The primary biostratigraphic objective for all Leg 192 sites was to date sediments immediately overlying basement and any intercalated with basalt. Our secondary objective was to estimate sedimentation rates and delineate the location and duration of unconformities and then to compare the results to those obtained from other sites drilled on the Ontong Java Plateau. After we established age estimates, our final objective was to determine paleoenvironments from recovered benthic assemblages and integrate the results with the plateau's history.
We used the timescale and bioevents of Berggren et al. (1995) for Cenozoic strata and the Gradstein et al. (1995) timescale for the Cretaceous. Absolute age estimates for Cretaceous calcareous nannofossil and planktonic foraminifer bioevents are from the database housed at the Energy and Geoscience Institute at the University of Utah. In addition, the recently published "Sequence Chronostratigraphic Chart" of Hardenbol et al. (1998), which was calibrated to the same Mesozoic time-scale used during Leg 192 (Gradstein et al., 1995), also provided estimates of absolute age for many Cretaceous microfossil bioevents.
We used the following zonal schemes at all Leg 192 sites: (1) Martini (1971) for the Cenozoic; (2) Sissingh (1977, 1978; CC Zones) for the Campanian-Maastrichtian; (3) Bergen and Sikora (1999) for the Coniacian-Santonian; and (4) Bralower et al. (1993, 1995) for the Aptian-Albian. Bralower et al. (1993) modified the original zonation published by Roth (1978; NC Zones). We also used Perch-Nielsen (1985), Jeremiah (1996), Bown (1998), and Burnett (1997) as references for taxonomy and stratigraphy. A large number of Cretaceous nannofossil events were derived from the data of Bergen (1998), Bergen and Sikora (1999), and J.A. Bergen (unpubl. data) from reference sections in North America, western Europe, and Tunisia.
We examined calcareous nannofossils by standard light microscope techniques using transmitted light and crossed polarizers at 1000×-1560× magnification. From smear slides, we recorded seven calcareous nannofossil abundance levels as follows:
Biofacies analyses were performed on the samples recovered, the objectives being to define paleoenvironments and to elucidate depositional conditions through time in conjunction with shipboard sedimentological work. Benthic foraminifers, although consistently rare relative to planktonic species, are the primary facies markers. However, our interpretations are based not only on paleoecologic evidence but include preservational and lithologic criteria as described for deep-water carbonates by Sikora et al. (1999).
Biostratigraphy for the Cenozoic section employed the planktonic foraminifer zonal time calibration of Berggren et al. (1995). Additional biostratigraphic ranges used are those defined by Kennett and Srinivasan (1983) for the Neogene and by Toumarkine and Luterbacher (1985) and Olsson et al. (1999) for the Paleogene. The paleobathymetric estimates of Van Morkhoven et al. (1986) are followed for deep-water benthic foraminifers.
The zonal timescale used for the Cretaceous section is that of Gradstein et al. (1995). The Upper Cretaceous biostratigraphic zonation is based upon a combination of that of Caron (1985) and Bergen and Sikora (1999). For the Lower Cretaceous, the zonation of Sliter (1989) is employed. Taxonomy for Lower Cretaceous planktonic foraminifers is that defined by Boudagher-Fadel et al. (1997). Paleobathymetric estimates for the Cretaceous sections follow Nyong and Olsson (1984) and Sikora and Olsson (1991).
Core catcher samples of ~20 cm3 (plus one additional sample per section, where necessary) were washed in tap water over a 63-µm mesh sieve to retrieve free specimens of microfossils. More indurated samples were first crushed using a mortar and pestle before washing. Before processing each sample, we soaked the sieves in a solution of methylene blue in order to stain any contaminants remaining from previous washes. Washed residues were dried in an oven at ~50°C. We examined the dried samples under a binocular microscope and recorded foraminifer faunal composition in qualitative terms based on an assessment of species observed in a random sample of 200-400 specimens from the >63-µm size fraction. Relative abundances were reported using the following categories:
Preservation of planktonic foraminifer assemblages was recorded as follows:
Our relative paleobathymetric estimates use classical continental margin terminology, such as shelf, slope and abyssal, because the paleobathymetric zonations employed were developed from such margins (e.g., Van Morkhoven et al., 1986). The classic paleoenvironmental definition in terms of water depth is employed:
Although this paleobathymetric scheme varies from the neritic-bathyal-abyssal classification used to describe plateau physiography, we employ it in order to avoid confusion in comparisons to other paleoenvironmental studies. Also, we emphasize that the paleobathymetric zonation used is relative, with water depths given only to define terminology and not intended as absolute values for benthic foraminifer water depth distribution.
We analyzed limestone samples in thin section, oriented across the bedding plane where possible. Identification of microfossils in thin section is primarily based upon the internal and external morphologies of the specimens but also employs wall thickness and structure, test size, and external ornamentation.