– For Battistini et al. (1975) a bioherm is a: “lens-shaped
organic reef. . . embedded in situ inside sedimentary
layers of different lithological nature. . . it may be
surrounded by a peripheral talus of biodetrital sediments,” whereas a biostrome is a “layered, bank like
organic reef of variable extension, creating no discontinuity inside the embedding sedimentary layers.”
– Kershaw (1994) introduces a number of complementary terms: the constructing organisms may be mostly
in situ (autobiostromes), or mostly as debris included
in the structure ( parabiostromes), with autoparabiostrome as intermediate. Conversely, allobiostromes
are formed of material derived from allochthonous
sources, for example skeletal plankton sedimented
onto the sea bed.
Discussion
Although these terms may be useful, there is no general
agreement on a complete definition taking into account
at one and the same time such different characteristics as
age, stratigraphic conformity or unconformity, along with
the autochtonous or allochtonous nature of deposited
organisms. As a consequence, many biologists and geographers use “bioherm” as a general term not only for major
biological build-ups such as algal rims or coral reefs (e.g.,
see Adey and Burke, 1976) but also for small-scale
organic build-ups, for which the word “biostrome” would
suit better.
Further difficulties come from the fact that relationships
with embedding layers are not visible on living formations. For example, an algal rim growing on the outer edge
of a coral reef is indeed a bioherm, or a part of it since it
takes an active part in the edification and protection of
the latter, but a thin algal layer coating a limestone or volcanic shore, or lining a vertical cliff without altering local
sedimentation should be called a biostrome even if both
formations are in continuity with one another.
Furthermore, actualists consider detrital accumulations
of dead shells and broken skeletal material (generally
mud-supported and harboring a completely different
infauna) as quite different from true build-ups or reefs
which are in situ developed formations.
For these reasons bioherm and biostrome cannot be put
into automatic correspondence with the highly diversified
terms used by actualist reef geologists or biologists, since
additional factors such as size of the buildup, thickness,
relation with the sea-surface, and resistance to waves must
be taken into account. Chronology is also important: most
reef complexes are vertical successions of layers
corresponding to successive glacial low and high sealevels.
Initially there was no size limitation in the definition
of bioherms and biostromes, and they vary from
microbioherms formed by a single colony to coral knolls
of decametric size. Nevertheless a general agreement
exists to limit the use of the latter words to structures
important enough to be easily positioned and mapped.
This, in turn, creates a problem since minor built structures
such as algal rims or coral knolls may be of metric or decametric size and difficult to cartography. In fact, bioherms
and biostromes are part of a same continuum.
Ecological factors are also important.
– Bioherms: mainly develop in favorable ecological conditions, growth is three-dimensional,and biodiversity
high to oligospecific. Sediment accumulation and diagenesis may be high, leading to bafflestone, framestone,
or boundstrome structures (sensu Bathurst 1971).
Bioherms growing near the surface may lead to true
wave-resistant reefs but deep-water coral formations
may be called bioherms but certainly not reefs. In the
same manner the thick calcareous algal build-ups
developing below À30 m (“coralligene”) in the Mediterranean might be called bioherms since, although
relatively thin (a few meters thick), they alter both bottom profile and sedimentation (Laborel 1961)
– Biostromes: often develop in adverse ecological conditions and their importance depends on such factors as
depth, exposure and hydrodynamics, temperature, and
sedimentation. They tend to be oligo or monospecific.
Algal rims and ridges are linked to strong exposure to
surf, as well as pavements of calcareous algae covering
the bottoms of reef passes with strong tidal currents.
Corals may also develop biostrome-like, for example
sheet-like patches of Siderastrea along West African
and Cape Verde Islands rocky coasts (Laborel, 1974),
or western tropical Atlantic Montastrea cavernosa
developing monospecific layers down to À60 m.
For the geologist, biostromes and bioherms are fossil formations - meaning that after their living phase they were
killed, eroded, or reworked and covered by younger sediments. Their detail morphology is therefore altered - erosion and sedimentation tending to erode reliefs and fill
lower points. This has an over-simplifying effect when
using fossil coral-reefs as sea-level markers. Blanchon
and Blakeway (2003) recently called attention to such
difficulties.
Conclusions
Unless bio-accumulated detrital mounds and layers are
taken out of the definition of bioherms and biostromes
and given a different name (a decision that only geologists
can take), and the status of small-scale build-ups is settled,
use of the words bioherm and biostrome should preferably
be restricted to the fossil formations for which they were
coined (whether their associated detrital facies, and other
types of detrital formations are included or not). Students
of living reefs are hence encouraged to prefer more general terms (such as “biological build-up,” “reef-like structure,” or “biogenic construction”) instead.
Bibliography
Adey W. H., and Burke, R. B., 1976. Holocene bioherms (algal
ridges and bank barrier reefs) of the eastern Caribbean. Geological Society of America Bulletin, 87, 95–109.
