Southern Australian submarine canyons: Their distribution and ages

Marine Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SOUTHERN AUSTRALIAN SUBMARINE CANYONS: THEIR DISTRIBUTION AND AGES C. C. VON DER BORCH

Horace Lamb Centre for Oceanographic Research, The Flinders University of South Australia, Bedford Park, S. A. (Australia) (Received July 29, 1967)

SUMMARY

A study of available echo sounding data, coupled with a recent Precision Depth Recorder section along the continental slope, has enabled detailed plotting of submarine canyon axes along the continental margin of southern Australia. Groups of large submarine canyons are confined to the continental slope in areas of shallow basement rock, opposite on-shore areas of Cambrian to Precambrian rocks. Areas of Tertiary basin development, both on-shore and on the continental shelf, lack a development of major submarine canyons on adjacent continental slopes. Relics of Early Tertiary or pre-Tertiary large-scale drainage systems on the ancient land surface of the Great Plateau, of southwestern Australia, may be related to the large submarine canyons. Results of the study indicate at least an Early Tertiary age for initial sub-aereal cutting of heads of the large canyons. Smaller canyons, occurring on slope areas in the vicinity of Tertiary basins, may be related to Pleistocene low sea level stands.

INTRODUCTION

Submarine canyons have long been a geological enigma. Their possible origin and age of formation have been subjects of controversy since their recognition as long ago as 1893. Early workers proposed a Pleistocene age for formation of canyons on continental slopes, suggesting sub-aereal cutting by rivers during low sea level stands related to glacial periods. Subsequent work, particularly off the Californian coast, produced strong evidence for a pre-Pleistocene origin of larger canyons, particularly considering the large volumes of terrigeneous sediments contained in abyssal fans at canyon mouths (MENARD, 1960). The southern coast of Australia is fronted by rocks varying in age from Archaean to Modern. Crystalline shield rocks crop out along the coastline in the southwest of the continent and at the central south coast, whilst Cretaceous and Tertiary sediMarine Geol., 6 (1968) 267-279

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ments occur in three major basins. Unusually large and well-developed submarine canyons occur in specific zones along the continental slope. Relationships of canyons with on-shore geology and morphology provide unique evidence for an estimation of the age of formation of southern Australian submarine canyons.

SOURCE OF DATA Fig.1 illustrates the bathymetry of the continental shelf and slope region off southern Australia. Ship track distribution forming the basis for contours is shown in fig.2 of CONOLLY and VON DER BORCH (1967), after which the present bathymetry has been somewhat modified. A more recent P.D.R. (precision depth recorder) traverse made by the author along approximately the 1,000 fathom contour from Perth to Adelaide, in addition to existing P.D.R. profiles collected by H.M.A.S.

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"Gascoyne", has made possible the plotting of submarine canyon axes along the continental margin from Perth in the west to longitude 142°E. Canyon contours shown in Fig,1 are in most cases somewhat diagrammatic, due to extreme topographic complexity coupled with sparse sounding data. Some canyons shown may be in fact considerably longer and more winding, and in some cases, canyons illustrated as neighbouring individuals may be mutual tributaries. However, along the actual ship tracks, canyon axes have been plotted in their true positions, with the exception of the Murray Group, in which case ship tracks could not be exactly reconciled with existing hydrographic information. In the case of the Perth Canyon (Fig.1 and 2) sufficient ship track data exist to allow relatively detailed contouring. It is uncertain whether the canyon cross-sections shown in Fig.3 are in all cases normal to canyon axes. This results in the fact that although canyon depths along the lines of section are meaningful, widths and wall slopes may be misleading. Distances quoted as miles in this publication refer to nautical miles. Depths are given in fathoms.

