Download The distribution and structure of coral reefs: one

Biological Journal of the Linnean Society (1983), 20: 11-38. With 24 figures
The distribution and structure of coral reefs:
one hundred years since Darwin
T. P. SCOFFIN AND J. E. DIXON
Grant Institute of Geology, University o f Edinburgh, Edinburgh EH9 3JW
Plate tectonic theory accounts for the steady subsidence of mid-plate oceanic islands by cooling of
the lithosphere and so provides a sound basis for Darwin’s theory of atoll formation. Now it is
evident that because the lithosphere behaves elastically in response to loads such as islands, more
localized subsidence and uplift patterns can also be explained. Tectonically active areas, where one
plate is subducted beneath another, are also likely to contain regions of marked uplift, but are less
amenable to modelling. These processes together provide a background motion framework for most
reef settings with rates of vertical movement of the order of a few millimetres per year.
Reef forms are greatly influenced by the configuration of their foundations. Holocene reef
foundations were essentially moulded by processes of deposition and erosion during the Pleistocene
when global sea level changes were often greater than I cm year-‘.
We are now developing a sufficient understanding of the rates and nature of reef processes of
growth and destruction to be able to see the manner in which the structural development of reefs
responds to the complex interplay of tectonic uplift and subsidence plus changes of sea level and
climate.
reefs -oceanic islands - atolls - uplift - subsidence - plate tectonics - tectonic
sea level changes - reef geomorphology - reef sedimentation.
KEY WORDS:-Coral
theory
-
C0NTE N TS
Introduction . . . . . . . . . . . . . . . . . . .
Darwin’s contribution.
. . . . . . . . . . . . . . .
The distribution of modern coral reefs . . . . . . . . . . . . .
Ridge and mid-plate volcanoes
. . . . . . . . . . . . .
Subduction zone setting . . . . . . . . . . . . . . .
Latitudinal implications of plate motion . . . . . . . . . . .
Long-term tectonic fluctuations: summary . . . . . . . . . . .
Gross structure of corals reefs: uplift and subsidence histories
. . . . . . .
Superficial features on modern reefs
. . . . . . . . . . . . .
Karst origin for the pre-Holocene surface . . . . . . . . . . .
Sedimentary origin (i.e. framework growth and sediment accumulation) for preHolocene surface . . . . . . . . . . . . . . . . .
An example of platform reef development on the Great Barrier Reef of Australia.
,
Summary remarks.
. . . . . . . . . . . . . . . . .
References. . . . . . . . . . . . . . . . . . . .
11
12
14
16
17
19
20
22
25
28
30
34
36
37
INTRODUCTION
Darwin’s book, T h e Struclure and Distribution o f Coral Reefs, published in 1842,
described the characteristics of reef building organisms and set out a profound
0024-4066/83/0500 1 I
+ 28 S03.00j0
I1
01983 The Linnean Society of London
T. P. SCOFFIN AND J. E. DIXON
12
theory that accounted for the major forms of reefs in the oceans and provided
explanations for their distribution. He spent many months on the project
reading every work on the islands of the Pacific and consulting many charts,
though the germ of the theory grew in his mind during his stay on the west coast
of South America before he had seen a true coral reef Uudd, 1890). In contrast,
modern advances in scientific understanding tend to be the result of the
calculated application of new technology, by teams of scientists working on
programmes of research, rather than the results of inspired deductions in the
manner of Darwin. Our approach in this article is to summarize the major
developments in the understanding of reef structure and distribution over recent
years indicating, where appropriate, the technological innovation that made
each breakthrough possible. I t is not our intention to catalogue all the various
theories of the formation of barrier reefs and atolls published after the
appearance of Darwin’s book; for this the reader is referred to the paper by
Steers & Stoddart (1977).
Over the last 30 years there have been major developments in earth science
that have significantly advanced our knowledge of coral reefs. The application
of geophysical techniques to the problem of the evolution of the oceans and the
ability to drill deep cores on land and at sea and apply refined dating methods
to them have helped to unravel the complex relative movements of land and sea
which are critical to an understanding of reef evolution. Vast quantities of new
data on the occurrence of organisms and sediments, and the nature of marine
processes have come from the refinement of techniques for making observations
underwater using SCUBA and submersibles. There is now widespread
agreement in sight on the rates of modern reef building and destroying
processes. These figures are currently being applied to reefs of the recent past to
model their development with the aid of computers. Long-term monitoring
experiments and remote sensing techniques using satellite imagery and seismic
interpretation, coupled with new computer-aided methods of data processing,
storage and retrieval are opening up a further new era in coral reef research.
Darwin’s contribution
Darwin noted that reef-builders prefer a rocky platform for a foundation, that
the actual surface of all reefs differs little from one to another, but that reefs can
take a variety of forms:
Atoll:
Barrier reef:
Fringing reef:
An annular structure with the absence of land within
its central expanse.
The reef is separated from land by a deep lagoon.
The reef is attached to the land.
He observed that the reef-building corals cannot flourish in water deeper than
about 55 m. It was therefore difficult to account for atolls which commonly
rose from unfathomable depths (of 3000 m or more) and equally difficult to
explain some barrier reef foundations. As it was unlikely that there were vast
numbers of sea bed mountains all at just the optimum depth beneath the water
surface for coral colonization, Darwin concluded there must have been
subsidence. His studies of modern volcanic rocks in western South America
opened his eyes to the great movements taking place on the earth’s surface over
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
13
relatively short periods. Here he found evidence for large scale crustal elevation
and so began to consider possible evidence for complementary crustal
subsidence elsewhere. He suggested that at different times new volcanic islands
form in the ocean, support a reef and subside. The subsidence theory brings
together in a single evolutionary concept the main kinds of reefs, fringing,
barrier and atoll, as successive stages of coral reef growth on foundering volcanic
island bases (Fig. 1). It further accounts for the deep lagoons on aLolls and
behind some barrier reefs. Passes on reef rims were considered to align with
former river channels, now drowned. Darwin invoked changes in the relative
movements of land and sea to account for features such as terraces and double
barriers. Employing the principle that atolls and barrier reefs indicate zones of
subsidence whereas fringing reefs and active volcanoes represent zones of uplift,
Darwin went on to produce a global map of oceanic areas of subsidence and
uplift (Fig. 2).
Darwin’s subsidence theory of atoll formation was confirmed in 1951 when
two holes were drilled which reached the volcanic rock basement beneath
Enewetak Atoll at depths of 1267m and 1405 m (Ladd et al., 1953). The
limestones recovered were all of shallow water origin demonstrating both
subsidence of the atoll and the upward growth of shallow water corals since
Eocene time, approximately 49 My B.P. (Schlanger, 1963). One contribution of
the modern theory of the earth’s crustal evolution to coral reef research is to
provide both an explanation for and a means of quantifying the subsidence of
the sea floor and the islands that rise up from it, that Darwin had correctly
deduced so many years earlier.
(B) Barrier
reef
+
- - {
-/ +
-/+
+
Figure 1. Sequential development-fringing
island.
+
+
+
+
+\---
reef, barrier reef, atoll-on
subsidence of volcanic
14
T. P. SCOFFIN AND J. E. DIXON
Figure 2. Darwin’s map of global areas of subsidence, represented by barrier reefs and atolls (open
circles here), and uplift, represented by fringing reefs and volcanic islands (black triangles)
superimposed on a map of crustal plates and their directions of relative motion. (After Darwin,
1842 and Rosen, 1982).
THE DISTRIBUTION OF MODERN CORAL REEFS
Reef-building corals have specific light, temperature, sedimentation and
substrate requirements for healthy growth in normal sea water (see Wells,
1957). These conditions restrict them to tropical latitudes where their principal
occurrences are on the warmer western sides of oceans located on shallow rocky
prominences (Fig. 3). The distribution of coral reefs relates essentially therefore
to the distribution of their continental and oceanic island foundations within the
western tropical oceans.
