Abstract

Pleistocene fanglomerates, extending southeastward from the Guadalupe Mountains, consist of predominantly micritic carbonate particles (limestone and dolostone) eroded from the Permian reef complex. Accessory particles consist of detrital monocrystalline grains of calcispar, quartz and chert. The fanglomerate is firmly cemented by low-magnesium calcite which was precipitated in the vadose zone. Predominantly micritic particles are coated by bilaminar films, consisting of an inner calcimicrite lamina and an outer rim of drusy, dogtooth spar. Bilaminar films do not completely occlude porosity in large neighboring interstices. Monocrystalline carbonate grains are concentrated in the sand-size fraction and are enveloped by thin, discontinuous micrite films and thick overgrowth aureoles which expand outward to occlude porosity in adjacent interstices. The following lines of evidence suggest that overgrowth aureoles on monocrystalline grains were precipitated at much greater rates than outer drusy rims of bilaminar film-coated grains: (1) both monocrystalline grains and bilaminar film-coated grains are enveloped by thin, inner micrite films, suggesting that outer drusy films are homologous with overgrowth aureoles; (2) overgrowth halos on monocrystalline grains are 3-10 times thicker than outer drusy rims of dogtooth spar on similar-sized bilaminar film-coated particles; (3) where overgrowth aureoles expand outward and engulf neighboring grains, growth of outer drusy rims is poorly developed or commonly precluded. Bilaminar films, especially outer drusy rims, commonly exhibit gravitational cementation, recording precipitation in pendulous vadose water films.

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