Interpretation of palaeomagnetic signals from palaeo-lake sediments of the Atacama (Chile) – evidence of geomagnetic excursions in C5Cn.1n?

Sediment cores recovered from the Playa Adamito Grande (PAG) claypan in the hyperarid core of the Atacama Desert (Chile) show a sequence of three main lithological units. Here we focus on the lowest unit, which was found below 29.2 m to the bottom of the core at a depth of 52 m. The silty-clayey sediment is frequently interspersed with gypsum-rich layers and is interpreted as a palaeo-lake deposit. At the top of this unit an erosion unconformity was identified at ca. 29.5 m depth. Two U/Pb zircon ages obtained from tephra layers provide chronological data at 31.49 m (16.27 ± 0.09 Ma) and 29.22 m (15.54 ± 0.03 Ma) sediment depth.  Below, none of the various dating methods employed yielded results. The timing and duration of the lake phase in the Atacama, were therefore attempted to be determined more precisely using magnetic polarity stratigraphy. The mineral magnetic association is composed of partially maghemised magnetite and Ti-magnetite in domain states that allow for retention of the magnetic signal over long geological time scales. However, the sediment sequence mainly exhibits normal polarity, interrupted only by thin layers with reversed polarity. Therefore, it is not readily possible to specify age constraints for the lake phase by correlation with the geomagnetic polarity time scale (GPTS). Some of the layers with reversed polarity coincide with layers rich in gypsum. Therefore, we discuss the nature of these short-lived reversed sections, considering possible post-depositional overprinting of the magnetic signal or the occurrence of hiatuses. However, our considerations show that the palaeomagnetic signal most likely represents a high-resolution record of field instabilities during the Miocene. This interesting case study thus provides new evidence for the occurrence of geomagnetic excursions during the Subchron C5Cn.1n (Middle Miocene).