Three exposures of Cambrian rocks occur as erosional remnants in the Libby Trough, Montana. The trough is a northwest-southeast striking faulted syncline, located between Thompson Falls and Libby, Montana. Three measured sections were obtained in the southernmost exposure, and one measured section each in the northern exposures. Nine hundred five samples were collected, stabbed, polished and described using a binocular microscope. Ninety representative thin sections were examined to verify the primary rock descriptions. Cambrian rocks throughout western Montana include a basal sandstone overlaid by a succession of two shale and two carbonate units in alternating sequence. Although the measured sections are incomplete, the carbonates can be subdivided into five lithologic units. Quartz arenites of the Flathead Formation (approximately 8 m thick) form the basal unit of the transgressive sequence. Shales with interbeds of shaley sandstones and shaley lime mudstones characterize the Wolsey Shale (approximately 75 m thick). The overlying carbonate sequence (Fishtrap dolomite) is about 915 m thick and is divided into five lithofacies.
- Pub Date:
- February 1979