ABSTRACT

Gypsum sand crystals of Laguna Madre are crystals of selenite of lens-like, discoidal shape that poikilotopically enclose dominantly terrigenous sand. They occur over an 85-km length of the lagoon as inferred from their presence in spoil islands formed during the excavation of the intracoastal waterway. These crystals are most abundant around the region known as the sand bulge.

Gypsum sand crystals show a spectrum from two intergrown crystals to clusters of more than 100 crystals, the latter of which may have either a random orientation of crystals or an irregular radial array. Clusters reach 50 cm in length and weigh up to 11 kg. Single crystals and small clusters commonly grew with the largest crystal perpendicular to or at a high angle to bedding, whereas large crystal clusters grew with their long dimension in the plane of bedding. In most sand crystals gypsum grew passively and merely filled pores: these samples have from 40 to 54 percent cement. Where gypsum precipitated rapidly it was displacive: these samples have from 60 to 70 percent "cement," and some cm-thick domains are completely free of sand. Growth of gypsum displaced rinds of micrite cement from their sand grain nuclei, sericite flakes from rock fragments, and fractured mollusc shells. SEM examination of sand grains and mollusc fragments from leached sand crystals reveals no evidence of grain corrosion or replacement. If the gypsum cement were to dissolve during further burial, no textural clue to its original presence will exist. Sr isotopic values (< 0.7084) of gypsum cement are below that expected for modern sea water, suggesting the release of non-radiogenic Sr from volcanic rock or carbonate rock detritus in the sediments. ¹⁸O values of Laguna Madre gypsum is heavier than expected for modern gypsum probably because of the fractionation effect of sulfate-reducing bacteria.

Laguna Madre became hypersaline only 210 years ago, so the gypsum sand crystals are very young, and they must have grown in years to tens of years. The presence of sand crystals to a depth of at least 4.5 m indicates the gypsum formed in the phreatic zone, and their increase in size with depth suggests they formed, in part, by seepage refluxion of lagoonal brine.