The nature of the pre-Holocene surface, John Brewer Reef, with implications for the interpretation of Holocene reef development

MARINE QEOLOGY INTERNATIONAL JOURNAl. OF M A M N E GEOLOG~ GEOCHEMISTRY AND GEOPHYSICS

ELSEVIER

Marine Geology 122 (1994) 63-79

The nature of the pre-Holocene surface, John Brewer Reef, with implications for the interpretation of Holocene reef development P.D. Walbran 1

Geology Department, James Cook University of North Queensland, Townsville, Qld. 4811, Australia Received 6 October 1992; revision accepted 22 September 1994

Abstract

Seismic reflection profiles trace out a steep-sided antecedent platform, assumed to be the Holocene/Pleistocene interface, beneath the post-glacial (Holocene) coralline and sediment veneer of John Brewer Reef, a small mid-shelf reef within the central Great Barrier Reef Province. This platform, defined by a semi-continuous seismic reflector, is characterised by three essentially planar surfaces, at - 2 0 m, - 3 2 m and - 4 0 m depth with respect to present low water datum. The - 2 0 m surface, which exhibits a maximum relief of _+2 m, is extensive and is postulated to represent either a senile reef which grew to maturity most probably during the 50 kyr interstadial, or the erosion and concurrent infilling, during this phase, of a n e a r - flat reefal structure which dates from an earlier - 20 m interstadial. The deeper surfaces are less extensive. That at - 3 2 m is considered to represent a fringing reef which became established on the flanks of the antecedent platform during a brief late-Pleistocene sea-level stillstand. The - 4 0 m surface appears to be predominantly erosional in origin and also of late-Pleistocene age. Whilst these surfaces are sites of major coral growth, Holocene reef development has occurred largely independent of antecedent platform morphology. Present reef morphology reflects the interaction between the rate of post-glacial sea-level rise, prevailing hydrodynamic conditions and biological activity, and bears no resemblance to that of the underlying antecedent surface. The present morphology of John Brewer Reef is consistent with a continuous post-glacial sea-level rise.

1. Introduction

Discussion regarding the nature of the substrate upon which Holocene coral reef growth has taken place dates back more than 150 years (see Hopley, 1982; Harvey, 1986, for reviews). The existence of an antecedent platform had been speculated for several decades (Hoffmeister and Ladd, 1944). However, it was the proposition by MacNeil (1954), later popularised by Purdy (1974a,b), that the surface m o r p h o l o g y observed in modern reefs is inherited from an underlying antecedent karst

tPresent address: 22 Kalang Road, Elanora Heights, NSW 2101, Australia. 0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00104-9

terrain, which provided the impetus for a revision in thinking on Holocene coral reef development. Purdy's study was restricted to reefs in British Honduras (Belize) but he considered it to be equally applicable to other parts of the world where modern reef growth occurs, including Australia's Great Barrier Reef (GBR). While an antecedent karst foundation for the G B R Province has been rejected (Hopley, 1982), most authors consider that the morphology of the underlying pre-Holocene surface has nonetheless influenced the surface morphology of individual Holocene reefs, especially in the early stages of reef development (e.g. Davies et al., 1977; Davies and Marshall, 1980; Harvey, 1980, 1986; Hopley, 1982; Searle, 1983; Isdale, 1991).

64

P.D. Walbran/Marine Geology 122 (1994) 63-79

This widely held view has relied heavily upon seismic reflection and drill-core data which suggest significant variability in the depth to the preHolocene (Pleistocene) surface across individual reef tops and from one reef to another throughout the GBR Province. Reef-top seismic reflection work has not previously been undertaken in the GBR Province. The use of high frequency seismic equipment in conjunction with a shallow-draft vessel allowed extensive profiling to be carried out across John Brewer Reef in the central GBR Province (Fig. 1). The profiles obtained provide the first detailed mapping of the Holocene/Pleistocene unconformity beneath a coral reef in the GBR Province. This paper describes the surface morphology of the antecedent foundation of John Brewer Reef and considers possible mechanisms for the formation of the surfaces. Implications arising from these findings on the interpretation of Holocene sea-level stillstands and controls on Holocene reef growth are also discussed.

J

~

2. Setting of John Brewer Reef John Brewer Reef is a small (6 km x 3 km), mature, lagoonal platform reef (sensu Hopley, 1982) situated at the inner margin of the outer shelf of the GBR approximately 70 km northeast of Townsville in central Queensland (Fig. 1). In this section of the GBR, the inter-reef tract is at 45-50 m water depth and reefs are well spaced: Lodestone Reef, 7 km to the south, the Slashers Reefs Complex, 7 km to the north and Hopkinsons Reef, 13 km to the northeast are those in closest proximity to John Brewer Reef (Fig. 1). Wind data collected on John Brewer Reef in the period June 1985-July 1986 suggest that winds with an easterly to southerly component occur for approximately 60% of the year and that winds from these directions typically average between 6.7 and 7.7 m/s (Kelvin Michael, pers. commun.). The tidal range on John Brewer Reef is 88% that of Townsville harbour (mean spring range 2.4 m, maximum spring range 4.0 m) with tidal heights

47"

148"

•

°. •, ~

~ '

i/

t ~ '

. ~j £'/' : ]

=::1 ,,

lg"

•

Ae".

"IY

•

-

,,oo.,.,..,s

k ',~

~

...,

]

~ John Brewer-o "P'~ Reef . ~ Lodestone

'" '

"

~'Hopkin,~on 14 Reef ,~ ~ Ir

R, e f /

whee,,r.

