On the basis of a petrographic study of regionally distributed rock samples, the Proterozoic Thelon Formation, Northwest Territories, is here subdivided into four lithofacies. Clay-mineral studies and computer-aided assessment of field-notes substantiate these subdivisions, three of which can be placed in a vertical stratigraphic succession.
Clay-mineral analysis, a study of quartz-grain types and review of the field data all indicate that quartz-rich sandstones of the Thelon Formation were derived mainly from kaolin-rich paleosols that were developed on granitoid and metamorphic rocks of the Churchill Structural Province.
The petrologic and field data confirms a fining-upward trend, culminating with a marine transgression, as proposed by Donaldson (1967).
In addition to recrystallized kaolinite, authigenic feldspar and diagenetic muscovites occur within the three main units of the Thelon Formation. The fourth subdivision, a petrographically distinct zone of possibly different age and/or origin, is characterized by alumina-rich dioctahedral chlorite in the sandstone matrix.
Recognition of the subdivisions, and refinement of petrologic and clay-mineral data for the Thelon Formation, provide a basis for more detailed studies of other, possibly correlative, Proterozoic sandstone units in the Canadian Shield.
Computer-processable field notes used in the study of field data were derived by conversion of the original field notes to a computable form. The system proved to be highly efficient, testifying to the potential of using field-note systems that are computer oriented. A field-note form was developed for future studies in the Thelon Basin and comparable Proterozoic basins, and recommendations for the use and derivation of computable field-note forms are presented herein.