PRELIMINARY-STUDY OF TONSTEINS OF THE PASTORA FORMATION (STEPHANIAN B) OF THE CINERA-MATALLANA COALFIELD, NORTHWESTERN SPAIN

The correlation of seams of the Pastora Formation in the Cinera-Matallana basin is affected by facies changes, seam discontinuities, such as seam splitting and joining, and tectonic features. Exploratory drillings in the Matallana syncline and in an underground exposure at the northern flank of the Llombera syncline prove for the first time the existence of six tonsteins. The investigation provided three kaolin tonsteins, one kaolinite/illite transition tonstein and two illite tonsteins, which are described in detail. The determination of coal rank gives evidence that there is no strict co-incidence with the degree of diagenesis or metamorphosis of the original volcanic ashes. Thus, further reasons for the alteration have to be considered, such as: varying compositions of the original volcanic ashes, biochemical and chemical processes after deposition, local hydrothermal metasomatism caused by the intrusion of basic magma or tectonic stress. Besides known faunal and facies marker horizons, these newly detected tonsteins represent first-class markers within the Pastora Formation. The possibility of using these layers for the correlation of strata and for palaeomorphological studies depends on the successful continuation of the search for additional tonsteins, as well as on the compilation and evaluation of subsequent investigations.