The micropaleontological biostratigraphy of Site 1123 is based mostly on the onboard study of core-catcher samples. Holes 1123A and 1123B samples were used for the uppermost part of the section, Hole 1123B samples for the middle part, and Hole 1123C samples for the lower part. Additional samples were taken from within selected cores to address specific age and paleoenvironmental questions. The absolute ages assigned to biostratigraphic datums follow the references listed in Tables T2, T3, T4, T5, all in the "Explanatory Notes" chapter; and Tables T5, T6, and T10 in this chapter.
Calcareous microfossils are diverse and abundant, but their preservation deteriorates gradually below 190 mbsf, especially in chalk. Strong differential dissolution in many samples has left only resistant species preferentially preserved. Radiolarians are generally abundant and well preserved throughout the sequence, except for three barren samples. Diatoms and silicoflagellates are present with good preservation in the upper 300 mbsf (uppermost Miocene to Pleistocene) and also in the upper Eocene/lower Oligocene interval recovered. Significant differential dissolution of planktonic foraminifers and calcareous nannofossils in Miocene and Pliocene sediments suggests that this site has frequently been bathed in corrosive bottom waters underneath the lysocline. In contrast, the late Eocene-early Oligocene calcareous microplankton fossils are relatively better preserved, despite their deep burial.
As this site is north of the Subtropical Convergence, the presence of diverse temperate and subtropical assemblages in samples facilitates correlation with both the global standard zonations of low latitudes and with the New Zealand stages. The sequence is well dated by more than 100 datum levels from various fossil groups. Age assignments given by the five microfossil groups are in close agreement. A summary of zonation and major datum levels is presented in Figure F16. All the fossil groups suggest that Site 1123 recovered a continuous Neogene sequence of ~587 m back to ~20-21.5 Ma. This Neogene sequence terminates at a regional Oligocene unconformity (Marshall Paraconformity of Carter and Landis, 1972), which at this site separates lower Miocene (20-21.5 Ma) from lowest Oligocene-uppermost Eocene (~33 to 35 Ma) sediments.
Reworked Paleogene and early Miocene nannofossils and radiolarians are common in the upper part of the section. The presence of allochthonous subantarctic diatoms in the upper Neogene sediments at this site documents the influence of sediment input by contour currents flowing along the northern slope of Chatham Rise. The sharp contrast in preservation of calcareous microfossils between the Paleogene and Neogene, the presence of frequently reworked nannofossils, diatoms, and radiolarians in the upper sections, as well as the existence of a big hiatus (33 to ~21.5 Ma missing), suggests that the onset of the Deep Western Boundary Current took place after 33 Ma.
Nannofossils in core catchers of Cores 181-1123A-1H to 17H, 181-1123B-18X to 52X, and 181-1123C-18X to 33X were examined. Additional samples were selected from within cores to increase the stratigraphic resolution. Nannofossils are abundant throughout the sequence, except for a few intervals where dissolution removed most of the species. Preservation of nannofossils is generally good in the top part of the sequence from Samples 181-1123A-1H-1, 40 cm, to 181-1123B-21X-CC (0-191.27 mbsf). Starting from 191 mbsf, the preservation deteriorates gradually. The identified species are tabulated in Table T4. Reworking is frequent throughout the sequence. The presence of many age-diagnostic species allows a detailed zonation of the sequence (Fig. F16). Twenty-two datum levels have been recognized. Their depths (mbsf) and age estimates are listed in Table T5. An age-depth relationship curve is shown in Figure F17. The upper 587.25 mbsf contains a continuous Neogene sequence (0 to ~21.6 Ma). A major disconformity is detected at ~587.2 mbsf between Samples 181-1123C-29X-2, 97 cm, and 29X-2, 114 cm. The sediments below this disconformity record a brief interval of the earliest Oligocene and latest Eocene (~33 to 35 Ma) (Figs. F16, F17). Compared to previous sites, the sedimentation rates of the Neogene sediments of this site are much lower (~33 m/m.y.).
The core catcher of the first core (Sample 181-1123A-1H-CC) indicates an age already older than the first age-diagnostic datum level, the first occurrence (FO) of Emiliania huxleyi, dated at 0.24 Ma (Naish et al., 1998). This datum was detected at Sample 181-1123A-1H-4, 75 cm, which marks the base of Zone NN21. Within Zone NN21, a reversal of dominance from Emiliania huxleyi to Gephyrocapsa oceanica occurs between Samples 181-1123A-1H-2, 7 cm, and 1H-2, 145 cm, indicating an age of 0.085 Ma (Berggren et al., 1995a) at ~2.26 mbsf.
Although a few Pseudoemiliania lacunosa are present in samples above 15.61 mbsf, the genuine FO of this marker species at the base of Zone NN20 is its first abundant occurrence in Sample 181-1123A-3H-CC (23.21 mbsf). Because of the low sedimentation rates (~3-4 cm/k.y.) and coarse sampling, we were not able to detect all the bioevent datum levels of the Pleistocene as was possible at previous sites. Instead, two datum levels marking the beginning of the Pleistocene were detected: the FO of the medium-sized Gephyrocapsa and the last occurrence (LO) of Calcidiscus macintyrei at Sample 181-1123A-8H-CC (72.02 mbsf). These two events have been calibrated using paleomagnetic data slightly younger than the Olduvai Normal Event, at ~1.67 and 1.6 Ma, respectively (Raffi et al., 1993; Raffi and Flores, 1995).
