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 no rtheastern Baja California, Mexico. The Tuff of San Felipe (new name) was e rupted from a vent east of the Sierra San Felipe of NE Baja California at c a. 12.6 Ma. This is the only widespread middle Miocene pyroclastic flow dep osit identified in northeastern Baja California. Its distinctive age and wi despread distribution make it an important marker horizon for structural re construction 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 pr eserved, 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 summ arizes all known information including petrography, geochemistry, geochrono logy, paleomagnetics, geographic distribution, and field appearance of this important tuff. It is a densely welded, crystal-rich, lithic-lapilli pyroc lastic Mow deposit, with 5-15% alkali feldspar, and can be 180 m thick in s ome 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 minimu m volume estimate for the deposit is 54 km(3). Some recent Ar-40/Ar-39 age determinations suggest that the tuff is about 12.6 Ma in age. In all locati ons studied, the Tuff of San Felipe has a unique, low-inclination, reversed magnetization, which may record a field transition or a geomagnetic excurs ion within reversed polarity subchron C5Ar.2r (12.401 to 12.678 Ma). This l ow-inclination magnetization, as well as the mineralogy and age, is key to correlating the tuff across the region, because deposits are highly disrupt ed 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 subseque nt development of the Pacific-North America plate boundary in the Gulf of C alifornia, and it will be important to identify this tuff in outcrops elsew here on the Baja California Peninsula and on the North America plate in Son ora. (C) 1999 Elsevier Science B.V. All rights reserved.