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Carbonate sedimentation on subtropical shelves around Lord Howe Island and Balls Pyramid, Southwest Pacific
Marine Geology 188 (2002) 333^349 www.elsevier.com/locate/margeo
Carbonate sedimentation on subtropical shelves around Lord Howe Island and Balls Pyramid, Southwest Paci¢c D.M. Kennedy a; , C.D. Woodro¡e a , B.G. Jones a , M.E. Dickson a , C.V.G. Phipps b a
School of Geosciences, University of Wollongong, Wollongong, NSW 2522, Australia b School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia
Received 4 July 2001; received in revised form 11 April 2002; accepted 23 May 2002
Abstract Lord Howe Island and Balls Pyramid are mid-oceanic basaltic islands, 20 km apart, in the Tasman Sea. Subaerial carbonates dating back at least 350 ka and a Holocene fringing reef occur on Lord Howe Island. No reefs or subaerial carbonate deposits occur on Balls Pyramid. Both islands sit near the centre of wide shelves that are on average 40^50 m deep with a distinct break of slope, between 70 and 100 m depth. The Lord Howe shelf is characterised by a discontinuous drowned ridge, which rises to 30 m depth and is located between the mid-shelf and shelf edge. It is composed of limestone and is interpreted as a fossil reef being veneered by Holocene coralline algae. Early to Middle Holocene branching-coral gravel is found in the lee of the fossil reef, indicating limited give-up reef growth during the Postglacial transgression. The surface sediments across the shelf are calcareous, except in close proximity to the island where volcanic content is significant. Coralline algae represent the dominant grain type, with minor amounts of coral. Bryozoans, Halimeda and foraminifera are common; however, they are not volumetrically important. Rhodoliths and molluscs occur near the shelf edge, and appear to have accumulated at times of lower sea level. These subtropical shelves are mid-oceanic examples of the transition between tropical and temperate carbonate sedimentation. They indicate the potential for carbonate production on broad planated shelves outside reef-forming seas. : 2002 Elsevier Science B.V. All rights reserved. Keywords: carbonate sediments; corals; rhodoliths; subtropical; planated shelf; Lord Howe Island; Tasman Sea
1. Introduction Extensive carbonate depositional environments occur within the tropics, characterised by chloro* Corresponding author. Present address: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand. E-mail address: [email protected] (D.M. Kennedy).
zoan assemblages in which coral and coralline algae are prominent (Stoddart, 1969; Milliman, 1974; Guilcher, 1988). Temperate carbonate depositional environments are dominated by foramol assemblages in which bryozoans, bivalves, red algae, benthic foraminifera and barnacles are characteristic components (Nelson, 1988). Within the subtropics a transition zone occurs between the coral-dominated associations of the warm
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waters and the cool-water foramol associations typical of temperate regions. Few of these subtropical transitional carbonate sedimentary environments have been described in detail. In the northern hemisphere this re£ects the relative scarcity of such regions. For instance, in the northwestern Atlantic Ocean, the warm Gulf Stream maintains a tropical assemblage of biota across 24^30‡N, even extending as far as Bermuda. In the northwestern Paci¢c Ocean the Ryukyu Islands exhibit a transition from inshore coral reefs to an outer shelf and upper slope dominated by coralline algae (Tsuji, 1993). In the southern hemisphere the transition is more clearly marked along several continental shelves. Along the coast of Brazil, there is a transition from tropical chlorozoan (coral and Halimeda) assemblages, through chloralgal (calcareous green algae) and rhodalgal (coralline algae) to temperate molechfor (mollusc, echinoid and foraminifera) associations (Carannante et al., 1988). Both tropical and temperate carbonate assemblages occur around the coast of mainland Australia. The tropical north contains extensive coral reefs (Fairbridge, 1971), while southern Australia is dominated by bioclastic temperate carbonates (James and Kendall, 1992) and calcitic carbonates, dominated by coralline algae, bryozoans, foraminifera and molluscs, occur in southwestern Australia (Collins, 1988), and southern New South Wales (Marshall and Davies, 1978). Recently the transitions on both the western and eastern shelves of Australia have been described. O¡ the continental shelf east of Fraser Island, southern Queensland, a distinct subtropical biota is composed principally of coralline algae, often in the form of rhodoliths (Marshall et al., 1998; Lund et al., 2000). The northern Rottnest shelf in western Australia is characterised by luxuriant stands of seagrasses and macrophytes growing on sediments of coralline algae and large foraminifera with scattered hardgrounds (James et al., 1999). The Lord Howe Island group and the shelves that surround them represent a subtropical transition in open ocean. The island chain is on a northward-migrating plate, and the islands, presently at the southernmost limit of coral reef for-
mation, are gradually moving into reef-forming seas. They have been planated largely outside regions in which coral reef growth is possible, and sitting on the margin of the Lord Howe Rise, appear to have undergone little subsidence. This has resulted in broad shelves on which the relationship between the tropical and temperate assemblages of calcareous organisms can be examined.
2. Regional setting 2.1. Geological characteristics Lord Howe Island and Balls Pyramid are located 600 km east of the Australian mainland on the western margin of the Lord Howe Rise, a rifted continental crustal block (Fig. 1). The islands are part of a seamount chain related to hot-spot volcanism (Hayes and Ringis, 1973; Willcox et al., 1980). Only the basalts of Lord Howe Island have been dated yielding an age of between 7.2 and 6.6 Ma (McDougall et al., 1981). Lord Howe Island contains the southernmost coral reef in the world along its western side (Slater and Phipps, 1977). The reef appears to have been characterised by rapid Middle Holocene growth and lagoonal in¢ll (Kennedy, 1999; Kennedy and Woodro¡e, 2000). Holocene coral reef growth also occurs on the atolls of Elizabeth and Middleton reefs, 200 and 250 km north of Lord Howe Island, respectively (Scho¢eld et al., 1983). Miocene and Quaternary shallow-water carbonates dominate the surface of seamounts to the north of Middleton Reef (Quilty, 1993). Calcarenites veneer the central section of Lord Howe Island below 100-m elevation and have a maximum age of at least 350 ka (Brooke, 1999; Price et al., 2001). No subaerial carbonate deposits occur on Balls Pyramid. 2.2. Oceanographic characteristics The East Australian Current dominates ocean circulation around the Lord Howe group (Boland and Church, 1981). The £ow is concentrated along the Australian continental shelf to 33‡S
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Fig. 1. Bathymetry of the shelves around Lord Howe Island and Balls Pyramid. Contour interval is 10 m. Inset: location of the islands.
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Fig. 2. Location of grab samples, piston core, seismic traverses and bathymetric pro¢les.
where it makes an eastward turn into the Tasman Sea. This eastward £ow forms the boundary between warmer northern waters and the southern temperate Tasman Current (Stanton, 1981; Mar-
tinez, 1994). The latitudinal position of the front varies from 30‡S in winter to 34‡S in summer (Hamon, 1962, 1968; Stanton, 1981). This means that Lord Howe Island and Balls Pyramid are
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subjected to alternating oceanographic conditions, with mean annual sea surface temperature varying between 18 and 23‡C (Veron and Done, 1979). The tidal range at Lord Howe Island is 1.5 m at springs and 0.8 m at neaps. GEOSAT altimeter data collected between 1995 and 1996 indicate that Lord Howe Island experiences mean signi¢cant wave heights of 2.3^2.5 m. This is in accordance with earlier SEASAT data (1978) which indicate average waves of 2.0^2.9 m (Allen, 1984). Following the predominant wind direction, easterly swells dominate in summer whereas southern swells dominate in winter. Storm swells can approach from the north during summer and autumn and from the west during autumn and spring.
3. Methods The bathymetry of the shelves surrounding Lord Howe Island and Balls Pyramid was examined on the basis of a compilation of soundings supplied under license by the Hydrographic O⁄ce of the Royal Australian Navy. The bathymetry was examined by interpolation of a grid using an inverse distance weighting algorithm and a triangulated irregular network across the shelves (using ESRI^Arcview GIS). The surface sediments on the shelves around Lord Howe Island and Balls Pyramid were sampled during cruise 12/98 aboard the CSIRO research oceanographic vessel Franklin in October 1998. A total of 73 sediment samples were collected using a Smith^McIntyre grab sampler (Fig. 2). Sampling sites were chosen to include the major sections of the shelf, however, poor weather restricted ship work especially in obtaining deeper samples and along the Balls Pyramid shelf. A piston core was also collected from the inner shelf. Locations were recorded using GPS. All samples were described brie£y in the ¢eld and sealed for later analysis. No chemical pre-treatment was carried out aboard ship. Continuous seismic re£ection pro¢ling was undertaken using an EG and G sparker 3-element array system (at 500 J) and a GeoAcoustics boomer system with
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ampli¢er and ¢lter (at 200 J) and a 10-element streamer of hydrophones. In the laboratory, sediments were initially washed in freshwater to remove salts and then oven-dried at 50^60‡C. Mud was virtually absent from the samples so wet sieving was unnecessary. The dried samples were sieved to remove the s 5.6-mm fraction with the remaining sample split, using a ri¥e box splitter, into separate fractions for size, component, carbonate and X-ray di¡raction (XRD) analysis. Subsamples of 70^100 g were dry sieved for 10 min at 5.6-mm, 2-mm and 0.063-mm intervals. Component analysis was undertaken by pointcounting grain mount thin sections under a petrographic microscope using a grid of 400 points. Components were identi¢ed using keys in Milliman (1974), Adams et al. (1984) and Sco⁄n (1987). Photographs of each main compositional group present in the slides were used as a reference to maintain identi¢cation consistency. A broad visual compositional estimate of the gravel fraction was made. Mineral composition was determined on powdered samples using a Phillips X-ray di¡ractometer. Each sample was scanned from 20 to 40‡ (2 theta) at 1‡min31 , using Cu^K radiation at 40 kv and 30 mA. Total carbonate content was calculated by acid digestion of approximately 10 g of sample in HCl. Radiocarbon dating was undertaken at the Waikato Radiocarbon Laboratory. Ages have been corrected for the marine reservoir e¡ect by subtracting 450 P 35 yr from the conventional age (Gillespie and Polach, 1979) and are reported in radiocarbon yr BP (Table 1).
