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doi:10.2204/iodp.proc.330.105.2012

Paleontology

The first core of Hole U1374A contained 6.64 m of unconsolidated pelagic sediment (Unit I), with an additional 0.62 m of volcanic sandstone at the bottom (upper portion of Subunit IIA). The unconsolidated sediment is composed of tan winnowed foraminiferal ooze with a small amount of fine fraction, suggesting deposition under relatively strong ocean-bottom currents. Low natural gamma radiation also indicates a paucity of clay minerals throughout the pelagic sedimentary interval of Unit I (see “Physical properties”). The sediment recovered in Unit I contains abundant planktonic foraminifers together with calcareous nannofossils in the fine fraction. Shipboard examination of the microfossils from Core 330-U1374-1R was conducted on samples recovered from section halves. Preliminary age estimations are summarized in Figure F10 and Tables T3 and T4.

Beneath the Pleistocene–Holocene unconsolidated sediment of Unit I, consolidated sequences of igneous and sedimentary rocks were recovered, including Units II, VII, IX, and XI. Smear slides and thin sections prepared from these sedimentary units and additional thin sections containing sediment recovered from igneous sequences were examined for microfossil biostratigraphy. Preliminary age estimations are summarized in Figure F10 and Tables T5 and T6. Although age-diagnostic microfossils were not observed in the barren Units III–XIX, the preliminary ages of Subunits IIB–IIC and IID were estimated to be late Maastrichtian and late Campanian, respectively.

Calcareous nannofossils

Unit I

Initial examination of the pelagic sediment of Unit I at Rigil Guyot revealed a fairly homogeneous assemblage, likely the result of the soupy character of the sediment, the sloshing of the unconsolidated material in the core liner, as well as some degree of mechanical disturbance caused by rotary coring, which together allowed for movement of the nannofossils through the length of the core. In general, the pelagic cap recovered in Hole U1374A contains calcareous nannofossil assemblages of Pleistocene to Holocene age (Zones CN13–CN15).

Samples of pelagic sediment (Unit I) were taken from the 99–100 cm intervals of Sections 330-U1374A-1R-1 through 1R-4, with an additional sample from Section 1R-5 at 30–31 cm. All nannofossil samples exhibit moderate preservation. Small gephyrocapsids are abundant in all samples and compose the majority of the assemblage relative to all other species. Because of their extremely small size, definite determination of individual species is best done with onshore scanning electron microscopy. The Pleistocene species Ceratolithus cristatus (Zones CN13–CN15) appears in all samples, though its occurrence is rare. The late Pleistocene form Ceratolithus telesmus (first occurrence in Subzone CN14b) was found in rare abundance in most samples (Table T3). In Sample 1R-5, 30–31 cm, it occurs with Helicosphaera sellii (last occurrence in Subzone CN13b). The nonconcurrent intervals of these two species support an interpretation of mechanical disturbance or reworking. Neogene background species Helicosphaera kamptneri and Calcidiscus leptoporus occur frequently throughout Unit I. Sediment that settled through the core liner into the core catcher of Section 1R-5 was examined for calcareous nannofossil content. Small Gephyrocapsa and Pseudoemiliania lacunosa compose the majority of the calcareous nannofossils present. Given this, the sediment of Unit I was preliminarily assigned to Zones CN13–CN15 (Pleistocene–Holocene).

Unit II

In addition to the uncemented pelagic sedimentary samples taken for shipboard analysis, samples of consolidated sediment from Cores 330-U1374A-2R and 3R in Unit II, obtained from razor blade scrapings, were examined. A sample of dark brownish fine-grained sandstone from Subunit IIC (Section 2R-4, 32 cm) was found to be nearly barren; two specimens of Micula concava were found. A sample obtained from volcaniclastic sandstone of Subunit IIC, which is interpreted to be burrow infilling in Subunit IID (Sample 3R-1, 70 cm; see below), contains other Cretaceous species in rare abundances. Preliminary investigation revealed the presence of Nephrolithus frequens, indicative of Zone CC26, among other Late Cretaceous species; therefore, the preliminary age of this sediment within Subunit IIC was assigned as latest Cretaceous (late Maastrichtian) (Table T5). Additional samples from this interval were taken for detailed postexpedition study in order to refine these biostratigraphic age assignments.

