The Tuff of San Felipe: an extensive middle Miocene pyroclastic flow deposit in Baja California, Mexico

We document the existence of a widespread Miocene ash-flow tuff sheet in northeastern Baja California, Mexico. The Tuff of San Felipe (new name) was erupted from a vent east of the Sierra San Felipe of NE Baja California at ca. 12.6 Ma. This is the only widespread middle Miocene pyroclastic flow deposit identified in northeastern Baja California. Its distinctive age and widespread distribution make it an important marker horizon for structural reconstruction of this part of the Gulf Extensional Province, which is on the Pacific plate. The vent position, near the modern Gulf of California coast, allows the possibility that exposures of the Tuff of San Felipe may be preserved east of the Gulf on the North America plate in Sonora, yielding a tie point for the past relative position of the two plates. This paper summarizes all known information including petrography, geochemistry, geochronology, paleomagnetics, geographic distribution, and field appearance of this important tuff. It is a densely welded, crystal-rich, lithic-lapilli pyroclastic flow deposit, with 5–15% alkali feldspar, and can be 180 m thick in some locations near the vent. The Tuff of San Felipe is >40 m thick up to 40 km SW of the vent and >10 m thick at least 25 km NNW of the vent. A minimum volume estimate for the deposit is 54 km3. Some recent 40Ar/39Ar age determinations suggest that the tuff is about 12.6 Ma in age. In all locations studied, the Tuff of San Felipe has a unique, low-inclination, reversed magnetization, which may record a field transition or a geomagnetic excursion within reversed polarity subchron C5Ar.2r (12.401 to 12.678 Ma). This low-inclination magnetization, as well as the mineralogy and age, is key to correlating the tuff across the region, because deposits are highly disrupted by subsequent normal faulting and outcrops are sparse and discontinuous away from the vent. The documentation of these characteristics is important because the Tuff of San Felipe is a key structural marker for the subsequent development of the Pacific–North America plate boundary in the Gulf of California, and it will be important to identify this tuff in outcrops elsewhere on the Baja California Peninsula and on the North America plate in Sonora.