6.5 Nonactualistic (Precambriam) Depositional Environments
287
cules (autotroph bacteria, e.g., methane bacteria), or of
living on organic molecules generated by abiotic chemical synthesis (fermentation bacteria). Stromatolites,
i.e., microbially precipitated layered structures, have
been preserved in several metamorphic series
(greenstone belts) as old as 2600 to 3500 Ma (Fig.
6.18). Their structure was most probably generated by
mats of blue-green algae (cyanobacteria). These
cyanobacteria may therefore have started to produce
oxygen as early as about 3500 Ma ago. Stromatolite
growth was restricted, however, to shallow subaqueous
environments.
With the beginning ofthe Proterozoic, micro-organisms inc1uding bacteria, a1gae, fungi, and possibly
1ichen-like plants became more abundant and diversified, but the more complex, multicellular metazoa obviously did not yet exist. In the period between 2300
and 600 Ma, the stromatolites were the most common,
widespread fossil structures, which built thick sequences of limestones, dolomites, and cherts in the
tidal and subtidal zone. The amplitude of single structures reached many meters (cf. Sect. 3.2.1). It appears
that since that time the importance of cyanobacteria
mats gradually dec1ined.
The evolution of multicellular metazoa did not start
until after about 1800 Ma. It is assumed that they required an atmosphere containing oxygen and an ozone
screen in the upper atmosphere to protect them from
ultra-violet radiation. The first organic-walled fossils,
inc1uding "possible dinoflagellates" appeared in the
early Proterozoic (900-800 Ma; Butterfieid and
Rainbird 1998). Early types of metazoa were not capable ofbuilding shells or skeletons. They inc1ude jelly
fish, worms, sponges, and soft corals and are summarized under the term "Ediacaran fauna", which was
first found in the Flinders Ranges in southern Australia
(e.g., Conway Morris 1990). The main evolution ofthe
metazoa occurred in the Phanerozoic; the formation of
hard skeletal parts by "biomineralization" started at the
PrecambrianiCambrian boundary.
Much has been written and speculated about the early forms
oflife (e.g. Schopf1983; Schopf and Packer 1987; Awrarnik
1989; Knoll 1992; Bengtson 1994; Grotzinger et al. 1995).
However, this text is not the place for a detailed discussion
of this important topic. In addition, one should bear in mind,
that not all of the older age determinations are accurate and
reliable. This uncertainty caused some discrepancies between
several data sets, e g. those for the stable carbon isotopes
(Karhu and Holland 1996). New findings ofmicrofossils and
improved techniques will refine and modify further our present knowledge.
6.5.5 Precambrian Sediments
Archean and Proterozoic rocks are known from all
continents and represent a major part ofthe so-called
shields or cratons. The most common rock associations are old, high-grade gneiss and low-grade
greenstone belt complexes. They inc1ude banded ironstone formations and various silicic1astic rocks, for
example turbidite sequences.
Relatively widespread sedimentary sequences accumulated on the platforms of Archean cratons, representing the nuc1ei of continents (Goodwin 1991), since
the Proterozoic (about 2500-650 Ma; Plumb 1991).
The depositional environment of these sediments was
mostly shallow-marine, tidal, or continental. In several
regions, these sediments are relatively little affected by
subsequent tectonism and metamorphism. Here, some
special sedimentary rocks are briefly discussed.
Carbonates
Precambrium carbonates result form chemical and
biochemical processes in contrast to Phanerozoic carbonates which originated predominantly from biogenic
skeletal partic1es. Calcareous skeletal material was not
available prior to the Phanerozoic (Fig. 6.18). In
Archean and early Proterozoic sequences, dolomites
prevail over limestones; later, limestones become more
important.
Chemical precipitation of carbonate as whitings was
favored by oversaturation of the ocean with respect to
calcite and aragonite (high HC0 3 -/Ca++ ratio).
Upwelling of waters rich in carbonate alkalinity may
have caused rapid precipitation of aragonite and calcite
in shallow water. Furthermore, uptake of CO2 by
cyanobacteria triggered carbonate precipitation (see
below). The dominance of dolomites in earlier times
may have been caused by a high Mg/Ca ratio of the
early (soda-rich?) ocean. Under the specific conditions
of this ocean, either primary or early diagenetic dolomite could form (cf. Sect. 13.3); even iron-carbonate
(siderite) has been quoted as a possible primary mineral. The preservation of lamination and other sedimentary structures in dolomites indicates very early
dolomitization. Late Proterozoic dolomites preferentially formed in tidal and lagoonal environments sirni1ar to their Phanerozoic counterparts.
All Precambrian carbonates are comparatively rich
in iron, manganese, and silica, and they may alternate
with chert layers (see below). Furthermore, some authors assurne that strontium-rich aragonite precipitated
from Proterozoic ocean water in both peritidal and
open marine subtidal environments.
Some Proterzoic carbonates show sedimentary
structures (e.g. ripple marks, cross-bedding) typical of
calcarenites and dolarenites. The sand-sized particles
were probably produced by mechanical fragmentation
of fine-grained carbonate larninae, originally precipitated between microbial mats or in shallow lagoons. In
addition, peloids, ooids and oncoids occur in these
environments. Many Proterozoic carbonate sequences
also contain relatively fine-grained carbonate breccias,
287
cules (autotroph bacteria, e.g., methane bacteria), or of
living on organic molecules generated by abiotic chemical synthesis (fermentation bacteria). Stromatolites,
i.e., microbially precipitated layered structures, have
been preserved in several metamorphic series
(greenstone belts) as old as 2600 to 3500 Ma (Fig.
