ABSTRACT
The Paradox Salt basin, located in southwestern Colorado and southeastern Utah, is bounded by the Uncompahgre uplift on the northeast, the San Juan basin contiguous to it on the southeast, the Defiance uplift on the south, the Black Mesa and Kaiparowits basins on the southwest, the Kaibab upwarp on the west, and the Uinta basin on the northwest. The Monument upwarp and the San Rafael swell are major associated structures which extend obliquely across the southwest and northwest parts of the Paradox Salt basin shelf, and exerted only slight sedimentational control within the confines of the basin prior to late Pennsylvanian time.
Major marine facies gradations present in Pennsylvanian and Permian strata comprise limestone, dolomite, gypsum, anhydrite, salt, gray shale, and black shale grading to black siltstone and black limestone, quartzose sandstone, feldspathic sandstone, arkose, and arkosic conglomerate. These stratal variations were most strongly influenced not only by the major source area in the Uncompahgre-San Luis highland but also by the rate and periodicity of subsidence of the Paradox geosyncline.
The structural history of the northwest-trending Paradox Salt basin, its shelves, and the ringing uplifts, is very complex, with epeirogenic movements prior to Pennsylvanian time responsible for the formation of widespread disconformities in pre-Pennsylvanian strata. Rapid subsidence to form the Paradox Salt basin (in a broad shelf area initially receiving allogenic sediments from the San Luis highland, the Defiance uplift, and the Kaibab upwarp, and carbonates generated within the basin area itself) was succeeded by the deposition of a unique sequence of evaporites which characterizes this most anomalous Pennsylvanian sequence known anywhere in North America. During evaporite deposition and continuing with major pulsations until Triassic time, the Uncompahgre uplift rose out of the northeast side of the Paradox basin to shed many cubic miles of low-grade graywacke sediments succeeded by arkose. Tn late Pennsylvanian and early Permian time, these coarse clastics interfingered with marinecarbonatesand fine-grainedclastics in the deeper part of the basin while the correlative shelf carbonates were being altered during numerous diastems and times of non-deposition along the southwest shelf of the Paradox basin. Epeirogenic uplift of the entire region began during early Permian time and brought the Paradox Salt basin area above sea-level, with coarse clastics moving into the continuously downwarping continental basin southwest of the Uncompahgre uplift.
Complexities of marine to non-marine sedimentation across the Permian-Pennsylvanian interface, with marine waters retreating northwestward from the region during early Permian time, have made the choice of a simple formational upper boundary for the Pennsylvanian system impossible. It is proposed that the Rico transition facies be considered a basal member of the Cutler formation despite the recognition that the Rico facies grades from Virgil to Hueco in age within the Paradox Salt basin region. The Hermosa formation is limited to the post-evaporite sequence below the Rico facies, whereas the Paradox evaporite sequence below the Hermosa formation is here considered a mappable formation. Pre-Paradox strata above the Molas shale are here given the new formation name “Pinkerton Trail” limestone inasmuch as it is a widespread mappable unit both in the subsurface of the Paradox Salt basin and on the surface in the San Juan Mountains.
