Site 1138 was drilled in 1141.4 m of water on the CKP ~150 km north-northwest of Site 747 (Leg 120). Approximately 650 m of Upper Cretaceous through Pleistocene carbonate and biosiliceous pelagic ooze and ~340 m of probable Upper Cretaceous shallow marine and terrestrial sediments overlie volcaniclastic rocks and basaltic lavas.
A relatively complete and expanded section (110 m) of Pliocene and Pleistocene biosiliceous sediments was recovered at this site. A comparable section has not been encountered during previous drilling on the Kerguelen Plateau. Abundant and well-preserved diatoms provide biostratigraphic control in this part of the section. Biogenic carbonate content increases below the upper Neogene, but abundance and preservation of calcareous microfossils fluctuates throughout the succession. The upper Miocene to Upper Cretaceous consists almost entirely of nannofossil ooze and chalk.
The Miocene and Paleogene sections are punctuated by hiatuses, but a complete K/T boundary may be present in Core 183-1193A-52R. Sedimentation rates are very high in the Pliocene and Pleistocene (28.5 m/m.y.) but decrease to between 9 and 13 m/m.y. in the Miocene and upper Paleogene. Sedimentation rates (Fig. F12) reach a minimum during the Paleocene (2.8 m/m.y.) and were also low during the Maastrichtian (4.9 m/m.y.). Slightly higher sediment accumulation rates (8.8 m/m.y.) characterize the lower Upper Cretaceous.
The oldest microfossils encountered are of Late Cretaceous age (early Turonian) and indicate a minimum basement age of ~92-93 Ma. These microfossils are in dark-gray and green horizons within highly bioturbated organic-rich chalks and clays, which are generally too lithified to adequately preserve age-diagnostic microfossils. Immediately beneath this sections is a ~1-m-thick marine black shale unit with 2.2 wt% organic carbon (see "Organic and Inorganic Geochemistry"). Nannofossils and planktonic foraminifer biostratigraphy suggest that this could represent the Coniacian-Turonian anoxic event (OAE2, the "Bonerelli" horizon; Schlanger and Jenkyns, 1976). The black terrestrial sediments below are rich in fossil plant remains, including common wood fragments, abundant sporangia, and pollen.
Calcareous nannofossils are largely absent in the Pliocene-Pleistocene diatomaceous clays and ooze of lithostratigraphic Unit I, but become common in Sample 183-1138A-12R-CC and abundant below that point until thick black shales are encountered near the bottom of the marine section (Core 183-1138A-69R). Preservation downhole is generally excellent downward to the K/T boundary in Core 183-1138A-52R. Below this boundary, preservation varies between good and moderate in Maastrichtian-Campanian chalks of Subunit IIIB down to Core 183-1138A-60R. From there to the bottom of the marine sedimentary sequence (Core 183-1138A-71R), preservation is good to poor depending on lithology. In general, below Core 183-1138A-61R, preservation deteriorates downhole in the clean, hard chalk until Core 183-1138A-63R, where recovery was also poor. Preservation then improves in the underlying darker greenish and blackish claystone laminae and interbeds that become more common downhole in Unit IV. We dated the lowermost pelagic sediment as early Turonian in age. Indeed, the Cenomanian/Turonian oceanic anoxic event may be represented by the black claystone at the bottom of Core 183-1138A-69R.
Specimens of Coccolithus pelagicus are few in Pleistocene core-catcher Samples 183-1138A-5R-CC and 7R-CC but are common in the mid-Pliocene Sample 183-1138A-9R-CC, where they are accompanied by small (2-5 µm) but very abundant taxa that include various species of Reticulofenestra. Coccolithus pelagicus are sparse in the upper Miocene Sample 183-1138A-11R-CC, common in Sample 183-1138A-12R-CC, where they are accompanied by Reticulofenestra perplexa, and abundant beginning in nannofossil clay of Subunit IIA. From this point down to Sample 183-1138A-20R-CC, we assigned the clay to the long-ranging Zone CN11-5b (for more detailed age determinations, see "Diatoms"). Dominance of the low-diversity assemblages alternated between C. pelagicus and R. perplexa, probably depending on paleoclimatic/paleoceanographic factors.
Large Cyclicargolithus floridanus floridanus, grading up in size to the larger Cyclicargolithus floridanus abisectus, appear in Sample 183-113A-21R-CC, constituting about 10% of the assemblage, but increase downhole to dominant proportions in Sample 183-1138A-24R-CC. In the latter sample, it is accompanied by Coccolithus miopelagicus, a few Calcidiscus leptopora/macintyrei, and rare Helicosphaera; the latter, plus rare Discoaster deflandrei in the superjacent core, indicate somewhat warmer surface waters. We assigned cores in this interval to the combined Zones CN5a-CN3. The next interval, from Cores 183-1138A-25R to 28R-CC, is dominated primarily by C. pelagicus and belongs to the lower Miocene Zones CN2-CN1, based on the absence of C. leptoporus/macintyrei.
