FACIES ANALYSIS OF THE GYPSUM UNITS IN THE UPPER MIOCENE BOLEO FORMATION IN THE SANTA ROSALÍA BASIN, BAJA CALIFORNIA SUR, MÉXICO

The opening of the Gulf of California is marked by deposition of the Upper Miocene Boleo Formation in the Santa Rosalía basin located along the central east coast of the Baja California Sur peninsula. The Boleo Fm overlies volcanic rocks, contains basal carbonates, gypsum, and conglomerate, and an upper sequence of Cu-bearing clastic sediments (Wilson & Rocha, 1955). Recent studies on the mineralogy of the gypsum showed evidence of strontianite, tenorite, Fe-, Mn- and Cu-Co oxides, and other minerals along the bedding planes of the gypsum units (Maher et al., 2016). These data suggest an influx of metal-bearing fluids into the evaporitic environment with periodic reducing conditions and that the gypsum deposition is coeval with the ore formation in the upper clastic Boleo Fm and not stratigraphically below in a “basal” positon. To determine the depositional setting of the gypsum within the Boleo Fm and relationship to mineralization, we studied the gypsum outcrops in Arroyo Boleo and at the Caopas gypsum mine north of Santa Rosalía. Drone technology was used to map the exposures and create geographically-referenced orthophotomosaics of the outcrops using Agisoft Photoscan software. We mapped the beds in the orthophoto images and measured the stratigraphic section in Arroyo Boleo. Samples from both locations were analyzed using petrographic, XRD, XRF, and SEM methods. From field observations, we defined three subaerial facies (clastic, laminated mud and gypsum with nodules, and gypsrudite and pedogenic gypsum) and three subaqueous facies (wavy laminated with bands of small selinite crystals, parallel laminated, and massive gypsum beds). These facies provide evidence of four major cycles from marine to freshwater environments in a restricted basin with water level and salinity fluxuations. Together with previous evidence of Cu-mineralization, our study suggests that clastic interbeds in the gypsum may correlate to periods of basin shallowing followed by basin subsidence and possible hydrothermal activity and mineralization.