Geology Reference
In-Depth Information
Mineralogical and diagenetic data.
Cold-water car-
bonates are dominated by skeletal grains consisting of
Low-Mg calcite (bryozoans, many bivalves) and/or
High-Mg calcite (coralline algae, echinoderms). Ara-
gonitic skeletons are rare and usually recorded by
molds. Authigenic glauconite and phosphorite grains
are common features of cold-water carbonates.
A striking feature of carbonate cold-water sediments
is their retarded and weak cementation and lithifica-
tion (Rao 1994). Diagenesis is predominantly destruc-
tive. The diagenetic potential is low, but intraskeletal
and interskeletal cementation occurs in temperate and
cold-water carbonates (Fig. 16.16). Mg-calcite cements
dominate in cold-water reefs.
Plate 147 CoolWater Coral Reef Limestones (Early Tertiary of Fakse, Denmark)
The Fakse quarries south of Copenhagen exhibit lowermost Tertiary limestones comprising bryozoan lime-
stones (-> 3) and coral limestones (-> 1). These limestones differ clearly in faunal criteria. The bulk of the
grains of the
bryozoan facies
consists of cheilostome and cyclostome bryozoans with a greatly varying bryo-
zoan/matrix ratio; colonial corals and hydrozoans are missing; octocorals are represented by large colonies, and
the associated fauna is low-diverse. The coral facies comprises several subfacies characterized by the dominat-
ing coral taxa, including the
Dendrophyllia
subfacies (-> 1) and the
Faksephyllia
subfacies (-> 4). Framebuilding
organisms were azooxanthellate, ahermatypic corals, stylasterinid hydrozoans (-> 5) and fan-shaped octocorals
(-> 6). Associated organisms include bryozoans, bivalves and gastropods, decapod arthropods, brachiopods,
echinoderms, serpulids and siliceous sponges. Borings within corals and shells are abundant. The diversity of
the framebuilding community is low, that of the associated fauna significantly high. Most of the sediment within
the mounds and surrounding the mounds is parautochthonous and consists of broken corals and the debris of
other skeletons (-> 5, 6).
A cold-water setting for the coral mounds is indicated by the low-diversity azooxanthellate coral community
whose generic composition corresponds to that of modern cool-water coral associations, the dominance of colo-
nial corals with arborescent to dendroid corallites and fragile branches, breakdown of coral framework predomi-
nantly by bioerosion, occurrence of open-marine planktonic organisms within the micritic sediments of the
mound and intermound facies, interfingering of coral mound-bearing strata with pelagic sediments, the pre-
dominance of heterotrophic organisms and the complete absence of algae.
1
Coral limestone.
The bulk of the mounds consists of micritic limestones with azooxanthellate, dendroid scleractinian
corals. Facies types are distinguished by the dominance of specific coral taxa. The
Dendrophyllia
(D) facies forms to-
gether with the
Faksephyllia
facies (-> 4) the core of the asymmetrical mounds. The originally aragonitic corals are
converted to Low-Mg calcite. Note open framework porosity (white arrow) and bioturbation (black arrows) indicating
firmground substrate consistency and slow lithification of the lime mud. The bioclastic debris within the matrix was
formed by the breakdown of coral skeletons caused by strong bioerosion.
2
Preservation dependent on skeletal mineralogy.
Cross section of the azooxanthellate coral
Dendrophyllia
candelabrum
Hennig, encrusted by different bryozoans (B). Note the good preservation of the calcitic bryozoan and the poor preserva-
tion of the originally aragonitic coral. The matrix is micrite.
3
Bryozoan limestone.
The base of the mounds was built by bryozoan limestone representing a typical 'bryomol' facies (see
Pl. 106/2, 3). The chalky, poorly lithified bioclastic packstone yields abundant cyclostome and cheilostome bryozoans
(white arrows) as well as octocorals (
Moltkia isis
Steenstrup, black arrows). Note the high open intra- and interskeletal
porosity (white areas).
4
Faksephyllia
facies. Some corallites of the dendroid colonies are partly dissolved (white areas). Note the fine-grained
bioclastic debris.
5
Hydrozoans.
Longitudinal and cross sections of hydrozoans. Fan-shaped stylasterinid hydrozoans are associated with
scleractinian corals. The branches of the bushy aragonitic skeletons exhibit ramified canals appearing as pores of different
size and function. Feeding polyps project through larger gastropores (arrow), prey-gathering polyps occupy dactylopores.
6
Octocorals.
Isolated internode of the octocoral
Moltkia lyelli
Nielsen, exhibiting the characteristic axial channel as well as
two exterior calyces. Modern octocorals occur in tropic and non-tropic oceans. They include sea pens, soft corals, sea
fans, organ pipe corals and precious corals. The skeleton is commonly internal, spicular, horny or calcitic and aragonitic.
Fossil records comprise calcareous spicules as well as axis structures consisting of horny nodes and calcitic internodes.
7
Fine-grained sediment.
Most of the sediment within the mounds is parautochthonous and is produced by the breakdown
and deposition of corals without transport (left). Allochthonous sedimentation at the slope of the mounds (grain-supported
coral debris, right) is uncommon.
-> 1, 4, 6, 7: Bernecker and Weidlich (1990)
