A geomagnetic polarity stratigraphy for the Middle and Upper Ordovician

Hounslow, M.W. and Harris, S.E. and Wójcik, K. and Nawrocki, J. and Ratcliffe, K.T. and Woodcock, N.H. and Montgomery, P. (2021) A geomagnetic polarity stratigraphy for the Middle and Upper Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology, 567: 110225. ISSN 0031-0182

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Abstract

Magnetostratigraphic studies of the Ordovician provide evidence for the nature of core-mantle boundary interactions, and provide means for dating and correlation across differing environmental regimes. We provide new magnetostratigraphic data from the Middle and Upper Ordovician, compiling this into a polarity chronostratigraphic scale for the Dapingian to Hirnantian interval. The new data are derived from the Backside Beck and Cheney Longville sections in Britain, the Mójcza section in Poland and two cores from Poland and Lithuania. The chronology is provided by existing biostratigraphy, principally based on chitinozoans and conodonts for the Ordovician. Correlations between sections are supported by carbon isotope stratigraphy linked to Baltic isotopic zonations, along with lithological and local magnetic susceptibility correlations in Polish cores. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured lithologies (marls and limestones) and magnetite in non-red mudstones and limestones. A positive reversal test (class C) in the Cheney Longville section and positive fold tests in the Backside Beck section provide validation of the isolation of a primary palaeomagnetic signal. Palaeomagnetic directions from cores were re-oriented using Kiaman-age and Brunhes overprints. These new datasets in combination with existing Middle Ordovician data provides a near-complete magnetic polarity chronostratigraphic scale through the Middle and Upper Ordovician. Brief normal-polarity magnetozones extend well into the later parts of what has been considered the Moyero Superchron, which started in the late Tremadocian. Reversal frequencies for the mid and late Ordovician are 1.7 and 1.5 Myr−1 respectively, although that for the late Ordovician may be an underestimate.