CANYONDISTRIBUTIONAND DESCRIPTION Fig.1 shows two main groups of large submarine canyons on the slope off southern Australia. One, the "Albany Group", occurs from longitude l15°E to 124°E, and the other, the "Murray Group", from longitude 135°E to 138°E. A group of smaller canyons occurs at the eastern boundary of the map. This has been named the "Bridgewater Group" after a canyon described by HOPKINS (1966). In addition, two areas of small canyons occur both at the western and eastern sections of the Great Australian Bight, marginal to the main canyon groups. The major portion of the slope opposite the Bight appears to be free of large-scale canyoning, from the Pasley Canyon (longitude 124°20'E) to the Ceduna Canyon (longitude 133°E). This relatively undissected zone includes the outer slope of the Ceduna Plateau. In addition to the above, at least one major canyon occurs off the west coast of Australia, the Perth Canyon, opposite the Swan River estuary. Depth data is sparse north of this areas and the presence of additional canyons unresolved. The entire continental slope from Perth to Bass Strait has now been traversed by at least one P.D.R. track, thus it is possible to describe cross-sectional characteristics of all existing canyons in this area. Selected portions of traverses are illustrated by the lettered cross sections in Fig.3, referring to positions shown on Fig.l. Crosssections have been traced directly from P.D.R. rolls. Perth Canyon

Details of the Perth Canyon are shown in Fig.2. Contours at 100-fathom intervals are justified in this case due to relatively high sounding density. Marine Geol., 6 (1968) 267-279

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The canyon head lies immediately seaward of the 100-fathom contour, in a region of relatively gentle gradient on the upper continental slope. The head is about 30 miles offshore from the estuary of the Swan River. At least two well-defined tributar~ canyons coalesce at about the 400-fathom contour, at which point the main axis takes a pronounced curve to the northwest. Forty miles seaward from the head, i~ a water depth of 1,500 fathoms, the canyon loses its identity, and sounding data i~ inadequate to resolve possible fan valley systems. Fig.3A-B illustrates a traverse of the canyon. Both tributaries have a vertical relief approaching 600 fathoms along this line, and a steep V-section is apparent in the southern tributary where the traverse crosses normal to the axis. From the Perth Canyon south to the Leeuwin Canyon, slope topography is exceptionally smooth and undissected, opposite the landward margin of the Naturaliste Plateau.

Leeuwin Canyon The Albany Group of canyons is defined in its western margin by the Leeuwin Canyon (Fig.3C-D). This has a " W " profile in section, probably due to the presence of two coalescing tributaries. At this line the canyon has a vertical relief of 800 fathoms and an apparent width of 13 miles.

D'Entrecasteaux Canyon Adjacent to the Leeuwin Canyon lies the D'Entrecasteaux Canyon (Fig.3C-D). A notable point concerning this canyon is the presence of a steep, fiat-floored valley on its eastern side. This suggests some form of sediment channelling, either sediment creep (DILL, 1962), or turbidity current activity (KuENEN, 1953). Vertical relief of this canyon is about 800 fathoms.

Broke Canyon This canyon has a rather complex cross-section, consisting of a series of ridges and valleys, with valley floors along the line of section occurring at varying levels (Fig.3C-D), possibly indicative of tributary systems. Vertical relief of the Broke Canyon is about 700 fathoms.

Wilson Canyon This canyon lies opposite the conspicuous Wilson Inlet, with the canyon head situated approximately 25 miles offshore. (Fig.3C-D).

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Albany Canyon The next canyon of note to the east is the Albany Canyon, which appears, along the line of section, to be a broad, mature trough (Fig.3D-E)with a head about 20 miles seaward of the drowned topography of King George Sound. Width of this canyon is 20 miles and vertical relief about 800 fathoms.

Vancouver Canyon Adjacent to the Albany Canyon lies the Vancouver Canyon (Fig.3D-E). On existing data it is uncertain whether this and the Albany Canyon are separate individuals or tributaries of a main trunk canyon. Vertical relief of this canyon is of the order of 600 fathoms. Eastward from the Vancouver Canyon the slope is dissected by a series of small steep V canyons (Fig.3, near E), with widths at the top varying from 2 to 3 miles and depths up to 300 fathoms.

Bremer Can),on The next large canyon to the east is the Bremer Canyon (Fig.3F-G). A welldefined flat floor is evident, about 2 miles in width.

Stokes Canyon Possibly due to the ship track being normal to the canyon axis in this case, the Stokes Canyon appears relatively narrow and steep, with a width at the top of 9 miles and a well-defined flat floor about 1 mile wide (Fig.3H-1).

