The Paradox<subtitle>A Pull-Apart Basin of Pennsylvanian Age</subtitle>

Abstract

Abstract The Paradox basin of the east-central Colorado Plateau Province is an intracratonic depression developed on continental crust. The elongate, northwest-trending, rhombic-shaped salt basin of Middle Pennsylvanian age is bounded on the northeast by the Uncompahgre and San Luis segments of the Ancestral Rocky Mountains and on the southwest by the less prominent Four Corners lineament. We have previously demonstrated that the basin sagged along intersecting basement fracture zones by strong east-west extension during Middle Pennsylvanian time. The master fracture system was the northwest-trending Olympic-Wichita structural lane that lies along the eastern margin of the Paradox salt. Oblique divergent strike-slip faulting along the Uncompahgre-San Luis segment created a tension-releasing bend where the Paradox pull-apart basin nucleated and subsequently developed throughout Middle Pennsylvanian time. Smaller sub-basins developed by orthogonal spreading along intersecting northeast-trending transform faults, where the rate of basin floor subsidence was related to combinations of normal and strike-slip faulting. The greater Paradox basin was episodically deepened during Middle Pennsylvanian time by rejuvenated extensional basement faulting. Vertical displacement was greatest along the Uncompahgre front, which caused tilting of the basin and contemporaneous development of an asymmetrically thick sedimentary sequence with great thicknesses of salt and arkose. By middle Desmoinesian time, during deposition of the Desert Creek stage, the rate of divergent strike-slip faulting slowed considerably. Folds caused by minor wrench movements provided shoaling conditions along the southwest shallow carbonate shelf where porous algal mounds developed. Meanwhile, continued tectonic movement and space reduction of the basin floor may have triggered salt flowage and diapirism in the deep eastern pull-apart trough. As wrench tectonism diminished in Late Desmoinesian-early Permian time, the eastern part of the basin continued to subside at a faster rate than the western part and was filled with marine and nonmarine sediments.