A sedimentary succession, ranging in age from the Late Cretaceous to Quaternary, was recovered at Site 1172 (East Tasmanian Plateau). Microfossils belonging to seven major groups were investigated. Combined results suggest that there are no major breaks in the sedimentary succession from the Pleistocene to the uppermost Oligocene. In contrast, it appears that most of the Oligocene is missing, whereas the Eocene-Oligocene transitional interval is present, albeit very condensed. Underlying the upper Eocene strata, a thick middle Eocene to upper Maastrichtian section was recovered with possible breaks in the lower middle Eocene and at the Paleocene/Eocene boundary (Fig. F15).
Siliceous microfossils are rare to absent in the Holocene to Pliocene interval but are common to abundant and well preserved in the Miocene. The thin Oligocene succession yields few radiolarians, whereas diatoms remain abundant downhole. Both groups are common to locally abundant in the Eocene of Hole 1172A with good preservation. The upper Paleocene to upper Maastrichtian interval is virtually barren of siliceous microfossils.
Planktonic foraminifers and calcareous nannofossils are generally abundant, with preservation ranging from moderate to good in the Neogene and Oligocene. Although less abundant, calcareous nannofossils remain consistently present until the middle Eocene, when abundance and preservation decrease dramatically. Below the middle Eocene (Hole 1172D), the Cenozoic succession is barren of calcareous nannofossils. Planktonic foraminifers are virtually absent below the middle/upper Eocene boundary. Well-preserved and reasonably diversified calcareous microfossils reappear in the upper Maastrichtian interval.
Calcareous benthic foraminifers are consistently present throughout the Neogene-Oligocene carbonate succession. The middle Eocene sequence yields only rare agglutinating species. However, calcareous and agglutinating taxa are present in the Paleocene to upper Maastrichtian interval.
Well-preserved organic walled dinoflagellate cysts (dinocysts) and few sporomorphs are present in the Holocene-Pleistocene interval of the recovered succession at Site 1172. The remaining Neogene to lower Oligocene strata are devoid of acid-resistant organic matter. Moderate to well-preserved dinocysts are the dominant constituent of palynological associations in the upper Paleocene to lowermost Oligocene part of the succession and are persistent below this interval. Well-preserved terrestrial palynomorphs dominate the lowermost part of Hole 1172D, attributed to the late Maastrichtian to middle Paleocene.
Sedimentation rates were relatively low in the Neogene to upper Eocene interval, ranging from 0.16 to 3.2 cm/k.y. (Fig. F15). In contrast to previous Leg 189 sites, sedimentation rates were also generally low in the middle Eocene to Maastrichtian interval (between 2.6 and 1.04 cm/k.y.) (see "Age Model and Sedimentation Rates").
The Pliocene/Pleistocene boundary is placed between Samples 189-1172A-1H-CC and 2H-CC on combined microfossil evidence. The upper/lower Pliocene boundary is placed between Samples 189-1172A-5H-CC and 6H-CC, whereas the Pliocene/Miocene boundary is recognized between Samples 189-1172A-9H-CC and 10H-CC. The upper/middle Miocene boundary falls within planktonic foraminiferal Zone SN8 and nannofossil Zone CN6 but cannot be located with accuracy on the basis of the present set of samples; it must, however, be present above Sample 189-1172A-20H-CC. The middle/lower Miocene boundary is inferred to be above Sample 189-1172A-33X-CC. The Miocene/Oligocene boundary falls within the uppermost part of planktonic foraminiferal Subzone SP14b (but is normally placed for convenience at the base of Zone SN1) and at the base of nannofossil Subzone CN1a. Planktonic foraminifers indicate the passage of this boundary between Samples 189-1172A-37X-CC and 38X-CC. In terms of nannofossils, the Oligocene/Miocene boundary is associated with the last occurrence (LO) of Reticulofenestra bisecta (23.9 Ma), which is recognized between Samples 189-1172A-36X-CC and 37X-CC. The boundary is generally placed just above this datum.
The carbonate sequence of Site 1172 is subdivided biostratigraphically chiefly on calcareous microfossil data. The few available siliceous microfossil datums are in broad agreement with the age assignments presented above. Palynomorphs have not been analyzed in sufficient detail and/or are absent in the Neogene.
The combined paleobiotic information suggests temperate to cool temperate conditions throughout the Neogene and a deep-marine (lower bathyal to upper abyssal), well-ventilated depositional setting. To a certain extent, the Neogene encountered at Site 1172 is quite comparable with the age-equivalent interval recovered at Site 1168. A conspicuous difference is the absence of palynomorphs in this interval. It appears that in comparison with Site 1168, water masses at Site 1172 were better ventilated and more oxygen-rich, resulting in the absence of organic matter in the lower Oligocene to Neogene sediments.
The upper/lower Oligocene boundary coincides with the base of planktonic foraminiferal Subzone SP14b and is placed between Samples 189-1172A-38X-CC and 39X-CC. Part of the upper and lower Oligocene appears to be missing, as the sediments correlated to the latter zone are resting on an interval assigned to the early Oligocene planktonic foraminiferal Zone AP13.
On nannofossil evidence, it appears that much of the interval from ~27 to ~36 Ma is missing at this site. Diatoms constrain the age of the Oligocene deposits to the interval between 30.24 and 28.8 Ma (between Samples 189-1172A-39X-3, 61-63 cm, and 37X-CC).
