A continuous hemipelagic sedimentary section reaching the middle Miocene (15 Ma) was recovered from Site 1085. The micropaleontological studies were carried out on core-catcher samples from Hole 1085A. Additional samples from within the cores were examined to improve the biostratigraphic resolution. A high-resolution biostratigraphy was developed using calcareous nannofossils and planktonic foraminifers. Sedimentation rates range from 1.5 to 13 cm/k.y. The lowest sedimentation rates are within the middle Mio-cene (1.5 cm/k.y.) and the highest are within the Pleistocene (13 cm/k.y.). Two other intervals with high sedimentation rates occur within the early part of the late Pliocene (7 cm/k.y.) and across the Miocene/Pliocene boundary (8 cm/k.y.).
Calcareous nannofossils were studied in core-catcher samples from Hole 1085A. Additional samples from within the top 11 cores (~top 100 mbsf) were examined close to datum events to improve the stratigraphic resolution. Nannofossils are abundant and well preserved throughout the entire section. Reworking (trace; early Plio-cene specimen) was limited only to Cores 175-1085A-8H through 11H.
Based on the oldest identified datum, Site 1085 terminated within Zone NN5 (middle Miocene). Of the 25 identified biohorizons, 16 are zonal boundary markers (Table 2). Within the sampling resolution, the sedimentation appears continuous throughout the entire section (Fig. 12). The nannofossil-derived biostratigraphy agrees with the magnetostratigraphy derived from this site (see "Paleomagnetism" section, this chapter).
Subzones NN21b and NN21a together occupy the top 7.6 mbsf of Hole 1085A. The first occurrence (FO) of the Emiliania huxleyi acme, the datum event for the Subzone NN21b/NN21a boundary, was identified within the top part of Core 175-1085A-2H (Samples 1H-CC to 2H-2, 90 cm) at the mean depth of 4.85 mbsf. Zone NN21 terminates between Samples 2H-2, 90 cm, and 2H-4, 90 cm (last occurrence [LO] of the Gephyrocapsa caribbeanica acme at 0.26 Ma), at the mean depth of 7.6 mbsf.
This 0.2-m.y. interval terminates at 14.9 mbsf (between Samples 175-1085A-2H-CC and 3H-2, 1420 cm), which is the mean depth of the LO of the Pseudoemiliania lacunosa datum event.
In addition to the zonal boundary events, five biohorizons were identified within this interval. Sedimentation rate estimates within the top part of this interval (Fig. 12B), from the Zone NN20/NN19 boundary (0.46 Ma) to the LO of the Small Gephyrocapsa acme (0.6 Ma), are the highest recorded over the entire section (17 cm/k.y.). The nannofossil-based stratigraphy of the early Pleistocene part of Hole 1085A agrees closely with the magnetostratigraphy between 40 and 90 mbsf (Fig. 12A). The base of Zone NN19 (1.95 Ma) was reached between Samples 175-1085A-10H-CC and 11H-3, 10 cm, at the mean depth of 91 mbsf.
Zones NN18 and NN17 were combined because the LOs of Discoaster pentaradiatus (NN18/NN17 zonal boundary event) and D. surculus (NN17/NN16 zonal boundary) were identified within the same sampling interval. A refined biostratigraphy based on a higher resolution sampling will be done on shore to precisely constrain the upper boundary of the short-duration Zone NN17 (from 2.45 to 2.55 Ma). The base of Zone NN17 was identified between Samples 175-1085A-12H-CC and 13H-CC at the mean depth of 113.1 mbsf.
This 1.27-m.y. interval is constrained within Hole 1085A between 113.1 and 188.8 mbsf. In addition to the zonal boundary events, a biohorizon dated at 2.83 Ma was identified between Samples 175-1085A-13H-CC and 14H-CC (LO of D. tamalis). Other useful diagnostic species for this zone are D. decorus, D. challengeri, and D. variabilis.
The top of this interval is defined by the LO of Reticulofenestra pseudoumbilica (3.82 Ma), a datum event identified between Samples 175-1085A-20H-CC and 21H-CC. The LO of D. pansus, found within Core 175-1085A-24H, is a noncalibrated event used to differentiate an upper (CN11b) from a lower (CN11a) subzone within Zone NN15. Amaurolithus delicatus, the last representative of the nonbirefringent ceratoliths within the Pliocene, is found as a rare component of the calcareous nannofossil assemblage representative of Zone NN15. The Zone NN15/NN14 boundary is defined by the LO of A. tricorniculatus (4.5 Ma), a datum identified at the mean depth of 226.4 mbsf (Samples 24H-CC through 25H-CC).