4. Results 4.1. Shelf bathymetry Lord Howe Island sits near the centre of a rhomboidal shelf with a width (west^east) of 24 km and a length (north^south) of 36 km. Balls Pyramid lies at the centre of a smaller shelf with a width (west^east) of 15 km and a length (north^
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south) of 22 km. The two shelves are separated by a trough that is on average 600 m deep. Both shelves are relatively £at with slopes of less than 1‡, in contrast to the precipitous slopes of the subaerial volcanic edi¢ces. There are distinct shelf breaks at around 70^100 m depth (Fig. 3). Slopes are steepest near the shelf break, typically 15^20‡ (max 30‡). The surface morphology of the Lord Howe shelf is quite complex, being characterised by a ridge at approximately 30 m depth almost encircling the island, between 1.5 and 8 km from shore, on its western, southern and eastern sides (Fig. 1). This ridge is most pronounced on the western, fringing reef, side of the island where it is 3 km wide. On the eastern side of the shelf the ridge is less continuous, located farther o¡shore from the island and characterised by a series of elongate high patches (Figs. 1 and 4). A trough occurs between the ridge and island, and is deepest (60 m) on the eastern side. The trough shallows on the southern and western sides of the shelf to around 40 m depth. On the southern margin of the shelf the ridge occurs adjacent to the shelf break. No ridge occurs along the northern edge of the shelf which gradually slopes towards the shelf break. A slight depression occurs close to the 350 m contour, however, this does not appear to be related to any ridge feature (Figs. 1 and 4). Based on morphology and surface lithology, as seen in several dives, the ridge is composed of limestone and is believed to be a fossil reef.
Morphology of the Balls Pyramid shelf is less complex. The shelf has a relatively uniform slope extending down to 50 m depth, whereafter the slope gradually increases towards the shelf break. A few isolated undulations of around 10-m relief occur in the central part of the shelf, in addition to the monolithic Balls Pyramid that rises to 550 m height (Fig. 1). 4.2. Seismic pro¢ling, lithology and sediment thickness Unfortunately neither sparker nor boomer seismic pro¢ling enabled di¡erentiation of lithological di¡erences or sediment thickness across the shelves. Selected seismic pro¢les are shown in Fig. 3, indicating a prominent break of slope on the shelf edge. The lack of seismic di¡erentiation between the fossil reef, which further diving con¢rmed to be composed of limestone, and the underlying basalt is attributed to the highly lithi¢ed nature and relatively small thickness of the limestone. This is in contrast to Holocene reef limestones and Pleistocene calcarenites which overlie basalt in the lagoon and which were clearly distinguishable on Uniboom seismic pro¢ling in the lagoon (Kennedy, 1999; Kennedy and Woodro¡e, 2000). The depth of sand on the inner shelf could not be discriminated in seismic pro¢les, presumably because it is not thick enough to be distinguished from the sea-£oor re£ector. One piston core on
Table 1 Radiocarbon dating results of material from the Lord Howe shelf Sample number
Laboratory code
Material
Water depth
PC1-206 LH-G18 LH-G48
Wk 7785 Wk 8995 Wk 8996
LH-G19
Wk 8997
LH-G16
Wk 8998
LH-G39
Wk 9004
Bulk sand Coral branch Large algal rhodolith Branching-coral gravel Disarticulated bivalves Small algal rhodoliths
Environmentally corrected radiocarbon age
(m)
Conventional radiocarbon age (yr BP)
50 31 117
4510 P 90 Modern 7150 P 90
4060 P 95 Modern 6700 P 95
36
8820 P 120
8370 P 125
110
660 P 130
210 P 135
64
3960 P 100
3510 P 105
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Fig. 3. Selected seismic pro¢les of the outer shelf showing a break of slope along the shelf edge.
the eastern side of the island penetrated 2.25 m into the sediment and was sand throughout whereas other attempted piston cores failed to obtain any sample. A radiocarbon date of
4060 P 95 yr was determined on bulk sand at a depth of 2.06 m (Table 1) and indicates accumulation at an average rate of 0.51 mm/yr. This accumulation rate is assumed to be a minimum.
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Fig. 4. Bathymetric pro¢les across the Lord Howe shelf, constructed from gridded bathymetric data. Location of pro¢les is shown in Fig. 2.
4.3. Sediment grain size Sediments recovered from across the two shelves are dominated by sand and gravel. Mudsized material was generally absent, the highest proportion (1.3% of the sample) being recorded in sample G47 at 158 m depth o¡ the Balls Pyramid platform. There is considerable variability in the nature of unconsolidated sediment size across the shelf, whereas most grab samples recovered a reasonable volume of sediment, a few contained little sediment at all, and the material recovered was predominantly gravel. In particular those samples from the surface of the fossil reef contained very coarse and angular
material that appeared to have been cemented to the substrate (samples G15, G66, G84 from the fossil reef and G52, G78 from water depths of 40^ 50 m on the front of the reef). Similarly, those samples recovered from close to the island in water depths of 20^30 m were dominated by pebbles or larger gravel. On the Balls shelf the coarsest samples were found both in the centre around the Pyramid and at the shelf break. The majority of samples are composed predominantly of sand. This is particularly the case in the depression between Lord Howe Island and the fossil reef, where samples were generally exclusively sand and all contained less than 25% gravel. Much of the surface of the Balls shelf also lacked
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Fig. 5. Constituent grain analyses of selected shelf samples.
gravel and is dominated by sand. In the piston core described above the sand size changed little with depth. 4.4. Mineralogy The shelf surface sediments are almost entirely calcareous with only 15 samples (20%) having a non-carbonate component over 0.5%. Volcanic rock fragments are the dominant non-carbonate component comprising a maximum of 9.5% of the sediment (sample G79). Siliceous sponge spicules were also present, but only in trace proportions ( 6 1%) in six samples, with a maximum of 4%
near the southern tip of Lord Howe Island (sample G55). Volcanic minerals occur close to the basaltic islands on both shelves. No gravel clasts of volcanic lithology were recovered, despite the prevalence of boulder beaches of volcanic clasts around the islands. On the Lord Howe shelf volcanic detritus was more common close to the island on the eastern, non-reefal side of the island. Only one sample, opposite a reef passage, contained basaltic minerals on the western side. Similarly, on the Balls Pyramid shelf, volcanic grains were concentrated around the Pyramid. One sample below the shelf break (G47, depth 158 m) contained a trace
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Fig. 6. Photographs of (a) sediments containing rhodoliths from the outer shelf (G39); (b) branching-coral gravel in a sandy matrix collected from just inside the fossil reef (G19); (c) Bathyarea shell beds below 100 m on the shelf slope (G16) and (d) typical sediment from just below the shelf break on Balls Pyramid showing the common occurrence of bryozoan, echinoid and foraminiferal detritus (G47).
of volcanic grains ( 6 0.5%) suggesting a possible outcrop of basalt or the shelf may be swept by strong currents that concentrate denser volcanic grains at this site. Siliceous spicules had a similar distribution to the volcanic minerals with two exceptions. The highest proportion (4.75%, sample G55) occurred on the fossil reef o¡ the southern part of Lord Howe Island, where live sponges were also recovered. Trace amounts were also found in two samples (G16, G47) taken below the shelf break. 4.5. Distribution of grain types The sediments on the shelves consist of the remains of crustose and free-living coralline algae, hermatypic corals, bryozoans, foraminifera, mol-
luscs, echinoids, Halimeda and recrystallised skeletal grains. Grains comprise whole skeletons or fragments, and gravel-sized clasts tend to be heavily encrusted by coralline algae. The relative proportion of component grain types in sediment samples is shown in Figs. 5 and 6. 4.5.1. Coralline algae Coralline algae, both branching and crustose, are common across the shelves with the proportion in the sand-sized fraction varying between 10 and 53%. The highest concentrations ( s 30%) occurred on plateau sections of both shelves and close to the fossil reef on the Lord Howe shelf. The inner shelf tends to contain the lowest proportions of coralline algae ( 6 20%) with samples collected below the shelf break also containing a
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low proportion of sand-sized coralline algal grains. This abundance is greater than that recorded along the continental shelf of eastern Australia (Marshall and Davies, 1978; Marshall et al., 1998). Two types of gravel-sized clasts can be identi¢ed; crustose sheets and rhodoliths. Crustose sheets tend to be composed of multiple layers of coralline algae growing over a coral/algal substrate and were only recovered from the surface of the fossil reef. The coralline algae were often live and growing in association with encrusting bryozoans and foraminifera, macroalgae, and less often Halimeda. Algal rhodoliths adopt three main forms, i.e. small (10^20 mm diameter) and large (up to 100 mm diameter) rounded, and branching. Most common is a well-rounded smooth form (Fig. 6a), with branching forms being the least common. The majority of rhodoliths contained a coral core; although in some cases mollusc material occurred. The smaller well-rounded forms tended to be found on the outer edge of both shelves. Some large accumulations were also recovered just landward of the fossil reef on the Lord Howe shelf (samples G87 and G88), in one case constituting over 50% of the sample. The branching rhodolith type was found across both shelves and tended to be associated with areas of abundant coral gravel where elongate coral clasts commonly form the core. The largest rhodoliths, up to 100 mm diameter, were found on the southern tip of the Lord Howe shelf, in depths of 50^120 m (Figs. 7 and 8). They did not have a live outer surface and tended to be highly bored, with the core being replaced by ¢ne mud, indicating several generations of reworking (see Focke and Gebelein, 1978). The outer surfaces were encrusted by bryozoans with a couple of small (3-mm) corals (dead) also being found. Radiocarbon dates on a large rhodolith and small rhodoliths returned ages of 6700 P 95 and 3510 P 105 yr BP, respectively, at the southern and northern edge of the Lord Howe shelf, respectively (Table 1). 4.5.2. Coral Coral is not a major component of the shelf
Fig. 7. Photographs of sections through rhodoliths from the outer shelf (G48) (scale bar is 10 mm).
