Geology of McLean County, North Dakota

McLean County, in west-central North Dakota on the east side of the Williston Basin, is covered by 8,500 to 13,000 feet of Paleozoic, Mesozoic, and Cenozoic rocks, which dip to the west at low angles. The Tertiary Tongue River and Sentinel Butte Formations are widely exposed in McLean County, and the Tertiary Cannonball Formation is exposed in a few places. Glacial drift, assigned to a new formation, the Coleharbor Formation, occurs throughout the county and reaches a maximum thickness of at least 400 feet in certain preglacial valleys. The eastern part of McLean County, part of the Missouri Coteau, is dominated by hilly topography on dead ice moraine with associated lake sediments, ice-contact gravel deposits, and areas of collapsed outwash. Much of the remainder of the county is part of the Coteau Slope and is characterized by gently rolling topography on ground moraine. The Missouri Trench, with steep bedrock slopes, forms the western boundary of the county. Glacial deposits of probable pre-Wisconsinan age were identified in McLean County, but little is known of either their exact age or the circumstances under which they were deposited. Glacial drift of probable Early Wisconsinan age covers most of the area of the Coteau Slope and Late Wisconsinan drift covers most of the Missouri Coteau. Both the Early and Late Wisconsinan glaciers stagnated as they thinned and receded from the area, resulting in hummocky areas of dead-ice moraine. The modern route of the Missouri River did not become firmly established until the Late Wisconsinan glacier receded from the area. This modern route of the river is a composite of several valley segments that range in age from preglacial to Late Wisconsinan

Quaternary geology of the Northern Great Plains

The Great Plains physiographic province lies east of the Rocky Mountains and extends from southern Alberta and Saskatchewan nearly to the United States-Mexico border. This chapter covers only the northern part of the unglaciated portion of this huge region, from Oklahoma almost to the United StatesCanada border, a portion that herein will be referred to simply as the Northern Great Plains (Fig. 1). This region is in the rain shadow of the Rocky Mountains. Isoheyets are roughly longitudinal, and mean annual precipitation decreases from about 750 mm at the southeastern margin to less than 380 mm in the western and northern parts (Fig. 2). Winters typically are cold with relatively little precipitation, mostly as snow; summers are hot with increased precipitation, chiefly associated with movement of Pacific and Arctic air masses into warm, humid air masses from the Gulf of Mexico. Vegetation is almost wholly prairie grassland, due to the semiarid, markedly seasonal climate. The Northern Great Plains is a large region of generally low relief sloping eastward from the Rocky Mountains toward the Missouri and Mississippi Rivers. Its basic bedrock structure is a broad syncline, punctuated by the Black Hills and a few smaller uplifts, and by structural basins such as the Williston, Powder River, and Denver-Julesburg Basins (Fig. 3). Its surface bedrock is chiefly Cretaceous and Tertiary sediments, with small areas of older rocks in the Black Hills, central Montana, and eastern parts of Wyoming, Kansas, and Oklahoma. During the Laramide orogeny (latest Cretaceous through Eocene), while the Rocky Mountains and Black Hills were rising, synorogenic sediments (frequently with large amounts of volcanic ash from volcanic centers in the Rocky Mountains) were deposited in the subsiding Denver-Julesburg, Powder River, and other basins. From Oligocene to Miocene time, sedimentation generally slowed with declining tectonism and volcanism in the Rocky Mountains. However, since the later Miocene, epeirogenic uplift, probably associated with the East Pacific Rise, affected the Great Plains and particularly the Rocky Mountains. During the last 10 m.y. the Rocky Mountain front has risen 1.5 to 2 km, and the eastern margin of the Great Plains 100 to 500 m (Gable and Hatton, 1983), with half to one-quarter of these amounts during the last 5 m.y. Thus, during the later Miocene the Great Plains became a huge aggrading piedmont sloping gently eastward from the Rocky Mountains and Black Hills, with generally eastward drainage, on which the Ogallala Formation and equivalents was deposited. The Ogallala underlies the High Plains Surface, the highest and oldest geomorphic surface preserved in this region. It has been completely eroded along some parts of the western margin of the region (e.g., the Colorado Piedmont), but eastward, it (and its equivalents, such as the Flaxville gravels in Montana) locally is preserved as caprock or buried by Quaternary sediments (Alden, 1924, 1932; Howard, 1960; Stanley, 1971, 1976; Pearl, 1971; Scott, 1982; Corner and Diffendal, 1983; Diffendal and Corner, 1984; Swinehart and others, 1985; Aber, 1985). During the Pliocene, regional aggradation slowly changed to dissection by the principal rivers. In the western part of the region the rivers flowed eastward, but the continental drainage divide Figure 3. Major bedrock structures of the Northern Great Plains. extended northeast from the Black Hills through central South Dakota, far south of its present position. The ancestral upper Missouri, Little Missouri, Yellowstone, and Cheyenne Rivers drained northeast to Hudson Bay, whereas the ancestral White, Platte, and Arkansas Rivers went to the Gulf of Mexico (Fig 4A). Their courses are marked by scattered surface and subsurface gravel remnants; in Montana and North Dakota, deposits of the preglacial Missouri River and its tributaries are buried deeply beneath glacial and other sediments (Howard, 1960; Bluemle, 1972)