The base of this 0.52-m.y. interval was identified between Samples 175-1085A-26H-CC and 27H-CC (FO of D. asymmetricus; 5.02 Ma). This interval is marked by the first downhole occurrence of Sphenolithus sp.
This 0.52-m.y. interval is constrained between the mean depths of 246.2 and 300.8 mbsf. Zone NN13 is combined with Zone NN12 because of the sparse occurrence of Ceratolithus spp. throughout this interval. The LO of D. quinqueramus, identified between Samples 175-1085A-32H-CC and 33H-CC, defines the NN12/NN11 zonal boundary.
This 3.06-m.y. interval is defined as the range of D. quinqueramus. The range of A. amplificus (LO between Samples 175-1085A-35X-CC and 36X-CC; FO between Samples 175-1085A-39X-CC and 40X-CC) was used to refine the age model of this interval.
The Zone NN11/NN10 boundary was identified at the mean depth of 415.8 mbsf (FO of D. quinqueramus; 8.6 Ma). The LO of D. bollii biohorizon (9.1 Ma; between Samples 175-1085A-46X-CC and 47X-CC) was used to further refine this interval.
This interval is defined as the range of D. hamatus. Its FO event (10.7 Ma) was identified at the mean depth of 512.2 mbsf (between Samples 175-1085A-54X-CC and 55X-CC). D. calcaris, a diagnostic species within nannofossil assemblages from Zone NN9, is present throughout this interval.
This interval extends from 10.7 to 11.8 Ma and therefore includes the Pliocene/Miocene boundary. The absence of Catinaster coalitus, whose FO is used to define the Zone NN8/NN7 boundary (11.3 Ma) from Hole 1085A, forced us to lump Zones NN8 and NN7 together. The LO of D. kugleri (11.5 Ma), identified between Samples 175-1085A-58X-CC and 59X-CC, might be used to approximate the Zone NN8/NN7 boundary.
The base of this interval is defined by the LO of Sphenolithus heteromorphus. This biohorizon was identified within a slumped sequence in Core 175-1085A-63X at the mean depth of 589.7 mbsf. Age control for the bottom part of Hole 1085A is therefore based on planktonic foraminiferal datums identified within Cores 175-1085A-62X and 64X. The presence of S. heteromorphus within the undisturbed bottom Core 64X, however, indicates that Hole 1085A terminated within Zone NN5.
Site 1085 contains abundant planktonic foraminifers and, unlike previous Leg 175 sites, is not affected significantly by dissolution downcore.
The uppermost assemblage (Sample 175-1085A-1H-CC) is dominated by Globigerina bulloides, Globorotalia inflata, and Neogloboquadrina pachyderma. Other species that are present include Globigerina quinqueloba, G. umbilicata, Globigerinella siphonifera, Globigerinoides ruber, G. sacculifer, G. crassaformis, G. hirsuta, G. scitula, G. truncatulinoides, N. dutertrei, and Orbulina universa. The presence of a warm-water species (including one endemic to the Indo-Pacific Ocean, G. hexagona) downcore (e.g., Sample 175-1085A-7H-CC) indicates transport of warm Indian Ocean water around the cape by the Agulhas Current.
The zonation was based on the temperate G. conoidea–G. sphericomiozea lineage, but the sporadic presence of warmer water fauna increased the number of datums, and good age control was achieved (Table 3). There is generally very good agreement among the datums with calcareous nannofossil biostratigraphy and magnetostratigraphy, despite the large sampling interval (usually every other core catcher). Because of time constraints, the complete assemblage could not be analyzed and, therefore, some of the FOs and LOs could not be precisely defined. The premature FOs and LOs in the age-depth plot (Fig. 12) are attributed to the large sampling interval. For example, the LO of G. cibaoensis is in Sample 175-1085A-35X-CC, at the same depth as the FO of G. sphericomiozea. G. sphericomiozea has a very brief range in the early Pliocene, and its last-appearance datum (LAD) is 5.6 Ma, 1 m.y. earlier than the LAD of G. cibaoensis. The true LO of G. cibaoensis at Hole 1085A is probably higher, between Samples 175-1085A-32H-CC, which did not contain G. cibaoensis, and 35X-CC.
Pleistocene Zone Pt1 (0–65 mbsf) encompasses Samples 175-1085A-1H-CC through 7H-CC (range of G. truncatulinoides).
Zones Pl5 and Pl6 (65–127 mbsf) were not differentiated at Site 1085 because the boundary between the two zones is defined based on (sub)tropical faunas that are not present (e.g., G. miocenica).
