252
Summary (Green Marine Clays)
- Green authigenic particles in marine sediments
comprise a variety of poorly defined minerals
of varying chemical composition. They contain
ferrous iron.
- These minerals form in slightly reducing environments close to the sea floor (glauconitization, verdinization, and chloritization).
The verdine facies develops in shelf sediments
off the mouths of subtropical to tropical rivers.
- Glauconitic minerals (glaucony facies) form
outside the influence of deltas in deeper water
(shelf edge, upper slope, tops of submarine
highs) in areas ofvery slow or interrupted sedimentation. They are not restricted to warm water or a specific climate zone.
6.2 Oolitic Ironstones
Oolitic ironstones are known throughout the
Phanerozoic, but seldom from Neogene sediments.
Stirred by substantial economic interest, much has
been written and speculated about these particular
deposits. In the last decades, however, mining of
these ore deposits has declined, because their iron
content is considerably lower than that of the large
Precambrian banded iron deposits (Sect. 6.5) which
are now exploited in several continents.
The genesis of oolitic ironstones is still controversial and
obviously not fully understood (e.g., Gygi 1981; Maynard
1983; Odin et al. 1988; Schneider and Walter 1988;
Dreesen 1989; Young and Taylor 1989; Kimberley 1979,
1994). The influence of sea-level changes has been discussed, e.g., by Hallam and Bradshaw (1979), Bayer et al.
(1985), Bayer (1989a) and Burkhalter (1995). Reasonable
modem analogs of the marine ironstones are missing, although iron ooids (oolites) were also found in lakes, for
example in Lake Chad, North Africa (Lemoalle and
Dupont 1976).
A special type of iron ooids has been observed in a
modem shallow-marine volcanic setting in Indonesia
(Heikoop et al. 1996). Here, the limonitic and siliceous
ooids are precipitated around hydrothermal vents in
andesitic rocks and cover only a small area.
Depositional Environment
Ancient iron oolites consist to a large degree of
"iron" ooids, which form pure oolitic layers or oolitic
clay beds. The ooids are sand-size grains consisting
of smaller nuclei surrounded by concentric layers of
goethite and/or chamosite. Their formation requires
repeated reworking under turbulent hydrodynamic
Chapter 6 Special Depositional Environments
conditions (Fig. 6.2). This is indicated by the shrinkage, fractioning, and overgrowth of individual ooids.
Such an environment should be permanently oxidizing, but
the presence of ferrous iron in chamosite, which is a
diagenetic chlorite mineral, indicates that the ooids developed partially within the sediment under reducing conditions.
An alternative hypothesis is given by Dahanayake and
Krumbein (1985), who assume that iron ooids form under
coatings of microbial films. Fecal pellets and algal-produced oncoid particles may be transformed into iron-rich
ooid size particles due to their organic matter content favoring a reducing microenvironment. The occurrence of
iron ooids and ferruginous microbialites in the same sequence, as observed in the Middle Jurassie of the Swiss
Jura mountains, suggests a common biogenie origin
(Burkhalter 1995).
Iron oolites commonly contain marine fossils and
exhibit sedimentary structures and depositional sequences charac~eristic of shallow-marine environments, As with oolitic carbonates, an environment is
needed where input or ultimate deposition of
terrigenous silicates is low or virtually absent. After
burial, the primary iron-rich sediment and its constituents are more or less modified by diagenetic overprint, including the precipitation of some pyrite
and/or siderite. Carbonate ooids may be replaced by
iron compounds (diagenetic ferruginization).
Provenance of Iron
A further problem is the source of iron present in
thick ironstones. Unlike the formation of carbonate
ooids, which are precipitated from sea (or lake) water
supersaturated with respect to calcium carbonate, the
iron required for the ironstones has to be delivered
either by rivers draining deeply weathered, vegetated
and quartz-depleted soils, or some other effective
mechanism.
Oxygen-free groundwater rich in ferrous iron, discharging
directly into the sea (Fig. 6.2), may be such a mechanism.
However, this process is probably not sufficient to produce
iron concentrations on the order required for the large and
widely extended ancient ore bodies. Thin (mostly $0.5 m)
Ordovician iron oolitic clay beds on the Baltic platform
may result from volcanic ash delivering both iron-rich clay
minerals and iron for the formation of iron ooids
(Sturesson 1999). These beds are intercalated with oolitic
limestones and can be traced over 1200 km from Norway
to Russia.
Rivers supply iron in the form of detrital iron-bearing
minerals or as mixed iron oxyhydrate-organic-matter
colloids. This process is accompanied by the transport of considerable quantities of other clastic materials into the sea. These cause relatively high sedimentation rates near the river mouths and thus dilution of
Summary (Green Marine Clays)
- Green authigenic particles in marine sediments
comprise a variety of poorly defined minerals
of varying chemical composition. They contain
ferrous iron.
