Abstract: Geochemistry, Hydrology, and Mineralogy of Noncarbonate Coastal Sabkha, Laguna Madre, Texas
The Sand Bulge area of the Laguna Madre flats is a silicoclastic coastal sabkha behind Padre Island on the Gulf Coast of southwest Texas. It is inundated sporadically by wind-tidal flooding of waters from the adjacent lagoon or the Intracoastal Waterway. Sabkha interstitial brines are recharged by infiltration of partly evaporated ponded flood waters; flow from the "freshwater lens" beneath Padre Island contributes slightly to sabkha groundwater. Manometric data indicate that the brine refluxes downward through the sabkha and laterally toward the Intracoastal Waterway. A combination of high lateral permeability, frequent recharge, and low evaporation rates through the sabkha surface prevents brines from reaching halite saturation.
Most of the sabkha surface is covered with blue-green algal mat. In brine ponds or within the algal mat, precipitation of magnesian calcite ("algal micrite"), induced by algal or bacterial action, removes much of the Ca++ from recharge waters before they reach the sabkha groundwater table. Thus, though brine salinities reach 240 parts per thousand (well into the gypsum-precipitation field for evaporating seawater), gypsum is not present in the sabkha sediments in this area, and geochemical analysis indicates that the brines are not gypsum saturated.
There is little dolomite in the sabkha sediment. Carbonate minerals present include low-magnesium calcite (molluscan shells), magnesian calcite ("algal micrite"), and aragonite (molluscan shells).
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC