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doi:10.2204/iodp.proc.330.106.2012

Paleontology

Rotary core barrel drilling in Hole U1375A retrieved only foraminiferal debris in the core catcher of Core 330-U1375A-1R, with all 8.5 m of sediment presumably flushed away during the drilling process. This sample contained <25 cm³ of sandy foraminiferal ooze with a small percentage of fine fraction, including nannofossils. The sediment sample recovered from the core catcher is assumed to be a mixture of sediment grains from the entire 8.5 m interval drilled and was assigned a preliminary age of latest Miocene–Holocene (Fig. F5; Tables T3, T4). In addition to soft-sediment analysis, smear slides and thin sections taken from the consolidated basalt conglomerate of Subunit IIA were analyzed for microfossil biostratigraphy. On the basis of samples taken from Core 2R, a preliminary Paleocene (Danian–Thanetian) age was assigned to Subunit IIA.

Hole U1375B was drilled ~300 m from Hole U1375A, and again only a small amount of rock cuttings (<10 cm³) and a very small amount of foraminiferal debris were recovered from the core catcher of Core 330-U1375B-1R. The extremely poor preservation of microfossils prevented age determination of these rock cuttings. The majority of the core in Hole U1375B is composed of microgabbro (Unit I; see “Igneous petrology and volcanology”).

Calcareous nannofossils

Unit I

In Hole U1375A on Achernar Guyot, one smear slide was prepared from the sediment of Unit I within the core catcher of Core 330-U1375A-1R. This smear slide contains a mixed assemblage of Neogene and Quaternary species. Present species include Discoaster variabilis, Helicosphaera sellii, Helicosphaera kamptneri, Ceratolithus cristatus, Umbilicosphaera sibogae, Coccolithus pelagicus, Calcidiscus leptoporus, Rhabdosphaera clavigera, Pseudoemiliania lacunosa, and small Gephyrocapsa. The preliminary age of this mixed assemblage is assigned to approximately Zones CN10–CN15 (uppermost Miocene to Holocene).

Unit II

Two additional samples were taken from Sections 330-U1375A-2R-1, 13 cm, and 2R-1, 32 cm, from the consolidated carbonate matrix of Subunit IIA (Fig. F3). However, calcareous nannofossils were present at rare abundance levels in both samples, and relatively poor preservation made immediate recognition of individual species difficult. In Sample 2R-1, 13 cm, specimens of Prinsius (Neobiscutum) dimorphosus were identified in association with apparent examples of other Prinsius sp. On the basis of the presence of P. dimorphosus a preliminary age of Zones CP1b–CP3 (range of P. dimorphosus) from the early Paleocene was assigned. Preservation in Sample 2R-1, 32 cm, is quite poor, and individual species will be identified during postexpedition study. Samples of Subunit IIB were completely barren of nannofossils.

Planktonic foraminifers

Unit I

Samples 330-U1375A-1R-CC and 330-U1375B-1R-CC (Unit I) contain <25 and 10 cm³ of sandy foraminiferal ooze, respectively, although most of the foraminiferal tests in Sample 330-U1375B-1R-CC were broken into pieces. Therefore, only Sample 330-U1375A-1R-CC was used for planktonic foraminiferal analyses. This sample contains Globorotalia (Globoconella) inflata, Globorotalia (Truncorotalia) crassaformis, and Globorotalia (Truncorotalia) truncatulinoides (Table T4). During the washing procedure only a small amount of fine fraction was washed away, indicating that this sediment is composed mainly of foraminiferal tests (>50%). Almost all foraminiferal specimens have whitish tests, and some are glassy. Although only a few individuals are filled with calcite cement, most show no sign of calcite overgrowth and cementation. In addition to the abundantly encountered species listed above, Globigerinoides extremus and Sphaeroidinellopsis cf. seminulina (indicating approximately earliest Pleistocene and approximately early Pliocene, respectively) were identified in this sample. Considering that no drilling mud was pumped during the drilling of Core 1R, Sample 1R-CC likely contains foraminiferal tests from all horizons of the 8.5 m interval drilled, explaining the co-occurrence of these heterochthonous species. On the basis of the occurrence of these planktonic foraminifers, the preliminary age assigned to this Unit I sample is late Miocene–Holocene (Fig. F5).

Unit II

In addition to this core catcher sample, thin sections taken from the consolidated conglomerate of Subunit IIA were examined. The matrix of this conglomerate is mostly composed of fossiliferous micrite. Although macrofossil bioclasts such as algae, bryozoans, and echinoderms are rare in the matrix, abundant foraminifers are preserved within thin sections. Triplicate thin sections were made from Samples 330-U1375A-2R-1, 26–29 cm, and 2R-1, 67–70 cm (Table T5). Sample 2R-1, 26–29 cm (8.76 mbsf), contains Acarinina spp., Globanomalina cf. pseudomenardii, Igorina sp. (Igorina albeari?), Morozovella sp. (Morozovella angulata?), and other planktonic foraminifers with globular and serial morphologies (Fig. F6). On the basis of the occurrence of these species, this sample was correlated to planktonic foraminiferal Zones P4a–P4c (Selandian–Thanetian). Sample 2R-1, 67–70 cm (9.17 mbsf), contains only a few planktonic foraminifers. Age-diagnostic species were not identified in this sample. Subunit IIB samples were completely barren of planktonic foraminifers.

Preliminary age estimation for Site U1375

On the basis of nannofossil and planktonic foraminiferal analyses, the preliminary age of the sandy foraminiferal ooze of Unit I was assigned to the late Miocene–Holocene. Because all sediment from Unit I, with the exception of <25 cm³ of sediment within the core catcher, was washed away during core retrieval, the stratigraphic distribution of microfossils could not be identified.

Although Sample 330-U1375A-2R-1, 13 cm (8.63 mbsf), from Subunit IIA contains Danian–Selandian (~60–64.5 Ma) nannofossils, Sample 2R-1, 26–29 cm (8.76 mbsf), yields an asynchronous suite of Selandian–Thanetian (55.9–55.2 Ma) planktonic foraminifers. Visual observation of hand specimens and thin section analyses indicate that the Subunit IIA conglomerate contains not only basalt grains but also allochthonous micrite grains within its matrix. The heterogeneity of the micritic portion of Subunit IIA may explain the disparity in age between calcareous nannofossils and planktonic foraminifers. In addition, provincialisms of microfossils should be taken into account. Nevertheless, both nannofossils and planktonic foraminifers indicate a preliminary Paleocene age for Subunit IIA. Precise postexpedition microfossil analyses will clarify the nature of the disparity of age estimation of Subunit IIA.

At the top of the basalt conglomerate of Unit II, a ferromanganese crust forms the boundary between Units I and II, consistent with a significant time gap between these units. Microfossil age determinations for the foraminiferal ooze of Unit I and the conglomerate of Unit II indicate that a sequence of >40 m.y. is missing (Fig. F5).

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