/

Reef

KeepefReef "

Davies Reef

~ ~

* . - "

.,41

" ~

/,w ~~

~.

ii~

)- - '

'

. ,

Fig. I. Locationof John BrewerReefwithin the central GBR.

~g"

P.D. Walbran/Marine Geology 122 (1994) 63 79

occurring approximately 15 minutes earlier than at Townsville (Keith Brazier, pers. commun.; Queensland Department of Transport, 1993). Modelling of current patterns on and around John Brewer Reef by Black and Gay (1987) indicated strongest currents are generated by water leaving the lagoon through the leeward and western openings and across the northwestern reef flat. The prograding sand sheets on the windward side of the lagoon correspond with the lowest reef-top current velocities. The western or northwestern slopes are convergence zones for northerly surface currents generated by the predominating weather patterns and the southerly longshelf current (Wolanski and Pickard, 1983; Williams et al., 1984; Black and Gay, 1987). Lagoonal flushing is likely to be the major means of water exchange between the John Brewer Reef lagoon and the open sea.

3. Methods

Extensive continuous seismic profiling (CSP) of John Brewer Reef was carried out using high frequency (3.5 kHz) seismic reflection equipment comprising a 360 J monopause acoustic transducer, a 10 element streamer and an EPC recorder. The use of a shallow-draft vessel allowed profiling across the reef top on high tides. A total distance of 25.90 nm (47.97 km) was profiled (Fig. 2a).

4. Interpretation of the seismic data

Despite problems arising from interference in the form of transmission pulses, the superimposition of multiples over the reef top and a partial lack of resolution over hard ground (coralline substrate), the seismic record collected in this study is sufficiently complete to allow accurate mapping of a strong, semi-continuous subsurface reflector across John Brewer Reef (Figs. 3-6). This reflector traces out a steep-sided structure with three planar surfaces, at water depths of approximately 20 m, 32 m and 40 m below present low water datum (LWD), which bear no physical resemblance to present reef morphology. Three important characteristics suggest that this reflector represents the subaerially eroded

65

Holocene/Pleistocene interface (Reflector A of Johnson et al., 1982): (1) It can be traced semi-continuously across John Brewer Reef and into the off-reef floor where its depth corresponds to the first major subsurface reflector, identified in CSP profiles, of the interreef tract (Walbran, 1991). (2) It is the only major reflector discernible below the reef top. (3) It occurs at the same depth as a unit of extreme penetrative resistance identified by Bock and Foruria (1985) as the Pleistocene surface in their dynamic probing investigation of the site proposed for a floating hotel resort ("Floatel") in the lagoon of John Brewer Reef (Fig. 2a). Bock and Foruria (p. 6) noted that "when compared with the Holocene layers, the Pleistocene sediments exhibit a substantially higher degree of cementation and strength". The upper surface of the antecedent platform, situated at a mean water depth of 20 m (Fig. 2a), is extensive, comprising the entire area underlying the John Brewer Reef lagoon and reef flat (Fig. 2b). Maximum relief within this surface is +_2 m. Seismic reflectors delineating the antecedent surface can be traced beneath elements of positive relief in present reef morphology, such a patch reefs, clearly demonstrating that initiation and subsequent development of such features has occurred independent of underlying substrate morphology (Figs. 3 and 4). A major terrace encircles the antecedent platform at water depths of between - 3 0 m to - 3 4 m with a mean depth of about - 3 2 m (Figs. 4 6). Invariably sloping towards its outer margin, this terrace is best developed in the western reef where it is several hundred metres wide (Fig. 4), and on the leeward side of the reef, particularly in the northeastern sector (Figs. 5 and 6). Seismic resolution is often poor beneath the thick post-glacial carbonate sequence that has been built up on this terrace and Reflector A is discontinuous. A smaller surface, at - 4 0 m, is most extensive on the southern and southeastern sides of the antecedent windward slope where it is up to about 30 m in width (Fig. 6). Further along the windward slope, in the vicinity of profile lines I and J (Fig. 2a), the - 4 0 m surface is reduced to no

P.D. Walbran/Marine Geology 122 (1994) 63-79

66

N

T

\.50

40x,,O0 20

~

,5!:o 0

1 km

\\

1 km

B Fig. 2. (a) CSP transects, John Brewer Reef, with depths (in metres) of the pre-Holocene surface relative to present low water datum. The windward moat vibracore site is also shown. (b) Surface contour diagram of the antecedent (Pleistocene) topography (Reflector A) underlying the post-glacial coralline substrate and sediment bodies of John Brewer Reef. Transect line A-B relates to cross sections in Fig. 8.

more than a notch whereas on the leeward slope, if present, it is masked by the fairly gentle gradient of Reflector A at this depth (Fig. 6).

Off the reef top, Reflector A drops rapidly from m to a depth of about - 5 0 m in an erosional scour (moat) at the base of the windward slope

-20

P.D. Walbran/Marine Geology 122 (1994) 63- 79

67

LWD~

Line A

.-

W

70

Ill

--

--

X

~

o I

",.

.

E

.....

z ......

,

-.-

[LWD

1 km I

]

Fig. 3. CSP line A, western lagoon to eastern inter-reef, John Brewer Reef. Due to the very shallow operating conditions across the reef top (1), the top 1 2 m of the patch reefs and the reef flat have been lost in the transmission pulse. Reflector A (A) can be traced semi-continuously across the reef top, becoming fainter or disappearing beneath patch reefs and the reef flat and reappearing off the reef as the - 4 0 m surface (2). The - 3 2 m terrace is not seen in this profile but is evinced by the windward marginal reef (3) which has grown from that terrace. Seaward of the - 4 0 m surface, Reflector A is situated at or very close to the present sea floor and only clearly re-emerges on the distal side of an erosional scour (moat) at the base of the windward slope (4). The first subsurface multiple (M) is shallower and generally easily distinguished from Reflector A.

