Towards the definition of AMS facies in the deposits of pyroclastic density currents

Michael H Ort, T. T. Newkirk, J. F. Vilas, J. A. Vazquez

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Abstract Anisotropy of magnetic susceptibility (AMS) provides a statistically robust technique to characterize the fabrics of deposits of pyroclastic density currents (PDCs). AMS fabrics in two types of pyroclastic deposits (small-volume phreatomagmatic currents in the Hopi Buttes volcanic field, Arizona, USA, and large-volume caldera-forming currents, Caviahue Caldera, Neuquén, Argentina) show similar patterns. Near the vent and in areas of high topographical roughness, AMS depositional fabrics are poorly grouped, with weak lineations and foliations. In a densely welded proximal ignimbrite, this fabric is overprinted by a foliation formed as the rock compacted and deformed. Medial deposits have moderate-strong AMS lineations and foliations. The most distal deposits have strong foliations but weak lineations. Based on these facies and existing models for pyroclastic density currents, deposition in the medial areas occurs from the strongly sheared, high-particle-concentration base of a density-stratified current. In proximal areas and where topography mixes this denser base upwards into the current, deposition occurs rapidly from a current with little uniformity to the shear, in which particles fall and collide in a chaotic fashion. Distal deposits are emplaced by a slowing or stalled current so that the dominant particle motion is vertical, leading to weak lineation and strong foliation.

Original languageEnglish (US)
Pages (from-to)205-226
Number of pages22
JournalGeological Society Special Publication
Volume396
DOIs
StatePublished - 2015

Fingerprint

magnetic anisotropy
foliation
Magnetic susceptibility
density current
lineation
Anisotropy
Current density
Deposits
caldera
pyroclastic deposit
particle motion
Vents
ignimbrite
Topography
roughness
Surface roughness
Rocks
topography
fabric
rock

ASJC Scopus subject areas

  • Ocean Engineering
  • Water Science and Technology
  • Geology

Cite this

Towards the definition of AMS facies in the deposits of pyroclastic density currents. / Ort, Michael H; Newkirk, T. T.; Vilas, J. F.; Vazquez, J. A.

In: Geological Society Special Publication, Vol. 396, 2015, p. 205-226.

Research output: Contribution to journalArticle

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