Paper No. 210-13
Presentation Time: 9:00 AM-6:30 PM
STRATIGRAPHY, PROVENANCE, AND DEPOSITION OF THE UPPER MIOCENE BOLEO FORMATION, SANTA ROSALÍA BASIN, BAJA CALIFORNIA SUR, MEXICO
Through the NSF-funded Baja Basins REU, we studied the Santa Rosalía basin of Baja California Sur, where new exposures created by mineral exploration provide unique opportunities to study syn-rift sedimentation and the early evolution of the Gulf of California. We present results of a detailed study of the Upper Miocene Boleo Formation between two copper-bearing ore beds (Mantos 3 and 2). Six sections were measured in the western basin margin. Samples were collected for petrographic, SEM, XRD, grain size, and facies analyses. We present a new framework for the middle Boleo Formation; petrographic analyses show a variety of compositions, including vitric tuff, crystal-lithic tuff, and volcaniclastic litharenite grading upward to volcaniclastic rudite. Examination of provenance and possible systematic variation with time is currently underway. From these data, the following eight lithofacies were defined: (A) yellow tuff; (B) thick-bedded conglomerate; (C) pebbly, massive sand; (B) red tuff used as a marker bed between Manto 3 and 2; (E) thin-bedded, pebble-filled channels interbedded with sand; (F) thick-bedded channels with deep erosional bases interbedded with sand; (G) small, cross-bedded sand channels interbedded with clay and silt; (H) interbedded sandstone and siltstone with a white-blue tuff layer near its base. Facies H contains a thin, bioturbated mudstone with organic matter. Facies A and H provide two new marker beds - fine-grained tuffs from which future U-Pb dating may provide new age constraints for the basin. XRD and modal analyses of the sediment suggest that the middle Boleo Formation is derived from local volcanics. Textural analysis and volcanic glass and pumice fragments in several sandstones indicates that the middle Boleo Formation was geographically near an active volcanic source. Petrographic and facies analyses suggest two models for the deposition of the Boleo Formation between Mantos 3 and 2; a fan-delta complex aggrading eastward and then abandoned by the deposition of Manto 2, or a braid delta with low-relief alluvial fans on the western basin margin. This framework and the identification of new marker beds will aid mining exploration in and between Mantos 3 and 2, while our depositional model could provide context for future studies of tectonics and sedimentation in syn-rift basins.