Provenance and maximum depositional ages of Upper Triassic and Jurassic sandstone, north-eastern Mexico

Abstract

Upper Triassic to lowest Upper Jurassic strata in north-eastern Mexico record surface processes during the early rifting that led to the opening of the Gulf of Mexico. Exposed near Ciudad Victoria, Tamaulipas and northwest to Galeana, Nuevo Leon, these continental deposits are called the Huizachal Group. Several key questions about these strata hamper their integration into a regional understanding of Late Triassic and Jurassic sediment routing and deposition. First, the depositional age of the stratigraphically lowest unit, the El Alamar Formation, is less well established than the depositional ages of the stratigraphically higher parts of the succession, leading to questions about the timinig of the onset of tectonic context, and regional correlation of these strata. Second, better knowledge of provenance and sediment routing, including fluvial transport distance, can help determine whether and when the depositional basin was split into subbasins versus connected by a throughgoing river system. This study uses sandstone detrital zircon U-Pb isotopic ages and clast compositions to constrain depositional ages and reconstruct tectonic setting and provenance. Detrital zircon U-Pb ages yielded a maximum depositional age of 210 Ma for the part of the El Alamar Formation exposed near Galeana, younger than previously determined. Although long-distance (>100 km) sediment transport cannot be ruled out, the sources for Huizachal Group sandstone framework grains and detrital zircon could have been entirely local, within about 100 km. Alluvial deposits and pyroclastic and lava flows likewise indicate transport of no more than 100 km. We therefore infer deposition in partitioned rift basins receiving sediment from local sources. These data and interpretations contribute to understanding surface processes during the initial rifting that eventually led to opening of the Gulf of Mexico.