288
deposited as tempestites, or larger intraclasts derived
from microbial mats and bounded by organic matter
(e.g. flat pebble conglomerates).
A specific, but rather enigmatic phenomenon ofProterozoic
carbonates are the so-called "molar tooth" structures, i.e. ±
vertical cracks filled with calcite spar during early diagenesis
and later deformed by compaction (e.g. Fairchild et al. 1997;
James et al. 1998).
For further details about Precambrian carbonates see, e.g.,
Tucker (1982), Knoll and Swett (1990), Peryt et al. (1990),
Fairchild (1991, 1993), Kah and Knoll (1996), Grotzinger
and Knoll (1995), Pflüger and Gresse (1996), Song and
Einseie (1996), Sugitani et al. (1998).
Stromatolites, Phosphorites
Stromatol ites. As already mentioned above, calcareous
and dolomitic stromatolites playa great part in Precambrian carbonate deposition. Thick and widespread
stromatolite-bearing carbonate sequences are very
common in Proterozoic sedimentary rocks. Theyacted
as very effective carbonate factories.
Stromatolites are built up by the filaments of
cyanobacteria and form a variety of structures ranging
from flat horizontal mats to cone-shaped and columnar
reef-like bodies. The microbial mats operated in two
ways: (1) they trapped chemically precipated carbonate
particles and (2) caused biochemical precipitation of
carbonate (mainly aragonite) by the uptake of CO 2
from sea water supersaturated with respect to calcium
carbonate. Microbial mats and stromatolites consist to
a great part of synsedimentary, in-situ formed micritic
carbonate cement and therefore became rapidly
lithified.
Examples ofthis mechanism have been described from modem alkaline lakes, which have low contents of calcium
(Kempe and Kazmierczak 1990).
An example ofProterozoic rocks rich in stromatolites is
the 4500 m thick sequence in the Peking area, China (Song
and Gao 1985; Song and Einseie 1996). These deposits consist predominantly of carbonates which accumulated in a
long-persisting subtidal, intertidal, and supratidal environment and thus reflect very stable tectonic and environmental
conditions. More information about stromatolites and further
examples offield exposures are described by Walter (1976),
Horodyski (1976), Grotzinger and Knoll (1995), and in the
references listed earlier.
Phosphorites. Due to its low calcium content, a sodarich (?) ocean may have contained relatively high concentrations of phosphate, as observed in present soda
lakes. Phosphorites in economic concentrations appear,
however, not before the late Proterozoic (Veizer
1988). In contrast to younger occurrences, these and
most Cambrian phosphorites are commonly
nonpelletal, i.e., a direct relationship to biogenic activity cannot be established. Thus, chemical precipitation
Chapter 6 Special Depositional Environments
of phosphorite at the sediment-water interface cannot
be excluded. A rise in the calcium content and minor
changes in the physico-chemical properties of sea water may have led to supersaturation with respect to
calcium phosphate. Diagenetically formed phosphorites caused by the release of phosphorus from
decomposition of stromatolitic algal tissue (cf. Sect.
5.3.6) may have acted as nuclei for precipitation of
phosphorite from sea water or interstitial water.
On emerging phosphate-bearing stromatolites
phosphorite mayaiso have formed crusts ("phoscrete")
similar to calcrete as observed in Cambrian carbonates
in Australia (Southgate 1986).
Phosphorites were described from Precambrian sequences in
several regions (e.g., Salop 1983, and references listed
above). In China, widespread, partially thick phosphorite
beds in association with cherts, dolomites, limestones, carbonaceous shales, and stromatolites are known from the early
and late Proterozoic (e.g., Sang and You 1988). They were
deposited on stable platforms which received little
terrigenous input.
Bedded Quartzites aud Cherts
Some of the oldest sedimentary sequences in the
Archean (<:3500 Ma) are characterized by metamorphic quartzite-amphibolite associations. The bedded
quartzites can reach 1000 m in thickness, and most of
them are recrystallized and do not show any internal
structures or clastic textures, such as the contours of
detrital grains (Salop 1983). These rocks can be interpreted as mineralogically and texturally mature sandstones, as known from younger, repeatedly recycled
quartz deposits. Some ofthem contain clastic interbeds
with zircon and sillimanite grains and high Ti contents.
Another group of quartzitic rocks are "banded
chert", chert nodules, and various phenomena of silicification. These features are often observed in Proterozoic carbonate sequences. As carbonate, the silica is
not of skeletal origin because opal-secreting organisrns, such as radiolaria, diatorns and sponges, did not
yet exist. The banded cherts appear to have been chemically precipitated as amorphous, water-rich silica layers in a similar way as observed in present-day highly
alkaline soda lakes of the Magadi type (Sect. 2.5).
Such an explanation supports the above mentioned
"soda ocean" theory, allowing the dissolution of high
amounts of silica. This could be precipitated in areas
of high evaporation, for example in marginal shallow
seas, or by lowering of the pR by fresh-water inflow
and/or decreasing water temperature.
Prirnary silica precipitation is assumed, e.g., for part of an
Archean chert sequence in Westem Australia (Sugitani et al.
