Geology Reference
In-Depth Information
16.2.4 How to Classify Reef Carbonates?
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Caution:
Note that the classification of rock
samples into bafflestones, bindstones and framestones
is not necessarily parallel to the assignment of the fos-
sils within the samples to reef-building guilds! Guilds
interpret the ecologic function of reefbuilders based on
defined parameters. Reef rock categories are highly
subjective. A bindstone sample may yield fossils be-
longing to the Binder Guild and to the Constructor
Guild. A framestone can be formed by assemblages of
Constructor-, Baffler- and Binder Guilds.
The unique position of reef carbonates is demonstrated
by their specific grouping within textural limestone
classifications that subdivide autochthonous reef car-
bonates with regard to the inferred role of sessile 'reef-
building' organisms for the formation of the reef rock
(Sect. 8.2; Fig. 8.1A). The differentiation of framestone,
bafflestone and bindstone (Embry and Klovan 1971)
is useful, but is not without problems.
Problems relate to linking the rock names with spe-
cific skeletal shapes (e.g. bafflestone with stalk-shaped
organisms and framestone with massive growth forms)
and the incorporation of process-related and necessar-
ily subjective terms in the classification. Using the terms
bafflestone, bindstone or framestone (Pl. 41 and Pl. 42),
or one of the other names listed in Box 8.1 requires
concise information on the paleoecological and tapho-
nomic criteria of supposed reef-builders. Otherwise,
not only will the selected name be wrong, but also the
environmental interpretation derived from the rock
name, if the observed fabric is believed to reflect eco-
logic constraints within ancient reefs (see Fig. 8.1B and
Fig. 8.1C).
16.2.5 Microfacies Approach to Reef Studies
Microfacies studies of reef limestones require large
samples that provide data on the higher reef-building
taxa present, and the major growth forms and orienta-
tions. A combination of quantitative studies using grids
and core sampling is recommended (Webb 1999).
In the field, reef limestones often appear as massive
or only coarse bedded lens-shaped or domed carbon-
ate bodies, varying in size from a few centimeters to
several hundreds of meters. Stacked reefs can achieve
thicknesses of tens to hundreds of meters. Reef lime-
Fig. 16.7.
Coral reef limestone with the rugose coral
Phillipsastraea.
Description and classification of this
sample must take into account growth forms of corals
(dendroid), narrow or wide distance between corallites
(close), mono- or polymict composition of reefbuilders
(monomict), proportions of reefbuilders/matrix/cement
(skeletons of reefbuilders and micrite matrix dominate;
carbonate cement is restricted to shelter voids between
corallites), composition of the matrix (peloidal micrite,
probably microbial) and cement types (thin radiaxial-fi-
brous cements and late blocky cement in a few voids).
The dendroid growth form and the micrite between the
corallites, would make the samples correspond to a baffle-
stone, but genetically it is a framestone because corals
and microbes formed a framework.
The sample comes from mounds (High Relief Car-
bonate Mud Mounds after the classification of Riding
2002), famous because of the variegated colors of the
lime mud (récif rouge) and a distinct depth-dependent
vertical succession of facies (Boulvain 1990). The
mounds were formed on a ramp. They exhibit marked
differences in dominant reefbuilders and microfacies:
Facies A formed below storm wave base is represented
by red, stromatactis and sponge-rich mounds. Facies B
developed on the top of facies A, characterized by pink
coral-stromatoporoid mud mounds formed below storm
wave base. Facies C starting development around the
lower base of the photic zone is represented by gray coral-
thrombolite-calcimicrobe mounds. The sample has been
taken from this facies. Late Devonian (Early Frasnian):
Beauchâteau Quarry, Philippeville, southern Belgium.
Width of the picture 5 cm.
