Sediments recovered from Site 1175 provide a continuous sedimentary record from the Quaternary (Subzone NN21b) to the Pliocene (NN18). Calcareous nannofossils were used for developing the biostratigraphic framework according to the zonation schemes of Gartner (1977) and Martini (1971) with zonal modifications proposed by Young (1998) (Table T8). The interval (core and section) and depth (mbsf) constraints of calcareous nannofossil events recognized at Site 1175 are listed in Table T9. For nannofossil ranges at Site 1175 see Table T10.
Hole 1175A was cored to 435.7 mbsf, recovering late Quaternary (Subzone NN21b) to late Pliocene nannofossil assemblages (Zone NN18). Calcareous nannofossils are abundant throughout the sequence and generally well preserved. The upper 200 m of the section is presumed to represent slump deposits. This implies several constraints using nannofossil last and first occurrence datums for age determination. Reworked nannofossils could indicate an older age than their actual time of deposition. Folded sediment strata may also lead to inaccurate sedimentation rates.
Sediments recovered from 7.11 to 385.84 mbsf (Samples 190-1175A-1H-CC to 41X-CC) yield Pleistocene nannofossil assemblages. Nannofossils are abundant in the Pleistocene and well preserved. Small placoliths such as Emiliania huxleyi and Gephyrocapsa spp. dominate the assemblage alongside numerous subordinate taxa present. Reworked Neogene taxa such as discoasterids, Reticulofenestra pseudoumbilicus, and Sphenolithus spp. were encountered frequently throughout the Pleistocene samples. Based on counts of 300 specimens per sample, the onset of the E. huxleyi acme Subzone NN21b (0.085 Ma) could be determined between Samples 190-1175A-3H-3, 75-76 cm, to 3H-CC. A further event to subdivide Subzone NN21a was the last occurrence of Helicosphaera inversa (0.14 Ma) between Samples 190-1175A-8H-CC and 9H-CC. The base of Subzone NN21a marked by the first occurrence of E. huxleyi (0.26 Ma) was observed between Samples 190-1175A-14H-CC and 15H-4, 75-76 cm. The Pleistocene assemblages older than 0.26 Ma are characterized by the dominance of gephyrocapsids. The interval from Samples 190-1175A-15H-4, 75-76 cm, to 20H-3, 78-79 cm, marked by the absence of both E. huxleyi and Pseudoemiliania lacunosa, was assigned to Zone NN20. The last occurrence of P. lacunosa (0.46 Ma) between Samples 190-1175B-20H-3, 78-79 cm, and 20H-CC defines the top of Zone NN19. The last occurrence of Reticulofenestra asanoi (0.8 Ma) between Samples 190-1175A-30X-3, 75-79 cm, and 30X-CC and its first occurrence (1.06 Ma) between Samples 190-1175A-32X-CC and 33X-4, 73-74 cm provide further datums to subdivide Zone NN19. Further analysis of relative abundances of the different Gephyrocapsa morphotypes may provide a more precise biostratigraphic resolution for Zone NN19.
Sediments cored from 390.25 to 435.69 mbsf (Samples 190-1175A-42X-2, 75-76 cm, to 47X-CC) are assigned a Pliocene age. The late Pliocene assemblages are dominated by reticulofenestrids and Pseudoemiliania spp. Nannofossils are well to moderately preserved and generally less abundant than in the Pleistocene sediments. The first occurrence of Gephyrocapsa oceanica (1.77 Ma) approximates the Pleistocene/Pliocene boundary. Common reworking of Pliocene species, mainly discoasterids and reticulofenestrids, makes the distinction of Zone NN18 difficult. The last occurrence of Discoaster brouweri, defining the top of Zone NN18, was observed between Samples 190-1175A-44X-CC and 46X-CC with fairly abundant and well-preserved D. brouweri specimens.