Morphology and basement of the Sahul Shelf

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MORPHOLOGY AND BASEMENT OF THE SAHUL SHELF j. j. VEEVERSAND TJ. H. VAN ANDEL Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T. (Australia) ; Scripps Institution of Oceanography, La Jolla, Calif. (U.S.A.)

(ReceivedNovember 1, 1966) (Resubmitted February 3, 1967)

SUMMARY By tracing lineaments from the land into the adjacent Sahul Shelf, previous authors speculated that the bathymetric rises on the shelf correspond to areas of relatively shallow Precambrian rocks, and a bathymetric depression to deeper Precambrian rocks. A recent aeromagnetic survey confirms these speculations: the bathymetric depression corresponds in detail with the axis of a non-magnetic section 6 km thick, and the bathymetric rises with crests of a thinner non-magnetic section. The shelf morphology is thus a subtle reflection of regional structure, and originated by the rejuvenation of very ancient structural patterns. In the adjacent Timor Trough, the submarine morphology does not correspond with the depth of the magnetic basement, indicating a different kind of structural history.

INTRODUCTION In the first geological review of the Sahul Shelf, FAIRBRIDGE(1953) divided the shelf into rises and depressions, which were found to lie opposite swells and basins on the adjacent land. From this arrangement, Fairbridge speculated that the structural histories of the shelf and the adjacent land were similar. Further weight was given to this view by our analysis (VANANDEL and VEEVERS,1965, 1967) of lineaments on the land (northwestern Australia) and on the Sahul Shelf, made possible by a new compilation of existing bathymetrical data (Fig.l). Besides the regional morphological elements recognized by Fairbridge (the Londonderry Rise, Bonaparte Depression, and Van Diemen Rise), we showed the existence of two other first-order elements: the Malita Shelf Valley and the Sahul Rise, and re-defined the Van Diemen Rise. In the absence of regional geophysical information, we speculated that the extension from the land onto the shelf of primary lineaments initiated in the Precambrian and later re-activated indicated that the basement of parts of the shelf is Precambrian, and that, in particular, the rises correspond to areas of relatively shallow Precambrian rocks, and the depression to deeper Precambrian rocks. Marine GeoL, 5 (1967) 293-298

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MORPHOLOGY AND MAGNETIC BASEMENT

Striking confirmation of these speculations is now available in the report of an aeromagnetic survey of the Sahul Shelf (JACQUEMIN, 1967). The chief discoveries of this survey 1 are as follows (Fig.2): as indicated by the isobaths of the magnetic basement, the onshore Bonaparte Gulf Basin extends seaward beneath the Sahul Shelf to cover an area of some 200,000 kmL The southwestern edge of the Bonaparte Gulf Basin lies a short distance north of the Kimberley coast, and the eastern edge passes west of Bathurst Island. The chief structure is a trough, 6 km deep, whose axis trends from north-northwest to north-northeast. The eastern part of the trough is cut by several faults, most of which parallel the isobaths, and crossed by three crests. The Marine Geol., 5 (1967) 293-298

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z The aeromagnetic survey was carried out from June to November 1965 by Compagnie G6n6rale de G6ophysique for Areo Limited and Australian Aquitaine Petroleum Pty Ltd. and was subsidized by the Commonwealth of Australia under the Petroleum Search Subsidy Acts. Flight-lines were flown at 600 m above sea level, and were spaced 3 km apart in bands of four, with 40 km between bands. A total of 35,550 km of acceptable profiles were flown in a rectangular grid. Radio navigation was by the Toran system. The instrument used was a cesium vapor magnetometer with digital recording; the accuracy was 0.1 ~. The depth estimates were given by the quantitative study of two types of anomaly: (a) wide anomalies of high intensity, which are assumed to be related to compartments of the basement extending to infinite depth; and (b)wide anomalies of low intensity, assumed to be related to a "thin sheet" of the basement. The determinations of the depth of the magnetic basement are estimated to have an accuracy of about 10%. Where checked against drill holes in the landward part of the Bonaparte Gulf Basin, the aeromagnetic determinations of depth of the magnetic basement were found to be shallower than the actual depth.

