TJEERD H. VAN ANDEL JOHN J. VEEVERS

SUBMARINE MORPHOLOGY OF THE SAHUL SHELF, NORTHWESTERN

Abstract: The Sahul Shelf, located between north- of which it probably is the submerged extension. western Australia and the Trough, consists This requires uplift, weathering, and denudation of a central basin surrounded by broad, shallow of the shelf in middle and late Tertiary. Subse- rises. Superimposed on the regional relief is a quently, the shelf was deformed to form the basin system of banks, terraces, and channels. The flat and rises. This deformation caused the original tops of banks and terraces form parts of several drainage to become antecedent. Lower surfaces regional, subhorizontal surfaces. The steplike to- were formed during Pleistocene low sea-level pography closely resembles the system of late stands. Cenozoic erosional surfaces on the adjacent land

area with the shelf edge. The Sahul Rise sepa- Introduction rates the Bonaparte Depression from the Timor The Sahul Shelf, off the northwest coast of Trough. The basin is connected with deep Australia, is a very wide continental shelf water by a long, narrow valley with a maximum which derives its interest primarily from its depth of 200 m, Malita Shelf Valley. position between the continental mass of The shelf edge is represented by a gradual Australia and the island arches and geosynclines change in slope, usually marked by a low cliff of Indonesia. An early study, based on con- at 110-130 m. A second shelf edge at approxi- ventional nautical charts, was made by Fair- mately 550 m, reported by Fairbridge (1953), bridge (1953). In 1960 and 1961, Scripps Insti- does not exist. Just beyond the edge of the tution of Oceanography made a detailed shelf on the upper continental slope are numer- bathymetric and sampling survey of the ous clusters of small, steep-sided banks, 1-5 km and the adjacent Timor Trough (van Andel in diameter; they rise from 200 to 400 m and and others, 1961; van Andel and Veevers, universally have flat tops at 20-25 m. Fair- in press) as part of the International Indian bridge (1953) has explained these banks as Ocean Expedition. The resulting information, strings of reefs, a broken barrier originally supplemented by detailed sounding data for formed in shallow water, and slowly grown up parts of the shelf provided by the Hydrogra- during gradual subsidence of the Timor Trough. pher, Royal Australian Navy, forms the basis Superimposed on this broad regional relief of this report. is a complex fine structure of banks, terraces, and channels. The tops of the banks are gener- Regional Morphology ally flat; their slopes are fairly steep and The regional morphology of the Sahul Shelf occasionally interrupted by terraces. The chan- is unusual (Fig. 1); instead of the flat, gently nels are closely spaced, subparallel or in the seaward-sloping plain of most shelves, the form of a braided pattern, and steep-walled Sahul Shelf contains a large central basin, the (Fig. 2). In most cases, the channels cut Bonaparte Depression, with a maximum depth through the rises, suggesting antecedent sys- of 120-140 m. This basin is reflected in the tems, and are much deeper in their central coast line by a deep embayment, the Joseph portions than at either end. They lose their Bonaparte Gulf. The Bonaparte Depression is identity in the Bonaparte Basin and on the surrounded by broad rises with crest depths outer shelf, in depths of approximately 100 m. of 40-50 m. The Van Diemen Rise, extending On the Van Diemen Rise, the bank and seaward from Melville and Bathurst Islands, channel topography is strikingly similar to the is the submerged continuation of these islands; morphology of Melville and Bathurst Islands. the Londonderry Rise connects the Kimberley On the Londonderry and Sahul Rises, the

Geological Society of America Bulletin, v. 76, p. 695-700, 4 figs., June 1965 695

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topography is more subdued, the differences in converge in the same direction. Surfaces III relief smaller, and the maximum elevations and IV (Figs. 3, 4) can be traced with confi- lower. The Bonaparte Depression is an area of dence over most of the shelf area. On the rises, gentle slopes or flat bottoms, with only a few they form terraces and valley floors; in the minor banks. The outer shelf is characterized Bonaparte Basin, they are developed as bank by flat bottoms; wide, low terraces; and shallow tops. Below surface IV, several other terrace channels. Aggraded surfaces predominate in the and valley-floor levels can be traced in the Joseph Bonaparte Gulf. basin, on the flanks of the rises, and on the

124° 126° 128° 130° Figure 1. Regional morphology of the Sahul Shelf, northwestern Australia

The most striking feature of the shelf outer shelf. Surface IV and lower surfaces topography is the widespread occurrence of converge markedly on the rises and diverge flat, subhorizontal surfaces, separated by short, toward the outer shelf and the center of the steeper slopes (Fig. 3). Gradients on the flat Bonaparte Depression. surfaces are of the order of 0.05 m/km with only minor irregularities. The surfaces occur Interpretation at numerous levels and are covered by uncon- The bank and channel topography might be solidated calcareous sediments. attributed to formation of platform reefs, ad- Many of these flat surfaces can be correlated justed by growth and marine erosion to various regionally on the basis of cross-correlation be- sea-level stands. A few active platform reefs tween sections, tracing on detailed charts, occur locally on the Sahul Shelf (Fairbridge, elevation, and sequential order. The higher 1950). However, on the whole, the bank mor- surfaces shown in Figures 3 and 4 (I and II) are phology is not similar to reef topography; little restricted to the rises where they form the tops reef material has been found in some 375 of higher banks. They dip northwestward and samples taken, and the system of sea-level

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changes needed to explain all surfaces is im- cise correlations are not yet obtainable. The probably complex. oldest and highest of these surfaces, variously Fairbridge (1953) has pointed out the simi- named the Australian, Bradshaw, or Tennant larity between topography of the shelf and the Creek surface, is probably of mid-Tertiary age relief of the adjacent land. The now available and deeply laterized, indicating a long period detailed knowledge of the shelf relief shows of peneplanation and weathering before uplift. that this similarity is very striking and strongly A Miocene marine-transgressive sediment, the