Zone Pl4 (127–147 mbsf) ranges from the LAD of D. altispira (3.09 Ma; Sample 175-1085A-15H-CC) to the LAD of S. seminulina (3.12 Ma; Sample 175-1085A-17H-CC). Zone Pl3 (147–203 mbsf) ranges from Sample 17H-CC (LO of S. seminulina) through 21H-CC (LO of G. margaritae).
The LAD of G. margaritae (3.58 Ma) and the LAD of G. nepenthes (4.18 Ma) define Zone PL2 (Samples 175-1085A-23H-CC through 29H-CC, 203–287 mbsf). G. margaritae is not present in Sample 25H-CC, but there is some evidence for dissolution (pyrite), and the species is very dissolution susceptible. Sample 29H-CC contains a specimen of G. cf. nepenthes that would render an age assignment to the older Zone Pl1, but the sample also contains G. sphericomiozea. The base of Zone Pl1 is approximated by the first-appearance datum (FAD) of G. sphericomiozea (Sample 29H-CC, 287 mbsf). Thus, Zones Pl1 and Pl2 are not differentiated.
The FAD of G. sphericomiozea (Sample 175-1085A-29H-CC) marks the Miocene/Pliocene boundary (5.6 Ma) as the top of Zone Mt10 (287–382 mbsf). The base of Zone Mt10 is defined as the FAD of G. conomiozea (7.12 Ma) and occurs in Sample 175-1085A-42X-CC. Zones Mt9 and Mt8 are not differentiated in this study and are bounded by the LAD of G. mayeri (top of Zone Mt9; 7.12, Ma) and the FAD of G. nepenthes (bottom of Zone Mt8; 11.8 Ma). Zones Mt9 and Mt8 are present in Samples 175-1085A-44X-CC through 56X-CC (382–555 mbsf), although the species are not present in every sample. Zone Mt7 ranges from the FAD of G. nepenthes to the LAD of G. peripheroronda (14.0 Ma) and is represented only in Sample 175-1085A-62X-CC (555–589 mbsf). The base of the underlying zone, subtropical Zone Mt6, was not reached. The lowermost portion of transitional Zone Mt6 is described by subtropical Zone M6. The top of Zone M6 is defined as the FAD of G. peripheroacuta. This species is found in Sample 175-1085A-64X-CC (589–604 mbsf) and constrains the age to 15.1–14.8 Ma. Sample 63X-CC is a slumped interval.
The benthic foraminiferal fauna of Site 1085 was studied in selected core-catcher samples from Hole 1085A. The overall abundance of benthic foraminifers was high throughout the studied interval, except for the lowermost core catcher (Sample 175-1085A-64X-CC; 604.29 mbsf), which contained rare to few benthic foraminifers (Table 4). The planktonic to benthic ratio at this site is very high, about ten times higher than at previous sites. Preservation is good throughout Hole 1085A.
The uppermost two samples (Samples 175-1085A-1H-CC [3.59 mbsf] and 3H-CC [23.19 mbsf]) are strongly dominated by Bulimina aculeata and Epistominella exigua (Table 4; Fig. 13). Farther downcore, the dominating species are Trifarina angulosa and Cassidulina laevigata (Sample175-1085A-5H-CC; 42.17 mbsf), Uvigerina hispidocostata (Sample 175-1085A-7H-CC; 61.03 mbsf), E. exigua and Gavelinopsis lobatulus (Sample 175-1085A-9H-CC; 80.12 mbsf), Uvigerina peregrina (Sample 175-1085A-11H-CC; 98.69 mbsf), Oridorsalis umbonatus (Sample 175-1085A-13H-CC; 117.82 mbsf), Hoeglundina elegans (Sample 175-1085A-15H-CC; 137.07 mbsf), U. hispidocostata and Sphaeroidina bulloides (Sample 175-1085A-17H-CC; 156.43 mbsf), and U. hispidocostata (Sample 175-1085A-19H-CC; 175.29 mbsf; see Table 4 and Fig. 13). Several of these species are more or less restricted to the peaks in relative abundance and are absent elsewhere in the studied sequence (i.e., B. aculeata, C. laevigata, G. lobatulus, T. angulosa, and U. peregrina).
The early Pliocene (175–280 mbsf) is dominated by Bolivina subaenarensis, Cibicidoides wuellerstorfi, and Uvigerina auberiana (Table 4; Fig. 13). The upper Miocene sequence is dominated by U. auberiana, Globocassidulina subglobosa, Cibicidoides pachyderma and in the lower part E. exigua and Melonis barleeanum (Table 4; Fig. 13).
