2.4 Volcaniclastic Sediments
Base surge deposits are commonly thin and irregular. They mantle to some extent the ground surface,
but tend to reach greater thicknesses in topographie
depressions (Fig. 2.25a). They predominantly consist
of poorly sorted sand to gravel-size partieles of various composition; juvenile elasts often make up only a
small proportion of the total sediment, while older
tephra and lithoelasts constitute the major part. The
sedimentary structures of base surge deposits at any
given locality are unidirectional. They inelude wavy
planar lamination, low-angle cross bedding, elimbing
dunes and antidunes, and chute-and-pool structures.
Characteristic features are low-angle truncations and
steeply inelined laminae draping obstaeles on the
stoss side (Fig. 2.25b and c). The occurrence of base
surge deposits is mostly limited to a few kilometers
from the source, after which they grade into laminated fallout layers.
The areal distribution of continental tephra deposits
follows some simple rules. Proximal tephra deposits
near the source tend to fill valleys on the slopes and
form coarse-grained alluvial fans at the base of the
volcanic structures (cf. Fig. 2.27b). At greater distances from the source (intermediate source, medial),
predominantly gravel- and sand-size volcanielastics
alternate with the deposits of braided streams, and/or
they are mixed with these fluvial beds. At distal
depositional sites, far from the source, sand- and siltsized volcanielastics are increasingly incorporated
into the deposits of fluvial plains or swept into lakes.
Finally, volcanielastic material can reach a marine
delta and the coast. In general, primary volcanielastic
beds are more dispersed over the landscape than the
linearly accumulating, purely fluvial sediments. Debris flows, sheet floods, and so-called
hyperconcentrated flows with a relatively high proportion of suspended material tend to generate broad
sheets of sand- and gravel-sized pyroelasts extending
laterally beyond the range of normal fluvial channels.
Airfall ash commonly covers even larger areas, but it
is easily redeposited by fluvial activity. If ash accumulates on isolated plateaus which receive no other
sediments and undergo little erosion, airfall deposits
may build up sequences of some thickness.
Figure 2.26 illustrates proximal to distal facies changes in a
Miocene fluvial to lacustrine environment in Washington
which was episodically affected by volcanic eruptions
(Smith 1988). In a time span of 5 Ma, a 350 m thick,
dacitic volcaniclasticlfluvial sequence accumulated which
Fig. 2.24. a Plinian volcanic eruption with gas thrust
and wind-driven convective plume. Tephra deposits
inelude ballistic bombs and blocks, lapilli, and volcanie ash. Primarily, ashfall deposits uniformly drape
the landscape (mantle bedding) and form distinct layers in lakes, but they are largely reworked and redistributed in coastal areas and shallow seas.
67
can be traced as far as 120 km to the east of the volcanic
centers. In the proximal depositional regime, volcaniclastic
sediments predominate, including debris flows, bedded
conglomerates produced by sheet floods, and crudely stratified, normally graded deposits from hyperconcentrated
flows. In addition, scour-and-fill bedded sandstones with
pebble lenses indicate transport conditions related to volcanic processes. These sedimentary structures are
characterized in transverse section by broad, low-angle
onlapping stratification and are thought to result from sheet
floods. All of these volcaniclastic-dominated facies types
pinch out at distances between 40 and 80 km from the
source (Fig. 2.26a), whereas thin volcanic ash layers remain unaffected.
In contrast, the siliciclastic-dominated beds, composed
mainly of material from various older rocks, display little
change with increasing distance from the paleovolcanoes,
but their proportion in the total basin fill increases distally.
they consist of clast-supported conglomerates, trough and
planar bedded channel sandstones, overbank sands tones,
and mudstones. Their fine-grained material is mostly altered and reworked volcaniclastics.
The interrelationship between extensive paleosols, volcanic ash layers (tuffs), and deposits from sheet floods and
debris flows, on the one hand, and laterally restricted fills
of concurrently incised channels, on the other hand, is
shown in Fig. 2.26c. Channels and soils formed in the relatively long inter-eruption periods, while rapidly deposited
syn-eruption sediments covered larger areas and partially
modified the paleo-drainage pattern.
2.4.3 Marine Tephra Deposits
Marine volcanielastic sediments are widespread and
have a high preservation potential in the rock record.
They may be derived either from sources on land or
from submarine volcanic activity. For those derived
from volcanic eruptions on land, generation and
mode of deposition was already mentioned in the
previous section. In coastal areas and in shallow seas,
thin fallout beds are commonly reworked and mixed
with normal marine sediments. In addition, intensive
bioturbation frequently masks the occurrence of thin
fallout layers in subaqueous environments. Mixed
shallow-water sediments and thicker accumulations
of tephra on the shelf may be transported by turbidity
currents into deeper water and form ash turbidites
which sometimes show "double grading" (Fig.
2.24b).
