Structure of the Western Continental Margin, New Zealand, and Challenger Plateau, Eastern Tasman Sea

JAMES E. ANDREWS Department of Oceanography, and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822 JAMES V. EADE New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, P.O. Box 8009, Wellington, New Zealand

ABSTRACT Lord Howe Rise separates the Tasman Basin from the New Caledonia Basin and a Egmont Terrace west of Cook Strait has basin and ridge province to the northeast. been built, principally during times of lowered Maximum depths along the top of the rise sea level, as a prograding continental shelf onto are between 500 and 1,500 m, with the shal- the Challenger Plateau at the southern end of lowest part of the rise (except for Lord Howe Lord Howe Rise. The plateau subsided below Island) at its southeast extremity, the Chal- sea level during early Eocene time, an event lenger Plateau. Most of the surface of the accompanied by slow upbuilding of the plateau. plateau is <800 m deep, and has a slightly Major outbuilding appeared only in early to convex surface rising to a minimum recorded middle Miocene time, following a rerouting of depth of 476 m. Bellona Gap, a major saddle major surface currents from the area. with a minimum depth of ,500 m, separates Challenger Plateau from the rest of the rise. INTRODUCTION Isolated pinnacles occur along the top of During September 1971, the R/V Kana Lord Howe Rise; several have been recorded Keoki of the University of Hawaii conducted from Challenger Plateau (Brodie, 1952; Houtz geophysical surveys for Deep Sea Drilling and others, 1967; van der Linden, 1973). Such Project (DSDP) sites along a track between pinnacles were crossed by Kana Keoki and Wellington, New Zealand, and Suva, Fiji. Glomar Challenger tracks; Figure 3 shows a Later, during drilling operations at these sites, typical example. Scour depressions around the a call at Wellington by the D/V Glomar bases of these features are common. Challenger resulted in lines roughly parallel to the Kana Keoki tracks (Fig. 1). Seismic SEDIMENTS profiles taken along these tracks are presented Sediments of the western shelf, central New in Figure 2. Zealand, have been described by McDougall and Brodie (1967), van der Linden (1969), BATHYMETRY and Norris (1973). The western continental shelf break is at Sediments of the inner shelf north of Cape ~175 m along most of New Zealand (Fig. 1), Egmont anfl off Cape Farewell are predomi- but west of Cook Strait is less distinct—at 300 nantly medium and fine sands of the present to 350 m. Here the shelf is at its widest (> 160 sedimentary cycle. Fine sand from the northern km), and the outer shelf slopes gently from end of South Island is being swept northeast <100 m down to Egmont Terrace which lies across Cook Strait by the D'Urville Current at an average depth of 250 m and is ~75 km and forms a belt between Capes Egmont and wide at its widest point. West of the terrace Farewell (McDougall and Brodie, 1967). the sea floor slopes down to the southeast end Sediments of the outer shelf off western New of Lord Howe Rise. Zealand are largely relict of times when sea

Geological Society of America Bulletin, v. 84, p. 3093-3100, 3 figs., September 1973 3093

