Illite-smectites in cored Dakota Group (Cretaceous) shales and bentonites of the northern Denver Basin, identified using a modification of Reynolds' and Hower's (1970) X-ray diffraction criteria, show decreasing sample-sample expandability ranges, lower modal expandability, and a change from random to ordered mixed-layering with increasing burial depths and temperatures. These are results characteristic of burial diagenesis as described by Perry and Hower (1970). Ordered illite-smectites from low-temperature wells and much of the kaolinite (+ or - chlorite, vermiculite) and discrete illite from all wells are interpreted as detrital. Episodic but frequent admixture of highly-expandable bentonitic illite-smectite with detrital clays is reflected by fluctuating expandabilities within individual cores and the lack of samples containing both highly-expandable illite-smectite and high proportions of other clay minerals. Differential diagenesis and/or differential sedimentation rates do not satisfactorily account for lower modal expandabilities and increased proportions of ordered illite-smectite observed in shales from "D" and "J" sand-stones compared with those from "Huntsman" and Skull Creek shales of equivalent temperatures. Instead, they are attributed to provenance differences which gave lower-expandability illite-smectites--and a "head start" on diagenetic trends--to sand-associated shales. Eastern and western provenances identified from heavy mineral studies (e.g., MacKenzie and Poole, 1962) for "D" and "J" sands were not matched by similar clay mineral provenances affecting shales. However, decreasing proportions of kaolinite (+ or - chlorite, vermiculite) encountered from northeastern to southeastern "D" locales are interpreted as the results of differing sources.