Permian and Triassic strata of the Middle Rocky Mountain region (Wyoming, southwestern Montana, eastern Idaho, and northern Utah) reflect geosynclinal conditions to the west and shelf environments to the east. Sequences of thousands of feet of strata in southeastern Idaho and northern Utah give way eastward to intervals a few hundred feet thick. Geosynclinal areas were sites of deposition of Permian phosphorite, chert, and dark mudstone, and of Triassic olive shale, tan siltstone, and grey limestone. A broadly comparable west to east marine-nonmarine transition characterizes both systems, with red beds dominating in eastern Wyoming.

The specific boundary between the two systems at any one locality is placed primarily on lithologic evidence. Throughout much of the region, the beds below the contact are notably resistant to erosion whereas those above the contact are not. Attitudes are typically conformable on each side of the contact, and evidences of pre-Triassic erosion are aspects of micro-relief on a generally flat surface of great extent. The contact commonly lacks a basal conglomerate, whereas minor intraformational conglomerates and discontinuity surfaces can be found within both Permian and Triassic sequences.

A change from carbonate to fine-grained, land-derived sediment typifies the contact through much of Wyoming. To the east, red beds overlie red beds, and to the west, each of the several Permian geosynclinal rock types can be found directly below Triassic shales and calareous siltstones. Sandstone forms the upper Permian beds in southwestern Montana. The Upper Permian strata commonly exhibit silicification, whereas the Lower Triassic beds do not. Bedded evaporites commonly occur in drab Lower Triassic strata of the shelf, whereas they are limited to the red bed sequence in the Permian.

Through most of the region, zone fossils are lacking in strata near the contact. For the most part, the youngest Permian faunas are either nondiagnostic, or are of Guadalupian aspect. In the Teton Pass area of northwestern Wyoming, a small collection of brachiopods suggests the possibility of post-Guadalupian Permian beds. Southeast Idaho has one of the thickest Lower Triassic marine sections in the world. It contains a notable sequence of Lower Triassic ammonoid zones, starting with an upper Otoceratan assembage, but ammonoids are absent in the shelf sequence. The Upper Permian and Lower Triassic strata contrast strikingly in diversity and abundance of fossils. Groups common in Perian strata but absent in Lower Triassic beds include bryozoans, productoids, seven bivalve superfamilies, echinoderms and scaphopods. Three bivalve general and linguloid brachiopods account for most of the Lower Triassic fossils.

Over the shelf, the youngest fossiliferous Permian strata were deposited in sublittoral to supralittoral carbonate environments, and a hiatus possibly representing several million years separates the two parallel sequences This area was probably subaerially exposed, but remained nearly at sea level. Erosion, solution, and soil formation were minimal, probably reflecting

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lack of relief and arid climate. To the west, mudstone deposition may have persisted in some localities during much or all of this time. Renewed deposition on the shelf took place in shallow water in which high salinities prevailed farther west than in Permian seas.

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