REPORT ON THE THIRD LEG OF THE JOINT CCOP/SOPAC AUSTRALIA, NEW ZEALAND, U.S. CRUISE OF THE R/V S.P. LEE (L7 -82-SP)

by

L. W. Kroenke

Hawaii Institute of Geophysics University of Hawaii Honolulu, Hawaii

Objectives

To participate in a multichannel seismic survey on board the R/V S.P. LEE (cruise L7 -82-SP)from Honiara, Solomon Islands to Rabaul, Papua New Guinea.

Itinerary

12 May Disembark from the R/V KANA KEOKI in Port Vila, Vanuatu 12-19 May In Vila 19 May Travel to Honiara.1, Solomon Islands and the R/V S.P. LEE. 19 May- 11 June On board the R/V S.P. LEE at sea. 11 June Arrival in Rabaul, Papua New Guinea 12 June By air to Port Moresby, Papua New Guinea 13 June By air to Sydney, Australia 14 June By air to Suva, Fiji 14 June - 28 June In Suva to complete reports 28 June Travel to Honolulu, Hawaii

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Activities

A multichannel seismic survey was conducted around the central Islands of the Solomon Island chain between 19 May and 11June using USGS vessel R/V S.P. LEE. The leg (L7 -82-SP) had been planned at a precruise meeting held in conjunction with the 10th Session of CCOP/SOPAC held in Port Vila, Vanuatu, 6-14 October 1981. This was the third leg of the S.P. LEE cruise to be jointly undertaken by CCOP/SOPAC, Australia, New Zealand and the United States as part of the CCOP/ SOPAC Work Programme (CCSP/SI.2: Evaluation of the hydrocarbon potential of the Central Solomon Trough [the Slot Area]). Led by co-chief scientists J. Vedder (USGS) and D. Tiffin (CCOP/SOPAC), scientists on board included representatives from CCOP/ SOPAC Technical Secretariat, Australia, New Zealand, Solomon Islands, and the United States (see Annex 1).

Results

Between 19 May and 6 June 1982 (139/0630Z), 52 multichannel seismic lines representing 1914 line miles (3490km) and 36 successful sonobuoy refraction lines were completed. Bottom sampling included 2 successful gravity corer and 7 successful rock dredge stations. The orientation of the seismic lines and their approximate locations are given in Annex 2. The location of the bottom sampling stations is given within the narrative that follows. A short, high resolution single channel survey was also undertaken of Rabaul Harbour just prior to the arrival of the R/V S.P. LEE in Rabaul. The success of the cruise is attributed to the technical proficiency, enthusiasm, and dedication of all participants in the leg. In particular the close rapport that developed between the scientific party and crew was instrumental in ensuring a successful voyage.

Geophysical Data Acquisition

The multichannel seismic reflection system used on the S.P. LEE consisted of a tuned array of 5 Bolt airguns with a combined capacity of 21.7 liters (l,326 cu.in),

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.a 2,400m, 24-group SEI streamer, and GUS 4300 digital recording instruments. Navigation was by means of Magnavox MX702A dual channel satellite navigation system supplemented by Radar ranges and visual bearings. High resolution reflection profiling data, using a 1200J EG&G Uniboom system, and 12 and 1.5kHz bathymetric data, both using Raytheon CESP III systems, were simultaneously acquired. Six Raytheon Line Scan Recorders, two of which were used as single channel air gun monitors, continuously recorded these data. Gravity and magnetic data were also acquired simultaneously, using a La Coste-Romberg gravimeter and a Geometries proton-precession magnetometer, respectively.

The data are to be processed by the U.S. Geological Survey in Menlo Park, CA. Velocity control for the seismic reflection data will be determined from stacking velocities obtained during processing and from sonobuoy seismic refraction data that were also periodically collected during the cruise.

Narrative

The R/V S.P. LEE departed Honiara, Solomon Islands, at 1730 Local time on 19 May (139/06302) to begin the multichannel seismic survey. The streamer was deployed along the outbound transit line (TRA). Both the Transit line and line 1 were located in Iron Bottom Sound. The first third of line 2 was located in Skylark Channel between Guada1canal and Florida Is. enroute from Iron Bottom Sound to Indispensable Strait. The last two thirds of line 2 and lines 3, 4, 5, 6, 7 and 8 were all located in Indispensable Strait between the Florida Island platform and the Island of . A somewhat irregular line, line 9 (141/01052-141/02032), was located in a passage across the Florida Island Platform just northwest of the Florida Group.

Line 10, north of Savo, and the first part of Line 11 were located at the southeastern end of (the Slot). The remainder of Line 11 passed the northwestern tip of Guada1canal, across the forearc horst between and the Russell Islands, over the inner trench wall, and possibly onto the easternmost

4 part of under thrusting Woodlark l3asin seafloor. Line 13, joined to line 11 by line 12, recrossed the same features in reverse order passing just to the Southeast of the Russell Islands.

