Neogene nannofossil biostratigraphy indicates a relatively complete stratigraphy with all zones from CN5 through CN15 (middle Miocene–Holocene) identified by their primary zonal fossils. Zones CN1–CN5 could not be easily distinguished because of the absence of the marker species Sphenolithus belemnos, Helicosphaera ampliaperta, and Discoaster kugleri. In addition, a number of CN subzones could not be recognized due to the absence of D. kugleri (Subzone CN5b), Discoaster loeblichii, Discoaster neorectus (Subzone CN8b), and Amaurolithus amplificus (subdivisions within Zone CN9) and an anomalously low last occurrence (LO) of Triquetrorhabdulus rugosus (Subzone CN10b). The absence of these zonal and subzonal markers is most likely due to rarity or ecological/biogeographic exclusion rather than significant missing time/sediment. The age-depth plot shows no obvious "benches" or datum event clusters that would indicate hiatuses, and the match with magnetostratigraphy is good (see Fig. F3; Evans et al., this volume). Shipboard planktonic foraminifer biostratigraphy also indicated a relatively complete stratigraphy (Bralower, Premoli Silva, Malone, et al., 2002).
Miocene sediments older than Zone CN5 are more difficult to assign to biozones due to the absence, or rare and sporadic occurrence, of primary marker species, as listed above. Sphenolithus heteromorphus, although present, is sporadically distributed, and sphenoliths and Helicosphaera are generally rare throughout the Miocene section. A number of secondary lower–middle Miocene datum events are included on the age-depth plot (Fig. F3) (e.g., the LO of Calcidiscus premacintyrei, the LO of Coccolithus miopelagicus, and the LO of Cyclicargolithus floridanus), but there is no single solution that honors all the data points. Most likely, the LO of S. heteromorphus is anomalously low because of ecological exclusion of sphenoliths. This interpretation is supported by the presence of the Zone CN4 discoasters, Discoaster muscicus and Discoaster petaliformis (Young, 1998), in Sample 198-1208A-34X-5, 114 cm, and the first occurrence (FO) of large Reticulofenestra pseudoumbilicus (Young, 1998) in Sample 198-1208A-33X-5, 120 cm, which marks the top of Zone CN4.
The lower Miocene–Paleocene section is a short (16 m), condensed section incorporating at least three major hiatuses (Figs. F2, F4). Nannofossil biostratigraphy is problematic in places due to mixing of stratigraphically incongruous assemblage components (Table T2). The mixing includes both upsection reworking and downsection bioturbation; the latter was prominent in this lithologic unit (Bralower, Premoli Silva, Malone, et al., 2002). Intriguingly, however, the unit appears to incorporate the Paleocene/Eocene boundary and the Eocene/Oligocene boundary carbonate pulse (e.g., Bralower, Premoli Silva, Malone, et al., 2002; Lyle, Wilson, Janecek, et al., 2002).
The upper Oligocene–lower Miocene section (~1.1 m) is characterized by low-diversity assemblages dominated by Discoaster deflandrei and C. floridanus. The FOs of Sphenolithus ciperoensis and Discoaster druggii; LOs of Reticulofenestra bisecta, S. ciperoensis, and Cyclicargolithus abisectus; and occurrence of Triquetrorhabdulus carinatus allow the identification of the subzones/intervals CP19–CN1a, CN1a, and CN1c–CN2.
The uppermost Eocene–lower Oligocene section (~5.5 m) can be assigned to Zone CP16 based on the absence of Eocene discoasters (e.g., Discoaster barbadiensis) and the presence of Isthmolithus recurvus and Reticulofenestra umbilicus. Subzones CP16a–CP16c can be identified using the first common occurrence of Clausicoccus subdistichus (Subzone CP16b) and the LO of Coccolithus formosus (Subzone CP16c). The base of Subzone CP16b is marked in the core by a prominent switch from dark brown, in places barren, claystones to lighter-colored, grayish orange nannofossil ooze (see fig. F15 in Chapter 4, Shipboard Scientific Party, 2002). This switch to carbonate accumulation may reflect a significant post-Eocene/Oligocene boundary deepening of the carbonate compensation depth (CCD) and is an event that is recognized elsewhere in the Pacific (van Andel, 1975; e.g., DSDP Sites 42, 70, 161, 162, and ODP Site 1217; Lyle, Wilson, Janecek, et al. 2002). This uppermost Eocene–lower Oligocene section is bounded top and bottom by hiatuses comprising Zones CP17–CP18 (lower Oligocene) and Zones CP12–CP15 (middle–upper Eocene), respectively.
The short (~0.75 m) upper Paleocene–lower Eocene section (see fig. F17 in Chapter 4, Shipboard Scientific Party, 2002) can be correlated with Zones CP8–CP11, based on the ranges of Discoaster multiradiatus, Discoaster diastypus, Rhomboaster bramlettei, Tribrachiatus orthostylus, and Discoaster lodoensis and the decline and LO of Fasciculithus (Bralower et al., 1995; Aubry et al., 1996; Schmitz et al., 1997; see also Bralower, Premoli Silva, Malone, et al., 2002). The increase in Zygrhablithus bijugatus, recorded at this level elsewhere on Shatsky Rise, was not seen at Site 1208 because this species was absent throughout. Its absence may be due to greater dissolution at this deeper site.
A number of the samples in this interval yielded mixed assemblages but consideration of range continuity, abundance, and overall assemblage character allowed construction of a relatively coherent stratigraphy (Fig. F3). Nevertheless, the following zonal designations should be viewed with some caution. Zone CP8 is identified using the FO of D. multiradiatus and Subzone CP9a the FOs of D. diastypus and R. bramlettei. Subzone CP9b is identified using the LO of Tribrachiatus contortus, although this taxon was extremely rare; however, further support is provided by the additional FOs of T. orthostylus (although problematic due to mixing) and Sphenolithus radians. Zone CP10 is recognized using the FO of D. lodoensis, and the LO of T. orthostylus, at the top of this interval, indicates the presence of Zone CP11. The FO of Toweius crassus is a problematic datum and was not identified at this site. The Paleocene–Eocene section lies unconformably on Upper Cretaceous sediments. Cretaceous biostratigraphy is presented in detail in Lees and Bown (this volume).