Chemical Remanent Magnetization of Red Beds and Synthetic Hematite

STOKKING, L. B., Texas A&M University, College Station, TX, and L. TAUXE, Scripps Institution of Oceanography, La Jolla, CA

Magnetic minerals produced during diagenesis add one or more components of chemical remanence to the net magnetization of the rock. The magnetic signature of chemically magnetized rocks may thus differ from the orientation of the Earth's magnetic field at the time of sediment deposition. Interpretation of paleomagnetic data obtained from chemically magnetized sediments requires an understanding of the way in which magnetic minerals record the orientation and intensity of the magnetic field applied during their growth and the reliability with which successive generations of these minerals grown in different applied field orientations record those fields. To understand the properties of chemical remanence produced during the growth of one or more generations of hematite, we precipitated hematite in known conditions of the applied magnetic field. Hysteresis properties, scanning electron microsopy, and thermogravimetric analysis demonstrate that the synthetic material is analogous to pigmentary hematite present in the Siwalik red beds of northern Pakistan; thermal demagnetization and isothermal remanence acquisition behavior of both are also similar. Hematite

precipitated in a single episode parallels the growth field, and preliminary work suggests a linear relationship between the field intensity applied during precipitation and the mineral's remanent intensity. The directional properties of hematite produced in successive generations are far more complex and argue for careful assessment of the reliability of paleomagnetic data from red beds. We show that magnetic fabric analysis is useful in this evaluation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)