Previous   |    Next

doi:10.2204/iodp.proc.342.106.2014

Biostratigraphy

Coring at Site U1405 recovered a 307 m thick sequence of Pleistocene to upper Oligocene clays, biosiliceous nannofossil ooze, and nannofossil ooze. Nannofossils and planktonic foraminifers are present in the uppermost brown foraminifer sandy clay and nannofossil ooze (Core 342-U1405A-1H; 0–6.18 mbsf) and indicate Quaternary to Pliocene ages (nannofossil Zones NN19–NN15/NN14). We were unable to provide biostratigraphic ages for the interval between 6.18 and 26.20 mbsf because of the absence of siliceous and calcareous microfossils. Below 26.20 mbsf, nannofossils, planktonic foraminifers, and radiolarians provide a well-defined biostratigraphy indicating upper lower Miocene to upper Oligocene sediment with relatively high sedimentation rates (1.4–10 cm/k.y.). Benthic foraminifers are generally rare throughout the entire sequence (the “present” category) but are well preserved and dominated by infaunal taxa, suggesting high organic matter flux to the seafloor.

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F16. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F17. Datum and zonal determinations from nannofossils, planktonic foraminifers, and radiolarians are in close agreement. The incorporated paleomagnetic data have helped to identify four relatively short hiatuses, each of ~1–2 m.y. duration, within the lower Miocene between 13.5 and 22 Ma. These hiatuses were not evident at the resolution of microfossil zones. Conversely, extremely high rates of sediment accumulation are inferred for the interval encompassing the Oligocene/Miocene boundary. Rates of 4.0–10.4 cm/k.y. are recorded between 22 and 24.5 Ma.

A summary of calcareous and siliceous microfossil abundance and preservation is given in Figure F18.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and additional working section half samples from Holes U1405A and U1405B. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Calcareous nannofossil occurrence data are shown in Table T4. Note that the distribution chart is based on shipboard study only and is, therefore, biased toward age-diagnostic species.

At Site U1405, the preservation of calcareous nannofossils is generally moderate and good but is occasionally poor in carbonate-poor intervals. Preservation is particularly good in pale, carbonate-rich beds around the Oligocene–Miocene transition (Fig. F18), evidenced by the presence of well-preserved spinose sphenoliths and holococcoliths (Fig. F19).

The uppermost sediment in Hole U1405A (Samples 342-U1405A-1H-1, 107 cm, to 1H-CC; 1.07–6.18 mbsf) contains abundant nannofossils indicative of Pleistocene–Pliocene Zones NN19–NN15/NN16 marked by the presence of Pseudoemiliania lacunosa (Sample 1H-1, 107 cm; 1.07 mbsf), top of Discoaster brouweri (Sample 1H-3, 107 cm; 4.07 mbsf), top of Discoaster surculus (Sample 1H-4, 60 cm; 5.10 mbsf), top of Reticulofenestra pseudoumbilicus (Sample 342-U1405A-1H-4, 120 cm; 5.70 mbsf), and top of Amaurolithus spp. and base of Ceratolithus rugosus (Sample 1H-CC; 6.18 mbsf). From Sections 342-U1405B-2H-CC through 3H-CC (11.90–25.72 mbsf), the sediment is noncalcareous and barren of nannofossils.

The interval from Sample 342-U1405A-4H-1, 100 cm, to 33H-CC (26.20–307.14 mbsf) contains frequent to abundant calcareous nannofossils ascribed to lower Miocene–upper Oligocene Zones NN4–NP25 primarily based on the top and base of Sphenolithus heteromorphus, top and base of Sphenolithus belemnos, base of Discoaster druggii, top of Sphenolithus delphix, and top of Sphenolithus ciperoensis. The Miocene/Oligocene boundary is approximated by the top of the short-ranging species S. delphix and top of Sphenolithus capricornutus (Fig. F19). S. delphix has a reported longevity of 100 k.y. (see Table T1 in the “Methods” chapter [Norris et al., 2014a]) and is found through a 27 m interval at Site U1405 (Samples 342-U1405A-19H-6, 46 cm, to 22H-4, 110 cm; 175.66–201.80 mbsf).

The Miocene/Oligocene boundary interval also contains at least one carbonate-rich layer with common to abundant Braarudosphaera (Fig. F19) and may include thin (1–2 mm), discrete Braarudosphaera ooze layers disrupted by bioturbation. Similar layers have been described from the lower Oligocene of the North and South Atlantic Ocean (Parker et al., 1985). Braarudosphaera is normally restricted to shelf seas (Bown, 2005), so these range expansions may be caused by unusual paleoceanographic conditions, such as increased upwelling or reconfiguration of circulation that sweeps shelf waters away from the margin (Kelly et al., 2003).

