Biostratigraphic control at Site 1192 was provided by shipboard analyses of calcareous nannoplankton and planktonic foraminifers from core catcher samples. Faunal assemblage changes in benthic foraminifers were also studied. The biostratigraphic results and zonal assignments of Holes 1192A and 1192B indicate a Pleistocene to middle lower Miocene sequence.
Calcareous nannofossils are generally abundant and moderately well preserved at Site 1192. No significant reworking of nannofossils was apparent, and all nannofossil events recognized occur in normal stratigraphic sequence in these sediments. Over a dozen nannofossil datums were determined based on examination of core catcher samples, providing modest biostratigraphic resolution for the Pleistocene through middle early Miocene.
Samples 194-1192A-1H-CC and 2H-CC contain abundant, well-preserved nannofossils, including common Reticulofenestra asanoi (Table T4). The presence of this species in the absence of Calcidiscus macintyrei indicates an age range of 0.88-1.7 Ma (Pleistocene) for these two samples.
Samples 194-1192A-3H-CC through 6H-CC contain late Pliocene index species. Reticulofenestra pseudoumbilica and Sphenolithus spp. were found in Samples 194-1192A-7H-CC through 9H-CC, and they allow the assignment of nannofossil Zones CN10-CN11 (early Pliocene) to this interval.
The Pliocene/Miocene boundary is drawn between Samples 194-1192A-9H-CC and 10H-CC based on the presence of Discoaster quinqueramus in the latter, whereas planktonic foraminifers place the boundary between Samples 1194-1192A-12H-CC and 13H-CC. Samples 194-1192A-10H-CC through 15H-CC also contain Discoaster surculus; thus, an age range of 5.6-7.5 Ma (Zone CN9) can be assigned to this interval. Samples 194-1192A-16H-CC and 17H-CC contain D. quinqueramus but no D. surculus and can be assigned to the lower part of Subzone CN9a with an age range of 7.5-8.5 Ma. Neither D. quinqueramus nor Discoaster neohamatus is present in Sample 194-1192A-19H-CC, and this absence suggests an age of CN8 (8.5-9.4 Ma). D. neohamatus was found in Samples 194-1192A-20H-CC and 21H-CC, indicating an age range of 9.4-11.9 Ma. Samples 194-1192A-24H-CC through 29H-CC contain Cyclicargolithus floridanus but no Sphenolithus heteromorphus, and thus can be assigned an age range between 11.9 and 13.6 Ma (Zone CN5, middle Miocene).
Samples 194-1192B-4H-CC and 5H-CC contain D. neohamatus but no D. quinqueramus or Discoaster hamatus, indicating these samples are within the lower part of Zone CN8. D. hamatus occurs in Samples 194-1192B-6H-CC through 8H-CC, and the range of this species defines Zone CN7 (9.5-10.5 Ma). Neither D. hamatus nor C. floridanus is present in Samples 194-1192B-9H-CC through 11H-CC, and this suggests an age range of 10.5-11.9 Ma for the interval. Samples 194-1192B-17X-CC through 2X-CC contain S. heteromorphus, resulting in an age range assignment of 13.8-18.2 Ma for this interval.
Hole 1192A (Cores 194-1192A-1H through 29H) represents an incomplete sequence ranging in age from the Pleistocene to the late Miocene; Zones N23-N18 and N16 were identified. Hole 1192B (Cores 194-1192B-1H through 22X) also represents an incomplete sequence of Pleistocene to early Miocene age; Zones N23-N22, N17-N16, N14, and N8-N7 were identified. A list of foraminifer datums from Holes 1192A and 1192B are given in Table T4.
Samples from Holes 1192A and 1192B, in general, contain good to moderate planktonic foraminifer preservation from the Pleistocene to late Miocene. Poor preservation is encountered at the base of the hemipelagic sequence (see "Lithostratigraphy and Sedimentology") in Sample 194-1192B-22X-CC, which affects the age assignment in these samples. Three of the samples, 194-1192B-13X-CC, 16X-CC, and 18X-CC, were barren of index fossils with low abundances of all planktonic foraminifers and high levels of siliciclastics. The first of these samples is coincident with the onset of significant lithification in the sediments.
In this hole, the Pleistocene is a relatively straightforward interval for planktonic foraminifer biostratigraphy. The base of Zone N22 is marked by the first appearance of Globorotalia truncatulinoides overlapping in range with its ancestral form Globorotalia tosaensis. G. truncatulinoides occurs in Samples 194-1192A-1H-CC and 2H-CC but not in 3H-CC, thus placing the base of the Pleistocene between Samples 2H-CC and 3H-CC.
Several planktonic foraminifer datums occur in the Pliocene section of Hole 1192A, and there are a number of index fossils defining the Pliocene in this hole, although they are not particularly abundant. G. tosaensis clearly defines Zone N21 with its first occurrence (FO). This zone fossil is present up to and including Sample 194-1192A-7H-CC, indicating that the cores down to this point are Zone N21 or younger. The last occurrence (LO) of Globigerinoides fistulosus coincides with the Zones N21/N22 boundary and occurs between Samples 194-1192A-2H-CC and 3H-CC. However, G. fistulosus does not occur in the other Zone N21 samples (i.e., 194-1192A-6H-CC and 7H-CC). The Dentoglobigerina altispira LO datum marks the Zone N20/N21 boundary and provides a useful marker in this hole. This zone fossil first occurs in Sample 194-1192A-7H-CC and is common throughout the rest of the section. The FO of Globorotalia miocenica, which defines the base of Zone N20, could not be identified with certainty, as this species is not clearly distinct from its ancestor Globorotalia pseudomiocenica at this site. The LO of Globigerina nepenthes marks the top of Zone N19 and is present in Samples 194-1192A-9H-1, 80-82 cm, and older.