Following a brief diversion to Honiara along lines 14 (north of Savo) and 15 (Florida to Guadalcanal) d longitudinal Transect of Iron Bottom and New Georgia Sound (line 16) was begun on 142/2222Z. Line 16 was run from Honiara to a point just north of Nggerasi Lagoon, New Georgia. From there a series of NE-SW lines, lines 17 through 31 (144/0650Z- 149/1654Z), progressively re-crossed New Georgia Sound from the northwest to the southeast finishing just off the Russell Islands.

Line 32, extending ENE from the Russells to Malaita again recrossed New Georgia Sound, the Florida Islands Platform (northwest of Florida) and Indispensable Strait. Line 33, 34, 35, 36 and 37 looped out across the Malaita Anticlinorium, passing the northwest tip of Malaita onto the lowermost slope of the Roncador Homocline before recrossing the anticlinorium on a long straight run back (line 37) across Indispensable Strait and the Florida Island Platform, ending in New Georgia Sound just north of Savo.

Line 38, longitudinally traversing the northeast side of New Georgia Sound, was terminated at 0600 Local time on Wednesday 2 June (152/18002) to begin bottom dredging and coring stations. Station 1 (at 08° 26'.47'N, 158° 55.15'E) was located on line 26 in a water depth of 1809m in the central part of New Georgia Sound northwest of Russell Island. A gravity corer, lowered onto the bottom at 153/00122 recovered 192cm of grey mud.

After scrubbing a site further basinward, Station 2 (at roughly 08°56.0'S, 158° 40.0'E) was located on line 26, on the southwestern edge of the Basin, west of Mborukua Island between the Russells and the New Georgia Group. A dredge lowering was

5 made at 153/07532 in 1109m of water. The haul recovered about one quarter of a chain bag of reef detritus (coral1ine algae) together with minor amounts of tuffaceous sediment.

Station 3 (at roughly 09° 12.1 'S, 158°47.7'E) was located on line 27 southwest of the Russel1 Islands, at the base of a 300m fault scarp fronting a tilted (rotated) block of stratified sediment at the top of the inner trench wal1. The scarp was dredged from 153/13002-14002 in water depths between 1250 and 984m. About l5kgms of greenish grey ashy siltstone was recovered.

Station 4a (at roughly 09°06.8S, 159°27.1 'E) was located along line 13 in water depths between 750 and 400m at the southern edge of the Russel1 Island platform on a prominent submarine scarp. During the first dredging attempt (Station 4) the rock dredge was lost. During the second attempt at the same site (Station 4a), the fore reef slope was dredged between 153/2125 and 154/0036Z. Approximately one half a chain bag ful1 of reef limestone together with a few fragments of branching coral was recovered.

Station 5 (at roughly 09°04.6'S, 159°29.2'E) was located along line 10 northeast of the Russel1 Islands in water depths between 1100 and 900m. The stratified base of the Island platform was dredged between 154/0217 and 0437Z. No rock samples were obtained except for a trace of mud on the dredge frame.

Station 6 (at roughly 08°56.7'5, 159°11.6'E) was located along line 30 on the north insular slope of the Russell Islands in 600 to 300m of water. The fore reef slope was dredged from 154/0840 to 0930Z and one sample of Mn coated reef limestone was recovered.

Station 7 (at roughly 8°40.6'S, 159°36.5'E) was located along line 30 on the southwest insular slope of in 900 to 300m of water. Apparently .another fore reef slope was dredged from 154/1540 to 1640Z. About one half of a

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Chain bag of recent coralline debris was recovered.

Station 8 (at 08°46.2'S, 159°36.5'E) was located along line 31 on the lower insular slope south of San Jorge Island in 1279m of water. A gravity corer was lowered into the bottom at 154/1830z and a 262cm core of olive grey, bioturbated, foram rich mud was recovered.

Station 9 (at roughly 08°29.2'5, 158° 16.2IE) was located along line 22 on a low submarine ridge north of New Georgia. The area around the base of the ridge was dredged from 155/0400 to 0551Z. The dredge haul contained mainly buff coloured, Mn coated, foram rich, fine-grained, silty sandstone which contained a few shell fragments and was extensively bored. Two samples of Mn coated branching coral were also recovered.

At 155/2030Z the multichannel seismic survey was resumed along line 39 near the point of departure from line 38. Line 39, however, had to be terminated prematurely at 156/0135Z to facilitate repair of the streamer and the longitudinal Slot traverse along the southwest side of Santa Isabel was completed on line 40. Nevertheless, sufficient overlap exists between lines 38, 39, and 40 to ensure complete coverage of that part of New Georgia Sound. Following the completion of line 40, a series of transverse crossings of the Sound between Choiseul and the New Georgia Group were begun along lines 41, 42, and 43. Along line 43 a circular high, charted in the bathymetry, was crossed north of Kolombangara.

Originally thought to be a submarine volcano (Hughes and Turner, 1977) the circular high appears to be a structural feature, formed by a tilted fault block i.e. a mid basin horst. The horst forms a saddle in west central New Georgia Sound which longitudinally divides the Central Solomon’s Trough of Katz (1980) [the Solomon Basin of De Broin et. al. (1977)] into two basins: the Shortland Basin (new name) to the northwest and the Russell Basin (new name) to the southeast.

