Geology Reference
In-Depth Information
Plate 51 Biogenic Encrustations
Biogenic encrustations originate from the growth of sessile organisms on various substrates. These epibionts
grow on animal hosts (epizoans) or on plant hosts (epiphytes, e.g. sea grass or soft algae). Epibionts from
ephemeral substrates are generally rare, in contrast to the considerable record of epibionts from hard substrates.
Crust-forming calcareous organisms occur on living and dead surfaces of skeletons, particularly in reef environ-
ments, and on lithified rock surfaces (e.g. hardgrounds, see Sect. 5.2.4.1).
Calcareous encrusters include microbes and algae (see Pl. 50), adherent foraminifera, sponges, corals, bryo-
zoans, serpulids and some mollusks. The frequency, composition and diversity of crust-building communities
have changed over time. Sessile foraminifera (-> 1, 2, 4, 7, 8) belonging to different suborders are common in
Mesozoic and Cenozoic reef and shelf faunas (Adams 1962). Some taxa contributed significantly to the forma-
tion of Tertiary reefs (Plaziat and Perrin 1992). Attached foraminifera are valuable proxies for sedimentation
rates, water energy and paleo-water depths (Ghose 1977; Martindale 1992).
Biogenic encrustations take part in the stabilization of sediment, the rapid lithification of reef slopes (Keim
and Schlager 1999), the formation of carbonate grains (-> 2, 4) and the construction of rigid reefs (-> 7). Major
controls on the occurrence and distribution of encrusters are substrate, availability, water energy and (low)
sedimentation rates. Succession analyses of encrusting associations provide clues to substrate types, sedimenta-
tion rates, paleo-water depths (Rasser and Piller 1997) and associated sea-level fluctuations (Reid and Macintyre
1988).
Biogenic encrustations can be more easily described by using the simple classification system proposed by
Taylor and Wilson (2002). Each term consists of two or three roots: The location of the colonizing organisms is
indicated by the prefix epi- or endo-. The second root refers to the type of the substrate (rock, -litho-; animal, -zoo-;
plant, -phyto-; or any organic hardpart of unknown or uncertain status, living or dead, -sclero-). The last root
describes the identity of the colonizing organism (animal, -zoan; plant, -phyte, or either, -biont). Examples are
given on this plate.
1
Attached agglutinated foraminifera (
Ammovertella
Cushman, Carboniferous-Permian). Note the flat lower surface of the
tests representing adaption to an encrusting mode of life (e.g. on not preserved, poorly calcified algal blades). The fora-
minifera were probably epiphytozoans. Early Permian: Carnic Alps, Austria.
2
Foraminiferal encrustations (
Tolypammina
Rhumbler) attached to shells. Episkeletozoan crust. Long-lasting encrusta-
tions lead to the formation of foraminiferal oncoids. Late Triassic (Carnian): Central Carinthia, Austria.
3
Encrustations of sphinctozoid sponges (
Uvanella
; arrows) on and between corals facilitate the construction of rigid reef
frameworks. Epizoozoan crust. Late Triassic (Hawasina Formation, Norian): Central Oman.
4
Formation of composite grains by encrustation of ooids with nubeculariid foraminifera and cyanobacteria. Nubeculariids
are miliolid porcelaneous foraminifera, often attached to sedimentary grains or shells. They contribute to the formation of
oncoids and Tertiary reef structures. The crust type can be classified as epilithozoan crust or more precisely as epizoozoan
crust. Late Triassic (Late Carnian): Central Carinthia, Austria.
5
Encrustations of serpulids around a vanished recrystallized structure, probably a shell infilled with peloids. Episkeletobiont
crust. The serpulid encrustation indicates the existence of a hard substrate. Early Cretaceous: Ostermünchen well, South-
ern Bavaria, Germany.
6
Repeated biogenic encrustations form characteristic sequences consisting of two and more taxonomically different organ-
isms. Example: Cystoporid bryozoans (
Dybowskiella
, B) growing upon (now recrystallized) solenoporacean red algae
(SO) are overgrown by sphinctozoid sponges (S). The relations between encrusting organisms include epiphytozoans
followed by episkeletozoan crust types. Middle Permian reef limestone: Straza near Bled, Slovenia.
7
Modern reef limestone. Note the interaction of different encrusting organisms in the formation of an autochthonous
boundstone structure: A meshwork resembling the microproblematicum
Bacinella
(B), interpreted as being of cyanobac-
teria origin is encrusted by corallinacean red algae (RA); followed by attached agglutinated foraminifera (F). The devel-
opment of the encrusters was disturbed several times by the influx of ooid sand. San Salvador, Bahamas.
8
Encrustation of foraminifera (
Palaeonubecularia
) on a phylloid alga. Epiphytozoan crust. Note the difference in the
preservation of the primary Mg-calcitic foraminifera and the primary aragonitic alga which has lost almost all its signifi-
cant features due to the recrystallization of its aragonitic skeleton. Early Permian: Forni Avoltri, Carnia, Italy.
9
Alternations of calcimicrobes (
Rothpletzella
, R, Pl. 53/8) and fossils interpreted alternatively as foraminifera or cyano-
bacteria (
Wetheredella
Wood, W) are important in the formation of Paleozoic oncoids. Many Early Paleozoic biogenic
crusts were built predominantly by microbes, sponges and stromatoporoids. Early Devonian (Emsian): Lake Wolaya,
Carnic Alps, Austria.
