Date of Award
Master of Science (MS)
The Flathead Formation, which is 4 to 60 metres thick in the middle and northern Rocky Mountains of Wyoming and Montana, contains cross-bedded and parallel-bedded, quartz sandstone. The formation contains marginal-marine and shallow-marine sediment that was deposited unconformably on Precambrian crystalline and sedimentary rock by an eastward-transgressing sea during middle Cambrian time.
This field study of the Flathead Formation on the perimeter of the Bighorn Basin, Beartooth Mountains, and Little Belt Mountains reveals that the formation consists of three intervals. The lower interval contains medium to very coarse, pebbly, cross-bedded sandstone and conglomerate. The middle interval contains medium to coarse, cross-bedded, quartz sandstone. Cross beds dip to the west in most places, but some cross beds dipping north, south, and east are present. The upper interval contains fine to coarse, parallel-bedded, quartz sandstone.
Trace fossils are present in the middle and upper intervals. Skclithos, Monocraterion, and Rusophycus have been identified. Horizontal tubes on bedding planes are the most common trace fossils. Large vertical tubes containing smaller Skolithos tubes and bedding deflected downward in a cone-in-cone fashion were formed by anemones moving upwards in their dwelling tubes. Inarticulate brachiopods are present in the upper interval in some places. The brachiopods, which are the first to be identified in the formation belong to three genera.
Comparison of the characteristics of the Flathead Formation with modern shallow-marine and marginal-marine environments indicates the sandstone was deposited in a high-energy environment with wave-induced currents. A barrier island was not present in most places.
The sandstone was deposited on a weathered Precambrian surface with 3 metres of relief, but up to 70 metres of relief was present in some places. The thickness of the Flathead Formation varies with the amount of surface relief, Resistant knobs of Precambrian rock existed as islands in the Owl Creek Mountain area.
Degenstein, Joel A., "Geology of the Flathead Formation (Middle Cambrian) on the perimeter of the Bighorn Basin, Beartooth Mountains, and Little Belt Mountains in Wyoming and Montana" (1978). Theses and Dissertations. 71.