Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D 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 3-D structure, nor their processes of formation or destruction are well understood, and outcrop...
Main Authors: | , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-05-01
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Series: | Solid Earth |
Online Access: | http://www.solid-earth.net/7/789/2016/se-7-789-2016.pdf |
Summary: | 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 m<sup>2</sup> 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. |
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ISSN: | 1869-9510 1869-9529 |