JIM C. STANDARD Dept. Geology and Geophysics, University of Sydney, Sydney, N.S.W., Australia Submarine Geology of the Tasman Sea Abstract: The physiographic features of the con- mum eastward development of the Australian tinental margin of eastern Australia, the Tasman continent. Lord Howe Rise is considered orogenic Basin, Lord Howe Rise, and the Coral Sea Platform in origin and probably of Early Paleozoic age. The are described and discussed geologically. Three Tasman Basin is a stable area underlain by per- guyots, each having more than 14,000 feet of relief manent ocean-type crust which may have acted as and a platform depth of less than 150 fathoms, are a nucleus for the eastward growth of the island mapped and described. arcs which lie between the Tasman Basin and the The present continental slope of southeastern South Pacific Basin. Australia west of the Tasman Basin marks the maxi- CONTENTS Introduction 1777 Figure Acknowledgments . 1777 1. Location map of the physiographic features of Bathymetry 1778 the Tasman Sea 1778 Physiographic features 1779 2. Profile from southeastern Australian coast to Continental margin 1779 Lord Howe Island; north-south profile of Tasman Basin 1781 guyots and east-west profile of Lord Howe Lord Howe Rise 1782 Rise 1780 Coral Sea Platform 1782 3. Profiles of continental shelf and slope of south- Geological interpretation 178? eastern Australia 1781 Tasman Basin 1783 4. East-west profile of Derwent Hunter Guyot . 1782 Seamounts and guyots 1784 Volcanic islands and reefs 1784 Plate Facing Lord Howe Rise and Coral Sea Platform 1785 1. Bathymetric map of the middle part of the Conclusions 1785 Tasman Sea 1782 References cited 1786 Table 1. Guyots, scamounts, banks, and reefs of the Tasman Sea 1783 (1958). Hess and Maxwell (1949) indicated no INTRODUCTION crustal thickness but suggested that the island The geology of the southwest Pacific, in- arcs of the southwest Pacific are the result of cluding the Tasman Sea, received much atten- progressive outward growth of the Australian tion around the turn of the century (Suess, continent from a nucleus in Western Australia. 1906; Gregory, 1910; Marshall, 1911). Benson Officer (1955) found that there was no con- (1923; 1924) gives an excellent review of many tinental-type crust in the southwest Pacific of these early papers and lists more than 400 and the crustal thickness of the Tasman Basin references. was only 5 km, the same as that of the South Most early writers believed in the existence Pacific Basin. Geological interpretation of re- of an ancient Australian continent extending cent oceanographic surveys conducted by the eastward from the present Australian coast to Royal Australian Navy supports Officer's con- the South Pacific Basin including both Fiji clusion. Islands and New Zealand. Among the more recent authors who have expressed a similar ACKNOWLEDGMENTS view are Bryan (1944), Macpherson (1946), The author thanks Commander J. H. S. de Jersey (1946), Gill (1952), Glaessner (1952), Osborn, Lieut. Commander R. S. Hardstaff, Gutenberg and Richter (1954), Marshall and and Mr. T. Steward of the Hydrographic Narain (1954). Fleming (1957), and Gill Office Royal Australian Navy, without whose Geological Society of America Bulletin, v. 72, p. 1777-1788, 4 figs., 1 pi., December 1961 1777 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/12/1777/3442070/i0016-7606-72-12-1777.pdf by guest on 28 September 2021 1778 J. C. STANDARD—TASMAN SEA co-operation this paper could not have been Royal Australian Navy has made it possible to written. Dr. A. A. Day and Dr. H. G. Wilshire construct a bathymetric map of part of the of the Department of Geology and Geophysics, Tasman Sea based on continuous echo-sounding University of Sydney, critically read the manu- profiles (PL 1). The portion of the Tasman Sea script, and Miss Jeannette Forsyth of the same which has been mapped in most detail is in- department drafted the maps and figures. cluded in the world-wide network of ' 'Plotting Areas for Ocean Soundings" 413 and 414. BATHYMETRY However, detailed information is not available Until recently there has been little ocean- for most of the Tasman Sea, and the maps pre- ographic knowledge of the Tasman Sea, most sented here are based upon information avail- of which was based on pre-1900 single, lead- able before June 1961. line soundings many of which were several The names of physiographic features used hundred miles apart. Recent work by the in this paper are those used by Brodie (1952; Figure 1. Location map of the physiographic features of the Tasman Sea Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/12/1777/3442070/i0016-7606-72-12-1777.pdf by guest on 28 September 2021 BATHYMETRY 1779 1958) and Wiseman and Ovey (1955). The pro- probably