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doi:10.2204/iodp.proc.330.103.2012

Paleontology

Rotary coring of Hole U1372A returned three cores of uncemented pelagic sediment (stratigraphic Unit I). Core 330-U1372A-1R recovered 0.1 m of sediment in the core catcher only, whereas Core 2R was 7.55 m long and Core 3R was 2.74 m long. The recovery rates for each core were 100%, 79%, and 72%, respectively. The retrieved sediment is tan in color and is mainly composed of planktonic foraminifers with only a small fine fraction, suggesting that this sediment is winnowed foraminiferal ooze formed under strong water currents that washed out the fine fractions (see “Sedimentology”). Low bulk density and natural gamma radiation also indicate a paucity of clay minerals in Cores 1R through 3R (see “Physical properties”). The late Miocene to Holocene sediment recovered in Unit I contains planktonic foraminifers in abundance, along with calcareous nannofossils in the fine fraction remnant. Shipboard examination of these microfossils from core catcher and supplementary core samples was conducted, and preliminary age estimates are summarized in Figure F14 and Tables T3, T4, and T5.

Beneath the uncemented sediment of Unit I, lithified breccia, limestone, and conglomerate were recovered from the interval between the top of Core 330-U1372A-4R (13.50 mbsf) and Section 8R-3, 63 cm (45.58 mbsf). Thin sections made from billets containing limestone or micritic sediment in Subunits IIA and IIB were examined with transmitted light for planktonic foraminifers. Preliminary age estimations are summarized in Figure F14 on the basis of abundant planktonic foraminifers, which indicate that this sediment was deposited during the later Cretaceous to early Paleocene.

Calcareous nannofossils

The soupy nature of the Unit I pelagic sediment capping Canopus Guyot required the core liner to be perforated and stood on end, allowing excess water to drain before curation. This allowed movement of nannofossils through the length of the core and may have resulted in the homogeneous assemblage in the samples examined. In general, the pelagic cap recovered from Hole U1372A contained nannofossil assemblages of Neogene and Quaternary age.

Unit I sediment from the core catchers of Cores 330-U1372A-1R through 3R was examined for calcareous nannofossil content. Core 1R is only 10 cm long and is thought to have been recovered at the mudline. Species present included Emiliania huxleyi and small gephyrocapsids assigned to Zone CN15 (Holocene) (Table T3).

Additional samples from Unit I were taken from the 0–1 cm interval of each section in Cores 330-U1372A-2R and 3R. All nannofossil samples examined exhibited dominantly moderate to good preservation. Six samples were taken from Core 2R, but the soupy nature of Core 2R caused the sediment to become homogenized during drilling and retrieval of the core. Abundant to common species include E. huxleyi and small gephyrocapsids, which compose the majority of the assemblage and are indicative of an age from Pleistocene to recent. However, because of their extremely small size, definite determination of individual species is best done on shore with scanning electron microscopy. Pseudoemiliania lacunosa and Rhabdosphaera clavigera are quite common. Ceratolithus cristatus was encountered in all samples, although its occurrence is rare. Neogene background species Helicosphaera kamptneri and Calcidiscus leptoporus are frequently found throughout. The presence of Ceratolithus telesmus in samples from Sections 2R-1 and 2R-2 preliminarily place these samples at Zone CN14b–CN15 (upper Pleistocene). The lower limit of the range of Core 2R was inferred by the presence of round Pseudoemiliania lacunosa (Emiliania annula Zone CN13a [lower Pleistocene]). Additional sampling for postexpedition examination will allow determination of the degree of overall core disturbance. The core catcher of Core 2R displayed the same disturbance as other samples from Core 2R, placing it also in Zones CN13–CN15 (mid–late Pleistocene).

One sample was taken from each section of Core 3R at 0–1 cm. These samples were disturbed and displayed a mixed assemblage, as in Core 330-U1372A-2R. Represented species in the two samples include Reticulofenestra pseudoumbilica, Sphenolithus abies, Sphenolithus neoabies, Amaurolithus delicatus, Amaurolithus tricorniculatus, Discoaster challengeri, Discoaster pentaradiatus, and Discoaster brouweri. On the basis of the abundance of R. pseudoumbilica and S. neoabies, along with the presence of A. delicatus and A. tricorniculatus, these samples were preliminarily assigned to Zone CN9–CN11 (late Miocene to mid-Pliocene). Because discoasters are present in Core 3R with ranges that extend past the base of Subzone CN11a, a disconformity or a very low sedimentation rate is possible between the top of Section 3R-1 and the sample taken from Section 3R-6. Examined sediment from the core catcher of Core 3R, toward the bottom of Unit I, is composed of an assemblage of species similar to that of the two samples from Sections 3R-1 and 3R-2. However, P. lacunosa is common; this may result from contamination, but no immediate explanation is postulated.

Planktonic foraminifers

Unit I

Planktonic foraminiferal biostratigraphy of Cores 330-U1372A-1R through 3R (Unit I) was based on core catcher samples. Zonal assignments are summarized in Figure F14 and Tables T4 and T5. Uncemented pelagic sediment from Cores 1R through 3R is correlated to the late Miocene to Holocene, with significant condensation between the late Miocene and mid-Pliocene. Because this sediment is mostly composed of sand-size foraminiferal tests with little clay matrix and is highly permeable, it is inferred that foraminiferal tests in this interval were reworked during drilling and retrieval of the core.