The LO of Discoaster brouweri in Sample 181-1123A-8H-CC (72.02 mbsf) marks the top of Zone NN19 at 1.96 Ma (Raffia and Flores, 1995). Sample 181-1123A-10H-CC (91.88 mbsf) contains several Discoaster species, which represent the last stock of discoasterids before they became extinct at about the Pliocene/Pleistocene boundary. The common occurrence of Discoaster pentaradiatus indicates that the sediments were deposited before 2.2 to 2.4 Ma (Wei, 1993). The top of the Zone NN17 thus is defined in Core 181-1123A-10H. The last appearance of D. surculus, which defines the top of the Zone NN16 (2.55 Ma), occurs in Sample 181-1123A-12H-CC (110.76 mbsf). On the other hand, the LO of Discoaster tamalis (2.76 Ma; Raffi and Flores, 1995) in Sample 181-1123A-11H-CC (101.14 mbsf) appears to be conflicting. Considering the possibility that D. tamalis might be reworked, we chose the LO of D. surculus at 2.55 Ma as the age marker for this interval (Table T5).
The LO of Reticulofenestra pseudoumbilicus (the top of NN15 at 3.82 Ma) (Raffi and Flores, 1995) in Sample 181-1123A-14H-CC (129.66 mbsf) and the FO of D. asymmetricus (the bottom of NN14 at 4.13 Ma, Shackleton et al., 1995) in Sample 181-1123A-16H-CC (148.66 mbsf) allow a detailed zonation of the Pliocene. Furthermore, the FO of Pseudoemiliania lacunosa (4.0 Ma; Gartner, 1990) in Sample 181-1123A-14H-CC provides another good age marker. The absence of several important marker species for the early Pliocene, including Ceratolithus rugosus and C. acutus, makes it difficult to differentiate Zone NN12 from NN13. The appearance of Discoaster quinqueramus in Sample 181-1123B-24X-CC (220.5 mbsf) indicates that the Pliocene/Miocene boundary lies slightly above this level.
Conventionally, the late Miocene in low-latitude regions can be subdivided into very fine zones based upon a series of fast-evolving Discoaster, Amaurolithus, and Catinaster species. However, the sporadic occurrence of Amaurolithus and Discoaster spp., aggravated by overgrowth, allows only partial recognition of the standard zones at this site. Nevertheless, several additional datum levels, which are not markers for the standard zonation, proved very useful for correlation in the mid-latitudes. For instance, the generally rare occurrence of Amaurolithus primus in this sequence casts doubt about the biochronologic value of its FO (7.39 Ma) in Sample 181-1123B-29X-CC (268.49 mbsf), but the LO (7.73 Ma) and FO (9.34 Ma) of a short-ranged species, Minylitha convallis, at 287.54 mbsf (Sample 181-1123B-31X-CC) and 326.02 mbsf (Sample 181-1123B-35X-CC), respectively, helps to confirm the utility of Amaurolithus primus as a useful marker, despite its low abundance.
The middle/late Miocene boundary (~11.2 Ma) is approximated by the LO of Coccolithus miopelagicus (10.94 Ma) (Backman and Raffi, 1997) in Sample 181-1123B-40X-CC (374.06 mbsf). The LO of Calcidiscus premacintyrei (12.65 Ma) (Raffi and Flores, 1995) occurs in Sample 181-1123B-43X-CC (399.52 mbsf). This level has been reported to be slightly older than the top of Zone NN6 (Young et al., 1994). The zonal marker separating Zone NN5 from NN6, the LO of Sphenolithus heteromorphus (13.52 Ma) (Backman and Raffi, 1997), was found in Sample 181-1123B-49X-CC (457.06 mbsf).
Most specimens of Discoaster spp. have been overgrown to the extent that species-level characters are unrecognizable. Not being able to differentiate to species, we only separated the Discoaster variabilis group from that of D. deflandrei. The transition from the D. deflandrei group to the D. variabilis group occurs at ~506 to 498 mbsf (Samples 181-1123C-20X-CC to 19X-CC), which marks generally the boundary between the early and middle Miocene. Although the early/middle Miocene boundary could be more precisely determined by the LO of Helicosphaera ampliaperta, which occurred slightly above the boundary, the virtual absence of this species in the sequence precludes its use as a marker at this site.
In the lower Miocene sequence, a few zonal markers were recognized. The LO of Sphenolithus belemnos in Sample 181-1123C-22X-6, 136 cm (526.26 mbsf), which defines the upper boundary of the Zone NN3, is a few meters lower than the FO of Sphenolithus heteromorphus in Sample 181-1123C-21X-CC (516.96 mbsf). Because neither S. belemnos nor S. heteromorphus were found in the particular sample (Sample 181-1123C-22X-5, 78 cm; 524.18 mbsf) that was taken between these two samples, we place the boundary between Zones NN3 and NN4 (18.3 Ma) right at this level (524.18 mbsf). Within Zone NN4, the FO of Calcidiscus premacintyrei in Sample 181-1123C-20X-3, 147 cm (504.48 mbsf), provides an additional age marker at 17.4 Ma.
The FO of S. belemnos was recorded in Sample 181-1123C-23X-CC (536.71 mbsf), indicating an age younger than 19.2 Ma (Berggren et al., 1995b). A major floral discontinuity was found between Samples 181-1123C-29X-2, 97 cm, and 29X-2, 114 cm. Early Oligocene assemblages occur below this floral break. Based upon the sedimentation rate of 20 m/m.y., calculated from the above three datum levels, the age of the oldest sediments immediately above the paraconformity is estimated to be 21.7 Ma.
