Tonsteins

Tonsteins are hard, dense, clay- or silt-rich layers that occur within coal-bearing sequences, typically within coal seams or immediately adjacent to them. The term arises from the German words ton (clay) and stein (stone), and in coal geology they are interpreted as diagenetically altered ash beds that were deposited from volcanic eruptions into peat-forming environments during coal formation.

Lithology and formation are characteristic: tonsteins are fine-grained and compact, often more resistant to weathering than surrounding mudstones. Their mineralogy is dominated by clay minerals such as illite and kaolinite, with minor quartz, feldspar, and iron oxides. In some occurrences they preserve plant microfossils such as spores and pollen, which aid in age dating of the associated coal seam.

Stratigraphic significance is a key feature of tonsteins. Because they originate from volcanic ash, tonsteins can act as regional marker beds that help correlate coal seams across basins. Their relatively uniform thickness and distinguishable composition make them useful for constructing coal-measure stratigraphy and for inferring the timing and extent of volcanic activity during coal formation.

Distribution and implications: tonsteins are reported in Carboniferous coal-bearing sequences in Europe and North America, where volcanic ash falls were common during peat formation. They are valuable both for regional correlation and for paleoenvironmental reconstructions, as the preserved microfossils and the ash-derived geochemistry provide insights into past volcanic events and sedimentation processes in coal-forming environments.