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doi:10.2204/iodp.proc.317.103.2011

Biostratigraphy

Calcareous nannofossils, diatoms, and planktonic and benthic foraminifers from core catcher samples from Holes U1351A and U1351B were examined to develop a shipboard biostratigraphic framework for recovered sediments (Fig. F24; Table T6). Biostratigraphic control was achieved primarily with calcareous nannofossils in the Pleistocene section and with planktonic foraminifers in the middle Pliocene to late Miocene section where species of the Globoconella miotumida lineage were well represented. Occasional infill sampling for calcareous nannofossils between core catchers helped to constrain some bioevents and evaluate abundance distributions of nannofossils across sequence boundaries. Diatoms were absent from all but one core catcher sample. The abundance of calcareous microfossils was variable, and several samples were barren. Benthic foraminifers were primarily used for determining paleowater depths and depositional environments. All depths in this section are reported in m CSF-A.

A Holocene–late Miocene succession was recovered at Site U1351. The Pliocene/Pleistocene boundary was identified between Samples 317-U1351B-18X-CC and 19X-CC (134.48–147.55 m), based on calcareous nannofossil and foraminiferal biostratigraphic evidence. The Miocene/Pliocene boundary was reliably constrained with planktonic foraminiferal evidence between Samples 317-U1351B-94X-CC and 95X-CC (819.12–822.69 m).

The sedimentary succession at Site U1351 was interrupted by two major unconformities. The first hiatus occurred between the Pleistocene and Pliocene (Samples 317-U1351B-18X-CC and 19X-CC [134.48–147.55 m]), where most, if not all, of the upper Pliocene was missing. The second was an intra-late Miocene unconformity between Samples 317-U1351B-113X-CC and 114X-CC (998.34–1005.34 m), where at least 3.4 m.y. was missing. Other hiatuses, identified on biostratigraphic and/or lithostratigraphic evidence, are shown in Figure F24.

Older, reworked material was evident in nannofossil assemblages throughout this succession, especially in the Pliocene and Miocene, where reworked material constituted a major component of the nannofossil assemblages. In addition, the Pliocene–Miocene sequence lacked all standard low-latitude nannofossil zonal markers except Reticulofenestra pseudoumbilicus.

Benthic foraminifers suggest that water depths varied throughout the Pleistocene but generally deepened downcore from inner to middle shelf depths to middle to outer shelf depths before shallowing to inner shelf depths in the middle Pliocene. Water depths then increased downcore to outer shelf to uppermost bathyal depths in the early Pliocene section, below 700 m. Outer shelf to uppermost bathyal water depths persisted throughout the cored Miocene succession, except for an excursion to middle to outer shelf depths in Core 317-U1351B-98X-CC (851.7 m).

Calcareous nannofossils

Nannofossil abundance was highly variable in the cored sections of Holes U1351A and U1351B (Table T7). Preservation was generally good to moderate and poor in only a few cases. Age assignment by means of nannofossils was problematic for the interval between Cores 317-U1351B-20X and 116X (160.16–1024.39 m), where standard warm-water zonal markers were absent. Nannofossil datums used for age determination for this site are summarized in Table T6.

Pleistocene

All sediments recovered from Hole U1351A (Samples 317-U1351A-1H-CC through 5H-CC [2.56–27.50 m]) were biostratigraphically zoned in NN21, the base of which was defined by the lowest occurrence (LO) of Emiliania huxleyi (Table T6; 0.29 Ma). The deepest core catcher sample for this hole (Sample 317-U1351A-6H-CC [28.00 m]) was barren of nannofossils. The frequent but not abundant presence of E. huxleyi likely places these samples in Zone NN21a, below the Holocene acme of E. huxleyi (i.e., >0.08 Ma).

