Development of strain fringes in sedimentary rocks: Evidence for deformation of Upper Ordovician glacial diamictites in the western Srednogorie Zone

The local formation of quartz strain fringes around authigenic pyrite crystals and chamosite strain fringes around detrital grains is described from Hirnantian glaciomarine diamictites (Sirman Formation) of the Palaeozoic Balkan Terrane, now cropping out in the Alpine western Srednogorie Zone. The fringes belong to the antitaxial type and are interpreted as products of synkinematic, low temperature growth in microdilatation sites as a result of diffusive mass-transfer processes. The relatively simple fringe geometry and fiber orientation reflect coaxial progressive deformation that can be broadly related to the Variscan orogeny. Performed XRD analysis of shales intercalated with the diamictites indicates that the Upper Ordovician glacial deposits underwent a high-grade diagenetic alteration. Therefore, this study reports a rare case of development of strain fringes in non-metamorphosed sedimentary rocks.