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Fig. 8. Underwater photographs of (a) the fossil reef surface in approximately 30 m of water depth, and (b) the inner shelf, Lord Howe shelf, showing rippled biogenic sand.
sediments, constituting generally less than 5% of the sediment, but reaching up to 12%. Corals tend to be most abundant on the inner parts of both shelves. On the Lord Howe shelf the highest proportions occurred around the fossil reef, whereas the highest concentrations on the Balls shelf were close to the pyramid. Coral gravel accounted for the greatest proportion of the coarse sediment in areas where rhodoliths were rare. Only a couple of live hermatypic corals were recovered in the
shallowest grab samples. The coral gravels tended to be highly abraded and sub- to well-rounded, often with a few mm thick coating of coralline algae (Fig. 6b). Signi¢cant concentrations of coral gravel were found immediately in the lee of the fossil reef composed primarily of Acropora species, representing material broken o¡ the reef during the Postglacial marine transgression (8370 P 125 yr BP; Table 1). Occasional live and dead, but mod-
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ern (Table 1), branching-coral colonies (possibly Pocillopora) occur on the shallowest parts of the fossil reef. The coral gravel on Balls Pyramid is composed of branches and well-rounded forms which are highly abraded and encrusted by coralline algae. 4.5.3. Halimeda Halimeda grains are ubiquitous on the Lord Howe and Balls shelves, with only three grab samples (G56, G74, G88) completely lacking grains. This is in contrast to the shallow lagoon where Halimeda is virtually absent (Kennedy, 1999). Sand-size Halimeda grains constituted between 0 and 10.25% of the sand. The highest proportions (8^10%) on the Lord Howe shelf occurred in the middle of the inner shelf (Fig. 5) with the lowest abundance occurring around the fossil reef. This is surprising given that the only live specimens of these algae were dredged from that location. 4.5.4. Molluscs and lithoclasts Mollusc grains were relatively common across the shelves accounting for 3.5^24% of the sandsized fraction. Pecten shells, up to 100 mm in diameter, were found on the outer shelf (Fig. 6a), with some encrusted and abraded specimens being found on the inner shelf. Valves of the boring bivalve Lithophaga occur extensively in areas with hard substrate. These molluscs are probably one of the most important erosive agents with extensive boring found in most larger clasts. The attached species Chama was also recovered in situ from the fossil reef. The largest proportion of mollusc material was found below 100 m depth on the shelf slope where shells such as Bathyarea (Fig. 6c) constituted the majority of the sediment. A radiocarbon age on disarticulated small bivalves from G16 of 210 P 135 yr BP implies contemporary sediment production. Lithoclasts were found across the shelf ranging in abundance from 1 to 20.5%. The majority of clasts were located in the inner shelf area and on top of the fossil reef. The lowest proportions of these grains tended to occur on the shelf edge and slope.
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4.5.5. Indeterminate On parts of the shelf up to 51% of the sediment was unidenti¢able (Fig. 5). These grains were not micritised, were generally transparent in plane polarised light, and tended to be more common in the ¢ner grain size fraction. The morphology and crystal structure of these grains suggest that they are skeletal in origin. The proportion of these grains was greatest on the inner Lord Howe shelf and the fossil reef but do not appear to correlate with the occurrence of any other identi¢able grain fraction. Due to the high proportion of indeterminable grains XRD analysis was conducted to assess the composition of the grains. High-Mg-calcite was the dominant mineral comprising 70% of the carbonate fraction in the majority of samples (range 46^84%). There appears to be no signi¢cant relationship between High-Mg-calcite content and location on the shelf. Aragonite composition is also fairly uniform across the shelf comprising 11^ 37%. The aragonite proportion tends to decrease close to the shelf break where it generally comprises less than 15%. Low-Mg-calcite composition is generally depth-related, ranging between 0 and 35%. On the shelf, proportions are generally low ( 6 20%). Below 100 m, proportions range between 15 and 35%. This may relate to the recrystallisation of relict aragonitic sediments on the shelf edge or, more probably, re£ects the increase in abundance of low-Mg foraminifera. 4.5.6. Other biogenic grains Foraminifera comprise up to 8% of the sediment, with the highest proportions occurring below the shelf break. A variety of species appears to be present; although shallow-water Baculogypsina, common within the lagoon on Lord Howe Island (Kennedy, 1999), was generally absent suggesting there is little sediment transfer out of the modern lagoon. Bryozoans were found in greatest abundance below the shelf break (Fig. 6d), comprising up to 6.25% of the sediment. Echinoid fragments were found across the entire shelf comprising between 0.25 and 1.5% (max. 3.25%). Gravel size spines were found in greatest concentrations close to the modern fringing reef, while live echinoids were sampled o¡ the fossil reef.
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5. Discussion Tropical and temperate shelf carbonate assemblages have been discriminated on the basis of their skeletal composition (Lees and Butler, 1972; Carannante et al., 1988). Hermatypic corals and Halimeda generally dominate tropical sediments while temperate sediments are generally dominated by bryozoans, molluscs and benthic foraminifera. The relative abundance of individual skeletal elements may be site-speci¢c and vary, but the assemblages tend to be characteristic of warm- and cool-water environments, respectively. Recent studies have emphasised the di⁄culty in assigning these classi¢cations in transitional subtropical shelf environments (Marshall et al., 1998). Transitional sediments have a broadly heterozoan assemblage (James, 1997) but with the important addition of coralline algae and large symbiont-bearing foraminifera (James et al., 1999). On the east Australian coastline, Marshall and Davies (1978) determined the transitional boundary to occur around 24‡S based on the occurrence of surface reefs. Marshall et al. (1998), however, have noted that some overlap of these assemblages occurs, with mid-shelf sediments o¡ Fraser Island (25‡S) being dominated by coralline algae associated with low species diversity of hermatypic corals, large benthic foraminifera, bryozoans, Halimeda and molluscs. A fringing reef occurs on the western side of Lord Howe Island (Slater and Phipps, 1977), and luxuriant coral communities occur around all sides of the island (Veron and Done, 1979; Harriott et al., 1995) as well as on Balls Pyramid. Coral grains are, however, poorly represented on the shelf apart from coral gravel concentrated around the fossil reef, which is presently producing little debris. On the Lord Howe Island and Balls Pyramid shelves coralline algae are the dominant sediment component with Halimeda and corals, although widespread, being minor components. Bryozoans are also not major sediment constituents. These islands are, therefore, a mid-oceanic transitional carbonate environment some 10‡ farther south than previously described. Modern sedimentation on the Lord Howe
Group can be divided into three major zones, the inner and outer shelf and shelf edge. On the Lord Howe shelf the inner and outer shelves are separated by the fossil reef which is generally delineated by the 30-m contour, while on the Balls shelf the absence of any reef feature means the inner shelf extends with little di¡erentiation to 50 m depth. The extent of the shelf edge zone is di⁄cult to determine given the sparse sampling below 100 m depth. However, the similarity of samples from both shelves suggests that it is extensive. The inner shelf zone is dominated by sand with gravel composition increasing towards the islands on both shelves as well as towards the fossil reef on the Lord Howe shelf. Biogenic sands appear to be related to the shallow-water benthic communities with some in situ production of bryozoans from isolated communities on the mobile sands. Halimeda concentrations are greatest in this zone which also has the lowest proportions of low-Mgcalcite (Fig. 8). The outer shelf zone occurs from the base of the fossil reef on the Lord Howe shelf and from a depth of 50 m on the Balls shelf. Rhodoliths (10^20 mm diameter) characterise the sediment and appear to be actively accreting, forming large ¢elds close to the shelf break. The exact nature of these ¢elds is hard to determine due to a lack of bottom photography. Local boat captains report surface currents of around 2.5 m/s and, using the threshold velocity for clast movement noted around Fraser Island (Harris et al., 1996), a bottom current of at least 0.55 m/s occurs. This zone also provides a suitable environment for Pecten growth with well-preserved shells being commonly found though not with the same high productivity as described on the outer shelf of the New South Wales coast (Ferland and Roy, 1997). The shelf edge zone tends to be ¢ner grained and is characterised by two types of gravel, i.e. large relict rhodoliths (up to 60 mm diameter) and small molluscs. The molluscs are modern dating at 210 P 35 yr BP. The highest proportion of low-Mg-calcite occurred in this zone related to either relict sediment or foraminifera. This would imply that a mix of relict and modern sediments occur within this zone.