In terms of planktonic foraminifers, the Eocene/Oligocene boundary cannot be recognized with accuracy at this site. An age of 30.24 Ma or younger can be assigned to the interval between Samples 189-1172A-39X-2, 3-5 cm, to 39X-3, 61-63 cm, on the basis of diatoms. An age of 27 to 36 Ma can be assigned to the interval between Samples 189-1172A-39X-2, 143 cm, and 39X-5, 63 cm, on the basis of nannofossils. The interval bounded by these latter samples (189-1172A-39X-2, 143 cm, and 39X-5, 63 cm) is barren of calcareous microfossils, whereas dinocysts and siliceous microfossils are consistently present. These groups suggest that the Eocene/Oligocene boundary (33.7 Ma) may be positioned between Samples 189-1172A-39X-3, 122-123 cm, and 39X-5, 22-24 cm. Combined information hence indicates that the succession, albeit condensed, is essentially complete across the Eocene-Oligocene transition.
The middle/upper Eocene boundary is recognized between Samples 189-1172A-39X-CC and 40X-CC. The bottom of Hole 1172A may be assigned to the middle middle Eocene on nannofossil and dinocyst evidence. Unfortunately, other microfossil groups cannot assist with an age assessment between the middle/upper Eocene and the K/T boundaries penetrated in Hole 1172D. Samples 189-1172D-5R-CC through 7R-CC are dated as early middle Eocene (43-49 Ma) based on nannofossil evidence, whereas Samples 189-1172D-14R-CC through 24R-CC are virtually barren of nannofossils or yield only a few specimens of nonage-diagnostic species. Dinocysts indicate the lower/middle Eocene boundary is close to Sample 189-1172D-12R-CC, whereas the Paleocene/Eocene boundary appears to be between Samples 189-1172D-19R-CC and 20R-CC. It appears that part of the lower middle Eocene is missing in Hole 1172D (Fig. F15).
Palynological evidence may be interpreted to indicate that the uppermost Paleocene and lowermost Eocene may be missing as well. Sample 189-1172D-23R-CC may be assigned to the Danian based on the common occurrence of the dinocyst Alisocysta reticulata (63-61 Ma), although unpublished reports suggest that in the Southern Hemisphere this species first appears in the uppermost Maastrichtian (P.S. Williumsen, pers. comm., 2000). The K/T boundary is presumed to be within Core 189-1172D-24R since Sample 189-1172D-24R-CC does not yield Tertiary dinocysts but does contain a typical Southern Hemisphere latest Maastrichtian assemblage. Detailed sampling within Section 189-1172D-24R-5 and subsequent palynological analysis revealed the K/T boundary to be between 189-1172D-24R-5, 77-79 and 44-46 cm, on the basis of the FO of Trithyrodinium evittii. The interval from Samples 189-1172D-25R-CC through 31R-CC contains Cretaceous nannofossils, including Arkhangelskilla cymbiformis, Cribosphaerella daniae, and Nephrolithus frequens. The last species is an index species for the late Maastrichtian, with an age range of 65-67 Ma in the Southern Ocean. Dinocysts from Sample 189-1172D-31R-CC indicate an age of 69 Ma or younger, whereas planktonic foraminifers confirm a Maastrichtian age for the bottom of Hole 1172D. Completeness of the K/T boundary succession will be assessed postcruise.
The Eocene-Oligocene transition is marked by a series of distinct stepwise environmental changes, reflecting increasingly cool conditions and coeval rapid deepening of the basin. A change from inner neritic depositional environments with marked freshwater influence and sluggish circulation to more offshore, deep-marine environments with increased ventilation and bottom-water current activity, marks the middle/upper Eocene boundary. Concomitant cooling is indicated by the increased numbers of endemic antarctic dinocyst species, whereas warmer episodes may be recognized as well. This trend continues throughout the upper Eocene and lowermost Oligocene (~34.0-33.3 Ma). Sediments and biota indicate increasing bottom-water ventilation and the appearance of conditions supporting highly productive surface waters, in outer-neritic to bathyal depositional settings, with increasingly cold conditions. The trend culminated in the early Oligocene (33-30 Ma) when rigorous ventilation, and generally oxygen-rich waters, precluded sedimentation of organic matter despite overall high surface-water productivity. The condensed calcareous sequence contains abundant siliceous microfossils and was deposited in an oceanic bathyal environment. The Paleocene to middle Eocene dinocyst assemblages are dominated by relatively warm water, (inner) neritic, eutrophic forms. Terrestrial palynomorphs increase in relative abundance downhole and throughout, indicating warm climates in this interval. This suggests even shallower marine conditions for the older part of the succession at Site 1172, with the influence of marked runoff. Absence of foraminifers, and even nannoplankton, in most parts of the middle Eocene to Paleocene confirms the attribution to marginal marine settings. More offshore settings return in the Maastrichtian section, indicated by the return of calcareous microfossils and more offshore dinocyst assemblages. Middle to late Eocene radiolarian faunas are mainly composed of endemic taxa, whereas warm-water taxa do occur sporadically as well.
All core-catcher samples plus additional samples from critical intervals were examined for calcareous nannofossils from all four holes at Site 1172. Neogene nannofossil assemblages are similar to those recovered at Site 1168. Several index species of discoasters and ceratoliths are present, and they contribute to a higher stratigraphic resolution than that for the more southern sites (Sites 1169, 1170, and 1171; Table T3). The Oligocene-Eocene transition also yields abundant, well-preserved nannofossils. From the mid-upper Eocene through the Paleocene, nannofossils are sporadically present with variable preservation and many barren intervals. Nannofossils are generally few to common in the upper Maastrichtian (Cores 189-1172D-25R through 31R).
The first occurrence (FO) of Emiliania huxleyi was not recorded in the first core-catcher sample at any of the three holes, indicating an age of older than 0.26 Ma for these samples. The LO of Pseudoemiliania lacunosa (0.46 Ma), marking the top of Zone NN19 (Martini, 1971) was recognized in all three holes (Table T4). The Pliocene/Pleistocene boundary is approximated by the LO of Discoaster brouweri (1.95 Ma).
