Date of Award

1995

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

First Advisor

Dr. Nels Forsman

Abstract

Large quantities of petrified wood occur in the Paleocene Sentinel Butte Formation in western North Dakota. An examination of this petrified wood has provided information about modes and degrees of fossilization, silica phases present, organic structure preservation, and parent tree type. In the study site, fossilized tree stumps were concentrated along one continuous clay horizon. Twenty petrified wood samples were obtained. Five representative samples were chosen for thin section characterization. Microscopic analyses revealed well preserved plant structures including rays, tracheids, and growth rings. These structures were suitable for identifying parent tree type, which was determined to be a gymnosperm. Silica phases involved in the petrification, include microcrystalline quartz (chert), fibrous chalcedony and megaquartz. Silica fills tracheids and also pervasively replaces organic material. Permineralization is the dominant mode of fossilization observed, with silica filling tracheids and other plant voids. Replacement of original plant material by silica is also seen. Larger fractures or veins are filled gradually with microcrystalline quartz, chalcedony, and megaquartz. Loss on ignition data supports the interpretation that observed plant structure retaining an organic composition. Variability observed in hand samples caused by differences in degree and types of fossilization. Laboratory analyses were conducted to determine if the continuous clay horizon containing the petrified stumps is a bentonite.

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