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level was much lower than at present (Mc- most part of piercement structures that pene- Dougall and Brodie, 1967)—there is a large trate the sedimentary section (Fig. 3). Reflec- zone of relict mud in the central part of the tors in older sediments are deflected upward shelf. North of Cape Egmont and off Cape around the structure, whereas reflectors in Farewell sediment is predominantly coarser younger sediments are deflected downward and sand than that on the inner shelf. The coarser parallel the scour depressions around the base sand lies at the shelf break and is linked by a of pinnacles. The change in deflection of narrow belt of sand that lies at the shelf break reflectors appears to occur at the depth of at the outer edge of Egmont Terrace. Shell reflector U (Fig. 2). beds and gravel layers lie immediately below the sand of the outer shelf. Cores from the CORRELATION AND AGE layers and dating of the shell beds indicate OF REFLECTORS these layers were deposited between 20,000 and 10,000 yr ago, mainly in nearshore and At Deep Sea Drilling Project site 207, middle-shelf water. reflector M corresponds best to the top of a Beyond the shelf break there is a rapid thin layer of disturbed sediment of middle transition to muddy sediment and, in even Miocene age. It is probably the sediment deeper water, to calcareous nannoplankton below this reflector that crop out on Lord ooze. Most of the Challenger Plateau is cov- Howe Rise farther north (<~28° S.); they have ered with foraminiferal ooze. been dated from a dredged sample as middle to late Miocene age (Menard, 1969). The re- STRUCTURE flector can be tentatively traced across to the Seismic profiles reveal two distinct structural plateau at the level of reflector A. Reflector U regions, Egmont Terrace and Challenger Pla- is correlated with the mid-Eocene to mid- teau. Egmont Terrace is typical of a prograding Miocene unconformity, and cannot be traced continental margin (Uchupi and Emery, 1965); across the plateau to Egmont Terrace. its up- and outbuilt structure is similar to The upper Eocene to lower Miocene marine continental margins in many other regions sequence of northwest Nelson, New Zealand, (Lewis, 1973). Growth has been primarily by consists largely of calcareous beds and limestone outbuilding. Upbuilding is maximal on the (Grindley, 1961). This sequence thins to the slope at each stage of outbuilding with minimal west on land and probably continues as a upbuilding on the shelf or deeper plateau. regular wedge on to the Challenger Plateau. Compressional structures within the prograd- The late Eocene to early Miocene time interval ing wedge are prominent on profile GC 1 in is probably represented on all three profiles by the region of the present shelf break and slope the thin wedge of sediment below reflector B. along with small-scale drag folds. They are also Major outbuilding probably began with in- present on the other two profiles, although less creased sediment supply resulting from early prominent. Miocene tectonism which increased the relief The sediment thickness of the terrace of adjacent land areas. Therefore, the age of exceeds 2 km, at 2.0 km per sec estimated the base of outbuilding (reflector B) is most velocity for the section. Reflector B is near likely early to middle Miocene. Since the the base of the up- and outbuilt terrace and is piercement structures of the outer plateau seen clearly only beneath its outer edge. Be- and Lord Howe Rise intrude sediments below tween the up- and outbuilt beds and basement reflector U and have been buried by sediments rocks there is a thin wedge of sediment (0.4 above reflector U, they represent middle sec) which pinches out toward the plateau. Eocene to middle Miocene volcanism. Activity Challenger Plateau is marked by a wedge of during this period in New Zealand was greatest sediment which thins over the high central during early and middle Miocene times (Well- part of the plateau. Sediment overlying base- man, 1956), and the piercement structures ment rocks thickens away from the top of the were most likely intruded at that time. An plateau. Maximum sediment thickness occurs early and middle Miocene age is also supported at the margins of the plateau and in down- by an increased input of volcanic ash and faulted basement depressions near Bellona Gap debris at the same time in sediments recovered (Fig- 1). from the South Fiji Basin (DSDP site 205) Pinnacles on Lord Howe Rise are the upper- (Burns and others, 1972).

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GC 2

0 KM 100

Figure 2. Line drawings of seismic reflection pro- profile compressed so that vertical and horizontal files along tracks GC 1, GC 2, and KK 1. Kana Keoki scales are same for all profiles.

