Site 1205, the third site to be occupied during Leg 197, was targeted at DSDP Site 432 (Leg 55) at the northwestern edge of Nintoku Seamount, a guyot or flat-topped volcanic complex in the central sector of the Emperor chain. Nintoku Seamount, at ˜41°N, is positioned approximately two-thirds the distance southward along the line of north-northeast–south-southeast–trending Emperor volcanoes extending from Meiji Seamount (˜53°N) in the north to Kammu Seamount (˜32°N) at the chain's southern terminus near the Hawaiian-Emperor bend. Like many of the Emperor Seamounts, Nintoku was named by Robert Dietz in 1954 after the 16th emperor of Japan.
To provide acoustic images of the stratigraphic and structural setting of the proposed site, a short underway geophysical survey was conducted. Hole 1205A was spudded ˜100 m southwest of Site 432 over what appeared to be a broad sediment-filled (˜70 m) swale in the surface of the main volcanic shield of Nintoku Seamount (Fig. F32). Coring sampled the entire sedimentary section before encountering basement at 42 mbsf, a depth similar to that reported at Site 432. Further penetration showed that the "sediment fill" was largely a stack of lava flows (˜95%) with interbedded soil horizons. Coring continued to a final depth of 326 mbsf.
Five sediment cores (only 2%–16% recovery) established that Nintoku Seamount's sediment carapace consists of sandstone and siltstone containing well-rounded to subrounded basalt clasts (Fig. F33), volcanic ash, and fossil fragments of molluscs, benthic foraminifers, bryozoans, and coralline red algae. These observations document a relatively shallow-water, high-energy depositional environment.
Drilling in Hole 1205A penetrated 283 m into the volcanic basement of Nintoku Seamount and recovered parts of at least 25 different lava flow units (Fig. F34). Little systematic variation with depth was recorded in average P-wave velocity, bulk density, grain density, and porosity, except for interbedded low-density, high-porosity soil horizons. It is presumed that these low-velocity interbeds are the underlying cause for the acoustically recorded layering in the upper 200–230 m of basement rock, below which the occurrence of soil horizons diminishes.
The age of the youngest volcanic rocks in Hole 1205A is constrained by nannofossils (Zone NP10) in the sediment immediately overlying basement to be older than 53.6–54.7 Ma, an age range that is just younger than a radiometric age of 56.2 basalt from nearby Hole 432A. The thickness and vesicularity of the flows from Site 1205, the presence of oxidized flow tops and soil horizons, together with the lack of pillow structure indicate that the flows erupted subaerially. They range from aphyric to highly plagioclase and olivine-phyric basalt. At 230–255 mbsf, two flows of tholeiitic basalt are intercalated within alkalic basalt flows. Above these flows the degree of alkalinity tends to increase upsection. Intraflow soil horizons are also most common in this interval, suggesting that eruption rates may have been lower during this period. Internal flow gradients within very thick lava flows have produced aligned groundmass crystals (Fig. F35) that help delineate flow structure. Clasts of hawaiite recovered in the basal conglomerate of the sedimentary section were not found as flow units within the underlying basement sequence.
Lavas from Nintoku Seamount have similar major element compositions to lavas erupted during the postshield stage of Hawaiian volcanoes such as Mauna Kea (Fig. F36). Slight differences in trace element composition between lavas from Nintoku Seamount and active Hawaiian volcanoes probably result from differences in source composition or variations in the degree of mantle melting (Fig. F37).
All the lava flows recovered at Site 1205 are only slightly altered except thin, highly weathered flow tops. The low-temperature (30°–60°C) alteration assemblage is homogeneous downhole and is composed of Fe oxyhydroxide, saponite and/or nontronite, celadonite, and zeolite. Veining is sparse, indicating small-scale fluid circulation. In contrast to the first two sites (1203 and 1204) drilled during Leg 197 at Detroit Seamount, K2O was not mobilized during alteration event(s) at Nintoku Seamount Site 1205.
Rock magnetic data (low-field magnetic susceptibilities, Koenigsberger ratios, and median destructive field values) obtained from oriented minicores suggest that the lava flows from Site 1205 carry a remanent magnetization suitable for paleolatitude analysis (Figs. F38, F39). Although the demagnetization characteristics of some samples indicate the need for thermal demagnetization studies, most samples yielded data suitable to make a preliminary determination of magnetic inclinations (Fig. F40).
Twenty-two independent paleomagnetic inclination groups were identified, yielding a mean inclination of –45.7° (+10.5°/–6.3°; 95% confidence interval) (Fig. F41). The preliminary mean inclination suggests a latitude of formation of an early Eocene Nintoku Seamount at 27.1° (+5.5°/–7.7°). This value, together with paleolatitudes from paleomagnetic analyses of basement rocks at DSDP Site 433 (Kono, 1980), Site 884 (Tarduno and Cottrell, 1997), and Sites 1203 and 1204 (Leg 197), form a consistent data set indicating southward motion of the Hawaiian hotspot from Late Cretaceous to early Tertiary time.