Thirty-four K-bentonites from the Marias River Formation (Upper Cretaceous) in the Montana disturbed belt have been studied in detail. A 2.5-m-thick K-bentonite is zoned mineralogically and chemically, the upper and lower contacts of the bed being more illitic and K-rich than the middle of the bentonite. K/Ar dates of illite/smectite separated from the bed are also zoned, yielding older ages at the contacts than at the center. A model for the formation of K-bentonites enclosed by K-rich shale is presented in which K-bentonites form from smectite bentonites during a thermal event (T = 100–200 °C). Potassium is derived from mineralogic breakdown (probably mica and K-feldspar) in the shale, and migrates by diffusion into the bentonite.