Figure 3. Intrusive structure on crest of Lord Howe at north end of profile GC 1. Rise north of DSDP site 207. Profile is east-west leg from outer neritic to upper bathyal depths DISCUSSION began in Maestrichtian time, and had con- Basement structures along KK 1 suggest cluded by late Eocene time (Kennett and major early graben development and possibly others, 1972); east of the plateau the marine minor extension along the rise-plateau bound- transgression in northwest Nelson began in late ary. Sediment fill in these structures and com- Eocene time (Grindley, 1961). Thus the pla- parison to the section on the rise (DSDP site teau sediments probably began accumulating 207) (Burns and others, 1972) suggest this at some point between these two times. Graben activity was pre-Miocene, most probably development near Bellona Gap is probably Eocene, age. contemporaneous with initiation of sedimenta- A comparison of the relatively central KK 1 tion. Minimal accumulation during early stages profile with GC 2, ~12 km to the south, and possibly reflects the effect of surface currents GC 1, 96 km to the north, indicates that sedi- through the region as postulated by Kennett ment accumulation is toward the plateau (in Andrews and others, 1973) and low relief margins despite increasing slopes and transport of contributing land areas (Grindley, 1961) of sediment into adjacent basins. Presumably, during Eocene-Oligocene time. Growth during winnowing by currents limits deposition of the this period was as a well-defined wedge across shallower portions of the plateau (depths near the length of the plateau. 500 m). The absence of evidence of mass With major rerouting of surface currents be- movements on the central line limits the possi- tween Australia and Antarctica during late bility of postdepositional removal of sediment. Oligocene time, rates of sediment accumulation The pattern presented in the profiles sug- slowly increased. Early Miocene tectonism in gests slow general upbuilding on an irregular, the region resulted in an increasing relief of faulted basement surface. Based on extrapola- adjacent land areas and a more rapid influx of tion from DSDP site 207, this probably began sediment. The intrusive structures of Chal- in early Eocene time with a marine transgres- lenger Plateau, Bellona Gap, and the rest of sion of the plateau. Subsidence at site 207 Lord Howe Rise appear to match this time

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frame and support the age of the period of 2. Simultaneously, basement faults were ac- increasing sedimentation. The middle Miocene tive, and graben development, showing differ- was the most active period with activity taper- ential subsidence and possibly minor extension, ing off by the end of the Miocene (van der formed the Bellona Gap. Lingen and others, 1973). 3. Following the marin: transgression, up- The major wedge of prograding sediments building began on the sea floor in the region of forming Egmont Terrace began to develop the present continental margin and the Chal- during low sea-level stands of late Miocene lenger Plateau. This upbuilding consisted of glacial periods (Kennett and others, 1972). a thin wedge of sediment extending to nearly Until that time the sea floor sloped gently 500 km offshore on a gently and evenly sloping toward Lord Howe Rise. The formation of the sea floor. prograding wedge of sediments marked the 4. Upbuilding was accelerated and outbuild- beginning of the formation of the continental ing began during early to middle Miocene shelf, shelf break, and continental slope in this time, following a rerouting of major surface cur- region as they are known today. rents from the area, increased elevation of Sediment patterns on the western shelf are adjacent land areas, and increasing volcanic largely controlled by coastal currents, especially activity. the D'Urville Current (Brodie, 1960) which 5. The up- and outbuilding was accompa- carries sediment from off the west coast of nied by basement subsidence beneath the de- South Island into Cook Strait. Currently, very veloping Egmont Terrace, and this may ac- little sediment is being deposited on the outer count for the present convex shape of Chal- shelf; similar conditions would have existed lenger Plater.u. Earlier subsidence shown by during late Tertiary and Pleistocene inter- drilling work on Lord Howe Rise was to the glacial periods. The relict coarse sediments of north, and it was followed by development of the shelf-break region were deposited during the structures near Bellona Gap. the last glaciation. At that time sea level was low enough to close Cook Strait between ACKNOWLEDGMENTS Egmont and Cape Farewell where depths were We thank the Deep Sea E'rilling Project for <100 m (Fig. 1). With the strait closed, the the D/V Glomar Challenger profiles. This work D'Urville Current would have carried sediment was supported in part by National Science north along the coast to Egmont Terrace. Here, Foundation Grant GA-28341 and by Office either convergence with the southward-moving of Naval Research Contract N00014-70-A- West Auckland Current or the widening of the 0016-0001. shelf would have caused the currents to slacken and deposit their load. Thus the Egmont Ter- REFERENCES CITED race has been formed by up- and outbuilding Andrews, J. E., Burns, R. E., Churkin, M., Jr., of sediment during the glacial periods. Davies, T. A., Dumitrica, P., Edwards, A. Internal compressive structures in the sedi- R., Galehouse, J. S., Kennett, J. P., Packham, ment section suggest that subsidence near the G. H., and van der Lingen, G. J., 1973, Deep region of maximum upbuilding (the prograding Sea Drilling Project: Leg 21, Tasman Sea- Coral Sea Preliminary Results, UNESCO surface) has continued to the present. Maxi- symposium on the oceanography of the South mum distortion is apparent on profile GC 1 Pacific: Wellington, New Zealand, 1973 (in along the northern margin of the plateau press). where sediment thickness is the greatest re- Bro.lie, J. W., 1952, Features of the seafloor west corded in these profiles. Here slump masses cf New Zealand: New Zealand Jour. Sci., are along the slope, and subsidence features v. B33, p. 373-384. are well developed in the section (see middle .960, Coastal surface currents around New section, Fig. 2). Distortion on the Miocene Zealand: New Zealand Jour. Geology and level (reflector A) is clearly present. Geophysics, v. 3, no. 2, p. 235-253. Burns, R. E., Andrews, J. E., Churkin, M., Jr., CONCLUSIONS Davies, T. A., Dumitrica, P., Edwards, A. R., Galehouse, J. S., Kennett, J. P., Packham, 1. Subsidence along the southern end of G. H., and van der Lingen, G. J., 1972, Lord Howe Rise and a consequent marine Glomar Challenger down under: Geotimes, west to east transgression occurred during v. 17, no. 5, p. 14-16. early to middle Eocene time. Grindiey, G. W., 1961, Sheet B, Golden Bay