(Figs. 2a and 6), an average gradient of 1 in 18, and to a depth of about - 4 3 m in the lee of the reef on a more gentle gradient of 1 in 29. Seaward of John Brewer Reef, Reflector A occurs very close to or may even coincide with the seafloor at about - 4 0 m (Walbran, 1991). Vibracores from the inter-reef tract in the vicinity of John Brewer Reef reveal Reflector A to be a stodgy, highly oxidised Pleistocene mud (Walbran, 1991).

5. Nature of the John Brewer Reef antecedent platform

The foundation of John Brewer Reef is a steepsided antecedent platform rising abruptly from a surrounding plain at about 40-45 m water depth.

Such a platform has been identified underlying many other coral reefs within the GBR Province (Orme and Flood, 1977; Harvey, 1980, 1986; Searle et al., 1981; Searle, 1983; Johnson and Searle, 1984) and the Holocene/Pleistocene unconformity reported at depths ranging from - 4 m to - 3 0 m below present sea level, with a clustering around - 1 7 m to - 2 2 m (Table 1). Reef-top cores from numerous GBR reefs have revealed that these antecedent platforms are older limestone reefs (Davies, 1974; Thom et al., 1978; Hopley et al., 1978, 1984; Davies and Marshall, 1979; Hopley, 1983a; Marshall, 1983a,b; Barnett, 1984; Johnson et al., 1984; Marshall and Davies, 1984; Davies et al., 1985). 14C and 2 3 ° y h / 2 3 4 U dating of central and southern GBR platforms revealed them to be Pleistocene in age (172-28

P.D. Walbran/Marine Geology 122 (1994) 63-79

68

LWD t

X

Line C

SW

LWD 1

SW

NE

..~

~

~

,

-

,~.

'~M ]

'-

0 I

="~",,~ X

rLWD

" :"

L30m

"P

1 km I

I

Fig. 4. CSP line C, central lagoon to western inter-reef, John Brewer Reef. Across the reef top (1) the first subsurface multiple (M) cuts across Reflector A (A) making the two readily distinguishable. The - 3 2 m terrace (2) is extensive in the western section of the reef and the seaward gradient can be clearly seen in this figure. A small mid-slope (proximal) sediment wedge (3) is banked up behind the marginal reef (4) which, on the western slope, is represented by fairly extensive, but low relief coral growth. The proximal sediment wedge is not contiguous with, but is situated at a similar depth to, a wedge on the leeward slope. Several metres of sediment have accumulated at the base of the marginal reef to produce an off-reef (distal) sediment body (5). Featureless Pleistocene(?) reflectors (P) can be seen below Reflector A in the inter-reef.

kyr; Them et al., 1978; Marshall, 1983b; Marshall and Davies, 1984; Hopley et al., 1984; Johnson et al., 1984; Table 1), consistent with similar features overseas (Thurber et al., 1965; Lalou et al., 1966; Veeh, 1966; Bloom et al., 1974; Chappell and Veeh, 1978; Marshall and Jacobsen, 1985). An indication of the nature and age of the John Brewer Reef Pleistocene surface is provided by carbonate rubble recovered from a vibracore which penetrated sediment at the base of the windward moat in the vicinity of line G (Fig. 2a). This rubble largely comprised coralline reef detritus which dated at 16.8-16.1 kyr B.P. (Walbran, 1991). These ages, which clearly do not relate to the Holocene transgression as sea level at that time was at least 120 m below that of present (Fig. 7), probably reflect contamination by younger

(Holocene) carbonates (Hopley, 1982). They do, however, point to a late Pleistocene age for the John Brewer Reef antecedent surface.

6. Origin of the - 2 0 m surface, John Brewer Reef

6.1. Aggradation versus degradation of a Pleistocene coral reef? The intersection of carbonate reef lithologies of Pleistocene age in drill cores from numerous reefs throughout the GBR Province and the presence of pre-Holocene coralline rubble in the John Brewer Reef windward moat provide strong circumstantial evidence that the platform underlying John Brewer Reef is a Pleistocene coral reef. If this assumption

P.D. Walbran/MarineGeology 122 (1994) 63-79 SE

Line

69

F

NW

-LWD

50m

SE

NW 1

'

: •

-,

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1~.

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.

L

--

~ . . . . . . .

•

,

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~

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. . . . . . .

' ::'. ~-~'-',-,':"~- ~ 50rn

0

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500

I

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I

Fig, 5. CSP line F, northeast sector 'notch', John Brewer Reef. The 'notch' has a sandy floor punctuated by large patch reefs and is bounded by a steep drop-off from the leeward lagoonal reef flat (1) and a patch reef representing the leeward slope marginal reef (2). The 'notch' is situated above a very broad section of the - 3 2 m terrace (A) which grades gently seaward.

