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doi:10.2204/iodp.proc.320321.108.2010

Biostratigraphy

At Site U1336 we recovered a 300 m thick sequence of middle Miocene to lower Oligocene nannofossil ooze and chalk with thin chert layers toward the base. Calcareous nannofossils are present throughout the succession, and there appears to be a complete sequence of nannofossil zones from Zone NN6 (middle Miocene) through Zone NP22 (lower Oligocene), except for Zone NN3, which could not be resolved. Radiolarians are present in the upper half of the section only, and Zones RN6–RP22 are present. Similarly, diatoms are only found in the upper cores and are poorly preserved. Planktonic foraminifers are present throughout the succession, and Zones N12 (Subzone M9b)–O1 are recognized. Planktonic foraminifers are abundant and moderately well preserved in the Miocene but less well preserved in the Oligocene. No preservational changes were associated with the sediment color shift from brown and very pale brown to light greenish gray. Ostracodes were occasionally observed in samples. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal determinations agree well, and an integrated calcareous and siliceous microfossil biozonation is shown in Figure F10. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F11. Benthic foraminifers are present throughout the section and indicate lower bathyal to abyssal paleodepths.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and core section samples from both holes. Depth positions (CSF) and age estimates of biostratigraphic marker events are presented in Table T3. Nannofossils are abundant and moderately well preserved in the Miocene with intervals of poor preservation. Preservation is poor to moderate through most of the Oligocene.

The uppermost sample investigated (Sample 320-U1336A-1H-1, 110 cm) contains Coronocyclus nitescens and Calcidiscus premacintyrei without Sphenolithus heteromorphus, indicating a Zone NN6 age. Sample 320-U1336B-1H-CC lacks C. nitescens and C. premacintyrei. The top common occurrence of Cyclicargolithus floridanus (lowermost Zone NN6) is recognized in Sample 320-U1336A-2H-6, 80 cm, just above the top S. hetermorphus (base of Zone NN6) in Sample 320-U1336A-3H-2, 90 cm. In Hole U1336B, both events occur within Core 320-U1336B-3H. Nannofossil Zones NN4 and NN5 cannot be differentiated in this succession because of the absence of the zonal marker Helicosphaera ampliaperta; however, the presence of Discoaster petaliformis between Samples 320-U1336A-3H-4, 90 cm, and 5H-4, 64 cm, indicates a position in upper Zones NN4 to NN5 (Young, 1999; Raffi et al., 2006). The base of D. petaliformis has been calibrated at 15.70 Ma at ODP Sites 925 and 926 (as Discoaster signus in Raffi et al., 2006). This event was observed in Sample 320-U1336A-5H-4, 64 cm. The intra–Zone NN4 datum top common Discoaster deflandrei occurs in Sample 320-U1336A-4H-4, 90 cm, at a level that appears to be too high in comparison with surrounding biostratigraphic datums. This may be due to difficulties in distinguishing D. deflandrei from Discoaster exilis in material where discoaster identification is impaired by relatively poor preservation. In Hole U1336B, the top common datum of D. deflandrei is quite distinct because of the lower sample resolution used; it occurs within Core 320-U1336B-5H.

The base of S. heteromorphus occurs in Sample 320-U1336A-7H-2, 50 cm, together with frequent Sphenolithus belemnos in Hole U1336A, and in Core 320-U1336B-7H. Zone NN3 is defined by the extinctions of S. belemnos (top) and Triquetrorhabdulus carinatus (bottom). As these events are only 1.5 m apart (i.e., in adjacent samples), this suggests that Zone NN3 is missing or confined to a condensed interval between Samples 320-U1336A-7H-2, 50 cm, and 7H-3, 53 cm. Alternatively, the ranges of one or both of these taxa may be modified in the equatorial Pacific Ocean.

