204 Paleozoic Tectonics of SW Laurentia Without Borders: Final Amalgamation of Pangea in the USA, Mexico, and South America

We invite contributions that explore the Paleozoic tectonic evolution of southwestern Laurentia during the final assembly of western Pangea. Research on Paleozoic tectonic belts in Gondwana, particularly from Mexico and South America, is especially welcome.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 217B

THE NUEVA INDEPENDENCIA FORMATION: THE FIRST RECORD OF UPPER TRIASSIC-LOWER JURASSIC MARINE FACIES AT SOUTHERN MEXICO

Paper number: 204-13
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

During the Late Triassic to Early Jurassic, the initial stages of Pangea's breakup triggered a widespread extensional tectonic regime. This process led to the separation of the pre-Mesozoic crystalline basement rocks of the paleo-Mexican continent, which were positioned along the western equatorial periphery of the supercontinent. The rifting-induced crustal thinning facilitated the development of multiple small, elongated basins throughout the Mexican territory, often delineated by fault-bounded basement uplifts. Within the La Concordia Jurassic basin, the established stratigraphy comprises the volcaniclastic deposits of the La Silla Formation and the fluvial-alluvial sediments of the Todos Santos Formation. This basin is structurally constrained to the southwest by the Motozintla Shear Zone, a recently recognized lithospheric-scale feature.

Based on field observations, petrographic analysis of sandstone and carbonate, and zircon U-Pb geochronology, we have documented the presence of a previously unrecorded Upper Triassic-Lower Jurassic deposit, informally named the Nueva Independencia Formation. Such unit, which underlies a continental succession, consists of transitional-marine facies, including conglomerates, limestones, and sandstones interbedded with organic-rich shales and siltstones. The carbonate deposits at the top of the unit include miliolids and articulated ostracod valves in a micritic matrix, which indicates low-energy conditions and a very shallow transitional-marine setting during deposition, most likely a lagoon environment. By analyzing its stratigraphic position and the ages derived from seven samples, we have constrained the formation's depositional age to the Rhaetian (Late Triassic) to Pliensbachian (Early Jurassic). Detrital zircon spectra from the Nueva Independencia Formation reveal seven primary age groups (>5% of ages), indicating multiple sediment sources: the Permo-Triassic Arc (~292-230 Ma), the Pan-African orogeny (~760–550 Ma), the Grenvillian province (~1250–950 Ma), the Amazon Craton (~3200–1800 Ma), the Ordo-Carboniferous Arc (~500–320 Ma), Jurassic magmatism (~200-178 Ma), and the Río Negro-Juruena Rodonian-San Ignácio provinces (~1800-1300 Ma). However, the zircons linked to South American sources are considered to be recycled from Paleozoic units exposed in the study area. The contrasting age data clusters suggest that the Nueva Independencia Formation was coeval with the earliest magmatism documented in the basin (La Silla Formation), indicating a more complex Late Triassic–Early Jurassic depositional system than previously understood. This provides crucial new insights into the separation of Gondwana and the pre-opening stage of the Gulf of Mexico.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412036

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