Geology of the Bonner quadrangle, Montana

Abstract

The geology of the Bonner quadrangle in western Montana was mapped principally to study the Missoula group of the Belt series, and the structure of the area. The type localities of the formations comprising this group are all in or near the Bonner quadrangle. These formations are here redefined and in part renamed. Like the rest of the Belt series, the Missoula group has been regarded heretofore as of late Precambrian age but in our opinion the results of the present study suggest that its upper part may be of Cambrian age. The oldest rocks exposed in the quadrangle belong to the Newland limestone, the local representative of the Piegan group. That group is characterized by the prevalence of calcite-rich rocks and is near the middle of the Belt series. Here clayey and sandy tan to gray limestone is the major component of the Newland limestone. Thin layers of very dark gray argillite are intercalated with the limestone; and noncarbonatic shale, sandy shale, and quartzite occur locally in units as much as 100 feet thick. The thickness is at least 4,000 feet. The Piegan group is succeeded upward by the Missoula group, which occupies most of the Bonner quadrangle. The Missoula group has an aggregate thickness of about 14,300 feet, and as subdivided here includes, from the base upward, the Miller Peak quartzite (including the Hellgate quartzite member), Bonner quartzite, McNamara argillite, Garnet Range quartzite, and Pilcher quartzite. All components of the Belt series within the quadrangle appear to have gradational contacts. Shallow-water features, such as mud cracks and ripple marks, are plentiful in argillite beds in both the Piegan and Missoula groups. The Miller Peak argillite is composed of red and green silty argillite and argillaceous quartzite, and locally a little pale-red and tan quartzite, and light-gray impure limestone. Some argillite units several hundred feet thick are of uniform color, but in others the colors alternate every few feet. Few argillite beds are free from clastic quartz which locally is sufficiently abundant to warrant naming the rocks clayey quartzite, or even quartzite. The quartzite forms isolated lenticular beds and lenses with some intercalated argillite. The Hellgate quartzite member, high in the formation, is one of the thickest of these lenses. It is about 1,200 feet thick, and has about 3,800 feet of argillite below it and 1,000 feet of argillite above. The Bonner quartzite (here newly named) corresponds to the middle member of the McNamara formation as previously defined. The formation is charac-189 190 CONTRIBUTIONS TO GENERAL GEOLOGY terized by its tendency to break into blocks speckled with grains of white, translucent, weathered feldspar. Pink vitreous quartzite predominates; the quartzite is commonly crossbedded and intercalated with minor amounts of green and red argillite. The formation is about 1,500 feet thick. The McNamara argillite, as here restricted, is composed largely of red and green metamorphosed clayey siltstone and minor amounts of argillite and quartzite. T...