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doi:10.2204/iodp.proc.311.103.2006

Biostratigraphy

Cores were recovered from Holes U1325B, U1325C, and U1325D. Hole U1325B contains a ~200 m thick Quaternary sedimentary sequence, and Hole U1325C contains a ~100 m thick sedimentary sequence of unknown age with a very thin Quaternary section at the top. The biostratigraphy determined for Site U1325 was based on an examination of all core catcher samples from Holes U1325B and U1325C (Table T2).

Diatoms

Diatoms are rare to very abundant and poorly to well preserved in Hole U1325B in the interval from ~9 to ~52 mbsf, where diatom assemblages are dominated by such marine species as Neodenticula seminae, Stephanopyxis dimorpha, Stephanopyxis spp., Thalassionema nitzschioides, and Thalassiosira spp. and are associated with abundant resting spores of the shallow-marine diatom genus Chaetoceros. In the interval from ~62 to ~115 mbsf, diatoms are very rare and poorly preserved in general, and it is rather difficult to note the predominance of either marine or nonmarine species except for few samples. Diatoms are very rare to abundant and poorly to moderately preserved in the interval from ~129 to ~198 mbsf, and the diatom assemblages are dominated by either marine species N. seminae, S. dimorpha, Stephanopyxis spp., T. nitzschioides, and Thalassiosira spp. or by the nonmarine diatom Aulacoseira granulata s.l. The abundances of all these species vary greatly from sample to sample. Abundant resting spores are frequently associated with marine diatoms in this interval as well. Upward-increasing patterns of nonmarine diatoms, which may reflect sedimentary cycles, are recognized in this interval. Diatoms are barren to common and poorly preserved in the interval from ~192 to ~299 mbsf in Hole U1325C, where diatom assemblages are dominated by the nonmarine diatom A. granulata s.l., if present at all.

The interval between the shallowest sample and Sample 311-U1325B-15X-CC (9.34–115.47 mbsf) contains N. seminae but lacks Proboscia curvirostris and was assigned to North Pacific Diatom (NPD) Zone 12 (N. seminae Zone; 0–0.3 Ma). The last occurrence (LO) of P. curvirostris was recognized in Sample 311-U1325B-16X-CC (129.49 mbsf), marking the NPD Zone 12/11 boundary (P. curvirostris Zone). The interval between the LO of P. curvirostris and Sample 311-U1325C-1X-CC (129.49–192.03 mbsf) contains P. curvirostris, but lacks Actinocyclus oculatus, and was assigned to NPD Zone 11 (P. curvirostris Zone; 0.3–1.0 Ma). The interval between Samples 311-U1325C-2X-CC and 15X-CC (199.08–299.12 mbsf) is either barren or contains no biostratigraphic zonal marker diatoms because of possible increased terrestrial input by turbidites. Therefore, no diatom zone and age assignments are possible for this interval. No abrupt changes in lithology or diatom abundance are observed between this interval and the overlying interval, suggesting it might be Pleistocene in age.

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