Esperance Canyon The large Esperance Canyon (Fig.3J-K) heads in about 60 fathoms of water about 20 miles offshore from the conspicuous bay of Esperance. The canyon head appears to notch the edge of the continental shelf, which breaks slope in this region at about 60 fathoms. Upper reaches of this canyon are situated about 10 miles west of Termination Island, the outermost of the extensive Recherche Archipelago. Relatively detailed contouring of the Admiralty chart of the area (Fig.4) suggests a relationship with Esperance Bay. A weU-defined trough extends seaward from Esperance to a depth of about 40 fathoms, where topography becomes smoothed by shelf sediments. It is probable that a sediment-filled channel may extend out to the head of the Esperance Canyon. The Esperance Canyon has a vertical relief of 1,000 fathoms. The section indicates a broad, trough-like profile about 20 to 25 miles in width, although this may be well in excess of the true value.

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Pasley Canyon The last notable canyon of the Albany Group, the Pasley Canyon, occurs at about longitude 124°E. This has a vertical relief of 600 fathoms (Fig.3L-N). From this point eastward, across the Great Australian Bight, the P.D.R, traverse indicates no canyons of size comparable to those of the Albany Group. Only minor channels with depths up to t00 fathoms are encountered. From longitude 126°E eastward, the continental slope has a relatively gentle gradient, forming the seaward margin of the Ceduna Plateau (CONOLLVand VON DER BORCH, 1967). Steep, small V

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canyons here give way to shallow troughs suggestive of fan-valleys. About four such troughs occur from longitude 126°E to 129°E, separated by broad, smooth interfluves. Troughs in this zone average 4 miles in width and 40 fathoms in vertical relief, with interfluves measuring up to twelve miles in width. From this evidence it seems unlikely that large-scale canyons occur on the relatively gentle continental slope to the north.

Ceduna Canyon The first conspicuous canyon eastward from the Pasley Canyon is the Ceduna Canyon, at about longitude 133°E (Fig.3N-O). This is a relatively small V canyon with a pronounced flat floor. The Ceduna Canyon has a vertical relief of 240 fathoms, and measures four miles in width at the top and 3/4 mile across the floor. Nine similar small, steep canyons occur between the Ceduna Canyon and position P (Fig. 1).

Murray Group The group of large canyons known as the "Murray Group" (SPR1GG, 1947, 1963) occurs between longitude 135°E and 138°E. The rugged nature of these canyons is evident from Fig.3, section P-Q, where one exceptional canyon has a vertical relief of 1,200 fathoms. Slope topography is extremely rough throughout the entire section, with a complexity of small, steep furrows and large, deep canyons. Several smaller canyons in the vicinity of the Murray Group lie partly within a Naval Loran-navigated survey, enabling detailed contours to be drawn of their upper reaches (VON DER BORCH, 1967). Data is sufficient in these cases to resolve two types of canyon heads, both amphitheatre and dendritic. P.D.R. traverses indicate that the last major canyon of the Murray Group occurs at long. 138°E (Fig. 1,Q). Further east only a few relatively small furrows occur until the Bridgewater Group is reached. The furrows have vertical relief of the order of 100 fathoms, in direct contrast to canyons of the Murray Group.

Bridgewater Group At the eastern margin of Fig.1 the Bridgewater Group of canyons is situated. This consists of four well-defined steep canyons having vertical relief up to 5{30 fathoms and widths of from 2 to 4 miles. The narrow V sections of the Bridgewater Canyons (Fig.3R-S) contrast markedly with the larger and broader canyons of the Albany and Murray Groups. HOPK1NS (1966) states that a reconnaissance seismic survey indicated many features in the shelf area which may be interpreted as either channel cut and fill or as filled submarine canyons. One example cited was a filled canyon three miles in width and 2,800 ft. deep, of possible Late Tertiary age.

