Magnetostratigraphy susceptibility of the Přídolian-Lochkovian (Silurian-Devonian) GSSP (Klonk, Czech Republic) and a coeval sequence in anti-atlas Morocco

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The magnetosusceptibility event and cyclostratigraphy (MSEC) record for the Přídolian-Lochkovian (Silurian-Devonian) Global Boundary Stratotype Section and Point (GSSP) (Klonk, Prague Basin, Czech Republic) is described and used to establish a magnetostratigraphy susceptibility profile for the GSSP. GSSP MSEC data are summarized into three magnetozones (MSZ). The Tmaň MSZ (Late Přídolian) with 13 MSSZs (MSSZ), the Klonk MSZ (latest Přídolian and earliest Lochkovian) with 17 MSSZs, and the Voskop MSZ (Early Lochkovian) with 7 MSSZs. The base of Klonk MSSZ 2 is coincident with the base of Lochkovian and the Devonian. The proposed magnetostratotype for the Přídolian-Lochkovian boundary is supported by MS data from a nearby core through the GSSP sequence and into the upper Ludlow. Three additional MSZs are recognized for the portion of the core sequence older than the GSSP boundary sequence. The extension of the MSZs and MSSZs away from the proposed magnetostratotype is tested by comparison with a Lochkovian sequence in the western Sahara of southeastern Morocco. MSZs are the result of global sea level fluctuations that alter base level and the pattern of erosion and influx of detrital paramagnetic mineral components into the marine environment. Large-scale transgressive and regressive patterns interpreted from the MSEC data establish that the Silurian-Devonian boundary in the Barrandian region falls between a short-lived transgressive pulse in the latest Přídolian and an equally short-lived regressive pulse in the earliest Lochkovian. MSSZs identified in the GSSP and core sequences are the result of variation in climate cycles and the resulting periodicity can explained by either the 38.9 Ka Silurian and Devonian equivalent of the modern 54 Ka obliquity cycle or the eccentricity cycle of 106 Ka periodicity. Use of the obliquity periodicity gives a duration for the Barrandian Lochkovian of 4.46 Ma which agrees with the most recent geochronometric estimate of 4.5 Ma for the Lochkovian. Use of the eccentricity periodicity gives a duration for the Barrandian Lochkovian of 12.16 Ma which agrees with the previous geochronometric estimates of 12.2 and 12.5 Ma for the Lochkovian. In any case the cyclic nature of the MSEC data established by the MSSZs can be explained by variations in the rate of supply of weathered terrigenous paramagnetic grains to the marine system. These variations in supply occurred due to climate changes resulting from the either the obliquity or eccentricity of the Earth's orbit. Knowledge of the periodicity of Lochkovian cycles in the Barrandian area allows estimation of the rates of sediment accumulation, the duration of fossil ranges, and the rates of evolution. Depending on the choice of obliquity or eccentricity to explain the cyclicity in the Barrandian Lochkovian, the estimated rate of sediment accumulation in the GSSP outcrop averaged either 51 or 140 Ka/m. The range of the graptoloid Monograptus uniformis angustidens, wholly contained within the Lochkovian sequence at the GSSP, is also estimated at either 51 or 140 Ka. © 2001 Elsevier Science B.V.

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Palaeogeography, Palaeoclimatology, Palaeoecology

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