In northern Webb County, Texas, the Upper Cretaceous San Miguel Formation of the Maverick basin contains sandstones that range in composition from feldspathic litharenites to lithic arkoses. Data include petrographic analysis of sidewall cores from two wells and cuttings from two additional wells. Volcanic rock fragments represent the predominant lithic constituents and consist of trachytic keratophyres. Plagioclase phenocrysts occur in a trachytic groundmass of albite-oligoclase laths and microlites.

Known Late Cretaceous volcanics, which have been reported from eruption centers to the north and northwest, consist of labradorite-olivine-basalt, nephelite-melilite "basalts," phonolite, nephelinite, and serpentinized basalts. This assemblage suggests that the keratophyric grains were derived from a different volcanic province. The most likely potential source areas for these keratophyric volcanic centers existed to the west or southwest, probably in Mexico. No Late Cretaceous keratophyric volcanics have been reported within this area. Perhaps such eruption centers did exist in Mexico and subsequently were buried or obscured by Cenozoic volcanic centers.

Sandstones from the superjacent Olmos Formation, in the same area, contain only 5 to 10% keratophyric rock fragments. This small percentage suggests that the keratophyric volcanic center had become dormant or extinct by the end of San Miguel "time," and the eroded volcanic centers were shedding much less debris during Olmos "time."

Major porosity-occluding cements in the San Miguel are chlorite, kaolinite, and ferroan calcite. Much secondary porosity was created by partial to complete solution of plagioclase and volcanic rock grains. In many intervals large primary and secondary voids are lined by chlorite and filled by kaolinite.

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