The Triassic(?) "Nazas Formation" of north-central Mexico is a sequence of volcanic flows, pyroclastic rocks, and volcaniclastic sediments. Outcrops occur in the Sierra del Rosario del Teyra, Guadalupe, and San Julian of northern Zacatecas. The Nazas is overlain unconformably by the Zuloaga Formation of Oxfordian age (Late Jurassic). A radiometric date of 230 ± 20 m.y. has been obtained for the Nazas volcanic rocks in the Villa Juarez uplift. Thus the Nazas Formation is tentatively correlated with the Huizachal Group of northeastern Mexico, the Eagle Mills Formation of the U.S. Gulf coastal province, and the Newark Supergroup of the Appalachian and Atlantic coastal provinces.

In the Sierra de San Julian, the sedimentary units of the Nazas include: (1) non-stratified, clayey matrix-supported pebble conglomerates; (2) planar-bedded and low-angle planar cross-stratified sandstones with interbedded siltstones; and (3) varved siltstones. A sequence of graded, sandy matrix-supported cobble-boulder conglomerates forms the uppermost unit in the Nazas.

Petrographic studies in progress suggest the following: (1) silicic-intermediate ashflow and airfall tuffs are volumetrically predominant; (2) sediments were derived mainly from the penecontemporaneous volcanic rocks, with a minor metamorphic source indicated by the presence of sand-size, finely polycrystalline quartz grains; and (3) the entire formation has had a complex diagenetic history. Hematite, calcite, and sericite are abundant alteration products.

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