The paper presents the results of composition examination of the uppermost of the Pogozhskaya rhythmic member from the northern near-slope fringes of the Caspian Region. The potassium-bearing horizon encountered there is determined to have resulted from the demineralization stage overlapping the carnallite and the bischofite stage deposits. In potassium salt accumulation, the major importance is attributed to the process of potassium ion concentration during the large-scale precipitation of salt-out halite associated with inflowing brine condensed to halite stage.