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The San Andres Formation, on the basis of cores from west Howard County, Texas, consists of a westward-prograding sequence in the Midland basin. It is composed of five principal facies: (1) intrabasinal to basinal limestones; (2) open-marine bivalve or crinoid wackestones and/or packstones; (3) open-marine fusulinid wackestones and/or packstones; (4) subtidal to intertidal lagoonal mudstones and/or wackestones; and (5) intertidal to supratidal carbonate-evaporite sabkha. Contacts between facies are uniformly gradational.
The formation shows a complex diagenetic history. Carbonate sediments have undergone pervasive dolomitization, marked principally by complete dolomitization of the mud matrix, leaching and destruction of allochems, and partial void fill by subhedral to euhedral dolomite rhombs. Matrix-crystal enlargement occurs at the upper end of the sequence, where the sabkha reaches its greatest development. Anhydrite is ubiquitous in the formation, and is well developed throughout the sequence. Primary crystal habits are chaotic, blocky rectangular, radial, and poikilotopic. Anhydrite has been subjected to substantial diagenesis, including replacement by length-slow chalcedony, lutecine, hematite, pyrite, sphalerite, and free sulfur. Two major zones of leaching are marked by nearly complete soluti n of anhydrite and precipitation of coarse blocky and fibrous calcite, with levels of porosity exceeding 20%. Rehydration of anhydrite to satin spar and pseudomorphic gypsum is common, particularly concurrent with zones of leaching. Although paragenetic indicators are sparse, poikilotopic and rectangular void-filling anhydrite apparently postdates precipitation of nodular anhydrite, and solution of evaporites postdates replacement of anhydrite by length-slow chalcedony, as indicated by loose spherulites in the bases of leached voids.
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