One hole was drilled at Site 1097, with 13.6% core recovery. Radiolarians, diatoms, and foraminifers were all present at this site, with abundances of each varying by sample (Fig. F14). Because of the glaciomarine nature of the sediments, reworking of microfossils is prevalent in most samples. Most of the marine muds within the diamicts in Hole 1097A, however, appear to have been deposited during interstadial conditions within the early Pliocene. The upper 125 m (Cores 178-1097A-1R through 15R) are reworked diamict sediments essentially barren of microfossils. Below 125 m (Core 178-1097A-16R), microfossils occur in intervals alternating with barren diamicts.

Samples 178-1097A-1R-CC and 2R-CC comprised rocks with some fine-grained sediment (diamict matrix) containing fragments of diatoms. The only age-diagnostic species were Fragilariopsis kerguelensis (present-3 Ma), Denticulopsis simonsenii s.l. (Miocene), and Denticulopsis delicata (Miocene). The obvious reworking of older sediment (concluded through observation of species of mixed ages and fragmentation of valves) and the low total abundance and diversity of diatoms recovered preclude any possible age inference (Fig. F15).

Only rocks, with no fine-grained sediments, were recovered from Cores 178-1097A-3R through 7R. The fine-grained diamict matrix of Sample 178-1097A-8R-CC contains a non-age diagnostic assemblage, consisting of Paralia sulcata, D. simonsenii s.l., D. delicata, Actinocyclus ingens, Thalassiosira complicata, Coscinodiscus spp., and Nitzschia/Fragilariopsis spp. The ages of these species range from Miocene to Pliocene, with most extending over a long time period. The absence of middle to late Pliocene and Pleistocene diatom species such as Fragilariopsis barronii (4.48-1.39 Ma), Fragilariopsis interfrigidaria (3.8-2.67 Ma), F. kerguelensis (3.3 Ma to Holocene), Thalassiosira insigna (3.4-2.57 Ma), and Thalassiosira vulnifica (3.17-2.28 Ma) suggest an early Pliocene age for the source sediment.

Pliocene diatoms, including Thalassiosira torokina, Thalassiosira oestrupii, Thalassiosira oliverana, and fragments of Thalassiosira fasciculata, occur within Samples 178-1097A-9R-CC through 11R-CC, where diatoms are more common than in overlying sediments. Published age ranges of some of the species observed help to constrain the age of deposition of this material (first occurrence [FO] of Thalassiosira inura, 4.85 Ma; FO of T. complicata, 4.62 Ma; FO of T. fasciculata, 4.48 Ma; FO of Fragilariopsis praeinterfrigidaria, 4.85 Ma). Using last occurrences in glacial sediments is rarely an option because of common reworking. Thus, the only age that can be determined from this sediment is an age younger than the FO of the youngest of the species observed (Figs. F14, F15).

Cores 178-1097A-16R, 19R, 23R, and 27R contained a diverse assemblage typical of the Thalassiosira inura Zone including T. inura, T. oestrupii, T. torokina, Fragilariopsis aurica, F. praeinterfrigidaria, Fragilariopsis praecurta, Actinocyclus karstenii, and Nitzschia reinholdii. With the inclusion of T. inura (the characteristic period assemblage) and the exclusion of F. barronii (the FO of this species marks the upper boundary of the T. inura Zone), it is likely that these sediments are from the T. inura Zone (Fig. F15).

Diatoms are rare with poor to moderate preservation below Core 178-1097A-37R. The neritic diatom species, Paralia sulcata, is dominant (more than 80%) in core-catcher samples from Cores 178-1097A-37R through 41R (Fig. F15). Overall diatom abundance and preservation decrease below this, but numbers of benthic diatoms increase slightly in samples below Core 178-1097A-41R as compared with overlying sediments.

The occurrence of the well-preserved epiphytic diatom species Arachnoidiscus ehrenbergii in sieved material (>63 µm) of Samples 178-1097A-42R-CC and 44R-CC, along with increasing numbers of benthic diatoms (including Cocconeis spp., Navicula spp., Gramatophora spp., and Delphineis spp.) in core-catcher samples below Sample 178-1097A-42R-CC, suggests a shallowing sequence to the bottom of the hole.

Figure F15 illustrates the ranges and abundances of selected diatom species in Hole 1097A. The samples that can be assigned to a particular zone (all within the T. inura Zone) are indicated. The change from many Stephanopyxis spp. in the upper part of the hole to a Paralia sulcata and benthic diatom species peak in the lower part of the hole is also shown.

Radiolarians were rare in core-catcher samples from Hole 1097A, and skeletons were broken. One reworked Cretaceous specimen was found in Sample 178-1097A-19R-CC. Age assignments were difficult because of the reworked nature of the sediments and the low abundances and poor preservation of radiolarians. Species typical of shelf environments (e.g., Spongotrochus glacialis, Spongopyle osculosa, and Porodiscid spp.) dominated the assemblages. The youngest marker species encountered was Acrosphaera cylindrica in Sample 178-1097A-10R-CC (92.1 mbsf), indicating an age of at least 0.610 Ma (Fig. F14). Species consistent with an age in the Upsilon Zone (Helotholus vema, Prunopyle titan, and Lampromitra coronata) were sporadic from Sample 178-1097A-12R-CC (111.5 mbsf) to 25R-CC (207.7 mbsf). The upper limit of the Upsilon Zone is placed in the barren interval of Core 178-1097A-11R (92.1-101.8 mbsf). The absence of H. vema below Sample 178-1097A-34R-CC (293.9 mbsf) suggests that the age of the sediment falls within the Tau Zone (older than 4.580 Ma). The boundary between the Upsilon and Tau Zones could be in the barren interval from Sample 178-1097A-26R-CC (217.3 mbsf) to 34R-CC (293.9 mbsf) (Fig. F14).

