Geology Reference
In-Depth Information
Plate 135 Microfacies of Ramp Environments: Middle Devonian of Graz (Austria)
The 'Barrandei Limestone' outcropping near Graz in Styria (Austria), named after a heliolitid tabulate coral
species (see Pl. 83/5, 6) is an example of carbonates deposited in near-coastal, subtidal ramp environments
influenced, but not controlled by siliciclastic influx.
The up to 40 m thick sequence consists of dark-blue to black limestones that alternate with calcareous marls
and shales. Both limestones and marls, yield abundant fossils, predominantly crinoids, brachiopods, tabulate
and rugose corals, and stromatoporoids. The limestones represent a low-energy mud facies with mudstones,
wackestones and packstones (-> 3), a fossiliferous high-energy mud facies (-> 1, 2, 4-7), a high-energy debris
facies with tempestites, and a reefoid facies with tabulate corals, stromatoporoids and udoteacean green algae.
Microfacies, fossils, and geochemical data indicate sedimentation on the inner part of a ramp with shallow shelf
lagoonal and somewhat deeper open-marine environments (Hubmann 1993). The input of clay, silt and land-
derived organic matter (Hubmann 2000) is not reflected by differences in the development of microfacies types
over time, but by the changes in the quantitative composition of skeletal grains and deviations of the normal
growth patterns of favositid tabulate corals. The crinoid columnals exhibit broad variations with regard to the
number and shape of central canals (-> 4-7) allow morphotypes to be differentiated. They have become increas-
ingly more important both in facies analyses and biostratigraphy (Le Menn 1987; Ebert 1994).
All samples shown are from dark, bedded Middle Devonian (Eifelian) limestones exposed at the Fürstenstand
on the Plabutsch Mountain west of Graz, Austria. This locality is characterized by a significantly high abun-
dance of fossils (Heritsch 1943). From the Middle Ages on, Devonian limestone has been used for building
material in the city of Graz.
1
Bioclastic coral-brachiopod wackestone/floatstone
. The limestone is characterized by diverse thick-shelled brachiopods
(B), dendroid tabulate corals, and crinoids (C). The black arrow points to a cross section of a brachiopod shell cut oblique
to the hinge line and showing the interior plates. The shell was infilled with sediment and coral fragments. The white
arrow points to infilling with large peloids (fecal pellets). Dendroid tabulate corals are represented by cross-sections of
Thamnopora
(TH) and
Syringopora
(SY). The inhomogeneous texture of the micritic-siltitic matrix indicates strong
burrowing. High-energy mud facies. This sample corresponds to Standard Microfacies SMF 9 and Facies Zone FZ 8 in
the Wilson Model, and to the Ramp Microfacies RMF 3 common in mid-ramp and outer-ramp settings (see Fig. 14.30).
The biotic assemblage of this facies type is different than the standardized Middle and Late Devonian shallow-marine
carbonates (Machel and Hunter 1994; Fig. 14.9 and Fig. 14.10).
2
Detail of the sample shown in -> 1.
Tabulate corals (cross and oblique sections of
Thamnopora
, TH), abundant crinoid
fragments (C) and brachiopod shells (B). The micritic matrix contains quartz silt.
3
Calcisphere packstone.
Circular and elliptical spar-filled sections are calcispheres, most probably algal spores. Note the
highly variable shapes and the wide size range of the spheres. The interpretation as algal spores is supported by the
common occurrence of udoteacean green algae in the studied section. The arrow points to a recrystallized udoteacean
thallus. Low-energy facies. In the standardized Devonian reef model (Machel and Hunter 1994), this facies type would be
assigned to facies I-f, characterizing sediments deposited in quiet water below the wave base in an off-reef setting. Using
a ramp model, this microfacies would be interpreted as a sediment deposited in an inner ramp, lagoonal environment
(Ramp Microfacies RMF 17; see Fig. 14.30). This interpretation is in agreement with the geological and geochemical
criteria of the Barrandei limestones. Using a platform model, the microfacies would correspond to SMF 18, characteriz-
ing a bank interior environment (FZ 8 of the Wilson model).
4
Characteristic fossils.
Cross-section of
Thamnopora boloniensis
(TH)
,
crinoidal columns and trilobites (TR). The black
arrow points to a thin-shelled brachiopod. The section resembles cross sections of vermetid gastropods and is reminescent
of a cross section of an ammonoid shell. However, these shells have different microstructures. This identification as a
brachiopod shell is based on the occurrence of fine pores within the shell. Note the strong biogenic encrustation of the
brachiopod by cyanobacteria (
Girvanella
). The white arrow points to the double-hole structure within a crinoid columnal
(C). High-energy facies.
5
Characteristic fossils.
Cross sections of crinoid columnals exhibiting dumbbell-shaped central structures. The variations
in the central structures allow columnal morphotypes to be distinguished. Types and abundance of these morphotypes
vary throughout the Paleozoic (Stukalina 1988). Tabulate corals are represented by two genera (
Thamnopora
, TH; and
longitudinal and cross sections of
Syringopora
, SY). Note the difference in the size of the corallites of
Thamnopora
as
compared with -> 4, indicating the existence of two species (-> 4:
Thamnopora boloniensis
; -> 5:
Thamnopora reticulata
).
6
Differentiation of crinoid elements.
Crinoid columnal exhibiting the canal pattern of
Cupressocrinites
characterized by
four peripheral canals separated from the central lumen.
Cupressocrinites
is a widely distributed genus known from the
Late Emsian to the Early Frasnian (Ures et al. 1999).
7
Crinoid packstone.
Accumulation of small crinoidal columnals with two small, closely set canals of equal size.
