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doi:10.2204/iodp.proc.320321.103.2010

Biostratigraphy

At Site U1331 we recovered a 187 m thick sequence of upper Oligocene–lower Eocene radiolarian clays, radiolarian clays with nannofossils, chert/porcellanite, and nannofossil ooze covered by Pliocene–Pleistocene clays. Radiolarian clays are the dominant lithology, with significant nannofossil ooze intervals in the Oligocene and lower Eocene. A poorly recovered chert/porcellanite sequence occurs in the uppermost lower Eocene–lower middle Eocene. Radiolarians are present through most of the section and are well preserved in the Eocene. They provide a coherent high-resolution biochronology. Calcareous nannofossils are abundant and poor to moderately well preserved when present. Nannofossil datum and zonal determinations agree well with the radiolarian biostratigraphy; an integrated calcareous and siliceous microfossil biozonation is shown in Figure F12. A detailed age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F13. Both radiolarian and nannofossil assemblages contain reworked older components, especially in the Oligocene section. Despite these allochthonous components, a coherent and ordered stratigraphy is apparent. Planktonic foraminifers are generally absent through most of the sequence but occur in several samples in the Oligocene and lower Eocene ooze lithologies. Benthic foraminifers are sporadically present and indicate lower bathyal to upper abyssal depths.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analyses of core catcher samples from all three holes (U1331A–U1331C) and from samples from each core section in Hole U1331A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5. Nannofossils are abundant in the pale nannofossil oozes of the Oligocene and lower Eocene and are consistently present through significant portions of the middle Eocene (from 75.30 to 96.18 m CSF and 110.50 to 119.58 m CSF in Hole U1331A), where radiolarian ooze with clay lithology dominates. However, barren intervals are frequent through the Eocene section and from 119.90 (Hole U1331A) to 185.32 (Hole U1331C) m CSF nannofossils are completely absent. Where present in the radiolarian clay lithologies the nannofossils are common to abundant, but typically etched, and characterized by abundant disaggregated and/or fragmented placolith shields. Discoasters present in these assemblages appear to be less susceptible to dissolution. In the nannofossil ooze lithology, nannofossil preservation is moderate to poor.

The red clay in the uppermost portion of the section (0–6.3 m CSF) is barren of calcareous nannofossils. The interval from Samples 320-U1331B-1H-5, 100 cm, to 320-U1331A-4H-1, 110 cm (7.00–25.30 m CSF), is carbonate rich, and nannofossil Zones NP23–NP21 are recognized using the base of Sphenolithus distentus in Sample 320-U1331A-2H-4, 70 cm (10.38 m CSF), top of Reticulofenestra umbilicus in Sample 320-U1331A-3H-6, 110 cm (23.29 m CSF), and top of Coccolithus formosus in Sample 320-U1331A-3H-CC (24.34 m CSF). Rare specimens resembling Sphenolithus ciperoensis were observed in Sample 320-U1331B-1H-CC, together with frequent Sphenolithus predistentus and Triquetrorhabdulus longus. We have not, however, applied the base of S. ciperoensis datum because of its absence from samples at similar, or shallower, depths from the top of Holes U1331A and U1331C. The rare occurrence of transitional S. distentus–S. ciperoensis morphologies suggests that the top of the Oligocene nannofossil-bearing lithology is in the vicinity of the Zone NP23/NP24 boundary. Reworked nannofossils of late Eocene to early Oligocene age are conspicuous between Samples 320-U1331A-2H-1, 147 cm (6.66 m CSF), and 2H-3, 70 cm (8.89 m CSF), coincident with coarse fining-upward sequences in the cores.

