Leg2A - 1(Hilina slump)

K208: Upper part of Hilina slump (J. Kimura )
Sept. 04, 2001

Results of Dive K208

Date: September 4th, 2001
Place: Cliff along a ridge on the upper slope of the Hilina Bench
Observer: Jun-Ichi KIMURA

Abstract
The dive is designed to observe geology along a rib on the upper slope in southwestern area of the Hilina bench. Thick pillow lava piles occurred along almost the entire course of the dive track. Observed exposures are “in situ” outcrops in which cut sections of pillow lavas. The pillow lava structures include rounded pillow lobes with radial joints, intact breccias that surround pillow lobes. Obtained samples from those outcrops are aphyric basalts with a compact structure. Some of them have glass rinds. The observations suggest that the ridge consists entirely of submarine lavas, and these lavas may overlie on consolidated volcaniclasic sandstone layers, which was observed in the lower slope of the Hilina in previous dives. The aphyric texture of the lavas does not resemble with any of lavas exposed in the northeastern upper slope of the Hilina bench. The pillow lavas could belong to alkali or transitional basalt suites of the old Kilauea stage.

要旨
このダイブは，ヒリナベンチの上部斜面西部にある小さな尾根の地質調査を行うことを目的とした．ピローラバがルート沿いのほとんど全ての露頭において確認された．ピローラバの露頭はほとんどがin situであり，溶岩の断面が観察された．ピローラバ構造は，ピローローブ，放射状節理，しまった角レキ状のマトリクスなどからなる．露頭から得られた試料は全て無斑晶質玄武岩で，気泡の少ないコンパクトな構造を示す．いくつかの試料にはガラスラインドがある．これらの観察事実から，この尾根はほぼ全面的にピローラバから構成されており，おそらくヒリナベンチ下部斜面に露出する砂岩をおおうものと考えられる．しかし，無斑晶質な溶岩の組織は，上部斜面東側に露出するいかなる溶岩とも記載岩石学的に一致しない．ピローラバはキラウエア火山の古期のアルカリ岩の可能性もある．

Purpose of Dive
The Hilina Bench of the main Hawaii Island has been thought being underlain by older stage volcanic products of Kilauea volcano. The Hilina area is located between Puna Ridge and Loihi Seamount, and consists of three individual topographic segments, lower slope, middle bench, and upper slope. The Kaiko and Shinkai dive operations done in 1998 and 1999 have revealed that the lower slope consists entirely of volcaniclastic sandstone and breccia layers. In contrast, the upper slope of the bench consists of transitional pillow lavas exposed in the eastern area, covered by thick sediment that originated from eruptives of the young shield stage volcanic activity of Kilauea. The purpose of this dive is to observe the geology along a NW-SE trending rib located in the southwestern area of the upper slope. We expect that there are some exposures of “in situ” lavas, which may be the continuation of the eastern submarine lavas or proximal facies of the lower slope sandstones. The boundary between transitional basalt and shield-stage tholeiitic basalts could be exposed in the former case.

Dive Results
The dive began at the bottom of a small NW-SE trending rib on the upper slope of the Hilina bench. The ROV/Kaiko landed on the flat of the Hilina bench at the depth of 2586m. A right colored sandy flat with small rock fragments was observed on it, and is thought to be recent sand sediments derived from the shoreline. A push core sample (Green) was obtained, and the recovery was about 3 cm long. Unidirectional current ripples with several tens of centimeter wave-length were observed, showing strong tide currents in this area. At depth of 2568m, a talus fragment was picked up (K208-1). It was covered with thin sand. At 2538m, the first outcrop of pillow lava covered by very thin sand layer appeared, and two lava samples were collected from the surface (209-#2A & #2B). Between 2538m and 2516m, the sand flat continued. A big outcrop of pillow lava occurred at 2516m and a lava sample was collected from the surface (K208-3). The outcrop continued up to 2370m, and beautiful cross-cut exposures of pillow lavas were observed. Observed pillow structures were rounded pillow lobes with radial cracks. Five samples at four localities were obtained, mainly pillow clast (K208-4 (2496m), -5A and -5B (2471m), -6 (2442m), and -7 (2371m)). Small depressions of about 10m deep occurred between sampling sites #5 and #6, #6 and #7, and #7 and #8. These depressions have directions of N80E almost sub parallel between each other. The directions are almost parallel to the slope direction. Between 2360m and 2348m, the slope became flatter, and shallow sand cover lay on the pillow outcrop surface. At depth 2348m, the pillow section appeared, and a sample (K208-8) was collected from a pillow lobe. Sediment cover occurred between 2309m and 2273m. At 2273m, stratified sand layers with one or two centimeters thick silt interbeds were exposed. Sampling attempted but failed, because the sediments were only weakly consolidated and large blocks could not be removed from the outcrop. A steep slope with pillow lava exposure re-appeared at 2243m depth. A pillow lava sample was obtained from a pillow lobe at the base of the outcrop (K208-9). This outcrop continued up to 2082m deep, and five samples were collected from the outcrop. These samples were K208-10 through to K208-14 (-10 (2214m), -11 (2204m), -12 (2182m), -13 (2153m), and -14 (2082m)). All but one are from pillow lobes, one (K208-13) is from jointed lava. The last sample, obtained from a pillow lobe surface, was difficult to remove from the outcrop because the jointed clast was intact, suggesting undisturbed nature of the outcrop. A small valley, between #12 and #13, has N60E direction. This small valley is topographically similar to those previously observed in the lower half of this rib. Sand flat occurred following location #13 and continued up to the dive-end point at 2075m deep.

In total of 16 lava samples were recovered from 13 outcrops and one sample from talus. A push core sample was obtained from the flat at the landing point. This sample may represent sand cover material observed along the dive, and also represent the recent sediment cover of the Hilina bench. Lava samples were all aphyric basalt with very compact structure. Vesiclation is totally poor and porosity could be less than several percent. No large phenocrystic minerals other than tiny olivines were contained. Most of the samples contain cross sections in a pillow lava lobe, because those clasts were collected from pillars surrounded by radial cooling cracks. Interiors of the pillow were massive and compact with small vesicules and the vesicles, become larger towards the pillow surface. Alignment of the vesicle orientations seems to be parallel to the surface of the pillows. Normally, the outer zones one to several centimeters thick are entirely compact, and glass rinds several millimeters thick surround the compact outer region. Surfaces of the glass rinds were generally smooth, but dimples with polygonal chilled clacks were developed.

It is obvious that the exposed outcrops are in place, but the loosely packed features (open radial joints developed between lava clasts in pillows) found in most of the outcops suggests disturbed or deformed nature of the rock masses. This deformation may have been developed during slumping of the mass. According to P. Lipman, none of the lavas found in the eastern upper slope of the Hilina area is lithologically identical to the lavas obtained by this dive. The eastern lavas are more porphylitic and chemically classified as transitional basalt. The petrographic difference between lavas from the two areas leaves the question open whether the western pillow lavas belong to early products of alkali basalt, transitional basalt or tholeiite from Kilauea. To solve the problem, we must await the results of dive observation at Papau Seamount and the results of chemical analyses.

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