Pyroclastic-surge deposits vary in size from those associated with large ash-flow sheets surrounding calderas to tuff rings formed during small phreatomagmatic explosions. This paper results from studies of pyroclastic-surge deposits at Crater Elegante, Sonora, Mexico; Peridot Mesa, Arizona; Coronado Mesa, Arizona; Ubehebe Crater, California; and the Bishop Tuff, California. Stratigraphic sections measured at varying distances from the vents are input for a Markov analysis of bed-form transitions, through which facies of pyroclastic surge are defined. The principal bed forms are massive beds, planar beds showing inverse grading, and sandwave beds with dunes, ripple and cross laminations, and antidunes. Markov analysis shows that sections are characterized by a dominance of either (1) sandwave and massive beds (sandwave facies); (2) planar, massive, and sandwave beds (massive facies); or (3) planar and massive beds (planar facies). Facies distribution maps demonstrate a systematic lateral variation away from the vent. Sandwave facies predominate in sections nearest the vent, massive facies dominate in sections at an intermediate distance from the vent, and planar facies occur in sections farthest from the vent. The spatial distribution of surge facies is compatible with a fluidization-deflation model of pyroclastic-surge transport and deposition. A pyroclastic-surge cloud that is initially fluidized at the vent deflates (defluidizes) as it moves laterally. During transport the cloud passes from a proximal viscous mode of flow characterized by deposition of the sandwave facies to a distal inertial mode of flow represented by deposition of the planar facies. The gradual transition from viscous to inertial flow is coincident with deposition of the massive facies. 177 on June 20, 2015 specialpapers.gsapubs.org Downloaded from

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