Radiolarians

Radiolarian biostratigraphy is based on analysis of selected core catcher samples from Hole U1405A. No samples from Hole U1405B were examined. Radiolarians are absent from the undated upper part of Hole U1405A. They are abundant and well preserved through the lower Miocene and upper Oligocene but are rare and poorly preserved in the lower Oligocene at Site U1405. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and the radiolarian distribution is shown in Table T6. Note that the distribution chart is based on shipboard study only and is, therefore, biased toward age-diagnostic species.

Uppermost Sections 342-U1405A-1H-CC through 3H-CC (6.18–25.72 mbsf) are barren or contain only rare and poorly preserved radiolarians.

Radiolarians are abundant and well preserved in Sections 4H-CC through 24H-CC (35.29–224.59 mbsf), and the assemblages can be correlated to lower Miocene–uppermost Oligocene radiolarian Zones RN4–RP22. Diatoms are also common to abundant in most samples examined. Sample 342-U1405A-4H-CC (35.29 mbsf) is correlated with the lower part of Zone RN4 based on the range overlap of Calocycletta costata and Carpocanopsis cingulata. Sample 6H-CC (54.14 mbsf) is correlated with the lower part of Zone RN3 based on the range overlap of Stichocorys wolffii and Dorcadospyris dentata. Sections 10H-CC through 12H-CC (92.19–110.85 mbsf) are correlated with Zone RN2 based on the range overlap of Stichocorys delmontensis and Dorcadospyris ateuchus. The top of Theocyrtis annosa is also identified in Sample 12H-CC (110.85 mbsf). Sections 15H-CC through 17H-CC (139.50–158.48 mbsf) are correlated with Zone RN1 based on the co-occurrence of Cyrtocapsella tetradica and Calocycletta virginis and the absence of S. delmontensis. Sections 19H-CC through 24H-CC (177.05–224.59 mbsf) are correlated with Zone RP22 based on the range overlap of Lychnocanium elongata and Artorphormis gracilis, as well as the absence of C. tetradica. Zone RP22 spans the Oligocene/Miocene boundary. Two intrazonal bioevents that have been found to help define the boundary in the low-latitude Pacific (Nigrini et al., 2006; Kamikuri et al., 2012), the base of Eucyrtidium diaphanes and the top of Dorcadospyris papilio, occur in Sample 22H-CC (206.11 mbsf).

Radiolarians are rare and poorly preserved in the interval 342-U1405A-25H-7, 0–5 cm, through 33X-CC (234.05–307.01 mbsf), and age-diagnostic taxa have not been identified.

Planktonic foraminifers

Core catchers and additional samples from Hole U1405A working section halves were examined. Samples contained, for the most part, diverse and well-preserved assemblages of planktonic foraminifers except for an interval in the lower Miocene (foraminifer Zones M2–M4), where samples were sporadically barren. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T7. The stratigraphic distribution of planktonic foraminifers is shown in Table T8 and Figure F18. The substantially improved abundance and preservation of planktonic foraminifers at Site U1405 (4285 mbsl) in comparison to Site U1404 (4747 mbsl) suggests that the mean position of the lysocline may have been located between these two sites throughout the early Miocene and Oligocene.

The uppermost interval from Samples 342-U1405A-1H-1, 105 cm, to 2H-CC (1.06–16.13 mbsf) contains Globorotalia truncatulinoides, indicative of Holocene–Pleistocene age sediment. Sections 342-U1405A-4H-1 through 19H-6 (35.26–175.40 mbsf) contain reasonably diverse and well-preserved lower Miocene assemblages containing the marker species Praeorbulina sicana, Globorotalia birnageae, Catapsydrax dissimilis, Globorotalia praescitula, Globoturborotalita angulisutularis, and Paragloborotalia kugleri and Globoquadrina dehiscens, indicating a continuous succession at the zonal level from Zone M5/M6 to Subzone M1a. Figure F20 shows a typical lower Miocene assemblage, including the distinctive dentoglobigerinids, and the first appearance of one of these, G. dehiscens, marks the Subzone M1a/M1b boundary. Figure F21 illustrates the remarkably good preservation of these lower Miocene planktonic foraminifers, revealing semiglassy taphonomy and even the presence of relict spines within the umbilici.

