Coralgal composition of drowned carbonate platforms in the Huon Gulf, Papua New Guinea; implications for lowstand reef development and drowning

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Marine Geology 204 (2004) 59^89 www.elsevier.com/locate/margeo

Coralgal composition of drowned carbonate platforms in the Huon Gulf, Papua New Guinea; implications for lowstand reef development and drowning Jody M. Webster a; , Laura Wallace a , Eli Silver a , Donald Potts b , Juan Carlos Braga c , Willem Renema d , Kristin Riker-Coleman e , Christina Gallup e b

a Department of Earth Sciences, University of California, Santa Cruz, CA 95064, USA Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064, USA c Departamento de Estratigra¢a y Paleontologia, Universidad de Granada, Spain d Nationaal Natuurhistorisch Museum, Leiden, The Netherlands e Department of Geological Sciences, Duluth, MN, USA

Received 19 February 2003; received in revised form 29 October 2003; accepted 17 November 2003

Abstract Collision between the South Bismarck plate and the northern edge of the Australian plate has produced an actively subsiding foreland basin in the western Huon Gulf. A series of drowned carbonate platforms and pinnacles are preserved on this margin due to a combination of this rapid subsidence and eustatic sea-level changes over the last 450 ka. We analyzed sedimentary and coralgal data from the platforms to better understand lowstand reef development and drowning in the Huon Gulf. The recovered limestones are divided into five main sedimentary facies: coral reef, coralline^foraminiferal nodule, coralline^foraminiferal crust, Halimeda, and planktonic foraminiferal limestones. Based on a comparison with modern analogues in the Indo-Pacific and elsewhere, we identified coral reef, deep fore-reef slope, deeper fore-reef slope, and pelagic/hemipelagic paleoenvironmental settings. An analysis of facies relationships and their paleoenvironmental meanings revealed lowstand corals reefs preserved at the top of the platforms that grew within V10 m of sea level. Two different coral assemblages were identified within this facies: (1) a shallow, high energy reef community characteristic of windward margins and limited to the deep platforms (1947, 2121, 2393 m), and (2) another shallow community but indicative of more moderate lower energy reef conditions and limited to the middle (1113, 1217, 1612 m) and shallow platforms (823 m). The change from high to lower energy reef growth conditions suggests that oceanographic/climatic conditions in the Huon Gulf have changed substantially through time, primarily through the closure of the Gulf as a result of tectonic rotation and uplift of the Huon Peninsula over the last 450 ka. Despite major environmental perturbations (i.e. relative sea-level and temperature changes) the platforms and the shallow water coral reefs exposed at the top have been able to re-establish themselves time and time again over the last 450 ka. We also identified two different incipient drowning scenarios influenced by the rate of relative sea-level rise. More rapid drowning in the middle and deep platforms produced a thin veneer of coralline^foraminiferal nodule and Halimeda limestones over the shallow coral reef material while the slower

* Corresponding author. Tel: +1-831-775-2072; Fax: +1-831-775-1620. E-mail address: [email protected] (J.M. Webster).

0025-3227 / 03 / $ ^ see front matter B 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0025-3227(03)00356-6

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drowning experienced by the shallowest platforms allowed thick coralline^foraminiferal crust limestones to develop. We recognize three main stages of platform development: (1) initiation and growth characterized by shallow coral reef growth as the platforms grew close to sea level during the lowstands, (2) incipient drowning marked by a shift to coralline^foraminiferal nodule, crust and Halimeda limestones as the platforms began to drown during rapid eustatic sea-level rise and continued subsidence, and (3) the complete drowning of the platforms characterized by platform ‘turn off’, increased bioerosion, Fe^Mn precipitation and pelagic/hemipelagic sedimentation as the platform surfaces finally drop below the photic zone. B 2003 Elsevier B.V. All rights reserved. Keywords: carbonate platform drowning; Pleistocene reef development; sea-level changes; lowstand; Papua New Guinea; coralgal composition

1. Introduction Our understanding of Pleistocene reef development in response to climatic and associated sealevel changes has been greatly enhanced over the last 30 years through extensive investigations of carbonate sequences on active margins in Barbados (Gallup et al., 2002), the Huon Peninsula (Chappell et al., 1996), East Indonesia (Hantoro et al., 1994), and the Ryukyu Islands (Nakamori et al., 1995a; Sagawa et al., 2001), rifted margins along the Red Sea (Gvirtzman, 1994; Plaziat et al., 1998), subsiding atolls (Camoin et al., 2001) and passive margins in the Great Barrier Reef (International Consortium for Great Barrier Reef Drilling, 2001; Webster and Davies, 2003) and the Florida Keys (Multer et al., 2002). For the most part, fossil reefs in these settings preserve a record of transgressive and highstand reef development, with lowstand growth either buried by subsequent reef growth or not preserved at all due to steep, shelf edge morphology. Submerged Pleistocene reefs on rapidly subsiding margins encapsulate a unique archive of lowstand reef development, drowning and associated climate change during transitions from glacial to interglacial conditions. Given the di⁄culties in access and the unique conditions required to preserve them there have been comparatively few comprehensive studies on such reefs (Hawaii : Moor and Campbell, 1987; Grigg et al., 2002; Jones, 1995; Fletcher et al., 2000 ; Papua New Guinea: Galewsky et al., 1996). To this end we imaged and sampled eight submerged platforms and pinnacles from the Huon Gulf on the north coast of Papua New Guinea (Fig. 1). These platforms represent the

submerged counterpart to the uplifted terraces exposed on the Huon Peninsula, studied so extensively by Chappell and others (e.g. Chappell, 1974; Chappell and Polach, 1976; Chappell et al., 1996; McCulloch and Esat, 2000; Chappell, 2002). Galewsky et al. (1996) and Galewsky (1998) suggested that the platforms preserved in the Huon Gulf drowned in response to a combination of rapid subsidence and eustatic sea-level rise. They calculated an average subsidence rate of 5.7 m/ka from a 348-ka shallow water coral dredged from the 1947-m platform. Using the last deglaciation as a model for older deglaciations, the average rate of sea-level rise was V10 m/ky, close to the maximum sustained vertical accretion rate for shallow coral reefs (V10^14 m/ky ; Buddemeier and Smith, 1988; Smith and Buddemeier, 1992). Furthermore, recent results from Barbados, the Huon Peninsula, and the Sunda Shelf indicate that the course of sea-level rise during the last deglaciation was in fact episodic, characterized by one, perhaps two very rapid, meter-scale (up to 20 m) sea-level rise events (meltwater pulse 1A and 1B) in excess of 45 m/ky in less than 6 500 years (Fairbanks, 1989; Bard et al., 1990; Chappell and Polach, 1991; Bard et al., 1996; Blanchon and Shaw, 1995; Hanebuth et al., 2000). Therefore, assuming the rates of sea-level rise during earlier deglaciations were at least similar to the last deglaciation, any such rise (and associated meltwater pulses) combined with rapid subsidence could easily drown even the healthiest coral reef. To better understand the relationship between carbonate platform development, subsidence and

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Fig. 1. (A) Regional map showing tectonic setting and location of the drowned platforms (after Wallace, 2002). The modern reefs (dashed line) fringe the Morobe coastline and extend east into the Solomon Sea forming the Trobriand platform. (B) Color 3-D bathymetry of the margin showing the structure and distribution of the platforms and pinnacles. The red arrows indicate the position of the ROV Jason dive sites. The yellow text represents the U/Th ages of the platforms currently dated.

climate change, we undertook a cruise to the Huon Gulf in August^September 2001. We mapped the drowned platforms using SeaBeam, side-scan sonar and then collected limestone samples using the Woods Hole ROV, Jason (Webster

et al., 2002). The bathymetric data, side scan data and limited dates indicate : (1) the presence of eight major platforms and numerous pinnacles that likely grew V450 and 60 ka in a backstepping fashion (Fig. 1A); (2) the morphology and

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age structure of the platforms is consistent with platform drowning as a result of rapid sea-level rise (and subsidence) during major deglaciations and other less dramatic interstadial events; (3) the platforms are composite features composed of pinnacles, banks and multiple terrace levels indicating an often complex history of platform growth and drowning; and (4) the platform tops preserve the signature and timing of platform drowning, despite experiencing substantial lateral modi¢cation. Wallace (2002) used the additional data to develop a numerical model incorporating variable £exural subsidence, reef growth and eustatic sealevel changes (see methods for model details).

From this model, Wallace (2002) estimated the timing of platform drowning (Fig. 2A) and suggested that the platforms and pinnacles drowned during early parts of the major deglaciations and smaller amplitude interstadial events (Fig. 2B) over the last 450 ka. U/Th dates from the 239-m (60.3 O 0.5 ka) and 1947-m (348 O 10 ka) platforms are close to the model ages. In this paper we present new coralgal and sedimentary data to better understand the development and drowning of carbonate platforms on the rapidly subsiding margin in the Huon Gulf. Our objectives are: (1) to document the coralgal and sedimentary composition of the drowned platforms, (2) to reconstruct the paleoenviron-

Fig. 2. (A) Depth and modeled age of the drowned carbonate platforms in the Huon Gulf, Papua New Guinea (after Wallace, 2002). (B) Eustatic sea-level for the last 500 ka showing the likely drowning ages (black circles) of the platforms. These times are based on a numerical model incorporating variable £exural subsidence, reef growth and eustatic sea-level changes (0^140 ka: Lambeck and Chappell, 2001; 141^350 ka: Lea et al., 2002; 351^500 ka: Imbrie et al., 1984). Note the Imbrie et al. (1984) curve is a deep marine N18 O record and not a direct measure of ice volume. Marine isotope stages (MIS) are after Imbrie et al. (1984).

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mental settings prior to and during drowning, (3) to characterize and evaluate the coralgal and sedimentary signatures of platform drowning, and (4) to discuss implications of this record for platform development, drowning, tectonics and climate change over the last 450 ka in the Huon Gulf.

