STRATIGRAPHY, SEDIMENTOLOGY, AGE, AND TECTONIC SIGNIFICANCE OF THE UPPER MIOCENE BOLEO FORMATION IN THE SANTA ROSALIA BASIN, BAJA CALIFORNIA SUR, MEXICO

The Santa Rosalia area in Baja California Sur contains a rich record of late Cenozoic volcanism, faulting, and sedimentation at the SW margin of the Gulf of California transform-rift. Important advances in our understanding of these processes have been made through many studies by Cathy Busby and her colleagues and students. The Boleo Formation is the oldest sedimentary unit in the Santa Rosalia basin, and unconformably overlies 9.7 – 8.8 Ma rift-transition volcanic rocks (Busby et al., 2020). The base of the Boleo Fm consists of discontinuous conglomerate and breccia overlain by a thin basal limestone that contains abundant marine fossils. The limestone is overlain by a thick siliciclastic sequence (conglomerate, sandstone, mudstone, and thin shale units rich in Cu and Mn ore (mantos)), locally with marine calcareous nannofossils, and stratigraphically equivalent gypsum. The thickness of the Boleo Fm ranges from near zero where it pinches out over structural highs beneath the Tirabuzón Fm, to 250-300 m in deeper parts of the basin. Growth structures record syn-depositional tilting to the NE on NW-striking, SW-dipping normal and oblique dextral-normal faults. Lithofacies change from thick proximal conglomerate in the SE through intermediate sandstone and siltstone, to distal gypsum in the NW. In Arroyo Infierno we observe a lateral change from coarse alluvial conglomerate in the SW to interbedded conglomerate and gypsum to nearly pure gypsum in the NE. Stratal and structural relationships indicate a complex 3D basin in which subsidence occurred by growth of a large monocline above the tip of a propagating NE-dipping normal fault that tilted the crust NE toward the paleo-Gulf of California. The base of the Boleo Fm was previously estimated to be ca. 7 Ma based on paleomagnetic data and an ⁴⁰Ar-³⁹Ar age of 6.76 ± 0.90 Ma on volcanic tuff (Holt et al., 2000). New zircon U-Pb data (this study) yield a revised age of 5.86 ± 0.12 Ma for the same tuff. Applying other detrital-zircon and paleomagnetic age constraints, and assuming an accumulation rate of ~1 mm/yr (typical for tectonically active basins), we conclude that the Boleo Fm accumulated in roughly 200-300 kyr sometime between 6.2 and 5.8 Ma. These results should be included in future models for the timing and processes of initial marine incursion into the paleo-Gulf of California.