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)

Late Holocene Eolian Activity in the Mineralogically Mature Nebraska Sand Hills

The age of sand dunes in the Nebraska Sand Hills has been controversial, with some investigators suggesting a full glacial age and others suggesting that they were last active in the late Holocene. New accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic rich sediments suggest that eolian sand deposition occurred at least twice in the past 3000 I4C yr B.P. in three widely separated localities and as many as three times in the past 800 14C yr at three other localities. These late Holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s Dust Bowl period, based on independent Great Plains climate records from lake sediments and tree rings. However, new geochemical data indicate that the Nebraska Sand Hills are mineralogically mature. Eolian sands in Nebraska have lower K feldspar (and K2O, Rb, and Ba) contents than most possible source sediments and lower K-feldspar contents than dunes of similar age in Colorado. The most likely explanation for mineralogical maturity is reduction of sand sized K feldspar to silt-sized particles via ballistic impacts due to strong winds over many cycles of eolian activity. Therefore, dunes of the Nebraska Sand Hills must have had a long history, probably extending over more than one glacial-interglacial cycle, and the potential for reactivation is high, with or without a future greenhouse warming