Date: August 25, 2002 Place: Southwest corner of Hilina slump, S of
Hawaii Island
18°
56.25’ N, 155° 7.04’ W
Pilot: Y. Sasaki Co-pilot: T. Yoshiume
Observer: Peter Lipman (U.S. Geological Survey)
Dive
#710 reached the sea floor at 4,430 m depth (11:40 HST) along the south base of
the lowest steep slope that offers relatively structurally intact exposures at
the southwest “corner” between the lower scarp and transverse boundary of
Hilina slump. The purpose of the
dive was to (1) observe the nature and composition of the basal deposits
flooring the deepest levels of the Hilina bench, directly downslope from the
summit of Kilauea; (2) evaluate the potential presence of Mauna Loa debris; and
(3) to search for large (datable) breccia clasts of extreme alkalic
compositions thus far only
identified in glass sandstones within the lower scarp. All exposures along the dive track were
massive breccia, sandstone, and other volcaniclastic rocks, containing diverse
clast types in glass-sand matrix.
Especially fine volcaniclastic outcrops were traversed at depths of
4375-4355 m and again at 4030-3900 m.
The dive was terminated at 3890 m depth (14:50 HST), about 45 minutes
earlier than planned because of deteriorating surface sea conditions. In total, the dive track was about 2400
m long, with 540 m of elevation change.
Despite the limited bottom time (~3 hr 10 min), this dive has filled a major gap in observations on the Hilina slump, providing the first observations and samples from the lowermost southwestern “corner” area. Once again, the absence of primary lavas in the Hilina bench stratigraphy is notable. The diverse clast types collected from breccias should provide important controls on the early growth of Kilauea, relative to Mauna Loa, especially if the recovered aphyric basalt clasts are alkalic in composition and Ar-Ar datable. Along with the previously determined relations for Papa’u Seamount, the large number of vesicular pahoehoe clasts in these deep breccias, that must have been subaerially derived and seem likely to have come from Mauna Loa, indicate that a clastic submarine apron from this volcano extends at shallow depth beneath much of the southwestern Hilina bench.
(1) 12:02-12:16. Rugged cliffs of massive breccia (15-20
m high) containing angular clasts, probably mainly basaltic lava (sample site
#2)
(2) 13:48-14:31. Massive cliffs of sandstone and
hyaloclastite containing small clasts in glass sand matrix, mostly too large to
image completely on either video or digital-camera; much giant angular talus
from the cliff outcrops (sample sites #5-6)
(3)
14:00-14:02. Bedded sandstone,
dipping about 30o southeast--best seen on camera #1 and digital
photos #43-50.
(4) 14:46-14:48. Subhorizontal breccia layers and interbedded sandstone. Breccia contains rounded clasts, of vesicular pahoehoe, some strongly reddish in color and obviously derived from subaerial sources (Site #7).
A key part of the first JAMSTEC Hawaii
project has been the submersible investigation of the Hilina slump, the
actively deforming southeast flank of the Kilauea volcano that is presently
moving seaward at rates up to 10 cm/yr. Submersible observations and samples
from the 1998-1999 and 2001 cruises showed that the lower bench of the Hilina
slump system, consists overwhelmingly of compositionally diverse volcaniclastic
rocks. Pillow lavas are confined
to slopes shallower than 3000 m depth. Submarine-erupted basalt clasts have
wide-ranging alkalic and transitional basalt compositions (to 41% SiO2,
10.8% alkalis), contrasting with present-day Kilauea tholeiites. The
volcaniclastic rocks provide a unique record of ancestral alkalic growth of an
archetypal hotspot volcano, including transition to its tholeiitic shield stage,
and associated slope-failure events.
A conspicuous NW-trending scarp,
aligned with Papa’u Seamount, bounds the slump terrain on its SW side but has
no subaerial expression. A dive in 2001 up the west flank of Papa’u Seamount
(K209) documents the unexpected conclusion that this elongate ridge consists
largely or entirely of breccia debris from Mauna Loa, and that volcaniclastic
deposits on the Hilina bench wrap depositionally around the seamount,
complicated by compressional deformation during gravitational spreading of the
island flank.
