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doi:10.2204/iodp.proc.317.105.2011

Biostratigraphy

Calcareous nannofossils, planktonic and benthic foraminifers, and diatoms from core catcher samples were used to create a shipboard biostratigraphic framework for Site U1353 (Fig. F15). Benthic foraminifers were also used to estimate paleowater depths. All depths in this section are reported in m CSF-A.

Site U1353 contained a Holocene to Miocene succession. Thirteen biostratigraphic events were recognized, mostly in the Pleistocene (Table T3). Pleistocene nannofossil abundances were high, and good preservation allowed for robust age control. Diatoms were sparse to absent at this site. A hiatus was recognized within the middle-early Pleistocene between Samples 317-U1353B-12H-CC and 14X-CC (80.12–80.77 m), where ~0.8 m.y. was missing.

The Pliocene/Pleistocene boundary was biostratigraphically picked between Samples 317-U1353B-21H-CC and 23H-CC (121.16–135.71 m), where part, if not all, of the upper Pliocene was missing. Biostratigraphic analysis of Site U1353 Pliocene and Miocene sediments was problematic for all microfossil groups because of either low abundances and/or the absence of key biostratigraphic markers. Below Sample 317-U1353B-60H-CC (257.69 m), nannofossil abundances dropped sharply and remained low for the rest of the downhole succession. Planktonic foraminifers were absent in most samples below this level. Shelfal benthic foraminifers were present in abundance throughout this succession but lacked reliable age markers.

Nevertheless, several important datums allowed for biostratigraphic constraint and critical correlation with Sites U1351 and U1352. The boundary between the middle and early Pliocene was recognized on the basis of a nannofossil datum between Samples 317-U1352B-27H-CC and 28H-1, 124 cm (149.68–150.64 m). The presence of an age-diagnostic planktonic foraminifer lower in the succession constrained the interval between 150.64 and 256.04 m to the early Pliocene (3.7–4.3 Ma). The Pliocene/Miocene boundary was not picked biostratigraphically.

Although there was no biostratigraphic evidence for late Miocene sediments at Site U1353, nearby Site U1351 contained an expanded late Miocene section, and seismic mapping supports the presence of a similar thick late Miocene interval at Site U1353. A nannofossil marker in Samples 317-U1353B-89X-CC and 90X-CC (518.66 and 528.87 m, respectively), dated at older than 12.03 Ma, suggested there is a substantial hiatus between Samples 88X-CC and 89X-CC (510.52–518.66 m), although the amount of time missing is unknown. Samples below 90X-CC were barren of calcareous nannofossils and planktonic foraminifers, except in the bottommost core catcher (Sample 98X-CC [604.60 m]), which contained an early to middle Miocene nannofossil assemblage. The reliability of this age is unknown because nannofossil reworking was common at other nearby sites and there was no other microfossil evidence to support the age.

Paleowater depths derived from benthic foraminifers ranged from subtidal to outer shelf throughout the Holocene–Miocene section. Pleistocene water depths generally fluctuated between subtidal to middle shelf, although a deepening to outer shelfal depths (correlated to the interval just above the middle-early Pleistocene hiatus) was noted in Samples 317-U1353B-10H-CC and 11H-CC (67.50–73.19 m). Pliocene water depths were generally subtidal to inner shelf but sometimes reached outer shelf depths in the early Pliocene. Middle-early Miocene water depths could not be interpreted reliably because of the low number of benthic foraminifers, but rare outer shelf and bathyal marker species were noted in some samples.

Calcareous nannofossils

Nannofossil abundances at Site U1353 ranged from barren to very abundant. Considerably more samples were barren at this site compared to outer shelf Site U1351 (Table T4). Preservation was variable and ranged from poor to good. Calcareous nannofossils provided reliable age control throughout the Pleistocene (Table T3). Below the Pleistocene, as at Sites U1351 and U1352, biostratigraphic markers used in mid- to low-latitude Neogene nannofossil zonations were absent (except Reticulofenestra pseudoumbilicus). Other species associated with the Reticulofenestra lineage were abundant and provided rough age control for the Pliocene section. Pliocene assemblages at Site U1353 were markedly different from those at Site U1351. Most notable was the absence of the middle to early Miocene and Oligocene reworking that was common in the early Pliocene–late Miocene section at Site U1351. This has significant implications for basinward and shelf-edge parallel sediment transport.