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It is apparent that an excellent degree of correlation exists between intenseh incised sections of the continental slope and areas of pre-Tertiary "basement" rock, on the adjacent shore. These zones coincide with areas of steep continental slope. The Albany Group begins in its western extremity opposite the seaward extrapolation of the Darling Fault, which separates Tertiary, Mesozoic and older sediments of the Perth Basin from the ancient crystalline rocks of the Australian Shield (Fig.l). AI least ten deep well-detined submarine canyons occur along the steep continental slope from this point to the Pasley Canyon, which marks the eastern boundary of the Albany Group. The Pasley Canyon lies opposite the eastern end of an obviously shallow basement area on the continental shelf, represented by the numerous islets of the Recherche Archipelago, all composed of crystalline basement rock. East of the Pasley Canyon the trend of the continental slope changes markedly from east1140

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west to northeast-southwest. Opposite this point on land lies the western boundary of the Tertiary Eucla Basin (MCWHAE et al., 1956), with its probable extension onto the continental shelf. The inflexion of the continental margin may be an expression of Tertiary downwarping during basin development. From this point to the Ceduna Canyon there is a notable absence of large canyons along the more gentle slope area, the canyon-free zone lying opposite the on-shore Tertiary basin. The Ceduna Canyon lies opposite the seaward extrapolation of the eastern boundary between the Eucla Basin and ancient crystalline rocks of the shield. Seismic and aeromagnetic data are not available at time of writing to indicate whether such extrapolation is justified, although it seems reasonable. From this point east to longitude 138°E an increasing number of canyons occurs along a zone of steep slope culminating in the Murray Group. The easternmost large canyon of this group lies at longitude 138°E. Recent aeromagnetic surveys along the continental shelf (HAEMATITE EXPLORATIONS, 1965) indicate shallow basement rocks west of this point, with downwarping of basement immediately to the east (Fig.l). The correspondence of largescale canyons on the slope to areas of pre-Tertiary rocks is once again obvious. In this case the pre-Tertiary rocks are the Upper Proterozoic to Cambrian sediments and intrusives of the Adelaide Geosyncline (GLAESSNER et al., 1958), with their southernmost accessible occurrence on small islets known as Young's Rocks, about 15 miles north of the Murray Group. The Bridgewater Group occurs on the slope opposite a deep sedimentary area known as the Otway Basin, containing Tertiary and older sediments. Aeromagnetic and seismic surveys indicate that basement depths in excess of 10,000 ft. extend beyond the edge of the continental shelf in this section (HAEMATITEEXPLORATIONS, 1965).

CANYON DISTRIBUTION WITH RESPECT TO PALAEO-DRAINAGE

Fig.5 illustrates the salt lake system and the ordered stream drainage of southwestern Australia. It is evident that a definite line of demarcation exists between the salt lake chains and the younger drainage. The former are restricted to the old land surface of the Great Plateau of western Australia, the surface of which dates back well into the Palaeozoic (JUTSON, 1934; MULCAnY, 1966; PRn~ER, 1966). The younger drainage dissects the outer margin of the Great Plateau. The surface of the Great Plateau has been extensively lateritized, with the age oflateritization being estimated to fie between Upper Eocene and Miocene (McWHAE et al., 1956). Uncertainty still exists on this age, but it is evident that lateritization took place well back in Tertiary times. Possibly related to the age of lateritization are the extensive chains of large salt lakes, which have been described on good evidence as being relics of vast river systems flowing in Tertiary times (GREGORY, 1914; BETTENAY, 1962). These are slightly incised into the laterite surface. Fig.5 shows a reconstruction of this ancient drainage system, after Bettenay, with heavy dashed Marine Geol., 6 (1968) 267-279

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lines indicating flow directions. It is apparent that a widespread system converges on the headwaters of the modern Swan-Avon Rivers near Perth, suggesting large scale stream discharge in those times into the vicinity off the Perth Canyon. Other salt lake drainage systems join headwaters of the Blackwood and Franktand Rivers (MuLcAHY, 1966) and a less well-defined group of broad salt lakes trends south towards Esperance Bay, suggesting ancient drainage towards that area (GREGORY, 1914). Large submarine canyons occur on the continental slope opposite these areas. At the present state of knowledge it is tempting to relate the initial phase of canyon formation to these Tertiary river systems. CANYON AGES

