Previous   |   Next

doi:10.2204/iodp.proc.320321.107.2010

Biostratigraphy

At Site U1335, we recovered a 420 m thick succession of Pleistocene to upper Oligocene sediments, including an expanded sequence of lower Miocene nannofossil ooze and chalk. A Pliocene–Pleistocene succession of nannofossil and radiolarian ooze was recovered from the top 29 m of the hole. This was deposited on top of 135 m of upper and middle Miocene nannofossil ooze and an expanded succession of 185 m of lower Miocene nannofossil ooze. A 71 m thick interval of upper Oligocene nannofossil chalk was recovered above basement. Radiolarians are present through most of the section apart from the basal 3 m of nannofossil chalk. They provide a coherent, high-resolution biochronology through a complete sequence of radiolarian zones from Zone RN14 (Pleistocene) to Zone RP21 (upper Oligocene). Calcareous nannofossils are present and moderately to well preserved through most of the succession. There appears to be a complete sequence of nannofossil zones from Zone NN20-21 (Pleistocene) to Zone NP25 (upper Oligocene). Planktonic foraminifers are generally well preserved throughout the section, enabling a complete sequence of planktonic foraminifer zones from Zone PT1a (Pleistocene) to Zone O6 (upper Oligocene) to be recognized. Nannofossil, radiolarian, and planktonic foraminifer datums are in good agreement. An integrated calcareous and siliceous microfossil biozonation is shown in Figure F13. A detailed age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F14. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples from both holes and from samples from most core sections of Hole U1335A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T4. Nannofossils are generally abundant and moderately to well preserved throughout but with some intervals of poor to moderate preservation in the upper Miocene (Cores 320-U1335-4H through 8H; Zones NN7–NN12).

Nannofossils are present and moderately to well preserved within the topmost sample (320-U1335A-1H-1, 40 cm) with an assemblage dominated by abundant Gephyrocapsa spp., Calcidiscus leptoporus, and Florisphaera profunda. This sample is assigned to an undifferentiated Zone NP20-21 because of the presence of questionable Emiliania huxleyi. The top of Pseudoemiliania lacunosa in Sample 320-U1335A-1H-3, 100 cm, marks the top of Zone NN19. A continuous succession of nannofossil datums (Table T4) suggests that there is near-continuous stratigraphy through the Pliocene–Pleistocene. It was not possible to reliably pick some of the additional datums within Zone NN19 that are based on size changes within the genus Gephyrocapsa. Because of relatively low sedimentation rates, however, several well-calibrated datums that are separated by 10–100 k.y. within the lower Pleistocene–Pliocene interval are found at the same level in the present section-resolution biostratigraphy (the top of Calcidiscus macintyrei at 1.61 Ma together with the top of Discoaster brouweri at 1.93 Ma, the top of Discoaster pentaradiatus at 2.39 Ma with the top of Discoaster surculus at 2.49 Ma, the top of Sphenolithus spp. at 3.54 Ma with the top of Reticulofenestra pseudoumbilicus at 3.70 Ma, the top of Ceratolithus acutus at 5.04 Ma with the top of Ceratolithus rugosus at 5.05 Ma, and the top of Triquetrorhabdulus rugosus at 5.28 Ma with the base of C. acutus at 5.35 Ma). Very low abundances or absence of some additional marker species also prevented the use of their associated datums, including the top of Helicosphaera sellii, base of common Discoaster triradiatus, and base of Ceratolithus larrymayeri. The Miocene/Pliocene boundary (5.33 Ma) is located between the datums at the top of T. rugosus at 5.28 Ma and base of C. acutus at 5.35 Ma, which both occur between Samples 320-U1335A-3H-CC and 4H-1, 66 cm.

