Stratigraphy of Paleocene and Pleistocene Sediments Exposed Along Lake Sakakawea in Eastern Dunn and Western and Central Mercer Counties, North Dakota

The bluffs of Lake Sakakawea, west- central North Dakota, provide excellent exposures of the Paleocene Sentinel Butte Formation and Pleistocene glacial deposits of the Coleharbor Group. The dominant sediment type in the Sentinel Butte Formation is thinly-bedded silt. Fine sand, clay, and lignite occur in lesser abundance. Lignite, clay, and peat are most commonly bounded by thinly-bedded silt. No lignite beds greater than five feet are present in the study area. The complete section of the Sentinel Butte Formation was not present in bluff exposures of the study area. The maximum exposed thickness of the Sentinel Butte Formation was 110 feet. Extreme lateral variability of lithology made detailed correlations exceedingly difficult, even across small distances such as from one bluff to the next. Glacial deposits of the Coleharbor Group are more common in bluff exposures in the eastern part of the study area and become less common and extensive towards the western edge of the study area. At least two separate units of pebble loam are present in the eastern part of the study area. Gravel and bedded silt crop out in the eastern part of the study area. Thicknesses of glacial drift in bluff exposures range from 0 to 30 feet. The glacial deposits seem to be restricted to former topographic low areas. Erosion by waves and ground water sapping is substantial, resulting in considerable land loss. This erosion is greater along the south shore of the lake

Geology of the Flathead Formation (Middle Cambrian) on the perimeter of the Bighorn Basin, Beartooth Mountains, and Little Belt Mountains in Wyoming and Montana

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