Sixty-seven meters of lowermost Miocene (Fig. F27) calcareous ooze (Cores 192-1184A-2R through 8R) was drilled at Site 1184. Integrated calcareous microfossil biostratigraphy demonstrates that a hiatus of nearly 1 m.y. duration (Fig. F28) is present between Core 192-1184A-4R and 5R. The poorly consolidated sediment recovered from Core 192-1184A-8R came from upsection. Nannofossil assemblages place most of the volcaniclastic material (Cores 192-1184A-13R through 45R) within the middle Eocene Zone NP16 of Martini (1971).
All samples examined contain abundant and well-preserved calcareous nannofossils and planktonic foraminifers. The calcareous microfossil biozonations used and our zonal assignments are illustrated in Figure F27. Table T5 is a summary of the main biostratigraphic events identified in Unit I, and Figure F28 (Ma values are from Berggren et al., 1995, and E. de Kaenel, pers. comm., 1998) is a time-depth plot.
All assemblages recovered from Unit I are from Zone NN2 of Martini (1971); Triquetrorhabdulus carinatus is present in the topmost sample examined (Sample 192-1184A-2R-1, 20-21 cm), and Discoaster druggii ranges to the base of Unit I. Additional nannofossil events can be used to subdivide Zone NN2 (Table T5; Fig. F27).
The boundary between Zones M1b and M2 of Berggren et al. (1995) is contained within the lowermost Miocene sediments. The two uppermost cores (Cores 192-1184A-2R to 3R) contain an assemblage typical of Zone M2 and are characterized by abundant species of Globoquadrina, Globigerinoides, and Dentoglobigerina, as well as the taxa Catapsydrax dissimilis, Jenkinsella siakensis, Globigerina venezuelana, and Globorotalia mendacis. Several appearances that have been placed near the base of Zone M2 (Kennett and Srinivasan, 1983; Bolli and Saunders, 1985) occur in Cores 192-1184A-4R and 5R (Table T5), indicating that this zone may be relatively complete. Zone M1b (Samples 192-1184A-5R-3, 20-21 cm, to 7R-CC, 0-10 cm) is marked by abundant Globorotalia kugleri, G. mendacis, Globigerina ciperoensis, and G. fariasi, as well as rare Globoquadrina dehiscens and G. praedehiscens. Sample 192-1184A-5R-3, 20-21 cm, marks the highest occurrences of both Globorotalia kugleri and Globigerina ciperoensis; the former species defines the M1b/M2 boundary.
Globally, the appearance of Globigerinoides species is variable and has been tied to the influx of warm, tropical water masses (Stainforth and Lamb, 1981; Bolli and Saunders, 1985). Its appearance at Site 1184 is relatively high in the section, in the lower part of Zone M2, where the genus is rare (i.e., the lowest occurrences of Globigerinoides primordius and G. parawoodi are in Sample 192-1184A-5R-1, 20-21 cm). A more common and diverse Globigerinoides assemblage is present at and above Section 192-1184A-3R-CC, where G. trilobus, G. quadrilobatus, and G. sacculifer enter the section. The reason for this delayed entry is unclear. Globoquadrina dehiscens is rare below the main Globigerinoides influx, possibly indicating a late entry of the tropical water mass typical of the Neogene Ontong Java Plateau region after cooling in the Oligocene. However, Globigerina venezuelana is common throughout the entire recovered lower Miocene interval and is a warm-water-mass indicator (e.g., Kennett and Srinavasan, 1983). Thus, either some unknown paleoecologic factor affected this site or the tropical preference of G. venezuelana has been overestimated.
We applied the Martini (1971) zonation to the rare and poorly preserved assemblages recovered throughout the volcaniclastic sequence in Cores 192-1184A-9R through 46R. We assume these assemblages are in situ. A sample from a thin chalk-filled fracture (Sample 192-1184A-9R-1, 5 cm) near the top of Unit II contains an early Miocene assemblage from Zone NN2, with Discoaster druggii. However, the characteristics of the assemblage (e.g., very high number of nannoliths) are very unlike those of any assemblage recovered from the overlying chalks. A sample from Core 192-1184A-10R yielded a single specimen of Ericsonia formosa and is no younger than earliest Oligocene (Zone NP21). Discoaster barbadiensis is present in Cores 192-1184A-11R and 12R, indicating an age no younger than latest Eocene (Zone NP20). Reticulofenestra umbilica ranges downward to near the base of the recovered section (Sample 192-1184A-45R-5, 12 cm), which indicates an age no older than the middle Eocene Zone NP16. Single specimens of Chiasmolithus minimus and Discoaster bifax recovered from Sample 192-1184A-13R-1, 59-60 cm, may indicate that nearly the entire volcaniclastic sequence belongs within the same nannofossil zone (NP16). Single specimens of Sphenolithus furcatolithoides in Sample 192-1184A-18R-6, 28-29 cm, and Discoaster gemmeus in Sample 192-1184A-25R-5, 68 cm, also were observed; both these species also went extinct within Zone NP16 (Perch-Nielsen, 1985). Although the total thickness of the volcaniclastic sediments is unknown, the deposition of >300 m of section within a single nannofossil zone implies a very rapid rate of deposition. It is therefore likely that much of the unit was deposited under comparatively deep-water conditions to provide the needed accommodation space. Cores 192-1184A-9R through 12R should be examined in more detail to determine if any of the volcaniclastic sediment is actually post-middle Eocene.
Unit II is barren of foraminifers.
Well-preserved benthic foraminifers recovered from the lower Mio-cene section represent two distinct assemblages. Intervals 192-1184A-2R-CC, 0-10 cm, to 3R-CC, 14-19 cm, may have been deposited at middle-slope depths. This possibility is indicated by the near absence of Bulimina jarvisi and the presence of Gyroidinoides soldanii, G. altispira, Hoeglundina elegans, and Sphaeroidina bulloides (Van Morkhoven et al., 1986). Samples 192-1184A-4R-CC, 0-5 cm, through 5R-3, 20-21 cm, contain a marked increase in benthic foraminifer diversity and abundance over those recovered from the underlying section. Globocassidulina moluccensis, Bulimina jarvisi, B. mexicana, Planulina rugosa, and Karreriella chapapotensis characterize the assemblage from this interval. In the absence of abyssal species, Bulimina jarvisi is an excellent lower-slope indicator (van Morkhoven et al., 1986); the other species present are consistent with this interpretation. The section below Sample 192-1184A-5R-3, 20-21 cm, contains rare benthic foraminifers, dominated by Stilostomella gracillima, Uvigerina senticosa, and Vulvulina spinosa, indicating a transition from the lower slope to an abyssal paleoenvironment. The uppermost 8 m of volcaniclastic rock is completely altered to pale brown Fe oxyhydroxide and clay, indicating weathering in an oxidizing (possibly subaerial) environment (see "Site 1184" in "Principal Results" in the "Leg Summary" chapter). Therefore, the overlying deep-water pelagic ooze indicates very rapid subsidence to near-abyssal water depths after the end of volcanism.