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Permian-Triassic Boundary in the Middle Rocky Mountains
Permian-Triassic strata of the Middle Rocky Mountains (Wyoming, southern Montana, eastern Idaho, northeastern Utah, and northern Colorado) reflect major subsidence in the miogeosyncline on the west and minor subsidence of the shelf to the east. Sequences of thousands of feet of strata in southeastern Idaho and northern Utah give way eastward to sequences a few hundred feet thick. Western areas were sites of deposition of Permian phosphorite, chert, and dark mudstone (Phosphoria Formation), and of Triassic olive shale, tan siltstone, and grey limestone (Dinwoody Formation). A broadly comparable west-to-east transition from normal marine to hypersaline environment characterizes both systems, with red beds dominating in eastern Wyoming.
Stratification is generally parallel on each side of the contact, and the only evidence of pre-Triassic erosion is commonly microrelief on a generally flat surface of great extent.
The specific boundary between the two systems at any one locality is placed primarily on lithologic evidence. A change from carbonate to fine-grained, land-derived sediment characterizes the contact through much of Wyoming. To the east, red beds and evaporites are overlain by similar red beds and evaporites; to the west, each of the several Phosphoria Formation rock types or equivalent Shedhorn Sandstone can be found directly below Triassic shale and calcareous siltstone. The Upper Permian strata commonly show silicification, whereas Lower Triassic beds do not. Bedded evaporite commonly occurs in drab Lower Triassic strata of the shelf, whereas it is limited to the Permian red-bed sequence in peripheral regions of the shelf.
Faunal control normally either confirms the boundary (western area), is absent above the boundary (central shelf), or is absent above and below it (eastern shelf). Youngest Permian faunas generally indicate a Guadalupian age, although a small collection of brachiopods from northwestern Wyoming suggests the possibility of post-Guadalupe Permian beds. The thick Triassic sequence in Idaho contains several Lower Triassic ammonoid zones, starting with a late Otoceratan assemblage. Upper Permian and Lower Triassic strata contrast strikingly in diversity and abundance of fossils. Groups common in Permian strata but absent in Lower Triassic beds include sponges, bryozoans, productoids, seven bivalve superfamilies, and scaphopods.
Over the shelf, the youngest Permian strata were deposited in sublittoral to supralittoral carbonate and evaporite environments. This area was probably sub-aerially exposed before deposition of basal Dinwoody strata but remained nearly at sea level. Erosion, dissolution, and soil formation were minimal, probably reflecting arid climate and lack of relief. To the west, deposition may have persisted in some localities during much or all of this time. Triassic strata indicate a westward shift of environments so that deposition of carbonate rocks prevailed in the miogeosyncline and high salinity prevailed farther west on the shelf than in Permian seas.
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