Geology Reference
In-Depth Information
Spiculites (Pl. 78/2) are sediments consisting pre-
dominantly of accumulations of sponge spicules. Ac-
cumulations are widely distributed in modern cold-wa-
ter environments. Spiculite mats consisting of an or-
ganic network of hexactinellid spicules and bryozoan
fragments are known from Arctic seamounts which are
covered by sponge-bryozoan bioconstructions (Henrich
et al. 1992) but they also occur in other, e.g. near-shore,
environments (Lane 1981).
Fossil spiculites are common in deep-marine set-
tings, both in basinal and slope position (Wiedenmayer
1980; Jach 2001; Pl. 112/1, Pl. 139/1).
Calcareous and muddy sediments with a high per-
centage of spicules are usually interpreted as deep or
cold-water deposits. However, today many siliceous
demosponges live in warm shallow waters, suggesting
that ancient spiculites could have originated in shal-
low-marine shelf and near-coast environments, too
(Gammon and James 2001). Accumulations of spicules
derived from siliceous sponges and spicule-moldic po-
rosity may contribute to the formation of reservoir rocks
in cherts (Saller 2001).
taxonomic units (Hexactinellida and Demospongiae
with spicular skeletons). More than 85 % of all species
of recent Porifera are 'soft siliceous sponges' with spicu-
lar supporting skeletons that are loosely embedded
within the soft body. The skeleton disintegrates after
the decay of the body. These sponges are already known
in Liassic deposits and are important constituents of
Jurassic and Cretaceous mud mounds. Hexactinellids
are exclusively marine animals, today mostly known
between 200 and 2000 meters, often living on soft sub-
strates. These sponges are abundant and diverse at shal-
lower depths in polar regions. Large mats of their spi-
cules provide a hard substrate for other organisms.
The second group of siliceous sponges is character-
ized by a rigid skeleton consisting of interconnected
spicula ('Lithistida' and dictyonal hexactinellids). These
sponges were important reef builders (Fig. 10.34).
Siliceous sponges and ancient reefs : Associations
between mound-building, benthic microbial commu-
nities and siliceous sponges are common in reef mounds
(Early Cambrian, Early-Middle Ordovician, Late Sil-
urian, Late Devonian, Late Mississippian, Late Permian,
Late Jurassic (Brunton and Dixon 1994). Thrombolite-
or stromatolite-forming calcimicrobes acted as con-
structors. Demosponges and hexactinellid sponges
acted as bafflers, binders and even as constructors of
reefs commonly formed below wave base. Hexactin-
ellids contributed to the formation of regionally re-
stricted reefs in the Late Triassic (Wendt et al. 1989)
and Early and Middle Jurassic, and wide ranging reefs
in the Late Jurassic (see Leinfelder et al. 2002 for a
review). The formation of these reefs was controlled
The following text summarizes the criteria and sig-
nificance of sponge groups commonly found in thin
sections of limestones. Glossaries of the morphologi-
cal terms used in describing sponges and extensive bib-
liographies can be found in Hooper and Van Soest
(2002).
Siliceous sponges
These fossils are characterized by spicula composed
of opal. They are polyphyletic and comprise two major
Fig. 10.35. The community in this reef
cave is represented by spherical demo-
spongid coralline sponges ( Astrosclera ,
the variously sized balls) and associated
with dendroid hydrozoans ( Stylaster ,
right), a colonial sphinctozoid sponge
( Vaceletia, top right corner) and various
other encrusting sponges (center bottom).
The bright orange-colored aragonitic
basal skeleton of Astrosclera encloses
siliceous spicules. Reef caves are differ-
ent in light, nutrients and sediment input
as compared with free reef surfaces. The
specifically adapted fauna of these cryp-
tic habitats yields some 'living fossils' e.g.
coralline sponges whose 'ultraconserva-
tive' constructional style is reminiscent
of Paleozoic and Mesozoic reef-building
organisms. Great Barrier Reef (Osprey
reef), western Australia. Courtesy of G.
Wörheide (Göttingen).
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