Sedimentology and Stratigraphy of the Upper Miocene El Boleo Formation, Santa Rosalía, Baja California, Mexico

The transtensional Upper Miocene Santa Rosalía basin, located in the east-central part of the Baja California Peninsula, consists of almost 500 m of non-marine to marine sedimentary deposits, and interbedded tuffaceous beds. The Santa Rosalía basin is a NW-SE elongated fault-bounded depocenter that records the sedimentation from Upper Miocene to Pleistocene time. The sequence is divided in El Boleo, La Gloria, Infierno and Santa Rosalía Formations. The lower most stratigraphic unit is the El Boleo Formation, a 200 to 300 m thick section composed in its lower part by a 1 to 5 m thick basal limestone and gypsum bodies followed by 170 to 300 m of clastic coarsening upward fan-delta, marine and nonmarine deposits. The upper clastic part of the El Boleo Formation show intraformational unconformities, synsedimentary folds and faults, and unidirectional sedimentary structures. These occur in at least three well organized upward coarsening cycles (90-100 m thick). Each cycle represents a prograding fan-delta deposit formed probably as consequence of large and repeated vertical movements of the basin floor with respect to the source areas. This activity is related to the early stage of the opening of the Gulf of California. Each cycle started with the deposition of a unit composed by laminar fine-grained sediments accumulated in an extensive area covered by shallow standing fresh water with periodic introduction of subaqueous debris flows. Each fine unit hosts Cu-Co-Zn ore bodies in the Santa Rosalía mining district. Lateral and vertical facies changes are present in each depositional cycle, involving proximal coarse sandstone and conglomerates through fine sandstone characterized by planar and low angle cross bedding, alternating with siltstone and mudstone with ripple lamination. Early, during the formation of the Santa Rosalía basin, two ancient depocenters located north-northwest and south-southeast of the basin were developed. These depocenters were filled by sediments during the first cycle, and were separated by a ridge formed by the volcanic rocks of the Comondú Formation.