THE HUIZACHAL GROUP IN NORTHEASTERN MEXICO: A BACK-ARC SUCCESSION RELATED TO EVOLUTION OF THE EARLY JURASSIC NAZAS ARC

The Huizachal Group in northeastern Mexico is composed of the Lower Jurassic La Boca Formation, and the Middle Jurassic La Joya Formation. An Upper Triassic unit recently defined as El Alamar Formation and considered by the first definition of the Huizachal Group as the basal part of the La Boca Formation is not considered in our model as part of the Group. El Alamar Formation represents a Late Triassic fluvial system, flowing in to the paleo-pacific ocean and feed the Potosí submarine fan. La Boca Formation represent continental sedimentation during the Early Jurassic including interlayered volcanic rocks exposed in the Huizachal Valley in Tamaulipas. To the west the Nazas Formation, consist of a volcanic succession interpreted as part of the Early Mesozoic arc of western North America. La Joya Formation postdates Early to Middle Jurassic volcanism in the region and represent the basal sequence deposited during the marine transgression after aperture of the Gulf of Mexico basin. While for decades the red beds of the Huizachal Group have been interpreted as extensional rift deposits associated with the opening of the Gulf of Mexico, the position and orientation of the basins, as well as the presence of volcanic rocks of the Nazas arc, show his relationship as a back-arc. Red beds of the Huizachal Group were deposited in two depocenters product of extension: 1) The Huizachal basin in Tamaulipas and 2) The Catorce basin in San Luis Potosí-Zacatecas. The Huizachal basin was filled by a more than 1500 m thick Early Jurassic typical back-arc sedimentary and volcanogenic succession of La Boca Formation, unconformable covered by a relative thin fining-upward sequence of alluvial-lagunar deposits of La Joya Formation. The Catorce Basin was filled, by a Middle Jurassic alluvial and accord to sedimentological features and palinological content shallow marine succession, up to 700 m thick (La Joya Formation) which rest unconformable on volcanic rocks of the Nazas arc. Such differences in time of extension-sedimentation in both recognized depocenters are interpreted as a possible westward migration of the arc related probably to rollback of the east dipping subducted plate and consequent westward migration of the back-arc. A tectonic evolution also compatible with the scarce U-Pb ages of the volcanic rocks, which become younger to the west.