2. Location and methods 2.1. Geological setting The study site is in the Huon Gulf located on the northern coast of Papua New Guinea (Fig. 1A). The Australian plate converges obliquely with the Paci¢c plate and has produced a complex and active system of micro-plates with associated vertical and lateral movements. In the Huon Gulf, the South Bismarck plate is colliding with and overriding the northern Australian continental margin (Pigram and Davies, 1987; Abbott et al., 1994) and a classic fore-deep has developed on the lower plate of the convergent zone in the Huon Gulf (Galewsky et al., 1996) (Fig. 1A). The resulting massive loading and subsidence has caused drowning of a series of carbonate platforms at 100^190 (von der Borch, 1972), 239, 625, 823, 1113, 1216, 1612, 1947, and 2121 m and pinnacles at 2393 m (Webster et al., 2002) (Fig. 1B). 2.2. Numerical model estimating platform drowning We used a numerical model developed by Wallace (2002) incorporating : (1) £exural subsidence of the foredeep due to the foreland-migrating load, (2) coral growth on the subsiding plate, and (3) changing eustatic sea levels, to estimate the drowning age of each platform. Typical of a plate being consumed at a subduction zone, subsidence rates in the Huon Gulf increase dramatically closer to the New Britain trench (closer to the ‘load’), therefore the platforms experienced a variable subsidence history. In the numerical model we use margin bathymetry and the age and location of the 1950-m and 240-m drowned platforms to essentially ‘grow’ and ‘drown’ reefs

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on a realistic £exurally sinking substrate. Coral reef vertical growth rates as a function of water depth are calculated using equations presented by Bosscher and Schlager (1992) and estimates of eustatic sea-level change over the last 500 kyr (Lambeck and Chappell, 2001; Lea et al., 2002; Imbrie et al., 1984). The model predicts the drowning age of each platform during rapid sealevel rise events (Fig. 2), and is in agreement with the conceptual models of platform drowning proposed by Schlager (1981). Wallace (2002) found that no single value for elastic thickness of the subsiding plate could explain both the £exural shape of the plate and the age and location of the 1950-m and 240-m platforms. Variable plate thickness can ¢t all the data quite well, with signi¢cant thinning towards the trench (from 16 to 10 km e¡ective elastic thickness). This thinning has implications for the rate of subsidence experienced by the platforms during growth and prior to complete drowning. The £exural subsidence history presented by Wallace (2002) suggests that while within the photic zone (V100 m), the younger, shallow platforms (underlain by the elastically thicker plate) subsided more slowly than the deep platforms, which lie on top of a thinner, weaker elastic plate. 2.3. Modern climatic and oceanographic setting The Huon Gulf lies within the in£uence of the NW monsoons (January^April) and SE trades (May^December) (von der Borch, 1972). However, there is no distinct energy regime with winds blowing from the SE and NE annually as well as in a diurnal local cycle (McAlpine et al., 1983). During the NW monsoon, seas are generally calm in the Huon Gulf. Although a short, choppy swell can develop during the SE trades, these prevailing conditions, plus the shape of the Gulf, and the positions of the Huon Peninsula and SE Trobriand platform, all act to shelter the Huon Gulf from much of the wave/wind energy from both NW and SE (Fig. 1A). Thus, the Huon Gulf usually experiences low to moderate energy conditions, particularly when compared with the more exposed Madang coast to the NW and the Trobriand platform to the SE.

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Table 1 Coral, coralline algal and foraminifera species identi¢ed from the drowned carbonate platforms in the Huon Gulf, Papua New Guinea Corals Family ACROPORIDAE Acropora sp.1346789 Acropora cytherea?8 Acropora grandis?9 Acropora hoeksemai89 Acropora humilis8 Acropora hyacinthus?48 Acropora muricata?6 Acropora palifera678 Acropora sp. (robusta group)8 Acropora spicifera?8 Astreopora sp.4 Montipora sp.12345678 Montipora cf. venosa7 Montipora cf. aequituberculata5 Montipora corbettensis?5 Montipora monasteriata345 Montipora foliosa?6 Montipora informis?8 Montipora tuberculosa47 Montipora undata?6 Family AGARICIIDAE Pachyseris speciosa5 Pavona varians5 Pavona clavus8 Pavona explanulata8 Pavona minuta?8 Family ASTROCOENIIDAE Stylocoeniella guentheri48 Family FAVIIDAE Cyphastrea sp.3568 Cyphastrea microphthalma356 Cyphastrea serailia6 Echinopora sp.6 Echinopora gemmacea?6 Echinopora hirsutissima?6 Favia sp.5678 Favia laxa67 Favia stelligera?5 Favites bestae?5 Favites chinensis6 Favites halicora5 Favites pentagona3 Goniastrea sp.?7 Leptastrea sp.5

Family FAVIIDAE Leptoria phrygia36 Montastrea curta?6 Montastrea multipunctata7 Montastrea salebrosa?6 Platygyra sp.8 Family FUNGIIDAE Fungia sp.6 Lithophyllon cf. lobata?5 Family MERULINIDAE Hydnophora exesa7 Family MUSSIDAE Lobophyllia?3 Scolymia?3 Family OCULINIDAE Galaxea sp.12578 Galaxea fascicularis7 Galaxea astreata?2 Family POCILLOPORIDAE Pocillopora sp.1368 Seriatopora sp.58 Stylophora pistillata16 Stylophora subseriata4 Family PORITIDAE Alveopora sp.2 Goniopora sp.6 Porites sp.123456789 Poritesaustraliensis?6 Porites lobata56 Porites horizontalata 57 Porites lichen35 Porites mayeri?8 Porites okinawensis4

Coralline algal

Large benthic forams

Family CORALLINACEAE

Family AMPHISTEGINIDAE Amphistegina sp.178 Amphistegina radiata1235 Amphistegina lessonii45

Subfamily Melobesioideae Mesophyllum sp.1234568 Lithoporella sp.123456789 Lithothamnion sp.12345689 Lithothamnion prolifa9 Subfamily Lithophylloidaeae Lithophyllum sp.1235678 Lithophyllum incrassatum18 Lithophyllum gr. pustulatum345679 Lithophyllum kotschyanum8 Subfamily Mastophoroideae Neogoniolithon sp.23456789 Neogoniolithon conicum45 Neogoniolithon fosliei457 Hydrolithon reinboldii12674 Hydrolithon onkodes2468 Spongites sp.45679 Subfamily Amphiroideae Amphiroa sp.*24678 Subfamily Corallinoideae Jania sp.*14 Corallina sp.*15 Family SPOROLITHACEAE Sporolithon sp.134568 Sporolithon episoredion6 Family PEYSSONNELIACAE Peyssonnelia sp.123456789 Polystrata sp.3

Family SIDERASTREIDAE Coscinaraea columna?4 Psammocora vaughani6 Psammocora explanulata6 Psammocora super¢cialis?8 Pseudosiderastrea tayamai?6 Siderastrea savignyana45678

Family NONIONIDEA Nonionellinae sp.4 Family SORITIDA Amphisorus sp.6 Family CALCARINIDAE Calcarina sp.156 Calcarina mayori5 Family NUMMULITIDAE Palaeonummulites sp.1 Cycloclypeus carpenteri1234567 Operculina sp.158 Heterostegina sp.134568 Heterostegina depressa14568 Heterostegina operculinoides1 Family ACERVULINIDAE Gypsina sp.1234568 Acervulina sp.1234568 Discogypsina sp.5 Miniacina sp.1234 Small benthic foraminifera Family TEXTULARIIDAE Textularia sp.1 Planktonic foraminifera Family GLOBOROTALIIDAE Globorotaeia sp.3 Globogerinoides ruber13

*geniculate coralline algae, superscript numbers indicate which platform the species were recovered from e.g. 1 239 m, 2 625 m, 3 823 m, 4 1113 m, 5 1216 m, 6 1612 m, 7 1947 m, 8 2121 m, 9 2393 m.

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2.4. Site selection and sampling Extensive bathymetric (SeaBeam) and side-scan sonar (DSL-120) data, processed while at sea, allowed us to select suitable dive targets at the platform tops. A separate paper based on this data detailing the structure and morphology of the platforms will be presented elsewhere. We recovered 333 samples totaling 1200 kg of limestone during 18 days of continuous Jason ROV operations. To record the paleoenvironmental signature prior to and during drowning, sampling focused on the elevated platform rim, the shallowest part of the platform. This also had the e¡ect of minimizing any error if samples were not in situ, presumably any transported and re-cemented samples would not have traveled far from the top of the platform. Wherever possible we attempted to sample in-situ limestones by breaking them o¡ with the manipulator arm or the edge of the ROV’s sampling platform. 2.5. Sample preparation and fossil identi¢cation Laboratory studies involved cutting, ultrasonically cleaning and microscopic identi¢cation of the samples to identify the corals and other organisms (e.g. coralline algae, foraminifera). Coral identi¢cation was based on Veron and Pichon (1976, 1979, 1982), Veron and Wallace (1984); Wallace (1991) ; Veron (1986) and Veron et al. (1977). Petrographic analysis of 125 thin sections was used to determine the texture, composition and relative abundance of the associated platform components such as coralline algae, planktonic, benthic and encrusting foraminifera, Halimeda, gastropods, bivalves, echinoderms, non-skeletal and terrigenous grains. Given their importance as indicators of paleoenvironments, special attention was given to the taxonomic identi¢cation of coralline algae (Ringeltaube and Harvey, 2000; Verheij and Erftemeijer, 1993; Woelkerling and Campbell, 1992; Woelkerling et al., 1993; Womersley, 1996) and large benthic foraminifera (Hallock and Glen, 1986; Hohenegger, 1995; Hohenegger et al., 1999, 2000). A combination of criteria was used to distinguish in-situ limestone samples from allochthonous rubble: (1) whether

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the sample was broken o¡ or collected loose, (2) the presence of dark Mn^Fe crusts on the exposed outer limestone surface indicating an original orientation, (3) lack of severe surface abrasion and rounding of coral colonies, (4) orientation of well preserved corallites, (5) orientation of acroporid and pocilloporid branches, (6) coral colonies or branches capped by thick (few cm) coralline algal crusts, and (7) presence of macroscopic and microscopic sediment geopetals in cavities and mollusc chambers and valves.