Nannofossil datums within the 46 m sequence of upper Miocene nannofossil ooze indicate a sedimentation rate of ~7 m/m.y., similar to that within the Pliocene–Pleistocene interval (Fig. F14). Additional datums within Zone NN11, based on Nicklithus amplificus (top of N. amplificus and abundance crossover of N. amplificus/T. rugosus), were not possible to place because of the very rare occurrence of N. amplificus, which was only observed in Sample 320-U1335A-4H-6, 66 cm. Catinaster calyculus was absent. The co-occurrence of the base of Discoaster hamatus, base of Catinaster coalitus, and top of Coccolithus miopelagicus datums between Samples 320-U1335A-7H-4, 82 cm, and 7H-6, 79 cm, marks the Zone NN8/NN9 boundary. Sedimentation rates also remain low (~7 m/m.y.) in the lower part of the upper Miocene. The middle/late Miocene boundary (11.60 Ma) is placed close to the top common Discoaster kugleri (11.58 Ma), which occurs at 72.8 m CSF between Samples 320-U1335A-8H-4, 90 cm, and 8H-6, 90 cm.

Sedimentation rates are significantly higher below the middle/late Miocene boundary: ~18 m/m.y. to ~19 Ma, increasing to ~33 m/m.y. from ~19 Ma through to the base of the hole in the upper Oligocene (~25 Ma). Almost all lower and middle Miocene datums and nannofossil zones were recognized, although the Zone NN4/NN5 boundary could not be accurately determined because of the absence of Helicosphaera ampliaperta in these sediments. The additional datum of the base of Discoaster petaliformis (15.70 Ma) (Discoaster signus in Raffi et al., 2006) was, however, recognized between Samples 320-U1335A-16H-CC and 17H-2, 70 cm, within the upper part of Zone NN4. At Site U1335 there is a short interval at the base of Zone NN4 between the base common occurrence of Sphenolithus heteromorphus and the top common occurrence of Sphenolithus belemnos, where both of these taxa are pre.sent at low abundances (cf. Young, 1999), some of which may represent intergrading morphologies between these two morphotypes. The intra–Zone NN2 bioevents of the base of H. ampliaperta and the abundance crossover of Helicosphaera euphratis/Helicosphaera carteri could not be recognized in this succession because of the absence of H. ampliaperta and patchy distribution of other Helicosphaera species. The additional event of the top acme of Triquetrorhabdulus carinatus (Raffi et al., 2006) is, however, easily identified toward the base of Zone NN2. Although the placement of the Zone NN1/NN2 boundary, as defined by the base of Discoaster druggii, is usually problematic because of the rarity and poorly defined nature of this taxon (Young, 1999), D. druggii is conspicuously present in Samples 320-U1335A-36H-CC and 37X-2, 90 cm. These two samples also bracket the base of Sphenolithus disbelemnos, which has been used as a proxy for the Zone NN1/NN2 boundary (Young, 1999); this may indicate that D. druggii has a short interval of increased abundance at the base of its range and may still be of some use in determining the Zone NN1/NN2 boundary.

The placement of the Oligocene/Miocene boundary (23.0 Ma), based on the planktonic foraminifer event of the base of P. kugleri at 348.6 m CSF, is in good agreement with the topmost Oligocene nannofossil event, the top of S. delphix at 349.7 m CSF between Samples 320-U1335A-37X-6, 50 cm, and 37X-CC. There are three nannofossil events closely spaced around the Zone NP25/NN1 boundary: top of Sphenolithus ciperoensis, abundance crossover of Triquetrorhabdulus longus/T. carinatus, and top common of Cyclicargolithus abisectus. At Sites U1332–U1335, there is no clear "acme" event of C. abisectus; however, it does appear to have a clear and consistent top occurrence, at least in the eastern equatorial Pacific, close to the Zone NP25/NN1 boundary at the same level as the top acme calibration on the ODP Leg 199 timescale (Lyle, Wilson, Janecek, et al., 2002).