The principles of plate tectonics developed over the last 20 years provide a
unifying theory explaining the origins and morphology of the ocean basins, and
the distribution of the continents and oceanic islands (see publications by
Wilson, 1963; Heirtzler, 1968; Bullard, 1969; Dewey, 1972; Burke & Wilson,
1976; for a general summary). The theory and its application have had a
profound and revolutionary effect on all aspects of earth science. We now
believe that the outer 100 km or so of the earth consists of rigid plates, the
‘lithosphere’, that are shifting relative to one another above a low-long-termstrength zone, the ‘asthenosphere’, in the earth’s upper mantle. The continents
are merely less dense portions of the lithosphere and are carried on the mobile
plates. New plate material is formed at spreading centres along ocean ridges by
Figure 3. Distribution of Recent hermatypic coral genera. (After Stoddart, 1969.)
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
15
the upwelling of the asthenosphere from which basaltic magma separates at the
ridge axis to form a 5 km thick oceanic crustal top layer to the plate. Plates are
consumed at subduction zones marked by deep ocean trenches where one plate
descends beneath the other into the asthenosphere. The spreading rates of new
crust vary from 1 to 10 cm year- on both sides of a ridge and can be measured
from the symmetrical sequences of dateable magnetic anomalies found on either
side of spreading centres in most of the world’s oceans. The Atlantic, Indian,
Arctic and Antarctic Oceans result from rifting of a large continent 180 to
120 My ago. Although the Pacific likewise has no crust now older than Upper
Jurassic it is the site of a much older ocean basin.
Shallow rocky prominences suitable for coral reef foundations are found at the
margins of continents (e.g. NE Australia) and continental fragments (e.g.
Malagasi) and also around oceanic volcanic islands. The distribution and the
lateral and vertical motions of these reef foundations are controlled by the
fundamental processes in the lithosphere and asthenosphere.
New oceanic crust created at a spreading ridge cools as it moves away from
the ridge coupled to its underlying upper mantle. The boundary separating
cooler rigid lithosphere above, from hotter plastic asthenosphere below, sinks
progressively lower into the mantle as the plate gets older further from the ridge.
Cool lithosphere is denser and thicker than hot lithosphere and since
gravitational equilibrium prevails, the ocean floor itself subsides in a way which
is so remarkably uniform that over large parts of all the world’s oceans, water
depth is proportional to the square root of the age of the sea floor. This steady
subsidence through cooling also affects the edges of the continents and any
attenuated or partially detached continental blocks created by a rifting episode.
Plate tectonics thus accounts for subsidence in the oceans. However, since the
mean depth of ocean spreading centres is around 2+ km, alternative mechanisms
must be sought for the initial elevation of suitable rocky substrates to sea level.
Continental fragments and margins present no problem. Many will have been
attenuated during rifting and be of such a thickness that their top surface will
subside slowly through mean sea level as isostatic gravitational equilibrium is
maintained on cooling (Fig. 4).In this way it is likely that major reef complexes
like the Bahamas and the Great Barrier Reef were built up to considerable
thicknesses.
Shallow water
Y
(may be oblique
m ridge trend)
Figure 4. Major zones of crustal uplift and subsidence.
T. P. SCOFFIN AND J. E. DIXON
16
Wholly oceanic edifices at or above sea level occur in a number of well
defined plate-tectonic settings (Fig. 4).
Ridge and mid-plate settings
(a) High lava-production sites on or very close to oceanic spreading centres, e.g.
Galapagos.
(b) Isolated island volcanoes in a mid-plate setting, e.g. Reunion.
(c) Linear chains of volcanic islands and associated sea-mounts, e.g. HawaiiEmperor chain.
Subduction zone settings
(d) Island-arc volcanic chains, e.g. Marianas, Tonga-Kermadec Arc.
(e) Frontal and fore-arc ridges forming islands or reefs and lying between the
volcanics and the trench in some arc systems, e.g. Barbados.
Ridge and mid-plate volcanoes
Distribution
Ridge and mid-plate volcanoes (a, b and c) are by definition located over
anomalous zones of high magma-production in the asthenosphere. Such zones
are reasonably described as ‘hot spots’ whatever their ultimate explanation. In
particular the existence of linear volcanic island chains like the Hawaiian
Islands (Fig. 5) with systematic age progressions from an active large island at
one end, through older eroded edifices to extinct submerged seamounts a t the
other, is most easily taken as evidence for persistent magma sources beneath the
plate which remains relatively fixed through time and over which the whole plate
system moves. The island chain is then the trace of the relative motion of plate
and ‘hot-spot’. Each island edifice has a finite life before it becomes too detached
from its source to be viable and subsidence ensues. Furthermore the parallelism
of several island chains in the Pacific, e.g. Tuamotu and Line Islands with the
Hawaii-Emperor chain (Fig. 5) prompted the conclusion that these (perhaps
Less than
40 My old
I
islands
0 Seamounts
L
c
More than
40 My old
0 Islands
o Seamounts
J
Figure 5. Island and seamount chains in the Pacific Ocean.
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
17
all) hot-spots together contributed a ‘reference-frame’. Upper limits on interhot-spot motion are close to minimum relative motion rates for plates. At
present the hot-spot model as a description of within-plate volcanic activity is
reasonably well accepted, and it provides a good explanation for many of the
gross features of Pacific island distribution.
Subsidence through plate cooling
Clearly any oceanic volcanoes once formed must move with the plate. As long
as volcanic activity continues then growth of the edifice can, in principle, keep
up with the inexorable subsidence of the surrounding sea floor and the island
will remain above sea level and possess a fringing reef if suitably located. Once
volcanic activity ceases, for example if the plate moves away from the magma
source in the underlying asthenosphere, or for any other reason, subsidence will
inevitably follow the now well known curves and the associated reefs evolve
through barrier and atoll stages if organic growth keeps pace.
Subsidence and uplgt from lithosphere-loading efects
O n a more local scale, the weight of an oceanic island is in part supported by
the strength of the oceanic lithosphere. Its response is to ‘sag’ and its behaviour
can be modelled as that of an elastic plate overlying a viscous fluid. Because of
the elastic behaviour of the lithosphere any local depression (moat) is
accompanied by a more extensive but less pronounced upward bulge (arch)
around the depression as the plate attempts to increase its radius of curvature at
the edge of the depression to reduce deviatoric stress in the plate. This upward
bulge can be concentric around a load such as an island, but a broad, linear or
arcuate bulge is formed seaward of ocean trenches in response to the down-bend
of the trench itself. The magnitude of these bulges around island loads can be
tens of metres and can spread tens of kilometres from the source. Trench ‘outer
highs’ are some 200 km broad.
The consequences for reef development can be marked. Active growth of one
volcanic edifice can lead to accelerated subsidence as it gets heavier. Subsidence
could to some degree be asymmetric. Conversely the loading effect of one island
can produce emergence of neighbouring islands through the elastic bulge effect
(McNutt & Menard, 1978).
Islands or seamounts approaching subduction zones will inevitably shallow
slightly 100-200 km out from the trench before beginning their descent to
trench depths. Scott & Rotondo (1983) have drawn up a dynamic model to
explain the gross configuration of reefs on the Pacific Plate (Fig. 6 ) . They have
indicated, with examples, how 1 1 fundamentally different island/atoll forms can
develop according to the nature and timing of the subsidence and uplift motions
we may expect in the mid-plate position. They point out that other
combinations are possible within the realm of plate tectonic theory alone. We
may of course expect changes in reef development if the foundations drift
laterally out of the tropics and coral growth is retarded, or if other outside
influences affect climate or relative sea level position, e.g. glaciation.
Subduction zone settings
Subduction zone environments-island-arc
and fore-arc ridges-are
subject
18
T. P. SCOFFIN AND J. E. DIXON
Figure 6. A model for island-atoll type development on the Pacific lithospheric plate. Vertical
arrows indicate relative rates of vertical movement (After Scott & Rotondo, 1983).
to quite distinct elevation and subsidence processes which do not in general
follow any simple path. At least three processes can lead to emergence of an
island or reef in the arc region itself
Frontal arc
It seems likely that the initiation of subduction in an ocean must be
accompanied and/or preceded by basic vulcanism associated with the initial
plate rupture. This process is poorly understood and in consequence the nature
of the crust in the core of the island arcs and the original substrate for coral
growth is unknown. This substrate for organic growth can apparently form a
very stable and longlasting slowly subsiding region some tens of kilometres
trench-wards from the volcanic centres themselves. The coral islands of Guam
and Saipan show (like the mid-plate atoll Enewetak), continuous coral growth
from at least the Eocene (40My). They may perhaps rest on a basement of
tectonically duplicated oceanic crust augmented by 'proto-arc' volcanics which
reached shallow depths.
