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doi:10.2204/iodp.proc.330.108.2012

Paleontology

Nannofossil foraminiferal ooze (Unit I), along with foraminiferal limestone and basaltic conglomerate (Unit II), were recovered from both Holes U1377A and U1377B on Hadar Guyot. In Hole U1377A, 6.1 m of advancement with Core 330-U1377A-1R retrieved <25 cm³ of the ooze, and 9.0 m of advancement with Core 2R recovered a 14 cm long section of the conglomerate of Unit II. In Hole U1377B, 9.1 m of sediment and sedimentary rock were presumed to have been penetrated with Core 330-U1377B-1R, but only 0.79 m of the ooze and 0.29 m of the conglomerate were recovered. The soft sediment of both holes is thought to have been mechanically disturbed during drilling and reworked by flushing of sediment within the core liner during retrieval. Consequently, it is assumed that a mixture of individual sequences within Unit I were recovered from Sections 330-U1377A-1R-CC and 330-U1377B-1R-1 and 1R-2. Although Units I and II show similar lithologies in Holes U1377A and U1377B, precise stratigraphic correlation of Units I and II between the two holes could not be accomplished because of very low recovery rates.

Calcareous nannofossils

The nannofossil foraminiferal ooze of Unit I recovered in Section 330-U1377A-1R-CC is composed of a mixture of yellowish-brown nannofossil ooze and white winnowed foraminiferal ooze. The winnowed foraminiferal ooze mainly consists of planktonic foraminifers with only a small percentage of fine faction. The nannofossil foraminiferal ooze contains a higher percentage of fine fraction compared to the winnowed foraminiferal ooze and to samples at all previous sites (U1372–U1376). The Unit I ooze recovered in Sections 330-U1377B-1R-1 and 1R-2 consists of pale yellow winnowed nannofossil foraminiferal ooze with a small amount fine fraction. Section 1R-2 is composed of limestone and basaltic conglomerate, but the core catcher of Core 1R contained washed foraminiferal ooze that was used for nannofossil and microfossil analyses. The pelagic ooze recovered from both Holes U1377A and U1377B indicates a preliminary age of mid-Pleistocene–Holocene (Fig. F5; Tables T2, T3, T4).

Hole U1377A

Two samples from Hole U1377A at Hadar Guyot were analyzed for calcareous nannofossil content. Core 330-U1377A-1R is estimated to have penetrated 6 m and has an assemblage dominated by small Gephyrocapsa but also contains G. caribbeanica and G. oceanica. Neogene background species Helicosphaera kamptneri and Calcidiscus leptoporus were frequently observed. Specimens of Reticulofenestra pseudoumbilica were seen along with late Pliocene Discoaster species, but they are few. On the basis of the dominant abundance of Gephyrocapsa and Pleistocene species, this sample is preliminarily assigned to Zones CN13–CN15 (mid-Pleistocene to Holocene).

The second sample from the core catcher of Core 330-U1377A-1R was tan nannofossil foraminiferal ooze with a higher percentage of fine fraction containing a virtually identical assemblage to that of the first sample. The only immediately observed difference between the two samples was the frequent occurrence of Ceratolithus cristatus in the assemblage of the tan sediment. It, too, is preliminarily assigned to Zones CN13–CN15 (mid-Pleistocene to Holocene).

Hole U1377B

The white winnowed nannofossil foraminiferal ooze sampled in Hole U1377B, as retrieved from the core catcher of Core 330-U1377B-1R, appears visually identical to the white foraminiferal ooze of Section 330-U1377A-1R-CC and also has a nearly identical nannofossil assemblage. One abundant specimen of Amaurolithus primus (Zones CN9b–CN11; late Miocene–early Pliocene) was found, but its presence is thought to result from disturbance during coring. The age of this sample is also given a preliminary assignment of Zones CN13–CN15 (mid-Pleistocene to Holocene).

Planktonic foraminifers

Unit I in both Holes U1377A and U1377B provided sufficient samples to study the planktonic foraminifers. In addition, thin sections made from Unit II rocks allowed successful identification of planktonic foraminifers of the limestone and basaltic conglomerate in Unit II. Planktonic foraminiferal faunas found in the thin sections made from Samples 330-U1377A-2R-1, 15–17 cm (6.25 mbsf), and 330-U1377B-1R-2, 8–12 cm (0.87 mbsf), indicate preliminary ages between middle and late Eocene and between late Paleocene and early Eocene, respectively (Fig. F5; Table T5).

Hole U1377A

One sample from Unit I, one from Unit II, and two from the igneous rocks in Unit III, were analyzed for planktonic foraminiferal biostratigraphy. Sample 330-U1377A-1R-CC (0.0 mbsf) of Unit I is composed of sandy foraminiferal ooze containing Globigerina bulloides, Globorotalia (Globoconella) inflata, Globorotalia (Truncorotalia) crassaformis, and Orbulina universa. More than 50% of washed grains are composed of planktonic foraminifers showing well-preserved whitish tests. In addition to these major species, Globigerinoides extremus, Globorotalia (Truncorotalia) truncatulinoides, and Sphaeroidinellopsis seminulina (indicative of older than early Pleistocene, younger than early Pleistocene, and older than early Pliocene, respectively) were observed in this sample. The stratigraphic ranges of these species do not overlap, which implies that the specimens were reworked from different stratigraphic positions because of drilling disturbance. The preliminary age of this sample is assigned to early Pliocene–Holocene (Fig. F5; Table T3).

