Paleomagnetism at DSDP Leg 85 Holes

On Leg 85, 16 holes were cored at five sites. Thirteen of the holes were cored with the hydraulic piston corer (HPC) or the variable-length hydraulic piston corer (VLHPC) or both; the remainder were rotary drilled. Partially duplicating stratigraphic sections were successfully recovered by hydraulic piston coring at Sites 572 to 575. Sub-bottom penetration was deepest (about 210 m) at HPC Hole 575A, which bottomed in lower Miocene sediments. Penetration by hydraulic piston coring was limited at all sites not by the failure of the corer to stroke out but by the excessive force (overpull) necessary to retrieve the core barrel from the hole. The sediments recovered are relatively uniform siliceous-calcareous oozes to calcareous ooze-chalks. Paleomagnetic measurements were made at all stratigraphic levels, but magnetostratigraphic sequences could be resolved only for the Pleistocene-Pliocene and for brief upper, middle, and lower Miocene sections. In the younger and less consolidated sediments, the declination often shows large-scale azimuthal rotations downcore. These smooth trends vary from core to core, indicating either rotation between the sediment and the core liner or the rotation of the core barrel during the coring or retrieval process. Thus, azimuthal orientation of the samples was impossible even though a Kuster azimuthal orientation tool was used during the hydraulic piston coring. At all sites, the downhole shift from mainly siliceous to mainly calcareous ooze-chalk coincided with a decrease in NRM intensity of at least one order of magnitude, to 1.0*10**-8 G. Diagenesis is the probable reason for this behavior, although the dilution of magnetic carriers as the result of higher accumulation rates may also be a factor. A tectonic analysis using data from samples with stable remanence indicates a northward plate motion of about 0.3 deg/m.y. during the last 18 m.y., a rate that agrees with most previous reconstructions of Pacific Plate motion.