Late Cretaceous bentonitic clays in the Kelkit Valley region of Turkey are composed of an alternation of limestone lenses and silicified tuff nodule-bearing pyroclastic rocks and their alteration products. Quartz, feldspar, biotite, trace amounts of augite together with pumice and volcanic rock fragments comprise the volcanogenic components. Diagenetic minerals are represented by clay, calcite, opal-CT, zeolite and dolomite. The clay fraction is dominated by smectite and lesser amounts of I-S, illite, chlorite and kaolinite. The d001 basal spacing of dioctahedral smectites ranges from 12.51 to 12.55 Å in Na-smectites and 14.97 to 15.52 Å in Ca-smectites. The CaO/Na2O ratio of smectites ranges from 0.15 to 19.50, and the interlayer Na and Ca contents are 0.22–0.30 in beidellites and 0.02–0.09 while those in montmorillonites are 0.01–0.13 and 0.03–0.15, respectively. The data obtained indicate that bentonites formed in a marine environment by the alteration of volcanic ash of rhyodacitic/dacitic and intermediate/acidic composition.