The Paradox is a northwest-southeast elongate structural and sedimentary basin, bounded on the east and northeast by the San Juan Mountains and the Uncompahgre

End_Page 1875------------------------------

Plateau of Colorado, on the south by the Defiance Plateau of Arizona and New Mexico, and on the west by the San Rafael swell of Utah. Approximately 20,000 feet of sediments are preserved within the Permo-Pennsylvanian basin. Surface exposures are mostly sediments of the Mesozoic System, represented by a few thousand feet of clastics. The La Plata, Carrizo, Abajo, and La Sal Mountains are Tertiary intrusives within the Paradox basin.

The thin early Paleozoic sediments transgressed easterly onto the northeast-southwest trending transcontinental arch with the Cambrian sandstones separated from the Devonian-Mississippian shelf carbonates by an Ordo-Silurian hiatus. Exposure of the Mississippian carbonates resulted in a karst-like regolith, the Molas Formation of Atokan and/or earliest Pennsylvanian age. Tectonic influence then gave a northwest-southeast structural grain to the Paradox basin, in which Pennsylvanian cyclic or rhythmic shelf carbonates, sapropelic "black shale" dolomites, evaporites and arkose-redbeds were deposited. Approximately 2,000 feet of shelf carbonates were deposited on edges of the basin where highlands were not present. Evaporites were deposited in the center and distal northwest end of the b sin and reach a present thickness of 10,000+ feet as the result of salt flowage in the cores of the intrusive salt anticlines. The many thin sapropelic dolomites are widespread throughout the basin and are the "time-markers" used for correlation. Clastics shed from the Uncompahgre and San Luis uplifts resulted in several thousand feet of arkose and redbeds ("Pennsylvanian" Cutler Formation) being deposited on the northeast and east flanks of the basin. These highlands persisted through Permian time and similar clastic deposition ("Permian" Cutler Formation) continued, depositing a thick wedge of arkose near the Uncompahgre front, which thins to the southwest to 2,000 feet of continental redbeds and eolian and marginal marine sandstones of the Cedar Mesa, Organ Rock, and De Chelly Formati ns.

Approximately 3,000 feet of Jurassic-Triassic continental shales and eolian sandstones were deposited over the Permo-Pennsylvanian basin and highlands. These sediments are exposed over the western two-thirds of the Paradox basin. Upper Cretaceous marine clastics, exposed at the eastern edge of the basin, are equivalent to a continental facies in western Utah.

The primary structural grain of the Paradox basin is a northwest-southeast lineation paralleling the Uncompahgre uplift of Permo-Pennsylvanian time. This alignment is illustrated best by the several salt anticlines. Laramide tectonics both rejuvenated the older trend and developed new structural lineations such as the north-south striking Monument upwarp. Regional uplift, coupled with the development of peripheral Tertiary basins, has placed the Permo-Pennsylvanian Paradox basin in a high structural and topographic position incised by deep superimposed drainage.

End_of_Article - Last_Page 1876------------