Page 132 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

subaerial composite volcano (Davies et al., 1978; Hummocky cross stratification is a characteristic
Kuenzi et al., 1979; Vessell and Davies, 1981). sedimentary structure displayed by storm-affected shelf
However, few studies have considered facies models sandstone deposits. As for lakes, pyroclastic flows and
specifically catering for the enormous and abrupt surges that travel across and come to rest on the sea
increases in particle supply, and the abundance of large, deliver pyroclasts directly to offshore shelf
low-density particles that are likely to accompany and sedimentation systems.
follow intermediate and large-magnitude explosive
silicic eruptions (Smith, 1991). Deep ocean settings (Fig. 59D) — Sedimentation is
dominated by suspension, flotation and mass flow
There are diverse sedimentary environments around processes. Volcaniclastic particles initially generated
active subaerial and subaqueous volcanoes that serve as and deposited near subaerial, shoaling and shallowly
permanent or temporary repositories of volcaniclastic submerged active volcanoes can ultimately reach deep
particles. The different environments are characterized ocean settings as a result of long-distance
by broad differences in the kinds of sedimentary resedimentation by a variety of water-supported mass
processes that operate. For example, traction transport flows, or transport in suspension. Pyroclasts delivered
and deposition dominate in most continental and directly to open ocean settings by primary volcanic
shallow subaqueous settings, whereas mass flows and transport processes are transported by surface currents
suspension dominate in below-wave-base, deep before finally settling from suspension or flotation.
subaqueous settings. These contrasts have an effect on
the volcanogenic sedimentary facies associations and
geometry (Fig. 59). In addition to particles delivered by
resedimentation and other sedimentary transport
processes, all these settings may receive pyroclasts
contributed directly by fallout from eruption columns
and widely wind-dispersed ash clouds. Subaerial and
shallow subaqueous settings in proximity to explosive
volcanoes can also include primary pyroclastic flow
deposits. The main sedimentary transport and
depositional processes in each of five such
environments are listed below.

Lakes (especially caldera lakes) (Fig. 59D) —
Volcaniclastic particles are transported to lakes by
rivers, in suspension or traction, by lahars and by
subaerial mass flows. Offshore deposition takes place
from suspension, turbidity currents and other types of
subaqueous mass flows. Pyroclastic flows and surges
may come to rest on the surfaces of lakes. Their
deposits will comprise some components that begin to
sink immediately (lithic fragments, hot pumice or
scoria, crystals) and others that settle from suspension
or flotation only very slowly (cold pumice or scoria,
ash). Lakes in calderas and near active cones also
receive slides and volcanic debris avalanches.

Alluvial environments (Fig. 59A) ─ Transport and
deposition of volcaniclastic particles in alluvial fans and
braidplains are dominated by fluvial traction currents,
hyperconcentrated flows and sheet floods, and subaerial
mass flows, especially cohesive debris flows and mud
flows.

Shoreline environments (Fig. 59B) — This setting is of
special importance for volcanoes near the coast and for
island volcanoes. Deposition can occur in deltas, barrier
island and lagoonal systems, and in near-offshore
settings. Sedimentation from traction currents and
suspension are dominant.

Offshore shelf environments (Fig. 59C) —
Sedimentation takes place from tidal, storm and ocean
currents in near-shore settings, and from suspension and
water-supported mass flows farther offshore.

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