The zone fossil Sphaeroidinella dehiscens is absent from Samples 194-1192A-13H-CC and older and generally infrequent from the Pliocene and Pleistocene samples. The FO of this species marks the base of Zone N18 and thus the Miocene/Pliocene boundary. It is possible to assign the Pliocene/Miocene boundary between Samples 194-1192A-12H-CC and 13H-CC, based on the overlap of the LO datum of Globorotalia plesiotumida near the base of Zone N19 and the FO datum of Globorotalia tumida near the base of Zone N18. The LO datum of G. dehiscens also clearly defines the top of Zone N18 as between Samples 194-1192A-12H-CC and 13H-CC. This must be compared with the nannofossil Miocene/Pliocene boundary between Samples 194-1192A-9H-CC and 10H-CC.
The LO of G. dehiscens occurs in Sample 194-1192A-13H-CC, which marks the Zones N18/N19 boundary. Samples 194-1192A-14H-CC through 24H-CC are assigned to Zones N17-N16, based on the absence of G. tumida and the consistent presence of G. plesiotumida and Neogloboquadrina acostaensis (FO datum marks the base of Zone N16).
Samples 194-1192A-25H-CC through 29H-CC occur in Zone N16 based on the LO datum of Globorotalia paralenguanensis (between 25H-CC and 26H-CC), which marks the Zone N16/N17 boundary. In these samples, G. plesiotumida and its ancestor Globorotalia merotumida become indistinguishable and are unreliable as datum markers.
Sample 194-1192B-1H-CC, from the first core in this hole, is assigned to Zones N22-N23 based on the presence of G. truncatulinoides. A core break occurs between Samples 194-1192B-1H-CC and 4H-CC. Samples 194-1192B-4H-CC through 11X-CC were assigned to Zone N17 based on the lack of G. tumida and the presence of G. pseudomiocenica and their relevant datums as discussed above. The top of Zone N16 is signified by the FO of G. plesiotumida, which is absent in Sample 194-1192B-12X-CC. This is corroborated by the appearance of the Paragloborotalia mayeri-siakensis morphological range. The LO of the latter morphospecies marks the Zone N14/N15 boundary. Between Samples 194-1192B-12X-CC and 16X-CC, the two end-member species occur intermittently. Therefore, age diagnosis is difficult because of the often barren or semibarren nature of these samples with respect to planktonic foraminifers.
For Samples 194-1192B-17X-CC to 32X-CC, a Zone N7-N8 date was attributed based on a tentatively identified Globigerinoides sicanus-bispherica lineage together with the descendant Praeorbulina curva datums. The P. curva LO denotes the Zone N8/N9 boundary, whereas the FA occurs near the base of Zone N8. The G. sicanus-bispherica zone spans the entire interval from the base of Zone N8 to the top of Zone N9. The morphological differences between G. sicanus-bispherica and P. curva are based on small variations in test sphericity and number of supplementary apertures (Kennett and Srinivasan, 1983). In addition, recrystallization of the tests made this a problematic determination.
Benthic foraminifers were relatively rare in all samples examined from Hole 1192A (Samples 194-1192A-1H-CC through 29H-CC). Specimens in Sample 194-1192A-1H-CC are distinct both in preservation and assemblage. The diverse assemblage of benthic taxa, which includes several porcellaneous genera (Pyrgo, Spiroloculina, Triloculina, and Quinqueloculina) indicates a mid-outer neritic (<200 m) paleoenvironment. In the rest of Hole 1192A, benthic foraminifers are relatively rare as compared to the dominant, well-preserved planktonic assemblage, although occasionally large Cibicidoides, nodosarids, and agglutinated benthics are conspicuous. The relatively diverse assemblages of rotaliid, buliminid, nodosarid, and agglutinated taxa are characteristic upper bathyal habitats.
Benthic foraminiferal assemblages in Hole 1192B are similar. Porcellaneous miliolid taxa are only found in any abundance in the top of Core 194-1192B-1H and in Sample 194-1192B-1H-CC. The presence of small, flat Amphistegina and Operculina, whose modern equivalents host algal endosymbionts (e.g., Hallock, 1999), along with several porcellaneous taxa in the Core 194-1192B-1H top, indicate a deep euphotic (~50-120 m) habitat depth (middle to outer neritic). Samples 194-1192B-4H-CC through 12X-CC are relatively similar to Samples 194-1192A-2H-CC through 22X-CC, with benthic foraminifers relatively rare compared to planktonic foraminifers. Large individual Cibicidoides are conspicuous in some cores. Preservation distinctly declines downhole, likely the result of reworking by bottom currents. Winnowing is inferred from observation in thin section Sample 194-1192B-1H-1, 28-32 cm, of planktonic foraminiferal tests that were infilled by micrite that was not observed outside the tests (Fig. F5; see also "Lithostratigraphy and Sedimentology"). Below Sample 194-1192B-13X-CC, both benthic and planktonic foraminifers are rare, probably as a result of winnowing of medium- and coarse-sand fractions, combined with poor preservation.