The intra–Zone NN2 datums, base of S. belemnos and top acme of T. carinatus (Raffi et al., 2006) are recognized in Samples 320-U1336A-8H-CC and 13H-CC, respectively. The lowermost occurrence of Discoaster druggii was observed in Samples 320-U1336A-14H-CC and 320-U1336B-14H-CC. Its rare and discontinuous distribution makes it difficult to exactly place the Zone NN1/NN2 boundary. The base of Sphenolithus disbelemnos occurs in Sample 320-U1336A-15H-CC and Sample 320-U1336B-15H-1, 10 cm. The top and base of S. delphix occurs in Samples 320-U1336A-17H-4, 90 cm, and 17H-6, 90 cm, respectively, although the species is rare and difficult to distinguish in these relatively poorly preserved assemblages. In Hole U1336B, S. delphix occurs from Sample 320-U1336B 17H-1, 150 cm, to 17H-3, 149 cm. The extinction of this species occurs just prior to (<100 k.y.) the Oligocene/Miocene boundary.

The Oligocene succession is predominantly nannofossil chalk with abundant nannofossils but poor to moderate preservation. The top of Zone NP25 is recognized by the top of Sphenolithus ciperoensis in Sample 320-U1336A-20H-CC. The intra–Zone NP24 abundance crossover from Triquetrorhabdulus longus to T. carinatus is difficult to place because of the poor preservation of these nannolith taxa—especially calcitic overgrowth of the slender rodlike taxa T. longus, which makes it indistinct from the thicker T. carinatus—but our initial analysis places it between Samples 320-U1336A-20H-CC and 20H-5, 100 cm, close to the top of S. ciperoensis datum. As at Sites U1332–U1335, the top of Cyclicargolithus abisectus was picked at this site in Sample 320-U1336A-19H-6, 70 cm, at a level consistent with the calibration for the top acme of C. abisectus (also Sites U1332–U1335). In Hole U1336B, this species was only observed in the lowermost core catcher sample (320-U1336B-20H-CC). The top of this species thus appears to have biostratigraphic potential in the paleoequatorial Pacific Ocean. The base of Zone NP25 is placed at the top of Sphenolithus distentus in Sample 320-U1336A-24X-3, 73 cm, and the base of Zone NP24 at the base of S. ciperoensis in Sample 320-U1336A-24X-5, 53 cm. The first appearance of S. distentus occurs within Zone NP23, in Sample 320-U1336A-29X-3, 40 cm. The top of Reticulofenestra umbilicus occurs in Sample 320-U1336A-35X-1, 34 cm, at the base of the sedimentary succession marking the base of Zone NP23.

Radiolarians

Radiolarian stratigraphy at Site U1336 (Table T4) spans the interval from just above the Zone RN6/RN5 boundary (middle Miocene) to the upper part of Zone RP22 (upper Oligocene). Below this level the sediments are barren of radiolarians. Above this level assemblages tend to have good to moderate preservation, with intermittent intervals of good preservation in Zones RN3 and RN4 (lower to middle Miocene). Species abundance and preservation are shown in Tables T5 and T6. One of the unusual aspects of this site was the recovery of well-preserved upper middle Miocene sediments practically at the seafloor. There is no sign of younger sedimentary material and only minor reworking of older microfossils into the sediments of Cores 320-U1336A-1H and 2H. Most of the other sites drilled in the region a few degrees north of the Pacific Equator encounter a layer of barren clay or radiolarian clay of variable thickness at the sediment surface. This clay usually contains radiolarian microfossils of widely ranging ages. The sediment surface at Site U1336 appears swept clean.

