Geoscience Reference
In-Depth Information
clasts and blocks of massive serpentinite. Massive serpentinite sporadically
occurred between 3,160 and 3,100 mbsl among rubbly talus. Between 3,100 and
3,080 mbsl (off-bottom point) was a mud-covered gentle slope, on which debris
blocks of semi-consolidated mudstone were partly observed.
4.13
Dredges
Rocks were first dredged from three sites on the Ohmachi Seamount (D731, D732,
and D794) during R/V “
Hakurei-Maru
” GH85-1 cruise in 1985 and GH86-1 cruise
in 1986. From D731 on the western slope of the main part of the seamount (Fig.
4
),
calcareous sandstone, volcaniclastic conglomerate with calcareous matrices, and
andesite cobbles were dredged. Andesites and calcareous conglomerate were
dated as Late Eocene to Early Oligocene (see Sec. 3.3). D732 on the southern slope
of the main part (Fig.
2
) yielded limestone of Late Eocene to earliest Oligocene
age (Nishimura et al.
1997
). Dredge hauls of D794 on the western scarp of the
peninsular part (Fig.
4
) were composed of andesite, dolerite, basaltic and andesitic
breccia, and sandstone. Two breccia samples contain the prehnite-pumpellyite facies
metamorphic minerals (Yuasa et al.
1992, 1999
).
Nine dredges (KT04-28 D06~D13, D24 and D25: Figs.
2
and
4
) were per-
formed during the cruise KT04-28 by R/V”
Tansei-Maru
” in 2004. Dredge sites
D06~D11 are located on the southwestern slope of the Ohmachi Seamount from
north to south, and the sites D12 and D13 is on the eastern slope. Location of D25
was nearly identical to D06, and D24 was close to D09. Serpentinites were col-
lected on sites D06, D07, D08, D09, D24, and D25. The other sites (D10~D13)
yielded either of dacite-andesite and their volcaniclastic varieties correlative to
the Paleogene, sandstone and mudstone to the Miocene, and unaltered basalt and
pumice presumably of Quaternary tephra or debris. Dredge hauls at D09 also
contained significant amounts of volcanic and sedimentary rocks in addition to
serpentinites. Serpentinites collected at D06, D07, and D25 were dominantly
schistose, whereas those at D08, D09, and D24 were mostly massive (Fig.
7
).
Sites D06 was set approximately the same site as lower parts of dives #609 and
#1064, and yielded a piece of Grt-Ep amphibolite (D06-1) in addition to domi-
nantly schistose serpentinite clasts.
5
Interpretation of Geologic Structures
A geological map for the fault scarp of the peninsular part of the Ohmachi
Seamount was constructed based on the dive and dredge results as shown in Fig.
4
.
Semi-quantitative orientation data of dips and strikes of foliation (schistosity) read
from video images are shown in Figs.
4
and
8
, with three quantitative in-situ
measurements of foliation; the method is briefly described in the appendix.
Search WWH ::
Custom Search