Geology Reference
In-Depth Information
9.2.2 Phanerozoic Encrusters
Encrusting organisms range in size from less than a
millimeter to several millimeters (cyanobacteria, fora-
minifera, many microproblematica) up to centimeters
(red algae, stromatoporoids, corals).
Micro-encrusters
are encrusting microfossils, often
occurring in close association with microbialites and
within marine and nonmarine oncoids (Sect. 4.2.4.1).
Cambrian and Ordovician encrusting communities
lived predominantly in sheltered reef cavities (Kobluk
1988) and are dominated by microbialites, particularly
cyanobacteria. Other common encrusters are bryozo-
ans and sponges. The differentiation between encrust-
ers occurring on exposed surfaces, cryptic habitats or
in shadowed cavities seems to have taken place in the
Ordovician. In Silurian and Devonian reef limestones,
biogenic crusts on the surface of benthic organisms are
composed of stromatoporoid sponges (Fig. 9.8), tabu-
late corals and porostromate cyanobacteria. Strong evi-
dence for spatial competition among ancient marine
hard substrate biotas comes from the study of over-
growth networks among Silurian bryozoans (Lidell and
Brett 1982; Taylor 1984). Microbes, algae, smaller fora-
minifera, bryozoans and chaetetid sponges are impor-
tant constituents of Carboniferous and Permian encrust-
ing communities.
Mesozoic reef and platform carbonates yield diverse
encruster communities whose taxonomic diversity and
ecologic complexity increase in the Late Jurassic and
the Cretaceous. Tertiary succession patterns resemble
those of modern crust communities, due to the domi-
nance of red algae, foraminifera, bryozoans, barnacles
and serpulids (Fig. 9.7 and Fig. 9.9).
Fig. 9.8.
Rugose corals (
Acanthophyllum
sp.) encrusted by
stromatoporoids (
Clathrocoilona
sp.) and thin layers com-
posed of porostromate cyanobacteria (
Girvanella
sp.; not rec-
ognizable in the picture). Stromatoporoids and cyanobacte-
ria form sheetlike crusts around the corals. The wide distri-
bution of this crust pattern in Middle Devonian platform car-
bonates suggests that encrustation took place during the life-
time of the coral. Middle Devonian: Letmathe, Sauerland,
Germany. Scale is 5 mm.
and the presence of biofilms. The available attachment
surface area is critical for the survival of crust-building
organisms. The biological potential of many encrust-
ers is strongly microbially controlled. Colonizer-inher-
ent factors are larval recruitment patterns and growth
strategies. Ecosystem-inherent factors include sedimen-
tation rates, water movement, predation/grazing pres-
sure and disturbance frequency.
In general, the dominance of encrusters with skel-
etons is reduced with depth, leading to complex
suc-
cession patterns
controlled by endogenic and exogenic
factors. The former includes reproduction and larval
strategies as well as competition for space and nutri-
ents. Exogenic controls are mainly the size of the en-
crusting surface and predation pressure.
Upper and lower surfaces of organisms acting as sub-
strates for encrusters are likely to be subject to differ-
ent light intensity, turbulence, grazing pressure and dis-
solved oxygen. These differences influence the distri-
bution of generalists and specialists and cause faunal
polarity between crusts developed upon upper and lower
surfaces of host organisms. Lower surfaces often ex-
hibit greater areal cover, higher density and a higher
diversity of encrusters than upper surfaces.
9.2.3 Significance of Encrustation Patterns
in Recognizing Depositional Settings and
Environmental Controls
The combined use of encrustation patterns, microbi-
alite types and bioerosion criteria can serve as a guide
to identifying inner and outer parts of carbonate plat-
forms and ramps, as well as interpreting paleo-water
depths and nutrient levels in ancient reefs. Prerequi-
sites for the successful application of these criteria are
(1) the knowledge of changes in the composition of
biogenic crusts during time, and (2) the differentiation
of micro-encrusters, if possible at low taxonomic lev-
els.
Changes in the composition of Triassic biotic crusts:
Middle and Late Triassic crusts exhibit distinct differ-
