UNDERGROUND FORCES: DEFORMATION PROCESSES IN GRAVEL DEPOSITS
Deformation bands in the gravel layers of the Eisenstadt-Sopron Basin form as a result of
heterogeneous displacement in the surrounding sediment. This is caused by gradients in the
deformation intensity, which occur both parallel and perpendicular to a fault. These findings
from a project funded by the Austrian Science Fund (FWF) will help scientists to reach a better
understanding of both basic geological processes and the formation and structure of oil and
water reservoirs.
(firmenpresse) - As impressively demonstrated by the Himalayas and Pacific oceanic trenches, tectonic
forces can really get things moving. However, even these dramatic geological
manifestations move just a few millimetres or centimetres per year. Other phenomena
associated with geological forces, known as deformation bands, are also subject to
movement on a similar scale. These bands arise in soft porous rocks, such as sandstone.
They occur where coarse-grained rocks are displaced by the shear forces of the overlying
or underlying rock horizons, or undergo a change in volume. In contrast to what is known
as a fault, in which the layer of rock ruptures, in the deformation bands, sediment grains
are merely fractured or reorganised. The porosity of the rock and, therefore, its permeability
to fluids, changes as a result of this process. Deformation bands thus contribute to the
formation and structure of oil or water reservoirs. Attaining a better understanding of their
effect on the surrounding rock is the aim of a project being carried out at the Department
for Geodynamics and Sedimentology at the University of Vienna.
IT'S ALL ABOUT THE GRAIN!
As part of this study, project leader Dr. Ulrike Exner and her team successfully
demonstrated that, due to their relatively coarse grain size, deformation bands in gravels in
the Eisenstadt-Sopron Basin near Lake Neusiedl on the Austrian-Hungarian border display
a gradient in the strain intensity. This gradient runs from the undeformed neighbouring rock
to the middle of the deformation band. As Dr. Exner outlines: "The stresses responsible for
this act perpendicular of the deformation band. However, we discovered that a
displacement gradient also exists parallel to the fault zone. The largest displacement can
be measured in the middle of the deformation band. It decreases above and below this
point towards the tips of the band." The presence of these two differently oriented
displacement gradients causes folding in the surrounding sediment layers.
REVERSE DRAG
Dr. Exner explains the further effects of these heterogeneous deformations in the rock as
follows: "The surrounding rock horizons begin to deform. This effect is known as reverse
drag. In closely-spaced deformation bands, such drags can even overlap. The resulting
geometrical patterns become increasingly complex." However, as Dr. Exner also states,
models exist that can explain these patterns: "The so-called domino model explains these
patterns in terms of the rotation of blocks of rock between the different deformation bands.
Because the rock is still soft and the deformation proceeds very slowly, the behaviour of
these blocks is ductile and they are easily deformed."
In the deformation bands examined as part of this project, the ratio between the
displacement of the layers of rock pushing against each other and the length of the
deformation bands is striking. These ratios, which range from 1:100 to 1:10, are unusually
large. According to Dr. Exner, this could facilitate the generation of reverse drag.
Despite the fact that the processes studied by Dr. Exner unfold below the surface of the
earth, her work is of direct and practical relevance to everyday life above ground:
deformation bands mainly form in porous rock which, due to the presence of numerous
pores, also acts as a reservoir for oil or water. Deformation bands alter porosity and can
therefore influence the extraction of oil or water. Moreover, the relevance of this FWF
project also extends to heavenly heights: the calcareous sandstone, also referred to as
Leithakalk (Miocene limestone found in Central Europe), from which St. Stephen's
Cathedral in Vienna is built, originates from the Eisenstadt-Sopron Basin. Its porosity - and
thus also its response to environmental impacts and protective measures - is also
influenced by deformation bands.
Data were presented at the "European Geosciences Union General Assembly 2010", 2.-7.
May in Vienna, Austria.
Image and text available from Moday, 21th June 2010, 9.00 a.m. CET onwards:
http://www.fwf.ac.at/en/public_relations/press/pv201006-en.html
Scientific Contact:
Dr. Ulrike Exner
University of Vienna
Department for Geodynamics and Sedimentology
Althanstrasse 14
1090 Vienna, Austria
T +43 / 650 / 35 66 948
E ulrike.exner(at)univie.ac.at
Austrian Science Fund (FWF):
Mag. Stefan Bernhardt
Haus der Forschung
Sensengasse 1
1090 Vienna, Austria
T +43 / 1 / 505 67 40 - 8111
E stefan.bernhardt(at)fwf.ac.at
W http://www.fwf.ac.at
Copy Editing & Distribution:
PR&D - Public Relations for Research & Education
Campus Vienna Biocenter 2
1030 Vienna, Austria
T +43 / 1 / 505 70 44
E contact(at)prd.at
W http://www.prd.at
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Datum: 22.06.2010 - 04:54 Uhr
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