The two holes drilled at Site 1204 (Hole 1204A: 51°11.68'N, 167°46.36'E, and Hole 1204B: 51°11.64'N, 167°46.42'E, respectively) are located toward the northern end of the summit platform of Detroit Seamount in international waters 18 km southeast of the Russian Exclusive Economic Zone. Prior to spudding in Hole 1204A, an underway geophysical survey was run to more fully characterize the stratigraphic and structural setting of the site. Crossing high-resolution seismic reflection profiles were collected using a single 80-in3 water gun firing at a rate of 6 s. Basement was cleanly imaged at the site as an upward-terminated sequence of discontinuous reflection events beneath a prominently layered set of reflectors representing sedimentary beds 850 m thick. The bulk of these sedimentary beds are part of the areally extensive Meiji drift sequence of Oligocene and younger age (Rea et al., 1995).
The location for Site 1204 was selected on the basis of two seismic lines digitally recorded during the underway survey and information from previously drilled Leg 145, Site 883, at virtually the same location (Fig. F23). Because lengths of drilling pipe were left behind at Site 883, Hole 1204A was moved 460 m southeast along one of the new seismic lines where a flat basement reflector could be distinguished. After a clogged bit forced us to abandon the first hole, Hole 1204B was spudded 100 m southeast of Hole 1204A.
Coring in Hole 1204A began beneath the Meiji drift sequence at a depth of 762 mbsf, where a relatively condensed sequence of chalk with volcanic ash containing middle Eocene to upper Paleocene nannofossils was recovered. Parts of this section could be correlated to the geomagnetic polarity timescale through shipboard paleomagnetic measurements. The lower Eocene carbonate units contain an interesting zone of sedimentary beds deformed and disrupted by sliding. Similar units are also preserved at Site 883 and at Site 884, which is at the base of the eastern flank of Detroit Seamount (Fig. F24). Ash in the sequence produces elevated magnetic susceptibility values that helped distinguish sedimentary subunits. The chalk beds overlie low-carbonate clay, diamictite, and ash-rich strata containing 71- to 76-Ma Campanian nannofossils (CC22/CC23). Coring began in Hole 1204B at a depth of 810 mbsf in sediment immediately overlying basement containing the same nannofossil assemblage as that found in Hole 1204A. This age suggests that the Site 1204 basement may be roughly coeval with that at Sites 1203 and 883.
The Site 1204 basement consists of basalt that occurs as pahoehoe lava flows constructed of multiple lobes (Fig. F25). The association of pahoehoe lava, carbonate sand, and low-density lapilli and vesicular basalt breccia (Fig. F26) suggests that these flows originated from subaerial vents and that the lava was emplaced in a nearshore environment. The sequence was altered at low temperature within the seafloor weathering and alkali mobility zone. Prominent reduced zones with saponite clay and secondary pyrite are intermixed with more common oxidized zones characterized by Fe oxyhydroxide. Particularly noteworthy are unaltered glassy margins of flow lobes (Figs. F27, F28). The lava flows recovered at Site 1204 have alkali basalt compositions, but with significantly lower abundances of incompatible elements than postshield-stage alkalic basalt lava flows erupted at Hawaiian volcanoes such as Mauna Kea. The abundance and ratios of certain trace elements (e.g., Ti and Zr) in the Site 1204 lavas fall along the trend for Suiko Seamount and most other Detroit Seamount compositions (Fig. F29).
Detailed shipboard paleomagnetic analyses of basalt at Site 1204 (Figs. F30, F31) yielded an average inclination of 58.9° (+5.8°/6.4°; 95% confidence level) and a preliminary paleolatitude estimate of 39.7° (+4.4°/3.7°). The estimated angular dispersion of the data, however, indicates that the full range of geomagnetic secular variation important for obtaining a high-resolution paleolatitude value has not been sampled. Nevertheless, the Site 1204 values are in agreement with our preliminary data collected at Site 1203 and data from Site 884 (a data set known to average secular variation) (Tarduno and Cottrell, 1997). Together, these data suggest that Detroit Seamount formed some 1500 km north of the present latitude of Hawaii.
Next Section | Table of Contents