Geology Reference
In-Depth Information
16 Realizing Depositional Constraints and Processes
Having learned how microfacies can be integrated into
facies models (Chap. 14) and how microfacies can be
translated into depositional settings (Chap. 15), the fol-
lowing chapter summarizes the potential of microfacies
for recognizing and interpreting major depositional con-
straints. These constraints are manifested by charac-
teristic sedimentation patterns and require the applica-
tion of specific facies analysis techniques.
acterization and simulation. Intensive analysis of out-
crop data, detailed facies studies of carbonate cycles,
and computer-based forward models have led to the
development of 3-D models describing depositional and
diagenetic patterns of carbonate rocks and predicting
carbonate reservoirs.
Both parts of the section start with a brief overview
of basic principles, continue with the discussion of rel-
evant microfacies data, and close with some examples
illustrating the advantage of thin-section studies for the
understanding of cyclic carbonates and for sequence-
stratigraphy interpretations.
Sect. 16.1 deals with cyclic carbonates and underlines
the benefit of microfacies for sequence stratigraphy.
Sect. 16.2 discusses methods applied to reef carbon-
ates and stresses the necessity of combining micro-
facies data and ecological characteristics.
Sect. 16.3 explains how painstaking microfacies stud-
ies assist in reconstructing vanished carbonate plat-
forms that are only recorded by pebbles and boul-
ders.
Sect. 16.4 includes case studies of ancient cold-water
carbonates attesting to the significance of micro-
facies criteria.
Sect. 16.5 treats ancient carbonates interpreted as prod-
ucts of marine seeps and vents.
Sect. 16.6 gives a short summary of the facies criteria
of mixed carbonate-siliciclastic paleoenvironments
and discusses limestone-marl sequences.
16.1.1 Cyclic Carbonates
A cycle is a group of rock units that occur in a certain
order, with one unit being frequently repeated through-
out the succession. A stratification cycle is a group of
beds which are regularly repeated (Schwarzacher 1987).
Regularity may be recorded by bed composition, re-
peated sequences of bed thicknesses, or a constant num-
ber of beds composing the cycles.
Cyclicity is a common feature of many limestone
successions and occurs across different carbonate en-
vironments from platforms to reefs and down to ba-
sins. Understanding the origin of cyclicity in platform
carbonates is crucial if cycles are to provide informa-
tion on climate, eustacy and local tectonics, and if they
are to be useful for high precision correlation. Many
cycles are ascribed to corresponding, high-frequency
and low amplitude sea-level changes. Cyclostratigraphy
based on orbital cycles is a well-established approach
in basin analysis (Fischer et al. 1990; House et al. 1995;
Schwarzacher 2000).
Pioneering studies of sedimentary cycles were un-
dertaken in the Pennsylvanian and Permian 'cyclo-
thems' of North America, followed by detailed and so-
phisticated investigations of Triassic platform carbon-
ates in the Alps, and Cretaceous pelagic carbonates in
different parts of the world. The term cyclothem , origi-
nally suggested for specific Late Paleozoic sedimen-
Many of the constraints on carbonate deposition as
well as the biological contribution to the formation of
carbonate rocks were subject to secular changes dur-
ing the Earth's history. Some of these changes, reflected
by the occurrence, frequency and distribution of micro-
facies criteria, are the topic of Sect. 16.7.
16.1 Cyclic Carbonates, Microfacies
and Sequence Stratigraphy
The objective of this section is to document the value
of microfacies in the study of cyclic carbonates within
the framework of sequence analysis. Carbonate se-
quence stratigraphy plays a major role in reservoir char-
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