The base of P. sicana indicates Zone M5 in Sample 342-U1405A-5H-6, 110–112 cm (43.31 mbsf). Subzone M4b is marked by the base of Fohsella birnageae, which occurs in Sample 342-U1405A-7H-4, 110–112 cm (59.31 mbsf). The tops of C. dissimilis and Catapsydrax unicavus mark the top of Zone M3 and are observed in Sample 342-U1405A-8H-CC (73.02 mbsf). The base of G. dehiscens occurs in Sample 17H-CC (158.48 mbsf). The base of P. kugleri (e.g., Fig. F21) marks the Subzone M1a/Zone O7 boundary (22.96 Ma) and is observed in Sample 18H-4, 110–112 cm (163.84 mbsf), marking a position just above the Oligocene/Miocene boundary (23.03 Ma).

Samples 342-U1405A-19H-CC through 30X-CC (177.05–272.73 mbsf) contain few to abundant well-preserved planktonic foraminifers of late Oligocene age. The tops of Paragloborotalia pseudokugleri (Sample 26H-4, 5–7 cm) and Globigerinoides primordius (Sample 342-U1405A-26H-CC) enabled delineation of upper Oligocene Zones O7 and O6.

Below Sample 342-U1405A-31X-1, 55–55 cm (280.85 mbsf), planktonic foraminifers are rare or absent, preventing zonal demarcation.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1405A and from working section half samples of Cores 342-U1405A-4H through 33X. Additional working section-half samples taken from Cores 1H and 2H were examined for preservation and relative abundance of total benthic foraminifers and individual morphogroups. Benthic foraminifers at this site are generally rare (the “present” category) relative to total sediment particles >150 μm (Fig. F18). Few samples in Cores 8H through 12H are barren of benthic foraminifers. Assemblages of Cores 4H through 32X are mainly represented by infaunal species, such as Cassidulina subglobosa, suggesting high primary production and organic matter flux to the seafloor.

Preservation of foraminifer tests is generally good to very good (Figs. F18, F22). The occurrences of benthic foraminifers at this site are shown in Tables T9 and T10.

Samples 342-U1405A-1H-CC and 2H-CC are dominated by a Pliocene–Pleistocene assemblage of the calcareous taxa Cibicidoides spp., Eponides regularis, Laticarinata pauperata, Oridorsalis umbonatus, and Pullenia spp. Agglutinated benthic foraminifers, such as Eggerella bradyi and Spiroplectammina sp., occur frequently.

Although benthic foraminifers occur almost continuously in Cores 342-U1405A-4H through 33X (27.70–301.54 mbsf), their abundance is low (the “present” category) throughout this interval. This lower Miocene to upper Oligocene sequence contains diverse calcareous benthic foraminifer assemblages of good to very good preservation. The assemblages are dominated by C. subglobosa and Stilostomella spp., including mainly Stilostomella gracillima, Stilostomella lepidula, and Stilostomella subspinosa. Abundant calcareous taxa are typically Anomalinoides sp., Dentalina sp., Epistominella umbonifera?, Gyroidinoides sp., Pullenia bulloides, Pullenia quinqueloba, and pleurostomellids. Agglutinated forms identified include abundant E. bradyi, whereas Bathysiphon sp. and Dorothia trochoides are subordinate.

Among calcareous species, infaunal taxa such as C. subglobosa, Dentalina sp., Gyroidinoides sp., P. bulloides, P. quinqueloba, and stilostomellids are notably abundant (Table T9), indicating a high organic matter flux to the seafloor. Abundant diatoms and radiolarians accompany the occurrence of these taxa, which implies high export production to the seafloor.

Benthic foraminifer assemblages at Site U1405 are strikingly similar to those at Site U1404, which is situated ~300 m deeper on J-Anomaly Ridge. Assemblages at Site U1405, however, lack Globobulimina pacifica, which is an indicator species for bottom water dysoxia. G. pacifica is common in the lower Miocene to uppermost Oligocene sequence at Site U1404 but occurs only sporadically as single specimens in Samples 342-U1405A-11H-6, 85–87 cm (100.06 mbsf); 14H-6, 70–72 cm (128.44 mbsf); and 18H-6, 86–88 cm (166.62 mbsf). The occurrence of assemblages indicative of high primary productivity and organic matter flux to the seafloor at both Sites U1404 and U1405 suggests a similar paleoenvironment, but with higher bottom water dissolved oxygen concentrations indicated at the shallower Site U1405.

The lowermost samples from Hole U1405A (Samples 33X-4, 92–94 cm, to 33X-CC; 304.63–307.01 mbsf) show significantly lower abundances of infaunal taxa and an increase in epifaunal taxa such as Anomalina sp. and Oridorsalis umbonatus, suggesting a lower organic matter flux to the seafloor than in the overlying sequence (27.70–301.54 mbsf).

Top of page   |    Previous   |    Next