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Leg1B (West to southwest Oahu)
K206: Southwest Oahu (H. Yokose)
Aug. 30, 2001
Abstract
In order to understand geological relationship
between flat-topped cone and steep pointed
volcanic cone, dive K206 was conducted one
of the flat-topped cone and steep pointed
cone complex in the area of southwest Oahu
on August 30, 2001. K206 traversed 2.2 km
east from the base of the western slope of
the flat-topped cone and up to the highest
place of the steep pointed cone.
The outcrops of the flat-topped cone are
composed mainly elongated pillow lavas, but
of the steep pointed cone are rubble slope
with bulbous pillow, which are cemented together
by thick ferromanganese crust (3 mm). We
could not observe the contact of the cones
due to covering of thick soft sediment. The
rocks collected from the flat topped cone
and from the steep pointed cone is also different.
The former is dense aphyric basalt and the
later is highly vesicular aphyric basalt.
This may suggest that geographical difference
between the cones correspond to volatile
content of the magma.
概要
火山平頂丘と火山尖頂丘の地質学的関係を理解する目的で，オアフ島の南西部に存在する火山平頂丘と尖頂丘複合体において，Ｋ206潜航調査を2001年の8月30日に実施した．Ｋ206潜航調査では，平頂丘の西側の基底部から尖頂丘の最位まで上昇する，東向きのコースを2.2km横断した．
平頂丘の露頭は，主に伸張した枕状溶岩からなるものの，尖頂丘の露頭は，球根状枕状溶岩を伴った岩塊の斜面からなる．これらの岩塊は，厚い鉄マンガンの殻（厚さ3mm）によってセメントされている. 厚くやわらかい堆積物に被覆されているため，我々は，両コーンの境界部を観察できなかった．平頂丘および尖頂丘から採取した岩石も異なっていた．前者は，緻密な無斑晶質玄武岩であり，後者は発泡度の高い無斑晶質玄武岩であった．これは，両コーンに認められる地形的な違いがマグマ中の揮発性成分量に対応する事を示しているのかもしれない．
Purpose
Relatively young lava fields with high acoustic
reflectivity, which means without covering
of thick soft sediment, have been recognized
in the west and northwest offshore of Oahu
based on the GROLIA image（Ｍoore et al.,
1987）. The topography of one of such field
indicates a large shield like volcano in
a rough bathymetric map. During ＫＲ01-12
cruise, we have mapped this shield like area
using Seabeam on RV Kairei.
The new Seabeam bathymetry revealed that
there are three different type of remarkable
topographic features in this area: (1) flat-plain
with strong acoustic reflection，(2) flat-topped
volcanic cone（ca.2.5ｋｍ in diameter and
200m in height），(3) steep pointed cone（ellipsoidal
in plain: major axis 2km, minor axis 0.5km;
200-400 m in height）. Volcanic topographies
like (2) and (3) above are similar to those
described in elsewhere（e.g., Clague et al.,
2001）．Submersible investigation of K203
revealed that the flat ocean floor with strong
acoustic reflection is sheet flows covered
with thin sediment and the steep pointed
cone is pyroclastic deposits cemented with
thick manganese coating.
Flat topped cones distributed in western
offshore of Oahu are different from other
area in their occurrence that they are accompanied
with steep pointed cone. In order to study
the geological relationship between flat-topped
cone and steep pointed cone, Ｋ206 was conducted
one of the flat-topped cone accompanied with
steep pointed cone.
Dive result
The course of K206 can be divided into three
different topographic features: flat ocean
floor，flat-topped cone, and steep pointed
cone.
Flat ocean floor
KAIKO landed on muddy bottom in the western
flank of one of the flat-topped cone located
on the west Oahu region (20o 38.81Ｎ, 158o
48.10). Many small rock chips (ca. 2cm in
diameter) are scattered on the flat ocean
floor. It looks like a shiny glassy fragment,
but are small rock fragment with manganese
coating. The number of the chips decreased
rapidly approaching to flank of the flat-topped
cone. The distribution area of the chips
correspond roughly to the highly reflective
area on sub-bottom profiler image. A highly
vesiculated aphyric basalt was collected
from this flat floor．
Flat topped cone
Coming up to the slope of flat topped cone,
we found many breccias of pebbly mudstone
on muddy bottom. They are covered with thick
manganese coating ( up to 3 mm). Probably,
these breccia were derived from slope failures
of the flat topped cone because the clasts
in pebbly mudstone are monomictic and dense
aphyric basalt which is the same rock as
pillow lava on the slope of flat topped cone.
