250
ever, much new information has become available in this
field (Odin and Matter 1981; Van Houten and Purucker
1984; Odin 1988; Odin and Morton 1988). Some important
aspects of our present knowledge are briefly summarized
here.
Remarks to the Nature of Greeu Particles
Green partieles in the oceans are authigenic aluminum silicates (clay minerals, mica) rich in iron and
magnesium. In addition, they may contain various
amounts of potassium and other cations. While magnesium and potassium are present in sea water in relatively high quantities, the concentration of iron in
oxygenated waters is extremely low. Therefore, normal sea water cannot be the source of iron.
Iron is delivered to the oceans by two principal
sources (Fig. 6.1): (l) by rivers from the continents
in the form of detrital partieles and colloids of iron
oxyhydrates, and (2) by volcanic activity within the
oceans, such as is common at mid-oceanic ridges,
island ares, and other volcanic islands. In the second
case, iron can be leached from basaltic rocks either
on the sea floor or from deeper sections of the oceanic crust by sea water circulating through basaltic
rocks and generating hydrothermal systems.
The green minerals contain either ferrous iron or
iron in both the ferrous and ferric states. Thus, their
formation is associated either with overall slightly
reducing conditions, or with reducing microenvironments within the sediment near the sea floor. The
authigenic growth of green pigment is referred to as
verdissement (Odin 1988) and includes glauconitization, verdinization, and chloritization.
Greenish Sediments in the Modern Oceans
In the present-day oceans, several zones of greenish
or brown, iron-rich sediment facies can be distinguished from shallow to deep water (Fig. 6.1):
- Pyrite and siderite occur in sediments in front of
rivers entering the sea. The iron is derived from detrital iron minerals which are deposited and frequently
redistributed in such areas. In conjunction with simultaneously delivered organic matter, reducing conditions are established within the sediment. Iron is
released from terrigenous material and made available as easily soluble ferrous iron to form early
diagenetic pyrite and siderite.
- Verdine jacies. In warm tropical to subtropical regions, this facies zone is found on the inner shelf in
front of the delta-front zone (at water depths of 20 to
60 m). The verdine facies occurs in areas of fairly
rapid sediment accumulation and is characterized by
various marine green clays ofthe phyllite group. This
Chapter 6 Special Depositional Environments
facies differs from the glaucony facies (see below) in
both the depositional environment and mineral composition. Similar to the glauconitic minerals, these
green clays are found as pigment in microtest chambers, fecal pellets, or in pores between other sediment particles. The green clays are predominantly
ferric and rich in magnesium.
This facies is known from modem shelf sediments off the
mouths of the rivers Amazon, Orinoco, Niger, Senegal,
Congo, and other tropical rivers (Odin et al. 1988a). Green
marine shales of considerable thickness are also common in
the ancient record, for example in the Lower Cretaceous.
- Iron ooids. If delta-front and inner shelf sediments
rich in iron are frequently reworked and redeposited
in areas some distance away from the zone of rapid
sedimentation, the ferrous iron compounds may become partially oxidized, transformed to ooids and
concentrated as oolitic iron deposits (see Sect. 6.2).
- Brown iron oxyhydrates. Detrital iron minerals
transported along the coast in shallow water are
chemically and/or biochemically alte red, particularly
so in tropical waters. The iron released is precipitated
as brown iron oxyhydrate (limonite, goethite).
- Glauconitic minerals commonly form in deeper
water along the shelf edge and upper slope or on the
tops of submarine highs, i.e., outside the influence of
deltaic or various other types of continuous sedimentation.
Glauconitization takes place in rather cold waters and
is a long-lasting process in environments of slow,
discontinuous deposition. The sediment should contain clay minerals and mica and develop microenvironments with slightly reducing conditions. As a result of the interaction between the sediment particles
and sea water, the original minerals are slowly altered to green grains or thin filrns on the surfaces of
larger particles or hardgrounds. Glauconitic minerals
may fill small cavities, for example the tests of
microfossils, or partially replace fecal pellets and
bioclasts. After deep burial, glauconitic minerals are
subject to modifications, particularly in their chemical composition.
Glauconitic minerals constitute a wide spectrum of minerals from K-poor glauconitic smectites to K-rich micas and
are known only from marine environments. Marine sediments characterized by the green pigment of these minerals
may be referred to as glaucony facies (Odin 1988).
- Iron-rich smectite. In the deep sea, iron is frequently concentrated in either iron-rich smectites or
ferromanganese nodules (Sect. 5.3.3). At least some
of the iron and manganese required for these sediments is delivered by volcanic activity (known as
juvenile iron).
