Abstract
Deposits of the approximately 2.1 ka, phreatomagmatic Ubehebe Crater form four main facies. Lapilli- and block-dominated beds occur within several hundred meters of the crater and transition outward into discontinuous lenses of lapilli and blocks; they are interpreted to have been deposited by ballistic processes associated with crater-forming explosions. Thinly bedded lapilli tuff/ash is found mainly within several hundred meters and laminated and cross-laminated ash extends at least 9 km from the crater center. Thinly bedded lapilli tuff and laminated and cross-laminated ash are interpreted to record multiple pyroclastic surges (dilute pyroclastic currents) sourced by phreatomagmatic explosions. Dune-form data provide possible constraints on the relationships between suspended load sedimentation and bed load transport that are consistent using two independent approaches. Massive lapilli ash/tuff beds occur in drainages below steep slopes and can extend up to approximately 1 km onto adjacent valley floors beneath large catchments. Although they are massive in texture, their grain size characteristics are shared with laminated and cross-laminated ash facies, with which they are interbedded. These are interpreted to record concentrated granular flows sourced by remobilized pyroclastic surge deposits, either during surge deposition or shortly after. These concentrated flows followed drainages independently of the pyroclastic surges. Ubehebe Crater has the dimensions of a typical maar ( approximately 800 m diameter), and there is no reason to suspect that its explosions and pyroclastic currents were somehow unique. Preservation of their thin distal deposits at distances up to 9 km is due to the arid setting, suggesting that hazard assessments for volcanic fields should use larger potential surge footprints than has been previously done. The deposits of surge-derived granular flows with their independent flow paths also represent a potential hazard at similar maars in areas with complex terrain.