The results of our biostratigraphic examinations are presented in Table T2, which includes columns for the lithologic units (Stephens, Kasahara, Acton, et al., 2003) to which the sample depth was assigned along with the Radiolaria occurrence interval/subinterval assignment as described above.
The most noteworthy observation of this investigation is the presence of well-preserved and rare to abundant Radiolaria in the six samples from Subintervals B3 and B4. The 45 taxa that collectively constitute this assemblage are Paleogene in age. Rare to common and consistent occurrences of Buryella clinata in these subintervals, as well as frequent trace to rare occurrences in the overlying and underlying sections, indicate an assignment to the uppermost lower Eocene Buryella clinata Zone. The only indication of a possible lower zonal assignment is the observation of a single specimen of Bekoma bidartensis in the sample at 4.59 mbsf, which is positioned above Subinterval B3 and probably reworked. The B. clinata Biochron has an age range of ~50.3–52.85 Ma (Sanfilippo and Nigrini, 1998). This zonal assignment, however, is not without ambiguities. Sanfilippo and Blome (2001) reported Thyrsocyrtis rhizodon occurring first at the base of the overlying lower middle Eocene Phormocyrtis striata striata Zone. This species has trace to common occurrences in four of the six samples from Subintervals B3–B4. Moreover, one whole specimen we identified as Theocorys anaclasta was observed in the sample at 6.18 mbsf, and other fragments of specimens tentatively assigned to this species and Theocorys acroria were observed at 6.18 and 6.49 mbsf. These occurrences force us to consider that Subintervals B3–B4 may belong to the P. s. striata Zone or that the entire assemblage contains mixed representatives of both zones. The P. s. striata Biochron has an age range of ~49–50.3 Ma (Sanfilippo and Nigrini, 1998). However, we prefer an assignment to the B. clinata Zone for Subintervals B3 and B4.
Taxonomic identifications are much more difficult in the sediments below Subintervals B3 and B4 because of extremely low abundances in Interval C and very poor preservation states in Interval D. Three of the seven samples from Interval C are barren of Radiolaria, and the remaining samples contain only trace or rare occurrences with poor to moderate preservation. Only six taxa were identified in these slides. However, traces of B. clinata occur in two of these samples, in association with P. s. striata and Sethocyrtis babylonis. A total of 19 taxa were identified in the six samples from Interval D, among them B. clinata at 37.47 and 37.67 mbsf. In the absence of younger or older index taxa, we tentatively assign Intervals C and D to the B. clinata Zone.
Components of the Radiolaria assemblage of Subintervals B3–B4 occur sporadically above Subinterval B3, and trace occurrences of B. clinata were observed as high as Subinterval A1. However, the Eocene fauna is admixed with Neogene taxa in the samples above Subinterval B3. Perhaps the most obvious evidence for diachronous faunas in the sediments at 5.19 mbsf is the presence of the collosphaerid, Siphonosphaera socialis, and Acrosphaera spinosa, Collosphaera murrayana, Clathrosphaera circumtexta, and Otosphaera polymorpha, which are restricted to sediments no older than Miocene in age. The diversity of the Neogene fauna increases upsection to the top of Interval B, and 40 Neogene taxa were identified. The age of this Neogene fauna is somewhat problematic. Lychnocanoma nipponica nipponica has a consistent occurrence in the four samples from Subintervals B1–B2. Morley and Nigrini (1995) reported the last occurrence ages of this species in the North Pacific as 6.25–9.6 Ma. This taxon would indicate that the Neogene Radiolaria assemblage between 3.01 and 3.85 mbsf is late Miocene in age; however, we are suspicious of the biostratigraphic reliability of this species. It is also possible that this Neogene assemblage is a composite of more than one fauna, and we must give greatest consideration to the youngest observed taxa. We conclude that the presence of Eucyrtidium matuyamai and Lamprocyrtis heteroporos in the sample at 3.15 mbsf are better indications of the age of this Neogene assemblage. Morley and Nigrini (1995) give ages of 1.0 and 2.0 Ma for the last occurrence and first occurrence, respectively, of E. matuyamai, using the timescale of Cande and Kent (1992). These authors give an age of 1.79 Ma for the last occurrence of L. heteroporos in the North Pacific. Based on the overlapping distributions of these two taxa in Hole 1223A, an age of 1.79–2.0 Ma is indicated for the Neogene Radiolaria assemblage in Subinterval B1. This biostratigraphic age estimate is in reasonably good agreement with the assignment of these sediments to Magnetochron C2N by Stephens, Kasahara, Acton, et al. (2003). However, Shackleton et al. (1995a) estimated somewhat different ages for biostratigraphic events based on ODP Leg 138 sites in the eastern equatorial Pacific and employing the timescale of Shackleton et al. (1995b). These authors found the age of 1.61 Ma for the last occurrence of L. heteroporos and 1.55 Ma for the first occurrence of Theocorythium trachelium trachelium. Although their occurrences are somewhat skewed, the presence these two taxa in Interval B suggests an age range of 1.55–1.61 Ma (Magnetochron C1r2r). Subjective bias in taxonomic identification, differing timescales, and disturbed stratigraphic ordering associated with turbidite sedimentation can account for the modest inconsistencies in this chronostratigraphy. In light of the many uncertainties in dating this material, we conclude that the age of Subintervals B1 and B2 is 1.55–2.0 Ma (late Pliocene to early Pleistocene), and the combined subintervals are assigned to the E. matuyamai Zone of Hays (1970).
Determining the age of the sediments in Interval A is very difficult because of the sparse occurrences of Radiolaria in poor preservation states. The more robust lattice construction of Paleogene taxa is favored for preservation over the more delicate Neogene skeletal lattice in sediments where SiO2 dissolution is pronounced. Thus, B. clinata is one of only 13 taxa that could be identified in Interval A, in association with three collosphaerid taxa and Orocena regalis. In the absence of index taxa with well-established stratigraphic ranges, we can only assign a late Neogene age to Interval A.