Soft‐sediment deformation (fluid escape) features in a coarse‐grained pyroclastic‐surge deposit, north‐central New Mexico

The Puye Formation in north‐central New Mexico is a very coarse‐grained fanglomerate which was deposited on the eastern flank of the Jemez caldera. Pyroclastic deposits occur within the Puye in the form of airfall pumice beds and the remnant of at least one pyroclastic‐surge deposit. This pyroclastic‐surge deposit shows the effects of fluidization and soft‐sediment deformation in the form of: (1) intrusive sedimentary plumes; (2) upwardly injected gravelly pipes; (3) ‘pocket structures’ similar to those of Postma (1983), and; (4) oversteepened and deformed cross‐stratification. Fluidization and soft‐sediment deformation resulted from a combination of the mechanical instability and high, possibly pressurized, fluid content of the deposit. This metastable condition was a consequence of the nature of the flow which deposited the sediment: a rapidly depositing, high‐velocity sediment gravity flow. The fluids in pyroclastic surges may be either gas or liquid. However, because of the coarse grain‐size of the fluidized sediment, it is suggested that liquids were responsible for the features described in this paper. Evidence also suggests that locally fluidization, liquefaction, and soft‐sediment deformation took place penecontemporaneously with deposition.