The LO of Discoaster pentaradiatus (2.4 Ma) and Discoaster surculus (2.51 Ma) mark the base of late Pliocene Zones NN18 and NN17 (Martini, 1971), respectively. The lower/upper Pliocene boundary is marked by the LO of Reticulofenestra psuedoumbilica (3.65 Ma).
The Miocene/Pliocene boundary is approximated by the LO of Discoaster quinqueramus (5.3 Ma) recognized in the first three holes at ~80 mbsf. Nannofossil biostratigraphic resolution for the Miocene is relatively low at this latitude compared with middle to low latitudes, with only several index species present.
The LO of Reticulofenestra bisecta (23.9 Ma) is recognized between Samples 189-1172A-36X-CC and 37X-CC. The Oligocene/Miocene boundary is generally placed just above this datum. Another Oligocene nannofossil datum, the LO of Chiasmolithus altus (26.1 Ma), is present one core lower, between Sample 189-1171A-37X-CC and 38X-CC.
The LOs of Reticulofenestra umbilica and Reticulofenestra reticulata are found between Samples 189-1172A-39X-2, 143 cm, and 39X-5, 63 cm. Several samples examined between these two samples are barren of calcareous nannofossils. The truncation of the stratigraphic ranges of R. umbilica and R. reticulata and an absence of the stratigraphic interval with Isthmolithus recurvus and Reticulofenestra oamaruensis suggest that much of the interval from ~27 through ~36 Ma appears to be missing at this site.
Two more Eocene nannofossil datums were determined with moderate precision—the LO of Chiasmolithus solitus (40.4 Ma) between Samples 189-1172A-42X-CC and 43X-CC and the FO of R. reticulata (42.0 Ma) between Samples 189-1172A-43X-CC and 44X-CC. The FO of R. umbilica (43.7 Ma) is located between Samples 189-1172A-45X-CC and 53R-CC. This uncertainty in a long stratigraphic interval is due to the fact that many of the samples within this interval are virtually barren of nannofossils.
Samples 189-1172D-5R-CC through 7R-CC are dated as the early middle Eocene (43-49 Ma) based on the presence of C. solitus and the absence of R. umbilica and Discoaster kuepperi. Samples 189-1172D-14R-CC through 24R-CC are barren of nannofossils or yield only a few specimens of long-ranging species.
The interval from Samples 189-1172D-25R through 31R-CC contains Maastrichtian nannofossils, including Arkhangelskilla cymbiformis, Cribosphaerella daniae, Eiffellithus turriseiffelii, Micula decussata, and Nephrolithus frequens. The last species is an index species for the late Maastrichtian, with an age range of 65-70 Ma in the Southern Ocean.
Shipboard examination of all core-catcher samples from Hole 1172A disclosed that sediments ranging in age from Maastrichtian to Quaternary were recovered at Site 1172. The planktonic foraminiferal assemblages are typical of cool temperate regions. An apparently complete Neogene to upper Oligocene section was recognized at Site 1172. The upper Oligocene is very thin with only Subzone SP14b being recognized. The upper Oligocene rests on the lower Oligocene (Zone AP13), indicating a stratigraphic break. Beneath this is an upper to middle Eocene sequence that can be recognized on planktonic foraminifers; an underlying lower Eocene to Paleocene section is barren of planktonics. The Maastrichtian can be recognized on planktonic foraminifers. The overall sequence, with the exception of the Paleocene and Cretaceous, including the hiatuses, is almost identical to that found at Site 1168.
Planktonic foraminifers provide little biostratigraphic control throughout much of the Paleogene sequence recovered at Site 1172. Almost all of the Oligocene is missing, with only a single sample from the uppermost part being recognized. From the upper Eocene of Hole 1172A down, planktonic foraminiferal assemblages are often depauperate and barren of planktonic foraminifers below Sample 189-1172A-43X-CC. Only in the upper Maastrichtian, planktonic foraminifers return (from Sample 189-1172D-25R-CC down) with a good assemblage of high-latitude Late Cretaceous species. Cretaceous species occur sporadically in samples to Sample 189-1172D-30R-CC. Sample 189-1172-31R-CC is barren. The stratigraphic distribution of species is given in Table T5. The core depths of the various planktonic foraminiferal datums at Site 1172 are given in Table T6. A brief discussion of the salient biostratigraphic findings is provided below.
The base of the Quaternary, as defined by the FO of Globorotalia truncatulinoides, is confined to the uppermost core in Holes 1172A (Sample 189-1172A-1H-CC). This appears to represent another example where the FO of G. truncatulinoides seems to be an unreliable datum in the Tasmanian region. The well-preserved assemblages are primarily temperate in character, being dominated by such cool-water species as Globigerina bulloides, Globorotalia crassaformis, Globorotalia inflata, and Neogloboquadrina pachyderma (sinistral).
The late Pliocene Zone SN13, as well as Subzones SN12b and SN12a, is well represented at Site 1172 (see Table T5). The abundant and well-preserved faunas are dominated by specimens belonging to the Globorotalia puncticulata/G. inflata plexus.
All of the late Miocene zones have been recognized at Site 1172, including Zone SN11. The only other record of this zone has been from Site 1168, where it is thin (just under 8 m). At Site 1170, where only this zone was missing, evidence from other microfossil groups also suggest the presence of a hiatus. At the other two sites, the absence of Zone SN11 and other zones also indicates a hiatus. At Site 1172 it is also thin, being recorded from only two samples (189-1172A-14H-CC and 15H-CC). Faunal preservation varies from good to moderate, and assemblages are common to abundant.
Zones SN5 to SN8, which are within the middle Miocene, are recognized with no conspicuous breaks. Over the interval of the upper part of Zone SN7 into Zone SN8, the faunas have typically moderate to good preservation with fairly low abundances reflecting some dissolution. In the lower part of Zone SP7, dissolution effects become less and both preservation and abundances improve. As with all other sites, specimens of Globigerinoides quadrilobatus are absent, but Globigerinoides trilobus was recorded from only one sample (189-1172A-31X-CC). Warm-water species with subtropical affinities are absent, which is surprising because the East Australian Current should have bathed this area at that time. Orbulina universa and Praeorbulina are both rare.
