70
reet observation. A further problem is the differentiation of primary from redistributed (epic1astic)
volcanigenic products.
The model in Fig. 2.25d is an attempt to describe
two characteristic stages of a submarine eruption.
During the most active phase of the volcano, large
volumes of tephra are ejected high into the overlying
water body. Some of the pyroc1astic material settles
back to the sea floor to form a subaqueous
pyroc1astic flow. Other proportions, such as highly
vesicular pumice, may rise to the water surface and
float until they come to rest along a coast. Finegrained ash can remain in suspension for considerable time and thus be widely dispersed by ocean currents. When the volcanic activity slows down, the
ejected material cannot maintain a steady pyroc1astic
flow. As a result, it accumulates near the volcanic
vent until the slope of the stored tephra fails. The
sliding mass frequently evolves into a debris flow or
turbidity current, by which the volcanic material is
intermittently transported into deeper water and deposited as debris flow deposits or ash turbidites. Towards the end of the eruptive phase, the turbidity currents become less frequent and carry finer ash. In
addition, the pelagic settling intervals may contain
some fine-grained ash which was distributed by normal oceanic currents.
Divers have directly observed submarine lava flows fonning pillow lava. However, it is not clear whether submarine
eruptions can generate hot pyroclastic flows and welded
tephra (ignimbrites). Welding in pyroclastic flow can occur
on island volcanoes and may continue when the flow enters
the sea.
Deep-sea ash layers commonly exhibit a sharp base and
a more gradational top. They are moderately affected by
bioturbation. They originate from both subaerial and submarine volcanic eruptions and occur several hundreds of
kilometers away from the source areas. Wind-driven ash
fallen on the sea surface may additionally be transported by
ocean currents. Bottom currents rework and redistribute
ash accumulated on the sea floor. Ash settled on top of
submarine highs can be carried by gravity mass movements
into deeper water.
Due to the variety of transport mechanisms (Fig.
2.27c), the areal distribution and thicknesses of marine ash layers can be very irregular. Around volcanie islands and seamounts, thick and widely extended
volcanic aprons are built up which consist of lava
flows, breccias, pyroc1astic flows, hyaloc1astites, and
debris flows. At greater distances from the source,
debris flows, mud flows, and ash turbidites may accumulate to great thicknesses and alternate with pelagic marine sediments. Submarine plateaus receive
airfall ash and water-suspended fine-grained ash distributed by surface currents. Bottom currents can redis tribute the tephra layers in various marine environments inc1uding the deep sea (cf. Chap. 5).
Chapter 2 Continental Sediments
2.4.4 V olumes and Transport Distances
ofTephra
Estimates on single large volcanic eruptions, which
occurred in historie, prehistoric, and Quaternary
times, yield volumes of ejected material ranging from
10 to some 1000 km 3 (Mount St. Helens 1980: 1-2
km 3 ). Their volcanic ash was disperses more than
1000 km from the source.
The approximate range of primary transport distances for various tephra is indicated in Fig. 2.27a.
Although these values greatly vary in relation to the
size of the volcano, the magnitude of eruption, the
type of magma, the relief of the land surface or sea
bottom around the eruption center, and other factors,
they c1early demonstrate the high potential of volcanie processes for contributing to the filling of sedimentary basins. In addition to primary transport,
loose volcanic1astic material is frequently reworked
and redeposited by fluvial, eolian, and marine proces ses and can thus principally be transported over
unlimited distances. However, much of this redistributed, fine-grained and mostly altered volcanic material is mixed with "normal" siliciclastic sediments
and remains unidentified.
Volcaniclastic sediments are easily recognized in the field
only up to a certain limit. This limit is set, for example, by
a minimum thickness of an ash layer or the occurrence of
characteristic primary sedimentary structures (cf. Fig.
2.25). The proportion of reworked pyroclastic material
should reach a certain amount to be seen readily in various
types of sandstones and conglomerates, debris flows and
turbidites. The examples in Figs. 2.26 and 2.27b,c demonstrate how certain characteristics of continental tephra deposits change with increasing distance from the source.
Most ofthe typical volcaniclastic beds pinch out after some
distance and are replaced by nonnal fluvial sandstones and
mudstones with decreasing proportions of reworked
pyroclastic material (see also below).
2.4.5 V olcaniclastic Sediments in Various Basin
Settings
The magma type erupting during volcanic activity
depends on the tectonic basin setting and may change
during basin evolution. Thus, the petrofacies of
tephra-bearing coarse c1astics and sandstones (based
on the occurrence of special mineral phases and their
ratios), can be used as a tool in recognizing the types
of volcanic1astics and their change with time. This
also affects the chemical characteristics of concurrently produced volcanic1astic and mixed volcaniclastic-c1astic sediments and may permit a stratigraphic correlation of rock types which vary in other
characteristics.