The interval from Samples 320-U1331A-4H-2, 70 cm (26.43 m CSF), to 9H-2, 60 cm (73.8 m CSF), is predominantly barren of calcareous nannofossils, but limited carbonate-bearing intervals containing nannofossils are present in each core. The tops of Discoaster saipanensis in Sample 320-U1331A-4H-7, 10 cm (33.30 m CSF), Chiasmolithus grandis in Sample 320-U1331A-6H-7, 9 cm (51.79 m CSF), and Chiasmolithus solitus in Sample 320-U1331A-8H-6, 50 cm (70.20 m CSF), and base of Dictyococcites bisectus in Sample 320-U1331A-6H-CC (52.60 m CSF) delineate nannofossil Zones NP22, NP21, and NP17 and an undifferentiated Zone NP18–NP20 interval. The Eocene/Oligocene boundary transition must lie between the top of C. formosus and top of D. saipanensis and is apparently complete at the resolution provided by the nannofossil biostratigraphy. Reworking is commonly found through this interval.

The interval from Samples 320-U1331A-9H-3, 60 cm (75.32 m CSF), to 13H-CC (119.58 m CSF) is largely nannofossiliferous. The base of R. umbilicus in Sample 320-U1331A-11H-4, 30 cm (94.68 m CSF), and top Nannotetrina fulgens in Sample 320-U1331A-13H-5, 70 cm (116.40 m CSF), indicate the presence of nannofossil Zones NP16 and NP15. These datums are further supported by the total range of Discoaster bifax (Samples 320-U1331A-8H-6, 50 cm, through 13H-2, 5 cm). The base and top of D. bifax were used by Bukry (1973) to define Subzone CP14a (along with alternative marker species), but modern calibrations are lacking and so these events were not included in the 320/321 timescale charts.

The interval from Samples 320-U1331A-14H-1, 70 cm (119.90 m CSF), to 320-U1331C-16H-CC (185.32 m CSF) is barren of calcareous nannofossils, although recovery in all three holes was poor between ~160 and 180 m CSF because of the presence of extensive chert/porcellanite horizons. A short interval of white nannofossil ooze, lying directly on basalt, was recovered at the base of Holes U1331A and U1331C (Cores 320-U1331A-22X and 320-U1331C-17H). The top of Tribrachiatus orthostylus, which defines the Zone NP12/NP13 boundary, is located in Sample 320-U1331C-17H-3, 109 cm. This basal nannofossil ooze contains Girgisia gammation, Discoaster lodoensis, and Toweius callosus, indicating a position in uppermost Zone NP12 to lowermost Zone NP13.

Radiolarians

Radiolarians are common to abundant throughout the recovered section, except in the basal carbonate, where they are absent (Table T6, T7, T8). In the upper (Oligocene) part of the section (Cores 320-U1331A-1H through 3H), radiolarians show distinct signs of dissolution and fragmentation. In the Eocene section, however, preservation is good. Even within and below the Eocene chert/porcellanite interval near the base of the section, radiolarians are frequently abundant and well preserved.

The radiolarian stratigraphy and datum levels shown in Table T9 are based on Nigrini et al. (2006). In this work, 12 new species were named and a substantial number of new biostratigraphic datums were correlated with the paleomagnetic stratigraphy of Sites 1218–1220. The application of this stratigraphy to Site U1331 was made difficult by extensive reworking of older radiolarians into younger assemblages. The upper ~10 m of the section is likely to be Quaternary in age, as indicated by the presence of Theocorythium trachelium in Sample 320-U1331A-1H-CC. However, the radiolarian assemblage in these sediments is dominated by species that range from Oligocene to early middle Eocene in age. Below this highly mixed interval, sediments are mid-Oligocene (Zone RP21) but also contain reworked lower Oligocene and Eocene species. To further complicate matters, the marker species for the base of the next younger zone (Lychnocanoma elongata; Zone RP22) is apparently present in the core catcher of Core 320-U1331A-2H but is not found in upper part of this core. The accompanying presence of Lithocyclia angusta and common Dorcadospyris circulus, as well as the absence of Acrocubus octopyle, Calocycletta robusta, and Dorcadospyris scambos, argue for placing the lower part of Core 320-U1331A-2H well within Zone RP21. We believe that this anomalous occurrence of what appears to be L. elongata may be a case of a brief iterative appearance, similar to that seen for A. octopyle and Didymocyrtis tubaria in Leg 199 material. Because of the pervasive mixing, first appearance datums are the more reliable.

