Volcanic and marine stratigraphy along the El Álamo Canyon, Santa Rosalía Basin, Baja California sur

This field trip highlights evidence of late Pliocene-early Pleistocene submarine and subaerial volcanism coeval with marine and marginal-marine sedimentation in the Santa Rosalía Basin in Baja California. The best exposures of these rocks occur at the El Álamo Canyon, which exhibits outcrops of the Tirabuzón and Infierno formations interbedded with submarine and subaerial volcanic and volcaniclastic deposits that are covered by the subaerial ignimbrites of Reforma and Aguajito calderas. Extensive field mapping and stratigraphy carried out in this canyon, aided with 40Ar/39Ar, and U-Pb geochronology, allowed us to divide the stratigraphy into three main sequences that, from base to top, are: (1) Santa Rosalía succession, (2) the Reforma caldera complex, and (3) El Aguajito caldera. This refined stratigraphy indicates that eight episodes of volcanism occurred between 2.5 and 1.36 Ma, during marine sedimentation in an internal continental shelf and in estuaries, coastal lagoons, or protected bays as supported by the fossil record. This sedimentary and volcanic interbedding suggests transgressions and regressions of the sea level, as well as tectonic uplifting. After the inception of volcanism in the Reforma caldera complex (1.29 Ma), the area emerged from these shallow seas followed by the formation of the Aguajito caldera (1.1 Ma), and then later on by the Tres Vírgenes volcanic complex (0.3 Ma). The last complex erupted a lava flow ca. 22 ka that so far stands as the youngest activity in the region. The magmatic evolution in the region is characterized by post-subduction calc-alkaline magmatism. Such magmatism is expressed as pure calc-alkaline rocks from El Aguajito-Reforma calderas, and as hybrid transitional magmas formed by adakitic rocks from the Tres Vírgenes volcanic complex.