The Pleistocene in Hole U1351B spanned Samples 317-U1351B-1H-CC through 19X-CC (7.71–147.55 m). The late Pleistocene E. huxleyi Zone (NN21a; Fig. F24) was recognized between Samples 317-U1351B-1H-CC and 5H-CC (7.71–32.34 m). The base of this event was dated at 0.29 Ma (Lourens et al., 2004); however, an acme of Gephyrocapsa aperta in Samples 317-U1351B-4H-CC and 5H-CC (28.17–32.34 m; dated at ~0.1 Ma) and marked reworking in Sample 317-U1351B-6H-CC (39.27 m) suggest an unconformable boundary. Zone NN20 (0.29–0.44 Ma), a gap zone demarcated by the LO of E. huxleyi at its top and the highest occurrence (HO) of Pseudoemiliania lacunosa at its base, was identified between Samples 317-U1351B-6H-CC and 10H-CC (39.27–69.83 m). The top of Zone NN19 (0.44 Ma) was robustly constrained between Samples 317-U1351B-10H-CC and 11H-1, 128 cm (69.83–70.98 m); however, its base (1.93 Ma) was loosely placed around Sample 20X-CC (160.16 m).

An unconformity was recognized between 76.92 and 90.02 m (Samples 317-U1351B-12H-CC and 13H-CC), evidenced by the concurrence of two nannofossil bioevents in Sample 317-U1351B-13H-CC: the HO of large gephyrocapsids (>5.5 µm; 1.26 Ma) and the HO of Helicosphaera sellii (1.34 Ma). In addition, Reticulofenestra asanoi, a common Pleistocene marker (0.84–1.14 Ma), and all large gephyrocapsids >5.5 µm (1.24–1.56 Ma) were missing from this section, suggesting a hiatus of at least 0.7 m.y.

Another hiatus was inferred between Samples 317-U1351B-16X-CC and 18X-CC (120.24–134.48 m), although the amount of time missing was not well constrained. This hiatus was evidenced by the absence of markers identifying the Calcidiscus macintyrei Zone and the abrupt disappearance of all Gephyrocapsa species below Sample 317-U1351B-16X-CC (i.e., there was no evidence of the downcore, large-to-small gradation associated with this genus in the early Pleistocene).

The Pliocene/Pleistocene boundary was identified between Samples 317-U1351B-18X-CC and 19X-CC (134.48–147.55 m) (Fig. F24). This boundary was also interpreted as unconformable, and planktonic foraminifers provided tentative evidence of a hiatus likely spanning the late Pliocene. The amount of time missing at this hiatus is uncertain.

Pliocene

A distinct increase in the amount of nannofossil reworking (mostly Miocene material) was evident between Cores 317-U1351B-20X-CC and 28X-CC (160.16–236.17 m). Bioevent marker species were poorly represented or absent, and samples were either barren or contained long-ranging taxa as well as reworked material. Benthic foraminifers suggest that deposition occurred in inner to middle shelf water depths, which is consistent with the low planktonic foraminiferal abundances typical of inner neritic environments. Calcareous nannoplankton prefer open oceanic conditions, and thus it is inferred that most of the nannofossil material in this interval is reworked. In addition, we inferred that standard zonal markers were inhibited by the influence of cold southern currents into the region because no low-latitude zonal markers were identified within this interval. Warm-water taxa including discoasters, ceratoliths, and amauroliths were absent, and only five Neogene sphenoliths were found.

The HOs of Sphenolithus spp. (3.45 Ma) and Reticulofenestra pseudoumbilicus (3.7 Ma) were identified between Samples 317-U1351B-28X-CC and 29X-CC (236.17–247.35 m). Their juxtaposition indicates the presence of a hiatus at this level. Below this hiatus, Samples 317-U1351B-29X-CC through 94X-CC (247.35–819.12 m) contained a substantial component of reworked late–early Miocene nannofossil taxa. Given the absence of standard Pliocene zonal markers, it was not possible to determine precise ages for this interval beyond a broad zonation.

Samples 317-U1351B-29X-CC through 78H-CC (247.35–678.99 m) contained late–middle Miocene reworked nannofossil material, whereas Samples 317-U1351B-78H-CC through 94X-CC (678.99–819.12 m) consisted of primarily middle–early Miocene and a minor component of Oligocene reworked material.