The nature and products of volcanic activity under
the sea are less known than those on land and must
be interpreted from their results rather than from dib Collapsing eruption column leads to pyroelastic
flow and the formation of ignimbrites and coignimbrite ash elouds with widespread fallout. c Idealized sections of pyroelastic flow deposits. (After
Sheridan 1979; Cas and Wright 1987; Schmincke
and Bogaard 1991). See text for further explanation
Base surge deposits are commonly thin and irregular. They mantle to some extent the ground surface,
but tend to reach greater thicknesses in topographie
depressions (Fig. 2.25a). They predominantly consist
of poorly sorted sand to gravel-size partieles of various composition; juvenile elasts often make up only a
small proportion of the total sediment, while older
tephra and lithoelasts constitute the major part. The
sedimentary structures of base surge deposits at any
given locality are unidirectional. They inelude wavy
planar lamination, low-angle cross bedding, elimbing
dunes and antidunes, and chute-and-pool structures.
Characteristic features are low-angle truncations and
steeply inelined laminae draping obstaeles on the
stoss side (Fig. 2.25b and c). The occurrence of base
surge deposits is mostly limited to a few kilometers
from the source, after which they grade into laminated fallout layers.
The areal distribution of continental tephra deposits
follows some simple rules. Proximal tephra deposits
near the source tend to fill valleys on the slopes and
form coarse-grained alluvial fans at the base of the
volcanic structures (cf. Fig. 2.27b). At greater distances from the source (intermediate source, medial),
predominantly gravel- and sand-size volcanielastics
alternate with the deposits of braided streams, and/or
they are mixed with these fluvial beds. At distal
depositional sites, far from the source, sand- and siltsized volcanielastics are increasingly incorporated
into the deposits of fluvial plains or swept into lakes.
Finally, volcanielastic material can reach a marine
delta and the coast. In general, primary volcanielastic
beds are more dispersed over the landscape than the
linearly accumulating, purely fluvial sediments. Debris flows, sheet floods, and so-called
hyperconcentrated flows with a relatively high proportion of suspended material tend to generate broad
sheets of sand- and gravel-sized pyroelasts extending
laterally beyond the range of normal fluvial channels.
Airfall ash commonly covers even larger areas, but it
is easily redeposited by fluvial activity. If ash accumulates on isolated plateaus which receive no other
sediments and undergo little erosion, airfall deposits
may build up sequences of some thickness.
Figure 2.26 illustrates proximal to distal facies changes in a
Miocene fluvial to lacustrine environment in Washington
which was episodically affected by volcanic eruptions
(Smith 1988). In a time span of 5 Ma, a 350 m thick,
dacitic volcaniclasticlfluvial sequence accumulated which
Fig. 2.24. a Plinian volcanic eruption with gas thrust
and wind-driven convective plume. Tephra deposits
inelude ballistic bombs and blocks, lapilli, and volcanie ash. Primarily, ashfall deposits uniformly drape
the landscape (mantle bedding) and form distinct layers in lakes, but they are largely reworked and redistributed in coastal areas and shallow seas.
67
can be traced as far as 120 km to the east of the volcanic
centers. In the proximal depositional regime, volcaniclastic
sediments predominate, including debris flows, bedded
conglomerates produced by sheet floods, and crudely stratified, normally graded deposits from hyperconcentrated
flows. In addition, scour-and-fill bedded sandstones with
pebble lenses indicate transport conditions related to volcanic processes. These sedimentary structures are
characterized in transverse section by broad, low-angle
onlapping stratification and are thought to result from sheet
floods. All of these volcaniclastic-dominated facies types
pinch out at distances between 40 and 80 km from the
source (Fig. 2.26a), whereas thin volcanic ash layers remain unaffected.
In contrast, the siliciclastic-dominated beds, composed
mainly of material from various older rocks, display little
change with increasing distance from the paleovolcanoes,
but their proportion in the total basin fill increases distally.
they consist of clast-supported conglomerates, trough and
planar bedded channel sandstones, overbank sands tones,
and mudstones. Their fine-grained material is mostly altered and reworked volcaniclastics.
The interrelationship between extensive paleosols, volcanic ash layers (tuffs), and deposits from sheet floods and
debris flows, on the one hand, and laterally restricted fills
of concurrently incised channels, on the other hand, is
shown in Fig. 2.26c. Channels and soils formed in the relatively long inter-eruption periods, while rapidly deposited
syn-eruption sediments covered larger areas and partially
modified the paleo-drainage pattern.
2.4.3 Marine Tephra Deposits
Marine volcanielastic sediments are widespread and
have a high preservation potential in the rock record.
They may be derived either from sources on land or
from submarine volcanic activity. For those derived
from volcanic eruptions on land, generation and
mode of deposition was already mentioned in the
previous section. In coastal areas and in shallow seas,
thin fallout beds are commonly reworked and mixed
with normal marine sediments. In addition, intensive
bioturbation frequently masks the occurrence of thin
fallout layers in subaqueous environments. Mixed
shallow-water sediments and thicker accumulations
of tephra on the shelf may be transported by turbidity
currents into deeper water and form ash turbidites
which sometimes show "double grading" (Fig.
2.24b).
The nature and products of volcanic activity under
the sea are less known than those on land and must
be interpreted from their results rather than from dib Collapsing eruption column leads to pyroelastic
flow and the formation of ignimbrites and coignimbrite ash elouds with widespread fallout. c Idealized sections of pyroelastic flow deposits. (After
Sheridan 1979; Cas and Wright 1987; Schmincke
and Bogaard 1991). See text for further explanation