Probable Burial Diagenetic and Provenance Effects on Dakota Group Clay Mineralogy, Denver Basin

Abstract

ABSTRACT 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 (± 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 (± chlorite, vermiculite) encountered from northeastern to southeastern D locales are interpreted as the results of differing sources.