Diagenesis of the Capitan Formation Forereef Facies (Permian, West Texas and New Mexico)
Leslie A. Melim, Peter A. Scholle, 1999. "Diagenesis of the Capitan Formation Forereef Facies (Permian, West Texas and New Mexico)", Geologic Framework of the Capitan Reef, Arthur H. Saller, Paul M. (Mitch) Harris, Brenda L. Kirkland, S. J. Mazzullo
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The diagenesis of the Capitan forereef can be divided into four overlapping episodes: 1) early marine diagenesis; 2) early burial diagenesis involving normal marine to hypersaline fluids; 3) late burial diagenesis; and 4) uplift related diagenesis. Early meteoric diagenesis was not identified in the Capitan forereef. Early marine diagenesis is limited to minor isopachous cement in upper forereef grainstones. The greatest amount of diagenesis in the Capitan forereef occurred during early burial diagenesis in marine to hypersaline pore fluids. The lower forereef facies was largely altered by marine pore fluids; fabrics include moldic porosity, fine blocky spar and overgrowth cementation, silicification, and rare aragonite neomorphism. The upper forereef facies, however, was nearly completely dolomitized by mesosaline fluids mainly derived by seepage reflux from the near-backreef carbonate lagoon facies. The middle forereef facies contains both styles of alteration. During and/or subsequent to dolomitization, hypersaline fluids completely cemented the reef and forereef with evaporites (gypsum and/or anhydrite). Deeper burial processes include stylolites and partial recrystal I ization of the early fabric-preserving dolomite to a fabric-destructive dolomite. During uplift, erosion of overlying Ochoan evaporites allowed the influx of meteoric water into the Capitan Formation. This led first to hydration of anhydrite to gypsum and kaolinitizarion of feldspars and later to complete dissolution of evaporites and precipitation of coarse blocky spars II and III in the resulting porosity.
Most diagenetic models focus on timing of alteration and the fluid composition. In the case of the Capitan forereef facies, however, the sedimentology was a major factor controlling the distribution of diagenetic fabrics. For example, debris-flow deposits (unsorted packstones to rudstones) are partially to completely dolomitized while interbedded turbidity-current deposits (graded packstones to wackestones) are limestone with fine blocky spar and aragonite alteration. The changing patterns of diagenesis from the upper forereef to the lower forereef are at least partially controlled by the change from a predominance of debris-flow deposits in the upper forereef to turbidity-current deposits in the lower forereef.
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The Capitan Formation of southeast New Mexico and west Texas contains one of the world's best exposed and most famous reefs. Depositional and diagenetic models derived from the Capitan have been used to interpret carbonate strata throughout the world. This volume contains 12 state-of the- art papers summarizing major new research on the Capitan, putting the Capitan into a modern statrigraphic, depositional, paleontologic, and diagenetic framework.