Geology Reference
In-Depth Information
Jurassic micro-encruster associations subdividing
carbonate ramp depositional environments:
Schmid
(1996) studying Late Jurassic reef and ramp carbon-
ates in different parts of Europe recognized five micro-
encruster associations characterized by composition and
diversity (Fig. 9.10).
Girvanella
associations within on-
coids are restricted to near coastal settings with fluctu-
ating salinities.The widespread
Bacinella-Lithocodium
association occurs in shallow lagoonal and reefal set-
tings affected by moderate environmental stress. Bal-
anced environmental conditions in shallow lagoonal and
reefal areas are indicated by high-diversity micro-en-
cruster associations consisting of porostromate mi-
crobes (
Cayeuxia, Girvanella
), red algae (
Marinella
)
and microproblematica (
Bacinella, Thaumatoporella
).
The
Tubiphytes-Koskinobullina
association is abundant
in mid-ramp positions, but also occurs in reef cavities.
Low-ramp and middle- to outer-ramp positions are char-
acterized by the low-diversity
Terebella
(a serpulid)-
Tubiphytes
association that thrives in low-energy envi-
ronments and appears to tolerate oxygen deficits at the
sea bottom.
Nutritional models derived from microbialites, mi-
cro-encrusters, bioerosion, and faunal diversity:
The
paper by Dupraz and Strasser (2002) on coral-microbi-
alite reefs from the Swiss Late Jurassic illustrates the
high potential of microbial structures, encrusters and
borings for evaluating changes in dominating nutrient
regimes. Micro-encrusters associated with microbialites
and reef-building corals include here red algae, fora-
minifera, bryozoans, serpulids and sponges. The semi-
quantitative micro- and macroscale distribution patterns
indicate three trophic structures that parallel the differ-
ent siliciclastic influx into the reef areas:
Phototrophic-
dominated structures
(indicated by
Lithocodium-Baci-
nella
and opportunistic serpulids and bryozoans) domi-
nate in moderately diverse coral reefs growing in poor
carbonate and nutrient-poor environments with only
low bioerosion.
Balanced phototrophic-heterotrophic
structures
developed in mixed siliciclastic platform en-
vironments and produced the most diversified coral
reefs with common thrombolites.
Heterotrophic struc-
tures
dominated in the case of strong siliciclastic accu-
mulation or a strong increase in nutrient availability;
thrombolites,
Terebella
and other serpulids
,
and bryo-
zoans are common. Bioerosion is moderate to high.
Fig. 9.9.
Encrustation of colonial scleractinian corals
by cor-
allinacean red algae (
Archaeolithothamnium gosaviense
Rothpletz). Both, the corals and the algae were attacked by
macroborers. The coral was strongly eroded and bored prior
to the overgrowth by the algae. The transition from mound-
like to erect growth indicates quiet-water conditions and low
and slow sediment input. Lagoonal back-reef environment.
Late Cretaceous (Coniacian): Northern Alps, Bad Reichenhall,
Bavaria, Germany. Scale is 2 mm. Courtesy of R. Höfling,
Erlangen.
ences in the association and diversity of organisms that
contributed to the formation of crusts in reef environ-
ments (Flügel 2002). Anisian crusts are rare but diverse;
their encrusting organisms comprise sphinctozoid
sponges, porostromate cyanobacteria, foraminifera, ser-
pulids and some microproblematica. The latter are com-
mon and sometimes abundant in younger Triassic but
also in Jurassic reef crusts. Crusts are frequent in Ladin-
ian and Early Carnian reefs; they are characterized by
various associations of sponges, microbes, and micro-
bially induced carbonate cement. The composition of
Late Carnian reefs differs significantly in that low-
growing sponges and porostromate cyanobacteria domi-
nate (Pl. 79/1). A complete change took place in the
Norian (Late Triassic): Biogenic crusts become highly
diverse and mainly built by microbialites (Pl. 116/2),
sphinctozoan and chaetetid sponges (Pl. 81/1), and mi-
cro-encruster associations, which also dominate in Late
Jurassic and Early Cretaceous reefs (e.g.
Lithocodium/
Bacinella;
Pl. 99/4, 5).
Late Cretaceous encruster associations dependent
on the reef
type:
Different encruster associations occur
in reefs built by different reef organisms as exempli-
fied by coral-stromatoporoid reefs, coral reefs, rudist
reefs and algal reefs (Moussavian 1992). Encrusters are
