STRONTIUM, BORON, OXYGEN, AND HYDROGEN ISOTOPE GEOCHEMISTRY OF BRINESFROM BASAL STRATA OF THE GULF-COAST SEDIMENTARY BASIN, USA

Significant spatial heterogeneities exist in the stable isotopic compo sition of saline formation waters from reservoirs of the Smackover For mation (Upper Jurassic). We focused on the southwest Arkansas shelf, a structurally simple portion of one of the interior basins of the nort hern Gulf Coast sedimentary basin. Here, faulting and facies changes j uxtapose dominantly oolitic carbonate strata against basal evaporites, red beds, and siliciclastics, as well as metamorphosed basement rocks . Brines from this area have exceptionally high Br and alkali element concentrations and have spatially heterogeneous hydrogen sulfide conce ntrations. Strontium, boron, oxygen, and hydrogen isotope compositions exhibit coherent relations with other aspects of brine geochemistry. Sr isotope compositions range from those expected for carbonates and e vaporites deposited from Jurassic seawater (0.7071 ) to radiogenic rat ios as high as 0.7107. Generally, most radiogenic Sr isotope values ar e associated with H2S-rich waters which also have elevated alkali elem ent (Li, B, K, Rb) concentrations. These alkali element-rich waters ar e associated with portions of the South Arkansas fault system which re ach basement. Boron isotope compositions are similarly heterogeneous, ranging from values of +26 to +50 parts per thousand. Brines with high est B contents are most depleted in B-11, consistent with boron input from brines generated from high-temperature siliciclastic diagenetic r eactions. Normalizing B contents to Br in the brines reveals a reasona ble mixing trend between a Dead Sea-type composition and Texas Gulf Co ast-type shale/sand reservoir waters. Oxygen and hydrogen isotope data exhibit regional variations which are controlled by meteoric water in vasion along the northern limb of the southwest Arkansas Fault, which has surface expression. Although oxygen isotope compositions are often near equilibrium with respect to reservoir carbonate, it is more diff icult to ascribe trends in deltaD values to local water-rock interacti on. The stable isotope trends can be explained either in terms of an e volved marine evaporite brine or a water evolved via water-rock intera ction, and mixing with meteoric water in the vicinity of the southwest Arkansas Fault. Spatial heterogeneity in isotopic composition of form ation waters from this relatively restricted study area suggests that assumptions of a homogeneous Sr reservoir in dating regional mineraliz ation events require careful assessment in ancient systems.