Geology Reference
In-Depth Information
rived carbonate sediment having some topographic
expression above the sea floor during growth (Stanton
1967). In practice, buildup has become an often used
term suggesting the existence of bodies of massive or
only coarsely bedded limestones genetically related or
not at all related to reefs, occurring within bedded car-
bonates or other lithologies.
Carboniferous interval (Pl. 8/6). Algae prevailed with
the onset of the Pennsylvanian (Pl. 56, Pl. 58/1).
• Middle Permian to early Late Triassic. Reef mounds,
mounds or reefs dominated by coralline calcareous
sponges (Pl. 42, Pl. 79, Pl. 145/1-3), corals (Pl. 116/2,
3) and microbes (Pl. 98/8).
• Latest Triassic to earliest Cretaceous: Shallow-wa-
ter reefs commonly dominated by scleractinian corals
(Pl. 84). Siliceous sponges important in Middle and
Late Jurassic mounds (Pl. 78/1, 4), bivalves in Jurassic
banks.
• Cretaceous: Reef mounds, reefs and biostromes
dominated either by rudist bivalves (Pl. 88) or corals.
• Cenozoic: Dominance of corals (Pl. 147/1, 4) and
coralline red algae (Pl. 54).
Seismic reefs
recognized by geophysics are defined
by geometry and reflection properties. They are often
not congruent in size and geometry with ecologic reefs
and mounds because they may also include massive
forereef debris as well as the backreef apron (Schlager
1992).
16.2.3 Reef Fossils
In fact, the temporal distribution of reefbuilders
through time is more complex in regard to number of
different reef-building associations and the predomi-
nance of a particular community. Statistical analysis
of PaleoReef data revealed a great variety in the diver-
sity of compositional reef types during the Ordovician,
the Viséan to Early Carnian, the Callovian to Early Ap-
tian, and the latest Cretaceous to Danian (Kiessling
2002).
Reef fossils comprise sessile, skeletonized organisms
contributing to the formation of reef structures, and
organisms using the diverse ecologic niches of reef
dwelling. Preservation of reef fossils is relatively good
in Paleozoic reef limestones because many skeletons
consist of calcite. Mesozoic and Cenozoic reefbuilders
with a prevailing aragonite skeleton mineralogy are
often poorly preserved and recorded by leached and
recrystallized fossils, or by open or closed molds. These
molds or recrystallized material also provide some clues
to the guild assignment and areal distribution of reef-
builders.
16.2.3.2 Reef Guilds: Ecologic Units
The reef guild concept was introduced, amplified and
exemplified by Fagerstrom (1987, 1988, 1991), Weid-
lich and Fagerstrom (1998), and Fagerstrom and Weid-
lich 1999a, 1999b). A guild is defined as a group of
species competing for the same class of environmental
resources without regard to taxonomic position. The
communities of organic reefs can be subdivided into
five functional units corresponding to guilds. Three
guilds contribute to the formation of reef frameworks
- the
Constructor Guild
, the
Baffler Guild
and the
Binder Guild
(see Fig. 16.6). The
Destroyer Guild
in-
cludes those organisms that are active in the biological
destruction of the reef framework by boring, rasping
and biting. The
Dweller Guild
consists of the organ-
isms living in the reef.
Criteria used in distinguishing guilds include growth
direction, growth form, skeletonization, transportabil-
ity and taphonomic features, and skeletal packing den-
sity and areal cover. Typical constructors and bafflers
have upward growth directions producing erect growth
skeletons. Binders have predominating lateral growth
producing relative thin, tabular-lenticular skeletons.
Constructors differ from bafflers in having much larger
and stronger skeletons, making the largest contribu-
16.2.3.1 Reef Biota: Compositional Changes
during Time
The dominant reef builders changed considerably
through time. Kiessling et al. (1999) and Kiessling
(2001) defined seven major Phanerozoic reef phases
based on prevailing high-ranked taxonomic composi-
tion and compositional reef types:
• Early Cambrian to Early Ordovician: Volumetric
predominance of microbes and microbial mounds, in
the Cambrian in association with archaeocyath sponges
(Pl. 82).
• Middle Ordovician to Frasnian: Dominance of stro-
matoporoid sponges (Pl. 80) and corals (Pl. 83). Reefs
and reef mounds prevailed. Bryozoans played a sig-
nificant role until the Silurian.
• Famennian to Early Permian: Metazoans except
bryozoans (Pl. 144/7, 8) were subordinate to microbes
and algae in reef construction. Reefs and reef mounds.
Microbes were more abundant in the Famennian-Early
