Date of this Version
USGS Geologic Investigations, Map I-2545
GEOLOGIC INVESTIGATIONS SERIES
Published by the U.S. Geological Survey, 1997
This map is necessarily generalized. For more detailed information, consult the geologic data stored in files of the Conservation and Survey Division, University of Nebraska-Lincoln. Other geologic maps at different scales that include all or parts of the map area are by Darton (1903a,b,c), Lugn (1939), Weeks and Gutentag (1981), and Weeks and otners (1988). Detailed groundwater investigations of parts of the map area were done by Wenzel and others (1946). Babcock and Visher (1951, 1952), Bjorklund (1957), Bjorklund and Brown (1957), Smith (1966, 1969), Smith and Souders (1971,1975). Souders (1986), Gottula (1993), and Verstraeten and others (1995).
The Scottsbluff 1° x 2° quadrangle, in western Nebraska, is in the northern part of the High Plains section of the Great Plains Physiographic Province (Fenneman, 1931). The map area is mostly a constructional plain that has beEn dissected principally since the Pliocene by the rivers and streams that drain the area. Most ofl the southern half of the map area contains the partly eroded remnant of the High Plains known as the Cheyenne Tableland, a region occupying about 3,600 mi2. Lodgepole Creek, and to a lesser extent Lawrence Fork, Ash Hollow, and tributaries of Pumpkin Creek. and the North Platte and South Platte Rivers have modified the topography of the tableland (Condra, 1946). The part of the High Plains north of the valley of the North Platte River is mostly in the Sand Hills but includes a small part of the Box Butte Tableland. Between the Cheyenne Tableland and the Box Butte Tableland and Sand Hills are the Pumpkin Creek valley and Wildcat Ridge, a remnant of the High Plains capping the deeply dissected divide separating Pumpkin Creek from the North Platte Valley. These parts of western Nebraska are historically important. because they were crossed by the Mormon Trail, Oregon Trail, Sidney-Deadwood Trail, the Pony Express (remnants of all of these can be seen today), and the Union Pacific Railroad along Lodgepole Creek. Famous landmarks include Ash Hollow, Chimney Rock, Jail and Courthouse Rocks, and Scotts Bluff. The geologic history of the rocks exposed in the map area began some 30 million years after the Cretaceous seas withdrew from the Western Interior Seaway, and the land was subjected to subaerial processes. [n the map area, the oldest Cenozoic rocks of the upper Eocene Chadron Formation were deposited in a paleovalley system (fig. 1) eroded into Cretaceous rocks. Upland areas during late Eocene time were mainly in the southwestern and western parts of the quadrangle. A north-trending branch of a paleovalley system along the east side of the quadrangle may have been controlled by the Rush Creek structure (Diffendal, 1980; Swinehart and others, 1985). After fluvial depOSition of the Chadron ceased, dominantly eolian volcaniclastic sediments (upper part of the Chadron Formation and the Brule Formation of the White River Group) were deposited during the late Eocene and early Oligocene. The middle part of the Brule Formation does, however, locally contain thick alluvial deposits (Blodgett, 1974; Swinehart and others, 1985). A narrow paleovalley was formed across the quadrangle before deposition in the late Oligocene of the Gering Formation (Arikaree Group). The long axis of Wildcat Ridge generally coincides with the paleovalley, which contains most of the Gering Formation in the quadrangle (Swinehart and others, 1985; Tedford and others, 1966). Although the Gering is mostly composed of fluvial sediments, most of the overlying upper Oligocene and lower Miocene Arikaree Group (Harrison and Monroe Creek Formations, undivided, and beds of the informal Camp Clarke unit) are composed of eolian volcaniclastic deposits. In contrast to the White River and Arikaree Groups, the Ogallala Group (middle and upper Miocene) is dominantly flUvial, paleovalley-fill depoSits. Most of the alluvial materiaJ was derived from erosion of rocks in the Southern Rocky Mountains (Stanley, 1976). The principal Ogallala paleovalley system extends across the southern part of the quadrangle (fig. 2). Toward the end of Ogallala Group depOSition, a southeast-trending paleovalley was eroded north of the present North Platte Valley and later filled with the informal Angora sand and gravel beds. The Broadwater Formation (Pliocene) consists principally of fluvial sand and gravel and fills a major paleovalley north of the present North Platte River (Swinehart and others. 1985; Swinehart and Diffenda), 1987). The North Platte River system probably began to form during erosion of the valley that was filled by the Angora sand and gravel beds (upper Miocene), but principal tributaries like Pumpkin Creek apparently developed later, during or after the early Pleistocene (Diffendal and Comer, 1983). From western Garden County westward across the quadrangle, the North Platte River has migrab~d generally southward and has entrenched several times, whereas Pumpkin Creek has migrated northward and has also entrenched several times. The map area is mostly covered by surficial depoSits composed chiefly of eolian sand, loess, colluvium, and alluvium (Mortlock and others, 1920; Hayes, 1921; Wolfanger and others, 1924; Clocker and others, 1962; Sautter and others, 1965; Yost and others, 1968; Helzer and others, 1985; and unpublished preliminary soil sutveys of Banner, Cheyenne, Garden, and Keith Counties being prepared by the Soil Conservation Service). Deformation has affected parts of the quadrangle (DeGraw, 1969; Diffendal, 1980; Sonnenberg and Weimer, 1981; Swinehart and others, 1985; Ahlbrandt and Groen, 1987). Surface rocks from Sidney Draw in southwestern Cheyenne County northeastward to southwestern Garden County are folded. Contour maps of the base of the Ogallala Group (fig. 2) and on the base of the Cenozoic rocks (fig. 1) show highs and lows that correspond with fold axes shown by Sonnenberg and Weimer (1981).
The authors thank the many farmers and ranchers who allowed us to work on their lands. We also thank v.L. Souders and FA. Smith (Conservation and SUlVey Division), J.J. Gottula (Nebraska Department of Environmental Quality), RG. Goodwin (Woodward-Clyde Consultants), and M.R. Voorhies and R.M. Hunt, Jr. (University of Nebraska State Museum).