Sample 330-U1372A-1R-CC, from the uppermost sediment of Hole U1372A (0–0.1 mbsf), contains Holocene planktonic foraminifers including Globorotalia (Globorotalia) tumida, Globorotalia (Truncorotalia) crassaformis, Globorotalia (Truncorotalia) truncatulinoides, and Sphaeroidinella dehiscens. However, Globorotalia (Truncorotalia) tosaensis was not observed in this sample. More than 50% of sand-size grains are composed of foraminiferal tests. Although some specimens are brownish and filled with calcareous cements, most show little evidence of dissolution and overgrowth and are predominantly white in color. On the basis of the occurrence of Gr. (T.) truncatulinoides and the absence of Gr. (T.) tosaensis, this sample from Unit I can be correlated to planktonic foraminiferal Zone PT1b (latest Pleistocene to Holocene) (Fig. F14).

Sample 330-U1372A-2R-CC (7.59–7.65 mbsf) contains Globigerina bulloides, Globorotalia (Globoconella) inflata, Gr. (T.) truncatulinoides, and Orbulina universa. In this sample, 20%–50% of sand-size grains are composed of foraminiferal tests. Nearly all specimens are filled with calcareous cement, and the sutures of many specimens are also filled with calcareous cement. In this sample, sediment grains are slightly cemented, and the washed residue contains aggregated granule-size grains. All of the species identified in this sample are modern species. Although Gr. (T.) tosaensis does not occur in this sample, the occurrence of Gr. (T.) truncatulinoides indicates that this sample can be correlated to the upper part of Zones PL6–PT1b. However, considering that overlying Section 1R-CC is correlated to Zone PT1b and the planktonic foraminiferal assemblage of this sample is different from that of Section 1R-CC (Table T4), Section 2R-CC might be correlated to the upper part of Zones PL6–PT1a (mid-Pleistocene) (Fig. F14).

Sample 330-U1372A-3R-CC (11.91–11.96 mbsf) contains Gg. bulloides, Globigerinoides immaturus, Globigerinoides sicanus, Globigerinoides extremus, Globorotalia (Globorotalia) merotumida, Globoquadrina dehiscens, and O. universa. Foraminiferal tests in this sample are again abundant in comparison to the total number of sediment grains. Preservation of planktonic foraminifers is good, despite the fact that the sediment is slightly cemented. On the other hand, some individuals have brownish tests with cement on their surfaces. Compared with samples from Sections 330-U1372A-1R-CC and 2R-CC, the number of occurring species in Sample 3R-CC is very high, and long-ranging species such as Gg. bulloides, O. universa, and Gs. immaturus are relatively abundant (Table T4). On the basis of the co-occurrence of Gr. (Gr.) tumida and Globigerinoides (Zeaglobigerina) nepenthes, this sample could be correlated to Zones PL1a–PL1b (upper Miocene to lower Pliocene). However, the biostratigraphic ranges of some species are correlated to Zones M5–M6, which indicates that some heterochthonous specimens were reworked into this sample. The high abundance of long-ranging species and the accumulation of pumiceous gravel with almost no clay fraction in this sample may indicate successive winnowing of fine fractions (see “Sedimentology”). The occurrence of these heterochthonous species implies the condensed nature in this sample. On the other hand, because Samples 2R-CC and 3R-CC are correlated to Zones PL6–PT1a and PL1a–PL1b, respectively, the sections between these samples (i.e., Sections 3R-1 and 3R-2) are tentatively correlated to Zones PL2–PL5 (Fig. F14).

Unit II

Planktonic foraminiferal faunas were also examined in thin sections from consolidated sequences of Subunits IIA and IIB. Although these consolidated sequences are mainly composed of basaltic breccia (see “Sedimentology”), foraminiferal tests are preserved within calcite cement or micritic matrixes. When species identification was difficult, general morphotypes were identified. Thin sections were examined from intervals 330-U1372A-4R-1, 37–41 cm (14.87 mbsf); 5R-1, 31–34 cm (18.31 mbsf); 5R-3, 16–20 cm (20.77 mbsf); 6R-1, 28–31 cm (23.78 mbsf); 7R-2, 105–111 cm (35.34 mbsf); and 8R-2, 24–27 cm (43.70 mbsf). Zonal assignments are summarized in Figure F14. The consolidated breccia from Section 330-U1372A-4R-1 in Subunit IIA is preliminarily correlated to the Danian based on the occurrence of only small globular and biserial foraminifers, whereas Subunit IIB contains characteristic double-keeled larger Cretaceous planktonic foraminifers having a range between the late Campanian and early Maastrichtian.

