Mechanisms of clay smear formation in unconsolidated sediments – insights from 3D observations of excavated normal faults

Clay smears in normal faults can form seals for hydrocarbons and groundwater, and their prediction in the subsurface is an important problem in applied and basic geoscience. However, neither their complex 3D structure, nor their processes of formation or destruction are well understood, and outcrop studies to date are mainly 2D. We present a 3D study of an excavated normal fault with clay smear, together with both source layers, in unlithified sand and clay of the Hambach open cast lignite mine in Germany. The faults formed at a depth of 150 m, and have Shale Gouge Ratios between 0.1 and 0.3. The fault zones are layered, with sheared sand, sheared clay and tectonically mixed sand-clay gouge. Thickness of clay smears in two excavated fault zones of 1.8 and 3.8 m2 is approximately log-normal, with values between 5 mm and 5 cm, without holes. The 3D thickness distribution is heterogeneous. We show that clay smears are strongly affected by R- and R'-shears, mostly at the footwall side. These shears can locally cross and offset clay smears, forming holes in the clay smear, while thinning of the clay smear by shearing in the fault core is less important. Thinnest parts of the clay smears are often located close to source layer cutoffs. Locally, the clay smear consists of overlapping patches of sheared clay, separated by sheared sand. More commonly, it is one amalgamated zone of shared sand and clay. Microscopic study of fault zone samples shows that grain-scale mixing can lead to thickening of the low permeability smears, which may lead to resealing of holes.