Hydrocarbon Resource Studies

in the Southwest Pacific,

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

GEOLOGICAL SURVEY OPEN-FilE REPORT83-293

Prepared in cooperation with the governments of Australia and New Zealand,the United Nations Committee for the Coordina tion of Joint Prospectingfor 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. GeologicalSurvey's Re-search Vessel S.P. Lee in 1982under the terms of an agreement between Australia, New Zealand,and the United States (ANZUS Tripartite Agreement) in associationwith the United Nations Com-mittee for the Coordination of Joint Prospecting for Mineral Resources in South Pacific OffshoreAreas. Data collectedon the cruise were first presented at the S.P. Lee postcruise meeting in Hon-olulu,Hawaiion August 19and 20, 1982,and further discussed at a meeting cosponsoredby the U.S.GeologicalSurvey and the Circum-PacificCouncilfor Energy and MineralResources, a sectionof theAmericanAssociationof Petroleum Geologists,in MenloPark, California,onApril 5, 1983.

PARTICIPATING SCIENTISTS

Leg 1 (L5-82-SP)*,April 2 to April 20, 1982 Leg 3 (L7-82-8P)*:May 19to June 11, 1982

Jon Childs USGS Larry Beyer USGSNeville Exon Bureau ofMineral Resources Canberra, Donna Blackman USGS

Australia, CCOP/SOP AC Terry Bruns USGSChris Gutmacher USGS GuyCochrane USGSRichard Herzer New Zealand GeologicalSurvey, Wellington, Jim Colwell Bureau ofMineral Resources,

New Zealand Canberra, AustraliaKay Kinoshita USGS Alan Cooper USGSDennis Mann USGS Frank Coulson SolomonIslands GeologicalSurvey,tTun U. Maung CCOP/SOPAC Honiara, SolomonIslandsGraig McHendrie USGS Kay Kinoshita USGSRobert Rowland USGS Greg Lewis USGSMark Sandstrom Australia National University, Canberra, Loren Kroenke CCOP/SOPAC

Australia MichaelMarlow USGStDavid W. Scholl USGS tDonald Tiffin CCOPISOPACBarbara Seekins USGS tJack Vedder USGSSione Soakai Ministry of Lands, Surveys and Natural RaymondWood New Zealand GeologicalSurvey,

Resources, Nuku'alofa, Tonga Lower Hutt, New ZealandAndrew J. Stevenson USGSTracy L. Vallier USGS

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

Donna Blackman USGSGuy Cochrane USGSJacque Daniel Officede la Recherche Scientifique

et Technique OutreMer, Noumea,New Caledonia

tDavid Falvey CCOP/SOPAC, Sydney University, Sydney,Australia

Michael Fisher USGStHo Gary Greene USGSCarolHiro~wa USGSMark Holmes USGSDavid Johnson James CookUniversity, Townsville,

AustraliaH.R. Katz New Zealand GeologicalSurvey,

Lower Hutt,New ZealandKay Kinoshita USGSGreg Lewis USGSAlexander Macfarlane Dept. of Geology,Mines and Rural Water

Supplies, Port Vila, VanuatuGreg Smith USGS

* cruise locating code .-t cochief scientist

II

CONTENTS

Page

Preface -_____________________________________IIIntroduction ------- ------------------------------ 1Acknowledgments ---------------------________________________________________________________________1Leg I-Tonga Ridge -- ----_______________________2

Leg 2--Central Basin of Vanuatu ----------------------------------------------------------------- 2Leg 3-Central Solomons Trough --------------------------------------------------------------------- 11Recommendations --------------------------------------------------------------------------------------- 15

I. Tonga Platform-Lau Ridge --------------------------------------------------------------- 20II. Vanuatu and Eastern Solomons Basins ----------------------------------------------------- 20

III. Solomon Islands and Bougainville, Papua New Guinea --------------------------------- 22IV. New Ireland Basin, Papua New Guinea ----------------------------------------------- 22V. North Fiji Basin ------------------------------------------------------------------------------ 23

References -------------------------------------------------------------------------------------- 23