due to an increase of sediments de- posed new names follow the rules suggested by posited by the rivers. A definite delta-type Wiseman and Ovey (1953). The name "Coral deposit is shown on the coastal charts of this Sea Platform" is proposed for the area north area at a depth of 50 fathoms. of the Tasman Basin. Wyrtki (1960) noted the eastward swing of Three guyots have been mapped. The the southeast Australian current away from the southern one, the Taupo Bank, was named by continent at about 32° S. latitude. The narrow- Fleming and Brodie, (1951, unpub. ms., New ness of the continental slope and shelf north of Zealand Oceanographical Committee, Welling- 32° S. latitude is possibly due to this current ton, N. Z.). The names Derwent Hunter which has prevented the accumulation of sedi- Guyot and Barcoo Guyot are proposed for the ments, in much the same way as the Gulf other two. The DERWENT HUNTER is a research Stream has off the east coast of Florida schooner belonging to the Australian Common- (Shepard, 1959). wealth Scientific Industrial Research Organiza- An 1890-foot well drilled in 1959 on Wreck tion which discovered the northern guyot in Island, in the southern part of the Great Barrier 1958. The BARCOO is a Royal Australian Navy Reef, passed through 530 feet of Recent coral- survey ship which discovered the central reef material. Tertiary sediments were pene- guyot in 1956. trated from 530 feet to 1795 feet (Mott, I960). Seven seamounts have been given the names The underlying bedrock is probably Paleozoic NT 1 (North Tasman seamount 1) through metamorphic rocks similar to those on the NT 7. The seamount NT 1, NT 2, and NT 3 mainland. A 732-foot well drilled on Heron were discovered in 1902 by theC. S. BRITTANIA Island, 6 nautical miles south of Wreck Island, while laying cable between Brisbane and Nor- revealed a marked increase in terrigenous mate- folk Island. David (1932) referred to these as rial at 500 feet. This was interpreted as repre- the Tasmantides Volcanoes. senting the base of Recent coral material and was thought to have been associated with the PHYSIOGRAPHIC FEATURES eustatic change of sea level of 300 feet during the last Pleistocene glacial period (Richards Continental Margin and Hill, 1942). This would indicate a post- The continental shelf between the Tasman Pleistocene subsidence for the continental Sea and the Australian continent ranges in shelf of about 200 feet. width from 40 miles in the north near Mary- No well-defined submarine canyon has been borough to 8 miles in the south near Jervis Bay found off the coast of Australia, probably be- (Fig. 1). This is noticeably less than the 200 cause of a lack of soundings rather than a lack miles of continental shelf near the southern end of canyons. Most of the coast of southeastern of the Great Barrier Reef. The average inclina- Australia has been surveyed by echo-sounding tion of the continental shelf along the Tasman methods to a depth of 100 fathoms, but these Sea is about 0°08', and the abrupt change in soundings were made along widely spaced slope, the shelf break, occurs at about 55 traverses at right angles to the coast, and small fathoms. These figures are similar to those canyons may have been missed. If such canyons recorded by Shepard (1948a) and Dietz and are found they will probably be the offshore Menard (1951) for other areas. The average type (Kuenen, 1950) located along the conti- continental slope of the Australian continent nental slope at a depth of more than 100 fath- along the Tasman Sea is about 6°, but further oms. Indications of possible canyons were found soundings will probably reveal steeper slopes off the mouth of the Hawkesbury River, off and scarp lines. As noted by Shepard (1959) Smoky Cape, and 12 miles northeast of Coffs the continental slope off southeastern Australia Harbour. The Tectonic Map of Australia, pub- may be classed as complex; in many places the lished by the Australian Bureau of Mineral lower portion of the slope is much steeper than Resources, shows two submarine canyons off the upper. The steepness of the lower slope is the coast: one is off the eastern end of Bass 15° in many places. This type of slope, typical Strait, the narrow body of shallow water of those leading into deep trenches, is also found (Jennings, 1959) separating the mainland from off the coast oi Japan and south of the Aleutian the island of Tasmania, and the other is off the Islands. mouth of the Swan River in Western Australia. A wider continental shelf is found off the However, these submarine canyons, like those mouth of the Hawkesburv and Hunter rivers, of Sprigg (1947) off South Australia and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/12/1777/3442070/i0016-7606-72-12-1777.pdf by guest on 28 September 2021 s.w.