Nannofossils from the chalky sediments below the paraconformity are generally abundant but have moderate to poor preservation. In the five samples below the paraconformity (Samples 181-1123C-29X-2, 114 cm, through 32X-2, 132 cm [587.39-616.42 mbsf]), the nannofossil assemblages are characterized by abundant Chiasmolithus spp. (difficult to be identified to the species level because of poor preservation), Dictyococcites abisectus, Reticulofenestra umbilicus, and Zygrhablithus bijugatus. The presence of Isthmolithus recurvus, a short-ranged species existing between 36 and ~32 Ma, constrains this interval to be in the early Oligocene to late Eocene. The occurrence of particularly abundant Clausicoccus spp. (listed as C. subdistichus in Table T4), centered around Sample 181-1123C-29X-CC (593.31 mbsf), suggests that this interval is in the acme zone of Clausicoccus, which has been dated to be 33.3 Ma (Berggren et al., 1995b). Discoaster saipanensis, whose last occurrence defines the top of Zone NP20, occurs in Sample 181-1123C-32X-2, 132 cm (616.42 mbsf). Therefore, the sequence below this level down to the bottom of the Hole 1123C (625.76 mbsf) is assigned to Zone NP20 (34.2-35.4 Ma), slightly lower than the Eocene/Oligocene boundary (Berggren et al., 1995b).
Throughout most of the Site 1123 section, the planktonic foraminiferal assemblages show evidence of variable, differential dissolution (Tables T6, T7). Benthic assemblages, which contain many relatively thick-walled taxa, appear more intact. However, this criterion may be misleading, since species content strongly varies between groups of samples, possibly the result of carbonate dissolution of benthic tests before their burial. Agglutinated benthic taxa are mostly rare to absent, except in Sample 181-1123C-22X-CC, where an acme of Eggerella bradyi occurs. Differential dissolution of planktonic foraminifer tests affected our ability to find all of the expected taxa of potential use in the biostratigraphy and led us to doubt the local stratigraphic distribution of some of the observed events. Also, the uncertainty attached to the age calibrations, coupled with the fact that the sampling depths of FO and LO events may be revised when samples in between core catchers are examined, demands caution in drawing detailed biochronological conclusions.
We are most confident in the correlation potential of those bioevents that occur within evolutionary lineages (especially the Globorotalia lineages), where both ancestor and descendant forms are present in assemblages, below and above the datum. First and last appearance datum with no clear ancestral or descendant forms present may be less reliable for correlation (e.g., FO G. praescitula, FO Zeaglobigerina nepenthes, FO Orbulina universa, FO O. suturalis, FO Globorotalia crassula, and FO G. truncatulinoides). Members of the G. fohsi and N. acostaensis lineages are essentially absent.
The younger part of the Quaternary interval is not easily recognized biostratigraphically by using foraminifers. The uppermost sediment (Samples 181-1123B-1H-1, 0-2 cm; and 1H-1, 18-20 cm) is younger than 0.45 Ma, based on the presence of Globorotalia hirsuta. Despite intensive searching, this species was not found in any lower samples. The planktonic foraminiferal assemblage in Sample 181-1123A-1H-CC (6 mbsf) is dominated by Globorotalia inflata (first common occurrence [FCO] in subantarctic ~0.7 Ma) and also has common Globorotalia truncatulinoides (FO in subantarctic ~0.8 Ma). This indicates a late Pleistocene age (younger than 0.7 Ma, New Zealand Castlecliffian Stage).
Globorotalia inflata is still dominant in Sample 181-1123A-2H-CC (16 mbsf), but here the first Globorotalia puncticuloides (LO ~0.6 Ma) occurs downhole. The dominant globorotaliid from Sample 181-1123A-3H-CC to 11H-CC is G. puncticuloides (last common occurrence [LCO] ~0.7 Ma, FO ~3.6 Ma). It co-occurs with Globorotalia inflata (FO 3.7 Ma). Samples 181-1123A-3H-CC to 5H-CC (23-44 mbsf) also contain Globorotalia truncatulinoides (FO ~0.8 Ma in the subantarctic, but earlier in the subtropics) suggesting an age of ~0.7-0.8 Ma for this interval, although it could be older, as this site is north of the STC. In Samples 181-1123A-2H-CC and 3H-CC, rare G. crassaformis hessi occurs. The latter is typical for the middle Pleistocene, the chronostratigraphic assignment is thus accepted for this interval. Samples down to 181-1123A-8H-CC (72 mbsf) are younger than 2.6 Ma because they also contain sporadic Globorotalia crassula (FO 2.6 Ma).
Samples 181-1123A-9H-CC and 10H-CC (82-92 mbsf) are late Pliocene (Mangapanian to early Nukumaruan Stages), because they contain a tightly constrained zone of dextral, unkeeled Globorotalia crassaformis (3.0-2.1 Ma).
Sample 181-1123A-11H-CC (101 mbsf) is mid-Pliocene (3.6-3.4 Ma, Waipipian Stage), based on the occurrence of Globorotalia puncticuloides (FO 3.6 Ma), G. crassaconica (LO 3.0 Ma), rare G. pliozea (LO ~3.4 Ma), and G. inflata (FO 3.7 Ma).
Sample 181-1123A-12H-CC (111 mbsf) is mid-Pliocene (~3.6-3.7 Ma, late Opoitian Stage), based on the occurrence of Globorotalia inflata triangula (FO 3.6 Ma), common G. pliozea (LCO 3.6 Ma), and G. puncticulata (LO 3.7 Ma).