3. Results 3.1. Sedimentary facies, coralgal composition and paleoenvironmental interpretations Detailed examination of the hand specimens and thin sections revealed ¢ve main sedimentary facies: coral reef, coralline algal^foraminiferal nodule, coralline algal^foraminiferal crust, Halimeda, and planktonic limestones. Characterized by their sedimentary textures and composition (e.g. coralgal assemblages, benthic foraminifera, other microfacies), these facies and their likely paleoenvironmental settings are discussed below and summarized in Tables 1 and 2. 3.1.1. Coral reef limestone This limestone is dominated by autochthonous hermatypic coral material forming in-situ coral frameworks de¢ned as ba¥estones (Fig. 3A), framestones (Fig. 3D) or bindstones (Fig. 4A). Associated components contributing to the framework or matrix material include non-geniculate coralline algae, large benthic foraminifera (e.g. Calcarina sp.), molluscs, echinoderms, the green algae Halimeda, and peloids. This facies also contains obvious allochthonous coral reef materials, including loose hermatypic corals, grainstones, rudstones and £oatstones. In-situ hermatypic corals are common to abundant in reef environments down to 50 m in the Indo-Paci¢c (Veron, 1986; Iryu et al., 1995; Done, 1982); therefore this facies was presumably deposited in depths 6 50 m. The paleoenvironmental setting of this facies can be determined

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De¢nition

Sedimentological features

Dominant components

Associated components

Paleoenvironmental interpretation

Coral reef limestone

Limestone dominated by hermatypic corals

In-situ coral framework (bindstones to framestones), and loose coral reef debris (grainstones, rudstones and £oatstones) Rudstones to £oatstones, cores of nodules mainly composed of coral fragments

Hermatypic corals*

Coralline algae** (mastophoroid), large benthic forams (e.g. Calcarina), molluscs, echinoderms, Halimeda and peloids

Reef £at to upper reef slope ( 6 20 m)

Non-geniculate coralline algae**(Melobesioid) and acervulinids

Large benthic forams (Amphistegina radiata, A. lessonii, Heterostegina depressa)

Deep fore-reef slope (V20^60 m)

Packstones to wackestones, extensive mud

Halimeda

Coralline algal^foraminiferal nod- Dominated by nodules ule limestone composed non-geniculate coralline algae and encrusting foraminifera Halimeda limestone Dominated by Halimeda segments Coralline algal^foraminiferal crust Dominated by overlapping limestone crusts of non-geniculate coralline algae and encrusting foraminifera Planktonic foraminifera limestone Dominated by planktonic foraminifera

Large benthic forams (Amphistegina radiata, A. lessonii, Operculina, Heterostegina depressa), coral fragments and molluscs Bindstone, forms a well Non-geniculate coralline Minor encrusting corals (e.g. Montipora, Porites), developed crustose framework algae** (Melobesioid) bryozoans, large benthic forams (Nummulites, and acervulinids Amphistegina, Cycloclypeus carpenteri, H. opercrulomoides) Mudstones to wackestones, Planktonic forams, Large benthic forams (Amphistegina radiata and commonly stained with Fe^Mn (ie. Globeroides), delicate A. lessonii, Cycloclypeus carpenteri and oxides and minor mud sized molluscs, small benthic Heterostegina depressa) terrigenous (quartz and rock forams (e.g. Textularia) fragments) Facies occurrence

Deep fore-reef slope (V20^60 m) Deeper fore-reef slope (build up) (V60^90 m) Outer shelf (20^120 m)

Coral assemblages*

Characteristics

Paleoenvironmental interpretation

Assemblage A

Dominated by robust branches or ridges of Acropora palifera, A. humilis group, and Coral reef limestone tabulate A. hyacinthus group (e.g. cytherea, spicifera), with associated encrusting Montipora sp. (e.g. M. tuberculosa, M. informis), and submassive to massive Porites sp. (e.g. P. horizontalata, P. lobata) and minor encrusting Siderastrea savignyana, Psammocora super¢cialis and faviids (e.g. Favia laxa, Montastrea multipunctata).

Reef, shallow ( 6 5 m), high energy, characteristic of windward margins, outer reef £at to upper reef slope

Assemblage B

Dominated by encrusting Montipora sp. (e.g. M. monasteriata, M. corbettensis, M. cf. Coral reef limestone aequituberculata), and Porites sp. (e.g. P. horizontalata) with associated faviids (Montastrea salebrosa?, M. curta?, Cyphastrea sp., Favia laxa, and Echinopora hirsutissima or E. gemmacea) and agariciids (Siderastriidea savignyana, Pseudosiderastrea tayamai? and Psammocora sp.).

Reef, shallow ( 6 10 m), low^moderate energy, upper reef slope

Non-geniculate coralline algal assemblages**

Characteristics

Facies occurrence

Paleoenvironmental interpretation

Mastophoroid assemblage (M)

Dominated by well developed crusts of Neogoniolithon fosliei and Hydrolithon onkodes and with associated Lithophyllum gr. pustulatum.

Coral reef limestone, interior crusts on some of the coralline^foraminiferal nodular limestone

Shallow ( 6 10^15 m) reef environments

Melobesioid assemblage (B)

Dominated by thin crusts of Mesophyllum sp., with associated Lithothamnion sp., Lithoporella sp., Sporolithon sp., and Peyssonnelia sp.

Coralline algal^foraminiferal crust limestone, coralline algal^foraminiferal nodular limestone

Non-reefal, deep water (V10^90 m)

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Sedimentary facies

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Table 2 A summary of the main sedimentary facies and coralgal assemblages from the drowned carbonate platforms in the Huon Gulf, Papua New Guinea

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more precisely from analysis of the coralgal growth forms and species composition. We identi¢ed 32 coral genera and 73 species within the coral reef limestone (Table 1). Based on the coral composition and comparison with modern reef zonation and ecology in Papua New Guinea and other areas from the Indo-Pacific, we identi¢ed two main coral assemblages and their paleoenvironmental settings within coral reef limestone prior to drowning (Table 2). Assemblage A is dominated by robust branches or ridges of Acropora palifera (Fig. 3A,B), the Acropora humilis group (Fig. 3C), Acropora grandis and the tabulate Acropora hyacinthus group with associated encrusting Montipora sp. (M. tuberculosa, M. informis), submassive to massive Porites sp. (P. horizontalata, P. lobata) (Fig. 3D) and minor encrusting colonies of Siderastrea sa-

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vignyana, Psammocora super¢cialis and faviids (Favia laxa, Montastrea multipunctata). Similar robust branching Acropora dominated communities are common on modern reef edges and upper reef slopes exposed to strong wave action and in water depths of 6 10 m in many parts of the Indo-Paci¢c (Great Barrier Reef: Done, 1982; Veron, 1986 ; Tahiti : Bard et al., 1996; Montaggioni et al., 1997 ; Mayotte: Camoin et al., 1997; Ryukyu Islands : Nakamori, 1986; Iryu et al., 1995; Webster et al., 1998). Data from modern reefs o¡ the Huon Peninsula and Madang on the north coast of Papua New Guinea indicate such Acropora palifera dominated communities are common in less than 5 m (Nakamori et al., 1994a, 1995b; Pandol¢ and Minchin, 1995). Thus, we interpret the paleoenvironment of assemblage A as a shallow (V0^5 m), high energy, outer reef

Fig. 3. Coral reef limestone showing examples of assemblage A corals. This assemblage is dominated by (A) robust branches; (B) ridges of Acropora palifera; (C) Acropora humilis?; and (D) submassive Porites horizontalata.

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Fig. 4. Coral reef limestone showing examples of assemblage B corals. This assemblage is dominated by (A) overlapping encrusting corals such as Montipora sp.; (B) Favites pentagona? (1), Pavona varians (2), and Montipora (3); (C) Montipora corbettensis? (1), Favites pentagona (2), and Montipora monasteriata? (3). Note the well preserved bivalve (Lithophaga? sp.) borings within the corals. (D) Overlapping plates or laminar of Echinopora hirsutissima or gemmacea are also common.

