ABSTRACT

The Late Jurassic Smackover Formation in the Mississippi salt basin consists of two 150 m thick shoaling-upward cycles, each capped by ooid grainstones. During deposition of the lower cycle, originally calcite ooids (preserved radial fabric, ¹⁸O = -3.8 PDB, ¹³C = 4.5 PDB, Sr²⁺ = 315 ppm) formed on the seaward side of the basin and former aragonite ooids (calcitized textures, ¹⁸O = -3.0 PDB, ¹³C = 5.5 PDB, Sr²⁺ = 1790 ppm) were precipitated on the landward side. In the upper cycle, originally calcite ooids (preserved radial fabric) were precipitated on both the seaward (¹⁸O = -2.5 PDB, ¹³C = 4.2 PDB, Sr²⁺ = 300 ppm) and the landward (¹⁸O = -2.5 PDB, ¹³C = 3.2 PDB, Sr²⁺ = 250 ppm) sides of the basin.

Because kinetic variables (Mg²⁺/Ca²⁺, rate of CO₃^2- supply, PO₄^3-, SO₄^2-) are incapable of totally preventing aragonite formation, we suggest that Smackover calcite ooids were precipitated from seawater with low carbonate saturation state (possibly undersaturated relative to aragonite). The shift from seaward calcite to landward aragonite ooids in the lower cycle was controlled by a shoreward increase in seawater salinity. The net effect of the salinity gradient was a landward increase in the carbonate saturation state in response to decreasing dissolved CO₂ and increasing CO₃^2-, Ca²⁺, and temperature. In seawater supersaturated with respect to both arago ite and calcite, kinetic variables favored dominance of aragonite over calcite. The landward increase in seawater salinity reflects extensive evaporation in an arid climate, resulting in antiestuarine circulation.

The monomineralogic (calcite) nature of ooids of the upper cycle suggests that the salinity gradient across the basin was not sufficient to alter the seawater saturation state. This is attributed to a less arid climate and/or a less restricted circulation.