is correct, the planarity of the antecedent surface may be explained in terms of a senile precursor which grew to sea level at about - 2 0 m with respect to present LWD during a Pleistocene marine transgression. A senile reef (i.e. a sedimentologically mature reef with infilled lagoon) may develop from a shallow platform (Hopley, 1982) thereby requiring only a relatively short period of time to reach sea level and achieve senility. Several examples of senile reefs are to be found in the GBR today (Hopley, 1982; Davies, 1983) and include reefs in Torres Strait and the Princess Charlotte Bay area (northern GBR), Wheeler Reef (central GBR) and Wreck Reef (Capricorn-Bunker Group, southern GBR). Alternatively, an erosional origin may be invoked as an explanation for the - 2 0 m surface. Only marine erosional agencies have the potential to plane a topographic high in what must have been a single cycle of landscape evolution. Established rates of lateral planation within the

intertidal zone vary between 0.05 mm yr -1 and 4.0 mm yr 1 for the GBR (Davies, 1983, p. 72). Submarine erosion of carbonate substrate proceeds at a rate 30 times slower than erosion within the intertidal zone (Davies, 1983). Based upon the most generous estimates of intertidal erosion rates, several hundred thousand years of stable sea level would be required to fully plane a pre-existing surface equivalent to that of the John Brewer Reef substrate. Full planation would not be necessary, however, had the initial surface been relatively flat, but somewhat irregular in nature. Such a situation might, for example, be represented by a mature lagoonal reef or reefal shoal dating from an earlier - 2 0 m interstadial (Fig. 7). Erosion and concurrent infilling across the top of the structure during a subsequent - 2 0 m stillstand would, given sufficient time, produce a level surface. The - 2 0 m antecedent surface is likely to have been produced on the occasion of the last - 2 0 m stillstand. According to current models for the

P.D. Walbran/Marine Geology 122 (1994) 63--79

70

G

Line

NW

,..=~~

~- ~--4.~-~

~

~.~,

~.,

SE

tLWD

80m

NW

~~_.=_

7

~

~

.....

8E

•

0 L

L

[LWD

80m

1 km ___1

Fig. 6. CSP line G, transect across John Brewer Reef from leeward slope to windward inter-reef. The relationship between the antecedent Pleistocene surface and the overlying post-glacial veneer, as discussed in the text, is highlighted in this profile. The offreef (distal) sediment wedge (1) has almost buried the small knoll of coral (2) that developed on the -40 m surface (3). Coral growth on the --32 m terrace (4) resulted in a marginal reef (5) of relatively low relief behind which sediment deposition has produced a mid-slope (proximal) sediment body (6). Beneath the reef top (7) Reflector A (A) is essentially flat with a maximum vertical relief of _+2 m. On the windward slope, extensive marginal reef development (8) on the -32 m terrace (9) has coalesced with coral growth (10) on the -40 m surface (11). The marginal reef may have attained sea level in the southwestern sector of the windward slope. Seaward of the -40 m surface, Reflector A is located at or close to the surface and only clearly re-emerges, beyond the erosional scour at the base of the windward slope (12), in the inter-reef (13). The top 1-2 m of the profile have been lost in the transmission pulse. region, sea level has stood at about - 2 0 m on four separate occasions during the previous 120 kyr, most recently during the 50 kyr interstadial ( B l o o m et al., 1974; Chappell, 1982; Hopley, 1982; Fig. 7). Support for a higher sea level at 30 kyr B.P. than has previously been suggested, however, comes f r o m G u l f St. Vincent, South Australia, where C a n n et al. (1988) identified a m a x i m u m transgression o f - 2 2 m at 31 kyr. The precise elevation o f the antecedent surface at the time o f its formation will be masked by any hydro-isostatic readjustment (loading and r e b o u n d ) o f the continental shelf or tectonic movement that might have occurred over the intervening period. H o p l e y (1983b), for example, estimated that d o w n w a r p i n g has resulted in a lowering o f the continental shelf off Townsville by at least 1-3 m since the onset o f the last marine transgression, whilst N a k a d a and L a m b e c k (1989) have postulated hydro-isostatic dislocation o f the north Queensland continental shelf by several metres

over the previous 20 kyr. The degree o f dislocation increases away from the coastline. Tectonism is not considered to have been a factor in determining topographic elevations along the north Queensland coast during the Holocene at least (Hopley, 1978). Acceptance o f the - 2 0 m platform as a planar or near-to-planar Pleistocene reef requires that the upper Pleistocene surface remained virtually unaffected by subaerial erosional processes for between 20 kyr and 40 kyr. The most quantitative estimates o f vertical subaerial erosion o f carbonate substrates have been carried out by Trudgill (1976, 1979) on A l d a b r a Atoll in the Indian Ocean where mean annual rainfall is 670 m m yr -1. Trudgill recorded an average vertical erosion rate o f 0 . 2 6 m m yr 1 over a two year period. Present rainfall in the central section o f the G B R is substantially greater than that recorded on Aldabra. However, pollen analyses from lake deposits in the A t h e r t o n Tableland 2 5 0 k m northwest o f Townsville (Kershaw, 1980) and interpretation o f

P.D. Walbran/Marine Geology 122 (1994) 63 79

7[

Table 1 Depths to Pleistocene surfaces and radiometric (14C) age ranges of upper Pleistocene foundations for reefs in the central GBR Province Reef

Depth to Pleistocene surface (m)

Age range of Pleistocene foundation (kyr)

Reference

Britomart Bowl Cockatoo Davies Darley Gable Grub Hedley Keeper Molar Myrmidon Redbill

25 26 30 7-18 25-26 10 17 18 30 24 10- 12 ca. 13-18 19 24 4 18

32.2 27.5

Stanley

15 22

Viper Wheeler

I 1-22 16 24

Davies and Hopley (1983); Johnson et al. (1984) Davies et al. (1985) Harvey (1980, 1986); Davies et al. (1985) Davies and Hopley (1983); Barnett (1984) Harvey ( 1980, 1986); Hopley (1982) Hopley (1982) Davies et al. ( 1985) Harvey ( 1980, 1986) Harvey ( 1980, 1986) Harvey ( 1980, 1986) Davies et al. (1985) Harvey (1980, 1986); Hopley (1982, 1983a); Hopley et al. (1984); Davies and Hopley (1983); Davies et al. (1985) Hopley (1982); Davies and Hopley (1983); Davies et al. (1985) Harvey ( 1980, 1986); Davies et al. (1985) Harvey (1977, 1980, 1986); Hopley (1982); Davies et al. (1985)

o~_PresentSeaL

;"°t 1

a

46.2 35.7

v

e

-40

i -6o ~. I o.. >= ~ -100 ~-120 ~ . -140 -160

I

? /

I, I It / / /

/

?