1998). It is indicated by abundant mosaic and spherulitic
microstructures in contrast to detrital quartzitic beds present
in the same sequence.
deposited as tempestites, or larger intraclasts derived
from microbial mats and bounded by organic matter
(e.g. flat pebble conglomerates).
A specific, but rather enigmatic phenomenon ofProterozoic
carbonates are the so-called "molar tooth" structures, i.e. ±
vertical cracks filled with calcite spar during early diagenesis
and later deformed by compaction (e.g. Fairchild et al. 1997;
James et al. 1998).
For further details about Precambrian carbonates see, e.g.,
Tucker (1982), Knoll and Swett (1990), Peryt et al. (1990),
Fairchild (1991, 1993), Kah and Knoll (1996), Grotzinger
and Knoll (1995), Pflüger and Gresse (1996), Song and
Einseie (1996), Sugitani et al. (1998).
Stromatolites, Phosphorites
Stromatol ites. As already mentioned above, calcareous
and dolomitic stromatolites playa great part in Precambrian carbonate deposition. Thick and widespread
stromatolite-bearing carbonate sequences are very
common in Proterozoic sedimentary rocks. Theyacted
as very effective carbonate factories.
Stromatolites are built up by the filaments of
cyanobacteria and form a variety of structures ranging
from flat horizontal mats to cone-shaped and columnar
reef-like bodies. The microbial mats operated in two
ways: (1) they trapped chemically precipated carbonate
particles and (2) caused biochemical precipitation of
carbonate (mainly aragonite) by the uptake of CO 2
from sea water supersaturated with respect to calcium
carbonate. Microbial mats and stromatolites consist to
a great part of synsedimentary, in-situ formed micritic
carbonate cement and therefore became rapidly
lithified.
Examples ofthis mechanism have been described from modem alkaline lakes, which have low contents of calcium
(Kempe and Kazmierczak 1990).
An example ofProterozoic rocks rich in stromatolites is
the 4500 m thick sequence in the Peking area, China (Song
and Gao 1985; Song and Einseie 1996). These deposits consist predominantly of carbonates which accumulated in a
long-persisting subtidal, intertidal, and supratidal environment and thus reflect very stable tectonic and environmental
conditions. More information about stromatolites and further
examples offield exposures are described by Walter (1976),
Horodyski (1976), Grotzinger and Knoll (1995), and in the
references listed earlier.
Phosphorites. Due to its low calcium content, a sodarich (?) ocean may have contained relatively high concentrations of phosphate, as observed in present soda
lakes. Phosphorites in economic concentrations appear,
however, not before the late Proterozoic (Veizer
1988). In contrast to younger occurrences, these and
most Cambrian phosphorites are commonly
nonpelletal, i.e., a direct relationship to biogenic activity cannot be established. Thus, chemical precipitation
Chapter 6 Special Depositional Environments
of phosphorite at the sediment-water interface cannot
be excluded. A rise in the calcium content and minor
changes in the physico-chemical properties of sea water may have led to supersaturation with respect to
calcium phosphate. Diagenetically formed phosphorites caused by the release of phosphorus from
decomposition of stromatolitic algal tissue (cf. Sect.
5.3.6) may have acted as nuclei for precipitation of
phosphorite from sea water or interstitial water.
On emerging phosphate-bearing stromatolites
phosphorite mayaiso have formed crusts ("phoscrete")
similar to calcrete as observed in Cambrian carbonates
in Australia (Southgate 1986).
Phosphorites were described from Precambrian sequences in
several regions (e.g., Salop 1983, and references listed
above). In China, widespread, partially thick phosphorite
beds in association with cherts, dolomites, limestones, carbonaceous shales, and stromatolites are known from the early
and late Proterozoic (e.g., Sang and You 1988). They were
deposited on stable platforms which received little
terrigenous input.
Bedded Quartzites aud Cherts
Some of the oldest sedimentary sequences in the
Archean (<:3500 Ma) are characterized by metamorphic quartzite-amphibolite associations. The bedded
quartzites can reach 1000 m in thickness, and most of
them are recrystallized and do not show any internal
structures or clastic textures, such as the contours of
detrital grains (Salop 1983). These rocks can be interpreted as mineralogically and texturally mature sandstones, as known from younger, repeatedly recycled
quartz deposits. Some ofthem contain clastic interbeds
with zircon and sillimanite grains and high Ti contents.
Another group of quartzitic rocks are "banded
chert", chert nodules, and various phenomena of silicification. These features are often observed in Proterozoic carbonate sequences. As carbonate, the silica is
not of skeletal origin because opal-secreting organisrns, such as radiolaria, diatorns and sponges, did not
yet exist. The banded cherts appear to have been chemically precipitated as amorphous, water-rich silica layers in a similar way as observed in present-day highly
alkaline soda lakes of the Magadi type (Sect. 2.5).
Such an explanation supports the above mentioned
"soda ocean" theory, allowing the dissolution of high
amounts of silica. This could be precipitated in areas
of high evaporation, for example in marginal shallow
seas, or by lowering of the pR by fresh-water inflow
and/or decreasing water temperature.
Prirnary silica precipitation is assumed, e.g., for part of an
Archean chert sequence in Westem Australia (Sugitani et al.
1998). It is indicated by abundant mosaic and spherulitic
microstructures in contrast to detrital quartzitic beds present
in the same sequence.