Planktonic foraminifers

Unit I

The planktonic foraminiferal biostratigraphy of Unit I was based on analysis of five samples from Sections 330-U1374A-1R-1, 1R-2, 1R-4, and 1R-5 that consist of sandy foraminiferal ooze (see “Sedimentology”). Only a small amount of fine fraction was washed away during the washing procedure, indicating that this sediment is composed mainly of foraminiferal tests. Indeed >50% of the washed sediment was composed of foraminiferal tests. Although Sample 1R-5, 60–62 cm (6.60 mbsf), is slightly consolidated and dark grayish brown, all other samples are unconsolidated and pale brown. Some foraminiferal tests in Sample 1R-5, 60–62 cm, display moderate preservation, with calcite overgrowth on test surfaces. With the exception of this sample, all others contain relatively well preserved foraminifers. Although foraminiferal specimens with glassy preservation are present, most tests are brownish in appearance. The chambers of all specimens are not filled with calcite, but the sutures of a few are. Foraminiferal preservation and abundance are shown in Table T4. Zonal assignments summarized in Figure F10 show correlation of Unit I to the Pleistocene–Holocene.

All samples examined from Unit I contain Globorotalia (Globoconella) inflata, Globorotalia (Truncorotalia) crassaformis, Globorotalia (Truncorotalia) truncatulinoides, and Orbulina universa. Additionally, Globigerina bulloides, Globigerinoides ruber, Globorotalia (Globorotalia) tumida, and Globorotalia (Hirsutella) scitula are present in some samples (Table T4). On the basis of the occurrence of Gr. (T.) truncatulinoides, Sample 330-U1374A-1R-5, 60–62 cm, is correlated to planktonic foraminiferal Zones PL6–PT1b (Pleistocene–Holocene) (Fig. F10).

Unit II

Consolidated volcanic sandstone (Sample 330-U1374A-1R-CC) from Subunit IIA was also analyzed. Although some benthic foraminifers and sea urchin spines were observed, no planktonic foraminiferal tests were found in the washed residue. In addition to these standard analyses, thin sections prepared from consolidated sediment were examined for planktonic foraminiferal content. Of 31 thin sections examined, only 1 thin section from Subunit IIB, 1 from Subunit IID, 2 from Subunit XIB, 2 from Unit XIV, and 1 from Unit XVI contained planktonic foraminifers (Table T6).

Subunit IIB

Sample 330-U1374A-2R-4, 27–32 cm (13.70 mbsf; Subunit IIB), contained Globotruncanita cf. conica, Globotruncanita cf. stuarti, and other planktonic foraminifers with globular and keeled morphologies. Although planktonic foraminifers are not common in thin sections, the preservation of those observed is relatively good (Fig. F11). Planktonic foraminifers are embedded in both the micritic part and the calcite cement. In addition, some gastropods and radiolarians were observed in the same sample. Planktonic foraminifers were also found in the infilling sediment within gastropod fossils, which exhibit a horizontal geopetal structure (see “Sedimentology”). Although zonal marker species were not found, the occurrence of Gl. cf. conica, which has a stratigraphic range from the Gansserina gansseri Zone to the Abathomphalus mayaroensis Zone, indicates that Subunit IIB, including this sample, can be correlated to the late Campanian–Maastrichtian (Caron, 1985). The overlying Subunit IIA contains no planktonic foraminifers or macrofossils, so the age of this subunit cannot be identified.

Subunit IID

Sample 330-U1374A-3R-1, 70–72 cm (15.10 mbsf), in Subunit IID contains Globotruncanella sp., Radotruncana cf. calcarata, and other planktonic foraminifers with globular and keeled morphologies. This sample is composed of limestone (see detailed description below) and contains many macrofossil bioclasts and common planktonic foraminifers in its micritic matrix. Although the foraminiferal specimen shown in Figure F12A is not sectioned in the equatorial plane, the spines at the posterior end of each chamber identify it as R. cf. calcarata. On the basis of this species’ occurrence, the preliminary age of Subunit IID was assigned as late Campanian (75.2–75.7 Ma) (Fig. F10).

Units V, VII, and X

Although some macrofossil bioclasts were found in Units V, VII, and X, no planktonic foraminifers were observed in Units III–X. Samples 330-U1374A-21R-4, 76–80 cm (111.34 mbsf), and 22R-1, 2–6 cm (111.22 mbsf), from Unit XI contain rare planktonic foraminifers with a globular morphology, but no zonal marker species were found (Table T6). Foraminifers with globular and biserial morphologies were also found from Unit XIV. However, most of these foraminifers are dissolved, making taxonomic identification difficult. Nonetheless, a few individuals are relatively well preserved, so these samples will be reanalyzed for further taxonomic and age identification postexpedition.

