Hole 1166A samples a section of ~360 m that is divided into five primary lithostratigraphic units. Fossil recovery is sporadic through the drill core, and well-constrained biostratigraphic age control is presently limited to narrow intervals. Units I and II are assigned Quaternary-late Pliocene and early Oligocene-late Eocene ages, respectively. Unit III contains late Eocene palynomorphs and Units IV and V contain Late Cretaceous (Turonian) pollen.
Unit I (0 to ~135 mbsf) is subdivided into four lithostratigraphic subunits. Subunit IA consists of poorly sorted sandy silty clay that is Holocene to possibly late middle Pleistocene in age. Subunit IB consists largely of diamicton and is assigned a broad age of Quaternary to late Pliocene because of poor biostratigraphic control. Subunit IC consists of interbedded poorly sorted sandy silt with lonestones and diatom-bearing clayey silt with dispersed granules and sand grains of late Pliocene age. Subunit ID consists of unfossiliferous diamicton and is not biostratigraphically constrained.
Unit II (~135 to ~160 mbsf) consists of interbedded diatom-bearing claystones and sands. Rich diatom assemblages recovered in this interval are assigned an age of earliest Oligocene-late Eocene.
Units III and IV (~160 to ~315 mbsf) contain sand, silt, and clay and include plant material and coal. No macrofossil material was identified in Unit V (~342.8-342.96 mbsf).
Numerous shipboard samples were taken from Units II-V (~135-343 mbsf) for shorebased palynological investigation. Well-preserved pollen and dinocryst assemblages were observed in initial analysis of this material. Detailed palynological results and age interpretations from Hole 1166A will be available in the Scientific Results volume and/or journal publications. Preliminary results give good age indications for these units.
Biostratigraphic zonal assignments and paleoenvironmental interpretations for Site 1166 are based on shipboard analysis of diatoms, radiolarians, foraminifers, and calcareous nannofossils. The results of these initial investigations are summarized in Table T3 and Figure F19 and are described below.
The Neogene section in Hole 1166A yielded moderately abundant faunas of planktonic foraminifers from Cores 188-1166A-1R through 12R. Samples 188-1166A-1R-CC, 2R-CC, and below 11R-CC are barren of foraminifers.
Planktonic faunas are dominated by Neogloboquadrina pachyderma (Ehrenberg), indicating that the entire section down to Sample 188-1166A-12R-1, 18-20 cm, is upper Miocene or younger (N. pachyderma Zone AN7, of Berggren et al., 1995). These faunas are those commonly found on a fully marine open shelf. Dissolution of specimens is not evident, but some specimens of N. pachyderma from the shallower parts of the section have some physical abrasion. Other than the abrasion, there is no evidence of reworking or biostratigraphic mixing of faunas.
Planktonic foraminifers are relatively common (several tens of specimens) in samples from Sections 188-1166A-1R-1 and 1R-2 but not from Sample 188-1166A-1R-CC. Four- and five-chambered forms of N. pachyderma were recorded separately in this interval. Planktonic foraminifers are also present but much less abundant in Section 188-1166A-3R-2 (base of section), where few specimens of N. pachyderma (Ehrenberg) are present, and in Sample 188-1166A-4R-CC. These specimens are gray in color and are slightly abraded but do not show any evidence of dissolution.
Both four- and five-chambered forms of N. pachyderma are dominant in Sample 188-1166A-5R-CC. This association is common in modern shelf faunas in the region (Quilty, 1985).
Samples 188-1166A-6R-CC, 7R-CC, and 9R-CC contain N. pachyderma, with or without Globigerina falconensis, but in low abundance. Sample 188-1166A-8R-CC is barren of foraminifers.
Several samples from Samples 188-1166A-5R-CC through 10R-CC contain small planktonic species in the 63- to 125-µm fraction.
Samples 188-1166A-10R-1, 11-13 cm; 10R-CC; 11R-1, 47-50 cm; and 11R-CC contain relatively diverse planktonic foraminiferal faunas, the best available in the Neogene at this site. These assemblages are dominated by the normal four-chambered form of N. pachyderma, and a few five-chambered forms are present. A small five-chambered form of Globorotalia (Tenuitella) (sensu Kennett and Srinivasan, 1983) is represented by several specimens, and a single specimen of Globigerinita parkerae (Bermudez) was recorded.