Reticulofenestra bisecta, ~10 to 12.5 µm in length, accompanied by C. floridanus abisectus of nearly the same length, marks the top of the Oligocene in Sample 183-1138A-29R-CC. Chiasmolithus altus dominates the assemblage in Sample 183-1138A-30R-CC, and the mid-Oligocene zone of that name extends down to Sample 183-1138A-35R-CC. The top stratigraphic occurrence of Zygrabdotus bijugatus was in Sample 183-113A-33R-CC, along with rare Sphenolithus moriformis; dominance alternates strongly among Cyclicargolithus, Coccolithus, Reticulofenestra, and Chiasmolithus in these cores, probably in response to changing paleoclimatic/paleoceanographic factors.
The co-occurrence of Reticulofenestra umbilica, Isthmolithus recurvus, Reticulofenestra oamaruensis, and Chiasmolithus oamaruensis in Sample 183-1138A-36R-CC indicates a hiatus between this and the superjacent core catcher. This assemblage belongs to the R. oamaruensis Zone, which straddles the Oligocene/Eocene boundary. Certain aspects of the assemblage, however, indicate that we can assign the sediments to the Eocene portion of the zone (Subzone ~CP15b). These include a preponderance of C. oamaruensis over its evolutionary descendent, C. altus, whereas Clausicoccus fenestratus are few in number. The latter bloomed during the earliest Oligocene, but was sparse and sporadic in its distribution during the latest Eocene (compare with the Eocene/Oligocene sequence at Deep Sea Drilling Project Leg 71, Site 511 on the Falkland Plateau and Ocean Drilling Program Leg 119 Hole 737B on the southern Kerguelen Plateau [Wise, 1983, table 1A; Wei and Thierstein, 1991, table 3]).
The next core catcher downhole, Sample 183-1138A-37R-CC, contains a middle Eocene assemblage assigned to Subzone CP14a, which indicates a substantial hiatus between this and the superjacent core catcher. The assemblages contain a few large Chiasmolithus grandis and Coccolithus eopelagicus (up to 22-24 µm) along with Chiasmolithus expansus, Chiasmolithus solitus, Blackites spinosus, Neococcolithites dubius, Sphenolithus moriformis, and a few six-rayed discoasters, which become common in Sample 183-1138-39R-CC. Specimens of Discoaster bifax appear downhole in Sample 183-1138A-41R-CC, but essentially disappear in the subjacent core catcher, although other discoasters increase. We noted Coronocyclus prinion in Samples 183-1138A-42R-CC and 44R-CC. Sample 183-1138A-43R-CC contains rare Nannotetrina cristata, which increase to few in Sample 183-1138A-44R-CC; this latter core also contains both Chiasmolithus gigas and R. umbilica. The co-occurrence of these two taxa has also been noted at other mid- to high-latitude sites (e.g., Applegate and Wise, 1987); thus, it is not possible to use the last uphole occurrence of C. gigas here as an unequivocal marker to subdivide Subzones CP13b and CP13c. Sample 183-1138A-44R-CC probably lies close to the Zone CP13/14 boundary.
Sample 183-1138A-45R-CC contains no R. umbilica, common C. gigas ranging in size from 14 to 20 µm, few N. cristata, and some discoasters. This sample was assigned to Subzone CP13b. Preservation diminishes downhole from good to moderate in Samples 183-1138A-46R-CC and 47R-CC because of dissolution, fragmentation, and overgrowth. The former sample contained no Reticulofenestra samodurovii, rare Orthostylus tribrachiatus (reworked?), rare Discoaster praebifax, Toweius magnicrassus, common Discoaster barbadiensis, Coccolithus formosus, and rare nine-rayed discoasters (Discoaster binodosus or Discoaster nonaradiatus), and probably lies close to the CP13a/12 zonal boundary, although it may contain some mixed or reworked taxa. Sample 183-1138A-4R-CC contains Sphenolithus radians, Discoaster lodoensis, Discoaster sublodoensis, Discoaster kuepperi, and Toweius magnicrassis and belongs to Zone CP12, probably the upper part.
A disconformity along which most or all of the lower Eocene is missing is apparently present between Samples 183-1138A-47R-CC and 48R-CC, so we tentatively assign the latter to Zone CP8. It contains Discoaster multiradiatus, Discoaster mohleri, Prinsius bisulcus, Toweius pertusus, Markalius inversus, and very abundant, simply constructed, small (3-5 µm) fasciculiths (probably Fasciculithus tympaniformis and Fasciculithus involutus). Sample 183-1138A-49R-CC contains larger F. tympaniformis (up to 8 µm), F. involutus, large Prinsius bisulcus (up to 11 µm), few Heliolithus sp., and abundant Prinsius martinii; we assigned the sample to Zone CP5. We assigned Sample 183-1138A-50R-CC to the upper Paleocene, high-latitude Zone NA6 (= ~CP3/CP4). It contains common Thoracosphaera operculata, large Chiasmolithus danicus, few C. bidens, very abundant Prinsius dimorphosus, common P. martinii, few Toweius pertusa, few Coccolithus subpertusa, Biscutum sp., Neochiastozygus concinnnus, and Placozygus sigmoides. The subjacent core-catcher sample contains rare fasciculiths (in place?), small Coccolithus pelagicus (6 µm), Chiasmolithus danicus, P. dimorphosus, but no Chiasmolithus bidens, and belongs to Zone NA5.
