CARBONATES WITHOUT A CAUSE: DECIPHERING THE ORIGIN OF UNIQUE CARBONATE LITHOLOGIES IN SALT DIAPIR CAPS

At the Gypsum Valley salt diapir in the Paradox Basin (CO/UT), we have discovered dolostones and limestones whose petrography and geochemistry do not match others in the region, and more significantly, do not fit any known model for the genesis of carbonates. These atypical lithologies, which are completely devoid of fossils, belong to a 15-35 m thick mélange between the gypsum-dominated salt diapir cap and the adjacent Triassic strata. Such mélanges at diapir margins are presently known to form due to a) halite dissolution in the cap of a salt diapir and the subsequent diagenetic replacement of gypsum with limestone when hydrocarbons migrate into the diapir cap and are oxidized by sulfate-reducing bacteria, and b) the accretion of non-evaporite lithologies from a layered evaporite sequence, including depositional carbonates of biotic and abiotic origin. Fluid migration and rock deformation due to salt movement relative to adjacent strata contribute to the geochemical inventory (e.g., silica content) and richness of rock fabrics in such mélanges, but do not entirely overprint the carbonates originally added to the salt diapir cap. We identified two such carbonates that are highly unique: a homogenous to finely-laminated, microcrystalline dolostone and a banded, coarse-crystalline limestone that shows striking similarity to zebra-dolostones described in the literature. Their carbon isotope signatures are incompatible with a hydrocarbon-oxidation origin, while clumped isotope signatures show elevated temperatures incompatible with the depositional origin. Thus, the presence of these atypical lithologies implies either the existence of a yet undiscovered carbonate rock formation mechanism, or so far unknown processes that lead to overprinting of carbon and oxygen isotope signatures of carbonate rocks. Evaporite deposition coincides with the formation of lithium-rich brines, which migrate into permeable units and are trapped by younger impermeable units likely disrupted by salt movement and deformation. Stratigraphically and structurally, the here studied carbonates are located at this decisive bottleneck in the migration pathway for fluids in the Paradox Basin, and thus they also have potential to serve as indicators for the location of lithium-bearing brines in diapir-adjacent sedimentary basins.