Recent studies have shown that the collapse of large volcanoes as a result of gravitational instability plays an important role in shaping volcanic environments. Detailed offshore bathymetric surveys of the Hawaiian Ridge (Moore et al., 1994), Reunion Island (Lenat et al., 1989), and the Canary Islands (Masson et al., 2002; Carracedo, 1999) reveal extremely large landslides. In the case of Hawaii, one of the Nuuanu Landslides caused by the collapse of Koolau Volcano on Oahu extends to 200 km from the island. In the Canary Islands, the debris extends to 30 km. Landslides on both island chains might have generated huge tsunamis (Moore, 1964; Moore and Moore, 1988; Moore et al., 1989). Herrero-Bervera et al. (2002) estimated the age of Nuuanu Landslides at 2.1-1.8 Ma. However, the size, the age, and the number of Nuuanu Landslides are still in question. Site 1223 is ~300 km from Oahu, ~100 km to the northeast of the presently defined Nuuanu Wailau debris field (Fig. F1). The objective of drilling at Site 1223 is to address the above questions.
A number of important results were obtained for Hole 1223A. One of the objectives of coring at this site was to determine if the Nuuanu Landslide occurred as a single or as a multistage event as indicated by the number of turbidites recovered. Several unconsolidated volcaniclastic turbidites of varying thickness were recovered in the first two cores from Hole 1223A. Eight of these were >10 cm thick at 1, 3, and 4 meters below seafloor (mbsf) for Unit 1, and 5.2, 6, 6.9, 7.3, and 7.9 mbsf for Unit 2. Many were <1 cm thick. Paleomagnetic data indicate that all but the uppermost turbidite have an age between 1.77 and 1.95 Ma. The top turbidite has an estimated age between 0.78 and 1.77 Ma.
A surprising discovery was the recovery of the two crystal vitric tuff layers. Preliminary geochemical analyses indicate these tuffs are geochemically similar to Hawaiian tholeiitic basalts. The olivines in the vitric tuff are fresh. Kink banding and fibrous structures were observed in some olivines. The fibrous structure may have been caused by crystallization of hematite. The kink banding and fibrous structures indicate that the source for these olivines may be mantle derived under shear stress.
Another important result is the identification of wairakite in the vitric tuff. Wairakite is stable at a temperature range from 200° to 300°C. This suggests that considerable heat was involved when the crystal vitric tuff was deposited. Tentative interpretations for the origin of crystal vitric tuffs are given in "Interpretation of Tuffs" in "Lithology."
The water depth at Site 1223 was 4245.8 m. We took two advanced piston corer (APC) and four extended core barrel (XCB) cores. We cored 41 m and recovered 23.54 m of core (57.4% recovery), with 12.7 m cored and 10.87 m recovered (85.6% recovery) with the APC and 28.3 m cored and 12.67 m recovered (44.8% recovery) with the XCB. After two APC cores, we switched to XCB coring. The use of the XCB system at these shallow depths and the long time needed for coring was unexpected as was the presence of lithified volcanic rocks. Core 200-1223A-5X was advanced only 1.0 m when it was recovered because of indications of jamming. Core 200-1223A-6X was advanced 8.0 m to a depth of 41.0 mbsf when it was recovered because of the time on site expiring.
We have identified 14 distinct lithologic units (Fig. F2). Unit 1 (0-5.11 mbsf) contains yellowish brown clay and volcaniclastic turbidites. Unit 2 (5.11-7.32 mbsf) consists of volcaniclastic turbidites only. Unit 3 (7.32-7.90 mbsf) is a thin layer of dark-brown clay. Unit 4 (7.90-10.78 mbsf) is unconsolidated black sand. Unit 5 (12.70-15.06 mbsf) is a crystal vitric tuff. Unit 6 (15.06-15.29 mbsf) is bioturbated claystone. Unit 7 (15.29-16.10 mbsf) is volcaniclastic sandy siltstone. Unit 8 (22.30-22.80 mbsf) is volcaniclastic silty claystone with carbonate granules. Unit 9 (22.80-22.91 mbsf) is volcaniclastic claystone. Unit 10 (22.91-24.92 mbsf) is volcaniclastic silty claystone. Unit 11 is vitric tuff. Subunit 11A (32.00-33.00 mbsf) is altered vitric tuff, highly disturbed by drilling. Subunit 11B (33.00-36.99 mbsf) is palagonitized crystal vitric tuff. Unit 12 (36.99-37.47) is volcaniclastic silty claystone with anhydrite vugs. Unit 13 (37.47-38.31 mbsf) is volcaniclastic silty claystone. Unit 14 (38.31-38.70 mbsf) is volcaniclastic clayey siltstone. Coring gaps of several meters exist between some of the cores, so additional units may exist or those identified may be thicker by several meters.