Samples from 181-1123A-13H-CC to 181-1123B-19X-CC (120-172 mbsf) are of early Pliocene age (3.7-5.2 Ma, Opoitian Stage), based on the occurrence of common Globorotalia puncticulata (FO 5.2 Ma, LO 3.7 Ma), and supported by the presence of G. pliozea (3.4-5.4 Ma). The lower part of this interval can be further subdivided, as Sample 181-1123A-18X-CC (164 mbsf) is of earliest Pliocene age (~4.7-4.8 Ma, early Opoitian Stage), based on the occurrence of Globorotalia crassaconica (FO 4.7 Ma), Globorotalia mons (LO 4.8 Ma), and Sphaeroidinella dehiscens (FO 4.8 Ma).
Sample 181-1123B-20X-4, 83-87 cm (177.65 mbsf), is latest Miocene (5.2-5.4 Ma, late Kapitean Stage), based on the occurrence of Globorotalia sphericomiozea (LO 5.2 Ma), G. juanai (LO 5.2 Ma in 181-1123B-20X-CC), and G. pliozea (FO 5.4 Ma). Sample 181-1123B-21X-CC (191 mbsf) is latest Miocene (5.5-5.6 Ma, middle Kapitean Stage), based on the presence of Globorotalia sphericomiozea (FO 5.6 Ma), G. mons (FO 5.5 Ma), and G. miotumida (LO 5.6 Ma). Also, no G. crassaformis was observed below this level.
Samples 181-1123B-22X-CC to 35X-CC (200-326 mbsf) are of undifferentiated late Miocene age (5.6-9.9 Ma), based on the dominance of sinistral Globorotalia miotumida (LO 5.6 Ma) and the occurrence of the distinctive Globoquadrina dehiscens (LO 9.9 Ma) in Sample 181-1123B-36X-CC (335 mbsf). G. conoidea Walters, the thick-walled form of G. miotumida Jenkins, is well represented. Within this long interval, the rare presence of Bolboforma pentaspinosa (LO 7 Ma) in Samples 181-1123B-27X-CC, 30-CC, and 32-CC assists to divide the upper Miocene interval into upper and lower parts. This is confirmed by the presence of Sphaeroidinellopsis paenedehiscens (FO ~8 Ma) in Sample 181-1123B-27X-CC (249 mbsf).
Samples 181-1123B-36X-CC to 40X-CC (336-374 mbsf) are earliest late Miocene (9.9-11.3 Ma, early Tongaporutuan Stage), based on the presence of Globoquadrina dehiscens (LO 9.9 Ma) and sporadic Neogloboquadrina pachyderma (FO 11.3 Ma; lowest occurrence in Sample 181-1123B-40X-CC). This agrees with the presence of Zeaglobigerina druryi (LO 11.3 Ma) in Samples 181-1123B-39X-CC and below. Sample 181-1123B-40X-CC contains the highest stratigraphic level of Catapsydrax parvulus, longer ranging in Miocene strata.
The assemblage in Sample 181-1123B-41X-CC is sparse and contains no species that are diagnostic of differentiating the middle from late Miocene. The presence of Neogloboquadrina nympha and Globorotalia miotumida suggests these samples are younger than 13.2 Ma.
Samples 181-1123B-42X-CC to 46X-CC (388-428 mbsf) are late middle Miocene (11.3-13.2 Ma, Waiauan Stage), based on the presence of Paragloborotalia mayeri (LO 10.8 Ma; highest occurrence in Sample 181-1123B-42X-CC) and Globorotalia miotumida (LO 13.2 Ma; lowest occurrence in Sample 181-1123B-46X-CC), together with Zeaglobigerina druryi (LO 11.3 Ma), Neogloboquadrina continuosa (LO ~11 Ma), Zeaglobigerina nepenthes (FO 11.8 Ma; lowest occurrence in Sample 181-1123B-42X-CC), and Globorotalia conica (LO ~13 Ma; highest occurrence in Sample 181-1123B-45X-CC). Sample 181-1123B-46X-CC is latest Lillburnian Stage (13-13.2 Ma) based on the presence of Globorotalia miotumida (FO 13.2 Ma) and Paragloborotalia partimlabiata (LO 13 Ma).
Samples 181-1123B-47X-CC to 50X-CC (437-469 mbsf) are early middle Miocene (15.8-13.2 Ma, Clifdenian and Lillburnian Stages) based on the presence of Globorotalia praemenardii (LO 13.2 Ma, FO 15.8 Ma), supported by the presence of Cibicidoides wuellerstorfi (FO 16.4 Ma; lowest occurrence in Sample 181-1123B-49X-CC) and more sporadic occurrence of Globorotalia amuria (FO 16 Ma; lowest occurrence in Sample 181-1123B-48X-CC). The single occurrence of one specimen of Globorotalia panda (FO 15 Ma) in Sample 181-1123B-48X-CC tentatively assists with subdivision within this interval. From Sample 181-1123B-47X-CC downhole, Globoquadrina praedehiscens is common, whereas Globorotalia mayeri is absent.
Samples 181-1123B-51X-CC to 181-1123C-20X-CC (480-506 mbsf) span the early to middle Miocene boundary (16.7-15.8 Ma, late Altonian and early Clifdenian Stages) based on the presence throughout of Globorotalia miozea (FO 16.7 Ma, LO 15.8 Ma) supported by the presence of Zeaglobigerina druryi (FO 17.4 Ma) and common Globoquadrina dehiscens (local absence zone 18.6-17 Ma). It is possible that this core-catcher sampling interval spans a slightly wider age interval, using the common presence of Catapsydrax stainforthi and the LO of Catapsydrax dissimilis in Sample 181-1123-20X-CC. The latter two events have LOs of ~16.4 and 17.3 Ma, respectively, in temperate-subtropical realms. On the other hand, it is possible that the datum of one or several of the taxa involved may have to be revised, once more precise ranges are known, using samples selected from between core-catchers. This interval can be subdivided by the presence of Paragloborotalia bella (LO 16.3 Ma) in Sample 181-1123C-19X-CC. Above is early Clifdenian Stage and below is late Altonian Stage.