(1st ed.), Geological map of New Zealand: the continental margin off the Atlantic coast Dept. Sei., Industrial Research, Wellington, of the United States: Am. Assoc. Petroleum New Zealand, scale 1:250,000. Geologists Bull., v. 51, p. 223-234. Houtz, R., Ewing, J., Ewing, M., Lonardi, A. G., van der Linden, W.J.M., 1969, Offshore sediments, 1967, Seismic reflection profiles of the New northwest Nelson, South Island, New Zealand: Zealand plateau: Jour. Geophys. Research, New Zealand Jour. Geology and Geophysics, v. 72, no. 18, p. 4713-4729. v. 12, no. 1, p. 87-103. Kennett, J. P., and others, 1972, Australian- 1973, Structure of the seafloor west of New Antarctic continental drift, paleocirculation Zealand: Jour. Geophys. Research (in press). changes and regional Oligocene unconformi- van der Lingen, G., Burns, R. E., Andrews, J. E., ties: Nature (Phys. Sei.), v. 239, no. 91, p. Churkin, M., Jr., Davies, T. A., Dumitrica, 51-55. P., Edwards, A. R., Galehouse, J. S., Kennett, Lewis, K. B., 1973, The continental terrace: Earth- J. P., and Packham, G. H., 1973, Lithostra- Sci. Rev. (in press). tigraphy of DSDP Leg 21, UNESCO sym- McDougall, J. C., and Brodie, J. W., 1967, Sedi- posium on the oceanography of the South ments of the western shelf, North Island, Pacific: Wellington, New Zealand, 1973 (in New Zealand: New Zealand Jour. Sei., no. press). 179, 56 p. Wellman, H. W., 1956, Structural outline of New Menard, H. W., 1969, Anatomy of an expedition: Zealand: New Zealand Jour. Sei., no. 21, 36 p. New York, McGraw-Hill Book Co., 255 p. Norris, R. M., 1973, Shell and gravel layers, MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY western continental shelf, New Zealand: New 5, 1973 Zealand Jour. Geology and Geophysics (in REVISED MANUSCRIPT RECEIVED MARCH 16, 1973 press). CONTRIBUTION NO. 539, HAWAII INSTITUTE OF Uchupi, E., and Emery, K. O., 1965, Structure of GEOPHYSICS, UNIVERSITY OF HAWAII

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