| Sumario: | We present the results of a stratigraphic study of an upper Pliocene to lower Pleistocene volcano-sedimentary succession exposed in and around the La Reforma and El Aguajito calderas, north of Santa Rosalia, Baja California Sur, Mexico. This succession is part of the pre-calderas substrate and is related to the sedimentary formations described in the Santa Rosalia basin. The succession has nine lithological units that includes effusive and pyroclastic deposits accumulated in subaqueous and subaerial settings, that include fossiliferous siliciclastic and carbonate rocks. These units were defined Based on previous studies, our stratigraphic descriptions, a regional correlation and we defined recognized these units were grouped into two stratigraphic successions that are: (1) The Infierno Formation dated 2.5 Ma near its base and 1.8 Ma atop. It is a volcano-sedimentary succession more complex than previously described that includes: Mesa de Enmedio ignimbrite (giant pumice block deposit), El Yaqui fossiliferous sandstone (lower Pleistocene), Cueva Amarilla ignimbrite (2.4 Ma), El Carrizo ignimbrite (1.89 Ma), El Azufre lava (1.8 Ma) and the Cueva de Los Fósiles fossiliferous sandstone. (2) The Pre-Reforma volcano-sedimentary succession that, rests unconformably over the Infierno Formation and consists of three units: El Contrabando volcano-sedimentary (1.43 Ma), Punta Candeleros lava (1.36 Ma) and the La Zorra volcano-sedimentary unit (1.35 Ma). The sedimentary units of the Infierno Formation contain abundant fossils with predominance of Pectinid bivalves of the Pectinidae family. The fossil record suggests that The Infierno Formation emplaced in tropical-subtropical waters, which varied from intertidal zones and sheltered bays, to inner shelf zones with less than 100 m depth.
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