Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 134 7. VOLCANICLASTIC SEDIMENTS OF THE NORTH AOBA BASIN: DEPOSITIONAL PROCESSES AND GEOLOGIC HISTORY1 Margaret R. Goud Collins2 ABSTRACT The tectonic and volcanological history of the central New Hebrides Island Arc since late Miocene time is reflected in the sedimentology of two Ocean Drilling Program (ODP) drill sites in the intra-arc North Aoba Basin. Sites 832 and 833 are located on the floor and the eastern margin, respectively, of a summit basin with water depth of over 3000 m. At each site, drilling reached depths greater than 1000 meters below seafloor (mbsf), and the cores represent the first opportunity to examine a thick section of intra-arc basin sediments. The cores are notable for thick sequences of volcanogenic sediments. Volcanism produced layers up to 42 m thick of unadulterated volcanic material, in sequences that grade upward from lapilli, to thick layers of volcanic sand, to fine ash and clay. The rapid erosion that accompanied volcanism and tectonic uplift of islands produced thick layers of breccia and conglomerate, featuring a wide variety of clasts of terrigenous and shallow-water origin, which serve as the basal layers of fining upward sequences. The sediments reflect four periods of island building. Volcaniclastic debris flows and coarse-grained turbidites, overlain by calcareous, fine-grained turbidites and hemipelagic sediments, were recovered from the deepest 240 m of Site 832. These sediments resulted from island building that I interpret as volcanism and uplift on Maewo Island in the late Miocene, with contributions of eroded sediment from Espiritu Santo Island. During the Pliocene, continuing erosion on Maewo Island and volcanism in the area contributed calcareous and volcanogenic debris to the hemipelagics, silty turbidites, and tephra collecting at both sites. Uplift and extension during the middle Pliocene are suggested by normal faulting in the sediments, and by an unconformity at Site 832. From latest Pliocene to early Pleistocene times, renewed uplift and volcanism on surrounding islands revived debris-flow deposition. Late Pleistocene sedimentation was dominated by volcanic ashflows as Central Chain volcanoes built depositional aprons. Tephra falls interbedded with hemipelagic sediments have dominated sedimentation in recent times. INTRODUCTION West of the New Hebrides Islands, the Australia-India Plate is being subducted to the east beneath the Pacific Plate at the New Hebrides Sites 832 and 833 are located on the floor of the intra-arc North Trench; the North Fiji Basin lies east of the arc. The Aoba basins are Aoba Basin, in water depths of 3089 and 2629 m, respectively (Fig. 1). opposite the d'Entrecasteaux Zone, an aseismic ridge and a parallel The volcaniclastic sediments recovered from the two drill holes, each seamount chain on the Australia-India Plate, which are being sub- of which reached sub-seafloor depths of more than 1000 m, offer a ducted at the New Hebrides Trench (Daniel, 1978). The basins are sedimentological record of the tectonic and volcanic history of the separated from the d'Entrecasteaux Zone by the Espiritu Santo and central New Hebrides Island Arc since the Miocene. The North Aoba Malakula islands, and are divided into northern and southern seg- Basin holes are less than 100 km from five volcanic islands, and rocks ments by Aoba Island, an active island-arc volcano (Fig. 1). exposed on the islands reveal three distinct periods of volcanism and The North Aoba Basin is surrounded by five volcanic islands, which uplift (Table 1). The cores, examined in the light of the geology of the were built by three episodes of volcanism and tectonism that created surrounding islands, provide a glimpse of the scale and duration of the the New Hebrides Island Arc. The volcanic islands of the arc are seafloor sedimentation processes associated with these three known grouped into the Western Belt, the Eastern Belt and the presently active tectonic and volcanic episodes. Conversely, because the basin's sedi- Central Chain (Fig. 1, Table 1; Carney and Macfarlane, 1982; Mac- mentation is controlled by the activity of the surrounding islands, the farlane et al., 1988). The island-arc history is described briefly here. cores offer the most continuous available record of the islands' history The oldest rocks in the Western Belt, which comprises Espiritu (Carey and Sigurdsson, 1984). Santo and Malakula islands, are volcaniclastic turbidites and breccias The proximity of Sites 832 and 833 to active volcanic islands and derived from a partly emergent volcanic arc active in the late Oligo- the number of sediment sources make this the best available record cene/early Miocene (Mitchell