Miocene

Despite massive reworking of Miocene nannofossils in this part of the section, the presence of the short-ranging species Scyphosphaera graphica and Scyphosphaera queenslandensis in Sample 317-U1351B-95X-CC (822.69 m) indicates a late Miocene age for this sample. This is consistent with ages derived from other fossil groups. The Miocene/Pliocene boundary was constrained to the interval between Samples 317-U1351B-94X-CC and 95X-CC (819.12–822.69 m), which is consistent with findings from both planktonic and benthic foraminifers. Samples 317-U1351B-95X-CC through 116X-CC (822.69–1024.39 m) showed evidence of some in situ material within assemblages, including scyphosphaerids, Sphenolithus neoabies (one specimen in Sample 317-U1351B-111X-CC [976.66 m]), and abundant well-preserved coccospheres (delicate articulated nannoplankton skeletons). No definitive nannofossil age could be assigned to the basal cored sediments.

Planktonic foraminifers

Planktonic foraminiferal biostratigraphy of the cored Holocene to late Miocene section of Site U1351 was based on the shipboard study of core catcher samples from Holes U1351A and U1351B (Tables T6, T8, T9). Absolute ages assigned to biostratigraphic datums follow the references listed in Table T3 in the "Methods" chapter. See Tables T10 and T11 and Figure F25 for planktonic foraminiferal abundances and an interpretation of oceanicity.

Holocene

The base of the Holocene was not identified biostratigraphically but was tentatively correlated with a distinctive lithologic boundary at Section 317-U1351B-1H-2, 108 cm (2.58 m), where greenish gray marly sands overlie gray calcareous muds.

Pleistocene

Planktonic foraminiferal assemblages were well preserved in the cored Pleistocene sections of Holes U1351A and U1351B. Specimens, mostly small thin-walled forms, were common to abundant in the upper part of the Pleistocene between Samples 317-U1351A-1H-CC and 6H-CC (2.56–28.00 m) and 317-U1351B-1H-CC and 9H-CC (7.71–65.91 m), where they composed 9%–56% of the total foraminiferal assemblage. Deposition generally occurred under inner to outer neritic oceanic conditions, except in Samples 317-U1351B-6H-CC (39.27 m) and 9H-CC (65.91 m), where planktonic abundances indicate extraneritic oceanic conditions (Fig. F25). Planktonic abundances were <10% in the lower part of the Pleistocene section between Samples 317-U1351B-10H-CC and 18X-CC (69.83–134.48 m), indicating deposition under inner neritic and sometimes sheltered inner neritic oceanic conditions (Tables T10, T11).

Assemblages in the Pleistocene section were dominated by Globigerina bulloides, Turborotalita cf. quinqueloba, and other small globogerinid species. Globoconella inflata, Neogloboquadrina pachyderma, Neogloboquadrina incompta, and Orbulina universa were also present in most of the section. Single specimens of the subtropical species Globigerinoides ruber and Globigerinella aequilateralis and a sinistrally coiled specimen of Truncorotalia truncatulinoides were also present in Sample 317-U1351B-4H-CC (28.17 m), the only sample in which these species were identified. The presence of Tr. truncatulinoides suggests that the uppermost part of the section is younger than 1.1 Ma (Table T6). Single specimens of Zeaglobigerina cf. woodi and Zeaglobigerina woodi in Samples 317-U1351B-19X-CC (147.55 m) and 21X-CC (168.48 m), respectively, mark the HO of the Zg. woodi group and indicate a minimum age of 2.7 Ma for the top of the cored Pliocene section. This suggests that the late Pliocene is missing, presumably at an unconformity between the Pliocene and Pleistocene.

Pliocene

Assemblages were generally well preserved in the middle Pliocene section of Hole U1351B, except in cemented horizons where tests were infilled with pyrite, glauconite, and sparry-calcite (Fig. F26). Planktonic foraminifers in this interval made up <6% of the total foraminiferal assemblage and included only small thin-walled forms typical of deposition under sheltered inner neritic oceanic conditions.

Middle Pliocene planktonic assemblages were dominated by Globigerina bulloides and other small Globigerina species. Turborotalita cf. quinqueloba, Neogloboquadrina pachyderma, Nq. incompta, and Globoconella inflata were also present, and Globoconella puncticuloides occurred sporadically below Sample 317-U1351B-20X-CC (160.16 m).

