Hydrocarbon Resource Studies in the Southwest Pacific, 1982

By H. Gary Greene, Florence L. Wong, and the scientific staff of the 1982 CCOP/SOPAC cruise

GEOLOGICAL SURVEY OPEN-FILE REPORT 83-293

Prepared in cooperation with the governments of Australia and New Zealand, the United Nations Committee for the Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas, and the Circum-Pacific Council for Energy and Mineral Resources

Second printing

1983 PREFACE This report summarizes the investigation conducted aboard the U.S. Geological Survey's Re- search Vessel S.P. Lee in 1982 under the terms of an agreement between Australia, New Zealand, and the United States (ANZUS Tripartite Agreement) in association with the United Nations Corn- mittee for the Coordination of Joint Prospecting for Mine] 'al Resources in South Pacific Offshore Areas. Data collected on the cruise were first presented at he S.P. Lee postcruise meeting in Hon- olulu, Hawaii on August 19 and 20, 1982, and further discussed at a meeting cosponsored by theie U.S. Geological Survey and the Circum-Pacific Council for Energy and Mineral Resources, a section of the American Association of Petroleum Geologists, in Menlo Par i, California, on April 5, 1983.

PARTICIPATING SCIENTISTS

Leg 1 (L5-82-SP)*, April 2 to April 20, 1982 Leg 3 (L7-82-SP)*: May 19 to June 11, 1982

Jon Childs USGS Liiiry Beyer USGS Neville Exon Bureau of Mineral Resources Canberra, Donna Blackman USGS Australia, CCOP/SOPAC Terry Bruns USGS Chris Gutmacher USGS GvyCochrane USGS Richard Herzer New Zealand Geological Survey, Wellington, Jim Colwell Bureau of Mineral Resources, New Zealand Canberra, Australia Kay Kinoshita USGS Alan Cooper USGS Dennis Mann USGS Frank Coulson Solomon Islands Geological Survey, tTun U. Maung CCOP/SOPAC Honiara, Solomon Islands Graig McHendrie USGS Kay Kinoshita USGS Robert Rowland USGS Greg Lewis USGS Mark Sandstrom Australia National University, Canberra, Loren Kroenke CCOP/SOPAC Australia Michael Marlow USGS tDavid W. Scholl USGS tDonald Tiffin CCOP/SOPAC Barbara Seekins USGS tJuck Vedder USGS Sione Soakai Ministry of Lands, Surveys and Natural Raymond Wood New Zealand Geological Survey, Resources, Nuku'alofa, Tonga Lower Hutt, New Zealand Andrew J. Stevenson USGS Tracy L. Vallier USGS

Leg 2 (L6-82-SP)*: April 28 to May 16, 1982

Donna Blackman USGS Guy Cochrane USGS Jacque Daniel Office de la Recherche Scientifique et Technique OutreMer, Noumea, New Caledonia tDavid Falvey CCOP/SOPAC, Sydney University, Sydney, Australia Michael Fisher USGS tH. Gary Greene USGS Carol Hirozawa USGS Mark Holmes USGS David Johnson James Cook University, Townsville, Australia H.R. Katz New Zealand Geological Survey, Lower Hutt,New Zealand Kay Kinoshita USGS Greg Lewis USGS Alexander Macfarlane Dept. of Geology, Mines and Rural Water Supplies, Port Vila, Vanuatu Greg Smith USGS

* cruise locating code t cochief scientist CONTENTS

Page

Preface -__. -_- _-._._ _.__--__-___.___..___.._.______.___. _____._. -_.___._..._._... ..- _. II

Acknowledgments ______1 Leg i _ Tonga Ridge ______2 Leg 2 Central Basin of Vanuatu ______- ____ 2 Leg 3 Central Solomons Trough 11

I. Tonga Platform-Lau Ridge -- - - _. _. . ._._.._.... 20 II. Vanuatu and Eastern Solomons Basins 20 III. Solomon Islands and Bougainville, Papua New Guinea 22 IV. New Ireland Basin, Papua New Guinea 22 V. North Fiji Basin ______23 ______23

ILLUSTRATIONS

Page

J.TPlPTTPT? IVxUXlI:* J.*1 xllUCA^rii"i«iY mar*lllaLP 2. Physiographic diagram of southwest Pacific with tracklines 3 3. Leg 1: Detailed tracklines and bathymetry of Tonga platform 4 4. Leg 1: Multichannel seismic line 8 across Tonga platform 5 5. Leg 1: Multichannel seismic line 11 across Tonga platform 6 T ,onr 1 TanriQ/*Vk man f\f i*ViA T^AYIGTJI nlat"frti*m ...... _ ...._ ... _ ...._ ... _ ...... _ . _ . _ . _ ... _ . _ . _...... _ ... _ .. 7 6 XJCg J.. J.Ov'^Jclv'll IIM*^ VX lill^ A Vllgc* ^FlAvl-Vl 111 7. Leg 2: Detailed tracklines and bathymetry of the Central Basin of Vanuatu 8 8. Leg 2: Structure and shelf sedimentary basins of the Central Basin of Vanuatu 10 9. Leg 2: Multichannel seismic line 10 across the New Hebrides Trench H 10. Leg 2: Isopach map of the Central Basin of Vanuatu 12 11. Leg 2: Single-channel seismic line 25 offshore of Espiritu Santo, Vanuatu 13 12. Leg 2: Single-channel seismic line 20A offshore of Espiritu Santo, Vanuatu 14 13. Leg 3: Detailed tracklines and bathymetry of the central Solomons Trough 16 14. Leg 3: Isopach map of the central Solomons Trough 18 15. Leg 3: Multichannel seismic line 30 between Santa Isabel and the Russell Islands 19 16. Leg 3: Rabaul harbor bathymetry and tracklines 19 17. Recommended tracklines for future work in southwest Pacific 21

in Hydrocarbon Resource Studies in the Southwest Pacific, 1982

By H. Gary Greene, Florence L. Wong, and the scientific staff of the 1982 CCOP/SOPAC cruise

