Capital Reef Complex (Permian), Guadalupe Mountains, Southwestern United States: A Classic Sedimentologic Model in Flux: ABSTRACT

The Capitan reef complex of west Texas and New Mexico has been an important sedimentologic model since a reef origin was proposed for the Capitan Limestone in 1929. The Capitan's magnificent exposures in the Guadalupe Mountain area; its large scale; its variety of carbonate, sandstone, and evaporite facies; and its relationship to major petroleum resources of the Permian basin have made it a justly famous sedimentary geologic model for academic and industrial geologists alike. Since 1950, extensive research has yielded markedly contrasting sedimentologic interpretations of key features, such as the nature and origin of the Capitan massive ("reef wall"); the back-reef pisolite, sandstone, and evaporite facies; the depositional profile of the shelf and shelf edge; the impor ance and magnitude of sea level fluctuations; and the role of submarine, vadose, and phreatic diagenesis. Early views of a barrier reef depositional profile have been replaced by a shelftop marginal mound profile, in which the mound's gentle crest coincided with the backreef pisolite and tepee facies. The Capitan massive, earlier considered an ecologic barrier reef, is now interpreted as an outstanding example of massive limestone formed at a submerged shelf edge where extensive submarine cementation lithified sponge wackestones and formed massive cement boundstones. Permian vadose diagenesis, earlier accorded much importance and inferred as being related to major sea level falls, appears negligible. Phreatic diagenesis by mixed meteoric and marine fluids was at least locally important in the Capitan massive and foreslope strata. The famous Guadalupe pisolite, interpreted until the mid-19 0s as lagoonal, and then widely accepted as Permian caliche of vadose origin, is now reinterpreted as largely synsedimentary, formed by subaqueous precipitation from hypersaline waters of a peritidal shelf crest; associated vadose fabrics are minor, overprint isopachous pisolite fabrics and are unrelated to major intraformational erosion surfaces. Subaerial erosion surfaces within the reef complex are largely localized high on the shelf marginal mound. Unequivocal evidence of emergence of the Capitan massive or its underlying foreslope has not been recognized, suggesting that any sea level lowering during Capitan deposition did not exceed a few tens of meters. Although a sabkha origin of the back-reef evaporites and eolian transportation of sand across a sabkha surface has conceptual appeal, a lagoonal origin of the evaporites and subaqueous deposition of the back-reef sandstone sheets better fits the available field evidence. Vast amounts of siliciclastic sand had to bypass the Capitan massive and upper foreslope facies, but neither channels nor scoured erosion surfaces have been identified. The Capitan reef complex can serve not only as a "world class" sedimentologic model, but also as a much studied model not yet well understood. The model is one whose scientific investigators have frequently been afflicted by over-reliance on Holocene models ("modern model mesmerism"), by overthrust of established authorities or dogma ("dogma reverence"), by uncritical acceptance of new concepts ("bandwagonitis"), by overuse of superficial "look-alike" features for End_Page 1441------------------------------ genetic analogies, and by just not discerning what was available for viewing in the field or laboratory. Capitan remains a model "in flux" awaiting more critical field and laboratory research. End_of_Article - Last_Page 1442------------