Site 1204 was the second Leg 197 site aimed at determining whether the Emperor Seamounts formed above a fixed or moving hotspot source that is presently located beneath Hawaii at ~19°N. (For more background on the hotspot hypothesis and the paleomagnetic test that is the primary objective of Leg 197, readers are directed to the "Leg 197 Summary" chapter and "Background and Scientific Objectives" in the "Site 1203" chapter). Site 1204 was targeted to be close to Site 883, drilled during Leg 145 in the summit region of Detroit Seamount (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). The recovered section from two holes drilled at Site 883 guided our site selection. Specifically, biostratigraphic analyses of claystone recovered above basement in Hole 883E constrain the basement to be in the same Late Cretaceous stage (i.e., Campanian) as that cored at Detroit Seamount Sites 884 and 1203 (Fig. F1). Therefore, several aspects of the hotspot test could be addressed by the recovery of a deeper basement section near Site 883.
Specifically, we sought to confirm and further refine paleomagnetic constraints available from Site 884, on the eastern flank of Detroit Seamount, by obtaining paleomagnetic data from a nearby summit site (Fig. F1). Prior paleomagnetic analyses of 81-Ma basalt (Keller et al., 1995) recovered at Site 884 indicate a paleolatitude of 36.2° +6.9°/-7.2° (95% confidence interval), suggesting considerable southward motion of the Hawaiian hotspot source since the Late Cretaceous (Tarduno and Cottrell, 1997; Cottrell and Tarduno, in press).
Drilling of two basement holes at Site 883 resulted in 37.8 and 26.7 m of penetration (in Holes 883E and 883F, respectively). Analyses of basalt samples from these holes indicate paleomagnetic inclination values compatible with those recorded at Site 884 (Cottrell and Tarduno, 2001). But the characteristic remanence directions from the lava flows recovered at Site 883 do not appear to sample the full range of expected secular variation of the geomagnetic field. Accordingly, we planned to acquire additional lava flows erupted over a time longer than those recovered in Holes 883E and 883F. The available biostratigraphic ages suggest that it may be possible to combine paleomagnetic data from several of the basement sites on Detroit Seamount to derive the high-resolution, time-averaged determination of paleolatitude needed for the hotspot test.
Drilling at Site 1204 was also intended to provide additional data for evaluating the geochemical evolution of the Hawaiian plume. Prior analyses of basalt samples from Site 883 have indicated a transitional tholeiitic to alkalic composition (Keller et al., 1995), but alteration in the rocks recovered limits their use for radiometric and some geochemical studies. A deeper sampling of less altered rocks could provide material useful for further insight into the possibility that the plume source changed composition with time (M. Regelous et al., unpubl. data) and whether mantle upwelling beneath a spreading ridge interacted with the hotspot source (Keller et al., 2000) during the Campanian.
Site 1204 was chosen along a preexisting analog seismic reflection line (Lonsdale et al., 1993) and two digital seismic reflection profiles collected by the JOIDES Resolution en route to the site (see "Operations" and "Underway Geophysics"). These surveys revealed a flat, cleanly imaged basement surface at Site 883 beneath ~850 m of sedimentary beds that are mainly composed of the Oligocene and younger Meiji drift deposits (Rea et al., 1995). Site 1204 was positioned ~0.5 km from Site 883 to avoid drilling pipe left at Site 883 during Leg 145 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). Coring at Site 1204 was planned to begin in the lower sedimentary section to both characterize the depositional environment of the basement-sediment transition and to determine the age of the transition using micropaleontological data.
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Ms 197IR-104