Island-arc volcanoes
Subduction of oceanic lithosphere results typically in andesitic vulcanism which
is generally sited on the upper plate above the point where the downgoing slab
is at a depth of some 120 km. The classic island-arc of active volcanoes results.
The origin of island-arc magmas is still not certain. They are often erupted
explosively and produce extensive ashes and tuffs interbedded with relatively
minor flows forming steep-sided unstable conical edifices. The scarcity of solid
lava, and the lack of consolidation make many such volcanoes poor substrates in
their more active phase. Long periods of quiescence can lead to rapid erosion to
sea level and favourable coral growth conditions.
Fore-arc ridges
Progressive subduction of oceanic lithosphere results in the scraping off of
sediments in the trench and the formation of an 'accretionary prism' between
trench and arc. When the sediment supply to the trench has been high, the
prism is well developed and can break the surface as a fore-arc ridge (Karig &
Sharman, 1975), e.g. Barbados and the Mentawai Islands off Sumatra.
Progressive uplift and tilting backwards towards the arc would be expected in a
simple accretion model but the real situation is more complex and not well
understood in detail.
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
19
At a point where an exceptional volume of lithosphere, such as a chain of
migrating volcanic islands, is consumed at the subduction zone, the motion of
that part of the overriding plate appears to be retarded relative to adjacent
portions of the plate leading edge. This mechanism could account for the
serrated configuration of island arc chains in the Western Pacific and is likely to
result in anomalous uplift patterns in the fore-arc region at the arc-chain
intersection.
Subsidence mechanisms in island-arcs
In the Western Pacific the most spectacular subsidence results from the
growth of 'back-arc basins'. The Mariana arc system has undergone three cycles
of back-arc basin formation since early Tertiary times, apparently beginning in
each case with a rift between the volcanic chain and the frontal arc. As the rift
widens and new sea floor is created the volcanoes become extinct and subside
rapidly to abyssal depths forming the 'remnant arcs' seen in the submarine West
Mariana and Palau-Kyushu ridges (Karig, 1972). After a volcanic hiatus a new
chain is established in the original location close to the frontal arc which itself
evidently remains undisturbed through the entire cycle. The cyclic character of
Western Pacific arc behaviour pndoubtedly adds an extra component ofcomplexity
to the subsidence-uplift history of active arcs.
Uplijit in subduction zone settings
The emplacement of intermediate and silicic magma bodies into the crust
above subduction zones is likely to result in long-term uplift as the crust becomes
thicker and less dense. Active continental margins are likely to show uplift
therefore, but intra-oceanic island arcs subjected to periodic back-arc rifting
may never reach the requisite crustal volume. Any resumption of volcanic
activity after a quiescent interval is likely to be accompanied by dilational and
thermal expansion effects which may possibly influence local sea level around
the arc. In general the uplift and subsidence patterns of island arcs are much less
amenable to modelling and prediction than those of islands in intra-oceanic
mid-plate settings.
Latitudinal implications of plate motion
Potassium/argon radiometric dating of basalts can give an indication of the
extent of lateral movement of volcanic islands riding on ocean plates, e.g. Hawaii
15 cm year- I , Austral Island 9 cm year- l , Marquesas Island 10 cm year- l .
Also there are dating and magnetic studies of successive basalts on volcanic
islands and atolls which show, for example, that Midway atoll has migrated
through 13" of latitude over 18 My (a movement of 1400 km) and is moving out
of the northern limit of coral seas whereas Pitcairn Island at 24"s is presently
reefless and moving along a NW-SE trajectory into reef seas at a rate of
11 cm year-' (Stoddart, 1976).
Studies of X-radiographs of coral structure (Fig. 7) reveal that coral growth
rate is reduced, along with a reduction in the areal coverage of corals, at higher
latitudes (Grigg, 1982, Fig. 8). The latitudinal limit beyond which coral growth
cannot keep pace with subsidence, and thus the threshold for atoll formation,
has been termed the Darwin Point (Grigg, 1982). The Darwin Point exists at
the northern end of the Hawaiian Archipelago at 29"N latitude; atolls and coral
T. P. SCOFFIN AND J. E. DIXON
20
Figure 7. X-radiographs of Montaslrea annularis corals revealing density banding representing
seasons.
islands transported north-west by tectonic movement on the Pacific plate appear
to have been drowned near the Darwin Point during the last 20My (Fig. 9).
We may expect Darwin Points in other parts of the world though not always at
the same latitude due to differences in ecological conditions, coral species
composition, island area, rates of erosion and tectonic histories.
Long-term tectonicJuctuations: summary
A global picture of zones of subsidence and uplift emerges, and the different
Laysan
19
I
I
I
I
20
21
22
23
I
I
24
25
*\
-..
1Midway
PRH
*
Lisianski,
I
I
26
,
27
Latitude (ON)
Figure 8. Reef accretion in kg CaCO, rn-? year-’ plotted against latitude for reefs of the Hawaiian
chain (after Grigg, 1982).
21
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
Subduction in
Kanchatko trench
'
I
II
54",
163'E
Reef coral growth ceases, I Fringing reefs
I become atolls
atolls drown to form guyots' continued
'
subsidence
1 contiwed subsdence,
I
I Subaeriol erosion, I
I subsidence, and I
I fringing reefs
1
I
19'
N, 155' W
Darwi
A
Melting
anornoly
OT 'hot spot'
Figure 9. Stages in reef history on the Hawaiian chain (after Grigg, 1982 and Scott & Rotondo,
1983).
mechanisms for generating emergent, or shallow, sea floor can be rationalized as
manifestations of plate-motion and subplate magma production. Active uplift is
likely to occur at mature volcanic arcs above continental margin or oceanic
subduction zones, in some fore-arc regions, and to a minor degree in mid-plate
settings through lithosphere-flexure effects. Subsidence is the inevitable fate of
oceanic lithosphere once created. Lithosphere-cooling models can account for it
very effectively and so allow anomalous departures from the normal curves to be
detected.
It must be emphasized that those subsidence and uplift trends that are
fundamentally thermal in origin are low-frequency and long-wavelength in
character and provide a background pattern of vertical motion on a millions of
years scale affecting hundreds of thousands of square kilometres. Lithospherecooling and subsidence affect the evolution of a single plate. World-wide bursts
of rapid sea floor spreading, can, over a several million year period, cause sea
water to be displaced by thermally expanded mid-ocean ridges, and sea level to
rise relative to land, in other areas. O n a shorter, but still 'long-term' time-scale,
elastic plate responses to point loads are likely to be one or two orders of
magnitude more rapid and affect areas a few tens of kilometres across.
Superimposed on these long-term thermal and mechanical effects are the much
higher-frequency fluctuations in sea level caused by global ice-volume
variations. In the fore-arc region of subduction zones local tectonic effects are
also likely to be often large and rapid enough to be the dominant source of
vertical movement.
Enewetak (Menard, 1964) and Midway atolls (Tracey, Ladd & Hoffmeister,
1948) on mature oceanic crust are estimated to be subsiding at rates of
0.02 mm year-', whereas Barbados (Mesolella et al., 1969) and the Huon
Peninsula, Papua New Guinea (Chappell, 1974) in tectonically active plate
convergence zones are currently undergoing uplift of rates of 0.3 mm year- I
and 3.0 mm year- respectively.
Now that many of the potential sources of the different components in the
uplift/subsidence pattern of individual reefs are recognized, the prime task in
22
T. P. SCOFFIN AND J. E. DIXON
any one area is to identify and disentangle them. Because we are at present still
emerging from a period of major short-term sea level fluctuations due to glacial
effects, these dominate most present day reef forms.
