GEOLOGIC MAP AND CROSS SECTION OF CONCEPCIÓN PENINSULA, GULF OF CALIFORNIA RIFT, BAJA CALIFORNIA SUR: BAJA BASINS NSF INTERNATIONAL RESEARCH EXPERIENCE FOR STUDENTS1

The Concepción Peninsula is a 40 X 14 km east-tilted horst block bounded along its west side by an active NNW-striking, west dipping normal fault zone (Bahìa Concepción fault zone), which in turn forms the east margin of the Concepcion Bay, one of the largest bays in the Gulf of California. This horst block exposes Cretaceous granitic basement overlain by Oligocene forearc and Miocene arc strata related to subduction of the Farallon plate, and Late Miocene rift strata associated with the opening of the Gulf of California. In January 2022 we mapped a 12 by 8 km area on the southeast side of the Concepción Peninsula using TouchGIS on a Google Earth base map, collecting 52 rock samples for petrographic and geochemical analysis, and plotting 104 GPS stations with rock descriptions. The samples and GPS data were used to create a geologic map in QGIS and a geologic cross section in Inkscape.

The southeast Concepción Peninsula exposes an 920 m thick east-dipping section that includes, from base to top: (1) 124 Ma granitic basement; (2) Oligocene El Salto formation (⁓300 m thick), with aeolian sandstones and three distinctive widespread ignimbrites; (3) ⁓300 m thick, purple, largely massive, coarse-grained volcanic debris flow deposit that is well indurated and heavily veined; (4) a peninsula-wide unconformity with substantially fresher volcanic rocks above it; above this unconformity lies (5) an ⁓200 m thick section of alkalic clinopyroxene ignimbrites and alkalic mafic to intermediate volcanic rocks, including the extensive, up to 85 m thick tuff of San Lino; (6) an ⁓90 m thick section of sheet forming alkalic mafic to intermediate lavas, locally cut by an aphyric rhyolite ignimbrite vent; and (7) an > 30 m thick (top eroded) section of mound-forming hornblende basaltic trachyandesite lavas.

The east dipping section is repeated by a series of ⁓north-south striking, west dipping normal faults that are parallel to the major active normal fault zone that bounds the peninsula on its west side. This series of faults shows up to 75 m of normal offsets that are easily estimated due to excellent exposures and distinctive stratigraphy.

¹This project recruited a different cohort of college juniors and seniors from the U.S. and Mexico each year for three years.