£at/upper reef slope environment, best developed on windward margins exposed to strong wave activity. Assemblage B is dominated by encrusting Montipora (M. monasteriata, M. corbettensis, M. cf. aequituberculata), and Porites sp. (P. horizontalata) with associated faviids (Montastrea salebrosa?, M. curta?, Cyphastrea sp., Favia laxa, Echinopora hirsutissima or E. gemmacea) and agariciids (Siderastrea savignyana, Pseudosiderastrea tayamai?, Psammocora sp.) (Fig. 4A^D). While the species assemblage is still indicative of shallow, perhaps upper-reef slope environments, this community probably developed in lower to moderate energy reef conditions, indicative of more sheltered margins, particularly when compared with assemblage A (Done, 1982; Veron, 1986; Montaggioni et al., 1997; Pandol¢ and Minchin, 1995). Signi¢cant nongeniculate coralline algal crusts

are associated with the abundant coral framework within this facies (Fig. 4A). These crusts are dominated by mastophoroids such as Neogoniolithon fosliei and Hydrolithon onkodes and associated Lithophyllum gr. pustulatum, and commonly occur as 2^5-mm crusts sandwiched between encrusting corals in assemblage B (Fig. 5A^C), and around branching corals. This same assemblage is common in shallow modern Indo-Paci¢c reefs ( 6 15 m) from the Great Barrier Reef (Borowitzka and Larkum, 1986; Adey, 1986), Tahiti, New Caledonia, Ryukyus (Montaggioni et al., 1997; Cabioch et al., 1999; Iryu et al., 1995), and Papua New Guinea (Matsuda et al., 1994). Thus, this mastophoroid assemblage (M) is characteristic of shallow (V 6 10^15 m) tropical reef environments. Summarizing, the coralgal composition indicates that the coral reef limestone was deposited

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Fig. 5. (A) Coral reef limestone showing abundant and well developed crusts of non-geniculate coralline algae (1). The mastophoroid assemblage is dominated by thick crusts of (B) Neogoniolithon sp.; (C) Neogoniolithon fosliei (1) and Hydrolithon onkodes (2).

in a shallow reef £at to upper reef slope environment, in probably 0^10 m of water, but certainly less than 20 m. 3.1.2. Coralline algal^foraminiferal nodule limestone This limestone is characterized by coral and minor mud fragments (up to 5 cm) encrusted by thin (0.2^4 mm) overlapping layers of non-geniculate coralline algae and encrusting acervulinid foraminifera (Gypsina or Acervulina ? sp.). Commonly, the coral fragments forming the nodule nuclei are extensively micritized, bored and in¢lled with peloidal micrite and minor bioclasts such as large benthic foraminifera (Amphistegina sp.) and molluscs. Texturally, this limestone ranges from rudstones to £oatstones (Fig. 6A,B), with abundant large benthic foraminifera, and minor contributions of Halimeda and mud making up the matrix.

The non-geniculate coralline algae forming the nodule crusts are dominated by a melobesioid assemblage (B), composed of thin crusts of Mesophyllum sp., with associated Lithothamnion sp., Lithoporella sp., Sporolithon sp., Peyssonnelia sp. and common acervulinids (Fig. 6B^D). Similar coralline assemblages exist on modern deepwater (Adey, 1986) platforms in the southern Great Barrier Reef and around Hawaii ranging from V10 to 90 m, but these are commonly observed below 60 m (Adey, 1979; Adey et al., 1982; Lund et al., 2000). In the Gulf of Mexico, Minnery et al. (1985) described analogous foraminifera^algal nodules currently forming at 50^75 m, while Reid and Macintyre (1988) recorded a similar facies forming at 30^60 m in the eastern Caribbean. Furthermore, on Florida’s outer shelf similar encrusting foraminifera/coralline nodules have been observed at 35^56 m in quiet water conditions (Prager and Ginsburg, 1989).

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Fig. 6. Coralline algal^foraminiferal nodule limestone. Texturally, these limestones range between rudstones (A) and £oatstones (B), with coral and mud nuclei, respectively. (C) Photomicrograph showing the internal structure of a nodule (similar to (A)), with a bored coral fragment forming the nucleus (1) and a thin inner crust of shallow water Hydrolithon onkodes (2), which is in turn encrusted by deepwater, non-reefal Mesophyllum sp. and Lithothamnion sp. (3). (D) Photomicrograph showing the internal structure of a nodule with a nucleus composed of micrite (1), and a well developed outer crust composed of deep water, nonreefal Lithothamnion sp., Sporolithon sp., Peyssonnelia sp., Lithporella sp. and abundant inter-layered acervulinid foraminifera (2). (E) Halimeda limestone with numerous segments or plates ‘£oating’ within a micrite matrix. (F) Photomicrograph of the Halimeda (1) dominated wackestone with abundant large benthic foraminifera Amphistegina sp. (2).

Large benthic foraminifera characterized by Amphistegina radiata, A. lessonii, and Heterostegina depressa commonly comprise the bioclasts making up the matrix in this facies. Recent work on the distribution and ecology of large benthic foraminifera in the Indo-Paci¢c (Indone-

sia: Renema and Troelstra, 2001 ; Ryukyu Islands: Hohenegger, 1995; Hohenegger et al., 1999, 2000) suggest that this assemblage usually lives on deep fore-reef slopes or shallow shelves between V20^50 m. While the coralline assemblages alone might suggest deep water conditions

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Fig. 7. (A) Coralline algal foraminiferal crust limestone characterized by alternating thin crusts of (B) non-geniculate algae dominated by the melobesioid assemblage (1), encrusting foraminifera (2), and micrite (3). (C) Associated with this facies are large benthic foraminifera such as Cycloclypeus carpenteri (1), encrusted here by a thin layer of Mesophyllum sp. (2). (D^G) Photomicrographs showing thin crusts of (D) Mesophyllum sp. (1), acervulinids (2), (E) Lithoporella sp., (F) Lithothamnion sp., and (G) Peyssonnelia sp. (1).

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(perhaps below 60 m), their characteristic nodule morphology and large benthic foraminiferal composition indicate that this nodule limestone was deposited in a deep, fore-reef slope (V20^60 m) setting. 3.1.3. Halimeda limestone This limestone is characterized by abundant Halimeda segments (up to 1 cm), associated with large benthic foraminifera (A. radiate, A. lessonii, Operculina and H. depressa), molluscs and coral fragments £oating within a matrix commonly composed of peloidal micrite (Fig. 6E,F). Texturally, this facies ranges from packstones to wackestones, and commonly grades into and is found at bathymetric levels similar to those of the coralline^foraminiferal nodule limestone. Recent studies have shown that Halimeda can accumulate as signi¢cant deposits in shallow lagoons and on deeper shelves, e.g. the Halimeda banks in the Great Barrier Reef, east Java Sea and Nicaraguan Rise (Marshall and Davies, 1988; Roberts et al., 1988; Hine et al., 1988).

Coralline algal dominated nodules are also found on the tops of these banks in the east Java Sea and the Nicaraguan Rise (Hine et al., 1988; Phipps and Roberts, 1988). Because of its co-occurrence with the nodule limestone and the similar composition of benthic foraminifera, we conclude that the Halimeda limestone was deposited in a similar deep fore-reef slope (V20^60 m) environment. 3.1.4. Coralline algal^foraminiferal crust limestone This limestone is characterized by well developed coralline algal^foraminiferal bindstones (Fig. 7A). Morphologically, they form overlapping non-geniculate coralline crusts of thin encrusting thalli and minor warty plants growing one over the other (Fig. 7B). Sandwiched between these layers are encrusting acervulinid foraminifera and layers of micrite and minor encrusting corals (Montipora sp. and Porites sp.), bryozoans, large benthic foraminifera (Palaeonummulites sp., Amphistegina sp., Cycloclypeus carpenteri and Heterostegina operculinoides) (Fig. 7C).

Fig. 8. (A) Photomicrograph of the planktonic foraminifera limestone composed of abundant planktonic foraminifera (1), delicate molluscs (2), smaller benthic foraminifera and minor terrigenous grains (e.g. quartz and rock fragments) (3). (B) Photomicrograph of associated large benthic foraminifera such as Cycloclypeus carpenteri (1), and Amphistegina radiata (2). (C) Photomicrograph showing planktonic limestone ¢lling a boring which cross cuts an encrusting acervulinid foraminifera (1).

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Similar to the nodule limestone, the corallines as characterized by a melobesioid assemblage (B) composed primarily of Mesophyllum sp. and associated Lithothamnion sp., Lithoporella sp., Sporolithon sp., and Peyssonnelia sp. (Fig. 7D^G). The thin encrusting to warty morphology of the algae suggests a low to moderate energy and probably deeper environment for accretion (Bosence, 1983, 1985; Womersley, 1996). Almost identical algal frameworks form modern buildups in deeper fore-reef settings in the Great Barrier Reef in the Capricorns (60^90 m) (Lund, 1994) and o¡ Fraser Island (90^110 m) (Marshall et al., 1998), o¡ New Caledonia (60^80 m) (Rio et al., 1991, and the Gulf of Mexico (70^90 m) (Minnery et al., 1985). In these settings, the deep water, tropical^subtropical buildups form low-lying, meterscale high, hummocky structures. Based on the similarities in structure, morphology and species composition, we suggest the coralline^foraminiferal crust limestone facies developed in a deeper fore-reef slope environment (V60^90 m). 3.1.5. Planktonic limestone This facies is dominated by planktonic forami-

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nifera (e.g. Globeroides), with delicate molluscs, small benthic foraminifera (Textularia sp.) and some large benthic foraminifera (Amphistegina radiata, A. lessonii, Cycloclypeus carpenteri, and Heterostegina depressa) (Fig. 8A^C). Texturally, this facies occurs as mudstones to wackestones, and is commonly stained by Fe^Mn oxides with minor mud-sized terrigenous grains (e.g. quartz and rock fragments) (Fig. 8A,C). This facies most commonly ¢lls cavities created by bioerosion or as centimeter-scale caps on the top surfaces of other facies. Cross cutting (Fig. 8C) and superposition relationships clearly indicate that deposition occurred after the development of the other facies. In some cases, bioerosion (by clionid sponges and bivalves) and in¢lling by micrite and planktonic limestone is so intense that the original depositional texture of the rock can be completely obscured. This and the other facies are often capped by well developed Mn and Fe crusts, that range from thin (1 mm) to centimeterscale bulbous crusts coating and commonly replacing much of the original depositional texture. Although it is di⁄cult to precisely de¢ne the paleobathymetry of this style of pelagic/hemipela-

Fig. 9. Idealized facies model showing the likely paleoenvironmental settings for the sedimentary facies recorded in the drowned platforms in the Huon Gulf, Papua New Guinea.

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Table 3 Morphology, composition and paleoenvironmental settings of the drowned carbonate platforms in the Huon Gulf, Papua New Guinea Platform depth

Platform morphology

239 m

Single platform

625 m

Composite (1) platform with (2) multiple pinnacles

1113 m

Composite platform showing two terrace levels

Composite platform, with two terrace levels and multiple pinnacles

Sample range

Sedimentary facies

Coral assemblages

Coralline algal assemblages

Paleoenvironmental settings

16

239^300 m

NA

Melobesioid (B)

Deeper fore-reef slope (build up) (V60^90 m)

31

(1) 625^698 m

(1) NA

(1) Melobesioid (B)

(1) Deeper fore-reef slope (build up) (V60^90 m)

(2) 726 m

Coralline algal^ foraminiferal crust limestone (1) Coralline algal^ foraminiferal crust limestone (2) Coral reef limestone.