The Eocene/Oligocene boundary is probably represented by a hiatus within the upper part of Core 320-U1331A-4H. The change in the assemblage from Zone RP20 in Core 320-U1331A-3H (Oligocene) to Zone RP19 in Core 4H is quite distinct in spite of the reworked older material. All radiolarian zones are represented down to Zone RP12 in Core 320-U1331A-16X; however, the amount of reworked material varies with depth (Fig. F14) as a result of the varying amount of downslope transport of sediments (see "Lithostratigraphy") and the overall sediment accumulation rate. Near basement the reworking of older microfossils is difficult to detect.

Zones RP10 and RP9 were recovered in Cores 320-U1331A-19X, 20X, and 320-U1331C-16H below the chert/porcellanite interval. The Zone RP9/RP8 boundary lies just below Sample 320-U1331C-16H-4, 96–103 cm. Zone RP8 is the oldest radiolarian zone recovered at Site U1331. Based on correlation with paleomagnetic stratigraphy, this zonal boundary lies at ~50.9 Ma (older than indicated in the "Methods" chapter). Based on this correlation, the oldest sediment recovered containing radiolarians must be slightly older than 51 Ma.

Diatoms

Diatoms were examined in core catcher samples from Hole U1331A. Diatoms are typically rare with poor preservation or are absent from the samples examined. The exception is Sample 320-U1331-3H-CC, which contains few diatoms with moderate preservation. The assemblage consists of Coscinodiscus marginatus, Coscinodiscus trochus, and Coscinodiscus excavatus. The occurrence of C. excavatus allows assignment of this sample to the lowermost Oligocene C. excavatus Zone.

Also of note is the interval from Samples 320-U1331A-13H-CC through 16H-CC, which contains rare to common diatoms with poor preservation. Sample 320-U1331A-13H-CC contains rare specimens of C. marginatus and Hemiaulus polymorphus. The occurrences of H. polymorphus and Triceratium schultzii in Sample 320-U1331A-15H-CC suggest an Eocene age for the sample. Sample 15H-CC is of note as the diatoms are common but dominated by a to be determined pennate diatom.

Planktonic foraminifers

Core catchers were sampled from all three holes at Site U1331, and additional samples were taken in Hole U1331A (approximately one per core) from any light-colored sediment intervals, which we assumed had a higher carbonate content. Samples were, for the most part, completely barren of planktonic foraminifers except for brief intervals in the Oligocene and early–middle Eocene when a carbonate-rich lithology was present. Thus, planktonic foraminifers were of limited use in the development of a high-resolution biostratigraphy and for correlation between holes. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T10. Abundances and estimates of preservation are presented in range chart format (Table T11). Where recovered, planktonic foraminifer assemblages are generally dissolved and moderately to poorly preserved, have relatively low species diversities (maximum = ~12 species), and include individuals that show evidence of recrystallization (i.e., are not glassy). The sporadic occurrence of planktonic foraminifers at this site is thought to be the result of dissolution because of the site's position either below or close to the CCD.

In Sample 320-U1331A-2H-4, 49–51 cm, a moderately preserved Oligocene assemblage was recovered containing the marker species Paragloborotalia opima, indicating planktonic foraminifer Zones upper O2 through O5. Further differentiation of biozones between Zones O2 and O5 was hindered by the absence of key taxa (i.e., Chiloguembelina cubensis, Subbotina angiporoides, Turborotalia ampliapertura, and Globigerina angulisuturalis). The long-ranging Eocene–Oligocene taxa Catapsydrax unicavus, Paragloborotalia nana, Turborotalia increbescens, Dentoglobigerina galavisi, and Globoquadrina euapertura are also present. In Sample 320-U1331B-1H-CC, a poorly preserved fauna was recovered containing P. nana, C. unicavus, G. euapertura, and Dentoglobigerina tripartita along with a number of paragloborotaliids, but the lack of age-diagnostic taxa prevented a zonal assignment beyond a very broad estimate, based on taxa ranges, of Zones E13–O6. Nannofossil and radiolarian zonal assignments NP24 and RP21, respectively, suggest that the assemblage in Sample 320-U1331B-1H-CC is upper Oligocene.

