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054906

DEPARTMENT OFMINERALS AND ENERGY

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS

1976/38

A REVIEW OF THE GEOLOGY AND GEOPHYSICS

OF THE COCOS ISLANDS AND COCOS RISE

by

D. Jongsma

cs2fj .3

The information contained in this report has been obtained

by the Department of Minerals and Energy

as part of the policy of the Australian Government to assist in

the exploration and development ofmineral resources. It may not be published in any form or used in a company prospectus or statementwithout the permission in writing of the Director,

Bureau of Mineral Resources, Geology and Geophysics.

1976/38

A REVIEW OF THE GEOLOGY AND GEOPHYSICS

OF THE COCOS ISLANDS AND COCOS RISE

by

D. Jongsma

CONTENTS

FOREWORD

SUMMARY

INTRODUCTION^ 1

INVESTIGATIONS^ 1

GEOLOGY AND GEOPHYSICS^ 1

PROSPECTIVITY^

2

REFERENCES^ 3

Figures

1. Locality of Cocos Islands

2. Bathyme try

3. Seismic profile

4. DSDP sites 211, 212, 213 summary

5. Tectonic summary diagram

6. Evolution of the Indian Ocean

FOREWORD

Reviews have been made of the geology and geophysics of most of

Australia's island territories, and other records in this series by the

same author are:

Record No. 1976/12

Record No. 1976/36

Record No. 1976/37

Record No. 1976/39

Record No. 1976/40

Record No. 1976/41

Lord Howe Rise and Norfolk Ridge

Macquarie Island and Macquarie

Ridge Complex

Christmas Island and Christmas Rise

Queensland Plateau

Area of Mellish, Frederick, Kenn

and Wreck Reefs and Cato Island

Marion Plateau

SUMMARY

The Cocos Islands lie in the northeastern Indian Ocean on a

northeasterly—trending line of seamounts termed the Cocos Rise. The

islands consist of coral atoll roefs built on a basaltic volcano. Sediments

on the Cocos Rise are very thin, ranging from 100 to 200 m thick. Prospectivity

of the islands and the rise is very low. No mineralization occurs on the

island, and the thickness of the sediments on the rise is insufficient for

petroleum to have accumulated.

INTRODUCTION

Cocos Islands lie in the northeastern Indian Ocean at 12°S and 97°E(Fig. 1). They are a group of low-lying coral islands of typical atoll form,

and were in fact the islands which Charles Darwin studied to formulate his

theory of atoll formation. The group consists of West, Horsburg, Direction,

Home, and South Islands. West Island supports an airstrip.

INVESTIGATIONS

Very little geological or geophysical work has been done on the

Cocos Islands. A magnetic survey was undertaken in 1946 (Chamberlain, 1960).

Several survey lines along which the magnetic field was measured converge on

the islands (Fig. 2). An underway seismic reflection profile was recorded

across Cocos Ridge during the D.V. Clomar Challenger's leg 22 (Veevers, 1974).

The R.V. Vitian dredged material from the sea-floor near the western end of

the Cocos Ridge (Bezrukov, 1973).

MORPHOLOGY

Cocos Island lie in an easterly-trending line of seamounts of which

Christmas Island is the northernmost. To the southwest this complex structure

lies the Ninetyeast Ridge. The detailed bathymetry shown in Figure 2 shows

that the Cocos Islands are situated on a seamount which rises from a depth of

about 5000 m with a gradient of 0.2. To the south of the Cocos Islands lies

the Umitalea Mary seamount which rises to 16 m from the sea surface.

GEOLOGY AND GEOPHYSICS

The Cocos Islands consist entirely of coral sand and limestone.

Magnetic surveying over the islands have revealed a magnetic anomaly of 250 nT

in vertical intensity (Chamberlain, 1960). This, together with the presence

of basalt and tuff pebble dredged from the western end of the Cocos Ridge

(Bezrukov, 1973), is good evidence that the islands are built on top of a

volcanic seamount.

A line drawing of a seismic profile across Cocos Rise taken during

leg 22 of the DSDP is shown in Figure 3. It indicates that Cocos Rise is

almost devoid of sediments and has a very rugged topography. The crest of

the rise contains some ponded acoustically-transparent sediments up to 200 m

thick in narrow valleys (Veevers, 1974).

90°E 110°E80°E I00°E

• 211 DSDP drill site

Isobaths in metres

— Track of Glamor Challenger, 1972

Profile illustrated in Fig.3

Record No. 1976/38 W/88- 74A

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LOCALITY OF COCOS ISLANDS

(After Bathymetric map of Indian Ocean, Central Headquarters,Geodesy and Cartography, State Geological Commission of USSR, Moscow, 1963

FIG.120° E

20° N

10°N

0°

20°5

300 5

Marine magnetic and bathymetric surveys Contour interval 500 m

93° E 94° 960 se° I00°E13°S

10°

1 2 0

14°

16°

180 s

FIG.2

BATHYMETRY AROUND THE COCOS ISLANDS

(After Sclater a Fisher, 1974)

Record No.1976/38^

W/B8-75A

FIG. 3

(See Fig./ for loco/ion of profile )

50^

100kmI^t^i^i^t^I

GLOMAR CHALLENGER SEISMIC REFLECTION PROFILE

ACROSS THE COCOS RISE

(From Veevers, 1974)

Record No. 976/38^

W/B8 -76 A

SITE 211 SITE 212 SITE 213

DSDP SITES 211, 212 AND 213

et al., 1974)

MAJOR LITHOLOGIC RESULTS OF

(After Sclater

Record No. 1976/38 W/B8 - 77A

1

1

FIG. 4

T.D. 521 m; 39 cores;174 m rec.

