CORAL CAYS OF THE CAPRICORN AND BUNKER GROUPS, GREAT BARRIER REEF PROVINCE, AUSTRALIA

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NO. 195.

CORAL CAYS OF THE CAPRICORN AND BUNKER GROUPS, GREAT BARRIER REEF PROVINCE, AUSTRALIA

by P. G. Flood

Issued by

THE SMITHSONIAN INSTITUTION Washington, D.C., U.S.A.

February 1977

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Fig. 1. Location of the Capricorn and Bunker Groups.

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CORAL CAYS OF THE CAPRICORN AND BUNKER GROUPS, GREAT BARRIER KEEP PROVINCE, AUSTRALIA

by P.G. Flood

Introduction

The islands and reefs of the Capricorn and Bunker Groups.are situated astride the Tropic of Capricorn at the southern end of the Great Barrier Reef Province and approximately 80 kilometres east of Gladstone which is situated on the central coast of Queensland (Fig. 1).

The Capricorn Group of islands consists of nine coral cays: North Island, Tryon Island, North West Island, Wi.lson Island, Wreck Island, Masthead Island, Heron Island, and One Tree Island. A tourist Resort and Marine Scientific Research Station have been established on Heron Island. A manned lighthouse operates at North Island and the

Australian Museum conducts a field research station on One Tree Island.

The Bunker Group consists of five coral cays: Lady Musgrave Island, Fairfax Islands (West and East), and Hoskyn Islands (West and East).

Morphological changes occurring between 1936 and 1973 are evident when comparing previous plans of these coral cays (Steers, 1938) with recent vertical aerial photographs. Changes are catagorised into two groups; those related to natural phenomena and secondly, those caused by human interference.

Previous work

The earliest scientific description of the Capricorn and Bunker Groups is that of Jukes (1847) who visited the area in 1843 on the voyage of H.M.S.

Ply.

Saville-Kent (1893) and Agassiz (1898) make brief references to the Groups. Steers (1937, 1938) provided the

first detailed descriptions and sketches of most islands of the Groups.

Department of Geology and Mineralogy, University of Queensland, Brisbane, Australia.

(Manuscript received July 1974 -- Eds.)

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The r e g i o n a l l o c a t i o n , bathymetry, and physiography of t h i s a r e a h a s been d i s c u s s e d by v a r i o u s a u t h o r s (Maxwell, 1968; Maiklem, 1968,

1970; Maxwell and Maiklem, 1964) and t h e i s l a n d v e g e t a t i o n h a s been mentioned by s e v e r a l workers (MacGillivray and Rodway, 1931; Fosberg and Thorne, 1961; Gillham,1963; C r i b b , 1965, 1969; Domm, 1 9 7 1 ) .

S t e e r s (1938) and Domm (1971) p r o v i d e an i n t r o d u c t i o n t o t h e Groups and both papers should be r e a d i n conjunction with t h i s a r t i c l e .

Reefs with c o r a l cays

Four d i s t i n c t r e e f t y p e s (Maxwell, 1968) occur w i t h i n t h e Capricorn and Bunker Groups ( F i g s . 2 , 3, 4 and 5 ) :

1. Platform Reefs:- Tyron, Wreck, North, Wilson, and E r s k i n e Reefs.

2 . Lagoonal Platform Reefs:- Heron, and One Tree Reefs.

3. Elongate P l a t f o r m Reefs:- North West, and Masthead Reefs.

4 . Closed Ring Reefs:- Lady Musgrave, F a i r f a x , and Hoskyn Reefs.

The r e a d e r i s r e f e r r e d t o Maiklem (1968) and Maxwell (1968) f o r d e t a i l e d d i s c u s s i o n s concerning t h e z o n a t i o n of i n d i v i d u a l r e e f t y p e s .

The c o r a l cays belong t o two d i s t i n c t t y p e s ( F a i r b r i d g e , 1 9 5 0 ) : 1. Vegetated sand cays:- North West, Heron, Masthead, E r s k i n e ,

Wilson, Wreck, Tryon, North, F a i r f a x (West), and Hoskyn (West).

2 . S h i n g l e cays:- One T r e e , Lady Musgrave, F a i r f a x ( E a s t ) , and Hoskyn ( E a s t ) .

The sand c a y s a r e a l l l o c a t e d t o t h e l e e of t h e i r r e e f f l a t , whereas t h e s h i n g l e c a y s , w i t h t h e e x c e p t i o n of Lady Musgrave, a r e l o c a t e d on t h e windward s i d e .