At Site 1097 all core-catcher samples, plus 10 additional core samples, were processed for foraminifers. Benthic foraminifers were present in all but five samples. Neogloboquadrina pachyderma sinistral was observed in Samples 178-1097A-36R-2, 85-87 (305.8 mbsf), and 44R-CC (365.4 mbsf). In addition, most samples examined contained reworked foraminifers and other biogenic components. Most reworked biogenic material is recognizable by dark discoloration and poor preservation. The amount of reworked biogenic materials observed in the sand fraction suggests that all samples record marine conditions that are near to, or beneath, a glacial ice front (Osterman, 1984). Even the most marine benthic foraminifer assemblage (Biofacies C; Table T2) implies conditions near grounded ice. Benthic foraminifers can provide no information about the previous existence or extent of an ice shelf at this site or farther inshore.

Three qualitative environmental biofacies are defined for Site 1097 (Table T2). It is important to recognize that there is great variability in the fauna preserved in these sediments. The benthic foraminifer and sedimentological records suggest rapid changes within small intervals of cored sediment. For example, replicate analyses of core-catcher Sample 178-1097A-44R-CC produced different values of foraminifer abundance and diversity, which implies that there were variable environments of deposition at this site within a small geographic area or time. The biofacies are defined broadly, however, and both analyses of Sample 178-1097A-44R-CC fall within the definition of Biofacies C.

Biofacies A consists predominantly of a reworked assemblage of benthic foraminifers. These samples contain <12 robust, yellow-colored foraminifer specimens often broken or filled with sediment, indicating transport and reworking. Also, this assemblage often contains additional worn, discolored, and replaced biogenic matter including Inoceramus prisms, mollusk shell fragments, and echinoderm spines and shell fragments. Benthic foraminifers of this biofacies consist almost exclusively of Globocassidulina subglobosa and Cassidulinoides parkerianus. This biofacies is believed to record glacial transport within a subglacial environment.

Biofacies B contains poor to moderately well-preserved and slightly more abundant foraminifers ranging from 12 to 30 foraminifers per sample. Better preservation of the foraminifers implies minimal glacial transport, but the lower numbers indicate less-marine conditions. Biofacies B is believed to record lower salinity shallow-water conditions or dilution of the foraminiferal fauna by higher sedimentation rates in an environment proximal to a glacier front. The low diversity of species (<5) indicates a lowered salinity.

Biofacies C consists of better preserved specimens of foraminifers, often in association with additional well-preserved, white, biogenic material of mollusk shells, sponge spicules, echinoderm spines, and fragments. Preservation of foraminifers in this facies is mixed, with both well-preserved and yellow reworked specimens, sometimes in the same sample. In five of the seven samples of this biofacies, the number of foraminifers ranges from 31 to 250 ( split) and the number of species from six to 13. The other two samples are placed in this biofacies based on excellent preservation of the foraminifers and associated biogenic material (fresh mollusk fragments). This biofacies is interpreted to record shallow-water marine conditions not exceeding a depth of 500 m.

Using these biofacies, Hole 1097A can be divided into four zones (Table T2). Biofacies A predominates in the upper 151 mbsf of Zone I. This interval, containing reworked marine components, is interpreted to be a subglacial till (see also "Lithostratigraphy"). Zone II, from Samples 178-1097A-23R-1W, 42-44 cm (179.32 mbsf), to 26R-CC (208.91 mbsf), contains all three biofacies. It is believed, however, to record an episode of more-marine sedimentation with glaciers nearby, which agrees with the diatom analysis of this hole. Zone III, from 218.3 to 287.7 mbsf, contains poor and rare assemblages of Biofacies A and may record an additional episode of subglacial conditions. Zone IV, in the lowest portion of Hole 1097A (288.8 mbsf to the bottom of the hole), includes Biofacies A, B, and C. It also records episodes of more-marine conditions alternating with episodes of low salinity and/or higher sedimentation rates that suggest closer proximity to a glacial front. The washed foraminifer samples in this lower zone contain the large diatom Arachnoidiscus ehrenbergii. This diatom lives on shallow-water marine grasses and presently is found in the shallower regions of the Antarctic Peninsula (Krebs, 1977). In addition, this diatom was recovered at the nearshore Sites 1098 and 1099. This supports a shallower continental shelf interpretation for Zone IV with respect to the other zones.

The benthic foraminifers of the more-marine intervals of Site 1097 are similar to Pliocene assemblages recorded in other areas of the Antarctica shelf. Ammoelphidiella antarctica, found throughout Zone IV, is reported to be restricted to the early to middle Pliocene in Deep Sea Drilling Project Site 273 (Hayes, Frakes, et al., 1975). Its occurrence in the Pecten Gravels of Wright Valley is bracketed by volcanic dates of 2.5 and 3.8 Ma (Webb, 1974). This species also occurs in Dry Valley Drilling Project Holes 10 and 11 in Taylor Valley (Ishman and Webb, 1988). Although all these faunas are similar to Site 1097, the diversity is much greater in the Ross Sea area (10 to 43 species), possibly because of deeper water in that area. The water depth of the Pliocene Taylor Fjord is estimated to have been 500-800 m. The lower diversity at Site 1097 suggests that the water depth here was <500 m in Zone II, the most marine interval, and <200 m in Zone IV.