Geology Reference
In-Depth Information
greatly upon the reliability of water-depth indicators
and needs bathymetrically well-characterized reference
beds that reflect a defined sea-level position. The inter-
pretation of cyclic sedimentation patterns requires
paleobathymetric pinpoints.
the upper subtidal zone. Depth range of udoteacean
green algae is broader and extends from a few tens of
centimeters to some tens of meters (Sect. 10.2.1.5). The
lower limit of the chlorocline (defined by the range of
modern non-dasyclad green algae) is about 100 m. In
the context of the facies interpretation of limestones,
this boundary is sometimes used to separate upper and
lower subtidal depositional settings. Corallinacean red
algae , common in Cretaceous and Cenozoic limestones,
occur both in shallow and deeper waters, in tropical
settings between 20 and 80 m, in cool-water settings
down to more than 200 m (Sect. 10.2.1.2). Squamaria-
cean red algae , known since the Early Cretaceous, oc-
cur at water depths down to about 50 m.
Stromatolites: The formation of laminated micro-
bial deposits is not as strongly light-dependent as the
depth distribution of calcareous green and red algae.
Ancient stromatolites were formed in depth ranges of
meters to tens of meters. Growth forms, the composi-
tion and lamination of stromatolites is used to differen-
tiate intertidal/subtidal and deeper subtidal environ-
ments (Sect. 9.1.4).
Benthic foraminiferal biofacies is useful for estimat-
ing paleobathymetry in Cenozoic and Mesozoic sub-
tidal shelf carbonates (Sect. 10.2.2). The estimates made
are based on biofacies models reflecting distribution in
the modern ocean. The bathymetric arrangement of
biofacies is regarded as a response to oceanic water-
mass structure. Some caution, however, is necessary
because ancient taxa may have had broader bathymet-
ric ranges.
Larger foraminifera: Many larger foraminifera can
be taken as good evidence of shallow photic environ-
ments because of their endosymbiotic associations
(Hottinger 1988).
Ratio of foraminiferal groups: Various diagrams
describing the proportion of benthic foraminifera (cal-
careous versus agglutinated groups; ratio of aggluti-
nated, calcareous and planktonic groups; plankton/
benthos ratio) are successfully used in estimating paleo-
depth conditions in shelf and deep-sea environments.
Ostracods: The group occurs from very shallow
waters to bathyal regions (Sect. 10.2.4.7). Many benthic
groups have developed morphological adaptions to life
on the deep and very deep aphotic ocean bottom, al-
lowing bathymetric zones to be distinguished (Benson
1984).
Zooxanthellate corals: Most modern scleractinian
corals with endosymbiotic primary producers occur in
shallow depths of a few tens of meters in water gener-
ally less than 100 m (Sect. 10.2.3.3). Fossil scleractin-
ian reef corals and coral reefs are often regarded as
evidence of well-lighted shallow environments. Con-
sidering the strong control of reef corals by factors other
Bathymetric subdivision of benthic marine settings
The morphology of the sea bottom and physically
or biologically defined boundaries are used to differ-
entiate environments situated at various water depths
(Sect. 2.3.1). Physically defined boundaries are given
by the tidal range, the fair-weather wave base and the
storm wave base. These levels are characterized by
changes in sediment composition and texture. Biologi-
cal boundaries are given by environmental constraints
that govern the depth distribution and zonation of
benthic organisms. The most important limiting eco-
logical factors are light (Sect. 12.1.4.1) and seawater
temperature (Sect. 12.1.6). The absolute depth of these
boundaries is exceedingly variable. Hydrostatic pres-
sure may be a major control in depth distribution of
benthic organisms when other parameters do not vary.
12.3.1 Hints to Paleowater Depths
Paleowater depths can be deduced from various evi-
dence, including paleontological and microfacies cri-
teria, mineralogical and geochemistry data, sedimen-
tological features, and stratigraphic and geomorpho-
logical aspects. Bathymetry and shallow-marine zona-
tion patterns were summarized by Liebau (1980), vari-
ous methodological approaches to paleobathymetric in-
terpretations by Hallam (1967) and Luterbacher (1984).
The following text lists some criteria that assist in the
assessment of bathymetric zones. Most features char-
acterize 'shallow' environments comprising intertidal
and subtidal shelves. Specific criteria of 'deep-water'
carbonates are discussed in Sect. 15.8.
Biological evidence
The evaluation of some of the criteria listed below
is an integrated part of microfacies analysis and SEM
studies (calcareous algae, larger foraminifera, micro-
borers, micro-encrusters). Other criteria require exten-
sive palecologically-oriented investigations.
Calcareous algae: Depth distribution patterns of
green and red algae are one of the best indicators of
ancient depositional depths because of their strong light
dependence. Untransported dasyclad green algae point
to very shallow depths (tens of centimeters) down to
only a few meters (Sect. 10.2.1.7). They record photic
bottom conditions and occur within the tidal zone and
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