Sample 320-U1331A-10H-2, 30–32 cm, contains a highly dissolved middle Eocene assemblage. The assemblage is dominated by robust globigerine forms. The genera Morozovelloides and Globigerinatheka are completely absent. The presence of Turborotalia pomeroli and Acarinina bullbrooki indicate that the assemblage falls within the upper part of planktonic foraminifer Zone E10 or within Zone E11, consistent with age estimates determined by nannofossil (Zone NP16) and radiolarian (Zone RP14) biostratigraphy. Single specimens of Turborotalia frontosa and Dentoglobigerina pseudovenezuelana found in Samples 320-U1331A-9H-CC and 10H-CC, respectively, indicate a broad planktonic foraminifer zonal range E7–E13 (middle Eocene) in keeping with the Zone E10/E11 assignment for Sample 320-U1331A-10H-2, 30–32 cm.

The nannofossil ooze recovered at the base of Hole U1331A in Sample 320-U1331A-22X-CC contains a moderately to poorly preserved assemblage of early Eocene planktonic foraminifers. Species identified in the assemblage include Morozovella aragonensis, Morozovella formosa, Morozovella lensiformis, Morozovella marginodentata, Acarinina pseudosubspherica, Acarinina soldadoensis, Acarinina wilcoxensis, Subbotina patagonica, Globanomalina planoconica, Cibicidoides grimsdalei, and Parasubbotina inaequespira. Based on the presence of M. aragonensis and Morozovella subbotinnae, this assemblage is assigned to planktonic foraminifer Zone E5, consistent with the nannofossil Zone NP12 assignment for this sample. Similarly, at the base of Hole U1331C in Sample 320-U1331C-17H-CC, the basal carbonate contains an early Eocene planktonic foraminifer assemblage of approximately the same age (planktonic foraminifer Zones E4–E5). However, in contrast to Hole U1331A, the planktonic foraminifer assemblage recovered in Sample 320-U1331C-17H-CC is highly dissolved and contains only a small number of whole specimens and a large number of broken planktonic foraminifers, as well as numerous agglutinated and calcareous benthic foraminifers and fish remains (teeth, otholiths, and bone fragments). Molds of planktonic foraminifers are also present but rapidly disintegrate when touched. Fauna is scarce but contains acarininids and subbotinids, whereas morozovellids are almost completely absent. Additional small sediment samples scraped from the surface of Section 320-U1331C-17H-3 between 77 and 105 cm during cleaning of the core surface contain moderately preserved foraminifer assemblages that were used to help further constrain the biostratigraphy. The assemblages are similar to those observed in Sample 320-U1331A-22X-CC (Table T10). Core 320-U1331C-17H is tentatively assigned to planktonic foraminifer Zone E5 based on the presence of M. subbotinae, M. aragonensis, M. formosa, and M. marginodentata and the absence of Subbotina velascoensis and Morozovella aequa. However, only a single highly dissolved specimen of M. aragonensis was identified (in Sample 320-U1331C-17H-CC), and the assemblages in general correspond to Zones E4–E5.