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Sea-level transgression across the shelf during the Holocene has greatly changed the oceanographic environment. At 12 ka the sea surface would have been at the shelf edge, £ooding the entire shelf to its current depth by 6.5 ka. The large rhodoliths along the shelf edge are no longer active and must have been deposited during a lower sea level. The size of these clasts, compared with the currently growing rhodoliths, suggests a more favourable environment for algal growth during the Early Holocene. The deposits of branching corals on the shelf appear to represent a period of greater sedimentation during the Early to Middle Holocene. A radiocarbon age of 8370 P 125 yr BP on branching coral from G19, contrasts with the modern date from the in situ clast in G18. SCUBA diving and grabs from the surface of the fossil reef indicate that these gravels could not be formed under modern conditions due to a sparse coral cover and their age. Thus it appears that an Early Holocene reef became established over the larger, fossil reef and gave-up. It terminated either as a result of temperature change or by drowning being replaced by coralline algal communities. As the top of the give-up reef is within modern established growth limits for corals in the tropics, a reduction in temperature would appear to be the primary factor in causing growth to cease. A similar change is observed within the modern reef, where much more proli¢c growth is indicated in the Middle rather than Late Holocene (Slater and Phipps, 1977; Kennedy and Woodro¡e, 2000). Over the longer term, the prominent fossil reef on the Lord Howe shelf indicates that much more proli¢c coral growth was possible than during the Holocene. The origin and age of the fossil reef is problematic. It occurs at a depth which would have been favourable for reef growth during marine oxygen^isotope stages 1, 3 and 5; however, it seems unlikely that such a large feature could have formed in the Late Quaternary. Reef limestones that have been dated to the Last Interglacial, substage 5e, occur beneath the modern lagoon at a depth of 2^4 m (Kennedy and Woodro¡e, 2000) while isolated coral clasts of similar age occur within a beach facies 2 m above
347
present sea level on the eastern side of the island (Woodro¡e et al., 1995). However, neither the modern reef nor the minor Last Interglacial reef limestones compare with the enormous submerged structure, in places 2^3 km wide, that occurs in 30-m water depth. It would appear that the fossil reef is older than marine oxygen^isotope stage 5e. In order for the reef to form it would have been necessary to planate the volcanic edi¢ce. This presumably started soon after the volcano erupted 6^7 Ma (McDougall et al., 1981), with marine abrasion occurring at a decelerating rate as the shelf widened. Indeed, truncation of cli¡ed shorelines of the islands would appear to be highly unlikely once a reef had been constructed to seaward. The reef might be Late Tertiary in age or, more likely, it may have been formed during a previous interglacial. Evidence for higher sea level, or more proli¢c reef growth in earlier interglacials has recently been described from both the Atlantic and Paci¢c reef provinces (Hearty et al., 1999; Stirling et al., 2001) and may have also occurred around Lord Howe. The continued northward movement of the Australian plate will progressively move these shelves into more tropical seas. Reef growth can be anticipated to become more widespread and tropical sediments will begin to dominate the shelf. The broad shelves that surround Lord Howe Island and Balls Pyramid developed because the volcanic edi¢ces were outside reef-forming seas and could be planated by marine abrasion, in contrast to the reef-encircled islands typical of reef-forming seas (Menard, 1986). Reefs protect the shores of volcanic islands or rim the margins of carbonate platforms, attenuating and dissipating wave energy. Mid-ocean shelves are consequently rare in the tropics. On linear island chains in which individual islands are carried out of reef-forming seas, rapid subsidence leads to drowning (Grigg and Epp, 1989). A transition from broad shelves with sediment suites composed of temperate carbonates into tropical reef sedimentary sequences may have been a feature of island chains in the past when there were more islands entering reef-forming seas (Wilson et al., 1998).
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6. Conclusions The sediments of the Lord Howe Island and Balls Pyramid shelves are neither distinctly tropical nor temperate. This is surprising given the abundant shallow-water reef development around Lord Howe Island. The carbonate sediments of the shelf are characteristic of transitional environments identi¢ed on the Australian mainland coast, but di¡er in that they are located in the open ocean away from any continental in£uences. Ocean temperature, perhaps also re£ecting water depth, would appear to be the main environmental factor determining the broad sediment composition. The depositional environment across these shelves has not been static. Coral and algal rhodolith growth appears to have been more abundant during the Early and Middle Holocene. The presence of these sediments suggests that there is little active deposition on the shelf or that modern carbonate productivity is low. Where a linear island chain is moving into reefforming seas a transition is likely to occur from a broad shelf with a temperate carbonate sediment suite, into a tropical reef framework. The subtropical sediments are the ¢rst stage of carbonate deposition over the underlying basalt. For those volcanic islands that may have been planated outside, but moved into, reef-forming seas, a similar transition may occur with tropical reefs over temperate carbonate sediments which in turn overlie a broad nearly-horizontal basalt surface.
Acknowledgements This study was funded by the Australian Research Council (grant to C.D.W. and B.G.J.) and extended earlier studies undertaken by C.V.G.P. Research was conducted with permission and support from the Lord Howe Island Board and the Lord Howe Island Marine Park Authority. The authors would like to thank the captain and crew of RV Franklin for their assistance during cruise 12/98 in the Tasman Sea. Further bathymetric data were provided under license from the Hydrographic O⁄ce of the Royal Aus-
tralian Navy. Vicki Harriott (James Cook University), David Mitchell (University of Sydney), Stewart Fallon, John Marshall and Eugene Wallensky (Australian National University), and Dean Hiscox (Lord Howe Island Board) provided valuable shipboard assistance. Ben Ackerman, David Carrie and Viki Lee are thanked for their contributions in sample preparation and analysis while thanks are due to Penny Williamson for the photographic plates.
References Adams, A.E., McKenzie, W.S., Guilford, C., 1984. Atlas of Sedimentary Rocks under the Microscope. Longman, Essex. Allen, T.D., 1984. An eye on ocean waves and currents: satellite-borne radars scan the sea surface. Phys. Bull. 35, 239^ 241. Boland, F.M., Church, J.A., 1981. The East Australian Current 1978. Deep Sea Res. 28A, 937^957. Brooke, B.P., 1999. Quaternary Stratigraphy and Evolution of Aeolianite on Lord Howe Island. Ph.D. Thesis. University of Wollongong, Wollongong, 530 pp. Carannante, G., Esteban, M., Milliman, J.D., Simone, L., 1988. Carbonate lithofacies as paleolatitude indicators: problems and limitations. Sediment. Geol. 60, 333^346. Collins, L.B., 1988. Sediments and history of the Rottnest Shelf, southwest Australia: a swell-dominated, non-tropical carbonate margin. Sediment. Geol. 60, 15^49. Fairbridge, R.W., 1971. Coral reefs of the Australian region. In: Jennings, J.N., Mabbutt, J.A. (Eds.), Landform Studies from Australia and New Guinea. ANU Press, Canberra, pp. 386^417. Ferland, M.A., Roy, P.S., 1997. Southeastern Australia: a sealevel dependent, cool-water carbonate margin. In: James, N.P., Clarke, J.D.A. (Eds.), Cool-Water Carbonates. SEPM Special Publication 56, pp. 37^52. Focke, J.W., Gebelein, C.D., 1978. Marine lithi¢cation of reef rock and rhodolites at a fore-reef slope locality (350 m) o¡ Bermuda. Geol. Mijnb. 57, 163^171. Gillespie, R., Polach, H.A., 1979. The suitability of marine shells for radiocarbon dating of Australian prehistory. In: Berger, R., Suess, H. (Eds.), Proceedings of the 9th International Conference on Radiocarbon. University of California Press, Berkeley, CA, pp. 404^421. Grigg, R.W., Epp, D., 1989. Critical depth for the survival of coral islands: e¡ects on Hawaiian Archipelago. Science 243, 638^641. Guilcher, A., 1988. Coral Reef Geomorphology. Wiley, Chichester, 228 pp. Hamon, B.V., 1962. The spectrums of mean sea level at Sydney, Co¡s Harbour, and Lord Howe Island. J. Geophys. Res. 67, 5147^5155.