Evidence of sporadic cave-in was found between Samples 317-U1353B-17H-CC and 65X-CC (106.80–290.38 m), most often recognized lithologically as downhole contamination of an upper shell-hash layer. Mud within this shell-hash layer contained Holocene–late Pleistocene species, most notably abundant Emiliania huxleyi. Core catcher samples containing caved or mixed assemblages are identified in the comments column in Table T4. To account for suspected downhole contamination, some events were recorded between several core catcher samples. Postcruise analysis of infill samples may better constrain these events.

Relative abundance counts were not carried out on six samples (317-U1353B-35H-CC, 44X-CC, 45H-CC, 46H-CC, 47H-CC, and 51H-CC) because the core catchers were composed of shell hash and a cursory examination of these samples revealed caved assemblages.

Holocene–Pleistocene

Although not identified biostratigraphically, the base of the Holocene was tentatively correlated with a distinct lithologic boundary at Section 317-U1353B-2H-2, 43 cm (8.43 m), where greenish gray marly sands overlie gray calcareous muds. Nannofossil abundances were highly variable in Pleistocene samples and ranged from rare to very abundant. Preservation was generally good, although a few assemblages were moderately preserved.

Samples 317-U1353A-1H-CC through 4H-CC (5.44–26.72 m) were zoned in NN21. The base of this zone was defined by the lowest occurrence (LO) of Emiliania huxleyi (0.29 Ma) between Samples 4H-CC and 5H-CC (26.72–34.08 m). This datum was correlated with the same stratigraphic level in Hole U1353B between Samples 317-U1353B-3H-CC and 4H-CC (26.06–27.67 m). Samples between 317-U1353A-5H-CC and 8H-CC (34.08–55.97 m) and 317-U1353B-4H-CC and 10H-CC (27.67–67.50 m) were zoned in NN20 (0.29–0.44 Ma), a gap zone between the LO of E. huxleyi and the highest occurrence (HO) of Pseudoemiliania lacunosa.

The HO of P. lacunosa (0.44 Ma; top of Zone NN19) was identified between Samples 317-U1353B-10H-CC and 11H-CC (67.50–73.19 m). Two datums were observed below this interval between Samples 12H-CC and 14X-CC (80.12–80.77 m): the LO of Gephyrocapsa omega (1.01 Ma) and the HO of Gephyrocapsa spp. >5.5 µm (1.26 Ma). The close proximity of these datums to the top of Zone NN19 suggests that there is a hiatus of ~0.8 m.y, although more refined postcruise analysis will be necessary to confirm this.

The HO of Helicosphaera sellii (1.34 Ma) was constrained between Samples 317-U1353B-14X-CC and 16X-CC (80.77–97.20 m). The LOs of Gephyrocapsa spp. >5.5 µm (1.56 Ma) and Gephyrocapsa spp. >4 µm (1.69 Ma) were identified between Samples 16X-CC and 18X-CC (97.20–108.44 m); however, because of the broad sample interval, it was difficult to interpret whether the close proximity of these datums represented a hiatus or a condensed section. The LO of Gephyrocapsa caribbeanica (1.73 Ma) was identified between Samples 20H-1, 107 cm, and 21H-CC (118.27–121.16 m).

Pliocene

The Pliocene/Pleistocene boundary was biostratigraphically picked between Samples 317-U1353B-21H-CC and 23H-CC (121.16–135.71 m). Sample 23H-CC contained specimens of Reticulofenestra ampla, whose HO is dated at 2.78 Ma (Kameo and Bralower, 2000), suggesting an unconformable boundary and the absence of upper Pliocene sediments. Sporadic occurrences of R. ampla were seen in the middle to lower Pleistocene succession but were interpreted as reworked because they occurred in samples with other material that was clearly identified as having been reworked. Below 135.71 m, R. ampla was ubiquitous.

Nannofossil abundances in the Pliocene section ranged from barren to few, and preservation ranged from poor to good. Biostratigraphic analyses of the Pliocene and Miocene sections from Site U1353 was problematic for all microfossil groups because of low abundances and/or the absence of age-diagnostic biostratigraphic markers. In addition, several Pliocene samples contained younger caved-in material (Table T4).