At the base of flat topped cone, we observed
pillow cones, sheet flow with columnar jointed
and pillow lavas. Many truncated pillow lavas,
especially tip of elongated pillows, indicate
a remarkable onion crack texture. Elongated
pillow lavas were dominated on the moderately
steep slope. Highly fractured flattened pillow
with approximately 25 cm thick were observed
on the steepest slope of the slope. The outcrops
of pillow lavas are always mantled with a
strata which is ca. 30 cm in thickness. Unfortunately,
we could not collect sample directly from
the strata due to cementation of thick manganese
coating. The strata draped largely on this
flat topped cone may be a deposit of slope
failure on the basis of the occurrence of
pebbly mudstone which derived from landslide
deposit and escarpment of outcrops. We tried
to collect push core sample on the flat top,
but the push core sampler could not penetrate
deeper than 5 cm. It shows that the thickness
of soft sediment on the flat top area is
very thin.
Steep pointed volcanic cone
Slope of the steep pointed cone is steeper
than that of the flat topped cone and are
covered mainly with talus which cemented
with thick manganese coating. Rock sample
collected on this steep slope included in
pebbly mudstone. We observed truncated pillow
just below the summit of the steep pointed
cone.
The summit of this steep pointed cone is
flat plain with small crater and is covered
with lava rubbles ranging from few cm to
50 cm in diameter．These lava rubbles are
also cemented with thick manganese coating.
We could not observe any bulbous pillow and
pillow cone on this summit. Because there
is no higher place than the summit, these
rubbles were originated in situ and are inferred
to preserve depositional environments.
Thickness of the mantling soft sediment is
also very thin( < 2cm) compared with the
flat ocean floor.
Rock sample
We collected two types of basaltic rocks
and pebbly mudstone. One is the dense aphyric
basalt that distribute on the slope of flat
topped cone. The other is highly vesiculated
(up to 20-30%) aphyric basalt that distributed
on the slope of steep pointed cone and flat
ocean floor. Vesicles of the outer part of
almost all highly vesiculated sample from
steep pointed cone are filled with pelagic
sediments. This suggests that these rocks
are not hyaloclastite but are epiclastic
rocks. Also in such epiclastic rocks, the
thickness of the manganese coating is up
to 3 mm.
Video highlights
9:47 -9:48: muddy bottom with many small
manganese clasts
10:29-10:32: breccia of pebbly mudstone
10:49-10:50: jointed sheet flow
10:58-10:59: pillow lava with spheroidal
crack
11:19-11:38: elongated pillow
13:02-13:04: rubble slope. Rocks are cemented
together by thick ferromanganese crust.
14:12-14:19:bulbous pillow and pillow breccia
time depth (m) altitude (m) heading (degree) Narrative Sample 9:25 3865 154 see water 9:29 vehicle was released 9:38 3983 13.1 153 see water 9:40 3999 0.1 86 on bottom, many rock chips on mud floor 9:42 3999 0.1 86 push core (red) in mud seds PC 1 (red) 9:44 3999 0.1 86 push core (blue) in mud seds PC2 (blue) 9:50 4000 1.5 88 many rock chips on mud floor 9:51 4002 0 105 sampled#1A put in box1 1A 9:56 4002 0.1 89 sampled#1B put in box1 1B 10:01 3999 2.4 59 many rock chips on mud floor 10:06 3997 2.1 160 many rock chips on mud floor 10:10 3997 0.1 148 sample #2 put in box2 2 10:12 3997 0.1 129 sample #3 put in box2 3 10:17 3993 0.8 92 mud bottom, many gravels 10:20 3993 0.1 95 sample #4 put in box3 one piece broke and fall in box2 4 10:26 3989 1.9 92 mud bottom, many gravels 10:31 3987 0.4 150 sampling attempt 10:35 3987 0.1 145 sample#5 put in box9 5 10:37 3983 2.1 130 outcrop, massive lava 10:41 3980 1.9 