290
6.18). The oldest laterites (2.2-2.0 Ga) have been
found in South Africa (Gutzmer and Beukes 1998)
which is in accordance with the age of paleosols indicating an increase in the atmospheric p02 (cf. Sect.
6.5.2).
Evaporites
The question whether or not significant evaporite deposits accumulated in the Archean is difficult to answer, because subsurface dissolution, diagenesis, and
metamorphism may have obscured and obliterated
most of them. Nevertheless, scarce halite or pseudomorphs of halite have been detected even in Archean
rocks. Relics of gypsum crystals and sulfate nodules
have been found in a chert-barite unit as old as 3500
Ma in Western Australia (Buick and Dunlop 1990).
However, thicker sulfate layers in combination with
halite could only precipitate after sufficient oxygen
was available for the oxidation ofhydrogen sulfide and
sulfur. Such sulfate deposits appear at the PaleoIMeso-Proterozoic boundary (~1.8 Ga) and gain in
importance in younger rock sequences (e.g. in the N eoProterozoic ).
6.5.6
Summary (Preeambrian Sediments)
- Various Precambrian sediment types reflect a nonactualistic depositional environment of the early
Earth: an atmosphere rich in carbon dioxide and
poor in oxygen, a high-alkaline, possibly soda-rich
stratified ocean with deep water rich in ferrous
iron.
- Red beds and the termination of iron loss in
paleosols indicate a significant rise in the
atmospheric p02 by about 2.2-2.0 Ga, caused by
increased production and burial of organic matter
(e.g. cyanobacteria).
- Supersaturation of the ocean with respect to
calcium and magnesium favored chemical and
biochemical carbonate precipitation mediated by
Chapter 6 Special Depositional Environments
See, e.g., Grotzinger and Kasting (1993), Warren (1997). If
the soda ocean concept is applied, the late appearance of
thick, massive gypsum in the upper Proterozoic may result
from the depletion of the earlier ocean in calcium (Kempe
and Degens 1985).
In the following period of the late Proterozoic and
early Cambrian, thick evaporites with high proportions
of halite, potassium and magnesium salts, and
dolomites accumulated. In addition, the Ca/Mg ratio in
carbonates tended to increase and their contents in
manganese and silica to decrease. Organic matter and
pyrite are present in Precambrian sedimentary rocks,
but their contribution to the composition of sediments
preserved in the geologic record becomes significantly
greater in the Phanerozoic.
Clastie Deposits
Apart from the special sediment types and environments mentioned in the previous sections, many Precambrian siliciclastic and volcaniclastic deposits
closely resemble their Phanerozoic counterparts.
Hence, facies models derived from modern environments are more and more used for the interpretation of
Precambrian depositional systems.
microbial mats (stromatolites). Primary or early
diagenetic dolomite, predominating in the Archean
and early Proterozoic, was later largely replaced
by limestones.
- Banded chert in carbonates may result from direct
or microbial mediated opal precipitation from sea
water rich in dissolved silica.
- The common banded ironstone formations appear
to result from fluctuations of the oceanic
chemocline, variations in the organic productivity,
and sea-Ievel changes.
- Significant amounts of evaporites (sulfates and
halite) are known since the N eoproterozoic. Some
older evaporites may have been removed by
dissolution and metamorphism.
6.18). The oldest laterites (2.2-2.0 Ga) have been
found in South Africa (Gutzmer and Beukes 1998)
which is in accordance with the age of paleosols indicating an increase in the atmospheric p02 (cf. Sect.
6.5.2).
Evaporites
The question whether or not significant evaporite deposits accumulated in the Archean is difficult to answer, because subsurface dissolution, diagenesis, and
metamorphism may have obscured and obliterated
most of them. Nevertheless, scarce halite or pseudomorphs of halite have been detected even in Archean
rocks. Relics of gypsum crystals and sulfate nodules
have been found in a chert-barite unit as old as 3500
Ma in Western Australia (Buick and Dunlop 1990).
However, thicker sulfate layers in combination with
halite could only precipitate after sufficient oxygen
was available for the oxidation ofhydrogen sulfide and
sulfur. Such sulfate deposits appear at the PaleoIMeso-Proterozoic boundary (~1.8 Ga) and gain in
importance in younger rock sequences (e.g. in the N eoProterozoic ).
6.5.6
Summary (Preeambrian Sediments)
- Various Precambrian sediment types reflect a nonactualistic depositional environment of the early
Earth: an atmosphere rich in carbon dioxide and
poor in oxygen, a high-alkaline, possibly soda-rich
stratified ocean with deep water rich in ferrous
iron.
- Red beds and the termination of iron loss in
paleosols indicate a significant rise in the
atmospheric p02 by about 2.2-2.0 Ga, caused by
increased production and burial of organic matter
(e.g. cyanobacteria).
- Supersaturation of the ocean with respect to
calcium and magnesium favored chemical and
biochemical carbonate precipitation mediated by
Chapter 6 Special Depositional Environments
See, e.g., Grotzinger and Kasting (1993), Warren (1997). If
the soda ocean concept is applied, the late appearance of
thick, massive gypsum in the upper Proterozoic may result
from the depletion of the earlier ocean in calcium (Kempe
and Degens 1985).
In the following period of the late Proterozoic and
early Cambrian, thick evaporites with high proportions
of halite, potassium and magnesium salts, and
dolomites accumulated. In addition, the Ca/Mg ratio in
carbonates tended to increase and their contents in
manganese and silica to decrease. Organic matter and
pyrite are present in Precambrian sedimentary rocks,
but their contribution to the composition of sediments
preserved in the geologic record becomes significantly
greater in the Phanerozoic.
Clastie Deposits
Apart from the special sediment types and environments mentioned in the previous sections, many Precambrian siliciclastic and volcaniclastic deposits
closely resemble their Phanerozoic counterparts.
Hence, facies models derived from modern environments are more and more used for the interpretation of
Precambrian depositional systems.
microbial mats (stromatolites). Primary or early
diagenetic dolomite, predominating in the Archean
and early Proterozoic, was later largely replaced
by limestones.
- Banded chert in carbonates may result from direct
or microbial mediated opal precipitation from sea
water rich in dissolved silica.
- The common banded ironstone formations appear
to result from fluctuations of the oceanic
chemocline, variations in the organic productivity,
and sea-Ievel changes.
- Significant amounts of evaporites (sulfates and
halite) are known since the N eoproterozoic. Some
older evaporites may have been removed by
dissolution and metamorphism.