IllUSTRATIONS

Page

FIGURE 1. Index map of study area ----------------------------------------------- 12. Physiographic diagram of southwest Pacific with tracklines ------------------------------------------------- 33. Leg 1: Detailed tracklines and bathymetry of Tonga platform ---------------------------------------------------- 44. Leg 1: Multichannel seismic line 8 across Tonga platform ------------------------------------------------------------- 55. Leg 1: Multichannel seismic line 11 across Tonga platform ---------------------------------------------------- 66. Leg 1: Isopach map of the Tonga platform --------------------------------------------------------------------- 77. Leg 2: Detailed tracklines and bathymetry of the Central Basin of Vanuatu ----------------------------------- 88. Leg 2: Structure and shelf sedimentary basins of the Central Basin of Vanuatu ------------------------------- 109. Leg 2: Multichannel seismic line 10 across the New Hebrides Trench ----------------------------------------- 11

10. Leg 2: Isopach map of the Central Basin of Vanuatu ---------------------------------------------------------- 1211. Leg 2: Single-channel seismic line 25 offshore of Espiritu Santo, Vanuatu ------------------------------------ 1312. Leg 2: Single-channel seismic line 20A offshore of Espiritu Santo, Vanuatu ----------------------------------- 1413. Leg 3: Detailed tracklines and bathymetry of the central Solomons Trough ------------.------------------------- 1614. Leg 3: Isopach map of the central Solomons Trough --------------------------------------------------------------- 1815. Leg 3: Multichannel seismic line 30 between Santa Isabel and the Russell Islands ------------------------- 1916. Leg 3: Rabaul harbor bathymetry and tracklines -------------------------------------------------------------------- 1917. Recommended tracklines for future work in southwest Pacific -_______________________________________________21

III

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. Fundswere 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 basicGeologicalSurvey) 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 SolomonIslands (fig.1). tions, someof the scientificstaff.The work was arranged under the ANZUS (Au- The data collectionphase of the joint programstralia, New Zealand, and United States) Tripar- was a three-leg, 60-day resources survey by thetite Agreement in associationwith 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 significantwork to be continued.gram of marine geoscientific research and min-eral resource studies in the South Pacificregion. ACKNOWLEDGMENTS

The U.S. Department of State was instrumen-tal in negotiating the agreement and establishing This report required the devotion and labor ofthe framework for the cooperative research. The many to meet the publication deadline. We areUSGS provided instruments and scientific and grateful to Tom Chase for his efforts in supplyingtechnical personnel. The Officeof Energy of the us with bathymetric and navigationcharts, and toUnited States Agency for International Develop- Phyllis Swenson who spent many hours draftingment (AID) provided funding for the implementa- the illustrations for this report. David Scholland

Jack Vedder reviewed the manuscript and contrib- dence that the forearc area, most of which isuted 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 theport from the ANZUS Tripartite and CCOPI platform section; (5) southwestward tracing (atSOPAC scientific participants. The studies were least 250 km) of an axis of early Tertiary vol-funded by the U.S. Agency for International De- canism that is situated along the Pacific side ofvelopment, the government of Australia, and the the platform and passes beneath or close to EuaU.S. Geological Survey and were conducted in Island; (6) acoustical tracking of the subductedcooperation with the governments of Australia, surface of the Pacific plate 25-30 km landward ofNew Zealand, and the United States. the inner trench wall (fig. 5); this slope is not un-

derlain by offscraped, low-velocity oceanicLEG 1, TONGA RIDGE pelagic deposits· and (7) the probable detection of

Departed Pago Pago, American Samoa, March 28 _ ' • •Arrived Nuku'afola, Tonga, March 31; departed, April 2 a magma chamber 3-4 km below a spreading aXIS

Arrived Suva, Fiji, April 20; departed, April 23 in Lau Basin (fig. 5).Arrived Port Vila, Elate, Vanuatu, April 26 A hensi aluati f h Icompre ensrve ev uation 0 t e petro eum

METHODS potential of the Tonga platform area must awaitthe results of laboratory analysis of rocks

Leg 1, conducted mainly on the southern Tonga dredged from the sea tloor 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 12kHz, 3.5 tifold retlection profiles.kHz, gravity, and magnetic profiles (fig. 2). In thearea, 2,421 km (1,307 n. mi.) of 24-fold, CDP seis-mic-retlection profiles, and 54 sonobuoy wide-angle LEG 2, CENTRAL BASIN OF VANUATUtl ti _ fraeti fil btain d (fi 3) Departed Port Vila, Elate, Vanuatu, April 28re ec on re Ion pro es were 0 e g. . Arrived Honiara, Guadalcanal, Solomon Islands, May 16

