Paleoenvironmental evolution and Tethyan transgression during the Valanginian: Insights from the Tonalá and San Miguel Papalutla Formations, Tlaxiaco Basin, Mexico

The Tlaxiaco Basin, located in southern Mexico within the Mixteca Terrane, preserves one of the most complete records of the breakup of Pangea. While its Jurassic units have been extensively studied in the context of rifting, this study focuses on the overlying Lower Cretaceous sedimentary succession, corresponding to the newly defined Tonalá Formation and the San Miguel Papalutla Formation. Through facies and microfacies analysis, gamma-ray spectroscopy, detrital zircon geochronology, and ammonite biostratigraphy, we reconstruct the paleoenvironmental evolution and sea-level fluctuations in the Tlaxiaco Basin during the Early Cretaceous. These formations document a transition from a carbonate platform to deeper marine basin environments, allowing for the precise identification of facies shifts associated with marine transgression events. One such transgression, recorded within the Neocomites neocomiensiformis ammonite Zone, correlates with the global third-order sequence KVa2 and reflects a Tethyan affinity. Sedimentological and U–Th geochemical data suggest warm, humid to tropical climatic conditions in the Tlaxiaco Bay during the Valanginian. The development of both formations occurred during a phase of thermal subsidence, resulting in a transgressive depositional pattern influenced by marine currents from the Tethys Sea. This refined stratigraphic and paleoenvironmental framework supports the interpretation of the Tlaxiaco Basin as a passive-margin system, contributing to broader models of Early Cretaceous eustatic dynamics and continental margin evolution. • The Tonalá Formation is a new lithostratigraphic unit of Valanginian age, supported by U–Pb zircon geochronology (136 ± 2 Ma) • First detailed facies analysis of Lower Cretaceous units in the Tlaxiaco Basin (Mixteca Terrane, southern Mexico) • Refines the stratigraphic framework of the Tlaxiaco Basin, resolving long-standing ambiguities in previous interpretations • A major Valanginian transgression in Neocomites neocomiensiformis Zone via facies and ammonite biostratigraphy