Neogene dinoflagellate cysts have been studied increasingly for the past two decades, particularly in the North Atlantic-western Europe region, using Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) material in a number of cases (e.g., Edwards, 1986; Powell, 1986a, 1986b; Mudie, 1987, 1989; Wrenn, 1988; Head et al., 1989a, 1989b, 1989c; de Vernal and Mudie, 1989; Head and Wrenn, 1992; de Verteuil and Norris, 1992, 1996; Head, 1993, 1994a, 1994b, 1997; Lentin et al., 1994; Matsuoka et al., 1997). These studies have provided a significant framework for Neogene biostratigraphy and paleoecology. In contrast, very few deep-sea boreholes have been studied from the North Pacific (e.g., Bujak, 1984).
Onshore sections including oil exploration boreholes in and around Japan have been the subject of recent progress in Cenozoic dinoflagellate cyst biostratigraphy, particularly through the correlation and dating of shallow-marine strata from sites where higher latitudes or diagenetic alteration have caused planktonic microfossils to be generally rare (Kurita and Matsuoka, 1994; Kurita and Obuse, 1994, 1997; Kurita and Kusunoki, 1997; Kurita and Yokoi, 2000; Kurita et al., 2000). These studies, however, have been conducted mostly on Paleogene sediments, and there remains little follow-up to the pioneering work on Neogene dinoflagellate cysts in this region by K. Matsuoka (e.g., Matsuoka, 1974, 1983; Matsuoka et al., 1987; Matsuoka and Bujak, 1988). Recently, Obuse and Kurita (1999) reported briefly on dinoflagellate cyst ranges and zonation of the late early Miocene to Pliocene onshore sections in northern Japan, using ages calibrated to detailed diatom biostratigraphy, thereby refining the biostratigraphy presented by Matsuoka et al. (1987). Continued evaluation and age calibration of the Neogene dinoflagellate zone boundaries and biohorizons, however, remains needed because all previous studies are based on composite sections from onshore localities remote from each other and with frequent hiatuses.
The present study aims to outline a biostratigraphy based on organic-walled dinoflagellate cyst assemblages from the lowest middle Miocene through uppermost lower Pliocene section of Hole 1151A, ODP Leg 186, drilled in the forearc area off the Sanriku Coast of northern Japan, northwestern Pacific (Fig. F1) (Sacks, Suyehiro, Acton, et al., 2000). We also discuss biohorizons that are potentially useful for regional correlation and apply a precise age calibration to them. Comparison is made between the present data set and the previous zonation scheme by Matsuoka et al. (1987) and by Obuse and Kurita (1999), both of which are based on onshore sections at various localities in northern Japan. Age calibration for the present study is based on the diatom biostratigraphy by Maruyama and Shiono (this volume).