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Discussion papers
https://doi.org/10.5194/se-2019-49
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-49
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Mar 2019

Research article | 25 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Deformation of intrasalt competent layers in different modes of salt tectonics

Mark G. Rowan1, Janos L. Urai2, J. Carl Fiduk3, and Peter A. Kukla4 Mark G. Rowan et al.
  • 1Rowan Consulting, Inc., Boulder, CO 80302, USA
  • 2Institute for Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, 52056 Aachen, Germany
  • 3Fiduk Consulting LLC, Houston, TX 77063, USA
  • 4Geological Institute, Energy and Mineral Resources, RWTH Aachen University, 52056 Aachen, Germany

Abstract. Layered evaporite sequences (LES) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different modes of salt tectonics using a combination of conceptual, numerical and analog models, and seismic data. In bedding-paralell extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become increasingly isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters while folded competent beds characterize salt pillows. Finally, in tall passive diapirs, stringers generated by intrasalt extension are rotated to near vertical in tectonic melanges during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are usually not imaged in tall passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near-vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Mark G. Rowan et al.
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Mark G. Rowan et al.
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Short summary
Ancient evaporite sequences were deposited as interlayered rocksalt, other evaporites, and non-evaporite rocks that have enormous differences in strength. Whereas the ductile layers flow during deformation, strong layers are folded and/or torn apart, with the intrasalt deformation dependent on the mode and history of salt tectonics. This has important implications for accurately imaging and interpreting subsurface seismic data and for drilling wells through evaporite sequences.
Ancient evaporite sequences were deposited as interlayered rocksalt, other evaporites, and...
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