Climatic vs. tectonic controls on peat accretion in non-marine setting; an example from the Žacléř Formation (Yeadonian–Bolsovian) in the Intra-Sudetic Basin (Czech Republic)

• Late Paleozoic continental basins of the Variscan orogenic belt.
• Sedimentary and hydrological history of peat swamps reconstructed.
• Tectonically generated accommodation space.
• Climatic oscillations responsible for cyclic pattern of coal-bearing strata.

The Žacléř coalfield in the western part of the Intra-Sudetic Basin is an example of coal accumulation in an alluvial valley deposited under a long-term (~ 3 m.y.) stable tectonic regime, with climatic oscillations possibly operating on several time scales. The resulting sedimentary record, about 600 m thick, encompasses about 60 coal-bearing cycles formed in a relatively small and localized part of the valley, whereas elsewhere a higher clastic input and/or lower rate of subsidence resulted in deposition of strata with very little coal. Available data suggest that tectonics produced accommodation for sediments and, via changes in topography and/or size of the source area, affected the volume and dispersal patterns of clastic material transported to the depocentre. Areas with long-term tectonic stability were suitable places for recognition of climatic signals in the sedimentary record, modified by the intensity of local clastic input. Climatic fluctuations generated coal-bearing cycles, probably through a variable intensity of precipitation which affected the density of vegetation cover across the landscape along with evaporation and groundwater table. These factors influenced the intensity of chemical weathering and hence regulated the volume and composition of clastics delivered to the depocentre. This in turn resulted in alternation of short periods of bedload-dominated deposition during the seasonally dry part of the cycle with periods of dominantly suspended load and organic (peat) deposition during the more humid (?everwet) part of the cycle. Estimated duration of the basic cycles (~ 46 k.y.) approximates the orbitally-driven axial tilt cyclicity. Basic cycles are arranged into two larger-scale cyclic patterns lasting for 270 k.y. and 1.5 m.y., respectively, which broadly correspond with the timing of glacial maxima and minima inferred from high-latitude glacial deposits of Gondwanan basins.All the coals of the Žacléř coalfield display similar petrographic composition dominated by collotelinite, and field observations as well as detailed petrographic study show that they formed in planar rheotrophic peat swamps where long-lasting peat accretion was episodically interrupted only by major floods on a century to millennial scale.