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NORTHWESTERN KANSAS.

The northwestern part of Kansas is underlain by a thick body of later Cretaceous shale, chalk, and sandstone, covered by a mantle of Tertiary sand, gravel, and grit. The Pierre shale and Niobrara chalk and shales are revealed in the larger valleys, and although to the eye the beds appear to lie flat they dip in various directions. In Decatur and Sheridan counties they slope to the northwest on the west side of an anticline whose axis lies near longitude 101°. In the

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FIGURE 4.-Diagram showing structure of Dakota sandstone in Syracuse and Lakin quadrangles, Kans. Contour lines and figures show elevation of Dakota sandstone above sea level.

counties to the west and south the dips are mostly to the north, into the deep basin in the extreme northwest corner of the State. In Gove County the beds lie nearly horizontal. Doubtless there are many local variations in the direction and rate of dip, but they are difficult to detect in the shales, especially as in wide areas these rocks are covered by the Tertiary deposits. Deep borings have been made at many places in western Kansas. All those of which information has been received are given in the following table:

Deep borings in Kansas west of longitude 100°.

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Deep borings in Kansas west of longitude 100°-Continued.

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The general geologic structure of Nebraska is shown in figure 5.

SOUTHEASTERN NEBRASKA,

The greater part of southeastern Nebraska is underlain by the Dakota sandstone, above which in places are the Graneros shale and Greenhorn limestone. Limestones and shales of Pennsylvanian age come to the surface in Cass, Otoe, Johnson, Gage, Pawnee, Richardson, Nemaha, and Lancaster counties. Nearly all of the upland is covered by glacial drift and loess, and the larger valleys are occupied by loess and alluvium. Owing to these surficial deposits, rock outcrops are får apart, and there is difficulty in determining the stratigraphy and structure. The Dakota sandstone and overlying shales dip west at a very low angle, and in places they may form domes, notably one a few miles west of Lincoln and another near Cortland. The underlying Carboniferous rocks appear to lie nearly horizontal or to dip to the west and south, but on close examination. the structure is found to be complex, with many local variations in direction and amount of dip. In a recent study of the rocks along Missouri, Platte, Weeping Water, and Big Nemaha rivers Condra and Bengtson have found a general rise of the strata toward an anticline whose crest is near Oreapolis and Plattsmouth, and there is a deep basin in Nemaha County.

Numerous deep borings have been made in southeastern Nebraska without finding oil or gas. The principal ones are given in the list on page 10. The deepest boring is the 3,010-foot hole at Ne

1 See contour map, Darton, N. H., Underground waters of a portion of southeastern Nebraska U. S. Geol. Survey Water-Supply Paper 12, pl. 20, 1898.

* Condra, G. E., and Bengtson, N. A., The Pennsylvanian formations of southeastern Nebraska Nebraska Acad. Sci. Pubs., vol. 9, No. 2, 1915.

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braska City, entirely in limestone, shale, and sandstone. The old 2.463-foot hole at Lincoln appears to have entered pre-Cambrian quartzite at 2,192 feet, but the identity of the rock is not fully established. The many deep artesian wells at Omaha yield large supplies of excellent water from sandstone under the Carboniferous limestones and shales.

NORTHEASTERN NEBRASKA.2

Most of the surface of northeastern Nebraska is covered by glacial drift or loess, and outcrops of rocks are scarce. The rocks consist of the Dakota sandstone overlain by strata of Benton and Niobrara age and underlain by Carboniferous limestones, which were penetrated by a hole at Ponca and by many deep artesian wells in Omaha. In general the rocks dip due west at a very low angle into a basin whose axis lies in Rock, Brown, and northwestern Cherry counties. Near and northwest of Niobrara the dip is to the southwest, into this basin. Doubtless there are other variations in the direction of dip, and possibly minor anticlines and synclines interrupt the general monocline.

Deep borings have been made at many places, which are listed below. In Knox, Cedar, Boyd, and Dixon counties there are many

'Darton, N. H., Preliminary report on the geology and underground water resources of the central Great Plains: U. S. Geol. Survey Prof. Paper 32, p. 283, 1905.

The geology of the northeasternmost countles is described by G. E. Condra (Geology and water resources of the Missouri River valley in northeastern Nebraska: U. S. Geol. Survey Water-Supply Paper 215, 1908).

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FIGURE 5.-General section across Nebraska from Omaha to the Rocky Mountains.

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artesian wells drawing from the Dakota sandstone at depths of 400 to 600 feet.

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The rocks exposed in central-southern Nebraska are the Dakota sandstone, 200 to 300 feet thick; shale and limestone of Benton age, about 270 feet thick to the east and probably 500 feet or more to the west; the Niobrara formation, 350 feet thick; the Pierre shale, which is at least 1,500 feet thick near the Colorado State line; and the cover of Tertiary grit and sand on the plateaus and alluvium in the valleys. The Dakota sandstone contains members of clay; the formations of Benton age comprise the Graneros shale at the top, the Greenhorn limestone, 25 to 50 feet thick, in the middle, and the Carlile shale at the bottom; the Niobrara has a basal member of chalky limestone, and the upper part is chalk and shale. Below the Dakota sandstone

to the cast is a thick succession of limestones and shales of Carboniferous age; to the west there are probably "Red Beds" of considerable thickness, but their stratigraphic relations are not known.

The following quotation from Condra's report1 gives the principal facts as to the structure of south-central Nebraska :

In southern Nebraska the rocks lie nearly horizontal, but, as shown in the section [fig. 6], with a general westerly inclination, interrupted near Cambridge by a low uplift or arch discovered by Mr. Darton several years ago. This arch develops in northwestern Kansas and extends northwestward across Nebraska. Its configuration is defined by the relations of the Niobrara formation, which is uplifted 200 feet or more near the crest of the arch, in the vicinity of Cambridge. From the crest the Pierre shale and 100 feet or more of the Niobrara have been eroded in the central and western portions of Furnas County. On the west side of the anticline the strata dip westward at an unknown but moderately rapid rate, so that in the extreme southwestern corner of Nebraska the top of the Niobrara formation is at a depth of about 2,000 feet, judging by the structure in adjoinIng portions of Colorado. East of the anticline there is a shallow syncline which holds Pierre shale from Arapahoe east to Naponee; thence eastward the strata rise gradually, and formations from Niobrara to Dakota outcrop in regular succession in the counties from Franklin to Jefferson. In general, the rate of inclination is regular and averages 3 feet in a mile. Locally, however, the rate is very much increased, especially in portions of Jefferson County. A local flexure is presented by the Greenhorn limestone in the southern portion of Jefferson County, and steep local dips of 10° or more occur to the southwest, in the exposure of Dakota sandstone south of Thompson, Nebr. The Dakota formation presents also a low anticline in the vicinity of Thompson and Gladstone, with the upper part of the arch removed by erosion. Faults, showing a few inches of displacement, occur in the Niobrara chalk in a number of places.

From the inclination of the rocks in the Medicine Creek valley northwest of Cambridge, it was believed that the axis of the anticline rises somewhat to the north and that a dome occurs in its crest near Stockville.

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1Condra, G. E., Geology and water resources of the Republican River valley and adjacent areas, Nebraska: U. S, Geol. Survey Water-Supply Paper 216, p. 11, 1908,

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