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doi:10.2204/iodp.proc.330.107.2012

Paleontology

Subsea camera observation during the spotting of Hole U1376A indicated that no apparent soft sediment was present in the immediate area of Hole U1376A on Burton Guyot. Rotary coring therefore retrieved only the consolidated sequences of stratigraphic Units I–IV. Volcaniclastic rock fragments from Cores 330-U1376A-1R and 2R of Subunit IC and limestone from Cores 3R and 4R of Subunit IIA were taken for microfossil analyses. Thin sections taken from Subunit IIA and the basaltic conglomerate of Subunit IIB were also used for planktonic foraminiferal biostratigraphy. Nannofossil biostratigraphy was supplemented by additional powdered samples from Cores 1R (Subunit IA) and 4R (Subunit IIA), yielding a preliminary age for the upper portion of Subunit IA of middle to late Miocene (Fig. F7; Table T2).

Additionally, macrofossils were found in Subunits IIA and IIB. Partial dissolution prevented taxonomic analyses of most of the macrofossils, but some preserved specimens occur locally, allowing a possible later Cretaceous age assignment for Subunit IIB, as discussed in “Macrofossils.”

Calcareous nannofossils

Calcareous nannofossil content was analyzed in nine samples from Hole U1376A. Six of the nine samples proved to be barren. Three of these were taken from the consolidated sediments in core catchers, including Samples 330-U1376A-1R-7-PAL (Subunit IC), 2R-6-PAL (Subunit IC), and 3R-6-PAL (Subunit IIA). The other three barren samples were taken (by scraping the rocks with a razor blade) from the limestone of Subunit IIA from Sections 330-U1376A-4R-1, 1 cm; 4R-4, 88 cm; and 4R-4, 107 cm.

The samples containing nannofossils were taken from Section 330-U1376A-1R-1 at 1 cm, 41 cm, and 76 cm, in Subunit IA. These discrete samples are composed of white chalk situated either directly above or below and in contact with a ferromanganese crust (Fig. F4), and they are isolated from each other by the volcanic sandstone/breccia material of Subunit IA (see “Sedimentology”). These observations suggest nannofossil assemblages in the chalk are free of any contaminating influence; nonconcurrent ranges observed in some chalk samples are best explained by their occurrence in condensed intervals, as clearly indicated by their association with the ferromanganese crusts. Sample 1R-1, 76 cm, contains Cyclicargolithus abisectus (Zones CP19–CN1a; late Oligocene–early Miocene) and discoasters such as Discoaster exilis (late early to early middle Miocene). Several examples of Triquetrorhabdulus rugosus place this sample at least in Zone CN4, and clear examples of Discoaster moorei co-occurring with likely specimens of Discoaster kugleri led to a preliminary minimum age assignment of Subzone CN5b (between middle and late Miocene).

Sample 330-U1376A-1R-1, 41 cm, in Subunit IA has slightly worse preservation, notably shown by increased overgrowth of discoasters. The Neogene background species Calcidiscus leptoporus is one of the most readily identifiable species present. This species occurs along with Discoaster asymmetricus, which is placed as low as Subzone CN8b by Perch-Nielsen (1985). This sample is assigned to Subzone CN9b (late Miocene) on the basis of unambiguous examples of Amaurolithus primus and the occurrence of Amaurolithus tricorniculatus.

The state of preservation in Sample 330-U1376A-1R-1, 1 cm, at the top of Subunit IA is very poor, and no zonal assignment is currently given. However, it is thought to be at least as young as (or most probably younger than) Sample 1R-1, 41 cm. Further shore-based research should allow for biostratigraphic refinement of these samples and age assignments.

Planktonic foraminifers

Only Samples 330-U1376A-1R-7, 49–50 cm (8.96 mbsf), and 2R-6, 121–122 cm (17.47 mbsf), from the consolidated volcanic sandstone of Subunit IC and Samples 3R-6, 117–120 cm (26.73 mbsf), and 4R-4, 36–39 cm (33.23 mbsf), from the consolidated limestone of Subunit IIA were used for planktonic foraminiferal biostratigraphy (Table T3). However, none of these samples contain planktonic foraminifers, making age estimation of these units impossible. In addition to these samples, four thin sections taken from Subunit IIA limestone and three thin sections taken from Subunit IIB basaltic conglomerate were examined (Table T4), but these were also barren (or had only scarce occurrences) of planktonic foraminifers. The Subunit IIA limestone is mainly composed of in situ calcareous algal framework and micrite grains containing macrofossil fragments such as bivalves, bryozoans, and echinoderms (see “Sedimentology”). Although Sample 3R-5, 99–103 cm (25.05 mbsf), contains one globular planktonic foraminifer, no other sample recovered from Subunit IIA contains any planktonic foraminifers. The Subunit IIB conglomerate consists of volcaniclastics and bioclasts, including annelids, bivalves, bryozoans, calcareous algae, echinoderms, and gastropods, which is indicative of a rocky shore environment (see “Sedimentology”). Sample 5R-1, 65–67 cm (38.85 mbsf), contains one planktonic foraminifer, but no planktonic foraminifers were found in the other thin sections from Subunit IIB. Because of the scarce occurrence of planktonic foraminifers, a planktonic foraminiferal zonal definition could not be made for Hole U1376A.

Macrofossils

The Subunit IIA limestone contains many molluscan fossils, of which most shell materials have been dissolved. However, Samples 330-U1376A-5R-3, 94–100 cm (41.65 mbsf), and 5R-3, 101–106 cm (41.72 mbsf), of Subunit IIB contain relatively well preserved molluscan fossils, possibly identifiable as later Cretaceous rudists. These fossils have centimeter-sized round shells composed of two shell layers (Fig. F8). Although their inner shell layers were dissolved, creating space between the sandstone infill and the outer shell layer, the outer shell layers are typically well preserved, which implies that the inner and outer shell layers were originally composed of aragonite and calcite, respectively. An outer surface of the inner shell layer of the specimen obtained from Sample 5R-3, 101–106 cm, shows fine striations and undulations indicative of an accretionary growth pattern (Fig. F9). However, key morphological characteristics for final identification of the taxonomy of these molluscan fossils (such as three-dimensional morphology and shell microstructures) await detailed postexpedition analyses using whole-round computed tomography scanning.

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