Pennsylvanian and Early Permian Paleogeography, Southern Colorado Plateau and Vicinity: ABSTRACT

Abstract

Abstract Pennsylvanian and Lower Permian sedimentary rocks of the southern Colorado Plateau have been the subject of much controversy concerning their correlation and origin. Correlation of rock sequences and major unconformities from the fossiliferous basins onto the adjacent shelves, where relatively unfossiliferous rocks were deposited, has provided a new interpretation in terms of regional stratigraphy and depositional analysis. The Supai Group was deposited in the Grand Canyon Embayment adjacent to the Cordilleran Geosyncline. Sediments of Morrowan, Atokan, Virgilian and Wolfcampian ages were deposited in a variety of offshore marine and shoreline environments. A persistent southwest-to-northeast-trending unstable area, herein named the Sedona Arch, confined all Supai sediments, except Virgilian, to the Grand Canyon and western Mogollon Rim regions; the other strata are truncated beneath the sub-Virgilian unconformity or undergo significant facies change across the arch. The Naco Group of Desmoinesian, Missourian, and Virgilian age was deposited on the Mogollon Shelf adjacent to the Pedregosa Basin. Except for Virgilian strata, these dominantly marine rocks are also truncated beneath the sub-Virgilian unconformity across the Sedona Arch. Thus the bulk of Supai and Naco rocks are not as closely related as most previous workers had thought. Mostly marine rocks of the Molas Formation and Hermosa Group were deposited in the Paradox Basin. The northwest-to-southeast-trending Kaibab Arch provided the barrier between these rocks and the Supai and Naco to the south. Except for several transitional units, the Supai, Naco, and Hermosa Groups were deposited in distinctly separate basins and the units do not generally intertongue. The Wolfcampian Esplanade and Cedar Mesa Sandstones and Halgaito Formation form a distinct eastward-thinning wedge of rocks west of the Sedona Arch. These high-energy marine shoreline and supratidal units grade westward into thick marine carbonate sequences of the Cordilleran Geosyncline. A widespread redbed sequence comprising the Hermit and Organ Rock Formations was deposited across the entire study area. The continental and possibly tidal deposits represent a sharp change in depositional regime from the underlying and overlying sandstone deposits. The youngest sequence of deposition studied consists of sandstone, redbed, evaporite, and carbonate rocks of the Schnebly Hill Formation and DeChelly Sandstone. These units are centered around the Holbrook Basin and represent a complex of shallow marine, high- and low-energy shoreline, eolian, and sabkha depositional environments. This depositional sequence was confined to an area east of Grand Canyon and south of the San Juan River by the Sedona and Kaibab Arches. Both formations grade upward or intertongue with the overlying Coconino Sandstone and related rocks.