7'

Depth of antecedent platform surfaces, John Brewer Reef

V ? 10 20 30 40 50 60 70 80 90 100 110 120 kyr BP

Fig. 7. Sea-level variation in northeast Queensland over the previous 120 kyr modified from Hopley (1982).

coastal plain soil and g e o m o r p h o l o g y in the Townsville region (Hopley, 1973; Hopley and Murtha, 1975) suggest a general reduction in temperatures and a dramatic reduction in precipitation in northeastern Queensland between approximately 60 and 8 kyr B.P. Subaerial erosion rates determined for A l d a b r a may, therefore, bear some

relationship to the situation in the central G B R following the last interstadial. Trudgill's figure o f 0.26 m m yr 1 represents an overall degradation o f a pre-existing planar surface by approximately 10.5 m in 40 kyr or 5 m in 20 kyr, significantly less than the m a x i m u m o f 19 m eroded during the last glacial to produce the present antecedent platforms in the southern G B R (Marshall, 1983b; Marshall and Davies, 1984). A n erosion rate o f the magnitude d o c u m e n t e d by Trudgill is able to account for the 4 m o f vertical relief evident in the John Brewer Reef antecedent surface if it was a p r o d u c t o f the 30 kyr sea-level high, but published sea-level curves for the G B R region indicate that sea level rose to only - 4 0 m on that occasion (Fig. 7). Clearly, if the - 2 0 m surface dates from the 50 kyr interstadial, as seems most likely, circumstances leading to antecedent surface morphologies in the southern G B R differed substantially from those on J o h n Brewer Reef. In addition to precipitation, the a m o u n t and rate o f subaerial erosion imposed on a carbonate platform is determined by the nature o f the substrate. The preservation o f the planar antecedent

72

P.D. Walbran/Marine Geology 122 (1994) 63 79

platform beneath John Brewer Reef may have been assisted by the formation of a protective calcrete (caliche) capping over the platform under conditions of reduced temperatures and precipitation in the region during the last glacial period. In vadose environments where evaporation exceeds precipitation on exposed limestone surfaces, unstable aragonite and high Mg-calcite phases are dissolved and reprecipitated in situ or near by to produce dense erosion-resistant calcrete (Monroe~ 1966: Read, 1976). Calcrete profiles up to 3 m thick have been reported from Barbados by James (1972). Although calcrete has been recorded at lhe top of Pleistocene lithologies in cores recovered from reefs within the GBR (Marshall, 1983a; Barnett, 1984; David Hopley, pers. commun.; David Johnson, pers. commun.), they appear to be generally thin. Bock (1984) and Bock and Foruria (1985) encountered a highly resistant layer, which they believed to represent the Holocene/Pleistocene disconformity, during their dynamic probing investigations on John Brewer Reef, Keeper Reef and others. The formation of an extensive calcrete deposit during the relatively arid climatic conditions of the late Pleistocene might have protected the surface from subaerial erosion to such a degree that its original morphology remained largely unaltered. 6.2. A Halimeda contribution to the - 2 0 m surUglc~'?

Actively accreting Halimeda bioherms have been documented in the lee of ribbon reefs in the northern GBR and on reefal shoals in the Swain Reefs Complex, southern GBR (Davies and Marshall, 1985; Marshall and Davies, 1988; Searle and Flood, 1988). These structures rise steeply from the sea floor to a maximum elevation of 20 m and are frequently flat-topped, thereby bearing a superficial resemblance to the John Brewer Reef antecedent platform. Consideration of the role of Halimeda in determining the morphology of the surface upon which Holocene coral growth has taken place is therefore warranted. An indication of the importance of Halimeda in determining the fabric of late Pleistocene reefs comes from Britomart and Davies Reefs in the

central

GBR

where

drill

cores

intersected

Halimeda-dominated lithofacies at the top of

the Pleistocene stratigraphy (Johnson et al., 1984; Barnett, 1984). Marshall (1983a) described Halimeda-rich limestones, up to 1.5 m thick and frequently comprising 30% Halimeda segments, from the top of the Pleistocene stratigraphy on reefs in the Capricorn-Bunker Group, southern GBR, whilst approximately 5 m of Halimeda detritus was recovered by David Hopley (pers. commun.) from beneath the coralline lithologies in a drill core from Raine Island in the far northern GBR. The age and lateral extent of this deposit has not as yet been determined but it sits on top of a coralline unit of probable Pleistocene age. Whilst acknowledging the importance of Halimeda in determining reef-top sediment budgets (Drew, 1983; Drew and Abel, 1983, 1985; Walbran, 1991), evidence from several sources suggests an actively accreting bioherm to be an unlikely origin for the John Brewer Reef antecedent platform: ( 1) Seismic profiles invariably show mound complexes containing numerous bioherms of varying sizes distributed over several square kilometres (Davies and Marshall, 1985; Marshall and Davies, 1988). Whilst biohermal complexes may cover areas greater than individual reefs, individual bioherms are significantly less extensive (up to 100 m × 150 m) than the John Brewer Reef antecedent platform. Further, the largest bioherms (in the northern GBR) are those located in the sheltered lee of shelf-edge ribbon reefs and they decrease in size westward (Davies and Marshall, 1985). (2) Drill core retrieved from a number of reef tops within the GBR have shown the antecedent platform underlying the post-glacial reef veneer to be coralline in nature (e.g. Davies, 1974; Marshall, 1983a,b; Johnson et al., 1984) while coralline rubble of pre-Holocene age was recovered from the base of John Brewer Reef windward slope (Walbran, 1991). This evidence is reinforced by seismic facies analyses which suggest that the preHolocene foundations are reefal limestones (Searle et al., 1981; Searle, 1983). (3) The internal structure of Halimeda bioherms is clearly visible in seismic reflection surveys (Searle and Flood, 1988). No such detail is apparent in the John Brewer Reef profiles, although this may

