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doi:10.2204/iodp.proc.320321.104.2010

Biostratigraphy

At Site U1332, we recovered a 148 m thick sequence of lower Miocene–lower middle Eocene radiolarian ooze, radiolarian clays with nannofossils, nannofossil ooze, and chert/porcellanite. The uppermost 10 m of clay is barren of calcareous microfossils and contains no age-diagnostic radiolarians. Nannofossil ooze is dominant in the Oligocene, and radiolarian ooze and clay are dominant in the Miocene and Eocene. A poorly recovered chert/porcellanite-rich sequence occurs in the 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 moderately well preserved in the Oligocene and poor to moderately well preserved in the Miocene and Eocene. Nannofossil datums and zonal determinations agree well with the radiolarian biostratigraphy; an integrated calcareous and siliceous microfossil biozonation is shown in Figure F10. A detailed age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F11. Planktonic foraminifers are rare through the Oligocene and absent in the Miocene and Eocene. Benthic foraminifers are present through most of the section but are rare in all but the Oligocene lithologies. They indicate lower bathyal to abyssal paleodepths.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples from all three holes and from additional samples from each core section, predominantly from Hole U1332A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Nannofossils are abundant in the nannofossil oozes of the Oligocene and are consistently present through the Eocene, excepting short barren intervals in Cores 320-U1332A-9H through 12H and 15X through 17X, where radiolarian ooze with clay lithology dominates. When present in radiolarian clays and basal dark brown clays, nannofossils are common to abundant, but typically etched, and characterized by abundant disaggregated and/or fragmented placolith shields. Discoasters are much less affected by etching and are virtually the only nannofossils present in several lower middle Eocene and lower Miocene samples. In the nannofossil ooze lithology, preservation is moderately good.

The clay of Unit I in the uppermost portion of the section (0–17.4 m CSF) is barren of calcareous nannofossils. The interval from Samples 320-U1332A-3H-4, 100 cm, to 4H-CC (18.90–32.95 m CSF) yields low diversity and relatively poorly preserved nannofossil assemblages dominated by Discoaster deflandrei and Triquetrorhabdulus carinatus. The presence of rare Sphenolithus delphix in Sample 320-U1332A-3H-CC (23.51 m CSF) is indicative of a short interval (23.1–23.2 Ma) within Zone NN1, very close to the Oligocene/Miocene boundary.

The upper Oligocene interval yields low diversity nannofossil assemblages, and the most distinct bioevent is the top of Sphenolithus predistentus in Sample 320-U1332A-5H-1, 80 cm (33.20 m CSF). The marker species Sphenolithus ciperoensis is rare and sporadically distributed through much of the upper Oligocene, so the Zone NP24/NN1 boundary cannot be determined. The crossover from Triquetrorhabdulus longus to T. carinatus is an intra–Zone NP25 event (24.7 Ma) and occurs between Samples 320-U1332A-4H-2, 60 cm, and 4H-3, 60 cm (25.0 and 26.50 m CSF). Very small specimens (<4 ┬Ám) of S. ciperoensis occur alongside rare Sphenolithus distentus between Samples 320-U1332A-5H-1, 80 cm, and 6H-3, 70 cm, but the two species cannot be reliably distinguished because of small size and the occurrence of intermediate morphologies. As a result, we have not differentiated Zones NP24 and NP23. The base of S. distentus is an intra–Zone NP23 datum (30 Ma) and occurs in Sample 320-U1332A-6H-5, 70 cm (48.60 m CSF).

The lower Oligocene Zones NP23 and NP22 are determined by the top of Reticulofenestra umbilicus in Sample 320-U1332A-7H-7, 80 cm (61.20 m CSF), and the top of Coccolithus formosus in Sample 320-U1332A-8H-5, 50 cm (67.40 m CSF). The Eocene/Oligocene boundary lies between the top of C. formosus and the top of Discoaster saipanensis, which occurs in Sample 320-U1332A-10H-3, 80 cm (83.70 m CSF). The boundary is apparently complete at the resolution provided by the nannofossil biostratigraphy. This interval is associated with a lithologic change from pale nannofossil ooze to brown radiolarian clay.

