Geology Reference
In-Depth Information
Ancient rhodoids and macroids
Most fossil rhodoids are known from Tertiary shelf
carbonates and reefs (Box 4.13). Coralline algae ap-
pear in the Late Jurassic. Only a few rhodoids have
been reported from the Cretaceous (e.g. Early Cenoma-
nian solenoporacean-corallinacean rhodoids, reef flat
facies). Late Carboniferous and Permian rhodoids (Pl.
56/8) differ from Cenozoic rhodoids in their preference
for near-coastal, very shallow settings.
Significance of rhodoids in microfacies analysis
Rhodalgal limestones represented by shelf carbon-
ates and reefs with large contributions of coralline al-
gae are, volumetrically, the most important facies in
the Miocene Mediterranean (Esteban 1996). The rhod-
algal-type grain associations are indicative of platforms
developing in temperate seas or in subtropical to tropic
areas with anomalous (e.g. cooler, eutrophic, upwelling)
conditions (Carannante and Simone 1996; Sect. 12.2).
Plate 12 Cyanoids and Rhodoids: Cyanobacterial and Red Algal Nodules
Cyanoids (-> 1) are a specific type of porostromate oncoids, characterized by recognizable tubular micro-
structures which can be attributed to calcified cyanobacteria. Cyanoids are common constituents of Paleozoic
shallow-marine limestones.
Rhodoids (rhodoliths; -> 6) are nodules formed by encrusting (mainly rhodophycean) red algae. The free-
living encrusting calcareous algae exhibit characteristic finely meshwork structures (see Pl. 54). Other organ-
isms contributing to the formation of nodules are encrusting foraminifera that form centimeter-sized macroids.
Living rhodoids occur in tropical, temperate and polar settings and are found from tidal zones down to more than
200 m. They contribute to the formation of platforms, ramps and reefs. Ancient rhodalgal-type grain associations
are often indicative of platforms formed in temperate seas or in subtropical to tropic areas, sometimes with
anomalous (e.g. cool-water, upwelling and nutrient-rich) conditions. Diagnostic descriptive criteria are the ex-
ternal shape, internal growth patterns, size ranges, and the biotic composition of the cortices. In addition, nuclei
types and the associated fauna are of importance. The growth form and shape are controlled by hydrodynamic
factors, the biotic composition by light intensity and competition with herbivorous organisms.
1
Cyanoids
, a group of skeletal oncoids formed by calcified porostromate cyanobacteria (see Pl. 53). Cores are platy phyl-
loid calcareous algae (see Pl. 58). Note the irregular discoidal shape of the oncoids and breaks in growth of layers. The
elliptical shape of the grains depends on the shape of the nuclei; the laminae form concentrically around the cores (Type
C oncoid, Radwanski and Szulszewski 1966). Oncoid sizes in the oncolitic bed vary between about 5 mm and 1 cm and
are within the range of piso- and macro-oncoids. Associated fossils are coated fusulinid foraminifera. SMF 13. Open-
marine inner platform. Early Permian (Asselian): Carnic Alps, Austria/Italy.
2
Micrite oncoids
produced by micritization of microbial oncoids, aggregate grains and peloids. Oncoid nuclei are shells
and foraminifera. Note solution void filled with silt (arrow). Open-marine platform. Early Jurassic (Pantokrator Lime-
stone): Peleponnesus, Greece.
3
Porostromate oncoids
composed of
Rothpletzella
( -> 3). These microfossils (attributed to green algae or to cyanobacte-
ria) are common constitutents of Devonian open-marine and semi-restricted shelf limestones and reefs. They occur to-
gether with lamellar stromatoporoids and contribute to the formation of boundstones. Note the alternation of black (with
tubular fossils) and white sparry laminae. Forereef slope. Late Devonian (Frasnian): Canning Basin, Australia.
4
Two-stage oncoid.
The nucleus, a chaetetid coralline sponge is surrounded by an unlaminated meshwork corresponding to
Bacinella
(imterpreted as microbial in origin). Lamination is only developed at the periphery of the spherical oncoid. The
grain is interlinked with other
Bacinella
oncoids, forming a framework texture together with corals and coralline sponges.
High-energy subtidal environment of a carbonate platform. Late Jurassic (Plassen Limestone, Tithonian): Krahstein,
Styria, Austria.
5
Modern soft lacustrine oncoid
composed of cyanobacteria and green algae. The asymmetrical shape and the dome-like
pattern of the laminae point to in-situ growth of the oncoid. The small micritic grains result from disintegration of the
oncoids by waves and frosting during winter. The oncoids from Lake Constance are restricted to near-shore areas. There
are two types, one smooth and hard, the other soft and exhibiting a spongy fabric. Soft oncoids occur embedded in a
substrate of a sandy flat covered by 0.5 to 1.5 m water. Gnadensee, Lake Constance, southern Germany.
6
Sphaeroidal, laminar rhodoid
formed by alternating crusts of corallinacean red algae (black) and encrusting foraminifera
(white chambers). Note serpulid worms (S). The nucleus of the rhodoid is a poritid coral. Shallow subtidal back-reef
environment. Early Tertiary: Lower Austria.
7
Brackish-water oncoids
. The elliptical oncoids were formed by weakly calcified cyanobacteria and algae. Smaller par-
ticles are algal peloids and coated grains. Note discontinuous laminae and shrinkage structures within the oncoids (ar-
row). The recrystallized nucleus was initially coated by dense dark and light concentric laminae. The layers of the second
growth stage follow the shape of the first growth stage. SMF 22. Coastal pond. Late Jurassic (Lusitanian): near Fatima,
Portugal.
