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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 950

Last Page: 950

Title: Chemical Evolution of Brines from Modern Coastal Marine Sabkha: ABSTRACT

Author(s): David T. Long, Robert Gudramovics

Article Type: Meeting abstract


Certain minerals in ancient clastic and carbonate rocks such as selenite and iron sulfides are commonly taken as indicators of a sabkha environment. The wind-tidal flat area of Laguna Madre, Texas (a silicoclastic sabkha), is a modern locus for the deposition of these minerals and thus affords an excellent opportunity to determine the controls on their deposition. The purpose of this research is to study the chemical evolution of the subsurface brines associated with the mineral deposition.

A system of 20 well sites has been established along the 22-km width of the tidal flats to investigate the chemistry and the hydrology of the sabkha system. The chemistry and peizometric potential of the waters were determined at each site from two wells (depths of 1.9 and 3.8 m) and from a shallow trench dug to intersect the water table. The chemical data from two sampling periods (August 1979 to March 1980) were reduced by computer. Using Br- as a conservative ion, the results of this study are as follows: (1) the concentrated waters (2 to 9 times the salinity of seawater) are typically NaCl solutions which are high in Mg2+, K+, and SO42- and low in Ca2+ and HCO-3; (2) the major source of water i from Laguna Madre with minor contributions from continental ground waters; (3) the mixing zone of the two waters is on the continental side of the chloride plateau; (4) the sabkha hydrology is dominated by wind-generated flood recharge with localized evidence of evaporitic pumping and reflux; (5) the chemistry of the brines primarily reflects the degree of evaporation of the Laguna Madre waters and the extent of flooding; (6) the brines are all undersaturated with respect to halite and supersaturated with respect to dolomite, but vary in saturation state with respect to gypsum and calcite.

Previous work in this area, and the two sampling periods of this study indicate minor changes in brine geochemistry as a function of season. In general, the chemical nature of the brines from the Laguna Madre sabkha is similar to brines of other active coastal sabkhas; variations can be attributed to differences in climate, geomorphology, and hydrology.

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