The deepest sample in Hole 1166A bearing planktonic species is Sample 188-1166A-12R-1, 18-20 cm. It contains a few specimens of N. pachyderma.
Samples of N. pachyderma and shell fragments from Sample 188-1166A-11R-1, 47-50 cm, have been selected for Sr dating and will provide adequate material for oxygen isotope studies.
The Neogene section in Hole 1166A yielded moderately abundant benthic foraminifers from Core 188-1166A-1R to Sample 188-1166A-12R-1, 18-20 cm.
The records presented here are likely to underestimate the benthic foraminiferal fauna because more delicate agglutinated forms (e.g., what appears to be Haplophragmoides sp.) were observed during examination of sediment samples under hand lens and stereobinocular microscope. These do not survive vigorous processing. Only the more robust forms were therefore recorded.
Benthic faunas are dominated by members of the Cassidulinacea (forms with granular crystalline walls): Globocassidulina, Cassidulina, and Ehrenbergina. Lagenid/nodosariid forms are absent, and other forms are rare. Three species of Globocassidulina are recognized here: Globocassidulina crassa (d'Orbigny), Globocassidulina subglobosa (Brady), and Globocassindulina biora (Crespin). The latter of these species is here classed as a separate species rather than incorporated into G. crassa, as was done by Milam and Anderson (1981).
Although Sample 188-1166A-1R-CC was barren of foraminifers, scattered specimens of Globocassidulina are present throughout the length of Core 188-1166A-1R. These are present in the lowest abundance of the three species recognized. Selected samples from this core, such as Samples 188-1166A-1R-1, 16-21 cm; 1R-1, 117-122 cm; and 1R-2, 9-11 cm, all contained benthic faunas, including sporadic specimens of Angulogerina earlandi (Parr).
Small faunas are present in Section 188-1166A-3R-2 (base of section) and in Samples 188-1166A-4R-CC; 5R-1, 26-31 cm; 5R-CC; 6R-CC; 7R-CC; 8R-1, 42-45 cm; and 9R-CC. Samples 188-1166A-10R-1, 11-13 cm; 10R-CC; and 11R-1, 47-50 cm, however, contain much larger, more diverse benthic faunas.
Several samples contain Uvigerina bassensis (Parr), which is present as single gray and slightly abraded specimens. It likely came from the same source as the planktonic specimens in these samples. The presence of the uvigerinids is evidence of some infauna, but their numbers are too low to draw meaningful conclusions about environmental significance.
Sample 188-1166A-10R-CC contained two species of Astrononion in addition to the species discussed in the preceding paragraphs.
Several samples contain enough foraminifers to allow planktonic percentage to be used as a depth indicator. Planktonic percentage in these samples is ~50%-80%, suggesting water depths corresponding to the outer continental shelf or deeper. Faunas with high globocassidulinid content (especially Globocassidulina) are widespread in the modern Prydz Bay in shallower, better oxygenated environments such as the Four Ladies Bank (Quilty, 1985). The outer continental shelf association also is consistent with the deep-shelf calcareous assemblage of Milam and Anderson (1981). These faunas are not expected from samples representing deeper parts of the region where siliceous mud and ooze (SMO of Harris et al., 1997) dominate. The agglutinated fauna normally found in SMO was not indicated in any of the faunas studied from Site 1166. A solitary sample, Sample 188-1166A-13R-1, 71-73 cm, contains a very small >63-µm residue—typical of that from SMO samples. This sample, however, yielded no foraminifers.
Two predominant lithologies are identified in Neogene cores at this site: massive diamicts and silty clays. An attempt was made to relate foraminiferal faunas to these different lithologies, but the faunal distributions bear little relationship to the lithologic changes.