The HO of Reticulofenestra pseudoumbilicus (3.70 Ma) was observed between Samples 317-U1353B-27H-CC and 28H-1, 124 cm (149.68–150.64 m). Helicosphaera sellii and H. cf. sellii were consistently present between Samples 55H-CC and 65X-CC (250.14–290.38 m), suggesting that Samples 28H-1, 124 cm, through 65X-CC (150.64–290.38 m) are most likely correlated with Zones NN15–NN12.

Of particular note is Sample 317-U1353B-56H-CC (251.63 m), which contained very abundant calcareous nannofossils. The high abundance of all calcareous microfossil groups, especially planktonic forms, suggests that the sample may represent a condensed high-stand deposit. A similar peak in planktonic and benthic foraminifer abundance was noted in the same sample. Below this level, the abundances of all microfossil groups were very low for the remainder of the cored section.

The interval between Samples 317-U1353B-66X-CC and 88X-CC (300.33–510.52 m) was dated early Pliocene to late Miocene, although the Miocene/Pliocene boundary could not be picked biostratigraphically. The assigned microfossil age was also supported by the presence of the gastropod Maorimactra ?chrydaea (Suter) in Sample 85X-CC (480.64 m). It is a marker species that is younger than 6.5 Ma.

Miocene

Miocene nannofossil abundances were variable, and, apart from four barren samples, abundances generally increased downhole and were common in the lowermost Samples 317-U1353B-97X-CC and 98X-CC (595.20 and 604.60 m, respectively).

Cyclicargolithus floridanus was observed in Samples 317-U1353B-89X-CC (518.66 m) and 90X-CC (528.87 m), indicating an age older than 12.03 Ma (Raffi et al., 2006). This suggests a substantial hiatus between Samples 88X-CC and 89X-CC (510.52–518.66 m), although the amount of time missing is unknown. Based on the presence of several specimens of Helicosphaera vedderi, the lowermost cored sample (317-U1353B-98X-CC [604.60 m]) has a middle-late early Miocene age.

Planktonic foraminifers

Holocene to Miocene planktonic foraminiferal biostratigraphy for Site U1353 was based on the examination of core catcher samples from Holes U1353A and U1353B (Tables T5, T6, T7, T8). Absolute ages assigned to biostratigraphic datums follow the references in Table T3 in the "Methods" chapter. Planktonic foraminifers were present in most samples in the Holocene–Pliocene succession and were generally well preserved, but samples in the lower part of the cored section were generally barren. For planktonic foraminiferal abundances and our interpretation of oceanicity, see Tables T5 and T6 and Figure F16.

Holocene

Although not identified biostratigraphically, the base of the Holocene was tentatively correlated with a distinct lithologic boundary at Section 317-U1353B-2H-2, 43 cm (8.43 m), a lithologic contrast where greenish gray marly sands overlie gray calcareous muds. Samples from this interval, including mudline Samples 317-U1353A-1H-1, 0–1 cm (0.01 m), and 317-U1353B-1H-1, 0–1 cm (0.01 m), and Samples 317-U1353A-1H-CC (5.44 m) and 317-U1353B-1H-CC (7.94 m), contained temperate planktonic foraminiferal assemblages consistent with present-day assemblages at the site. The underlying Sample 317-U1353A-2H-CC (9.18 m) had fewer temperate planktonic indicators and contained a high-energy, shallow inner shelf benthic foraminiferal fauna, a shallow-water mollusk assemblage, and garnet-bearing sand, which suggest a lowstand near-shore deposit.

Pleistocene

Planktonic foraminiferal assemblages in the Pleistocene sections of Holes U1353A and U1353B were characterized by small, thin-walled neritic forms. Planktonic abundance and diversity were very low (<5%) in the uppermost part of the Pleistocene succession between Samples 317-U1353A-3H-CC and 4H-CC (17.95–26.72 m) and 317-U1353B-2H-CC and 5H-CC (16.90–35.53 m), indicating deposition occurred under sheltered neritic conditions. Abundances were slightly higher in Samples 317-U1353A-5H-CC through 8H-CC (34.08–55.97 m) and 317-U1353B-6H-CC through 8H-CC (43.15–56.84 m), reaching a maximum of 31% in Sample 317-U1353B-7H-CC (48.91 m), where deposition occurred under extraneritic conditions. This was interpreted as a possible highstand. In the lowermost part of the Pleistocene section below this level, abundances were generally <5%, indicating that deposition occurred under sheltered inner neritic conditions.

