BIOSTRATIGRAPHY

Preliminary age assignments were established using core-catcher samples. Samples from elsewhere in the cores were examined when a more refined age determination was necessary. Two microfossil groups were examined for biostratigraphic purposes: calcareous nannofossils and planktonic foraminifers. Benthic foraminifers were used to estimate paleobathymetry. Sample positions and the abundance, preservation, and age or zone for each fossil group were recorded on barrel sheets for each core.

The time scales used in this work were those of Berggren, Kent, and van Couvering (1985), Berggren et al. (1985), and Harland et al. (1990) for the Neogene; Cretaceous and Paleogene age assignments are from Berggren et al. (1985), Berggren, Kent, and Flynn (1985), Hallam et al. (1985), and Harland et al. (1990) (Fig. 5, Fig. 6, Fig. 7, Fig. 8). The age estimates of Cenozoic calcareous nannofossil and planktonic biohorizons have been derived primarily from the geomagnetic polarity time scales (GPTS) of Berggren, Kent, and van Couvering (1985), Berggren et al. (1985), Berggren, Kent, and Flynn (1985), and Hallam et al. (1985). In Table 1 and Table 2, we summarize the nannofossil and planktonic foraminifer datums, respectively, used for Leg 149.

Mesozoic Zonation

The Mesozoic zonal schemes of Sissingh (1977) and Roth (1978) were used with additional subdivisions to increase biostratigraphic resolution. No Mesozoic nannofossil zonation has been developed that can provide worldwide application. Further refinement of the sequence of observed biostratigraphic events was provided by Perch-Nielsen (1979; 1985) and Bralower and Siesser (1992).

Cenozoic Zonation

Ages have been presented in a zonal framework. For convenience,we referred to the zonation and code number of Martini (1971). For Zone NN19, the subzoned scheme from Rio et al. (1990) was used (Subzones NN19A-NN19F). Some important species used by Bukry (1973, 1975), Gartner (1977), and code numbers of Okada and Bukry (1980), as well as newly proposed datum levels, have been incorporated into the Leg 149 stratigraphic framework. Primary and secondary biostratigraphic-event zonal markers for the Cenozoic and Mesozoic are shown in Table 1 and in Figure 8.

Methods

Calcareous nannofossil assemblages were described from smear slides prepared for each core-catcher sample and for as many samples in between as time permitted on board ship. Low-sedimentation-rate (condensed) zones or suspected hiatuses were sampled as closely as was practical. Turbidite sequences were sampled in the hemipelagic and overlying pelagic intervals, and special attention was paid to facies changes. Standard preparation techniques were used throughout. Smear slide examination was done exclusively with a light microscope, using whatever optical configuration yielded useful results. In all cases, a magnification of 1250 was used to estimate abundance semiquantitatively.

R (rare) = 1 specimen per 51-200 fields of view (2-5 specimens total);

F (few) = 1 specimen per 11-50 fields of view (6-15 specimens total);

C (common) = 1 specimen per 2-10 fields of view (16-250 specimens total);

A (abundant) = 1-10 specimens per field of view (251-500 specimens total); and

V (very abundant) = more than 10 specimens per field of view (>500 specimens total).

Estimation of total abundance of calcareous nannofossils for each sample was as follows:

B (barren);

R (rare) = 2-10 specimens for 500 fields of view (about three traverses);

F (few) = 11-50 specimens for 500 fields of view;

C (common) = 51-2000 specimens for 500 fields of view;

A (abundant) = 2,001-20,000 specimens for 500 fields of view; and

V (very abundant) = >20,000 specimens for 500 fields of view.

The qualitative evaluation of the preservation of calcareous nannofossils was recorded as good (G), moderate (M), or poor (P). These categories represent subjective impressions with approximately the following meaning:

P (poor) = Severe dissolution, fragmentation and/or overgrowth has occurred; primary features may have been destroyed and many specimens cannot be identified at the species level;

M (moderate) = Dissolution and/or overgrowth are evident; a significant proportion (up to 25%) of the specimens cannot be identified to species with absolute certainty; and

G (good) = Little or no evidence of dissolution and/or overgrowth is seen; diagnostic characteristics are preserved and nearly all specimens (about 95%) can be identified.

Planktonic Foraminifers

The Cenozoic zonal scheme given by Blow (1969), with slight modifications by Kennett and Srinivasan (1983) and Berggren, Kent, and van Couvering (1985), Berggren et al. (1985) for the Neogene, that of Bolli and Saunders (1985) for the Oligocene to Holocene, and Toumarkine and Luterbacher's (1985) scheme for the Paleocene to Eocene, were employed here. Cretaceous zonations are based on Sigal (1977). Biostratigraphic-event zonal markers for the Cenozoic are shown in Table 2.

Methods

Samples of approximately 10 cm³ were soaked either in plain tap water or in a Calgon solution and washed through a 63-m sieve. Samples were rinsed with methanol and then dried on a hot plate. The >125-m fraction was examined for the planktonic foraminifers, and the 63- to 125-m fraction was studied for the zonal markers, if they were absent in the larger size fractions.

The abundance of planktonic foraminifers is defined as follows:

B (barren);

R (rare) = <10 specimens;

F (few) = 10-100 specimens;

C (common) = 101-500 specimens; and

A (abundant) = >500 specimens.

Preservation characteristics were divided into three categories:

P (poor) = almost all specimens were dissolved or broken and fragments dominated;

M (moderate) = 30%-90% of specimens showed dissolved or broken chambers; and

G (good) = >90% of specimens were well preserved and unbro-ken.

Benthic Foraminifers

The abundance of benthic foraminifers is defined as follows:

B (barren);

R (rare) = <10 specimens;

C (common) = 10-100 specimens;

and A (abundant) = > 100 specimens.

The three classes of preservation of benthic foraminifers were the same as those for the planktonic foraminifers.

Paleobathymetric Methods

Benthic foraminifers were examined from the >63-m size fraction. Paleobathymetry estimated using the smaller benthic foraminifers was primarily based on van Morkhoven et al.'s (1986) depth zonations. Bathymetric ranges are as follows: neritic (0-200 m) is divided into upper (0-30 m), middle (30-100 m), and lower (100-200 m); bathyal is divided into upper (200-600 m), middle (600-1000 m), and lower (1000-2000 m); and abyssal (>2000 m).