Geology Reference
In-Depth Information
Sect. 10.2.1.5 ( Nuia, Pl. 98/1) and Sect. 15.1.2 ( Micro-
codium , Pl. 128/4).
Sedimentological significance: In modern shallow-
marine, low- to moderate energetic environments udo-
teaceans are a major source of calcium carbonate mud
from postmortem disintegrations of their skeletons. Hal-
imeda is abundant (1) in tropical lagoonal environments
(Hillis 1991) similar to fossil counterparts, (2) in reef
and forereef environments, and (3) on deep slopes,
forming banks and reefs (Drew 1983; Marshall and
Davies 1988). Ancient Halimeda -form algae contrib-
uted to sediment production, locally in the Late Trias-
sic (Pl. 57/3) and more commonly in the Cretaceous
(Pl. 57/6) and significantly since the Early Tertiary. The
sedimentological role of Paleozoic udoteaceans seems
to be restricted to some Ordovician and Devonian shelf
carbonates (Pl. 57/4, 5).
Fig. 10.12. Gymnocodiacean limestone. Various sections
through segments of Gymnocodium bellerophontis Rothpletz.
These algae were poorly calcified in the central part (which
is commonly filled with sediment) and strongly calcified in
the peripheral cortex, where the branched filaments may ter-
minate in the form of funnels or bowls, appearing as polygo-
nal structures in tangential sections. Faint relics of medullary
filaments occur within the grayish interior of two sections.
Middle Permian: Balya, Anatolia, Turkey. Scale is 1 mm.
Gymnocodiacean algae
Systematic position: These algae were originally re-
garded as codiaceans, but later attributed to the red al-
gae because of some similarities of the reproductive
structures to those in a genus of recent chaetangiacean
red algae (Elliott 1955). The affinity of the group is not
yet settled. A red algal affinity has been accepted by
many authors, but was recently questioned again in fa-
vor of a possible assignment to green algae (Mu 1991;
Bucur 1994). The gymnocodiaceans resemble fossil
erect udoteaceans in growth form, internal structure,
and palecological and paleogeographical distribution,
but seem to differ in containing internal reproductive
organs (Mu and Riding 1983).
Morphology: The extinct algae have erect, branched
or unbranched, segmented or unsegmented cylindrical
thalli, consisting of cylindrical, oval, or pyriform seg-
ments. The internal structure of these segments is char-
acterized by a medullar zone and peripheral cortical
zones. Calcification (primary aragonite) was signifi-
cantly stronger in the cortical than in the medullar zone;
the center of the thalli, therefore is commonly filled
with sediment or cement. Medullar filaments run par-
allel along the axis, cortical filaments are branched sev-
eral times and form a distinct pattern. Reproductive
structures are represented by oval or spherical cavities
in the cortex or in the area between the cortex and me-
dulla.
Distribution : The records of this group range from
the Middle Devonian (Mamet et al. 1994) to the Late
Tertiary, with most abundant occurrences at the Late
Permian (Mu 1991) and common occurrences in some
Cretaceous carbonates (Elliott 1958; Bucur 1994).
Limestones composed more or less completely of gym-
nocodiacean algae are common in the Middle and Late
Permian of the Tethys (e.g. Dolomite Mountains, South
Tyrol) and most abundant in the Capitanian and the
Early Changsiangian stages.
Fig. 10.11. Longitudinal section of
Permocalculus . This gymnocodiacean
genus is of rock-building importance
in Permian and Cretaceous carbonates.
Late Permian: Bergama, northwestern
Anatolia, Turkey. Scale is 1 mm.
Sedimentological significance : During the Late Per-
mian, gymnocodiaceans were important sediment pro-
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