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Characteristics and origin of subaqueous pumice-rich pyroclastic facies: Ohanapecosh formation (USA) and Dogashima formation (Japan)

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Jutzeler, M (2012) Characteristics and origin of subaqueous pumice-rich pyroclastic facies: Ohanapecosh formation (USA) and Dogashima formation (Japan). PhD thesis, University of Tasmania.

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Abstract

This thesis discusses the processes that generate subaqueous, pumice-rich pyroclastic
facies in below wave-base environments. The principal method is field-based facies
analysis. I also present grain size distributions and hydraulic sorting ratios of selected
samples using a new technique that combines image analysis and functional stereology.
Two Tertiary volcaniclastic successions have been studied. The Ohanapecosh Formation
(Washington State, United States) is dominated by subaqueous pumice-rich facies
derived from subaerial explosive eruptions and subaqueous sediment remobilisation
in a continental basin environment. The Dogashima Formation (Izu Peninsula, Japan)
contains well-preserved examples of subaqueous pumice-rich facies produced by
subaqueous explosive eruptions and below wave-base sediment remobilisation in an
oceanic arc setting.
The common processes of lithification and welding prevent quantification of grain size
by conventional sieving for most clastic rocks in the geological record. In addition, the
true grain size distribution of clastic rocks is finer grained than its representation in a
random 2D section. I show that image analysis combined with functional stereology can
be used to infer 3D volume fractions and weight percent of clast populations >0.25-2 mm
from 2D cross-sectional images. Data from synthetic rocks correlate well with results
from sieving of the same samples while still unconsolidated. The method can be applied
to any type of coarse grained clastic rock, regardless of age, and therefore has a wide
application in volcanology and clastic sedimentology.
The >800-m-thick Ohanapecosh Formation records voluminous sedimentation of volcanic
clasts during the Eocene-Oligocene in the Central Cascades. Most volcaniclastic beds
are dominated by angular pumice clasts and fiamme of intermediate composition, now
entirely devitrified and altered. Very thick to extremely thick (1–50 m) and very thin to
thick (0.001–1 m) beds are laterally continuous and have even thickness; erosion surfaces,
cross-beds and other traction structures are almost entirely absent, which strongly
suggests a below wave-base environment of deposition for most of the succession.
The Chinook Pass Member is mostly composed of extremely thick, graded, matrixsupported,
pumice-and-fiamme-rich beds that commonly include a coarse basal breccia
comprising sub-rounded dense clasts. The abundance of angular pumice clasts and
extreme thickness suggest that this facies was generated by magmatic volatile-driven
explosive eruptions, and the sub-rounded dense clasts were probably rounded above
wave-base. Thus, these beds are interpreted to have been deposited in a below wave-base
setting by subaerial pyroclastic flows that crossed the shoreline, and transformed into
eruption-fed, water-supported subaqueous volcaniclastic density currents. Reversely to
normally graded pumice breccia facies that contains sub-rounded pumice clasts, wood
and accretionary lapilli is interpreted to have formed by settling from pumice rafts, also
related to subaerial explosive eruptions. The White Pass Member chiefly contains massive iii
to normally graded volcanic breccia and coarse volcanic breccia that suggest deposition
from subaqueous high-concentration density currents and subaqueous debris flows. The
abundance of angular pumice clasts suggests minor reworking above wave-base. Very
thin to thick interbeds of fine sandstone to mudstone are interpreted to be derived from
subaqueous and subaerial sources, and to have mostly been deposited from low density
turbidity currents and suspension. The presence of shallow basaltic intrusions and mafic
volcanic breccia composed of scoria lapilli and that contains rare beds of accretionary
lapilli indicate the presence of intra-basinal scoria cones that may have been partly
subaerial. The lateral transition to thinner and finer-grained facies in the Johnson Creek
Member and western part of the White Pass Member suggests that the principal sources
were to the east of the preserved exposures of the Ohanapecosh Formation.
The Pliocene Dogashima Formation (Izu Peninsula, Japan) is composed of three
volcaniclastic sequences erupted under water. Dogashima 1 is mostly composed of
pumice breccia, shard-rich siltstone, and cross-bedded and planar bedded pumice
breccia/sandstone. The base of Dogashima 2 is dominated by very thick, clast-supported,
massive grey andesite breccia composed of very coarse andesite clasts with quenched
margins. It is gradationally overlain by very thick, clast-supported white pumice breccia.