On the basis of foraminiferal faunas, the Neogene section can be divided into three intervals:
Nearly all core-catcher samples from Cores 188-1166A-1R through 39R were barren of calcareous nannofossils. Sample 188-1166A-21R-CC contained a very rare, poorly preserved specimen of a middle Eocene to earliest Miocene taxon, Reticulofenestra daviesii. A single specimen of Coccolithus pelagicus and two heavily overgrown specimens of Reticulofenestra sp. were also noted. It cannot be determined whether these specimens are in situ, reworked, or possible contaminants. A probable contaminant specimen of the Oligocene-Miocene taxon, Reticulofenestra hesslandii, was also noted in Sample 188-1166A-31R-CC.
The absence of nannofossils at this locality is consistent with that reported by Wei and Thierstein (1991) for the lower part of the nearby Prydz Bay Site 742 (Shipboard Scientific Party, 1989c; ODP Leg 119). Diatoms are present in the upper part of the section in Hole 1166A, which indicates that conditions were conducive to marine phytoplankton productivity at this site through at least some intervals of the late Eocene-earliest Oligocene and Pliocene. However, during these periods, nearshore surface waters were not favorable for nannoplankton production.
Quaternary, Pliocene, and lowermost Oligocene-upper Eocene diatom assemblages are recognized in Hole 1166A. Fossil diatom occurrence, however, is generally sporadic throughout the section, with abundant and well-preserved assemblages present only in limited intervals.
Initial biostratigraphic analysis for diatoms was carried out primarily on core-catcher samples. Because of a high degree of lithologic variation over narrow intervals (see "Lithostratigraphy"), discrete core intervals containing fine-grained sediments were preferentially sampled in an effort to identify more fossiliferous horizons. Coarser grained facies were also examined in some intervals, but these samples generally contained very rare poorly preserved diatoms.
Three distinct siliceous microfossil assemblages are noted in Hole 1166A: Quaternary sediments were recovered in Core 188-1166A-1R, upper Pliocene marine diatoms are present in two silt beds interbedded with poorly sorted silts with lonestones in Core 188-1166A-13R, and lowermost Oligocene-upper Eocene marine sediments are identified in Cores 188-1166A-15R and 16R. A major disconformity (~30 m.y.) within Section 188-1166A-15R-2 is interpreted to separate upper Pliocene and lowermost Oligocene-upper Eocene strata.
Below Sample 188-1166A-17R-2, 58-59 cm (153.48 mbsf), no diatoms were recovered. Diatoms may be absent in this interval because of a lack of marine diatom productivity during deposition or because of dissolution. Alternatively, these strata represent estuarine or nonmarine facies deposited in an environment not inhabited by diatoms.
The diatom zonation of Harwood and Maruyama (1992) and Winter and Harwood (1997) was applied to Quaternary and Pliocene cores of Hole 1166A, and the Paleogene cores are currently left unzoned. Diatom zonal datums recognized in Hole 1166A are listed in Table T3.
Core 188-1166A-1R is characterized by silt-rich sediments that contain poorly preserved diatoms; however, an abundant and well-preserved diatom assemblage is present in a biosiliceous sponge-spicule horizon in Sample 188-1166A-1R-2, 72-73 cm (2.22 mbsf). This assemblage consists entirely of extant Southern Ocean diatoms. The absence of the fossil species Actinocyclus ingens in this sample (last occurrence [LO] = 0.66 Ma) indicates that the interval above 2.22 mbsf is within the Holocene-middle to upper Pleistocene Thalassiosira lentiginosa Zone. This sample also contains abundant sea ice-associated Fragilariopsis curta, which is abundant in modern surface-sediment samples near Site 1166 (Stockwell et al., 1991; Taylor et al., 1997).
Moderately abundant and well-preserved diatoms are present in Sample 188-1166A-1R-CC (3.02 mbsf). This sample is also assigned to the T. lentiginosa Zone, based on the absence of A. ingens. Diatoms in this sample represent an extant assemblage, except for the common occurrence of well-preserved Thalassiosira torokina ("late" form). The LO of this taxon is documented at 1.8 Ma in the Southern Ocean (Harwood and Maruyama, 1992) but is known to range into the T. lentiginosa Zone (0-0.66 Ma) in the CRP-1 drill core in the southeastern Ross Sea (Bohaty et al., 1998). The observed occurrence of T. torokina in T. lentiginosa Zone sediments in Hole 1166A and CRP-1, therefore, represents a diachronous LO between Antarctic shelf (Prydz Bay and Ross Sea) and Southern Ocean (Kerguelen Plateau) locations. At the current time, the presence of T. torokina in this sample can be taken only to indicate that the assemblage is not entirely extant.