THE GROSS STRUCTURE OF CORAL REEFS: UPLIFT AND SUBSIDENCE HISTORIES
As initial coral growth is governed by the distribution of rocky prominences
we may anticipate that a linear shelf edge will provide foundations for a linear
reef and a circular island will promote an annular reef structure. For volcanic
islands it is clear that where coral growth can keep pace with subsidence a
fringing reef will develop into a barrier reef which in turn will develop into an
atoll with a deep central lagoon, as Darwin predicted. The reefal development
along the Society chain shows this trend in a north-westerly direction away from
the present igneous centre as the volcanic islands progressively cool and subside
with age (Fig. 10). Where an island has been uplifted for example by the
loading of adjacent lithosphere, a veneer of coral growth coats the core. If this
movement of land relative to sea is unidirectional then the emergent reef is
progressively younger towards the present sea level where modern corals fringe the
land. The widest atolls are to be expected where the slow drifting plate motion
has kept a slowly subsiding volcanic foundation within the tropical zone for a
long period. Rapid subsidence of an atoll would produce a thick limestone
accumulation only if calcium carbonate deposition kept pace. Several atolls
have been drilled proving thick subsurface reefal deposits of up to Tertiary age
(e.g. Bikini, Enewetak, Midway). The oldest known Pacific atoll is Darwin
Guyot between Hawaii and the Marshall Islands which rises from a depth of
6000 m to 1370 m below sea level. The limestone rim (of unknown thickness)
contains Cretaceous rudists and encloses a central depression 20 m deep (Ladd,
Newman & Sohl, 1974). Older atolls may still be found though we may assume
that many of the reefs that initiated on volcanic islands that formed early in the
ocean's evolution have been carried on the migrating ocean crust to be
consumed or scraped off in subduction zones at plate boundaries. This explains
why few fossil reefs outcropping on continents are of the order of thickness of the
(Cretaceous to Recent) living atolls (Rosen, 1982).
Studies on atolls and reefs in different settings reflect the interplay of longand short-term processes. Our present understanding of the petrography of
limestones when co-ordinated with radiometric dating allows us to reconstruct
the recent history of atolls from cores and uplifted reefs from exposures. For
example Bikini and Enewetak, mid-plate islands on mature steadily subsiding
oceanic crust, have large quantities of original aragonitic skeletons separated by
layers of altered (recrystallized and dissolved) limestone, suggesting periods of
emergence and freshwater 'solution. These unconformities can be correlated
from Bikini to Enewetak. The recrystallized limestones have isotopic
compositions in accordance with alteration by fresh water during emergence
(Gross & Tracey, 1966). The solution unconformities occur as follows: one at
the top of the Eocene, one at the top of the lower Miocene and one at the top of
the upper Miocene (this is probably Pleistocene). Miocene sediments also
contain pollen and snails indicative of high islands. Ladd (1977) interpreted the
history of Bikini and Enewetak to be:
(1) Basaltic volcanoes were built above sea level in pre-Tertiary time.
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
23
Figure 10. Atoll development on the Society Island chain, Pacific Ocean.
(2) Partial truncations by wave action after volcanic activity.
(3) Subsidence commenced and coral growth commenced in late Eocene.
(4) Reefs were elevated at least three times in Tertiary by sea level changes.
Parts of reefs stood as high islands then.
( 5 ) Pleistocene glaciation caused sea level lows, which may have deepened
lagoons and passes and also formed terraces.
The uplifted terraces of Barbados, in an active fore-arc region, reveal a
complex history of sea level changes superimposed on a steady uplift of the island
(Mesolella et al., 1969) (Figs 11, 12, 13, 14). This has resulted in the capping
Pleistocene reefs being broadly progressively younger towards sea level with, in
detail, a few minor reversals. Many of the reef terraces show a development of
deep fore-reef a t the foot of the cliff (massive corals of Montastrea and Diploria),
extending up through an intermediate depth zone (of broken Acropora cervicornis
24
T. P. SCOFFIN AND J. E. DIXON
Figure 1 1 . Geological map of Barbados. The white area represents the Pleistocene coral cap with
the black lines the terrace edge cliffs.
branches), to the reef crest (of large Acropora palmata branches) at the top of the
cliff, with back reef sands (and local fringing reefs) occupying the bulk of the
terrace top. This faunal succession was repeated at most new still-stands of sea
(Mesolella, Sealy & Matthews, 1970).
Reversals of movement are generally more common than Darwin realised.
Scott & Rotondo ( 1983) have categorized thermal and mechanical ‘events’
which may affect a generally subsiding mid-plate island and so lead to
temporary reversal. Mangaia in the Cook Islands is an example where 70 m
high Miocene reefal limestones rim a dissected volcano. Subsidence followed by
uplift is known from active areas such at the Izu-Bonin arc south of Japan in the
islands of Kita-Daito, Minami-Daito and Olino-Daito Jima (Konishi, Omura &
Nakamichi, 1974). The vertical movements raise fringing reefs above sea level
while elevating formerly deeply submerged rocky substrates to suitable levels for
Figure 12. West-east schematic section through northern Barbados.
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
25
Figures 13 & 14. Fig. 13. Aerial view of the west coast of Barbados, showing modern reef and
terraces (sugar field covered) of emerged Pleistocene reefs. Fig. 14. Sea cliff and notched sea stack of
127 000-year-old reef complex below which is 83 000-year-old reef limestone. Pleistocene reef
terraces, Barbados.
reef-builders to colonize, thus developing a new lower phase of fringing reef such
as is found in parts of the Lesser Antilles (Adey & Bure, 1977).
In general during each period of stability or still-stand of the sea, corals grow
up to sea level and then outwards over the lateral margin of the reef developing
a platform. A sequence of still-stands of different elevation results in a succession
of abandoned reef terraces, that are naturally subaerial after predominant uplift
and submarine after predominant subsidence.
There is good reason to expect that the rate of sea level rise will affect the
actual form of the reef. From a study of the stratigraphy of raised reefs of Huon
Peninsula, New Guinea and Atauro Island Timor, and many radiometric ages,
Chappell (1974) has been able to develop a growth history of Pleistocene reefs.
He later (1980) attempted to model, by computer, the growth form of these
reefs by calculating the reaction of coral growth to the four main environmental
stresses (light, hydrodynamic stress, sediment flux and subaerial exposure)
during the different rates of sea level rise. In the model, sea level rises at rates of
0.0 cm year- and 0.5 cm year- induce formation ofa fringing reefwhereas a rate
of 1 cm year- develops a barrier and lagoon (Fig. 15).
THE SURFICIAL FEATURES ON MODERN REEFS
Solution unconformities within cores and the stepped geomorphology of most
modern reefs indicates the intermittent nature of movements of land relative to
sea. Darwin explained terraces and double barriers by changes in rates of
26
( I ) Stable sea level
Figure 15. Computer model of reef growth according to different rates of sea level rise (after
Chappell, 1980).
subsidence for which we now have possible global mechanisms. What Darwin
did not appreciate was that global sea level changes caused by the advance and
retreat of ice-sheets have frequently been the dominant factor in controlling the
growth and form of Recent reefs in all but the most tectonically active areas.
During the Pleistocene, as polar ice sheets expanded and retreated, sea level
fell and rose with amplitudes of 100 m or more for at least 2 My, a phenomenon
that Darwin could not have appreciated. The indications are that during low
sea stands (maximum ice) the shallow tropical seas were still warm enough to
support corals. Emiliani’s ( 1955) calculations show temperature oscillations of
surface tropical sea water of no more than 6°C through which many corals
would survive. Obviously more would perish in the extreme marginal seas. The
recolonization of marginal seas would be influenced by currents and the
longevity of the coral’s larval stage.
When the sea level fell, though many individual reefs were exposed and their
corals exterminated, many species migrated to lower elevations to return on the
following advance, with best reef development occurring at the optimum depths
for coral growth.
The last interglacial period (about 125 000 years B . P . ) , when sea surface temperatures were similar to todays, produced global sea levels comparable to, or
a metre or so higher than, those in the modern oceans. During the
following glaciation (when sea fell to a maximum of - 150 m at about
18 000 years B . P . ) there were four main periods of transgression giving interstadia1 reef growth at - 14 m (105 000 years B . P . ) , at -20 m (84 000 years
B . P . ) , at -28 m (60000 years B . P . ) , and at -34 m (40000 years B.P.) (Bloom,
1974; Chappell & Veeh, 1978) (Fig. 16). Between and after these interstadials
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
l o r Modern
27
sea level
x 000 years ap
Figure 16. Sea level curve for the last 140000 years (after Bloom et al., 1974 and Hopley, 1982).
the upper portions of reefs were exposed to the atmosphere until the completion
of the Holocene transgression (i.e. a total of about 100000 years). Rates of
subaerial solution of about 0.1 mm year- ' for exposed reefal limestones
(Trudgill, 1976) and aboui three times this figure for intertidal erosion
(predominantly organic) indicate that profound changes are to be expected in
the surface morphology of the last interglacial and interstadial reefs by the next
period of reef colonization in the Holocene. The Holocene transgression took
place at a rate of 10 m 1000 years-' up to 8000 years B.P., at 5 m 1000 years-'
between 8000 and 5000 years B.P., at 2 m 1000 years-' between 5000 and 2000
years B.P. and at 1 m 1000 years-' since then. Sea level has been at its present
position for about 2000-3000 years, approximately 0.1 yo of the duration of the
Pleistocene (Stoddart, 1973).
