Geology Reference
In-Depth Information
and a few hundred thousand years (for fourth-order)
are grouped into third-order cycles formed on a scale
of about 1 to 10 million years. Second-order cycles have
durations ranging from 10 to 100 Ma and first-order
cycles last several hundreds of million years.
First- and second-order cycles attributed to long-
term sea-level changes and tectonics (breakup of su-
percontinents, changes in spreading rates and in the
flexural rigidity of the lithosphere) will not control the
internal architecture of carbonate sequences, because
the rates of the amplitudes of sea-level fluctuation (1-
2 cm/1000 years) are two orders of magnitude less than
carbonate sedimentation rates (10-1000 cm/1000
years). The formation of sedimentary cycles in the third,
fourth and fifth order bands have been explained by
Milankovitch cycles, but also by autocyclic factors and
tectonic controls. Causes of third-order cycles are the
waxing and waning of polar ice caps (glacio-eustasy)
and changes in the volume of ocean basins (tectono-
eustasy). Shallow-marine carbonate platforms and
ramps as well as pelagic carbonates record third-order,
fourth-order and fifth-order eustatic cycles and depo-
sitional sequences (see Sect. 16.1.2.1).
•
Statistical methods:
Cyclicity in carbonates may not
be obvious when looking at the changes in lithology,
texture or biotic composition. In order to recognize hid-
den cyclicity patterns statistical methods are used.
These methods include correlation analysis, principle
component analysis and spectral analysis looking at
time series in terms of their frequency composition (e.g.
Pelosi and Raspini 1993; Gischler et al. 1994). Tradi-
tional time-series analysis assumes that the cyclic record
is complete, so that each recorded stratigraphic rhythm
corresponds to one pulse of the cycle-producing mecha-
nism. The main assumptions are equal periodicity per
cycle and no gaps in the data set.
Power spectral analysis is applied to look for cy-
clicity and detect orbital-climatic forcing (see papers
in De Boer and Smith 1994). These methods are ap-
plied to platform and ramp carbonates as well as pe-
lagic carbonates. The basic data used comprise thick-
ness, stacking and composition of strata as well as the
distribution of paleontological or geochemical data.
Cycle-thickness data can be analyzed for its direction-
ality or randomness (Drummond and Wilkinson 1993;
Grötsch 1996). Regarding the sedimentation rate as
constant and periodicity as variable, Schwarzacher and
Fischer (1982) have calculated periodic cyclicity from
a statistical analysis of the thickness of the sequences.
Spectral power spectra based on a fast Fourier trans-
formation of cycle thickness to frequency (cycles per
1000 years) indicate whether the sedimentary cyclicity
is within the Milankovitch spectrum or not (Schwar-
zacher 1991) and aid in the discussion of the recog-
nized cyclicity in terms of eccentricity, obliquity and
precession cycles (Goldhammer et al. 1987, 1990;
Osleger 1991). The techniques of time series analysis
are described in Einsele et al. (1991), Sprenger and Ten
Kate (1993) and Schwarzacher (1993).
Recognition of cyclicity:
Cycles and sequences can
be recognized
by changes within the sedimentary suc-
cession
expressed by:
•
Lithology
(alternation of limestone and dolomite,
or limestone and marls; insoluble residue; see Sect.
13.1.1.1).
•
Bedding
(recurrent changes in bed thickness, bun-
dling or composition). Sedimentary rhythms could be
reflected by couplets of strata or groups of couplets.
•
Facies
including depositional and diagenetic micro-
facies (see the following text).
•
Fossils
(distribution, association, frequency). Ichno-
fossils and burrowing fabrics in pelagic carbonates are
particularly useful in determining sea-level positions
(Savrda and Bottjer 1994).
•
Geochemistry
including fluctuations in stable iso-
topes (Sect. 13.2.2), major and minor elements (Sect.
13.2.1), and content of organic matter (Sect. 13.3).
Carbon as well as oxygen isotopes are used to deci-
pher the cyclicity of shallow-water micrites, but car-
bon values may offer more reliable data than oxygen
values, which are more strongly affected by diagen-
esis. Distinct differences in the isotope data of the mi-
critic matrix of tidal and subtidal limestones reflect cy-
clic changes in environmental conditions or different
diagenetic controls (Joachimski 1991; Jimenez de
Cisneros and Vera 1993).
16.1.1.2 Microfacies and Cyclic Carbonates
Variations in microfacies criteria within a sedimen-
tary succession (e.g. a cycle or a parasequence) indi-
cate changes in environmental controls and depositional
rates. Changes in sea-level position (e.g. shallowing-
upward) are connected with shifts in depositional wa-
ter depths, which in turn can be associated with changes
in hydrodynamic conditions (e.g. low-energy to high-
energy). Changes in depositional rates influenced by
sea-level fluctuations may lead to the formation of
boundaries between depositional units (sequences)
pointing to significant alterations and/or breaks in sedi-
mentation. These breaks are expressed by submarine
discontinuities and unconformities, and by subaerial
