstand systems tract underscores the growing rejection of a sequence-stratigraphic paradigm that wave-dominated coasts form primarily during periods of progradation, with tide-dominated coasts being limited or favored during periods of transgression. Finally, tide-dominated deltas have been much more poorly recognized in the rock record than wave-dominated coasts and river-dominated deltas, but the previously unrecognized presence of one in the Windy Hill Sandstone suggests that other tide-dominated deltas might be similarly unrecognized in the rock record of epicontinental seas.