Why dating Lake Pannon sediments is a pain: contradictory magnetic polarities due to multiple generations of diagenetic greigite

Abstract

Dating of Late Miocene (Pannonian) Lake Pannon sediments has proven difficult to date due to the endemic nature of biota, scarcity of reliable radio isotopic data and generally inconsistent, erratic results of previous paleomagnetic studies. Here we present an investigation into the viability of the Late Miocene Lake Pannon sediments for a paleomagnetic correlation using samples from three recently drilled well-cores (PAET-26, PAET-27 and PAET-30) from the Paks region, Hungary. We found that significant intervals of the cores contain contradictory magnetic polarities within the same horizon. These multi-polarity samples consist of a medium temperature (MT) and high temperature (HT) component, in many cases with anti-parallel directions. Our rock-magnetic results indicate that both components are carried by the magnetic iron sulphide greigite. Thus, alternating field demagnetization results in directions equal to the dominant component intensity wise, which is variable throughout the cores, resulting in erratic, incorrect polarity patterns. Separating the greigite components is preferably done by using a combination of thermal and alternating field demagnetization. The HT component represents the ChRM, and is of early diagenetic origin, whilst the MT component records a delayed acquisition and thus is of late diagnetic origin. Using the directions obtained from the HT component two correlations are proposed for PAET-30 for the interval from 160 to 442.5 m DBS. Option A puts the age at 7.140 to 8.108 Ma, option B at 8.769 to 11.056 Ma. Depth-age plots shows the average sedimentation rates are 29 cm/kyr for A and 12 cm/kyr for B.