INTRODUCTION tion and operation of the investigation. Funds were also provided by the government of Au­ From April 2 to June 11, 1982, the USGS (U.S. stralia. CCOP/SOPAC in Fiji served as the basic Geological Survey) conducted a hydrocarbon re­ coordinator and provided, along with Australia, source investigation in the offshore areas of New Zealand, and CCOP/SOPAC member na­ Tonga, Vanuatu, and the Solomon Islands (fig. 1). tions, some of the scientific staff. The work was arranged under the ANZUS (Au­ The data collection phase of the joint program stralia, New Zealand, and United States) Tripar­ was a three-leg, 60-day resources survey by the tite Agreement in association with CCOP/SOPAC Research Vessel S.P. Lee. This report sum­ (United Nations Committee for the Coordination marizes that investigation, gives tentative re­ of Joint Prospecting for Mineral Resources in sults, outlines the future work schedule, and pro­ South Pacific Offshore Areas), as part of a pro­ poses significant work to be continued. gram of marine geoscientific research and min­ eral resource studies in the South Pacific region. ACKNOWLEDGMENTS The U.S. Department of State was instrumen­ tal in negotiating the agreement and establishing This report required the devotion and labor of the framework for the cooperative research. The many to meet the publication deadline. We are USGS provided instruments and scientific and grateful to Tom Chase for his efforts in supplying technical personnel. The Office of Energy of the us with bathymetric and navigation charts, and to United States Agency for International Develop­ Phyllis Swenson who spent many hours drafting ment (AID) provided funding for the implementa­ the illustrations for this report. David Scholl and

SOUTH SOLOMON ISLANDS PACIFIC OCEAN SANTA CRUZ ISLANDS

BANKS SAMOA ISLANDS ISLANDS

VANUATU ISLANDS "" ISLANDS

TONGA ISLANDS

AUSTRALIA

FIGURE 1. Location map of the South Pacific study area. Jack Vedder reviewed the manuscript and contrib­ dence that the forearc area, most of which is uted significantly to the improvement of the text. deeply submerged, may have been a source ter- We also appreciate the encouragement and sup­ rane for some of the volcaniclastic deposits of the port from the ANZUS Tripartite and CCOP/ platfioim section; (5) southwestward tracing (at SOP AC scientific participants. The studies were least 5550 km) of an axis of early Tertiary vol- funded by the U.S. Agency for International De­ camsmi that is situated along the Pacific side of velopment, the government of Australia, and the the platform and passes beneath or close to Eua U.S. Geological Survey and were conducted in Island (6) acoustical tracking of the subducted cooperation with the governments of Australia, surfac 3 of the Pacific plate 25-30 km landward of New Zealand, and the United States. the inner trench wall (fig. 5); this slope is not un- derlain by offscraped, low-velocity oceanic LEG I.TONGA RIDGE pelagi deposits; and (7) the probable detection of Departed Pago Pago, American Samoa, March 28 Arrived Nuku'alofa, Tonga, March 31; departed, April 2 a magina chamber 3-4 km below a spreading axis Arrived Suva, Fiji, April 20; departed, April 23 in Lau Basin (fig. 5). Arrived Port Vila, Efate, Vanuatu, April 26 A comprehensive evaluation of the petroleum METHODS potential of the Tonga platform area must await the ssults of laboratory analysis of rocks Leg 1, conducted mainly on the southern Tonga dredg d from the sea floor and sampled on land, platform up to 250 km south of Tongatapu, Tonga, and the interpretation of the fully processed mul- included over 5,700 km (3,078 n. mi.) of 12 kHz, 3.5 tifold reflection profiles. kHz, gravity, and magnetic profiles (fig. 2). In the area, 2,421 km (1,307 n. mi.) of 24-fold, CDP seis­ mic-reflection profiles, and 54 sonobuoy wide-angle LEG 2, CENTRAL BASIN OF VANUATU Departed Port Vila, Efate, Vanuatu, April 28 reflection-refraction profiles were obtained (fig. 3). Arrived Honiara, Guadalcanal, Solomon Islands, May 16 Also, at 13 sampling stations 9 dredge hauls and 4 gravity cores were taken. A brief (12-hr) sampling METHODS visit was included at Ata Island during data collec­ Leg 2, conducted in the Central Basin of Van- tion in the platform area (Scholl and others, 1982). uatu, ncluded 2,520 km (1,361 n. mi.) of 24-fold, Geophysical data were also gathered offshore CDP multi-channel seismic-reflection profiles (fig. of the western margin of the island of Viti Levu, 7). In addition, 622 km (336 n. mi.) of high-resolu- Fiji and over Baravi Basin at the request of the tion Uniboom seismic-reflection profiles and nearly Fiji Mineral Resources Department, and along 4,5001m (2,430 n. mi.) of 3.5 kHz very high resolu- the Lee's virtual due-west track from Suva to tion scismic-reflection, 12 kHz bathymetric, mag- Port Vila (fig.2). netic, and gravity profiles were collected, and 33 GENERAL RESULTS sonobuoy wide-angle reflection refraction profiles were obtained (Greene and others, 1982, 1983; fig. Initial results include the following data taken 2). Three sampling stations comprised one dredge from Maung and others (1982) and Scholl and statior and two gravity core stations. Geophysical others (1983): (1) acoustical resolution of a west­ trackline spacing was generally 5-10 km (2.7-5.4 n. ward-thickening, extensionally deformed plat­ mi.) apart, and in some localities spacing was as form section (<4.5 km/s; 2-3 km thick) of chiefly close as 1 km (0.5 n. mi.) apart (fig. 7). Neogene and younger volcaniclastic and lime­ stone beds overlying a basement of arc-type igne­ GENERAL RESULTS ous rocks (>5 km/s) (figs. 4, 6); (2) identification of a regional unconformity of late Miocene age in Preliminary onboard data reduction and inter­ the upper part of the platform section that possi­ pretation resulted in (1) the delineation of three bly reflects thermal uplift of the ridge just prior small shelf sedimentary basins east of Malekula to the opening of the Lau Basin; (3) identification and Espiritu Santo; (2) estimation of minimum of large, moundlike buildups in the lower part of sedimentary thickness in the intra-arc Central the platform section that may be reefal or possi­ Basin; and (3) the discovery of the northward bly igneous masses (fig. 4); (4) recognition of evi­ continuation of the southern New Hebrides 160 E 170 E AREA OF U.S.G.S.-UNITED NATIONS RESOURCE EVALUATION EXPEDITION TO THE SOUTHWEST PACIFIC

