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 112

samples were taken from an outcrop at 3,225 mbsl. Serpentinites and associated
ultramafic cumulates were covered by semi-consolidated mud between 3,129 and
3,079 mbsl.

4.4 Dive 6K#575

Above the debris apron containing float stones of limestone and massive serpen-
tinite, outcrops of massive serpentinite occurred between 3,360 and 3,290 mbsl.
Foliated rocks, which were not sampled but now inferred as schistose serpentinite,
were observed at 3,236 mbsl. The lowermost locality of volcanic rocks lies at 3,212
mbsl, and sporadic occurrences of volcanic rocks and volcanic conglomerate con-
tinued until 2,614 mbsl. The uppermost section between 2,600 and 2,460 mbsl
consisted of stratified sedimentary rocks, parts of which were confirmed as massive
or thin-bedded mudstone samples.

4.5 Dive 6K#608

Gentle debris slope containing blocks of dacite continued between the basin floor
at 3,440 and 3,400 mbsl. Between 3,400 and 3,100 mbsl, the slope had many cliffs
of andesite lava. Gentler mud-covered slope with sporadic exposures of andesite
continued until the off-bottom point at 2,840 mbsl. Andesite lava was generally
fractured.

4.6 Dive 6K#609

The lowermost part of the slope comprised a debris apron with local landslide
scarps (Fig. 5e ). Collected debris was composed of schistose serpentinite and
amphibole schist (6K#609R002 and R004-R006: Ep-Ab amphibolite). Outcrops of
schistose serpentinite sporadically occurred on generally mud-covered lower slope
between 3,350 and 3,240 mbsl. Serpentinite on outcrops were generally intact with
scarce joints, and with apparently weak foliations gently dipping east to southeast.
Lineations were nearly horizontal in schistose serpentinite samples whose orienta-
tion can be restored by video records. A float of amphibole schist (#609R012:
Grt-Zo amphibolite) was collected at the foot of a serpentinite outcrop at 3,340
mbsl. The middle terrace between 3,240 and 3,220 mbsl was a mud-covered gentle
slope, on which floated blocks of serpentinite and volcanic rocks were scattered.
The shallowest occurrence of serpentinite debris was at 3,175 mbsl within the basal
talus of the upper slope. At 3,155 mbsl occurred an outcrop of horizontally strati-
fied tuff breccia with polymict andesite clasts (Fig. 5f ).