The Nueva Independencia Formation: the first record of lower Jurassic transitional to shallow-marine facies in southern Mexico

During the Late Triassic to Early Jurassic, the initial break-up of Pangea led to a widespread extensional tectonic regime. This extension resulted in the formation of numerous small, fault-bounded basins across Mexico. Within the Concordia basin, located in Chiapas in southern Mexico, the Jurassic succession was hitherto thought to comprise only volcaniclastic and continental sedimentary rocks. Based on detailed field observations, petrographic analysis of sandstone and carbonate rocks, and zircon U-Pb geochronology, the present work describes and formally proposes a newly discovered sedimentary unit, named the Nueva Independencia Formation. This unit is composed of conglomerate, limestone, and sandstone interbedded with organic-rich shale and siltstone rocks, representing a transitional to shallow marine environment. Its stratigraphic position and U-Pb geochronological data constrain the formation’s depositional age to the Sinemurian to Pliensbachian (Early Jurassic). Detrital zircon spectra reveal seven primary age groups from multiple sources. The contrasting age data clusters suggest that the Nueva Independencia Formation was coeval with the earliest magmatism documented in the basin, indicating previously unrecognized complexity of the Concordia basin depositional system during Early Jurassic time. This provides significant insights into the separation of the Yucatán Block from Laurentia prior to the opening of the Gulf of Mexico.