Global Paleomagnetic Correlation of the Blake Polarity Episode

HERRERO-BERVERA, EMILIO, and CHARLES E. HELSLEY, Southeast Hawaii Institute of Geophysics, Honolulu, HI

We have studied a high-resolution paleomagnetic record of a magnetic polarity episode recovered from a 20-m section of diatomaceous lacustrine sediments from Pringle Falls near La Pine, Oregon. A total of 982 specimens were collected at about 5-cm intervals from the 20-m section. The age of this record is at present constrained between 0.6 Ma and 18,000 yr based upon a pollen date. The characteristic magnetization of the samples was detemined by alternating field (AF) demagnetization to 15 mT. The average normal polarity stable declination and inclination of the nonmagnetic polarity episode sampled section (61 degrees; alpha(95) = 2.6) is statistically equivalent to that expected at this site for a geocentric axial dipole (62 degrees). The record provides a Virtual Geomagnetic Pole (V P) path that identifies a well-defined clockwise northeast-southwest loop with a radius of 60 degrees, followed by a second younger feature that traces out a counterclockwise loop 15 degrees in radius. Interpretation of the natural remanent magnetization/anhysteretic remanent magnetization (NRM/ARM) ratios for the samples indicates that the field intensity may have fallen and then increased above the average dipole field intensity. The low-intensity interval ends before the onset of the polarity episode as defined by directions and may suggest that the episode is a result of an unstable regeneration of the field after a significant lowering of overall field intensity. These two characteristic features are similar to the two features present in the Blake geomagnetic polarity episode at 11 ,000 and 105,000 Ma observed in different parts of the world.

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