Tonstein occurrences in the Raton coal field, Colfax County, New Mexico
— Bruce F. Bohor and C. L. Pillmore

Tonstein (from the German; literally translated as "clay-stone") partings are well known to European coal geologists. Williamson (1970, p. 119) defined them as:

 

.. dense mudstones containing kaolinite aggregates and crys-tals within essentially similar matrices. Usually occurring as thin beds less than 6 cm, they are typically developed in the coal measures facies. Although rare, they are of widespread distribution and form isochronous correlative horizons which have been extensively utilized in the European coalfields." Tonsteins have not been widely recognized in American coal strata, and only sparse reports of their presence have appeared in geologic literature of this country. The first American worker to describe a tonstein parting in coal in the western United States was apparently Rogers (1914), although he did not identify it as such. Much later, Hoehne (1959) discussed some tonstein-like partings found in North America. Seiders (1965) pointed out the similarity between a flint clay parting in the Fire Clay Coal Bed in eastern Kentucky and European tonsteins, and ascribed a volcanic origin to the former. Asquith (1968) noted the presence of large vermicular kaolinite crysyals in a clay parting associated with coal in the Almond Formation (Upper Cretaceous) of Wyoming, but Williamson (1969) was the first to point out that this occurrence was probably a tonstein.

 

Recently, tonsteins have been discovered in many Cretaceous and Tertiary coals of the Rocky Mountain region (Bohor and others, 1976). Field and mineralogical evidence indicate that the kaolinitic partings in these coals are actually altered volcanic ash layers. Knowledge of this mode of origin enhances the potential usefulness of tonsteins in correlation. This report documents the occurrence of tonstein layers in some coals of the Raton coal field, the New Mexico portion of the Raton Basin (Fig. 1), and presents evidence for their volcanic origin. It does not discuss all the tonstein layers extant in the coal field, nor does it attempt to make any correlations using them. Because coal deposits are lenticular in the Raton coal field, the stratigraphy and correlations of individual coal beds present complex problems. We hope that this discussion of tonsteins will indicate their usefulness to geologists as a new tool for correlating coal beds in this area.