Volcanic material is present throughout the stratigraphic column. Although we identified 14 distinct lithologic units, they may be grouped into three main types of lithologies: (1) unconsolidated clay and volcanic sediments, (2) weakly consolidated claystones and siltstones, and (3) crystal vitric tuffs. The turbidites are concentrated in the upper 12.7 m of the core. The tuffs have a volcanic fraction that ranges from 45% to 100%. The main constituents are (in order of increasing abundance) glassy shards, vitric fragments, olivine phenocrysts and clasts, plagioclase, palagonitized glass, lithic fragments, and clinopyroxene clasts. A MgO-rich olivine and Ca-rich plagioclase composition indicates equilibrium with mafic magmas. The mudstones and siltstones are found both in the middle and the bottom of the sections cored; they have high contents of clay, indicating detrital sources, and they have a low but variable volcanic fraction (~1%-25%). The silty claystone at the bottom of the palagonitized crystal vitric tuff (Subunit 11B) is characterized by the presence of relatively large (up to 3.5 mm) vugs filled by anhydrite, a mineral associated with hydrothermally altered basic rocks. The vitric tuffs were found just below the sand and at the top and in the middle of the mudstones and siltstones cored. The "nonvolcanic" fraction is composed of claystone clasts, micritic clasts, and, more rarely, radiolarians (generally <1%).
Whole-rock inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses were conducted on the two vitric tuffs, as well as several of the siltstones and claystones. The tuff's major components are SiO2 (47.6-49.6 wt%), TiO2 (~2 wt%), Al2O3 (11.3-11.9 wt%), Fe2O3 (11.3-12.7 wt%), MgO (12.4-15.8 wt%), CaO (6.74-7.17 wt%), Na2O (2.19-3.07 wt%), K2O (0.47-0.83 wt%), and P2O3 (0.16-0.23 wt%). Its trace elements are Ba (50-70 ppm), Sr (22-330 ppm), Y (~20 ppm), Zr (~120 ppm), and Ni (~430-580 ppm). The high MgO concentrations are not surprising because of the high percentage of olivine present. Siltstones and claystones show similar values as those from the above tuffs. The geochemistries of whole-rock crystal vitric tuff, siltstone, and claystone were compared with the basalt glass geochemistry of mid-ocean-ridge basalt (MORB), Kilauea tholeiitic basalt, Haleakala alkali basalt, North Arch alkali basalt, and Koolau tholeiitic basalt. The crystal vitric tuff, siltstones, and claystones have the most in common, geochemically, with the Hawaiian tholeiitic lavas. There are some ambiguities in the measurements and samples, however, because the tuff contains clay minerals that may affect the chemical compositions.
Several of the claystones and siltstones contain effervescing white vugs. X-ray diffraction (XRD) analysis of the filling from Sample 200-1223A-6X-4, 0-20 cm, gave a complex spectrum. The majority of the material in the vugs is anhydrite and a Ca sulfate. Additional components include paragonite, wairakite, and analcime, with pumpellyite of varying compositions as a minor component. Grains from two intervals in the turbidite were analyzed by XRD (200-1223A-1H-5, 94-95 cm and 114-115 cm). The upper interval has brown grains with dominant XRD peaks at wavelengths consistent with phillipsite. In addition, some of the smaller peaks have spectra consistent with clay minerals, mainly smectite and illite and minor plagioclase. Therefore, the composition of the material analyzed is mainly phillipsite with clay minerals and minor plagioclase. The lower interval has white granules that give sharp XRD peaks at wavelengths consistent with calcite and some lower-amplitude peaks interpreted to be plagioclase.
Wairakite was originally found in hot springs in the geothermal fields of Wairakei in New Zealand and of Onikobe in Japan (Miyashiro, 1973). It has also been found in hydrothermal areas in the Mariana Trough (Natland and Hekinian, 1982). Wairakite is stable at a temperature range from 200° to 300°C. This suggests that considerable heat was involved when the crystal vitric tuff was deposited sometime thereafter.
Some of the olivine clasts in the tuffaceous layers show kink banding. Many mantle rocks such as lherzolites and dunites show kink banding in olivine crystals. Also, tectonized peridotites often show such microstructures (e.g., Ishii et al., 1992). The kink bands are formed where shear is applied to the olivine crystal as shown by Kirby's deformation experiment (Kirby, 1983). Dislocations by shear stress in the olivine crystals generate the kink bands. Another interesting feature is the mass of fibrous lines identified in the olivine crystals. Iron oxide crystallization may be the cause of this structure. The presence of kink bands and fibrous structures indicates that olivine crystals in the crystal vitric tuffs may have been subjected to tectonic deformation.
