Abstract:
ODP Site 752A (ODP Leg 125) lies on the eastern margin of the Izu-Bonin forearc basin, about halfway between the active volcanic arc and the Izu-Bonin Trench in the western Pacific Ocean. Borehole 782A, which is 476.8m deep, was fully cored and described. A simple subdivision into two lithologic units is made: one sedimentary and the other volcanic basement. The stratigraphic age of the sediments is Pleistocene to middle Eocene. The generally low abundance of planktonic Foraminifera was observed in pre-Quaternary cores. The results of foraminiferal study from Borehole 782A indicate the sedimentary sequence between the Quaternary (zone P (sub 22) ) and the late middle Eocene (zone P (sub 14) ). There are S zones that might be divided based on the zonation of Bolli et al. Of these zones, the Catapsydrax dissimilis zone (zone N5) and the Catapsydrax stainforthi zone (zone N6) were found in sample 125-782A-35X-31X, showing the existence of lower Miocene sediments. The absence of zones P15-lower P16 and upper N3-N4, N7-N11, show the hiatuses between the late Eocene/middle Eocene, early Miocene/late Oligocene, and middle Miocene/early Miocene. Temperate-to-warm subtropical planktonic Foraminifera dominate the whole sedimentary sequence. The five stages of temperature change in this area can be identified based on the abundance and diversity of Foraminifera and the ratios of planktonic and benthonic Foraminifera. They are: I, late middle Eocene to early Miocene, II, middle middle Miocene to early late Miocene, III, middle late Miocene to early early Pliocene, IV, middle early Pliocene to middle middle Pliocene, and V, late middle Pliocene to Pleistocene. Stages I, III, and V are relatively warm and Stages II and IV are relatively temperate.