HR: 0800h
AN: GP11D-0880    [Abstracts]
TI: The Achilles Heel of Red Bed Paleomagnetism
AU: * Larson, E
EM: elarson7@msn.com
AF: Dept of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AU: Walker, T
EM: twalker@colorado.edu
AF: Dept of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AU: Patterson, P
AF: EXXOM Research, 3616 Richmond Ave., Houston, TX 77046 United States
AB: To obtain a paleomagnetic pole position from a group of red bed samples requires the extraction of the single-component `characteristic' remanence (a direction generally considered to be acquired at or within a few hundred years after deposition). The accepted method of extraction of this remanence, which has been in vogue for the last 40 years, involves total thermal demagnetization of the samples in a series of steps. Usually, at some temperature during this procedure, a sample's remanence (as displayed on a vector diagram), will begin to follow a straight-line path to zero intensity. This path direction is identified as that of the `characteristic' remanence of that sample; the temperature of initiation of straight-line decay is designated as the point at which all secondary components have been preferentially removed. The mean 'characteristic' remanence is then obtained from all of the sample `characteristic' directions. The underlying, significant assumption in this technique is that any secondary components reside in grains with lower maximum blocking temperatures than those carrying the `characteristic' remanence. Straight-line decay of remanence to zero, however, can also arise in another, highly plausible, way that has been completely excluded from consideration by red-bed adherents. As a result of long-term remanence acquisition, a sample may contain multiple remanence components that reside in grains of overlapping blocking temperatures. In this case, stepwise thermal demagnetization will produce proportional decay of all components, such that remanence decay will also follow a straight-line path to zero; a `characteristic' remanence component, therefore, cannot be distinguished from the others. During standard red-bed analysis, once straight-line remanence decay begins, the researcher, in reality, loses all ability to distinguish single- from multiple-component behavior. Yet, because of the restriction of the method, he/she will interpret any straight-line remanence decay to be the unique result of single-component, `characteristic' remanence behavior. This fundamental problem in analysis technique primarily will be demonstrated through the study of a fabricated two-component sample and a section of Triassic red beds in Wyoming that previously has been reported, in two separate classic red bed studies, to contain only single-component `characteristic' remanence but that unequivocally can be shown to contain three, greatly divergent major components.
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting