Page 107 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

(lahars); not produced in abundance in this setting,
(3) entry of subaerial pyroclastic flows into the sea or volcanogenic sediments may nevertheless be finally
into lakes and transformation to subaqueous, water- deposited there, if resedimented by mass flows or
supported volcaniclastic mass flows; transported by wind from subaerial sources nearby.
(4) subaerial pyroclastic flows and surges flowing across
and coming to rest on the surface of lakes or the sea; Transport and deposition of volcaniclastic
(5) explosive eruptions at subaqueous (submarine or particles
sublacustrine) volcanoes;
(6) rapid, essentially syn-eruptive resedimentation of More or less similar processes of transport and
pyroclastic deposits temporarily stored in shoreline deposition operate in the formation of primary
environments or on the flanks of subaqueous pyroclastic, resedimented syn-eruptive volcaniclastic
volcanoes. and volcanogenic sedimentary deposits. Regardless of
the clast-forming mechanism, each case essentially
Volcanogenic sedimentary deposits involves particles and interstitial fluid. Although the
physical properties of volcaniclastic particles vary
In volcanic terranes, all surface deposits - lavas, welded widely, and the interstitial fluid can be gas (volcanic gas,
pyroclastic deposits, other variably welded or air, steam, or a mixture) or liquid (water, watery mud,
consolidated volcaniclastic deposits, non-volcanic rocks muddy water), volcaniclastic deposits display a limited
- are subject to chemical and physical weathering and range of sedimentary structures. Volcaniclastic deposits
erosion. New particles created solely by surface are reviewed here firstly in terms of the depositional
weathering and erosion are termed epiclasts. However, processes that are evident from outcrop and lithofacies
weathering and erosion of pre-existing, poorly or non- information, and then in terms of textures and facies
welded primary volcaniclastic deposits can simply associations that help distinguish among the different
release the original pyroclasts or autoclasts and rapidly genetic categories.
provide large volumes of recycled particles. As a result,
only a proportion of the particles present in Transport of volcaniclastic particles from the site of
volcanosedimentary systems are true epiclasts: the rest, initial fragmentation to depositional sites may be
often the vast majority, are pyroclasts or autoclasts, continuous with a primary volcanic clast-forming process,
some freshly erupted and some liberated from poorly such as an explosive eruption, or involve surface
consolidated primary volcaniclastic deposits. All sedimentary transporting agents, such as wind, water or
particle types are available for transport or reworking by ice, or else involve both these processes in rapid
wind, water or ice. Final deposits are commonly mixtures succession. In each case, there are three broad
and, therefore, more appropriately identified as categories of clast transport processes:
volcanogenic sedimentary, rather than epiclastic
deposits. Mass-flow transport — groups of clasts, or clasts plus
interstitial fluid (air, water, volcanic gas) move together
In order to distinguish volcanogenic sedimentary and interact; mass flows vary widely in rheology and
from resedimented syn-eruptive volcaniclastic particle concentration;
deposits, we add the requirement that there is evidence
of significant transport or reworking prior to deposition, Traction transport — clasts are entrained in moving
and/or evidence that final deposition significantly post- interstitial fluid (air, water, volcanic gas) and are free to
dated eruption. Hallmarks of transport, reworking and behave independently;
post-eruptive resedimentation are modification of
primary clast shapes, especially rounding, admixtures Suspension transport — clasts are fully suspended in
of non-volcanic particles, mixtures of widely interstitial fluid (air, water, volcanic gas).
compositionally different volcanic particles, particles
that show the effects of weathering or partial diagenetic Each of these categories of transport processes produces
alteration, and association with non-volcanic a suite of characteristic textures and structures in the
sedimentary facies. resulting deposits. Transport in each mode can be
strongly influenced by the particle concentration and
Volcanogenic sediments are best known in subaerial the properties and behaviour of the interstitial fluid
volcanic terranes, where weathering and erosion are (air, water, muddy water, watery mud, volcanic gas,
vigorously active during and following eruptions. steam, ice), or else can occur primarily in response to
Such terranes commonly include a wide spectrum gravity acting directly on the particles or paniculate
of sedimentary environments (e.g. fluvial, alluvial, aggregate, regardless of the presence or character of an
lacustrine, shoreline) and eruptions may affect interstitial fluid. Particle concentration and other
sedimentation in neighbouring non-volcanic terranes, properties of the particle-fluid system fluctuate with
and in offshore submarine shelf and deeper water time and are rarely homogeneous throughout the system,
settings. In below-wave-base subaqueous settings, especially during deposition; therefore continuous
weathering and erosion are in general very limited. Mass sedimentation may involve alternation from one mode
wasting processes (rock falls, slides and slumps) along to the other, locally or temporarily. All three modes
fault scarps and on the flanks of island volcanoes and operate in subaerial and subaqueous settings, and are
seamounts constitute an important exception. Although
involved in formation of pyroclastic, resedimented syn-
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