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The Paradox basin of the east-central Colorado Plateau province is an elongate rhombic evaporite basin of Middle Pennsylvanian age. It is bounded on the northeast by the Uncompahgre and San Luis segments of the Ancestral Rockies. The northwest-trending basin sagged along preexisting basement rifts by strong east-west extension during Desmoinesian time. The dominant zone of weakness was the northwesterly Olympic-Wichita basement lineament that lies along the eastern margin of the salt basin and the southwestern front of the Uncompahgre and San Luis uplifts. Less prominent northwest and northeast shear zones are ubiquitous, but are especially well developed in basement and Paleozoic rocks underlying the San Juan basin and the southwest shelf of the Paradox basin. It is a cl ssic pull-apart basin developed along anastomosing wrench faults which first developed at about 1,700 m.y.B.P., and were rejuvenated repeatedly throughout the Paleozoic.
Initial subsidence and associated sedimentation occurred in the southeastern divergent termination of the basin in Atokan and early Desmoinesian time, when the Grenadier and Sneffels horst blocks were elevated to form the San Luis uplift. The quartzite-dominated horsts shed large volumes of relatively nonfeldspathic quartzose sediments into shallow seaways of northern New Mexico and southwestern Colorado. By mid-Desmoinesian time, the true Uncompahgre uplift was elevated to the north, and shed vast accumulations of arkose into the eastern margins of the basin. Meanwhile, cyclic evaporites, dominated by salt, accumulated in the structurally restricted basin. The southwestern shallow shelf of the basin developed along a broad zone of basement rifting, extending across the northern San J an basin, the Four Corners platform, and northwest to beyond the Henry Mountains. The Four Corners lineament served to terminate salt deposition along the structural shelf and create local shoaling conditions to host myriad algal bioherms that would become prolific petroleum reservoirs. These fields include the giant Aneth field and the recently developed Bug field complex. Unlike the dramatic events along
the eastern Paradox basin, tectonic involvement of carbonate sedimentation on the southwest shelf was very subtle. Sea-floor topography of a few feet over deep-seated faults would have been sufficient to localize biohermal development. Subtle tectonically induced topography on the present Monument upwarp and San Rafael swell limited sedimentation along the western basin margin.
The northwesterly rift fabric of the basin is transected by the conjugate northeast-trending basement fractures of the Colorado lineament. The most important of these underlies the length of the Colorado River, and terminates or offsets the major salt diapirs of the eastern basin. Northwest of the structure, the Paradox basin becomes compressional rather than extensional as to the south, due to convergent termination of the pull-apart basin; and marine sedimentation becomes rapidly limited to the narrow Oquirrh sag between the Emery and Uncompahgre uplifts. Also, the large influx of arkose from the Uncompahgre becomes much younger (Early Permian). Every aspect of sedimentation (clastic, evaporite, and carbonate) in the Paradox basin was greatly influenced by contemporaneous rejuvenati n of the basement tectonic fabric.
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