We measured bulk density using gamma ray attenuation (GRA), magnetic susceptibility, natural gamma ray (NGR), and compressional velocity (VP) on whole-core sections with the multisensor track (MST). Moisture and density and VP were also measured on selected individual samples. The yellowish brown clay in Core 200-1223A-1H has 83% porosity and a VP of 1.5 km/s. GRA density values from 0 to 10 mbsf gradually increase downhole from 1.2 to 2.2 g/cm3. The GRA densities agree with the bulk densities of the individual measurements, except for Core 200-1223A-3X, where GRA densities of the vitric tuff average 1.8 g/cm3 but bulk densities are slightly higher. Corresponding VP values for the vitric tuff in Core 200-1223A-3X average ~3.3 km/s. Bulk density, grain density, and VP for the siltstone in Core 200-1223A-4X are 1.6-2.0 g/cm3, 2.8 g/cm3, and 1.8-3.3 km/s, respectively. The vitric tuff in Core 200-1223A-6X has a 2.1-g/cm3 bulk density, a 2.6-g/cm3 grain density, and a 4.0-km/s VP . Grain densities of vitric tuff in Core 200-1223A-6X are lower than those in Core 3X because of the higher levels of alteration in Core 6X. The VP of the crystal vitric tuffs in Unit 5 (just below sand) and Unit 11 are ~3 and ~4 km/s, respectively. However, the bulk densities for both are similar, at ~2.2 g/cm3. The VP for the upper crystal vitric tuff deviates from the generally expected compressional velocity-density relationship (e.g., Johnston and Christensen, 1997), possibly because of the weak consolidation of Unit 5. VP and density increase with depth within the turbidites of Units 1 and 2, and the gradients can be associated with the graded bedding in the turbidites. Using this velocity and density increase with depth, we can identify the presence of the turbidite layers.
The magnetostratigraphy for Hole 1223A appears to record all the major chrons and subchrons from Chron C1n (the Brunhes Chron; 0.0-0.780 Ma) through Chron 2r (1.95-2.581 Ma). The Brunhes normal polarity interval spans only the top 14 cm of Core 200-1223A-1H, which is thinner than expected by ~1 m based on prior piston coring in the vicinity. Thus, we may not have recovered the very upper meter or so of the sedimentary section, or sedimentation rates may vary locally. The top and base of the normal polarity interval interpreted as Subchron C1r.1n (Jaramillo Subchron; 0.99-1.07 Ma) are at 0.79 and 1.23 mbsf, respectively. The top and base of the normal polarity interval interpreted as Chron C2n (the Olduvai Chron; 1.77-1.95 Ma) are at 2.02 and ~7 mbsf, respectively. All core recovered below ~7 mbsf appears to be of reversed polarity, which is interpreted to be the upper part of Chron C2r, possibly with the entire interval lying within Subchron C2r.1r (1.95-2.14 Ma).
Microbiological analyses from Site 1223 were conducted on sediments and tuffs. Most probable number (MPN) series were prepared from all samples in order to determine the concentration of sulfate-reducing as well as fermentative bacteria. From Site 1223, twenty-five distinct microbial colonies could be isolated to pure cultures and could be further characterized as facultatively anaerobic organisms forming stable cell aggregates under appropriate conditions.
One of the objectives of coring at this site was to try to determine if the Nuuanu Landslide occurred as a single or as a multistage event by inference from the number of turbidites recovered. Several unconsolidated volcaniclastic turbidites of varying thickness were recovered. Paleomagnetic data indicate that all but the uppermost turbidite have an age between 1.77 and 1.95 Ma; the top turbidite has an estimated age between 0.78 and 1.77 Ma. A surprising discovery was the recovery of the two crystal vitric tuff layers. Preliminary geochemical analyses indicate these tuffs are tholeiitic basalts that have geochemical similarities to Hawaiian tholeiites; olivine accumulations have preferentially enriched the MgO content of the bulk ICP-AES analyses. However, there remain some other possibilities for the sources of crystal vitric tuffs, such as a part of the Hawaiian Arch or a nearby seamount. The genesis of crystal vitric tuffs can be complicated. Questions arise about why they are indurated so close to the seafloor; why they are so glassy; why they are so rich in fresh olivine; why they include kink banding and fibrous structures in the olivine crystals; and why they were warm when emplaced.
Preliminary results are summarized below:
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
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