Magnetic susceptibility (MS) and gamma-ray spectrometry (GRS) stratigraphy were used for correlation and characterization of eight Silurian-Devonian (S-D) sections in the Prague Synclinorium (Czech Republic). They represent two different facies developments: lower subtidal to upper slope deposits and slope-to-basin-floor distal calciturbidites. Sections from relatively shallow-and deep-water sections are easy to compare and correlate separately, although the detailed relationship between these two facies is still not entirely clear and correlations between the two settings are difficult. This may be due to sharp facies transitions and presence of stratigraphic gaps. The MS and GRS stratigraphic variations combined with sedimentologic data have been also used for reconstruction of the evolution of the sedimentary environment. The beds close above the S-D boundary show noticeably enhanced MS magnitudes but weak natural gamma-ray emissions. It may correspond to an increased amount of terrigenous magnetic material occurring with short-term shallowing (sedimentological evidence). In deep-water sections the uppermost Silurian is characterized by high MS and GRS values. It corresponds to a supply of recycled sediment to the lower wedge which occurred during the late Pridoli regression phase. The basal Devonian beds correspond to gradual deepening, but the overlying sequences reflect other shallowing episodes which are expressed in increasing MS and gamma ray activity of rocks. The MS and GRS fluctuations are interpreted as a result of local subsidence of the sea bottom along synsedimentary growthfaults and/or a biotic event rather than of eustatic sea-level changes.

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The Ordovician to Middle Devonian Prague Basin, Bohemian Massif, represents the shallowest crust of the Variscan orogen corresponding toc.1–4 km palaeodepth. The basin was inverted and multiply deformed during the Late Devonian to early Carboniferous Variscan orogeny, and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a Tibetan-type orogenic plateau. The northeastern part of the Prague Basin is a simple syncline cross-cut by reverse/thrust faults and represents a doubly vergent compressional fan accommodatingc.10–19 % ~NW–SE shortening, only minor syncline axis-parallel extension and significant crustal thickening. The compressional structures were locally overprinted by vertical shortening, kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen's interior atc. 346–337 Ma. On a larger scale, the deformation history of the Prague Syncline is consistent with building significant palaeoelevation during Variscan plate convergence. Based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the Bohemian Massif, we argue for a differentiated within-plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones. The plateau growth, involving such complex three-dimensional internal deformations, was terminated by its collapse driven by multiple interlinked processes including gravity, voluminous magma emplacement and thermal softening in the hinterland, and far-field plate-boundary forces resulting from the relative dextral motion of Gondwana and Laurussia.

The first international stratotype, selected according to modern stratigraphic principles, is the section with the Silurian-Devonian boundary at Klonk in the Barrandian area, Czech Republic. Its approvement at the XXIV IGC in Montreal, 1972 deeply influenced the further development of stratigraphic principles and praxis in achieving an objective base for definition of global chronostratigraphic units, later followed in establishments of GSSP in different parts of the global stratigraphic column. The procedure of the establishment, present status and correlative value of the Silurian-Devonian boundary stratotype are reviewed, confirming the suitability of its selection.

The MS and GRS stratigraphic records of three Lochkovian-Pragian boundary sections representing different depositional conditions have been evaluated. All studied sections are characterized by increased MS magnitudes from Lochkovian to Pragian. Another prominent characteristic is a turnover in the Th/U ratio; the Pragian magnitudes are usually 2-4 times higher than the Lochkovian ones. Based on a combined physical stratigraphic and sedimentological approach a previously reported regressive event at this time interval is ascribed to climate warming accompanied by enhanced carbonate productivity while eustatic sea-level changes probably played only a minor role, which was locally also combined with synsedimentary uplift. • Key words: Devonian, Lochkovian-Pragian boundary, Prague Synclinorium, magnetic susceptibility, gamma-ray spectrometry. Birmingham, Aston Webb A Block, Edgbaston, Birmingham B15 2TT, United Kingdom; f.vacek@bham.ac.uk Physical stratigraphic methods (magnetic susceptibility and gamma-ray spectrometry) have been applied worldwide during last fifteen years in various areas and stratigraphic levels. In the Czech Republic this approach has been used mainly in the Devonian of the Prague Synclinorium (central Bohemia; e.g., Slavík et al. František Vacek, School of Geography, Earth & Environmental Sciences, University of

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