133 sample#6 put in box3 6 10:48 3972 1.7 164 sample#7 put in box4 7 10:49 3967 7.7 102 columnar joint 10:50 3967 4.8 137 massive lava? outcrop 10:56 3968 2.8 172 sample#8 put in box5 8 10:58 3952 6.1 98 pillow lava out crop 10:59 3944 2.5 87 some rock chips on mud bottom 11:03 3913 3.5 89 muddy floor 11:04 3906 3.2 90 massive lava? Outcrop 11:07 3906 0.3 47 sample#9 put in box6 9 11:10 3891 2.8 86 pillow lava outcrop 11:13 3886 1.3 121 sample#10 put in box3 10 11:15 3882 1.8 69 muddy bottom 11:17 3878 3.8 69 pillow lava outcrop 11:24 3867 3.9 67 pillow lava outcrop 11:27 3868 1.7 94 sample #11 put in box6(30cm) 11 11:28 3863 5.3 86 outcrop pillow lava 11:33 3849 1.6 51 pillow outcrop 11:35 3848 1.2 60 sample#12 put in box5(10cm) 12 11:39 3840 7.3 80 outcrop 11:44 3806 1.4 89 massive lava 11:53 3795 1.8 89 muddy floor 12:00 3793 1.4 89 muddy floor 12:04 3786 1.2 87 KAIREI at a speed of 0.3 knot 12:05 3786 1.2 87 KAIREI speed up to 0.7 knot 12:11 3783 0.9 89 muddy flat floor 12:15 3779 2.4 86 muddy floor with rock chips 12:20 3774 3.4 89 muddy floor with rock chips 12:24 3775 1.4 85 2 sea cucumbers on muddy floor 12:25 3775 0.3 91 sea bunny 12:31 3772 1.9 88 muddy floor with rock chips 12:34 3771 1.1 89 Mn crust under sediment 12:36 3769 1.2 89 still travelling across flat top of cone 12:44 3769 2.7 71 high reflectivity zone appears 100 m away to the left on sonar 12:49 3775 0.1 112 push core taken in mud (green) PC 3 (green) 12:51 3775 2.3 74 still sediment covered, but only a few cm thick (as seen by short push core) 12:52 3778 0.8 71 gravelly surface with less mud 12:56 3775 2.4 68 another, brighter, bigger reflector to the left on sonar 12:57 3777 0.9 31 slightly coarser bottom rubble 12:58 3777 1.7 29 heading directly for big reflector 13:01 3770 5.8 40 heading up steep slope, scattered outcrop (?) rocks 13:02 3773 1.4 34 sampling attempt on small pillow outcrop, difficulty may indicate thick Mn 13:07 3773 1.9 56 still trying to sample 13:09 3771 4.1 60 moving again (no sample) 13:10 3768 3 54 much bigger pillow (?) outcrop 13:12 3769 1.9 32 sampling attempt on big pillow outcrop 13:14 3762 3 70 going upslope, some platy material in with pillow-like structures 13:15 3757 2.6 71 gravelly surface with less mud 13:17 3756 3.6 32 contemplating sampling three v. large loose blocks 13:19 3758 1.3 38 sampling attempt 13:20 3758 1.3 38 sample #13 put in box5(30cm) 13 13:22 3738 5.6 73 steep outcrop of pillows 13:23 3724 3.9 68 slope parallel covering 13:25 3723 2.1 43 sampling attempt on big pillow outcrop 13:26 3723 2.1 54 sample #14 put in box7(big egg) 14 13:28 3721 2.2 73 crusty covering on top of mud - Mn? 13:33 3717 48 sample #15 fell into box two (smallish mud chunk) 15 13:34 3717 48 sample #16a in box 8 (large fist) 16a 13:36 3717 1.2 49 #16b in box 8 (much smaller) 16b 13:39 3717 1.2 49 #16c in box 8 (starfish, big pear size) 16c 13:40 3692 4.1 82 sediments and scattered block covering steep slope 13:41 3679 2.8 57 pillow outcrop 13:43 3669 3.2 57 still on steep pillow outcrop 13:45 3664 3.3 60 sample #17 in box 9 (starfish on small <5 cm) 17 13:49 3649 2.5 45 sample #18 in box 7 (big one, 30 cm) 18 13:50 3641 4.1 60 more pillows 13:53 3616 1.5 51 reached the top 13:56 3616 1.6 46 sampling attempt 13:59 3614 0.4 67 yellow push core in small sediment ponds 14:02 3611 2 63 mummy-shaped pillow 14:07 3608 1.9 63 flat-topped blocks - old lava lake surface? 14:08 3608 2 63 sample#19 in box 8 19 14:10 3605 1.9 121 travelling across flat ridge at top of cone 14:12 3605 1.4 123 near low reflectivity zone - crater? 14:13 3607 1.3 46 edge of depression 14:15 3619 0.8 105 rubbly floor 14:17 3610 208 leaving bottom
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