10 S

20 S

CO

40°S

FIGURE 2. Physiographic map with tracklines covered by Research Vessel S.P.Lee during the 1982 South Pacific surveys. Physiography by Tau R. Alpha. 177°W 176°W 175°W 174°W 173°W 21°S 21°S

SEGMENTS PLOTTED IN FIGS. 2 &

TOPOGRAPHY *f Ike TONGA REGION Btl«..i 21 23'S 1.JI73 177 W

177°W 176°W 175°W 174°W 173°W

FIGURE 3. Index map of southern Tonga platform showing tracklines of 24-fold multichannel data, locations of profile segments (that is, A-B through I-J) displayed on figures 4 and 5, positions of dredge stations where submarine outcrops were sampled, and depth contours in meters (based on velocity-corrected soundings). VOLCANIC SANDSTONE VOLCANIC BASEMENT MIDDLE MODERN TOF'UA AND LIMESTONE BEDS: MWVOLCANIC ARC PLIOCENE AND YOUNGER TONGA PLATFORM SECTION EOCENE (>40 MILLION YEARS) SE A.I' l«3 MILLION YEARS)

PLIOCENE AND X'S LINE 1 1 YOUNGER BEDS/ ./

FIGURE 4. Annotated segment of multichannel line 8 (A-B), which traverses the crestal platform of the Tonga Ridge, and mutually crossing segments of lines 11 and 14, which reveal a subsurface moundlike structure of possible igneous or reefal origin (see figure 3 for locations). The platform section noted along line 8 is constructed of volcaniclastic and limestone beds of post-Eocene age that, in general, are characterized by acoustic velocities less than about 4.5 km/s. The section's underlying acoustic basement is presumably arc-type volcanic and intrusive rocks of Eocene and older age. The upper, more coherently layered strata of the platform section is composed of Pliocene and younger beds; the underlying moundlike buildup within the lower platform section is probably associated with Tertiary beds of Miocene age. NW SE G LINE 11 H 0 .

LAU BASIN - 750 SURFACE OF LIQUID MELT (?) 3-4KM BELOW SEA FLOOR -1500

-2250

3000

WATER DEPTH IN METERS

-6000

PACIFIC OCEAN CRUST BELOW TRENCH SLOPE -7500

WATER - DEPTH IN METERS

-9000

^-9750

-10500

SEC 16 -I FIGURE 5. Annotated segments of multichannel line 11. Segment G-H reveals a subsurface upper crustal reflection horizon near the eastern side of Lau Basin that may be the top of a magma chamber underlying a spreading center. Segment I-J, recorded over the lower part of the landward slope of the Tonga Trench, reveals a deep, subsurface reflection horizon that is presumed to be the top of subducted Pacific oceanic crust. On a true-depth s< jction, the horizon actually dips downward beneath the Tonga Ridge, but a prominent velocity pull-up effect induced by high- velocity rocks underlying the lower slope causes the hori- zon to ascend to the west on this reflection-time section. Trench (fig. 8). Tentative interpretation of the of this arc is nearly devoid of sediment and that single-channel seismic-reflection data indicates the accretionary wedge is covered by only a ven­ that regionally the islands are deformed in a com­ eer of sediment (Fisher and others, 1982; fig. 9). plex structural pattern. These structures may be Rocks within this wedge have high acoustic vel­ related to the complex tectonic evolution of the ocities (4-5 km/s), and these rocks cause faint re­ arc that involves orthogonal subduction, oblique flections that dip arcward. Oceanic crust can be subduction, and arc reversal through middle and traced beneath this wedge for 30 km. late Miocene times. The three shelf or shelf-edge sedimentary ba­ Multichannel seismic data from this investiga­ sins that were mapped all lie in water depths of tion indicate that the trench near the central part less than 1,000 m and have sediment thicknesses

