FACIES VARIATION OF ERUPTION UNITS PRODUCED BY THE PASSAGE OF SINGLE PYROCLASTIC SURGE CURRENTS, HOPI BUTTES VOLCANIC FIELD, USA

Abstract

Base surges are a significant volcanic hazard associated with phreatomagmatism, but their transport and depositional dynamics are incompletely understood. Stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from Haskie tuff ring, Hopi Buttes volcanic field, USA, extend to > 1 km from the vent and show similar downcurrent changes in lithofacies. Within 200 m of the vent, eruption units are primarily disorganized lapilli-tuff that is rich in juvenile pyroclasts. At distances ≥ 200 m from the vent, eruption units transform into a couplet of juvenile-rich lapilli that grades upward into bedform-bearing ash tuff. At distances ≥ 900 m, upper and lower beds of the eruption unit couplet transform into planar-stratified beds. At distances ≥ 1100 m, couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar-bedded ash tuff. The collapse of dense eruption columns following explosive magma-wet sediment interaction generated turbulent surge currents. As the surges traveled away from the Haskie vent, they expanded and partitioned into a basal juvenile-rich traction carpet and overlying accidental-rich ash cloud surge. Near the limit of their runout, the surges cooled and lost their pyroclastic load, leaving planar beds of accidental-rich ash. The deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics. © 2006 Elsevier B.V. All rights reserved.