--> ABSTRACT: Shallow-Water Gypsum in Castile Formation--Significance and Implications, by Alan C. Kendall and Gill M. Harwood; #91022 (1989)

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Shallow-Water Gypsum in Castile Formation--Significance and Implications

Alan C. Kendall, Gill M. Harwood

The Ochoan (Upper Permian) Castile Formation of the Delaware basin (west Texas and New Mexico) is commonly used to illustrate the necessity for surface reflux in marine evaporite basins and as a textbook example of deep-water evaporite sedimentation in a barred basin. The presence of basin-wide, millimeter-thick calcite/anhydrite laminae and salinity cycles (laminar anhydrite at the base passing gradationally up into nodular anhydrite and laminar halite) have generated depositional models involving a brine-filled basin (up to 600 m deep) that exhibited periodic fluctuations in salinity. Such models have been widely applied to other evaporite basins.

Discovery of pseudomorphs of bottom-grown gypsum crystals in basin-wide intervals of nodular anhydrite in the lower parts of the Castile Formation throws into doubt these classical interpretations. The crystals indicate bottom waters at times were supersaturated with respect to gypsum and imply (at times when nodular anhydrite intervals formed) deposition occurred in waters that were, at most, only a few tens of meters deep. Much of the associated laminar anhydrite is similarly reinterpreted as being of shallow-water origin. At these times the basin must have been isolated and almost completely desiccated. Surface reflux of concentrated brine out of the basin would have been impossible. Reflux must have occurred by seepage through the basin floor or flanks (promoting dolomitization?). Desiccation of the basin would have created a large hydraulic head (initially over 0.5 km), leading to the possibility that much of the basin influx was ground and formation waters derived from the slightly older (Guadalupian) evaporitic marginal platforms. The Castile Formation thus may represent, in part, a recycled evaporite deposit.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.