The following b r i e f d e s c r i p t i o n s a r e provided t o a s s i s t

i n t e r p r e t a t i o n of t h e v e r t i c a l a e r i a l photographs ( P l a t e s 1-9) and t o a l l o w comparisons t o be drawn betv:sen p r e s e n t cay morphology and t h a t i l l u s t r a t e d i n s k e t c h e s made by t h e Geographical Expedition t o t h e Great B a r r i e r Reef i n 1936 (Figs. 6 and 7 ) .

Capricorn Group Heron I s l a n d ( P l a t e 1)

A sandy beach, 15-30 m e t r e s wide a t low t i d e , surrounds t h e cay.

-

.

Exposed beachrock o c c u r s on t h e s o u t h e r n , n o r t h e r n , and e a s t e r n beaches. Vegetation c o n s i s t s of a c e n t r a l zone of P i s o n i a g r a n d i s and an o u t e r zone of Casuarina e q u i s e t i f o l i a , Scaevola t a c c a d a , and T o u r n e f o r t i a a r q e n t e a . Pandanus t e c t o r i u s o c c u r s over t h e c e n t r e of t h e cay. A T o u r i s t R e s o r t and Research S t a t i o n have been developed on t h e western end of t h e cay.

Atoll Research Bulletin No. 195. Flood, P.G.Feb. 1977

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Changes o c c u r r i n g s i n c e 1936 a r e r e l a t e d t o t h e n a t u r a l elements and t o huinan i n t e r f e r e n c e . Sand e r o s i o n on t h e n o r t h e r n and e a s t e r n beaches h a s exposed new occurrences c f beachrock. The c o n s t r u c t i o n of a r e t a i n i n g w a l l and a b o a t harbour have d r a s t i c a l l y a l t e r e d t h e e e s t e r n end of t h e cay.

North West Isl-and ( P l a t e 2 )

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Th4.s i s t h e 1.arges.L sand cay of t h e Group. A sandy beach surrounds t h e cay and beachrock i s on1.y exposed a t t h e e a s t e r n end.

Vegetation i s s i m i l a r t o t h a t o c c u r r i n g on Heron I s l a n d .

The e a s t e r n a r e a of exposed beachrock i s s m a l l e r now t h a n i n 1.936 and t h e southwestern o u t c r o p i s no l o n g e r v i s i b l e .

Masthead IsLand ( P l a t e

--

3 )

Beachrock i s w e l l developed alonq t h e s o u t h e r n beach and an _.

occurrence of o l d e r beachrock i s s i t u a t e d some d i s t a n c e from t h e beach on t h e northwestern c o r n e r of t h e cay. V e g e t a t i o n i s s i m i l a r t o t h a t o c c u r r i n g on IIeron i s l a n d . Pric1cl.y P e a r , .introduced from t h e

MainLarid, i s now w e l l e s t a b l i s h e d over t h e e n t i r e cay.

Considerably more beachrock i.s now exposed on t h e s o u t h e r n beach and a spj.t h a s formed on t h e northwestern c o r n e r . The s h i n g l e and sand s p i t which was prominent i n 1936 h a s d i s a p p e a r e d .

One Tree I s l a n d ( P l a t e 4 )

- -

^.--

T h i s i s a s h i n q l e cay.

-

Veqetation c o n s i s t s of s c a t t e r e d

Tourne.fortia a r g e n t e a and Scaevola taccada with several. small groves of P i s o n i a g r a n d i s . A small. pond o:E bracki.sh water i s s i t u a t e d n e a r t h e c e n t r e of t i l e cay. A ternporary F i e l d Research S t a t i o n conducted by t h e A u s t r a l i a n Museum i s l o c a t e d towards t h e n o r t h e a s t e r n c o r n e r o f t h e cay.

The s t r a i - g h t a1i.gnmen.t of t h e western beach i n 1936 h a s been a l t e r e d by th.e a d d i t i o n of a sma1.l s p i t n e a r t h e northwestern end of t h e beach. The northwestern c o r n e r of t h e cay h a s extended towards t h e lagoon.

T'ryon 1sl.and ( P l a t e 5 )

Deachrock o c c u r s along t h e n o r t h w e s t e r n and s o u t h e a s t e r n beaches The cay is covered w i t h dense v e g e t a t i o n i n t h e same z o n a t i o n s a s on t h e l a r g e r sand c a y s .

