Permian Evaporites, Western Colorado Plateau, Southwestern Utah and Northwestern Arizona: ABSTRACT

Abstract

Permian rocks of the western Colorado Plateau contain five evaporite units that are separated by one marine sandstone and four limestones in a cyclic relationship. These evaporites were deposited during regressions in the Permian seas that allowed sabkhas to develop between the shallow-marine shelf of the Cordilleran geocline and the nonmarine sediments that were being deposited along the transcontinental arch during Early Permian time. The lowest Permian unit containing gypsum is the Pakoon Formation which represents the first regressive cycle in the Lower Permian. Above the Pakoon Formation is the Quantoweap Sandstone deposited in a shallow-marine environment. Overlying the Quantoweap Sandstone is gypsum and gypsiferous siltstone of the Seligman Member of the Toroweap F rmation, representing the second regression. During this regression a sabkha developed along what is now the western margin of the Colorado Plateau. A marine transgression followed depositing the limestone of the Brady Canyon Member in shallow-marine conditions. The third and largest marine regression produced the gypsum of the Woods Ranch Member of the Toroweap Formation while an eolian environment was depositing the Coconino Sandstone on the east. The last major transgression partly dissolved the gypsum of the Woods Ranch Member, locally generating the erosional unconformity between the Toroweap and Kaibab Formations. Along the present margin of the Colorado Plateau the regression of the Permian sea, which deposited the limestone of the Fossil Mountain Member, marked the development of the final sabkha represented by the gypsum of the Harrisburg Member in the area north of Grand Canyon. The deposition of gypsum in the Harrisburg Member was interrupted by a minor transgression that destroyed some of the underlying gypsum by dissolution and deposited a marine limestone. Alternating gypsum and limestone units are present between the limestone deposited by the minor marine transgressions and the Permo-Triassic boundary, suggesting that fluctuations between sabkha and tidal flat environments occurred before the final regression of the Permian sea. End_of_Article - Last_Page 613------------