The K/T boundary apparently lies within Core 183-1138A-52R and may be reasonably complete. A reconnaissance study within this core indicated that the boundary probably lies within Section 183-1138A-52R-3 at a color change at 127 cm, where the section passes downhole from a greenish glauconitic chalk to a whitish chalk. The apparent contact has been bioturbated, but the section may have been expanded by the input of fine clastic material derived from volcanic parent material (see "Lithostratigraphy"). The nannofossils in Section 183-1138A-52R-3 above this contact consist predominantly of Cretaceous taxa. However, we noted a well-preserved specimen of the Tertiary boundary index taxon, Biantholithus sparsus, in Sample 183-1138A-52R-3, 60 cm. Two specimens of the Tertiary incoming taxon Cruciplacolithus tenuis are also present at 183-1138A-52R-3, 30 cm. Cruciplacolithus primus and C. tenuis are abundant 3 cm from the top of the section.
Samples 183-1138A-52R-CC and 53R-CC contain well-preserved Cribrosphaerella daniae (centers intact) and abundant Nephrolithus frequens miniporus, the respective nominate taxa for the subzone and zone to which we assigned these cores. The next two core-catcher samples downhole (Samples 183-1138A-54R-CC and 55R-CC) exhibit highly variable preservation, with a few well-preserved specimens shown among much micritic carbonate. No Nephrolithus are present, perhaps a function of the generally poor preservation. Specimens of Biscutum coronum are rare, whereas Biscutum magnum and Reinhardtites levis are few to common, indicating that these cores belong to the B. coronum Zone (assuming that the B. coronum are not reworked and the assemblages are not mixed). If this assignment is correct, then a disconformity separates Core 183-1138A-54R from the superjacent Core 53R.
Biscutum coronum was equal in numbers to B. magnum in Samples 183-1138A-56R-CC and 57R-CC, which exhibit relatively good preservation and contain common Neocrepidolithus watkinsii. The latter is also represented by numerous detached spines in this and the subjacent Sample 183-1138A-58R-CC, where it is accompanied by Aspidolithus parcus expansus (indicating the A. parcus expansus Subzone of the B. coronum Zone [upper Campanian]). Sample 183-1138A-58R-CC also contains extraordinary numbers of Micula decussata, a circumstance that cannot be attributed solely to dissolution of other taxa because the preservation of this assemblage is no worse than others in this part of the column.
We also assigned Samples 183-1138A-60R-CC and 61R -CC to the A. parcus expansus Subzone based on the presence of the nominate species, N. watkinsii and R. levis. Preservation, however, deteriorates downsection in the more lithified, poorly recovered sediment; thus, we give no zonal assignment for Sample 183-1138A-62R-CC. The next core downhole contains Eiffellithus eximius plus R. levis; we assign it to the respective zone and subzone that bear those names (upper Campanian). Sample 183-1138A-64R-CC is too poorly preserved to date; however, Sample 183-1138A-65R-1, 30-32 cm, taken within the core, contains a better preserved assemblage belonging to the R. levis Subzone.
As a result of poor preservation in core-catcher samples toward the bottom of the hole, we assigned zones for the rest of the geologic section primarily on samples taken from within the cores. In Cores 183-1168A-66R to 68R, we found the best-preserved nannofossils in the darker colored lithologies, first greenish and then blackish going downhole, in which increased glauconite and clay contents apparently inhibited diagenesis.
Sample 183-1168A-66R-3, 22-24 cm, contains a dissolution-resistant, residual assemblage consisting of M. decussata, a single specimen of Lithastrinus moratus (= L. septenarius), and two specimens of Marthasterites furcatus, and we tentatively assigned it to the Lithastrinus moratus Subzone of the Eprolithus floralis Zone (lower Santonian); however, we noted no E. floralis. Nevertheless, a hiatus appears to exist between this and the last datable overlying sample. Sample 183-1138A-66R-4, 66-67 cm, from the next core section downhole, contains several Thiersteinia ecclesiastica, Micula concava, common Helicolithus trabeculatus, and a single specimen of Marthasterites furcatus. We assigned it to the upper portion of the T. ecclesiastica Zone (lowermost Santonian). The core-catcher sample from Core 183-1138A-66R lacks M. concava but does have M. decussata, and we consider it late Coniacian in age.
