198
These requirements are often met in deep seas far
away from land masses. However, siliceous beds can
also form in shallow waters.
Areal Distribution of Biosiliceous Sediments
In the modem oceans biosiliceous sediments mainly
occur at high latitudes far away from the coasts.
Here, diatomaceous oozes are deposited in two wide,
circumpolar belts, particularly around the Antarctic
and in the northern Pacific. Sediments very rich in
diatoms and poor in carbonate are also found in several regions of present-day coastal upwelling, for
example off southwest Africa, Peru, Baja California,
and in the Gulf of California. During the Miocene,
great parts of the coastal waters around the Pacific
(e.g., along North America) were characterized by
rapid deposition of diatomaceous oozes (known as
Monterey Formation).
Another, less weIl developed belt displaying
biosiliceous sediments is observed in zones of equatorial upwelling. Here, the sediments are dominated
by radiolarians (see above) and contain variable
amounts of biogenic carbonate. At the lateral transition to red clay below the CCD, carbonate is dissolved. This leads to a relative enrichment of
biogenic silica.
The widespread radiolarites in the Triassic-Liassic and
Middle und Upper Jurassic Tethyan Ocean probably were
also associated with an equatorial zone of high fertility
(Jenkyns and Winterer 1982; Hein and Parrish 1987; Hein
and Obradovic 1989). In addition, the CCD must have
been very high, because radiolarites were found not only in
deep basins, but also on subsiding carbonate platforms and
outer shelves. A simplified model of such a scenario is
shown in Fig. 5.4f. Since prolific planktonic carbonate
production, tending to raise the CCD, is not known prior to
the Late Jurassic, the high CCD was probably caused by
large, widely extended carbonate platforms bordering the
northem and southem rim of this east-west-oriented seaway. They extracted and fixed most of the carbonate available in the Tethyan Ocean.
In zones of coastal upwelling with bottom waters low
in oxygen, Neogene and Recent siliceous oozes are
frequently laminated and rich in organic matter. They
form thick, dark layers and may contain intercalations of calcareous beds, carbonate nodules and
phosphorite (see below). Siliceous oozes deposited in
deeper water, for example on mid-oceanic ridges and
in ponded basins on oceanic crust, tend to become
fully oxidized and therefore red-colored (Fig. 5.5).
Bioturbation in such layers is sparse or absent, because there is not enough organic matter left to live
on.
Chapter 5 Oceanic Sediments
Silica Diagenesis and Bedded Cherts
Silica Diagenesis. Because of the relatively good solubility of opaline silica, which is enhanced with increasing temperature at growing burial depths,
biosiliceous sediments undergo marked diagenetic
modifications. Near the sea floor and up to several
hundred meters subbottom, siliceous oozes normally
remain more or less unchanged and maintain a very
high porosity (75 to 90%). Below this depth range,
which depends significantly on the temperature gradient and other factors, the original skeletal material
(opal-A) is dissolved and reprecipitated as
cristobalite and tridymite (opal-CT). Sediments rich
in this phase are still porous and are referred to as
porcellanite. Later, with increasing temperature and
burial depth, these Sial modifications are in turn
transformed into quartz (chalcedony). Coeval with
these mineral transformations, the porosity of the
siliceous sediments is drastically reduced in two
steps (Fig. 5.6a, cf. Chap. 13).
First the intratest porosity of the microfossils is diminished,
and then the normal pore space between the grains is filled
with quartz cement provided by dissolution of opaline si 1ica. The first skeletons to be destroyed are the delicate tests
of diatoms, which are followed by the more robust radiolarians and sponge needles. These processes have been
described by many authors in detail (e.g., Lancelot 1973;
Calvert 1974; Kastner 1981; Pisciotto 1981; Isaacs et al
1983; Thein and von Rad 1987; Füchtbauer 1988; Hein
and Obradovic 1989; Ruiz-Ortiz et al. 1989; Tada 1991).
Layers particularly rich in primary Si0 2 andlor layers
more porous than others may be the loci of preferential
reprecipitation of quartz. Thus, they are transformed into
chert beds of secondarily enhanced SiO 2 concentration
(Fig. 5.6a). Migration of Si0 2 and growth of chert bands
and nodules cease when the sediments become impermeable as a result of cementation.
In Mesozoic-Cenozoic clayey siliceous oozes drilled
below the present sea floor, the transformation of opal-A
into opal-CT and quartz took place about 50 Ma after their
deposition. This is valid for average values of sedimentation rates and thermal gradients (Thein and von Rad 1987).
However, the silica in calcareous oozes, diatomaceous
oozes, and oozes associated with reactive volcanic material
can be transformed into porcellanite and quartz after
shorter time periods, particularly if the thermal gradient is
high.
Part of the silica released from microfossils mayaIso
be utilized to form clay minerals (zeolites) if sufficient AI l 0 3 , alkali and alkaline earth ions are available in the pore water. Thus the formation of chert
bands or nodules (see below) is hindered or completely prevented if the opal content in the primary
sediment was limited. In the case of silica-poor calcareous oozes which cannot provide enough additional ions for the generation of zeolites, the silica is
diagenetically concentrated in chert nodules (Fig.
5.6b).
