We present exceptional 3-D outcrop data of clay smears in freshly cut trenches of Miocene unconsolidated deltaic clay–sand sequences in an active lignite quarry in the Lower Rhine Embayment. We aim to identify and investigate processes that entrain clay into a fault or breach of formed clay smear. We show the importance of host rock deformation, grain-scale mixing and source layering on clay smear geometry by providing 2-D cross sections and 3-D thickness data of complete clay smear sequences.