EVIDENCE OF THE FIRST MARINE INCURSION IN THE SANTA ROSALIA BASIN, THROUGH THE DISTRIBUTION OF THE BASAL LIMESTONE IN THE UPPER MIOCENE BOLEO FORMATION

The Boleo Fm of Late Miocene age is a sedimentary deposit related to the opening of the Gulf of California consisting of basal units of conglomerate, limestone, gypsum, and fossiliferous sandstone overlain by clastic sequences hosting copper ore in the eastern-central part of the Baja California Peninsula. We collected samples for petrographic and mineralogical analyses of the Boleo Basal limestone from six stratigraphic sections in the area of Santa Agueda and El Morro to determine the environment of deposition. Seven units were defined, including: 1) conglomerate and breccia, 2) massive beds, 3) spongy-like texture with Mn vesicles, 4) thrombolitic (cottony texture) and stromatolite units, 5) tufa, 6) marine fossiliferous limestone, and 7) limestone with structures resembling gas-escape pipes. We mapped the distribution of this Basal Limestone facies to reconstruct the paleoenvironmental setting. On the topographic highs of the deposit, hydrothermal activity produced deposition of a massive to spongy like textures unit with Mn. The breccia unit may represent the location of hydrothermal vents. The limestone with vertical gas-escape structures and chalcedony are associated with these rocks. Laterally, in the topographic lows of the deposit, we find the thrombolites and stromatolites associated with shallow marine environment. Fossil gastropods, bivalves, and microfossils with lenticular distribution suggest a tidal channel system deposition in brackish to marine conditions. The marine fossiliferous limestone provides evidence of marine transgression into the basin. According to field relationships, the marine sequence was deposited horizontally into an area where hydrothermal springs produced limestones with some original dip and were subsequently deformed along local faults. This research shows the difference in the deposition of the studied outcrops within the limestone, where the tufa and massive to spongy like texture corresponds to a brackish-fresh water hydrothermal environment in a subtidal channel system. Furthermore, the unit with thrombolites, stromatolites and marine fossiliferous limestone suggest a seawater incursion into the basin related to the early rifting of the Gulf of California. This study is part of the NSF-funded Baja Basins REU project.