Pure compaction bands in the naturally deformed flysch sandstones of the Silesian Nappe (SE Poland): early markers of tectonic shortening

Deformation in sandstones often takes the form of tabular strain localization structures known as deformation bands. To date, only a few localities have been documented with naturally occurring deformation bands that primarily localize compactant failure i.e. pure compaction bands (PCBs).

The PCBs are found in the Oligocene flysch sequence of the (Lower) Krosno beds within the Otryt sandstones of the Silesian Nappe, the Outer Carpathians (SE Poland). The PCBs are perpendicular to the bedding planes and occur within the folded strata. The PCBs were formed in very fine-grained to very coarse-grained sandstones of moderate or poor sorting. The spacing of PCBs is on average 7 mm and the average thickness of a band is 0.7 mm. The PCBs’ microstructures originated due to a range of mechanisms which include pore collapse resulting in compact grain packing and disaggregation, including kinking and cataclasis. The type of resultant microstructure shows a strong relationship with the textural parameters of the host rock. The frequency of cataclasis increases with grain size, whereas kinking is related to the moderate sorting of the host rock. The structural restoration of beds to the primary horizontal position indicates that the PCBs formed prior to the folding and recorded the SW-NE directed shortening which is consistent with the subsequent folding. The results of the mechanical modelling and stratigraphic constraints suggest that the PCBs formed under shallow burial depths <1 km.

The occurrence of hydrocarbons and calcite veins within some fractured PCBs shows that the PCBs might have played an important role in fluid transport during the subsequent evolution of the fold-and-thrust belt of the Outer Carpathians.

Acknowledgements: This research was funded by National Science Centre, Poland (grant number: 2018/31/N/ST10/02486).