Geology Reference
In-Depth Information
All the families except the Beresellaceae are non-
septate. Because the Beresellaceae differ from the typi-
cal dasyclads by the horizontal and vertical septation
of the thalli and the absence of laterals, some authors
consider the group as separate order of green algae.
ically and correlated. Generic determinations require
evaluation of the following criteria: The shape and sub-
division of the thalli; aspondyl, euspondyl or meta-
spondyl arrangement of laterals; the shape of the later-
als; and the position of reproductive organs. If these
criteria and the extensive stratigraphic overviews of
dasyclads are used, nonspecialists will also be able to
determine genera and higher systematic groups. A de-
termination of species should be left to specialists.
How to determine dasyclad algae?
Many people
studying the microfacies of carbonate rocks are quite
satisfied to attribute algal bioclasts to the dasyclad
group, and postulate that the sample indicates deposi-
tion in shallow or very shallow warm water of normal
salinity. This is a very generalized statement that can
be significantly enhanced if the dasyclads are differen-
tiated at a genus and/or species level. Further system-
atic determinations are also necessary if algal-bearing
platform carbonates are to be subdivided stratigraph-
Distribution
of Phanerozoic dasyclads (Fig. 10.16):
The first dasyclads are known from the Late Cambrian,
but the group remains subordinate in contrast to cy-
anobacterians and other algal groups from the Ordovi-
cian to the Devonian. The first diversification took place
during the Carboniferous and Permian, a total renewal
Text continued on p. 442
Plate 61 Triassic Dasyclad Green Algae
No calcareous algae are known from the Early Triassic. Dasyclad algae are common in Tethyan Middle and Late
Triassic shallow-marine carbonates. Triassic dasyclads display environmentally controlled distribution patterns
that are used in the spatial subdivision of lagoonal platform carbonates (Senowbari-Daryan and Schäfer 1979;
Lobitzer et al. 1990). Middle Triassic dasyclads contributed significantly to the accumulation of grain- and
packstones. Regional stratigraphical zonations of Tethyan carbonates are based both on algal assemblages and
on the time-range of species (Ott 1972). A revised zonation has been worked out by Piros et al. (1994, 2002),
who established a detailed zonation for the Middle Triassic and Carnian.
1
Bioclastic grainstone composed of dasyclad algae, aggregate grains (AG), a few solenoporacean algae (S) and corals (C).
Dasyclads are
Diplopora annulata
(Schafhäutl) (D) and
Teutloporella
herculea
(Stoppani) (T). See Bucur and Enos
(2001) for a discussion of the taxonomy of
Diplopora
-like dasyclads. Identification of them requires evaluation of the
criteria seen in cross sections (CS) and longitudinal sections (LS). Note the occurrence of several marine cement genera-
tions. The sample is a good example of SMF 18 (see Pl. 122). Paleoenvironmental interpretation: Deposition in high-
energy environment (indicated by lack of micrite and grain-support fabric), water depth of a few meters (as compared
with depth distribution of modern dasyclads and inferred from the occurrence of aggregate grains), warm water (related to
the abundance of dasyclads), unstable sand bottom (high primary intergranular porosity, incipient marine cementation).
The sample represents lagoonal platform limestones formed within a back-reef area. SMF 18-D
ASY
. Middle Triassic
(Wetterstein Limestone, Late Ladinian): Amtmanngalgen, Gesäuse Mountains, Styria, Austria.
2
Dasycladad grainstone. Cross sections of
Heteroporella zankli
(Ott) characterized by large sporangia arranged in rings.
Dasyclads (D) are associated with porostromate cyanobacteria (C). Note high primary intergranular porosity now oc-
cluded by isopachous acicular marine-phreatic cements (MPC) and sparry coarse burial cements (CBC). Paleoenviron-
mental interpretation: High water energy (grainstone fabric, reworked cyanobacteria nodules), water depth a few meters
(as compared with the average distribution of modern dasyclads and calcified cyanobacteria), warm water (chloralgal
association, see Pl. 105/2), instable sand substrate. The sample comes from outer lagoonal carbonates deposited behind a
major reef complex formed by coralline sponges and corals. Late Triassic (Norian to Rhaetian): Cuzzo di Lupo, Palermo
Mountains, northwestern Sicily, Italy.
3
Dasyclad grainstone composed of
Teutloporella nodosa
(Schafhäutl). Diagnostic criteria are shown in longitudinal (LS)
and oblique sections. The long thalli are annulated. The aspondyle branches become thinner toward the outside, forming
a hairlike structure (white arrow). Note distinct circumgranular isopachous cements (black arrows) consisting of acicular
crystals. These cements were formed in marine phreatic environments. Paleoenvironmental interpretation: Shallow-ma-
rine warm-water carbonate, formed in high-energy areas. The high and still open interskeletal porosity (P) can be ex-
plained by strong meteoric phreatic diagenesis. The sample comes from platform shoals formed by a parautochthonous
accumulation of algae.
Teutloporella
is a facies-diagnostic fossil; it characterizes subtidal lagoonal deposits as well as
proximal reef detritus areas (Ott 1967; Piros et al. 1994). Stratigraphic range of the species: Ladinian and Early Carnian.
Middle Triassic (Late Ladinian): Calabria, Southern Apennines, Italy.
