In subsurface mudrocks of the Oligocene Frio Formation in South Texas, a substantial proportion of silt-size quartz (10-62 mu m) manifests cathodoluminescence (CL) that varies greatly in intensity within each particle, giving rise to textures that differ greatly from grain to grain. In contrast, most quartz grains of sand size (>= 62 mu m) have CL that is relatively homogeneous within individual grains. A substantial percentage of the quartz within silt grains is very weakly luminescent. No depth trend in the occurrence of luminescent textures in detrital quartz is observed, suggesting that the component of dark, possibly low-temperature silt-size quartz is not related to chemical processes in the present burial setting. Paleogene rhyolites of West Texas are a partial model for the source rocks that supplied sediment to the Frio Formation. Phenocrystic quartz in these volcanics has uniform CL similar to that observed in the Frio sand fraction, whereas complex CL structure seen in the associated groundmass bears many similarities to the CL textures observed in Frio silts. This observation further supports the idea that CL structure of detrital quartz in the Frio is most likely an inherited feature. If the quartz with dark CL is of low-temperature origin, it would help to elucidate the relatively 18 O-enriched isotopic values reported for mudrock quartz in the Frio.