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doi:10.2204/iodp.proc.307.105.2006

Biostratigraphy

Core catcher samples from each core of Holes U1318A and U1318B were examined for nannofossils, and Hole U1318A was examined for planktonic and benthic foraminifers. In several intervals, higher resolution sampling was performed in order to better constrain hiatus horizons. Nannofossils are moderately to well preserved in all sediments, except in Sections 307-U1318B-21X-CC (195.5 mbsf) and 27X-CC (241.1 mbsf), which display poor preservation. The preservation of planktonic foraminifers in Hole U1318A is very good to moderate. Samples from the upper part of Hole U1318A (Samples 307-U1318A-1H-CC, 2H-CC, 4H-CC, 7H-CC, and 8H-CC) contain low abundances of planktonic foraminifers, but in other samples they are common to very abundant.

Calcareous nannofossils

The middle to upper Pleistocene E. huxleyi Zone (recent–0.26 Ma) is represented by material recovered from Sections 307-U1318A-1H-CC (8.9 mbsf) through 8H-CC (73.6 mbsf) (Fig. F6; Table T3). These sections contain an abundance of reworked Late Cretaceous nannofossils. The lower Pleistocene small Gephyrocapsa Zone (0.96–1.22 Ma) is found only in Sample 307-U1318A-9H-CC (84.6 mbsf) (Fig. F6; Table T3).

The sediment at 104.3 mbsf (Sample 307-U1318A-11H-CC) in Hole U1318A yields two diagnostic genera, Sphenolithus spp. and Reticulofenestra spp., with a well-established last occurrence at 3.6 Ma (Table T3). This indicates that sediments below 104.3 mbsf in Hole U1318A are at least older than late Pliocene. The absence of traditional marker taxa made it difficult to derive an age for the underlying sediment in Sections 307-U1318A-11H-CC to 13H-CC. We very tentatively assign an age to this sediment that falls within the early middle Miocene. This assignment is based on the presence of taxa characteristic of this time period, which include Cyclicargolithus floridanus, Reticulofenestra gartnerii, and Helicosphaera scissura. At 86.2 mbsf in Hole U1318B, there is an obvious hiatus recorded both in the nannofossil record and as an erosion surface in the lithologic record. Results of high-resolution sampling in Hole U1318B locate the horizon of this hiatus between Samples 307-U1318B-10H-4, 70 cm (86.20 mbsf) and 10H-4, 83 cm (86.33 mbsf), which coincides with the lithostratigraphic boundary between Units 2 and 3 (see “Lithostratigraphy”).

The lower middle Miocene Zone CN3 is represented in Samples 307-U1318B-14H-CC (127.3 mbsf) through 20X-CC (186.0 mbsf) (Fig. F6). The sediments of this interval commonly yield the diagnostic taxa Sphenolithus hetermorphus, Helicosphaera ampliaperta, Discoaster deflandrei, R. gartnerii, and C. floridanus (Fig. F6; Table T3).

Preservation of nannofossils in Samples 307-U1318B-21X-CC (195.5 mbsf) and 27X-CC (241.1 mbsf) was very poor, and an age datum could not be derived for this sediment.

Planktonic foraminifers

The sediments in Hole U1318A yield Pleistocene–Miocene planktonic foraminifers (Fig. F7; Table T4). The Cenozoic mid-latitude Zones SN14 and SN13 (Jenkins, 1985, 1993), were assigned for sediment above Section 307-U1318A-12H-CC (112.5 mbsf) by the occurrence of Globorotalia inflata (Fig. F7; Table T4). This interval includes the boundary between the upper Pliocene (Zone SN13) and Pleistocene (Zone SN14) (Fig. F6). We could not, however, specify the boundary horizon because of the lack of index species. Sample 307-U1318A-12H-CC also includes Miocene taxa (Praeorbulina circularis and Globoquadrina dehiscens) that may have been reworked, and its age was tentatively regarded as Pliocene–Pleistocene because of the occurrence of G. inflata and Globorotalia crassaformis. Sample 307-U1318A-13H-CC (121.3 mbsf) lacks age-diagnostic species (Fig. F7). In the lowermost two samples (307-U1318A-14H-CC and 15H-CC; 128.2 and 139.1 mbsf), the Pliocene–Pleistocene species (G. inflata and G. crassaformis) are absent, and the assemblage is dominated by the Miocene taxa. Their age is 14.8–16.0 Ma, as determined by the occurrence of Praeorbulina circularis.

The planktonic foraminifer assemblage in lithostratigraphic Unit 1 in Hole U1318A is dominated by cool to temperate taxa Globigerina bulloides and Neogloboquadrina pachyderma (dextral and sinistral). Lithostratigraphic Units 2 and 3 more commonly yield the warmer water taxa G. crassaformis, Globorotalia crassula, G. inflata, Globorotalia scitula, Orbulina universa, and Globigerinoides spp.

Benthic Foraminifers

Sediments from Hole U1318A yield similar assemblages of benthic foraminifers as sediment from the other two sites (Fig. F8). Shallow-water species, such as Elphidium excavatum and Islandiella norcrossi, are more common in lithostratigraphic Unit 1 than in Units 2 and 3. Otherwise, there is no significant difference in assemblage among the three lithostratigraphic units. Species diversity is largest in Unit 3. Species associated with cold seepage, Bulimmina marginata and Bulimmina sp. A, are common throughout Hole U1318A (Fig. F8).

Discussion

An overview of the biostratigraphy in Holes U1318A and U1318B indicates that the ages of lithostratigraphic Units 1 and 2 are late and early Pleistocene, respectively, although the sediments contain large amounts of reworked Late Cretaceous nannofossils (Fig. F9). The hiatus at 86.7 mbsf in Hole U1318A (Fig. F6) defines the boundary between lower Pleistocene and possible Pliocene–Miocene sediment, according to calcareous nannofossils. This hiatus, however, corresponds to the boundary of seismic Units P3 and P2 (Fig. F9) and can not be confirmed in the planktonic foraminifer biostratigraphy (Fig. F6). This inconsistency will be examined by further biostratigraphic investigation.

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