Numerical modelling of clast rotation during soft-sediment deformation: a case study in Miocene delta deposits

  • M. Hölzel
  • B. Grasemann
  • M. Wagreich


A numerical model for a rotated clast in a sedimentary matrix is presented, quantifying the deformation in associated soft-sediment deformation structures. All the structures occur in a southwards prograding deltaic sequence within the Miocene Ingering Formation, deposited at the northern margin of the Fohnsdorf Basin (Eastern Alps, Austria). Debris flow and pelitic strata contain boudins, pinch-and-swell structures, ptygmatic folds, rotated top-to-S reverse faults and rigid clasts, developed under different stress conditions within the same layers. The deformation around a 24×10 cm trapezoid-shaped rigid clast, resembling the δ-clast geometry in metamorphic rocks, has been modelled using a 2D finite element modelling software. Under the chosen initial and boundary conditions the rotational behaviour of the clast mainly depends on the proportions of pure and simple shear; best fitting results were attained with a dominantly pure shear deformation (~65–85%), with stretching parallel and shortening normal to the bedding. In this specific model set-up, the initial sedimentary thickness is reduced by 30%, explained by stretching due to sediment creeping and compaction. The high amount of pure shear deformation proposed is compatible with the observed layer-parallel boudinage and pinch-and-swell structures. Rotated faults and ptygmatic folds were caused by the minor component of bedding-parallel simple shear.


Soft-sediment deformation Numerical modelling Rigid clast Neogene 

List of symbols

\( \tau _{{ij}} \)

deviatoric components of the stress tensor

\( \ifmmode\expandafter\dot\else\expandafter\.\fi{\varepsilon }_{{ij}} \)

components of the strain rate tensor

i, j

reference to two horizontal and vertical Cartesian directions





xi, xj

spatial directions





Fieldwork was funded by the Austrian Science Fund (FWF-Project P-14370-GEO: Stratigraphic Simulation in Neogene Sedimentary Basins) and scholarships of the University of Vienna and the Stratigraphic Commission of the Austrian Academy of Sciences. T. Barr, G. Houseman and C.L. Evans are thanked for providing the software BASIL used in this work. L. Plan, R. Hinsch and A.H. Rice are thanked for discussion and helpful remarks. W. Piller and H. von Eynatten improved the clarity of the final manuscript and are gratefully acknowledged.


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Department of Geodynamics and SedimentologyUniversity of ViennaViennaAustria

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