177°W 176°W 175°W 174°W 173°W 21°S

22°S - - 22°S

23°S - 23°S

177°W 176°W 175°W 174°W 173°W

NW LAU BASIN Intrusive contact TONGA RIDGE

TOFUA ARC 1 - - 1 2 - -2 3 - -3 4 - ~~ BACKARC -4 BASEMENT: 6 - " " " ARC BASEMENT: EOCENE -5 LATE CENOZOIC KM KM

VE = 4:1 50km

FIGURE 6. Diagrammatic representation of isopach contours, based on multichannel and sonobuoy refraction velocities and depth calculations, showing the thickness of the platform section in kilometers. Lower cross-section, from Lau Basin to the eastern edge of the southern Tonga platform, is a cartoon or schematic drawing illustrating presumed structural and stratigraphic re­ lations of regional importance (based on sonobuoy refraction data and multichannel reflection profile 11; see figure 3). Platform section has presumably been truncated by extensional rifting along western side of ridge. Young arc basement is shown in­ truding the truncated edge of the section. The missing or western part of the section presumably underlies Lau Ridge, approx­ imately 300 km to the west.

FIGURE 7. Trackline map and bathymetry, Leg 2, Vanuatu. KEY

2000 -" Bathymetric contours

Survey Vessel tracks

Major Fault/Lineament

Trench -dashed where continued from this survey

Older sedimentary rocks forming faulted ridge

Insular shelf sedimentary basin (Possible exploration target)

Offshore extension of Big Bay-Jordan River sedimentary basin

^2000-

169 C 170 c

FIGURE 8. Generalized outline of shelf or shelf-edge sedimentary basins and primary structural features delineated during Leg 2. Bathymetry in meters.

10 greater than 2 km (fig. 10). Structures capable of LEG 3, CENTRAL SOLOMONS TROUGH Departed Honiara, Guadalcanal, Solomon Islands, May 19 trapping oil and gas are present in all three ba­ Arrived Rabaul, New Britain, Papua New Guinea, June 11 sins. Sedimentary rocks identified within these smaller shelf-edge basins appear to be locally con­ METHODS nected with the deeper intra-arc basin (that is, the Central Basin), thereby forming potential Leg 3, conducted in the Solomon Islands trough pathways for petroleum migration from thick area, included approximately 3,700 km (1,998 n. sedimentary sections in deep-water marine mi.) of 24-fold, CDP multichannel and about source beds to shallow-water reservoir beds and 3,200 km (1,728 n. mi.) of high-resolution Uni- traps (fig. 11). boom seismic-reflection profiles in the central The main part of Central Basin contains about and western Solomon Islands area (fig. 13). For 5 km of rocks that have velocities less than 5 km/s the entire leg, including transit lines, about 5,500 (fig. 10). Preliminarily processed multichannel km (2,970 n. mi.) of 3.5 kHz high-resolution, seis­ seismic data indicate that three main seismic- mic-reflection, magnetic, gravity, and 12 kHz stratigraphic units compose the basin fill, all bathymetric profiles were collected. In addition separated by unconformities. They consist from 36 wide-angle seismic reflection-refraction oldest to youngest of an older, poorly bedded sonobuoy profiles were obtained. Nine dredge sedimentary unit resting on acoustic basement hauls were made, and two gravity cores were along the margins of the basin and everywhere taken. Dredge hauls ranged in water depths from overlain by a well-layered sedimentary unit that approximately 1,200 m to 300 m. Trackline spac­ is in turn overlain by a thin layer of well-bedded ing was generally between 5 and 10 km (2.7 and unconsolidated sediments (fig. 12). Several large 5.4 n. mi.). structures deform the youngest units of this fill; In addition to the resource survey specified in these structures do not appear to have igneous the ANZUS Tripartite Agreement, the USGS rocks at their core. made an 8-hour survey in Rabaul harbor, New The three shelf-edge basins identified in the Britain, on June 11 at the request of CCOP/ survey are considered promising sites for further SOP AC and the Rabaul Volcanological Institute investigations of hydrocarbon potential. They are (fig. 14). Reconnaissance bathymetric data were equal in size and sediment thickness to some of acquired, and the Rabaul seismograph network the smaller hydrocarbon producing basins found was tested using the airgun system aboard the elsewhere in the world (for example, western Lee as an artificial seismic source. Also, a coast of the United States). The water depths sonobuoy refraction profile was recorded, and in­ overlying all of these shelf-edge basins are shal­ termediate penetration and high-resolution Un- low enough for drilling with current technology. iboom seismic-reflection profiles were collected.

w 0 5 10 KM LINE 10

New Hebrides Trench

FIGURE 9. Uninterpreted multichannel record of line 10, Leg 2, across the New Hebrides Trench.

11 166 169*

ESPIRITUV:': :-. SANTO A * AOBA

FIGURE 10. Preliminary isopach map of sediments overlying acoustic basement rocks of 5 km/s seismic veloc­ ity in the Central Basin of Vanuatu. Dots are approximate sonobuoy locations where basement reflectors were recorded. Contours are sediment thickness in kilomete: -s; numbered dots indicate deepest recorded sediment thickness. 750 COcr UJ UJ 1500-

2250 L6-82-SP LINE 25

0 10 KILOMETERS

Vertical Exaggeration = 6.0

750- co a: UJ r- UJ 1500-

2250 FIGURE 11. Single-channel record and line drawing of line 25, Leg 2, which crosses the shelf basin off northern Espiritu Santo. Symbols: Q, Quaternary; Qr, reef deposits; T?pb, poorly bedded Tertiary strata, age uncertain; QTwl, well- bedded Tertiary strata, ?, age uncertain; Tos, older sediment.

