Field studies in the Huizachal anticlinorium, Sierra Madre Oriental, near Ciudad Victoria, Tamaulipas, Mexico, disclose that the Huizachal group (redbeds), formerly undivided, contains at least two mappable sequences, herein named La Joya formation and La Boca formation.
The La Boca formation consists mostly of red and green claystones, mudstones, siltstones, sandstones, and conglomerates and rests unconformably on (1) crystalline rocks of unknown age and (2) deformed Paleozoic strata lithically similar to those of the Marathon uplift (Texas). It is considerably folded and faulted and underlies either the La Joya formation (redbeds) or the overlying Zuloaga limestone (Late Jurassic) with angularity up to almost 90°.
The La Joya includes (1) a lithically variable basal conglomerate, (2) a thin limestone and limestone conglomerate, (3) red mudstones, claystones, siltstones, and fine quartzose sandstones which grade upward into red, pink, or green, fine to very coarse, cross-bedded, quartzose sandstones and red, pink, or green conglomerates. The La Joya possesses conformable to low angular unconformable relations with the superjacent Zuloaga limestone; it rests with considerable angularity on the La Boca formation and on deformed sediments of late Paleozoic age. At some places it overlies crystalline rocks probably Paleozoic and Precambrian in age.
Fossil plants from the upper part of the La Boca formation in Novillo Canyon indicate that this part of the group is probably late Triassic in age. The La Joya thus appears to be Jurassic in age, its general relations to the Zuloaga limestone (of definite Late Jurassic age) being such as to suggest that it might be Middle or Late Jurassic in age.