Macrofossils

In addition to planktonic foraminifers, three ammonoid fossils were discovered in Subunits IIC and IID (Figs. F13, F14, F15). Thin Section 112, taken from Sample 330-U1374A-2R-4, 88–91 cm (14.31 mbsf), in Subunit IIC contains one juvenile ammonoid fossil (Fig. F13). Although its outer shell wall is dissolved, at least two septa were observed. The shell diameter of this individual is 1.1 mm. Because of its size and the divergent septa observed, implying proseptum, this specimen appears to be a hatchling. The second ammonoid fossil from Subunit IIC was found in Sample 3R-1, 23 cm (14.63 mbsf) (Fig. F14), and is preserved underneath a convex-upward bivalve fragment. Three septa and the shell wall of this specimen were visible on the section-half surface. Because this sample was found in the archive half only, further nondestructive analysis will be conducted postexpedition. The third ammonoid fossil was found within the limestone of Subunit IID in Sample 3R-1, 69 cm (15.09 mbsf) (Fig. F15). The diameter of this specimen is estimated to be ~1 cm. Two suture lines were clearly observed on the surface of the working half of the core. The taxonomy of this fossil will be examined postexpedition. The occurrence of ammonoid fossils indicates that Subunits IIC and IID were deposited prior to the end of the Cretaceous, which is consistent with the occurrence of the late Campanian–Maastrichtian foraminifers from Subunit IIB.

Lithostratigraphy and biostratigraphy of Subunits IIC and IID

A complex contact relation was observed between the limestone of Subunit IID (15.05–15.31 mbsf) and the sandstone of Subunit IIC (13.78–15.05 mbsf), which both overlies Subunit IID and also appears in two burrow infills in the limestone (Fig. F16). Within these two subunits both late Maastrichtian nannofossils (Subunit IIC in burrow infill at 15.10 mbsf) and late Campanian planktonic foraminifers (Subunit IID at 15.10 mbsf) were identified. Although these two samples were collected from the same depth, the stratigraphic ranges of these microfossils (Zone CC26 for nannofossils and R. calcarata Zone for planktonic foraminifers) do not overlap (Fig. F17). Subunits IIC and IID are underlain by a basaltic boulder/cobble interval (Subunit IIE) that in some places is coated with relatively thick ferromanganese crusts. On the basis of its texture, color, and chemical composition, the limestone (Ls) in this 11 cm interval can be divided into five sublithologies next to the sandstone (Ss) of Subunit IIC and the basalt cobbles (Ba) of Subunit IIE and manganese crusts (Mn) (Fig. F16). Ls1a consists of fossiliferous micritic limestone from which Thin Section 113 (Sample 330-U1374A-3R-1W, 70–72 cm) was made. Brownish-colored Ls1b, composed of phosphatized limestone, overlies Ls1a. Ls1b is overlain by ferromanganese encrustations with a possible erosional surface, which is overlain by Ls2, including a fragment of the ferromanganese crust and two basalt pebbles. Although not clearly seen, the boundary between Ls2 and overlying Ls3 might again be defined by the ferromanganese encrustations. This limestone sequence from Ls1a–Ls1b to Ls3 is overlain by volcaniclastic sandstone of Subunit IIC at a potential erosional surface. The most interesting feature in this limestone sequence is the quasi-trapezoidal window developed at interval 3R-1, 68–70 cm (Fig. F16). This window is filled with volcaniclastic sandstone of Subunit IIC and Ls4 limestone, and the upper boundary is again encrusted by a thin ferromanganese layer. On the basis of thin section analyses, the lower boundary between Ls1a and Ls4 shows dissolution and a reprecipitation texture, which indicates subaerial dissolution and intertidal cementation (see “Sedimentology”).