T.D. 447m;67m rec.

15 cores;

6243 m19 ° 11.3S^99°17.8.E^ 10°12.7S^93°53.8 E

5611 m

100

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7.0.172-5 m5^19 cores;

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(See Fig.' for location of sites)

482

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-2-

Deep-sea drilling in the region (Von der Borch et al., 1974) shows

that Cocos Rise is surrounded by oceanic crust of Lower Tertiary age (60-70

my.). Figure 1 shows the location of DSDP sites in the eastern Indian Ocean

and a summary of the results is shown in Figure 4. Site 213 which lies tothe northwest of the Cocos Islands in a water depth of 5535 m, penetrated152 m of oozes and clays. Basement at this site consists of basaltic pillow

lava and is overlain by sediments of upper Palaeocene age (55-57 m.y.). Site212 to the south of the Cocos Islands is situated on a fracture zone; the age

of the oceanic crust was estimated to be mid-Cretaceous (about 91 m.y.) from

average accumulation rates of the overlying clay. About 500 m of clays and

oozes overlies the oceanic crust at this site. Site 211 to the east of the

Cocos Islands gave a basement age of Late Cretaceous (78 m.y.). At this site

435 m of oozes, clay, and ash overlies an altered amphibole -bearing basalt

basement.

The results of both magnetic surveys and the DSDP program were

studied by Sclater & Fisher (1974). From the identifiable linear magnetic

anomalies and the drilling results, they concluded that the sea-floor of the

Cocos Basin and the West Australian Basin becomes progressively younger north-

wards. This implies a spreading centre in the northeast Indian Ocean which

Sclater & Fisher (op. cit.) postulate to have been consumed at the Java Trench.

Figure 5 and 6 summarize the present tectonic pattern and the evolution of the

Indian Ocean.

The origin of the line of seamounts which include the Cocos Islands

and Christmas Island is still somewhat of an enigma. The fact that the sea-

mounts, like the Hawaiian-Emperor island chain, lie in a line suggests that

they may be the result of the lithospheric plate moving over a 'hot spot' or

a plume rising from deep in the mantle (Morgan, 1971).

TROSPECTIVITY

The mineral potential of the Cocos Islands and the surrounding area

appears to be virtually nil. No mineralization is associated with the volcanics

of the underlying seamount and no phosphate rock occurs on the islands.

Hydrocarbon accumulation in appreciable quantities is not likely as Cocos

Rise and the surrounding sea-floor have only a thin sedimentary cover.

Identified magnetic anomaly

Tentatively identified magneticanomaly

Known fracture zone

FIG. 580°E^

90°E^

10 0 °E

TECTONIC SUMMARY OF THE EASTERN INDIAN OCEAN

(After Scloter a Fisher, 1974)

Record No. 1976/38^

W/B 8-72-1 A

FIG. 6

53 m.y.B.P

ANOM-22

60^ 60

60

30°N

00

30°3

BO m.y.B.P.

tet*:.218 ..1 IP;

t•■•..../-.T34 17.

tit

f‘d o.217

100m.y.B.P.

Active spreading centre

Known fracture tone

Projected fracture zone

— Identified magnetic anomaly

0212 DSDP drill Site from Leg 22

TECTONIC EVOLUTION OF THE INDIAN OCEAN

(after Sclater a Fisher, 1974)Record No.1976/313^ W/B8-73—IA

-3-

REPMENCES

BEZRUKOV, P.L., 1973 - Principal scientific results of the 54th cruise of the

R.V. Vitian in the Indian and Pacific Oceans. (Feb-May 1973).

Oceanolog7 1 13(5), 761 -66.

CHAMBERLAIN, N.G., 1960 - Cocos Island magnetic Survey, 1946. Bur. Miner.

Resour. Aust. Rec. 1960/124 (unpubl.).

MORGAN, W.J., 1971 - Convection plumes in the lower mantle. Nature, 230, 42-43.

SCLATER, J.G., & FISHER, R.L., 1974 - Evolution of the east central Indian Ocean,

with emphasis on the tectonic setting of the Ninetyeast Ridge. Geol.

Soc. Am. Bull., 85, 683-702.

SCLATER, J.G., & VONDERBORCH, C.C., et al., 1974 - Regional synthesis of the

Deep Sea Drilling results from leg 22 in the eastern Indian Ocean.

In VON DER BORCH C.C. et al. - Initial Reports of the Deep Sea Drilling

Project, 22, 815-31. Washington, U.S. Govt Printing Office.

VEEVERS, J.J., 1974 - Seismic profiles made underway on Leg 22. In VON DER

BORCH, C.C. et al. - Initial Reports of the D.S.D.P., 22, 351-67.

Washington, U.S. Govt Printing Office.

YONDER BOHM, C.C., & SCLAITR, J.G., et al. 1974 Initial Reports of the Deep

Sea Drilling Project, 22. Washington, U.S. Govt Printing Office.