Geology Reference
In-Depth Information
often used to describe textural changes over time in the
controls of microfacies types (Carozzi 1988).
Up to now few but thought-provoking grain-size
analyses of ancient carbonate rocks exist. The follow-
ing case studies demonstrate the potential of this ap-
proach. Turbidites are discussed in Sect.15.7.3.4
Case studies: Grain-size analyses of Late Jurassic plat-
form and slope carbonates from the Alps
Late Jurassic carbonates in the central and western
Northern Calcareous Alps (Austria) offer an excellent
opportunity to discuss the value of grain sizes for dif-
ferentiating of ancient depositional environments. Sev-
eral isolated Bahamian-type carbonate platforms (Plassen
Formation) in the central Northern Alps are connected
with deep-marine basinal sediments (Oberalm Forma-
tion) by slopes consisting of allochthonous carbonates
represented by mass-flow sediments (Barmstein For-
mation: Steiger 1981) and fine-grained breccia (Tressen-
stein Formation; Hötzl 1966). Another open-marine
carbonate platform of approximately the same age is
the Sulzfluh limestone (Ott 1969) at the boundary be-
tween Switzerland and Austria. This limestone has been
interpreted as inter- and subtidal deposition but can
sometimes be reinterpreted as beach sediment in the
light of grain-size patterns.
The microfacies of these limestones has been stud-
ied in detail. Grain-size analyses were applied to bio-
and lithoclastic calcarenites. The methods used are: in-
terpretation of cumulative curves, parameter sorting and
parameter diagrams as well as Passega diagrams.
Fig. 6.5.
Inclusive standard deviation
σ
i
versus mean
of
twenty samples from the Late Jurassic Sulzfluh limestone.
Most biolithoclastic calcarenites (Fig. 6.6A) are moderately
well to moderately sorted and fall within field B. Field B
characterizes the pattern produced by calcareous bioclastic
sands from a modern carbonate beach in Yucatan (Folk and
Robles 1964). Field A shows the pattern of samples from ad-
jacent shallow submerged areas. Sorting classification after
Friedman (1962). Note: Another often used sorting classifi-
cation (Folk and Robles 1958) differs somewhat in boundary
values for moderately sorted samples. Modified from Ott
(1969).
Passega diagram shown in Fig. 6.6B. This pattern cor-
responds to beach sediments, too.
Does this interpretation make any sense? The biotic
composition (green algae, abundant porostromate cy-
anobacteria) and strong micritization indicate the ex-
istence of a very shallow sea. Wide grain-size ranges
and moderate sorting can be explained by mixing and
reworking of sedimentary particles. Many samples
show evidence of early marine-vadose diagenesis
(gravitative cements, internal silt). The co-occurrence
of skeletal grains and black pebbles (Sect. 4.2.8) points
to the existence of a very near vegetated coast line.
These criteria fit into the mental image of a sunny hu-
mid environment with sand cays at the margin of an
open-marine subtropical platform.
Sulzfluh limestone:
The carbonate platform exhib-
its two major facies, predominantly mud-supported
limestones and predominantly grain-supported lime-
stones. The latter consist of grain- and rudstones rich
in skeletal grains (calcareous algae, foraminifera, mol-
lusks, corals) and synsedimentary clasts. The grain sizes
of these biolithoclastic carbonates were studied using
the mean, standard deviation, skewness and kurtosis.
Parameter diagrams (Fig. 6.5) were compared with the
Passega diagram (Fig. 6.6B).
For modern calcareous sands, the sinusoidal pattern
in the sorting versus mean diagram observed in the
Sulzfluh samples (Fig. 6.5) is explained as a function
of the average grain size and indicates mixing of vari-
ously sized grain populations (Folk 1962). This pattern
is common in beach sediments, but also occurs in other
shallow- and deep-marine settings (Hubert 1964). The
same Sulzfluh samples have been used to construct the
Tressenstein limestone and Barmstein limestone:
Most Tressenstein limestones are characterized by fine-
grained lithoclastic calcareous breccias and coarse cal-
carenites consisting of poorly rounded, microfacially
strongly variable clasts. Common fossils are corals,
chaetetid sponges, calcareous algae and foraminifera.
The composition of the Barmstein limestones, which
intercalate with the Tressenstein limestones and occur
as distinct beds within the basinal Oberalm limestones,
is similar to the Tressenstein limestone. Significant dif-
ferences between Tressenstein and Barmstein lime-
stones relate to the size and sorting of the lithoclasts as
well as sedimentary structures, which point to the depo-
