Geology Reference
In-Depth Information
are defined as those anthozoans that secrete massive
external calcium carbonate skeletons. These corals in-
clude the Rugosa, Tabulata and Scleractinia as well as
some minor groups. For definitions of these groups see
Box 10.6.
ing with or without benefit of algal symbiosis (Schuh-
macher and Zibrowius 1985). Zooxanthellate corals are
restricted both bathymetrically and geographically by
their algal symbionts and are therefore concentrated in
shallow tropical and subtropical waters where they build
coral reefs. Azooxanthellate corals (Pl. 147/1, 2, 4) oc-
cur both in shallow and deep environments, but are more
abundant at greater water depths of several hundreds
of meters and more. Deep-water scleractinian corals
with constructional frameworks (e.g.
Lophelia
, Pl. 4/
3) form extended reef structures (Sect. 2.4.4.3). Azoo-
xanthellate deep-water corals may have existed since
the Early Mesozoic (Stanley and Cairns 1988).
Ecology:
Major controls on the distribution of mod-
ern scleractinian corals include surface water tempera-
ture, light, symbiosis, and surface circulation (see Veron
1995 for a critical discussion). The symbiosis of corals
and endosymbiotic algae has many physiological ad-
vantages for corals: enhancement of calcification, re-
moval of metabolic waste, and concentration and recy-
cling of nutrients. The term zooxantellae refers to en-
dosymbiotic photosynthesizing dinoflagellates, red and
green algae as well as cyanobacteria. The terms zoo-
xanthellate and azooxanthellate applies to corals liv-
Skeletal mineralogy, microstructure and preserva-
tion:
The originally aragonitic skeletons of scleractin-
ian corals become unstable once removed from marine
Plate 84 Jurassic Scleractinian Corals
Scleractinia or hexacorals (because of the sixfold symmetry of septal insertion) are colonial or solitary corals
with an aragonitic skeleton. The group became diversified and widely distributed in reefs and shelf environ-
ments in the Late Triassic and continued to expand during the Jurassic and Cretaceous. They were important reef
builders in the Late Jurassic and during the mid and Late Cretaceous. The diversity and importance as reef-
builders increased starting with the Eocene and reached a peak in the Miocene, when most of the modern reef
corals originated. The plate displays
growth types and morphological characteristics
of Jurassic scleractinian
colonial corals. In massive skeletons adjacent corallites may correspond to the astraeoid type (without walls
separating individual corallites, septa in contact), maeandroid type (-> 3) or cerioid type (-> 6). The corallites of
bushy, fasciculate colonies may be arranged parallel to each other (phaceloid, -> 1, 5) or branch irregularly in a
tree-like pattern. Jurassic corals, constructing frameworks and baffling fine-grained sediment, contributed to the
formation of reefs in high-energy shelf and platform-margin environments as well as on ramps. Environmental
demands on Jurassic corals might have been considerably different from those of modern scleractinians (Leinfelder
et al. 2002).
1
Growth type.
Transversal section of
Stylosmilia corallina
Koby. The colony consists of separated non-integrated coral-
lites which are more or less parallel (phaceloid growth type). Late Jurassic (Tithonian): Madonie Mountains, Sicily, Italy.
2
Reef corals:
Transversal section of
Calamophylliopsis flabellum
(Michelin). The coral colony is encrusted (black arrows)
and bored (B). The peloidal bindstone texture with spar-filled fenestrae (F) between the corallites indicates microbial
binding of bioclastic material. Middle to Late Jurassic. Pierre Chatel, south of Grenoble, France.
3
Growth type.
Oblique section of
Myriophyllia rastellina
(Michelin). The closely integrated chains of corallites of the
maeandroid colony are arranged in linear series (black arrows) within the same walls. The exsert edges of the septa and
walls form a ridge (white arrows). The structure is caused by intratentacular budding where new polypes and corallites are
formed through division within the parent corallites. Same locality as -> 1.
4
Reworked corals.
The thin section exhibits an allochthonous assemblage of broken and redeposited colonial corals, in-
cluding
Isastraea
sp., I,
Mesomorpha
sp., M, and
Thamnasteria gracilis
(Münster), T. Note the absence of definite walls
in
Thamnasteria
and
Mesomorpha
; the corallite centers are closely united by confluent septa. In thin sections coral debris
can be recognized not only in fragments exhibiting skeletal elements but also in bioclasts smaller than 0.20 mm (Stanton
and Flügel 1989). Jurassic exotic blocks floating in Cenozoic deposits: Northern margin of the Alps, Salzburg, Austria.
5
Reef corals:
Transversal section of
Thecosmilia longimana
(Quenstedt). The bushy colony consists of laterally free coral-
lites forming tufts of parallel corallites (phaceloid growth type). Note the biogenic encrustation (arrows) and the complete
filling of the interspaces between the corallites by binding and encrusting organisms contributing to the formation of an
organic framework. Late Jurassic: Same locality as -> 1.
6
Growth type.
Transversal section of
Latistraea foulassensis
Beauvais characterized by closely appressed prismatic coral-
lites whose walls are fused to each other (cerioid growth type). Late Jurassic: Untersberg, Salzburg, Austria.
-> 1-6: Determination by D. Turnsek (Ljubljana)
