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doi:10.2204/iodp.proc.307.104.2006

Biostratigraphy

Core catcher samples from each core of Holes U1317A, U1317B, U1317C, and U1317D were examined for nannofossils, and Hole U1317A was examined for planktonic foraminifers in order to provide a shipboard biostratigraphic framework for recovered sediment. In Hole U1317D, additional nannofossil samples were collected at higher resolution from the middle part of the core and examined in order to obtain better constraints for the uniformity and age of the mound base.

Nannofossils are moderately to well preserved in all sediments. Planktonic foraminifers are moderate to abundant throughout the hole and their preservation in Hole U1317A is good to moderate.

Calcareous nannofossils

The lower Pleistocene (0.46–1.22 Ma) is assigned for the upper part of the sediments recovered in Hole U1317A (Section 307-U1317A-1H-CC through Core 8H; 0–73.0 mbsf) (Fig. F8). We designated this age based on the common occurrence of the nannofossil species small Gephyrocapsa (last occurrence [LO] = 1.22 Ma) and Pseudoemiliania lacunosa (LO = 0.46 Ma) (Table T2). In Sample 307-U1317A-1H-CC, the very abundant occurrence of small Gephyrocapsa indicates that this interval can be older than the top of the small Gephyrocapsa Zone (0.96 Ma; Table T2). From Core 307-U1317A-2H to Section 11H-CC, the abundance of small Gephryrocapsa decreases significantly, and the age assignment for these intervals is tentative.

The lower Pleistocene to upper Pliocene Calcidiscus macintyrei Zone (1.59–1.95 Ma) is assigned for the lower part of lithostratigraphic Unit 1 of Cores 307-U1317A-9H through 14H (73.0–130.0 mbsf). C. macintyrei is common in this interval, and we used its occurrence as the criterion for the age designation of this interval. The LO of this species is 1.59 Ma and designates the top of the C. macintyrei Zone (1.59–1.95 Ma). The base of the C. macintyrei Zone (1.59–1.95 Ma) is defined by the LO of Discoaster brouwerii. In Holes U1317A, U1317B, and U1317C, this species is absent. This age assignment, however, remains tentative because of the presence of significant reworking in several sections (Fig. F9).

In several intervals (Fig. F9), we tentatively identified rare specimens of Emiliania huxleyi, which has an age range of Holocene to 0.26 Ma. It is often difficult to accurately identify this species under the light microscope. We decided that these specimens were too rare and questionable to be used as a criterion for age assignment. Further research using scanning electron microscope is needed to the resolve this issue.

The sediment at 130.5 mbsf in Hole U1317A and 145.8–216.7 mbsf in Hole U1317D commonly yields Sphenolithus spp. and Reticulofenestra spp., both of which have well-established LOs at 3.6 Ma. The presence of these genera indicates a sedimentary succession age older than late Pliocene. Sphenolithus heteromorphus (13.6–18.2 Ma) and Helicosphaera ampliaperta (LO = 15.6 Ma) occur in many horizons of lithostratigraphic Unit 2, although their occurrences in Hole U1317A are few (Fig. F9). Though not dominant, they are important species that define the age corresponding to early–middle Miocene (15.6–18.3 Ma). Occurrence of these species was recognized to 216.7 mbsf in Hole U1317D.

Planktonic foraminifers

We identified several age-diagnostic species in sediments of Hole U1317A (Table T3; see Fig. F14 in the “Site U1316” chapter). The upper part of lithostratigraphic Unit 1 was correlated to the middle–late Pleistocene subtropical and temperate Subzone Pt1b (Berggren et al., 1995a, 1995b) by finding Globorotalia tosaensis in Sample 307-1317A-7H-CC (63.7 mbsf). We assume that this horizon is the LO of G. tosaensis and the bottom of Subzone Pt1b (Fig. F8). Globorotalia inflata was recorded throughout this part.

The mid-latitude Zones SN14 and SN13, as defined by Jenkins (1985, 1993), were applied to the lower part of lithostratigraphic Unit 1 in Hole U1317A. These zones correspond to the lower Pleistocene and upper Pliocene by presence of Globorotalia crassaformis (Figs. F8, F10); however, we could not separate Zones SN13 and SN14 because the sediment in Hole U1317A poorly yields the index species. The base of Zone SN14 is defined by the first appearance of Globorotalia truncatulinoides, but this species was not found except in Sample 307-1317A-1H-CC (6.4 mbsf). The age of lithostratigraphic Unit 2 in Hole U1317A is assigned to Miocene (Biozones M1–M14) (Fig. F8) by the occurrence of Globoquadrina dehiscens (Fig. F10).

The planktonic foraminifer assemblage from Hole U1317A is poor in cool- to temperate-water taxa, such as Globigerina bulloides and Neogloboquadrina incompta. A typical cold-water species, Neogloboquadrina pachyderma, was only identified from Sample 307-U1317A-5H-CC (44.7 mbsf) (Fig. F10). Lithostratigraphic Unit 1 commonly yields warmer water taxa G. inflata, Globorotalia scitula, and Orbulina universa. These assemblages indicate warmer conditions than those of Unit 1 in Hole U1316A.

Benthic foraminifers

Sediments from Hole U1317A yield similar assemblages of benthic foraminifers as sediments from Hole U1316A (Figs. F17, F18 in the “Site U1316” chapter). A major difference is the rare occurrence of shallow-water species, such as Elphidium excavatum and Islandiella norcrossi (Fig. F11), which are common in lithostratigraphic Unit 1 in Hole U1316A. Species associated with cold seepage, Bulimmina marginata and Bulimmina sp. A, are less common than in Hole U1316A (Fig. F18 in the “Site U1316” chapter).

Discussion

An overview of the biostratigraphy in Holes U1317A and U1317D indicates that lithostratigraphic Unit 1 is Pleistocene and contains abundant reworked Late Cretaceous nannofossils (Table T4). The age of the top of lithostratigraphic Unit 2 is older than early Pliocene (3.6 Ma) by calcareous nannofossils and Miocene by planktonic foraminifers (Fig. F8). Unit 2 below Sample 307-U1317D-9R-CC (184.0 mbsf) is early–middle Miocene based on calcareous nannofossil biostratigraphy.

There is an inconsistency in age assignment of the upper part of Unit 1. Planktonic foraminfer biostratigraphy indicates an age younger than 0.65 Ma was assigned for sediments above Sample 307-U1317D-7H-CC, yet based on calcareous nannofossils, it is much older (0.96–1.22 Ma) (Fig. F8). The reason for this inconsistency between calcareous nannofossil age and planktonic foraminifer age is still uncertain, and further research is necessary.

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