Radiolarians

The radiolarian stratigraphy at Site U1335 (Fig. F13; Table T5) spans the interval from Zone RN14 (lower Pleistocene) in Core 320-U1335-1H to the uppermost part of Zone RP21 (upper Oligocene) in the upper part of Core 320-U1335-44X (Samples 320-U1335-44X-2, 105–107 cm, and 44X-4, 105–107 cm) (Tables T6, T7). The assemblages are generally well preserved throughout the recovered section; however, there are several intervals in which the preservation is only moderate. Samples with moderate preservation are most commonly found in the upper part of the section within Core 320-U1335-7H (lowermost upper Miocene) and in Cores 320-U1335-12H through 15H (middle Miocene), 25H through 29H (lower Miocene), and 37H and 38H (lowermost lower Miocene). This intermittent decrease in preservation is not clearly associated with any particular lithologic observation, such as the occurrence of turbidites or color changes. Nor is it associated with reworked older microfossils. Reworked older radiolarians are found mixed within younger assemblages most commonly in Cores 320-U1335-1H through 4H, an interval with lower average accumulation rates (Fig. F14). In the remainder of the section, reworking of older microfossils is minor and found mainly in the lower middle Miocene part of the section.

The lack of a clear association of preservation and reworking with the occurrence of turbidites in the section (see "Lithostratigraphy") suggests that the material transported by these turbidites was nearly the same age as the levels at which they were deposited. However, the frequent occurrence of these turbidites could have an effect on the upper limit (last occurrence) of some of the stratigraphically important species (Table T5) by extending their apparent range upsection. Therefore, the first occurrence of a species is generally considered a more reliable datum in Zones RP18 through RP20.

In the expanded lower Miocene and Oligocene section it becomes clear that some of the species used in the radiolarian stratigraphy are discontinuous in their appearance. This was noticed in the Leg 199 material in such common and robust species as Acrocubus octopylus and Didymocyrtis tubaria (Nigrini et al., 2006). In Site U1335 we also see the discontinuous occurrences of these species, as well as in Dorcadospyris ateuchus and Lychnocanoma elongata. This may reflect large variation in the abundance of these species with time and changing ecologic conditions. However, there is also the possibility that at least some of these intermittent disappearances of a species may reflect genetic changes in the lineage that give rise to either "iterative evolution" or changing ecological preferences.

As at Sites U1331–U1334, the lower part of the succession (Cores 320-U1335A-40X through the upper part of 44X) has only moderate radiolarian preservation and the lowermost samples (Samples 320-U1335A-44X-CC through 45X-CC) are completely barren of radiolarians.

Diatoms

Diatoms were examined in core catcher samples from Hole U1335A and represent the interval from the Fragilariopsis reinholdii Zone through the Rocella gelida Zone of Barron (1985, 2006) and Barron et al. (2006). Diatoms range in abundance from rare to abundant. Diatom preservation is variable but generally moderate to good. Several intervals as discussed below reflect much poorer preservation, whereas others are better. Typical zonal indicators are not always present, requiring the use of secondary markers for zonal placement. Specific zonal assignments are as follows.

Sample 320-U1335A-1H-CC contains a Pleistocene diatom assemblage typified by Actinocyclus ellipticus, Asteromphalus elegans, Azpeitia nodulifera, Hemidiscus cuneiformis, Nitzschia fossilis, F. reinholdii, Thalassiosira eccentrica, and Fragilariopsis doliolus. The occurrences of F. reinholdii with F. doliolus allows placement of this sample into the F. reinholdii Zone.

Sample 320-U1335A-2H-CC is assigned to the Nitzschia jouseae Zone based on the occurrence of N. jouseae without Rhizosolenia praebergonii. The R. praebergonii Zone was not observed as a result of sample spacing.

The Thalassiosira convexa Zone is represented in Samples 320-U1335A-3H-CC and 4H-CC. Sample 320-U1335A-3H-CC is assigned to the upper portion of this zone based on the occurrences of Fragilariopsis cylindrica, T. convexa var. aspinosa, and Thalassiosira oestrupii without N. jouseae. The suggestion of this sample being equivalent to the upper portion of this zone is based on the presence of specimens transitional between F. cylindrica and N. jouseae. Sample 320-U1335A-4H-CC is assigned to Subzone A of the T. convexa Zone based on the occurrences of Thalassiosira praeconvexa, Nitzschia miocenica, T. convexa var. aspinosa, and Asterolampra acutiloba. The N. miocenica Zone was not observed because of sample spacing.

