Pre-Late jurassic, fossil-bearing volcanic and sedimentary red beds of Huizachal Canyon, Tamaulipas, Mexico

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Huizachal Canyon, one of a series of generally east-west trending canyons in Tamauli- pas, northeastern Mexico, exposes a pre-Late Jurassic sequence of fossiliferous pyroclastic and epiclastic rocks, the Huizachal Group. Heretofore considered a sedimentary package associated with either metamorphic or intrusive rocks, structural relationships and petro- graphic studies presented here indicate that the Huizachal Group was in fact deposited unconformably upon an older, undescribed, sequence of pyroclastic rocks. Four igneous units are recognized in the steeply dipping older volcaniclastic sequence (respectively, units A-D): a complex suite of pyroclastic flows, accretionary lapilli tuff(s), and lava flows (unit A); a homogeneous, fine-grained felsitic rock (unit B); a sequence of conglomerates (unit C); and a mixed assemblage of rocks including mafic-to-intermediate composition lava flows and intercalated conglomerate and tuff (unit D). Most of these rocks have undergone extensive late-stage or postdepositional silicification, but relatively immobile trace elements demonstrate that these rocks range from subalkaline basalt to rhyolite. The younger, relatively flat-lying Huizachal Group overlies these rocks in angular unconformity. The fossil assemblage comes from a <10-m-thick sequence in the lower part of the Huizachal Group, which also is the result of pyroclastic volcanic deposition. Some of the organisms entrained within these tuff(s) were reworked by volcanic processes; others appear to have been actively trapped in a manner analogous to Pompeii. New U-Pb isotopic data from zircon in a volcaniclastic rock from the lowest part of the Huizachal Group (La Boca Formation) yields an age of 189 ± 0.2 Ma (analytical error). The sedimentary rocks immediately above this unit contain fossils considered to be Early Jurassic in age. Thus, the zircon isotopic age agrees with, and is supportive of, the age estimates based upon fossil vertebrates. Trace element geochemistry of the volcanic units is strongly suggestive of subalkalic ocean-continent Andean volcanism. Thus, the volcanic and sedimentary rocks of Huizachal Canyon were most likely deposited in a convergent plate margin setting instead of an exten- sional rift system as previously proposed. © 2005 Geological Society of America.

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Special Paper of the Geological Society of America