Also, at 13 sampling stations 9 dredge hauls and 4gravity cores were taken. A brief (12-hr) sampling METHODS

visit was included at Ata Island during data collec- L 2 d ted i th C tral B . f V. . d 2) eg ,conuc m e en asmo an-tion m the ~latform area (Scholl an others, 198. uatu, included 2,520 Ian (1,361 n. mi.) of 24-fold,Geophysical data were also gathered offshore CDP ult· hann I . . tl t· til (fi. . . . m i-c e SeISmIC-reec Ion pro es g.

of the western margm of the Island of VItI Levu, 7) I additi 622 km (336 i.) f hi h . I _F··· d over Baravi B . t th t f th . n I ion, n. mi. 0 g -reso uIJI an over aravi asm a e reques 0 e t· Uniboo .. tl etl til d I... . Ion m m seismic-re e Ion pro es an near yFIJI Mmeral Resources Department, and along 4 500Ian (2 430 i.) f 35kHz hi h I _th L' irt al d t t k fr S t' ,n. rm, o. very Ig reso ue ee 8 VI U ue-wes rae om uva 0 ti .. tl t· 12 kHz b th t .P rt V·I (fi 2) on seisnnc-re ec ion, a yme nc, mag-o I a g. . netic, and gravity profiles were collected, and 33

GENERAL RESULTS sonobuoy wide-angle retlection refraction profileswere obtained (Greene and others, 1982, 1983; fig.

Initial results include the following data taken 2). Three sampling stations comprised one dredgefrom Maung and others (1982) and Scholl and station and two gravity core stations. Geophysicalothers (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 asform section «4.5 kmls; 2-3 km thick) of chiefly close as 1 Ian (0.5 n. mi.) apart (fig. 7).Neogene and younger volcaniclastic and lime-stone beds overlying a basement of arc-type igne- GENERAL RESULTSous rocks (>5 km/s) (figs. 4, 6); (2) identificationof 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 threebly reflects thermal uplift of the ridge just prior small shelf sedimentary basins east of Malekulato the opening of the Lau Basin; (3) identification and Espiritu Santo; (2) estimation of minimumof large, moundlike buildups in the lower part of sedimentary thickness in the intra-arc Centralthe platform section that may be reefal or possi- Basin; and (3) the discovery of the northwardbly igneous masses (fig. 4); (4) recognition of evi- continuation of the southern New Hebrides

2

Trench (fig. 8). Tentative interpretation of the of this arc is nearly devoid of sediment and thatsingle-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 kmls), and these rocks cause faint re-arc that involves orthogonal subduction, oblique flections that dip arcward, Oceanic crust can besubduction, 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 oftion indicate that the trench near the central part less than 1,000 m and have sediment thicknesses

greater than 2 km (fig. 10). Structures capable of LEG 3, CENTRAL SOLOMONS TROUGHtrapping oil and gas are present in all three ba- Dep.arted Honiara, Guadalcanal, Solomon Islands, May 19

Arnved Rabaul, New Britain, Papua New GUinea, June 11sins. Sedimentary rocks identified within thesesmaller shelf-edge basins appear to be locally con- METHODSnected with the deeper intra-arc basin (that is,the Central Basin), thereby forming potential Leg 3, conducted in the Solomon Islands troughpathways 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 aboutsource 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). For5km 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 kHzstratigraphic units compose the basin fill, all bathymetric profiles were collected. In addition

,s separated by unconformities. They consist from 36 wide-angle seismic reflection -refractionoldest to youngest of an older, poorly bedded sonobuoy profiles were obtained. Nine dredgesedimentary unit resting on acoustic basement hauls were made, and two gravity cores werealong the margins of the basin and everywhere taken. Dredge hauls ranged in water depths fromoverlain 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 andunconsolidated sediments (fig. 12). Several large 5.4 n. mi.).structures deform the youngest units of this fill; In addition to the resource survey specified inthese structures do not appear to have igneous the ANZUS Tripartite Agreement, the USGSrocks 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 SOPAC and the Rabaul Volcanological Instituteinvestigations of hydrocarbon potential. They are (fig. 14). Reconnaissance bathymetric data wereequal in size and sediment thickness to some of acquired, and the Rabaul seismograph networkthe smaller hydrocarbon producing basins found was tested using the airgun system aboard theelsewhere in the world (for example, western Lee as an artificial seismic source. Also, acoast 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.

