Clay Smear Processes in Unlithified Clay-sand Sequences of the Hambach Lignite Mine

Clay smears are highly variable three-dimensional structures and the controlling factors are still poorly understood. We present an outcrop based study in unlithified sand-clay sequences of the Hambach open-pit lignite mine, Germany. With the help of an excavator we dug four trenches across a fault with up to 1.5 m displacement. In cross-sections we could study clay smear thicknesses distributions and related host rock deformations (R- and R'-shears). Incremental slicing of cross-sections allowed the creation of a 3D thickness map showing that the thickness of the clay smear is not related to the down-dip position along the fault. Other sections showed that holes in the clay smear are the result of shear bands cross-cutting the clay smear rather than of strain thinning. Furthermore we show the importance of grain-scale mixing on clay smear thickness and continuity. In multilayer sequences a wide shear zone causes the formation of a continuous sand-smear between clay smears, forming an effective up-fault fluid pathway. Excavated clay smear surfaces show the variety of structures forming the smear including re-sheared clay fragments and clay-noses with the potential to thicken the clay smear and reseal discontinuous smears.