The Silurian stage boundaries in Bulgaria: the challenge of the Aeronian/Telychian (Llandovery) boundary

The upper Aeronian to lower Telychian (Llandovery Series, lower Silurian) strata in the parastratotype for the Saltar Formation (chert-shale succession) are documented in terms of their lithologies and graptolite biostratigraphy. The studied section Asaritsa сrops out in the western sector of the Stara Planina Mountains (Svoge Unit of the Srednogorie Zone). In section Asaritsa, the highest Aeronian graptolitic strata are separated from the lowest Telychian graptolitic strata by a graptolite-barren interval of pale-coloured shale. Based on the graptolites documented below and above this interval, as well as on its correlation to sections studied in detail from the Czech Republic and Spain, the immediate underlay of the pale-coloured shale is referred to the uppermost part of the sedgwickii Zone, the complete pale-coloured shale interval, or its greatest part, to the lower guerichi Zone, and the directly overlying it succession to the upper part of the guerichi Zone. The Aeronian/Telychian boundary is assumed to lie at the base of this pale-coloured shale package, or within its lower part. The study provides documentation of Metaclimacograptus undulatus, Parapetalolithus praecedens, Rastrites carnicus, Rivagraptus bellulus, and illustrations of Stimulograptus for the first time in Bulgaria.