The occurrences of Nitzschia porteri and F. cylindrica without N. miocenica allows placement of Sample U1335A-5H-CC into the N. porteri Zone. Thalassiosira burckliana was not observed in the sample, suggesting placement of the sample in Subzone B of the N. porteri Zone.

Sample 320-U1335A-6H-CC is assigned to the Thalassiosira yabei Zone based on the occurrences of T. yabei, A. ellipticus var. javanica, and A. nodulifera var. cyclopsa. T. burckliana was not observed in the sample, suggesting placement of the sample in Subzone A of the T. yabei Zone.

The last occurrence of Denticulopsis simonsenii was recognized to have a chronostratigraphic occurrence similar to that of Actinocyclus moronensis and as such was identified as a useful datum for determining the top of the A. moronensis Zone. The common occurrence of D. simonsenii in Sample 320-U1335A-7H-CC suggests placement of this sample in the A. moronensis Zone.

The co-occurrence of Craspedodiscus coscinodiscus and Coscinodiscus gigas var. diorama allows placement of Sample 320-U1335A-8H-CC in the C. coscinodiscus Zone. The occurrence of A. ellipticus var. spiralis and Rossiella praepaleacea without H. cuneiformis suggests placement in the middle portion of this zone.

Samples 320-U1335A-9H-CC and 10H-CC are assigned to the C. gigas var. diorama Zone. The zonal assignment of Sample 320-U1335A-9H-CC is tentative. This sample is characterized by the presences of D. simonsenii, Rossiella paleacea, Cavitatus jouseanus, C. coscinodiscus, Crucidenticula punctata, and C. gigas var. diorama. The absence of Coscinodiscus temperi var. delicata suggests placement in the C. gigas var. diorama Zone, whereas the absence of Cestodiscus pulchellus suggests placement in the younger C. coscinodiscus Zone. Sample 320-U1335A-10H-CC is assigned to the C. gigas var. diorama Zone based on the occurrence of Annellus californicus and C. pulchellus without Coscinodiscus lewisianus.

Samples 320-U1335A-11H-CC and 12H-CC are assigned to the C. lewisianus Zone based on the occurrence of C. lewisianus. The occurrence of Thalassiosira tappanae in Sample 320-U1335A-12H-CC supports this zonal assignment and suggests that this sample is equivalent to the lower portion of this zone.

The interval from Samples 320-U1335A-13H-CC through 18H-CC contains few to common diatoms with poor to moderate preservation. Cestodiscus peplum, the marker for the Coscinodiscus peplum/C.  lewisianus boundary, was not observed. As such, zonal assignment for this interval is tentative. Samples 320-U1335A-13H-CC through 15H-CC are assigned to the lowermost portion of the C. lewisianus Zone through Subzone B of the C. peplum Zone based on stratigraphic position. Zonal diagnostic species were not observed in this interval. The occurrence of A. californicus in Samples 320-U1335A-16H-CC and 17H-CC allow placement of these samples into Subzone A of the C. peplum Zone. The occurrence of Cavitatus miocenica in Sample 320-U1335A-18H-CC suggests placement of this sample in Subzone B of the Crucidenticulopsis nicobarica Zone.

Sample 320-U1335A-19H-CC contains Crucidenticula kanayae, Coscinodiscus blysmos, C. jouseanus, Thalassiosira fraga, and C. lewisianus var. similis. The occurrences of C. kanayae and C. blysmos without C. miocenica suggest that this sample approximates the C. peplum/C. nicobarica Zonal boundary.

Samples 320-U1335A-20H-CC and 21H-CC are assigned to the C. nicobarica Zone. The occurrence of Raphidodiscus marylandicus and C. miocenica in Sample 320-U1335A-20H-CC suggests placement of this sample in the lower portion of Subzone B of the C. nicobarica Zone or older. The co-occurrence of C. lewisianus var. robustus and C. lewisianus var. similis in Sample 320-U1335A-21H-CC suggests assignment of this sample into the upper portion of Subzone A of the C. nicobarica Zone.

