Geology Reference
In-Depth Information
Cyanoids occurring in different sedimentary environ-
ments are characterized by different criteria that can
be used in recognizing changes in sea level. Shallowing
is recorded by stromatolitic crusts overgrowing the
oncoids, changes in the size, shape and composition of
oncoids, and differences in depositional textures
(Ratcliffe 1988). Rhodoids formed in shallow and deep
waters and under different hydrodynamic conditions
differ in composition, shape and size (Sect. 4.2.4.2).
These differences can be used for estimating shallow-
ing- and deepening-upward patterns. Burrow systems
and taphofacies are sensitive indicators of shallowing
and deepening both in shallow and deep environments
(Brett 1995; Monaco and Giannetti 2002).
served as detritus and aragonitic skeletons; (2) detrital
sand facies (grainstones) with abundant coated reef
debris; and (3) baffler facies (wackestones, packstones)
distinguished from the other facies types by the high
amount of micrite and small amount of sediment-baf-
fling algae.
The multivariate data set was reduced by principal
component analysis. Using component loadings as a
relative index of framework production, vertical aggra-
dation reef cycles controlled by regional subsidence
rate, a third-order change in eustatic sea level, and su-
perimposed high-frequency fluctuations in the Milanko-
vitch band were recognized.
Sea-level fluctuations were also documented by the
changing abundance of marine and meteoric carbon-
ate cement types. The boundaries of depositional cycles
and diagenetic cementation cycles do not coincide be-
cause cementation lags behind sedimentation and early
diagenetic meteoric overprint may extend several
meters down into the sediment.
Cyclicity in reefs
Sea-level fluctuations are major controls on the
growth of shallow-water reefs. Reef-building organ-
isms respond to shallowing and deepening of the reef
top with changes in growth forms and guilds, succes-
sion patterns and the rate of carbonate production. Cy-
clic sea-level fluctuations can be recognized in ancient
reefs using reef guilds and reef diagenesis, as demon-
strated by Grötsch (1994) and Grötsch et al. (1994)
studying an Early Cretaceous reef in Slovenia formed
by corals and various encrusting organisms.
Shallow-marine and deep-marine cyclicity
Relationships between cyclic sedimentation on car-
bonate platforms and cyclicity in basinal carbonates
are reflected by
• prograding bioclastic wedges of depositional se-
quences in outer platform settings correlating with
packages of basinal limestone beds and bioclastic
clinoforms (e.g. Quesne and Ferry 1994);
• correlation of shallow-marine parasequence patterns
with those of basinal carbonate cycles derived from
compositional and geochemical data (Elder et al. 1994);
• composition of calciturbidites (e.g. Reijmer et al.
1994; Sect. 15.7.5.2). Microfacies analysis indicates
where the material has come from and whether a cy-
clic pattern exists. Answering the last questions requires
statistical treatment of frequency data using spectral
analysis and time analysis;
• compositional analyses of allochthonous sediment
intercalated within pelagic and hemipelagic beds,
caused by highstand shedding (Sect. 16.1.2.2).
Methods:
The section studied in Slovenia comprises
133 m massive coarse-grained reefal limestones and
fine-grained lagoonal limestones. Samples were taken
with an equal-distance spacing of 0.5 m. A peel with
an area of 16 cm
2
was prepared from each sample. In
order to set up a semiquantitative index to trace car-
bonate production per time unit, different environment-
sensitive variables have been considered:
• Relative abundance of reef guilds (constructor,
binder, baffler, dweller, destroyer; see Sect 16.2.3.2 for
discussion of the guild concept).
• Primary content of biogenic aragonite comprising
completely or partly aragonitic and now recrystallized
organisms.
• Textural types according to Dunham (1962). These
qualitative facies variables were transformed into 'fa-
cies numbers' that increase with increasing sparry ce-
ment and decreasing micrite content, increasing grain
size and increasing water energy.
Pelagic cyclicity
Recognizing and interpreting pelagic carbonate
cycles with respect to climatic variations and sea-level
fluctuations demands a combination of sedimentologi-
cal, geochemical and paleontological data and the
evaluation of these data by statistical analyses, e.g. spec-
tral analysis. Commonly used basic criteria are strati-
Results:
From thin section analysis three facies types
can be distinguished: (1) In-place and detrital frame-
work facies (grainstones, rudstones, boundstones) char-
acterized by abundant reef-building organisms pre-