Zones SN2 and SN4 are not recognized at Site 1172. Zone SN4 is based on the presence of G. trilobus without Praeorbulina curva, and as noted above, G. trilobus has been identified only from the middle Miocene at this site, so its absence does not indicate a hiatus. Zone SP2 also has not been recognized here, but as the whole of the lower Miocene at this site is thin (38 m), it may be present within a core and so not sampled. As at most of the other sites Globoquadrina dehiscens is rare, but Zone SN1 is recognizable.
The entire upper Oligocene, which encompasses some 6.8 m.y., is missing at Site 1172. The early Miocene Zone SN1 (Sample 189-1172A-26X-CC) unconformably overlies sediments containing Subbotina angiporoides, indicating an early Oligocene age (Zone AP13) for Sample 189-1172A-27X-CC. This hiatus is correlative to that at Site 1170, but there Subzone SP14b overlies Zone AP13. By contrast, the Oligocene-Miocene transition appears to be continuous at Site 1168, where the upper Oligocene (Zone SP14) is ~300 m thick.
The lower Oligocene succession at Site 1172 is the thinnest of all the sites, being recognized in a single sample (189-1172A-39X-2, 5-7 cm) from 356.15 mbsf. Here S. angiporoides is present without Subbotina linaperta or Globigerinatheka index. Sample 189-1172A-39X-CC contains both S. angiporoides and S. linaperta, indicating the late Eocene Zone AP11. Thus, the lower Oligocene must be confined to Core 189-1172A-39X.
The Eocene/Oligocene boundary has not been recognized at Site 1172, but as noted above, it must be within Core 189-1172A-39X. Further details await the results of shore-based studies.
Late and middle Eocene faunas are present at Site 1172. Although these are very well preserved, their abundance declines from the highest sample where they are rare to Sample 189-1172D-7R-CC, where only a trace is present. The presence of G. index in Sample 189-1172D-2R-CC indicates Zone AP12. Samples 189-1172A-39X-CC and 3R-CC have been referred to Zone AP12 on the appearance of S. linaperta. Acarinina primitiva is the only planktonic foraminifer species found in Sample 189-1172D-7R-CC, indicating a level within the interval of Zones AP7 to AP10. Below this, the samples are barren of planktonic foraminifers.
Late Cretaceous assemblages are encountered in Sample 189-1172D-25R-CC. They are low-diversity and low abundance assemblages that are well preserved. The faunas are typical of the Austral Realm assemblages from the subantarctic region (Huber, 1991). The absence of key zonal taxa (such as Abathomphalus mayaroensis) precludes assignment to the global Late Cretaceous zonal scheme. However, the presence of Rugoglobigerina rugosa indicates a Campanian to Maastrichtian age. Sample 189-1172D-26R-CC contains the same assemblage but is less well preserved. Samples below this are generally composed of single species in low numbers. None were found in Sample 189-1172D-31R-CC.
Similar to Sites 1170 and 1171, benthic foraminiferal assemblages at Site 1172 change from neritic to upper bathyal faunas in the Paleocene and Eocene to lower bathyal to abyssal faunas in the Oligocene and Neogene. The transition, however, is not abrupt. Reconstruction of water depths in the Cretaceous to middle middle Eocene is limited by very restricted faunas, but these suggest a change from upper bathyal to neritic depths. This trend is reversed in the upper middle Eocene, when a barren interval is followed by upper bathyal faunas. The uppermost Oligocene is most likely still marked by middle to lower bathyal depths, whereas the Neogene interval is dominated by lower bathyal to upper abyssal faunas.
The finely agglutinated foraminifer Spiroplectammina is the most persistent genus in the Cretaceous to middle Eocene interval. Van Morkhoven et al. (1986) describe S. spectabilis as having an upper depth range of middle to upper bathyal in the Paleocene and upper bathyal to outer neritic in the Eocene. The presence of planktonic foraminifers and calcareous benthic foraminifers belonging to the genera Bulimina and Anomalinoides in the uppermost Cretaceous (Samples 189-1172D-26R-CC to 25R-CC) suggest that, at least for this interval, more open marine conditions must have prevailed and that water depths of 200-400 m were likely. The Paleocene to upper middle Eocene samples are almost without exception virtually barren (189-1172D-23R-CC through 5R-CC) or entirely barren (189-1172A-4R-CC and 52X-CC through 44X-CC). Limited occurrences of Spiroplectammina spp. and other agglutinated taxa (e.g., Glomospira spp. and Reophax spp.) suggest a restricted environment. It has to be noted that sample spacing is wide and that more diverse and diagnostic faunas might be found within the cores. In this interval, two samples (189-1172A-56X-CC and 53X-CC) were found to contain Elphidium, a genus only found at neritic depths. In summary, this implies a shallowing of water depths from upper bathyal at the end of the Cretaceous to neritic in the middle middle Eocene.
The barren interval near the end of the middle Eocene is followed by a mostly calcareous benthic fauna with Cibicidoides spp., Globocassidulina spp., and costate Uvigerina spp. (Samples 189-1172D-3R-CC through 2R-CC and Samples 189-1172A-43X-CC through 39X-CC). The presence of rare planktonic foraminifers supports the conclusion that this interval was deposited in upper bathyal water depths. Most of the upper Eocene and Oligocene section is highly condensed, and more detailed sampling will provide information on the history of subsidence during this crucial interval. However, the presence of Hanzawaia mantaensis in Sample 189-1172A-37X-CC suggests that the latest Oligocene was still marked by bathyal water depths. Van Morkhoven et al. (1986) describe this species from upper to middle, and occasionally neritic, depths, but the co-occurrence with Cibicidoides mundulus and Fontbotia wuellerstorfi implies lower bathyal depths.