Planktonic foraminiferal assemblages in the Pleistocene section were largely composed of Globigerina bulloides, Turborotalita quinqueloba, and related forms. Globoconella inflata, Neogloboquadrina pachyderma, and Neogloboquadrina incompta were also commonly present, and Orbulina universa, Neogloboquadrina dutertrei, and Truncorotalia truncatulinoides occurred sporadically along with rare Globigerinita glutinata. A single specimen of the subtropical species Globigerinella aequilateralis was also present in Sample 317-U1353B-7H-CC (48.91 m).

Too few age-diagnostic planktonic foraminifers were found for reliable dating, but the presence of Truncorotalia truncatulinoides in Samples 317-U1353A-5H-CC (34.08 m) and 317-U1353B-4H-CC (27.67 m) indicated that the uppermost part of the section was younger than 1.1 Ma. The base of the New Zealand Haweran Stage (0.34 Ma) was not identified with planktonic foraminiferal evidence. It was, however, recognized on the basis of calcareous nannofossil dating between Samples 317-U1353A-5H-CC and 6H-CC, above the Pleistocene (Castlecliffian) benthic foraminiferal marker HO Siphotextularia wairoana, which occurs between Samples 317-U1353B-8H-CC and 9H-CC (56.84–61.90 m).

Pliocene

Planktonic foraminifers were present in about half of the Pliocene samples examined between Samples 317-U1353B-21H-CC and 59H-CC (121.16–256.04 m), and abundances seldom reached >5% of the total foraminiferal assemblage. Planktonic assemblages were generally composed of small, thin-walled neritic forms, and diversity was low except in Sample 56X-CC (251.63 m), where nine species were identified. A peak in calcareous nannofossil abundance was noted in the same sample.

Planktonic foraminiferal assemblages in the Pliocene succession were characterized by Globigerina bulloides and closely related forms. Turborotalita quinqueloba and Neogloboquadrina incompta were also present in most of the section. Globoconella inflata, Zeaglobigerina woodi, and Neogloboquadrina pachyderma also occurred sporadically.

The Pliocene section contained few planktonic foraminiferal markers, and the bioevents recognized were poorly constrained because of the sporadic nature of the planktonic foraminiferal record. The HO of the middle Pliocene marker Zeaglobigerina woodi (2.7 Ma) was noted between Samples 317-U1353B-24H-CC and 25H-CC (139.67–142.86 m).

Early Pliocene–Miocene

Planktonic foraminifers were extremely rare in the early Pliocene to Miocene section between Samples 317-U1353B-60H-CC and 98X-CC (257.69–604.60 m). Species present in this interval were not age diagnostic. The absence of planktonic foraminifers in most of this interval is an enigma, especially given the abundance and good preservation of predominantly low-energy, inner to middle shelf benthic foraminiferal assemblages. Although its causal mechanism is unknown, we speculate that the absence of planktonic foraminifers may be due to a predominantly offshore wind pattern.

The LO of Globoconella inflata was found between Samples 317-U1353B-59H-CC and 60H-CC (256.04–257.69 m), indicating that the section above this level was no older than 4.3 Ma. Although it was not identified, the base of the Pliocene occurred somewhere between Pliocene Sample 59H-CC (256.04 m) and where Miocene calcareous nannofossils were first noted in Sample 317-U1353B-89X-CC (518.66 m).

Benthic foraminifers

Eighty-three core catcher samples from Holes U1353A and U1353B were examined (Table T9) for benthic foraminifers. Four core catcher samples were not used for age and paleowater depth estimates because they contained caved calcareous nannofossil assemblages (see Table T4, comments column).

Benthic foraminifers were dominant among all microfossils in the 150–1000 µm size fraction, and preservation was good in the Pleistocene and ranged between poor and good in the Pliocene and Miocene. Benthic foraminiferal bioevents useful for age control are listed in Table T3.

Holocene–Miocene

The HO of Siphotextularia wairoana (0.34 Ma) was observed between Samples 317-U1353B-8H-CC and 9H-CC (56.84–61.90 m). The HO of Bolivinita pliozea (~0.6 Ma) was picked between Samples 317-U1353B-10H-CC and 11H-CC (67.50–73.19 m). These datums are consistent with calcareous nannofossil and planktonic foraminiferal ages.

