ABSTRACT 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 40 Ar/ 39 Ar, 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.

Abstract Extensional tectonics in the Gulf of California region occurred during middle- to early-late Miocene time. One of the best examples is found at Bahía Concepciόn, which is the largest fault-bound bay on the peninsular gulf coast of Baja California. The extensional basin confined to Bahía Concepciόn developed within an accommodation zone related to the late Miocene east-west extension in the broader Gulf of California region. This contribution is focused on the large, central part of the gulf region along the peninsular axis, sometimes referred to as the Baja California Central Domain (BCCD). The mainly volcanic Comondú Group is extensively exposed throughout the BCCD , but on a local basis it occurs as huge andesite blocks associated with stratified units that characteristically dip in opposite directions. This is a direct result of a main east-west extensional episode that generated normal faults on the surface and listric detachment faults at depth. Half grabens are the most common result of such extensional episodes in the BCCD . This event has produced subsidence, creating depocenters for mostly nearshore marine basins. The oldest marine sedimentary units present in the BCCD are the late Miocene–early Pliocene Tirabuzón Formation. Less typically, on Peninsula Concepciόn and at Punta San Antonio, a late Miocene extensional episode has resulted in up-thrown granodiorite basement along bounding faults. Extension on the Bahía Concepciόn zone was responsible for development of a half-graben structure first fully flooded in late Pliocene time. The tectono-sedimentary evolution of the adjacent Santa Rosalía and Bahía Concepción areas is recorded by three stratigraphic stages: (1) pre-rift strata represented by the Comondú Group; (2) a syn-rift stage containing syntectonic siliciclastic deposits; and (3) post-rift strata, represented by the late Miocene to Pliocene flat-lying to low-angle marine sedimentary units. The three stratigraphic stages are interpreted from outcrops in this region.