The purpose of the harbor survey was to (1) de- metamorphosed, has velocities of 4.8-5.8 kmls; atermine if local uplift is taking place in the har- deeper layer, which has a velocity of 6.4-7.5 kmls,bor, (2) test the sensitivity of the Rabaul seis- is 3-5 km below the acoustic basement. Thesemograph network, and (3) obtain refraction vel- higher velocity rocks may include deeper crustalocities of the rocks in the harbor area to calibrate igneous rocks similar to those observed in islandthe 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). OffshoreGENERAL RESULTS from northern Guadalcanal, the basin previously

identified in Iron Bottom Sound (newly namedThe 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 theprovince, 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 Straitderlain by early Tertiary volcanic and volcaniclas- and is separated from Iron Bottom Basin by thetic rocks; and the Volcanic province, which in- Nggela Sule Island platform; it is bounded to theeludes 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 theridge underlain by shallow, high-velocity rocks Russell and Shortland Basins consist principallyand incised by a deep intra-arc basin known as of volcaniclastic rocks and reef-detritus limestonethe 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, andkened 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(800-1,800m), 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 wellmay 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 pIes from each of the dredge stations indicatefor the sedimentary section and crustal layers Quaternary ages, chiefly in the range 0.2 to 1.6were 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 two45 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 ofothers, 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 em, respectively, oflike 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 atthe Shortland Basin and the Russell Basin (fig. intervals in both cores.15). The late Cenozoic Russell Basin is an elon-gate (60x 250 km) asymmetrical crustal depres- RECOMMENDA nONSsion that contains a 1-5 km sedimentary section(fig. 15). Velocities of the crustal rocks beneath To assist CCOP/SOPAC in further evaluation ofthe sedimentary section range from 4.8-7.5 km/s. hydrocarbon, as well as mineral, resources, futureThe 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

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 programMineral Resources Conference. At this meeting, should include the flanks, which may comprisemany of the cruise participants as well as other in- outcrops revealing substantial middle and lateterested scientists discussed the initial scientific Cenozoic subsidence, and crestal areas of bothresults, the status of data processing and distribu- Tonga and Lau Ridges. Resolution of the historytion, the scientific commitments and coordination, of vertical tectonism of these two geanticlinaland future work and concluded that future investi- ridges is particularly significant in assessing theirgations would be required to more fully assess the hydrocarbon potential, which, if present at all, isresource potential of the 1982study area and adja- likely contained in porous reefal masses of earlycent areas. Their conclusions, which were funda- and middle Tertiary age.mentally logical extensions of the 1982 work, Investigation of the possibility that polymetalformed the basis for the following recommends- sulfides may be forming above the magma cham-tions, presented first at the Eleventh CCOPf bers (?) identified in Lau Basin immediately westSOPAC 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 II. v u tu and Eastem Solomon 1 I nds BasinsLau Ridge, which is underlain by a coeval if not an a (continuation of Leg 2~asyngenetic section (figs. 2, 17).Reflection seismol-ogy is especially needed over the far southern re- Data collected during Leg 2 (fig. 17) delineatedgion of the Tonga platform (south of 23.5 S. to three sedimentary basins on the western margin ofabout 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 drocarbon and mineral potential. Before their po-the ridges separated by Lau Basin in this area in- tential can be defined and assessment of the area'sdicates the rifting event that split them may have geological hazards made, additional studies arebeen less severe (rapid) and possibly occurred needed. These include rock-sampling and heatflowlater 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, andcumstances may enhance the hydrocarbon poten- hydrocarbon maturation history.tial of the ridge's far southern platform section. Previously gathered seismic-reflection dataValuable resource information can also be ex- north of the Banks Islands indicate the presencetracted by extending the collection of geophysical there of other sedimentary basins (Katz, 1980).data farther northward along the Lau Ridge. These basins probably are Miocene to PleistoceneFurther investigation in the Tonga-Lau region in age and therefore possibly have resource poten-would in general be aimed at clarifying the early tial. It would be particularly useful to extend theand mid-Tertiary history of this area, which 1982 multichannel seismic-reflection grid to thewould significantly advance the evaluation of its relatively shallow shelf region that lies betweenoverall resource potential. the Torres Islands and the Santa Cruz IslandAdditional sampling is also needed in the group to determine the presence of sedimentary

southern Tonga platform area, as well as along basins (fig. 17).the adjacent Lau Ridge. The sampling program A 2O-daysurvey is recommended: 10days of sea-carried out during the present investigation was floor sampling and heatflow measurements withinassembled from shipboard analysis of seismic-re- the 1982 survey area and 10 days of multichannelflection records. By studying these records in seismic-reflection surveying in the northern Tor- .greater detail, sites for another dredging pro- res-Santa Cruz region. In addition, detailed seis-gram can be selected to sample more comprehen- mic-reflection surveying is needed along the west-sively the platform's exposed sedimentary se- ern Central Basin to further define potential struc-

