Abstract

Structural and isopachous maps and stratigraphic cross sections of Pennsylvanian strata in the Paradox Basin verified the presence of 29 mappable salt cycles and their lateral correlation with shelf-margin carbonate units. Paradox strata are Desmoinesian Cherokee in age and comprise five major stratal units and 26 salt cycles. The Pre-Paradox Lime Ridge section, of Atokan age, may contain five additional salt cycles nearer the middle of the basin. The post-Paradox Ismay section is almost wholly of Desmoinesian Marmaton age and comprises three identifiable cycles.

The evaporite-carbonate transition of successively younger cycles shows shelfward overstep of subjacent transition zones, suggesting concentric expansion of the basin. The expansion is controlled by downwarping along subparallel hinge-line trends which were initially the sites of monoclinal folding. Folding was later followed by faulting. Movements along these faulted flexures were subject to periodic rejuvenation related to renewed subsidence in the middle of the basin. Stratigraphic throw on the faults decreases shelfward, and earliest movement is along the faults nearest the middle of the basin.

The post-Kinderhookian–pre-Desmoinesian Paradox shelf was generally emergent, deeply eroded, and covered by the red regolithic Molas Formation. To the west, the Molas may be Mississippian in age, although it is Morrowan and Atokan in the Paradox region. A Molas-Lime Ridge marine sequence in the northwestern Paradox Basin is considered to be equivalent to the Belden of the Maroon Basin, a correlation which suggests stratigraphic continuity of the Belden between the Maroon and Paradox Basins.

Subsidence of the Paradox shelf at the end of Atokan time, and contemporaneous uplift of the margin, created a sag which became the site of evaporite deposition. Bordered on the west by the carbonate-covered, submerged Piute platform-welt on the seaward side, and the clastic-fringed San Luis–Front Range uplift on the cratonic landward side, the pan-shaped sag is now divided into two basin segments by the Uncompahgre Uplift.

Several unconformities are recognized within the Pennsylvanian sequence, few of which extend into the middle of the basin. Successively younger erosion surfaces migrated shelfward, suggesting concentric expansion of the peripheral shelf. Each cyclic sequence terminated with peripheral uplift and erosion, suggesting that cyclic deposition was tectonically controlled.

Subsidence, apparently initiated by loss of subcrustal support, resulted in an ovate sag. As the area of subsidence increased, a compressional ridge developed near the middle of the basin. This early arch is coincident in position with the present Uncompahgre Uplift and was ancestral to the present-day feature. Growth of the arch ceased at the end of Alkali Gulch time, when compression attendant upon subsidence was relieved by basin expansion. Compression adequate for deformation was again developed by the end of Desert Creek time, and the Uncompahgre arch became an influence upon sedimentation during Ismay time. At this time rupture along the southwestern flank of the Uncompahgre, and continued uplift, created a tilted fault block which divided the region into a southern Paradox Basin and a northern Maroon Basin.

Crustal failure along the Uncompahgre front caused a major realignment of forces throughout the basin. This realignment, or “basin collapse,” resulted in uplift along the margin. In the Paradox Basin, the realignment resulted in lateral shift along flex trends, folding, as well as thrust faulting of Pennsylvanian rocks and the twisting of pre-existing folded structures.