192
(3) The biogenic component is only slightly diluted
by terrigenous material, resulting in pelagic calcareous or siliceous oozes, chalks, limestones,
diatomites, radiolarites etc., usually with a sedimentation rate in the order of ;:;2 cmlka. These sediments
are particularly sensitive to relatively small environmental changes and therefore frequently produce
rhythmic and cyclic sequences (cf. Sects. 7.1 and
7.9).
(4) Partial or entire dissolution of the biogenic component and a very low influx of terrigenous matter
produce pelagic clay (usually reddish brown, known
as red deep-sea clay) and a very low net sedimentation rate (;:; 1 cmlka).
Under euxinic conditions and in poorly oxygenated
areas (cf. Sect. 10.3), the sediments of groups (1) to
(3) can be transformed into "black shales" of rather
differing composition. Group (4) is frequently associated with the occurrence of manganese nodules
(see below).
5.3.2 Deep-Sea Carbonates and
Carbonate Dissolution
Biogenic Carbonate Production
Planktonic carbonate is produced mainly by coccolithophores (nannoplankton), foraminifera, and freeswirnrning molluscs over wide areas of the tropical,
subtropical and temperate zones of the modem
oceans. It is less abundant at high latitudes where
cool temperatures deteriorate the living conditions
for these organisms and, in addition, promote their
dissolution in carbonate-undersaturated waters.
Oceanic carbonate production is (1) a function of
the evolution of life, i.e., the efficiency of organisrns
to extract calcium carbonate from sea water, (2) the
availability of calcium delivered by rivers from the
continents, or other (minor) sources, and (3) the recycling of calcium and nutrients (particularly nitrogen
and phosphorus) by the oceanic circulation system.
In addition, plankton growth depends (4) on the presence of special organic compounds, such as vitamins.
These "micronutrients" are mainly produced by bacteria and required by many phytoplankton species.
A summary on these and other growth factors is given by
Grant Gross (1980); rates of carbonate production are discussed in Sect. 10.2.
Over the Earth's history there have been times with low
and high global marine production and preservation of carbonate. A peak in shallow-marine carbonate production
occurred in the middle to late Paleozoic; an additional era
of high carbonate production began around 100 Ma ago
which has persisted into present times. Since that time the
fixation of calcium carbonate as calcite by marine plankton
has become an important factor.
Chapter 5 Oceanic Sediments
The Chalk Problem
Y oung, soft, calcareous muds are commonly described as calcareous oozes. Hemipelagic to pelagic
oozes are transformed into chalk as a result of incomplete, moderate diagenesis (e.g., Neugebauer 1974;
Zijlstra 1995; see also Sect. 13.1).
The term chalk refers to a variety of ancient, white to light
grey calcareous marine sediments, which are still porous,
friable, and composed predominantly of tiny, calcitic skeletal remains, such as coccolithophorides. Apart from pure
chalk, there are also chalks containing sands, larger shells,
glauconite, pyrite, phosphorite (see sections below), and
considerable proportions of clay (marly chalk) and organic
matter (bituminous chalk).
Although chalk consists predominantly of planktonic
skeletal remains, it was formed not only in the deep
sea above the calcite compensation depth, but also in
shallow seas, particularly in the upper Cretaceous.
The formation of chalk on shelves and in epicontinental seas was favored by a high sea-level stand and
low influx of terrigenous material, high production of
calcitic nannoplankton, and scarcity of aragonitic
skeletal remains which commonly promote lithification and cementation. Wide and stable shelves, as
weIl as flooded continental regions were exposed to
wind patterns and current systems providing nutrients
for planktonic productivity. The sedimentation rates
of chalk often exceed 10 cmlka.
The presence of abundant sighted ostracods in chalk suggests that the late Cretaceous sea floor in Alabama and
Mississippi was within the photic zone, i.e., not deeper
than 65 to 90 m (Puckett 1991). Most of the typical, widespread chalk deposits in Europe, North America, in the
Middle East, and Australia were generated during this period (Hattin 1986b, 1989; Hancock 1989; Zijlstra 1995). A
pure chalk facies may grade landward into marly chalk and
mudstone or, in high-energy environments, into shelly
chalk and skeletallimestone. Basinward, the chalk tends to
become laminated and richer in organic matter. On slopes
and in special morphological depressions, part of the material forming chalk may be derived from shallower water
and be redeposited by mass flows and turbidity currents
(Sect. 5.4). On submarine highs where continuous chalk
deposition is prevented by wave and current actions, or
hampered by low sea level stands, the chalk sequences are
frequently condensed and contain hardgrounds and omission surfaces. Many chalk sequences contain chert and
show rhythrnic bedding in the Milankovitch frequency
band (cf, Sect. 7.9).
Carbonate Dissolution
The modem oceans and their living commumtles,
including those in shaIlow-water environments (Sect.