The zonal assignment for Sample 320-U1335A-22H-CC is tentative given the minimal number of zonal indicators observed in this sample. The occurrence of Triceratium pileus suggests assignment to the lower portion of Subzone A of the C. nicobarica Zone or to the T. pileus Zone. Samples 320-U1335A-23H-CC and 24H-CC are assigned to the T. pileus Zone based on the occurrence of Actinocyclus radionovae in both samples. The occurrence of Thalassiosira spinosa in Sample 320-U1335A-23H-CC supports this zonal assignment.

The occurrences of Coscinodiscus rhombicus and T. fraga in Samples 320-U1335A-25H-CC and 26H-CC allow assignment of these samples to the Craspedodiscus elegans Zone. Other species observed include T. spinosa, C. miocenica, and C. lewisianus.

Zonal assignments for Samples 320-U1335A-27H-CC through 30H-CC are tentative, as this interval is characterized by diatoms with poor to moderate preservation. Diatom fragments are typical in this interval. Species observed include C. rhombicus, C. lewisianus, C. miocenica, and A. radionovae. The occurrence of Actinocyclus hajosiae in Sample 320-U1335A-30H-CC suggests that this section is equivalent to the lower portion of this zone or upper portion of the Rossiella fennerae Zone. The absence of Bogorovia veniamini in this sample and the occurrence of B. veniamini in Sample 320-U1335A-31-CC stratigraphically below this interval supports the C. elegans Zone assignment for Sample 320-U1335A-30H-CC.

The R. fennerae Zone is assigned to Samples 320-U1335A-31H-CC through 36H-CC based on the occurrence of B. veniamini with R. fennerae. Subzones were not differentiated within this interval given the amount of fragmentation. The assemblage is characterized by Coscinodiscus barronii, C. rhombicus, B. veniamini, R. fennerae, C. miocenica, and C. jouseanus. R. paleacea occurs in Section 320-U1335A-32H-CC, suggesting placement in the upper portion of this zone.

Samples 320-U1335A-37X-CC through 42X-CC are assigned to the R. gelida Zone based on the occurrence of R. gelida. The occurrence of Rocella schraderi in Sample 320-U1335A-40X-CC suggests that Samples 320-U1335A-37X-CC through 40X-CC are equivalent to the upper portion of this zone. The occurrence of C. lewisianus var. rhomboides and Craspedodiscus baronii without R. schraderi in Sample 320-U1335A-42X-CC suggests the equivalent of the lower portion of the zone.

Sample 320-U1335A-43X-CC is assigned to the B. veniamini Zone based on the occurrence of Rocella vigilans and Cestodiscus kugleri without R. gelida. Diatoms were not observed in Sample 320-U1335A-44X-CC.

Planktonic foraminifers

Micropaleontological samples from the core catchers were taken from both holes of Site U1335. High-resolution biostratigraphy was undertaken on additional samples from Hole U1335A (two per core). Planktonic foraminifer biostratigraphy indicates a nearly continuous succession of zones ranging from the Pleistocene (Zone PT1a) throughout the upper Oligocene (Zone O6) (Fig. F13), which agree well with calcareous nannofossil and radiolarian biostratigraphy (Fig. F14). Preservation and abundance of planktonic foraminifers are generally good from the Pliocene through Miocene with samples frequently containing >70% planktonic foraminifers, but there are exceptions within this interval and lower preservation and abundances of planktonic foraminifers are recorded during the late Oligocene. The depths of the main datum biostratigraphic events and age estimates are shown in Table T8. Preservation and presence of planktonic foraminifers are shown in Table T9.