13 750-

15OO-i

OJ225CH u

300CH

3750-

45OO- -6

L6-82-SP o 10 I LINE 2OA KILOMETERS Vertical Exaggeration = 8.0

O- O

750- -1

150O-

-3

u

3000- -4

3750- -5

-6 45OO-

FlGURE 12. Single-channel record and line drawing of line 20A, Leg 2 across the northern Central Basin of Vanuatu. Profile sishows ponded sediment behind a faulted ridge of oktor sedif MHtary«tary rocks located on the eastern shelf of Es- piritu Santo. Symbols: Qls, landslide deposits; QTwb, well-bedded Tertkury strata; TV, Tertiary volcanic rocks; others as in figure 11. 14 The purpose of the harbor survey was to (1) de­ metamorphosed, has velocities of 4.8-5.8 km/s; a termine if local uplift is taking place in the har­ deeper layer, which has a velocity of 6.4-7.5 km/s, bor, (2) test the sensitivity of the Rabaul seis­ is 3-5 km below the acoustic basement. These mograph network, and (3) obtain refraction vel­ higher velocity rocks may include deeper crustal ocities of the rocks in the harbor area to calibrate igneous rocks similar to those observed in island the seismograph network. Upon completion of outcrops in the Central province. the harbor survey, the Lee docked at Rabaul and In addition to the Russell and Shortland Ba­ thus ended the USGS-CCOP/SOPAC survey. sins, two other sedimentary basins have been de­ fined and named in the region (fig. 15). Offshore

GENERAL RESULTS from northern Guadalcanal, the basin previously identified in Iron Bottom Sound (newly named The Solomon Islands region encompasses three Iron Bottom Basin) is bounded by Savo Island to geologic provinces (Coleman, 1976): the Pacific the northwest, Guadalcanal to the south, and the province, which is underlain by obducted portions Nggela Sule Island platform to the northeast. of the Ontong-Java Plateau on the northeast Another sedimentary basin (newly named the In­ flank; the Central province, which is largely un­ dispensable Basin) lies in Indispensable Strait derlain by early Tertiary volcanic and volcaniclas­ and is separated from Iron Bottom Basin by the tic rocks; and the Volcanic province, which in­ Nggela Sule Island platform; it is bounded to the cludes the late Cenozoic volcanic island arc on the northeast by Malaita and the Malaita anti- southwest flank. The structure of the Solomon Is­ clinorium. land region is believed to consist of a volcanic The samples dredged from the perimeter of the ridge underlain by shallow, high-velocity rocks Russell and Shortland Basins consist principally and incised by a deep intra-arc basin known as of volcaniclastic rocks and reef-detritus limestone the central Solomons Trough (Katz, 1980; fig. 15). (Vedder and others, 1983). In general, the vol­ The ridge is flanked by oceanic crustal rocks of caniclastic rocks include poorly sorted foraminif- the Woodlark Basin on the southwest and thic­ eral sandstone, sandy calcilutite, lapilli tuff, and kened transitional crustal rocks of the Ontong- ashy mudstone that range from indurated to fri­ Java Plateau on the northeast. The deep-water able. The limestone is largely recemented and (800-1,800 m), intra-arc sedimentary basin lies be­ partly recrystallized coralline and algal reef de­ tween the Volcanic and Central provinces and tritus that suggests downslope transport as well may contain more than 5 km of sedimentary sec­ as submergence since deposition. Both the vol­ tion at places (fig. 15). caniclastic rocks and the limestones have low Both wide-angle sonobuoy reflection and re­ porosity. Calcareous nannofossils in selected sam­ fraction data were recorded, and velocity profiles ples from each of the dredge stations indicate for the sedimentary section and crustal layers Quaternary ages, chiefly in the range 0.2 to 1.6 were computed from these data. Seismic refrac­ million years before present (David Bukry, writ­ tion arrivals, routinely recorded to a range of 37- ten commun., 1982). Radiocarbon ages on two 45 km, provided maximum sedimentary layer limestone samples are > 31,000 years before pre­ depth estimates of 10-12 km beneath the island sent (J. B. Colwell, written commun., 1982). arc and adjacent ocean basins (Cooper and The two gravity cores from water depths of others, 1982; fig. 16). 1,810 and 1,279 m in the central Russell Basin re­ Two subsidiary basins separated by a horst- covered 192 and 262 cm, respectively, of like feature constitute the central Solomons Holocene laminated to bioturbated foraminifer- Trough. These newly named structural basins are rich mud. Thin layers of ashy material occur at the Shortland Basin and the Russell Basin (fig. intervals in both cores. 15). The late Cenozoic Russell Basin is an elon­ gate (60x250 km) asymmetrical crustal depres­ RECOMMENDATIONS sion that contains a 1-5 km sedimentary section (fig. 15). Velocities of the crustal rocks beneath To assist CCOP/SOPAC in further evaluation of the sedimentary section range from 4.8-7.5 km/s. hydrocarbon, as well as mineral, resources, future The acoustic basement, thought to be volcanic studies in the region of this recent survey are re­ and volcaniclastic rocks that are partly commended (fig. 17). These recommendations

15 llll. - NnlfMk. \1 WCi TOPOGRAPHY »f the SOLOMON ISLANI S REGION .f"^".. *"" rater riMnll»«

FIGURE 13. Trackline map and bathymetry for* Leg 3, Solomon Islands. T*POCRAPIT ( tk« ML«MN ISLANM REGION Hvm CM-ITS **t UT3»-Wn TI Ckm. IA Irak tmf,mi » A CW^

FIGURE 13. Continued.

