Geology Reference
In-Depth Information
Distribution: Predominantly Middle Carboniferous
to Late Permian. A few records are known from the
Middle Devonian and the Late Triassic (Reid 1986;
Mamet and Preat 1983). The algae flourished during
the Late Pennsylvanian and Early Permian (Asselian
and Sakmarian).
eral decades of meters. Banks are more extended and
may cover areas of more than 50 km 2 . Location and
development of phylloid algal mounds seems to be con-
trolled by paleocurrent directions and nutrient levels.
Phylloid algae acted mainly as important sediment
bafflers, but some of them have also built reefs by con-
structing organic frameworks (Samankassou and West
2002). Some phylloid algae (e.g. Eugonophyllum ) ex-
hibit a felt of aragonite needles interspersed between
small parallel medullar tubes, similar to Halimeda .
These needles might have been a source for carbonate
muds of mounds and banks.
Sedimentological significance: Phylloid algae are
dominant rock constituents in bedded shelf carbonates
and contribute significantly to the formation of Penn-
sylvanian and Early Permian mud mounds, reef
mounds, and banks. Mounds and banks dominated in
shelf settings and at shelf margins (Wahlman 2002).
Phylloid mounds are characterized by flat bases, undu-
latory surfaces and high micrite content. Rapid growth
and recovery rates of phylloid algae were ideal for life
in settings strongly controlled by short-term sea-level
fluctuations. Stacked mounds reach a thickness of sev-
Laterally extended phylloid algal buildups are ma-
jor hydrocarbon reservoirs in the southwestern United
States (Montgomery et al. 1999) because of high intra-
skeletal and interskeletal porosities and common sub-
aerial exposure. The depositional texture of the phyl-
Plate 58 Phylloid Algae
'Phylloid algae' is a non-taxonomic term referring to predominantly Late Paleozoic calcareous algae character-
ized by leaf-like growth forms (Fig. 10.13). The group includes udoteacean green algae (Pl. 57), ancestral red
algae (Pl. 56) as well as squamariacean red algae with erect or inclined growth forms. Viewed in cross-sections
in limestones, phylloid algae appear as thin sinuous lines, sometimes looking like 'potato-chips'. Many phylloid
algae are recrystallized and lack sufficient internal features for taxonomic identification. Epifluorescence pe-
trography assists in overcoming this difficulty (Dawson 1992). Generally, the central (medullary) part of the
blades is filled with carbonate cement (-> 5). The very poor preservation as well as rare records indicate a
primary aragonite mineralogy. Taxonomically important criteria are the shape and arrangement of cortical branches
(-> 2, 4 and 5). Blades can be several centimeters long and up to 1 mm thick.
1 Phylloid algal floatstone. All bent, curved and straight elements are recrystallized algal blades. Mollusk shells differ in the
shape and lack of peripheral pores. Many algal blades are bored (B). Other biotic elements are scarce (gastropods - G, and
rare echinoderm plates - E). The chaotic distribution of the blades resembles the edgewise fabrics of conglomerates, and
points to reworking and deposition caused by storm currents. Edgewise fabrics have been described mainly from peritidal
environments, but also occur in subtidal settings, e.g. as storm or swell lags deposited in shoreface environments, lateral
accretion lags deposited in tidal channels, or as transgressive lags formed by ripping up of the underlying strata as the sea
level rose during platform drowning (Demicco and Hardie 1994). SMF 5. Late Pennsylvanian (Lower Pseudoschwage-
rina beds, Gzhelian): Carnic Alps, Austria.
2 Neoanchicodium Endo. Blades with poorly calcified tubes and one or two rows of ellipsoidal cells in the peripheral zone
appearing as a 'ring of pearls' in transverse sections (arrow). The genus is common in Early Permian open-marine plat-
form carbonates. Early Permian shelf platform: Forni Avoltri, Carnia, Italy.
3 Eugonophyllum Konishi and Wray, an udoteacean green alga. The thallus consists of large blades standing erect on the
substrate. The alga is most commonly preserved as isolated and often heavily recrystallized fragments. Filaments in the
central medulla are often obliterated by recrystallization. The blades are strongly encrusted by foraminifera (arrows).
E: Epimastopora (see Pl. 60/4). Early Permian: Carnic Alps, Austria.
4 Ivanovia Khvorova. The central (medullary) zone is recrystallized. The well-preserved cortical zone exhibits thin branches
oriented perpendicular to the medullary zone. The sample comes from phylloid algal packstones overlying a Archaeo-
lithoporella-Tubiphytes reef. Middle Permian (Wordian): Djebel Tebaga, southern Tunisia.
5 Eugonophyllum mulderi Racz. Well-preserved cortical zone (C) with crescent-shaped branches. The medulla (M) is oblit-
erated by dissolution and cement filling. E: Echinoderm plate. Late Carboniferous (Gzhelian): Hüttenkofel, Carnic Alps,
Austria.
-> 4: Toomey 1991
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