The topmost Samples 320-U1335A-1H-3, 100–102 cm, and 1H-5, 42–44 cm, are assigned to the first Pleistocene Zone PT1a that is distinguished between the top of G. fistulosus in Sample 320-U1335A-1H-CC and the top of Globorotalia (Truncorotalia) tosaensis. Accurate assignment of Zones PL1–PL6 is difficult because although planktonic foraminifers are abundant and assemblages are diverse (frequently >20 species), a number of age-diagnostic marker species were not found or were too rare to be employed as reliable datums (e.g., Globoturborotalita nepenthes, Globorotalia [Hirsutella] margaritae, and Dentoglobigerina altispira). Furthermore, in Core 320-U1335A-1H and Sample 320-U1335-2H-CC the <250 µm size fraction is composed almost entirely of planktonic foraminifer fragments. In these same cores there is also evidence of dissolution/fragmentation with only the "skeleton" of some species being preserved (i.e., individuals of the subgenus Menardella are frequently observed with only the keel and limbate sutures and missing walls of the final whorl chambers). Consequently, these assemblages are biased toward more robust taxa (i.e., Globorotalia tumida, Pulleniatina spp., and Sphaeroidinella dehiscens). In such cases, both primary datums and additional age-diagnostic species were employed to assign zones. Zone PL6 was identified between the LO of Globorotalia pseudomiocenica in Sample 320-U1335B-2H-CC (3.43 m CSF) and the top of G. fistulosus. Zones PL4–PL5 were identified between the top of Sphaeroidinellopsis seminulina in Sample 320-U1335A-3H-4, 38–40 cm, and the top of Globoturborotalita woodi and G. pseudomiocenica in Sample 320-U1335A-2H-2, 104–106 cm. It was not possible to further subdivide the zones because of the absence of the age-diagnostic taxon D. altispira within the high-resolution samples taken from Zones PL4 and PL5 in this sediment interval. Moreover, the last occurrence of this taxon, which is easily identifiable and often abundant, is recorded ~20 m deeper in Holes U1335A and U1335B than expected based on previous calibration studies in the equatorial Pacific that utilize the top of D. altispira as a datum event to define the Zone PL4/PL5 boundary (Chaisson and Pearson, 1997) at 3.47 Ma. Zones PL1–PL3 were identified between the base of G. tumida (Sample 320-U1335A-3H-CC; 28.13 m CSF) and the top of S. seminulina (Sample 320-U1335A-2H-4, 104–106 cm; 14.44 m CSF). The absence or rarity of G. (Hirsutella) margaritae and G. nepenthes prevented further subdivision. However, a number of supplementary age-diagnostic events were identified, including the base of S. dehiscens at 5.54 Ma (Chaisson and Pearson, 1997) and the top of Sphaeroidinellopsis kochi at 4.53 Ma (Curry et al., 1995), indicating the presence of Zone PL1.

Upper Miocene Zone M14 was determined between the lowest occurrence of G. tumida in Section 320-U1335A-3H-CC and the base of G. (Hirsutella) margaritae in Sample 320-U1335A-4H-CC. The first occurrence of Globorotalia plesiotumida is recorded in Sample 320-U1335A-6H-2, 38–40 cm, and defines the base of Zone M13b. Zones M13a and M12 were not divided because of the rare occurrence of Neogloboquadrina acostaensis toward the base of its stratigraphic range. Zone M11 was delimited by the top of Paragloborotalia mayeri (10.57 Ma) in Sample 320-U1335A-7H-6, 95–97 cm, and the base of Globoturborotalita decoraperta (11.49 Ma) in Section 320-U1335A-8H-CC, which falls just above the base of Zone M11. Above the last occurrence of Globorotalia (Fohsella) fohsi s.l. in Sample 320-U1335A-9H-2, 38–40 cm, is Zone M10 (N13). Zone M9b/N12 is defined by the total range of Globorotalia (Fohsella) fohsi robusta between Samples 320-U1335A-9H-4, 38–40 cm, and 11H-CC. Zone M9a/N12 was identified in Sample 320-U1335A-12H-2, 38–40 cm, by the base of G. (Fohsella) fohsi s.l. The base of Zone N10 was defined by the first occurrence of Globorotalia (Fohsella) "praefohsi." Secondary datum events such as the tops of Clavatorella bermudezi (13.82 Ma), Globorotalia (Fohsella) peripheroronda (13.80 Ma), and Globorotalia praescitula (13.73 Ma) take place within Zone M7 (N10–N11). The base of Zone M7 (N10) is defined by the base of Globorotalia (Fohsella) peripheroacuta. The base of Praeorbulina sicana occurs in Sample 320-U1335A-18H-CC, defining Zone M5, but further differentiation of Zones M5 and M6 was problematic given the absence of Praeorbulina glomerosa, Orbulina spp., and Globigerinatella insueta, which could be explained by the low preservation potential of these taxa. The latter may also account for the very shallow base of C. bermudezi recorded here. As a result, the exact position of some of the datums belonging to these zones might be slightly biased. The base of Globorotalia (Menardella) archeomenardii at 16.26 Ma (Curry et al., 1995) was identified in Sample 320-U1335A-16H-CC, indicating topmost Zone M5b or Zone M6/N9. The top of Catapsydrax dissimilis that defines the base of Zone M4 was difficult to determine because of low abundance toward the top of its stratigraphic range, but it was identified in Sample 320-U1335B-20H-CC. The absence of G. insueta prevented accurate determination of the M2/M3 zonal boundary. The datum event marked by the top of Globoquadrina binaiensis (19.09 Ma) was identified in Sample 320-U1335A-20H-CC (185.19 m CSF) and falls toward the very top of the undifferentiated Zones M2–M3, higher than expected. Zone M1b was determined by the co-occurrence of P. kugleri, Paragloborotalia pseudokugleri, and Globoquadrina dehiscens between Samples 320-U1335A-28H-CC and 33H-CC. Zone M1a occurs between the bases of G. dehiscens in Sample 320-U1335A-33H-CC at 22.4 Ma and P. kugleri in Sample 320-U1335A-37X-4, 136–138 cm, at 23.0 Ma. The Oligocene/Miocene boundary was also well defined in Hole U1335B between Samples 320-U1335B-37X-CC and 38X-CC using the base of P. kugleri. The lowest zone defined is Zone O6 in the late Oligocene and the presence of P. pseudokugleri indicates that only the upper part of the zone is present.