In this Section, the relationship between the tectonic basin setting and the contribution of primary
reet observation. A further problem is the differentiation of primary from redistributed (epic1astic)
volcanigenic products.
The model in Fig. 2.25d is an attempt to describe
two characteristic stages of a submarine eruption.
During the most active phase of the volcano, large
volumes of tephra are ejected high into the overlying
water body. Some of the pyroc1astic material settles
back to the sea floor to form a subaqueous
pyroc1astic flow. Other proportions, such as highly
vesicular pumice, may rise to the water surface and
float until they come to rest along a coast. Finegrained ash can remain in suspension for considerable time and thus be widely dispersed by ocean currents. When the volcanic activity slows down, the
ejected material cannot maintain a steady pyroc1astic
flow. As a result, it accumulates near the volcanic
vent until the slope of the stored tephra fails. The
sliding mass frequently evolves into a debris flow or
turbidity current, by which the volcanic material is
intermittently transported into deeper water and deposited as debris flow deposits or ash turbidites. Towards the end of the eruptive phase, the turbidity currents become less frequent and carry finer ash. In
addition, the pelagic settling intervals may contain
some fine-grained ash which was distributed by normal oceanic currents.
Divers have directly observed submarine lava flows fonning pillow lava. However, it is not clear whether submarine
eruptions can generate hot pyroclastic flows and welded
tephra (ignimbrites). Welding in pyroclastic flow can occur
on island volcanoes and may continue when the flow enters
the sea.
Deep-sea ash layers commonly exhibit a sharp base and
a more gradational top. They are moderately affected by
bioturbation. They originate from both subaerial and submarine volcanic eruptions and occur several hundreds of
kilometers away from the source areas. Wind-driven ash
fallen on the sea surface may additionally be transported by
ocean currents. Bottom currents rework and redistribute
ash accumulated on the sea floor. Ash settled on top of
submarine highs can be carried by gravity mass movements
into deeper water.
Due to the variety of transport mechanisms (Fig.
2.27c), the areal distribution and thicknesses of marine ash layers can be very irregular. Around volcanie islands and seamounts, thick and widely extended
volcanic aprons are built up which consist of lava
flows, breccias, pyroc1astic flows, hyaloc1astites, and
debris flows. At greater distances from the source,
debris flows, mud flows, and ash turbidites may accumulate to great thicknesses and alternate with pelagic marine sediments. Submarine plateaus receive
airfall ash and water-suspended fine-grained ash distributed by surface currents. Bottom currents can redis tribute the tephra layers in various marine environments inc1uding the deep sea (cf. Chap. 5).
Chapter 2 Continental Sediments
2.4.4 V olumes and Transport Distances
ofTephra
Estimates on single large volcanic eruptions, which
occurred in historie, prehistoric, and Quaternary
times, yield volumes of ejected material ranging from
10 to some 1000 km 3 (Mount St. Helens 1980: 1-2
km 3 ). Their volcanic ash was disperses more than
1000 km from the source.
The approximate range of primary transport distances for various tephra is indicated in Fig. 2.27a.
Although these values greatly vary in relation to the
size of the volcano, the magnitude of eruption, the
type of magma, the relief of the land surface or sea
bottom around the eruption center, and other factors,
they c1early demonstrate the high potential of volcanie processes for contributing to the filling of sedimentary basins. In addition to primary transport,
loose volcanic1astic material is frequently reworked
and redeposited by fluvial, eolian, and marine proces ses and can thus principally be transported over
unlimited distances. However, much of this redistributed, fine-grained and mostly altered volcanic material is mixed with "normal" siliciclastic sediments
and remains unidentified.
Volcaniclastic sediments are easily recognized in the field
only up to a certain limit. This limit is set, for example, by
a minimum thickness of an ash layer or the occurrence of
characteristic primary sedimentary structures (cf. Fig.
2.25). The proportion of reworked pyroclastic material
should reach a certain amount to be seen readily in various
types of sandstones and conglomerates, debris flows and
turbidites. The examples in Figs. 2.26 and 2.27b,c demonstrate how certain characteristics of continental tephra deposits change with increasing distance from the source.
Most ofthe typical volcaniclastic beds pinch out after some
distance and are replaced by nonnal fluvial sandstones and
mudstones with decreasing proportions of reworked
pyroclastic material (see also below).
2.4.5 V olcaniclastic Sediments in Various Basin
Settings
The magma type erupting during volcanic activity
depends on the tectonic basin setting and may change
during basin evolution. Thus, the petrofacies of
tephra-bearing coarse c1astics and sandstones (based
on the occurrence of special mineral phases and their
ratios), can be used as a tool in recognizing the types
of volcanic1astics and their change with time. This
also affects the chemical characteristics of concurrently produced volcanic1astic and mixed volcaniclastic-c1astic sediments and may permit a stratigraphic correlation of rock types which vary in other
characteristics.
In this Section, the relationship between the tectonic basin setting and the contribution of primary