Sample 183-1138A-67R-2, 34-35 cm, yielded long-rayed Lithastrinus septenarius (L. moratus according to Varol, 1992) in the absence of M. decussata, and we consider it mid-Coniacian in age. Below this level, the rays of the these lithastrinids shorten and straighten as the long-rayed "septenarid" morphotype is traced back to its ancestral "moratid" form, then, ultimately, to the precursor Eprolithus lineage. We picked the point at which the latter transition occurred subjectively, using the light microscope on board ship, and the choice depends on the species concepts of the observer. The sole nannofossil paleontologist on this hard-rock leg considered this transition to have occurred at this level in the cored sequence, although some specimens in Section 183-1138A-68R-5 appeared to exhibit some rotation when one focused through them. This transition will have to be examined more closely with the scanning electron microscope onshore. We noted no Eiffellithus eximius below Sample 183-1138A-67R-2, 34-35 cm; however, Eprolithus eptapetalus and possibly Eprolithus rarus are present; thus, we regard the core-catcher sample (183-1138A-67R-CC) as probably mid- to late Turonian in age.
We examined 11 samples from Core 183-1138A-69R to ascertain the locations of the highest concentrations and best-preserved nannofossils. In this core, the highest nannofossil concentrations were in the lighter colored layers and laminae, which diminished in number and thickness downhole, apparently disappearing completely in a ~1-m-thick organic-rich zeolitic claystone at the base of Section 183-1138A-69R-5. Preservation was good in all samples, but best in the darker nannofossil-bearing lithologies.
Sample 183-1138A-69R-1, 25-27 cm, contains common, seven-rayed E. eptapetalus and E. floralis as well as Prediscosphaera avita (about 6 µm in diameter), Gartnerago obliquum, and a single specimen of Stoverius achylosus. The uppermost range of S. achylosus is given as lower Cenomanian by Perch-Nielsen (1985, fig. 70), but it has been recorded as rare in the Turonian of the Naturaliste Plateau Site 258 by both Thierstein (1974) and Watkins et al. (1996). The extinction of C. achylosus is considered to be a reliable global biostratigraphic event. It is placed in the lower upper Turonian by J. Bergen (pers. comm., 1999), but somewhat lower (below the lower/middle Turonian boundary) by Burnett (1998, fig. 6.3).
The evolutionary first occurrence datum (base) of the seven-rayed E. eptapetalus was noted in Sample 183-1138A-69R-3, 93-95 cm, which we consider no younger than mid-Turonian in age. We recorded the first downhole occurrence (top) of eight-rayed forms attributed to E. octopetalus (= Ephrolithus sp. 2 of Perch-Nielsen, 1985) in Sample 183-1138A-69R-4, 68-70 cm. Perch-Nielsen (1985, fig. 54) and Varol (1992) suggest a stratigraphic range of Cenomanian to Turonian for this taxon, whereas Burnett (1998, fig. 6.3) restricts it to the lower-middle Turonian. This plus the absence of E. eptapetalus confines this sample to the lower Turonian.
Sample 183-1138A-69R-5, 13-15 cm, contains an assemblage dominated by Watznaueria barnesae, E. floralis s.l., Parhabdolithus embergeri, and an assortment of small zygoliths. Fragments of Eiffellithus turriseiffelii were noted, but no Quadrum gartneri or any other miculid morphotype. The age of this sample could well be earliest Turonian. Sample 183-1138A-69R-5, 81-83 cm, from a massive black shale at the base of the section, is barren of nannofossils. This black shale has a total organic content of 2.2% (see "Organic and Inorganic Geochemistry").
The core catcher from Core 183-1138A-70R is an orange-colored glauconitic, sandy packstone and clay that bears serpulid worm tubes. In a smear-slide preparation, it yielded a large amount of carbonate cement, some zeolite, and no nannofossils. The core catcher from Core 183-1138A-71R is also barren, but a sample from a pectin-bearing, glauconitic sandstone at Section 183-1138A-71R-2, 32 cm, contains a few well-preserved nannofossils somewhat similar to those from Sample 183-1138A-69R-5, 81-83 cm, but also with Seribiscutum primitivum, whole E. touriseiffelii, and small (5-µm diameter) prediscosphaerids. This sample needs further study before we can determine its age.
The ages we determined for Core 183-1138A-69R and the nature of the black shale unit at the bottom of Section 183-1138A-69R-5 (see "Lithostratigraphy") suggest that the Cenomanian/Turonian boundary anoxic event (Schlanger and Jenkyns, 1976) may be preserved at the bottom of this core. Stoverius achylosus, which ranges from the Cenomanian into the Turonian, is present in Section 183-1138A-69R-5 above the black shale; however, middle Turonian taxa such as E. eptapetalus, are absent. Another important index species that is apparently absent is Q. gartneri (= M. decussata of some authors), which only made its first evolutionary appearance during the early Turonian. This species, however, appears as "few" to "rare" in the early part of its range in this region (Thierstein, 1974; Watkins et al., 1997), and we could have missed it during the shipboard examination of the cores. Thus, such negative evidence only suggests, but does not prove, the existence of a Cenomanian/Turonian boundary black shale at this site. Significantly, however, no characteristic Cenomanian taxa such as Corolithion kennedyi, Axopodorhabdus albianus, or Microstaurius chiasta, were noted.
