Sediments recovered from Hole 1177A provide a sedimentary record from the Pliocene through the early 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 T8). Abundance and preservation of calcareous nannofossils vary throughout the sequence. The nannofossil events recognized in Site 1177 are reported in Table T9. The epoch boundaries have been placed as in Table T10. For nannofossil ranges in Site 1177 see Table T11.
Hole 1177A was washed down to 300.20 mbsf. The sediments recovered from Hole 1177A range in age from Zone NN18 of the Pliocene to Zones NN4-NN2 of the early Miocene. Cores 190-1177A-1R to 8R bear mostly moderately to well-preserved nannofossil assemblages. Below this interval, from Core 190-1177A-9R to the bottom of the hole (832.13 mbsf), nannofossils are present only in low numbers and are poorly preserved except for Cores 190-1177A-18R to 23R with moderately preserved specimens. In general, assemblages are affected by both strong overgrowth and etching. Some intervals appear barren of nannofossils, inhibiting a zonal assignment for large sections of Site 1177. Frequent reworking of older nannofossils throughout the sequence, giving the sediments a seemingly older age, imposes further constraints on age determination.
The Pliocene sediments retrieved from Hole 1177A contain mostly moderately to well-preserved nannofossils (Samples 190-1177A-1R-CC [308.65 mbsf] to 7R- CC [367.11 mbsf]). The abundance of nannofossils is generally very low. The Pliocene nannofossil assemblages are dominated by different morphotypes of reticulofenestrids and discoasterids; associated are mainly Coccolithus spp., Calcidiscus spp., and sphenoliths. Reworked specimens were encountered frequently.
Sample 190-1177A-1R-CC contains Discoaster brouweri, the last occurrence of which defines the top of Zone NN18 (1.95 Ma) and thus assigns the top of the cored interval to be of Pliocene age. The top of Zone NN17, as marked by the last occurrence of Discoaster pentaradiatus, was recorded between Samples 190-1177A-4R-3, 68-69 cm, and 4R-CC. The last occurrence of Discoaster surculus (2.55 Ma), which defines the top of Zone NN16, was observed between Samples 190-1177A-6R-CC and 7R-4, 75-76 cm. The event defining the top of Zone NN14, the last occurrence of Amaurolithus spp. (4.0 Ma), was recorded between Samples 190-1177A-12R-CC and 13R-3, 75-76 cm. At this site Discoaster asymmetricus was only sporadically present, and most of the samples between 385.26 and 433.33 mbsf are depleted of nannofossils. Therefore, its first common occurrence, which marks the top of Zone NN13, could not be determined. The same problem applies to the base of Zone NN13, indicated by the first occurrence of Ceratolithus cristatus. Sample 190-1177A-14R-3, 70-71 cm, yields Discoaster quinqueramus, the last occurrence of which marks the top of Subzone NN11b (5.54 Ma). This event approximates the Pliocene/Miocene boundary (5.32 Ma).
Miocene sediments (425.99 ± 7.34 to 832.13 mbsf) recovered from Hole 1177A yield poorly preserved assemblages. The Miocene nannofossils are of low abundance. Discoasterids, sphenoliths, and Reticulofenestra spp. are strongly altered by overgrowth. Reworking of nannofossils is common throughout the Miocene sediments. Reworked Paleogene specimens, mostly Sphenolithus spp., are confined to intervals between Samples 190-1177A-34R-CC and 35-CC (617.26-628.82 mbsf) and from Samples 190-1177A-43R-2, 66-67 cm, to 46R-2, 40-41 cm (705.86-734.40 mbsf) and exhibit strong overgrowth.
The top of Subzone NN11a as marked by the first occurrence of Amaurolithus primus (7.2 Ma) could not be identified because of its scarceness throughout the sequence. The presence of Discoaster bergenii, which is confined to a short interval (7.8-8.6 Ma) within Subzone NN11a in Sample 190-1177A-23R-1, 74-75 cm, provides an alternative datum event. The same sample yields the first D. quinqueramus specimens. The absence of nannofossils, except for rare and scattered specimens of Reticulofenestra pseudoumbilicus (>7 µm), or zonal marker species precludes a precise zonal assignment for the interval between Samples 190-1177A-23R-CC and 32R-1, 68-69 cm. The last occurrence of R. pseudoumbilicus (>7 µm), marking the top of Subzone NN10a, was observed between Samples 190-1177A-23R-CC and 26R-1, 49-50 cm. Bounding the barren interval, the last occurrence of Coccolithus miopelagicus (10.9 Ma), the presence of which is basically restricted to Zone NN7 (Young, 1998), was recorded between Samples 190-1177A-32R-1, 68-69 cm, and 32R-CC (598.54 mbsf). This event approximates the boundary between the late and middle Miocene (11.2 Ma). The first occurrence of Discoaster kugleri marking the top of NN6 (11.8 Ma) was recorded between Samples 190-1177A-36R-1, 40-41 cm, and 36R-CC. The last occurrence of Sphenolithus heteromorphus (13.6 Ma), defining the top of Zone NN5, was recorded between Samples 190-1177A-39R-CC and 40R-1, 71-72 cm, as strongly altered by overgrowth and dissolution. The top of Zone NN4 (15.6 Ma), indicated by the last occurrence of Helicosphaera ampliaperta, was recorded between Samples 190-1177A-47R-2, 76-77 cm, and 47R-CC. The last occurrence of Discoaster deflandrei, which approximates the middle to lower Pliocene boundary, could not be determined because of the common reworking of D. deflandrei. Specimens of Sphenolithus belemnos, the range of which defines Zone NN3, were not identified, probably because of the poor state of preservation of the sphenoliths. The last appearance of Discoaster druggii (18.6 Ma), an event to occur within Zone NN3, was observed between Samples 190-1177A-50R-CC and 54R-CC. The event to define the base of Zone NN2 is the first occurrence of D. druggii (23.1 Ma). Because D. druggii is still present in the lowermost sample above basement (190-1177A-56R-2, 35-36 cm), the basal part of the cored sequence is assigned to Zones NN4-NN2 of the early Miocene.