From the above evidence, it appears that zones of large canyons on the continental slope of southern Australia correlate very welt with on-shore areas of preTertiary rocks. These large canyons would thus pre-date at least the youngest basin deposits of the Eucla and Otway Basins, suggesting an age greater than lower Miocene. It has been suggested by SHEPARD (1963) that upper reaches of submarine canyons on the continental slopes of the world have been cut by sub-aereal drainage, during low relative stands of sea level. A possible connection between ancient drainage patterns on the lateritized Great Plateau of western Australia and the canyons in that area indicates that canyon development off southern Australia may be related in age to the ancient drainage system and therefore to the humid period of lateritization. Age of lateritization is uncertain, although it undoubtedly occurred well back in the Tertiary, possibly between Upper Eocene and Miocene. Continental fluviatile deposits of mid to Upper Eocene age (GLAESSNERand PARKIN, 1958) are widespread in Tertiary basin deposits on shore north of the Murray Canyons. These stream deposits are related to an intensely pluvial period. Such a pluvial period, occurring during low relative stands of sea level, may have caused initial canyon cutting. The absence of large submarine canyons opposite areas of Tertiary basin development may be explained by sediment filling of pre-existing canyons. A more likely explanation, however, would be the fact that such areas which were tectonicalty negative during Tertiary times would not have been above sea level, particularly in the critical area of the continental slope, during the periods of intense drainage and dissection. Small canyons described for these regions, such as the Bridgewater Canyons and the Ceduna Canyon, may indeed be younger phenomena, related to Pleistocene low sea level stands. ACKNOWLEDGEMENTS

The author is indebted to the Royal Australian Navy for providing facilities Marine Geol., 6 (1968) 267-279

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aboard H.M.A.S. "Diamantina" during cruise Dm 1/67, from Fremantle to Adelaide, In addition the Hydrographic Office made available all plotting sheets and P.D.R. rolls for the area. The study was carried out under the auspices of an Australian Research Grants Committee grant and a Flinders University fellowship, at the Horace Lamb Centre for Oceanographic Research, Flinders University, South Australia.

REFERENCES BETTENAY, E., 1962. The salt lake systems and their associated aeolian features in the semi-arid regions of Western Australia. J. Soil Sei., 13 : 10-17. CONOLLY,J. R. and Vow DER BORCH, C. C., 1967. Sedimentation and physiography of the sea floor south of Australia. Sediment. Geol., 1 (2) : 181-220. DILL, R. F., 1962. Sedimentary and erosional features of submarine canyon heads. In: D. S. GORSLINE (Editor), Proc. First National Coastal Shallow Water Res. Con/i, Nat. Sci. Found. Off. Naval Res., p.531. GLAESSNER, M. F. and PARKIN, L. W., 1958. The geology of south Australia. J. Geol. Soc. Australia, 5 (2) : 1-163. GREGORY, J. W., 1914. The lake system of Westralia. Geograph. J., 43 : 656-664. HAEMAT1TE EXPLORATIONS, 1965. Bass Strait and Encounter Bay aeromagnetic survey, 1960-1961. Commonwealth Australia, Bur. Min. Res., Publ., 60 : 37 pp. HOVKINS, B. M., 1966. Submarine canyons. Tech. Bull., Broken Hill Proprietary, 26 : 39-43. JUTSON, J. T., 1934. The physiography of Western Australia. Bull. Geol. Surv. W. Australia, 95 : 1-150. KUENEN, PH. H., 1953. Origin and classification of submarine canyons. Bull. Geol. Soc. Ant., 64 : 1295-1314. McWHAE, J. R. H., PLAYFORD, P. E., LINDER, A. W., GLENISTER, B. F. and BALME, B. E., 1956. The stratigraphy of Western Australia. J. Geol. Soe. Australia, 4 : 1-161. MENARI~, H. W., 1960. Possible pre-Pleistocene deep sea fans off Central California. Bull. Geol. Soe. Am., 71 : 1271-1278. MULCAHY, M. J., 1966. Landscapes, laterites and soils in southwestern Australia. In: Landform Studies from Australia and New Guinea. Austral. Nat. Univ. Press, Canberra, pp.211-230. PRIDER, R. T., 1966. The lateritized surface of Western Australia. Australian J. Sci., 28 : 443-451. SHEPARD, F. P., 1963. Submarine Geology. Harper and Row, New York, N.Y., 557 pp. SPRIGG, R. C., 1947. Submarine canyons of the New Guinea and south Australian coasts. Trans. Roy. So¢. S. Australia, 71 • 293-310. SVRIGG, R. C., 1963. New structural discoveries off Australia's southern coast. Australian Oil Gas J., 1963 : 32-42. VON DER BORCH, C. C., 1967. South Australian submarine canyons. Australian J. Sei., in press.

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