P.D. Walbran/Marine Geology 122 (1994) 63-79

be attributable to the formation of a highly resistant calcrete capping during subaerial exposure, as previously postulated, or to masking by multiples in the seismic record. (4) A maximum vertical accretion rate of 3.5 m kyr-1 has been estimated for the Halimeda bioherms in the Swain Reefs Complex (Searle and Flood, 1988). Assuming a comparable rate of aggradation for late Pleistocene analogues, interglacial oscillations during this period were of insufficient duration (< 10 kyr) to allow the bioherm to attain the same vertical and lateral dimensions as that of the John Brewer Reef antecedent platform. (5) Seismic data suggest a strong correlation between the siting of modern Halimeda bioherms and coral reefs and the morphology of the substrate upon which each has developed. On adjacent shoals in the Swain Reefs Complex, m o d e m Halimeda bioherms occur on flat-lying surfaces of probable Pleistocene age, whereas coral reef development has taken place on surfaces exhibiting marked topographic relief and assumed to be Pleistocene reefs (Searle and Flood, 1988; D. Searle, pers. commun.). A similar relationship has been described by Orme et al. (1978) in the northern GBR and by Phipps and Roberts (1988) on Kalukalukuang Bank in the eastern Java Sea, Indonesia. The alternative to an aggradational origin, i.e. that the John Brewer Reef antecedent platform represents a Halimeda biohermal complex in which the areas between individual bioherms have been infilled with material derived from the top of the bioherms during a sea-level stillstand, also seems untenable, essentially for those reasons outlined above. In particular, the size and distribution of bioherms in the GBR Province today (Davies and Marshall, 1985; Marshall and Davies, 1988) indicate the very large volume of sediment that would be required to fill the intervening areas. Halimeda growth-rates (Drew, 1983; Drew and Abel, 1983) appear not to allow for the production of sufficient sediment volumes by late Pleistocene bioherms. Levelling and infilling, during a - 2 0 m stillstand, of a biohermal complex dating from an earlier, higher Pleistocene sea level (Fig. 7) may also

73

account for the - 2 0 m surface, although such an origin is, once again, considered improbable.

7. Origin of the - 3 2 m and - 4 0 m surfaces, John Brewer Reef

The - 3 2 m and - 4 0 m surfaces have a number of features (an absence of vertical relief, a seaward gradient and an occurrence around the perimeter of the reef) compatible with them representing fringing reefs which grew at or close to sea level on the exposed flanks of the antecedent platform during stillstands in sea-level fluctuation. A similar feature, identified at - 4 5 m in the lee of Wardle Reef 200 km north of Townsville, was interpreted by Searle et al. (1981) to be an inter-tidal reef flat of possible late Pleistocene age. Today, many of the siliciclastic continental islands within the GBR Province have associated fringing reefs (Hopley, 1982) which are, in some cases, at least as extensive as the Pleistocene surfaces on John Brewer Reef. Current sea-level models for the GBR Province recognise two interglacial peaks at about - 4 0 m, but none at - 3 0 m, within the late Pleistocene (Bloom et al., 1974; Chappell, 1982; Hopley, 1982; Fig. 7). Formation of a fringing reef at - 4 0 m during one of these peaks, 40 kyr B.P. or 30 kyr B.P., is a possibility. As noted earlier, evidence from South Australia points to a higher sea level at 30 kyr B.P. (Cann et al., 1988) and this may be account for the terrace at - 3 2 m. Alternately, both fringing reefs may have formed during brief stillstands within phases of late Pleistocene sealevel oscillation. Conceivably, the - 3 2 m and - 4 0 m surfaces may be of erosional origin and formed during stillstands; on one of the last occasions sea level fell from - 2 0 m (50 kyr or 30 kyr B.P.), during a - 3 0 m or - 4 0 m sea-level high (40 kyr or 30 kyr B.P.), or during the most recent marine transgression. Time constraints, based upon published erosional rates for carbonates in intertidal settings (Davies, 1983), do not allow for the erosion of such extensive platform as that present at - 3 2 m during late Pleistocene fluctuations or the recent transgression. Moreover, the extensive development of this terrace on the relatively protected

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P.D. Walbran/Marine Geology 122 (1994) 63 79

northern and western sides of the reef would seem to negate an erosional origin, although an erosional component cannot be completely discounted, particularly in the exposed windward sector of the antecedent platform where the terrace is more planar and exhibits less relief than on the sheltered leeward side. In contrast to the - 3 2 m surface, that at - 4 0 m is best developed on the southeastern side of the antecedent platform and greatly reduced on the leeward side. Erosional coral reef terraces, with widths comparable to that of the John Brewer Reef - 4 0 m surface, have been described from the Huon Peninsular in Papua New Guinea (Ota et al., 1993). These terraces formed within the surf zone and relate to uplift of the peninsular during the Holocene. The - 4 0 m surface is likewise considered to be primarily erosional in origin, but of late Pleistocene age. Palynological data revealed carbonaceous siliciclastic mud units at the top of inter-reef vibracores to represent paludal depositional environments adjacent to or within tracts of mangrove forest which were established along the windward margin of John Brewer Reef approximately 10 kyr B.P. (Walbran, 1991; J. Grindrod, pets. commun.). The presence of an early Holocene mangrove facies at 42 43 m depth with respect to present LWD demonstrates low-energy conditions predominated across this section of inter-reef tract when sea level was at - 4 0 m during the most recent marine transgression and negates a postglacial erosional origin for the - 4 0 m surface.

