Stratigraphic Relations Within Upper Chugwater Group (Triassic), Wyoming

Abstract

Detailed stratigraphic studies of the upper Chugwater Group (Triassic) in Wyoming permit well-defined lithologic correlation of units. Such correlation is necessary to reconstruct the paleogeography of the Triassic Wyoming shelf. The lower part of the Jelm Formation north of the type section in southeastern Wyoming correlates with the upper Crow Mountain Formation of central Wyoming; a tongue of the Jelm overlies the Crow Mountain in central and western Wyoming. The Ankareh Formation of western Wyoming is correlative with the Jelm tongue and with the Popo Agie Formation. The Alcova Limestone Member of the Crow Mountain is a tongue that extends southeastward from the upper Thaynes Formation in western Wyoming for approximately 250 mi. During deposition of the Crow Mountain, much of the Wyoming shelf was flooded by a shallow sea. Streams emptying into the sea in southeastern Wyoming deposited the basal Jelm and delivered sediment into the basin. A general regression followed and fluvial and fluvial-lacustrine sediment of the Jelm and Popo Agie was deposited over the marine Crow Mountain. Marine conditions prevailed longest in western Wyoming. Accumulation conditions were rather uniform on the Wyoming shelf during the Triassic, as indicated by the wide extent of thin stratigraphic units. After deposition of the Popo Agie, the Wyoming shelf was gently folded. Truncation before deposition of the Nugget Sandstone produced a regional unconformity. The folding and truncation that occurred between deposition of the Popo Agie and deposition of the Nugget suggest that the Nugget is Jurassic, rather than Late Triassic (?). The only major erosional break in the Wyoming Triassic is at the top of the Crow Mountain. Throughout central Wyoming, a disconformity separates the Crow Mountain and Jelm Formations. The writers suggest that at least part of Middle Triassic time is represented by this disconformity and the Jelm.