5. Site 12051

Shipboard Scientific Party2

BACKGROUND AND SCIENTIFIC OBJECTIVES

Nintoku Seamount occupies a central position in the Emperor Seamount chain of volcanoes and an important point in the paleolatitude history of the Hawaiian hotspot (see the "Leg 197 Summary" chapter). The structure of the seamount is that of an elongate volcanic edifice, aligned north-northwest along the Emperor trend, with two prominent ridges trending southwest and south-southwest as far as 100 km from the main center (Fig. F1). Undoubtedly, Nintoku Seamount is a plexus of coalesced volcanoes (Greene et al., 1980), much like many of the larger seamounts in this chain, whose positions are controlled to some degree by the fabric of the underlying ocean lithosphere. The Nintoku system is, however, clearly isolated from Yomei Seamount, ~100 km to the north, and Jingu Seamount, ~200 km to south, by abyssal depths.

In seismic reflection profile, the main edifice of Nintoku Seamount rises steeply from >5000 m as predominantly unsedimented volcanic slopes to a thinly sedimented (10-200 m thick), gently domed summit region between ~1400 and 1200 m that covers ~3400 km2 (Greene et al., 1980). From analysis of seismic reflection survey data (Dalrymple et al., 1980a) and core material recovered by drilling at Site 432 (Jackson, Koizumi, et al., 1980), the shipboard party of Deep Sea Drilling Project (DSDP) Leg 55 proposed that Nintoku Seamount was in an intermediate atoll stage (no lagoon but fringing reefs and banks and extensive carbonate bank interiors) before subsidence removed the island below the wave base. It is further speculated that a few small remnant volcanic peaks pierce the shallow-water bank interior deposits.

Nintoku Seamount apparently remained at or above sea level long enough to be almost completely flattened by subaerial erosion and wave action. Reefs were not identified in seismic profiles, but fragmental material of the bryozoan-algal facies was recovered, documenting shallow-water sedimentary conditions (McKenzie et al., 1980). Importantly, the sedimentary record suggests deposition in waters cooler than the present tropical reef belt (20 latitude). Shallow-water carbonate sedimentation ceased in Paleogene time (Jackson, Koizumi, et al., 1980).

Site 432 was located on the northwestern edge of the summit region of Nintoku Seamount in a gently sloping area mapped as terrace deposits. Although the sediment thickness was predicted to be 80 m from the site surveys, drilling intersected basement at only 42 meters below seafloor (mbsf). Poorly recovered sedimentary deposits are shallow-water biogenic carbonate beds typical of proximal depositional terraces flanking reefs and banks and fossiliferous volcanic sand (Jackson, Koizumi, et al., 1980). Drilling at Site 432 penetrated 32 m of volcanic basement (74 mbsf total) before terminating because of hole caving and damage to the bottom-hole assembly (BHA).

Site 1205 (4120.00N, 17022.70E) was located 100 m southwest of DSDP Site 432, in 1310 m water depth, where previous drilling had reached basement beneath Paleocene lagoonal deposits. We elected to return to Site 432 for a number of reasons. First, previous drilling at Site 432 had intersected reasonably unaltered basalt with good remanent magnetic properties (Kono, 1980), but insufficient lava flows were sampled to adequately determine paleolatitude. Hence, deeper drilling promised to achieve this goal, potentially providing a time-averaged paleolatitude for the Leg 197 hotspot motion test. Second, a short presite survey around Site 432 (see "Underway Geophysics") revealed a simple basement structure suitable for deeper drilling; nearby sites were found to have thinner apparent sediment cover. Third, the composition of previously recovered lava flows (alkali basalt to hawaiite) is most consistent with eruption during a postshield phase of volcano construction. Our petrochemical objectives are best met by recovery of tholeiitic lavas, which we hoped lay beneath the known section.

Another aspect of Nintoku Seamount's character deserves attention: the age of the sampled volcanic rocks in the overall age progression along the Emperor Seamounts volcanic lineament. With reference to Figure F1 in the "Leg 197 Summary" chapter, it is apparent that the reported radiometric age for alkali basalt flows from Site 432 (55-56 Ma) (Dalrymple et al., 1980b) is younger than expected if the Emperor Seamounts form a linear age progression (Clague and Dalrymple, 1989). This is consistent with the postshield classification of Nintoku Seamount capping lavas. Deeper drilling, we predicted, would penetrate erosional surfaces and sample the older, main sequence of shield-building lava flows.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.

2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 197IR-105

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