The novel results of the 1998-99 and 2001 dives onto the deformed flanks and adjacent seafloor of Hawaii provided motivation to return to this area for a few additional dives with the SHINKAI 6500 in 2002. The presence of alkalic pillow basalts on the upper slope, above the mid-slope bench (dive K208) documents that the ancestral alkalic edifice of Kilauea was large and projected into shallow water. The evidence from Papa’u Seamount indicates that an old volcaniclastic apron derived from Mauna Loa underlies much of the Hilina bench. The presence of indurated volcanic sandstones throughout the deep portions of the south flank of Kilauea suggest that the distal slopes of Hawaiian volcanoes are largely composed of sediment. This possibility has implications for the mechanical strength and long-term stability of the deforming flanks. However, the occurrence of primary volcanic rocks upslope of the sedimentary strata on Kilauea indicates that the transition from volcanic to clastic environment must be relatively complicated.. In order to interpret the evolution of the islands, and the kinematics of the deforming flanks, it is important to determine the deepest parts of the Hilina bench, especially directly downslope from the summit area of Kilauea where magma first rises from the mantle.
1. To assess the nature and
composition of the basal deposits flooring the deepest levels of the Hilina
Bench, in the “corner” area directly downslope from the summit of Kilauea.
2. To evaluate the potential presence
of Mauna Loa debris in deep parts of this deformed volcanic apron, flanking the
south side of Hawaii Island.
3. To search for large (datable) breccia
clasts of the unusual extreme alkalic compositions (phonotephrite, foidite,
nephelenite), thus far only identified in glass sandstones within the lower
scarp of the Hilina slump
2 – sample baskets
1 – sample container with lid
4 – push corers
In
addition to the fixed-mounted still camera, which was set to take photos every
30 sec, 84 images were made with the digital camera, mostly from the port-side,
and copilot Yoshiume exposed three rolls of 35 mm film from the starboard
porthole. A list of the digital
images, with estimated exposure times and evaluation of image quality, is
included at end of this report.
Dive #710 was targeted at the lowest
steep slope that promised relatively structurally intact exposures at the
southwest corner between the lower scarp and transverse boundary of Hilina
slump. Bathymetric expression of somewhat deeper slopes to the southeast
suggests that these areas may have been involved in downslope slumping, and
stratigraphic sequence may have been obscured and/or repeated. The planned
landing point was at 4,400 m, directly at the base of the best defined steep
ridge inboard from these potentially slumped masses, and the initial plan was
to proceed directly upslope as far as time permitted, observing and sampling
the best possible outcrop exposures.
Emphasis was to be on collecting basaltic clasts from breccias that
would be sufficiently large to analyze in detail for composition, and
potentially for age determination if alkalic compositions were identified. The entire dive track (4400-~3700 would
be deeper than any samples obtained previous from dives in adjacent flanks of
the Hilina slump.
SeaBeam maps collected on this and
previous surveys of this area show steep slopes down to 4600 m depth, with
multiple ridges separated by troughs that mark relatively young slump
break-away scarps. The dive track
started up the most westerly ridge, adjacent to a large landslide amphitheater
that was site of K91 dive in 1998, then traversed into the eastern flank of
this headwall.
The submersible reached bottom at 11:40
at 4437 m depth, exactly as planned directly at the base of a steep ridge, and
started up on a course of 020° over talus runs covered by varying thickness of
sediment (digital image #4). Some
talus runs have been actively relatively recently and lack sediment cover. A “reserve” sample (#1), collected from
talus at 11:56 (4400 m), is dense aphanitic basalt that may be alkalic in
composition. The talus runs led to
large outcrops of massive breccia beautifully exposed from about 4375 to 4355 m
depth (digital images #8-9). Some
joint surfaces were coated with thick white material (zeolite?; digital images
#5-7). Most viewed outcrops were
massive and structureless, but in one place thin beds of fine sand separate
debris-flow breccia units, and are offset by complex micro faults (digital
images #10). Three clast samples
(#2A, B, C) collected from a breccia outcrop at 4368 m were all olivine-phyric
basalt that may be transitional or tholeiitic in composition (from Mauna Loa?).
Above the sample site, upper steep
parts of the ridge were sediment covered, and only local mud-draped talus was
present up to a water depth of 4120 m.
Unintentionally, the submersible course drifted slightly too far to the
east, into a small trough that channeled debris from upper parts of the slope.
There, an area of coarse angular talus was sampled (#3A, B, C, D, E). The talus clasts at site #3 should
provide information mainly on the adjacent slopes within 200-300 m higher, above
which the small trough dies out.
The five samples collected included: three clasts of sandstone (two glassy, one greenish
altered), a vesicular pahoehoe-type basalt (likely subaerial), and a small
fragment of dense aphanitic basalt with a glass rind.