Paleowater depths

Paleowater depths derived from benthic foraminifers ranged from subtidal to outer shelf environments throughout the Holocene to Miocene section. Pleistocene paleowater depths fluctuated between subtidal and middle shelf water depths, although a notable deepening to outer shelf depths (correlated to the interval just above the middle/early Pleistocene hiatus) was noted in Samples 317-U1353B-10H-CC and 11H-CC (67.50–73.19 m). Pliocene water depths were generally subtidal to inner shelf but ranged down to outer shelf in the early Pliocene. Middle–early Miocene paleowater depths could not be reliably interpreted because of the low numbers of benthic foraminifers, but rare outer shelf and bathyal marker species were noted in some samples. Estimates of paleowater depths are given in Fig. F17. Depth-zone terminology is given in Figure F7 in the "Methods" chapter.

Holocene–Miocene

Two alternating benthic foraminiferal assemblages were seen in the Pleistocene section between Samples 317-U1353B-1H-CC and 20H-CC (7.94–119.16 m). One assemblage consisted mainly of Zeaflorilus parri (shallow inner shelf) and Elphidium charlottense (estuarine–subtidal) and was associated with Quinqueloculina incisa (inner shelf), suggesting a shallow inner shelf environment. The other assemblage consisted of Notorotalia zelandica (inner to middle shelf), Nonionella flemingi (mid-outer shelf), and Anomalinoides sphericus, implying a deeper depositional environment down to outer shelf. Water depths fluctuated from subtidal–shallow inner shelf to middle–outer shelf, with higher variability and frequency in the upper part of the section.

The Pliocene/Pleistocene boundary was placed between Samples 317-U1353B-21H-CC and 23H-CC (121.16–135.71 m) on the basis of calcareous nannofossil evidence. In these samples, subtidal to shallow inner shelf species Elphidium charlottense and Haynesina depressula were present. These species have also been reported in the sheltered, slightly brackish seaward part of enclosed harbors or inlets in shallow, subtidal environments (Hayward et al., 1999).

In the Pliocene–Miocene section between Samples 317-U1353B-25H-CC and 83X-CC (142.86–461.26 m), Astrononion spp. and inner to middle shelf species Notorotalia inornata and N. zelandica were dominant. The genus Astrononion has been reported in sheltered inner to middle shelf environments in New Zealand (Hayward et al., 1999).

Inner to middle shelf (and occasionally shallower or deeper) environments were inferred for most of the Pliocene–late Miocene section. The subtidal to shallow inner shelf species Elphidium charlottense occurred sporadically in the upper and lower parts of the section, whereas the outer shelf to upper bathyal species Uvigerina rodleyi, Anomalinoides sphericus, Anomalinoides parvumbilius, and Globocassidulina subglobosa were observed in the middle part of section.

Between Samples 317-U1353B-85X-CC and 90X-CC (480.64–528.87 m), Notorotalia spp., Bolivina spp., and inner shelf species N. aucklandica were abundant, in association with possible outer shelf species A. parvumbilius, where deposition occurred under inner to outer shelf water depths. Rare outer shelf and bathyal marker species were also found in some samples from the lower part of the cored section.

Between Samples 317-U1353B-91X-CC and 98X-CC (537.83–604.60 m), paleowater depths were not estimated because of the paucity and/or poor preservation of benthic foraminifers.

Diatoms

All core catcher samples from Hole U1353A (Samples 317-U1353A-1H-CC through 8H-CC [5.44–55.97 m]) were examined for diatoms (Table T10). Diatoms were only found in Sample 1H-CC (5.44 m). This sample included common, moderately preserved extant coastal taxa but no age-diagnostic species.

Hole U1353B core catcher samples were largely devoid of diatoms (Table T10). Sample 317-U1353B-1H-CC (7.94 m) contained common diatoms and an assemblage similar to that recognized in Hole U1353A, indicating marine deposition influenced by coastal upwelling and river input. Other samples had rare diatom valves, but nannofossil evidence suggested these samples contained "caved" material, and thus they were not used for diatom biostratigraphy.

Macrofossils

Macrofossils were examined in cored sediments from all Site U1353 holes. Provisional identification, age, and habitat preference are provided in Table T11.

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