6. Site 12061

Shipboard Scientific Party2

BACKGROUND AND SCIENTIFIC OBJECTIVES

Koko Seamount lies at the southern end of the Emperor Seamount chain of volcanoes, ~200 km north of the major inflection ("bend") in the Hawaiian-Emperor lineament. It is also the youngest site in our transect to investigate the paleolatitude history of the Hawaiian hotspot (see the "Leg 197 Summary" chapter). The seamount was named for the 58th emperor of Japan (A.D. 885-887) by Davies et al. (1972), who reported the results of bathymetric and seismic surveys and the contents of two dredge hauls (Thomas Washington's Aries-7 cruise) (see "Underway Geophysics"). The gross structure of the seamount is that of an elongate volcanic edifice, aligned northwest-southeast, oblique to the main trend of the Emperor Seamounts. A prominent south-trending ridge extends ~50 km from the summit area toward Kimmei Seamount (Fig. F1). Undoubtedly, the basement pedestal of Koko Seamount is a plexus of coalesced volcanoes, much like many of the larger seamounts in this chain, including Nintoku Seamount (Site 1205), but Koko Seamount is covered by a thick carbonate cap that obscures definition of distinct volcanic centers (Davies et al., 1972). The complex is, however, clearly isolated by abyssal depths from Ojin Seamount, ~200 km to the north, and Kimmei Seamount, ~100 km to the south (Fig. F1).

In seismic reflection profile, the main edifice of Koko Seamount rises steeply from >5000 m as predominantly unsedimented 7-9 volcanic slopes to a sedimented summit region that climbs gradually from ~1500 m to a broad plateau of ~500 m depth (Greene et al., 1980). The area of the summit plateau is ~5800 km2. Davies et al. (1972) first identified Koko Seamount as a guyot. From analysis of seismic reflection survey data (Dalrymple et al., 1980) and core material recovered during drilling at Deep Sea Drilling Project (DSDP) Sites 308 and 309 (Larson, Moberly, et al., 1975), Greene et al. (1980) proposed that Koko Seamount was a mature carbonate bank or atoll with a nearly circular lagoon and fringing reefs surrounded by depositional terraces. The presence of bryozoans, coral, and ooliths indicates a warm-water carbonate ecosystem (Larson, Moberly, et al., 1975), and the presumed thickness of these deposits (>200 m) suggests that this guyot remained in warm, shallow water longer than any of the other Emperor Seamounts. McKenzie et al. (1980) applied the Schlanger and Konishi (1975) shallow-water carbonate facies model to speculate that in Eocene time Koko Seamount occupied a paleolatitude corresponding to a modern 20 to 22N, at the boundary between massive coral-algal reef complexes and reef complexes mixed with bryozoan-algal biostromes, sand, and bioclastic debris.

The volcanic surface of Koko Seamount apparently remained at or above sea level long enough to be almost completely flattened by subaerial erosion and wave action. Extensive carbonate sedimentation kept up with subsidence of the volcanic complex until the island moved north out of the latitude range of carbonate reef growth. Once rapid carbonate sedimentation ceased as the seamount slipped below the depth of the photosynthetically supported growth, most of the constructional carbonate relief appears to have been unmodified by erosion (Greene et al., 1980). However, high-resolution reflection profiles gathered during Leg 197's presite survey for Site 1206 revealed bed forms typical of drift deposits. Evidently, bottom currents flowing over the submerged Koko Guyot have been active in redistributing particulate carbonate and volcaniclastic debris across its summit plateau. (Copies of the reflection records can be requested from the Ocean Drilling Program (ODP) Data Librarian; see the "Related Leg Data" contents list for contact information.)

DSDP Sites 308 and 309 (Larson, Moberly, et al., 1975) are located on the southeastern and southwestern edges of the summit region of Koko Seamount, respectively, in the gently sloping area mapped as terrace deposits (Greene et al., 1980). Sea conditions forced the termination of drilling at Site 308 after penetration to 68.5 meters below seafloor (mbsf) through lower Eocene calcareous volcaniclastic siltstone and sandstone. At Site 309 drilling terminated at 12 mbsf, owing to loss of a part of the bottom-hole assembly (BHA) in hard carbonate-cemented volcaniclastic sediment.

Site 1206 (3455.55N, 17208.75E) is located 6.2 km south-southwest of DSDP Site 308, in 1540 m water depth. Site 308 had been targeted during DSDP Leg 32 for a paleolatitude test of the motion of the Hawaiian hotspot, but drilling did not reach basement. We elected to return to Site 308 for a number of reasons. First, a clear basement sequence of reflectors was imaged on existing seismic reflection profiles that promised to provide the required shield-building lava flows ideal for paleomagnetic and petrochemical objectives. We planned a short presite survey around Site 308 (see "Underway Geophysics") to examine basement structure near the proposed drill site. Second, the composition of dredged rocks (alkalic basalt, trachyte, and nepheline phonolite) that have been radiometrically dated (whole-rock 40Ar-39Ar) (Clague and Dalrymple, 1973) is most consistent with eruption during a postshield phase of volcano construction. Hence, the measured age, 48.1 0.8 Ma, is likely to underestimate the age of the main sequence of shield-forming lava flows by an unknown amount (see Fig. F1 in the "Leg 197 Summary" chapter). Deeper drilling in the vicinity of Site 308, 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-106

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