%'he a r e a s of outcropping beachrock have a l t e r e d c o n s i d e r a b l y s i n c e 1936. No exposures can be seen on t h e n o r t h e a s t e r n beach, whereas t h e exposures on t h e northwestern and southwestern beaches have i n c r e a s e d i.n a r e a .

Wreck 1sl.iind ( P l a t e 5 )

--

Aeachrock i s well develoued alonq t h e s o u t h e r n beach. V e q e t a t i o n i s simil.ar t o t h a t on Tryon i s l a n d e x c e p t t h a t t h e P i s o n i a g r a n d i s t o r e s t i s l e s s w e l l developed.

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No a p p a r e n t changes have o c c u r r e d s i n c e 1936.

North I s l a n d ( P l a t e 6 )

T h i s i s t h e s m a l l e s t c a y o f t h e Group. A manned l i g h t h o u s e o p e r a t e s on t h i s i s l a n d .

S t e e r s (1938) d o e s n o t d e s c r i b e t h i s i s l a n d . Wilson I s l a n d ( P l a t e 6 )

T h i s i s l a n d i s n o t a t r u e sand c a y because o f t h e q u a n t i t y of c o a r s e coral. s h i n g l e p r e s e n t i n t h e sediments. Beachrock i s well-- developed a l o n g t h e s o u t h e a s t e r n beach. Pandanus t e c t o r i u s i s t h e dominant vege-tation w i t h Casuarina e q u i s e t i f o l i a r e s t r i c t e d t o t h e western s i d e o f t h e cay.

S i g n i f i c a n t changes have o c c u r r e d s i n c e 1936. Considerable quant'ties of sand have moved, exposing beachrock on t h e n o r t h e r n beach and c o v e r i n g formerly exposed beachrock on t h e s o u t h e r n beach.

The sandy beach on t h e w e s t e r n s i d e h a s i n c r e a s e d i n a r e a . E r s k i n e I s l a n d

Beachrock i s w e l l developed on t h e w e s t e r n and northwestern beaches. The v e g e t a t i o n d i f f e r s from t h a t p r e s e n t on t h e o t h e r cays i n t h a t Pandanus t e c t o r i u s and Casuarina e q u i s e t i f o l i a a r e a b s e n t , and t h e majh v e g e t a t i o n c o n s i s t s of T o u r n e f o r t i a a r g e n t e a , Scaevola

t a c c a d a , and s t u n t e d P i s o n i a g r a n d i s .

The cay h a s i n c r e a s e d i n s i z e s i n c e 1936 and sand now c o v e r s most of t h e f o r m e r l y exposed beachrock.

Bunker Group Lady

- Musgrave I s l a n d ( P l a t e 7 )

Thi.s isl.and i s t h e o n l y s h i n g l e c a y s i t u a t e d on t h e leeward r e e f f l a t . Beachrock i s exposed a l o n g t h e n o r t h e a s t e r n and e a s t e r n

beaches and an o u t c r o p o f l i t h i f i e d c o r a l conglomerate, s i m i l a r t o t h a t forming t h e c o r e o f t h e c a y , o c c u r s n e a r t h e s o u t h e a s t e r n c o r n e r . Vegetation c o n s i s t s of P i s o n i a g r a n d i s , T o u r n e f o r t i a a r g e n t e a ,

Casuarina e q u i s e t i f o l i a , and Pandanus t e c t o r i u s . The vegeta-tion i s l e s s dense t h a n t h a t of t h e l a r g e r sand c a y s o f t h e Capricorn Group.

A small pond of b r a c k i s h w a t e r i s l o c a t e d towards t h e s o u t h e r n end of t h e cay.

The shapes o f t h e s o u t h e a s t e r n and e a s t e r n beaches have a l t e r e d s i n c e 1936. E r o s i o n h a s exposed a l a r g e r a r e a of beachrock on t h e s o u t h e r n beach and t h e eroded sand h a s been d e p o s i t e d on t h e e a s t e r n beach p a r t l y o b s c u r i n g t h e beachrock exposed i n 1936 and forming a w e l l - d e f i n e d beach on t h e n o r t h e a s t e r n c o r n e r of t h e cay.

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Fairfax Islands (Plate 8)

The eastern cay is composed of shingle and the western of sand and shingle. Interference with the vegetation of the eastern cay occurred as a result of phosphate mining and secondly during the period when the Australian Military Forces used the area as a bombing target. The cay is no longer used for either purposes. Vegetation is dominated by Pisonia yrandis which is restricted to the centre of the island. Two brackish pools are located towards the eastern end of the island.

