Mineralogical, chemical, and isotopic changes were found as depth increased in texturally homogenous sediment recovered from six 300-m (985-ft) boreholes on the outer shelf and slope of the northwestern Gulf of Mexico. In 2 of the 3 boreholes on the shelf and in all 3 boreholes on the slope, sediments of late Pleistocene age were found to be directly in contact with salt at the top of piercement structures. The other borehole was drilled on the flank of a salt intrusion also penetrating late Pleistocene sediment. In sediments over the top of the salt, the abundance of expandable clays (smectite) compared with nonexpandable clays (illite) decreases with depth. Within the carbonate fraction, δ13C values range from –2% near the salt-sediment interface to 1% at the surface. This deviation is apparently a response to reprecipitation adjacent to the salt-sediment interface, with lighter isotopes derived from oxidation of the isotopically light organic matter. No mineralogical, chemical, or isotopic trends were found in sediments on the flank of the salt intrusion.
Clay transformation caused by heat of the salt stock releases a significant amount of intercellular water (≈ 200 L/m3 or 1.5 gal/ft3), which dissolves the leading edge of the salt intrusion leaving a residual concentration of granular anhydrite. High sulfate concentration in the presence of organic material sets chemical conditions by which sulfate is reduced to sulfide and bicarbonate is formed. The subsequent buildup of bicarbonate leads to precipitation of calcite and also formation of more water to continue dissolution of salt. Isotopic data of the carbonate fraction in sediment above the salt mass supports this model.
The most commonly accepted model of cap rock formation requires the intrusion of a salt stock into a flowing aquifer, a unit which supplies water needed to dissolve salt concentrate, the anhydrite, and provide the subsequent chemical environment for gypsum and calcite formation. In the proposed model, water is the result of diagenesis caused by sediment-salt interaction. Cap rock formation then may occur well below base level and well out on the continental slope.