Anomalous Geomagnetic Field Behavior Recorded by Glacial Sediments from Northwestern Patagonia, Argentina

Abstract

Glacial sediments and two volcanic flow units from northwestern Patagonia, Argentina were sampled for paleomagnetic measurements as part of an ongoing program to date and correlate the glacial stratigraphy. Samples taken from a large ball and pillow structure in the Anfiteatro Moraine are normal polarity and pass the fold test suggesting that the magnetization was locked in prior to the formation of the structure. Samples from Pichileufu-age glaciolacustrine sediments have anomalously low inclinations and show little movement of the magnetic vector over a 400 to 500 year period if rhythmites are interpreted to be varves. Tills in the Malleo valley, 120km north of Bariloche, that are tentatively correlated with the Pichileufu-age sediments are bracketed by normal polarity basalt flows from Pino Santo and Volcan Lanin. Anisotropy of magnetic susceptibility results from the Pichileufu and Anfiteatro Moraine sediments indicate that both units have a primary depositional fabric. A redeposition experiment of Pichileufu lake sediment suggests that it is capable of recording steep inclination geomagnetic fields. The Pichileufu sediments are interpreted to have recorded anomalous low inclination geomagnetic field behavior which is not characterized by the dramatic looping usually associated with geomagnetic excursions or polarity transitions. These observations may be explained by local dominance of a standing focus of the nondipole field for at least 400 to 500 years in the early- to mid-Brunhes normal polarity epoch.