Geology Reference
In-Depth Information
Fig. 10.36.
Coralline sponges.
The thin-section photograph shows a characteristic reef assemblage consisting of non-
segmented (inozoan, I) sponges, chambered sponges (sphinctozoans, S), and chaetetid coralline sponges (C). Note the
competition for space, a common feature in reefs built by coralline sponges. Late Triassic (Zlambach beds, Rhaetian):
Northern Calcareous Alps, Austria.
by an interplay of oscillating sea-level and fluctuating
oxygen-minimum zones. The former favored the ex-
pansion of microbes, the latter in conjunction with suc-
cessive highstands supported the growth of sponges in
deeper water on foreslopes and drowned platforms
(Reitner and Neuweiler 1995).
eton, which may then be recorded only as voids con-
taining some relicts of sponge structures. Stagnant ba-
sins are suitable environments for the dissolution of
spicules, which in turn may be responsible for the chert
formation in limestones.
The diagenesis of siliceous sponge spicules follows
different paths (Gaillard 1983; Lang and Steiger 1984;
Brachert et al. 1987). These include the infilling of the
axial canals of the spicules with micritic sediment, iron
and manganese oxides and cement, followed by the dis-
solution of spicules and infilling of the molds by High-
Mg calcite cement (Pl. 78/3) or by fine-grained sedi-
ment obscuring the former skeleton.
Fossil siliceous sponges are frequently enclosed by
or associated with micritic or peloidal 'crusts'. Calcifi-
cation of decaying tissue of siliceous sponges leads to
the formation of various automicrite types that may mir-
ror the shape of the sponge body (Warnke 1995). Ma-
jor processes in sponge diagenesis and in the forma-
tion of sponge buildups are (a) early lithification and
Siliceous sponge diagenesis:
In many limestones the
skeleton of siliceous sponges is preserved as calcite
pseudomorphs. As shown for Jurassic hexactinellids,
sponges from different depositional environments re-
flecting different hydrodynamic regimes are preserved
differently (Brachert 1991). Preservation is best in low-
energy basinal carbonates and bedded limestones
formed between sponge mounds due to slow silica dis-
solution and rapid micrite cementation. As water tur-
bulence (and water temperature) increases, dissolution
accelerates with respect to micrite cementation. This
leads to a complete calcitization of spicules in shallow
and higher-energy areas, or the dissolution of the skel-
Text continued on p. 504
