Paleomagnetic secular field variation and rock magnetism of some early holocene (<9900BP) postglacial lacustrine sediments near Thunder Bay, Northwestern Ontario, Canada

Abstract

Paleosecular magnetic changes through a 1.5m section of post-glacial lacustrine deposits are strongly influenced by mineralogy and differential compaction. The sediments chiefly comprise clay and, in the lower one third part of the section, rhythmites which vary from ~2mm to ~Scm in thickness. 125 paleomagnetic specimens were collected in total from the 1.5m section. Anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization identify a magnetic fabric with maximum susceptibility parallel to the bedding plane of the sediments and with a preferred northwest-southeast axis, probably indicating paleo-current alignment. Minimum susceptibility represents the pole to bedding, due to grain alignment. Incremental acquisition and demagnetization of isothermal and anhysteretic magnetizations and the orthogonal three-axis test indicate that the sediments contain two magnetic phases with different coercivities. Magnetic hysteresis measurements (clays n=226; silts n=37) show that clay is dominated by single domain magnetite and hematite (means of M5=58.47±9.22Am2 , Mrs=17.12±27.22Am2 , Hc=21.09±7.69mT, Hcr=62.04±4.09mT) whereas silt is dominated by pseudo-single domain and single domain magnetite (means of M5=681.0±395.9Am2 , Mrs=163.2±84.75Am2 , Hc=26.07±2.94mT, Hcr=56.08±3.17mT). The silt is dominated by magnetite, whereas the clay carries both hematite and magnetite.