The western cay features an elongated sand spit that supports vegetation on its western extremity. Vegetation is similar to that occurring on the larger sand cays of the Capricorn Group. A tin shed, erected by the Australian Navy when the eastern cay was used for

bombing practice, is located in the centre of the island. This shed has been occupied for the past three years by Julie Booth who is studying the behaviour of marine turtles.

Steers (1937) mentions the islands. However, he did not provide any sketch of the cays.

Hoskyn Islands (Plate 9)

The eastern cay is composed of shingle and supports vegetation similar to, although less well-developed than, that of Lady Musgrave Island. The western cay is composed of sand and its vegetation is similar to that of the larger sand cays Capricorn Group.

Both cays have increased in size since 1936.

Influences causing morphological changes, 1936-1973

Most cays have changed morphologically as a result of continued erosion of sand on the eastern, southern, and southeastern beaches and subsequent deposition of sand on the western and northwestern beaches.

The Southeast Trade Wind which blows for approximately nine months of the year causes this natural pattern to be common within this region

(Flood, in press) and accounts for the nett westerly migration of sand on each cay.

Human interference is responsible for significant changes at Heron Island (Pla-te 10). A concrete retaining wall constructed in the early 1960's on the northwestern corner of the island is responsible for erosion of the western beach (Fig. 8). The wall alignment

reflects and refracts onto the western beach those waves approaching the island from a northwesterly and northeasterly direction and considerably increases the erosive energy of waves in that area.

The erosion problem was increased in 1967 by the dredging of a boat harbour into the reef flat to provide access to the tourist

resort (Plate 10, fig. 2). Even before cyclone "Emily" (April, 1972)

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the retaining walls of the harbour were breached in several places, allowing sand from the western end of the island

to

move into the harbour and to the deeper water beyond (Plate 10, fig. 3). The boat harbour was redredged in late 1972. Approximately 20,&0 cubic metres of sand were placed adjacent to the concrete retaining wall on the northwestern corner of the island in an attempt to lessen its erosive influence (Plate 10, fig.

4 ) .

The sand is migrating

westward by longshore drift under the influence of the Southeast Trade.

Acknowledgements

The author gratefully acknowledges the use of research facilities at the Department of Geology and Mineralogy, University of Queensland, and at the Heron Island Research Station. I am particularly grateful to Dr. G.R. Orme who organised the fieldtrips and aerial reconnaissance of most cays of the Capricorn and Bunker Groups in 1972. The

Co-ordinator-General's Department, Queensland is thanked for reproductions used in Plates 2 and 3. The officers of the No. 2 Sqdn. Royal Australian Air Force, Amberley generously supplied the aerial photographs of the coral cays illustrated in Plates 1 and 4 to 9 inclusive.

References

Agassiz, A. 1899. A visit to the Great Barrier Reef of Australia in the steamer "Croyden" during April and May, 1896. Bull. Harvard Mus. Comp. Zoology 26: 1-203.

Cribb, A.B. 1965. The marine and terrestrial vegetation of Wilson Island, Great Barrier Reef. Proc. R. Soc. Qd. 77: 53-65.

Cribb, A.B. 1969. The vegetation of North West Island. Qd

Nat.

19:

85-93.

Domm, S.D. 1971. The Uninhabited Cays of the Capricorn Group, Great Barrier Reef, Australia. Atoll Res. Bull. 142: 1-27

Fairbridge, R.W. 1950. Recent and Pleistocene coral reefs of Australia.

J.

Geol. 58: 330-401.

Flood, P.G. in.press. Sand movement on Heron Island - a vegetated sand cay, Great Barrier Reef Province, Australia. Proc. 2nd Intern. Symp. Corals and Coral Reefs, 1973.

Fosberg, F.R. and Thorne, R.F. 1961. Vascular plants of Heron Island, pp. 5-13 in Fosberg et dl., Heron Island, Capricorn Group,

Australia. Atoll Res. Bull. 82: 1-16.

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Gillham, M.E. 1963. Coral cay v e g e t a t i o n , Heron I s l a n d , G r e a t B a r r i e r Reef. Proc. R . Soc. Qd 73: 79-92.

Jukes, J.B. 1847. N a r r a t i v e of t h e . S u r v e y i n g Voyage o f H.M.s. ~ l y . Boone, London. 1: 424 pp.

MacGillivray, W.D.K. and Rodway, F.A. 1931. P l a n t s on i s l a n d s of t h e Bunker and Capricorn Groups. R e p . G t B a r r i e r R e e f Comm.