On the other hand, the gradual transitional upward in Section 183-1138A-69R-5 from black claystone essentially or totally devoid of nannofossils into laminae and then beds containing progressively more calcareous nannofossils and other calcareous microfossils might be expected during the ventilation of an anoxic deposition environment (e.g., see Frontispiece, DSDP Initial Reports Volume 71, Pt. 1; Ludwig, Krasheninnikov, et al., 1983), one which could have existed in the nearshore shelf region during a major transgression. Such a major global transgression has been postulated as the immediate cause of the Cenomanian/Turonian boundary anoxic event and, along with the progressive subsidence of the volcanic platform of the central Kerguelen Plateau, could explain the lithologic succession from nearshore, oxidized sediments to the meter-thick black shale. We will test this hypothesis during the shore-based research on these cores.
Planktonic foraminifers from the Neogene, Paleogene, and Late Cretaceous are comparable to those encountered during previous drilling on the Kerguelen Plateau, in particular the central Kerguelen Plateau Site 747 (Leg 120). We apply the Neogene Kerguelen (NK) zonal system of Berggren (1992) developed specifically for assemblages in this region to the Pleistocene to Miocene sections (see "Biostratigraphy" in the "Explanatory Notes" chapter). The Paleogene can be characterized in terms of the Antarctic (AP) zonal scheme of Stott and Kennett (1990; modified by Huber, 1991, and Berggren, 1992). A modified version of the Cita et al. (1997) Southern Ocean Late Cretaceous Scheme is useful for biostratigraphic correlation of the Upper Cretaceous, principally the Maastrichtian and uppermost Campanian sediments. Biostratigraphic control is reduced below this interval because of poor microfossil preservation, hiatuses, and low recovery. Unusually good preservation of planktonic foraminifers in thin clay horizons of Santonian and Turonian age may help to improve existing zonal schemes at these key intervals.
We examined planktonic foraminifers in core-catcher samples from all sediment cores (Cores 183-1138A-1R through 73R), as well as additional samples within cores where necessary, to locate major stratigraphic boundaries. Below the thick Pliocene-Pleistocene siliceous ooze section, in which planktonic foraminifers were generally well preserved, preservation varies considerably downhole. Preservation is reasonable in the Miocene but diminishes in the Paleogene and fluctuates between poor and moderate in the Oligocene, diminishing further in the Eocene and Paleocene as the effects of dissolution increase. Preservation is slightly better in uppermost Cretaceous sediment for several cores but is highly variable and often poor in older material.
The abundance of planktonic foraminifers varies highly in the biosiliceous ooze, ranging from relatively common to virtually absent during the Pliocene and Pleistocene. These fluctuations are reflected in the calcium carbonate profile, which shows large scatter in carbonate values within diatom-rich Unit I (see "Organic and Inorganic Geochemistry"). Biogenic carbonate content is much higher in the middle Miocene because siliceous microfossils are less abundant. Upper Paleogene sediment contains common to abundant planktonic foraminifers, but abundance falls in the middle Eocene and Paleocene section. Planktonic foraminifers are mostly sparse in the Lower Cretaceous sediment owing to diagenesis and dilution by clay sediments. Examination of additional samples in clay-rich layers within the cores allowed us to determine the age and composition of the sediments in this interval. Quality of preservation and abundance of planktonic foraminifers is very good in these samples. Planktonic foraminifers are essentially absent from the basal Core 183-1138A-69R black shale, as well as from the underlying glauconite sands and wood-bearing claystones and sandstones at the bottom of the sedimentary sequence.
The top 121.6 m of biosiliceous ooze (Cores 183-1138A-1R through 13R) is characterized by a low-diversity planktonic foraminiferal assemblage dominated by sinistrally coiled Neogloboquadrina pachyderma and Globigerina bulloides. This assemblage is typical of the subantarctic late Neogene (Pliocene, Pleistocene, and latest Miocene) and falls within Zone NK7. Less common elements of the fauna are Globorotalia puncticulata, Turborotalia quinquelobula, and Globigerinita ulva. Diatom biostratigraphy provides greater biostratigraphic control within this interval.
Diversity is also relatively low in the upper Miocene, and foraminifers are often diluted by abundant diatoms, sponge spicules, and variable quantities of fibrous gypsum crystals. Samples 183-1138A-14R-CC through 19R contain common Globorotalia scitula and globigerinids, including G. bulloides and Gobigerina woodi. Based on the absence of N. pachyderma, we assigned these samples to the mid- and upper Miocene Zones NK5-NK6. Globigerinids are larger and more abundant in Sample 183-1138A-20R-CC. In this sample G. bulloides, G. woodi, and Globigerina falconensis are accompanied by Globorotalia praescitula and Globorotalia miozea. A similar range of species is present in the next core downhole, Sample 183-1138A-21R-CC, but forms are generally smaller and globorotalids dominate over globigerinids, perhaps indicating somewhat warmer water. Based on the presence of G. miozea, we assign these cores to the lower middle Miocene Zone NK4.
