Alluvial Fans, Loess Plains, Lakes, and Distributive Fluvial Systems: Depositional Systems of the Permian-Triassic Red Beds and Evaporites of Wyoming (USA)

Abstract

The end-Permian ecological crisis was a period of warming and aridity reaching extreme levels by the end Permian and not recovering before the Middle Triassic. The Permian Goose Egg Formation and the Chugwater Group’s Early Triassic Red Peak Formation and Middle Triassic Alcova Limestone are a continuous or nearly continuous succession of rocks deposited during this warm, arid time in Wyoming. Comprised of red beds, evaporites, and rare carbonates, these rocks have not been subjected to detailed sedimentological investigations in the modern era. Previous interpretations of marine and marginal marine deposition have largely been based on the laterally extensive nature of bedding and the presence of gypsum and carbonate. However, chemical sedimentary rocks can form in diverse environments, and laterally extensive beds are common for paleosols, lakes, and loess. This dissertation is a detailed sedimentologic study undertaken to reconstruct depositional environments of the Permo-Triassic red beds and evaporites of Wyoming. For the first time, this study evaluates this succession’s relationship to similar successions in Pangea’s midcontinent. 3D models from areal imagery, centimeter-scale measured sections, detailed petrology, and microanalysis were utilized at central and eastern Wyoming locations to reconstruct environments, climate, and life from the Goose Egg Formation, the Red Peak Formation, and the Alcova Limestone. The Goose Egg Formation was deposited in perennial and ephemeral saline lakes and associated continental facies. Eolian processes were an important sediment transport mechanism, with windblown silt and gypsum common. The Red Peak Formation was deposited in distal alluvial fans and ephemeral saline lakes. The red pigmentation of the Red Peak Formation is derived from: (1) impact reddening of grains during eolian saltation; (2) iron oxide cement or concretions formed in acid saline lakes and transported by the wind into the Red Peak Formation; and (3) in-situ pedogenic weathering in a near-surface oxygenated environment. The teleconnection to the acid saline lacustrine systems suggests acid saline systems were located upwind. The Alcova Limestone was deposited in perennial and ephemeral saline lakes after the groundwater table rose, likely from increased precipitation or decreased evaporation. At least some of the Alcova Limestone was likely originally deposited as gypsum and diagenetically altered to calcium carbonate after burial. This study documents that the red bed, evaporite, and carbonate succession in the late Permian and Early Triassic in Wyoming were deposited in a continental environment of lakes and distal alluvial fans. Aridity increases during Goose Egg and into Red Peak time.