Page 130 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

together, forming even more markedly bimodal deposits Suspension sedimentation associated with
(40.3-4). subaqueous volcaniclastic mass flows

Distal water-settled ash deposits are composed of shards Turbulence accompanying subaqueous volcaniclastic
and crystal fragments, and are graded from coarser mass flows produces dilute suspensions of fine
crystal-rich bases to finer shard-rich tops, or are volcaniclastic particles in the enclosing body of water.
massive. They typically occur in thin (centimeter to tens If quiet conditions are restored, particles eventually
of centimeters) but very widespread (thousands of settle from suspension and are preserved as a layer of
square kilometers) intervals within other deep laminated or massive, volcaniclastic mud capping the
subaqueous sedimentary deposits (Ninkovich et al., mass-flow deposit (40.1, 40.5-6). Alternatively, the
1978; Ledbetter and Sparks, 1979; Sparks and Huang, suspension may be affected by currents that delay
1980). Primary vitriclastic textures are commonly settling and result in transport and sedimentation of the
modified or destroyed during glass crystallization and finer components separately from the parent mass-flow
diagenesis, and ancient, lithified, fine-grained, water- deposit. Thus, volcaniclastic suspension deposits
settled fall layers have a cherry or flinty appearance. associated with subaqueous mass flows may
gradationally overlie the coeval parent mass-flow
Coarser, proximal, syn-eruptive water-settled fallout deposit, or else may be deposited with other non-
deposits are parallel stratified, with beds being volcanic suspension sediments such as muds or biogenic
internally graded or massive, and bimodal, with coarser oozes. They are moderately well sorted, commonly
pumice clasts occurring together with finer lithic clasts graded, dominated by relative fine grain sizes (sand and
and crystals (Dimroth and Yamagishi, 1987; Cashman finer), and may include a significant admixture of non-
and Fiske, 1991). Cashman and Fiske (1991) described volcanic particles such as clay and biogenic particles
Late Miocene volcaniclastic deposits thought in part to simultaneously settled from the water column.
be proximal (2-3 km from source) water-settled fallout
(41). The deposits consist of both dense and pumiceous, Sedimentation in volcanic terranes
intermediate composition, juvenile clasts, crystals and
lithic fragments. Thermoremanent magnetization studies In non-volcanic sedimentary environments, facies and
show that juvenile andesite blocks in the base of the facies variations are dependent on several interrelated
sequence were emplaced hot (450-500°C — Tamura et controls (Reading, 1986): sedimentary processes of
al., 1991) and fossils in enclosing sedimentary facies erosion, transport and deposition; grain size, volume
constrain the environment of deposition to shallow and rate of the sediment supply; climate, especially
submarine (Cashman and Fiske, 1991). The temperature, precipitation and wind; subsidence rate and
combination of vesicular and hot, dense juvenile clasts the influence of regional tectonics on sediment sources;
suggests that the volcaniclastic deposits were related to sea level changes; biological activity; and, for biogenic
explosive activity accompanying submarine dome and chemical sediments, the water chemistry. In active
extrusion. volcanic terranes, there are additional special controls
on sedimentation, for example:
The unit studied by Cashman and Fiske (1991) has a 1. Eruptions strongly influence sedimentary processes
dense-clast-rich, poorly sorted breccia at the base, and sediment supply.
overlain gradationally by diffusely stratified to graded, 2. Because steep slopes and earthquakes are very
pumiceous breccia and sandstone, with an interval of common, slope failure events are especially important.
cross-bedded pumiceous sandstone at the top (41.1). In 3. Volcano-tectonism, in particular, faults, local rapid
the graded part of the section, the grain size distribution uplift and subsidence, causes frequent and sudden
is distinctly bimodal, and pumice:lithic clast diameter changes to sedimentation.
ratios are consistent with settling of a pumice-crystal- 4. Some volcanic processes are constructional and
lithic clast mixture through water. However, the overall rapidly create and modify topography and drainage.
character of the enclosing unit suggests an alternative
interpretation which is also consistent with the grain These special volcanic controls have conspicuous
size distribution. The unit shows the internal effects on grain size distributions, sedimentary
organisation of deposits from a coarse, high particle structures and facies architecture of resedimented and
concentration, volcaniclastic mass flow (cf. Lowe, volcanogenic sedimentary deposits. Grain size
1982). The diffusely stratified-graded interval analysed distributions are affected because eruptions can
by Cashman and Fiske (1991) could reflect rapid instantly produce abundant clasts of either a very
aggradation by fallout from suspension at the base of a narrow or an exceptionally wide grain size range, and
decelerating, high particle concentration mass flow. clasts that vary widely in shape and in density.
Notwithstanding the possibility of an alternative Sedimentary structures are affected because aggradation
interpretation, Cashman and Fiske (1991) have rates can be abnormally high, and inhibit or modify the
convincingly proven and quantified the characteristic ordered development of sedimentary structures. Water-
bimodality and fines-depletion of water-settled supported and gravity-driven volcaniclastic mass flows
pumiceous volcaniclastic aggregates. are common in a wide range of subaerial and
subaqueous volcanic settings. Grain size grading and
sorting are often weakly developed in pumice- or scoria-
rich volcanogenic sedimentary deposits, because clasts

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