In this paper three samples of tuff, Sarmatian in age were analyzed from Bukova Glava locality on Mt. Krndija. The aim of this study was to determine the structural and mineralogical characteristics, compare the obtained results with the similar deposits of the North Croatian Basin and to reconstruct the type of magma and potential sources of materials. Analyzes of granulometric composition, analysis of carbonate content, separation of heavy and light mineral fraction and diffraction X-ray analysis were performed on these samples. The results of granulometric data show that these are medium to poorly sorted samples where two of them are named like mud, and one of them like silt. Carbonate content is very low in the analyzed samples. Within the heavy minerals assemblage, barite, jarosite, clinozosite, epidote and zoisite dominate, while zircon, apatite and garnet are less represented. The composition of the heavy mineral assemblage indicates a weak or completely excluded terrigenous input to the marine environment during the Sarmatian period. Clay minerals from the smectite group were identified by X-ray diffraction. It is assumed that smectites within the analyzed samples were formed by devitrification of volcanic glass. The samples of three analyzed tuff probably belong to the neutral rock series which is a characteristic of the active continental margin located in the Eastern Carpathians, which is considered to be the source of pyroclastic material.