Origin and Modification of Magnetic Fabric in Fine-Grained Detrital Sediment by Depositional and Post-Depositional Processes

Abstract

Glaciolacustrine varved clay of late Wisconsinan age in western New York has stable remanent magnetization and anisotropic magnetic susceptibility (AMS). The results of rock magnetic tests demonstrate that remanence is carried by interacting single domain grains of magnetite, but coarse multidomain grains of magnetite are also present. As in many fine-grained detrital sediments, remanent inclination is anomalously shallow, given the latitude of the area of deposition and the existence of a geocentric dipole field at the time of deposition. The AMS consists of a foliation that is gently inclined to bedding and a weaker lineation in the plane of foliation. Independence of magnetic fabric and direction of remanence can be demonstrated by comparing remanence and AMS at closely spaced sites in the same beds and also between widely spaced sites of different age. The magnetic fabric suggests that deposition is accompanied by transient density currents which align large multidomain grains; magnetic alignment of single domain grains occurs later in a dilute slurry at rest on the surface of deposition, rather than during settling in still water. Compaction after deposition results in decrease of remanent inclination and increase in magnetic foliation. Remanent declination and magnetic lineation are unaffected by compaction.