Sample 181-1123C-21X-CC (517 mbsf) is early Miocene (18.6-16.7 Ma, middle Altonian Stage) based on the occurrence of common Globorotalia zealandica (acme zone 18.6-16.7 Ma), Sphaeroidinellopsis disjuncta (FO 18.5 Ma), and absence of Globoquadrina dehiscens (local absence zone 18.6-17 Ma). Several Bolboforma taxa are present in this sample.
Sample 181-1123C-22X-CC has a sparse, partly dissolved planktonic assemblage, largely composed of Catapsydrax dissimilis and abundant benthic foraminifers.
Samples 181-1123C-23X-CC, 25X-2, 32-34 cm (limestone boulder in debris flow), 27X-CC, and 28X-CC (537-584 mbsf) contain Globorotalia incognita (FO 21.6, LO 18.5 Ma), indicative of an early Miocene age (21.6-18.5 Ma, late Otaian and early Altonian Stages), supported by the presence of Zeaglobigerina connecta (FO ~22.2 Ma). Other samples within this interval have sparse assemblages of nonspecific probable early Miocene taxa, containing mostly Catapsydrax dissimilis.
Samples 181-1123C-29X-CC and 30X-CC (593-600 mbsf) are latest Eocene to early Oligocene (32-34.3 Ma; early Whaingaroan Stage) based on the occurrence of abundant Subbotina angiporoides (LO 30 Ma) and Paragloborotalia gemma (LO 32 Ma, FO 35 Ma), and absence of Globigerinatheka index (LO 34.3 Ma). Sample 181-1123C-30X-CC contains rare Reticulophragmium aff. amplectens, known to extend into the Oligocene of the Norwegian Sea.
Samples 181-1123C-31X-CC to 33X-CC (611-626 mbsf) are late Eocene in age (Runangan to Bortonian Stages), based on the occurrence of abundant Globigerinatheka index (LO 34.3 Ma), Subbotina linaperta, and Porticulosphaera semiinvoluta (LO 35.3 Ma, FO 38.4 Ma). In New Zealand, the latter species' recorded range is restricted to the Runangan Stage (dated ~35.5-34.3 Ma); hence, Hole 1123C probably bottomed in limestones of 35.3-35.5 Ma.
Sample 181-1123C-31X-CC also contains Paragloborotalia gemma (FO 35 Ma), indicating that the top of this interval is 34.3-35 Ma (late Runangan Stage). Present in Sample 181-1123C-33X-CC is the distinctive, cosmopolitan, deep-water benthic Nuttallides truempyi, which has a New Zealand LO of 37 Ma (Bortonian Stage), but extends to the top of the Eocene elsewhere. At DSDP Site 277, Nuttallides truempyi's youngest occurrence is in the early Kaiatan Stage (near the base of NP19, ~36 Ma, Hollis et al., 1997). The basal Sample 181-1123C-32C-XX also contains incomplete specimens of Spiroplectammina indistinguishable from S. spectabilis. This taxon is widely recorded in Eocene strata in the circum-North Atlantic and becomes extinct at the top of the Eocene (Kaminski et al., 1989).
The following is a summary of foraminiferal ages in terms of the New Zealand stage classification, and of local chronological calibration of these stages, according to Table T2, in the "Explanatory Notes" chapter (see also Fig. F16 in this chapter):
Diatoms (Table T8) are present in varying abundance in the Pleistocene and are common in the Pliocene to uppermost Miocene sediments. In the upper to lower Miocene, diatoms are absent or so poorly preserved and rare that they are not useful for biostratigraphic dating. Only in the lower Oligocene to upper Eocene sediments below the unconformity recovered within Core 181-1123C-29X (587.3 mbsf) are diatoms again common.
In spite of the common abundance of diatoms in upper Neogene sediments, stratigraphic marker species of the middle to low latitudes are rare and occur inconsistently. Reworking of older diatoms is also found in practically all samples, making biostratigraphic evaluation difficult. The few species that could be used for biostratigraphic assignment were Nitzschia denticuloides, Denticulopsis praedimorpha, Hemidiscus ovalis, Nitzschia reinholdii, and Thalassiosira convexa (Fig. F16). In addition, the Hemidiscus karstenii acme, which is found in the Antarctic and subantarctic regions in the Pleistocene, seems to be present in this area as well. Whether the presence of H. karstenii occurs at the same time in both hemispheres, and whether it results from the northward displacement of subantarctic diatoms has to be tested with more detailed studies later and by correlation of its abundance fluctuations against the oxygen isotope record at this site.
The relatively large and dissolution-resistant marker species Cestodiscus reticulatus was not found in the lower Oligocene sediments. Fragments of a large Coscinodiscus species, which may belong to Coscinodiscus excavatus, were encountered in Samples 181-1123-29X-CC and 30X-CC. This, together with the occurrence of species of the genus Rocella belonging to the complex of R. vigilans, place these cores into the early Oligocene. Samples 181-1123-31X-CC to 33X-CC are characterized by the presence of Hemiaulus polycystinorum var. mesolepta, Hemiaulus caracteristicus, Riedelia lyriformi, and Riedelia claviger, which place these cores into the middle to late Eocene.
