Page 128 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Ground surge deposits and ash cloud surge deposits are subaerial depositional settings, although they can be
primarily distinguished by their stratigraphic generated from shallowly submerged vents and travel
relationships with associated pyroclastic flow deposits: across water. Base surge deposits indicate proximity to
ground surge deposits underlie the associated flow, a volcanic vent. Surge deposits thus provide narrow
whereas ash cloud surge deposits overlie, are constraints on depositional settings of ancient volcanic
interbedded with or occur as lateral facies equivalents of sequences. However, they resemble non-primary
associated pyroclastic flow deposits. In both instances, volcaniclastic deposits, and it is therefore imperative to
the components of these deposits are closely similar to establish beyond doubt a primary origin.
those of the associated flow, and both may show
evidence for having been emplaced hot. Thickness Suspension transport and volcaniclastic
variations with distance from source are unpredictable, suspension deposits
except for vent-derived ground surge deposits that
become thinner distally. Suspension refers to transport of particles fully
supported by buoyancy and fluid turbulence within the
Distinguishing among the different genetic types of fluid (water, muddy water, air, volcanic gas). Whether
pyroclastic surge deposits is usually difficult and relies particles are suspended within the flow or moved by
heavily on context. Pyroclastic surge deposits also have traction (as bedload or by saltation) depends on their
features in common with other sorts of primary density relative to the fluid and on the velocity of the
pyroclastic deposits (especially bedded, fine-grained, flow. Relatively rapid flows of dense fluid are able to
fallout deposits), and with reworked volcanogenic carry coarse grains in suspension. Deposition takes
sediments (McPhie, 1987; Smith and Katzman, 1991). place when the flows decelerate and the suspended
Reworking of pyroclasts by water or wind may produce grains settle through the fluid under the influence of
planar bedded and cross-bedded pyroclast-rich deposits gravity. Very fine particles (fine volcanic ash, clay
that superficially resemble primary surge deposits. platelets) settle only when the fluid reaches a standstill.
However, water-reworked cross-bedded pyroclast-rich
deposits commonly contain appreciably rounded or Explosive volcanic eruptions eject pyroclasts, which fall
abraded clasts and significant amounts of admixed out from suspension in eruption columns and dilute ash
epiclastic (volcanic or non-volcanic) detritus and are clouds in the atmosphere or from suspension in water.
lacking in fine ash components. They show no Suspension sedimentation of fine pyroclasts also
systematic changes related to distance from a volcanic accompanies and follows the subaerial emplacement of
vent, and are also distinguishable by the context. Wind- pyroclastic flows (Layer 3 deposits) and pyroclastic
reworked cross-bedded pyroclast-rich deposits are well- surges. Subaqueous volcaniclastic mass flows are
sorted, lack both the finer (ash) and coarser (lapilli) associated with dilute suspensions of fine particles that
clasts usually present in surge deposits, contain eventually settle from the water column.
admixtures of non-volcanic particles and show
distribution, thickness, and palaeoflow patterns that Suspension sedimentation is an important depositional
reflect regional wind directions rather than point sources process in below-wave-base subaqueous settings,
(such as volcanic vents). especially quiet, deep-water settings that occur in deep
lakes and ocean basins (Fig. 48). The deposits are
Dimensions of pyroclastic surge deposits characteristically fine grained (mud; <0.0625 mm) and
may be massive or planar laminated. In these settings,
Single, base surge-producing eruptions involve very suspension sediments are closely associated with
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small volumes of pyroclasts (<0.01 km ), most of subaqueous mass-flow deposits.
which are deposited within 3-5 km of the source vent.
The maximum thickness (generally <1 m) of bed-sets Differences in textures and lithofacies among the
occurs closest to source, thinning to a few centimeters various types of subaerial pyroclastic fallout and water-
distally. A series of base surge eruptions repeated in settled volcaniclastic suspension deposits are subtle.
rapid succession may build up near-vent deposits several Criteria for distinguishing subaerial fallout from syn-
tens of metres thick and produce a total erupted volume eruptive water-settled fallout deposits are still being
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of about 0.1 km . developed (Cashman and Fiske, 1991). In ancient
volcanic sequences, the presence of volcaniclastic
Ground surge and ash cloud surge deposits are typically suspension deposits does not uniquely constrain the
less than 1-2 m thick, and thickness variations can be depositional setting, nor the genetic process (primary
marked over short distances. Some ash cloud surge pyroclastic, syn-eruptive resedimentation, or post-
deposits thicken away from the source of the parent eruptive sedimentation). Correct interpretation depends
pyroclastic flow (Fisher, 1979). Areal extents of discrete on consideration of the character of enclosing facies.
ground surge or ash cloud surge deposits are very poorly
known but, presumably, are at least as great as the extent
of the associated pyroclastic flow deposits, and therefore Pyroclastic fall deposits (39)
likely to range up to hundreds of square kilometers. Subaerial fall deposits can be generated by the entire
range of explosive eruptions (magmatic,
Significance phreatomagmatic and phreatic) and by magmas of any
composition. Pyroclasts ejected explosively into the
Primary pyroclastic surge deposits are restricted to
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