Magnetostratigraphy susceptibility of the Pridolian-Lochkovian (Silurian-Devonian) GSSP (Klonk, Czech Republic) and a coeval sequence in Anti-Atlas Morocco

The magnetosusceptibility event and cyclostratigraphy (MSEC) record for the Pridolian-Lochkovian (Silurian-Devonian) Global Boundary Stratotype Sectio n and Point (GSSP) (Klonk, Prague Basin, Czech Republic) is described and u sed to establish a magnetostratigraphy susceptibility profile for the GSSP, GSSP MSEC data are summarized into three magnetozones (MSZ). The Tman MSZ (Late Pridolian) with 13 MSSZS (MSSZ), the Klonk MSZ (latest Pridolian and earliest Lochkovian) with 17 MSSZs, and the Voskop MSZ (Early Lochkovian) w ith 7 MSSZs. The base of Klonk MSSZ 2 is coincident with the base of Lochko vian and the Devonian. The proposed magnetostratotype for the Pridolian-Loc hkovian boundary is supported by MS data from a nearby core through the GSS P 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 magnetostratoty pe 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 comp onents into the marine environment. Large-scale transgressive and regressiv e patterns interpreted from the MSEC data establish that the Silurian-Devon ian boundary in the Barrandian region falls between a short-lived transgres sive pulse in the latest Pridolian and an equally short-lived regressive pu lse in the earliest Lochkovian. MSSZs identified in the GSSP and core seque nces are the result of variation in climate cycles and the resulting period icity 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 per iodicity. Use of the obliquity periodicity gives a duration for the Barrand ian 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 wi th the previous geochronometric estimates of 12.2 and 12.5 Ma for the Lochk ovian. In any case the cyclic nature of the MSEC data established by the MS SZs can he explained by variations in the rate of supply of weathered terri genous 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 al lows estimation of the rates of sediment accumulation, the duration of foss il 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 eithe r 51 or 140 Ka/m. The range of the graptoloid Monograptus uniformis angusti dens, wholly contained within the Lochkovian sequence at the GSSP, is also estimated at either 51 or 140 Ka. (C) 2001 Elsevier Science B.V. All rights reserved.