The middle Miocene sequence is dominated by C. pachyderma, C. wuellerstorfi, E. exigua, G. subglobosa, and Oridorsalis umbon-atus (Table 4; Fig. 13).
The species Anomalinoides semicribratus was found in Samples 175-1085A-53X-CC through 64X-CC (497.97–604.29 mbsf; see Table 4; Fig. 13). This species has a reported LO in the middle Miocene (N12), but forms transitional from A. semicribratus to Ano-malinoides globulosus occur in Zones N13–N14 (Van Morkhoven, et al., 1986). In Hole 1085A, Anomalinoides globulosus has its FO in Sample 175-1085A-49X-CC (459.81 mbsf; see Table 4 and Fig. 13).
The species Bulimina tuxpamensis was found in the two lowermost core catchers (Samples 175-1085A-63X-CC [594.60 mbsf] and 64X-CC [604.29 mbsf]). This species has a known range from the late Paleocene through early middle Miocene (N9), but has been recorded in middle Miocene Zones N10–N13 (Van Morkhoven, et al., 1986), which further confirms the middle Miocene age for the base of Hole 1085A.
Radiolarians are generally rare and show signs of dissolution in almost all samples examined down to Sample 175-1085A-15H-CC, although concentrations by the sample preparation often produces high abundances of radiolarians in the assemblage slides. From Sample 175-1085A-16H-CC to the deepest core catcher, radiolarians are virtually absent (Table 5).
The absence of Axoprunum angelinum indicates that the uppermost Samples 175-1085A-1H-CC and 2H-CC belong to Zone NR1 of Caulet (1991). It was not possible to determine a zone for samples below Sample 2H-CC because of the scarcity or absence of age-diagnostic taxa. The FO of Cycladophora davisiana indicates an age of 2.70 Ma for Sample 12H-CC.
Axoprunum stauraxonium and the Ellipsoxiphus attractus group are common in Samples 175-1085A-1H-CC, 2H-CC, 3H-CC, 7H-CC, 9H-CC, 15H-CC, 16H-CC, 23H-CC, 31H-CC, 34X-CC, and 35X-CC, indicating low productivity under subtropical warm-water conditions. The radiolarian assemblages that are characterized by common C. davisiana suggest upwelling conditions for Samples 175-1085A-4H-CC, 6H-CC, 8H-CC, and 10H-CC. Thus, intermittent upwellings seem to have occurred in a warm-water condition through the last ~3 m.y. in the Northern Cape Basin area. The occurrence of an Antarctic species Cycladophora pliocenica in Sample 175-1085A-14H-CC indicates an influence of cooler current into the area.
Core-catcher samples from Hole 1085A were analyzed for their diatom content. Samples were prepared as smear slides and were acid-cleaned. The treated samples were washed with distilled water and sieved through a 20-µm sieve. Diatoms are absent in most core-catcher samples. From 304.90 mbsf (just below the Pliocene/Mio-cene boundary) through the end of the hole (604.29 mbsf; middle Miocene), the sediments are barren of diatoms. Trace amounts are found in Samples 175-1085A-1H-CC (3.59 mbsf) through 9H-CC (80.12 mbsf), 27H-CC (250.87 mbsf), 29H-CC (269.64 mbsf), 31H-CC (288.98 mbsf), 32H-CC (296.87), 35X-CC (324.57 mbsf), 39X-CC (363.27 mbsf), and 48X-CC (450.25 mbsf). It is only the interval between Samples 10H-7, 47 cm (89.17 mbsf), and 14H-CC (127.45 mbsf) that contains rare to few diatoms. It is interesting to note that this interval includes a mixed Thalassiothrix antarctica–rich assemblage (composed of Southern Ocean species, Chaetoceros spores, warm-oceanic species, and abundant nannofossils) similar to that at Site 1084, although overall abundances are much lower. As was the case for Site 1084, the interval at Hole 1085A occurs in the late Pliocene (~1.8–2.6 Ma).
Sponge spicules are particularly abundant in the Pleistocene and upper Pliocene sediments, from Sample 175-1085A-1H-CC to Sample 15H-CC, 3.59 to 137.07 mbsf, and are absent from Sample 175-1085A-36X-CC (333.90 mbsf) to the end of the hole. One interesting feature at Site 1085 is that dinoflagellate cysts are common in the upper Miocene sequence, between Samples 175-1085A-38X-CC (353.57 mbsf) and 51X-CC (478.77 mbsf), with an approximate age of 6–10 Ma.