THE GEOLOGICAL AND GEOPHYSICAL SETTING NEAR SITE 462 1

DSDP Site 462 is situated in the northern Nauru Basin in the western central Pacific Ocean between the eastern Caroline Islands and the Marshall Islands (Fig. 1). The primary objectives at this site were to sample Mesozoic sediments and the underlying Mesozoic oceanic crust. Previous work (Larson, 1976) has established a magnetic anomaly sequence (Ml6 to M25) in the central region of the Nauru Basin. South of Nauru Island, the magnetic anomalies are indistinct. North of Anomaly M25, in the northern region of the basin, the magnetic anomalies are of low amplitude, but have a distinct lineation parallel to the M16 to M25 sequence to the south (Fig. 2). Reflection profiles in the Nauru Basin show approximately 0.3 to 0.5 seconds of sediments, characterized by numerous reflectors, overlying a highly reflective "basement." Deeper reflectors are sometimes observed, however, on low-frequency reflection profile and variable-angle sonobuoy records (Houtz, 1976; Larson, 1976), suggesting that up to a kilometer of consolidated sediments or flat-lying volcanic rocks may be present beneath the upper 300 to 500 meters of unconsolidated sediments. A further seismic characteristic of the Nauru Basin, mentioned by Larson (1976) and Houtz (1976) and observed on existing unpublished HIG data, is an unusual lack of refracted arrivals observed on sonobuoy stations. Larson (1976) suggested that the lack of refractions may result from a velocity contrast across the limestone/basalt boundary which is too small to propagate refractions but large enough to produce a reflection. Houtz (1976) has suggested that the lack of refractions may be caused by alternating bands of volcanics and sediments, which would make a layer too incoherent to produce a refraction.