The Eocene nannofossil Zones NP18–NP20 through NP14 are recognized using the top of Chiasmolithus grandis in Sample 320-U1332A-11H-6, 70 cm (97.60 m CSF); top of Chiasmolithus solitus in Sample 320-U1332A-13H-1, 140 cm (109.80 m CSF); the total range of Nannotetrina fulgens from Samples 320-U1332A-15X-2, 137 cm (128.77 m CSF), to 16X-2, 39 cm (137.39 m CSF); and the presence of Discoaster sublodoensis in Sample 320-U1332B-18X-CC (146.21 m CSF). The base of Dictyococcites bisectus, total range of Discoaster bifax, top and base of Nannotetrina, and top of Discoaster lodoensis datums were also useful in supporting these zonal determinations.

The dark brown clays resting on basalt in Hole U1332A are mostly barren of nannofossils, but several samples from Holes U1332B and U1332C contain age-diagnostic taxa indicative of Zone NP14. Samples 320-U1332B-18X-2, 48 cm (145.88 m CSF), and 18X-CC (146.21 m CSF) contain dissolution-affected assemblages that nevertheless contain common D. lodoensis and rare D. sublodoensis. In Hole U1332C, Sample 320-U1332C-18X-CC (147.25 m CSF) contains an etched assemblage without D. lodoensis but with rare Nannotetrina. These observations within a succession that was poorly recovered and which is largely barren of nannofossils suggests that the top of D. lodoensis and base of Nannotetrina datums occur within these lowermost sediments, indicating an age between 48.0 and 48.4 Ma.

Radiolarians

Radiolarian stratigraphy at Site U1332 spans the interval between Zone RN1 (base of the lower Miocene) and the upper part of Zone RP13 (middle Eocene) (Tables T4, T5, T6). At the top of the section the first two cores are barren of radiolarians (Table T7). The third core (Sample 320-U1332A-3H, 93–95 cm) contains a highly mixed assemblage ranging in age from middle Eocene through Miocene. The youngest species found in this mixture (Theocorythium vetulum) has a first appearance at ~7 Ma. This sample is probably from the upper Miocene but could be mixed with still younger, nonfossiliferous sediments. Preservation of the lower Miocene and Oligocene assemblages is generally poor to moderate, with the common occurrence of reworked, older microfossils.

Preservation improves somewhat in the lower Oligocene (Zone RP20); however, near the base of the Oligocene the sequence of first and last appearances of species within Zone RP20 seems to be repeated in the lower part of Core 320-U1332A-8H and the upper part of Core 9H. The base of Core 9H (Sample 320-U1332A-9H, 92–98 cm) is within the upper Eocene Zone RP19. There is substantial reworking of older middle Eocene microfossils in the Eocene part of Core 9H extending down into Cores 320-U1332A-10H and 11H. Radiolarians are generally abundant and well preserved throughout the Eocene section.

The oldest radiolarian-bearing sediments are found in Cores 320-U1332C-16X and 17X (Zone RP13). The lowermost cores in all three holes at this site are barren of radiolarians.

Diatoms

Diatoms were examined in core catcher samples from Holes U1332A–U1332C. The examined sequence represents the interval extending from the Bogorovia veniamini Zone through the Coscinodiscus excavatus Zone of Barron (1985, 2006) and Barron et al. (2004). Diatoms range in abundance from rare to abundant. Diatom preservation is variable but is generally poor to moderate. The intervals from Cores 320-U1332A-1H and 2H, 320-U1332B-1H and 2H, and U1332C-1H are barren of or contain rare diatoms. The interval is unzoned.

The interval from Samples 320-U1332A-3H-CC through 5H-CC is assigned to the B. veniamini Zone based on the occurrence of B. veniamini in these samples without Rocella gelida. Supporting these zonal assignments is the occurrence of Cavitatus jouseanus and Rocella vigilans in Sample 320-U1332A-5H-2, 115–116 cm; the occurrence of Rossiella symmetrica in Sample 320-U1332A-4H-4, 110–111 cm; and the occurrences of Cestodiscus kugleri, R. vigilans, and C. jouseanus in Samples 320-U1332A-4H-CC and 320-U1332C-5H-CC.

