Geomagnetic polarity during the early Silurian: The first magnetostratigraphy of the Llandovery

Abstract

Magnetostratigraphic studies in the Silurian are absent, and what is understood about the geomagnetic polarity during this time is based on polarity bias-type data from palaeopole-type studies. We provide the first composite magnetic polarity record through the Lower Silurian (Llandovery) from the magnetostratigraphy of six sections. These are integrated with graptolite biostratigraphy and some carbon isotope chemostratigraphy. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured mudstones and mostly magnetite in non-red lithologies. The influence of possible tectonic disruption of the fabric is assessed using anisotropy of magnetic susceptibility. Only the most thermally mature section at Backside Beck shows the imprint of initial tectonic fabric formation. The Llandovery is divided into 6 major normal-reverse-polarity chron couplets (referred to as LL1 to LL6). An additional longer, exclusively normal polarity interval (referred to as WE1n), beginning in Telychian Stage slice Te3, runs into the lower Sheinwoodian. Within these five polarity couplets there are 10 further submagnetozones, and 10 tentative submagnetozones. Average reversal frequency (including the tentative submagnetozones) was ca. 3.0 Myr−1 in the Early Silurian, which is probably an underestimate, due to insufficient sampling density in some parts of the Rhuddanian and Aeronian. This reversal frequency is similar to that in the late Cenozoic, indicating the future potential utility of magnetostratigraphy for high-resolution correlation and dating in the Early Silurian.