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doi:10.2204/iodp.proc.330.104.2012

Paleontology

Despite the fact that a veneer of unconsolidated pelagic sediment was observed on the seafloor before spotting Hole U1373A (using the vibration-isolated television camera; see “Operations”) Core 330-U1373A-1R was composed of consolidated basalt conglomerate (Unit I; see “Sedimentology”). Nonetheless, a small sample of sand- and granule-size angular grains with planktonic foraminiferal tests and nannofossils was recovered from the core catcher of Core 1R. These grains are composed of basalt fragments, feldspar crystals, foraminiferal tests, glass fragments, ferromanganese crust, olivine crystals, and some other minor crystals. Linear scratches were observed on the surface of many of these lithic grains, indicating mechanical abrasion during drilling. Because no drilling mud was pumped during the drilling of Cores 1R, 2R, or 3R, these sand- and granule-size grains are assumed to be a mixture of cuttings from Core 1R and surface sediment. The preliminary age assigned to these cuttings is Pleistocene–Holocene.

Instead of conducting microfossil analyses on soft sediment, which is standard procedure, we analyzed thin sections for microfossil biostratigraphy in the consolidated sequences of Units I and III. In addition to these microfossil analyses, macrofossils embedded in Subunit IIIB were examined. These macrofossils were identified as Flemingostrea sp. (an oyster), which has a long biostratigraphic age range of Late Cretaceous–Miocene and provides the preliminary age of Subunit IIIB. Preliminary age assignments are summarized in Figure F8 and Tables T3 and T4.

Calcareous nannofossils

Nannofossil analysis in Hole U1373A was restricted to a small percentage of sediment (no more than 50 cm³) recovered at the bottom of the core catcher of Core 330-U1373A-1R. No stratigraphic unit or subunit was therefore assigned to this small amount of recovered material. This material is assumed to have settled out from the seawater in the core liner and likely represents unsampled oozy sediment observed during site spotting on top of the drilled guyot. The observed assemblage contained Neogene background species Helicosphaera kamptneri and Calcidiscus leptoporus. Common species include small Gephyrocapsa, Pseudoemiliania lacunosa, and Emiliania huxleyi. Rare specimens of Gephyrocapsa caribbeanica were also observed, the first occurrence of which suggests a lower limit of Subzone CN13b. On the basis of this lower CN13b limit and the disturbed nature of the recovered sediment, a range of Zones CN13b–CN15 (lower Pleistocene to Holocene) was assigned as the age of the sediment overlying Subunit IA (Fig. F8; Table T3).

Planktonic foraminifers

Rock cuttings from Core 330-U1373A-1R and the less consolidated basalt conglomerate of Subunit IIIB were used for planktonic foraminiferal analyses. Zonal assignments are summarized in Figure F8 and Table T4. Sample 1R-3, 14–16 cm (2.43–2.45 mbsf), from Unit I contains Globorotalia (Globoconella) inflata, Globorotalia (Truncorotalia) crassaformis, Globorotalia (Truncorotalia) truncatulinoides, and Sphaeroidinella dehiscens (Table T4). Although many grains in the cuttings show linear tool marks on their surfaces under binocular microscope observation, relatively well preserved foraminifers were found in those cuttings. Some foraminiferal tests are brownish, but most are free from calcite cement and are white. Foraminiferal tests compose 10%–20% of the grains. On the basis of the occurrence of Gr. (T.) truncatulinoides, the preliminary age for this sample is correlated to Zone PL6–PT1b (Pleistocene–Holocene). However, because the sample was derived from a very small amount of surface sediment, no stratigraphic unit or subunit was assigned to this material.

Sample 330-U1373A-4R-1, 99–100 cm (19.56–19.60 mbsf), of Subunit IIIB contains no planktonic foraminifers. The disaggregated grains of this sample are mainly composed of altered basalt, altered glass, and calcite cement fragments with minor bioclasts. Among these bioclasts, calcareous algae and sea urchin spines are common, with a few benthic foraminifers. All of these benthic foraminifers are filled with calcite cement, and their tests are often dissolved.

In addition to these analyses, thin sections taken from Samples 330-U1373A-1R-1W, 8–12 cm; 1R-2W, 7–9 cm; and 1R-3W, 66–70 cm (Unit I); 3R-3W, 10–13 cm (Subunit IIIA); 3R-3W, 126–129 cm, and 4R-1W, 72–74 cm (Subunit IIIB); 5R-1W, 33–36 cm (Subunit IIIC); and 6R-2W, 39–43 cm (Subunit IIID) were examined. Samples from Units I and III contain abundant bioclasts within the volcaniclastic matrix (see “Sedimentology”). Benthic foraminifers, bivalve fragments, bryozoans, calcareous algae, and sea urchin spines were generally identified in these samples. On the other hand, only Samples 1R-1W, 8–12 cm, and 1R-2W, 7–9 cm, from Unit I rarely contain planktonic foraminifers. However, none of these planktonic foraminifers were sectioned in the axial plane in thin section, which is required for species identification. In addition, some of the planktonic foraminifers were fractured, and shell microstructures such as pores were not visible in any of the specimens, indicating that the preservation of foraminiferal tests is poor in Unit I. Because of the tests’ poor preservation, species identification was difficult. Although one single-keeled morphotype was identified in Sample 1R-1W, 8–12 cm (Subunit IA), all other observed foraminifers in Units I and III show globular or biserial morphologies. Therefore, the preliminary ages of Units I and III could not be identified by onboard analyses and will be examined postexpedition.

Macrofossils

Well-diversified bioclasts including macrofossils occur in Subunit IIIB. Although most of the macrofossils are fragmented and unidentifiable, some well-preserved fossils were found in Section 330-U1373A-4R-1 (Fig. F9). Shell height of the specimen shown in Figure F9A is >4 cm, and the valve is slightly convex in the outer mold of the anterior view. Multiple imbrications were identified on the anterior and external views (Fig. F9A, F9B). Thin section observations of the specimen shown in Figure F9B indicate that the multiple imbrications are composed of numerous prismatic shell layers (Fig. F10). On the basis of these morphological characteristics, these individuals were identified as Flemingostrea sp., a marine oyster. The total range of this genus is from the Late Cretaceous to Miocene, and its acme was in the Late Cretaceous (Stenzel et al., 1971). Therefore, Subunit IIIB can be correlated to this long interval. However, because the occurrence of this genus has been reported in the Eocene of New Zealand (Beu and Raine, 2009), Subunit IIIB might be correlated to the Eocene (55.8–33.9 Ma) as well.

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