At this time (12:45), because of the
lack of outcrop exposures, a decision was made to change course to the
northwest (to 320o) and
traverse toward the steep east flank of a large landslide slump scarp. During the traverse, small outcrops
with slope-parallel crude bedding were encountered at 13:35 (digital images
#11-12), in which fairly well-sorted breccia layers alternated with well-sorted
sand. Clast samples from one
exposure (#4A, B, C, D) included three blocks of vesicular pahoehoe, one of
which was deep red and oxidized, that must have been subaerially erupted. These may have originally come from
Mauna Loa; they resemble clast samples collected in 2001 from Papa’u Seamount
(dive K209).
At about 13:50, huge massive knobs and
cliffs of strongly indurated rock were encountered (digital images #14-18; one
isolated knob was at least 25 m high and appeared unfractured and
structureless, but was too large to image effectively with either video or
still camera (note video voice commentary). Massive angular talus at the base of this knob (digital
images #19-21) yielded sample #5, a dense rock containing small angular clasts
of devitrified aphanitic basalt in glass-sand matrix. Some of the more rounded talus at the base of continuing
rugged outcrops shows sufficient weathering to display fragmental hyaloclastite
textures (digital images #33-42); other angular structureless-appearing talus
blocks and outcrops (e.g. at 14:13) may be better sorted sandstone (digital
images #23-25). Sample #6,
collected at 14:10, was densely indurated hyaloclastite, similar to #5. Further along these superb rugged
outcrops that required great driving skill by the pilots, cliff exposures
containing strongly jointed rocks showed crude bedding dipping to the east,
roughly down the regional slope of the Hilina lower scarp (digital images
#43-50). In places (~12:26),
strongly jointed outcrops of massive rocks (sandstone?) contain angular
fractures coated by irregular patches of zeolite (digital images #55-61). Further upslope (~14:27-28), more
massive jointed and bedded exposures appear to consist massive breccia layers,
separated by thinner sandstone beds (digital images #63-77). Overall, these rugged outcrops were
continuous exposed over a vertical distance of about 125 m.
Above a minor bench covered by talus
and mud, additional large outcrops displayed subhorizontal beds of breccia
interlayered with more finely bedded sands with a subhorizontal orientation
(digital images #78-84). These
probably are somewhat younger deposits than the massive cliffs imaged just
below, based on the lesser lithification and flatter bedding. The breccia beds contain obvious
diverse clast types, some strongly reddish in color, and the four clasts of
sample #7 are all vesicular pahoehoe of obvious subaerial origin, probably from
Mauna Loa.
The dive was terminated at 14:50 hrs,
somewhat early due to deteriorating surface sea state.
This dive has filled a major gap in observations on the Hilina slump, providing observations and samples from the lowermost southwestern “corner” area. The lowermost outcrop samples of S710 are about 900 m below the lowest (talus) samples collected during the adjacent dive K91 in 1998, and the even highest samples (at 3890 m) are at just about the depth as the lowest outcrops imaged (but not sampled) during dive K91. Once again, the absence of primary lavas in the Hilina bench stratigraphy is notable. The diverse clast types collected should provide important controls on the early growth of Kilauea, relative to Mauna Loa, especially if Ar-Ar datable alkalic clasts were recovered. Along with the previously determined relations for Papa’u Seamount, the large number of vesicular pahoehoe clasts in these deep breccias, that must have been subaerially erupted and seem likely to have come from Mauna Loa, indicate the likelihood that a clastic submarine apron from this volcano extends at shallow depth beneath much of the western Hilina bench.
