 |
| Titel |
Bombs, welded spatter, rockfall and cross-cutting breccia enclosed in avalanche deposits 300 m deep in a debris-filled vent (diatreme), Hopi Buttes, Arizona |
| VerfasserIn |
James White, Nathalie Lefebvre, Bruce Kjarsgaard |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079692
|
|
|
|
|
|
| Zusammenfassung |
| Diatremes are debris-filled vents that are surprisingly large relative to the small maar
volcanoes that are their surface expression. Field characteristics of well-exposed diatreme
deposits in the Hopi Buttes volcanic field, in Arizona, USA, challenge existing diatreme
models, but may provide insight into the broader behavior of magma plumbing systems
feeding small basaltic volcanoes.
Standing Rocks East is a volcanic "neck" rising 35 m above the adjacent land
surface. It was previously identified as the deposit of a "root zone", i.e. the fragmental
zone at the base of a diatreme structure, based on the depth of exposure, textural
diversity of its deposits, irregular dikes that terminate within it, and its small footprint
relative to a nearby diatreme remnant. Painstaking mapping in a new study reveals:
(1) most of the diatreme structure at the level of the "neck" is filled by a coarse
country-rock breccia, which contains blocks sourced both from as far as 200 m
below exposure, and as much as 300 m above it at the paleosurface; (2) a zone of
juvenile-rich heterolithic lapilli tuff, with domainal map-view variations in deposit
granulometry and componentry were emplaced after the country-rock breccia but before
the rocks of the neck; (3) the neck comprises an architecturally complex range of
deposits in which metres-wide subvertical sheets dominated by coherent basaltic rock
cut, locally with surface wrinkes and clast imprints, and locally grade outward
into, subhorizontally layered domains, up to several metres in extent, of breccia
and welded spatter including large isolated boulders of mixed pyroclastic and host
mud/mudrock that deformed adjacent spattery deposits. From these relationships
we draw these conclusions. (A) The neck is not a root zone, because it is entirely
enclosed within earlier deposits in the diatreme structure – it is not at the bottom
of this diatreme structure, and hence represents an intra-diatreme fragmentation
zone. (B) This fragmentation zone formed after coarse country-rock breccias in
which blocks spanning hundreds of metres of stratigraphy below and above current
exposure level, filled a deep vent structure, and after these breccias were cut through by
explosively emplaced heterolithic lapilli tuff. (C) Weak fragmentation of fluid magma
at the irregular and unstable floor of a deep crater pocked by subcraters allowed
emplacement of the bewildering sequence of steeply to shallowly dipping, coherent to
welded to fully fragmental, mixed, and locally hot-deposit-deforming rocks of the
neck.
During development even of this small volcano, there was great diversity of activity. The
country rock breccia requires excavation of a deep, largely open, crater at the same time that
large country-rock blocks were brought upward from their source. Heterolithic tuff is from
effective and repeated explosive fragmentation events, while the neck comprises
small-volume deposits of material emplaced at the base of a deep open crater. The small
volume of solidified magma here indicates formation of a large diatreme structure despite
limited magma supply; at times transfer of magma elsewhere along the feeding system
probably facilitated deepening of explosion sites by drawing magma downward in the
structure. |
|
|
|
|
|
|