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D.M. Kennedy et al. / Marine Geology 188 (2002) 333^349 Hamon, B.V., 1968. Spectrum of sea level at Lord Howe Island in relation to circulation. J. Geophys. Res. 73, 6925^ 6927. Harriott, V.J., Harrison, P.L., Banks, S.A., 1995. The coral communities of Lord Howe Island. Mar. Freshw. Res. 46, 457^465. Harris, P.T., Tsuji, Y., Marshall, J.F., Davies, P.J., Honda, N., Matsuda, H., 1996. Sand and rhodolith-gravel entrainment on the mid- to outer-shelf under a western boundary current: Fraser Island continental shelf, eastern Australia. Mar. Geol. 129, 313^330. Hayes, D.E., Ringis, J., 1973. Sea£oor spreading in the Tasman Sea. Nature 243, 454^458. Hearty, P.J., Kindler, P., Cheng, H., Edwards, R.L., 1999. A +20m middle Pleistocene sea-level highstand (Bermuda and the Bahamas) due to partial collapse of Antartic ice. Geology 27, 375^378. James, N.P., 1997. The cool-water carbonate depositional realm. In: James, N.P., Clarke, J.A.D. (Eds.), Cool-Water Carbonates. SEPM Special Publication 56, pp. 1^20. James, N.P., Collins, L.B., Bone, Y., Hallock, P., 1999. Subtropical carbonates in a temperate realm: modern sediments on the southwest Australian shelf. J. Sediment. Res. 69, 1297^1321. James, N.P., Kendall, A.C., 1992. Introduction to carbonate and evaporite facies models. In: Walker, R.G., James, N.P. (Eds.), Facies models: response to sea level change. Geological Association of Canada, Toronto, pp. 265^275. Kennedy, D.M., 1999. Reef growth and lagoonal sedimentation at high latitudes, Lord Howe Island, Australia. PhD thesis, University of Wollongong, Wollongong, 252 pp. Kennedy, D.M., Woodro¡e, C.D., 2000. Holocene lagoonal sedimentation at the latitudinal limits of reef growth, Lord Howe Island, Tasman Sea. Marine Geology 169, 287^304. Lees, A., Butler, A.T., 1972. Modern temperate-water and warm-water shelf carbonate sediments contrasted. Marine Geology 13, M67^M73. Lund, M., Davies, P.J., Braga, J.C., 2000. Coralline algal nodules o¡ Fraser Island, eastern Australia. Facies 42, 25^34. Marshall, J.F., Davies, P.J., 1978. Skeletal carbonate variation on the continental shelf of eastern Australia. BMR Journal of Australian Geology and Geophysics 3, 85^92. Marshall, J.F., Tsuji, Y., Matsuda, H., Davies, P.J., Iryu, Y., Honda, N., Satoh, Y., 1998. Quaternary and Tertiary subtropical carbonate platform development on the continental margin of southern Queensland, Australia. Int. Assoc. Sedimentol. Spec. Publ. 25, 163^195. Martinez, J.I., 1994. Late Pleistocene palaeoceanography of the Tasman Sea: implications for the dynamics of the warm pool in the western Paci¢c. Palaeogeogr., Palaeoclim., Palaeoecol. 112, 19^62. McDougall, I., Embleton, B.J.J., Stone, D.B., 1981. Origin and
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evolution of Lord Howe Island, southwest Paci¢c Ocean. J. Geol. Soc. Aust. 28, 155^176. Menard, H.W., 1986. Islands. Scienti¢c American Library, New York, 230 pp. Milliman, J.D., 1974. Marine Carbonates. Springer-Verlag, Berlin, 375 pp. Nelson, C.S., 1988. An introductory perspective on non-tropical shelf carbonates. Sediment. Geol. 60, 3^12. Price, D.M., Brooke, B.P., Woodro¡e, C.D., 2001. Thermoluminescence dating of aeolianites from Lord Howe Island and south-west Western Australia. Quat. Sci. Rev. 20, 841^846. Quilty, P.G., 1993. Tasmantid and Lord Howe seamounts: biostratigraphy and palaeoceanographic signi¢cance. Alcheringa 17, 27^53. Scho¢eld, D.W., Slater, R.A., Phipps, C.V.G., 1983. Holocene erosion of Elizabeth Reef, Tasman Sea, Australia. In: Harris, P.M. (Ed.), Carbonate buildups: a core workshop. SEPM, Dallas, pp. 558^577. Sco⁄n, T.P., 1987. An Introduction to Carbonate Sediments and Rocks. Chapman and Hall, New York, 274 pp. Slater, R.A., Phipps, C.V.G., 1977. A preliminary report on the coral reefs of Lord Howe Island and Elizabeth Reef, Australia. Proceedings of the 3rd International Coral Reef Symposium 2, pp. 313^318. Stanton, B.R., 1981. An oceanographic survey of the Tasman Front. N.Z. J. Mar. Freshw. Res. 15, 289^297. Stirling, C.H., Esat, T.M., Lambeck, K., McCulloch, M.T., Blake, S.G., Lee, D.-C., Halliday, A.N., 2001. Orbital forcing of the marine isotope stage 9 interglacial. Science 291, 290^293. Stoddart, D.R., 1969. Ecology and morphology of recent coral reefs. Ecol. Rev. 44, 433^498. Tsuji, Y., 1993. Tide in£uenced high energy environments and rhodolith-associated carbonate deposition on the outer shelf and slope o¡ the Miyako Islands, southern Ryukyu Island arc, Japan. Mar. Geol. 113, 255^271. Veron, J.E.N., Done, T.J., 1979. Corals and coral communities of Lord Howe Island. Aust. J. Mar. Freshw. Res. 30, 203^ 236. Willcox, J.B., Symonds, P.A., Hinz, K., Bennett, D., 1980. Lord Howe rise, Tasman Sea: preliminary geophysical results and petroleum prospects. BMR J. Aust. Geol. Geophys. 5, 225^236. Wilson, P.A., Jenkyns, H.C., Elder¢eld, H., Larson, R.L., 1998. The paradox of drowned carbonate platforms and the origin of Cretaceous Paci¢c guyots. Nature 392, 889^ 894. Woodro¡e, C.D., Murray-Wallace, C.V., Bryant, E.A., Brooke, B., Heijnis, H., Price, D., 1995. Late Quaternary sea-level highstands in the Tasman Sea: evidence from Lord Howe Island. Mar. Geol. 125, 61^72.
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Carbonate sedimentation on subtropical shelves around Lord Howe Island and Balls Pyramid, Southwest Paci¢c D.M. Kennedy a; , C.D. Woodro¡e a , B.G. Jones a , M.E. Dickson a , C.V.G. Phipps b a
School of Geosciences, University of Wollongong, Wollongong, NSW 2522, Australia b School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia
Received 4 July 2001; received in revised form 11 April 2002; accepted 23 May 2002
Abstract Lord Howe Island and Balls Pyramid are mid-oceanic basaltic islands, 20 km apart, in the Tasman Sea. Subaerial carbonates dating back at least 350 ka and a Holocene fringing reef occur on Lord Howe Island. No reefs or subaerial carbonate deposits occur on Balls Pyramid. Both islands sit near the centre of wide shelves that are on average 40^50 m deep with a distinct break of slope, between 70 and 100 m depth. The Lord Howe shelf is characterised by a discontinuous drowned ridge, which rises to 30 m depth and is located between the mid-shelf and shelf edge. It is composed of limestone and is interpreted as a fossil reef being veneered by Holocene coralline algae. Early to Middle Holocene branching-coral gravel is found in the lee of the fossil reef, indicating limited give-up reef growth during the Postglacial transgression. The surface sediments across the shelf are calcareous, except in close proximity to the island where volcanic content is significant. Coralline algae represent the dominant grain type, with minor amounts of coral. Bryozoans, Halimeda and foraminifera are common; however, they are not volumetrically important. Rhodoliths and molluscs occur near the shelf edge, and appear to have accumulated at times of lower sea level. These subtropical shelves are mid-oceanic examples of the transition between tropical and temperate carbonate sedimentation. They indicate the potential for carbonate production on broad planated shelves outside reef-forming seas. : 2002 Elsevier Science B.V. All rights reserved. Keywords: carbonate sediments; corals; rhodoliths; subtropical; planated shelf; Lord Howe Island; Tasman Sea
1. Introduction Extensive carbonate depositional environments occur within the tropics, characterised by chloro* Corresponding author. Present address: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand. E-mail address: [email protected] (D.M. Kennedy).
zoan assemblages in which coral and coralline algae are prominent (Stoddart, 1969; Milliman, 1974; Guilcher, 1988). Temperate carbonate depositional environments are dominated by foramol assemblages in which bryozoans, bivalves, red algae, benthic foraminifera and barnacles are characteristic components (Nelson, 1988). Within the subtropics a transition zone occurs between the coral-dominated associations of the warm
0025-3227 / 02 / $ ^ see front matter : 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 4 0 6 - 1
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waters and the cool-water foramol associations typical of temperate regions. Few of these subtropical transitional carbonate sedimentary environments have been described in detail. In the northern hemisphere this re£ects the relative scarcity of such regions. For instance, in the northwestern Atlantic Ocean, the warm Gulf Stream maintains a tropical assemblage of biota across 24^30‡N, even extending as far as Bermuda. In the northwestern Paci¢c Ocean the Ryukyu Islands exhibit a transition from inshore coral reefs to an outer shelf and upper slope dominated by coralline algae (Tsuji, 1993). In the southern hemisphere the transition is more clearly marked along several continental shelves. Along the coast of Brazil, there is a transition from tropical chlorozoan (coral and Halimeda) assemblages, through chloralgal (calcareous green algae) and rhodalgal (coralline algae) to temperate molechfor (mollusc, echinoid and foraminifera) associations (Carannante et al., 1988). Both tropical and temperate carbonate assemblages occur around the coast of mainland Australia. The tropical north contains extensive coral reefs (Fairbridge, 1971), while southern Australia is dominated by bioclastic temperate carbonates (James and Kendall, 1992) and calcitic carbonates, dominated by coralline algae, bryozoans, foraminifera and molluscs, occur in southwestern Australia (Collins, 1988), and southern New South Wales (Marshall and Davies, 1978). Recently the transitions on both the western and eastern shelves of Australia have been described. O¡ the continental shelf east of Fraser Island, southern Queensland, a distinct subtropical biota is composed principally of coralline algae, often in the form of rhodoliths (Marshall et al., 1998; Lund et al., 2000). The northern Rottnest shelf in western Australia is characterised by luxuriant stands of seagrasses and macrophytes growing on sediments of coralline algae and large foraminifera with scattered hardgrounds (James et al., 1999). The Lord Howe Island group and the shelves that surround them represent a subtropical transition in open ocean. The island chain is on a northward-migrating plate, and the islands, presently at the southernmost limit of coral reef for-
mation, are gradually moving into reef-forming seas. They have been planated largely outside regions in which coral reef growth is possible, and sitting on the margin of the Lord Howe Rise, appear to have undergone little subsidence. This has resulted in broad shelves on which the relationship between the tropical and temperate assemblages of calcareous organisms can be examined.