17 I52°IO'E I52°I2'E I52°I4'E I52°I6'E T 4- - 4°I2'S

0920 BLANCHE

BAY

4 - 4° 18' S

NEW BRITAIN ISLAND

FIGURE 14. ^Trackline map, Leg 3, Rabaul harbor area New Britain, Papua New Guinea.

18 8°-

156' 158 160 ( FIGURE 15. Structural basins and preliminary isopach map of sediment overlying basement rock in the Solomon Islands. Dots are sonobuoy locations at which basement refractors were recorded. Contours are sediment thickness in kilometers.

Mo SEC V.E.= 2.5

KILOMETERS

FIGURE 16. Uninterpreted multichannel profile, Leg 3, line 30 between Russell Islands and the southeast end of Santa Isabel. See figure 13 for location.

19 were developed at a postcruise meeting held in quence; .without this program, questions regard­ Honolulu, Hawaii, August 19-20, 1982, in conjunc­ ing the Ridge's geologic history and resource po­ tion with the Third Circum-Pacific Energy and tential cannot be resolved. The sampling program Mineral Resources Conference. At this meeting, should include the flanks, which may comprise many of the cruise participants as well as other in­ outcrop^ revealing substantial middle and late terested scientists discussed the initial scientific Cenozoiq subsidence, and crestal areas of both results, the status of data processing and distribu­ Tonga afid Lau Ridges. Resolution of the history tion, the scientific commitments and coordination, of vertical tectonism of these two geanticlinal and future work and concluded that future investi­ ridges isj particularly significant in assessing their gations would be required to more fully assess the hydrocarbon potential, which, if present at all, is resource potential of the 1982 study area and adja­ likely contained in porous reefal masses of early cent areas. Their conclusions, which were funda­ and middle Tertiary age. mentally logical extensions of the 1982 work, Investigation of the possibility that polymetal formed the basis for the following recommenda­ sulfides may be forming above the magma cham- tions, presented first at the Eleventh CCOP/ bers (?) identified in Lau Basin immediately west SOPAC Session held at Wellington, New Zealand, of the Tonga platform is also needed. This recon­ in November 1982. naissance exploration will involve the collection

I. Tonga Platform-Lau Ridge (continuation of Leg 1) of close -line bathymetric data, bottom photo- graphs, water samples for helium-isotope Multichannel seismic-reflection work to supple­ anomalies, and samples of young lava. ment that of Leg 1 is recommended in the south­ ern Tonga platform area and over the adjacent M. Vanuatu and Eastern Solomon Islands Basins Lau Ridge, which is underlain by a coeval if not (continuation of Leg 2) syngenetic section (figs. 2, 17). Reflection seismol­ ogy is especially needed over the far southern re­ Data collected during Leg 2 (fig. 17) delineated gion of the Tonga platform (south of 23.5 S. to three sedimentary basins on the western margin of about 27 S.), which still is nearly unstudied, and the Central Basin of Vanuatu that may have hy- the closely adjacent Lau Ridge. The closeness of drocarboii and mineral potential. Before their po­ the ridges separated by Lau Basin in this area in­ tential c^n be defined and assessment of the area's dicates the rifting event that split them may have geological hazards made, additional studies are been less severe (rapid) and possibly occurred needed. |These include rock-sampling and heatflow later than the event that rifted the southern measurements to determine source-rock potential, Tonga platform area to the north. These cir­ past sedimentary depositional environments, and cumstances may enhance the hydrocarbon poten­ hydrocarbon maturation history. tial of the ridge's far southern platform section. Previously gathered seismic-reflection data Valuable resource information can also be ex­ north of the Banks Islands indicate the presence tracted by extending the collection of geophysical there oi| other sedimentary basins (Katz, 1980). data farther northward along the Lau Ridge. These basins probably are Miocene to Pleistocene Further investigation in the Tonga-Lau region in age ajid therefore possibly have resource poten­ would in general be aimed at clarifying the early tial. It Would be particularly useful to extend the and mid-Tertiary history of this area, which 1982 mijiltichannel seismic-reflection grid to the would significantly advance the evaluation of its relatively shallow shelf region that lies between overall resource potential. the Torjres Islands and the Santa Cruz Island Additional sampling is also needed in the group to determine the presence of sedimentary southern Tonga platform area, as well as along basins (fjig. 17). the adjacent Lau Ridge. The sampling program A 20-day survey is recommended: 10 days of sea- carried out during the present investigation was floor sampling and heatflow measurements within assembled from shipboard analysis of seismic-re­ the 1982 survey area and 10 days of multichannel flection records. By studying these records in seismic-reflection surveying in the northern Tor- greater detail, sites for another dredging pro­ res-Santja Cruz region. In addition, detailed seis­ gram can be selected to sample more comprehen­ mic-refit jction surveying is needed along the west­ sively the platform's exposed sedimentary se­ ern Cen :ral Basin to further define potential struc-

20 160°E 170"E AREA OF U.S.G.S.-UNITED NATIONS RESOURCE EVALUATION EXPEDITION TO THE SOUTHWEST PACIFIC

10 S

10 s

20 S

20 S

.. " -

30°S

40°S

^ . Xj Proposed detailed survey \^^ Recently completed survey lines , « . .^^^ TV Refers to proposal ill Pr<>Posed reconnaissance survey - - - Proposed transit lines number in text

