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Clay smears in normal faults can form seals for hydrocarbons and groundwater,and their prediction in the subsurface is an important problem in applied andbasic geoscience. However, neither their complex 3-D structure, nor theirprocesses of formation or destruction are well understood, and outcropstudies to date are mainly 2-D. We present a 3-D study of an excavated normalfault with clay smear, together with both source layers, in unlithified sandand clay of the Hambach open-cast lignite mine in Germany. The faults formedat 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 tectonicallymixed sand–clay gouge. The thickness of clay smears in two excavated fault zonesof 1.8 and 3.8 m<sup>2</sup> is approximately log-normal, with values between5 mm and 5 cm, without holes. The 3-D thickness distribution isheterogeneous. We show that clay smears are strongly affected by R andR' shears, mostly at the footwall side. These shears can locally cross andoffset clay smears, forming holes in the clay smear, while thinning of theclay smear by shearing in the fault core is less important. The thinnest parts ofthe clay smears are often located close to source layer cut-offs. Locally, theclay smear consists of overlapping patches of sheared clay, separated bysheared sand. More commonly, it is one amalgamated zone of sheared sand andclay. A microscopic study of fault-zone samples shows that grain-scale mixingcan lead to thickening of the low permeability smears, which may lead toresealing of holes

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