The preservation of planktonic foraminifers was good in the upper part of the Pliocene section of Hole U1351B, and moderately preserved, recrystallized, and pyrite-filled assemblages occurred in sandy horizons lower in the section. In the upper part of the section between Samples 317-U1351B-30X-CC and 69X-CC (255.06–591.90 m), planktonic foraminifers composed <18% of the total foraminiferal assemblage and included mostly small thin-walled forms, with an increasing number of thicker walled forms downcore. Deposition occurred generally under inner neritic and sheltered inner neritic oceanic conditions, except for Samples 317-U1351B-29X-CC (247.35 m), 30X-CC (255.06 m), and 55X-CC (486.50 m), which were deposited under outer neritic oceanic conditions. In the lowermost Pliocene section between Samples 317-U1351B-55X-CC and 94X-CC (486.50–819.12 m), planktonic abundances increased (Fig. F25) to a high of 54% at the base of the section. The maximum size of planktonics also increased downhole, coincident with increased numbers of thicker walled forms.

Early Pliocene planktonic foraminiferal assemblages were dominated by Globigerina bulloides and other small globogerinids. Neogloboquadrina pachyderma, Nq. incompta, and Turborotalita cf. quinqueloba also occurred sporadically, with rare specimens of Orbulina universa, Globigerinita glutinata, and Zeaglobigerina woodi s.l. Globoconella puncticuloides and typical 3–3½-chambered specimens of Gc. inflata were also present in small numbers down to Samples 317-U1351B-43X-CC (372.13 m) and 44X-CC (382.57 m), respectively. Typical four-chambered populations of Gc. puncticulata s.s. were relatively common in the lowermost Pliocene section from Samples 317-U1351B-44X-CC through 94X-CC (382.57–819.12 m), immediately above the HO of Globoconella sphericomiozea, where 5% or more of fully grown specimens in populations of Gc. puncticulata and Gc. sphericomiozea had weak keels on the last formed chamber. In this region of the southwest Pacific, the HO of Gc. puncticulata s.s. is dated at ~4.3 Ma, and the HO of Gc. sphericomiozea is dated at 5.30 Ma (Cooper, 2004).

Miocene

The preservation of late Miocene planktonic foraminifers was generally moderate to poor. Foraminiferal tests were mostly recrystallized, and chambers were infilled with sparry-calcite (Fig. F26), especially in the early late Miocene interval from Samples 317-U1351B-114X-CC through 116X-CC (1005.34–1024.39 m), where zeolites were noted by the shipboard sedimentology team in interstitial cements. Planktonic abundances ranged from 17% to 89%, and planktonic specimens were often thick-walled and sometimes encrusted.

Late Miocene planktonic assemblages were dominated by Globigerina, including Gg. bulloides. Species of Globoconella useful for subdividing and dating the late Miocene were also common, and Neogloboquadrina pachyderma, Hirsutella scitula, Orbulina universa, and Zeaglobigerina woodi s.l. occurred sporadically in the section. The HO of Globoconella conomiozea s.s., where 95% or more of fully grown specimens in populations of Gc. sphericomiozea and Gc. conomiozea had keels, was constrained between Samples 317-U1351B-100X-CC and 101X-CC (877.62–882.55 m). Typical populations of Gc. conomiozea s.s. were present down to Sample 317-U1351B-113X-CC (998.34 m), immediately above the HOs of the early late Miocene species Globoconella miotumida and Hirsutella panda between Samples 317-U1351B-113X-CC and 114X-CC (998.34–1005.34 m) and the highest common occurrence (HCO) of Hirsutella panda and the HO of Paragloborotalia mayeri s.l. within Samples 317-U1351B-114X-CC and 115X-CC (1005.34–1014.56 m). The juxtaposition of Gc. conomiozea in the sample above that in which these events occurred suggests a major unconformity with a minimum of 3.4 m.y. missing between Samples 317-U1351B-113X-CC and 114X-CC (998.34–1005.34 m). The planktonic foraminiferal assemblage in Sample 317-U1351B-116X-CC (1024.39 m) indicates a late Miocene age of 10.60–10.91 Ma at the bottom of the hole.