The downsection decrease in preservation and ultimate disappearance of the radiolarians below Core 320-U1336A-19H appears to be associated with dissolution and reprecipitation of the biogenic silica as intergranular cement and chert. It has been proposed that the dissolution of biogenic silica is the source of silica forming porcellinite and chert, and on crust <65 Ma in age, almost all cherts in the Pacific lie <150 m above basement (Moore, 2008b). Chert encountered at this site (see "Lithostratigraphy") fits within these criteria. What is unusual about silica dissolution at this site is that it extends so far up into the section. Normally the silica-free zone (the zone of the sediments at the base of a sedimentary section that is devoid of siliceous microfossils) extends only ≤40 m above the basement in Pacific sediments (Moore, 2008a). Here the silica-free zone extends >120 m above basement, something that is usually seen only in sites on the older crust of the western Pacific. Moore (2008a, 2008b) proposes that the dissolution of the silica in basal sedimentary sections is associated with the circulation of warm hydrothermal waters in the upper oceanic crust that extends into the lower sediments when they are cut by permeable fractures and faults. Such small-offset faults appear to be present in the section near this site (Fig. F2). These faults, combined with possible higher heat flow so close to a major fracture zone (located 10 km south of the site), may have aided in a much more pervasive dissolution of the biogenic silica in the basal sediments at Site U1336.

The topmost part of the section corresponds to the lowest part of Zone RN6 (uppermost middle Miocene) that is based on the presence of Diarutus pettersoni and Dorcadospyris alata (Samples 320-U1366A-1H-4, 104–106 cm, and 320-U1336B-1H-CC). Only core catcher samples were investigated in Hole U1336B. Zone RN5 is well distinguished by four datum events (Table T4). The Zone RN5/RN4 boundary occurs between Samples 320-U1336A-4H-2, 105–107 cm, and 4H-4, 105–107 cm, and in Core 320-U1336B-5H. This boundary is defined by the evolutionary transition from Dorcadospyris dentata to D. alata.

In Hole U1336A, the Zone RN4/ RN3 boundary was recognized by the base of Calocycletta (Calocyclissima) costata between Samples 320-U1336A-6H-4, 105–107 cm, and 6H-CC. The base of Zone RN3 is distinguished by the evolutionary appearance of Stychocorys wolffii between Samples 320-U1336A-8H-2, 105–107 cm, and 8H-4, 105–107 cm. The top of Zone RN1, defined by the top of Theocyrtis anossa, occurs between Samples 320-U1336A-12H-2, 105–107 cm, and 12H-4, 105–107 cm, and in Core 320-U1336B-14H. The base of Zone RN1 is determined by the base of Cyrtocapsella tetrapera between Sample 320-U1336A-15H-4, 105–107 cm, and Sample 320-U1336A-15H-CC and in Core 320-U1336B-15H. The lower sections contain Lychnocanomma elongata, indicating Zone RP22. Eight radiolarian events are observed within Zone RP22 in Hole U1336A.

The Oligocene/Miocene boundary occurs between Sample 320-U1336A-16H-CC and Sample 320-U1336A-17H-4, 105–107 cm, as suggested by the presence of Dorcadospyris cyclacantha, which has a short stratigraphic range across the Oligocene/Miocene boundary. It appears at 23.29 Ma and disappears just after the Oligocene/Miocene boundary at 22.98 Ma. In Hole U1336B, D. cyclacantha was observed only in Sample U1336B-17H-CC.

Diatoms

The diatom stratigraphy in Hole U1336B (Table T7) spans the interval from just above the C. peplum zone (middle Miocene) in Core 320-U1336B-1H to the lowermost part of C. nicobarica zone (upper lower Miocene) in Sample 320-U1336B-7H-CC. Below Sample 320-U1336B-7H-CC, the sediments are barren of diatoms. Above this level the valves tend to be mostly poorly preserved. Sample 320-U1336B-1H-CC contains the highest diversity with C. pulchellus as dominant component, accompanied by C. jouseana, and T. yabei. Fragments of the large centric diatoms Ethmodiscus are present in the upper part of Hole U1336B.