8. Evolutionary model Surface morphology is a useful parameter in establishing relative ages of the three John Brewer Reef antecedent platform surfaces. The lack of positive relief on the - 3 2 m and - 4 0 m surfaces suggests that at - 2 0 m to be the oldest. Had the deeper surfaces existed at the time the - 2 0 m surface developed some relief, in the form of patch reefs, would most likely be evident in these surfaces. This is particularly so on the windward side of the reef where marginal reef development, analogous to that now found on the windward slope,

would have certainly been initiated. Similar rationale can be applied in assigning an older age to the - 3 2 m terrace over the - 4 0 m surface. This being the case, the most plausible scenario for the late-Pleistocene/Holocene evolution of John Brewer Reef, set against the established sea-level history for the region (Fig. 7), is as depicted in Fig. 8.

9. Recognition of post-glacial stillstands Published sea-level curves for the east Australia region typically show a continuous, rapid rise for the most recent marine transgression (Bloom et al., 1974; Thorn and Chappell, 1975; Chappell, 1982; Hopley, 1982; Thorn and Roy, 1983; Grindrod and Rhodes, 1984). By contrast, Carter and Johnson (1986) and Carter et al. (1986) described a series of "reef and shoreline features" on the GBR shelf which they attributed to an episodic sea-level rise during the last post-glacial transgression. Among those recognised by these authors were breaks at - 3 9 m, - 2 8 m and - 2 3 m which correspond closely to the depth of the antecedent surfaces on John Brewer Reef. Previously, Maxwell (1968, 1973) and Maxwell and Swinchatt (1970) described "shoreline features" at - 3 7 m, - 2 9 m and - 18 m. Whilst the data presented in this paper do not exclude the possibility of post-glacial transgressive sea-level stillstands at the depths suggested by Carter and his co-authors, the morphological features attributed by them to post-glacial stillstands may merely be Holocene expressions of the underlying Pleistocene foundation. The interpretation of surficial features is difficult without an appreciation of the subsurface morphology. No individual, persistent feature on John Brewer Reef unequivocally relates to a postglacial transgressive sea-level stillstand, with the exception of limits imposed by present sea level on vertical reef growth. The Holocene morphology of John Brewer Reef is consistent with a continuous rise in sea level, although possibly subject to fluctuations in the rate of rise, in the period between 10 and 8 kyr B.P., when reef

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P.D. Walbran/Marine Geology 122 (1994) 63 79

a Windward Side

Leeward Side A

-32 m surface

0 O~

-~E m y t--a

B

20

4o 60

b A 0 O~

E

~ v r-E3

20

4o

6O 0 i

Post-glacial reef framework i i i Post-glacial carbonate sediment

.... --

1 km i

i

J

i

i

Late-Pleistocene sea levels - --

Present sea level

Pleistocene foundation

Fig. 8. Schematic representation of the late-Pleistocene/Holocene history of John Brewer Reef. Location of transect line A--B is shown in Fig. 2b. (a) Phase 1: Formation of the - 2 0 m surface, as a senile reef or via the concurrent erosion and infilling of a preexisting near-flat reefal structure, during the 50 kyr interstadial; Phase 2: Growth of a fringing reef at - 32 m during a stiUstand as sea level fell from - 2 0 m 50 kyr B.P.; Phase 3: Production of the - 4 0 m surface via erosion and possible fringing reef development during the 40 kyr or 30 kyr interstadial. (b) Phase 4: Establishment of post-glacial coralline framework and carbonate sediment bodies and establishment of present morphology subsequent to the onset of the most recent marine transgression. Post-glacial (< 18 kyr B.P.) structures: 1 = Inter-reef, western and leeward sides; 2 = off-reef (distal) sediment wedge; 3 = western and leeward marginal reefs; 4=mid-slope (proximal) sediment wedge; 5=lagoon; 6=windward reef flat; 7=windward marginal reef; 8 = windward moat; 9 = inter-reef, windward side. Late-Pleistocene structures: 1 0 = - 4 0 m surface, western and leeward slopes; 1 1 = - 3 2 m terrace, western and leeward slopes; 12 = - 2 0 m surface; 13 = - 3 2 m terrace, windward slope; 14= --40 m surface, windward slope. Subsurface morphology of post-glacial reef-top structures is speculative.

growth was initiated in the central GBR and 6 . 5 - 6 k y r B.P., w h e n it s t a b i l i s e d a t its p r e s e n t level ( T h o r n a n d C h a p p e l l , 1975; C h a p p e l l et al., 1983; D a v i e s et al., 1 9 8 5 ) .

