a limited veneer of reef coral – especially in troughs.” Little
sedimentologic data on the Halimeda sediments and no dating have been published.
The bathymetry, seismic structure, submersible examination and photography, sedimentology, and biology of
Halimeda banks in the region east of Cooktown
(Figure 1) have been described by Davies and coworkers
(Davies and Marshall, 1985; Phipps, Davies and Hopley,
1985; Marshall and Davies, 1988). They report that
between 15.10S and 15.35S in the lee of Ribbons 2–7,
the “reefless” tract supports a luxurious growth of
Halimeda that during the Holocene have developed
Halimeda bioherms in water depths between 30 and 50 m.
In the southern Great Barrier Reef region, Halimeda
bioherms have been reported from the Swains reefs
region, west of Frigate Shoals in water depths of
20–32 m (Figure 1) (Searle and Flood, 1988).
The Sahul shelf
On the Sahul Shoals, Halimeda are integral components of
large “banks” such as Big Bank, Snow White, Happy,
Grumpy and Udang (Rees et al., 2007). The banks are generally flat topped with few bioherms in the sense of those
described from the Great Barrier Reef.
The Java Sea bioherms
In the Java Sea, Phipps and Roberts (1988) describe
Halimeda bioherms on K Bank in the eastern part of
the Makassar Straits (Figure 3). They are similar to
those described from the Great Barrier Reef, if not
a little larger.
The Miskita bank/channel bioherms
Several generations of Halimeda bioherms occur on the
shelf northeast of Nicaragua (Figure 4), as seen mainly
on multichannel seismic sections.
Descriptive features of Halimeda bioherms
In the Great Barrier Reef, Halimeda bioherms occur over
large areas behind the outer reefs, as “fields” or “complexes” of inter-fingering or inter-merging individual
elongate bioherms, 150 m long (N–S) and 100 m wide
(W–E), slightly flat topped and sloping away in all directions at 5–15
(Figure 5a). The most striking surface feature is a forest of soft green algae, sometimes 50 cm
thick on the tops and thinning down the sides of each
bioherm. This forest is dominated by various species of
Caulerpa and Halimeda. Structurally, it is similar to rain
forest in that there is a thick undergrowth on a sandy/gravely
surface, a middle layer of less dense cover and a top canopy.
This forest is an ephemeral feature, appearing and
disappearing within months. Mollusks, foraminifera, and
bryozoans form integral and important parts of the epifauna
with Halimeda. These collectively mantle the surface of the
top and sides of the bioherms but stop suddenly 3–5 m
above the troughs between adjacent bioherms.
Seismic data shows in the Great Barrier Reef and the
Java Sea, the bioherms are up to 15 m thick in the GBR
(Figure 5a, b þ c) but are substantially thicker (þ20 m)
in the Makassar examples (Figure 5d). In the northern
Great Barrier Reef, the bioherms sit on a prominent seismic reflector (Figure 5a þ c) which dips from around
33 m immediately west of the reefs to around 65 m some
5–10 km west of the reefs. The bioherms themselves also
decrease in size and increase in depth in the same direction. The reflector in Figure 5a þ c, is a leached skeletal,
limestone whose vugs are partially filled with soil. The
geometry of the bioherm complexes bear no relation to
the geometry of the prominent reflector. A similar seismic
reflector also underpins the bioherms in the Makassar
Straits (Figure 5d).
Bedding is the commonest internal structural feature
(Figure 5a–d); internally bioherms are sometimes bedded,
sometimes transparent and often multigenerational. Individual bioherm complexes show bedding suggestive of
several generations of growth. Tops are frequently seismically transparent. Sometimes, there are two seismic
facies – a lower indistinctly bedded unit that merges laterally with a biostromal sheet facies and an upper bedded
facies forming the upper two thirds of the banks.
Sedimentologically, as seen in cores (Figure 6), the
bioherms are comprised of Halimeda sands and gravels
in an olive green matrix. Large foraminifera, Marginopora
vertebralis and Alveoinella quoyi are subordinate but
important parts of the gravel fraction, as are mollusks
and bryozoans. The Halimeda are dominantly H. opuntia,
v. hederacea, and H. copiosa with subordinate amounts of
H. fragilis, H. discoidea, and H. gracilis. Texturally the
bioherms are gravelly, sandy muds throughout but with
slight textural variations down the core. At the surface,
the gravel fraction (largely Halimeda) comprises 50%,
this dropping to 20–30% within 1 m of the top. Deeper
in the core the gravel fraction either remains at 20–30%
or drops even further to less than 10%. The sand fraction
in most cores stands at 20–30% and is comprised dominantly of broken Halimeda leaves with contributions also
from foraminifera, bryozoans, and mollusks. Mud in the
cores measures around 10% in the top half meter of the
cores and then increases substantially to between 30 and
50% throughout the rest of the core. Approximately half
of this mud fraction is carbonate and half non-carbonate
or terrigenous (quartz, kaolinite, and smectite). However,
in the Swains Reefs bioherms, the mud matrix is totally
carbonate and devoid of terrigenous material, reflecting
the distance of these bioherms from the Australian mainland. In the Java Sea, Phipps and Roberts (1988) describe
the bioherms as disarticulated Halimeda plates with a fine
grained matrix of foram-rich carbonate mud (usually less
than 40%), only a small percentage of which is noncarbonate (volcanic shards and siliceous spicules). In
some cores, Halimeda plates form a disorientated opentextured accumulation. Locally, within cores, sediments
542
HALIMEDA BIOHERMS
sedimentologic data on the Halimeda sediments and no dating have been published.
