Internal Architecture of an Upper Cretaceous Shelf-Ridge Sandstone, Hanna Basin, Southern Wyoming

Abstract

The informally named Seminoe sandstone constitutes a Campanian-age shelf-ridge sandstone within the Haystack Mountains Formation, Mesaverde Group, of the Hanna basin. To determine its external geometry, vertical sequence organization, and internal lithofacies architecture, thirty vertical sections with a lateral spacing of 0.3 to 4 km were made along a section line perpendicular to the paleocurrent trend of the sand body. The Seminoe sandstone forms a lenticular sand body of 35 km wide {times} 30 m thick (23 {times} 100 ft) enclosed within a marine mudstone (a tongue of the Steele Shale). The sand body has a southeastward (basinward) facing asymmetry and a gradational base. In vertical profile the sand body is characterized by a large-scale coarsening-upward sequence. Shelf mudstones pass gradationally through storm sheet sandstones and siltstones into trough and tabular cross-stratified sandstone. Paleocurrents from the sand body are unidirectional toward the southwest (subparallel to the paleoshoreline) and are interpreted to record deposition under a geostrophic storm current regime. Internally, the sand body is characterized by a series of large-scale, imbricate, lithofacies units. Individual units are separated by low-angle (1{degree}), laterally extensive (5-15 km), basinward-dipping abandonment surfaces. The units are interpreted to record successive episodes of active shelf-ridge sedimentation andmore » abandonment in response to relative sea level changes.« less