The absence of Rouxia spp. in Samples 188-1166A-1R-2, 72-73 cm, and 1R-CC further suggests a latest Quaternary age for this interval. The LO of Rouxia spp., however, on the Antarctic shelf is not well constrained. Bohaty et al. (1998) record the presence of Rouxia leventerae in (lower?) T. lentiginosa Zone sediments in the CRP-1 drill core. In Southern Ocean sections, Abbott (1974) and Akiba (1982) document a LO of Rouxia spp. at ~0.35 Ma and Pichon (1985) notes a LO within Stage 6 (~0.15 Ma).
Sample 188-1166A-1R-CC contains more abundant Fragilariopsis kerguelensis specimens than are found in surface sediments near Site 1166 today. This could represent either more open-water conditions or an increase in the influence of winds affecting the Prydz Bay gyre that may have pushed the open-water assemblage farther into the shelf than today.
Site 1166 was overridden by grounding ice during the last glacial maximum (LGM) (Domack et al., 1998). The change in diatom assemblages between the sandy silty clay above 2.92 mbsf (e.g., Sample 188-1166A-1R-2, 72-73 cm [2.22 mbsf]) and the diamict below (e.g., Sample 188-1166A-1R-CC [3.02 mbsf]) may represent a disconformity as a result of this glacial advance. If this is the case, the sandy silty sediment above 2.92 mbsf represents deposition since the LGM.
Diatoms are low in abundance and are predominantly broken within diamict between 3.02 and 113.84 mbsf. These may be glacially reworked diatoms in lodgment till, or alternatively, they could be reworked or deposited in situ within waterlain till. Detailed sedimentological analysis may enable identification of these facies and allow comment on the mode of diatom deposition. Nevertheless, the presence of diatoms within the diamict helps constrain the age of these deposits. Sample 188-1166A-2R-CC (10.64 mbsf) contains F. kerguelensis (first occurrence [FO] = 3.1 Ma), indicating that the diamict above 10.64 mbsf is <3.1 Ma in age. Further work on rare diatoms in Sample 188-1166A-12R-CC (106.37 mbsf) may also provide biostratigraphic information enabling age constraint for the diamicts above this level.
Upper Pliocene marine diatoms are present in two greenish gray silt horizons in Core 188-1166A-13R that are interbedded with poorly sorted sandy siltstones. The lighter gray-colored silt horizons in this core contain abundant and well-preserved diatoms (e.g., Samples 188-1166A-13R-1, 70-71 cm [114.00 mbsf]; and 13R-2, 8-10 cm [114.88 mbsf]). The upper silt bed (~113.95 to ~114.10 mbsf) is placed within the T. kolbei Zone of Harwood and Maruyama (1992), as indicated by the presence of T. kolbei (LO = 1.9 Ma) and the absence of T. vulnifica (LO = 2.3 Ma). T. kolbei Zone sediments were previously recovered nearby from Site 742 (Mahood and Barron, 1996) and may represent a correlative horizon to that recovered in Hole 1166A.
The age of the lower silt bed (~114.50 to ~115.15 mbsf) is constrained by the presence of T. vulnifica (FO = 3.2 Ma). In the Southern Ocean zonal scheme, the full range of T. vulnifica represents the top of the T. vulnifica Zone and the base of the Thalassiosira insigna-T. vulnifica Zone (of Harwood and Maruyama, 1992). The presence of T. insigna would further indicate that the T. vulnifica Zone is disconformably absent, placing this interval in the T. insigna-T. vulnifica Zone. The absence of the subzonal datum Fragilariopsis weaveri (LO = 2.8 Ma) also constrains this interval to the T. insigna-T. vulnifica Subzone "b." The absence of the T. vulnifica Zone, therefore, suggests a disconformity between ~114.10 and ~114.50 mbsf. Sharp lithologic contacts occur with changes in grain size at 114.40 mbsf and sediment color at 114.50 mbsf; these levels may represent erosive events indicated by the absence of the T. vulnifica Zone.