3 ( 7 ) : 58-63.

Maiklem, W.R. 1968. TI-@ C a p r i c o r n Reef Complex, G r e a t B a r r i e r R e e f , A u s t r a l i a . J. sedim. P e t r o l . 3 8 ( 3 ) : 785-798.

Maiklem, W.R. 1970. Carbonate Sediments i n t h e Capricorn Reef

Complex, G r e a t B a r r i e r Reef, A u s t r a l i a . J. sedim. P e t r o l . 4 0 ( 1 ) : 55-80.

Maxwell, W.G.H. 1968. A t l a s of t h e G r e a t B a r r i e r Reef. E l s e v i e r , Amsterdam. 258 pp.

Maxwell, W.G.H. and Maiklem, ^W.R. 1964. L i t h o f a c i e s a n a l y s i s , s o u t h e r n p a r t o f t h e Great B a r r i e r Reef. Pap. Dep. Geol. Univ.

Qd 5 ( 1 1 ) : 1-21.

S a v i l l e - K e n t , W. 1893. The Great B a r r i e r R e e f o f A u s t r a l i a . A l l e n , London. 387 pp.

S t e e r s , J . A . 1937. The c o r a l i s l a n d s and a s s o c i a t e d f e a t u r e s of t h e G r e a t B a r r i e r Reefs. Geogrl J. 89: 1-28, 119-146.

1938. D e t a i l e d n o t e s on t h e i s l a n d s surveyed and examined by t h e Geographical E x p e d i t i o n t o t h e Great B a r r i e r Reef i n 1936. Rep. G t B a r r i e r Reef Comm. 4 ( 3 ) : 51-96.

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.

ONE T R E E REEF

M A S T H E A D RE

P i g . Jlccf zol-;;ii:.ioil: ~ T I E 'i'rc'e, lili~si:I?i::i?tl, lic'rc~>n, and Nort:ll West i i c c f s .

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LAGOONAI- P L A T F O R M R E E F

CLOSED R I N G R E E F

F i g . 4 . P l , y : ; : i . ~ ~ g r i ~ ~ ~ ~ ~ . ~ . c z o n a t i o n of t h e r e e f types of tile C a ~ i r i c o r r i anri iiliriker Groups ( n ~ o c l j f i r d fron Max~\rc:l.i, 1968, p. 7 0 6 ) .

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BUNKER GROUP

/

H O S K Y N R E E F

23O48'30" 0 501

152"18'00" ^LMETRES ^.^-²

--

F A I R F A X R E E F

Fig. 5. Reef z o n a t i o n ; Hoskyn, Fairfax, and Lady Musgrave ~ e e f s

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MEWON I S L A N D I ^{ONE TREE} !Sl._ANi:)

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m e t r e s

0

-

5 0 August 196C

Wall extended 1964-6

I

October 196E

I

^eroding^bank

M a y 1970

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;oncrele retaining wail constructed o Augusl 1960

n,

r

August 1 9 6 d

~c Artificial spit form6 from dredging spoil

4 6 lo 12

October 1 9 6 7

loot h o r b o u r

A p r i l 1 9 7 2

F i g . 8 . S e q u e n c e o f e v e n t s c o n n e c t e d w i t h e r o s i o n o f t h e w e s t e r n e n d o f Heron I s l a n d ( c o n t o u r s a r e i n f e e t , l f t . = 3 0 . 5 c m ) . Atoll Research Bulletin No. 195. Flood, P.G.Feb. 1977

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WRECK ISLP.ND

PLATE 5

Plate 5. Tryon Island and Wreck iisland. Vertj.ca:!. aerial p!iol,oqr2;~ii taken

25.9.73.

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NORTH ISL-AND Q 3 0 0 M E T R E S

i..*-L- A-&--,.,,.

W I L S O N I S L A N D 2 5 . 9 . 7 3

PLATE 6

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FAIRFAX I S L A N D S 0 300 METRES 25.9.73 PLATE 8

P l a t e 8. F a i r f a x I s l a n d s . V e r t i c a l a e r i a l photographs t a k e n 25.9.73.

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PLATE 9

Pl~ate 9. Hoskyn I s l a n d s . V e r t i c a l a e r i a l p h o t o g r a p h s taken 25.9.73

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CORAL CAYS OF THE CAPRICORN AND BUNKER GROUPS, GREAT BARRIER REEF PROVINCE, AUSTRALIA

NO. 195.