Globorotalia zealandica and Paragloborotalia incognita in Samples 183-1138A-21R-CC to 24R-CC indicate an early to middle Miocene age (Zone NK3-NK4). Samples 183-1138A-25R-CC and 26R-CC are characterized by an early Miocene (Zone NK2) fauna composed of catapsydracids, Paragloborotalia incognita, and rare Globigerina brazieri. P. incognita is absent in the core catcher of Core 183-1138A-27R. We assign this and the subjacent sample accordingly to the lower Miocene Zone NK1.
Globigerina euapertura, the nominate taxon of the upper Oligocene Zone AP16, appears in Sample 183-1138A-29R-CC, accompanied by catapsydracids, tenuitellids, G. brazieri, Globorotaloides suteri, and various indeterminate globigerinids of late Oligocene affinities. The next two samples downhole contain similar assemblages, and we assign all to combined Zones AP16-AP15. The downhole last appearance datums (LADs) of Chiloguembelina cubensis and Subbotina angiporoides are in Core 183-1138A-33R. The presence of these taxa in Samples 183-1138A-34R-CC to 36R-CC, and absence of Globigerinatheka index and characteristic Eocene Acarinina spp. is evidence for a mid- to early Oligocene age.
The next interval, Samples 183-1138A-37R-CC to 43R-CC, is characterized by low dominance-high diversity assemblages of middle Eocene (Zone AP10-AP11) forms including C. cubensis, Subbotina linaperta, Pseudohastigerina micra, G. index, and low numbers of small acarininids. This middle Eocene fauna directly underlying cores containing a typical lower Oligocene assemblage indicates a hiatus between Sections 183-1138A-36R-CC and 37R-CC. Sample 183-1138A-38R-CC contains large, abundant, and particularly spherical forms of G. index and Globigerinatheka subconglobata, commonly possessing numerous bullae and supplementary apertures similar to the New Zealand forms (Jenkins, 1971). At low latitudes the G. subconglobata-Globigerinatheka beckmanni group show a similar trend toward development of spherical tests. The two species become very common in a short interval in the middle Eocene (tropical Biozone P13). These comparable evolutionary events may be synchronous and useful for cross-latitude biostratigraphic correlation. Alternatively, increased species diversity and morphological variability in the genus may be related to local climatic variation and warming of surface waters.
Preservation diminishes downhole from moderate to poor in Samples 183-1138A-39R-CC and 46R-CC because of dissolution and fragmentation until planktonic foraminifers are indistinguishable in Sample 183-1138A-47R-CC. Preservation is poor to moderate in Samples 183-1138A-48R-CC and 51R-CC. Acarinina mckannai, Chiloguembelina spp., Globanomalina compressus, Globanomalina australiformis, and small morozovellids, in the absence of G. index, P. micra, and Acarinina primitva, indicate an early Eocene-late Paleocene (Zones AP5-AP6) age. Preservation diminishes again in the two subjacent core-catcher samples. We tentatively assign these samples to the upper Paleocene. A poorly preserved, low-diversity assemblage of small globigerinid and biserial forms, including Eoglobigerina spp., Chiloguembelina crinita, and Zeuvigerina teuria are in Sample 183-1138A-51R-CC. Based on the presence of these taxa and the absence of acarininids and well-developed Globanomalina spp., we assign this sample to the lower Paleocene Zone AP1b. A possible expanded K/T boundary occurs in Core 183-1138A-52R.
Cretaceous foraminifers are in Cores 183-1138A-52R-CC through 69R-CC. The Maastrichtian fauna is identical to that at other Kerguelen Plateau and South Atlantic high-latitude sites, although the uppermost Maastrictian Pseudotextularia elegans Zone may be missing at Site 1140. Samples 183-1138A-52R-CC and 53R-CC contains moderately well-preserved Globigerinelloides subcarinatus, Globigerinelloides multispina, Heterohelix globulosa, Heterohelix planata, Hedgerbella sliteri, Globotruncanella petaloidea, Abathmophallus mayeroensis, and Archeoglobigerina australis, an assemblage characteristic of Huber's (1992) G. subcarinatus Subzone. The downhole first appearance datum of G. subcarinatus is in Core 183-1138A-54R. Therefore, we assign the next sample downhole, Sample 183-1138A-54R-CC, to the lower Maastrichtian G. petaloidea Subzone. Abathmophalus, the nominate genus for the middle and upper Maastrichtian, is absent from core catchers, except Sample 183-1138A-53R-CC, in which A. mayaroensis is present.
