A.C. Demagnetization of Unstable Triassic Keuper Marls from S.W. England

Abstract

Summary In 1953, 329 specimen disks taken from 35 rock samples covering a stratigraphical thickness of 170 ft of Keuper Marl (Triassic) were collected from Sidmouth, Devonshire, and the results were described by Creer (1955, 1957a) in which an analysis of the resultant directions of natural remanent magnetization indicated the presence of three separate components, there called ‘?rimary”, “secondary” and “temporary”. Re-measurement of 65 of these rock disks from 26 of the 35 rock samples after storage in field free space for 3 years indicates that the temporary component of magnetization has decayed away. In further experiments, the same 65 rock disks were demagnetized in alternating magnetic fields of peak value up to I 200 oersteds. The directions of magnetization which were originally strung out along the great circle between the primary and secondary directions progressively coalesce into two opposed groups of magnetization as the intensity of the demagnetizing field is increased. It appears that in many cases the secondary magnetization has been almost completely destroyed and in the remainder of cases considerably reduced in comparison with the primary. The method of a.c. demagnetization is described.