RECONSTRUCTING THE DEPOSITIONAL ENVIRONMENTS OF INTERBEDDED GYPSUM AND CLASTIC SEDIMENTS OF THE UPPER MIOCENE BOLEO FORMATION, BAJA CALIFORNIA SUR, MÉXICO

More than 100 m of gypsum deposited in the Upper Miocene Boleo formation is currently being excavated at the Caopas mine north of the town of Santa Rosalía in BCS, México. The mine exposures provide a unique opportunity to study the gypsum and interbedded clastic layers and to determine the depositional environments of the evaporitic sequence. We measured a stratigraphic section of the upper 50 m of gypsum along dipping beds of a syncline in the mine and used a photomosaic from drone photography to map the units. The lower 50 m of section were described in the field and thicknesses estimated from a mine map. Samples were collected from each unit for petrographic, XRD, and SEM/EDS analyses. The lower section, which is devoid of clastic sedimentary interbeds, appears diagenetically converted to anhydrite with burial and gypsified with uplift. The top of the section is marked by a prominent, ferrigenous red gypsum with karst features that denotes a period of weathering. Above the red gypsum, six clastic sedimentary cycles with abundant Mn- and Fe-rich and minor Cu-rich mineralization were observed interfingering with the gypsum layers. Laminated gypsum consists of alternating layers of gypsum, with organic carbon, calcite, Mn- and Fe-enriched gypsum deposited from a stratified brine whose surface waters were seasonally freshened. The water depth shallows up section with the deposition of wavy laminated and lenticular bedded gypsum. The gypsum cycle becomes subaerial with altered alabastrine gypsum and gypsrudite. Clastic layers include interbedded red mud with parallel laminated and nodular gypsum deposited in a sabkha overlain by fluvial channels and overbank sediment. Gypsum slump blocks within the clastic layers and discordant soft-sediment deformation (seismites) in the gypsum layers indicate synsedimentary deformation. The sedimentary shallowing-upward cycles are likely caused by pulses of fault-driven basin subsidence. The presence of Mn and Cu mineralization in the upper 50 m of the section suggests that the evaporite deposition is coeval with the Lucifer and Boleo mineralization. Understanding mineralization-evaporite association within the Boleo deposit might aid to identify the main ore forming events.