The Ordovician‒Silurian boundary beds in the El Pintado section (Sierra Morena de Sevilla Global UNESCO Geopark, SW Spain)

The El Pintado 1 section of the Ossa-Morena Zone (Valle syncline, SW Spain) includes a rather continuous stratigraphic succession ranging from the Upper Ordovician to the Devonian, including remarkably fossiliferous black graptolitic shales that begin their sedimentation in the basal Silurian Akidograptus ascensus graptolite Biozone. The investigation for the location of the Ordovician–Silurian boundary in previous beds has produced negative results, due to the existence of a probable basal gap equivalent to the lower part of the aforementioned biozone, and the absence of stratigraphically relevant fossils, such as graptolites or conodonts, that could provide a high-resolution biostratigraphy to the Hirnantian succession. Nonetheless, some interesting records of rare shelly fossils in the Valle Shale (Hirnantian) are presented, and the Rhuddanian graptolite biozonation in the lower 38 m of the Lower Graptolite Shale (Rhuddanian to Ludfordian) is discussed.