Site 1247 is located on the eastern flank of the summit of southern Hydrate Ridge. Two holes were drilled at Site 1247, and one hole (Hole 1247B) was cored from the seafloor to 220 mbsf. The biostratigraphy determined for Site 1247 was based on examination of diatoms and calcareous nannofossils from all core catcher samples from Holes 1247B. Calcareous nannofossils were also examined in smear slide samples collected by the sedimentologists.
Hole 1247B yields few to common and poorly to moderately preserved diatoms, except in the intervals from Samples 204-1247B-22H-CC to 24H-CC, where diatoms are rare. Diatom assemblages in Hole 1247B are dominated by species such as Stephanopyxis dimorpha, Stephanopyxis spp., Neodenticula seminae, and Thalassionema nitzschioides. Warm-water species, such as Fragilariopsis doliolus and Thalassiosira oestrupii, are frequently present throughout the sediment sequence recovered.
The interval from the seafloor to 35.02 mbsf contains N. seminae but does not contain Proboscia curvirostris. This interval was assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae Zone). P. curvirostris is present almost continuously from Samples 204-1247B-6H-CC to 27X-CC (44.37-220.57 mbsf). The last occurrence (LO) of P. curvirostris was placed between Samples 204-1247B-5H-CC (35.02 mbsf) and 6H-CC (44.47 mbsf). Rare Actinocyclus oculatus is present from Samples 204-1247B-10H-CC (81.90 mbsf) and 13H-CC (106.72 mbsf); thus, the LO of A. oculatus was placed between Samples 9H-CC (72.26 mbsf) and 10H-CC (81.90 mbsf). The interval below the LO of A. oculatus, from 81.90 mbsf to the bottom of Hole 1247B, was assigned to NPD 10 (A. oculatus Zone) based on the presence of A. oculatus and the absence of Neodenticula koizumii in this interval. The presence of P. curvirostris in the interval from 44.37 mbsf to the bottom of Hole 1247B means that the sediment sequence at Site 1247 is younger than 1.6 Ma.
The calcareous nannofossil biostratigraphy for Site 1247 was based on an investigation of all core catcher samples from Hole 1247B. In addition, the smear slides taken for sedimentologic descriptions, which were distributed throughout the cores, were also examined. Generally, the intervals are barren or have trace abundances of calcareous nannofossils from the seafloor down to 44.37 mbsf, whereas the intervals are rarely to commonly present from 47.35 to 220.57 mbsf, except in a few short intervals (e.g., 90.49-100.40 mbsf) that contain trace calcareous nannofossils.
The core catcher samples taken from the interval of 3.55-22.93 mbsf (Samples 204-1247B-1H-CC to 3H-CC) contain no or trace calcareous nannofossils. Emiliania huxleyi was found in a few smear slides produced for the sedimentological study from the same interval. The LO of E. huxleyi was placed in Sample 204-1247B-5H-CC (35.02 mbsf); consequently, the interval from the seafloor to 35.02 mbsf was assigned to Zone NN21. The underlying interval between Samples 204-1247B-6H-5, 36 cm, and 7H-3, 80 cm (40.46-47.35 mbsf), does not contain Pseudoemiliania lacunosa. A few P. lacunosa specimens are infrequently present from 47.35 to 53.08 mbsf; reworked early Pleistocene species, such as Calcidiscus macintyrei, Helicosphaera sellii, and large-sized Gephyrocapsa lumina, are also present in rare abundance. Therefore, the trace and infrequent presence of P. lacunosa in this interval could also be the result of reworking. The reworking of this interval is supported by the sedimentologic data, which suggests that the frequency of turbidites increased toward Horizon Y at 60 mbsf (see "Lithostratigraphy"). On the other hand, P. lacunosa is present in rare abundance from 63.08 mbsf down to the bottom of Hole 1247B, allowing us to place the LO of P. lacunosa at 63.08 mbsf. Consequently, the zonal boundary between NN20 and NN19 was placed at 63.08 mbsf. Abundant, various, small Gephyrocapsa species are present in the interval from 107.59 to 151.36 mbsf (Samples 204-1247B-14H-1, 89 cm, to 20X-6, 57 cm), assigning this interval to the mid-Pleistocene small Gephyrocapsa spp. Acme Zone (1.0-1.2 Ma). In consequence, the depths of 107.59 and 151.36 mbsf were determined as the upper level (1.0 Ma) and the lower level (1.2 Ma) of the small Gephyrocapsa spp. Acme Zone, respectively. The LO of C. macintyrei (1.59 Ma) was recognized in Sample 204-1247B-22X-CC (171.97 mbsf). Rare to common G. lumina is present in Sample 204-1247B-27X-CC (220.57 mbsf), assigning the bottom of Hole 1247B to the lower part of Pleistocene Zone NN19, with an age younger than 1.67 Ma.
Based on analysis of diatoms and calcareous nannofossils from Hole 1247B, seven microfossil events (two diatom and five nannofossil events) were recognized (Table T2; Fig. F11). The age of the bottom sediments (220.57 mbsf) in Hole 1247B was estimated to be younger than 1.6 Ma, based on diatom assemblage, or to be younger than 1.67 Ma, based on nannofossil assemblage. This means that the 220-m-thick sequence at Site 1247 is early Pleistocene-Holocene in age.
We estimated the linear sedimentation rates using these events. The sedimentation rates are 13 cm/k.y. for the interval from the top of 1247B to 60 mbsf, 9 cm/k.y. for the interval from 60 to 110 mbsf, 22 cm/k.y. for the interval from 110 to 150 mbsf, and 4 cm/k.y. for the interval from 150 to 170 mbsf.