A notable difference between the Neogene benthic foraminiferal assemblages at this site and at previous sites is the early occurrence of F. wuellerstorfi. Both C. mundulus and F. wuellerstorfi are present from the uppermost Oligocene to lowermost upper Pliocene (Samples 189-1172A-37X-CC through 5H-CC), with the exception that C. mundulus is likely absent across the Miocene/Pliocene boundary (Samples 189-1172A-14H-CC through 8H-CC). Similar to Site 1171, Melonis barleeanus is present in the lower Neogene, but it plays a much lesser role than at Sites 1168 to 1170. Sigmoilopsis schlumbergeri is consistently present within the Pliocene and Pleistocene (Samples 189-1172A-11H-CC through 1H-CC). This is unlike Site 1170, where the few specimens were very badly preserved. Site 1172 shows high abundances of Uvigerina hispida and Uvigerina peregrina from the uppermost Pliocene to the topmost samples (189-1172A-7H-CC to 1H-CC). Similar abundance patterns were not recorded at the other sites, but in the same interval Chilostomella oolina is an abundant species. This species is also recorded in one sample near the top at Site 1168, in the topmost samples of Site 1169, and in the top sample at both Sites 1170 and 1171.
Most samples in the biogenic carbonate sequence (189-1172A-38X-CC to 1X-CC) contain a few ostracode valves. In the Cretaceous-Paleocene sequence, only one valve is recorded. Bolboforma spp. are absent.
Radiolarians are common in the Miocene and Eocene but are absent or rare in the Holocene through Pliocene and Oligocene intervals. No radiolarians were recovered from the Paleocene to Cretaceous. Standard radiolarian zones cannot be applied for the radiolarian sequence of Site 1172 because of the near absence of age-diagnostic species (Table T7). Characteristics of the radiolarian assemblages are summarized below.
The Holocene through Pliocene radiolarian faunas (Samples 189-1172A-1H-CC through 12H-CC, 189-1172B-1H-CC through 12H-CC, and 189-1172C-1H-CC through 12H-CC) are mainly composed of only two or three spumellarian species, including Stylacontarium acquilonium, and are identical to the same age interval of Site 1168. The Miocene radiolarians of Site 1172 are marked by an absence of antarctic species. The faunas are very similar to those of age-equivalent intervals of Sites 1170 and 1171. Differences are expressed in terms of the continuous occurrence of temperate artiscinid forms and the sporadic occurrence of tropical artiscinid species at Site 1172. Samples 189-1172A-13H-CC through 26X-CC, 189-1172B-13H-CC through 22H-CC, and 189-1172C-13H-CC through 22H-CC are assigned to the late Miocene. The Miocene/Pliocene boundary is placed approximately at the last consistent occurrence (LCO) of Stichocorys delmontensis at 5.18-6.90 Ma, and the boundary between the upper and middle Miocene by the last abundant occurrence (LAO) of Cyrtocapsella japonica at 11.6 Ma. The C. japonica LAO is a global event and is also recognized at Sites 1168, 1170, and 1171. At Site 1172 this bioevent is not recorded between the LO of C. japonica (at 9.9-10.6 Ma) in Sample 189-1172A-21H-CC and the LAO of Cyrtocapsella tetrapera (at 12.5 Ma) in Sample 189-1172A-27X-CC. A hiatus or condensed zone from 11.6 through 12.5 Ma may, therefore, be placed between Samples 189-1172A-26X-CC and 27X-CC.
Samples 189-1172A-32X-CC through 37X-CC are barren of radiolarians or yield only a few specimens. Oligocene faunas have been recorded from one sample only (189-1172A-38X-CC). The late and middle Eocene radiolarians consist mainly of antarctic species associated with a few tropical species. Age-diagnostic species for the antarctic, however, are sporadic. Eocene subantarctic or temperate faunas have never been reported, so those recovered from Site 1172 are unique in linking tropical and antarctic faunas and provide new insights into their evolution.
The LO of Lychnocanoma amphitrite (at 32.8 Ma), which is a presumed synchronous bioevent in the tropical regions, is recognized between Samples 189-1172A-39X-CC and 40X-CC, although the Eocene/Oligocene boundary (at 33.7 Ma) is placed within Core 189-1172A-39X based on other evidence. This discrepancy is probably caused by the diachronicity of the LO of L. amphitrite between the subantarctic and the tropics. Another bioevent, the FO of Eucyrtidium antiquum, is known to occur at 32.96 Ma in the antarctic. This bioevent is recorded in Sample 189-1172A-40X-CC. Hence, both bioevents are apparently older at Site 1172 than in antarctic and tropical regions. Samples 189-1172D-2R-CC and 3R-CC are late Eocene in age, based on the absence of E. antiquum and the co-occurrences of Eucyrtidium cheni and Eucyrtidium spinosum.
The middle/upper Eocene boundary is placed between Samples 189-1172A-46X-CC and 48X-CC, as the FO of L. amphitrite (at 36.4-37.3 Ma) is recognized below Sample 189-1172A-46X-CC and the LO of Spongatractus pachystylus (at 37.3-38.8 Ma) is above Sample 189-1172A-49X-CC. Samples 189-1172A-49X-CC through 56X-CC and 189-1172D-4R-CC through 14R-CC are correlated to the middle Eocene. The FO of Dictyomitra amygdala (approximately lower-middle Eocene in the North Pacific; Shilov, 1995) is recognized between Samples 189-1172D-14R-CC and 15R-CC. The middle Eocene Samples 189-1172A-51X-CC and 52X-CC commonly contain two lower Eocene reworked species: Amphisphaera goruna and Spongatractus balbis. In other middle Eocene samples of Site 1172A, no significant amounts of reworked radiolarians are found.
