The Vanuatu Island Arc: An Outline of the Stratigraphy, Structure, and Petrology

Six long-range seismic refraction profiles were collected with ocean-bottom seismometers and a pneumatic seismic source over the central New Hebrides Island Arc during the EVAX (1982) and EVAXIII (1986) cruises. Four profiles are located in the sedimentary intra-arc North Aoba Basin and two others were shot on the western border of the North Fiji Basin. The crust of the western border of the North Fiji Basin is 8 km thick and has seismic velocities with clear oceanic affinities. The crust beneath the North Aoba Basin is 11 to 13 km thick. The sediment fill reaches 4 km in thickness in the central part of the basin and thins toward its eastern and western borders. A 200-m-thick high velocity layer, with a seismic velocity of 4 km s" 1

Section: Geological and Tectonic Settingmentioning

confidence: 99%

Sedimentary and Crustal Structure of the North Aoba Basin from Seismic Refraction Data

Pontoise¹,

Charvis²,

Gérard³

1994

“…Lithologic sequences recovered from Hole 829A are similar to those encountered at Site 828 on the northern flank of NDR (Fig. 36) Carney et al, 1985). Tectonic Unit D of Site 829, which comprises about 90 m of highly fractured Oligocene chalk, was probably thickened by thrust faulting.…”

Section: Discussion and Interpretationmentioning

confidence: 57%

Site 829

1992

hr Universal Time Coordinated (UTC). After 10 days and 1.5 hr on site drilling three holes (Holes 829A, 829B, and 829C) we departed Site 829 at 1230 UTC on 5 November 1990. We were forced to abandon Hole 829A at 590.3 meters below seafloor (mbsf) because of hole collapse and filling problems. Hole 829B was aborted because a core barrel bent on the steeply sloping seafloor. Hole 829C was completed in a hard horizon (58.4 mbsf) where the advanced piston coring (APC) system could penetrate no farther. Site 829 (proposed Site DEZ-2) is located within the collision zone of the d'Entrecasteaux Zone (DEZ) along the forearc slope of the New Hebrides Island Arc where the North d'Entrecasteaux Ridge (NDR) impinges upon the arc slope, approximately 3 km south of Site 827 and about 35 km west of Espiritu Santo Island, Vanuatu. This site was located very close to the original proposed site (DEZ-2) along a single-channel seismic reflection line surveyed by the JOIDES Resolution prior to spudding in at Site 827. The site selected lies on a very narrow (~200-m-wide) shelf protruding out from a 10° slope. We were able to spud in without difficulty on our first try and cored to 590.3 mbsf, recovering 197.44 m of core for a recovery rate of 33.4%. Hole 829B was drilled and cored to 19.5 mbsf, recovering 15.59 m for an 80% recovery rate; Hole 829C was drilled and cored to 58.4 mbsf, recovering 52.67 m of core for a 90.2% recovery rate. The purpose of drilling a second hole at this site was to recover cores with the APC in the upper 100 m of section that was poorly recovered during drilling with the rotary core barrel in Hole 829A. Oriented cores for paleomagnetic analyses and structural examinations were taken and whole-round core samples for both fluid analyses and physical properties measurements were obtained. This was our second attempt to drill to the décollement in the DEZ-New Hebrides Island Arc collision zone. Although hole collapse prevented the penetration of the décollement, we successfully cored the overlying thrust sheets and accretionary prism and may have entered the upper reaches of the décollement zone. Tectonic

“…Coudray (1976) reported the occurrence of a 120-m-thick reef tract, dated 1 to 1.5 Ma., beneath the modern New Caledonian reef province. Carney et al (1985) also stated that successive regressive fringing reef complexes developed around the flanks of various uplifted Vanuatu islands throughout late Pliocene-Pleistocene time. The foraminiferal data presented here indicate that (1) climatic and environmental reef growth-inhibiting conditions cannot have operated throughout the early and intermediate Pleistocene; (2) reef-like bodies may have formed repeatedly on the platform as the slope was prograding.…”

Section: Discussion: Implications For the Growth History Of The Centrmentioning

confidence: 99%

Shallow-Water Foraminiferal Taphocoenoses at Site 821: Implications for the Pleistocene Evolution of the Central Great Barrier Reef Shelf, Northeast Australia

Montaggioni¹,

Vénec-Peyré²

1993

Reworked shallow-water foraminifers that settled on the upper slope of the central Great Barrier Reef at Site 821 (water depth, 212.6 m) were used as indicators of the paleoclimatic and paleoenvironmental conditions that have controlled the Pleistocene evolution of the adjacent platform. Throughout the 400-m-thick sequence drilled, the nature, composition, and distribution of the shallow-water foraminiferal assemblages studied indicate that (1) all the species recorded are at present living in diverse tropical, reef-related areas of the Indo-Pacific and Atlantic provinces; (2) the composition of the microfaunal taphocoenoses is almost identical between the different stratigraphic intervals studied and the modern Great Barrier Reef environments; (3) inner-neritic, tropical environments have continued to develop since the middle Pleistocene; (4) high-to moderate-energy platform edges occurred repeatedly throughout Pleistocene time. These factors may suggest that, since the beginning of the Pleistocene, several reef-like tracts have grown successively on the central area of the northeastern Australian shelf edge. These tracts probably had a sufficiently evolved morphological zonation to act as shelters for foraminiferal biocoenoses of high species diversity.