FIGURE 17. Recommended survey areas of the USGS in the South Pacific. tural and stratigraphic traps and to assess possible tion of the arc. geologic hazards. Before the hydrocarbon potential of the sea-floor Preliminary interpretation of the 1982 Lee data areas of the Solomon Islands can be fully have shown two other areas in Vanuatu that evaluated,I ' additional multichannel tracklines are should be studied in more detail, preferably with required^ particularly in the Indispensable Strait- seismic-reflection and rock-sampling techniques: San Cristobal area and in the western part of the (1) the relatively shallow shelf area south of Shortlanfi Basin. Sea-floor sampling needs to be Malekula and Efate Islands, where sedimentary done, based on the recently collected geophysical basins have been identified, and (2) along the data, to correlate acoustic stratigraphy with rock flanks of Maewo and Pentecost Islands, where types th^t make up the thick sedimentary bodies. structures indicating hydrocarbon traps may exist. It is estimated that approximately 30 days of Studies of the deep structure in the Vanuatu and ship time are needed to accomplish the work re- eastern Solomon island arc province may be possi­ commen^ed for the Solomons area. This time could ble using a two-ship operation. Should it be possi­ be divided into three 10-day phases, with the time ble to coordinate two ships in the region, it is re­ devoted ito geophysical work and sea-floor sampl­ commended that an arc transect involving expand­ ing beintf divided equally. ing seismic spreads parallel to the arc plus con­ Recommendations IV and V represent adjoining stant offset spreads across the arc be undertaken. areas wfiere regional geological and geophysical It would also be desirable to collect wide-beam study cati significantly increase resource assess­ sidescan sonar survey data in the Vanuatu region ment dapa; scientists attending the Lee post-sur­ to better define the bedrock sea-floor exposures. vey meeting in Honolulu in August 1982 recom­ mended this work be initiated under the same III. Solomon Islands and Bougainville, Papua New Guinea, Tripartite Agreement and scientific guidelines as Basins (continuation of Leg 3) for the present study.

Additional geophysical work, in particular multi­ IV. New Ireland Basin, Papua New Guinea channel seismic-reflection surveying, and sea-floor sampling are needed to adequately evaluate the re­ The N|ew Ireland Basin is a continuation of the source potential in the West Melanesian arc. Al­ Vanuatu,, Solomon Islands, and Bougainville Ba­ though more information to determine the nature sins associated with the Melanesian arc. It extends of the tectonic framework would also be desirable, for about 900 km from eastern New Ireland to the recommendations here are confined to those western, Manus Island and lies mostly offshore, relating directly to resource potential. south of the islands. It averages 160 km in width A multichannel seismic survey of the southeast­ and has a sedimentary section that is locally ern part of Indispensable Strait and the basin thicker ,han 5 km. Water depths range from 0 to areas north and east of San Cristobal Island are re­ 3,000m The basin is truncated on the southwest quired to complete the present work. This work by a syj tern of transform faults, and much of the should have first priority and should be extended sedimentary and carbonate sequence pinches out eastward to correlate with the present investiga­ against ;he Northeast Ridge, which was the outer tion in Vanuatu and the New Hebrides arc (fig. arc of the West Melanesian Trough during 17). Several tracklines toward Santa Cruz Islands Oligocer e time. from the vicinity of San Cristobal would be suffi­ Nevil e Exon of the Australian Bureau of Min­ cient to accomplish a reconnaissance survey. eral Resources and Donald Tiffin of CCOP/SOPAC Bottom sampling is necessary to determine the made C9nsiderable progress in assessing the hy- age, nature, and stratigraphic relation of rocks be­ drocartym potential of the New Ireland Basin from neath the arc, particularly in the central Solomons seismic-reflection surveys carried out by CCOP/ Trough, but also in other areas of the arc including SOPAC Gulf Oil, ORSTROM, (Office de la Re­ Indispensable Strait and the region east of cherche Scientifique et Technique Outre-Mer) and Bougainville and north of Choiseul. Inasmuch as the Australian Bureau of Mineral Resources. the offshore basins in this region represent much Land geology and interpretation of present greater areas than the islands, and may reflect offshorej seismic-reflection data (Exon and Tiffin, much different histories, sampling should be useful 1983) indicate that the basin comprises a thick in revealing resource potential as well as the evolu­ Oligocene volcaniclastic sequence overlain by as

