Magnetostratigraphy of the Lower Carboniferous

Kamenikova, Tereza (2020) Magnetostratigraphy of the Lower Carboniferous. Masters thesis, UNSPECIFIED.

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Abstract

Geomagnetic polarity reversals studied by magnetostratigraphy have been described well in the Cenozoic and Mesozoic, unfortunately, there are still many data gaps in the Palaeozoic where the magnetostratigraphic record is not easy to correlate or is unknown. One of those is during the Carboniferous System. The magnetostratigraphy of Carboniferous rocks is quite complicated, since overprints produced during the Kiaman Superchron (late Carboniferous-mid Permian) tend to be the strongest components and primary remanence very weak or completely overprinted. This study focused on sediments from S. Cumbria, in North-West England. The studied limestones were on the western edge of the Craven Basin and the NE fringe of the Irish Sea Basin. Two localities in South Cumbria were sampled, the Martin Limestone Formation in Meathop Quarry (50 m profile) and Urswick Limestone Fm in Trowbarrow Quarry (183m) together covering the lower and upper Visean, with a gap in the middle Visean. The sedimentation rate is approx. 30 kyrs/m. Detailed profiles were hand sampled at ~one-meter intervals, conducted with magnetic susceptibility sampling at ~25 centimetres intervals. For paleomagnetic measurements, most samples were thermally demagnetized up to 300-400°C and then AF (alternating field) demagnetization was used. This best differentiated the original magnetic signal from the later Kiaman overprint, and limited thermal alteration. For the time correlation, biostratigraphy using foraminifera was used for Meathop Quarry and carbon isotopes for Trowbarrow Quarry. Results show a Carboniferous dual polarity magnetisation clearly distinguished from the younger Kiaman component. The first detailed magnetostratigraphy through a part of the Visean is presented. The dominant carrier of remanence is magnetite at Meathop Quarry (over 40% of samples) with 7 reversals. At Trowbarrow Quarry, magnetite is dominant in the reversed polarity samples (50% of samples) and hematite for normal polarity samples (over 35% of samples), showing 18 reversals. These results fill a gap in magnetostratigraphic research provding the first detailed magnetostratigraphic study of the Lower Carboniferous in Europe.