Samples 320-U1332A-6H-CC and 320-U1332C-6H-CC are assigned to the R. vigilans Zone based on the occurrence of R. vigilans without B. veniamini. The occurrence of Kozloviella minor and C. jouseanus in Sample 320-U1332C-6H-CC suggests placement of this sample into Subzone C of this zone, but such a zonal assignment is tentative given the typical poor state of diatom preservation. The Cestodiscus trochus Zone was not recognized because of sample spacing and/or preservation.

The C. excavatus Zone is recognized from Samples 320-U1332A-7H-4, 110–111 cm, through 8H-CC based on the occurrence of C. excavates in this interval. The occurrence of C. trochus and Cestodiscus robustus in occasional samples through this interval support this zonal assignment.

Diatoms are typically rare or absent from samples below Core 320-U1332A-8H. The exceptions are Samples 320-U1332A-13H-2, 100–101 cm, and 320-U1332C-13H-CC, which contain few diatoms with poor preservation. The assemblage is representative of the middle Eocene. Sample 320-U1332A-13H-2, 100–101 cm, contains specimens of Triceratium brachitium and Hemiaulus spp. Sample 320-U1332C-13H-CC contains specimens of Triceratium inconspicum and Hemiaulus spp.

Planktonic foraminifers

Core catcher samples were analyzed from all three holes, and additional samples were taken in Hole U1332A (two per core) from any light-colored sediment intervals, which we assumed had a higher carbonate content. Planktonic foraminifers are absent from the Miocene and Eocene sediments but are consistently present in the Oligocene (from Zone O6 to the latest Eocene/earliest Oligocene); however, distinction of zones between Zone O2 and the late Eocene was hindered by the absence of age-diagnostic taxa of Hantkenina spp., Turborotalia cerroazuloensis, and Pseudohastigerina naguewichensis. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T8. Taxon ranges and abundances are shown in Table T9. Planktonic foraminifer assemblages show good to moderate preservation in the upper Oligocene, but both preservation and abundance of planktonic foraminifers decrease downcore. We note that a higher diversity of Oligocene taxa is recorded at Site U1332 than at Site U1331, ~20 versus 12 species, respectively, which is consistent with the better preservation of fauna observed at this site (see Table T9).

The lowermost part of planktonic foraminifer Zone O6 was inferred between Samples 320-U1332A-3H-7, 75–77 cm, and 4H-7, 38–40 cm, and in Sample 320-U1332B-5H-CC based on the presence of a number of different Dentoglobigerina spp., which range to the latest Oligocene (Olsson et al., 2006), and the absence of Paragloborotalia opima, Paragloborotalia kugleri, and Paragloborotalia pseudokugleri. A Zone O6 assignment for these samples is also consistent with nannofossil (Zone NP24) and radiolarian (Zone RP21) determinations. The presence of the biostratigraphic marker species P. opima indicates the presence of planktonic foraminifer Zones O2–O5 between Samples 320-U1332A-5H-2, 100–102 cm, and 6H-CC, in Sample 320-U1332B-6H-CC, and in Samples 320-U1332C-4H-CC through 5H-CC. The general absence of age-diagnostic Oligocene taxa, assumed to be dissolution susceptible based on their general absence here and during biostratigraphic investigations during ODP Leg 199 (Shipboard Scientific Party, 2002a), hindered further differentiation of the Oligocene. An exception is in Sample 320-U1332C-5H-CC where Globigerina angulisuturalis and P. opima were both found, which allowed the identification of planktonic foraminifer Zones O4 and O5. However, the absence of Chiloguembelina cubensis prevented further differentiation between Zones O4 and O5. Typical Oligocene taxa identified in samples were Catapsydrax dissimilis, Catapsydrax unicavus, Dentoglobigerina tripartita, Dentoglobigerina galavisi, Dentoglobigerina pseudovenezuelana, Globoquadrina euapertura, Globoquadrina venezuelana, Paragloborotalia nana, Subbotina angiporoides, Subbotina utilisindex, and Turborotalia increbescens. Below Samples 320-U1332A-6H-CC, 320-U1332B-7H-CC, and 320-U1332C-6H-CC, the low abundance of planktonic foraminifers coupled with the absence of age-diagnostic taxa meant that the assemblage could only be assigned to a broad zonal range (e.g., Zones E13–O2).