Samples 183-1138A-54R-CC and 55R-CC contain a slightly different assemblage consisting of Archeoglobigerina australis, Rugoglobotruncana circumnodifer, Globotruncana arca, G. petaloidea, and rare Abathmophalus intermedius. This fauna appears to be older than that found in the overlying sample. Nannofossil studies indicate a possible hiatus between Samples 183-1138A-53R-CC and 54R-CC.
Planktonic foraminifers are highly fragmented and dissolved in Sample 183-1138A-56R-CC, but preservation improves slightly downhole so that we were able to identify key zonal markers. In Samples 183-1138A-57R-CC and 58R-CC we recognize Globigerinelloides impensus, the nominate taxon for the upper Campanian zone bearing this name. Also present is H. globulosa, A. australis, and a small planispiral form resembling G. impensus, comparable to Globigerinelloides sp. recorded by Huber (1990, pl. 1, figs. 8, 9) and Quilty (1992, pl. 1, figs 22, 23). Inoceramid prisms are found in these sample in varying quantities.
The G. impensus Total Range Zone is a useful and consistently recognizable biozone in Upper Cretaceous sediments on the Kerguelen Plateau. The position of the upper boundary of this zone, however, is ambiguous. In Leg 183 cores, we observe the LAD of G. impensus above rather than below the Aspidiolithus parcus expansus nannofossil datum (74.6 Ma) within magnetic Chron C33n and have modified the Upper Cretaceous G. impensus Zone accordingly (see Fig. F6D in the "Explanatory Notes" chapter).
Preservation varies considerably downhole in Unit IV (Cores 183-1138A-59R through 69R) as diagenesis increases. Low recovery and poor microfossil preservation in Cores 183-1138A-59R through 65R prevented us from assigning zones. Recovery was greater in Cores 183-1138A-66R through 69R; we interpreted ages where we could examine samples from carefully selected clay-rich horizons within the chalks and nannofossil claystones. We recovered relatively well-preserved assemblages of mid-Cretaceous planktonic foraminifers from these clay-rich intervals. The fauna is more diverse and richer in keeled forms than other mid- to lower Late Cretaceous faunas described from the South Atlantic (Huber, 1990; Sliter, 1977; Krasheninikov and Basov, 1983). The assemblage is comparable to Upper Cretaceous Austral realm faunas (Huber, 1992), but we also recognize affinities with more temperate Cretaceous faunas such as those from the Exmouth Plateau (Wonders, 1992). Paleogeographic reconstructions suggest that the Exmouth Plateau, which at present lies at about 17°S, was more than 10° south of this position during the Late Cretaceous.
Planktonic foraminifers are abundant, but only moderately to poorly preserved in Sample 1138A-66R-4, 99-102 cm. In addition to abundant Hedbergella planispira and Heterohelix spp., relatively rare but large double-keeled forms are present which, although difficult to identify because of adhering carbonate obscuring surface details, probably belong to the genera Dicarinella and Globotruncana. Based on the presence of the Whitinella baltica and the absence of Archeoglobigerina cretacea, we assign this sample to the Coniacian-Turonian W. baltica Zone.
We also assign the next interval of samples downhole to the W. baltica Zone, but the planktonic foraminiferal assemblage shows some differences in faunal composition. Sample 183-1138A-67R-4, 68-71 cm, contains Heterohelix spp., W. baltica, Whitinella archeocretacea, Whiteinella paradubia, and common double-keeled forms comparable to Dicarinella imbricata and Globutruncana spp. Samples 1138A-68R-1, 102-104 cm, and 68R-4, 5-7 cm, contain H. planispira, W. baltica, and less rare keeled forms. The double-keeled forms are rarely intact, commonly occurring as detached umbilical and spiral halves. Also common in these samples are small, compressed forms with probable hedbergellid or praeglobotruncanid affinities.
Sample 183-1138A-69R-1, 25-27 cm, contains abundant inoceramid prisms and a rather poorly preserved fauna that includes Whitinella spp., H. planispira, and probable Praeglobotruncana stephanii. We tentatively assign these samples to the upper part of the Praeglobotruncana spp. Zone (early Turonian). Forms with widely spaced double keels are missing from the last two samples above the essentially microfossil-barren black shale (Samples 183-1138A-69R-5, 14-16 cm, and 69R-5, 81-83 cm). Sample 183-1138A-69R-5, 14-16 cm, is the better preserved of the two and contains a diverse assemblage of planktonic foraminifers that includes Dicarinella imbracata, Praeglobotruncana sp., W. baltica, and rare Shackoina cenomana. We also assign these samples to the upper Praeglobotruncana spp. Zone because of the absence of characteristic Cenomanian forms. The presence of this lower Turonian assemblage directly above the black shale in Section 183-1138A-69R-5 suggests this horizon may represent the Cenomanian-Turonian oceanic anoxic event.
