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Structural Geology
No attempt was made to construct a map showing primarily the attitude of rock layers. To do so would be a simple engineering project, as there are numerous good "key beds," and the intervals between prominent limestone beds are remarkably constant. Geologists of several oil-producing companies have worked in the two counties and it is very probable that accurate information concerning geologic structure is available to those persons most interested in having it. For that reason the structure is discussed in only a general way and from the viewpoint of those features that were obvious to me when I made the areal geologic map.
The consolidated rocks are structurally included in the Prairie Plains monocline, i.e., beds dipping westward or slightly north of westward from the Ozark plateau. The general inclination is about 30 feet per mile. This dipping of beds can be plainly observed; for example, the Cottonwood limestone high on the hills at Manhattan disappears under the alluvial filling of Kansas River valley a short distance west of Eureka Lake (Odd Fellows' Home), and the Fort Riley limestone descends nearer and nearer the floor of the river valley as one goes westward from Junction City along Smoky Hill river. Other beds of stratified rock are approximately parallel to those just cited. There are minor distortions in the position of the layers of rock, and hence in places the general westward dip is steepened or locally the beds dip in some other direction. In southeastern Riley County there is a structural dome or anticline and the valley of Deep Creek is superposed upon it. This upwarping of strata is nearly in superposition on the buried Nemaha Mountains and is a part of the anticlinal structure that crosses the state above the buried mountains. From the distribution of outcrops in relation to topography it seems that the crest of the fold in Deep Creek valley is probably a few miles southeast of Zeandale, but the alluvium of Deep Creek valley has obscured the bedrock rock over an extensive area, so the statement must be qualified. Exposures in the Flint Hills west of Zeandale show a steep westward dip and the rocks there lie upon the west limb of the Deep Creek fold. Plate, 4A shows this condition.
The buried mountains occupy a band a few miles wide crossing Kansas from Cowley County to Nemaha County, along which the surface of the very ,ancient crystalline rocks, above which were deposited younger stratified sediments, is much higher and hence much nearer the present land surface than it is on either side. The mountains were described by Moore and Haynes (1917, p. 173) and are well known to all students of the subsurface geology of the Midcontinent region. The Nemaha Mountains extend well into Oklahoma and into Nebraska. The mountains were named Nemaha because they are not many hundreds of feet below the surface in Nemaha County, Kansas. Many geologists have discussed the probable origin of the superposed folds in relation to the buried topography. It is very probable that the flexures in rock above the buried hills and valleys are at least partly due to differential compaction and settling of the younger rock layers above the rigid crystalline rocks following the uneven surface below. It is likewise probable that the distortion is partly due to tangential compressive forces acting on the rocks. Bass (1929, pp. 126-128) has discussed the origin of structural features in southern Kansas, and inasmuch as structural features above the buried hills in Riley County are very similar, his discussions should be consulted by those who are concerned with the origin of the folds in the area of this report.
There is a noticeable arching of beds in the vicinity of Winkler, in northern Riley County in the southeast part of T. 6 S., R. 6 E. Near the town, exposures of Florence limestone in the Barneston formation show that the Wreford limestone beds must lie well below the surface of the alluvium of the flood plain of Fancy Creek; but only a short distance westward the Wreford formation lies well above the almost flat flood plain, and still farther westward it dips below the stream again. The same reversal of dips can be plainly seen in the bluffs forming the walls of Fancy Creek valley and even more plainly along streams tributary to Fancy Creek on the north. As the base of the Wreford formation is indicated on the map showing areal geology, the arch near Winkler is indicated on the map by an inlier of the Wreford formation along Fancy Creek. The fold described here is a part of the Abilene anticline, which extends from a point near Kingman in Kingman County in the south-central part of Kansas northward into Nebraska. In Nebraska it is commonly called the Barneston arch. Probably it is no more acute near Winkler than elsewhere along its axis, although elsewhere the flat topography may not reveal the arching so well.
A considerable amount of faulting on a minor scale has occurred in the region of the Abilene arch. In several road cuts, especially in T. 6 S., R. 6 E., exposures of Towanda and Winfield beds show many small normal faults, having a few feet of throw. Reverse faults, one of which has an apparent throw of about 11.5 feet, are exposed in a railroad cut in sec. 1, T. 9 S., R. 4 E. There the apparent angle of faulting is about 22 degrees, and the strata are overthrust to the westward. Because many of the better exposures in an area of several square miles exhibit faulting, it, may be assumed that there are many concealed faults. Plate 17 shows the faulted rock layers in sec. 1, T. 9 S., R. 4 E.
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Kansas Geological Survey, Geology of Riley and Geary Counties
Web version Nov. 2000. Original publication date Dec. 1941.
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