22 much as 2 km of Miocene platform and reefal car­ tichannel lines be taken across the central and bonate beds, and a similar thickness of younger southern part of the North Fiji Basin (fig. 17). In volcaniclastics and biogenic sediment. Should the addition, sea-floor sampling should be done along offshore Miocene sequence prove to consist largely the north and west margins of Fiji and on selected of carbonates, the New Ireland Basin would be sites along the tying traverses. Approximately, 20- considered to have good hydrocarbon potential. 30 days of ship time would be required for this in­ To further evaluate the basin, which has not vestigation. been drilled and from which few bottom samples have been recovered, it is suggested that a three- REFERENCES week research program be carried out, consisting of approximately 40 percent multichannel seismic- Coleman, P. J., 1976, A re-evaluation of the Solomon Islands as reflection surveying and 60 percent sampling. The an arc system: United Nations Committee for the Coordi­ nation of Joint Prospecting for Mineral Resources in South seismic surveying should concentrate on the east­ Pacific Offshore Areas, Technical Bulletin 2, p. 132-140 ern and western areas where no multichannel data (IDOE Workshop, Suva, 1975). exist (fig. 17). The sampling should consist largely Cooper, A. K., Wood, R. A., Bruns. T. R., and Marlow, M. S., of dredging and, if possible, heatflow work to as­ 1982, Crustal structure of the Solomon Islands arc from sess the present day thermal gradient in the basin. sonobuoy refraction data [abs.]: EOS, Transactions of the American Geophysical Union, v. 63, p. 1121. The dredging would be designed to sample the Exon, N. F. and Tiffin, D. L., 1983, Geology of offshore New older sequences identified seismically, and espe­ Ireland Basin in northern Papua New Guinea, and its pe­ cially the carbonate sequence. Dredging should troleum prospects: Transactions, Third Circum-Pacific also be carried out, if possible, on steep slopes Conference for Energy and Minerals, August 1982, Hon­ along the faulted southwest margin of the basin, olulu, Hawaii [in press]. Falvey, D. A., 1978, Analysis of palaeomagnetic data from the around the uplifted Tabar-Feni Islands in the New Hebrides: Bulletin of the Australian Society of Explo­ northeast, and possibly in the West Melanesian ration Geophysicists, v. 9, p. 117-123. Trench. Fisher, M. A., Falvey, D. A., and Greene, H. G., 1982, Investi­ The proposed New Ireland Basin study would gation of the central New Hebrides arc, using multichannel not only greatly increase knowledge of the geol­ seismic data [abs.]: EOS, Transactions of the American Geophysical Union, v. 63, p. 1120. ogy, geophysics, and resource potential of the Greene, H. G., Falvey, D. A., Macfarlane, A., Cochrane, G., basin, but also would aid in the understanding of Daniel, J., Fisher, M. A., Holmes, M. L., Kate, H. R., the tectonic framework in this highly complex re­ Smith, G. L., and Pragnell, J., 1982, Preliminary results of gion, which involves at least four crustal plates. Leg 2 Lee cruise in Central Basin of Vanuatu to assess hy­ drocarbon potential and geologic evolution of New Heb­ rides Arc basins, southwest Pacific [abs.]: American As­ V. North Fiji Basin sociation of Petroleum Geologists Bulletin, v. 66, p. 969. A reconnaissance geophysical survey and sampl­ Greene, H. G., Falvey, D. A., Macfarlane, A., Cochrane, G., Daniel, J., Fisher, M. A., Holmes, M. L., Johnson, D., ing program within the North Fiji Basin (fig. 17), Katz, H. R., and Smith, G. L., 1983, Preliminary report on especially in its southernmost area that geologi­ the geology, structure, and resource potential of the Cen­ cally connects Fiji with Vanuatu, is highly desira­ tral Basin of Vanuatu: Transactions, Third Circum-Pacific ble. It has been shown (Falvey, 1978) that the Fiji Energy and Minerals Conference, August 1982, Honolulu, Islands and the New Hebrides arc regions were Hawaii [in press]. Katz, H. R., 1980, Basin development in the Solomon Islands once adjoining structures and that they were sepa­ and their petroleum potential: United Nations Committee rated beginning in late Oligocene to early Miocene for the Coordination of Joint Prospecting for Mineral Re­ time to form the North Fiji Basin. This hypothesis sources in South Pacific Offshore Areas, Symposium on Pe­ suggests that the sedimentary basins located in troleum Potential in Island Arcs, Small Ocean Basins, Sub­ Bligh Waters of Fiji, which have been explored for merged Margins and Related Areas, Technical Bulletin No. 3, p. 59-75. oil and gas, may have evolved adjacent to and con­ Maung, T. U., Exon, N. F., Sandstrom, M. W., Herzer, R. H., temporaneously with the Central Basin of Van­ Vallier, T. L., Stevenson, A. J., Childs, J., and Scholl, D. uatu. To confirm or discount this hypothesis and W., 1982, Tonga Ridge, initial results of new geological and thereby more completely assess the resource po­ geophysical studies [abs.]: EOS, Transactions of the Amer­ tential of both regions, it is proposed that multi­ ican Geophysical Union, v. 63, p. 1121. Scholl, D. W., Maung, T. U, Vallier, T. L., Childs, J., Steven­ channel seismic reflection profiles and other son, A. J., Exon, N. F., Herzer, R. H., Sandstrom, M. W., geophysical data be collected along the north and and Soakai, S., 1982, Preliminary results of geophysical western margins of Fiji and that several long mul­ and geological studies to assess resource potential and

23 geologic evolution of central Tonga Ridge and summit plat­ fining T. R., Colwell, J. B., Cooper, A. K., Kroenke, L., form (21-24° latitude) [abs.]: American Association of Pe­ Marlow, M. S., and Wood, R. A., 1982, Preliminary results troleum Geologists Bulletin, v. 66, p. 983. of Lejj 3 Lee cruise basin development and resource po­ Scholl, D. W., Vallier, T. L., Maung, T. U., Exon, N. F., tential of central Solomons Trough [abs.]: American As­ Herzer, R. H., Sandstrom, M. W., Stevenson, A. J., sociation of Petroleum Geologists Bulletin, v.66, p.986. Mann, D. M. and Childs, J., 1983, Initial cruise report, Vedder, Jf. G., Tiffin, D. L., Kroenke, L., Colwell, J. B., SOPAC Leg 1 (cruise L5-82) southern Tonga platform: Cooper, A. K., Beyer, L. A., Bruns, T. R., Coulson, F. I. Transactions, Third Circum-Pacific Energy and Mineral E., Marlow, M. S., and Wood, R. A., 1983, Preliminary re­ Resources Conference, August 1982, Honolulu, Hawaii [in sults bf Leg 3 Lee cruise, Solomon Islands: Transactions, press]. Third| Circum-Pacific Energy and Minerals Conference, Vedder, J. G., Tiffin, D. L., Coulson, F. I. E., Beyer, L. A., August 1982, Honolulu, Hawaii [in press].

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