Samples 189-1172D-15R-CC through 22R-CC contain rare, poorly preserved unidentifiable radiolarians. Samples 189-1172D-23R-CC and 24R-CC are barren of radiolarians.
Core-catcher material from Holes 1172A and 1172D was analyzed for diatoms, sponge spicules, and silicoflagellates. Samples were treated with 40% HCl to remove the carbonate component. Intervals rich in organic matter were treated with up to 40% H2O2 prior to acid treatment.
Diatoms are confined to discrete intervals at Site 1172. Abundance data for diatoms, sponge spicules, and silicoflagellates are presented in Table T8. Samples 189-1172A-1H-CC through 12H-CC are barren or contain "few" diatoms with moderate to good preservation. There is an abrupt downhole increase in biogenic silica between Samples 189-1172A-12H-CC (110.05 mbsf) and 13H-CC (120.76 mbsf). Samples 189-1172A-13H-CC through 18H-CC contain an abundant, well-preserved, and diverse open-ocean flora. Throughout this interval, there are fluctuations in the dominance of subantarctic and temperate/warm taxa, suggesting meridional shifts in the position of the Subtropical Convergence at this site during the late Miocene. Samples 189-1172A-19X-CC through 28X-CC contain less abundant, lower diversity floras, whereas Samples 189-1172A-29X-CC through 37X-CC are barren or contain only rare diatom occurrences.
There is a second abrupt downhole increase in biogenic silica between Samples 189-1171A-37X-CC (345.10 mbsf) and 38X-CC (353.76 mbsf). Samples 189-1172A-38X-CC and 39X-2, 3-5 cm, and 189-1171D-2R-CC (early Oligocene) contain a well-preserved open-ocean flora plus a noteworthy proportion (~30%) of well-preserved neritic diatoms. The Eocene section (Samples 189-1172A-39X-CC and below) contains an abundant and generally well-preserved neritic flora in association with open-ocean taxa in varying proportions (~2%-10% of the total assemblage, see below). Similar neritic assemblages to those observed at Sites 1170 and 1171 are present at Site 1172. Although Pyxilla is generally broken throughout the sequence, diatom preservation is mostly very good. In particular, whole frustules and chains (e.g., Paralia sulcata vars.) remain intact, suggesting a quiescent environment and/or sluggish circulation. Eocene and early Oligocene neritic diatoms are considered to be in situ because of such preservation and their high abundance. Samples 189-1172A-39X-CC through 41X-CC (upper middle to upper Eocene) contain ~2% open-ocean diatoms, whereas Samples 189-1172A-42X-CC through 49X-CC (middle Eocene) contain a slightly greater proportion of open-ocean taxa, decreasing downhole from ~10% to 5% of the total assemblage. This information suggests that during the late Eocene, the depositional setting may have shallowed or become slightly more restricted again before opening to deeper waters. Samples 189-1172-50X-CC through 53X-CC comprise an entirely neritic diatom flora with no open-ocean influences. This is also supported by dinocyst evidence (see "Palynology"). High abundances of high productivity indicators (e.g., Chaetoceros resting spores) found in Samples 189-1172A-42X-CC through 53X-CC suggest eutrophic conditions for the middle Eocene (see also "Palynology"). Samples 189-1172A-54X-CC and below contain only rare occurrences or are barren of diatoms. However, pyritized diatoms are present in low abundance at the K/T boundary. Initial analyses suggest a shallow (neritic) marine setting by the common occurrence of Stephanopyxis spp.
The sporadic occurrence of planktonic diatoms in the Oligocene to Quaternary interval curtails any detailed diatom biostratigraphy for this interval. Three bioevents only are recognized in Hole 1172A (see Table T9). These are the LO of Denticulopsis dimorpha (10.7 Ma), placed between core-catcher samples of Cores 189-1172A-23H and 24H, and the FO of Rocella vigilans (30.24 Ma) and Cavitatus (Synedra) jouseanus (30.64 Ma), both placed between Samples 189-1172A-39X-2, 3-5 cm, (356.14 mbsf) and 39X-3, 61-63 cm (358.22 mbsf). Diatom biostratigraphic markers are not present in the Eocene to Cretaceous sediments (Holes 1172A and 1172D).
Onboard palynological analysis included a few selected core-catcher and core samples from Site 1172, comprising critical intervals from Hole 1172A to assist in dating and/or to assess the potential for future, more detailed studies. Similarly, a selection of core-catcher and core samples was analyzed from Hole 1172D (cf. Table T10).
Samples 189-1172A-1H-CC and 4H-CC yielded abundant (189-1172A-1H-CC) to a few (189-1172A-4H-CC) Pleistocene-Pliocene dinoflagellate cysts, dominated by Protoperidinium cysts like Brigantedinium spp. and Multispinula quanta (Sample 189-1172A-1H-CC) or oligotrophic warm-water representative of Impagidinium (Sample 189-1172A-4H-CC). This indicates good potential for future detailed studies of the younger Neogene.
Further analysis of samples from Hole 1172A was limited to the presumed Eocene-Oligocene transition (Cores 189-1172A-37X to 40X) and the lowermost Cores 189-1172A-50X to 56X to provide (additional) age control. Palynomorphs are consistently present in great abundance from Sample 189-1172D-39R-2, 18-20 cm, down. Above this horizon, samples are devoid of acid-resistant organic matter. The Eocene-Oligocene succession of Hole 1172A is similar to those recorded at Sites 1170 and 1171. Also in this case, the lower Oligocene is palynologically barren and thus provides further evidence of the inception of the influence of well-oxygenated (bottom) water masses.
