EL ALAMAR FORMATION: THE ONLY TRIASSIC SUCCESSION OF THE HUIZACHAL GROUP IN NORTHEASTERN MEXICO

Redbeds outcropping in the Sierra Madre Oriental in Nuevo Leon and Tamaulipas are defined as the Huizachal Group, consisting of the upper Triassic-lower Jurassic Huizachal or La Boca Formation, and the unconformably overlaying middle to upper Jurassic La Joya Formation. According to field and analytical evidence including petrography, geochemistry and detrital zircon provenance in addition to the scarce reported flora, it is clear the existence of a Late Triassic fluvial sequence, informally named by the present research as El Alamar formation. This is an older fluvial succession outcropping in El Alamar Canyon and the Galeana region in Nuevo León, as well as in the northern part of the Huizachal Peregrina anticlinorium in Tamaulipas, and is different from the unconformably overlying La Boca Formation which includes coeval volcanic products that yielded Early Jurassic U-Pb zircon ages. The name El Alamar formation is derived from El Alamar Canyon in the Sierra de Pablillo, Nuevo León, where a section of more than 350 m of cyclic conglomeratic sandstones to mudstones is exposed. El Alamar formation overlies unconformably Paleozoic metamorphic, sedimentary and magmatic rocks; south of Galeana Nuevo León, it is well exposed along the Federal highway 58 (Km 76.5). This rock unit is absent because of no deposition in the Miquihuana-Aramberri area in southern Nuevo León and Tamaulipas, where Paleozoic schists are well exposed, directly underlying Jurassic and Cretaceous marine beds. The criteria used to separate El Alamar and La Boca formations are the absence of volcanic rocks in the Triassic sequence (El Alamar formation) and the common occurrence of such materials in the overlaying early Jurassic La Boca Formation. In addition a general gray-green and yellow to dark brown color is distinctive of El Alamar formation. This is in contrast to a consistent red-purple and red brown color of the overlying Jurassic strata. The presence of abundant petrified wood is also characteristic of the Triassic succession. The biostratigraphic age is determined on the basis of scarce flora as typical for the Late Triassic, more comparable to Carnian-Norian floras known from North America. An upper Triassic age of deposition is also in agreement with early Triassic maximal deposition ages indicated by the detrital zircon geochronology.