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SITE 1206

Principal Results

The last and most southern drilling site occupied during Leg 197, Site 1206, was located on the southeastern side of the lower summit terrace of Koko Seamount. Koko Seamount is a flat-topped seamount or guyot with a crowning ornamentation of small reefal bodies. It rises from the abyssal floor (˜5000 m) of the western subtropical Pacific at 35.3°N, thus just north of the "bend" (˜32°N) in the ˜5000-km-long Hawaiian-Emperor Seamount chain. The seamount was named by Thomas Davies and colleagues in 1972 for the 58th emperor of Japan (Davies et al., 1972).

A short seismic survey was conducted to locate a suitable structural and stratigraphic setting for Site 1206, which was initially targeted for the vicinity of DSDP Site 308 drilled in 1973 during Leg 32 (Larson, Moberly, et al., 1975). Weather conditions prevented Leg 32 from reaching basement during drilling at this site, which penetrated to a depth of 68.5 m in Koko Seamount's sediment cover. In consideration of the short time remaining in Leg 197 to conduct basement coring at Koko Seamount, a location was sought where a thin sedimentary section covered its main shield-building volcanic mass. Site 1206, at a water depth of ˜1545 m, was located ˜6.2 km south of Site 308 in an area where the surficial section of acoustically layered material, presumably all or mostly sedimentary beds, was less than one-half that recorded at Site 308 (Fig. F42). At Site 1206 (34°55.55'N; 172°8.75'E), basement was reached at a subsurface depth of 57 m. Coring continued to a depth of 335 mbsf, or 278 m into basement.

The top 57 m of sediment was washed with the core barrel in place. Recovered debris in the wash core included fragments of fossiliferous calcarenite and calcareous mudstone and siltstone indicative of shallow-water depositional settings. The lower part of the wash core recovered a 15- to 20-cm-long section of laminated, shell debris–bearing mudstone containing a nannofossil assemblage typical of Zones NP14 and NP15, of the early to middle Eocene (43.5–49.7 Ma). This age is similar to a radiometric analysis reported for a dredged rock (48.1 Ma) (Clague and Dalrymple, 1973) from Koko Seamount. Although volcaniclastic beds commonly rich in shell fragments were recovered from Hole 1206A in the underlying sequence of volcanic basement rock, none of these deposits contained identifiable nannofossils.

Lava flows in Hole 1206A dominated in number over volcaniclastic beds and thin lenses of calcarenite (Fig. F43). Many lavas are pahoehoe flows interbedded with subordinate a'a units that show evidence of subaerial extrusion. A large degree of scatter with depth was recorded in bulk density, grain density, and porosity of these basement rocks. Although P-wave velocity varies widely (2215–4820 m/s) with depth, velocity correlates strongly with bulk density, and thus inversely with the degree of vesicularity. The bulk of the basalt flows are aphyric to olivine-phyric lava (Fig. F44) and tholeiitic or transitional to alkalic in composition. With respect to major and trace element geochemistry, the basaltic lavas from Koko Seamount resemble those drilled during DSDP Leg 55 at Suiko Seamount (Figs. F45, F46).

All the basalt flows recovered at Site 1206 are slightly altered and show a homogeneous assemblage of Fe oxyhydroxide, clay (brown clay, saponite, aliettite, and celadonite), carbonate, and zeolite. Unaltered olivine was characteristic of many of tholeiitic lava flows. The occurrence of aliettite (alternating talc and saponite-layered smectite), which expands in contact with water, caused the mechanical disintegration of some massive basalt intervals. Possibly, this phenomenon led to the onset of unstable hole conditions that, with respect to probable tool loss, made logging too risky to conduct.

Low-field magnetic susceptibilities, Koenigsberger ratios, and high median destructive field values (Figs. F47, F48, F49) suggested that the lava flow units from Hole 1206A carry a stable remanent magnetization and are suitable for the determination of preliminary paleomagnetic inclinations. Three early Tertiary polarity chrons were recognized within the lava flow units. Geomagnetic polarity reversals were not found at other Leg 197 sites. At Koko Seamount, 14 independent paleomagnetic inclination groups were identified, yielding a mean inclination of 38.5° (+8.4°/–10.9°; 95% confidence interval) (Fig. F50). The preliminary mean inclination of the lava flow units suggests a paleolatitude of 21.7° (+6.4°/–7.0°) for Koko Seamount.

The paleolatitude result gained at Koko Seamount strengthens confidence in the correctness of the implications of the paleomagnetically determined latitude of formation for Nintoku, Suiko, and Detroit Seamounts. These volcanic edifices, each located progressively north of Koko Seamount, also formed paleomagnetically progressively farther north of the present location of the Hawaii hotspot (˜19°N). Thus, singularly and as a linear group, the latitudes of formation of these four Emperor Seamounts establish that to reach its present position, the Hawaii hotspot moved rapidly southward during the Late Cretaceous and early Tertiary (i.e., from ˜81 to 43 Ma).

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