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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 It may not be published in any form or used in a company prospectus or statementwithout the permission in writing of the Director,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
1
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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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-
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7.0.172-5 m5^19 cores;
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- 288•5•••• 300
- 318.5
_ 4001- 402.5
•- 430.5
(See Fig.' for location of sites)
482
- 500
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411.5
428.6
-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.