Paleomagnetism of Deep Sea Drilling Project Leg 85 Sediments: Neogene Magnetostratigraphy and Tectonic History of the Central Equatorial Pacific

Abstract

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 ò. 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°/m.y. during the last 18 m.y., a rate that agrees with most previous reconstructions of Pacific Plate motion.

Understanding Bulk Sediment Stable Isotope Records in the Eastern Equatorial Pacific, From Seven Million Years Ago to Present Day

“…The map is modified from Reghellin et al (). Backtracked sites location at 7 Ma, and migration paths are from Weinreich and Theyer () (Site 573), Mayer et al () (Site 851) and Pälike et al () (Site U1338). Dashed box delimitates the area of strong wind‐driven equatorial upwelling, situated at ±2° latitude of the equator and west of the Galapagos Islands (Fiedler & Lavin, ).…”

The Magnetostratigraphy of Northwest Pacific Sediments, Deep Sea Drilling Project Leg 86

“…Other potential error sources, such as the depositional mode or a compaction of the sediments and an inappropriately averaged secular field variation, should be negligible in the present case. As in most other recent studies on DSDP materials (Tauxe et al, 1984;Weinreich and Theyer, in press), the method developed by Briden and Ward (1966) and refined by Kono (1980) has been used to estimate true mean inclinations from the paleomagnetic data.…”

“…the specific constraints imposed by this drilling method often reduced the quality and amount of the retrieved sediments and, consequently, the resolution of their magnetostratigraphies. On the other hand, very successful paleomagnetic studies have been completed on HPC materials recovered in the Caribbean (Kent and Spariosu, 1982a), the South Atlantic (Tauxe et al, 1984;Chave, 1984) and the Equatorial Pacific (Kent and Spariosu, 1982b;Weinreich and Theyer, in press). …”

Paleomagnetism of Ocean Drilling Program Leg 101 Sediments: Magnetostratigraphy, Magnetic Diagenesis, and Paleolatitudes

“…1) Although it was expected that the carbonate sediments of the Bahamas might be only weakly magnetic and thus difficult to Paleomagnetically measure and analyze, the promise of exceptional biostratigraphic and stable isotope age constraints on these sediments provided further impetus for this study. Moreover, much of the coring was done using the advanced hydraulic piston corer (APC); numerous recent paleomagnetic studies have shown that sediments recovered with a similar system (HPC) used during DSDP drilling yielded excellent paleomagnetic results (e.g., Spariosu, 1982a, 1982b;Tauxe et al, 1984;Chave, 1984;Weinreich and Theyer, 1985). The objectives of this study were twofold.…”

“…Moreover, much of the coring was done using the advanced hydraulic piston corer (APC); numerous recent paleomagnetic studies have shown that sediments recovered with a similar system (HPC) used during DSDP drilling yielded excellent paleomagnetic results (e.g., Spariosu, 1982a, 1982b;Tauxe et al, 1984;Chave, 1984;Weinreich and Theyer, 1985). The objectives of this study were twofold.…”