Shelf-Edge Carbonate-Redbed Transitions, Red Peak and Thaynes Formations (Triassic), Western Wyoming and Adjacent Idaho: ABSTRACT

Stratigraphic correlations between the shallow, generally open-marine deposits (dominantly limestone) of the Triassic miogeosyncline in eastern Idaho (Thaynes Formation) and shelf redbeds in west-central Wyoming (Red Peak and Crow Mountain Formations) have been uncertain. The principal, most widely recognized marker bed in the Wyoming sequence--the Alcova Limestone Member--has been correlated variously with many different parts of the Thaynes, as well as with beds considered younger than the Thaynes. Our studies indicate that the Alcova is a marine tongue extending eastward from the sandstone and limestone unit of the upper part of the Thaynes. Carbonate tongues of the Thaynes below the sandstone and limestone unit also extend eastward into redbeds and evaporites in weste n Wyoming, but none of them extends as far as the sandstone and limestone unit. Red Peak-Thaynes correlations lead to the reconstruction of an average paleogeography for western Wyoming and adjacent Idaho. In eastern Idaho and westernmost Wyoming, carbonate mounds and oolite shoals composed of bivalves, algae, and oolites lined the shelf edge. Shallow, agitated water is indicated by the carbonate associations. Restricted, shallow-marine conditions, characterized by gypsum and early diagenetic dolomite, extended eastward from the banks into western Wyoming. Throughout west-central and central Wyoming, shallow-marine and paralic conditions prevailed during deposition of the redbeds (dominantly siltstone) of most of the Red Peak Formation. Despite differences in age and tectonic setting, the facies relations and the reconstructed paleogeography resemble the Permian shelf-basin sequence of West Texas. This similarity suggests the possibility that oil and End_Page 864------------------------------ gas accumulations are present along the shelf edge although deformation may have allowed hydrocarbons to escape. End_of_Article - Last_Page 865------------