Abstract: Geochemical and Textural Indications of Open System Behavior During Evaporite Diagenesis, Louann Salt, Southwestern Alabama

Rachel A. Eustice, Lynton S. Land, Lawrence E. Mack

Trace element and ⁸⁷Sr/⁸⁶Sr analyses of halite, sylvite, and fluid inclusions from the Middle Jurassic Louann salt indicate that the salt diagenesis is heterogeneous and that bedded salt deposits are not closed geochemical systems during burial.

The Louann salt from the Champian Klepac #1 core from 11,941 to 11,173 ft, SW Alabama, can be subdivided into 4 zones in ascending order: a basal sheared zone, a chevron halite zone, a potash zone, and a chevron halite cap. Br concentrations of halite increase from 65 ppm in the basal sheared zone to 206 ppm at the base of the potash zone, consistent with the progressive evaporation of seawater to bittern salt precipitation. ⁸⁷Sr/⁸⁶Sr ratios of anhydrite inclusions in halite (0.70694-0.70767) and fluid inclusions (0.70723-0.70814) from these zones are only slightly more radiogenic than Callovian seawater (approx. 0.7070).

Rb, Sr, and Br concentrations of halite and sylvite separates from the potash zone are not consistent with precipitation as primary or secondary marine phases. ⁸⁷Sr/⁸⁶Sr ratios on halite (0.70765 to 0.71007), sylvite (0.70886 to 0. 71653), and fluid inclusions (0.70777 to 0.70997) from this zone are much more radiogenic than Callovian seawater. In addition, Rb/Sr model ages of halite and sylvite range from 1 Ma to 4200 Ma, without clustering near the presumed Callovian age of the salt (approx. l60 Ma). This indicates the salt has been an open system, with more intense open system behavior concentrated in the mineralogically heterogeneous potash zone. Diagenesis of the Louann salt and the associated bitterns helps explain the high Rb and Br concentrations, and the radiogenic ⁸⁷Sr/⁸⁶Sr ratios of brines from the Mississippi salt basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994