The bathymetry, seismic structure, submersible examination and photography, sedimentology, and biology of
Halimeda banks in the region east of Cooktown
(Figure 1) have been described by Davies and coworkers
(Davies and Marshall, 1985; Phipps, Davies and Hopley,
1985; Marshall and Davies, 1988). They report that
between 15.10S and 15.35S in the lee of Ribbons 2–7,
the “reefless” tract supports a luxurious growth of
Halimeda that during the Holocene have developed
Halimeda bioherms in water depths between 30 and 50 m.
In the southern Great Barrier Reef region, Halimeda
bioherms have been reported from the Swains reefs
region, west of Frigate Shoals in water depths of
20–32 m (Figure 1) (Searle and Flood, 1988).
The Sahul shelf
On the Sahul Shoals, Halimeda are integral components of
large “banks” such as Big Bank, Snow White, Happy,
Grumpy and Udang (Rees et al., 2007). The banks are generally flat topped with few bioherms in the sense of those
described from the Great Barrier Reef.
The Java Sea bioherms
In the Java Sea, Phipps and Roberts (1988) describe
Halimeda bioherms on K Bank in the eastern part of
the Makassar Straits (Figure 3). They are similar to
those described from the Great Barrier Reef, if not
a little larger.
The Miskita bank/channel bioherms
Several generations of Halimeda bioherms occur on the
shelf northeast of Nicaragua (Figure 4), as seen mainly
on multichannel seismic sections.
Descriptive features of Halimeda bioherms
In the Great Barrier Reef, Halimeda bioherms occur over
large areas behind the outer reefs, as “fields” or “complexes” of inter-fingering or inter-merging individual
elongate bioherms, 150 m long (N–S) and 100 m wide
(W–E), slightly flat topped and sloping away in all directions at 5–15
(Figure 5a). The most striking surface feature is a forest of soft green algae, sometimes 50 cm
thick on the tops and thinning down the sides of each
bioherm. This forest is dominated by various species of
Caulerpa and Halimeda. Structurally, it is similar to rain
forest in that there is a thick undergrowth on a sandy/gravely
surface, a middle layer of less dense cover and a top canopy.
This forest is an ephemeral feature, appearing and
disappearing within months. Mollusks, foraminifera, and
bryozoans form integral and important parts of the epifauna
with Halimeda. These collectively mantle the surface of the
top and sides of the bioherms but stop suddenly 3–5 m
above the troughs between adjacent bioherms.
Seismic data shows in the Great Barrier Reef and the
Java Sea, the bioherms are up to 15 m thick in the GBR
(Figure 5a, b þ c) but are substantially thicker (þ20 m)
in the Makassar examples (Figure 5d). In the northern
Great Barrier Reef, the bioherms sit on a prominent seismic reflector (Figure 5a þ c) which dips from around
33 m immediately west of the reefs to around 65 m some
5–10 km west of the reefs. The bioherms themselves also
decrease in size and increase in depth in the same direction. The reflector in Figure 5a þ c, is a leached skeletal,
limestone whose vugs are partially filled with soil. The
geometry of the bioherm complexes bear no relation to
the geometry of the prominent reflector. A similar seismic
reflector also underpins the bioherms in the Makassar
Straits (Figure 5d).
Bedding is the commonest internal structural feature
(Figure 5a–d); internally bioherms are sometimes bedded,
sometimes transparent and often multigenerational. Individual bioherm complexes show bedding suggestive of
several generations of growth. Tops are frequently seismically transparent. Sometimes, there are two seismic
facies – a lower indistinctly bedded unit that merges laterally with a biostromal sheet facies and an upper bedded
facies forming the upper two thirds of the banks.
Sedimentologically, as seen in cores (Figure 6), the
bioherms are comprised of Halimeda sands and gravels
in an olive green matrix. Large foraminifera, Marginopora
vertebralis and Alveoinella quoyi are subordinate but
important parts of the gravel fraction, as are mollusks
and bryozoans. The Halimeda are dominantly H. opuntia,
v. hederacea, and H. copiosa with subordinate amounts of
H. fragilis, H. discoidea, and H. gracilis. Texturally the
bioherms are gravelly, sandy muds throughout but with
slight textural variations down the core. At the surface,
the gravel fraction (largely Halimeda) comprises 50%,
this dropping to 20–30% within 1 m of the top. Deeper
in the core the gravel fraction either remains at 20–30%
or drops even further to less than 10%. The sand fraction
in most cores stands at 20–30% and is comprised dominantly of broken Halimeda leaves with contributions also
from foraminifera, bryozoans, and mollusks. Mud in the
cores measures around 10% in the top half meter of the
cores and then increases substantially to between 30 and
50% throughout the rest of the core. Approximately half
of this mud fraction is carbonate and half non-carbonate
or terrigenous (quartz, kaolinite, and smectite). However,
in the Swains Reefs bioherms, the mud matrix is totally
carbonate and devoid of terrigenous material, reflecting
the distance of these bioherms from the Australian mainland. In the Java Sea, Phipps and Roberts (1988) describe
the bioherms as disarticulated Halimeda plates with a fine
grained matrix of foram-rich carbonate mud (usually less
than 40%), only a small percentage of which is noncarbonate (volcanic shards and siliceous spicules). In
some cores, Halimeda plates form a disorientated opentextured accumulation. Locally, within cores, sediments
542
HALIMEDA BIOHERMS