20

tural and stratigraphic traps and to assess possible tion ofthe arc.geologichazards. Before the hydrocarbonpotential of the sea-floorPreliminary interpretation of the 1982 Lee data areas of the Solomon Islands can be fully

have shown two other areas in Vanuatu that evaluated, additional multichannel tracklines areshould 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 Shortland Basin. Sea-floor sampling needs to beMalekula and Efate Islands, where sedimentary done, based on the recently collectedgeophysicalbasins have been identified, and (2) along the data, to correlate acoustic stratigraphy with rockflanks of Maewo and Pentecost Islands, where types that makeup the thick sedimentary bodies.structures indicatinghydrocarbontraps mayexist. It is estimated that approximately 30 days ofStudies of the deep structure in the Vanuatu and ship time are needed to accomplishthe work re-

eastern Solomonisland arc provincemay be possi- commendedfor the Solomonsarea. This time could .ble using a two-ship operation. Should it be possi- be dividedinto three lO-dayphases, with the timeble to coordinate two ships in the region, it is re- devoted to geophysicalwork and sea-floor sampl-commendedthat an arc transect involvingexpand- ingbeingdividedequally.ing seismic spreads parallel to the arc plus con- RecommendationsIV and V represent adjoiningstant offset spreads across the arc be undertaken. areas where regional geological and geophysicalIt would also be desirable to collect wide-beam study can significantly increase resource assess-sidescan sonar survey data in the Vanuatu region ment data; scientists attending the Lee post-sur-to better definethe bedrocksea-floorexposures. vey meeting in Honolulu in August 1982 recom-

mended this work be initiated under the sameIII. Solomon Islands and Bougainville, Papua New Guinea, Tripartite Agreement and scientific guidelines as

Basins <continuation of Lea 3) for the present study.

Addition~ &:eophysi~work, ~ particular multi- IV. New Ireland Basin, Papua New Guineachannel seismic-reflectionsurveymg, and sea-floorsamplingare needed to adequately evaluate the re- The New Ireland Basin is a continuation of thesource potential in the West Melanesianarc. Al- Vanuatu, SolomonIslands, and BougainvilleBa-though more information to determine the nature sinsassociatedwith the Melanesianarc. It extendsof the tectonic framework wouldalso be desirable, for about 900 km from eastern New Ireland tothe 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 widthA multichannelseismicsurvey of the southeast- and has a sedimentary section that is locally

ern part of Indispensable Strait and the basin thicker than 5 km. Water depths range from 0 toareas north and east of San Cristobal Island are re- 3,000m. The basin is truncated on the southwestquired to complete the present work. This work by a system of transform faults, and much of theshould have first priority and should be extended sedimentary and carbonate sequence pinches outeastward to correlate with the present investiga- against the Northeast Ridge, whichwas the outertion in Vanuatu and the New Hebrides arc (fig. arc of the West Melanesian Trough during17). Several tracklines toward Santa Cruz Islands Oligocenetime.from the vicinity of San Cristobal·would be suffi- Neville Exon of the Australian Bureau of Min-cient to accomplisha reconnaissancesurvey. eral Resourcesand DonaldTiffinofCCOP/SOPACBottom sampling is necessary to determine the made considerable progress in assessing the hy-

age, nature, and stratigraphic relation of rocks be- drocarbonpotential of the New Ireland Basin fromneath the are, particularly in the central Solomons seismic-reflectionsurveys carried out by CCOPITrough, but also in other areas of the arc including SOPAC, Gulf Oil, ORSTROM, (Officede 1a Re-Indispensable Strait and the region east of chercheScientifiqueet TechniqueOutre-Mer) andBougainvilleand north of Choiseul. Inasmuch as the AustralianBureau ofMineralResources.the offshore basins in this region represent much Land geology and interpretation of presentgreater areas than the islands, and may reflect offshore seismic-reflectiondata (Exon and Tiffin,muchdifferent histories, samplingshouldbe useful 1983) indicate that the basin comprises a thickin revealing resource potential as wellas the evolu- Oligocenevolcaniclastic sequence overlain by as

22

much as 2 km of Miocene platform and reefal car- tichannel lines be taken across the central andbonate beds, and a similar thickness of younger southern part of the North Fiji Basin (fig. 17). Involcaniclastics and biogenic sediment. Should the addition, sea-floor sampling should be done alongoffshore Miocene sequence prove to consist largely the north and west margins of Fiji and on selectedof 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 sampleshave been recovered, it is suggested that a three- REFERENCESweek research program be carried out, consistingof approximately 40 percent multichannel seismic- Coleman, P. J., 1976, ~ re-eval~tion ofth~ Solomon Islands ~reflection surveying and 60 percent sampling. The an ~c sysu:m: Umted ~atlOns C.omnnttee for the. Coordi-

