Soft sediment deformation in dry pyroclastic deposits at Ubehebe Crater, Death Valley, California

Abstract

Soft sediment deformation structures are common in fine-grained pyroclastic deposits and are often taken, along with other characteristics, to indicate that deposits were emplaced in a wet and cohesive state. At Ubehebe Crater (Death Valley, California, USA), deposits were emplaced by multiple explosions, both directly from pyroclastic surges and by rapid remobilization of fresh, fine-ash-rich deposits off steep slopes as local granular flows. With the exception of the soft sediment deformation structures themselves, there is no evidence of wet deposition. We conclude that deformation was a result of destabilization of fresh, fine-grained deposits with elevated pore-gas pressure and dry cohesive forces. Soft sediment deformation alone is not sufficient to determine whether parent pyroclastic surges contained liquid water and caused wet deposition of strata.

Large-scale volcanic deposit fluidization by dilute pyroclastic density currents

“…We think that it was the cause of the concentrated surge-derived pyroclastic flows observed at Montserrat in 1997 14,15 and our model is fully compatible with the 'ash boiling' described 15 . It is also compatible with observations at Ubehebe Crater 13,19 and particularly the conclusion that the deposit was inflated and soft shortly after deposition 19 . The fact that surgederived pyroclastic flows are not very commonly described in the literature is probably due to the difficulty of identifying them, since the topography generally causes these secondary concentrated flows to move into channel bottoms and blend with primary concentrated flows in process, making it difficult to distinguish between the two.…”

“…At a much larger scale, the fluidity could affect extensive areas, up to several hundred square metres, and, on slopes, the remobilized deposit could evolve into a concentrated flow able to move hundreds to thousands of metres independent of the parent dilute PDC. This phenomenon, called surge-derived pyroclastic flow, was observed particularly well at Montserrat in 1997 14,15 , and where also documented at Mount St Helens 16,17 , Montagne Pelée 18 , Merapi 11 and at phreatomagmatic Ubehebe Crater 12,13,19 . Among the various mechanisms evoked for localised fluidisation (ref 12 and therein), the role of pore gases is thought to be highly influential.…”

“…For deposits of dilute PDC and surge-derived pyroclastic flows, K has been estimated between 10 -11 and 10 -10 m² although the finest particles found in these deposits (< 20 m) can significantly lower this value 11,20,38,39 . Dilute PDC deposits is typically of some centimetres to tens of centimetres thick 11,19,20 . Because they may overlie previous deposits into which the gas can diffuse, we have explored deposit thicknesses H of up to 10 m. We have also explored a large range of deposit densities , between 500 to The maximal static gas pressure at the base of a moderate PDC and its unloading time (i.e.…”

Lateral Extent of Pyroclastic Surge Deposits at Ubehebe Crater (Death Valley, California) and Implications for Hazards in Monogenetic Volcanic Fields

“…Many phreatomagmatic pyroclastic surges may have temperatures of ∼100–300°C as they begin to propagate outward. For example, plastering of damp ash onto surfaces at Taal and Capelinhos during historical eruptions further indicates sub‐boiling temperatures (Waters & Fisher, 1971), although we see no evidence that the Ubehebe Crater surges were wet (Valentine et al., 2021).…”