Geochemistry, hydrology and mineralogy of the Laguna Madre flats, south Texas




Amdurer, Michael

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The Land Cut Area of the Laguna Madre Flats is a silicoclastic coastal sabkha that lies behind Padre Island on the Gulf Coast of southwest Texas. It is sporadically inundated by wind-tidal flooding of waters from the adjacent lagoon and the Intracoastal Waterway. Sabkha interstitial brines are recharged by infiltration of partially-evaporated ponded flood waters ("reflux"); flow from the fresh water lens beneath Padre Island contributes somewhat to sabkha ground water. Manometric data indicate that the brine refluxes downward through the sabkha and laterally toward the Intracoastal Waterway. Some of the brine may flow seaward through incised Pleistocene stream valleys. Most of the sabkha in this area is covered with blue-green algal mat. Precipitation of microcrystalline magnesian calcite ("algal micrite") in brine ponds or within the algal mat, induced by algal or bacterial action, removes much of the calcium from recharge waters before they reach the sabkha watertable. Thus, though brine salinities reach 240 per mil (well into the gypsum-precipitating field for evaporating seawater), gypsum does not occur in the sabkha sediments in this area and geochemical analysis indicates that the brines are not saturated with respect to gypsum. No dolomite was detected in the sabkha sediment. Carbonate minerals present include low-Mg calcite (molluscan shells), magnesian calcite ("algal micrite" and micritized shell fragments) and aragonite (molluscan shells)