(2) Assemblage B?

(2) Mastophoroid (M)

(1) 823^837 m

(1) Coral reef limestone

(1) Assemblage B

(1) Mastophoroid (M)

(2) 837^840 m

(2) Coralline algal^ foraminiferal crust limestone (1) Coral reef limestone

(2) NA

(2) Melobesioid (B)

(2) Reef, shallow (V 6 10 m), low^moderate energy, upper reef slope (1) Reef, shallow (V 6 10 m), low^moderate energy, upper reef slope (2) Deeper fore-reef slope (build up) (V60^90 m)

(1) Assemblage B

(1) Mastophoroid (M)

(2) NA

(2) Melobesioid (B)

(3) Assemblage B

(3) Mastophoroid (M)

(3) Reef, shallow (V 6 10 m), low^moderate energy, upper reef slope

(1) NA

(1) Melobesioid (B)

(1) Deep fore-reef slope (V20^60 m)

(2) 1218^1223 m

(2) Coralline algal^ foraminiferal nodule limestone (3) Coral reef limestone. NB loose material derived from the top of the platform (1) Coralline algal^ foraminiferal nodule limestone (2) Halimeda limestone

(2) NA

(2) NA

(3) 1264^1305 m

(3) Coral reef limestone

(3) Assemblage B

(2) Mastophoroid (M)

(1) 1612^1682 m

(1) Coral reef limestone

(1) Assemblage B

(1) Mastophoroid (M)

(2) 1623^1650 m

(2) NA

(2) Melobesioid (B)

(3) 1649^1682 m

(2) Coralline algal^ foraminiferal nodule limestone (3) Halimeda limestone

(2) Deep fore-reef slope (V20^60 m) (3) Reef, shallow (V 6 10 m), low-moderate energy, upper reef slope (1) Reef, shallow (V 6 10 m), low-moderate energy, upper reef slope (2) Deep fore-reef slope (V20^60 m)

(3) NA

(3) NA

1947^1995 m

Coral reef limestone

Assemblage A

Mastophoroid (M)

58

45

(1) 1130^1145 m

(2) 1137 m

(3) 1145^1174 m

1216 m

1612 m

1947 m

Single platform with pinnacles

Single platform with pinnacles

Composite platform, with two terrace levels

54

47

46

(1) 1216^1217 m

(1) Reef, shallow (V 6 10 m), low^moderate energy, upper reef slope (2) Deep fore-reef slope (V20^60 m)

(3) Deep fore-reef slope (V20^60 m) Reef, shallow, (V 6 5 m), high energy, outer reef £at to upper reef slope

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823 m

Sample No.

2393 m

Pinnacle with two terrace levels

8

2393^2463 m

Coral reef limestone

Assemblage A

Mastophoroid (M)

(5) Reef, shallow (V 6 10 m), low^moderate energy, upper reef slope (1) Reef, shallow, (V 6 5 m), high energy, outer reef £at to upper reef slope (5) Assemblage B

(5) Mastophoroid (M)

(4) 2121m (platform)

(4) Coralline algal^ foraminiferal nodule limestone (5) 2136^2173 m (W platform) (5) Coral reef limestone

(4) NA

(4) Melobesioid (B)

(3) Reef, shallow, (V 6 5 m), high energy, characteristic of windward margins, outer reef £at to upper reef slope (4) Deep fore-reef slope (V20^60 m) (3) Assemblage A

(3) Mastophoroid (M)

(2) 2054^2076 m (pinnacle)

(2) Coralline algal^ foraminiferal nodule limestone (3) 2121^2124 m (E platform) (3) Coral reef limestone

(2) NA

(2) Melobesioid (B)

(1) Reef, shallow, (V 6 5 m), high energy, outer reef £at to upper reef slope (2) Deep fore-reef slope (V20^60 m) (1) Mastophoroid (M) (1) Assemblage A 39 Composite platform with two terrace levels and multiple pinnacles 2121 m

(1) 2054^2076 m (pinnacle)

(1) Coral reef limestone

Paleoenvironmental settings Coralline algal assemblages Coral assemblages Sedimentary facies Sample range Sample No. Platform morphology Platform depth

Table 3 Morphology, composition and paleoenvironmental settings of the drowned carbonate platforms in the Huon Gulf, Papua New Guinea

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gic facies, the texture and composition suggest deposition in non-reefal, deep open shelf settings (V s 60 m). These ¢ve facies and their paleoenvironmental settings are summarized in Fig. 9. This ¢gure represents an idealized facies model ¢tting all the observed facies into a realistic paleoenvironmental scheme. It is from this framework that we describe and interpret the facies distribution and paleoenvironmental signi¢cance for each of the drowned platforms. 3.2. Platform composition, facies distribution and paleoenvironmental signi¢cance The major variations in sedimentary facies, their relationships and implications for the paleoenvironmental settings of each platform are discussed below and summarized in Table 3. The results and discussion are organized by their increasing current bathymetric positions : (1) shallow platforms (239, 625, 823 m), (2) middle platforms (1113, 1217, 1612 m), and (3) deep platforms (1947, 2121, 2397 m). 3.2.1. Shallow platforms 3.2.1.1. The 239-m platform. This platform is dominated by the coralline^foraminiferal crust facies and is recorded at depths ranging from 239 to 300 m. Although it is not possible to estimate the stratigraphic thickness, a 50-cm-thick sample was recovered from the top of the platform, indicating substantial accretion. The characteristic crustose morphology, melobesioid coralline assemblages (B) and acervulinids indicate the top of this platform developed in a deeper fore-reef slope environment (V60^90 m) prior to complete platform drowning. The general lack of corals, apart from a few coral fragments (Galaxea astreata ?, Porites sp. and Montipora sp.), also suggests a non-reefal environment. 3.2.1.2. The 625-m platform. Sampling focused on the platform edge (625^698 m) and the base of a seaward pinnacle rising to 550 m. The platform edge is again dominated by the coralline^ foraminiferal crust facies, indicating a deeper

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fore-reef slope environment (V60^90 m). Coral reef limestone facies composed of assemblage B corals (fragments of Galaxea, Mussidae, Porites sp. and Leptoria phrygia) and encrusted by a mastophoroid coralline crusts were recovered loose from the base of the pinnacle. Side scan images show well developed debris ¢elds adjacent to and on the £anks of the steep pinnacles (Webster et al., 2002), indicating that the pinnacles represent a di¡erent phase of platform growth characterized by coral reef growing in shallow (V 6 10 m), low to moderate energy, upper reef slope settings. 3.2.1.3. The 823-m platform. Coral reef limestones are common near the top (823^840 m) of this platform. Assemblage B corals (encrusting Montipora monasteriata, Porites lichen, Favites pentagona, Cyphastrea microphthalma, tabulate Acropora sp.) and crusts of mastophoroid corallines (M) indicate that the top of this platform developed in shallow (V 6 10 m), low to moderate energy, upper reef slope settings. At almost the same bathymetric level, numerous samples of coralline^foraminiferal crust limestone (V60^90 m) were found, indicating platform deepening during a later phase of platform growth. 3.2.2. Middle platforms 3.2.2.1. The 1113-m platform. This platform is dominated by coral reef limestone. Between 1135 and 1145 m in-situ assemblage B corals (encrusting Montipora monasteriata, M. tuberculosa, Porites sp. and minor Pavona explanulata, Favites halicora, Stylocoeniella guentheri, branching Stylophora subseriata, Coscinaraea columna ? and tabulate Acropora hyacinthus) and mastophoroid coralline crusts dominate. This coralgal assemblage indicates the top of the platform grew in shallow (V 6 10 m), low^moderate energy, upper reef slope conditions. Some coralline algal^foraminiferal nodule limestones were also recorded from the top of the platform (1137 m). Close examination of the nodules revealed an important sequence of non-geniculate coralline algal crusts (Fig. 6C). Typically, the inner crusts directly overlying the coral nuclei (Fig. 6C, 1) are composed of the shallow water, mastophoroid assemblage

(Neogoniolithon sp.) (Fig. 6C, 2) while the outer layers are composed of the deep fore-reef slope (V20^60 m) melobesioids (Sporolithon sp., Lithothamnion sp., Mesophyllum sp., and Lithophyllum. gr. pustulatum) (Fig. 6C, 3). This sequence implies either: (1) transport of shallow coral reef material to a deeper depositional site, or (2) deepening in place. From 1145 to 1174 m, allochthonous corals of the same taxonomic composition as the top of the platform were found. 3.2.2.2. The 1217-m platform. The top of the platform, between 1216 and 1223 m, is composed of mixed Halimeda and coralline^foraminiferal nodule limestones. The composition of coralline algae (melobesioid assemblage) and benthic foraminifera indicates that they developed in a deep fore-reef slope (V20^60 m) environment. From 1264 to 1305 m, the platform is dominated by coral reef limestones composed of assemblage B corals (encrusting Montipora monasteriata, M. corbettensis, M. cf. aequituberculata, and encrusting P. lichen and submassive/massive P. horizontalata, P. lobata) with associated encrusting or foliacous Pavona varians, Pachyseris speciosa, Favia stelligera ?, Favites pentagona, and Favites bestae?), and well developed mastophoroid coralline crusts. This coralgal assemblage indicates shallow (V 6 10 m), low^moderate energy, upper reef slope conditions. The occurrence of deep (V20^ 60 m), fore-reef slope environment facies bathymetrically above the coral limestone is consistent with platform deepening. 3.2.2.3. The 1612-m platform. Coral reef limestone is common between 1612 and 1682 m and is characterized by assemblage B corals (encrusting Montipora undata? M. cf. monastriata, massive/encrusting Porites sp., submassive/encrusting Montastrea salebrosa ?, M. curta?, Cyphastrea, Favia laxa, and Echinopora hirsutissima, E. gemmacea and Siderastrea savignyana, Pseudosiderastrea tayamai?, Psammocora sp.) and abundant mastophoroid coralline crusts (V 6 10 m). Mixed Halimeda and coralline algal^foraminiferal nodule limestones were also collected at similar bathymetric depths. Like the 1113-m platform, the inner crusts of these nodules are composed of the