|
DIVE S710 |
|
Aug. 25,2002 |
|
|
BASAL SW CORNER OF HILINA BENCH |
|
|
|
VIDEO LOG |
|
|
Recorded by J.-I. Kimura [with
additions by P. Lipman] |
|
Time |
Sub
head angle |
Depth
(m) |
X |
Y |
Description |
|
11:39 |
59 |
4420 |
|
|
descending |
|
11:40 |
9 |
4437 |
|
|
see seafloor, talus or mud |
|
11:42 |
33 |
4437 |
-1010 |
-80 |
landed on seafloor, talus runs of
varying activity, some with mud, some no sediment |
|
11:50 |
16 |
4402 |
|
|
Slope-parallel weakly indurated
breccia; stop for sampling |
|
11:56 |
52 |
4400 |
-930 |
-110 |
Sample #1:
rock sample from talus deposit |
|
11:59 |
23 |
4395 |
|
|
coarse talus deposit |
|
12:01 |
12 |
4378 |
|
|
coarse talus deposit draped with mud |
|
12:02 |
8 |
4364 |
|
|
Rugged cliffs of well indurated massive
breccia; irregular white (zeolite?)-filled fractures |
|
12:09 |
|
4368 |
|
|
stop for sampling |
|
12:14 |
29 |
4368 |
-890 |
-90 |
Sample #2A,B.C: three clast samples from breccia |
|
12:16 |
15 |
4355 |
|
|
Top of breccia cliffs, into mud with
talus breccia |
|
12:19 |
11 |
4329 |
|
|
thick mud |
|
12:26 |
13 |
4297 |
|
|
talus breccia covered by thick mud,
diameter of the breccia increase with time |
|
12:29 |
12 |
4278 |
|
|
muddy seafloor |
|
12.32 |
12 |
4254 |
|
|
talus breccia covered by thick mud,
diameter of the breccia increase with time |
|
12:35 |
16 |
4243 |
-490 |
-30 |
position confirmed, mud seafloor |
|
12:37 |
14 |
4236 |
|
|
talus breccia covered by thick mud |
|
12:41 |
11 |
4221 |
|
|
talus breccia |
|
12:44 |
11 |
4207 |
-330 |
10 |
position confirmed, talus breccia
covered with mud |
|
12:46 |
11 |
4192 |
|
|
heading set at 10 degrees |
|
12:49 |
12 |
4174 |
|
|
talus breccia covered with thick mud |
|
12:52 |
40 |
4168 |
|
|
talus breccia covered with thick mud |
|
12:54 |
40 |
4151 |
10 |
160 |
very coarse talus breccia polygonal
shape, mud drape |
|
12:59 |
5 |
4121 |
|
|
stop for sampling |
|
13:06 |
2 |
4119 |
140 |
140 |
samples #3A, B, C (yellow), D (sm
black), E (sm black): from talus run |
|
13:09 |
321 |
4107 |
|
|
change heading; coarse talus
breccia/mud deposit occurred alternatively |
|
13:14 |
321 |
4066 |
200 |
70 |
muddy seafloor |
|
13:18 |
310 |
4041 |
|
|
heading set at 310 degrees |
|
13:19 |
311 |
4040 |
|
|
talus breccia (subrounded) covered with
mud |
|
13:24 |
312 |
4023 |
420 |
-180 |
Small breccia outcrop, with
slope-parallel crude bedding; positioning |
|
13:36 |
81 |
4022 |
470 |
-210 |
sample #4A,B,C,D: rounded clasts, from small breccia outcrop |
|
13:37 |
310 |
4022 |
|
|
heading set at 310 degrees |
|
13:43 |
316 |
4028 |
|
|
Small breccia outcrop; crude
subhorizontal bedding (camera #1) |
|
13:44 |
209 |
4027 |
|
|
Large well bedded outcrop of finely
bedded slope-mantle sediment |
|
13:46 |
311 |
4029 |
|
|
coarse talus deposit, subrounded in
shape, clast supported |
|
13:50 |
314 |
4032 |
|
|
Talus from huge massive outcrops (ss?
not visable in video); stop for
sampling |
|
13:52 |
55 |
4031 |
660 |
-420 |
sample #5:
steeply (45 degrees) inclined bedded sst? |
|
14:01 |
33 |
4013 |
|
|
bedded sst outcrop, dippin ~30o
E |
|
14:02 |
22 |
3998 |
|
|
coarse blocky talus deposit |
|
14:07 |
37 |
3964 |
|
|
Massive hyaloclastite outcrops; stop
for sampling |
|
14:10 |
37 |
3964 |
760 |
-390 |
sample #6:
volcanicrasticbreccia? |
|
14:12 |
67 |
3958 |
|
|
coarse blocky talus deposit |
|
14:13 |
68 |
3940 |
|
|
More massive, angular jointed outcrops
(sandstone?) |
|
14:21 |
46 |
3927 |
890 |
-410 |
postioning confimed |
|
14:23 |
18 |
3935 |
|
|
very coarse talus breccia polygonal
shape |
|
14:24 |
44 |
3927 |
|
|
jointed massive sst, vertical open
joints |
|
14:27 |
31 |
3907 |
|
|
massive sst |
|
14:29 |
31 |
3900 |
|
|
breccia bg. sst? |
|
14:30 |
40 |
3894 |
|
|
muddy seafloor |
|
14:34 |
40 |
3890 |
|
|
talus covered with mud |
|
14:35 |
24 |
3888 |
|
|
Large breccia outcrop, subhorizontal
beds; some reddish rounded clasts (camera #1) |
|
14:37 |
55 |
3888 |
1020 |
-400 |
sample #7A,B,C,D |
|
14:49 |
83 |
3888 |
|
|
leave from the bottom |