Reworking of Eocene and Oligocene diatom valves (e.g., of the species Paralia architecturalis, Pyxilla reticulata, Triceratium barbadense, T. kanayae, and T. inconspicuum var. trilobata) is observed throughout the Neogene, especially in the more clay-rich, green-gray sediment intervals. Occasionally, however, Miocene and Pliocene diatoms also are reworked into younger sediments.
Silicoflagellates are not common, but nevertheless provide age-diagnostic markers such as Mesocena quadrangula in the Pleistocene, Dictyocha neonautica, the characteristic species for the D. fibula Zone, which straddles the Miocene/Pliocene boundary, Mesocena diodon borderlandensis, which is characteristic of the late Miocene D. brevispina Zone, and Mesocena diodon diodon and Paradictyocha apiculata, which are characteristic of the Miocene C. triacantha Zone. Reworked species among the silicoflagellates include Corbisema inermis minor, Dictyocha spinosa, Mesocena oamaruensis, M. occidentalis, Naviculopsis foliacea foliacea, and Valacerta tumidula.
Radiolarian biostratigraphy at Site 1123 is based on the examination of 70 core-catcher samples and two core samples (Table T9). Radiolarian faunas are generally abundant and well preserved throughout the section (Samples 181-1123A-1H-CC to 181-1123C-33X-CC, 0-625.76 mbsf). Extensive reworking of Paleocene and Eocene radiolarians is common throughout the section, and middle Miocene radiolarians occur frequently in the upper part of the section. Only three out of the processed samples proved to be barren (Samples 181-1123B-38X-CC, 50X-CC, and 181-1123C-23X-CC). Radiolarian datum levels applied at Site 1123 are shown in Table T10, and an age-depth plot using these radiolarian datum levels is shown in Figure F18. The radiolarian zones used at this site are of the low-latitude zonation of Sanfilippo and Nigrini (1998) and partly of the middle-latitude zonation of Foreman (1975) and Morley (1985).
The last occurrence of Stylatractus universus (LO 0.46 Ma) occurs in Sample 181-1123A-3H-CC, and this species was not found in the upper two samples (181-1123A-1H-CC and 2H-CC). The uppermost interval (0-23 mbsf), therefore, is younger than 0.46 Ma. In Sample 181-1123A-4H-CC (34.7 mbsf), the last occurrence of Axoprunum angelinum was also recognized. Stylatractus universus has been considered as a junior synonym of Axoprunum angelinum by some authors, or vice versa. The former taxon has been frequently recorded in the Southern Hemisphere, whereas the latter tends to occur in the Northern Hemisphere. However, examination of samples at this site reveals that both forms co-exist in the same samples at the upper part of the section. They are morphotypically different and A. angelinum has a much longer stratigraphic range. For this reason, both species are listed as legitimate taxa and used for correlation at this site. The samples between 181-1123A-3H-CC and 5H-CC (23-44 mbsf) are assigned to the Stylatractus universus Zone, based on the occurrence of a single specimen of Eucyrtidium matuyamai in Sample 181-1123A-6H-CC.
Sample 181-1123A-7H-CC (63 mbsf) contains the FO datum of Theocorythium trachelium (FO 1.6-1.7 Ma in the equatorial Pacific) and the LO of Eucyrtidium calvertense (LO 1.92 Ma in the subantarctic). A middle to early Pleistocene age (0.46-1.8 Ma) is indicated for the interval between Samples 181-1123A-3H-CC and 7H-CC (23-63 mbsf).
Sample 181-1123B-16H-CC indicates an age of 3.9 Ma, based on the FO of Amphirhopalum ypsilon, which is consistently present in the upper section. Sample 181-1123B-17H-CC (158.57 mbsf) also indicates an age of 3.7 Ma, based on the LO of Lychnodictyum audax.
The interval between Samples 181-1123A-15H-CC and 181-1123B-21X-CC (140-191 mbsf) is assigned to the Sphaeropyle langii Zone of Foreman (1975), based on the last occurrence of Stichocorys peregrina (LO ~2.9 Ma) in Sample 181-1123A-15H-CC and the FO of Sphaeropyle langii (6.0-6.2 Ma) in Sample 181-1123B-21X-CC. A Pliocene and latest Miocene age is indicated for this interval.
Sample 181-1123B-19X-CC (172 mbsf) is of latest Miocene age (5.0-5.2 Ma) based on the Dictyophimus splendens (LO 5.0-5.2 Ma in the North Pacific). It first occurs in Sample 181-1123B-27X-CC and last occurs in Sample 181-1123B-19X-CC, and is, therefore, a good short- range marker species (172-250 mbsf) in this section. In addition, Lychnocanoma parallelipes (LO 5.6 Ma; FO 6.8-7.3 Ma in the Northwest Pacific) also shows a short range from Samples 181-1123B-21X-CC to 29X-CC (191-268 mbsf), which reinforces the age assignment of latest Miocene (5.2-7.3 Ma) for this interval.
Samples between 181-1123B-21X-CC and 32X-CC (191-296 mbsf) are assigned to the Stichocorys peregrina Zone of Foreman (1975), the base of which is defined by the evolutionary transition (7.7 Ma in the northwest Pacific) from Stichocorys delmontensis to Stichocorys peregrina in Sample 181-1123B-32X-CC. This zone spans the time interval from 5.0-6.2 to 7.7 Ma, and its base is diachronous between low and middle latitudes (e.g., 6.71 Ma; base of the Stichocorys peregrina (RN9) Zone of Sanfilippo and Nigrini, 1998).
