DETAILED CHRONOSTRATIGRAPHIC UNDERSTANDING OF SYN-RIFT LITHOFACIES IN THE TODOS SANTOS BASIN, MEXICO

The break-up of equatorial western Pangea was driven by the interplay of Atlantic rifting and subduction-related tectonics along the proto-Pacific margin, starting in the Triassic and leading to the formation of extensional basins, including the Gulf of Mexico (GOM), which had a pivotal role in the stratigraphic record of both the southern US and eastern and southern Mexico. These Mexican basins emerged during the early Mesozoic and predate and overlap the timing of GOM opening, hence the detailed chronostratigraphic record of these basins is critical for understanding the timing of crustal extension, stratigraphic succession, and the underlying tectonic drivers during this period.

In southern Mexico, the Todos Santos Group (TSG) provides valuable insights into the evolution of Jurassic syn-rift deposition, recording the initial break-up of westernmost Pangea and the later GOM opening. While the lithostratigraphic division of the TSG served as a framework for the Todos Santos Basin (TSB), detailed Detrital Zircon (DZ) U-Pb data show that different lithostratigraphic units exhibit similar deposition ages, highlighting the importance of defining a chronostratigraphic framework to elucidate the syn-rift facies evolution.

Systematic new DZ U-Pb data, focusing on the youngest zircon ages, enabled us to determine robust Maximal Depositional Ages (MDAs) for different lithofacies distributed throughout the TSB and to define a new chronostratigraphic framework. These constraints are based on provenance DZ data, augmented by 30 to 40 euhedral zircons, significantly enhancing the robustness of the MDAs and the resolution of the chronostratigraphic framework of the sedimentary successions. The new MDAs reveal that the chronostratigraphy of the TSG is linked with three distinct rifting episodes that occurred during the Sinemurian, Toarcian, and Callovian. Two of these episodes predate the GOM opening and can be attributed to paleo-Pacific back-arc extension and the opening of the Central Atlantic. Moreover, the detailed DZ MDAs show that different lithofacies can be coeval across the TSB, and should not be used to define a stratigraphic succession. In summary, these data show both the power of the DZ MDA estimates to establish a chronostratigraphic syn-rift succession and clearly show that most of the TSB stratigraphy predates GOM opening.