84
Sources and Composition of Marine Sediments
Wales into Newfoundland and Appalachia mark an ancient Atlantic seaway (Fig.
1.19).
The older the carbonate deposits, the more recrystallized they tend to be. Aragonite is replaced by calcite in this process, and many original structures of fossils are
destroyed through "diagenesis". "Dolomitization" is an extreme form of diagenetic
alteration, whereby every second Ca-ion is eventually replaced by Mg.
The shallow water limestones of the geologic record, produced in the shelf environments of ancient oceans, commonly contain an admixture of siliceous rocks, as
layers or nodular masses of chert arranged along horizons parallel to the bedding.
Mineralogically, this material is, in the main, finely crystalline quartz; in Tertiary
sediments it may be cryptocrystalline, or it may be still amorphous in parts. The
origin of these flinty masses has long been a mystery (although this did not prevent
their being used for tool-making in the Stone Age, of course). In the Miocene Monterey Formation of California, the source of the silica is shells of planktonic diatoms.
However, in shelf limestones the source may well have been siliceous sponges. Such
sponges are abundant on shelves and upper slopes in the present ocean wherever
there are silica-rich waters, that is, in high latitudes and in upwelling areas.
Why is there no evidence for incipient chert formation in modern shelf carbonate
areas?
The reason is that silicate concentrations are very low in present tropical waters.
Diatoms and sponges precipitate silica, but their skeletons are delicate and are
quickly redissolved in the highly undersaturated seawater, which is stripped of its
silicate by diatoms in upwelling regions. Could ancient oceans have had higher
concentrations of silicate in the water? This is quite possible, if such oceans were
much less fertile than the present ones. Also, it has been suggested that a high level
of volcanic activity favored the formation of siliceous sedimentary rocks by supplying large amounts of silicate to seawater. For the Eocene, this idea has been
offered to explain the abundace of cherts in deep-sea carbonates. These cherts caused
considerable difficulties for deep-sea drilling, since they quickly destroy the drill bit,
and impede recovery of sediment cores.
3.6.3 Plankton Organisms contribute a significant amount of material to slope sediments (Fig. 3.8). Deep-sea carbonates are made almost entirely of planktonic remains
(see Chap. 8). Shelf sediments, especially those of the geologic past, can also contain
considerable amounts of planktonic remains. For example, the English chalk is made
largely of the remains of planktonic calcareous algae, the coccolithophorids (which
produce the little platelets called coccoliths). The role of diatoms in supplying opal to
margin sediments has been mentioned.
In general, the remains of planktonic organisms increase relatively to those of
benthic organisms in the offshore direction. A well-known application of this principle is the determination of the ratio of planktonic to benthic foraminifera in the
mapping of marine facies. In the deep sea, the ratio is greater than 10: 1. On the edge
of the shelf it is close to 50:50. In the Persian Gulf, a somewhat restricted shelf sea,
the plankton-benthos ratio is even lower: about 3:7 near the entrance, and less than
I: 10 in the interior. Thus, the planktonic remains are typical for the open ocean.
Sources and Composition of Marine Sediments
Wales into Newfoundland and Appalachia mark an ancient Atlantic seaway (Fig.
1.19).
The older the carbonate deposits, the more recrystallized they tend to be. Aragonite is replaced by calcite in this process, and many original structures of fossils are
destroyed through "diagenesis". "Dolomitization" is an extreme form of diagenetic
alteration, whereby every second Ca-ion is eventually replaced by Mg.
The shallow water limestones of the geologic record, produced in the shelf environments of ancient oceans, commonly contain an admixture of siliceous rocks, as
layers or nodular masses of chert arranged along horizons parallel to the bedding.
Mineralogically, this material is, in the main, finely crystalline quartz; in Tertiary
sediments it may be cryptocrystalline, or it may be still amorphous in parts. The
origin of these flinty masses has long been a mystery (although this did not prevent
their being used for tool-making in the Stone Age, of course). In the Miocene Monterey Formation of California, the source of the silica is shells of planktonic diatoms.
However, in shelf limestones the source may well have been siliceous sponges. Such
sponges are abundant on shelves and upper slopes in the present ocean wherever
there are silica-rich waters, that is, in high latitudes and in upwelling areas.
Why is there no evidence for incipient chert formation in modern shelf carbonate
areas?
The reason is that silicate concentrations are very low in present tropical waters.
Diatoms and sponges precipitate silica, but their skeletons are delicate and are
quickly redissolved in the highly undersaturated seawater, which is stripped of its
silicate by diatoms in upwelling regions. Could ancient oceans have had higher
concentrations of silicate in the water? This is quite possible, if such oceans were
much less fertile than the present ones. Also, it has been suggested that a high level
of volcanic activity favored the formation of siliceous sedimentary rocks by supplying large amounts of silicate to seawater. For the Eocene, this idea has been
offered to explain the abundace of cherts in deep-sea carbonates. These cherts caused
considerable difficulties for deep-sea drilling, since they quickly destroy the drill bit,
and impede recovery of sediment cores.
3.6.3 Plankton Organisms contribute a significant amount of material to slope sediments (Fig. 3.8). Deep-sea carbonates are made almost entirely of planktonic remains
(see Chap. 8). Shelf sediments, especially those of the geologic past, can also contain
considerable amounts of planktonic remains. For example, the English chalk is made
largely of the remains of planktonic calcareous algae, the coccolithophorids (which
produce the little platelets called coccoliths). The role of diatoms in supplying opal to
margin sediments has been mentioned.
In general, the remains of planktonic organisms increase relatively to those of
benthic organisms in the offshore direction. A well-known application of this principle is the determination of the ratio of planktonic to benthic foraminifera in the
mapping of marine facies. In the deep sea, the ratio is greater than 10: 1. On the edge
of the shelf it is close to 50:50. In the Persian Gulf, a somewhat restricted shelf sea,
the plankton-benthos ratio is even lower: about 3:7 near the entrance, and less than
I: 10 in the interior. Thus, the planktonic remains are typical for the open ocean.