A thin section of Unit II foraminiferal limestone from Sample 330-U1377A-2R-1, 15–17 cm (6.25 mbsf), contains Acarinina sp., Globigerinatheka sp., and Subbotina spp. (Fig. F6). The test material of these individuals is relatively well preserved. In fact, the pore structure and layering of tests are clearly visible in the section surface (Fig. F6). The stratigraphic range of genus Globigerinatheka indicates that this sample is preliminarily correlated to planktonic foraminiferal Zones E8–E16 (middle–late Eocene) (Fig. F5; Table T5).

Although Unit III is composed of basaltic basement, rock cuttings recovered from Sample 330-U1377A-3R-1, 116–121 cm (16.26 mbsf), contain echinoderm fragments with no planktonic foraminifers, indicative of a relatively shallow marine environment. Therefore, though not recovered, a shallow-marine or hemipelagic sequence should be present between the Unit I nannofossil foraminiferal ooze and the Unit III basalt.

Hole U1377B

Unconsolidated foraminiferal ooze of Unit I and cuttings from the cores drilled into the consolidated sequence of trachybasalt (Unit III) were sampled for planktonic foraminiferal biostratigraphy. Although Unit II (Section 330-U1377B-1R-2) is composed of foraminiferal limestone and basaltic conglomerate, a small amount of washed sandy foraminiferal ooze (likely derived from Unit I) was recovered at 28–29 cm in Section 1R-2 (10.7 mbsf). This sample contains Gr. (G.) inflata, Gr. (T.) crassaformis, and Gr. (T.) truncatulinoides. More than 50% of the washed-down sand-size grains are composed of planktonic foraminifers. The occurrence of these species indicates a preliminary age of middle Pleistocene–Holocene. Nonetheless, Gs. extremus and Ss. Seminulina (indicative of older than early Pleistocene and older than early Pliocene, respectively) co-occur in this sample. As was observed in Hole U1377A, this sample also encloses reworked individuals from various other horizons. The preliminary age of this sample was assigned to early Pliocene–Holocene (Fig. F5; Table T4). Cuttings from Sample 2R-3, 20–40 cm (12.20 mbsf), contain no biogenic fragments.

A thin section of Unit II consolidated limestone from Sample 330-U1377B-1R-2, 8–12 cm (0.89 mbsf), contains Acarinina sp., Morozovella sp. (Morozovella cf. acuta?), Subbotina spp., and a few echinoderm fragments (Fig. F7). Although precise species identification needs to be refined postexpedition, on the basis of the occurrence of M. cf. acuta this sample could be correlated to Zones P4b–E2 (late Paleocene–early Eocene) (Fig. F5; Table T5).

Preliminary age estimation for Site U1377

Both nannofossil and planktonic foraminiferal faunas recovered from Unit I ooze are composed of a mixture of Holocene, Pleistocene, and Pliocene species because of drilling disturbance. Although the mixed nature of these microfossil faunas and the low recovery rate prevent stratigraphic correlation of Unit I between Holes U1377A and U1377B, the preliminary age of Unit I in both holes is assigned to the early Pliocene–Holocene. A partly older age of this unit may, however, be indicated by the occurrence of a specimen of Amaurolithus primus (late Miocene–early Pliocene) in Section 330-U1377A-1R-CC.

A thin section made from Unit II in Hole U1377A contains predominantly planktonic foraminifers, whereas one from Hole U1377B contains not only planktonic foraminifers but also larger benthic foraminifers and macrofossils. This indicates that the depositional depth of Unit II recovered in Hole U1377B was shallower than that recovered in Hole U1377A, which is in agreement with the interpretation that distinct levels of Unit II were recovered in each hole (see “Sedimentology”). The planktonic foraminiferal genus Globigerinatheka found in Sample 330-U1377A-2R-1, 15–17 cm, evolved during the middle Eocene and went extinct at the end of the Eocene (Pearson et al., 2006). Although precise species identification was not made on board, this generic range indicates a preliminary age between middle and late Eocene for Unit II of Hole U1377A. On the other hand, Morozovella sp. (Morozovella cf. acuta?) found in Unit II of Hole U1377B has a biostratigraphic range of Zones P4b–E2 (Pearson et al., 2006), indicative of between the late Paleocene and early Eocene, which is slightly older than the age of Unit II in Hole U1377A. Although the low recovery rate of Unit II in both holes prevents precise stratigraphic correlation, the age discrepancy between the Unit II limestone in Hole U1377A and that in Hole U1377B implies that either distinct layers of Unit II were sampled in each hole or a few diachronous limestone horizons were developed on Hadar Guyot. The former is more likely, as indicated by lithostratigraphic relationships based on observation of ghost Core 330-U1377B-3G (see “Sedimentology”) and the close proximity of the holes. Although the low recovery rate prevents precise stratigraphic constraint, a ~30 m.y. unconformity may exist between Units I and II.

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