Abstract

Stratigraphic correlations between the shallow, generally open-marine deposits of the Triassic miogeosyncline in eastern Idaho (Thaynes Formation) and shelf deposits in Wyoming (Red Peak and Crow Mountain Formations) have been uncertain. The most widely recognized marker bed in the Wyoming sequence, the Alcova Limestone Member, has been correlated variously with many different parts of the Thaynes, and also 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. This resolution of the problem is possible only through the combined use of surface and subsurface stratigraphic sections, sedimentary petrography, paleontology, bedding types, and sedimentary structures.

Red Peak-Thaynes correlations permit reconstruction of a generalized paleogeography. In eastern Idaho and westernmost Wyoming, carbonate mounds and oolite shoals lined the shelf edge. Restricted, shallow-marine conditions extended eastward from the banks into western Wyoming. Throughout west-central and central Wyoming, shallow-marine and paralic conditions prevailed during deposition of most of the Red Peak.

Despite differences in age and tectonic setting, the facies relations and reconstructed paleogeography resemble those of the Permian shelf-basin sequence of West Texas. This similarity suggests the possibility of oil and gas accumulations along the shelf edge, although subsequent deformation may have allowed hydrocarbons to escape.

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