Louann salt geochemistry (Gulf of Mexico sedimentary basin, U.S.A.): A preliminary synthesis

Abstract

Samples of Jurassic salt and anhydrite were obtained from throughout the Gulf of Mexico sedimentary basin. Salt samples have been analyzed for bromide content and 87 Sr 86 Sr of anhydrite inclusions, and anhydrite for 87 Sr 86 Sr . Most samples of diapiric salt contain <60 ppm Br, the minimum value expected for first-cycle precipitation of marine halite. In addition, 87 Sr 86 Sr ratios are commonly radiogenic relative to the ratio in presumed coeval Jurassic (Callovian) seawater. Both these observations suggest that during deformation, salt undergoes extensive rock-water interaction, losing bromide to the reacting brines, and incorporating radiogenic Sr. Similar behavior of anhydrite during deformation with respect to Sr is also observed. Samples of deeply buried salt exhibit bromide concentrations consistent with the precipitation of marine halite, but include several samples with extremely high values. Few 87 Sr 86 Sr ratios, however, seem to be characteristic of coeval Callovian seawater. Samples with normal bromide content but radiogenic Sr suggest primary deposition from, or recrystallization in the presence of radiogenic fluids with marine-like bromide content. Samples with extremely high bromide content and radiogenic Sr values suggest recrystallization in fluids with elevated bromide content due to loss of bittern phases or extensive rock-water interaction of bedded salt during deep burial. The data suggest a dynamic system in which physical modification of evaporites during deformation and deep burial is accompanied by chemical modification.