GEOCHEMISTRY AND THE ORIGIN OF DOLOMITES OF THE UPPER JURASSIC SMACKOVER FORMATION IN THE MISSISSIPPI SALT BASIN

Two dolomite types with distinct geochemical compositions are found in the Smackover Formation in the Mississippi Salt Basin. The first occurs in the basin margin, parallel to the paleoshoreline. This dolomite is fine-crystalline and has high d¹³C and d¹⁸O compositions averaging 5.0‰, and 4.1‰, respectively (isotope values are reported relative to PDB). The second Smackover Formation dolomite type occurs on high-rise salt structures. This dolomite is course crystalline, and its d¹³C and d¹⁸O compositions average 4.1‰, and -2.2‰, respectively. The d¹³C and d¹⁸O values of dolomite associated with anhydrite layers of the overlying Buckner Formation (4.4‰, and -0.3‰, respectively) are between the two Smackover Formation dolomites. The shoreline-parallel dolomite of the Smackover Formation and dolomites of the Buckner Formation did not have any effect on porosity and permeability development. Whereas dolomitization of the Smackover Formation on high-rise salt structures created excellent porosity in permeability.

Geological, geographical distribution, and isotopic compositions of dolomites in the Smackover and Buckner formations indicate three different processes of dolomitization. The association of the Buckner dolomite with anhydrite layers indicates that it formed from a seawater that had reached anhydrite saturation in the Buckner lagoon. The Smackover Formation dolomite that occurs on high-rise salt structures formed by the seepage of Mg-rich brine into Smackover grainstones through subaerial exposure surfaces on crests of salt domes. This brine was derived by evaporation of the Buckner seawater in lagoonal environments between salt domes. The shoreline parallel dolomite of the Smackover Formation may have formed by extremely evaporated seawater in salt ponds along the paleoshoreline.