Accessories of the Dolostone Associated with Salt Diapirs as a Key to Decipher Its Origin

The Moab Valley salt wall formed during the late Mississippian to Early Permian due to mobile salt from a layered evaporite sequence, the Paradox Formation which was deposited in a flexural foreland basin associated with the Uncompahgre uplift. Today, surface exposure of the salt walls includes gypsum beds which are overlain by banded dolostones that appear to interfinger with siliciclastic sediments from the Triassic Chinle Formation. A straightforward interpretation of this field relationship is to consider the gypsum as caprock belonging to the salt wall and the dolostones as a lacustrine deposit belonging to the Chinle Formation. However, other lacustrine deposits belonging to the Chinle Formation that are not associated with salt walls display limestone, which raises the question why at the Moab location, dolostone was formed. Typically, high magnesium-calcium ratios are needed to drive dolomite formation, a condition that can be achieved in lakes in semi-arid by the precipitation of gypsum (calcium sulfate). This opens the possibility that the gypsum found at the Moab Valley salt wall is rather a lacustrine sediment deposited concurrently with the dolostone and the siliciclastics from the Chinle formation than caprock of the salt diapir.

To test this hypothesis, we dissolve gypsum and dolostone samples, extracting insoluble residue that comprises minerals and organic matter. Comparison of the inventory of constituents will reveal if the two lithologies were formed contemporaneously or at different times.