The massive grey andesite breccia is confined to a palæo-valley eroded into beds of
Dogashima 1. The white pumice breccia is hydraulically sorted and stratified in proximity
to the wall of this palæo-valley, and is stratified and finer grained in the adjacent overbank
setting. The top of Dogashima 2 is dominated by very thick cross-bedded pumice
breccia-conglomerate and planar bedded pumice breccia that contains coarse pumice
clasts. Dogashima 2 has an erosive contact with overall monomictic andesite breccia
of Dogashima 3. The similar mineralogy and composition of white andesite pumice
and grey andesite clasts in Dogashima 1 and 2 suggests they were co-magmatic and
erupted from the same vent. Dogashima 2 is interpreted to record explosive destruction
of a subaqueous hot lava dome by a subaqueous, magmatic volatile-driven explosive
eruption. Most of the products of this eruption were deposited in two gradational units
from cohesionless, water-supported volcaniclastic density currents. Coarse pumice clasts
and ash present in overlying planar beds were settled from suspension. This sequence
demonstrates that lava or dome effusion on the sea floor can switch to an open-vent,
pumice-forming, magmatic volatile-driven explosive activity, as in subaerial analogues.
Pumice breccia of Dogashima 1 is interpreted to be the product of precursory explosive
activity, whereas Dogashima 3 records a late, dome-building episode. Pumice breccia
and dome-related clasts indicate cyclic effusive and explosive activity throughout the
Dogashima Formation. Cross-bedded pumice breccia/breccia conglomerate facies in the
Dogashima Formation are most likely to be products of resedimentation of pumiceous
aggregates, and development and destruction of subaqueous dune fields in a below
wave-base, canyon setting. I apply the image analysis and functional stereology method to pumiceous volcaniclastic
rocks of the Ohanapecosh and Dogashima formations and the Manukau Sub-Group
(New Zealand) and the Sierra La Primavera caldera (Mexico). Samples from these waterlain
successions were grouped into three broad facies, on the basis of bed thickness,
abundance of matrix and clast size sorting. The volume of pumice clasts, dense clasts
and matrix (<2 mm), modal grain size distribution and hydraulic sorting ratio between
pumice and dense clasts were used to characterise these three facies. On plots of sorting
versus median diameter, the three facies overlap, which suggests that all three facies
have overall good hydraulic sorting in their coarse clasts (>2 mm), and that the pumice
clasts were fully waterlogged during transport and deposition. In addition, the studied
subaqueous volcaniclastic samples overlap with the fields defined by matrix-free subaerial
pyroclastic flow deposits, which confirms that outputs from functional stereology and
conventional sieving give comparable results.
The volcaniclastic successions in the Ohanapecosh and the Dogashima formations
include good examples of subaqueous, pumice-rich pyroclastic facies that were erupted
onland or under water, and deposited in below wave-base settings. Eruption-fed facies
generated by subaqueous pumice-forming explosive eruptions are exemplified by the
extremely thick and graded sequence in Dogashima 2 (Dogashima Formation), which
contains dense clasts that were deposited hot. Numerous beds of graded, extremely
thick, pumice-rich facies in the Ohanapecosh Formation are interpreted to be deposited
from eruption-fed, water-supported high concentration density currents that were fed by
subaerial pyroclastic flows, primarily because rounded dense clasts, accretionary lapilli
and wood in these and associated facies imply that the source vents were subaerial.
Resedimentation events can occur during eruptions and after, especially after eruptions
that produce large volumes of pyroclasts in unstable environments. However,
resedimented facies can be difficult to distinguish from eruption-fed facies in below
wave-base successions, essentially because clast reworking below wave-base is minimal
during resedimentation. In addition, eruption-fed facies can contain clasts that were
previously abraded in above wave-base settings, for example during long-distance
transport in subaerial pyroclastic flows, in pumice rafts (e.g. Chinook Pass Member,
Ohanapecosh Formation) or during transport in the dense-clast-dominated bedload of
subaqueous volcaniclastic density currents (e.g. Dogashima 2). Short-distance transport
in pyroclastic flows might not be able to abrade pumice clasts (e.g. the eruption-fed facies
in the Chinook Pass Member, Ohanapecosh Formation).

Item Type: Thesis (PhD)
Keywords: subaqueous volcanology, explosive volcanology, Dogashima, Ohanapecosh, grain size distribution, functional steredology, subaqueouspumice, pyroclastic facies
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Date Deposited: 14 Jan 2013 00:55
Last Modified: 11 Mar 2016 05:53
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