Geology Reference
In-Depth Information
bathymetric range most probably comprising both
photic and dysphotic settings. The common occurrence
of Stromatactis mounds on deep ramps is regarded as
clue for water depths of more than 100 m.
•
Bioturbation fabrics
reflect differences in nutrients,
oxygen and substrate that may differ at different water
depths. Abundance and burrowing types are used to
distinguish shallow- and deep-water environments
(Sect. 5.1.4).
Sedimentological evidence
•
Sedimentary structures:
Physical depth boundaries,
e.g. wave base and storm wave base are reflected by
specific sedimentary structures as well as microfacies
criteria (Sect. 12.1.2.1; Demicco and Hardie 1994).
Stratigraphic evidence
•
Estimates of absolute paleowater depth
values are
rare, but may be calculated using stratigraphic evidence
if the original paleorelief between the platform and the
basin is preserved and compaction is considered (Kenter
1990).
Mineralogical evidence
•
Carbonate minerals:
High hydrostatic pressure, low
water temperature and high partial pressure of CO
2
lead
to dissolution patterns of aragonite and calcite that char-
acterize deep-water levels (Sect. 2.4.5.6).
•
Authigenic minerals:
Syndepositional diagenetic
minerals, e.g. glauconite, chamosite or phosphorite are
often used in defining water depths although this ap-
proach is strongly disputed (Amorosi 1992).
12.3.2 Case Study: Assessing the Water
Depth of a Carbonate Ramp
Fig. 12.12 and Pl. 108 demonstrate the potential of
calcareous algae and other biota in recognizing rela-
Fig. 12.12.
Depth zonation
along a Late Viséan ramp
profile in the Béchar Basin,
Western Algeria. The depth
zonation is based on the dis-
tribution of algal assem-
blages and other biota and
the recurrence of distinct
associations in an analogue
outcrop study of micro-
facies, paleontological data
and limestone texture. Note
the different distribution of
green and red algae. Most
bedded grainstones and
packstones overlying the
mound facies originated in
the euphotic zone corre-
sponding to water depths
within the meter- to about
20 m range. Most mounds
consisting of bafflestones,
wackestones and mudstones
were formed in the aphotic
zone characterized by the
absence of algae. The num-
bers refer to bathymetric
zones defined by qualitative
and quantitative biotic com-
position. Oolitic and bioclastic carbonates formed in zone 1 exhibit large-scale cross stratification. Zone 2 was dominated by
crinoids which were frequently reworked by storms. Zone 3 is characterized by brachiopod shell banks above the coral
thicket of zone 4. The upper part of zone 5 shows evidence of reworking. The zones 5 to 7 are parts of the mound facies. The
ramp is differentiated not only by biota but also by the dominant sediment: Oolitic and bioclastic sands prevail on the upper
ramp, crinoid banks on the mid ramp, and massive dome-shaped mounds on the deep ramp. The reservoir potential occurs in
ooid and crinoid grainstones overlying the mounds, formed in the bathymetric zones 1 and 2 (Madi et al. 2000).
Width of the bars indicate relative abundance of major fossil groups. FWB - fair-weather wave base, SWB - storm wave
base. After Madi et al. (1996).