10. Controls on Holocene reef growth E s t a b l i s h m e n t o f c o r a l o n t h e t h r e e p l a n a r surf a c e s (sites o f m a j o r c o r a l g r o w t h ) o n t h e a n t e c e d -

76

P.D. Walbran/Marine Geology 122 (1994) 63 79

ent platform, and the post-glacial sea-level history of the GBR are the key elements in the Holocene development of John Brewer Reef. The limited development of coral at - 4 0 m, in the form of patch reefs ("bommies"), may be attributable to a combination of a rapid rise in sea level commencing about 10 kyr B.P., which outpaced coral growth, and burial beneath sediment derived from the reef up-slope, particularly that on the - 3 2 m platform. Today, Holocene coral growth at - 4 0 m on the leeward slope is represented by small knolls that are virtually buried by Holocene sediment wedges. The greatest development of coral on the - 3 2 m platform has occurred along the southern side of the reef, particularly in the southeast, where the marginal reef has grown to within 2 m of present sea level, and the northwest on the leeward slope. Local hydrodynamic conditions would appear to offer the most plausible explanation for the enhanced development of coral in these areas: Coral productivity is favoured by fairly turbulent, clear water conditions with most prolific coral growth occurring in those areas most influenced by wave action and surface currents (Kinsey and Davies, 1979; Done, 1982). On John Brewer Reef, these areas are the windward margin and, to a lesser extent, the northwest leeward slope where converging currents winnow the mud fraction from the surface sediment (Walbran, 1991). If it is assumed that reef growth in the central GBR lagged behind the Holocene sea-level rise by up to 2 kyr (Davies et al., 1985), substantial coral growth would not have commenced on the - 2 0 m surface until 8 7 kyr B.P. and probably reached present sea level between about 6 2 kyr B.P. (Davies et al., 1983, 1985). Drill cores taken through reef tops frequently show that, rather than being a continuous coralgal section, the Holocene sequence consists, in part, of carbonate detritus (Davies and Hopley, 1983; Johnson et al., 1984; Davies et al., 1985). Without a series of radiocarbon dates from drill core, it is not possible to estimate Holocene coral growth rates for John Brewer Reef. The figures quoted point to a reeftop accretion rate, comprising both framework aggradation and sediment accumulation, of 3.3-10.0 m kyr 1. This compares to accretion

rates of 3-5 m kyr -1 obtained by Smith and Kinsey (1976) based upon calcium carbonate production rates, an average growth rate of 3 9 m kyr-1 determined by Davies et al. (1983) from 14C dating of drill cores from four reefs within the central GBR and an overall Holocene accretionary rate of 5 m kyr-1 for the GBR, as estimated by Hopley (1984). An indication of Holocene coral growth rates on John Brewer Reef can be gauged from the windward marginal reef situated in the vicinity of profile line H (Fig. 2a). This tract has grown vertically from a pre-Holocene terrace at - 3 2 m to approach the present upper limit of coral growth on John Brewer Reef, a total elevation of approximately 30 m. Vertical accretion from the - 3 2 m terrace has occurred over a minimum period of 2 kyr (i.e. kept pace with sea-level rise) to a maximum period of 8 kyr (i.e. has only recently caught up to sea level) representing an average growth rate of between 15.0 m kyr ~ and 3.8 m kyr -~. As it is unlikely vertical growth on any reef in the GBR managed to keep pace with sea-level rise (Hopley, 1984; Carter et al., 1986) a rate of 15 m kyr -~ is almost certainly too high. Davies and Hopley (1983) report an average vertical accretion rate for windward margins of 4 6 m kyr--1 (coral head facies may grow at up to 7 m kyr -1 and framework facies at up to 16 m kyr i) whereas vertical accretion rates of between 2.6 m kyr and 10.2 m kyr-1 have been documented by Davies et al. (1985) on mid-shelf reefs in the central GBR.

11. Conclusions

(1) Post-glacial coral reef development on John Brewer Reef has taken place on a antecedent platform in which an extensive and essentially planar surface can be identified in seismic reflection profiles at 20 m depth with respect to present LWD. Two smaller surfaces are present on the flanks of the platform at 32 m and 40 m water depth. (2) The planarity of the - 2 0 m surface may be explained in terms of a senile reef which grew to sea level from an shallow underlying platform. Alternatively, the - 2 0 m surface may represent a

P.D. Walbran/Marine Geology 122 (1994) 63-79

situation in which irregularities in an older, previously exposed, near-flat reefal structure have been infilled with material eroded from more windward sections of the structure. The - 2 0 m surface is likely to have been a p r o d u c t of either the 50 kyr or, less likely, 30 kyr interstadial. The - 3 2 m terrace appears to represent a relict fringing reef. The - 4 0 m surface m a y similarly represent a relict fringing reef, a l t h o u g h a p r e d o m i n a n t l y erosional origin seems more reasonable. Both the - 3 2 m and -40 m surfaces are of p r o b a b l e late Pleistocene age. (3) B e y o n d p r o v i d i n g the substrate for H o l o c e n e coral growth, the a n t e c e d e n t surface has h a d little influence in d e t e r m i n i n g present reef m o r p h o l o g y ; post-glacial reef growth a n d m o r p h o l o g y m a y occur i n d e p e n d e n t l y of the c o l o n i s a t i o n substrate surface m o r p h o l o g y . (4) The present surface m o r p h o l o g y of J o h n Brewer R e e f appears consistent with the c o n t e n t i o n that sea-level rise d u r i n g the H o l o c e n e transgression was a c o n t i n u o u s event a n d n o t p u n c t u a t e d by a series o f short stillstands.

Acknowledgements I a m i n d e b t e d to Bob H e n d e r s o n for his guidance a n d e n c o u r a g e m e n t d u r i n g the course of this work. I gratefully acknowledge the assistance in the field provided by K e v i n H o o p e r a n d G r a n t Kelly. I t h a n k Bob H e n d e r s o n , D a v i d Hopley, Bob Carter a n d two a n o n y m o u s reviewers for constructive c o m m e n t s o n drafts of the m a n u script. Chris W i l s o n generously f o u n d the time to c o m p u t e r draft Figs. 2 a n d 8.

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