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Solid Earth An interactive open-access journal of the European Geosciences Union

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Solid Earth, 7, 789-815, 2016
https://doi.org/10.5194/se-7-789-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
18 May 2016
Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults
Michael Kettermann1, Sebastian Thronberens1,a, Oscar Juarez2, Janos Lajos Urai1, Martin Ziegler2, Sven Asmus3, and Ulrich Krüger3 1Structural Geology, Tectonics and Geomechanics Energy and Mineral Resources Group, RWTH Aachen University, Lochnerstraße 4–20, 52056 Aachen, Germany
2Chair of Geotechnical Engineering, RWTH Aachen University, Aachen, Germany
3RWE Power AG, Cologne, Germany
anow at: Weatherford International plc, Hanover, Germany
Abstract. 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 3-D structure, nor their processes of formation or destruction are well understood, and outcrop studies to date are mainly 2-D. We present a 3-D 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. The 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 3-D 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. The thinnest parts of the clay smears are often located close to source layer cut-offs. Locally, the clay smear consists of overlapping patches of sheared clay, separated by sheared sand. More commonly, it is one amalgamated zone of sheared sand and clay. A 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.

Citation: Kettermann, M., Thronberens, S., Juarez, O., Urai, J. L., Ziegler, M., Asmus, S., and Krüger, U.: Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults, Solid Earth, 7, 789-815, https://doi.org/10.5194/se-7-789-2016, 2016.
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Short summary
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.
We present exceptional 3-D outcrop data of clay smears in freshly cut trenches of Miocene...
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