Finding Unconformities in Lacustrine Sediments Using Continuously-Sampled Paleomagnetic Records

NEGRINI, ROBERT M., California State University, Bakersfield, CA, and STEVEN P. LUND, University of Southern California, Los Angeles, CA

Comparisons of detailed paleomagnetic records from two separate Pleistocene lake basins demonstrates the potential utilization of such records to detect unconformities representing time gaps of as little as a few hundred years. One of the records is from the Wilson Creek lacustrine sediments located near Mono Lake in California. The other record is from lacustrine sediments exposed in the Ana River Canyon near Summer Lake, Oregon. Both are reliable records of paleomagnetic field directions representing a time interval from 30,000 to 15,000 yr B.P. The Wilson Creek record appears to be continuous, based on the presence of five complete waveforms which have periods of 3000-4000 yr. In the Summer Lake record all or part of the oldest four waveforms are clearly preserved. The stratigraphi position of two of the three conspicuous gaps in the Summer Lake record

corresponds exactly to the stratigraphic positions of geologic features commonly associated with erosional events or depositional hiatuses.

The potential for using this method to detect unconformities in older sediments using this method depends critically upon whether or not "type" secular variation records can be reliably established for older time intervals. The potential for using this method on cores depends on the ability to recover reliable declination information from core samples. These topics and the complementary use of independent rock magnetic parameters such as susceptibility will be discussed.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)