In Hole U1331B, toothpick samples were taken from coarse-grained layers identified in Samples 320-U1331B-1H-5, 116 cm, 1H-7, 72 cm, 10H-2, 9 cm, 11H-2, 89 cm, and 11H-3, 18 cm, to determine the composition of the layers. Samples from coarse layers consistently yielded fish teeth, fish bone, and benthic and planktonic foraminifers in the >250 µm size fraction (with a planktonic:benthic foraminifer ratio of ~1:2). In Samples 320-U1331B-1H-5, 116 cm, and 1H-7, 72 cm, only a few species such as P. nana and C. unicavus were identified, indicating a middle Eocene through Oligocene age. In Samples 320-U1331B-10H-2, 9 cm, 11H-2, 89 cm, and 11H-3, 18 cm, the presence of P. nana, C. unicavus, Turborotalia possagnoensis, and Globigerinatheka index indicates planktonic foraminifer Zones E11–E15, a range consistent with nannofossil and radiolarian biostratigraphic determinations. The ratio of planktonic to benthic foraminifers was higher in the <250 µm size fraction than in the >250 µm size fraction.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in all three holes of Site U1331. Benthic foraminifers at this site were sporadically present, but they were common in several samples (e.g., 320-U1331A-2H-CC, 10H-CC, and 22H-CC). Preservation of foraminifer tests was generally moderate to poor. The occurrence of benthic foraminifers at this site is shown in Table T12.

The uppermost sample in Hole U1331A (Sample 320-U1331A-1H-CC; 5.14 m CSF) contains no benthic foraminifers. In Sample 320-U1331A-2H-CC (15.02 m CSF), Oridorsalis umbonatus, Nuttallides umbonifera, Cibicidoides mundulus, and Globocassidulina subglobosa are common, and Cibicidoides havanensis and C. grimsdalei are subordinate. A similar fauna was found in Samples 320-U1331B-1H-CC, 2H-CC, and 320-U1331C-1H-CC. Preservation of foraminifer tests is good to moderate. These Oligocene taxa indicate lower bathyal and abyssal paleodepths (van Morkhoven et al., 1986).

Samples 320-U1331A-3H-CC through 19X-CC (24.34–162.36 m CSF) contained only sporadic benthic foraminifers. Agglutinated forms, such as Rhizammina spp., Spiroplectammina spectabilis, and Paratrochamminoides spp., are found with some poorly preserved calcareous forms. Calcareous forms identified include O. umbonatus, Nuttallides truempyi, Cibicidoides eoceanus, and C. grimsdalei and are found in Samples 320-U1331A-10H-CC (90.51 m CSF), 320-U1331B-8H-CC (77.19 m CSF), 9H-CC (81.61 m CSF), and 320-U1331C-10H-CC (110.92 m CSF). Preservation of these foraminifer tests is moderate. These faunas suggest lower bathyal to abyssal paleodepth at this site in the middle Eocene.

Similar faunas were also found in Samples 320-U1331A-10H-7, 44–46 cm, and 10H-7, 47–49 cm (scrape samples), taken from a coarse-grained bed. N. truempyi, O. umbonatus, and C. grimsdalei are common, and G. subglobosa, Gyroidinoides spp., Anomalinoides sp., and Alabamina sp. are subordinate. There is no marked difference in the faunal association between these scrape and core catcher samples. Qualitative observation suggests that the test size of benthic foraminifers in Sample 320-U1331A-10H-7, 47–49 cm, is slightly larger than that observed in Sample 10H-7, 44–46 cm. In addition, planktonic foraminifers were more abundant in these coarse-grained layers compared to other Eocene samples. The ratio of planktonic/planktonic + benthic foraminifers increased uphole (from 14 to 31) through this interval. These features suggest that calcareous foraminifers might have been transported by a turbidity current to their current location and were preserved by rapid burial, but there is no evidence that the transport is from a significantly shallower depth zone.

Early Eocene Sample 320-U1331A-22H-CC (187.18 m CSF) contains rare O. umbonatus and N. truempyi. In contrast, Sample 320-U1331C-17H-CC (189.51 m CSF) contains common N. truempyi and Gyroidinoides spp. with G. subglobosa and O. umbonatus subordinate. Calcite overgrowths were commonly observed on the test surface of calcareous forms. These taxa suggest a lower bathyal to abyssal paleodepth.

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