5.3 Hemipelagic and Pelagic Sediments
The formation of these nodules appears to have started
rather early in diagenesis, because their cores are frequently richer in Si02 than their outer rims. More features
of silica and carbonate diagenesis are shown in Fig. 5.6c
and d. Precipitation of both phases can be more or less coeval, that is, chert may form the center of a carbonate nodule, or pre-existing carbonate concretions may influence
the formation and shape of chert. In this respect it is important whether or not carbonate was originally present as unstable aragonite or high-Mg calcite in some quantity.
In clayey and marly sediments, gravitational compaction
continues after the formation of concretions, in special
cases deforming chert "veinlets" (Fig. 5.6c and d). Siliceous turbidites and calcareous turbidites rich in opaline
silica are affected by diagenesis in a way similar to that
described for pelagic sediments (Fig. 5.6e; e.g. Elorza and
Bustillo 1989) ..
Bedded cherts. The silica of lithified biosiliceous
sediments is frequently concentrated in chert bands
and layers of chert nodules. Some of these features
are of primary origin, others are caused by
diagenesis. In contrast to layers deposited from pelagic settling, many rhythmically bedded radiolarites
represent bioclastic, fine-grained turbidites deposited
by low-density turbidity currents.
Individual beds displaya sharp lower boundary and some
grading (either in grain size or in composition; cf. Sect.
5.4.2). They are derived from the slopes of submarine
highs (ridges, hills, deep plateaus; Fig. 5.5) which receive
little terrigenous material (e.g. Barrett 1982). If they accumulate below the CCD, the siliceous turbidites are devoid
of carbonate.
These types of red or greenish colored, banded chert
layers rnay alternate with more or less siliceous red
or greenish muds tones and are usually interpreted as
deep-water deposits.
Carbonate-bearing biosiliceous sediments and
cherty pelagic limestones indicate a depositional
depth above the CCD or between the aragonite and
calcite compensation depth (Fig. 5.5; Sect. 5.3.2).
However, successions of purely pelagic, biosiliceous
sediments overlain by siliceous carbonates and pure
carbonates do not necessarily signify shallowing sequences, but can originate from a drop in the CCD as
well (Fig. 5.5a through e). Similarly, pelagic limestones followed by chert layers are not always the
result of subsidence and consequent deepening of the
basin in question.
In particular, radiolarites in ancient rocks are frequently
ungraded and show well developed laminations and lowangle cross-bedding. Because it is unlikely that beds with
these sedimentary structures have been deposited from
suspension currents, they are assurned to originate from
normal traction currents at the sea bottom, which sorted
and concentrated hydrodynamically equivalent partic\es
(e.g., Barrett 1982; Gursky 1988; Ruiz-Ortiz et al. 1989;
Vecsei et al. 1989). Due to subsequent strong compaction
of the highly porous primary beds, the initially steeper an201
gles of the cross-laminae have been modified. Such beds
may have formed at different water depths and may thus
contain carbonate and siliciclastic material (Fig. 5.5).
5.3.6 Summary (Hemipelagic and Pelagic
Sediments)
- Hemipelagic and pelagic deep-sea sediments
consist of allochthonous terrestrial components
(river-borne and eolian dust) and autochthonous planktonic biogenic components (skeletal carbonate and opaline silica).
- Transport, deposition, and erosion of clay- and
silt-sized particles are affected by the formation
of aggregates.
- Calcareous or siliceous oozes accumulate in
regions of very low terrestrial input. Chalk also
formed in shallow waters.
- A great part of the planktonic production of
biogenic carbonate and silica is dissolved in the
deep sea (e.g. below the CCD), depending on
the balance between production in surface waters and the supply of Ca and Si by external
sources.
- Below the CCD, red clays ± manganese nodules are common.
- The sedimentation rates of pelagic sediments
are in the order of 0.5 to 5 cm/ka.
5.3.7 Marine Phosphorites
Sources of phosphorus
and phosphate precipitation
Sediments containing phosphorite horizons and
larger phosphorite deposits have attracted geologists
for a long time. These features generally represent
significant indicators of a certain depositional environment, and some of them are of considerable economic interest. Phosphorites are known from continental margin settings, epicontinental seas,
seamounts and guyots, and oceanic islands. The latter
occurrences of phosphorite are mostly associated
with guano provided by sea birds; they are not discussed further here.
The phosphate phase of the common phosphorites
consists of the mineral francolite, i.e., carbonate
jluorapatite. This mineral contains variable amounts
of trace elements, particularly uranium and rare earth
elements, which have been studied in detail by geochemists.