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shallow water, mastophoroid assemblage (Hydrolithon onkodes, Spongites sp.) while the outer layers are composed of the deep fore-reef slope (V20^60 m) melobesioid assemblage and associated acervulinids. Again, this indicates either: (1) the transport of shallow coral reef material to a deeper depositional site, or (2) deepening in place. The co-occurrence of the deep, fore-reef slope (V20^60 m) facies with shallow coral limestones indicates platform deepening. 3.2.3. Deep water platforms 3.2.3.1. The 1947-m platform. This platform is entirely dominated by coral reef limestone consisting of assemblage A corals (e.g. robust branching and ridge-like Acropora palifera, submassive/encrusting Montipora tuberculosa, with associated massive and submassive Porites horizontalata, Siderastrea savignyana, Favia laxa?, Montastrea multipunctata) and mastophoroid coralline crusts. This coralgal community is typical of very shallow ( 6 5 m), upper reef slope/reef crest environments and is characteristic of windward margins. Nakamori et al. (1994a, 1995a, 1995b) observed a similar assemblage in the modern and uplifted Holocene and Pleistocene reefs of the Huon Peninsula and considered this community to be indicative of high energy, shallow (3^5 m), middle reef slope environments. 3.2.3.2. The 2121-m platform. Sampling focused on the platform and a single pinnacle above the platform. The pinnacle (2054^2076 m) has both coral reef limestones (assemblage A corals and mastophoroid coralline algae assemblages) and coralline algal^foraminiferal nodule limestones at similar bathymetric levels. Therefore, the pinnacle records the likely transition from shallow ( 6 5 m), upper reef slope/reef crest to deep fore-reef slope (V20^60 m) environments. Both the western and eastern parts of the platform are dominated by coral reef limestone. However, analysis of the coral assemblages reveals important di¡erences. The west platform (2136^2173 m) is composed of assemblage B corals (e.g. encrusting Montipora informis, Psammocora super¢cialis, Stylocoeniella guentheri, massive Porites

77

sp.) indicating a shallow (V0^10 m), low^moderate energy, upper reef slope environment. Side scan imagery from the western part of the platform suggests that it was probably sheltered locally by the seaward portion of the platform (Webster et al., 2002). In contrast, the eastern platform (2121^2124 m) is dominated by coral assemblage A (robust branching/digitate Acropora humilis group, tabulate Acropora hyacinthus group, encrusting Montipora sp., massive Pavona clavus? and Porites sp.) with associated mastophoroid coralline crusts. This coralgal assemblage suggests a shallow (V 6 3 m), high energy environment, characteristic of windward margins (e.g. outer reef £at to upper reef slope) such as those described by Nakamori et al. (1994b, 1995b). Coralline algal^foraminiferal nodule limestones were also recovered from the top of the eastern platform (2121^2124 m). Examination of these samples revealed a clear deepening sequence (Fig. 10A^D). For example, coralline algal^foraminiferal nodule limestones with the concentric layers of melobesioid coralline crusts (e.g. Mesophyllum sp.) and acervulinids (Fig. 10B) grade vertically into a 1^2-cm-thick mudstone composed of peloids, larger benthic foraminifera (Amphistegina sp.), mollusc and bryozoan fragments (Fig. 10C). The top of this sequence is marked by a sharp boundary between peloidal mud and a thin (1-cm) cap of planktonic foraminifera limestone composed of Fe^Mn stained planktonic foraminifera, delicate mollusc and terrigenous grains (Fig. 10D). The occurrence of the deep, fore-reef slope (V20^60 m) nodule facies on top of the shallow coral limestones indicates platform deepening and possible drowning. Furthermore, the sharp boundary (Fig. 10D, 1) at the top of the nodule limestone combined with the very different nature of the sediments indicates a hiatus between the deposition of the coralline algal^ foraminiferal nodule and the planktonic limestone. 3.2.3.3. The 2393-m pinnacle. The few samples collected from this deep pinnacle are composed of coral reef limestone and are dominated by assemblage A corals (e.g. robust branching Acropora palifera, Acropora grandis, and massive Por-

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Fig. 10. Drowning signature recorded in the 2121-m platform. (A) Hand specimen showing an upward sequence from coralline algal^foraminiferal nodule limestone (B), to peloidal mud (C), to planktonic foraminifera limestone (D). (B) Photomicrograph showing the internal structure of a nodule with a bored and micritized coral(?) nucleus (1). This in turn is encrusted by concentric layers of coralline algal algae (e.g. Mesophyllum sp.) (2) and acervulinids (3). (C) Photomicrograph showing a peloidal (1) mudstone with large benthic foraminifera (Amphistegina sp.) (2). (D) Photomicrograph showing the sharp contact (1) between the peloidal mudstone and the overlying planktonic foraminifera limestone composed of planktonic foraminifera (2) and terrigenous grains (3).

ites sp.). This coralgal community is typical of shallow ( 6 5 m), upper reef slope/reef crest environments, characteristic of windward margins, exposed to strong wave activity.

4. Discussion The sedimentary and coralgal data indicate that the platforms record three main stages of development : (1) initiation and growth, characterized by shallow coral reef development as the platforms grew close to sea-level during sea-level lowstands,

(2) incipient drowning marked by a shift to coralline algal^foraminiferal nodule, crust and Halimeda limestones as platforms began to drown during rapid eustatic sea-level rises and continued subsidence, and (3) the complete drowning of the platforms characterized by platform ‘turn o¡’, followed by increased bioerosion, Fe^Mn precipitation, and pelagic/hemipelagic sedimentation as the platforms ¢nally drop below the photic zone. We discuss these stages of platform development and drowning, and their implications for climate change and tectonic subsidence in the Huon Gulf over the last V450 ka.

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4.1. Initiation and growth of the platforms 4.1.1. Platform initiation and growth The development of carbonate platforms in foreland basins is controlled by the growth potential of the carbonate system (in£uenced by nutrients, salinity, temperature, sediment input, energy, etc.), accommodation space and relative sealevel £uctuations, which in turn are a function of tectonic subsidence and superimposed eustatic sea-level changes (Galewsky et al., 1996; Dorobek, 1995). While we have no direct data detailing the initiation of the platform development in the Huon Gulf, numerical modeling by Wallace (2002) suggests that the platforms probably initiated growth as eustatic sea level stabilized sometime during the highstand phase of each major sea-level cycle. Depending on the relationship between the subsequent eustatic sea-level fall and rise during small interstadial events, subsidence rates and platform growth rates, the platforms may have experienced repeated short intervals of reef growth, subaerial exposure and platform drowning. This would have the e¡ect of producing complex, stacked platform sequences with signi¢cant facies variations separated by drowning and subaerial exposure horizons. A separate paper modeling the internal structure and composition of each platform will be presented elsewhere. Ultimately, the only way to test these models is to collect high-resolution seismic data and then drill the platforms. Regardless of their internal con¢guration, by the time eustatic sea level fall stabilized during the major lowstands a thick sequence of carbonates had accumulated (100^200 m), the top of which is dominated by coral reef limestone. 4.1.2. Lowstand coral reef development and paleoenvironmental implications This period of platform development is characterized by lowstand coral reef development. Over 73 species (Table 1) were recovered from these lowstand reefs, more than half of that reported from their highstand counterparts uplifted on the Huon Peninsula (Pandol¢, 1995). The Huon Gulf total, although lower, represents a diverse coral fauna given the extremely limited and random sampling strategy possible using an ROV.

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The coralgal assemblages (A and B) constrain fairly precisely the paleobathymetry of the coral reef dominated stage (Table 2). Based on comparison with modern reefs in Papua New Guinea and the Indo-Paci¢c, we conclude that the top of the platforms grew within 20 m of relative sea level. In fact, the data suggest that the shallow (823 m), middle platforms (1113, 1216, 1612 m) probably grew within 10 m of sea level, while the deeper platforms (1947, 2121, 2397 m) were within 5 m. Accepting the ecological limitations of our data, we suggest that despite experiencing major environmental perturbations (e.g. rapid relative sea-level rise, drowning, subaerial exposure, seasurface temperature £uctuations) over the last 450 ka, reef growth in the Huon Gulf has been able to re-establish itself time and time again, producing thick (100^200 m) platforms, the tops of which are dominated by shallow coral assemblages similar to modern Indo-Paci¢c reefs. This ability of coral reefs to grow and re-establish themselves despite seemingly catastrophic events has been observed in the successive Pleistocene uplifted reefs from the Huon Peninsula, Barbados (Jackson, 1992; Pandol¢, 1995; Nakamori et al., 1994a) and more recently in a series of stacked reefs below the Great Barrier Reef (Webster and Davies, 2003). The second major conclusion to emerge from this study is the clear di¡erence in coral assemblages between the shallow, middle and deep platforms. The shallow and middle platforms are characterized by assemblage B corals (shallow, low to moderate energy conditions) while the deep platforms are dominated by assemblage A corals (shallow, very high energy conditions). Although little is known about modern fringing reefs along the Morobe coastline, the present oceanographic conditions are not consistent with a very high energy, windward margin (von der Borch, 1972; McAlpine et al., 1983). A likely modern analogue for this style of high energy reef growth occurs on the north coast of Papua New Guinea at Madang and the Huon Peninsula. For example, the exposed outer reefs of the Madang Lagoon and the reefs fringing the Huon Peninsula are composed of very similar coral assemblages to the assemblage A corals we recov-

