efficiently from about 80% porosity on the sea floor to
about 35–40% at about 500–600 m depth (Day-Stirrat
et al. 2012). This may be due partly to poor sorting.
Kaolinite is the coarsest grained of the clay minerals
and will therefore be deposited in the most proximal
parts of deltas and shorefaces while illite and smectite
will be transported further out into the more distal parts
(Fig. 6.1). As sedimentation progrades into deep water
on a continental margin illitic and smectitic clay will
form the base and grade upwards into more kaoliniterich sediments (Bjørlykke 2011).
From deltas and the shelf edge, clayey sediments as
well as sand may be transported down the slope by
turbidity currents or debris flows. In the most distal
shelf or deep-water facies, sedimentation rates can be
very low and aeolian dust may make up much of the
sediment deposited. This is particularly true offshore
dry areas like the Sahara in West Africa. This mud is
rich in fine-grained smectite and illite, often with iron
oxides which may serve as an important nutrient for
organic production in the South Atlantic Ocean. Volcanic ash may be deposited over large areas. During
transgressions, extensive layers of mud may be deposited on shelves along the basin margin. This mud may
later be eroded and supplied to the basin during periods
of uplift, so that the prograding regressive sequences
may be rich in clay. Weathering of basic rocks like
anorthosites and gabbroid rocks proceeds rapidly and
produces almost only clay minerals and very little sand,
because of the lack of quartz.
6.3
Silica (SiO 2 ) Deposits
Silica which is liberated through weathering goes into
solution as silicic acid (H 4 SiO 4 ). Even if quartz and
also feldspar have relatively low solubility, large
quantities of silica are transported by rivers out into
the sea. The total amount of dissolved silica added
annually to the sea is estimated to be about 4 Â 10
8
tonnes. Some silica is also introduced from the midoceanic ridges, but is probably of only very modest
significance compared with the fluvial input. An equal
amount of silica must be removed for the seawater
composition to remain constant. Silica is removed
biologically as biogenic silica, and through the formation of silicate minerals in the sea. Radiolaria and
diatoms are particularly efficient at removing silica.
As a result, even though quartz is only very slightly
soluble in seawater (3–6 ppm SiO 2 ), surface water
(seawater) is usually undersaturated with respect to
quartz. This is because the organisms can precipitate
silica from seawater even if the concentration of SiO 2
is far less than 1 ppm. While amorphous silica has a
solubility of about 150 ppm, cristobalite and tridymite
have a solubility of 6–15 ppm, depending on the
degree of order in the crystals. The most important
silica-producing organisms are:
Phytoplankton: diatoms and silicoflagellates
Zooplankton: radiolaria
Silica sponges
These organisms are built up of amorphous silica
(opal A). The total production of organic silica in the
oceans has been estimated to be from 2 Â 10
10 up to
10
11 tonnes/year. The largest contribution comes from
diatoms, and also a very large percentage (50–70%) of
the total primary production of carbon (2 Â 10
10
tonnes/year) is ascribable to diatoms consisting of
about 60% silica and 40% carbon.
The flux of silica into the oceans from the continents
is far less than the organic production of siliceous
organisms. The approximately 4 Â 10
8 tonnes/year
brought by rivers only represents about 1% of all
organic silica precipitation. Some silica is added to the
sea through submarine volcanism along the oceanic
spreading ridges. Seawater is being circulated
(convected) through hot basalt, dissolving silica and
precipitating sulphides from the sulphate in the seawater. Even with this source of silica, the supply to the
ocean water can not balance the amounts precipitated
biologically. Since we must assume that the composition of seawater has been relatively constant, this means
that only about 1% of the overall organic silica production is retained in sedimentary deposits. Most of the
silica from plankton redissolves in the undersaturated
seawater before it reaches the bottom, and some also
dissolves on the seafloor and diffuses up into the water
column. Consequently it is only when the rate of
organic precipitation is higher than the rate of solution
that we find deposition of organic silica.
Diatoms dissolve because seawater is undersaturated with respect to silica, and large quantities of
organic material can thereby be released without oxidation taking place. Organic matter produced by the
solution of diatoms thus constitutes a large part of the
total organic matter accumulated. Phosphates, nitrogen and various trace metals are also released through
the disintegration of plankton as they sink through the
water column. These recycled nutrients can once more
provide the basis for organic production when water
6 Mudrocks, Shales, Silica Deposits and Evaporites
219
about 35–40% at about 500–600 m depth (Day-Stirrat
et al. 2012). This may be due partly to poor sorting.