- Celadonite or celadonitic minerals are typical clay
minerals rich in ferric iron and potassium, which
ever, much new information has become available in this
field (Odin and Matter 1981; Van Houten and Purucker
1984; Odin 1988; Odin and Morton 1988). Some important
aspects of our present knowledge are briefly summarized
here.
Remarks to the Nature of Greeu Particles
Green partieles in the oceans are authigenic aluminum silicates (clay minerals, mica) rich in iron and
magnesium. In addition, they may contain various
amounts of potassium and other cations. While magnesium and potassium are present in sea water in relatively high quantities, the concentration of iron in
oxygenated waters is extremely low. Therefore, normal sea water cannot be the source of iron.
Iron is delivered to the oceans by two principal
sources (Fig. 6.1): (l) by rivers from the continents
in the form of detrital partieles and colloids of iron
oxyhydrates, and (2) by volcanic activity within the
oceans, such as is common at mid-oceanic ridges,
island ares, and other volcanic islands. In the second
case, iron can be leached from basaltic rocks either
on the sea floor or from deeper sections of the oceanic crust by sea water circulating through basaltic
rocks and generating hydrothermal systems.
The green minerals contain either ferrous iron or
iron in both the ferrous and ferric states. Thus, their
formation is associated either with overall slightly
reducing conditions, or with reducing microenvironments within the sediment near the sea floor. The
authigenic growth of green pigment is referred to as
verdissement (Odin 1988) and includes glauconitization, verdinization, and chloritization.
Greenish Sediments in the Modern Oceans
In the present-day oceans, several zones of greenish
or brown, iron-rich sediment facies can be distinguished from shallow to deep water (Fig. 6.1):
- Pyrite and siderite occur in sediments in front of
rivers entering the sea. The iron is derived from detrital iron minerals which are deposited and frequently
redistributed in such areas. In conjunction with simultaneously delivered organic matter, reducing conditions are established within the sediment. Iron is
released from terrigenous material and made available as easily soluble ferrous iron to form early
diagenetic pyrite and siderite.
- Verdine jacies. In warm tropical to subtropical regions, this facies zone is found on the inner shelf in
front of the delta-front zone (at water depths of 20 to
60 m). The verdine facies occurs in areas of fairly
rapid sediment accumulation and is characterized by
various marine green clays ofthe phyllite group. This
Chapter 6 Special Depositional Environments
facies differs from the glaucony facies (see below) in
both the depositional environment and mineral composition. Similar to the glauconitic minerals, these
green clays are found as pigment in microtest chambers, fecal pellets, or in pores between other sediment particles. The green clays are predominantly
ferric and rich in magnesium.
This facies is known from modem shelf sediments off the
mouths of the rivers Amazon, Orinoco, Niger, Senegal,
Congo, and other tropical rivers (Odin et al. 1988a). Green
marine shales of considerable thickness are also common in
the ancient record, for example in the Lower Cretaceous.
- Iron ooids. If delta-front and inner shelf sediments
rich in iron are frequently reworked and redeposited
in areas some distance away from the zone of rapid
sedimentation, the ferrous iron compounds may become partially oxidized, transformed to ooids and
concentrated as oolitic iron deposits (see Sect. 6.2).
- Brown iron oxyhydrates. Detrital iron minerals
transported along the coast in shallow water are
chemically and/or biochemically alte red, particularly
so in tropical waters. The iron released is precipitated
as brown iron oxyhydrate (limonite, goethite).
- Glauconitic minerals commonly form in deeper
water along the shelf edge and upper slope or on the
tops of submarine highs, i.e., outside the influence of
deltaic or various other types of continuous sedimentation.
Glauconitization takes place in rather cold waters and
is a long-lasting process in environments of slow,
discontinuous deposition. The sediment should contain clay minerals and mica and develop microenvironments with slightly reducing conditions. As a result of the interaction between the sediment particles
and sea water, the original minerals are slowly altered to green grains or thin filrns on the surfaces of
larger particles or hardgrounds. Glauconitic minerals
may fill small cavities, for example the tests of
microfossils, or partially replace fecal pellets and
bioclasts. After deep burial, glauconitic minerals are
subject to modifications, particularly in their chemical composition.
Glauconitic minerals constitute a wide spectrum of minerals from K-poor glauconitic smectites to K-rich micas and
are known only from marine environments. Marine sediments characterized by the green pigment of these minerals
may be referred to as glaucony facies (Odin 1988).
- Iron-rich smectite. In the deep sea, iron is frequently concentrated in either iron-rich smectites or
ferromanganese nodules (Sect. 5.3.3). At least some
of the iron and manganese required for these sediments is delivered by volcanic activity (known as
juvenile iron).
- Celadonite or celadonitic minerals are typical clay
minerals rich in ferric iron and potassium, which