344
8 ALLOCHTHONOUS SEDIMENTS
In concluding this review of clay minerals, the following points should be noted. The
clay mineral suite that is found in a particular rock at a particular time is a result of four
main variables. The nature of the source rock controls the input of clay minerals. There
is a higher probability of kaolinite and illite forming from a granite source than from a
volcanic hinterland. Similarly, the more intense the weathering the higher the probability of kaolinite occurring at the expense of illite. Size-sorting during transportation
may also segregate the various clay mineral species. Kaolinite crystals tend to be larger
than illites and illite to be larger than montmorillonite. This fact is complicated, however, by the tendency of clay particles to form floccules when they are carried from
acidic freshwater to neutral seawater. Simultaneously, various transformations of clay
minerals take place within seawater both during transportation and early burial (Keller, 1970).
Thus while most studies of modern marine coasts show some regular zonation of clay
minerals, it is hard to demonstrate whether this results from differential transportation
or incipient diagenesis (e.g., Porrenga, 1966). More exhaustive accounts of clays and
clay minerals will be found in Muller (1967), Grim (1968), Millot (1970), Mortland and
Farmer (1978), Potter et al. (1980), Shaw (1980), and Chamley (1989).
8.4 PYROCLASTIC SEDIMENTS
Figure 8.3 showed how the allochthonous sediments are divisible into the claystones
(defined by their grain size and clay mineral composition), the siliciclastic sands of terrigenous silica minerals, and the volcaniclastic sediments. The volcaniclastic sediments
are relatively rare by volume in the earth's crust, but deserve mention for the sake of
completeness. Many minerals of volcanic rock are unstable at surface temperatures and
pressures. For this reason, therefore, detritus derived from volcanic activity is commonly
only preserved interbedded with lava flows and can seldom survive detrital transportation far from the volcanic center from which it came (Orton, 1996). The volcaniclastic
sediments can be classified into three groups according to their particle size.
Agglomerates are the counterpart to conglomerates. They are formed both by explosive eruptions and by scree movement of volcanic detritus both within a caldera and on
the flanks of volcanoes. Sand-grade volcaniclastic sediment is of two types. Erosional
volcaniclastic sands are produced by normal subaerial or subaqueous processes acting
on eruptive rocks. The pyroclastic sediments in contrast are ejected into the atmosphere
during volcanic eruptions. Pyroclasts include, therefore, "bombs" which fall close to the
vent, sands which fall around the vent for a distance of kilometers, and dust which may
be carried into the upper atmosphere and transported around the world. In many cases
it is impossible to distinguish whether an ancient volcaniclastic sediment was produced
by normal erosion of lavas or by pyroclastic action. Large clasts cannot normally be
transported far from their source. Pumice provides an exception to this because it floats.
As illustrated by the frontispiece of this book, pumice can be transported across whole
oceans.
Volcaniclastic sands are generally referred to as tufts or ashes. They may be subaerial
or subaqueous. Volcaniclastic sands are composed essentially of crystals, glass, and rock
fragments (Plate 3B). The crystals are of minerals associated with the eruption, such as
olivine and quartz. Glass occurs both as globules and angular irregularly shaped shards.
8 ALLOCHTHONOUS SEDIMENTS
In concluding this review of clay minerals, the following points should be noted. The
clay mineral suite that is found in a particular rock at a particular time is a result of four
main variables. The nature of the source rock controls the input of clay minerals. There
is a higher probability of kaolinite and illite forming from a granite source than from a
volcanic hinterland. Similarly, the more intense the weathering the higher the probability of kaolinite occurring at the expense of illite. Size-sorting during transportation
may also segregate the various clay mineral species. Kaolinite crystals tend to be larger
than illites and illite to be larger than montmorillonite. This fact is complicated, however, by the tendency of clay particles to form floccules when they are carried from
acidic freshwater to neutral seawater. Simultaneously, various transformations of clay
minerals take place within seawater both during transportation and early burial (Keller, 1970).
Thus while most studies of modern marine coasts show some regular zonation of clay
minerals, it is hard to demonstrate whether this results from differential transportation
or incipient diagenesis (e.g., Porrenga, 1966). More exhaustive accounts of clays and
clay minerals will be found in Muller (1967), Grim (1968), Millot (1970), Mortland and
Farmer (1978), Potter et al. (1980), Shaw (1980), and Chamley (1989).
8.4 PYROCLASTIC SEDIMENTS
Figure 8.3 showed how the allochthonous sediments are divisible into the claystones
(defined by their grain size and clay mineral composition), the siliciclastic sands of terrigenous silica minerals, and the volcaniclastic sediments. The volcaniclastic sediments
are relatively rare by volume in the earth's crust, but deserve mention for the sake of
completeness. Many minerals of volcanic rock are unstable at surface temperatures and
pressures. For this reason, therefore, detritus derived from volcanic activity is commonly
only preserved interbedded with lava flows and can seldom survive detrital transportation far from the volcanic center from which it came (Orton, 1996). The volcaniclastic
sediments can be classified into three groups according to their particle size.
Agglomerates are the counterpart to conglomerates. They are formed both by explosive eruptions and by scree movement of volcanic detritus both within a caldera and on
the flanks of volcanoes. Sand-grade volcaniclastic sediment is of two types. Erosional
volcaniclastic sands are produced by normal subaerial or subaqueous processes acting
on eruptive rocks. The pyroclastic sediments in contrast are ejected into the atmosphere
during volcanic eruptions. Pyroclasts include, therefore, "bombs" which fall close to the
vent, sands which fall around the vent for a distance of kilometers, and dust which may
be carried into the upper atmosphere and transported around the world. In many cases
it is impossible to distinguish whether an ancient volcaniclastic sediment was produced
by normal erosion of lavas or by pyroclastic action. Large clasts cannot normally be
transported far from their source. Pumice provides an exception to this because it floats.
As illustrated by the frontispiece of this book, pumice can be transported across whole
oceans.
Volcaniclastic sands are generally referred to as tufts or ashes. They may be subaerial
or subaqueous. Volcaniclastic sands are composed essentially of crystals, glass, and rock
fragments (Plate 3B). The crystals are of minerals associated with the eruption, such as
olivine and quartz. Glass occurs both as globules and angular irregularly shaped shards.