tonstein

noun2

1. •
   A hard, compact, kaolinite-rich claystone formed by the diagenetic alteration of volcanic ash, usually occurring as thin, laterally persistent layers interbedded with coal seams.

   • Tonsteins, which are altered volcanic ash, mark instantaneous events over large areas and thus are excellent time markers for basin analysis. — 1990
2. •
   Any indurated argillaceous rock that stands out within coal measures as a distinctive marker horizon.

   • In Figure 13, all data points of the tonstein samples fall within the CHARAC range. — 2025