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A rhyolitic volcanic ash bed about 0.3 m thick is exposed in a roadcut along Texas Highway 193 near Mount Blanco in the upper part of a sequence of Pleistocene sedimentary deposits at the type locality of the Blanco Formation, about 59 km northeast of Lubbock, Texas. This ash, here named informally the Guaje ash bed, has chemical and petrographic characteristics closely resembling those of the rhyolitic air-fall tephra (Guaje Pumice Bed) that directly underlies ash flows of Pleistocene age in the Jemez Mountains of northern New Mexico. The Guaje Pumice Bed and the ash flows belong to the Otowi Member of the Bandelier Tuff. Properties common to the Guaje ash bed and the Guaje Pumice Bed include: refractive index of glass, 1.497–1.498; microphenocrysts of quartz, sanidine (Or42–44), ferrohedenbergite (Fe51Ca42Mg7), chevkinite, allanite, zircon, and magnetite. Chemical composition of the glass of the Guaje ash bed matches that of the Guaje Pumice Bed for all major elements except K and Na and for trace elements determined by standard chemical analyses, atomic absorption, and neutron activation. Paleomagnetic measurements indicate that the ash has reverse depositional remanent magnetization. Glass shards of the ash have a fission-track age of about 1.4 ± 0.2 m. y. Sanidine from the Guaje Pumice Bed and its genetically related ash-flow sheet in the Jemez Mountains was K-Ar dated at about 1.4 m. y. by R. R. Doell and his colleagues in 1968. Correlation of the Guaje ash bed with the radiometrically dated Guaje Pumice Bed establishes a minimum age of about 1.4 m. y. for the Blanco Formation.

Volcanic ash deposits whose source is the Cascade Mountains area were correlated on the basis of 19 elemental abundances obtained by instrumental neutron activation analysis (INAA). After activation of glassy separates in a TRIGA reactor, gammaray spectra were obtained and analyzed with computer programs. The elements Na, Sm, Sc, Fe, Ce, Hf, and Th were determined with relative standard deviations less than 5%; the precision for La, Co, Eu, Yb, Cs, Ba, and Lu was less than 17%; larger errors were obtained for Rb, Ta, Nd, Tb, and Cr. A statistical method was developed for correlation on the basis of relative elemental compositions unique to the ash deposits. Elemental abundances of Mazama glassy separates were independent of distance from the source. The site to site chemical variability of crystal rich Glacier Peak and St. Helens ash layers was greater than for Mazama and Newberry ashes. The Rb, Yb, Lu, Th, and Ta contents in Newberry glass were more than twice those in Mazama glass. The concentrations of trace elements in Glacier Peak and St. Helens ashes generally were less than one-half those in Mazama glass. The presence of Mazama ash has been confirmed at sites in Oregon, Washington, Alberta, and in sediments of the Pacific Ocean.