6.18). Their structure was most probably generated by
mats of blue-green algae (cyanobacteria). These
cyanobacteria may therefore have started to produce
oxygen as early as about 3500 Ma ago. Stromatolite
growth was restricted, however, to shallow subaqueous
environments.
With the beginning ofthe Proterozoic, micro-organisms inc1uding bacteria, a1gae, fungi, and possibly
1ichen-like plants became more abundant and diversified, but the more complex, multicellular metazoa obviously did not yet exist. In the period between 2300
and 600 Ma, the stromatolites were the most common,
widespread fossil structures, which built thick sequences of limestones, dolomites, and cherts in the
tidal and subtidal zone. The amplitude of single structures reached many meters (cf. Sect. 3.2.1). It appears
that since that time the importance of cyanobacteria
mats gradually dec1ined.
The evolution of multicellular metazoa did not start
until after about 1800 Ma. It is assumed that they required an atmosphere containing oxygen and an ozone
screen in the upper atmosphere to protect them from
ultra-violet radiation. The first organic-walled fossils,
inc1uding "possible dinoflagellates" appeared in the
early Proterozoic (900-800 Ma; Butterfieid and
Rainbird 1998). Early types of metazoa were not capable ofbuilding shells or skeletons. They inc1ude jelly
fish, worms, sponges, and soft corals and are summarized under the term "Ediacaran fauna", which was
first found in the Flinders Ranges in southern Australia
(e.g., Conway Morris 1990). The main evolution ofthe
metazoa occurred in the Phanerozoic; the formation of
hard skeletal parts by "biomineralization" started at the
PrecambrianiCambrian boundary.
Much has been written and speculated about the early forms
oflife (e.g. Schopf1983; Schopf and Packer 1987; Awrarnik
1989; Knoll 1992; Bengtson 1994; Grotzinger et al. 1995).
However, this text is not the place for a detailed discussion
of this important topic. In addition, one should bear in mind,
that not all of the older age determinations are accurate and
reliable. This uncertainty caused some discrepancies between
several data sets, e g. those for the stable carbon isotopes
(Karhu and Holland 1996). New findings ofmicrofossils and
improved techniques will refine and modify further our present knowledge.
6.5.5 Precambrian Sediments
Archean and Proterozoic rocks are known from all
continents and represent a major part ofthe so-called
shields or cratons. The most common rock associations are old, high-grade gneiss and low-grade
greenstone belt complexes. They inc1ude banded ironstone formations and various silicic1astic rocks, for
example turbidite sequences.
Relatively widespread sedimentary sequences accumulated on the platforms of Archean cratons, representing the nuc1ei of continents (Goodwin 1991), since
the Proterozoic (about 2500-650 Ma; Plumb 1991).
The depositional environment of these sediments was
mostly shallow-marine, tidal, or continental. In several
regions, these sediments are relatively little affected by
subsequent tectonism and metamorphism. Here, some
special sedimentary rocks are briefly discussed.
Carbonates
Precambrium carbonates result form chemical and
biochemical processes in contrast to Phanerozoic carbonates which originated predominantly from biogenic
skeletal partic1es. Calcareous skeletal material was not
available prior to the Phanerozoic (Fig. 6.18). In
Archean and early Proterozoic sequences, dolomites
prevail over limestones; later, limestones become more
important.
Chemical precipitation of carbonate as whitings was
favored by oversaturation of the ocean with respect to
calcite and aragonite (high HC0 3 -/Ca++ ratio).
Upwelling of waters rich in carbonate alkalinity may
have caused rapid precipitation of aragonite and calcite
in shallow water. Furthermore, uptake of CO2 by
cyanobacteria triggered carbonate precipitation (see
below). The dominance of dolomites in earlier times
may have been caused by a high Mg/Ca ratio of the
early (soda-rich?) ocean. Under the specific conditions
of this ocean, either primary or early diagenetic dolomite could form (cf. Sect. 13.3); even iron-carbonate
(siderite) has been quoted as a possible primary mineral. The preservation of lamination and other sedimentary structures in dolomites indicates very early
dolomitization. Late Proterozoic dolomites preferentially formed in tidal and lagoonal environments sirni1ar to their Phanerozoic counterparts.
All Precambrian carbonates are comparatively rich
in iron, manganese, and silica, and they may alternate
with chert layers (see below). Furthermore, some authors assurne that strontium-rich aragonite precipitated
from Proterozoic ocean water in both peritidal and
open marine subtidal environments.
Some Proterzoic carbonates show sedimentary
structures (e.g. ripple marks, cross-bedding) typical of
calcarenites and dolarenites. The sand-sized particles
were probably produced by mechanical fragmentation
of fine-grained carbonate larninae, originally precipitated between microbial mats or in shallow lagoons. In
addition, peloids, ooids and oncoids occur in these
environments. Many Proterozoic carbonate sequences
also contain relatively fine-grained carbonate breccias,