Planktonic foraminifers

A high-resolution planktonic foraminifer biostratigraphy was generated at Site U1336 using core catchers and supplementary additional samples, usually two per core. The sedimentary succession at this site ranges from Subzone M9b (Zone N12) (middle Miocene) to Zones O1–O2 (lower Oligocene) (Fig. F10; Table T8), which agrees well with calcareous nannofossil and radiolarian biostratigraphy (Fig. F11). Species abundance and preservation are shown in Tables T9 and T10. The preservation and abundance of planktonic foraminifers is moderate in the upper part of the succession and deteriorates to poor downcore with extensive infilling and encrustation. Preservation of planktonic foraminifer tests is inferior to nearby equatorial Pacific Ocean sites drilled during DSDP Leg 9 (Jenkins and Orr, 1972). Planktonic foraminifer tests can account for >70% of the total residue in each sample, but some intervals are characterized by lower abundance and assemblages are dominated by dissolution-resistant forms (i.e., Samples 320-U1336A-6H-4, 38–40 cm, and 320-U1336B-17H-3, 80–82 cm); in such cases identification of datum events is difficult. As a result of the extensive infilling we applied a very broad definition to Globoquadrina venezuelana, which is dominant to abundant in most samples. Preservation is poor in the lowest part of the sedimentary succession consisting of nannofossil chalk. Planktonic foraminifers are encrusted with calcite in this section and it is difficult to identify and/or estimate the percentage of planktonic foraminifers in each sample because the washing process failed to successfully extract all the foraminifers from the chalk.

A continuous record of the middle to lower Miocene is observed from Subzone M9b (Zone N12) to Subzone M1a. Subzone M9a through Zone M8 (Zone N12) are distinguished on the basis of several datum events within the Globorotalia (Fohsella) fohsi group (Table T8). The base of Globorotalia (Fohsella) fohsi robusta occurred in Samples 320-U1336A-1H-3, 38–40 cm, and 320-U1336B-1H-CC. The Zone M7/M8 (N11/N12) boundary is defined by the base of G. (Fohsella) fohsi sensu lato in Samples 320-U1336A-2H-CC and 320-U1336B-2H-CC. The base of Zone N11 is indicated by the base of Globorotalia (Fohsella) "praefohsi" in Samples 320-U1336A-3H-5, 38–40 cm, and 320-U1336B-3H-CC.

It was not possible to fully constrain the boundary between Zones M6–M7 (N9–N10) because the base of Globorotalia (Fohsella) peripheroacuta (14.24 Ma) on which it is defined appears to occur deeper than the base of Orbulina spp. (14.74 Ma), which separates Zones M6 and M5. Higher resolution samples in Cores 320-U1336A-3H and 4H may be needed in order to fully resolve these zonal boundaries. However, Zone M5 is identified between the base of Orbulina spp. in Samples 320-U1336A-3H-CC and 320-U1336B-3H-CC and the base of Praeorbulina sicana in Samples 320-U1336A-6H-2, 38–40 cm, and 320-U1336B-6H-CC. The rarity of Catapsydrax dissimilis in Hole U1336A made it difficult to distinguish the Zone M3/M4 boundary; however, in Hole U1336B the top of C. dissimilis was constrained to the interval between Samples 320-U1336B-7H-CC and 8H-CC. It was not possible to identify the boundary between Zones M3 and M2 because of the absence of Globigerinatella insueta. The top of P. kugleri defines the top of Subzone M1b and was found between Samples 320-U1336A-10H-CC and 11H-2, 30–32 cm. The base of Subzone M1b was between Samples 320-U1336A-15H-2, 38–40 cm, and 15H-4, 38–40 cm, using the base of Globoquadrina dehiscens. The co-occurrence of P. kugleri, Paragloborotalia pseudokugleri, and G. dehiscens in Sample 320-U1336A-11H-2, 38–40 cm, and Samples 320-U1336B-12-CC, 13-CC, and 14-CC indicate Subzone M1b.

The base of Zone M1 is marked by the base of P. kugleri and is employed to approximate the Oligocene/Miocene boundary (Steininger et al., 1997). It occurs between Samples 320-U1336A-16H-CC and 17H-2, 38–40 cm, and between Samples 320-U1336B-16H-1, 52–54 cm, and 17H-3, 80–82 cm. Only specimens with a subacute periphery and more than six chambers in the final whorl were considered to be P. kugleri (see Pearson and Wade, 2009, for discussion). Sample 320-U1336B-16H-CC contained many reworked specimens of early and middle Oligocene age and could not be used to provide further constraint on the base of Zone M1.

