SUPRA-SALT SYNDEPOSITIONAL FOLDING IN THE JURASSIC MORRISON FORMATION, BIG GYPSUM VALLEY, COLORADO

The Paradox Basin exposes salt diapirs that form elongate “salt walls”. The salt wall in Gypsum Valley, CO, provides a unique opportunity to study the exposed contacts of salt with Pennsylvanian-Cretaceous strata. Most exposures of sediments that interact with salt are hundreds of meters from the contacts. In Gypsum Valley, a set of tight folds within the Jurassic Morrison Formation are preserved along the diapir margins. These are best exposed at the southeastern end of Big Gypsum Valley and progressively open to the northwest. It was previously thought that the Morrison Formation came into contact with the salt after dissolution due to faulting, creating the observed folds. However, these folds are incompatible with the standard diapir models. Our new hypothesis is that the Morrison Formation was deposited directly on the salt and the beds folded during deposition due to the movement of the underlying salt.

A detailed map and 3D outcrop model along the diapir margin show synclines and subhorizontal Morrison beds in contact with the salt. The lower beds of the Morrison onlap the tilted strata flanking the diapir, indicating continued subsidence of the flanking minibasins. Basal strata contain diapir-derived detritus, reflecting erosion of the diapir. These include 1 cm long, ½ cm thick green clay chips and Paradox Formation limestone clasts. Correlated stratigraphic sections show that Morrison strata thicken into the axes of synclines and thin on anticlines. This indicates that the folds in the Morrison were syndepositional and associated with salt movement during its deposition. Bed shortening through folding is evident in the folds. 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 northwestern end of Gypsum Valley has four sites that are possible outcrop analogues of reservoirs and traps. These include the crests of anticlines on units deposited on the salt, horn traps, pinch-out traps, and stratigraphic traps where there is an unconformity between the Morrison and the older Wingate and Chinle Formations. 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.