The lower Tansill Formation (Permian) in the vicinity of Carlsbad Caverns, New Mexico, is a basinward prograding sequence of back-reef carbonate rocks equivalent to the Capitan reef. Toward the north (shelfward), the facies tract consists of outer-shelf skeletal-intraclast grainstone, shelf-crest pisolite, and lagoonal pelletal dolomite. Diagenesis occurred by means of four phases that reflect the changing hydrologic regime. Square-ended ray cement (phase I), now calcite or dolomite, but originally aragonite, was the product of marine waters. Calcitized ray cement from the reef has retained up to 3,500 ppm strontium and has a relatively heavy carbon and oxygen isotopic composition. Back-reef examples are lower in strontium and lighter isotopically, reflecting neomorphism nder more open conditions by meteoric water.

Soon after deposition, outer-shelf and shelf-crest sediment was periodically exposed to fresh water (phase II). Calcite cement that is the product of this early meteoric-schizohaline regime is nonluminescent, has low or variable magnesium content, and has an intermediate isotopic composition. Contemporaneous diagenetic processes include solution, conversion of aragonite to calcite, and schizohaline dolomitization. Perhaps reflecting its genesis, the strontium content of Tansill dolomite increases shelfward from the shelf crest, which was the area of meteoric recharge. As back-reef carbonate deposits became buried, meteoric conditions became more stable in the shallow subsurface (phase III). Phase III calcite cement is luminescent, uniformly low in magnesium, and has a relatively light isotopic composition. Upon erosion of the overlying sediment, the rocks were subjected to near-surface meteoric flushing, resulting in solution and minor cementation (phase IV).

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