2. Regional setting 2.1. Geological characteristics Lord Howe Island and Balls Pyramid are located 600 km east of the Australian mainland on the western margin of the Lord Howe Rise, a rifted continental crustal block (Fig. 1). The islands are part of a seamount chain related to hot-spot volcanism (Hayes and Ringis, 1973; Willcox et al., 1980). Only the basalts of Lord Howe Island have been dated yielding an age of between 7.2 and 6.6 Ma (McDougall et al., 1981). Lord Howe Island contains the southernmost coral reef in the world along its western side (Slater and Phipps, 1977). The reef appears to have been characterised by rapid Middle Holocene growth and lagoonal in¢ll (Kennedy, 1999; Kennedy and Woodro¡e, 2000). Holocene coral reef growth also occurs on the atolls of Elizabeth and Middleton reefs, 200 and 250 km north of Lord Howe Island, respectively (Scho¢eld et al., 1983). Miocene and Quaternary shallow-water carbonates dominate the surface of seamounts to the north of Middleton Reef (Quilty, 1993). Calcarenites veneer the central section of Lord Howe Island below 100-m elevation and have a maximum age of at least 350 ka (Brooke, 1999; Price et al., 2001). No subaerial carbonate deposits occur on Balls Pyramid. 2.2. Oceanographic characteristics The East Australian Current dominates ocean circulation around the Lord Howe group (Boland and Church, 1981). The £ow is concentrated along the Australian continental shelf to 33‡S
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335
Fig. 1. Bathymetry of the shelves around Lord Howe Island and Balls Pyramid. Contour interval is 10 m. Inset: location of the islands.
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Fig. 2. Location of grab samples, piston core, seismic traverses and bathymetric pro¢les.
where it makes an eastward turn into the Tasman Sea. This eastward £ow forms the boundary between warmer northern waters and the southern temperate Tasman Current (Stanton, 1981; Mar-
tinez, 1994). The latitudinal position of the front varies from 30‡S in winter to 34‡S in summer (Hamon, 1962, 1968; Stanton, 1981). This means that Lord Howe Island and Balls Pyramid are
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subjected to alternating oceanographic conditions, with mean annual sea surface temperature varying between 18 and 23‡C (Veron and Done, 1979). The tidal range at Lord Howe Island is 1.5 m at springs and 0.8 m at neaps. GEOSAT altimeter data collected between 1995 and 1996 indicate that Lord Howe Island experiences mean signi¢cant wave heights of 2.3^2.5 m. This is in accordance with earlier SEASAT data (1978) which indicate average waves of 2.0^2.9 m (Allen, 1984). Following the predominant wind direction, easterly swells dominate in summer whereas southern swells dominate in winter. Storm swells can approach from the north during summer and autumn and from the west during autumn and spring.
3. Methods The bathymetry of the shelves surrounding Lord Howe Island and Balls Pyramid was examined on the basis of a compilation of soundings supplied under license by the Hydrographic O⁄ce of the Royal Australian Navy. The bathymetry was examined by interpolation of a grid using an inverse distance weighting algorithm and a triangulated irregular network across the shelves (using ESRI^Arcview GIS). The surface sediments on the shelves around Lord Howe Island and Balls Pyramid were sampled during cruise 12/98 aboard the CSIRO research oceanographic vessel Franklin in October 1998. A total of 73 sediment samples were collected using a Smith^McIntyre grab sampler (Fig. 2). Sampling sites were chosen to include the major sections of the shelf, however, poor weather restricted ship work especially in obtaining deeper samples and along the Balls Pyramid shelf. A piston core was also collected from the inner shelf. Locations were recorded using GPS. All samples were described brie£y in the ¢eld and sealed for later analysis. No chemical pre-treatment was carried out aboard ship. Continuous seismic re£ection pro¢ling was undertaken using an EG and G sparker 3-element array system (at 500 J) and a GeoAcoustics boomer system with
337
ampli¢er and ¢lter (at 200 J) and a 10-element streamer of hydrophones. In the laboratory, sediments were initially washed in freshwater to remove salts and then oven-dried at 50^60‡C. Mud was virtually absent from the samples so wet sieving was unnecessary. The dried samples were sieved to remove the s 5.6-mm fraction with the remaining sample split, using a ri¥e box splitter, into separate fractions for size, component, carbonate and X-ray di¡raction (XRD) analysis. Subsamples of 70^100 g were dry sieved for 10 min at 5.6-mm, 2-mm and 0.063-mm intervals. Component analysis was undertaken by pointcounting grain mount thin sections under a petrographic microscope using a grid of 400 points. Components were identi¢ed using keys in Milliman (1974), Adams et al. (1984) and Sco⁄n (1987). Photographs of each main compositional group present in the slides were used as a reference to maintain identi¢cation consistency. A broad visual compositional estimate of the gravel fraction was made. Mineral composition was determined on powdered samples using a Phillips X-ray di¡ractometer. Each sample was scanned from 20 to 40‡ (2 theta) at 1‡min31 , using Cu^K radiation at 40 kv and 30 mA. Total carbonate content was calculated by acid digestion of approximately 10 g of sample in HCl. Radiocarbon dating was undertaken at the Waikato Radiocarbon Laboratory. Ages have been corrected for the marine reservoir e¡ect by subtracting 450 P 35 yr from the conventional age (Gillespie and Polach, 1979) and are reported in radiocarbon yr BP (Table 1).
4. Results 4.1. Shelf bathymetry Lord Howe Island sits near the centre of a rhomboidal shelf with a width (west^east) of 24 km and a length (north^south) of 36 km. Balls Pyramid lies at the centre of a smaller shelf with a width (west^east) of 15 km and a length (north^
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south) of 22 km. The two shelves are separated by a trough that is on average 600 m deep. Both shelves are relatively £at with slopes of less than 1‡, in contrast to the precipitous slopes of the subaerial volcanic edi¢ces. There are distinct shelf breaks at around 70^100 m depth (Fig. 3). Slopes are steepest near the shelf break, typically 15^20‡ (max 30‡). The surface morphology of the Lord Howe shelf is quite complex, being characterised by a ridge at approximately 30 m depth almost encircling the island, between 1.5 and 8 km from shore, on its western, southern and eastern sides (Fig. 1). This ridge is most pronounced on the western, fringing reef, side of the island where it is 3 km wide. On the eastern side of the shelf the ridge is less continuous, located farther o¡shore from the island and characterised by a series of elongate high patches (Figs. 1 and 4). A trough occurs between the ridge and island, and is deepest (60 m) on the eastern side. The trough shallows on the southern and western sides of the shelf to around 40 m depth. On the southern margin of the shelf the ridge occurs adjacent to the shelf break. No ridge occurs along the northern edge of the shelf which gradually slopes towards the shelf break. A slight depression occurs close to the 350 m contour, however, this does not appear to be related to any ridge feature (Figs. 1 and 4). Based on morphology and surface lithology, as seen in several dives, the ridge is composed of limestone and is believed to be a fossil reef.
Morphology of the Balls Pyramid shelf is less complex. The shelf has a relatively uniform slope extending down to 50 m depth, whereafter the slope gradually increases towards the shelf break. A few isolated undulations of around 10-m relief occur in the central part of the shelf, in addition to the monolithic Balls Pyramid that rises to 550 m height (Fig. 1). 4.2. Seismic pro¢ling, lithology and sediment thickness Unfortunately neither sparker nor boomer seismic pro¢ling enabled di¡erentiation of lithological di¡erences or sediment thickness across the shelves. Selected seismic pro¢les are shown in Fig. 3, indicating a prominent break of slope on the shelf edge. The lack of seismic di¡erentiation between the fossil reef, which further diving con¢rmed to be composed of limestone, and the underlying basalt is attributed to the highly lithi¢ed nature and relatively small thickness of the limestone. This is in contrast to Holocene reef limestones and Pleistocene calcarenites which overlie basalt in the lagoon and which were clearly distinguishable on Uniboom seismic pro¢ling in the lagoon (Kennedy, 1999; Kennedy and Woodro¡e, 2000). The depth of sand on the inner shelf could not be discriminated in seismic pro¢les, presumably because it is not thick enough to be distinguished from the sea-£oor re£ector. One piston core on
Table 1 Radiocarbon dating results of material from the Lord Howe shelf Sample number
Laboratory code
Material
Water depth
PC1-206 LH-G18 LH-G48
Wk 7785 Wk 8995 Wk 8996
LH-G19
Wk 8997
LH-G16
Wk 8998
LH-G39
Wk 9004
Bulk sand Coral branch Large algal rhodolith Branching-coral gravel Disarticulated bivalves Small algal rhodoliths
Environmentally corrected radiocarbon age
(m)
Conventional radiocarbon age (yr BP)
50 31 117
4510 P 90 Modern 7150 P 90
4060 P 95 Modern 6700 P 95
36
8820 P 120
8370 P 125
110
660 P 130
210 P 135
64
3960 P 100
3510 P 105
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339
Fig. 3. Selected seismic pro¢les of the outer shelf showing a break of slope along the shelf edge.
the eastern side of the island penetrated 2.25 m into the sediment and was sand throughout whereas other attempted piston cores failed to obtain any sample. A radiocarbon date of
4060 P 95 yr was determined on bulk sand at a depth of 2.06 m (Table 1) and indicates accumulation at an average rate of 0.51 mm/yr. This accumulation rate is assumed to be a minimum.
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Fig. 4. Bathymetric pro¢les across the Lord Howe shelf, constructed from gridded bathymetric data. Location of pro¢les is shown in Fig. 2.
