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| Titel |
Relative timing of cementation and deformation band formation in Miocene calcareous sands (Eisenstadt Basin, Austria) |
| VerfasserIn |
M. Lommatzsch, U. Exner, S. Gier, M. Harzhauser  |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068402
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| Schlagwörter |
Geologie, Tektonik, Deformation, Gestein, Sediment, Kalksand, Sedimentologie, Spektroskopie, Geochemie, Miozän, Neogen, Eisenstädter Becken, Wiener Becken |
| Geograf. Schlagwort |
Österreich |
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| Zusammenfassung |
| In this study we investigated deformation bands in unconsolidated, arkosic sands and
calcareous sands of Miocene age (Vienna Basin, Austria) by using sedimentological,
geochemical and spectroscopic techniques. Deformation bands are tabular fault zones in high
porosity rocks and sediments characterized by small displacements caused by reduction of
porosity through grain rotation, translation and/or fracturing. We performed XRD, SEM,
EPMA, grain size analyses and light-microscopy measurements to quantify the influence of
sedimentological properties, chemical composition and alterations on the petrophysical
properties and deformation mechanisms. The timing of deformation bands relative to
cementation may be relevant in relation to the quality and connectivity of hydrocarbon or
groundwater reservoirs.
The investigated outcrop in a sand pit near Eisenstadt, eastern Austria, exposes numerous
conjugated deformation bands, some of which are formed in uncemented, coarse sands, while
others occur in the overlying carbonate-bearing, cemented sands. These deformation bands
formed at low burial depth and are related to the nearby Eisenstadt Fault. With regard to the
mineral content and cementation the deformation bands differs in shape, displacement, mean
grain size und porosity. To determine the deformation mechanisms and the resulting
microstructures and porosity distribution of the deformation bands we observed the
microstructures by using SEM and light-microscopy. We used grain size and XRD
analyses to estimate the influence of mean grain size and mineral content on the
displacement, thickness and porosity variations on deformation bands in the different
lithologies.
The coarse host sediment mainly consists of detrital quartz, biotite, albite, sericite,
chlorite, muscovite grains and metamorphic lithoclasts. The calcareous sands additionally
contain calcitic bioclasts, and the pore space is cemented by a blocky, fine grained cement
coating the individual grains. Previous studies of the uncemented sands revealed that the
deformation bands have the same mineral content as the adjacent host sediment, but show an
enrichment of phyllosilicates in the pore space. The microstructures indicate a grain size
reduction by grain flaking and rotation in deformation bands with small offsets (0.5
- 8 cm), and an increasing grain fracturing and disaggregation of clasts at larger
displacements (up to 60 cm). At the unconsolidated, arkosic sands the porosity reduction in
deformation bands is dominated by a cataclasis of sericitised albite grains and the
smearing of mica schists into the pore space. The measured reduction in porosity of
up to 40% is associated with a permeability reduction. In order to constrain the
influence of cementation on the mechanical properties, we collected samples from the
carbonate-bearing, cemented lithologies. We identified two types of deformation bands,
which can be distinguished by the timing of cementation relative to the deformation
band formation. One type of deformation bands shows fractured cement coatings,
indicating a deformation event postdating the cementation. In contrast, a second
type of deformation bands shows a cementation of the previously fractured detrital
siliciclastic grains. Notably, the porosity is significantly lower in the second type,
while the first type shows increased porosity relative to the cemented host sediment. |
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