The Jurassic Louann salt was the first significant sedimentary unit to accumulate in the Gulf of Mexico sedimentary basin. Br/Cl and 87Sr/86Sr ratios of halite from a single core into the top of the formation record the evaporation of normal seawater to bittern stage. The bittern zone today consists of intergrown halite and sylvite. The Br and Rb contents of the solid phases, along with 87Sr/86Sr ratios and Rb/Sr systematics, are inconsistent with precipitation of the existing phases from seawater evaporated in Jurassic time. Rather, petrography and fluid inclusion and solid phase chemistry from the bittern zone is consistent with postdepositional water/rock interaction which diagenetically modified a marine bittern assemblage to halite + sylvite. The chemistry of the Br- and Rb-rich saline formation waters characteristic of this area today, likewise, may reflect water/evaporite interaction during burial.