The rise of sea level (in places over 14 mm year-') was too fast for many
shallow water corals to keep pace, they died and became drowned reefs. (For
example the abandoned reefs common at -20 to -30 m on the flanks of the
Bahama Banks (Hine & Neumann, 1977).) Some corals may have kept pace
and grown essentially vertically since the last glaciation, but unquestionably,
most of the living, shallow-water coral reefs started anew, on features of positive
topographic relief that were prominent at the time of the Recent rise of the sea.
So the foundations of most of our living reefs either existed as positive features
throughout the exposure of the last glaciation or were formed during that
period.
The results of drilling through modern reefs reveal that the Holocene
increment has, in the main, been a very insignificant veneer of only a few metres
above the pre-Holocene surface. Thicknesses vary from 4 to 24 m for the Great
Barrier Reef (Harvey, 1981) giving average vertical growth rates of about
5 mm year-' for the Holocene. Regions of faster lowering of land relative to sea
in the Holocene have greater average vertical growth rates and the thicknesses
of Holocene reef accretion vary accordingly; for example, maximum Holocene
thicknesses of 33 m are reported for the Atlantic (Mcintyre, Burke &
Stuckenrath, 1977) and 15 m for Pacific atolls (Adey, 1978). The thicknesses
and corresponding rates of accretion for Holocene reefs do vary considerably
according to which environment of the reef is cored. However, even though the
28
T. P. SCOFFIN AND J. E. DIXON
Holocene increment on a modern reef may be thin, it does not necessarily follow
that the reefs present configuration closely reflects that of the pre-Holocene
surface. Areas of minimum Recent subsidence may favour extensive lateral reef
growth and sedimentation thus obscuring the form of the underlying
foundations. Davies & Kinsey (1977) have shown from studies of One Tree Reef
in the southern part of the Great Barrier Reef that here post-glacial growth dates
from only about 9000 years B.P., on a surface 20-25 m below present sea level.
They have shown that at present day steady sea level, little or no vertical
growth potential can be realized and that accrued material must represent
lateral growth and the progradation of sand sheets with lagoon infilling.
The latest additions to the relief of the pre-Holocene foundation were
moulded during the last glacial period when sea level was low. Two opposing
processes modelled this surface, one of removal of material (by freshwater
solution from above and by biological erosion at the sides) and one of the
accumulation of materials (by subaerial sedimentation and, for brief periods,
submarine sedimentation and reef growth during interstadials) . Most preHolocene surfaces therefore result from a combination of the effects of processes
that produce negative and positive topographic relief.
Karst origin for the pre-Holocene surface
Several authors (Hoffmeister & Ladd, 1945; MacNeil, 1954; Bloom, 1974;
Purdy, 1974) have suggested that the result of weathering and erosion during
Pleistocene glaciations was not to plane off the foundations of reefs at wave base
(as originally proposed by Daly in his glacial control theory of 1910, 1915) but
rather to produce jagged karst landscapes on the emerged limestone terrains by
acid freshwater solution. The occurrences of drowned sink holes (Fig. 17) 100 m
deep on several carbonate platforms endorses the solution theory and indicates
the potential for the development of considerable solution relief. Holocene reefs
would then form thin veneers on the prominences of karst platforms following
the rising sea. Purdy (1974) expanded the solution experiments of MacNeil
(1954) using blocks of pure limestone and raining acid and, with the aid of
Figure 17. Blue hole in supra-tidal zone, Andros Bahamas (photograph courtesy of Conrad
Neumann).
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
29
geophysical subsurface data, explained the basic configuration of numerous
Recent reef provinces by the karst control theory. "All that is apparently
required is a large surface area of gently sloping beds that is bordered on one or
more sides by a relatively steep slope. The dissolving action of meteoric water
differentially lowers the central area relative to that immediately adjacent to the
steep slopes and results in a partially or completely rimmed solution basin.
Subsequent rise in sea level permits coral colonization of both the solution rim
and the residual karst prominences within the basin. The resulting barrier reef
or atoll with its satellite lagoon reef, is thus formed without recourse to a prior
history of reef development" (Fig. 18) (Purdy, 1974). Purdy further
demonstrated that depending upon circumstances, including among others the
original slope of the exposed surface, the solution rim of an atoll may be either
continuous or characterized on one side or another by numerous breaches. If
annual rainfall intensity is sufficiently high (i.e. tropical) a conical karst
topography will develop within the solution basin (e.g. Bikini Atoll); if it is low
(i.e. temperate) a doline dominated landscape will result (e.g. Bahama Banks). In
the case of barrier reefs a karst marginal plain originally developed by the effect of
high rainfall on a subaerially exposed seaward-sloping contact between
overlying carbonates and underlying non-carbonate rocks (Fig. 18). The
seaward retreat of a limestone wall produces a marginal plain with remnant
towers (e.g. southern barrier reef of Belize). The karst control theory as
propounded by Purdy, explains drowned atolls as drowned karst topography;
reef passes as drainage breaches in the solution rim; faros as karst products of
breaching; limestone islands peripheral to platforms as exposures of the fossil
drainage divide; and spurs and grooves as expressions of lapies. Though there
may be a strong morphological similarity between karst erosion forms and coral
reef surface features, the karst antecedent platform mechanism of atoll formation
does not exclude subsidence as a major control on reef form.
Atolls
Barrier reefs
Rain woter
Roin woter
4.4 4 4 4 4 4
Dtssolutiop effects
Former sea level mln.
max.
min.
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
Maximum dissolution
Karst morainal olain
Conical Korst
-
Minimum dissolution
I
Conical Karst
olution rim
Limestone
m Non-carbonate foundation
eza Karst plain alluvium
0 Marine sediments
Figure 18. Diagrammatic evolution of atolls and barrier reefs according to antecedent karst theory
(after Purdy, 1974).
30
T. P. SCOFFIN AND J. E. DIXON
Hopley (1982) has presented a model for the evolution of a shelf reef from the
last interglacial to the present which is derived principally from an interplay of
karst erosion during sea level lows and coral growth during sea level highs
(Fig. 19). This model goes a long way towards explaining many of the features
displayed on shelf reefs though it is perhaps too simplistic, not taking into
account the various other processes such as marine erosion and coastal
sedimentation that must have operated during fluctuating sea levels.
Sedimentary origin (i.e. framework growth and sediment
accumulation) f o r pre-Holocene surface
Features of positive topographic relief which, later, during the Holocene
transgression, have proved suitable substrates for coral colonization, were
produced by terrestrial and submarine sedimentary processes:
(1) Features formed during sea level highs (i.e. during interglacial or interstadial periods)
(a) Growth features at one stand of sea: We can learn something of former reef
growth patterns by examination of modern reef growth. However, even though
the reef may be very small, unless we drill it and date the core we can never be
certain that the structure results from only one increment (Holocene) of growth.
For this reason we should perhaps examine the growth of a single coral colony
and extrapolate (with the addition of other features such as loose sediments) to
Figure 19. Idc:alized model for the evolution of a shelf reef from the last interglacia11 to the present
(after Hopley, 1982).
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
31
reefs. Growth of a massive coral from the locus of one polyp will (all else being
favourable) develop into the form of a hemisphere till sea level is reached.
Upward growth stops at the air/water interface and only lateral growth
continues as a micro-atoll forms (Scoffin & Stoddart, 1978). The same gross
form is to be expected for the lagoonal patch reef. Once the sea/air interface is
reached growth will be lateral over the talus around the reef periphery. A
prevailing wind may at this stage develop distinctive windward and leeward
margins. The central portions of the lagoonal patch reef having no sand-free
zones on which to grow will interfere with one another and eventually die, be
bio-eroded and become sand covered. As with micro-atolls, individual patch
reefs will expand laterally and coalesce. Eventually large platform reefs will
result having shallow sand-coated centres with isolated vestiges of living coral
and a lip of actively growing coral around the scallop-shaped margin. O n a
stable platform with still sea level, a lagoon may thus become filled with coral
patches and their sediment.