.. . nation of J omt Prospectmg for Mineral Resources m South. SeISmICsurveymg 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 st~cture of the Solomon Island~ arc fromsess the present day thermal gradient in the basin. sonob~oy refractlo~ data ~abs.]: EOS, Transactions of the

Th d d . uld be desi d t I th American Geophysical Union, v. 63, p. 1121.e re gmg w~ . eSI~e. 0 samp e e 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-Pacificalso be carried out, if possible, on steep slopes Conference .~o~Energy and Minerals, August 1982, Hon-along the faulted southwest margin of the basin olulu, Hawaii [m press).

. ..' Falvey, D. A., 1978, Analysis of palaeomagnetic data from thearound the uplifted. Ta~ar- Fern Islands in ~he New Hebrides: Bulletin of the Australian Society of Explo-northeast, and possibly m 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 multichannelnot only greatly increase knowledge of the geol- seismic ?ata [a~s.]: EOS, Transactions of the American

hvsi d t tial f th Geophysical Umon, v. 63, p. 1120.ogy! geop ySICS, an ~es.ource po en I ~ e Greene, H. G., Falvey, D. A., Macfarlane, A., Cochrane, G.,basm, but also would aid m the understandmg of Daniel, J., Fisher, M. A., Holmes, M. L., Katz, H. R.,the tectonic framework in this highly complex re- Smith, G. L., and Pragnell, J., 1982, Preliminary results ofgion, 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-V. North Fiji Basin rides Arc basins, southwest Pacific [abs.]: American As-

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.,

. ithi th N rth F'" B . (fi 17) Daniel, J., Fisher, M. A., Holmes, M. L., Johnson, D.,mg p~ogr~ ~ n e 0 IJI asm g..' Katz, H. R., and Smith, G. L., 1983, Preliminary report onespecially m 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-Pacificble. 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).

di .. t t d th t th _ Katz, H. R., 1980, Basin development in the Solomon Islandsonce a Jo~g s ;tIc ures ~ a ey were. sepa and their petroleum potential: United Nations Committeerated beginning m 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.

'1 d hid di t t d 3, p. 59-75.Ol an gas, may ~ve evo ve a jacen 0 an 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 andthereby 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.channel seismic refl~ction profiles and other Scholl, D. W., Maung, T. D, Vallier, T. L., Childs, J., Steven-

h . al d son, A. J., Exon, N. F., Herzer, R. H., Sandstrom, M. W.,geop ySIC ata be collected along the north and and Soakai, S., 1982, Preliminary results of geophysicalwestern margins of Fiji and that several long mul- and geological studies to assess resource potential and

geologicevolutionofcentral TongaRidgeand summitplat- Bruns, T. R., Colwell,J. B., Cooper, A. K., Kroenke, L.,fonn (21-24°latitude) [abs.]: AmericanAssociationof Pe- Marlow,M.S.,and Wood,R.A., 1982,PreliminaryresultstroleumGeologistsBulletin,v. 66,p. 983. of Leg 3 Lee eruise-basin developmentand resource po-

Scholl,D. W., Vallier, T. L., Maung, T. U., Exon, N. F., tential of central SolomonsTrough [abs.]: AmericanAs-Herzer, R. H., Sandstrom, M. W., Stevenson, A. J., sociationofPetroleumGeologistsBulletin,v.66,p.986.Mann, D. M. and Childs, J., 1983, Initial cruise report, Vedder, J. G., Tiffin, D. L., Kroenke, L., Colwell,J. B.,SOPAC Leg 1 (cruise L5-82)southern Tonga platfonn: Cooper,A. K., Beyer, L. A., Bruns, T. R., Coulson,F. I.Transactions, Third Circum-PacificEnergy and Mineral E., Marlow,M.S., andWood,R. A., 1983,Preliminaryre-ResourcesConference,August 1982,Honolulu,Hawaii[in sults of Leg 3 Lee cruise, SolomonIslands: Transactions,press]. Third Circum-PacificEnergy and Minerals Conference,

Vedder, J. G., Tiffin, D. L., Coulson, F. I. E., Beyer, L. A., August1982,Honolulu,Hawaii[inpress].

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