DISTRIBUTION AND DEPOSITIONAL MODEL OF THE CINTA COLORADA MARKER BED OF THE COPPER-RICH BOLEO FORMATION, SANTA ROSALíA BASIN, BAJA CALIFORNIA SUR, MéXICO

The Boleo formation of Late Miocene age is found at the base of the Santa Rosalía basin in the central region of the Baja California peninsula. It marks the incipient opening of the Gulf of California rift system and records the evolution of the basin margin. The Boleo formation consists largely of interbedded tuff and tuffaceous conglomerate, sandstone, and mudstone, a fairly persistent basal marine limestone, scattered gypsum lenses, and a few fossiliferous sandstone layers (Wilson, 1955). The Cinta Colorada is a red bed within the Boleo formation between two copper ore deposits (Manto 3 and Manto 2), making it a significant stratigraphic marker. The Cinta Colorada varies in texture and thickness within its depositional range, and the nature of the bed is not well understood. Field investigations were conducted in the western portion of the Santa Rosalia basin on the Minera Boleo property. GPS location, elevation, bed thickness, bed descriptions, and samples were collected and recorded at 47 outcrops. The bed thickness varies from 9 cm to 250 cm. The distinctive uniform basal layer of the Cinta Colorada has an average thickness of 5.5 cm. QGIS software was used to render an interpolation map using the field-collected elevation and thickness data. The interpolation layers (hypsometric and isopach) were combined with the locations of the bed from a geo-referenced geologic map of Wilson and Rocha (1946). These data show that the bed covers an area of approximately 7.5 km². The Cinta Colorada appears primarily to be an air-fall tuff at its base with a possible high-density debris flow at the top. The bed’s sandy ash likely fell from the eruption plume within a few kilometers of the source vent while the finer ash was probably carried away by the winds. Some portions of the Cinta Colorada contain larger angular lapilli- to block-sized fragments of pumice and other volcanic clasts, suggesting the Cinta Colorada may be, in part, an ash-flow deposit. In addition, local fine-grained portions of the Cinta Colorada contain possible plant fragments suggesting deposition was in a basinal marsh or delta. Remobilization of some of the upper parts of the Cinta Colorada to lower-lying areas may account for the variation in thickness. This research was carried out as part of the NSF-funded Baja Basins Research Experience for Undergraduates.