Seafloor Geology of the Basement Serpentinite Body in the Ohmachi Seamount (Izu-Bonin Arc) as Exhumed Parts of a Subduction Zone Within the Philippine Sea - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 110

Minor amounts of limestone and calcareous sandstone were dredged. Nishimura
( 1992 ) dated calcareous sandstone containing andesite pebbles (dredged from
D731: Fig. 4 ) as Late Eocene based on larger benthic foraminifera. Nishimura et al.
( 1997 ) gave 87 Sr/ 86 Sr stratigraphic ages for two limestone samples. A limestone
dredged at site D732 (Fig. 2 ) was dated as 34.23 ± 0.78 Ma (Late Eocene to earliest
Oligocene). Calcareous matrix of a conglomerate dredged at site D731 was dated
as 39.65 ± 0.71 Ma (Late Eocene). These Eocene limestones were dredged from the
upper slope of the seamount, and are inferred to be intercalated within or capping
the Paleogene volcanic rocks. Floats of foraminiferal limestone were also found in
the debris apron near the basin floor. One of them (6K#1065R003) also contained
dissolved molds of colonial corals (Fig. 5g ).

3.4 Miocene Turbidite

Alternating beds of siltstone, sandstone, and conglomerate were observed on the
upper slope (ca. 2,650-2,400 mbsl) of dive track 6K#341. These beds were con-
sidered as turbidite in terms of graded beds with parallel and cross laminae
(Yuasa et al. 1998, 1999 ). Siltstone yielded planktonic foraminifers indicating a
late Early Miocene age (N8: ~16Ma; Yuasa et al. 1998 ). Turbidite alternation of
sandstone and siltstone were also collected as float stones in debris apron near the
basin floor (3,381 mbsl) during dive 6K#1064 (Fig. 5h ).

3.5 Soft Mud Beds

On the western slope (fault scarp) of the Ohmachi Seamount, semi-consolidated
beds of soft and cohesive mud occasionally occurred in outcrops. They are com-
posed of several meters thick beds parallel to the slope surface, with jointed escarp-
ments at each termination. Their surfaces are coated by thin ferro-manganese
oxides. Open burrows are common. The mud contains abundant coccoliths accom-
panied by vitric tuff, foraminferal and radiolarian remnants, and rock fragments in
cases. They are considered as post-rifting slope cover sediments, whose exposure
presumably resulted from active erosion by landslide and bottom currents.

4 Dive and Dredge Results

Four scientific cruises (YK96-11, YK00-08, YK01-04, and YK08-05) by JAMSTEC
R/V Yokosuka with 12 dives in total have been performed on the Ohmachi Seamount
using submersible “ Shinkai 6500 ”. All submersible dive tracks were set on the
western slope (fault scarp) of the seamount (Fig. 4 ).