Benthic foraminifers

Benthic foraminifers (in the 150–1000 µm size fraction of washed samples) were examined in 114 core catcher samples from Hole U1351A and U1351B (Table T12). The preservation of benthic foraminifers was generally good to moderate in most samples, except in the lower part of the cored section between Samples 317-U1351B-90X-CC and 116X-CC (778.52–1024.39 m), where it was poor. The abundance of benthic foraminifers among a composite group of microfossils was dominant to abundant in the cored Holocene to upper Miocene section (see "Biostratigraphy" in the "Methods" chapter). Stratigraphic occurrences of age-diagnostic benthic foraminifers are shown in Table T6.

Holocene–Pleistocene

Benthic foraminifers were examined in six core catcher samples from Hole U1351A (Samples 317-U1351A-1H-CC through 6H-CC [2.56–28.00 m]) and 108 core catcher samples from Hole U1351B (Samples 317-U1351B-1H-CC through 116H-CC [7.71–1024.39 m]). Among the age-diagnostic benthic foraminifers, Notorotalia zelandica occurred abundantly throughout this hole, except in Sample 317-U1351A-3H-CC (18.15 m). Notorotalia finlayi, which is common today in shallow water depths of the New Zealand region, occurred only in Sample 317-U1351A-6H-CC (28.00 m), where it was rare. Only one possible reworked, broken specimen of Bolivinita pliozea, whose HO is associated with the mid-Pleistocene Transition (0.6 Ma) in New Zealand waters (e.g., Hayward, 2001), was found in the lowermost Sample 317-U1351A-6H-CC (28.00 m). A middle Pleistocene age (<0.6 Ma) was given to the bottom of Hole U1351A.

In Hole U1351B, the HO of the mid-Pleistocene marker species Bolivinita pliozea was observed between Samples 317-U1351B-7H-CC and 8H-CC (47.83–54.46 m). The LO of the extant taxon Notorotalia zelandica was noted between Samples 317-U1351B-23X-CC and 24X-CC (180.13–189.75 m) and the HO of Haeuslerella parri was noted between Samples 317-U1351B-28X-CC and 29X-CC (236.17–247.35 m). Hornibrook et al. (1989) reported the appearance of N. zelandica in the late Pliocene at the base of the New Zealand Nukumaruan Stage (2.40 Ma) and the disappearance of H. parri in the early Pleistocene at the top of the Nukumaruan Stage (1.63 Ma).

Pliocene

The HO of Notorotalia hurupiensis was recognized between Samples 317-U1351B-33X-CC and 34X-CC (278.35–286.38 m). The LO of Notorotalia finlayi was observed between Samples 317-U1351B-57H-CC and 59H-CC (507.60–510.49 m). These datums are correlated with the base of the New Zealand Waipipian Stage (3.62 Ma). The HOs of Notorotalia taranakia and Haeuslerella morgani were identified between Samples 317-U1351B-83X-CC and 84X-CC (716.94–726.71 m), supporting an early Pliocene age (New Zealand Opotian Stage [3.62–5.30 Ma]) for this interval.

Miocene

Three benthic bioevents were noted in the late Miocene interval. The HO of Texturalia miozea was observed between Samples 317-U1351B-96X-CC and 97X-CC (832.15–847.79 m). This event is questionably dated as late Miocene (New Zealand upper Tongaporutuan Stage [6.67–8.95 Ma]). The LO of Haeuslerella parri was identified between Samples 317-U1351B-99X-CC and 100X-CC (861.39–877.62 m), and the LO of Bolivinita pliozea (including Bolivinita cf. pliozea) was recognized between Samples 317-U1351B-114X-CC and 115X-CC (1005.34–1014.56 m), New Zealand Kapitean and upper Tongaporutuan Stages, respectively.

Paleowater depths

Interpreted paleowater depths based on presence, absence, and frequency of key benthic foraminiferal species or genera are shown in Figure F27. Terminology for depth zones is given in Figure F9 in the "Methods" chapter.

The most commonly occurring benthic group was the diverse Notorotalia group, which composed >30% of benthic assemblages in most of the cored sequence. The Notorotalia group includes N. aucklandica, N. zelandica, N. finlayi, N. inornata, N. hornibrooki, N. hurupiensis, N. taranakia, and N. profunda. Species abundances within this group varied in the studied section and provided valuable water depth information. Other ubiquitous taxa included Nonionella flemingi, Astrononion sp., Anomalinoides sphericus, shallow-water miliolids, and textularids (e.g., Haeuslerella). Uvigerinids, cassidulinids, and buliminids were generally less common.

