Geology Reference
In-Depth Information
Fig. 2.1.
Settings of carbonate depositional environments.
list at the end of this section). The strength of these
papers is that they outline the main criteria of recent
marine and non-marine carbonates and their major con-
trols in the context of interpreting ancient carbonate
rocks. The information included in these fundamental
texts is crucial for microfacies analyses. The 'Micro-
facies of Carbonate Rocks' does not intend to dupli-
cate this information, but focuses on those data which
are specifically relevant for microfacies studies.
2.3 Classification of Marine
Environments
Marine environments are classified into the benthic,
for the sea bottom, and the pelagic, pertaining to the
water mass. There is no universally accepted scheme
of subdivision of marine environments which is equally
acknowledged by biologists, oceanographers and ge-
ologists. Fig. 2.2 summarizes categories that are fre-
quently used. The categories are predominantly based
on schemes proposed by Hedgpeth (1957) and Edwards
(1979), who discuss marine terminology.
2.2 Carbonate Sediments Originate
on Land and in the Sea
2.3.1 Boundary Levels
Carbonate sediments originate on land and in the sea.
They are formed in three major settings: On the conti-
nents, within the transitional area between land and sea,
and in the shallow and deep sea. Today only around
10 % of marine carbonate production takes place in
shallow seas. 90% of the modern carbonate produc-
tion is related to the deposition of calcitic plankton in
the deep sea. These proportions were very different
during most parts of the Phanerozoic.
Major depositional environments are shown in Fig.
2.1. Terrestrial and aquatic non-marine carbonates are
receiving increasingly more attention from geologists
because of their economic importance and their sig-
nificance in paleoenvironmental analyses. These car-
bonates are discussed in Sect. 15.1 to 15.4. Owing to
their very shallow-water setting, marginal-marine car-
bonates have been studied in great detail and offer a
wealth of information on coastal sedimentation pro-
cesses and diagenesis.
About 70% of microfacies studies dealing with ma-
rine carbonates concerns shallow-marine carbonates
formed on the shelf and near the shelf break. Future
research should be focused on the huge deep-marine
carbonate depositional regions on the continental slopes.
Several levels at the sea bottom and within the water
column are commonly used in a vertical subdivision
of marginal-marine and marine environments (Box
2.1). Microfacies analysis provides the basic data for
recognizing these levels in ancient carbonate rocks (see
Chap. 12). Essential critical interfaces that control sedi-
mentary patterns and the distribution of organisms
are:
(1) The lower and the upper boundaries of the tides
(control the distribution of organisms), (2) the base of
the photic zone (controls the distribution of light-de-
pendent phototrophic organisms), (3) the base of the
zone of wave abrasion (above which bottom currents
and wave action may lead to erosion and cementation),
(4) the base of the action of storms on the sea bottom,
(5) the O
2
minimum zone (strongly limiting life on and
in the sea bottom), (6) the thermocline (the layer of
water that is too cold for most carbonate-producing or-
ganisms), and (7) the pycnocline (the layer of water
where salinity is too high for most organisms). High
and low tide, wave base and storm wave base are used
as basic boundaries in the classification of the major shal-
low-marine environments.