Discrimination of eolian and pyroclastic-surge processes in the generation of cross-bedded tuffs, Jemez Mountains volcanic field, New Mexico

Abstract

Discrimination of pyroclastic deposits emplaced by volcanic processes and reworked pyroclastic sediments is a primary objective of studies in volcaniclastic strata. Cross-bedded tuffs of both eolian and pyroclastic-surge origins are identified within the late Miocene Peralta Tuff Member of the Bearhead Rhyolite, in north-central New Mexico. Distribution, paleoflow directions, grain-size characteristics, and depositional structures permit discrimination of these two similar facies. Eolian tuffs exhibit structures diagnostic of eolian sands, are relatively well sorted, and lack both the fine ash and coarse lapilli and bombs typically found in surge deposits. Discrimination between eolian reworked and primary surge pyroclastic deposits is critical for volcanic hazard studies near modern volcanoes and for correct interpretation of proximity to vents in ancient volcaniclastic sequences.