The existence of pyroclastic kaolin coal-tonsteins (tonsteins, smectites) from Upper Carboniferous strata of the coal mining region of Zonguldak and Amasra, north-western Turkey is reported. In the area of Zonguldak, tonsteins were discovered within the seams Pic II and Piric (Asma 5 Mine) of the Kozlu Formation (Westphalian A). In the area of Amasra, tonsteins occur in the seams Kalin and Tavan (Tarlaagzi Mine) of the Karadon Formation (Westphalian B, C). Thin sections were prepared to investigate mineral composition and texture; detailed descriptions are presented. Kaolinite is the predominant mineral. In the Tavan tonstein montmorillonite-illite is also present. Pyrogenic phenocrysts prove that the tonsteins are alteration products of volcanic ashes. Therefore, a wide lateral distribution of these layers can be expected. These tonsteins represent very suitable stratigraphic markers in the Upper Carboniferous strata of northwestern Turkey. (C) 2000 Elsevier Science B.V. All rights reserved.