Estonian graptolitic argillite (also known as dictyonema argillite, dictyonema oil shale, dictyonema shale or alum shale) was laid down during the Early Ordovician under a marine environment.
Estonian graptolitic argillite (also known as dictyonema argillite, dictyonema oil shale, dictyonema shale or alum shale) is a marinite-type of black shale. Graptolitic argillite was laid down during the Early Ordovician under a marine environment.