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in three Site U1332 holes. Benthic foraminifers occurred continuously in calcareous nannofossil ooze of the Oligocene, whereas they were generally rare in radiolarian ooze of the Eocene. Occurrence of benthic foraminifers at this site is shown in Table T10.

The upper two samples in Hole U1332A (320-U1332A-1H-CC and 2H-CC; 3.86–13.62 m CSF) did not contain benthic foraminifers. In the interval from Samples 320-U1332A-3H-CC through 7H-CC (23.51–61.49 m CSF), Nuttallides umbonifer, Oridorsalis umbonatus, Cibicidoides mundulus, Globocassidulina subglobosa, and Gyroidinoides spp. were common and Cibicidoides havanensis and Cibidoides grimsdalei were subordinate. O. umbonatus and Cibicidoides spp. were generally common in the lower part of the interval (maximum = 24% and 16%, respectively), whereas N. umbonifer was abundant in the upper part of the interval (maximum = 22%). A similar faunal transition was recognized in Hole U1332B (Samples 320-U1332B-4H-CC through 8H-CC; 29.61–67.47 m CSF) and Hole U1332C (Samples 320-U1332C-4H-CC through 9H-CC; 36.52–75.91 m CSF). In addition, tube-shaped agglutinated forms (e.g., Rhizammina spp.) and Reophax spp. were sometimes abundant (maximum = 47% and 13%, respectively) in the uppermost part of the interval (e.g., Sample 320-U1332B-3H-CC; 19.97 m CSF). Preservation of foraminifer tests is very good to good. These faunal compositions indicate lower bathyal and abyssal paleodepths during the Oligocene, based on van Morkhoven et al. (1986). The Oligocene fauna are characterized by abundant calcareous hyaline forms, such as N. umbonifer, O. umbonatus, C. mundulus, G. subglobosa, and Gyroidinoides spp., and are similar to those observed in previous studies in the eastern equatorial Pacific (Site 573, Thomas, 1985; Sites 1218 and 1219, Nomura and Takata, 2005). However, assemblages dominated by agglutinated foraminifers occur much earlier (late Oligocene) at this site than at Sites 1218 and 1219. This temporal offset may be represent a preservational bias caused by the greater water depth at this site than those found at Sites 1218 and 1219.

Samples 320-U1332A-10H-CC through 15X-CC (89.89–132.93 m CSF) rarely contain benthic foraminifers. Agglutinated forms, such as Rhizammina spp. and Spiroplectammina spectabilis, were found with some calcareous hyaline taxa (e.g., Siphonodosaria antillea). Preservation of these tests was poor. Similar occurrences were also recognized in Samples 320-U1332A-9H-CC through 17X-CC (77.09–135.06 m CSF) and in Samples 320-U1332A-10H-CC through 17X-CC (85.53–139.91 m CSF). Calcareous hyaline forms, such as O. umbonatus, Nuttallides truempyi, Cibicidoides eocanus, and C. grimsdalei, were present at least in two horizons, Samples 320-U1332A-11H-CC (98.52 m CSF) and 13H-CC (118.38 m CSF). Similar occurrences were also recognized in Samples 320-U1332B-13H-CC (110.76 m CSF) and 10H-CC through 17X-CC (85.53–139.91 m CSF). However, preservation of calcareous foraminifer tests was poor in the lower part of the study interval. These fauna suggest lower bathyal to abyssal paleodepth at this site in the middle to late Eocene. Faunal associations of these calcareous taxa in the middle to late Eocene are similar to those observed at Site U1331 and previous preliminary studies in the eastern equatorial Pacific (Shipboard Scientific Party, 2002a). Common occurrences of these calcareous foraminifers in Hole U1332A roughly coincide with high-carbonate intervals (see "Lithostratigraphy" and "Geochemistry") that may be related to carbonate accumulation events noted by Lyle et al. (2005).