Characteristics and origin of deformation structures within lignite seams – a case study from Polish opencast mines

Abstract

The present paper discusses the genesis of some deformation structures within Miocene lignite seams in Poland. These meso- to macro-scale deformation structures are the best developed in areas where the thickest lignite seams are exploited, that is, in the Belchatow, Turow and Lubstow lignite opencast mines. They show a wide variety of structures such as high-angle inclined strata, normal and reverse faults and folds, fractures and clastic dikes. These deformation structures were not analysed in terms of the structural development of the lignite-bearing areas, but rather in the context of their origin. The high-angle inclination of clastic sedimentary strata clearly formed mainly as a result of vertical tectonic movements in the sub-Cenozoic basement, their currently measured dip angle being significantly higher than their natural angle of repose. All the observed normal and reverse faults cutting the Miocene strata are a clear example of syn- and/or post-depositional tectonics. The normal faults clearly indicate tensional tectonics while the reverse faults reflect compressional tectonics. In turn, the majority of folds occurring within the lignite seams are a result of peat/lignite compaction. However, one of the folds studied is predominantly the result of vertical displacements in the sub-Cenozoic basement. The en echelon fracture pattern may additionally indicate the horizontal component of these displacements. Finally, the most spectacular and debatable structures are clastic dikes. In this study the genesis of some of them is linked with the evolution of the graben’s marginal fault.