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Pisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T., Barron, J., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 127/128, Pt. 114. LATE NEOGENE SILICOFLAGELLATES AND EBRIDIANS FROM LEG 128, SEA OF JAPAN1Hsin-Yi Ling2ABSTRACTDatum levels and biostratigraphy, including a new zonation of late Neogene silicoflagellates and ebridians, are presented,based on analysis of submarine sediments collected from Sites 798 and 799, Sea of Japan, during the Leg 128 cruise.INTRODUCTIONSilicoflagellates and ebridians play an important role in biostrati-graphy, next only to diatoms in their consistent occurrence in subma-rine sediments from the Sea of Japan recovered during the Leg 128cruise of the Ocean Drilling Program (ODP). Furthermore, the de-tailed shipboard paleomagnetic analysis established the "absoluteages" for their biostratigraphic events.MATERIALS AND METHODSThe Oceanographic data for Leg 128 (Fig. 1) are Site 798, at37°03832'N, 134°79976"E, in 903.1 m water depth, and Site 799, at39°22046'N, 133°86685'E, in 2073.0 m water depth.The procedures for sample preparation and the methods used todesignate the location of illustrated specimens in the strewn slides aswell as the relative abundance of taxa within each sample are essen-tially the same as described in the Deep Sea Drilling Project (DSDP)Leg 19 report (Ling, 1973). All microslides used for the presentinvestigation, including the figured specimens, will be depositedpermanently in the Micropaleontology Collection of the Departmentof Geology, Northern Illinois University.LATE NEOGENE SILICOFLAGELLATE/EBRIDIANZONATION FROM THE SEA OF JAPANIn this section, the development of silicoflagellate/ebridian zona-tion from the Sea of Japan is reviewed in conjunction with that of thenorthwest Pacific, followed by the detailed documentation of datumlevels based on recent results and the presentation of a new zonationin the Sea of Japan.Previous ZonationThe biostratigraphic zonation of silicoflagellates from the mid-latitude North Pacific was proposed by Bukry (1973a) for DSDP Site173 off California in correlation with the coccolith zones (Fig. 2).After the successful biostratigraphic application of a combination ofsilicoflagellates and ebridians during DSDP Leg 19, Ling (1973)presented a combined zonation from the high-latitude North Pacific,which he later revised based on the analysis of Leg 31 subsurfacesediments, including from the Sea of Japan (Ling, 1975). Age assign-ments for these zones were made by correlation with co-occurringdiatoms (Koizumi, 1975). Further examination of Northwest Pacificsediments revealed that the last appearance datum (LAD) of Dictyo-cha subarctios occurs slightly above the Brunhes/Matuyama ChronPisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T., Barron, J., et al., 1992. Proc.ODP, Sci. Results, 127/128, Pt. 1: College Station, TX (Ocean Drilling Program).2 Department of Geology, Northern Illinois University, DeKalb, IL 60115, U.S.A.boundary of approximately 0.7 Ma (Ling, 1976). Hays et al. (1969)and Saito and Burckle (1977) reported the last occurrence of Meso-cena cf. elliptica between the Brunhes Chron and Jaramillo Subchron,which in the eastern Pacific is at about 0.73 Ma.Barron (1980) incorporated silicoflagellate events into his diatomzonation for DSDP Leg 57 sediments from the Northwest Pacific,correlated them with the paleomagnetic stratigraphy, and listed theestimated "absolute ages." The silicoflagellate datums included are:LAD Mesocena quadrangula between Olduvai and Jaramillo, 1.5 MaLAD Distephanus pseudofibulaOverlap of D. pseudofibula and Thalassiosira antiqua (diatom)Overlap of D. pseudofibula and Mesocena hexagonaLater Bukry and Monechi (1985) proposed a silicoflagellate zonationbased on DSDP Leg 86 samples, also from the Northwest Pacific (Fig.2). It should be pointed out that they left most of the Pliocene, the entirelate Miocene, and most of the Middle Miocene unzoned. As discussed inthe following, these zonal gaps can now be satisfactorily filled for the Seaof Japan by a combination of silicoflagellates and ebridians.Kobayashi (1988) recognized 11 silicoflagellate datum levels and12 zones for the Neogene interval from the Japan Sea coastal regionand DSDP Hole 43 8A, located in the northwest corner of the NorthPacific. His uppermost three zones, Distephanus jimlingii, Para-mesocena circulus, and Dictyocha neopseudofibula in ascendingorder, which represent the entire Pliocene, can be generally followedduring the present study.Datum LevelsThe datum levels recognized from the Sea of Japan (Fig. 3) arediscussed in ascending order. They are grouped in two categories: firstorder (designated by bold letters), which have been recognized pre-viously and/or correlated with the occurrence of other groups of bothcalcareous and siliceous microfossils and the estimated "absoluteage" based on magnetostratigraphy, including Leg 128 shipboardanalysis (Ingle, Suyehiro, von Breymann, et al., 1990); second order(designated by plain letters), which have been observed but requirefurther observation, or the datum has not been correlated with othermicrofossil data or magnetic reversal history.First-appearance datum (FAD) of Distephanus jimlingiiAlthough Kobayashi (1988) applied this datum level to draw theMiocene-Pliocene boundary, this datum could not be followed due toits inconsistent distribution in the present Leg 128 samples.FAD of Dictyocha neopseudofibulaKobayashi (1988) recognized the datum level as the base of hisuppermost (12) zone and the early/late Pliocene boundary. This datumlevel occurs between Samples 128-798B-34X-5, 30-32 cm, and237H. Y. LINGFigure 1. Bathymetry (in meters) of the Sea of Japan. Sites 794,798, and 799 drilled during Leg 128 are shown byopen circles. Sites 299-302 drilled during DSDP Leg 31 are indicated by solid circles, and Sites 794-797 drilledduring Leg 127 are shown by solid circles. Site 794 was drilled during both Legs 127 and 128.128-798B-34X-CC at 321.16-321.95 m below seafloor (mbsf). Fur-thermore, Kobayashi^ observation of the slight upward extension ofthe geological range of Paramesocena circulus above this datum isin agreement with observation of Hole 798B samples.This datum would be more reliable if a similar occurrence wereobserved in other subsurface sections or if the distribution of Para-mesocena circulus were more consistent.Last-appearance datum (LAD) of Distephanus jimlingiiThe LAD of D. jimlingii (as Cannopilus


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