The full range of T. vulnifica, alternatively, represents the top of the T. vulnifica Zone and the base of the T. striata-T. vulnifica Zone, which is defined from Antarctic Shelf cores in the Ross Sea (Winter and Harwood, 1997). These zones are divided by the LO of T. striata. Using this zonation, the absence of T. striata and the presence of T. vulnifica, therefore, place the interval between ~114.50 and ~115.15 mbsf within the T. vulnifica Zone (of Winter and Harwood, 1997), indicating that the "upper" and "lower" silt horizons are conformable.
The conflict between the two zonal interpretations for the "lower" silt bed arises from the biostratigraphic application of the LO of T. insigna. The range of this taxon does not overlap with T. vulnifica in the Ross Sea but does so in the Southern Ocean (Winter and Harwood, 1997) and in Hole 1166A. In Hole 745B (ODP Leg 119), the LO of T. insigna occurs above the LO of T. vulnifica (Baldauf and Barron, 1991). Further work is required to investigate the biostratigraphic range of T. insigna and validate its use as a zonal marker. Additionally, some taxonomic confusion exists between T. insigna and other morphologies, such as Thalassiosira insigna/inura (transitional form), which was observed as high as 114.00 mbsf (Sample 188-1166A-13R-1, 70-71 cm) within T. kolbei Zone sediments. Diatoms similar in appearance to T. insigna/inura (transitional form) were also illustrated from the T. kolbei Zone of Hole 742A but were included under the designation of Thalassiosira oliverana (Mahood and Barron, 1996).
Thalassiosira elliptipora also appears to have a different range on the Antarctic shelf than in more northerly deep-sea sections. On the Kerguelen Plateau, the FO is nearly contemporaneous with the LO of T. vulnifica at 2.3 Ma (Harwood and Maruyama, 1992). At Site 1166, however, T. elliptipora occurs in moderate abundance within the range of T. vulnifica. This is consistent with observations from the Ross Sea, where the range of T. elliptipora is entirely within the T. vulnifica and T. striata-T. vulnifica Zones (Winter and Harwood, 1997)
From preliminary observations, the upper Pliocene diatom assemblages in Hole 1166A appear to contain a relatively higher component of Eucampia antarctica var. recta winter intercalary valves than winter terminal valves. The opposite is found in Prydz Bay today (Fryxell, 1991). The ratio between these valves has been used to reconstruct winter sea-ice conditions in marine sediments from the Kerguelen Plateau (Kaczmarska et al., 1993). At Site 1166, this ratio may suggest a lower winter sea-ice concentration during the late Pliocene than today. Also consistent with this observation is a low relative abundance of the sea-ice-associated taxon F. curta, which suggests a lower summer sea-ice concentration than today.
An interval of diamict is present between Samples 188-1166A-14R-CC (131.88 mbsf) and 15R-2, 118-120 cm (135.23 mbsf). This interval is barren of diatoms and consequently has been left unzoned.
A major change in lithology and mineralogical composition (see "Lithostratigraphy") occurs at Section 188-1166A-15R-2, ~8 cm (~135.65 mbsf); this level is interpreted as representing a major disconformity. Abundant and well-preserved lowermost Oligocene-upper Eocene diatom assemblages occur below this level between 135.73 and 151.60 mbsf.
