6.5 Nonactualistic (Precambriam) Depositional Environments
289
Without advocating the soda ocean theory, Drever et al.
(1988) have pointe out that the silica concentration of the
early ocean was about 20 times greater than that ofthe modem ocean (6 ppm) which is depleted in silica by the abundant
growth of diatoms and radiolaria.
Regular, wide-extended chert layers probably resulted
from episodic silica precipitation as mentioned above.
However, it cannot be excluded that microbial mats
absorbed !imited amounts of silica from sea water
which later formed banded chert. It is also possible
that si!ica migrated from sea water into the sediment
by downward diffusion. This process may have fed the
growth of chert nodules c10se to the sediment-water
interface.
The depositional environment of these types of
chert was very shallow or tidal. The chert layers are
!ittle compacted and often associated with dolomite.
For further details see, e.g. Maliva et al. (1989) and
Siever (1992).
Banded Ironstone Formations
A third, frequently discussed phenomenon of the Precambrian is the occurrence of thick and widespread,
economically important banded irons tone formations
(itabirites). They occur on all contir..ents and represent
the most important iron ores exploited in the modem
world.
Most of these deposits have an age between 2600
and 1800 Ma with a maximum in iron deposition
around 2400 to 2300 Ma, but some are older or younger. One can mainly distinguish two types of ironstones:
(1) Banded ironstones.
(2) Granular ironstones.
In their typical facies, the banded irons tones consist of
alternating thin layers of chert and red, iron-rich beds
composed of hematite, magnetite, siderite and iron
silicates. The lateral persistence of the banding is not
very well known.
The granular ironstones are composed of sand- to
gravel-size detrital partic1es, mostly peloids and
intrac1asts of hematite and chert. Both types can occur
in the same sequence and often alternate with wellsorted quartz sand and oolithic grains. Sedimentary
structures in these beds indicate wave- and current
action typical of shallow seas.
In addition to the primary signature of the
depositional environment, the mineralogical composition ofthese rocks is affected by subsequent metamorphism and finally by weathering processes at the land
surface. Tropical to subtropical weathering enhances
the iron content of these deposits, but it destroys other
characteristics of these rocks.
The coexistence of chert, iron carbonate, iron silicates, and iron oxyhydrates (as precursors of hematite
and magnetite) reflect precipitation from an alkaline,
alternatively reducing and oxidizing solution rich in
dissolved silica and ferrous iron. The thin-bedded
strata of Proterozoic ironstones probably formed in
widely extended shallow seas, some possibly in the
tidal zone. Most workers agree in the assumption that
the early ocean must have been stratified with a
chemocline between somewhat oxygenated surface
waters and anoxie deeper water rich in ferrous iron.
Iron oxyhydrate was precipitated along the chemocline
or in regions of upwelling. Repeated rise and fall of
the chemoc1ine around the level of the shallow sea
floor or fluctuations in the location and intensity of
coastal upwelling caused iron precipitation to turn on
or shut off in these regions. In addition, algal blooms
may have periodically produced oxygen which was
immediately used up by the oxidation and precipitation
of iron. Long-term second-order sea-Ievel highstands
(cf. Sect. 7.8) may have generated major peaks in iron
deposition with periodicities of 20 to 125 Ma.
Some occurrences of iron-rich beds are associated
with deposits of deeper water, mafic tuffs and subaqueous volcanic exhalations which may have delivered dissolved iron and silica. The source of iron for
the huge ironstone deposits accumulated in a relatively
short time period, is not c1ear. Some workers argue
that terrestrial sources may have not been sufficient
and therefore assume increased hydrothermal activity
and leaching of basaltic rocks along mid-oceanic
ridges. Accompanying c1astic beds can contain pyrite.
The oldest Archean representatives ofbanded ironformations are more difficult to explain because all
types of microfossils were probably scarce. Some of
these ironstones are thought to be of volcanic origin.
Ofthe many publications on the banded ironstone formations
a few only can be mentioned (e.g. Cloud 1973; Breitkopf
1988; Holland 1984; Zhu et al. 1988; Drever et al. 1988;
Veizer 1988; Grotzinger and Knoll 1994). The overprint by
metamorphism and subsequent weathering processes is described, e.g., by Trendall and Morris (1983), Weggen and
Valeton (1990); the possiblc impact of sea-Ievel changes and
hydrothermal activity by Isley (1995) and Simonson and
Hassler (1996).
Red Beds
Red beds (cf. Sect. 6.3) only became abundant after
the period of banded iron formation. This may be related to the fact that the early Earth was dominated by
oceans and the area of emerged continents was limited.
In addition, a great part of the red beds formed on the
continents may have been eroded later. Nevertheless,
many workers have assumed that the appearance of
extensive red beds coincided with a substantial increase in the oxygen content of the atmosphere (Fig.
