VOLCANIC ASH LAYERS IN THE JAPAN SEA : TEPHROCHRONOLOGY OF SITES 798 AND 799 1

Exceptional records of explosive volcanic activity were collected at Sites 798 and 799 on Leg 128 in the Japan Sea at the Oki Ridge and Kita-Yamato Trough, respectively. Ash deposition consists of 1-mmto 20-cm-thick pyroclastic-fall layers dated from mid-Miocene to late Quaternary and of one 27-m-thick pyroclastic-flow sequence dated at about 12.5 Ma. Tephra layers are classified into four sedimentary types: homogeneous, graded, heterogeneous, and turbiditic. They originated from the Japanese volcanic arc (shards, bubble wall fragments, and micropumices of rhyolitic to dacitic composition) or from the volcanic islands of the Japan Sea (micropumices and microscoriae of trachytic to phonolitic composition). The pyroclastic-flow deposit is correlated with the Onnagawa Formation in northern Japan. A total of nine volcano-tectonic phases are distinguished: four in the Miocene, two in the Pliocene, and three in the Quaternary. The tephra record may aid in determining the precise timing of the Yamato Basin backarc evolution. INTRODUCTION Leg 128 of the Ocean Drilling Program (ODP) was devoted to probing the history and structure of the Japan Sea and particularly to assessing the timing and dynamics of the opening of the marginal basin (Ingle, Suyehiro, von Breymann, et al., 1990). The record of the explosive activity can be read, with certain limitations, in the distribution of ash in deep-sea sediments (Donnelly, 1974; Hein and Scholl, 1978; Pouclet et al., 1990). Exceptional records of volcanic activity from the neighboring volcanic arcs and islands were collected in the central and south Japan Sea at Sites 798 and 799 (Fig. 1). LEG 128 ASH RECORD At Site 798 on Oki Ridge in the southern Yamato Basin, an almost complete representation was acquired from the Quaternary to the middle Pliocene, by combining data from different holes. This was possible because of the continuous sedimentation without any severe turbation or repetitions by slumping, the excellent preservation of ash, and the fairly good recovery. At Site 799, in the Kita-Yamato Trough, a very satisfying record was recovered from the Quaternary to the lower Pliocene in Hole 799 A, in spite of some slumping disturbance. In the Pliocene strata, nonrecovered tephra layers were accurately recognized and enumerated by means of the formation microscanner (FMS) logging tool. Upper and middle Miocene strata were retrieved in Hole 799B with a less satisfactory recovery. In this latter hole, the ashes are highly altered but still recognizable. Most of the volcaniclastic material consists of pyroclastic-fall layers produced by subaerial explosive activities (fine shards and micropumices). Epiclastic debris in the form of reworked and corroded fragments of glass and minerals are found. No hyaloclastites were observed at either Site 798 or 799. At present, upper atmospheric wind directions are north to south in the winter, south to north in the summer, and are variable during the spring and fall. The wide distribution of late-Quaternary marker tephras of known origin in and around Japan (Machida, 1981; Furuta et al., 1986; Machida and Arai, 1988) indicates that significant quantities of tephra may have arrived at Sites 798 and 799 from a large 1 Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al., 1992. Proc. ODP, Sci. Results, 127/128, Pt. 2: College Station, TX (Ocean Drilling Program). 2 Géotectonique et Pétrologie, Faculté des Sciences, Université, BP 6759, F45067 Orleans Cedex 2, France. 3 Marine Geology, Research Laboratory, Department of Geology, University of Toronto, Toronto, Ontario, M5S 3B1 Canada. number of explosive eruptions in many directions up to distances of at least 500 km. The most likely sources are the larger acidic volcanoes of the Japan arc (e.g., Kikai, Aira, Unzen, Aso, Toya), the nearby volcanic islands (e.g., Ulleung, Oki-Dogo), and the explosive volcanoes of North Korea (e. g., Baegdusan). A prominent 27-m-thick sequence of pyroclastic flows of rhyolitic composition was recovered near the bottom of Hole 799B. The ash deposits consist mainly of micropumices. They originated from a nearby explosive volcano that may have been as close as the Yamato Bank. PETROGRAPHIC ASPECTS The petrography of individual ash layers is detailed in Ingle, Suyehiro, von Breymann, et al., 1990). Results are summarized here. Ash-fall Layers A total of 113 ash layers was encountered at Site 798 from the Quaternary to the middle Pliocene, and as many as 256 layers were found at Site 799 from the Quaternary to the middle Miocene. Uthology Layers range from 1 mm to 20 cm in thickness. At Site 798, 65% of the layers are less than 0.4 cm thick, and 10% are more than 5 cm thick. At Site 799, the proportion is 47% thinner layers and 6% thicker layers. The higher frequency of the medium-thick layers may indicate the proximity of island volcanoes of moderate explosive activity. The average grain size of the volcanic layers is silt to very fine sand (<125 μm). Coarser grain sizes of fine sand (>125 μm) characterize some medium and thick layers. By definition, a tephra layer contains at least 60% volcaniclastic material (lithic, vitric, crystal) (Explanatory Notes, Ingle, Syehiro, von Breymann, et al., 1990). No lithic fragments were recognized. The glass fractions consist of shards, bubble wall fragments, micropumices, and microscoriae. Many shards exhibit characteristic Uand Y-shapes (tricuspate) that have resulted from disruption of highly vesicular pumices. Vesicles are elongated to tubular in the micropumices; they are spherical or irregular in the microscoriae and correlate with a lower vesicularity and the presence of enclosed magmatic phenocrysts. The crystal fractions consist of fragments or euhedral grains of quartz, alkali feldspar, plagioclase, biotite, amphibole, pyroxene, and scarce oxides. The proportion of crystals ranges from 2% to 10% in the glass-rich beds and from 10% to 30% in the crystal-rich beds.