Late Triassic stratigraphy and facies from northeastern Mexico: Tectonic setting and provenance

摘要

Triassic strata in northeastern Mexico distributed over an area of 120,000 km2 have received relatively little attention in paleogeographic and tectonic reconstructions of western equatorial Pangea. Triassic marine sequences of the Zacatecas Formation in western San Luis Potosí and Zacatecas constitute facies corresponding to different sections of a submarine fan system previously described as the Potosí fan and were deposited on the paleo-Pacific margin of Pangea. New geochronologic data and field observations permit revision of the stratigraphy from Late Triassic to Early Jurassic fluvial deposits of the Huizachal Group that crop out in Nuevo León and Tamaulipas and indicate a link between the lower part of this succession (Triassic strata) and the Potosí fan. This work proposes and defines the El Alamar formation, which represents the only Triassic strata of the Huizachal Group (lower part of the Late Triassic–Early Jurassic La Boca Formation), and is interpreted as a continental succession that records a major fluvial system draining equatorial Pangea and flowing west into the Potosí fan. Petrographic and geochemical studies indicate that both Triassic successions, the Zacatecas Formation (marine) and El Alamar formation (continental) have continental block and recycled orogenic provenances. U-Pb detrital zircon geochronology by laser ablation–multicollector–inductively coupled plasma–mass spectrometry show three main zircon age populations: (1) Grenvillian (1300–900 Ma); (2) Pan-African (700–500 Ma); and (3) dominant Permian–Triassic (280–240 Ma). The presence of these zircons displays evidence of sources in Grenvillian Oaxaquia block, Pan-African terrains such as Yucatan and southeastern Texas, and a prominent contribution from the Permian–Triassic east Mexico magmatic arc. Notable is the absence of detrital contributions from the southwestern North American craton. The geochronological data thus argue against proposed southeastward displacement of Triassic successions and their basement to their current position as proposed by the Mojave-Sonora megashear hypothesis. We propose that continental Triassic strata were deposited in eastern Mexico before the opening of the Gulf of Mexico basin, and are thus autochthonous and transported detritus toward the ancient Pacific margin into the Potosí fan, which is also autochthonous.