The rims of patch reefs, and also fore-reef slopes will develop a profile related
to the ecological zonation of the reef dwellers and to sedimentation patterns. An
elevated algal ridge may develop on the seaward margins of reefs at the
intertidal level which may owe its existence and hence form, to the relative
absence of algal grazers in this zone. O n the fore-reef slopes, coral growth will be
restricted in those hollows where loose sediment tends to be funnelled. Spurs of
coral framework and grooves of debris may thus develop on reef slopes. There is
evidence that these features may be orientated according to the direction of wave
action (Munk & Sargent, 1954; Roberts, 1974). Submarine fans of reef debris
will spread out from canyons at the foot of the reef slope.
Eventually a steady-state condition is reached which produces characteristic
sediment facies associated with the mature reef (Longman, 1981) which are as
follows (basin to landward): (1) distal talus; (2) proximal talus; (3) reef slope;
(4) reef framework; ( 5 ) reef crest; (6) reef flat; (7) back reef sand.
The three-dimensional lateral and vertical development of these facies is then
a function of the skeletal structure and ecological requirements of the reef
builders, the prevailing physical conditions and the relative position of land and
sea.
(b) Sedimentation patterns on reefs: Coral reefs produce vast amounts of loose
carbonate sediment by the mechanical and biological breakdown of framework
and the accumulation, with or without disintegration, of calcareous reefdwellers. Coarse stable fragments may prove ideal substrates for further coral
colonization whereas relatively fine mobile particles will restrict the growth of
sedentary reef-builders unless the sediment accumulation is stabilized by
cementation.
Ladd (1950) pointed out that on a living reef perhaps only about 10% of the
overall volume of the structure is in situ framework, the remainder (90%) is loose
sediment. This sediment is subject to the prevailing hydrodynamic regime. Not
only does loose sediment predominate in the volume of a reef but also large-scale
sedimentary structures (such as washover fans or boulder tracts (Fig. 20)) can
be built in a fraction of the time taken to build equivalent sized structures by
purely framework growth. Visual observations in deep waters from submersibles
(Land & Moore, 1977; James & Ginsburg, 1979) reveal that huge blocks of reef
framework fall under gravity down fore-reef slopes to accumulate on ledges or at
32
T. P. SCOFFIN AND J. E. DIXON
Figures 20-22. Fig. 20. Boulder tract on the leeward margin of Howick Island, Northern Great
Barrier Reef. Fig. 21. Aerial mosaic of the reefs of the north-west margin of Bermuda atoll. Width of
view c. 15 km. Fig. 22. deltaic reefs of the Nothern Great Barrier Reef. Width of view c. 2 km.
the toe of the reef. Evidence from fossil reefs shows that with time the reef
framework builds out over such talus deposits during a stillstand or regressional
setting. Exceptional wave forces are capable of carrying some large blocks up on
to the reef flat and major storms may build an arcuate deposit of the boulder
tract (Fig. 20).
The influence of storms on the thickets of branching corals on the windward
margins of many Pacific reefs is to produce a rampart, or ridge, of coral stick
rubble on the windward front of the reef flat. These ramparts have an
asymmetrical wave form and can build up to several metres in height. Ramparts
may progressively migrate to leeward by wave action and the coral debris be
eventually swept off the leeward side of the reef or into the lagoon to build a
wedge of sediment. Ramparts may be stabilized by the intertidal precipitation
of high magnesium calcite to form intertidal rocks (rampart rocks, platforms or
bassett edges, see Scoffin & MacLean, 1978). Commonly successive ramparts
are separated by a moat on the front of the reef flat. Such superficial features
could provide ideal foundations on submergence for certain small-scale double
barriers.
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
33
Once the reef has reached sea level the waves built by the prevailing winds
will carry loose sediment to leeward. The windward reef margins thus develop a
smooth arcuate form. The refraction of waves around the sides of reefs will
produce a confluence of wave-induced currents at the leeward margin and cause
sediment to be deposited as a leeward spit or cay (Scoffin et al., 1978).
(2) Features formed during sea level lows (i.e. during glacial periods)
Calcareous and non-calcareous sediment will be transported and deposited at
the coastal zone by marine, fluvial and subaerial processes during sea level lows
to produce topographic features which may later be colonized by reef growth
when sea level rises.
Strandline features, such as beaches (or beach rock) and spits, have been
suggested by Maxwell (1968) to provide the foundations of some of the Great
Barrier Reefs of Australia, as they correlate in depth closely with known former
levels of sea. Garrett & Scoffin (1972) considered that the rims of some atolls
may, during low sea levels, have developed barrier islands (with a supply of
sediment from lower reef terraces) which when subjected to storm action would
produce spillover lobes, washover fans and sluice channel features. They
recognized similar morphological features on the now coral-covered margins of
the Bermuda atoll rim (Fig. 21) and suggested that the barrier island features
produced during earlier positions of sea level may have provided a foundation
for modern coral growth. I t is theoretically easier to explain the plan
morphology of some barrier reef structures by a veneer of growth on truncated
barrier islands than by 100% coral growth from the shelf sea bed.
Where swift tidal currents sweep large quantities of sediment between islands
(or reefs) tidal deltas may develop. The characteristic form of several of the
barrier reefs in the Swains and Northern Province areas of the Great Barrier
Reef of Australia (Fig. 22) led Maxwell (1970) to suggest that tidal delta
features would be the foundation of modern reef growth. One of the arguments
against such a theory has been the great depths of the inter-reef channels which
could not have been produced by tidal scouring during one phase of formation
(Harvey, 1981) .
Large volumes of loose sediment on or near beaches would, under appropriate
climatic conditions, build dunes orientated according to prevailing winds.
Stanley & Swift (1967) suggested an aeolian dune foundation for some of
Bermuda’s reefs, where they noted dune bedded limestone cores to Holocene
reefs.
Many of the arguments relating modern reef form to former sedimentary
accumulations suffer from a lack of substantial subsurface evidence. Though
tantalizingly suggestive, a close similarity of morphologies alone cannot now be
regarded as conclusive proof of a particular origin for the reef form.
Confirmatory evidence is provided by (a) subsurface seismic profiles which
reveal the three-dimensional nature of the foundations and (b) well data which
can provide concrete evidence of the type and age of underlying material. Choi
& Ginsburg (1982) have used to excellent effect both these techniques to show
that the foundations of many reefs in the southernmost Belize lagoon, British
Honduras, are founded on coastal plain sediments which owe their form to
fluvial processes. They noted that the residual elevations of siliclastic
sediments-channel banks, bars and slopes of islands-were favoured sites for
initial accumulations of carbonates (Fig. 23).
2
T. P. SCOFFIN AND J. E. DIXON
34
0
l
2 krn
I
I
I
I
Figure 23. Subcrop maps of part of the southernmost Belize lagoon. T, Pt, Pt, and Pm, are
accoustic units. T, represents early Pleistocene limestone, Pt, and Pt, represents early to middle
Pleistocene siliciclastic fluvial sediments, Pm , is late Pleistocene reefal sediment superimposed to
reveal close relationships with underlying morphology (after Choi & Ginsburg, 1982).
A n example of platform reef development on the
Great Barrier Reef of Australia
As a consequence of the extent of study of reef morphology, surface structure
and sediments plus the accruing data on subsurface from seismics and boreholes
there are now several models of reef growth being produced for the well studied
areas such as the Great Barrier Reef.