BIOHERMS AND BIOSTROMES
157
organic reef. . . embedded in situ inside sedimentary
layers of different lithological nature. . . it may be
surrounded by a peripheral talus of biodetrital sediments,” whereas a biostrome is a “layered, bank like
organic reef of variable extension, creating no discontinuity inside the embedding sedimentary layers.”
– Kershaw (1994) introduces a number of complementary terms: the constructing organisms may be mostly
in situ (autobiostromes), or mostly as debris included
in the structure ( parabiostromes), with autoparabiostrome as intermediate. Conversely, allobiostromes
are formed of material derived from allochthonous
sources, for example skeletal plankton sedimented
onto the sea bed.
Discussion
Although these terms may be useful, there is no general
agreement on a complete definition taking into account
at one and the same time such different characteristics as
age, stratigraphic conformity or unconformity, along with
the autochtonous or allochtonous nature of deposited
organisms. As a consequence, many biologists and geographers use “bioherm” as a general term not only for major
biological build-ups such as algal rims or coral reefs (e.g.,
see Adey and Burke, 1976) but also for small-scale
organic build-ups, for which the word “biostrome” would
suit better.
Further difficulties come from the fact that relationships
with embedding layers are not visible on living formations. For example, an algal rim growing on the outer edge
of a coral reef is indeed a bioherm, or a part of it since it
takes an active part in the edification and protection of
the latter, but a thin algal layer coating a limestone or volcanic shore, or lining a vertical cliff without altering local
sedimentation should be called a biostrome even if both
formations are in continuity with one another.
Furthermore, actualists consider detrital accumulations
of dead shells and broken skeletal material (generally
mud-supported and harboring a completely different
infauna) as quite different from true build-ups or reefs
which are in situ developed formations.
For these reasons bioherm and biostrome cannot be put
into automatic correspondence with the highly diversified
terms used by actualist reef geologists or biologists, since
additional factors such as size of the buildup, thickness,
relation with the sea-surface, and resistance to waves must
be taken into account. Chronology is also important: most
reef complexes are vertical successions of layers
corresponding to successive glacial low and high sealevels.
Initially there was no size limitation in the definition
of bioherms and biostromes, and they vary from
microbioherms formed by a single colony to coral knolls
of decametric size. Nevertheless a general agreement
exists to limit the use of the latter words to structures
important enough to be easily positioned and mapped.
This, in turn, creates a problem since minor built structures
such as algal rims or coral knolls may be of metric or decametric size and difficult to cartography. In fact, bioherms
and biostromes are part of a same continuum.
Ecological factors are also important.
– Bioherms: mainly develop in favorable ecological conditions, growth is three-dimensional,and biodiversity
high to oligospecific. Sediment accumulation and diagenesis may be high, leading to bafflestone, framestone,
or boundstrome structures (sensu Bathurst 1971).
Bioherms growing near the surface may lead to true
wave-resistant reefs but deep-water coral formations
may be called bioherms but certainly not reefs. In the
same manner the thick calcareous algal build-ups
developing below À30 m (“coralligene”) in the Mediterranean might be called bioherms since, although
relatively thin (a few meters thick), they alter both bottom profile and sedimentation (Laborel 1961)
– Biostromes: often develop in adverse ecological conditions and their importance depends on such factors as
depth, exposure and hydrodynamics, temperature, and
sedimentation. They tend to be oligo or monospecific.
Algal rims and ridges are linked to strong exposure to
surf, as well as pavements of calcareous algae covering
the bottoms of reef passes with strong tidal currents.
Corals may also develop biostrome-like, for example
sheet-like patches of Siderastrea along West African
and Cape Verde Islands rocky coasts (Laborel, 1974),
or western tropical Atlantic Montastrea cavernosa
developing monospecific layers down to À60 m.
For the geologist, biostromes and bioherms are fossil formations - meaning that after their living phase they were
killed, eroded, or reworked and covered by younger sediments. Their detail morphology is therefore altered - erosion and sedimentation tending to erode reliefs and fill
lower points. This has an over-simplifying effect when
using fossil coral-reefs as sea-level markers. Blanchon
and Blakeway (2003) recently called attention to such
difficulties.
Conclusions
Unless bio-accumulated detrital mounds and layers are
taken out of the definition of bioherms and biostromes
and given a different name (a decision that only geologists
can take), and the status of small-scale build-ups is settled,
use of the words bioherm and biostrome should preferably
be restricted to the fossil formations for which they were
coined (whether their associated detrital facies, and other
types of detrital formations are included or not). Students
of living reefs are hence encouraged to prefer more general terms (such as “biological build-up,” “reef-like structure,” or “biogenic construction”) instead.
Bibliography
Adey W. H., and Burke, R. B., 1976. Holocene bioherms (algal
ridges and bank barrier reefs) of the eastern Caribbean. Geological Society of America Bulletin, 87, 95–109.
BIOHERMS AND BIOSTROMES
157