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Cenozoic tectonic evolution of the main lignite-rich grabens in Poland. Part 1. Tectonic stages

Widera Marek

Acta Geologica Polonica

2024

Vol. 74, no. 1

art. no. e2

Understanding the Cenozoic tectonic evolution of grabens rich in lignite is important in the context of the accumulation of ~40–650 m of peat, as well as the exploitation of later formed lignite seams with a thickness of ~20–250 m. Six such areas were selected for a detailed palaeotectonic analysis: the Gostyń, Szamotuły, Legnica, Zittau, Lubstów, and Kleszczów grabens. During the analysis, borehole data were used, taking into account the compaction of peat at the transition to lignite, in order to reconstruct the magnitude of the total subsidence. This made it possible to distinguish between regional (covering areas also outside the grabens) and local (occurring only in the grabens) tectonic movements, and among the latter, tectonic and compactional subsidence. The hypothetical palaeosurface of the mires was reconstructed based on the lignite decompaction. As a result, it was possible to determine whether the examined peat/lignite seams underwent post-depositional uplift and/or subsidence. Between one (Gostyń Graben) and four (Zittau Basin and Kleszczów Graben) stages of tectonic subsidence were distinguished in the studied lignite-bearing areas. In the case of the Zittau Basin, as well as the Lubstów and Kleszczów grabens, post-depositional stages of tectonic uplift were also indicated. Like the boundaries of lithostratigraphic units, the successive stages of the Cenozoic tectonic development of the examined grabens are diachronic.

Cenozoic tectonic evolution of the main lignite-rich grabens in Poland. Part 2. Tectonics versus autocompaction and compaction

Widera Marek

Acta Geologica Polonica

2024

Vol. 74, no. 2

art. no. e7

This paper is a continuation of the previous one (Widera, 2024. Acta Geologica Polonica, 74 (1), e2). A new, alternative interpretation of the tectonic development of two lignite-rich deposits in the Lubstów and Kleszczów grabens in central Poland is presented. The maximum thickness of lignite mined from both deposits is >86 and>250 m, respectively. These grabens were selected for detailed tectonic analysis because syn-depositional or post-depositional tectonic uplift is undeniably evident. The current study focuses on the distinction between tectonic subsidence/uplift and autocompactional subsidence, and on the timing of their occurrence. Such a research approach allows for the presentation of new conceptual models of Cenozoic tectonic evolution during the formation of the third, very thick, Ścinawa lignite seam (ŚLS-3) and the second Lusatian lignite seam (LLS-2). As a result, it is shown here that the magnitude of both the downward and upward tectonic movements are significantly smaller than previously thought. This new interpretation is also confirmed by the low rank of lignite coalification and the net calorific value of the ŚLS-3 and LLS-2.