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Site 840 was drilled in 754.4 m of water during Ocean Drilling Program Leg 135 to provide a better understanding of the geologic history of the Tonga forearc during the Late Cenozoic and to permit investigation of the nature and age of the regional unconformity known as Horizon "A". The cores recovered are the only ones that have been drilled in the marine environment on the southern Tonga Platform. Detailed study of these cores, combined with interpretation of sedimentary structures from Formation MicroScanner (FMS) images indicate two tectonic events of uplift and rotation of the Tonga Platform. Sedimentologic evidence of a significant seismic reflector interpreted as Horizon "A" was recognized at 500 m below seafloor (mbsf) in the Site 840 cores. This corresponds to an age of 6.3 Ma for this event of uplift of the outer forearc and westward tilting of the Tonga Platform. The second tectonic event identified by this study, at 5.1 Ma, is marked by significant changes in the sedimentation rate, lithology, sedimentary structures and carbonate content of the sediments and by changes in the preservation and abundance of foraminifers. The sediments below 260 mbsf are dominated by late Miocene volcaniclastic turbidites interbedded with thin intervals of heavily bioturbated pelagic marlstones. These sediments were deposited on an east-dipping forearc slope, at or near the foraminiferal lysocline, estimated to be not less than 2000 m during the late Miocene. Significant uplift of the outer forearc and consequent northwest tilting of the southern Tonga Platform occurred at approximately 5.1 Ma. At this time, the southern Tonga Platform assumed its present configuration and shallow bathymetry. Sediments younger than 5.1 Ma accumulated on this shallow-water platform in water depths similar to those found there today. INTRODUCTION Site 840 was one of two Tonga forearc sites drilled on ODP Leg 135. It lies in 754.4 m of water and is located approximately 45 km east-northeast of 'Ata Island and about 130 km south-southwest of the islands of Tongatapu and 'Eua (Fig. 1). The site is located in a region informally known as the south central Tonga Platform that forms the crest of the Tonga Ridge south of 21°S. The Tonga Ridge is defined by the 1000-m isobath and extends for more than 1400 km. It is a broad, flat-lying, north-northeast-trending feature between the west-dipping Tonga Trench subduction zone to the east and the active Tofua Arc to the west. Site 840 was intended to provide information on the sedimentologic and tectonic history of the Tonga arc and forearc throughout the Middle and Late Cenozoic. Specifically, the recovered sediments were intended to document the uplift and subsidence history of the forearc since the Middle Cenozoic. The regional, locally angular unconformity known as Horizon "A" (Herzer and Exon, 1985) was a drilling objective. This feature was identified and mapped based on single and multichannel seismic surveys (Fig. 2) and was estimated to lie between 450 and 500 mbsf at Site 840 (Shipboard Scientific Party, 1992). This unconformity was reasoned to reflect a tectonic event coincident with the initiation of rifting and opening of the Lau (backarc) Basin (Parson, Hawkins, Allan, et al., 1992). It was anticipated that recovery of this interval would permit an investigation of the nature and age of the tectonic event. One of the significant aspects of the drilling and logging of Site 840 was the unexpected predominance of resedimented material. Before Leg 135, it was anticipated that the rocks and sediments recovered would be shallow-water platform carbonates, possibly even reefal material, interbedded with some volcaniclastics (Parson, Hawkins, Allan, et al., 1992). However, the strata recovered are predominantly gravity flow deposits in the form of volcaniclastic turbidites and debris flows. These are interbedded with thin intervals of pelagic or hemipela1 Hawkins, J., Parson, L., Allan, J., et al, 1994. Proc. ODP, Sci. Results, 135: College Station, TX (Ocean Drilling Program). 2 Department of Geosciences, University of Tulsa, 600 South College, Tulsa, OK 74104, U.S.A. gic marlstones. No evidence of a shallow-water carbonate platform at or near Site 840 was recovered. Strata recovered at Site 840 record the evolution of the Tonga forearc since the late Miocene. In this chapter we present a description of the autochthonous and allochthonous sediments recovered from this site. Our discussion includes an interpretation of the probable depositional environment of these sediments, an evaluation of the tectonic implications of the turbidites, an estimation of the probable paleowater depth, and an assessment of the age and significance of Horizon "A" at this location. METHODS Forty thin-sections from various lithologies were prepared and examined. Samples from the most carbonate-rich pelagic and hemipelagic marls, particularly those containing biogenic structures, received special attention. All samples were mounted with an ultraviolet-cure epoxy so that the process could be conducted without heating the samples. This was done to preserve the natural fluorescence of the sediments, particularly the biogenic carbonate fraction. Because carbonate material is more prone to fluoresce than volcaniclastic material, ultraviolet and blue-violet epifluorescent microscopy was used in conjunction with standard petrography to resolve the presence and relative degree of preservation of biogenic carbonate grains (Dravis and Yurewica, 1985). This technique also proved helpful in revealing biogenic structures in thin-section because of the concentration of pelagic carbonate material in infilled burrows. Eighty smear slides were prepared to compare the abundance and preservation of biogenic carbonate grains (microfossils) from within infilled burrows to those within the host sediment. Because overall core recovery from Site 840 was only 40% (Table 1), Formation MicroScanner (FMS) logging data were used to complement the study of primary sedimentary and biogenic structures in the forearc deposits. These images can be used to resolve individual turbidite units because of the variation in resistivity from the base to the top of the interval (McLeod, this volume). Processed data from FMS logs acquired from Hole 840B were reformatted for use on a Sun workstation. Schlumberger FMS Image Examiner software was used to identify individual turbidites. Color

Late Miocene Sedimentation History of the Tonga Forearc at Site 840