Within the RN9 Zone, Stylacontarium aquilonium first occurs in Sample 181-1123B-22X-CC (201 mbsf), and the datum of rapid increase in abundance of Stichocorys peregrina (RI event at 6.8-7.3 Ma) is recognized in Sample 181-1123B-29X-CC (268.5 mbsf). The interval between 296 and 326 mbsf is of middle late Miocene (8.2-9.7 Ma) age, based on the presence of Didymocyrtis antepenultima (LO 8.2 Ma, FO 9.7 Ma) in Samples 181-1123B-32X-CC and 35X-CC.
Cyrtocapsella japonica is sporadically present in varying abundances in the lower part of the section (181-1123B-41X-CC to 181-1123C-28X-CC). Its LO (9.9 Ma) and acme (10.2 Ma) horizons are recorded in Samples 181-1123B-41X-CC and 42X-CC, respectively. However, these horizons may occur higher than this apparent position, because the paucity of these forms or barren radiolarian occurrence in the higher Samples from 181-1123B-37X-CC to 39X-CC (345-364 mbsf).
The interval between Samples 181-1123B-42X-CC and 45X-CC (388-415.6 mbsf) contains abundant Stichocorys delmontensis, Cyrtocapsella tetrapera, Cyrtocapsella japonica, Lithopera neotera, and Didymocyrtis laticonus, which is assigned to the Diartus petterssoni (RN6) Zone of Sanfilippo and Nigrini (1998). The LO of Cyrtocapsella cornuta occurs in Sample 181-1123B-45X-CC (417 mbsf), which indicates an age of 11.6-12.3 Ma.
Common Cyrtocapsella tetrapera occurs consistently throughout the lower part of the section from Samples 181-1123B-44X-CC to 181-1123C-27X-CC (408.3-574.96 mbsf) and decreases rapidly between 408.3-399.52 mbsf, which suggests that Sample 181-1123B-44X-CC is of late middle Miocene age, based upon the dated age of 12.6 Ma for this rapid decrease event of this species.
Samples 181-1123B-46X-CC and 49X-CC (428-457 mbsf) are middle Miocene in age, based on faunas characterized by Botrystrobus miralestensis, Eucyrtidium punctatum, Cyrtocapsella tetrapera, Theocorys redondoensis, Lithopera renzae, and Theocorys spongoconum. The interval is assigned to the Dorcadospyris alata (RN5) Zone of Sanfilippo and Nigrini (1998).
In Sample 181-1123C-19X-CC (498 mbsf), the radiolarian fauna is characterized by the presence of common Cyrtocapsella tetrapera, common Cyrtocapsella cornuta, rare Cyrtocapsella japonica, and few Eucyrtidium punctatum (FO 17.02 Ma), together with rare Carpocanopsis bramlettei, Dorcadospyris alata, rare Stichocorys wolfii, Theocorys spongoconum, and few Phormocyrtis alexandrae. In addition, the LO of Phormocyrtis alexandrae (= Eucyrtidium sp. B of Sakai) is placed in the same sample, whereas this species is known to occur in the lower half of the Dorcadospyris alata Zone (Sakai, 1980). Furthermore, Phormocyrtis alexandrae and Theocorys spongoconum co-occur in the basal part of the Dorcadospyris alata Zone (Sakai, 1980). This suggests that the sample is early to middle Miocene (15-17 Ma) in age, and it correlates with the basal part of the Dorcadospyris alata (RN5) Zone and the Calocycletta costata (RN4) Zone of Sanfilippo and Nigrini (1998).
The interval between Samples 181-1123C-20X-CC and 23X-CC (506-536.7 mbsf) yields few radiolarians, including Cyrtocapsella tetrapera and Cyrtocapsella japonica, which suggests an age of middle to lower Miocene.
The interval between Samples 181-1123C-24X-CC and 27X-CC (546-575 mbsf) contains rare, but some age-diagnostic species including Stichocorys delmontensis (FO 20.53 Ma), Cyrtocapsella tetrapera (FO 23.62 Ma), Dorcadospyris alata, Lychonocanoma elongata (FO 24.6 Ma), Dorcadospyris mahulangi, Anthocyrtidium marieae, and Phormocyrtis alexandrae. This interval indicates an early Miocene age (17.5-20 Ma), because the last three species are known to occur from the lower Miocene Puriri Formation, Kaipara Harbour, New Zealand (O'Connor, 1994, 1997a, 1997b) and are correlated to the upper Stichocorys delmontensis Zone or lower Stichocorys wolffii Zone.
An age-depth plot for Site 1123 was constructed as shown in Figure F18, by using radiolarian datum levels including the FO, LO, RD (rapid decrease horizon), RI (rapid increase horizon), and an evolutionary transition. Accordingly, the sedimentation rate for the section (0-417 mbsf) is ~34 m/m.y., which is concordant with the rates calculated based upon other microfossil biochronologies and magnetostratigraphy.
Samples 181-1123C-29X-CC and 30X-CC, from a white to light gray limestone, contain well-preserved radiolarian assemblages dominated by Stylacontarium bispiculum, Lophocyrtis longiventer, Lophocyrtis dumitricai, and Aphetocyrtis gnomabox. This fauna is assigned to the Axoprunum(?) irregularis Zone of Takemura and Ling (1997), based on the rare occurrence of Eucyrtidium antiquum (FO 32.8-33.1 Ma), which indicates an early Oligocene age (~33 Ma).