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ered from the deep platforms in the Huon Gulf (Pandol¢ and Minchin, 1995; Nakamori et al., 1994b). Forming the eastern extension of modern reefs on the Morobe coastline, the more exposed Trobriand platform in the Solomon Sea (Fig. 1A) is another possible analogue for modern high energy reef growth. This platform forms a broad shallow ( 6 100 m) area rimmed by carbonate reefs; it is relatively stable recording only minor subsidence (von der Borch, 1972). Summarizing, the fossil coral data suggest that

paleoenvironmental conditions have changed substantially in the Huon Gulf over the past V450 ka, with a shift from high energy conditions, characteristic of windward reef margins to lower more moderate energy reef conditions V350^300 ka. How can we explain this apparent oceanographic change ? We propose two di¡erent possible mechanisms : (1) closure of the Huon Gulf due to the rotation and uplift of the Huon Peninsula, and/or (2) variation in more regional factors such as the position of the Intertropical Convergence Zone

Fig. 11. (A). Position of the Papua New Guinea paleo-shoreline, trench and reef growth at 450 ka (green line) relative to their modern positions (gray lines). (B^F) These data in 100-ka time slices along with likely wind and wave vectors from the NW, NE and SE. These ¢gures were constructed by rotating the South Bismarck plate (upper plate) relative to the Australian plate (lower plate) which is held ¢xed here about a well known pole of rotation between the two plates (After Wallace, 2002). For each ¢gure the position of the modern coastline is shown as a light gray line.

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(ITCZ) and the intensity of successive lowstand periods. 4.1.3. Tectonic closure of the Huon Gulf Recent tectonic reconstructions of the last 450 ka of vertical and lateral plate motions in the Huon Gulf (Wallace, 2002) provide some clues to explaining the change in coral assemblage patterns. Fig. 11A shows the likely position of the Huon Gulf shoreline 450 ka accounting for the rotation and uplift of the Huon Peninsula as the South Bismarck plate collides with and overrides the Australian plate (Abbott et al., 1994; Wallace, 2002). We speculate that oceanographic conditions in the Gulf were considerably di¡erent at this time as a result of this past shoreline con¢guration. Further, continued tectonic reorganization over the last 450 ka has acted to close or narrow the Huon Gulf, causing a signi¢cant shift in oceanographic conditions. This process is illustrated in a series of expanded diagrams showing the likely position of the present and paleo-coastlines, trench, reef and likely wind/wave vectors in£uencing reef development at 450-ka, 350-ka, 250-ka, 150-ka, and 0-ka time steps (Fig. 11B^F). 4.1.3.1. 450^350 ka. The situation of the Huon Gulf area in this period is illustrated in Fig. 11B,C. Its shoreline con¢guration was considerably di¡erent 450 ka ago. Our best estimates suggest that the SE coastline of the Huon Peninsula was 40^50 km farther NW than its current position. Furthermore, given the average published uplift rates (V2 m/ky; Chappell et al., 1996; Abbott et al., 1994), the Huon Peninsula was V1 km lower in elevation. The di¡erent shoreline and lower elevation would have opened the Huon Gulf to substantially more wind and wave energy, from the N, NE and NW during the monsoon season. In this more open oceanographic setting, coral reefs in the Huon Gulf would have experienced the higher energy conditions, characteristic of windward margins (assemblage A) that appear to be recorded in the deeper pinnacles and platforms (2397 and 2121 m). Plate motion continued over the next 100 ka without much modi¢cation of oceanographic conditions, so that the 1947-m platform is also characterized by high energy reef growth.

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4.1.3.2. 250, 150 and 0 ka. The situation of the Huon Gulf area at these time steps is illustrated in Fig. 11D^F. By 250 ka, the continued uplift and rotation of the Huon Peninsula had narrowed the Huon Gulf considerably. This caused a major shift in oceanographic conditions towards the more sheltered (from NW, N and NE directions), lower energy settings characterized by the assemblage B corals which dominate the middle and shallow platforms (1650, 1212, 1113, and 823 m). The period between 350 and 300 ka may represent an important oceanographic threshold in the Huon Gulf, because it is from this time that assemblage B corals (lower energy) have dominated the tops of the platforms. Since the Huon coastline approached its current con¢guration, the Gulf has experienced only low to moderate energy conditions, particularly when compared with the more exposed conditions of the Madang, northern Huon Peninsula, and Trobriand platform reefs. 4.1.4. Regional changes in climate/oceanography over the last V450 ka We favor tectonic closure because it is a more direct mechanism for changing oceanographic conditions in the Huon Gulf. However, we cannot completely rule out the in£uence of more regional variations in climate, such as shifts in the ITCZ or changes in the intensity and magnitude of lowstands. The ITCZ is a major feature a¡ecting atmospheric circulation in the Paci¢c Ocean. It represents the near-equatorial region where air masses from the Northern and Southern hemispheres meet. Studies in the equatorial Paci¢c by Rea (1994) recorded variations in the accumulation rate and grain size of aeolian dust. Over Cenozoic time scales, these variations are thought to re£ect changing climate regimes (i.e. trade winds) associated with shifts in the latitudinal position of the ITCZ. Recent results from the eastern equatorial Paci¢c indicate that over smaller time scales the position of the ITCZ shifts a few degrees further south during the glacials and retreats north during the interglacials (Rea, pers. commun.). Although the ITCZ is less distinct in the western Paci¢c, we speculate that variations in how far it shifts south during the glacial periods might in£u-

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ence winds and/or the distribution and frequency of storms in the Huon Gulf. While there is no direct evidence of this happening in the western Paci¢c, there are strong indications of a major change in the climate regime of the equatorial Paci¢c V300 ka (i.e. mid-Brunhes climate event) (Chuey et al., 1987; Pisias and Rea, 1988; Rea, 1994). Several paleoclimate proxies (e.g. eolian grain size) indicate increased amplitude variations from 850^300 ka, characterized by increased wind intensities when compared with period 300^0 ka (Pisias and Rea, 1988). Long-term oxygen isotopic records from the western equatorial Paci¢c (Ontong Java) indicate that the intensity and magnitude of the lowstands has varied over the last 450 ka (Lea et al., in press). Recent Mg/Ca paleothermometry from the same location indicates that the SST values for marine isotope stages (MIS) 12 and 10 are signi¢cantly lower than those for MIS 8, 6, 4, and 2 (Lea et al., in press). These data provide additional evidence of a major shift in climate mode V300^350 ky. In this case the Mg/Ca data indicate more intense lowstands before V300^350 ka (Lea et al., in press). It is possible that any change in lowstand conditions (e.g. SST) could have had a corresponding impact on regional climate/oceanographic conditions. Given that the tops of platforms probably grew mainly during lowstands, reef growth could have been in£uenced by climate variations between the different lowstand oceanographic conditions before and after V350 ka in the Huon Gulf. E¡ects of such regional mechanisms would probably be enhanced by the tectonically more open Huon Gulf of the past, and it is entirely possible that these three main factors (tectonics, position of the ITCZ and the lowstand climate conditions) acted in concert to in£uence coral reef growth in the Gulf over the last 450 ka. 4.2. Incipient platform drowning 4.2.1. Sedimentary and coralgal signatures of platform drowning The sedimentary and coralgal data from the tops of the platforms provide compelling evidence of incipient platform drowning in response to rap-

id eustatic sea-level rise and subsidence. Platform growth switches from a coral reef dominated stage (0^20 m) to a deeper fore-reef stage (20^90 m) characterized by coralline algal^foraminiferal nodule, crust and Halimeda limestones. Examining the data (Table 3), two main facies patterns emerge : (1) the coral reef limestones on deep and middle platforms are capped by a thin veneer of coralline algal^foraminiferal nodule and Halimeda limestones, and (2) the shallow platforms are capped with signi¢cant buildups of coralline algal^foraminiferal crust limestone. Based on the slow vertical accretion rates of crustose coralline algae frameworks (V0.04 mm/yr; Reid and Macintyre, 1988; Bosence, 1983; Lund, 1994), it is very unlikely that the entire platform structure (i.e. 100^ 200 m) thickness was built entirely of this facies. We suggest that the coralline algal^foraminiferal crust limestone to be a well developed buildup over the drowned coral reef limestone and perhaps over a thin veneer of coralline algal^foraminiferal nodule and Halimeda limestones. Based on the facies composition and distribution, we identi¢ed two incipient drowning scenarios perhaps controlled by the rate of relative sealevel rise. Furthermore, we speculate that the difference in subsidence rates between the deep and shallow parts of the margin may in£uence the incipient drowning signatures. 4.2.2. Incipient drowning ^ rapid relative sea-level rise An average subsidence rate of 5.7 m/ky has been estimated for the 1947-m platform (Galewsky et al., 1996). After taking the £exural history of the margin into account, Wallace (2002) estimated that instantaneous subsidence rate, the rate experienced by the platform during the growth and incipient platform drowning, to be V4 m/ ky. For the deeper and middle platforms we speculate that rapid eustatic sea-level rise combined with rapid subsidence acted to quickly drown the shallow coral reef limestone. The resulting uneven and perhaps partially unlithi¢ed reef surface was probably re-worked. In this deep, fore-reef setting (20^60 m), a thin and patchy cover of coralline algal^foraminiferal nodule and Halimeda limestones developed (Fig. 6). The presence of

