Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

F.R. Karner


Sedimentary materials of the Sentinel Butte Formation have been petrographically examined using optical and scanning electron micro scope/microprobe techniques. The formation consists of fine-grained materials which generally are classified mineralogically as volcanic litharenites or feldspathic litharenites. Most rock units in the formation are siltstones and mudstones. Multiple source rock types, including volcanic, metamorphic, and sedimentary, are represented by mineralogic constituents, but volcanic rock fragments are most abundant. Petrographic distinctions between basal and uppermost sandstone units suggest that a change in sediment supply took place near the end of Sentinel Butte time. Authigenic cement development appears concentrated in more porous and permeable sandstone units. A general pattern of cement development is suggested; pore-lining montmorillonite precipitation preceded pore-filling zeolite development, which was followed by calcite or dolomite growth.

A widespread volcanic ash and bentonite unit in the formation indicates that volcanism accompanied Paleocene sedimentation and that volcanic glass can be preserved for longer periods of geologic time than commonly thought possible. The manner of preservation of the bentonite/ ash unit makes it ideally suited for testing the usefulness of chemical correlation procedures for bentonites developed in terrestrial settings. Petrographic comparison of the Sentinel Butte bentonite/ash with other claystone units may yet reveal the presence of other bentonites in Paleocene strata. Chemical correlation of newly discovered bentonites may lead to an improved understanding of the time-stratigraphic framework of the Fort Union Group.

Sodium montmorillonite is the most abundant clay mineral in the formation. Other clay minerals, including kaolinite and illite, are minor. Detrital and authigenic montmorillonite appears to be distinguishable on the basis of discriminant analysis of major element composition. Comparisons of lignitic samples suggest that lignite precursor material representing various stages of coalification is present in the formation. Similar characterization studies of other Fort Union Group rocks eventually may lead to the determination of petrofacies and sediment dispersal patterns.

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