The diatom assemblage between the lower part of Core 188-1166A-15R and the upper part of Core 17R is characterized by the presence of the following taxa: Distephanosira (Melosira) architecturalis, Eurossia irregularis, Hemiaulus caracteristicus, Hemiaulus dissimilis, Hemiaulus incisus, Kannoa hastata, Pterotheca danica, Pyxilla reticulata, Stictodiscus kittonianus, Stephanopyxis grunowii, Stephanopyxis oamaruensis, Stephanopyxis splendidus, Stephanopyxis superba, and Vulcanella hannae. In addition, a number of characteristic ebridian, silicoflagellate, and chrysopphyte-cyst taxa were identified in this assemblage, including Archaeosphaeridium tasmaniae, Archaeosphaeridium australensis, Ammodochium ampulla (double loricate skeleton), Ebriopsis crenulata (loricate skeleton), Ebriopsis crenulata (nonloricate skeleton), Ebrinula paradoxa, Parebriopsis fallax, and Pseudammodochium dictyoides (single skeleton).
The age of the interval between 142.45 and 148.26 mbsf is partially constrained by the presence of H. caracteristicus, which indicates an age of greater than ~33 Ma, based on its LO within Chron C13n in Hole 744B on the southern Kerguelen Plateau (Baldauf and Barron, 1991). The absence of several taxa characteristic of the middle Eocene, such as Trinacria cornuta and Craspedodiscus moellerii (Gombos, 1983), further suggests a lower age limit of less than ~37 Ma. The lowermost Oligocene-upper Eocene section of Hole 1166A is left unzoned at the present time; there are very few Southern Ocean reference sections that contain well-preserved middle to upper Eocene diatom assemblages, and zonal schemes are under development.
Similar lowermost Oligocene-upper Eocene assemblages are present in the interval from ~366 to ~500 mbsf in the CIROS-1 drill core in McMurdo Sound (Harwood, 1989) and in Hole 739C in Cores 119-739C-25R through 38R in Prydz Bay (Barron and Mahood, 1993; Mahood et al., 1993). Siliceous-microfossil taxa common to assemblages in all three of these sections include E. crenulata (loricate skeleton), E. irregularis, H. caracteristicus, Kisseleviella sp. G (of Scherer et al., in press), P. dictyoides (single skeleton), P. danica, S. splendidus, S. kittonianus, and V. hannae. The assemblage present in Hole 1166A, however, does not contain Rhizosolenia oligocaenica, Skeletonema utriculosa, Skeletonemopsis mahoodii, or Sphynctolethus pacificus, which are present in CIROS-1 and in Hole 737C. This suggests the interval recovered in Hole 1166A may be slightly older than the CIROS-1 and Hole 739C sections. The presence of Pseudorutilaria monile (absent in CIROS-1 and Hole 739C) and the common presence of P. danica and D. architecturalis further support this assumption, but further detailed analyses of additional samples will be required to completely characterize the assemblage and confirm the absence of specific taxa in Hole 1166A.
Lowermost Oligocene-upper Eocene assemblages documented in Cores 188-1166A-15R through 17R are almost exclusively neritic/planktonic in character, suggesting paleowater depths of >50 m. Additionally, the high abundance of planktonic diatoms, particularly in Samples 188-1166A-15R-CC and 188-1167A-16R-4, 130-131 cm, indicates an open-marine environment. The high biogenic opal content of these sediments also suggests that the surface-water conditions were euphotic and that turbidity, due to terrigenous sediment influx, was low. These conditions differ from those in a marine setting proximal to river mouth or polythermal/wet-based glaciers, which would be influenced by a high sediment influx. Diatom assemblages in this interval, therefore, indicate that the site was located at a distal position relative to the ice margin—that is, if glaciers were present at the time of deposition.
Radiolarian faunas are present in only a few samples in Hole 1166A. The radiolarian zones proposed by Lazarus (1990, 1992) for Neogene high-latitude sections (see "Radiolarians" in the "Explanatory Notes" chapter) were applied to the radiolarian-bearing samples from this hole. The radiolarian zones and associated datums recognized in Hole 1166A are outlined in Table T3 and Figure F19.
