Sediments recovered from Site 1178 provide a sedimentary record from the Pleistocene through the late Miocene. Calcareous nannofossils were used for developing the biostratigraphic framework according to the zonation schemes of Martini (1971) with zonal modifications proposed by Young (1998) (Table T7). Abundance and preservation of calcareous nannofossils vary throughout the sequence. The interval (core and section) and depth (mbsf) constraints of calcareous nannofossil events recognized at Site 1178 are listed in Table T8. The epoch boundaries have been placed as in Table T9. For nannofossil ranges, see Table T10.
Hole 1178A recovered sediments to 410.35 mbsf. The sediments recovered from Hole 1178A range in age from Subzone NN21a of the Pleistocene to Zone NN13 of the Pliocene. Cores 190-1178A-1H to 9X bear well-preserved nannofossil assemblages. Below this interval to Core 190-1178A-39X, nannofossils are less abundant and are mostly moderately preserved. Only Samples 190-1178A-22X-CC and 44X-CC were barren of nannofossils. Toward the bottom of the hole, nannofossils are present in low numbers and are poorly preserved. In Sample 190-1178A-42X-CC, the assemblage is strongly affected by calcite overgrowth. Common reworking of nannofossils takes place throughout the sequence giving the sediments a seemingly older age, which imposes further constraints on age determination. The sediments retrieved yield a broad range of deformation structures (see "Structural Geology") leading to a repetition of biostratigraphic events and thus to a disturbed biostratigraphic succession. The common occurrence of an unidentified species with morphological features similar to large Gephyrocapsa (>4 µM), possibly overgrown Reticulofenestra, was observed from Sample 190-1178A-23X-CC to the bottom of Hole 1178B.
The Pleistocene sediments retrieved from Hole 1178A contain mostly well-preserved nannofossils (Samples 190-1178A-1H-CC [2.89 mbsf] to 2H-CC [12.31 mbsf]). The abundance of nannofossils is generally very high. The Pleistocene nannofossil assemblages are dominated by Emiliania huxleyi and different morphotypes of Gephyrocapsa. Reworked Pliocene to Miocene taxa are present in high numbers and are generally well preserved.
Sample 190-1178A-1H-CC contains a nannofossil assemblage dominated by E. huxleyi and small Gephyrocapsa indicative of Subzone NN21a. Zone NN20 was not recognized. The last occurrence of Pseudoemiliania lacunosa, which marks the top of Zone NN19 (0.46 Ma), was recorded between Samples 190-1178A-1H-CC and 2H-CC. The latter sample also contains Gephyrocapsa oceanica, the first occurrence of which (1.8 Ma) approximates the Pleistocene/Pliocene boundary (1.88 Ma).
Sediments recovered from 17.31 to 193.37 mbsf bear Pliocene nannofossil assemblages. In the upper part of the Pliocene section (Samples 190-1178A-3H-CC to 9X-CC), nannofossils are abundant and very well preserved. Reworked Miocene specimens, mainly discoasterids, appear throughout the section. The last occurrence of Discoaster surculus, which defines the top of Zone NN16 (2.55 Ma), is recorded between Samples 190-1178A-2H-CC and 3H-CC. The top of Zone NN15, as marked by the last occurrence of Reticulofenestra pseudoumbilicus (>7 µm), was recorded between Samples 190-1178A-6H-CC and 7H-CC. The last occurrence of Amaurolithus spp. (4.0 Ma), between Samples 190-1178A-13X-CC and 14X-4, 75-76 cm, marks the top of Zone NN14. Specimens of Discoaster asymmetricus, of which the first common occurrence defines the top of NN13, are only rarely present. Thus, this event cannot be assessed. The first appearance of Ceratolithus cristatus is used to identify the top of the negatively defined Zone NN12. Since C. cristatus is still present in Sample 190-1178A-22X-3, 57-58 cm, the interval between 118.85 and 193.37 mbsf is placed in Zones NN14-NN13.
Miocene sediments retrieved from Hole 1178A (204.53-410.35 mbsf) yield generally moderately to poorly preserved assemblages. Below the barren Sample 190-1178A-22X-CC at 199.05 mbsf, sediments were assigned a late Miocene age because of the presence of well-preserved specimens of Discoaster quinqueramus. The range of this species is used to define Zone NN11 of the late Miocene. Here it co-occurs with Reticulefenestra pseudoumbilicus (>7 µm), the presence of which in that zone is confined to Subzone NN11b, assigning these sediments to Subzone NN11b. Specimens of the Miocene species Triquetrorhabdulus rugosus were found in Sample 190-1178A-24X-CC. Amaurolithus amplificus is confined to a short range (6.05-6.8 Ma) within Subzone NN11b and was observed in Sample 190-1178A-35X-CC. The last occurrence of Minylitha convallis, an event to take place within Subzone NN11a, was observed between Samples 190-1178A-39X-CC and 40X-CC. The first occurrence of Amaurolithus primus, marking the top of Subzone NN11a (7.2 Ma), was found between Samples 190-1178A-38X-CC and 39X-1, 19-20 cm.
Samples analyzed from Hole 1178B (399.0-673.17 mbsf) are of late Miocene age. The duplication of biostratigraphic intervals is observed in Hole 1178B and can probably be linked to several domains of deformation structures identified (see "Structural Geology"). The repetition of biostratigraphic events is difficult to exactly define because reworking is common in these sediments. The nannofossil assemblages are generally moderately preserved except for some samples in which nannofossils are altered by strong overgrowth (Samples 190-1178B-3R-CC, 9R-CC, 16R-CC, and 30R-CC. Only a few samples are actually barren of nannofossils (Samples 190-1178B-7R-4, 69-70 cm, and 11R-3, 74-75 cm).
Whereas the uppermost samples of Hole 1178B down to Sample 190-1178B-4R-CC are assigned to Subzone NN11a of the late Miocene because of the presence of D. quinqueramus and M. convallis, which are confined to an interval from Subzone NN11a to Zone NN9, Sample 190-1178B-5R-3, 12-13 cm, yields Amaurolithus, R. pseudoumbilicus (>7 µm), and D. quinqueramus, which indicates an assignment to Subzone NN11b. This implies a shift from older to younger sediments. Amaurolithus is present down to Sample 190-1178B-15R-CC, where its first occurrence (7.4 Ma) is placed. The interval from Sample 190-1178B-16R to 29R-CC contains abundant R. pseudoumbilicus (>7 µm). D. quinqueramus is absent in this interval, indicating an assignment to Subzone NN10a, which contains markedly older sediments. Then Amaurolithus spp. and D. quinqueramus reappear between Samples 190-1178B-29R-CC and 30R-CC. Amaurolithus is confined to Zones NN11B-NN14, marking a shift toward a younger age. The investigation of additional samples may lead to the identification of additional age reversals.