Chemical remagnetization and clay diagenesis: testing the hypothesis in the Cretaceous sedimentary rocks of northwestern Montana

• Published in: Physics and chemistry of the earth. Parts A/B/C Vol. 27; no. 25; pp. 1131 - 1139
• Main Authors: Gill, J.D, Elmore, R.D, Engel, M.H
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.2002
• Subjects: Authigenesis; Clays; Diagenesis; Magnetite; Remagnetization; USA, Montana, Sweetgrass Arch; USA, Montana, Thrust Belt Province; Remagnetization; Clays; Diagenesis; Magnetite; Authigenesis
• ISSN: 1474-7065; 1873-5193
• DOI: 10.1016/S1474-7065(02)00108-0

Although the migration of fluids is a likely agent of remagnetization for some chemical remanent magnetizations (CRMs), widespread CRMs, which occur in rocks that have not been altered by externally derived fluids, need to explained by another mechanism. We are testing clay diagenesis as a remagnetization mechanism for such CRMs by comparing results from Mesozoic strata in the disturbed belt of Montana where the rocks contain ordered illite/smectite that formed by moderate heating as a result of thrust loading, with equivalent strata on the adjacent Sweetgrass Arch which contain unaltered smectite-rich clay mineral assemblages. The results indicate that the magnetization in the rocks in the Sweetgrass Arch is weak and dominated by a modern viscous component. In contrast, the disturbed belt rocks have higher magnetic intensities and contain a prefolding or early synfolding, reversed tertiary magnetization that is interpreted to be a CRM residing in magnetite and perhaps pyrrhotite. A presence–absence test and the timing of acquisition for the CRM suggest that magnetite authigenesis could be related to the smectite-to-illite conversion.