Geology Reference
In-Depth Information
16.2 Understanding Reef Carbonates
reefs, true reefs) are aquatic biosedimentary deposits
formed essentially in place by procaryote and eucary-
ote skeletal or non-skeletal organisms. This category
also includes structures often called
framework reefs.
Frameworks may be built by large in-place skeletons
being in contact with small-scaled in-situ skeletons (mi-
cro-framework or detrital framework; Weidlich and
Flügel 1995) or with non-skeletal microbes contribut-
ing to the formation of automicrites (Webb 1996).
•
Carbonate mud mounds
are carbonate mud-domi-
nated (micrite and fine-silt) deposits with topographic
relief. Carbonate mounds differ from ecologic reefs in
the scarcity of in-place skeletons, stromatolites and
thrombolites, but probably represent a continuum of
shared ecologies and sedimentary characteristics (Wood
2001). They are the least well understood subjects for
reef studies owing to their often genetic obscurity and
potential heterogeneity, as shown by Bridges et al.
(1995) and Kopaska-Merkel and Haywick (2001).
Reef limestones are different. They differ from other
carbonate rocks by the strong biological control on their
formation. This control in turn is responsible for spe-
cific sedimentation patterns and cementation types. The
evolution of reefs and reef organisms, and the devel-
opment of reef attributes during time has been discussed
in several texts (Fagerstrom 1987; Webb 1996; Wood
1999; Stanley 2001; Kiessling et al. 2002). Secular
changes and variations, e.g. in main reef-building
groups, the composition of reef communities, reef abun-
dance, paleolatitudinal distribution, or reef setting, are
evident.
Modern reefs have been discussed in Sect. 2.4.3.4
and Sect. 2.4.4.3. Reefs are formed in tropical and non-
tropical, warm-water and cold-water environments (see
Tab. 2.3). The following text is focused on ancient reefs
and the question how reef limestones should be de-
scribed with regard to microfacies and paleontological
criteria.
In practice, the terms mud mound and reef mound
are widely used. Both types are characterized by re-
lief.
Mud mounds
consist of a high amount of fine-
grained carbonate (some authors suggest it is more than
50% of the rock volume).
Reef mounds
(or skeletal
mounds) are defined by bioclastic lime mud with mi-
nor amounts of organic bindings.
All these terms refer to autochthonous, biologically
controlled deposits. Geometrically similar mound-
shaped structures may, however, also develop by al-
lochthonous accumulation of skeletal grains, e.g. cri-
noid particles or fine-grained reef debris. These cal-
careous bioclastic deposits have been referred to as
hydrodynamic reefs.
A careful inspection of sedimentary structures must
clarify whether the deposit is an allochthonous, cur-
rent-swept and transported deposit or an in-place ac-
cumulation.
Crinoid limestones are used here as an example to
reveal their varying origin. In-place degradation of
crinoids or other skeletons (e.g. segmented algae) to-
gether with the baffling of sediment and stabilization
of the substrate by microbes can contribute to the for-
mation of mounds. Crinoid particles tend to be trans-
ported, and can also form dunelike structures without
a genetic relationship to reefs or carbonate mounds.
Hydrodynamic controls on the deposition of sand-sized
reef debris of randomly scattered skeletal grains in reefs
are possible (Fagerstrom and Weidlich 1999).
16.2.1 What is a Reef?
Definition.
Riding (2002) presented a lucid and pre-
cise overview of what the term 'reef' means to geolo-
gists and which of the many names used in describing
ancient structures interpreted as reefs are meaningful.
He defined a reef as 'a calcareous deposit created by
essentially in place sessile organisms'.
This broad definition aims to encompass reefs of all
types, ignoring their scale, and unites structures that
fundamentally are products of substrate colonization
and controls by sessile organisms. Riding's definition
agrees widely with the definition proposed by Flügel
and Flügel-Kahler (1992) and the slightly modified use
in the topic on 'Phanerozoic Reef Patterns' (Kiessling
et al. 2002): 'Laterally confined biogenic structures,
developed by the growth or activity of sessile benthic
organisms and topographic relief and (inferred) rigid-
ity'.
Only these broad definitions allow a comparative
analysis of reef development and patterns in space and
time.
16.2.2 Reef Types
Beginning in the 1950's, a differentiation of two major
categories became fashionable:
'organic reefs'
and
'mounds'
•
Reefs and mounds are encompassed within the term
buildup,
originally defined as carbonate masses repre-
senting in-place accumulations of largely skeleton-de-
Organic reefs
(also called ecologic reefs, biological
