Abstract

Studies of anhydrite in Permian shelf sediments of Texas and New Mexico indicate similarities to Recent gypsum deposits of Laguna Madre, coastal S. Texas. Permian anhydrite occurs with greatest frequency within cyclical clastic and carbonate shelf sediments. The most common form (other than of pore-filling or replacement origin) is the nodule. Nodules are packed to varying degrees, ranging from widely spaced individuals to dense zones which have been misinterpreted as beds. Two types of truly bedded anhydrite occur: thin discrete layers, and laminated beds. In Laguna Madre, rosettes, bladed crystals, and thin beds of gypsum occur below the sediment surface. Thin sheets of lagoon water periodically are blown over mud flat areas where evaporation markedly increases salinity. The concentrated brines percolate downward, supplying hypersaline ground water from which the gypsum is precipitated. The discovery of Permian anhydrite nodules that are pseudomorphous after euhedral gypsum and which closely resemble Recent gypsum crystals and crystal aggregates suggests a similar mode of origin.

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