Sample 188-1166A-1R-2, 70-72 cm, contains moderately preserved radiolarians and is assigned to the Chi to Psi Zones (1.9-0.0 Ma). Triceraspyris antarctica (Haecker) and Lithelium nautilodes Popofsky both appear in the lower part of the Chi Zone (Lazarus, 1992), and both taxa are common in this sample. Absent from the sample, however, are Cycladophora pliocenica, which has a LO in the middle of the Chi Zone, and Pterocanium c. trilobum, which has a LO at the top of the Chi Zone (or bottom of the Psi Zone), thus suggesting that this sample could be constrained to the Psi Zone. Also present in this sample are Spongotrochus? glacialis, Antarctissa denticulata, Antarctissa cyclindrica, and Phorticum clevei, all high-latitude species consistent with assignment to the Chi and/or Psi Zones.
Radiolarians in Samples 188-1166A-13R-1, 150 cm, and 13R-2, 5 cm, are very rare and poorly preserved, but Antarctissa strelkovi is present. This taxon ranges from the base of the Tau Zone (6.1 Ma) to the top of the Upsilon Zone (2.4 Ma); therefore, these samples are assigned to the Tau or Upsilon Zones (Lazarus, 1992).
It is not likely that radiolarian biostratigraphic resolution will improve in Hole 1166A, as the more promising samples from within the core have already been processed (e.g., 15 samples from Cores 188-1166A-1H, 13H, and 15H) and are barren of radiolarians.
Preliminary palynological examination of Site 1166 found palynomorphs of the middle Nothofagidites aspersus Zone of the Gippsland Basin of southeastern Australia in Units II and III and of the Phyllocladidites mawsonii Zone from the Gippsland Basin in Units IV and V (O'Brien et al., in press). These palynomorphs date Units II and III as mid to late Eocene and Unit IV as Turonian. Additionally, Unit V is of Turonian-Santonian(?) age.
Initial micropaleontological analysis of Hole 1166A was carried out for diatoms, radiolarians, foraminifers, and calcareous nannofossils. Diatoms are present in limited core intervals and provide the primary basis for initial biostratigraphic age estimates. Three distinct diatom assemblages were noted of Quaternary, Pliocene, and late Eocene-earliest Oligocene age. Extant Quaternary diatoms are present at 2.12 mbsf and indicate an age of <0.66 Ma. This assemblage may extend down to a lithological contact at 2.92 mbsf. Two upper Pliocene horizons of diatomaceous clay are present from ~113.95 to ~114.10 mbsf and from ~114.50 to ~115.15 mbsf. Diatoms in these intervals belong to the T. kolbei (1.8-2.2 Ma) and T. vulnifica (Antarctic shelf) or T. insigna-T. vulnifica (Southern Ocean) Zones (2.2-3.2 Ma), respectively. In support of these assignments, preliminary radiolarian data constrain the age of the lower Pliocene bed to >2.4 Ma.
A distinct change in lithology occurs at ~135.65 mbsf, which most likely represents a major disconformity. Diatoms beneath this change, between 135.73 and 153.48 mbsf, are early Oligocene-late Eocene in age (~33 to ~37 Ma). No diatoms were recovered below 153.48 mbsf.
Planktonic foraminifers are common in two intervals in the upper ~90 m of Hole 1166A, and these assemblages are consistent with age interpretations derived from diatom and radiolarian biostratigraphy. Foraminiferal populations indicate that during intervals in the late Pliocene sea-ice concentrations were less than today. Diatoms suggest that marine conditions existed at Site 1166 during intervals in the earliest Oligocene to late Eocene; the dominance of planktonic diatoms in this interval also suggests that waters were >50 m deep at distal position relative to the ice margin (that is, if glaciers were present). Calcareous nannofossils were largely absent in Hole 1166A.
Initial postcruise analyses of palynofloras document well-preserved dinoflagellate and pollen assemblages in Units II-V. These results indicate a middle to late Eocene age for Units II and III and Late Cretaceous (Turonian-Santonian) age for Units IV and V.
Figure F20 is a plot of age vs. depth for Hole 1166A. Seven diatom and six radiolarian datums were used (Table T3), along with one paleomagnetic polarity tie, to construct this age model. Four intervals of Quaternary, late Pliocene, earliest Oligocene-late Eocene, and Turonian age are recognized. A linear sedimentation rate is not assumed between the well-constrained sections because unconformities may exist in these intervals.