In the upper part of Zone O6, robust dentoglobigerinids and paragloborotaliids are more abundant. The upper part of Zone O6 is indicated by the presence of P. pseudokugleri. The 150–250 µm size fraction contains assemblages abundant and diverse in paragloborotaliids, but they are rare in the >250 µm size fraction. This is consistent with the size change in paragloborotaliids documented at Site 1218 (Wade et al., 2007). Frequent specimens of Dentoglobigerina galavisi, Dentoglobigerina tripartita, G. venezuelana, and Paragloborotalia nana are found in lowermost part of Zone O6. The top of Paragloborotalia opima marks the top of Zone O5, and whereas at previous sites it was not observed, the lower boundary of this zone is fully identified in Hole U1336A by the top of the common occurrence of Chiloguembelina cubensis (28.0 Ma) that occurs between Samples 320-U1336A-25X-2, 45–47 cm, and 25X-CC. The base of Globigerina angulisuturalis defines the base of Zone O4 and occurs between Samples 320-U1336A-27X-CC and 28X-2, 20–22 cm. Zone O3 is partially distinguished by the co-occurrence of Subbotina angiporoides, Turborotalia ampliapertura, and P. opima (i.e., Sample 320-U1336A-29X-CC). Zones O1 and O2 at the base of the succession are not fully constrained, although the lack of P. opima and the presence of P. nana, C. dissimilis, Dentoglobigerina spp., and Turborotalia ampliapertura indicate that the lower Oligocene is preserved. Further age control is difficult because of the poor preservation of the planktonic foraminifers in the deeper part of the section.

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1336A and U1336B. The occurrence of benthic foraminifers at this site is shown in Tables T11 and T12. The sample intervals from Samples 320-U1336A-1H-CC through 35X-CC (8.22–299.61 m CSF) and 320-U1336B-1H-CC through 20H-CC (1.68–174.01 m CSF) contain benthic foraminifers continuously, although abundances are low overall. Samples 320-U1336A-34X-CC (294.10 m CSF), within a chert interval, and 6H-CC (49.87 m CSF), within a radiolarian ooze, contain rare benthic foraminifers. Benthic to planktonic foraminifer ratios are ~1%–2% in Samples 320-U1336B-1H-CC through 16H-CC (8.22–136.86 m CSF), except in Sample 320-U1336B-6H-CC (49.87 m CSF), where the benthic to planktonic foraminifer ratio rises to 25%. In Samples 320-U1336B-16H-CC through 20H-CC (136.81–174.01 m CSF), benthic to planktonic foraminifer ratios increase to 5%–10%. Fish teeth and ostracodes are present in most of the core catcher samples examined. Benthic foraminifer tests are moderately well preserved in the upper part of Site U1336 (Samples 320-U1336A-1H-CC through 19H-CC, 8.22–170.63 m CSF; and 320-U1336B-1H-CC through 20H-CC, 1.68–174.01 m CSF), but preservation deteriorates and tests are frequently abraded and infilled below this level.

The Oligocene to middle Miocene benthic foraminifer assemblage at Site U1336 is relatively diverse and indicates oligotrophic, lower bathyal to abyssal paleodepths. Characteristic taxa are Cibicidoides mundulus, Globocassidulina subglobosa, Gyroidinoides spp., Laticarinina pauperata, Nuttallides umbonifer, Oridorsalis umbonatus, Pullenia bulloides, and Siphonodosaria spp. Brizalina pusilla is abundant (39%) in Sample 320-U1336A-30X-CC (264.48 m CSF). The benthic foraminifer assemblage at Site U1336 closely resembles Oligocene and early Miocene assemblages previously described from the eastern equatorial Pacific Ocean (ODP Site 573, Thomas, 1985; ODP Sites 1218 and 1219, Takata and Nomura, 2005). The assemblage from Site U1336 also shows close affinity to Oligocene and middle Miocene assemblages from Sites U1332–U1334, although N. umbonifer occurs less frequently at Site U1336 than at these other sites.