The formation of these nodules appears to have started
rather early in diagenesis, because their cores are frequently richer in Si02 than their outer rims. More features
of silica and carbonate diagenesis are shown in Fig. 5.6c
and d. Precipitation of both phases can be more or less coeval, that is, chert may form the center of a carbonate nodule, or pre-existing carbonate concretions may influence
the formation and shape of chert. In this respect it is important whether or not carbonate was originally present as unstable aragonite or high-Mg calcite in some quantity.
In clayey and marly sediments, gravitational compaction
continues after the formation of concretions, in special
cases deforming chert "veinlets" (Fig. 5.6c and d). Siliceous turbidites and calcareous turbidites rich in opaline
silica are affected by diagenesis in a way similar to that
described for pelagic sediments (Fig. 5.6e; e.g. Elorza and
Bustillo 1989) ..
Bedded cherts. The silica of lithified biosiliceous
sediments is frequently concentrated in chert bands
and layers of chert nodules. Some of these features
are of primary origin, others are caused by
diagenesis. In contrast to layers deposited from pelagic settling, many rhythmically bedded radiolarites
represent bioclastic, fine-grained turbidites deposited
by low-density turbidity currents.
Individual beds displaya sharp lower boundary and some
grading (either in grain size or in composition; cf. Sect.
5.4.2). They are derived from the slopes of submarine
highs (ridges, hills, deep plateaus; Fig. 5.5) which receive
little terrigenous material (e.g. Barrett 1982). If they accumulate below the CCD, the siliceous turbidites are devoid
of carbonate.
These types of red or greenish colored, banded chert
layers rnay alternate with more or less siliceous red
or greenish muds tones and are usually interpreted as
deep-water deposits.
Carbonate-bearing biosiliceous sediments and
cherty pelagic limestones indicate a depositional
depth above the CCD or between the aragonite and
calcite compensation depth (Fig. 5.5; Sect. 5.3.2).
However, successions of purely pelagic, biosiliceous
sediments overlain by siliceous carbonates and pure
carbonates do not necessarily signify shallowing sequences, but can originate from a drop in the CCD as
well (Fig. 5.5a through e). Similarly, pelagic limestones followed by chert layers are not always the
result of subsidence and consequent deepening of the
basin in question.
In particular, radiolarites in ancient rocks are frequently
ungraded and show well developed laminations and lowangle cross-bedding. Because it is unlikely that beds with
these sedimentary structures have been deposited from
suspension currents, they are assurned to originate from
normal traction currents at the sea bottom, which sorted
and concentrated hydrodynamically equivalent partic\es
(e.g., Barrett 1982; Gursky 1988; Ruiz-Ortiz et al. 1989;
Vecsei et al. 1989). Due to subsequent strong compaction
of the highly porous primary beds, the initially steeper an201
gles of the cross-laminae have been modified. Such beds
may have formed at different water depths and may thus
contain carbonate and siliciclastic material (Fig. 5.5).
5.3.6 Summary (Hemipelagic and Pelagic
Sediments)
- Hemipelagic and pelagic deep-sea sediments
consist of allochthonous terrestrial components
(river-borne and eolian dust) and autochthonous planktonic biogenic components (skeletal carbonate and opaline silica).
- Transport, deposition, and erosion of clay- and
silt-sized particles are affected by the formation
of aggregates.
- Calcareous or siliceous oozes accumulate in
regions of very low terrestrial input. Chalk also
formed in shallow waters.
- A great part of the planktonic production of
biogenic carbonate and silica is dissolved in the
deep sea (e.g. below the CCD), depending on
the balance between production in surface waters and the supply of Ca and Si by external
sources.
- Below the CCD, red clays ± manganese nodules are common.
- The sedimentation rates of pelagic sediments
are in the order of 0.5 to 5 cm/ka.
5.3.7 Marine Phosphorites
Sources of phosphorus
and phosphate precipitation
Sediments containing phosphorite horizons and
larger phosphorite deposits have attracted geologists
for a long time. These features generally represent
significant indicators of a certain depositional environment, and some of them are of considerable economic interest. Phosphorites are known from continental margin settings, epicontinental seas,
seamounts and guyots, and oceanic islands. The latter
occurrences of phosphorite are mostly associated
with guano provided by sea birds; they are not discussed further here.
The phosphate phase of the common phosphorites
consists of the mineral francolite, i.e., carbonate
jluorapatite. This mineral contains variable amounts
of trace elements, particularly uranium and rare earth
elements, which have been studied in detail by geochemists.