Fairbridge (1950) produced a classification of the Great Barrier Reef which
was later modified by Maxwell ( 1968) which was based on organic and sedimentary
growth of reefs in response to prevailing wind and wave conditions
during a single period of relatively stable eustatic sea level. Fairbridge (1980)
suggested that the downwind detrital horns produced on the flanks of reefs by
winds would develop to form open and then closed lagoons, which would finally
be infilled by sediment and organic growth. Maxwell (1968) maintained that
organic reefs once initiated would expand in directions controlled by the
hydrodynamic-bathymetric-biological balance. We now know that the
Holocene phase of reef growth is too thin for all these features to have developed
during one still-stand of the sea. Much of the morphology of present reefs is
derived from the morphology of the antecedent platforms from which they
grow. Jell & Flood (1977) suggested that a t first the antecedent morphology is
mimicked and later masked by Holocene growth. The rise and fall of sea level
relative to an upward growing reef is seen as producing a succession of reef types
which may be reversed by a change in direction of the rise or fall of relative sea
DISTRIBUTION AND STRUCTURE OF CORAL REEFS
35
level. Thus planar surfaces produce platform reefs, antecedent platforms with
shallow depressions produce lagoonal platform reefs, and broad surfaces with
very deep central depressions produce closed ring reefs. It is now thought
however that reef growth is not uniform over the whole antecedent platform
once it has drowned. Maximum growth can be expected on the windward reef
margins, and later reefs spread to leeward over detrital sediment accumulations
derived from the windward reef. Hopley (1982) has drawn up a classification for
platform reefs which incorporates the various morphologies seen on the
Queensland Shelf in a progressive model from juvenile through mature to
senile (Fig. 24). In the juvenile phases initial colonization takes place on the
antecedent foundation, and upward growth of the reef lags behind sea level rise,
enhancing the original relief of the foundation. (Longman, 1981; Hopley, 1982).
In the mature phases reefs reach modern sea level and develop reef flats over the
highs in the antecedent surface and especially around the reef margins. Hopley
notes that one or more lagoons typify this phase but in the later stages of
(I) Unmodified antecedent
platform reef
Minor Karst
depressions
oc+ isolated pinnacle
Central depression
‘.-; :;;--.-.,-*----.
,a
c
5,. o
\,
‘.$
Halimedo veneer
...,’..
‘s--t’*._-I
( I V ) Crescentic reefs
Aligned coral zone
fI
Logoons infilig
ove and spur zone
anded reef fiat
Shallow moated area
with microutolls
Lee side sand
slope
Figure 24. Classification of shelf reefs based on a medium-sized antecedent platform, for the Great
Barrier Reef (after Hopley, 1982).
36
T. P. SCOFFIN ANDJ. E. DIXON
maturity lateral transport of sediment from the productive margins leads to
masking of initial relief forms and widening of reef flats. The senile phase is
reached when lagoons are completely filled and lateral movement of sediment
from the windward margins is leading to progradation to leeward (Hopley,
1982). The planar reef type (senile) are common in the near-shore zone of the
Great Barrier Reef. Storm-produced ramparts are less easily removed by waves
from reef flats in this relatively sheltered inshore portion of the Great Barrier
Reef, and they become lithified by cements of intertidal and spray origins
enhancing the supra-tidal construction of these reefs (Scoffin et al., 1978).
Hopley’s model is based on a shallow pre-Holocene foundation and a single
marine transgression of the Holocene. It is easy to see how complex a model
could be developed if one took into consideration several marine transgressions
of the Pleistocene-each
one with its concomitant reef growth, sediment
production, movement and deposition, coupled with the intervening regressions
when freshwater solution, fluvial and aeolian transport and deposition
prevailed.
SUMMARY REMARKS
Since many, if not most, modern reef complexes initiated growth at least as
long ago as the early Pleistocene (or even Tertiary) it is without doubt that they
have spent long periods exposed to the atmosphere with consequent freshwater
solution and long periods beneath the sea when conditions for healthy growth
were favourable. During both sea level highs and lows, sediments could be
moved by marine, fluvial and aeolian processes and thus further modify the
gross configuration of the reef. The final important variable is that of vertical
tectonic movements brought about by crustal plate evolution as outlined earlier.
Over the last few million years when modern reefs were developing, these
various processes were interacting and moulding the form. The rates of each
process would vary according to the local tectonic, climatic and ecological
factors and we may expect different processes to be dominant for different
lengths of time in different areas. It is for this reason that it is difficult to apply a
model for reef morphology worked out for one area, universally. For example
we may expect the influence of karst erosion to be greater in regions of greater
rainfall; karst-related configuration to be less apparent where vast quantities of
sediment movement (or lateral reef growth) has taken place during the
Holocene; regions of rapid subsidence to have thicker Holocene reefs but also
deeper lagoons and to reflect the last glacial morphology more accurately
than reefs in regions of uplift where lateral development by corals and sediment
may have dominated; regions prone to tropical storms to have reefs whose
configurations are different to those in sheltered zones; and so on.
The last 100 years has seen a much greater understanding of.the overall
processes which may influence reef distribution and structure. We now are
collecting data on the present rates of these processes resulting from both longterm low frequency and short-term high frequency events, and piecing together
the effects of their interactions. Soon we will be able to determine the nature
and rates of past processes that built fossil reefs and in turn, from the study of
three-dimensional structure, the geologist will be able to advise on the likely
physical and biological consequences of unbalancing or catastrophic events.
DISTRIBUTION AND STRUCTURE O F CORAL REEFS
37
ACKNOWLEDGEMENTS
T. P. Scoffin acknowledges support for his coral reef work from the Natural
Environment Research Council. Grant Nos: G R 3 1442 and G R 346 17.
REFERENCES
ADEY, W. H., 1978. Coral reef morphogenesis: a multidimensional model. Science, 202: 831-837.
ADEY, W. & BURKE, R., 1977. Holocene biohenns of the Lesser Antilles-geologic control of development, 421 pp.
Tulsa, Oklahoma: American Association of Petroleum Geologists, Memoirs: Seventh Caribbean Geological
Congress.
BLOOM, A. L., 1974. Geomorphology of reef complexes. In L. F. Laporte (Ed.), Reefs in Space and Time: 1-8.
Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, Special Publication, 18.
BLOOM, A. L., BROEKER, W. S., CHAPPELL, J., MATTHEWS, R. L. & MESOLELLA, K. J,, 1974.
Quaternary sea level fluctuations on a tectonic coast: new Th*30/U2s'dates from New Guinea. Quaternary
Research, 4: 185-205.
BULLARD, E., 1969. The origin of the oceans. Scient$c American, 221: 66-75.
BURKE, K. C. & WILSON, J. T., 1976. Hot spots on the Earth's surface. Scientijic American, 235: 46-57.
CHAPPELL, J., 1974. Geology of coral terraces, Huon Peninsula, New Guinea: A study of Quaternary
tectonic movements and sea level changes. Geological Society of America, Bulletin, 85: 553-570.
CHAPPELL, J., 1980. Coral morphology diversity and reef growth. Nature, 286: 249-252.
CHAPPELL, J. & VEEH, H. H., 1978. Late Quaternary tectonic movements and sea level changes at Timor
and Atauro Island. Geological Society of America, Bulletin, 89: 356-368.
CHOI, D. R. & GINSBURG, R. N., 1982. Siliciclastic foundations of Quaternary reefs in the southernmost
Belize Lagoon, British Honduras. Geological Society of America, Bulletin, 93: 116-126.
DALY, R. A., 1910. Pleistocene glaciation and the coral reef problem. American Journal of Science, Series 4 , 30:
297-308.
DALY, R. A., 1915. The glacial control theory of coral reefs. Proceedings of the American Academy of Science, 51:
155-25 I .
DARWIN, C. R., 1842. The Structure and Distribution ofcoral Reefs, 214 pp. London: Smith, Elder & Co.
DAVIES, P. J. & KINSEY, D. W., 1977. Holocene reef growth-One Tree Island, Great Barrier Reef.
Marine Geology, 24: MI-MII.
DEWEY, J. F., 1972. Plate tectonics. Scientgc American, 226: 56-68.
EMILIANI, C., 1955. Pleistocene temperatures. J o u m l of Geologv, 63: 538-578.
FAIRBRIDGE, R. W., 1950. Recent and Pleistocene coral reefs of Australia. Journal of Geology, 58: 330-401.
GARRETT, P. & SCOFFIN, T . P., 1972. Sedimentation on Bermuda's atoll rim. proceedings of the Third
International Coral Reef Symposium, 2: 87-97. Miami: University of Miami.
GRIGG, R. W., 1982. Darwin point: a threshold for atoll formation. Coral Reefs, I: 29-34.
GROSS, M. G. & TRACEY, J. I. Jr, 1966. Oxygen and carbon isotopic composition of limestones and
dolomites, Bikini and Eniwetok Atolls. Science, 151: 1082-1084.
