3 – The Great Barrier Reef in Time and Space: Geology and Palaeobiology
21
Holocene transgression from the Last Glacial Maximum
(18 ka) to the current high sea level stand. There is also
radiometric evidence from the GBR that the Holocene
reef growth was superimposed upon relic Pleistocene
reef topography from the last interglacial age.
In other tectonically stable parts of the world this last
interglacial reef is well documented at about 2–6 m
a.s.l. For example, the last interglacial reef (125 ka, 5 m
a.s.l.) is emergent along the West Australian Ningaloo
coast, where it is extensively preserved in near desert
conditions (Fig. 3.2 D). Whereas the north Queensland
coast is well endowed with evidence of Pleistocene
shorelines in the form of beach rock, dunes, dune foundations, and beach ridges (e.g. Cowley beach near Innisfail), there are relatively few occurrences providing
surficial expression of last interglacial reef framework
from the GBR (e.g. Stradbroke Island, Evan’s Head, Lord
Howe Island, Saibai Island in the Torres Strait, Digby Island). One possible explanation for this is that the northeast coast’s moist airflow from onshore tradewinds has
weathered and eroded the emergent 125 ka GBR reef below the current high sea level such that they now only
exist as a base for Holocene reef growth. But perhaps the
last interglacial reef did not everywhere grow to the high
sea level. One recent explanation is that vertical movements in the form of hydro-isostasy or tectonic lowering
are the major factors.
Because of the Milankovitch cycles discussed earlier, sea level fluctuations are not confined to ‘glacial’
(i.e. Last Glacial Maximum [LGM] 18 ka) and ‘interglacial’ (i.e. Last Interglacial [LI] 125 ka) periods; smaller
scale fluctuations are referred to as ‘stadial’ (temporary
ice advance) and ‘interstadial’ (temporary ice retreat)
times. Abundant studies carried out in tropical seas
correlate the growth of ‘wave resistant organic structures’ such as coral reefs with sea level transgressions.
We therefore might expect to see transgressive reef deposits developed during the smaller scale sea level
changes between the high stand LI and the low stand
LGM (Fig. 3.2C). But is there any evidence for GBR
reef-building during these lower sea levels?
The nature of low sea-stand reefs has been studied in
the Huon Gulf in Papua New Guinea. Here are found
similar rates of accumulations and coral communities that
were not unlike their high sea level stand counterparts
in the adjacent raised reef terraces of the Huon Peninsula, PNG. Regardless, reefs must be seen as dynamic
and fluid, reacting to sea level throughout the major and
minor Pleistocene fluctuations in sea level (Fig. 3.2A).
On the GBR, there is a history of investigation of the
terraces and positive-relief features on the continental
shelf and margin for evidence of lower sea levels.
‘Wave-cut’ terraces have been recorded in the southern
GBR at 175 m and in the central GBR at 113 m,
88 m and 75 m, where they were interpreted to correspond to postglacial shorelines. Submerged reefs, terraces and notches have been consistently recorded on
single beam echo sounder transects across the southern
GBR shelf edge but so far there is insufficient evidence
on the spatial distribution of these features to make accurate comparisons against sea level curves.
Recent investigations by marine geologists using
multibeam echo sounders have revealed that drowned
reefs extend for hundreds of kilometres along the GBR
outer shelf edge in 40 m to 70 m depth. They appear to be submerged ‘barrier reefs’ approximately
200 m wide and are comprised of two parallel ridges
of eroded limestone pinnacles (Fig. 3.3A). These
drowned shelf-edge reefs might be an important archive of past climate and sea level changes, and potentially provide predictive tools for GBR coral
community response to future climate changes. It is
now also possible to map shelf depth palaeo-drainage
in greater detail than ever before (Fig. 3.3B). Very recent work has extended the occurrence of these submerged shelf-edge reefs as far south as the northern
end of the Swain Reefs.
In previous decades the inherent difficulty of remote underwater exploration has restricted the usefulness of this work. A submarine terrace might represent
a constructional feature—an interstadial reef—but it
might also represent an erosional feature—a wave cut
cliff or bench. Modern acoustic techniques involving
multibeam sonar hold great promise for finally illuminating the inter-reef and shelf-edge stories by combining high resolution 3-dimensional structure with an
ability to map its regional extent.
