CARBONATES ADJACENT TO GYPSUM IN THE PARADOX BASIN - OH CAPROCK, WHEREFORE ART THOU?

Proper identification of carbonate caprock is vital to provide context for salt diapirism and tectonics, and to prevent erroneous interpretation of subsurface and outcrop geology. Salt walls in the Paradox Basin are often flanked by carbonate units, located between gypsum caprock belonging to the salt diapir and sediments deposited during salt diapirism. These carbonate units could be A) upturned strata, B) stringers in the gypsum/anhydrite-dominated caprock, C) a product of carbonate dissolution-reprecipitation within the diapir, D) authigenic caprock derived from hydrocarbon oxidation coupled to microbial sulfate reduction, or E) an accumulation of carbonates in a basin adjacent to an exposed diapir (i.e. sabkha carbonates, bioskeletal carbonates, carbonates precipitated in the water column as well as mixing of calcium/magnesium- with carbonate-rich fluids).

Determining the origin of these diapir-flanking carbonates is not straightforward, and requires a combination of petrographic (e.g. fossils, accessory minerals, presence of dead oil paragenetic sequence), stratigraphic (e.g. interfingering with sedimentary strata adjacent to the diapir), and geochemical (e.g. trace element and isotope composition) criteria. In the Paradox Basin at the Gypsum Valley salt diapir (CO), we identified lateral, authigenic carbonate caprock previously misidentified as sedimentary carbonate belonging to the diapir-adjacent stratigraphy. However, not all carbonate outcrops directly adjacent to the Gypsum Valley can automatically be deemed caprock. Moreover, the carbonate caprock identified by us cannot be considered “typical”. Unlike its well-studied siblings from the U.S. Gulf Coast salt dome province, a significant portion of the carbonate is dolomite, and the carbon isotope signatures are much heavier than what is expected for hydrocarbon-derived carbonate.

Here, we address the following questions: 1) How abundant is carbonate caprock at salt diapirs in the Paradox Basin, 2) what conditions led to the formation of the caprock with unique geochemical characteristics at the Gypsum Valley salt wall, and 3) what are the implications of outcropping carbonates for the interpretation of the subsurface geology?