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doi:10.2204/iodp.proc.307.103.2006

Biostratigraphy

Core catcher samples from each core of Holes U1316A, U1316B, and U1316C were examined for nannofossils, and Hole U1316A was examined for planktonic foraminifers in order to provide a shipboard biostratigraphic framework for recovered sediment. In several intervals, additional high-resolution sampling was performed in order to better constrain ages in stratigraphically important horizons. Nannofossils are moderately to well preserved in all sediments, except for Samples 307-U1316A-10H-CC (79.8 mbsf) and 15-CC (95.5 mbsf), which are barren of nannofossils. The preservation of planktonic foraminifers in Hole U1316A ranges from good to moderate. Samples from Section 307-U1316A-15X-CC (95.5 mbsf) contain low abundances of planktonic foraminifers, but in other samples the abundances are moderate to abundant.

Calcareous nannofossils

The middle–upper Pleistocene E. huxleyi Zone is represented by material recovered in Samples 307-U1316A-1H-CC (6.8 mbsf) through 4H-CC (35.9 mbsf) (Figs. F12, F13). These sections contain abundant reworked Late Cretaceous nannofossils. The middle Pleistocene Pseudomiliania lacunosa Zone (0.46–0.96 Ma) is found only in Sample 307-U1316A-5H-CC (44.8 mbsf) (Fig. F13). The upper Pliocene to lower Pleistocene Calcidiscus macintyrei Zone (1.59–1.95 Ma) was recovered only in Sample 307-U1316A-6H-CC (54.9 mbsf) (Fig. F13). There is a likely hiatus between Samples 307-U1316A-5H-CC (lithostratigraphic Subunit 2A) and 6H-CC (Subunit 2B).

The lower–middle Miocene Zone CN3 is represented in Sample 307-U1316A-7H-CC (57.8 mbsf) (Fig. F13). There is a significant hiatus of ~12 m.y. between Samples 307-U1316A-6H-CC (Unit 2) and 7H-CC (Unit 3). Results of high-resolution sampling specify this hiatus between Samples 307-U1316A-7H-1, 25 cm (55.05 mbsf), and 7H-127 cm (55.07 mbsf), at the boundary of two lithostratigraphic units (see “Lithostratigraphy”).

The common occurrences of several age-diagnostic nannofossils identify the interval from Sample 307-U1316A-7H-CC (57.8 mbsf) through 21H-CC (132.2 mbsf) as lower–middle Miocene (Unit 3). These diagnostic taxa include Sphenolithus heteromorphus, Helicosphaera ampliaperta, Discoaster deflandrei, Reticulofenestra gartnerii, and Cyclicargolithus floridanus (Fig. F13; Table T2). The preservation of nannofossils is generally poor to moderate in the upper part of the unit (from Section 307-U1316A-7H-CC [57.8 mbsf] through 15X-CC [95.5 mbsf]) but is good in the lower part of the unit (from Section 307-U1316A-16X-CC [108.5 mbsf] through 21X-CC [132.2 mbsf]). Reworking is not common.

Hole U1316C was used to fill in stratigraphic gaps, as it spans from 49.2 to 142.0 mbsf and was further useful for examining the hiatus at 55.06 mbsf in Hole U1316A. In addition to samples from core catchers, nannofossil biostratigraphy was derived from samples located within the actual core (Table T3). Nannofossil biostratigraphy indicates that lower–middle Miocene sediment spans the lower part of Hole U1316C (49.2–143.2 mbsf) (Table T3).

Planktonic foraminifers

The planktonic foraminifer biostratigraphy identified two different ages in Hole U1316A. These are Zones SN14 and M5~6 (Jenkins, 1985, 1993), which correspond to the Pleistocene and early–middle Miocene, respectively (Fig. F12; Table T4). Globorotalia truncatulinoides (Fig. F14) was found in Sample 307-U1316-6H-CC (54.9 mbsf) but was not observed in any other samples. The FAD of G. truncatulinoides defining the base of Zone SN14 was tentatively placed just above this horizon. Globorotalia inflata also has its first appearance in Sample 307-U1316-6H-CC (54.9 mbsf). This appearance indicates ages younger than late Pliocene (Zone SN13). Age of the sediment below Sample 307-U1316-7H-CC is assigned to the early–middle Miocene by the occurrence of Praeobulina sicanus (Figs. F14, F15).

The Pleistocene planktonic foraminifer assemblage from Hole U1316A is dominated by cool- to temperate-water taxa Globigerina bulloides, Neogloboquadrina incompta, and Neogloboquadrina pachyderma (dextral and sinistral) (Figs. F14, F15). It is noteworthy that typical cold-water species N. pachyderma commonly occurs in Unit 1 (Fig. F16). Warmer water taxa, Globorotalia crassaformis, Globorotalia crassula, G. inflata, Globorotalia scitula, G. truncatulinoides, Orbulina universa, and Globigerinoides sp. are less frequent.

Benthic foraminifers

Sediments from Hole U1316A yield diverse assemblages of benthic foraminifers (Fig. F17). Shallow-water species, such as Elphidium excavatum and Islandiella norcrossi are common in lithostratigraphic Unit 1 but rare in Units 2 and 3. Otherwise, there is no significant difference in assemblages among the three lithostratigraphic units. Species associated with cold seepage, Bulimmina marginata and Bulimmina sp. A, are common throughout Hole U1316A (Fig. F18).

Discussion

An overview of the nannofossil biostratigraphy in Holes U1316A, U1316B, and U1316C indicates the uppermost 44 m of sediment is middle–upper Pleistocene (<0.26 Ma) and contains an abundant amount of reworked Late Cretaceous nannofossils. Below this section lies a thin package of uppermost Pliocene to middle Pleistocene sediment. At the boundary between lithostratigraphic Units 2 and 3 (55.05 mbsf), a hiatus was recognized ranging from early–middle Miocene to latest Pliocene. The lower middle Miocene sediment continues to the base of Holes U1316A, U1316B, and U1316C (Fig. F12).

Planktonic foraminifer biostratigraphy indicates that sediments recovered from Hole U1316A range in age from Quaternary to early–middle Miocene (Fig. F12). The planktonic foraminifers indicate a hiatus at 55.05 mbsf, consisting of planktonic foraminifer Zones M4 to SN13 of Jenkins (1983, 1995). There is no inconsistency with calcareous nannofossil biostratigraphy (Fig. F12).

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