Despite the limitations of technology, four decades
of exploration combined with the new hydrographic
charts do tell us one thing. The interstadial GBR does
21
Holocene transgression from the Last Glacial Maximum
(18 ka) to the current high sea level stand. There is also
radiometric evidence from the GBR that the Holocene
reef growth was superimposed upon relic Pleistocene
reef topography from the last interglacial age.
In other tectonically stable parts of the world this last
interglacial reef is well documented at about 2–6 m
a.s.l. For example, the last interglacial reef (125 ka, 5 m
a.s.l.) is emergent along the West Australian Ningaloo
coast, where it is extensively preserved in near desert
conditions (Fig. 3.2 D). Whereas the north Queensland
coast is well endowed with evidence of Pleistocene
shorelines in the form of beach rock, dunes, dune foundations, and beach ridges (e.g. Cowley beach near Innisfail), there are relatively few occurrences providing
surficial expression of last interglacial reef framework
from the GBR (e.g. Stradbroke Island, Evan’s Head, Lord
Howe Island, Saibai Island in the Torres Strait, Digby Island). One possible explanation for this is that the northeast coast’s moist airflow from onshore tradewinds has
weathered and eroded the emergent 125 ka GBR reef below the current high sea level such that they now only
exist as a base for Holocene reef growth. But perhaps the
last interglacial reef did not everywhere grow to the high
sea level. One recent explanation is that vertical movements in the form of hydro-isostasy or tectonic lowering
are the major factors.
Because of the Milankovitch cycles discussed earlier, sea level fluctuations are not confined to ‘glacial’
(i.e. Last Glacial Maximum [LGM] 18 ka) and ‘interglacial’ (i.e. Last Interglacial [LI] 125 ka) periods; smaller
scale fluctuations are referred to as ‘stadial’ (temporary
ice advance) and ‘interstadial’ (temporary ice retreat)
times. Abundant studies carried out in tropical seas
correlate the growth of ‘wave resistant organic structures’ such as coral reefs with sea level transgressions.
We therefore might expect to see transgressive reef deposits developed during the smaller scale sea level
changes between the high stand LI and the low stand
LGM (Fig. 3.2C). But is there any evidence for GBR
reef-building during these lower sea levels?
The nature of low sea-stand reefs has been studied in
the Huon Gulf in Papua New Guinea. Here are found
similar rates of accumulations and coral communities that
were not unlike their high sea level stand counterparts
in the adjacent raised reef terraces of the Huon Peninsula, PNG. Regardless, reefs must be seen as dynamic
and fluid, reacting to sea level throughout the major and
minor Pleistocene fluctuations in sea level (Fig. 3.2A).
On the GBR, there is a history of investigation of the
terraces and positive-relief features on the continental
shelf and margin for evidence of lower sea levels.
‘Wave-cut’ terraces have been recorded in the southern
GBR at 175 m and in the central GBR at 113 m,
88 m and 75 m, where they were interpreted to correspond to postglacial shorelines. Submerged reefs, terraces and notches have been consistently recorded on
single beam echo sounder transects across the southern
GBR shelf edge but so far there is insufficient evidence
on the spatial distribution of these features to make accurate comparisons against sea level curves.
Recent investigations by marine geologists using
multibeam echo sounders have revealed that drowned
reefs extend for hundreds of kilometres along the GBR
outer shelf edge in 40 m to 70 m depth. They appear to be submerged ‘barrier reefs’ approximately
200 m wide and are comprised of two parallel ridges
of eroded limestone pinnacles (Fig. 3.3A). These
drowned shelf-edge reefs might be an important archive of past climate and sea level changes, and potentially provide predictive tools for GBR coral
community response to future climate changes. It is
now also possible to map shelf depth palaeo-drainage
in greater detail than ever before (Fig. 3.3B). Very recent work has extended the occurrence of these submerged shelf-edge reefs as far south as the northern
end of the Swain Reefs.
In previous decades the inherent difficulty of remote underwater exploration has restricted the usefulness of this work. A submarine terrace might represent
a constructional feature—an interstadial reef—but it
might also represent an erosional feature—a wave cut
cliff or bench. Modern acoustic techniques involving
multibeam sonar hold great promise for finally illuminating the inter-reef and shelf-edge stories by combining high resolution 3-dimensional structure with an
ability to map its regional extent.
Despite the limitations of technology, four decades
of exploration combined with the new hydrographic
charts do tell us one thing. The interstadial GBR does