The occurrence of R. cf. calcarata from Sample 330-U1374A-3R-1, 70 cm, indicates that the Ls1a limestone was deposited during the late Campanian (75.2–75.7 Ma) (Fig. F17). Although the expected paleolatitude of this seamount is relatively high (likely between 45°–51°S; Koppers et al., 2010), the occurrence of keeled globotruncaniid foraminifers indicates that the generalized tropical-temperate biostratigraphic zonal scheme is applicable to this site (Caron, 1985). Smear slides made from Sample 3R-1-NANNO 1, 70 cm (α in Fig. F16A), in Ls1a (Subunit IID) are barren of nannofossils, but Sample 3R-1-NANNO 2, 70 cm (β in Fig. F16A), in Ss (Subunit IIC) contains N. frequens, indicating nannofossil Zone CC26, which suggests that the volcaniclastic sandstone infills found in the quasi-trapezoidal window were deposited in the latest Maastrichtian (65.0–65.8 Ma) (Figs. F16, F17). Because the first occurrence datum of N. frequens is regarded as globally diachronous, it is correlated to Zones CC25–CC26 in the southern high latitudes (Watkins et al., 1996; Bown, 1998) (Fig. F17). Therefore, the maximum age of the volcaniclastic sandstone infills, together with overlying Subunit IIC, are assigned to the late Maastrichtian (65.0–69.2 Ma). The biostratigraphic age of the Subunit IID limestone and the overlying Subunit IIC volcaniclastic sandstone indicates that the completion of this highly condensed limestone-volcaniclastic cap sequence took 6–9 m.y. (Fig. F17).

At least three eustatic sea level falls were identified in the late Campanian–Maastrichtian at ~72.0, ~68.5, and ~66 Ma (Fig. F17). The potential multiple dissolution cavities or erosional surfaces developed in the Subunit IID limestone may be correlated to these eustatic sea level fluctuations. Considering the significant lithologic change between the Subunit IID limestone sequence and the Subunit IIC volcaniclastic sandstone, the erosional contact between the two may be correlated to the largest sea level fall at ~66 Ma. Precise age estimation and evolution of the depositional environment need to be further analyzed with postexpedition research.

Preliminary age estimation for Site U1374

Unit I of Site U1374 includes material from the Pleistocene–Holocene only (Fig. F10), which differs from the late Miocene–Holocene range of Unit I at Site U1372 on Canopus Guyot (see Fig. F14 in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). Although their ages are slightly different, the sedimentary covers of both seamounts are composed of winnowed foraminiferal ooze with only a small percentage of fine fraction. Preservation of foraminiferal tests and foraminiferal abundance in comparison to the total sediment grains at these two sites are also quite similar, indicating that both sites have diachronously equivalent sequences of pelagic sedimentation.

The age of Subunit IIA at this site is as yet undetermined with microfossil biostratigraphy. However, the lithology and grain assemblage of Subunit IIA are comparable to those of Subunits IIB and IIC, suggesting that Subunits IIC–IIA may represent a successive sequence (see “Sedimentology”). The occurrence of a thick ferromanganese deposit at the top of Subunit IIA (Section 330-U1374A-1R-5, 64 cm) most likely indicates a time gap between unconsolidated Unit I and consolidated Subunit IIA. Because of the large difference in age, a sharp unconformity (possibly <60 m.y.) is likely present between the uncemented pelagic sediment of Unit I and the multicolored volcanic sandstone of Subunit IIA.

Planktonic foraminiferal biostratigraphy indicates that Subunit IIB is correlated to Ga. gansseri–A. mayaroensis Zone (upper Campanian–Maastrichtian). Nonetheless, nannofossil N. frequens, indicative of Zones CC25–CC26 (upper Maastrichtian), was found in underlying Subunit IIC, which indicates that the age of Subunit IIB is correlated to the late Maastrichtian (Fig. F10).

Because the age of the volcaniclastic sandstone of Subunit IIC and the limestone of Subunit IID can be estimated as late Maastrichtian and late Campanian, respectively, a significant time gap was identified between Subunits IIC and IID. Subunit IID is mainly composed of basalt pebbles, pure limestone, phosphatized limestone, and ferromanganese crusts (Fig. F16). The composition of this limestone is quite similar to that of Subunit IIB at Site U1372. Although Subunit IIB at Site U1372 was recovered as fragmented pieces and the Subunit IIB/IIC boundary was not recovered, it is also composed of basalt pebbles, pure limestone, phosphatized limestone, and ferromanganese crusts (see “Sedimentology” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]; see also XL3_EVAL.PDF in XRF in “Supplementary material”). Although the preliminary ages of these limestones differ slightly, the Subunit IID limestone at Site U1374 potentially might be correlated to the Subunit IIB limestone at Site U1372. If this correlation is possible, the phosphatization and ferromanganese encrustation in both limestones might be considered synchronous events. Further discussion of this synchronicity and the cause of the phosphatization and encrustation events requires more precise age determinations and correlations. These analyses will be conducted postexpedition.

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