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During Leg 60 of the Deep Sea Drilling Project, 17 holes were drilled at 10 sites in the Mariana Trench and Trough region of the western Pacific from March to May 1978. Samples recovered from these holes range in age from late middle or late Eocene to late Pleistocene, as determined by calcareous nannoplankton. Late Cretaceous specimens, together with mixed Cenozoic assemblages (Eocene through Pleistocene), were recorded for the first time in samples recovered from the west wall of the Mariana Trench. Detailed biostratigraphic studies reveal several hiatuses in the Oligocene and Miocene intervals at two sites in the fore-arc basin east of the Mariana Ridge (Sites 458 and 459). The greatest of these shows 4 m.y. to be missing between late Miocene and late Pliocene at Site 458, while a time span of about 7 m.y. is absent at the same horizon at Site 459. A nannoplankton zonation, which is largely an emendation of that proposed by Bukry (1973b, 1975) for low latitudes and by Ellis (1979), is described and compared with the zonation used by Martini (in press) on Leg 59. INTRODUCTION On Leg 60 of the Deep Sea Drilling Project, we occupied 10 sites and drilled 17 holes across the Mariana Trench, island arc, and Trough in the western Pacific from March to May 1978 (Figure 1). A total of 560 samples was examined by light microscope, and from these 17 critical samples were selected for detailed study by scanning electron microscopy. The samples range in age from late middle or late Eocene to late Pleistocene. Several reworked Late Cretaceous nannoplankton specimens, together with mixed Cenozoic assemblages, were observed in samples recovered from the west (arc-side) wall of the Mariana Trench (Site 460). This is the first occurrence of Mesozoic fossils from the western margin of the Mariana Trench. NANNOPLANKTON ZONATION The zonation used for nannoplankton age determinations during Leg 60 is emended from that proposed by Bukry (1973b, 1975) for low latitudes and by Ellis (1979) (Figure 2). The absolute age estimates and epoch boundary placement are also after Bukry (1973b, 1975), except for the Miocene/Pliocene boundary and the ages of the adjacent subzones. These conform to the position proposed by Ellis and Lohman (1979). The nannoplankton zonation utilized on Leg 60 is different from that used on Leg 59 (Martini, in press). A conference between Martini and Ellis resulted in a graphic comparison of the two zonation schemes (Figure 3). Although a different absolute time framework was employed by Martini (in press) than is used here, the relationship of zonedefining species to one another remains the same in both schemes. The following zonation description presents a brief definition or emendation of each zone or subzone, lists the nannoplankton species used to recognize the zonal limits, and compares the zone definition with that of Martini (1971, in press) and Bukry (1973b, 1975) where appropriate. Many of the emendations are necessary because of local influences affecting species occurrence relationships. Consequently, a coded zonation scheme is presented for application of Leg 60 biostratiInitial Reports of the Deep Sea Drilling Project, Volume 60. graphic results and possibly other western Pacific nannoplankton data. A comparison of the WPN zones and subzones proposed here with those of Bukry (1973b, 1975) is shown in Figure 2. Those zones that have been emended are designated with an asterisk. WPN-12b Subzone Definition. Top—Chiasmolithus grandis, last occurrence. Base—Dictyococcites bisectus, first occurrence. Remarks. This subzone has a different, older base than Bukry's (1973b, 1975) Discoaster saipanensis Subzone. WPN-13a Subzone Definition. Top—Isthmolithus recurvus, first occurrence. Base—Chiasmolithus grandis, last occurrence. WPN-13b Subzone Definition. Top—Discoaster barbadiensis and D. saipanensis, last occurrence. Base—Isthmolithus recurvus, first occurrence. Remarks. This subzone is equal to the /. recurvus Subzone of Bukry (1975). WPN-14a Subzone Definition. Top—Coccolithus subdistichus acme, last occurrence. Base—Discoaster barbadiensis and D. saipanensis, last occurrence. WPN-14b Subzone Definition. Top—Coccolithus formosus, last occurrence. Base— Coccolithus subdistichus acme, last occurrence. Remarks. This subzone is equal to the C. formosus Subzone of Bukry (1973b), and together with the Coccolithus subdistichus Subzone it is equal to the Ericsonial subdistichus Zone (NP 21) of Martini (1971, in press). WPN-14c Subzone Definition. Top—Reticulofenestra hillae and R. umbilica, last occurrence. Base—Coccolithus formosus, last occurrence. Remarks. This subzone is equal to the Helicosphaera reticulata Zone (NP 22) of Martini (1971, in press) and the R. hillae Subzone of Bukry (1973b). WPN-15 Zone Definition. Top—Sphenolithus distentus, Cyclicargolithus abisectus, and Triquetrorhabdulus carinatus, first occurrence. Base— Reticulofenestra hillae and R. umbilica, last occurrence. Remarks. The last occurrence of Bramletteius serraculoides also occurs at the top of this zone at Site 458. This zone is equal to the lower part of the Sphenolithus predistentus Zone (NP 23) of Martini

Calcareous Nannoplankton BiostratigraphyDeep Sea Drilling Project Leg 60

Calcareous Nannoplankton BiostratigraphyDeep Sea Drilling Project Leg 60