Geology Reference
In-Depth Information
than light and symbiosis, these interpretations argue for
support by additional criteria (e.g. growth form pat-
terns, community structure). Zooxanthellate symbio-
sis may have been independently established several
times in scleractinian coral groups. The existence of
zooxanthellate symbiosis in Paleozoic rugose corals is
strongly debated. SEPM Special Publication 72 (Kiess-
ling et al. 2002) contains much data on paleolatitudinal
distribution and the occurrence of Phanerozoic reef cor-
als in shallow or non-shallow environments. These data
should be checked.
•
Distributional patterns:
Water depth on its own does
not directly control distributional patterns, but bathy-
metric inferences may exist from environmental vari-
ables (e.g. temperature, salinity, substrate) that limit dis-
tribution with increasing depth. Benthic organisms ex-
hibit distinct distributional patterns from the shoreline
over the shelf to the shelf break. Gradient analyses of
fossil benthic assemblages (e.g. mollusks, brachiopods)
provide indications of relative water depths.
•
Zonation patterns of microborers:
Endolithic micro-
borers are excellent paleobathymetric indicators for eu-
photic and dysphotic depths of shelves, reefs and ramps
and also allow aphotic regions to be recognized (Sect.
9.3.4; Fig. 9.16). Zonation patterns of ancient endo-
liths show a high coincidence between paleodepths
derived from the composition of the endolithic asso-
ciations and the paleodepths accounting for the paleo-
geographical situation and facies distribution, indepen-
dent of the age of the examples. Comparative associa-
tions occurring in comparative paleowater depths are
known from the Early Paleozoic to the recent.
•
Micro-encrusters:
Distributional patterns of milli-
meter- to centimeter-sized encrustations resulting from
the growth of various heterotrophic and autotrophic
organisms offer a good chance for recognizing bathy-
metrically different environments of carbonate ramps
above the fair-weather wave base, between the wave
base and the storm wave base, and below the storm
wave base (Sect. 9.2.3; Fig. 9.10).
•
Trace-fossil communities
are strongly bathymetri-
cally controlled due to their behavioral response to a
bathymetric gradient in the food supply and oxygen.
Paleobathymetric estimations based solely on trace fos-
sil assemblages can have pitfalls, because factors other
than nutrients (e.g. oxygenation and sedimentation
rates) are also major controls on the composition of the
communities (see Frey et al. 1990 for a discussion).
everywhere. If you use the prevailing grain types of a
limestone as bathymetric indicators, you should con-
sider that the place of origin of sedimentary particles
and the depth of the final resting place can differ.
•
Ooids:
Accumulations of calcareous tangentially-
structured ooids originate today in very shallow and
shallow tropical waters down to a few meters of water
depth (down to about 15 m). See Sect. 4.2.5.
•
Aggregate grains:
Depth distribution is similar to
that of ooids. Many grapestones are formed in very shal-
low depths of only a few tens of centimeters.
•
Cortoids:
The existence of micrite envelopes around
skeletal grains is not an unmistakable indication of shal-
low-water deposition (Sect. 4.2.3), but ancient grain-
stones consisting of a high percentage of cortoids oc-
curring in association with calcareous green algae and
mollusk grains can be taken as proof of shallow sub-
tidal depths in euphotic and dysphotic environments
within a range of some tens of centimeter to a few
meters.
•
Oncoids:
Marine carbonate oncoids are formed in
the shallow euphotic and dysphotic zone, both in inter-
tidal and subtidal environments (Sect. 4.2.4.1). Cyan-
oids point to water depths within the meter range. Dif-
ferences in biotic composition (algae, microbes, fora-
minifera), lamination and nuclei offer possibilities of
distinguishing oncoids formed in shallow or deeper set-
tings (Box 4.10). Ferruginous or phosphatic non-car-
bonate oncoids are candidates for 'deepwater' but not
all candidates pass the test.
•
Rhodoids
occur from intertidal to deep subtidal en-
vironments (Sect. 4.2.4.2). They originate in shallow
and deep settings and often characterize the deeper part
of the photic zone and the dysphotic zone below the
wave base. The depth range of modern tropical rhodoids
comprises tens of meters, often around 80 m, but may
be extended to more than 180 m.
Macroids are common at depths of several tens of
meters. Non-tropical rhodoids are formed at various
depths. Temperate-cool water rhodoids occur between
about 5 and 150 m with a maximum between 20-40 m;
polar cold-water rhodoids are known from depths of
about 60 to 100 m.
Ancient rhodoids bear a high potential as paleodepth
indicators, if the biotic composition and the growth
strategies are thoroughly analyzed.
Depositional fabrics
•
Open-space and fenestral fabrics
are excellent
paleodepth indicators, but require detailed studies of
the subcategories and the fabric types (Sect. 5.1.5).
Birdseyes are good clues to water depths within a cen-
timeter range; stromatactis-type structures have a wider
Microfacies evidence
Specific grain types and fabrics can be used to dif-
ferentiate very shallow, shallow, deeper and deep depo-
sitional settings. Fine-grained micritic matrix originates