4.3. Sediment grain size Sediments recovered from across the two shelves are dominated by sand and gravel. Mudsized material was generally absent, the highest proportion (1.3% of the sample) being recorded in sample G47 at 158 m depth o¡ the Balls Pyramid platform. There is considerable variability in the nature of unconsolidated sediment size across the shelf, whereas most grab samples recovered a reasonable volume of sediment, a few contained little sediment at all, and the material recovered was predominantly gravel. In particular those samples from the surface of the fossil reef contained very coarse and angular
material that appeared to have been cemented to the substrate (samples G15, G66, G84 from the fossil reef and G52, G78 from water depths of 40^ 50 m on the front of the reef). Similarly, those samples recovered from close to the island in water depths of 20^30 m were dominated by pebbles or larger gravel. On the Balls shelf the coarsest samples were found both in the centre around the Pyramid and at the shelf break. The majority of samples are composed predominantly of sand. This is particularly the case in the depression between Lord Howe Island and the fossil reef, where samples were generally exclusively sand and all contained less than 25% gravel. Much of the surface of the Balls shelf also lacked
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Fig. 5. Constituent grain analyses of selected shelf samples.
gravel and is dominated by sand. In the piston core described above the sand size changed little with depth. 4.4. Mineralogy The shelf surface sediments are almost entirely calcareous with only 15 samples (20%) having a non-carbonate component over 0.5%. Volcanic rock fragments are the dominant non-carbonate component comprising a maximum of 9.5% of the sediment (sample G79). Siliceous sponge spicules were also present, but only in trace proportions ( 6 1%) in six samples, with a maximum of 4%
near the southern tip of Lord Howe Island (sample G55). Volcanic minerals occur close to the basaltic islands on both shelves. No gravel clasts of volcanic lithology were recovered, despite the prevalence of boulder beaches of volcanic clasts around the islands. On the Lord Howe shelf volcanic detritus was more common close to the island on the eastern, non-reefal side of the island. Only one sample, opposite a reef passage, contained basaltic minerals on the western side. Similarly, on the Balls Pyramid shelf, volcanic grains were concentrated around the Pyramid. One sample below the shelf break (G47, depth 158 m) contained a trace
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Fig. 6. Photographs of (a) sediments containing rhodoliths from the outer shelf (G39); (b) branching-coral gravel in a sandy matrix collected from just inside the fossil reef (G19); (c) Bathyarea shell beds below 100 m on the shelf slope (G16) and (d) typical sediment from just below the shelf break on Balls Pyramid showing the common occurrence of bryozoan, echinoid and foraminiferal detritus (G47).
of volcanic grains ( 6 0.5%) suggesting a possible outcrop of basalt or the shelf may be swept by strong currents that concentrate denser volcanic grains at this site. Siliceous spicules had a similar distribution to the volcanic minerals with two exceptions. The highest proportion (4.75%, sample G55) occurred on the fossil reef o¡ the southern part of Lord Howe Island, where live sponges were also recovered. Trace amounts were also found in two samples (G16, G47) taken below the shelf break. 4.5. Distribution of grain types The sediments on the shelves consist of the remains of crustose and free-living coralline algae, hermatypic corals, bryozoans, foraminifera, mol-
luscs, echinoids, Halimeda and recrystallised skeletal grains. Grains comprise whole skeletons or fragments, and gravel-sized clasts tend to be heavily encrusted by coralline algae. The relative proportion of component grain types in sediment samples is shown in Figs. 5 and 6. 4.5.1. Coralline algae Coralline algae, both branching and crustose, are common across the shelves with the proportion in the sand-sized fraction varying between 10 and 53%. The highest concentrations ( s 30%) occurred on plateau sections of both shelves and close to the fossil reef on the Lord Howe shelf. The inner shelf tends to contain the lowest proportions of coralline algae ( 6 20%) with samples collected below the shelf break also containing a
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low proportion of sand-sized coralline algal grains. This abundance is greater than that recorded along the continental shelf of eastern Australia (Marshall and Davies, 1978; Marshall et al., 1998). Two types of gravel-sized clasts can be identi¢ed; crustose sheets and rhodoliths. Crustose sheets tend to be composed of multiple layers of coralline algae growing over a coral/algal substrate and were only recovered from the surface of the fossil reef. The coralline algae were often live and growing in association with encrusting bryozoans and foraminifera, macroalgae, and less often Halimeda. Algal rhodoliths adopt three main forms, i.e. small (10^20 mm diameter) and large (up to 100 mm diameter) rounded, and branching. Most common is a well-rounded smooth form (Fig. 6a), with branching forms being the least common. The majority of rhodoliths contained a coral core; although in some cases mollusc material occurred. The smaller well-rounded forms tended to be found on the outer edge of both shelves. Some large accumulations were also recovered just landward of the fossil reef on the Lord Howe shelf (samples G87 and G88), in one case constituting over 50% of the sample. The branching rhodolith type was found across both shelves and tended to be associated with areas of abundant coral gravel where elongate coral clasts commonly form the core. The largest rhodoliths, up to 100 mm diameter, were found on the southern tip of the Lord Howe shelf, in depths of 50^120 m (Figs. 7 and 8). They did not have a live outer surface and tended to be highly bored, with the core being replaced by ¢ne mud, indicating several generations of reworking (see Focke and Gebelein, 1978). The outer surfaces were encrusted by bryozoans with a couple of small (3-mm) corals (dead) also being found. Radiocarbon dates on a large rhodolith and small rhodoliths returned ages of 6700 P 95 and 3510 P 105 yr BP, respectively, at the southern and northern edge of the Lord Howe shelf, respectively (Table 1). 4.5.2. Coral Coral is not a major component of the shelf
Fig. 7. Photographs of sections through rhodoliths from the outer shelf (G48) (scale bar is 10 mm).
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Fig. 8. Underwater photographs of (a) the fossil reef surface in approximately 30 m of water depth, and (b) the inner shelf, Lord Howe shelf, showing rippled biogenic sand.
sediments, constituting generally less than 5% of the sediment, but reaching up to 12%. Corals tend to be most abundant on the inner parts of both shelves. On the Lord Howe shelf the highest proportions occurred around the fossil reef, whereas the highest concentrations on the Balls shelf were close to the pyramid. Coral gravel accounted for the greatest proportion of the coarse sediment in areas where rhodoliths were rare. Only a couple of live hermatypic corals were recovered in the
shallowest grab samples. The coral gravels tended to be highly abraded and sub- to well-rounded, often with a few mm thick coating of coralline algae (Fig. 6b). Signi¢cant concentrations of coral gravel were found immediately in the lee of the fossil reef composed primarily of Acropora species, representing material broken o¡ the reef during the Postglacial marine transgression (8370 P 125 yr BP; Table 1). Occasional live and dead, but mod-
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ern (Table 1), branching-coral colonies (possibly Pocillopora) occur on the shallowest parts of the fossil reef. The coral gravel on Balls Pyramid is composed of branches and well-rounded forms which are highly abraded and encrusted by coralline algae. 4.5.3. Halimeda Halimeda grains are ubiquitous on the Lord Howe and Balls shelves, with only three grab samples (G56, G74, G88) completely lacking grains. This is in contrast to the shallow lagoon where Halimeda is virtually absent (Kennedy, 1999). Sand-size Halimeda grains constituted between 0 and 10.25% of the sand. The highest proportions (8^10%) on the Lord Howe shelf occurred in the middle of the inner shelf (Fig. 5) with the lowest abundance occurring around the fossil reef. This is surprising given that the only live specimens of these algae were dredged from that location. 4.5.4. Molluscs and lithoclasts Mollusc grains were relatively common across the shelves accounting for 3.5^24% of the sandsized fraction. Pecten shells, up to 100 mm in diameter, were found on the outer shelf (Fig. 6a), with some encrusted and abraded specimens being found on the inner shelf. Valves of the boring bivalve Lithophaga occur extensively in areas with hard substrate. These molluscs are probably one of the most important erosive agents with extensive boring found in most larger clasts. The attached species Chama was also recovered in situ from the fossil reef. The largest proportion of mollusc material was found below 100 m depth on the shelf slope where shells such as Bathyarea (Fig. 6c) constituted the majority of the sediment. A radiocarbon age on disarticulated small bivalves from G16 of 210 P 135 yr BP implies contemporary sediment production. Lithoclasts were found across the shelf ranging in abundance from 1 to 20.5%. The majority of clasts were located in the inner shelf area and on top of the fossil reef. The lowest proportions of these grains tended to occur on the shelf edge and slope.
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4.5.5. Indeterminate On parts of the shelf up to 51% of the sediment was unidenti¢able (Fig. 5). These grains were not micritised, were generally transparent in plane polarised light, and tended to be more common in the ¢ner grain size fraction. The morphology and crystal structure of these grains suggest that they are skeletal in origin. The proportion of these grains was greatest on the inner Lord Howe shelf and the fossil reef but do not appear to correlate with the occurrence of any other identi¢able grain fraction. Due to the high proportion of indeterminable grains XRD analysis was conducted to assess the composition of the grains. High-Mg-calcite was the dominant mineral comprising 70% of the carbonate fraction in the majority of samples (range 46^84%). There appears to be no signi¢cant relationship between High-Mg-calcite content and location on the shelf. Aragonite composition is also fairly uniform across the shelf comprising 11^ 37%. The aragonite proportion tends to decrease close to the shelf break where it generally comprises less than 15%. Low-Mg-calcite composition is generally depth-related, ranging between 0 and 35%. On the shelf, proportions are generally low ( 6 20%). Below 100 m, proportions range between 15 and 35%. This may relate to the recrystallisation of relict aragonitic sediments on the shelf edge or, more probably, re£ects the increase in abundance of low-Mg foraminifera. 4.5.6. Other biogenic grains Foraminifera comprise up to 8% of the sediment, with the highest proportions occurring below the shelf break. A variety of species appears to be present; although shallow-water Baculogypsina, common within the lagoon on Lord Howe Island (Kennedy, 1999), was generally absent suggesting there is little sediment transfer out of the modern lagoon. Bryozoans were found in greatest abundance below the shelf break (Fig. 6d), comprising up to 6.25% of the sediment. Echinoid fragments were found across the entire shelf comprising between 0.25 and 1.5% (max. 3.25%). Gravel size spines were found in greatest concentrations close to the modern fringing reef, while live echinoids were sampled o¡ the fossil reef.