Paleowater depths inferred from depth-restricted taxa suggest inner shelf and middle shelf environments for most of the Holocene–late Pleistocene interval in Hole U1351A, shallowing to subtidal in the lowermost part. The fauna is characterized by the inner to middle shelf species Notorotalia zelandica, N. hornibrooki, and Quinqueloculina auberiana in Samples 317-U1351A-1H-CC through 5H-CC (2.56–27.50 m). In Samples 317-U1351A-2H-CC (11.44 m) and 4H-CC (25.08 m), the middle to outer shelf indicator Anomalinoides sphericus was also abundant. Benthic faunas in Sample 317-U1351A-6H-CC were dominated by the estuarine to subtidal indicator Elphidium charlottense and associated Notorotalia and miliolids.

The Holocene–middle Pliocene section of Hole U1351B was dominated by taxa indicative of inner to middle shelf environments (Samples 317-U1351B-1H-CC through 30X-CC [7.71–255.06 m]), but inner or middle to outer shelf environments were occasionally noted in this interval. Taxa included inner to middle shelf species Notorotalia zelandica, N. aucklandica, N. hornibrooki, Notorotalia depressa, and Quinqueloculina auberiana. The latter taxon is occasionally associated with the estuarine to subtidal indicator Elphidium charlottense and the middle to outer shelf indicator Anomalinoides sphericus.

The late early Pliocene interval between Samples 317-U1351B-34X-CC and 65H-CC (286.39–563.80 m) contained abundant Notorotalia hurupiensis and the extant inner to middle shelf species N. aucklandica. Anomalinoides sphericus co-occurred with the above taxa in the lower part of the interval (Samples 317-U1351B-34X-CC through 43X-CC [286.39–372.13 m]). Thus, the paleowater depth for most of this section was estimated as inner to middle shelf and slightly deeper for the upper part of the section.

In the earliest Pliocene–late Miocene (Samples 317-U1351B-66X-CC through 116X-CC [564.07–1024.39 m]), the deepwater species Notorotalia profunda was dominant. This species occurred in association with Notorotalia hurupiensis, Nonionella flemingi, Astrononion sp., and other foraminifers common in bathyal depths, such as Uvigerina spp., Pullenia spp., and Melonis spp. Outer shelf to uppermost bathyal environments characterize the succession between Samples 317-U1351B-82X-CC and 116X-CC (707.27–1024.39 m), except in Sample 98X-CC (851.69 m) where there is an excursion to middle to outer shelf water depths.

The downhole transition from shelfal to bathyal faunas was poorly defined in terms of benthic assemblages but was inferred to occur between Samples 317-U1351B-69X-CC and 82X-CC (591.90–707.27 m).

Diatoms

All core catcher samples from the Holocene–late Pleistocene succession in Hole U1351A (Samples 317-U1351A-1H-CC through 6H-CC [2.56–28.00 m]) were examined for diatoms, but they were only found in Sample 317-U1351A-1H-CC (2.56 m) (Table T13). Diatoms were common and moderately preserved in this sample, and the assemblage was dominated by Paralia sulcata (99%), with minor Thalassionema nitzschioides, resting spores of Chaetoceros and Hyalodiscus sp. (living in today's coastal regions). A fragment of the freshwater and brackish water genus Pinnularia, which is presumed to have been transported from the mouth of a river, was also found in this sample.

Core catcher and calcareous concretion samples (124 in total) from Holocene–upper Miocene Hole U1351B sediments were examined for diatoms (Table T13). Although diatoms occurred in most samples, they were severely etched. Other samples were barren, most likely because of dissolution. An exception is Sample 317-U1351B-13H-CC (90.02 m), a hard silt taken from inside bivalve shells. Most of the preserved diatoms were fragments, but a biostratigraphically useful species, Thalassiosira fasciculata (HO = 0.56–0.61 Ma; LO = 4.30–4.69 Ma), was identified along with an extant cosmopolitan species, Coscinodiscus radiatus. No diatoms were found in any concretion sampled.

Macrofossils

Macrofossils were examined in cored sediments from all Site U1351 holes. Provisional identification, age, and habitat preferences are provided in Table T14.

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