Geology Reference
In-Depth Information
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correlation by physical tracing of laterally continu-
ous bedding surface of platform tops and platform
flanks;
of a particular depositional environment without taxo-
nomic treatment or at least the consideration of eco-
morphological groupings. Seek out cooperations with
paleontologists!
•
biostratigraphic correlation, limited by ecological
and preservation factors;
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well-log marker, e.g. condensation horizons, light
stable isotopes, element chemostratigraphy, foramin-
iferal abundance, organic carbon (see Kauffman
1988).
15.7.5.2 Case Study: Late Triassic of the
Gosaukamm Region, Austria
The potential of microfacies analyses for interpreting
platform-to-basin relations is exemplified by impres-
sive case studies dealing with Late Triassic (Norian and
Rhaetian) depositional environments in the Northern
Calcareous Alps (Fig. 15.29). Shallow-marine environ-
ments are represented by the Dachstein Formation con-
sisting of thick series of platform interior carbonates
with marginal coralline sponge and coral reefs. The
rhythmically bedded platform carbonates display the
'Lofer cycles' reflecting sea-level fluctuations of dif-
ferent orders (see Sect. 16.1). The platform margin
passes into the deeper marine Hallstatt basin filled with
marls and pelagic carbonates and gravity flow depos-
its including calciturbidites. The periplatform 'Pedata/
Pötschen beds', representing the basinal equivalent of
the Dachstein Formation, are characterized by an abun-
dance of calciturbidites and submarine slides.
These formations are exposed in the Gosaukamm
area about 60 km southeast of Salzburg, Austria. Micro-
facies and paleontological data of the Dachstein lime-
stone are well known, allowing the paleoenvironmen-
tal significance and distribution of the shallow-marine
biota in different parts of the platform and the reefs
(Wurm 1982) to be evaluated. Reijmer and Everaars
(1991) and Reijmer et al. (1991) studied a 95 m thick
section of the Late Norian Pedata/Pötschen limestone
exhibiting 810 calciturbidite beds. The Lacke section
comprises mud-, pack- and grainstones, alternating with
calcisiltites and marls. The mudstone to grainstone beds
show grading and sharp contacts and correspond to tur-
bidites. Calcisiltites and marls represent the pelagic
background sediments.
Point-counter analyses of samples taken from the
top and bottom, in thicker beds also from the middle
part of each turbidite, were the basis for the statistical
treatment of the frequency data of grains and matrix.
The constituents encountered in thin sections were sum-
marized into facies-diagnostic groups and assigned to
seven point-counter groups (clusters) that characterize
particular paleoenvironments of the suggested source
areas on the platform (e.g. platform interior), within
the reef (e.g. shallow reef, deeper forereef) and in the
open ocean. Summary statistics revealed a close rela-
tionship between the biota present on the Dachstein
platform and the composition of the calciturbidites.
The
principal assumption
in tracing platform-basin
relationships by compositional logs
is that composi-
tional variations in redeposited shallow-water material
within basinal sediments adjacent to platforms reflect
variations in water depths at the top of the platform
and sea-level fluctuations as demonstrated for Late
Quaternary interglacial and glacial calciturbidites dif-
fering in grain composition (Reijmer et al. 1988; Haak
and Schlager 1989). The abundance increase and de-
crease in specific grains may indicate evolutionary
stages of platforms, e.g. drowning of platforms associ-
ated with a decrease in ooids, or prograding and ag-
grading of platforms by an increase in the abundance
of coarse skeletal sands.
The intensity of shallow-water sediment export may
be different at different flanks of the source area, e.g.
at the opposite sides of a reef (Betzler et al. 1995). The
timing of strong shedding is controversial:
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Shedding during sea-level highstands (Droxler and
Schlager 1985; Reijmer et al. 1988; Haak and
Schlager 1989; Glaser and Droxler 1991; Schlager
1992).
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Shedding during the late highstand (Shanmugan and
Moiola 1988).
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Shedding during sea-level lowstand.
The
basic prerequisite
for the success of the Grain
Composition Log method is the
knowledge of the spe-
cific occurrence of grain categories and fossils in
spe-
cific source areas,
e.g. specific platform and reef envi-
ronments. This knowledge presupposes that the distri-
bution patterns and their ecological controls of plat-
form and reef organisms of a particular time slice are
already well established. Since distribution patterns of
benthic organisms have changed over time, composi-
tional logs of calciturbidites will be only interpreted
successfully if paleontologically well-founded zona-
tion patterns for the specific time slice are available
(Reijmer and Everaars 1991; Reijmer et al. 1991; Everts
1991, 1999).
Caution:
Similar or identical biota may
indicate platform interior environments for one time
slice, and marginal platform environments for another
time slice. Some common fossil groups (e.g. echino-
derms, brachiopods) can hardly be used as indicators
