Characterization and Geochemistry of Carbonate Caprock Associated with the Gypsum Valley Salt Wall, Paradox Basin, Colorado: Implications for Understanding Lateral Caprock Emplacement

On the northeastern side of Gypsum Valley salt wall in the Paradox Basin of Colorado, discontinuous exposures of layered gypsum and carbonate rocks were previously mapped as the contact between diapiric Paradox Formation and marine limestone of the Pennsylvanian Honaker Trail Formation. Utilizing new and existing mapping, petrographic and isotopic geochemical analyses, this study reinterprets this zone to represent lateral gypsum and carbonate caprock that formed in the Triassic during passive rise of the Gypsum Valley salt wall. Gypsum Valley lateral carbonate caprock can be distinguished from Pennsylvanian and Permian depositional carbonate strata exposed elsewhere on the salt wall by the recognition of caprock distinctive fabrics (i.e. massive, porphyritic, layered and brecciated), the lack of fossils or sedimentary structures, the lack of depositional interfingering with the adjacent outboard Triassic Chinle Formation stratigraphy, and its carbon isotopic signature (\u3c –6 ‰) that reflects the contribution of isotopically light carbon from the oxidation of hydrocarbons during caprock formation. Additionally, the preliminary results from the lateral carbonate caprock produced a U-Pb age of 211 ±16Ma, which is consistent with the upper Triassic age suggested by the association with the Chinle Formation. The orientation of the lateral caprock layering parallels that of the adjacent halokinetically drape-folded Chinle strata, which contains fluvial channel conglomerate-bearing, caprock-derived clasts. This confirms that the carbonate caprock had already formed in the Triassic in a crestal position on top of the Gypsum Valley salt wall during Chinle deposition and was subsequently rotated by halokinetic drape folding to its flanking position during continued passive diapirism. In addition to the reinterpretation of the studied gypsum and carbonate units as lateral caprock, the term “capstone” was introduced when discussing a particular caprock lithology, as a distinction from the term “caprock” that is generally used in reference to the entire rock body. A capstone classification is proposed based on the recognition of four distinct megascopic fabric types: 1) massive: consisting of a homogeneous mineralogy and texture; 2) porphyritic: comprising two distinct crystal sizes; 3) layered: subdivisions based on layer size that includes: microlaminated (1–3 mm), laminated (3–10 mm), and banded (\u3e 10 mm); and 4) brecciated: these units are subdivided based on the degree of separation between brecciated capstone fragments, which may be close, loose, or spatially independent and subdivided into crackle breccia, mosaic breccia, and disorganized breccia, respectively. Paragenetic relationships indicate micritic dolomite to be the original carbonate capstone fabric formed during the Triassic. Thus, the additional capstone fabrics and mineralogies exposed today signify the diagenetic alteration of the original dolomite capstone, likely by multiple fluid flow events. Therefore, it is possible to use the evolution of capstone fabrics as an archive of fluid flow at the salt-sediment interface