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thin shallow water, mastophoroid coralline crusts around coral fragments, which are in turn overlain by crusts of deep water melobesioid corallines, may be direct evidence for such rapid drowning. 4.2.3. Incipient drowning ^ slow relative sea-level rise For the shallowest platforms we speculate that the slower subsidence rates experienced by this part of the margin might play a role in the dominance of the coralline algal^foraminiferal limestone on the tops of these platforms. The 239-m platform has been dated at 60.3 ka O 0.5 when, according to recent reconstructions, eustatic sealevel was V51 m below the present day level (Lambeck and Chappell, 2001). On base of this evidence combined with new paleobathymetric constraints on platform depth (60^90 m) we estimate an average subsidence rate of between 1.6^ 2.1 m/ky for this platform. The instantaneous rate accounting for £exural subsidence is probably similar given the shorter time frame (e.g. V60 ka as opposed to 348 ka for the 1947-m platform) (Wallace, 2002). In this scenario, after initial drowning of the coral reef, the platform surface remained within this deep to deeper, fore-reef setting (20^90 m) longer than the middle and deep platforms, assuming eustatic sea-level rise and the growth potential of the platforms were similar. As a result, signi¢cant deep water coralline algal^foraminiferal buildups had time to develop (Fig. 7). Similar coralline algal^foraminiferal crust facies are observed on the 150-m reef around Hawaii, and developed after the drowning of the coral reef during the last deglaciation (Webster et al., 2004). 4.3. Complete platform drowning With continued eustatic sea-level rise and subsidence, the platform surface ¢nally sinks below the photic zone of intensive carbonate production. Beneath this depth ( s 90^120 m) bioerosion, Fe^ Mn precipitation and pelagic/hemipelagic sedimentary processes dominate. 4.3.1. Bioerosion Multiple phases of sponge, worm and mollusc

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boring were present in many samples. Although the cross cutting relationships are complex, we recognize two stages: (1) pre-platform drowning bioerosion, and (2) incipient and post drowning bioerosion. The coral reef limestone contains clear evidence of syndepostional boring. For example, complete specimens of the boring bivalves Lithophaga? sp. (Fig. 4C) can still be seen within their cavities, and likely formed in shallow reef environments (Perry, 1998). Furthermore, numerous borings in this stage of bioerosion are ¢lled with micrite and associated coral and mollusc fragments but lack planktonic foraminifera. With the development of the deeper non-reefal facies (coralline algal^foraminiferal nodule, crust and Halimeda limestone) boring is extensive, with multiple cross cutting in¢lls (Fig. 8C). After the platforms drowned completely bioerosion continues and perhaps dominates. Planktonic limestone commonly ¢lls the borings, in some cases with associated large benthic foraminifera (Cycloclypeus carpenteri). The bathymetric range of C. carpenteri in the Paci¢c is 60^120 m (Iryu et al., 1995; Hohenegger et al., 2000), constraining at least some of these in¢lls to those depths. However, most of these in¢lls are composed of planktonic foraminifera, smaller benthic foraminifera and delicate molluscs, all of which probably continued to accumulate as the platforms subsided to their current depths. Unlithi¢ed olive green pelagic muds routinely ¢ll cavities in samples from all platform depths, providing evidence of continued pelagic sedimentation. 4.3.2. Iron^manganese coatings Thick Fe^Mn crusts (few cm) are common on the outer surfaces and on surfaces buried within cavities of many limestone samples from the Huon Gulf. The timing, chemistry, growth rate and depth at which these crusts develop depend on a range of factors (e.g. exposure time, ocean chemistry) (Hein et al., 2000). James and Ginsburg (1979) observed that Fe^Mn crusts on limestones recovered from the fore-reef slope o¡ the Belize barrier reef system occurred below 40 m. Further, they considered that Mn coatings were more common in the deeper reaches of the margin, below V150 m. Camoin et al., 1998 recov-

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ered similar crusts (phosphate^Mn) from drowned atolls in the NW Paci¢c. They considered these crusts probably formed as the guyot dropped below the photic zone ( s 100 m). Therefore, it is likely that the Fe^Mn crusts covering the submerged platforms in the Huon Gulf began to precipitate from around 100 m, after the complete drowning of the platform. Eventually the platform surface is also colonized by diverse living or recently dead deep ( s 120 m) water communities of ahermatypic corals (Madrepora arbuscula), bryozoans, sponges, echinoderms, molluscs and anemones. In summary, the combined a¡ects of signi¢cant bioerosion, in¢lling and Fe^Mn precipitation probably intensi¢es following the lowering of the platforms below the photic zone (V100^120 m). These processes continue as the platforms reach their current bathymetric levels, sometimes so intense that the original depositional texture of the carbonate material is completely destroyed. 4.4. Model of platform development and drowning Summarizing all available data we now propose a general model of platform development and drowning. The model illustrates the development and drowning of a single platform, during an idealized 100-ky eustatic sea-level cycle and the likely sedimentary and coralgal responses (Fig. 12). 4.4.1. Stage 1 ^ Initiation and growth of the platform The platform initiates growth sometime during the highstand and continues through to lowstand. Depending on rates of eustatic sea-level change, subsidence and platform growth, the platform potentially experiences short intervals of subaerial exposure and platform drowning. This has the a¡ect of producing a complex, stacked platform sequence with signi¢cant facies variations and subaerial exposure horizons (dashed green line on Fig. 12). Eventually, eustatic sea-level fall stabilized during the lowstand period, having produced a thick sequence of carbonates, the top of which is dominated by shallow water coral reef limestone.

4.4.2. Stage 2 ^ Incipient platform drowning Rapid eustatic sea-level rise during the ¢rst part of the transgression drowns the coral reef stage of platform growth. Depending on the rate of relative sea-level rise (eustatic sea-level rise and subsidence) two di¡erent signatures of platform deepening and incipient drowning develop (Fig. 12, 2a,b). In the deep fore-reef slope environment (20^60 m), coralline algal^foraminiferal nodule and Halimeda limestones develop as patchy veneer on top of the drowned coral reef surface on the middle and deep platforms. The shallower platforms experience slower rates of relative sealevel rise (e.g. subsidence rate is less than half the rate in the deeper platforms), and under these conditions signi¢cant coralline algal^foraminiferal crust limestones develop (V60^90 m) (Fig. 8). 4.4.3. Stage 3 ^ Complete platform Subsidence and transgression continue, ¢nally lowering the platform surface below the photic zone (90^120 m). All signi¢cant platform production turns o¡ and the platform drowns completely. Now erosional and diagenetic processes dominate with multiple phases of boring/in¢lling and minor planktonic foraminiferal limestone deposited in the in¢lls and as thin caps. Signi¢cant Fe^Mn crusts develop along with a diverse living or recently dead deep ( s 120 m) water communities (e.g. ahermatypic corals, worms, bryozoans, sponges, echinoderms and molluscs). Subsidence continues until the platforms reach their current depth.

5. Summary and conclusions The drowned platforms in the Huon Gulf record a detailed history of carbonate platform growth, drowning and backstepping in response to the combined a¡ects of rapid subsidence and eustatic sea-level change. Based on a detailed examination of sedimentary, coralgal and paleoenvironmental data we draw the following conclusions : (1) The drowned platforms record ¢ve main sedimentary facies: coral reef, coralline algal^foraminiferal nodule, coralline algal^foraminiferal

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Fig. 12. Summary model showing development and drowning of an idealized platform in the Huon Gulf. Eustatic sea-level curve represents an idealized 100-ka sea-level cycle with the platform growth period shown as a green line. The red circle is the possible timing of each stage of platform drowning. See Fig. 9 for explanation of sedimentary facies symbols.

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crust, Halimeda, and planktonic foraminiferal limestones. They represent a range of paleoenvironments from reef, deep fore-reef slope, deeper fore-reef slope and pelagic/hemipelagic settings. (2) The tops of the platforms are characterized by coral reef limestones that formed during lowstands and grew within V10 m of sea level. (3) Despite major environmental perturbations (e.g. relative sea-level and temperature changes), the platforms and the shallow water coral reefs exposed at the top have been able to re-establish themselves time and time again over the last 450 ka. (4) Variation in coral assemblages between the platforms indicates that the oceanographic conditions in the Huon Gulf have changed over the last 450 ka from very high energy conditions, characteristic of windward margins (assemblage A), to lower, more moderate energy conditions (assemblage B), indicative of more sheltered margins. This is due to tectonic closure of the Huon Gulf related to the rotation and uplift of the Huon Peninsula with possible ampli¢cation as a result of variations in more regional oceanographic/climate factors. (5) The platforms record clear signatures of incipient platform drowning, caused by eustatic sealevel rises associated with deglaciation and continued subsidence. We identi¢ed two di¡erent incipient drowning scenarios, perhaps in£uenced by the di¡ering subsidence rates across the margin. More rapid relative sea-level rise in the middle and deep platforms produced a thin veneer of coralline algal^foraminiferal nodule and Halimeda limestones over shallow coral reef material, while the slower rates of the shallowest platforms allowed thick buildups of coralline algal^foraminiferal crust limestone to develop. (6) Finally, we identi¢ed three main stages of platform development : (1) initiation and growth characterized by shallow coral reef growth as the platforms grew close to sea level during the lowstands, (2) incipient drowning marked by a shift to coralline algal^foraminiferal nodule, crust and Halimeda limestones as the platforms began to drown during rapid eustatic sea-level rise and continued subsidence, and (3) the complete drowning of the platforms characterized by platform ‘turn

o¡’, followed by increased bioerosion, Fe^Mn precipitation and pelagic/hemipelagic sedimentation as the platforms ¢nally drop below the photic zone.

Acknowledgements We thank the captain and crew of RV Melville for their support during the Huon Gulf Cruise, and the crew of the ROV Jason for their outstanding work during the Jason’s last operational mission. We thank Bruce Applegate for processing the DSL-120 data, and David Caress and Jenny Paduan from the Monterey Bay Aquarium Research Institute for reprocessing the SeaBeam data. The initial draft was improved greatly by discussions with Lisa Sloan, Christina Ravelo and David Clague. We especially thank Hugh Davies for his assistance within Papua New Guinea. Finally, we thank Lucien Montaggioni, Toru Nakamori and Guy Cabioch for their detailed and helpful reviews. The project was funded by the US National Science Foundation (Grant No. OCE9907153).

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