Benthic foraminifers

Benthic foraminifers from core catcher samples were examined semiquantitatively in all holes at Site U1335. Benthic foraminifers occurred almost continuously, except in Sample 320-U1335B-3H-CC and several other samples in the early Miocene and late Oligocene. The occurrence of benthic foraminifers at this site is shown in Table T10.

From Samples 320-U1335A-1H-CC to 9H-CC (8.84–85.05 m CSF), Oridorsalis umbonatus, Cibicidoides mundulus, and Globocassidulina subglobosa are common with a few subordinate Anomalinoides sp. A, Gyroidinoides spp., Latcarinina pauperata, Melonis pompilioides, and Melonis barleeanum. A similar fauna is found between Samples 320-U1335B-1H-CC and 9H-CC (3.46–79.96 m CSF). Preservation of the foraminifer tests is moderate to good. Pliocene and middle Miocene taxa recorded here indicate lower bathyal and abyssal paleodepths (van Morkhoven et al., 1986).

Samples 320-U1335A-10H-CC through 44X-CC (104.56–417.80 m CSF) generally contain common O. umbonatus, C. mundulus, G. subglobosa, and Gyroidinoides spp. with a few subordinate Anomalinoides sp. A, Astrononion echolsi, L. pauperata, and Pullenia spp. in several horizons. Several samples in the lower part of the interval (early Miocene and late Oligocene) were barren of benthic foraminifers (Samples 320-U1335A-28H-CC, 29H-CC, and 37X-CC; 265.99, 275.59, and 350.04 m CSF, respectively) or rare (Samples 320-U1335A-32H-CC, 33H-CC, and 44X-CC; 297.98, 313.51, and 394.90 m CSF, respectively), where present assemblages in these samples are often dominated by small individuals. Preservation of the foraminifer tests is moderate to good. Similar benthic foraminifer assemblages were also recognized in Hole U1335B (Samples 320-U1335B-10H-CC through 45X-CC; 89.06–411.88 m CSF), including a number of barren or rare horizons of benthic foraminifers (Samples 320-U1335B-32H-CC, 35H-CC, 36H-CC, and 43X-CC; 297.98, 326.87, 336.55, and 394.90 m, respectively); however, these were not always contemporaneous with those observed in Hole U1335A. These faunal compositions indicate lower bathyal and abyssal paleodepths during the middle Miocene to late Oligocene.

Top of page   |   Previous   |   Next