Effects of Diagenesis on Rock Magnetic Properties of Sediments and Sedimentary Rocks

Abstract

Rock magnetic methods are sensitive indicators of the state of iron in a sediment, and changes in rock magnetic properties can provide information about diagenetic processes in sediments. One means by which this can be done is to compare the alternating field demagnetization of the natural remanent magnetization (NRM) with the alternating field demagnetization of a laboratory induced saturation isothermal remanent magnetization (SIRM). Data collected from various types of sediments and sedimentary rocks that have been studied in the Paleomagnetism Laboratory of the University of California-Davis indicate that unlithified lake sediments of Holocene or Pleistocene age have NRM-SIRM intensity ratios that range from 5 {times} 10{sup {minus}2} to 5 {times} 10{sup {minus}3}, and that on log-log plots, the intensity ratios are concave upward slightly. For Neogene and Late Cretaceous sedimentary rocks, the ratios are closer to 1 {times} 10{sup {minus}3}, and the curvature is considerably less systematic. These results imply that lithification may lead to the destruction or demagnetization of a significant fraction of the magnetic carriers as well as to changes in the grain size distribution of the remaining carriers. On the other hand, chemical alteration of sediments appears to produce NRM:SIRM ratios that are three to five timesmore » higher than those of equivalent unaltered sediments. This result is corroborated by studies of the rock magnetic properties of individual tephra layers that can be found in both altered and unaltered states in large, drained lacustrine basins.« less