Dinocysts are generally the most abundant category of palynomorphs in productive samples from Hole 1172A. In addition, pollen, spores, foraminifer organic linings, and acritarchs are present, albeit generally in low relative abundances. The few identified sporomorph taxa may be compared with established records from New Zealand and Australia and indicate cooling and increasing humidity from the middle middle Eocene to the late Eocene. Moreover, as stated for previous sites, again they are comparable to those reported by Mohr (1990) from approximately age-equivalent sediments from across Antarctica.
Palynomorphs are generally well preserved, and dinocyst concentrations are high, but assemblages are often of relatively low diversity for Eocene standards and may be totally dominated by a single taxon. In terms of quantitative distribution patterns, the middle to upper Eocene interval is quite comparable to the correlative successions of Holes 1170D and 1171D.
Middle to late Eocene (and earliest Oligocene) Hole 1172D assemblages are approximately identical to those reported from age-equivalent records from nearby sites and even to presumed coeval successions recovered from the other side of Antarctica, as well as those from Northern Hemisphere high-latitude sites (see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter and "Palynology" in "Biostratigraphy" in the "Site 1171" chapter). Applied dinocyst events are listed in Table T11. For the middle to late Eocene, the dinocyst datums are reasonably consistent with results from the calcareous nannofossils from Hole 1172D.
The palynological succession of the Eocene-Oligocene transition (viz., the interval from Sample 189-1172A-38X-CC to 40R-CC) is quite comparable with that of Hole 1170D. The youngest available productive sample of the Eocene-Oligocene interval in Hole 1172A (189-1172A-39X-6, 18-20 cm) contains a few operculae of Enneadocysta partridgei, as well as several specimens of Stoveracysta kakanuiensis. Representatives of an undescribed Protoperidinioid genus, provisionally assigned to Brigantedinium?, dominate the assemblage. The latter phenomenon indicates a highly productive setting, as motile stages of this group feed predominantly on diatoms. The typical composition of the sample, together with the very few (parts of) specimens of E. partridgei, would suggest an age close to 33.3 Ma or slightly younger (i.e., near the extinction of E. partridgei; see also "Palynology" in "Biostratigraphy" in the "Site 1170" chapter and "Palynology" in "Biostratigraphy" in the "Site 1171" chapter). Overlying samples are barren, and thus, no interpretations can be provided on possible missing section.
The underlying Sample 189-1172A-39X-5, 22-24 cm, yields an assemblage typical for the early late Eocene (~35.5 Ma) with warm-water influence exemplified by the presence of Schematophora speciosa, Aireiana verrucosa, and Hemiplacophora semilunifera. Representatives of the cosmopolitan Deflandrea phosphoritica group dominate this sample. The dinocyst assemblage of this sample suggests eutrophic neritic marine conditions (see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter and "Palynology" in "Biostratigraphy" in the "Site 1171" chapter). The palynological results from samples from the so-called "glauconitic unit" suggest its relative stratigraphic completeness across the Eocene-Oligocene transition interval. In a sample taken from just below the inception of the dominantly glauconitic aspect (189-1172D-39X-6, 64-66 cm), endemic antarctic species and/or bipolar high-latitude taxa are dominant (notably Spinidinium macmurdoense), suggesting relatively cool conditions. The assemblage, with consistent Alterbidinium distinctum, indicates an age of ~36.5 Ma for this sample. The underlying Sample 189-1172A-39X-CC is again dominated by cosmopolitan taxa.
The palynological succession of the Eocene-Oligocene transition is, thus, strongly suggestive of a stepwise series of changes from the late middle Eocene to the earliest Oligocene. Relatively warm conditions in inner neritic environments, with eutrophic conditions caused by river input, rather rapidly evolve into an apparently bathyal setting with high surface-water productivity and cool temperate to cold conditions in a stepwise pattern (see also "Diatoms, Silicoflagellates, and Sponge Spicules").
The top of the middle Eocene is recognized by the FAO of A. distinctum (between Samples 189-1172A-50X-CC and 40X-CC; ~37 Ma). The age of the base of Hole 1172A (Sample 189-1172A-56X-CC) is difficult to assess. The base appears to lie just below the onset of the Enneadocysta-acme sensu Raine et al. (1997) at ~43 Ma, whereas it is positioned below the FO of Cerebrocysta bartonensis. This sample is also marked by the presence of a conspicuous morphotype of the Deflandrea phosphoritica group, "Deflandrea convexa." Apparently, the environmental conditions associated with the development of this morphotype occurred during the middle middle Eocene of New Zealand (Wilson, 1988). The available information would indicate that the base of Hole 1172A has an age of ~44 Ma. Combined with results from Hole 1172D, a possible hiatus straddling the lower middle Eocene is indicated by the conjunction of the LO of Wilsonidinium ornatum and Hystrichokolpoma spinosa between Samples 189-1172D-12R-CC and 189-1172A-56X-CC.
Palynomorphs are consistently present in selected samples from Hole 1172D, albeit in low abundances in some cases. Dinocysts are common, and locally abundant, in productive samples from Hole 1172D and are assigned to the late Maastrichtian to early Eocene. In addition, pollen, spores, foraminifer organic linings, and acritarchs are also common in the upper samples and are persistent downhole. In the lowermost portion of Hole 1170D, in the upper Paleocene to Maastrichtian, sporomorphs are quantitatively significant and locally dominant (Table T10). These late Maastrichtian-Paleocene assemblages throughout are indicative of warm climates. Palynomorphs are generally well preserved and dinocyst assemblages are often of relatively low diversity for Eocene standards and may be totally dominated by a single taxon. In terms of quantitative distribution patterns, the lower Eocene to Paleocene interval is quite different from the correlative successions of Hole 1171D. Notably, representatives of Glaphyrocysta are particularly abundant in the Paleocene section of Hole 1172D, whereas they were only in the background in presumed coeval assemblages from Hole 1171D.
