13
µm (R. umbilicus),
13-10 µm, <10-6 µm, and <6 µm.This biostratigraphic investigation is based on the observation of one to five samples in each core section. A greater number of samples were observed at zonation boundaries. Smear slides were prepared for each of the recovered samples using Norland Optical Adhesive as the mounting medium. Slides were routinely examined under an Olympus binocular polarizing microscope (BX type) with a magnification of 1500x. Several thousand coccolith and discoaster specimens were identified. In addition, ~200 specimens were counted at random and listed in order to determine the relative abundances of the species and their stratigraphic range (Takayama and Sato, 1987; Takayama, 1993). After this examination, these slides were continuously scanned for the presence of other important species.
Preservation of nannofossils was recorded as follows:
G = good; no dissolution or overgrowth;M = moderate; slight to moderate dissolution or overgrowth; and
Three calcareous nannofossil abundance levels are recorded as follows:
A = abundant; >50 specimens per field of view;
I examined coccoliths in the general size of genus Reticulofenestra, Cribrocentrum, Dictyococcites, and Toweius throughout their range. In this report, these genera are tentatively classified into three or four groups. The concept of classification is presented below
It is generally agreed
that the smallest size of Reticulofenestra
umbilicus ranges from 13 to 14 µm
(Backman and Hermelin, 1986). I classify reticulofenestrids into four groups
according to size: 13
µm (R. umbilicus),
13-10 µm, <10-6 µm, and <6 µm.
According to Perch-Nielsen
(1985), the smallest size of Cribrocentrum
reticulatum is 6 µm. I classify Cribrocentrum
to three groups according to size: 10
µm (C. reticulatum),
10-6 µm (C. reticulatum),
and <6 µm (Cribrocentrum
spp).
It is generally accepted
that the smallest Dictyococcites
bisectus size is 10 µm (Bralower and Mutterlose, 1995). I classify Dictyococcites
into three groups according to size: 10
µm (D. bisectus),
10-5 µm, and <5 µm.
Toweius crassus has been the subject of many taxonomic controversies (Wei, 1993; Gallagher, 1989; Perch-Nielsen, 1985). According to Wei (1993), T. crassus ranges from 8 to 12 µm in length and is elliptical to sub-elliptical (but never round) in shape. In samples from Site 1051, T. crassus can not be distinguished from Toweius callosus because of gradational form. Two types of Toweius <4 µm in size were recognized in Hole 1051A. In this report, tentative classification of genus Toweius is offered to supplement conventional classification, subdivided by size and situation of the central opening, as follows: Toweius sp. A is <4 µm has no central opening, and Toweius sp. B is <4 µm and has an opening.
Recovered cores from Holes 1051A and 1052A are described briefly as follows (Figs. F2, F3):
0-20 mbsf is composed mainly of nannofossil ooze, 5%-10% calcareous chalk, with clay;120-380 mbsf is composed mainly of nannofossil chalk and calcareous chalk, with clay; and
380-530 mbsf is composed mainly of nannofossil chalk and calcareous chalk, with clay.
0-132 mbsf is composed mainly of nannofossil ooze, 20% siliceous ooze, with 10% clay; and132-170 mbsf is composed mainly of nannofossil chalk, with chert.