Kaolinite is the coarsest grained of the clay minerals
and will therefore be deposited in the most proximal
parts of deltas and shorefaces while illite and smectite
will be transported further out into the more distal parts
(Fig. 6.1). As sedimentation progrades into deep water
on a continental margin illitic and smectitic clay will
form the base and grade upwards into more kaoliniterich sediments (Bjørlykke 2011).
From deltas and the shelf edge, clayey sediments as
well as sand may be transported down the slope by
turbidity currents or debris flows. In the most distal
shelf or deep-water facies, sedimentation rates can be
very low and aeolian dust may make up much of the
sediment deposited. This is particularly true offshore
dry areas like the Sahara in West Africa. This mud is
rich in fine-grained smectite and illite, often with iron
oxides which may serve as an important nutrient for
organic production in the South Atlantic Ocean. Volcanic ash may be deposited over large areas. During
transgressions, extensive layers of mud may be deposited on shelves along the basin margin. This mud may
later be eroded and supplied to the basin during periods
of uplift, so that the prograding regressive sequences
may be rich in clay. Weathering of basic rocks like
anorthosites and gabbroid rocks proceeds rapidly and
produces almost only clay minerals and very little sand,
because of the lack of quartz.
6.3
Silica (SiO 2 ) Deposits
Silica which is liberated through weathering goes into
solution as silicic acid (H 4 SiO 4 ). Even if quartz and
also feldspar have relatively low solubility, large
quantities of silica are transported by rivers out into
the sea. The total amount of dissolved silica added
annually to the sea is estimated to be about 4 Â 10
8
tonnes. Some silica is also introduced from the midoceanic ridges, but is probably of only very modest
significance compared with the fluvial input. An equal
amount of silica must be removed for the seawater
composition to remain constant. Silica is removed
biologically as biogenic silica, and through the formation of silicate minerals in the sea. Radiolaria and
diatoms are particularly efficient at removing silica.
As a result, even though quartz is only very slightly
soluble in seawater (3–6 ppm SiO 2 ), surface water
(seawater) is usually undersaturated with respect to
quartz. This is because the organisms can precipitate
silica from seawater even if the concentration of SiO 2
is far less than 1 ppm. While amorphous silica has a
solubility of about 150 ppm, cristobalite and tridymite
have a solubility of 6–15 ppm, depending on the
degree of order in the crystals. The most important
silica-producing organisms are:
Phytoplankton: diatoms and silicoflagellates
Zooplankton: radiolaria
Silica sponges
These organisms are built up of amorphous silica
(opal A). The total production of organic silica in the
oceans has been estimated to be from 2 Â 10
10 up to
10
11 tonnes/year. The largest contribution comes from
diatoms, and also a very large percentage (50–70%) of
the total primary production of carbon (2 Â 10
10
tonnes/year) is ascribable to diatoms consisting of
about 60% silica and 40% carbon.
The flux of silica into the oceans from the continents
is far less than the organic production of siliceous
organisms. The approximately 4 Â 10
8 tonnes/year
brought by rivers only represents about 1% of all
organic silica precipitation. Some silica is added to the
sea through submarine volcanism along the oceanic
spreading ridges. Seawater is being circulated
(convected) through hot basalt, dissolving silica and
precipitating sulphides from the sulphate in the seawater. Even with this source of silica, the supply to the
ocean water can not balance the amounts precipitated
biologically. Since we must assume that the composition of seawater has been relatively constant, this means
that only about 1% of the overall organic silica production is retained in sedimentary deposits. Most of the
silica from plankton redissolves in the undersaturated
seawater before it reaches the bottom, and some also
dissolves on the seafloor and diffuses up into the water
column. Consequently it is only when the rate of
organic precipitation is higher than the rate of solution
that we find deposition of organic silica.
Diatoms dissolve because seawater is undersaturated with respect to silica, and large quantities of
organic material can thereby be released without oxidation taking place. Organic matter produced by the
solution of diatoms thus constitutes a large part of the
total organic matter accumulated. Phosphates, nitrogen and various trace metals are also released through
the disintegration of plankton as they sink through the
water column. These recycled nutrients can once more
provide the basis for organic production when water
6 Mudrocks, Shales, Silica Deposits and Evaporites
219