Paper No. 25
Presentation Time: 9:00 AM-6:30 PM


KNAPP, Jonathan P. and BENISON, Kathleen C., Department of Geology and Geography, West Virginia University, Morgantown, WV 26506-6300,

Permo-Triassic red beds in the northern Rocky Mountains have posed a challenge to stratigraphers and sedimentologists for over a century. Notoriously hard to interpret due to paucity of fossils and chronologic indicators, and containing no major economic minerals, these redbeds have received significantly less scientific scrutiny than other Paleozoic and Mesozoic rocks of the region. Most previous workers have interpreted this succession as tidal flat deposits due to the presence of gypsum and carbonates. However, our observations raise significant questions as to this interpretation.

A 104 meter thick section of exposed red beds assigned to the Permo-Triassic Goose Egg and Chugwater Formations 15 miles north of Rawlins, Wyoming was measured with centimeter-scale resolution. The succession is divided into three units, from lowest to highest in the section: (1) 92.3 meters of red alternating siltstone and sandstone; (2) 2.11 meters of carbonate and gypsum typified by an laminae and algal boundstone; and (3) 8.6 meters of brick red, pink, and yellow fine to course sandstone with some conglomerate.

Distinctive features of unit 1 include ripple marks, bioturbation, cross bedding, rip-up clasts, halite casts, cracks, blocky peds, cross-bedding, and rare burrows and root features in stacked successions of fining upward cycles with slightly erosional bases. Unit 2 is typified by flat to upward doming laminations with minor amounts of disturbed bedding. Unit 3 has abundant cross-bedding, some large scale rip-up clasts, bioturbation, and a high degree of lateral discontinuity. Erosional surfaces are common. Based on the observed sediment structures, preliminary interpretations of environment of deposition suggests a paleosol, fluvial, and lacustrine setting.

This study is part of a regional survey to re-examine red beds. Detailed study of Permo-Triassic redbeds and associated evaporites and carbonates will lead to better understanding of Pangaean environments and climates.