Origin of Tuff Deposits in the Lower Miocene Lospe Formation, Santa Maria Basin, California

The Lospe Formation contains at least five mappable tuff units (17-18 Ma) which were erupted during initial stages of Neogene Santa Maria basin subsidence. Individual tuff units are lenticular, as much as 15-20 m thick, and 1-3 km wide; they were deposited predominantly in a lacustrine setting. Subaqueous deposition is indicated by facies of the interbedded nonvolcanic Lospe Formation. The lowermost Lospe Tuff unit, however, which overlies Jurassic basement, is interpreted as a subaerial deposit. Each subaqueous tuff unit contains two or more eruption units. Each eruption unit consists of three zones which are, from base to top: (1) massive vitric tuff comprising about 50% of the eruption unit, (2) thin- to medium-bedded vitric tuff with pumice concentrations at the tops of beds and mud drapes between beds, and (3) a thin-bedded interval of massive to planar laminated tuffaceous siltstone-mudstone. The predominance of delicate cuspate vitric shards and pumice, and the near absence of nonvolcanic detritus indicates that little or no reworking of the ash occurred prior to deposition. The Lospe tuffs are predominantly distal pyroclastic flow (zones 1 and 2) and pyroclastic turbidite (zones 2 and 3) deposits, derived from subaerial magmatic eruptions. A possible source for the Lospemore » tuffs is located at Tranquillon Mountain, 30 km to the south in the westernmost Transverse Range, where 17-18 Ma proximal pyroclastic deposits of welded lithic tuff breccia and thick pumiceous fallout tuffs are present. The similar ages, stratigraphic positions, and petrology, as well as the lateral facies relations suggests a correlation between the Tranquillon volcanic center and the Lospe tuff units. The authors are currently testing this hypothesis on the basis of geochemical and isotopic analyses.« less