Abstract: The study area covers volcanic-volcanosedimentary units of Eocene age in the Sivas-Ulaş area. Thepyroclastic (tuffaceous claystone / siltsone / sandstone, crystal ash tuff) and volcanic (basalt, basalticandesite, andesite) rocks of the Karacalar member from Kaleköy formation include volcanogenic(plagioclase, augite, hornblende, biotite), diagenetic (K-feldspar, mixed-layered chlorite-smectite / C-S,chlorite, smectite, analcime) and post-volcanic (calcite, dolomite, quartz) minerals. The volcanogenic(plagioclase), diagenetic (K-feldspar, C-S, chlorite, smectite), post-volcanic (quartz, calcite, dolomite) anddetrital (illite) minerals were observed in the epiclastic (shale, siltstone, calcareous siltstone, sandstone, calcareous sandstone) and chemical (limestone, gypsum) rocks of the Yapalı member from this formation.C-S and chlorite amounts increase, but illite content decreases in the vertical direction of the formation.C-S + K-feldspar zoning is widely developed by due to the interaction between sea-water and volcanicglass in basic-intermediate composition, on the basis of optic and electron microscopes and also X-raysdata. This zone corresponds to the deeper parts of the Sivas basin in the Eocene period and show verticallya transition in zeolite zone in approximately northern parts of the basin (Yavu area).