Sediments that compose Units V and VI (Cores 183-1138A-70R through 72R) do not contain planktonic foraminifers. Sample 183-1138A-70R-CC is composed of abundant carbonate grains and zeolite crystals and is barren of fossils except for a few small calcite tubes that are considered to be the mineralized linings of serpulid worm burrows. The rusty brown glauconite sand in the subjacent core (Sample 183-1138A-70R-2, 12-18 cm) contains pectin shells and rare benthic foraminifers, registering the earliest marine sediments above a thin layer of terrestrial sediments and igneous basement.
We looked for Neogene diatoms in the smear slides that were prepared from core-catcher samples for calcareous nannofossil studies. In some cases, diatom preservation was good and biostratigraphic control by coccoliths was poor because of low-diversity, high-latitude assemblages. We did not attempt to prepare proper samples for diatom studies, and our preliminary results could be greatly improved by shore-based study by a diatom specialist.
Cores 183-1138A-1R and 2R contained abundant Thalassiosira lentiginosa but no Actinocyclus ingens, and we assigned them to the upper Quaternary Thalassiosira lentiginosa Zone. A. ingens is quite abundant in Sample 183-1138A-3R-CC; we assign this sample and the next four cores downhole to the zone of that name. Fragilariopsis barronii is abundant in Samples 183-1138A-5R-CC through 7R-CC (perhaps as high as 4R-CC), which indicates the lower portion of the A. ingens Zone (= the Nitzschia kerguelensis Zone of Harwood and Maruyama, 1992).
A disconformity apparently lies above the next core catcher because Sample 183-1138A-8R-CC contains abundant Thalassiosira insigna and belongs to the upper Pliocene T. insigna-Thalassiosira vulnifica Zone. Sample 183-1138A-9R-CC yielded Nitzschia clementii, Thalassiosira inura, Nitzschia reinholdii, T. oestrupii, N. barronii, and Fragilariopsis interfrigidaria. We assigned it and the next two cores downhole to the mid-Pliocene F. interfrigidaria Zone.
Another disconformity probably lies between Samples 183-1138A-11R-CC and 12R-CC. We assign the latter sample as well as Sample 183-1138A-13R-CC to the N. reinholdii Zone. Besides the nominate species, Sample 183-1138A-13R-CC contains Nitzschia aurica, Rhizosolenia hebetata group, Simonseniella barboi, Hemidiscus ovalis, and Thalassiosira oliverana. Sample 183-1138A-14R-CC contains Denticulopsis hustedtii, whereas Sample 183-1138A-15R-CC yielded that taxon plus rare Nitzschia donahuensis and few A. kennettii. Sample 183-1138A-15R-CC, therefore, belongs to the Asteromphalus kennettii Zone, as do probably the next two core-catcher samples downhole, although we did not discern the nominate species in the smear slides. In this region, however, the occurrence of A. kennettii can be rare and sporadic except in the uppermost part of its range (Harwood and Maruyama, 1992, table 14).
Sample 183-1138A-18R-CC contains common Denticulopsis dimorpha, indicating that we reached the top of the zone of that name. The next core-catcher sample downhole yielded Denticulopsis lauta, Denticulopsis hustedtii, common Nitzschia dentiduloides, and abundant Denticulopsis dimorpha, and we assigned it to the middle Miocene Denticulopsis praedimorpha-N. denticuloides Zone. Sample 183-1138A-20R-CC probably belongs to this zone or lower in the biostratigraphic column; D. hustedtii and N. denticuloides are present but D. dimorpha is absent in this core. Below this point in the hole, pennate diatoms become rare in the smear slides; Denticulopsis maccollumii is present in Samples 183-1138A-22R-CC and 23R-CC, but we could not assign zones confidently because of the lack of diatoms in the smear slides; at this point in the hole they are strongly diluted by calcareous nannofossils.
Preparation for analyzing plant fossils included four-fraction wet sieving with 750-, 250-, 125-, and 45-µm sieves, followed by air drying for 16 days. We then picked individual pieces from dry-sieved fractions under the binocular microscope.
The pieces range in size from a few millimeters to 3 cm. The state of fossilization/permineralization ranges up to coal. Most pieces are covered with a light brown glaze of iron-bearing, low-temperature altered material and are very well rounded. Preservation ranges from good to poor, down to nearly amorphous black remains.
We found parts of fern axes that probably belong to different species, within particularly well-preserved vessels. We also noted leaves of different sizes. Rarely, leaves were even preserved with sporangia, and in one example we found the top part of a young, enrolled frond. Fronds are unique features of ferns.
The material also contains gymnosperm remains, probably of more than one species. These include wood fragments, parts of seeds, and cone scales. We could not classify other pieces found, such as epiderms or spines, at sea. The material is promising for other kinds of shore-based analyses, such as various sectioning techniques, cuticular analysis, and electron microscope observations.
With proper specimen preparation onshore, the material should also be useful for palynomorphological biostratigraphy. The core-catcher material of Core 183-1138A-69R-CC seems to contain a rich flora of spores; we also observed some in the core catcher of Core 183-1138A-71R.