Widespread occurrence of greigite in the sediments of Lake Pannon: Implications for environment and magnetostratigraphy

Abstract Lake Pannon was a large lake at the southern margin of the European plate, which separated from the Central Paratethys at the Sarmatian–Pannonian boundary and was filled completely with sediment during the Pliocene. The separation produced an endemic fauna that makes classical geological correlation problematic with sediments outside the lake. Magnetostratigraphic correlation could potentially solve the problem, and several long cores from deep-drillings have been analysed for polarity information. In the course of these studies, primary magnetite was assumed to carry the natural remanent magnetization (NRM). Later, when outcrops of dark-grey clays and marls of Lake Pannon were sampled for palaeomagnetic investigations of tectonic processes, greigite was discovered as the carrier of the NRM at several localities. This motivated a systematic study of the fine-grained sediments of Lake Pannon in order to identify the principal carriers of the NRM, to relate them to depositional environments, and to assess the importance of neoformation of magnetic minerals. The concentration of magnetic minerals is low in the fine-grained sediments of Lake Pannon. This made extraction of the magnetic minerals impossible, so they were identified with magnetic methods. Magnetic experiments suggest that greigite is the most common magnetic mineral in the Lake Pannon sediments. Magnetite, when it occurs, is often accompanied by pyrite. Haematite is rare. The studied sediments represent three time intervals: 11–9.3 Ma, 8–7 Ma and 6–4 Ma. In the 11–9.3 Ma sediments, greigite occurs as an original (early diagenetic) magnetic mineral. During this period, the climate was warm and humid; the lake was fairly deep and stratified; high organic productivity resulted in oxygen-depleted bottom waters. Burial was rapid, due to intensive erosion from the surrounding Carpathian and Alpine mountains. Magnetite-bearing sediments deposited during this period do not carry a coherent palaeomagnetic signal, which suggests that the magnetite was neoformed. The 8–7 Ma sediments were deposited in sublittoral, upper delta plain and swamp environments, in humid conditions. They are also greigite-bearing. During the third investigated time period (6–4 Ma), the climate was arid, the lake was of low salinity, and was restricted to the southern part of the Pannonian basin. These sediments were deposited in a distal environment (low sedimentation rate). The characteristic magnetic mineral of this period is magnetite, which most probably has a detrital origin. Widespread greigite, which is always of diagenetic origin, suspected neoformation of magnetite in the older parts of Lake Pannon, and occasionally opposite polarities connected to greigite and magnetite when they occur together in the present study are important for magnetostratigraphic interpretation. They imply that detailed magnetic mineralogical investigations and documentation of the consistency of the NRMs (by working on fully oriented samples) are essential in magnetostratigraphic studies of molasse sediments.