HARVEY, N., 1981. Seismic inuestigatiom of a pre-Holocene substrate beneath modern reefs in the Great Barrier Reef
Province. Unpublished Ph.D. Thesis, James Cook University of North Queensland, 329pp.
HEIRTZLER, J. R., 1968. Sea-floor spreading Scientific American, 219: 60-70.
HINE, A. C. & NEUMANN, A. C., 1977. Shallow carbonate-bank-margin growth and structure, Little
Bahama Bank, Bahamas. American Association of Petroleum Geologists, Bulletin, 61: 376-406.
HOFFMEISTER,J. E. & LADD, H. S., 1945. Solution effects on elevated limestone terraces. Geological Society
of America, Bulletin, 56: 809-818.
HOPLEY, D., 1982. The Geomorphology of the Great Barrier Reef, 453pp. New York :John Wiley & Sons.
JAMES, N. R. & GINSBURG, R. N., 1979. The seaward margin of Belize Barrier and atoll reefs. International
Association of Sedimentologists, Special Publications No. 3, 191 pp. Oxford: Blackwell.
JELL, J. S. & FLOOD, P., 1978. Guide to the geology of reefs of the Capricorn and Bunker Groups, Great
Barrier Reef Province, with special reference to Heron Reef. Papers, Department of Geology, University of
Queensland, 8: 1-85.
JUDD, J. W., 1890. Critical introduction. In C. R. Darwin On the structure and distribution of coral reefs; also
geological observations on the volcanic islands and parts of South America visited during the voyage of H.M.S. Beagle:
157-165. (Minerva ed.) London: Ward Lock.
KARIG, D. E., 1972. Remnant arcs. Geological Society of America, Bulletin 83: 1057-1068.
KARIG, D. E. & SHARMAN, G. F., 1975. Subduction and accretion in trenches. Geological Society of America,
Bulletin, 86: 377-389.
KONISHI, K., OMURA, A. & NAKAMICHI, O., 1974. Radiometric coral ages and sea level records from
the Late Quaternary reef complexes of Ryukyu Islands. Proceedings of the Second International Coral Reef
Symposium, 2: 595-613. Brisbane: Great Barrier Reef Committee.
LADD, H. S., 1950. Recent reefs. Amcrican Association of Petroleum Geologists, Bulletin, 3:203-214.
38
T. P. SCOFFIN AND J. E. DIXON
LADD, H. S., 1977. Types of coral reefs and their distributions. I n 0. A. Jones & R. Endean (Eds), Biology
and Geology of Coral Reefs Vol. I V Geology 2: 1-19. New York: Academic Press.
LADD, H. S., INGERSON, E., TOWNSEND, R. C., RUSSELL, M . & STEPHENSON, H. K., 1953.
Drilling on Eniwetok Atoll, Marshall Islands. American Association of Petroleum Geologists, Bulletin, 37:
225 1-2280.
LADD, H. S., NEWMAN, W. A. & SOHL, N. F., 1974. Darwin guyot, the Pacific’s oldest atoll. Proceedings of
the Second International Coral Reef Symposium, 2: 51 3-522. Brisbane: Great Barrier Reef Committee.
LAND, L. S. & MOORE, C. H., Jr, 1977. Deep fore reef and upper island slope, North Jamaica. I n S. H.
Frost, M. P. Weiss & J. B. Saunders (Eds), Reefs and Related Carbonates-Ecology and Sedimentology. Studies in
Geology, No.4: 53-65. Tulsa, Oklahoma: American Association of Petroleum Geologists.
LONGMAN, M. W., 1981. A process approach to recognizing facies of reef complexes. Society of Economic
Paleontologists and Mineralogists, Special Publication, XI: 1-7.
MACINTYRE, 1. G., BURKE, R . B. & STUCKENRATH, R., 1977. Thickest recorded Holocene reef
section in the Western Atlantic: Isla Perez core hole, Alcran Reef, Mexico. Geology, 5: 749-754.
MACNEIL, F. S., 1954. The shape of atolls: an inheritance from subaerial erosion forms. American Journal oj’
Science, 252: 402-427.
MAXWELL, W. G. H., 1968. Atlas of the Great Barrier Reef, 258pp. Amsterdam: Elsevier.
MAXWELL, W. G. H., 1970. Deltaic patterns on reefs. Deep-sea Research, 17: 1005-1018.
MCNUTT, M. & MENARD, H. W., 1978. Lithospheric flexure and uplifted atolls. Journal of Geophysical
Research, 83: 1206- I2 12.
MENARD, H. W., 1964. Marine Geolou of the PaciJiC, 271pp. New York: McCraw-Hill.
MESOLELLA, K. J., MATTHEWS, R. K., BROECKER, W. S. & THURBER, D. L., 1969. The
astronomical theory of climatic change: Barbados data. Journal of Geology, 77: 250-274.
MESOLELLA, K. J., SEALY, H. A. & MATTHEWS, R. K., 1970. Facies geometries within Pleistocene reefs
of Barbados, West Indies. American Association of Petroleum Geologists, Bulletin, 54: 1899-191 7.
MUNK, W. H. & SARGENT, M. C., 1954. Adjustment of Bikini Atoll to ocean waves. United States Geological
Surucy, Professional Paper, 260-C: 275-280.
PURDY, E. G., 1974. Reef configurations: cause and effect. In L. F. Laporte (Ed.), Reefs in Space and Time.
Society of Economic Paleontologists and Mineralogists, Special Publication, 18: 9-76.
ROBERTS, H. H., 1974. Variability of reefs with regard to changes in wave power around an island.
Proceedings of the Second Inftmational Coral R e d Symposium, 2: 497-512. Brisbane: Great Barrier Reef
Committee.
ROSEN, B. R., 1982. Darwin, Coral Reefs and Global Geology. Bioscience, 32: 519-525.
SCHLANGER, S. O., 1963. Subsurface geology of Eniwetok Atoll. United States Geological Suruv, Professional
Paper, 260-BB: 991-1066,
SCOFFIN, T. P. & MACLEAN, R. F., 1978. Exposed limestones of the Northern Province of the Great
Barrier Reef. Philosophical Transactions of the Royal Society of London, A291: 119-138.
SCOFFIN, T . P. & STODDART, D. R., 1978. The nature and significance of microatolls. Philosophical
Transactionr of the Royal Sociey of London, 8 2 8 4 : 99-122.
SCOFFIN, T . P., STODDART, D. R., MACLEAN, R. F. & FLOOD, P. G., 1978. The Recent development
of the reefs in the Northern Province of the Great Barrier Reef. Philosophical Transactions of the Royal Sociep of
London, B284: 129-1 39.
SCOTT, G. A. J. & ROTONDO, G. M., 1983. A model to explain the differences between Pacific plate
island-atoll types. Coral Reefs, I : 139-150.
STANLEY, D. J. & SWIFT, D. J. P., 1967. Bermuda’s southern aeolianite reef tract. Science, 157: 677-681,
STEERS, J. A. & STODDART, D. R., 1977. The origin of fringing reefs, barrier reefs and atolls. I n 0. A.
Jones & R . Endean (Eds), Biology and Geology of Coral Reefs Vol. I V Geology 2: 21-57. New York: Academic
Press.
STODDART, D. R., 1969. Ecology and morphology of Recent coral reefs. Biological Review, 44: 433-498.
STODDART, D. R., 1973, Coral reefs: the last two millions years. Geograph, 58: 313-3123,
STODDART, D. R., 1976. Continuity and crisis in the reef community. Micronesica, 12(1): 1-9.
TRACEY, J. I. Jr, LADD, H. S. & HOFFMEISTER, J. E., 1948. Reefs of Bikini, Marshall Islands. Geological
Sociely of America, Bulletin, 59: 861-878.
TRUDGILL, S. T., 1976. The subaerial and subsoil erosion of limestones on Aldabra Atoll, Indian Ocean.
Zeitschrij fur Geomorphologie, Supplementband, 26: 20 1-2 10.
WELLS, J. W., 1957. Coral reefs. In J. W. Hedgpeth (Ed.), Treatise on Marine Ecology and Paleoecolopy Vol. I
Ecology. Geological Socieb of America, Memoirs, 67:609-63 I .
WILSON, J. T., 1963. Continental drift. Scient$c American, 208: 86-100.