IDENTIFICATION AND CLASSIFICATION OF LATERAL CARBONATE CAPROCK: GYPSUM VALLEY SALT WALL, PARADOX BASIN, COLORADO

Discontinuous exposures of layered gypsum and carbonate rocks at the Gypsum Valley (GV) salt wall in the Paradox Basin were previously mapped as the Penn. Honaker Trail Limestone. Utilizing new mapping, petrographic and geochemical analyses, we reinterpret this zone to represent lateral caprock that formed during passive rise of the GV salt wall. GV lateral carbonate caprockcan be distinguished from depositional carbonate strata exposed elsewhere on the salt wall by the recognition of caprock distinctive fabrics, the lack of fossils or sedimentary structures, the lack of depositional interfingering with the adjacent outboard Triassic Chinle Fm, and its carbon isotopic signature (< –6 ‰) that reflects the contribution of isotopically light carbon during caprock formation. The orientation of the lateral caprock layering parallels that of the adjacent halokinetically drape-folded Chinle strata, which contains fluvial channel conglomerates bearing caprock-derived clasts. This confirms that the carbonate caprock had already formed in the Triassic, in a crestal position on top of the GV salt wall during Chinle deposition and was subsequently rotated by halokinetic drape folding to its flanking position during continued passive diapirism.

The term capstonewas introduced when discussing a particular caprock lithology, differing from term caprockthat is 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 (1-3mm), laminated (3-10mm), and banded (>10mm);and 4) brecciated, which is subdivided based on the degree of separation between brecciated capstone fragments:crackle, mosaic, and disorganized breccia. Paragenetic relationships indicate micritic dolomite to be the original carbonate capstone formed in 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.