Graptolite biostratigraphy of the Lower Silurian (Llandovery) shelf deposits of the Western Iberian Cordillera, Spain

Abstract

Eight Lower Silurian graptolite biozones (triangulatus, convolutus, linnaei, turriculatus, crispus, griestoniensis, tullbergi and spiralis) and three subzones (runcinatus–gemmatus, palmeus and hispanicus) have been recognized in the Sierra Menera, Nevera, and Tremedal massifs in the Castilian Branch of the Iberian Cordillera (Western Iberian Cordillera). Early Silurian, Rhuddanian low-diversity normalograptid faunas are also present and these, together with the rich graptolite faunas of the Aeronian triangulatus and convolutus biozones, come from black, shaly intercalations within the quartzose sandstones of the upper part of the Los Puertos Quartzite. Telychian graptolite biozones have been recognized in the succeeding black-shale sequence of the Bádenas Formation. Diachroneity of the transition from sandstones to black shales is dated by graptolites. It ranges from about the base to at least the top of the linnaei Biozone. We suggest that the Lower Silurian black shales of the Western Iberian Cordillera were deposited in a shallow, shelf environment, not much deeper than that of presumably storm-influenced sandstones of the Los Puertos Quartzite.