Boso TTT-Type Triple Junction: Formation of Miocene to Quaternary Accretionary Prisms and Present-Day Gravitational Collapse - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 61

3 Diatom Analysis

3.1 Materials, Method, and Results

ROV KAIKO-10K dives 10K#148 and #151, during cruise KR99-10 of
JAMSTEC's R/V KAIREI , investigated the toe of the landward slope of the
trench on the Honshu-arc side of the Boso triple junction at depths of about
9,200-8,900 m (Figs. 3 and 5 ). The ROV dive routes crossed the foot of the
accretionary prism, where the seafloor topography is the result of submarine
landslides (Figs. 5 and 6 ). The slopes and cliffs expose soft bedded formations
that contain scattered cobbles of harder rocks (Figs. 5 and 6 ) (Oji et al. 2009 ).
These exposures were sampled using a manipulator arm and push corer installed
on ROV KAIKO-10K . All samples (except two hard rock samples) broke up in
the sample basket.
Diatom analyses of nine samples were performed (Table 1 ). Samples 10K#148
R1-5 (2, 5, 11, 12), 10K#148 R-2, and 10K#151 R-1 were rock samples. Samples
10K#151 C-1, C-2, and 10K#151 C-1 were core-top samples. Samples 10K#148
R-2 and 10K#151 R-1 were relatively hard, indurated mudstone float clasts taken
from the slope and contained ash and diatom fragments. The other samples were
soft diatomaceous mud or mudstone that broke up during collection with the
manipulator arm and became mixed in the sample box; the sample numbers for
these are therefore somewhat arbitrary. The outcrops of steeply dipping or subhori-
zontal beds shown in Figs. 6 and 7 are probably exposures of such soft sedimentary
rocks, which probably represent turbidite deposits.
Unprocessed strewn slides were prepared for each sample following the method
described by Akiba ( 1986 ). One hundred diatom valves were counted for each slide
at 600× magnification. After routine counting, each slide was scanned in its entirety
to find diatoms missed during counting. Diatoms found as fragments were also
recorded. Resting spores of the genus Chaetoceros were counted separately.
Complete diatom valves were found in eight samples; sample 10K#148 R-2
contained only highly fragmented valves (Table 2 ). Diatom assemblages were a
mixture of cold- and warm-water species, except for sample 10K#151 R-1, which
was dominated by cold-water diatoms.

3.2 Diatom Biostratigraphy

Because the samples contained cold- and warm-water diatom assemblages, both
Neogene North Pacific middle to high latitude diatom zonation (Akiba 1986 ;
Yanagisawa and Akiba 1998 ) and low-latitude zonation (Barron 1985 ) can be used
for age determination. Table 1 shows the diatom zones and ages for the samples
analyzed, and Fig. 8 show the 25 most significant species.