Paleomagnetic results from the Cambrian and Ordovician sediments of Bornholm (Denmark) and Southern Sweden and paleogeographical implications for Baltica

Abstract

If apparent polar wander paths (APWP) cross, the question arises how to prove the older magnetization to be primary and not just a younger overprint. This problem is typically met in areas affected by percolating mineralizing fluids and/or heating due to a younger regional igneous activity. The Permian magnetic overprint is the classical example. Earlier paleomagnetic studies over the Lowermost Cambrian Nekso Sandstone Fm (NSF) of Bornholm (Denmark) yielded a characteristic remanent magnetization (ChRM) similar to the Permian directions for Baltica. Since a possible reason could be a chemical overprint, we checked whether this phenomenon did take place on a regional scale. Some samples therefore were collected from other Lower Cambrian clastics of Bornholm and Southern Scandinavia. In result we show that the well‐grouped and stable ChRM of the NSF contrasts with fairly chaotic, soft, and badly preserved magnetizations of the Balka, Hardeberga, Mickwitzia, and Lingulid sandstones of Bornholm and Southern Sweden, thus not indicating widespread paleomagnetic overprint. We demonstrate that the ChRM of the NSF is most probably of syndepositional/early diagenetic origin and its similarity to the Permian direction for Baltica is only casual. We propose a normal polarity and a near‐equatorial position on the Southern Hemisphere for Baltica in the early Cambrian time, as well as a more complicated trend of the APWP for this paleocontinent than envisaged by other authors. Paleomagnetic results from the Arenigian limestones of the Laesaa Formation (Bornholm) that yield excellently defined but most probably only secondary components are also presented.