Relatively little is known from the Maastrichtian to the middle Eocene in terms of southern high-latitude dinocysts. Most meaningful information is derived from New Zealand and Antarctica (Seymour Island) sites (e.g., Wilson, 1988; Wrenn and Hart, 1988). However, these dinocyst successions still await chronostratigraphic calibration. Applied dinocyst events are listed in Table T11. For the Paleocene to lower Eocene interval, dinocysts are the sole source of biostratigraphic information (also see Fig. F15).
Assemblages indicative of a late early Eocene age characterize the topmost analyzed sample from Hole 1172D, Sample 189-1172D-12R-CC, with the presence of Wilsonidinium ornatum (the LO at ~50 Ma). Assemblages from the Samples 189-1172D-13R-CC and 19R-CC are not particularly age diagnostic. A late Paleocene age (~58 Ma) is indicated for Sample 189-1172D-20R-CC based on the occurrence of Cerodinium boloniensis and associated Cerodinium morphotypes. A possible hiatus is indicated by a rather abrupt compositional change of the palynological association between Samples 189-1172D-20R-CC and 21R-CC, where dominating sporomorphs give way to dominating dinocysts. In addition, the classic (and globally identified) oldest Apectodinium acme (or any Apectodinium acme) was not recorded in the available material, indicating that Paleocene-Eocene transitional strata may be missing.
Sample 189-1172D-23R-CC may be assigned to the Danian on the basis of the co-occurrence of the dinocysts Alisocysta reticulata and Trithyrodinium evittii. The K/T boundary is presumed to be within Core 189-1172D-24R since Sample 189-1172D-24R-CC does not yield Tertiary dinocysts but does contain a typical Southern Hemisphere latest Maastrichtian assemblage, dominated by Manumiella druggii. Subsequent palynological analyses of available samples from Section 189-1172D-24R-5 revealed the K/T boundary to be between interval 189-1172D-24R-5, 79-77 and 46-44 cm, on the basis of the FO of T. evittii. The lowermost sample analyzed, Sample 189-1172D-31R-CC, still contains abundant M. druggii. It also yields representatives of undescribed species of Spiniferites and Impagidinium and the age-diagnostic taxon Acanthaulax wilsonii. Combined with nannofossil information, microfossil assemblages indicate an age of younger than 67 Ma for this sample.
The quantitative dinocyst distribution in the Eocene generally indicates somewhat restricted, eutrophic, neritic conditions throughout the succession. For further discussion, see "Palynology" in "Biostratigraphy" in the "Site 1170" chapter chapter because the assemblages in this site are comparable to those in Site 1172. The late Maastrichtian to middle Eocene dinocyst assemblages are dominated by relatively warm water, (inner) neritic, eutrophic forms. Terrestrial palynomorphs increase in relative abundance downhole, and throughout they indicate warm climates in this interval. This suggests shallow marine conditions for the older part of the succession at Site 1172, with marked influence of runoff. Only the lowermost Sample 189-1172D-31R-CC yields significant numbers of the typically offshore taxon Impagidinium. The upper Maastrichtian succession may hence represent a regressive sequence.
The combined microfossil biostratigraphy at Site 1172 yielded 74 bioevents with age significance. Principal trends through this section are shown in Figure F15. Datums are from the combined microfossil bioevents from Holes 1172A, 1172B, 1172C, and 1172D and 59 magnetic polarity datums (see "Paleomagnetism"). The bioevents (Table T12) are comprised of 22 FO events, 48 LO events, three acme event, and one LAO event. All events are plotted according to their mean depth at Site 1172 and by their ages as defined in "Biostratigraphy" in the "Explanatory Notes" chapter). The FO events may have been estimated to be too shallow and the LO events may have been estimated as being deep, based on the limited sampling interval. Stratigraphic positions of these datums will be refined with further study. See individual microfossil group discussion for more detailed bioevent data.
Sedimentation rates are moderate from the middle middle Miocene to the Pleistocene (2.0-3.2 cm/k.y.) with no obvious hiatuses. This section of the age-depth plot is well constrained by 25 bioevents and 35 paleomagnetism datums.
Sedimentation rates dropped abruptly to 0.43 cm/k.y. for the remainder of the Miocene to the upper lower Oligocene. No hiatuses are suggested by the 19 datums that define this interval. The lower Oligocene to upper Eocene interval shows an even slower sedimentation rate (0.16 cm/k.y.). A condensed section or intermingling of short periods of sedimentation and times of nondeposition are clearly indicated through the carbonate-glauconitic siltstone transition (see "Lithostratigraphy"). Nine bioevents spanning 6.5 Ma are within 8.98 m. Though no major fossil group provides age control through this boundary, all major groups contribute to interpreting this key interval.
Sedimentation rates increase to 1.8 cm/k.y. across the middle/upper Eocene boundary. Biostratigraphic control is poor through the remainder of Site 1172 because of the lack of consistent microfossil appearances, except for the dinocysts, which provide the only age control through the Paleogene. An ~3-m.y. hiatus is inferred across the lower/middle Eocene boundary at 550 mbsf. A correlative hiatus was also recognized at Site 1171 (see "Age Model and Sedimentation Rates" in "Biostratigraphy" in the "Site 1171" chapter). Sedimentation rates are 1.04 cm/k.y. until the lowermost Paleocene, where another hiatus is indicated by dinocyst datums. A change in lithology is also seen in this interval (see "Lithostratigraphy"). The length of this hiatus is unclear and requires shore-based analysis for clarification.
Sedimentation rates double in the upper Maastrichtian (2.6 cm/k.y.), which spans the bottom 80 m of core. Calcareous nannofossils, moderately preserved in this section, and dinocysts provide the only age control for this section. Detailed shore-based work will provide additional data.