- These minerals form in slightly reducing environments close to the sea floor (glauconitization, verdinization, and chloritization).
The verdine facies develops in shelf sediments
off the mouths of subtropical to tropical rivers.
- Glauconitic minerals (glaucony facies) form
outside the influence of deltas in deeper water
(shelf edge, upper slope, tops of submarine
highs) in areas ofvery slow or interrupted sedimentation. They are not restricted to warm water or a specific climate zone.
6.2 Oolitic Ironstones
Oolitic ironstones are known throughout the
Phanerozoic, but seldom from Neogene sediments.
Stirred by substantial economic interest, much has
been written and speculated about these particular
deposits. In the last decades, however, mining of
these ore deposits has declined, because their iron
content is considerably lower than that of the large
Precambrian banded iron deposits (Sect. 6.5) which
are now exploited in several continents.
The genesis of oolitic ironstones is still controversial and
obviously not fully understood (e.g., Gygi 1981; Maynard
1983; Odin et al. 1988; Schneider and Walter 1988;
Dreesen 1989; Young and Taylor 1989; Kimberley 1979,
1994). The influence of sea-level changes has been discussed, e.g., by Hallam and Bradshaw (1979), Bayer et al.
(1985), Bayer (1989a) and Burkhalter (1995). Reasonable
modem analogs of the marine ironstones are missing, although iron ooids (oolites) were also found in lakes, for
example in Lake Chad, North Africa (Lemoalle and
Dupont 1976).
A special type of iron ooids has been observed in a
modem shallow-marine volcanic setting in Indonesia
(Heikoop et al. 1996). Here, the limonitic and siliceous
ooids are precipitated around hydrothermal vents in
andesitic rocks and cover only a small area.
Depositional Environment
Ancient iron oolites consist to a large degree of
"iron" ooids, which form pure oolitic layers or oolitic
clay beds. The ooids are sand-size grains consisting
of smaller nuclei surrounded by concentric layers of
goethite and/or chamosite. Their formation requires
repeated reworking under turbulent hydrodynamic
Chapter 6 Special Depositional Environments
conditions (Fig. 6.2). This is indicated by the shrinkage, fractioning, and overgrowth of individual ooids.
Such an environment should be permanently oxidizing, but
the presence of ferrous iron in chamosite, which is a
diagenetic chlorite mineral, indicates that the ooids developed partially within the sediment under reducing conditions.
An alternative hypothesis is given by Dahanayake and
Krumbein (1985), who assume that iron ooids form under
coatings of microbial films. Fecal pellets and algal-produced oncoid particles may be transformed into iron-rich
ooid size particles due to their organic matter content favoring a reducing microenvironment. The occurrence of
iron ooids and ferruginous microbialites in the same sequence, as observed in the Middle Jurassie of the Swiss
Jura mountains, suggests a common biogenie origin
(Burkhalter 1995).
Iron oolites commonly contain marine fossils and
exhibit sedimentary structures and depositional sequences charac~eristic of shallow-marine environments, As with oolitic carbonates, an environment is
needed where input or ultimate deposition of
terrigenous silicates is low or virtually absent. After
burial, the primary iron-rich sediment and its constituents are more or less modified by diagenetic overprint, including the precipitation of some pyrite
and/or siderite. Carbonate ooids may be replaced by
iron compounds (diagenetic ferruginization).
Provenance of Iron
A further problem is the source of iron present in
thick ironstones. Unlike the formation of carbonate
ooids, which are precipitated from sea (or lake) water
supersaturated with respect to calcium carbonate, the
iron required for the ironstones has to be delivered
either by rivers draining deeply weathered, vegetated
and quartz-depleted soils, or some other effective
mechanism.
Oxygen-free groundwater rich in ferrous iron, discharging
directly into the sea (Fig. 6.2), may be such a mechanism.
However, this process is probably not sufficient to produce
iron concentrations on the order required for the large and
widely extended ancient ore bodies. Thin (mostly $0.5 m)
Ordovician iron oolitic clay beds on the Baltic platform
may result from volcanic ash delivering both iron-rich clay
minerals and iron for the formation of iron ooids
(Sturesson 1999). These beds are intercalated with oolitic
limestones and can be traced over 1200 km from Norway
to Russia.
Rivers supply iron in the form of detrital iron-bearing
minerals or as mixed iron oxyhydrate-organic-matter
colloids. This process is accompanied by the transport of considerable quantities of other clastic materials into the sea. These cause relatively high sedimentation rates near the river mouths and thus dilution of