Supra-salt Syndepositional Folding Within the Jurassic Morrison Formation, Big Gypsum Valley, Colorado
The Paradox Basin, along the Utah and Colorado border, exposes salt diapirs that form elongate “salt walls”. Most exposures of sediments that were deposited during salt movement are hundreds of meters from the contacts. However, in the southeastern part of the Gypsum Valley diapir, a set of tight folds within the Jurassic Morrison Formation are preserved along the diapir margins where they overlie salt. These are best exposed at the southeastern end of Big Gypsum Valley. Previous interpretations suggested that the Morrison Formation folding and faulting occurred during dissolution of the diapir. However, field mapping reported here reveals that the Morrison Formation was deposited on the salt and the beds folded during deposition due to the movement of the underlying salt. Detailed maps and 3D outcrop models of one presumed paleo-canyon, and four areas where folds are well exposed, show anticline-syncline couples and sub-horizontal Morrison beds in contact with the salt. The lower unit of the Morrison onlap the tilted strata flanking the diapir and the folded strata are found in the third stratigraphic unit. The onlap indicates continued subsidence of the flanking minibasins that exceeded the rate of deposition. Correlated stratigraphic sections across the folds show that Morrison strata thin and onlap the flanks of synclines. Indicating that thickening in synclines is a depositional phenomena rather than structural thickening through deformation after deposition. Basal Morrison strata in direct contact with Paradox gypsic caprock contain diapir-derived clasts, reflecting erosion of the diapir margin by the first channels that flowed there. On the southeastern section of the study area, the Morrison Formation incises at least 4 m into the underlying strata and is inferred to be the eroded remnant of a paleo canyon. The canyon is incised into upturned Pennsylvanian rocks that form a “megaflap” that must have formed a topographic high during Morrison deposition. The megaflap underwent erosion during salt-tectonic uplift prior to Morrison deposition. The lack of faulting in some areas indicates that much of the Morrison is in place and has not been dropped into its present location through diapir dissolution collapse as has been previously believed. The findings contribute to creating models that try to define the resulting stratigraphy when salt is the main driver of sedimentation patterns. Potential reservoirs can be delineated on both sides of the diapir margin within the folded strata. Attaining a better understanding of the observed geometries will also serve as a basis for offshore exploration, where seismic imaging of salt diapirs is poor, making drilling hazardous for the equipment and employees involved.
Soltero, Alondra, "Supra-salt Syndepositional Folding Within the Jurassic Morrison Formation, Big Gypsum Valley, Colorado" (2020). ETD Collection for University of Texas, El Paso. AAI27964685.