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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. The fanglomerate was cemented in the vadose zone. Predominantly micritic particles are coated by bilaminar films, consisting of inner calcimicrite laminae and outer rims 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 over rowth 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 rims are homologous with overgrowth halos; (2) overgrowth rims on monocrystalline grains are 3-10 times thicker than outer rims of dogtooth spar on similar-sized, bilaminar film-coated particles; (3) where overgrowth halos expand outward and engulf neighboring grains, outer drusy rims are missing or poorly developed.
Bilaminar films, especially outer drusy rims, thicken downward from lower grain boundaries into interstices in response to a gravitational effect (downward thickening of water films).
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