Ep. Moldovanyi et al., STRONTIUM, BORON, OXYGEN, AND HYDROGEN ISOTOPE GEOCHEMISTRY OF BRINESFROM BASAL STRATA OF THE GULF-COAST SEDIMENTARY BASIN, USA, Geochimica et cosmochimica acta, 57(9), 1993, pp. 2083-2099
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.