TECTONIC AND STRATIGRAPHIC EVOLUTION OF SHATSKY RISEShatsky Rise is a broad elevation in the west-central Pacific (Fig. 1) with an area of about 7.5 x 105 km2, roughly comparable to the size of California. The rise consists of three prominent highs arranged in a northeasterly trend. The southern high, Shatsky Plateau, is the largest with a length of about 700 km and a width of about 300 km. All previous DSDP and ODP drill sites are located on the southern high (Fig. 2).
The northeasterly trend of the rise lies along the trace of a triple junction (Hilde et al., 1976). The regional magnetic anomalies of the abyssal Pacific seafloor surrounding Shatsky Rise exhibit a nearly orthogonal pattern (Sager et al., 1988; Nakanishi et al., 1992) with the presumed intersection of the anomalies near the crest of the rise. The rise is bracketed by magnetic polarity Zones CM21 and CM10 (Fig. 2). Nakanishi et al. (1989) proposed that Shatsky Rise formed by a magmatic pulse before polarity Chron CM20 in the Tithonian (Late Jurassic) at a hot-spot triple-junction intersection. The subsequent flow of lava ceased at about polarity Chron CM12 in the Valanginian (Early Cretaceous).
Pelagic sedimentation on Shatsky Rise has been more-or-less continuous since at least the Early Cretaceous and shows a moderately stratified section overlying the acoustic basement. Sedimentation appears to have been interrupted by episodic erosional events identified by previous drilling (Sliter and Brown, 1993). Some of the resulting unconformities show up as major reflectors observed in seismic records. Quaternary channeling of the sediment pile is evident on seismic records, and in some places Lower Cretaceous sediments crop out at the seafloor (Sliter et al., 1990). Divergence of deep reflectors suggests that there was more rapid basement subsidence in the Early Cretaceous prior to the deposition of Upper Cretaceous and Cenozoic sediments.
The subsidence history of Shatsky Rise is not well known. Thierstein (1979) back-tracked Sites 305 and 306 (currently at 2903 and 3416 m, respectively) to close to 1000 m paleodepth in the Early Cretaceous. Recently, rudists, corals, and echinoid spines were dredged from atop the southern high at 3000 m (Sager et al., 1999), suggesting paleodepths close to sea level. Detailed studies of benthic foraminiferal assemblages are required to more accurately constrain the subsidence history of Shatsky Rise.
All eight previous DSDP and ODP drill sites on Shatsky Rise were located on the southern rise (Fig. 2). During Leg 6, drilling at Sites 47 and 48 atop the rise reached Maastrichtian cherty chalk and at Sites 49 and 50 on the lower flank, Berriasian cherty chalk was recovered. During Leg 32, 640 m of section was penetrated at Site 305 before drilling was terminated in the early Barremian, whereas drilling at Site 306 reached the Berriasian at 475 m (Fig. 3). Scraps of organic carbon-rich sediments of Aptian age were recovered at Sites 305 and 306 (Larson, Moberly, et al., 1975). Additional fragments of carbonaceous shale from Core 37 at Site 305 are early Cenomanian to early Turonian in age and apparently underlie a major unconformity that extends to the Coniacian. Drilling at Site 577 and, most recently, piston coring at Site 810 terminated in the Maastrichtian. Thus, Shatsky Rise has the stratigraphic targets needed for the recovery of sediments from the desired Cretaceous and Paleogene intervals.
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