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western part is more complex, consisting of a broad area of deep magnetic basement bounded southward by a slope cut by faults and crossed by two crests and northwestward by a long crest. The magnetic basement slopes southeastward from Timor to a depth exceeding 6 kin, and then rises to the crest which marks the northern part of the Bonaparte Gulf Basin. A comparison of Fig.1 and 2 shows that the first-order morphological elements of the sheld correspond to elements of the magnetic basement. The following correspondences are noted: (1) The axis of the main morphological depression (Bonaparte Depression and Malita Shelf Valley) corresponds with the axis of the offshore Bonaparte Gulf Basin. The correspondence of the Malita Shelf Valley with the northern part of the structural axis is particularly close. (2) The axis of part of the Londonderry Rise corresponds with a swell in the magnetic basement. (3) The western part of the Bonaparte Depression corresponds with an area of deep magnetic basement. (4) Parts of the poorly-known Sahul Rise correspond with swells in the magnetic basement. The only notable discrepancy between morphology and basement depth exists between the floor of the Timor Trough and the deepest magnetic basement in this area. The deepest magnetic basement lies beneath the continental slope halfway between the edge of the Sahul Shelf and the axis of the Timor Trough. This discrepancy seems to indicate that the present Timor Trough is a young geological feature-perhaps as young as Pleistocene-- which has not existed long enough to accumulate a great thickness of sediment along its present axis. Parts of the southeastern flank of the Timor Trough have a gentle slope and are smooth, suggesting a thick sequence of probably Cenozoic sediment (VAN ANDEL and VEEVERS, 1967). This discrepancy is possibly yet another point of contrast between the short and eventful tectonic history of Timor and the Timor Trough and the long, monotonous tectonic history of northwestern Australia and presumably also the Sahul Shelf. The identity of the magnetic basement and its age or ages are not certainly known, but checks against the onshore part of the Bonaparte Gulf Basin indicate that the magnetic basement coincides with either a layer or layers high in the Precambrian, or the surface of the Precambrian, or the Lower Cambrian Antrim Plateau Volcanics. Together with other recent geophysical surveys with electrosonic profilers (SMtxn, 1966) the aeromagnetic survey confirms the speculation of ~NOAKES et al. (1952) that the Paleozoic and Mesozoic Bonaparte Gulf Basin extends across the Sahul Shelf to cover an area of some 200,000 kmz. The correspondence of submarine morphology with the surface of the magnetic basement shows that the present shelf morphology is a subtle reflection of regional structure and that the similarity in size and shape of the present Bonaparte Depression "to pre-Cambrian and Paleozoic basins on the adjacent continent suggests a rejuvenation of very ancient structural patterns" (VAN ANDEL et al., 1962). Earlier VAN ANDEL and VEEVERS(1965) developed FAIRBRIDGE'S (1953) view that the stepped Marine Geol., 5 I1967~ 2 9 3 - 2 9 8

MORPHOLOGY AND BASEMENTOF THE SAHULSHELF

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surfaces of the Sahul Shelf are continuous with those of the adjacent region of northwestern Australia. The new structural information provided by the aeromagnetic survey shows furthermore that the similarity between the shelf and the adjacent continent extends in depth. The correspondence or parallelism between regional morphology and structure cannot be explained, as FAIRBRIDGE(1953) believed, by regarding the rises as areas of Precambrian rock perhaps with thin caps of younger sediment and the depressions as areas of thick Phanerozoic sediment. According to the aeromagnetic survey, most of the rises contain non-magnetic sections only a little thinner than those in adjacent depressions. The conclusion seems inescapable that the morphology parallels structures formed during the latest of a sequence of long-continued movements. Blockfaulting which is the dominant expression of structure in the landward part of the Bonaparte Gulf Basin (TRAVES, 1955; GU1LLAUME, 1966; VEEVERSand ROBERTS, in preparation)--rather than folding is the more likely means whereby morphology will correspond with structure. That this may be so in the Sahul Shelf area is indicated by the fact that many faults (Fig.2) parallel the strike of the magnetic basement. This does not necessarily imply that folds are lacking in the Sahul Shelf area; in this area folds, if any, are probably secondary structures stemming from primary fault structures.

ACKNOWLEDGEMENTS

Publication by the senior author was authorized by the Director, Bureau of Mineral Resources, Geology and Geophysics. Contribution from Scripps Institution of Oceanography, University of California. We thank A. A. Opik, J. E. Thompson, and K. A. Townley for criticism of the manuscript. Participation of the junior author was made possible by Contract Nonr-2216(23) of the Office of Naval Research. The research has been supported in part by National Science Foundation grants G 14103 and GP-350.

REFERENCES FAIRBRIDGE,R. W., 1953. The Sahul Shelf, northern Australia; its structure and geological relationships. J. Roy. Soc. W. Australia, 37 : 1-33. GUmLAUME,R. E. F., 1966. Petroleum geology in the Bonaparte Gulf Basin, N.T. 8th Comm. Mineral Metallurgical Congr., 5 : 183-196. JACQUEMIN, M., 1967. Timor Sea aeromagnetic survey. Bur. Mineral Resources Australia Petrol. Search Subsidy Acts, Pabl., in press. NOAKES, L. C., OPn% A. A. and CR~PtN, I., 1952. Bonaparte Gulf Basin, northwestern Australia. Congr. GdoL Intern., Compt. Rend., 19e, Algiers, 1952, Symp. S~r. Gondwana, pp.91-106. SMITH,E. R., 1966. Timor Sea Joseph Bonaparte Gulf marine gravity and seismic Sparkarray survey, northwest Australia 1965. Bur. Mineral Resources Australia, Records, 1966/72 (unpublished). TRAVES, D. M., 1955. The geology of the Ord-Victoria region, northern Australia. Bur. Mineral Resources Australia, Bull., 27 : 1-133. Marine GeoL, 5 (1967) 293-298

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VAN ANDEL, TJ. H. and VEEVERS,J. J., 1965. Submarine morphology of the Sahul Shelf, northwestern Australia. Bull. Geol. Soe. Am., 76 : 695-700. VAN ANDEL, TJ. H. and VEEVERS,J. J., 1967. Morphology and sediments of the Timor Sea. Bur. Mineral Resources Australia, Bull., 83 : 1-173. VAN ANDEL, TJ. H., CURRAV, J. R. and VEEVERS,J. J., 1962. Recent carbonate sediments of the Sahul Shelf - - northwestern Australia. Natl. Sci. Found, O)fice Naval Res., Coastal Shallow Water Res. Conf., pp.564-567. VEEVERS, J. J. and ROBERTS, J., in preparation. Geology of the Bonaparte Gulf Basin. Bur. Mineral

Resources A ustralia,'Bull.,r.97.

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