Permian Castile Sr isotope data: Readme file

NAME OF DATA SET: Strontium isotope data, Castile Anhydrite, Upper Perimian, Delaware Basin

CONTRIBUTOR: Walter E. Dean, U.S. Geological Survey, Denver

SUGGESTED DATA CITATION/ORIGINAL REFERENCE FOR DATA:

ORIGINAL REFERENCE FOR BACKGROUND:

GEOGRAPHIC REGION: western Texas, New Mexico

PERIOD OF RECORD: 254 Ma (Latest Permian Period).

LIST OF FILES:

• Castile_Srdata.txt
• Castile_Srmethods.txt

DESCRIPTION:

The Late Permian (early Ochoan) Castile Formation is a major evaporite sequence (~10,000 km3) of calcite, anhydrite, and halite in west Texas and southeastern New Mexico. Traditionally the Castile brine has been considered to have been derived from seawater. This tradition has recently been challenged by two versions of the closed basin-drawdown model. They call for deposition from a mixed brine, in part marine and in large part nonmarine. They propose drawdown of as much as 500 m to form a major sink for ground water issuing from the surrounding Capitan reef-complex. A large fraction of the solute within the brine body is inferred to have been recycled from older Permian evaporites on the surrounding shelf.

Strontium isotope analyses show no evidence that meteoric ground-water was contributed to the Castile brine. From a stratigraphic, geographic, and lithologic array of 65 samples of anhydrite, gypsum, and calcite, 59 have an 87Sr/86Sr ratio of 0.706923 (delta SW of -225.0), a ratio that is the same as that of strontium in early Ochoan ocean-water. If considerable (>15%) influx of meteoric water had occurred, enough continental strontium would have been introduced to have resulted in higher ratios.

Low bromide values (20-40 ppm) in Castile halite, which have been used to argue for meteoric influx and for recycled salt, probably resulted from diagenesis. During shallow burial by halite, centimeter-sized, bottom-grown crystals of gypsum were altered to nodular anhydrite. The rising water of dehydration caused the halite to recrystalize. During the recrystalization, some bromide was expelled.

Despite the large volume of water that evaporated annually from its surface (~52 km3/yr, assuming an evaporation rate of 2 m/yr), the Castile brine-body never completely desiccated. The surrounding shelf was flat, hot, and generally dry. It probably could not have supplied a significant volume of meteoric spring-water to the basin over tens of thousands of years. More likely, during the entire history of the evaporite sequence, influx was dominantly marine. Marine ground-water flowed through the Capitan Formation into the evaporite basin along its southern and possibly western margin probably with a rate of flow that was usually fast enough to prevent major drawdown of the brine surface.

Core Locations:

UNM Cowden 2: section 34, township 1, block 64, Survey T & P.R.R. Co., Culbertson CO, Texas.

UNM Cowden 4: section 33, township 2, block 62, Survey T & PT-2S, Culbertson CO, Texas.

UNM Phillips 1: section3, block 110, Public School Land, Culbertson CO, Texas

Union University "37" 4: section 7, block 2, University Lands Survey, Winkler CO, Texas

Stateline outcrop: SE sec. 28, T. 26S, R. 24E, Eddy Co., New Mexico

"Castile_Srdata.txt" file contains the Sr isotope data of Castile samples as tab-delimited ASCII text.

Variable names:

• Sample Sample identification number
• Member Member of the Castile Formation as defined by Anderson et al. 1972.
• Delta SW Sr-isotope composition relative to sea water as defined by:
  sw = (87Sr/86Sr unknown - 87Sr/86Sr modern seawater) x105 (see also "Castile Srmethods.txt" file
• Lithology composition of sampel (e.g. anhydrite, gypsum, calcite)

"Castile_Srmethods.txt" file contains a description of the geochemical methods used.