and Warden, 1971; Carney et al., 1985). of the arc-basin sedimentary processes resulting from volcanic and The early Miocene sediments are intruded by dikes and sills that bear tectonic activity. The cores contain interbedded debris flows, tur- phenocrysts of the same minerals found in the adjacent volcaniclastic bidites, ashfall deposits, and hemipelagic sediments (Fig. 2) compa- sediments (Mitchell and Warden, 1971). Volcanism continued through rable to the sedimentary record found in drilling on the margins of the middle Miocene, after which subsidence and faulting created a backarc basins (Klein, 1985; Carey and Sigurdsson, 1984). north-trending graben that was filled with at least 4000 m of volcano- genic graywackes and pelagic sediments that are now exposed on SITE SETTING Espiritu Santo and Malakula (Fig. 3; Carney et al., 1985). Uplift and erosion after about 11 Ma was followed by resubmergence in the late The Aoba basins lie within the New Hebrides Island Arc, and form Miocene, so that the middle Miocene pelagic sediments are overlain an isolated deep water area within the arc's summit platform (Fig. 1). unconformably by late Miocene/early Pliocene (8.5-5 Ma) hemipelagic and pelagic mudstones and turbidites (Fig. 3; Mitchell and Warden, 1971; Carney and Macfarlane, 1982). By the late Pliocene, the Western Greene, H.G., Collot, J.-Y, Stokking, L.B., et al., 1994. Proc. ODP, Sci. Results, Belt had been uplifted, collecting an upper Pliocene-lower Pleisto- 134: College Station, TX (Ocean Drilling Program). cene terrigenous, volcaniclastic sequence and a cap of Pleistocene- " Coastal Research Center, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. (Present address: Office of the Chief Scientist, National Oceanic and Holocene age reef limestone that documents hundreds of meters of Atmospheric Administration, Department of Commerce. Washington, D.C. 20230, U.S.A.) uplift during the late Quaternary (Taylor, 1992). •~••J M.R. GOUD COLLINS Western Belt ||||[||||Central Chain f - *^j Eastern Belt Figure 1. A. Bathymetric map of the intra-arc basins of the New Hebrides Island Arc, showing positions of Sites 827-833. B. Regional framework. Table 1. Islands surrounding the North Aoba Basin: distance to drill sites and age Santa Maria dated at 1.8 Ma), and they display no raised terraces or of exposed volcanic rocks. other signs of uplift (Carney and Macfarlane, 1982). Minimum Minimum distance to distance to METHODS New Hebrides Arc Age of exposed Site 832 Site 833 Island geological class volcanic rocks (km) (km) For this study, a detailed description and interpretation of the em- Espiritu Santo Western Belt late Oligocene- 55 86 placement process for every sediment layer of every core was made, (Queiros Peninsula) middle Miocene Santa Maria Central Chain latest Pliocene-present 50 70 using reexaminations of the core photographs, shipboard descriptions, Mere Lava Central Chain late Pleistocene—Holocene 69 45 and selected smear slides. The characteristics and thicknesses of each Maewo Eastern Belt late Miocene-early Pliocene 56 20 Aoba Central Chain late Pleistocene-present 60 45 sedimentary layer, defined by changes in lithology and appearance of bedding planes, were entered into a database so that variations with * From Carney and Macfarlane (1982). time and depth could be compared. Layers were differentiated by grain size, lithology, and appearance of bedding planes. The maximum thick- The Eastern Belt, which includes Maewo and Pentecost islands, ness of a single layer was one core-length, or 9.5 m. displays arc-derived volcaniclastic bathyal sediment as old as the The descriptions presented in this paper draw from and synthesize, early to middle Miocene (14-8 Ma; Carney et al., 1985). In contrast but also expand upon and quantify, those generated aboard the JOIDES to the Western Belt, which was subsiding in the late Miocene, the Resolution during Leg 134 (Collot, Greene, Stokking, et al., 1992). Eastern Belt was being uplifted. Foraminiferal mudstones dated at Emphasis is placed on the processes that operated in depositing each 7-4 Ma indicate when the Eastern Belt rose above the Carbonate unit, and lithologic information is offered only insofar as necessary to Compensation Depth (CCD); these mudstones are overlain by and explain those conclusions. Complete descriptions of the cores and their interbedded with a series of pillow lavas (Fig. 3; Mallick, 1973; lithologies can be found in the Leg 134 Initial Reports (Collot, Greene, Carney and Macfarlane, 1982). Volcanism ended between 3 and 4 Ma Stokking, et al., 1992). (Carney et al., 1985), and the volcanic rocks were covered by pelagic sediments. The Eastern Belt was uplifted and tilted in late Pliocene Classification