High‐resolution rock magnetic cyclostratigraphy in an Eocene flysch, Spanish Pyrenees

Abstract

A rock magnetic cyclostratigraphy, based on anhysteretic remanent magnetization (ARM) intensity variations, was developed for the Eocene Arguis Formation in the Spanish Pyrenees. The Arguis Formation was sampled for ARM cyclostratigraphy, rock magnetic, and paleomagnetic analyses. Rock magnetic measurements indicate that the dominant magnetic mineral controlling the ARM cyclostratigraphy is depositional magnetite. Using thermal demagnetization, a detailed magnetostratigraphy was developed for the Arguis to provide an absolute time framework for the ARM cyclostratigraphy. The magnetostratigraphy is carried by a combination of depositional magnetite and secondary iron sulfides. Spectral analysis of the magnetostratigraphically scaled ARM time series reveals the presence of significant Milankovitch frequencies including eccentricity, obliquity, and precession. The ARM time series was tuned to the orbital eccentricity model for the Eocene. Coherency analysis indicates that the eccentricity‐tuned ARM is in phase with October–November insolation for this site's paleolatitude. Varying amounts of terrigenous input delivered by a fluvial source at orbital time scales, during the rainy season, and diluted by a relatively constant input of marine carbonate is the most likely explanation for the cyclicity of the ARM record. The absolute age resolution of the ARM chronostratigraphy is 1% or less based on an age offset with the reference chronology that was used for the Eocene. The offset in age between the two chronologies may be due, in part, to the uncertainty in the chron ages for this part of the Eocene. The relative age resolution within the ARM chronostratigraphy is much better.