Dictyonema Shale

second stratigraphic horizon from the bottom of the Silurian deposits, found in Leningrad Oblast and the Estonian SSR. It is represented by clay shales that are rich in organic matter and contain graptolitic fossils (Dictyonema flabelliformis Hall) and more rarely brachiopod fossils (O bolus appolinis Eichw). Thickness varies from 0.12 to 4 m. The Dictyonema shale corresponds to the upper part of the Tremadocian series.