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The Shortland Basin part of New Georgia Sound was surveyed from 157/06302 to 158/21122, along a series of roughly North-South transverse crossings (lines 44-51 inclusive). The multichannel survey ended at 159/17302 with the completion of a longitudinal tie line (line 52) joining the Shortland Basin survey to the Russell Basin Survey. After completion of line 52, sufficient cruise time remained to undertake two dredge stations within the confines of the Shortland Basin.

Station 10, at roughly 07°30.4'S, 156°08.5'E was located along line 51 in water depths between 825 and 685m. The base of a submarine ridge, appearing to over thrust the crest of the frontal arc horst west of Vella Lavella in the New Georgia Group, was dredged between 160/0700 and 07452. Approximately one fifth of a chain bag of olive grey foram rich, volcaniclastic sanstone was recovered together with sandy calcilutite (foram rich) and one clast of leucocratic volcanic rock (welded tuff?).

Station 11, at roughly 07°29.0'S, 156°08.2'E, was located along line 51 on water depths between 700 and 600m. The upper slope of the same ridge dredged at station 10 was dredged between 160/0840 and 09002. A full chain bag of Mn coated fine grained well cemented calcarenite together with a few samples of soft greenish grey, foram rich sandy mudstone, a few dense calcareous branching corals and a crinoid were recovered.

Summary

Four multichannel lines (1, 14, 15 and the beginning of 16) were located in Iron Bottom Sound. The Sound outlines a sedimentary basin, Iron Bottom Basin (new name), which is bounded by Savo Island to the northwest, Guadalcanal to the South and the Florida Island platform to the northeast.

Ten lines (2-8, 32, 33 and 38) were located in Indispensable Strait. The Strait coincides with Indispensable Basin (new name), which extends from the Florida Island platform northeast to the Island of Malaita and contains the deeply buried, tightly folded southwest end flank of the Malaita Anticlinorium. The northwestern end of the basin abuts against Santa Isabel; the southeastern end is not as well defined possibly terminating south of Small Malaita (North of San Cristobal, west of Uki Island).

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North of Indispensable l3asin, six lines (33-37 and part of 38) extend northward across the Malaita Anticlinorium and onto the south flank of the Roncador Homocline (the outer slope of the North Solomon Trench). South of Indispensable Basin, four lines (2, 9, 32 and 38) cross the Florida Islands Platform: line 2 through the Sealark Channel south of Nughu Island; lines 9, 32 and 38 northwest of Vatilau in the Florida Group. Five lines (11- 13, 26, and 27) extend across the frontal arc horst, southwestward over the inner trench wall, and onto the floor of Woodlark Basin. Lines 26 and 27 also crossed the Pocklington Ridge and Trough, grazing the flank of the Louisade Rise.

Twenty three lines (part of 11, part of 13, most of 16, 17-25, parts of 26 and 27, 29- 31, part of 32, and 40-42) were located in the southeastern end of New Georgia Sound between Santa Isabel and the frontal arc horst capped by the New Georgia and Russell Islands herein called the Russell Basin. The basin extends from Savo Island northwest to the small midbasin horst between Choiseul and the New Georgia Group north of Kolombangara Island. Nine lines (44-52) were located in the north western end of New Georgia Sound, i.e. the Shortland Basin extending from the mid basin horst to the Shortland Islands.

ANNEX I Scientific Party R/V S.P. LEE Cruise L7 -82-SP Central Solomons Trough

Scientists

Jack Vedder Co-Chief Scientist, USGS, Menlo Park, California Don Tiffin Co-Chief Scientist, CCOP/SOPAC, Suva, Fiji Larry Beyer USGS Menlo Park, California Donna Blackman USGS Menlo Park, California Terry Bruns USGS Menlo Park, California Guy Cochrane USGS Menlo Park, California Jim Colwell Bureau of Mineral Resources, Canberra, Australia Alan Cooper USGS Menlo Park, California Frank Coulson Solomon Islands Geological Survey, Honiara, Solomon Islands Gregory Lewis USGS Menlo Park, California Kay Kinoshita USGS Menlo Park, California Loren Kroenke Hawaii Institute of Geophysics, Honolulu Hawaii (CCOP/SOPAC) Michael Marlow USGS Menlo Park, California Raymond Wood New Zealand Geological Survey, Lower Hutt, New Zealand

Technicians

Dave Hogg Electronics Technician, USGS Menlo Park, California Larry Kooker Electronics Technician, USGS Menlo Park, California Kevin O'Toole Marine Technician, USGS Menlo Park, California Paul Wenberg Marine Technician, USGS Menlo Park, California

Watch Assignments

Chief of Watch Mike Marlow, Alan Cooper, Don Tiffin/Jack Vedder Multi-channel Guy Cochrane, Terry Bruns, Donna Blackman Navigators Kay Kinoshita (lead), Larry Beyer, Gregg Lewis/Don Tiffin Geophysics Larry Beyer (gravity & magnetic), Loren Kroenke, Frank Coulson, Jim Colwell, Raymond Wood

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