Subunit IIA

Sample 330-U1372A-4R-1, 37–41 cm (14.87 mbsf), from Subunit IIA contains foraminifers with only globular and biserial forms, and no keeled forms were observed (Fig. F15). Abundant planktonic foraminifers were observed in this single thin section, but their globular forms notably have a maximum diameter of <200 µm. Additionally, no inoceramid shell fragments were identified. Most Cretaceous planktonic foraminifers were extinct at the Cretaceous/Paleogene boundary (Caron, 1985; Olsson et al., 1999), and the test size of planktonic foraminifers became significantly smaller in the Danian (Olsson et al., 1999). On the other hand, during the evolutionary history of planktonic foraminifers, test size increased and keeled species appeared again in the mid-Paleocene (Olsson et al., 1999). Considering that Subunit IIB (see below for details) was assigned to between the late Campanian and early Maastrichtian, test size observed in this sample was significantly smaller, and only globular and biserial planktonic foraminifers were identified, a preliminary age for this sample can be assigned to the Danian.

Subunit IIB

A thin section taken from Sample 330-U1372A-5R-1, 32–34 cm (18.34 mbsf), of Subunit IIB was observed for planktonic foraminiferal biostratigraphy. This sample contains abundant planktonic foraminifers showing wide morphological diversity, such as globular, biserial, planispiral, single-keeled, and double-keeled morphologies (Fig. F16). Although almost all individuals were obliquely sectioned at their marginal part in thin section, one specimen sectioned at the axial plane was observed. On the basis of its morphological characters, this specimen might be identified as Globotruncanita cf. conica (Fig. F16A). Inoceramid shell fragments were also observed in this sample (Fig. F17). The biostratigraphic range of Gl. conica spans the Gansserina gansseri Zone to the Abathomphalus mayaroensis Zone (upper Campanian to Cretaceous/Paleogene boundary) (Caron, 1985). On the other hand, inoceramid bivalves became extinct near the base of the A. mayaroensis Zone (~69 Ma) (MacLeod and Orr, 1993). Therefore, this sample from Subunit IIB is preliminarily correlated to the Ga. gansseri Zone (upper Campanian to lower Maastrichtian) (~69–73 Ma) (Fig. F14).

Subunits IIC, IID, and IIE

Although Sample 330-U1372A-5R-3, 16–20 cm (20.77 mbsf), contains one keeled specimen, the majority of the foraminifers in Subunits IIC and IID have globular and biserial forms in all samples from Cores 5R through 7R. The maximum diameter of the globular planktonic foraminifers in these samples is always >250 µm. Although inoceramid shell fragments indicate that the later Jurassic–later Cretaceous period occurred from Sample 8R-2, 24–27 cm (43.70 mbsf), no age-diagnostic planktonic foraminifers were identified in Subunits IIC, IID, or IIE. Therefore, it is impossible to make a precise age assignment until postexpedition analyses are complete.

Preliminary age estimation for Site U1372

Unit I

The age of the Unit I sandy foraminiferal ooze on the top of Canopus Guyot is constrained on the basis of nannofossil and planktonic foraminiferal biostratigraphy (Fig. F14). Because recovered sediment is highly waterlogged and reworked within the core liners during drilling and retrieval of the cores, mixed microfossil assemblages were observed in Cores 330-U1372A-1R and 2R. Nonetheless, both nannofossil and planktonic foraminiferal assemblages indicate mid-Pleistocene to Holocene age for Cores 1R and 2R.

Core 330-U1372A-3R is correlated to the late Miocene to mid-Pliocene. Although a disconformity may have been identified between Sections 2R-6 and 3R-1 on the basis of nannofossil biostratigraphy of the core catcher and the additional sample from each section, this possible disconformity was not identified by planktonic foraminiferal biostratigraphy from the core catcher samples. On the other hand, Sample 3R-CC contains not only late Miocene to early Pliocene planktonic foraminiferal species but also mid-Miocene species. Considering the occurrence of this mixed assemblage and accumulation of rounded pumiceous gravel in this sample (see “Sedimentology”), significant reworking and condensation is assumed to have occurred during deposition. Precise age assignment and biostratigraphic zonation of Unit I will be investigated postexpedition.

Unit II

Although positive evidence identifying the age of Subunit IIA has not yet been found, the following observations allow assignment of this subunit to the Danian: (1) underlying Subunit IIB is correlated between the late Campanian and early Maastrichtian on the basis of planktonic foraminiferal and inoceramid assemblages, (2) only planktonic foraminifers showing globular and biserial morphologies were found from Subunit IIA, and (3) the maximum diameter of planktonic foraminifers found in Subunit IIA is <200 µm. However, a ferromanganese crust found at the top of Subunit IIB may be indicative of a significant time gap between the deposition of Subunits IIA and IIB (see “Sedimentology”). A record of the Cretaceous/Paleogene boundary may occur in the ferromanganese crust or have been eroded prior to deposition of Subunit IIA.

Considering the age of Units I and II, a substantial time gap (at least 40 m.y.) is expected between the deposition of these units. Lithologic and lithification differences observed between Unit I (foraminiferal ooze) and Unit II (mostly consolidated basalt breccia; see “Sedimentology”) and the assigned preliminary age of Unit I and Subunit IIA suggest that Subunit IIA is unconformably overlain by Unit I.

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