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Structural Analysis of Disrupted Carbonate Caprock Underlying a Salt Shoulder, Gypsum Valley Salt Wall, Paradox Basin, Colorado

Abstract

Salt shoulders are defined as abrupt inward steps of a diapir margin on a steeply rising passive diapir. They commonly form anticlinal structures that can serve as significant hydrocarbon traps. Shoulders have recently been identified at Gypsum Valley Salt Wall (GVSW), Paradox Basin CO, where Triassic strata of the fluvial Chinle Formation onlap the shoulder in a series of halokinetic sequences. Hydrocarbons migrated into the shoulder carrying sulfate-reducing microbes, locally altering GVSW’s gypsic caprock to carbonate (calcite and dolomite). Geologic mapping of the caprock along the shoulder at Gypsum Valley shows a disruption of horizontally banded caprock into highly variable orientations, reflecting faulting and folding within the caprock. The disrupted zone can be divided into distinct polygonal shaped structural domains that vary in style and trend of deformation as well as capstone fabric type. Differing capstone fabric types can be described as massive, porphyritic, layered (microlaminated, laminated, or banded), or brecciated (crackle, mosaic, or disorganized). Domain boundary zones yield fetid, hydrocarbon-bearing carbonate cements filling fractures ranging from 0.5-2 meters in width. To determine the origin of the caprock deformation, we constructed detailed structural and capstone fabric type maps that document the distribution and relationships of caprock fabrics, fracture fill, and cement type to structures. The causes of carbonate caprock disruption include: 1.) continued halite and caprock dissolution and associated karst collapse, 2.) extension and radial faults associated with continued passive rise of the inboard margin of the shoulder, and 3.) folding and faulting associated with both early and late stage shoulder formation and collapse. Because caprock is part of the salt body, understanding how and where caprock deformation has occurred along the salt wall will allow us to decipher the controls of salt shoulder formation and ultimately the processes involved in halokinesis that are not extractable from the salt itself.