Elemental and isotope geochemistry of Smackover Formation brines in Alabama and Arkansas: Implications for fluid mixing and water-rock interaction

Kelly Kreitzer; Ming-Kuo Lee; Haibo Zou; Yang Wang; Andrew L. Masterson; Ann S. Ojeda; Billor Zeki; Natalia Malina; Gregory Guthrie; Bonnie McDevitt; Ann Arnold; Demi Durham

doi:10.1016/j.apgeochem.2026.107086

USGS · 70283351

Elemental and isotope geochemistry of Smackover Formation brines in Alabama and Arkansas: Implications for fluid mixing and water-rock interaction

Kelly Kreitzer·Ming-Kuo Lee·Haibo Zou·Yang Wang·Andrew L. Masterson·Ann S. Ojeda·Billor Zeki·Natalia Malina·Gregory Guthrie·Bonnie McDevitt·Ann Arnold·Demi Durham

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Abstract

This study investigates the geochemical characteristics of brines within the Upper Jurassic Smackover Formation in Alabama and Arkansas through elemental and isotopic analyses and geochemical modeling to reveal the extent and nature of fluid mixing and water-rock interactions. Smackover Formation brines exhibit Na-Cl-Ca hydrogeochemical facies and sulfate depletion relative to seawater. Lower sulfate concentrations, coupled with high H 2 S concentrations in some brines (0-364 mg/L) at temperatures >80°C, suggest that thermochemical sulfate reduction occurred. Cl/Br and Na/Br ratios indicate that salinity was controlled by the mixing of meteoric water with evaporated seawater that had reached halite saturation, rather than by halite dissolution. The δ 18 O and δ 2 H values of the brines also suggest mixing between meteoric water and evaporated seawater that had reached gypsum and halite saturation. Geochemical modeling of seawater evaporation predicts a Cl-Na or Cl-Mg facies after halite saturation, contrasting with the observed Cl-Ca dominance of the Smackover Formation brines. The Ca-enriched brines may have evolved from Cl-Mg-dominated evaporated seawater through dolomitization of limestone. The 87 Sr/ 86 Sr ratios range from 0.70740 to 0.71048, higher than the average of Jurassic seawater (0.707). The non-linear relationship between 87 Sr/ 86 Sr and 1000/Sr suggests alteration of brine geochemistry by fluid mixing or by inputs from Sr and 87 Sr-enriched rock sources. Lithium concentrations are positively correlated with 87 Sr/ 86 Sr ratios and TDS in brines enriched in these constituents. These correlations suggest deeper-seated geogenic sources and have implications for Li exploration in the northern Gulf Coast basin. Normal faults associated with the pre-Jurassic topography may have provided potential pathways for brine migration into the Smackover Formation under the influence of overpressure or thermohaline convection in the Gulf Coast.

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Applied Geochemistry Article

Source-reported bounding extent: 30.292893915779075° to 32.16780840975727° latitude; -88.44707143598633° to -86.6170519897454° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Kelly Kreitzer, Ming-Kuo Lee, Haibo Zou, Yang Wang, Andrew L. Masterson, Ann S. Ojeda, Billor Zeki, Natalia Malina, Gregory Guthrie, Bonnie McDevitt, Ann Arnold, Demi Durham. 2026-09-25. Elemental and isotope geochemistry of Smackover Formation brines in Alabama and Arkansas: Implications for fluid mixing and water-rock interaction. https://doi.org/10.1016/j.apgeochem.2026.107086

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