Abstract

The Sand Bulge area of the Laguna Madre Flats is a silicoclastic coastal sabkha lying between Padre Island and the Intracoastal Waterway on the Gulf coast of southwest Texas. The sabkha is sporadically inundated by wind-tidal flooding of water from an adjacent lagoon (Laguna Madre). Saline interstitial brines are formed by infiltration of partially evaporated ponded flood water. Manometric data indicate that the brine seeps downward through the sabkha and laterally, west toward the Intracoastal Waterway (ICWW). Some of the brine may flow seaward (east) through incised Pleistocene stream valleys. Most of the sabkha in this area is covered with blue-green algal mat. Precipitation of microcrystalline Mg-calcite ("algal micrite") in brine ponds or within the algal mat, induced by algal or bacterial action, removes much of the calcium from recharge water before it reaches the sabkha watertable. Thus, though brine salinities reach 240 per mil, low calcium concentrations prevent the precipitation of gypsum, and thermodynamic calculations indicate that the brines are undersaturated with respect to gypsum. Gypsum does not presently occur in the sabkha sediments in this area, contrary to earlier observations (Fisk, 1959). It is suggested that emplacement of the ICWW altered the rate and direction of water flow through the sabkha, leading to less saline brines and possibly to the dissolution of previously formed gypsum.

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