Magnetic anisotropy of Cretaceous deep sea sedimentary rocks from the Pacific plate

Bijaksana, Satria (1991) Magnetic anisotropy of Cretaceous deep sea sedimentary rocks from the Pacific plate. Masters thesis, Memorial University of Newfoundland.

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Abstract

Palaeomagnetism and magnetic anisotropy are measured for Cretaceous deep sea sedimentary rocks cored from the Pacific plate by the Deep Sea Drilling Project. The objective is to use magnetic anisotropy to test whether sediment compaction during burial is responsible for the shallowing of remanence inclination reported previously for these rocks. -- The present palaeomagnetic study confirms that except at site 463 remanence inclination is shallower than expected. Careful alternating field demagnetization shows that the inclination shallowing is not likely due to a shallow overprint. -- Anisotropy of anhysteretic susceptibility (AAS) measurements reported in this thesis show that most specimens (except those from site 463) have a high degree of magnetic anisotropy (averaging 15%) with strong foliation in the bedding plane as would be expected from sediment compaction. Correlation found between degree of magnetic anisotropy and density for both claystones and limestones suggests that the anisotropy was enhanced by compaction. For the claystone specimens, the hypothesis of sediment compaction causing inclination shallowing is supported by a correlation found between the inclination shallowing and the AAS degree of anisotropy. No such correlation is found for the limestones perhaps because they magnetized earlier in the compaction process. The claystone results suggest that at mid-palaeolatitudes claystones with AAS degree of anisotropy of about 15% may show a compaction-induced inclination error of 5° leading to an underestimation of palaeolatitude of 4°. -- Both claystones and limestones seem to show an inclination shallowing of about 12°, which is independent of anisotropy and thus is probably not compaction-induced. Perhaps inaccuracies in the apparent polar wander path for the Pacific plate are responsible. -- The axis of minimum anhysteretic susceptibility deviates an average of only 5° (± 3°) from vertical orientation in the DSDP specimens with about 15% anisotropy. Perhaps AAS could be used to locate the bedding plane in massive sediments. -- The magnetic susceptibility of most specimens was too weak for its anisotropy to be reliably measured. However, the susceptibility anisotropy of the stronger specimens did resemble the AAS but was weaker. -- Key words: palaeomagnetism; Pacific plate; Cretaceous; deep-sea sedimentary rocks; remanence inclination shallowing; magnetic anisotropy; sediment compaction.

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