The less than 2-micron fraction of deltaic sediments of the central North Slope, Arctic Alaska, were analyzed by X-ray diffraction. In almost all samples illite is the predominant clay mineral; smectite, chlorite, and kaolinite are present in minor amounts. In the Colville Delta, there is a notable increase in the illite/smectite ratio and a decrease in the smectite/kaolinite ratio from the fluvial channels to the saline fluviomarine and marine regions adjacent to the estuarine mouth. These changes in clay mineral assemblages presumably are due to reconstitution

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in the saline environment, through K⁺ adsorption and/or cation exchange, of either degraded illites or mixed-layered illite-smectite derived from nonsaline fluvial channels. This conclusion is supported by results of laboratory studies conducted on fresh and brackish water clays with seawater. The observed environmental variations in clay mineral assemblages may have important implications on paleogeographic interpretations.

There are two clay-mineral zones in the shallow-marine facies of the deltaic complex. Clays west of Oliktok Point have markedly lower illite/smectite and illite/kaolinite ratios than clays east of the Point. These lateral variations probably are related to differences in clay-mineral sources rather than depositional environments.

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