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5. Discussion Tropical and temperate shelf carbonate assemblages have been discriminated on the basis of their skeletal composition (Lees and Butler, 1972; Carannante et al., 1988). Hermatypic corals and Halimeda generally dominate tropical sediments while temperate sediments are generally dominated by bryozoans, molluscs and benthic foraminifera. The relative abundance of individual skeletal elements may be site-speci¢c and vary, but the assemblages tend to be characteristic of warm- and cool-water environments, respectively. Recent studies have emphasised the di⁄culty in assigning these classi¢cations in transitional subtropical shelf environments (Marshall et al., 1998). Transitional sediments have a broadly heterozoan assemblage (James, 1997) but with the important addition of coralline algae and large symbiont-bearing foraminifera (James et al., 1999). On the east Australian coastline, Marshall and Davies (1978) determined the transitional boundary to occur around 24‡S based on the occurrence of surface reefs. Marshall et al. (1998), however, have noted that some overlap of these assemblages occurs, with mid-shelf sediments o¡ Fraser Island (25‡S) being dominated by coralline algae associated with low species diversity of hermatypic corals, large benthic foraminifera, bryozoans, Halimeda and molluscs. A fringing reef occurs on the western side of Lord Howe Island (Slater and Phipps, 1977), and luxuriant coral communities occur around all sides of the island (Veron and Done, 1979; Harriott et al., 1995) as well as on Balls Pyramid. Coral grains are, however, poorly represented on the shelf apart from coral gravel concentrated around the fossil reef, which is presently producing little debris. On the Lord Howe Island and Balls Pyramid shelves coralline algae are the dominant sediment component with Halimeda and corals, although widespread, being minor components. Bryozoans are also not major sediment constituents. These islands are, therefore, a mid-oceanic transitional carbonate environment some 10‡ farther south than previously described. Modern sedimentation on the Lord Howe
Group can be divided into three major zones, the inner and outer shelf and shelf edge. On the Lord Howe shelf the inner and outer shelves are separated by the fossil reef which is generally delineated by the 30-m contour, while on the Balls shelf the absence of any reef feature means the inner shelf extends with little di¡erentiation to 50 m depth. The extent of the shelf edge zone is di⁄cult to determine given the sparse sampling below 100 m depth. However, the similarity of samples from both shelves suggests that it is extensive. The inner shelf zone is dominated by sand with gravel composition increasing towards the islands on both shelves as well as towards the fossil reef on the Lord Howe shelf. Biogenic sands appear to be related to the shallow-water benthic communities with some in situ production of bryozoans from isolated communities on the mobile sands. Halimeda concentrations are greatest in this zone which also has the lowest proportions of low-Mgcalcite (Fig. 8). The outer shelf zone occurs from the base of the fossil reef on the Lord Howe shelf and from a depth of 50 m on the Balls shelf. Rhodoliths (10^20 mm diameter) characterise the sediment and appear to be actively accreting, forming large ¢elds close to the shelf break. The exact nature of these ¢elds is hard to determine due to a lack of bottom photography. Local boat captains report surface currents of around 2.5 m/s and, using the threshold velocity for clast movement noted around Fraser Island (Harris et al., 1996), a bottom current of at least 0.55 m/s occurs. This zone also provides a suitable environment for Pecten growth with well-preserved shells being commonly found though not with the same high productivity as described on the outer shelf of the New South Wales coast (Ferland and Roy, 1997). The shelf edge zone tends to be ¢ner grained and is characterised by two types of gravel, i.e. large relict rhodoliths (up to 60 mm diameter) and small molluscs. The molluscs are modern dating at 210 P 35 yr BP. The highest proportion of low-Mg-calcite occurred in this zone related to either relict sediment or foraminifera. This would imply that a mix of relict and modern sediments occur within this zone.
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Sea-level transgression across the shelf during the Holocene has greatly changed the oceanographic environment. At 12 ka the sea surface would have been at the shelf edge, £ooding the entire shelf to its current depth by 6.5 ka. The large rhodoliths along the shelf edge are no longer active and must have been deposited during a lower sea level. The size of these clasts, compared with the currently growing rhodoliths, suggests a more favourable environment for algal growth during the Early Holocene. The deposits of branching corals on the shelf appear to represent a period of greater sedimentation during the Early to Middle Holocene. A radiocarbon age of 8370 P 125 yr BP on branching coral from G19, contrasts with the modern date from the in situ clast in G18. SCUBA diving and grabs from the surface of the fossil reef indicate that these gravels could not be formed under modern conditions due to a sparse coral cover and their age. Thus it appears that an Early Holocene reef became established over the larger, fossil reef and gave-up. It terminated either as a result of temperature change or by drowning being replaced by coralline algal communities. As the top of the give-up reef is within modern established growth limits for corals in the tropics, a reduction in temperature would appear to be the primary factor in causing growth to cease. A similar change is observed within the modern reef, where much more proli¢c growth is indicated in the Middle rather than Late Holocene (Slater and Phipps, 1977; Kennedy and Woodro¡e, 2000). Over the longer term, the prominent fossil reef on the Lord Howe shelf indicates that much more proli¢c coral growth was possible than during the Holocene. The origin and age of the fossil reef is problematic. It occurs at a depth which would have been favourable for reef growth during marine oxygen^isotope stages 1, 3 and 5; however, it seems unlikely that such a large feature could have formed in the Late Quaternary. Reef limestones that have been dated to the Last Interglacial, substage 5e, occur beneath the modern lagoon at a depth of 2^4 m (Kennedy and Woodro¡e, 2000) while isolated coral clasts of similar age occur within a beach facies 2 m above
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present sea level on the eastern side of the island (Woodro¡e et al., 1995). However, neither the modern reef nor the minor Last Interglacial reef limestones compare with the enormous submerged structure, in places 2^3 km wide, that occurs in 30-m water depth. It would appear that the fossil reef is older than marine oxygen^isotope stage 5e. In order for the reef to form it would have been necessary to planate the volcanic edi¢ce. This presumably started soon after the volcano erupted 6^7 Ma (McDougall et al., 1981), with marine abrasion occurring at a decelerating rate as the shelf widened. Indeed, truncation of cli¡ed shorelines of the islands would appear to be highly unlikely once a reef had been constructed to seaward. The reef might be Late Tertiary in age or, more likely, it may have been formed during a previous interglacial. Evidence for higher sea level, or more proli¢c reef growth in earlier interglacials has recently been described from both the Atlantic and Paci¢c reef provinces (Hearty et al., 1999; Stirling et al., 2001) and may have also occurred around Lord Howe. The continued northward movement of the Australian plate will progressively move these shelves into more tropical seas. Reef growth can be anticipated to become more widespread and tropical sediments will begin to dominate the shelf. The broad shelves that surround Lord Howe Island and Balls Pyramid developed because the volcanic edi¢ces were outside reef-forming seas and could be planated by marine abrasion, in contrast to the reef-encircled islands typical of reef-forming seas (Menard, 1986). Reefs protect the shores of volcanic islands or rim the margins of carbonate platforms, attenuating and dissipating wave energy. Mid-ocean shelves are consequently rare in the tropics. On linear island chains in which individual islands are carried out of reef-forming seas, rapid subsidence leads to drowning (Grigg and Epp, 1989). A transition from broad shelves with sediment suites composed of temperate carbonates into tropical reef sedimentary sequences may have been a feature of island chains in the past when there were more islands entering reef-forming seas (Wilson et al., 1998).
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6. Conclusions The sediments of the Lord Howe Island and Balls Pyramid shelves are neither distinctly tropical nor temperate. This is surprising given the abundant shallow-water reef development around Lord Howe Island. The carbonate sediments of the shelf are characteristic of transitional environments identi¢ed on the Australian mainland coast, but di¡er in that they are located in the open ocean away from any continental in£uences. Ocean temperature, perhaps also re£ecting water depth, would appear to be the main environmental factor determining the broad sediment composition. The depositional environment across these shelves has not been static. Coral and algal rhodolith growth appears to have been more abundant during the Early and Middle Holocene. The presence of these sediments suggests that there is little active deposition on the shelf or that modern carbonate productivity is low. Where a linear island chain is moving into reefforming seas a transition is likely to occur from a broad shelf with a temperate carbonate sediment suite, into a tropical reef framework. The subtropical sediments are the ¢rst stage of carbonate deposition over the underlying basalt. For those volcanic islands that may have been planated outside, but moved into, reef-forming seas, a similar transition may occur with tropical reefs over temperate carbonate sediments which in turn overlie a broad nearly-horizontal basalt surface.
Acknowledgements This study was funded by the Australian Research Council (grant to C.D.W. and B.G.J.) and extended earlier studies undertaken by C.V.G.P. Research was conducted with permission and support from the Lord Howe Island Board and the Lord Howe Island Marine Park Authority. The authors would like to thank the captain and crew of RV Franklin for their assistance during cruise 12/98 in the Tasman Sea. Further bathymetric data were provided under license from the Hydrographic O⁄ce of the Royal Aus-
tralian Navy. Vicki Harriott (James Cook University), David Mitchell (University of Sydney), Stewart Fallon, John Marshall and Eugene Wallensky (Australian National University), and Dean Hiscox (Lord Howe Island Board) provided valuable shipboard assistance. Ben Ackerman, David Carrie and Viki Lee are thanked for their contributions in sample preparation and analysis while thanks are due to Penny Williamson for the photographic plates.
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