Page 121 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 49 Turbidity current deposits (turbidites). (A) Low-density (classical) turbidite, showing the Bouma divisions
(a to e). (B) Sandy, high-density turbidite, showing deposits from the high density stage (divisions S 1-3) and from
the residual low density stage (divisions T e,d,t). Modified from Lowe (1982) and Stow (1986).

Coarse particles in gravelly high-density turbidity Deposits from volcaniclastic turbidity currents show
currents are probably transported in a basal, highly many of the textural and structural features of their
concentrated traction carpet and in suspension at the non-volcanic equivalents (29, 30, 31, 32). The principal
base of the flow. When flow velocity declines differences arise in cases involving pumice-rich flows.
sufficiently, the traction carpet freezes and suspended The low density of pumiceous particles means that
clasts are sedimented very rapidly. Deposits thus have a coarse pumice particles are deposited together with
basal, reversely graded traction carpet layer (R 2), markedly finer, non-vesicular particles such as crystal
succeeded by a normally graded suspension fragments, resulting in much poorer sorting in deposits.
sedimentation layer (R 3) (Fig. 37). This process leaves a Furthermore, pyroclastic eruptions effectively supply
residual sandy high-density turbidity current that may enormous volumes of particles instantaneously and
rework the underlying gravel or else continue downslope have the potential to generate very large-scale, far-
and deposit independently. traveled turbidity currents. The term megaturbidite is
sometimes used for deposits from voluminous
Volcaniclastic turbidites volcaniclastic turbidity currents (29.4, 30).
Volcaniclastic megaturbidite sedimentation units can
Volcaniclastic turbidity currents are responsible for be of the order of 100 m thick and include abundant,
resedimentation of a wide variety of unconsolidated, coarse, dense components (Fig. 44).
primary volcaniclastic and volcanogenic sedimentary
deposits that are initially temporarily deposited in Significance
shallow subaqueous shelf and delta settings. In this
case, they are generated by slumping of unconsolidated Turbidites, in general, are diagnostic of subaqueous,
deposits, triggered by earthquakes, rapid loading or rapid below-wave-base depositional settings, and provide a
changes in pore fluid pressure. They are also fed directly, very valuable constraint on interpretations of ancient
from subaerial settings by syn-eruptive transformation of volcanic sequences. However, they do not independently
pyroclastic flows, volcanic debris avalanches, discriminate lacustrine from submarine settings and
volcaniclastic debris flows and lahars that transgress cannot be used alone to give precise water depths. The
shorelines, and from explosive eruptions at submerged components in volcaniclastic turbidites give
vents. In some cases, such syn-eruptive deposits are information on the character, composition, and setting
overlain by thinner bedded volcaniclastic turbidite of the source volcanic terrane, and whether it was
sequences characterized by upward thinning and fining active or inactive (33). Shapes of volcanic clasts in syn-
bed thickness/grain size profiles (Bull and Cas, 1991). eruptive volcaniclastic turbidites strongly reflect
These subaqueous sequences develop in response to the original fragmentation processes. In post-eruptive deposits,
inundation of a subaerial fluvial or deltaic system during clast shape also records the history of reworking and
a pyroclastic eruption. They record the gradual post- transport during temporary storage in subaerial or
eruptive readjustment of the sedimentary transport shallow-water settings, prior to final deposition below
and depositional processes by means of mass-flow wave base.
resedimentation.

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