These requirements are often met in deep seas far
away from land masses. However, siliceous beds can
also form in shallow waters.
Areal Distribution of Biosiliceous Sediments
In the modem oceans biosiliceous sediments mainly
occur at high latitudes far away from the coasts.
Here, diatomaceous oozes are deposited in two wide,
circumpolar belts, particularly around the Antarctic
and in the northern Pacific. Sediments very rich in
diatoms and poor in carbonate are also found in several regions of present-day coastal upwelling, for
example off southwest Africa, Peru, Baja California,
and in the Gulf of California. During the Miocene,
great parts of the coastal waters around the Pacific
(e.g., along North America) were characterized by
rapid deposition of diatomaceous oozes (known as
Monterey Formation).
Another, less weIl developed belt displaying
biosiliceous sediments is observed in zones of equatorial upwelling. Here, the sediments are dominated
by radiolarians (see above) and contain variable
amounts of biogenic carbonate. At the lateral transition to red clay below the CCD, carbonate is dissolved. This leads to a relative enrichment of
biogenic silica.
The widespread radiolarites in the Triassic-Liassic and
Middle und Upper Jurassic Tethyan Ocean probably were
also associated with an equatorial zone of high fertility
(Jenkyns and Winterer 1982; Hein and Parrish 1987; Hein
and Obradovic 1989). In addition, the CCD must have
been very high, because radiolarites were found not only in
deep basins, but also on subsiding carbonate platforms and
outer shelves. A simplified model of such a scenario is
shown in Fig. 5.4f. Since prolific planktonic carbonate
production, tending to raise the CCD, is not known prior to
the Late Jurassic, the high CCD was probably caused by
large, widely extended carbonate platforms bordering the
northem and southem rim of this east-west-oriented seaway. They extracted and fixed most of the carbonate available in the Tethyan Ocean.
In zones of coastal upwelling with bottom waters low
in oxygen, Neogene and Recent siliceous oozes are
frequently laminated and rich in organic matter. They
form thick, dark layers and may contain intercalations of calcareous beds, carbonate nodules and
phosphorite (see below). Siliceous oozes deposited in
deeper water, for example on mid-oceanic ridges and
in ponded basins on oceanic crust, tend to become
fully oxidized and therefore red-colored (Fig. 5.5).
Bioturbation in such layers is sparse or absent, because there is not enough organic matter left to live
on.
Chapter 5 Oceanic Sediments
Silica Diagenesis and Bedded Cherts
Silica Diagenesis. Because of the relatively good solubility of opaline silica, which is enhanced with increasing temperature at growing burial depths,
biosiliceous sediments undergo marked diagenetic
modifications. Near the sea floor and up to several
hundred meters subbottom, siliceous oozes normally
remain more or less unchanged and maintain a very
high porosity (75 to 90%). Below this depth range,
which depends significantly on the temperature gradient and other factors, the original skeletal material
(opal-A) is dissolved and reprecipitated as
cristobalite and tridymite (opal-CT). Sediments rich
in this phase are still porous and are referred to as
porcellanite. Later, with increasing temperature and
burial depth, these Sial modifications are in turn
transformed into quartz (chalcedony). Coeval with
these mineral transformations, the porosity of the
siliceous sediments is drastically reduced in two
steps (Fig. 5.6a, cf. Chap. 13).
First the intratest porosity of the microfossils is diminished,
and then the normal pore space between the grains is filled
with quartz cement provided by dissolution of opaline si 1ica. The first skeletons to be destroyed are the delicate tests
of diatoms, which are followed by the more robust radiolarians and sponge needles. These processes have been
described by many authors in detail (e.g., Lancelot 1973;
Calvert 1974; Kastner 1981; Pisciotto 1981; Isaacs et al
1983; Thein and von Rad 1987; Füchtbauer 1988; Hein
and Obradovic 1989; Ruiz-Ortiz et al. 1989; Tada 1991).
Layers particularly rich in primary Si0 2 andlor layers
more porous than others may be the loci of preferential
reprecipitation of quartz. Thus, they are transformed into
chert beds of secondarily enhanced SiO 2 concentration
(Fig. 5.6a). Migration of Si0 2 and growth of chert bands
and nodules cease when the sediments become impermeable as a result of cementation.
In Mesozoic-Cenozoic clayey siliceous oozes drilled
below the present sea floor, the transformation of opal-A
into opal-CT and quartz took place about 50 Ma after their
deposition. This is valid for average values of sedimentation rates and thermal gradients (Thein and von Rad 1987).
However, the silica in calcareous oozes, diatomaceous
oozes, and oozes associated with reactive volcanic material
can be transformed into porcellanite and quartz after
shorter time periods, particularly if the thermal gradient is
high.
Part of the silica released from microfossils mayaIso
be utilized to form clay minerals (zeolites) if sufficient AI l 0 3 , alkali and alkaline earth ions are available in the pore water. Thus the formation of chert
bands or nodules (see below) is hindered or completely prevented if the opal content in the primary
sediment was limited. In the case of silica-poor calcareous oozes which cannot provide enough additional ions for the generation of zeolites, the silica is
diagenetically concentrated in chert nodules (Fig.
5.6b).