Figure 2. Bank and channel topography of the Sahul Shelf, northwestern Australia, showing only those banks that have closed contours greater than 40 m above the surrounding terrain. Arrows indicate detailed cross section in Figure 3. One degree latitude equals 60 nautical miles.

suggests that both are of the same origin. Thus, White Mountain formation, occurs in places the shelf relief would be due to subaerial on the laterite and predates the uplift. Strip- weathering, erosion and peneplanation, and ping of parts of the laterite resulted in the subsequent submergence with little sedimen- formation of a slightly younger, much less tation. This hypothesis, although plausible, deeply weathered lower surface, the Wave Hill cannot be proved at this time, but its conse- or Maranboy surface. Even lower are several quences are interesting enough to explore cycles of partly erosional, partly depositional briefly in this paper. plains which are generally attributed to Pleisto- Various authors (Hays, unpub. rept., Bur. cene sea-level fluctuations. Tentatively, it is Min. Res. Australia; King, 1949, Paterson, in suggested that the Australian and Wave Hill press; Wright, 1963) have described series of surfaces are the landward equivalents of Sahul late Tertiary and Quaternary erosion surfaces Shelf surfaces I and II, whereas surface III and on the adjacent land. Several of these are lower surfaces represent the seaward extensions apparently continuous over large portions of of the Pleistocene cycles. western and northern Australia although pre- All shelf surfaces follow the regional pattern

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SPACING OF SURFACES 0 50 NAUT. MILES

Figure 4. Spacing between surfaces II and III and surfaces III and IV (see Fig. 3) over the Sahul Shelf, northwestern Australia

of rises and depressions. If the hypothesis just Because there is no convergence of surfaces presented is correct, this regional relief is I/II or II/III over the rises (Fig. 4), it may be probably due to epeiric deformation during or assumed that no deformation took place until after the formation and uplift of the erosional the time of uplift of surface III. The channel surfaces. Deformation of the Wave Hill surface system of the Van Diemen Rise, now ante- has been noted by Hays (unpub. rept., Bur. cedent, may represent the original northerly Min. Res., Australia) and appears to be well drainage on these surfaces. When the Sahul aligned with the regional structure of the shelf. and Van Diemen Rises developed, drainage

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was diverted to the west. Subsequent uplift of sequence of late Cenozoic sediments may occur the Londonderry Rise converted these channels here. During the period of emergence, the also into an antecedent system. shelf-edge banks formed a fringe of reefs just Surfaces III and IV and, even more marked- beyond the hinge line between continental ly, those below IV show clear divergences uplift and geosynclinal subsidence. toward low areas and convergences over the If this hypothesis, admittedly speculative, rises (Fig. 4). Hence, deformation probably should prove to be correct through further began between surfaces III and IV, possibly in geophysical work and drilling, this late Ceno- the early Pleistocene, and has continued since zoic history would represent one more cycle that time. in a long series of shallow marine transgressions The hypothesis of subaerial origin, if correct, followed by uplifts and erosion along structural requires that during most of its Cenozoic lines quite similar to the present ones (Fair- history the Sahul Shelf was above sea level and bridge, 1953). intermittently rising. Unless we assume that successive Pleistocene minima of sea level were Acknowledgments progressively lower, this rise must have con- This investigation forms part of the Scripps tinued until relatively late in the Pleistocene. Institution of Oceanography participation in Then it was followed by a rather abrupt sub- the International Indian Ocean Expedition sidence of some 120 m. Because the original and has been supported by the Office of Naval relief seems well preserved, not much sedimen- Research, contract Nonr 2216(01), and the tation can have taken place since the sub- National Science Foundation Grants NSF- sidence. The shelf appears to be mainly a zone G14103 and GP-350. The authors are indebted of sediment bypassing. The smooth, long, gentle to Joseph R. Curray and H. W. Menard for southeastern slope of the Timor Trough, on the advice and review of the manuscript. other hand, suggests that a thick, continuous

References Cited Fairbridge, R. W., 1950, Recent and Pleistocene coral reefs of Australia: Jour. Geol., v. 58, no. 4, p. 330-401 • • 1953, The Sahul Shelf, northern Australia; its structure and geological relationships: Royal Soc. Western Australia, Jour., v. 37, p. 1-33 King, L. C., 1949, The cyclic land surfaces of Australia: Melbourne, Royal Soc. Victoria, Proc., v. 62, p. 79-95 Paterson, S. J., in press, Geomorphology of the Ord-Victoria region: Commonwealth Sci. and Indus. Research Organization, Land Research Ser. van Andel, Tj. H., and Veevers, J. J., in press, Morphology and sediments of the Timor Se;i: Bur. of Min. Res., Australia, Bull. van Andel, Tj. H., Curray, J. R., and Veevers, J. J., 1961, Recent carbonate sediments of the Sahul Shelf, northwestern Australia: Coastal and Shallow Water Research Conf., Natl. Sci. Found, and Office of Naval Research, p. 564-567 Wright, R. I., 1963, Deep weathering and erosion surfaces in the Daly River basin, : Jour. Geol. Soc. Australia, v. 10, 1, p. 151-163

UNIVERSITY OF CALIFORNIA, SCRIPPS INSTITUTION OF OCEANOGRAPHY, LA JOLLA, CALIF. BUREAU OF MINERAL RESOURCES, CANBERRA, AUSTRALIA MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 21, 1964 CONTRIBUTION FROM SCRIPPS INSTITUTION OF OCEANOGRAPHY, LA JOLLA, CALIF.

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