289
Without advocating the soda ocean theory, Drever et al.
(1988) have pointe out that the silica concentration of the
early ocean was about 20 times greater than that ofthe modem ocean (6 ppm) which is depleted in silica by the abundant
growth of diatoms and radiolaria.
Regular, wide-extended chert layers probably resulted
from episodic silica precipitation as mentioned above.
However, it cannot be excluded that microbial mats
absorbed !imited amounts of silica from sea water
which later formed banded chert. It is also possible
that si!ica migrated from sea water into the sediment
by downward diffusion. This process may have fed the
growth of chert nodules c10se to the sediment-water
interface.
The depositional environment of these types of
chert was very shallow or tidal. The chert layers are
!ittle compacted and often associated with dolomite.
For further details see, e.g. Maliva et al. (1989) and
Siever (1992).
Banded Ironstone Formations
A third, frequently discussed phenomenon of the Precambrian is the occurrence of thick and widespread,
economically important banded irons tone formations
(itabirites). They occur on all contir..ents and represent
the most important iron ores exploited in the modem
world.
Most of these deposits have an age between 2600
and 1800 Ma with a maximum in iron deposition
around 2400 to 2300 Ma, but some are older or younger. One can mainly distinguish two types of ironstones:
(1) Banded ironstones.
(2) Granular ironstones.
In their typical facies, the banded irons tones consist of
alternating thin layers of chert and red, iron-rich beds
composed of hematite, magnetite, siderite and iron
silicates. The lateral persistence of the banding is not
very well known.
The granular ironstones are composed of sand- to
gravel-size detrital partic1es, mostly peloids and
intrac1asts of hematite and chert. Both types can occur
in the same sequence and often alternate with wellsorted quartz sand and oolithic grains. Sedimentary
structures in these beds indicate wave- and current
action typical of shallow seas.
In addition to the primary signature of the
depositional environment, the mineralogical composition ofthese rocks is affected by subsequent metamorphism and finally by weathering processes at the land
surface. Tropical to subtropical weathering enhances
the iron content of these deposits, but it destroys other
characteristics of these rocks.
The coexistence of chert, iron carbonate, iron silicates, and iron oxyhydrates (as precursors of hematite
and magnetite) reflect precipitation from an alkaline,
alternatively reducing and oxidizing solution rich in
dissolved silica and ferrous iron. The thin-bedded
strata of Proterozoic ironstones probably formed in
widely extended shallow seas, some possibly in the
tidal zone. Most workers agree in the assumption that
the early ocean must have been stratified with a
chemocline between somewhat oxygenated surface
waters and anoxie deeper water rich in ferrous iron.
Iron oxyhydrate was precipitated along the chemocline
or in regions of upwelling. Repeated rise and fall of
the chemoc1ine around the level of the shallow sea
floor or fluctuations in the location and intensity of
coastal upwelling caused iron precipitation to turn on
or shut off in these regions. In addition, algal blooms
may have periodically produced oxygen which was
immediately used up by the oxidation and precipitation
of iron. Long-term second-order sea-Ievel highstands
(cf. Sect. 7.8) may have generated major peaks in iron
deposition with periodicities of 20 to 125 Ma.
Some occurrences of iron-rich beds are associated
with deposits of deeper water, mafic tuffs and subaqueous volcanic exhalations which may have delivered dissolved iron and silica. The source of iron for
the huge ironstone deposits accumulated in a relatively
short time period, is not c1ear. Some workers argue
that terrestrial sources may have not been sufficient
and therefore assume increased hydrothermal activity
and leaching of basaltic rocks along mid-oceanic
ridges. Accompanying c1astic beds can contain pyrite.
The oldest Archean representatives ofbanded ironformations are more difficult to explain because all
types of microfossils were probably scarce. Some of
these ironstones are thought to be of volcanic origin.
Ofthe many publications on the banded ironstone formations
a few only can be mentioned (e.g. Cloud 1973; Breitkopf
1988; Holland 1984; Zhu et al. 1988; Drever et al. 1988;
Veizer 1988; Grotzinger and Knoll 1994). The overprint by
metamorphism and subsequent weathering processes is described, e.g., by Trendall and Morris (1983), Weggen and
Valeton (1990); the possiblc impact of sea-Ievel changes and
hydrothermal activity by Isley (1995) and Simonson and
Hassler (1996).
Red Beds
Red beds (cf. Sect. 6.3) only became abundant after
the period of banded iron formation. This may be related to the fact that the early Earth was dominated by
oceans and the area of emerged continents was limited.
In addition, a great part of the red beds formed on the
continents may have been eroded later. Nevertheless,
many workers have assumed that the appearance of
extensive red beds coincided with a substantial increase in the oxygen content of the atmosphere (Fig.