EARLY TO MIDDLE JURASSIC JURASSIC RIFTING IN SOUTHEAST MEXICO AND IMPLICATIONS FOR GULF OF MEXICO OPENING

The Gulf of Mexico (GOM) is a major basin in the tectonic evolution of North America, whose evolution was recorded in the offshore and onshore stratigraphic record of southern USA and east and south Mexico. Early Mesozoic basins in eastern Mexico have been linked to break-up of Pangea and GOM opening. However, the lack of constraints on timing and architectural evolution of early syn-rift basins in southeast Mexico limits our understanding of early GOM evolution. The early Mesozoic siliciclastic Todos Santos Group (TSG) records the evolution of pre-Oxfordian syn-rift deposition, linked to western Pangea break-up and subsequent GOM opening. New stratigraphic, petrographic, and detrital zircon (DZ) U-Pb provenance data elucidate the TSG stratigraphy and link them to other early Mesozoic basinal records.

The Concordia Basin basement is formed by the late Paleozoic Chiapas Massif Metamorphic Complex (CMMC), Paso Hondo, Grupera, and Santa Rosa Formations. Early Jurassic volcanics and siltstone of La Silla Fm, overlay the basement and are overlain by syn-rift siltstone and sandstone of El Diamante member and coarse to medium sandstone of the Jericó member. Lithological differences and local angular unconformities link them to syn-tectonic deposition after La Silla Fm and prior to deposition of siltstone and anhydrite of the Oxfordian San Ricardo Fm.

Zircon U-Pb data for La Silla volcanism yields Sinemurian to Pliensbachian ages. Maximal depositional ages (MDAs) of 176-179 Ma for El Diamante member and 193-187 Ma for the Jerico member suggest deposition of TSG rocks as early as the Toarcian. Toarcian MDAs in strata below the Pliensbachian-Sinemurian Jerico member suggest a progressive unroofing sequence and erosion of La Silla Fm. Proterozoic and Paleozoic DZ age signatures for El Diamante member switch to predominantly Permo-Triassic DZ ages in the Jerico member, indicating a change in sediment sources during unroofing from the CMMC to Chiapas Batholith and Jurassic Nazas arc.

La Silla volcanism in the Concordia Basin matches the volcanic record and MDA’s of syn-rift sequences in east Mexico implying coeval magmatism linked to Early Jurassic rifting and back-arc extension. El Diamante Toarcian MDA suggests that such rocks were linked to late back-arc extension along the western margin of Pangea prior to opening of the GOM.