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Origin of Supra-Salt Synclines in the "Post Diapiric" Jurassic Morrison Formation, Big Gypsum Valley, Colorado


The Jurassic Morrison Formation, composed of two members - the Salt Wash and Brushy Basin - can be found surrounding and partially burying the Gypsum Valley diapir in the Paradox Basin, Colorado. The diapir began passively rising during the Late Pennsylvanian/Early Permian and continued to rise differentially along the length of the salt wall until the late Jurassic, when the Morrison Formation was deposited. The diapir was breached during Neogene erosion of the Colorado Plateau, exposing the diapir caprock. Folds within the Morrison Formation are preserved along the diapir margins where the Morrison overlies the salt. These are best exposed at the southeastern part of Big Gypsum Valley as a series of tight folds that get progressively more open to the northwest. A detailed map along the diapir margin shows synclines and subhorizontal Morrison beds in direct contact with the salt. The lower beds of the Morrison onlap the tilted strata flanking the diapir, indicating continued minibasin subsidence and diapir rise that continued into Salt Wash time. Basal strata of the Morrison contains clasts of diapir and other flanking strata reflecting erosion of the diapir margin by the first channels that flowed across the diapir. These included 1 cm long, ½ cm thick green-gray clay chips and Paradox Formation limestone clasts that can be observed at the base of Morrison sandstone channels ranging in thickness from 0.5 m - 2 m. Measured sections have been completed on both the northeastern and southwestern part of Big Gypsum Valley. These, along with the tracing of beds along the diapir margin, better displayed a change in bed thickness, which thinned into the axes of synclines and thinned on anticlines. Thus indicating the folds in the Morrison were syndepositional and associated with diapir subsidence or movement during early Morrison deposition. The initial results of this study indicate that diapir and salt movement continued through at least Basal Salt wash time. 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. Faulting and rotation of the Morrison in other areas indicates that solution collapse may have been locally important. Constraining the mechanisms that formed these structures will help improve our understanding on how salt diapirs function and their impact on the surrounding strata, which could ultimately serve as traps for resources such as oil and gas.