Samples 181-1123C-31X-CC and 32X-CC (611-617 mbsf) contain common Eucyrtidium spinosum (LO 31.7-31.9 Ma) and rare Eucyrtidium antiquum (FO 32.8-33.1 Ma) together with abundant Lophocyrtis longiventer and few Lophocyrtis aspera. The two samples are assigned to the lower Axoprunum (?) irregularis Zone and upper Eucyrtidium spinosum Zone of Takemura and Ling (1997) because Eucyrtidium spinosum co-occurs with Eucyrtidium antiquum in Sample 181-1123C-31X-CC but not in Sample 181-1123C-32X-CC. The estimated age for these samples is placed at about the Oligocene/Eocene boundary (31.7-33.5 Ma).
Sample 181-1123C-33X-CC (625.76 mbsf) yields a radiolarian fauna containing abundant Lychnocanoma amphitrite, Calocyslas (?) nakasekoi, Lophocyrtis dumitricai, and Dicolocapsa microcephala, indicative of a late Eocene age.
Only in the uppermost part (upper Pleistocene) of the hole are the foraminiferal assemblages well preserved and lacking indications of selective dissolution to varying degrees. It is clear that the site was below the lysocline for much of the Miocene and Pliocene. In many instances, the remaining foraminiferal assemblages are particularly sparse and required washing large core-catcher samples to obtain ~100 planktonic specimens. A preliminary study of pairs of dark-colored (cool hemipelagic) and light-colored (warm pelagic) lithologies within the lower Miocene and Pliocene indicates that planktonic foraminiferal percentages and abundances are lower in the darker lithologies than in the lighter ones. This suggests that dissolution was stronger during the cooler periods than during the warmer ones and, possibly, that planktonic productivity was higher during the warmer intervals. Dissolution is prevalent, however, throughout both cool and warm intervals.
Many of the planktonic assemblages lack or have very few representatives of dissolution-susceptible species (not always thin-walled). For example, particularly severe corrosion by dissolution removed or broke most specimens of Globigerina, Globigerinoides, Praeorbulina, Orbulina, and Globorotalia panda in the middle Miocene, and Globorotalia praescitula in the early Miocene.
Many of the early Miocene assemblages are dominated by thick-walled Catapsydrax, a taxon that in adjacent New Zealand rarely composes more than 10% of even a deep-water planktonic assemblage of the same age. Typically, the Neogene planktonic assemblages contain thick-walled, less porous, larger planktonics (e.g., the genera Globorotalia, Sphaeroidinellopsis, Neogloboquadrina, and Catapsydrax) that appear to have survived dissolution on the seafloor and become preferentially concentrated in the assemblage. These large forms are accompanied by an apparently random sprinkling of often thinner walled, smaller planktonic species that have perhaps survived by being moved down into the sediment by bioturbation soon after their arrival on the seafloor. Fragmented, partially dissolved, planktonic tests and chambers are common in most samples. Many of the surviving larger specimens have the last, less calcified chamber dissolved (e.g., Globorotalia miozea lineage forms).
The evidence suggests that, throughout most of the Neogene, the site was swept by corrosive bottom waters. This is most marked in the Miocene, where many assemblages have fewer than 10% planktonic foraminifers, compared with a normal oceanic assemblage above the lysocline, which would have >99% planktonic forms.
Site 1123 planktonic assemblages exhibit considerable evidence of their location north of the Subtropical Convergence. Numerous subtropical and temperate taxa not found in previous sites to the south (Sites 1119-1122), occur here: Catapsydrax parvulus, C. stainforthi, Globigerinoides bolli, G. conglobatus, G. ruber, Globorotalia juanai, G. tumida, G. tosaensis, Neogloboquadrina acostaensis, Sphaeroidinella dehiscens, Sphaeroidinellopsis paenedehiscens, Zeaglobigerina druryi, and Z. nepenthes.
The early Oligocene and late Eocene foraminiferal assemblages have more normal planktonic foraminiferal compositions in both abundance (~99% of foraminifers) and diversity, reflecting oceanic conditions at lower bathyal depths.
The middle Miocene to Holocene benthic foraminiferal assemblages are relatively constant and typical of mid-bathyal to upper abyssal depths. Common taxa include Chilostomella, Cibicidoides pachyderma, C. wuellerstorfi, Eggerella bradyi, Epistominella exigua, Globocassidulina, Gyroidina, Laticarinina pauperata, Martinotiella communis, Melonis barleeanum, M. pompilioides, Nodosaria longiscata, Oridorsalis umbonatus, Pyrgo murrhina, Pullenia bulloides, P. quinqueloba, Quinqueloculina venusta, Sigmoilopsis schlumbergeri, Sphaeroidina bulloides, and Uvigerina dirupta. It is difficult to determine to what degree dissolution has modified the benthic assemblages, but undoubtedly selective dissolution has removed many thin-walled or more porous taxa and concentrated the more solution-resistant taxa (e.g., Oridorsalis). Much detailed quantitative study will be required to determine significant benthic assemblage changes through the section that may have been influenced by oceanographic changes.
Significant changes in the benthic foraminiferal assemblages are apparent in the upper Eocene, such as the loss of common Nuttallides truempyi, and in the upper lower Miocene, such as the loss of Planulina renzi and the appearance of Cibicidoides wuellerstorfi.
The clasts within the debris flow in Cores 181-1123C-24 and 25 contain foraminiferal assemblages of early Miocene age and of bathyal to abyssal depth, similar characteristics to the fauna of the section into which they were emplaced.
The diatom assemblages are temperate to subtropical and show two special characteristics: (1) dominance by high-productivity species belonging to Thalassionema and Thalassiothrix, along with resting spores of Chaetoceros; (2) displaced subantarctic diatoms, which are inferred to have been entrained into the sediments by the contour current formed by the upper Lower Circumpolar Deep Water throughout the latest Miocene to Pleistocene. In the Paleogene sediments, no such influence is recognizable.