August 27, 2001

This site summary was received on 24 August from Drs. Robert Duncan and John Tarduno, Co-Chief Scientists aboard JOIDES Resolution, Leg 197.

Shipboard Scientific Party, Leg 197

SITE SUMMARY, SITE 1206

The final and most southern drilling site occupied by Leg 197, Site 1206, was located on the southeastern side of the lower summit terrace of Koko seamount. Koko 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 deg. N, thus just north of the "bend"(~32 deg. N) in the ~5000-km- long Hawaiian-Emperor seamount chain. The seamount was named by Thom Davies and colleagues in 1972 for the 58th Emperor of Japan. Drilling at Koko was intended to complete the systematic testing of the widely accepted notion that each of the Emperor volcanoes had been constructed above the Hawaii hotspot at a stationary location (~19 deg. N).

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. Weather conditions prevented drilling from reaching basement at this site, but limited penetration to a depth of 68.5 meters provided information on Koko's sediment cover. Site 1206, at a water depth of ~1545 m, was located approximately 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. At Site 1206 (34 deg 55.55'N; 172 deg 8.75' E), basement was reached at a depth of 57 mbsf. Coring continued to a depth of 335 mbsf, or 278 m into basement.

The top 57 meters of sediment was washed with the core barrel in place. Recovered debris in the wash core included pieces of fossiliferous calcarenite and calcareous mudstone, siltstone, and lapilli breccia 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 Zone NP14-15, of the early to middle Eocene (43.5-49.7 Ma). This age is similar to a radiometric age for a lava (48.1 Ma) dredged from Koko seamount. Although volcaniclastic beds commonly rich in shell fragments were recovered from Site 1206 in the underlying sequence of volcanic basement rock, none of these deposits contained identifiable nannofossils.

Lava flows at Hole 1206A dominated in number over volcaniclastic beds and thin lenses of calcarenite. Many lavas are pahoehoe flows interbedded with subordinate a'a units that show evidence of subaerial eruption near a subsiding shoreline. A large degree of variability with depth was recorded in bulk density, grain density, and porosity of these basement rocks. Although P-wave velocity varies widely (2215 to 4820 m/s), velocity correlates strongly with bulk density, and thus inversely with the degree of vesicularity. The bulk of the flows are aphyric to olivine-phyric basalt of tholeiitic composition. A few alkali basalt lava flows were identified. With respect to major and trace element geochemistry, the basaltic lavas from Koko resemble those drilled during DSDP Leg 55 at Suiko seamount (45 deg N).

All the lava flows recovered at Site 1206 are slightly altered and show a similar assemblage of Fe-oxyhydroxide, clay (brown clay, saponite, aliettite, celadonite), carbonate and zeolite. Unaltered olivine was found in many of tholeiitic lava flows. The occurrence of aliettite (a mixed-layered clay containing smectite and talc), which expands in contact with water, caused the mechanical disintegration of some massive basalt intervals. Possibly, this phenomena 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 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, fourteen independent paleomagnetic inclination groups were identified, yielding a mean inclination of 38.5o (+8.4o/-10.9o; 95% confidence interval). The preliminary mean inclination of the lava flow units suggests a paleolatitude of 21.7o (+6.4o/-7.0o) for Koko seamount.

The paleolatitude result gained at Koko seamount confirms the implications of previously determined paleolatitudes for the 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 deg. N). This consistent trend and overall precision of paleolatitude data from the Emperor seamounts establish that the Hawaii hotspot moved rapidly southward during the Late Cretaceous and early Tertiary, (i.e., from ~81 to 43 Ma) to reach its present position.