Lignite cleat studies from the first Middle-Polish (first Lusatian) lignite seam in central Poland

Abstract

Some Miocene lignite seams are characterized by the presence of natural discontinuities, so-called cleats. Most often they are opening-mode fractures, consisting of two orthogonal sets (face and butt), both almost perpendicular to the bedding. This paper determines distributions of cleat orientation, spacing, and aperture from the first group of the Middle-Polish (first Lusatian) lignite seams (MPLS-1), which has been exploited, inter alia, in the Jóźwin IIB and Kazimierz N opencast mines in central Poland. All observations and measurements were conducted at macroscopic scale.The cleats' mean orientations are NE–SW and NW–SE, dipping at a high angle >75°. The first set of fractures refers to the face cleats and the second one to the butt cleats. The spacing of face cleats is from 10.0 to 17.2cm (averaging 12.65cm) and the spacing of butt cleats is from 9.3 to 24.3cm (averaging 13.75cm). On the other hand, aperture measurements of these cleats range from 0.2 to 1.9cm (averaging 0.68cm) and from 0.1 to 1.5cm (averaging 0.42cm), respectively. The obtained results clearly indicate that face cleat orientations (NW–SE) are strictly parallel to the elongation of the main tectonic structures in the study area. Their origin may be explained in at least two ways, that is, by tectonic folding (the maximum principal stress, σ1, was horizontal) or by salt diapirism (the maximum principal stress, σ1, was vertical). In both cases the intermediate principal stress, σ2, was parallel to the face cleat strikes, namely NW–SE, although the well-developed butt cleats, in the context of regional geology, provide additional evidence for the creation of an orthogonal system of fractures (cleats) in the cover of salt structures. Thus, the NW–SE direction is characteristic of both the extent of the main tectonic units in central Poland and the face cleat orientation in MPLS-1.