Magnetostratigraphy of the Visean (Early Carboniferous): North-Western England, Cumbria

In the past, extracting Carboniferous magnetisations from sediments in northern England has proved problematic due to widespread Kiaman-age reverse polarity remagnetisation. Today, it may be possible to better constrain the primary signal using modern magnetometers that have greater sensitivity, by choosing sections affected by less burial-heating, and by using a refined demagnetisation technique. To test this, we studied multiple (partly overlapping) sections in limestone units in southern Cumbria, to constrain a magnetostratigraphy through most of the Visean (late Chadian-Asbian). The Visean in this area is roughly 650 m thick and composed of grainstones, packstones and wackstones, with minor amounts of shale. Age control is based on the late Visean carbon isotope excursion, along with foraminiferal biostratigraphy. Palaeomagnetic hand sampling was conducted at ~1 m spacing (~22 ka spacing), with chips collected every ~25 cm (~6 ka spacing) for magnetic susceptibility and cyclostratigraphic assessment. Combined thermal and alternating field demagnetisation was found to be the most successful in revealing the primary magnetisation in a majority of limestones where the primary magnetic carrier was magnetite. Locally, hematite was found to be the primary magnetic carrier, and was mostly associated with paleosols and some dolostones.

Whilst a majority of samples were found to have a large Kiaman-age secondary overprint, we additionally found weak (often <10 micro A/m) dual polarity early Carboniferous magnetisations, once this remagnetisation was removed. However, great circle behaviour due to blocking spectra overlap between the Kiaman and early Carboniferous components, can be significant. Here, we present our results and provide an assessment on how to better constrain Carboniferous magnetostratigraphy moving forward.