3.4), extract considerably more calcium carbonate
per unit time than can be replaced by river influx of
(3) The biogenic component is only slightly diluted
by terrigenous material, resulting in pelagic calcareous or siliceous oozes, chalks, limestones,
diatomites, radiolarites etc., usually with a sedimentation rate in the order of ;:;2 cmlka. These sediments
are particularly sensitive to relatively small environmental changes and therefore frequently produce
rhythmic and cyclic sequences (cf. Sects. 7.1 and
7.9).
(4) Partial or entire dissolution of the biogenic component and a very low influx of terrigenous matter
produce pelagic clay (usually reddish brown, known
as red deep-sea clay) and a very low net sedimentation rate (;:; 1 cmlka).
Under euxinic conditions and in poorly oxygenated
areas (cf. Sect. 10.3), the sediments of groups (1) to
(3) can be transformed into "black shales" of rather
differing composition. Group (4) is frequently associated with the occurrence of manganese nodules
(see below).
5.3.2 Deep-Sea Carbonates and
Carbonate Dissolution
Biogenic Carbonate Production
Planktonic carbonate is produced mainly by coccolithophores (nannoplankton), foraminifera, and freeswirnrning molluscs over wide areas of the tropical,
subtropical and temperate zones of the modem
oceans. It is less abundant at high latitudes where
cool temperatures deteriorate the living conditions
for these organisms and, in addition, promote their
dissolution in carbonate-undersaturated waters.
Oceanic carbonate production is (1) a function of
the evolution of life, i.e., the efficiency of organisrns
to extract calcium carbonate from sea water, (2) the
availability of calcium delivered by rivers from the
continents, or other (minor) sources, and (3) the recycling of calcium and nutrients (particularly nitrogen
and phosphorus) by the oceanic circulation system.
In addition, plankton growth depends (4) on the presence of special organic compounds, such as vitamins.
These "micronutrients" are mainly produced by bacteria and required by many phytoplankton species.
A summary on these and other growth factors is given by
Grant Gross (1980); rates of carbonate production are discussed in Sect. 10.2.
Over the Earth's history there have been times with low
and high global marine production and preservation of carbonate. A peak in shallow-marine carbonate production
occurred in the middle to late Paleozoic; an additional era
of high carbonate production began around 100 Ma ago
which has persisted into present times. Since that time the
fixation of calcium carbonate as calcite by marine plankton
has become an important factor.
Chapter 5 Oceanic Sediments
The Chalk Problem
Y oung, soft, calcareous muds are commonly described as calcareous oozes. Hemipelagic to pelagic
oozes are transformed into chalk as a result of incomplete, moderate diagenesis (e.g., Neugebauer 1974;
Zijlstra 1995; see also Sect. 13.1).
The term chalk refers to a variety of ancient, white to light
grey calcareous marine sediments, which are still porous,
friable, and composed predominantly of tiny, calcitic skeletal remains, such as coccolithophorides. Apart from pure
chalk, there are also chalks containing sands, larger shells,
glauconite, pyrite, phosphorite (see sections below), and
considerable proportions of clay (marly chalk) and organic
matter (bituminous chalk).
Although chalk consists predominantly of planktonic
skeletal remains, it was formed not only in the deep
sea above the calcite compensation depth, but also in
shallow seas, particularly in the upper Cretaceous.
The formation of chalk on shelves and in epicontinental seas was favored by a high sea-level stand and
low influx of terrigenous material, high production of
calcitic nannoplankton, and scarcity of aragonitic
skeletal remains which commonly promote lithification and cementation. Wide and stable shelves, as
weIl as flooded continental regions were exposed to
wind patterns and current systems providing nutrients
for planktonic productivity. The sedimentation rates
of chalk often exceed 10 cmlka.
The presence of abundant sighted ostracods in chalk suggests that the late Cretaceous sea floor in Alabama and
Mississippi was within the photic zone, i.e., not deeper
than 65 to 90 m (Puckett 1991). Most of the typical, widespread chalk deposits in Europe, North America, in the
Middle East, and Australia were generated during this period (Hattin 1986b, 1989; Hancock 1989; Zijlstra 1995). A
pure chalk facies may grade landward into marly chalk and
mudstone or, in high-energy environments, into shelly
chalk and skeletallimestone. Basinward, the chalk tends to
become laminated and richer in organic matter. On slopes
and in special morphological depressions, part of the material forming chalk may be derived from shallower water
and be redeposited by mass flows and turbidity currents
(Sect. 5.4). On submarine highs where continuous chalk
deposition is prevented by wave and current actions, or
hampered by low sea level stands, the chalk sequences are
frequently condensed and contain hardgrounds and omission surfaces. Many chalk sequences contain chert and
show rhythrnic bedding in the Milankovitch frequency
band (cf, Sect. 7.9).
Carbonate Dissolution
The modem oceans and their living commumtles,
including those in shaIlow-water environments (Sect.
3.4), extract considerably more calcium carbonate
per unit time than can be replaced by river influx of