Geologic History and Geomorphology
Paleozoic EraRocks of Paleozoic age do not crop out in northwestern Kansas, but many facts concerning them and the Paleozoic history of this region are known from deep tests drilled for oil and gas. The known geologic history of this area starts with erosion of the pre-Cambrian basement rocks that occur below the Paleozoic strata. This ancient erosion surface was submerged below sea level and marine sediments were deposited upon it. Throughout much of Paleozoic time the area was successively submerged and elevated. Marine sediments accumulated during periods when the surface was below sea level, and these deposits were subsequently eroded during periods of emergence. The lower Paleozoic rocks consist for the most part of marine limestone, shale, and sandstone.
Thomas County lies just off the southwest flank of the prominent regional structure in the Paleozoic rocks known as the Central Kansas uplift, and the pre-Cambrian surface below the county is less than 2,500 feet below sea level. According to Moore and Jewett (1942) an important structural event occurred in this area between Devonian and Mississippian deposition. This consisted of a regional arching of the strata along a northwest-southeast axis and is indicated by the fact that pre-Mississippian erosion truncated the earlier Paleozoic rocks and stripped off all the beds down to the Arbuckle limestone. Although data from the immediate vicinity of Thomas County are not conclusive as to such a history, it is inferred from relationships farther east, where many more data are available, that Thomas County is included in the general area of the structure designated as the Ellis arch. This period of uplift and subsequent erosion is believed to have been followed by marine inundation and resulting deposition of the Mississippian strata over this part of Kansas.
The rocks of northwestern Kansas were again uplifted and warped along this same general structural trend at the close of Mississipian time or during early Pennsylvanian time to form the structural feature now recognized as the Central Kansas uplift. Moore and Jewett (1942) show that this structure in the central part of Kansas is nearly coincident with the earlier structure they designated as the Ellis arch. In the northwestern part of the state, however, they indicate that the two structures do not occupy the same location. The Ellis arch trended to the west-northwest across this part of the state and included all of Thomas County, but the Central Kansas uplift trends north-northwest and includes the northeastern corner of Sheridan County to the east of Thomas County.
The Mississippian strata believed to have existed across the top of the Central Kansas uplift were largely stripped away by early Pennsylvanian erosion, but more than 250 feet of these rocks remain beneath Thomas County. Coarse clastic sediments accumulated along the flanks of the uplift as a result of this period of erosion, and it is believed that they may have been contemporaneous with the denudation deposits that were spread out toward the east from the ancestral Rocky Mountains.
The sea again invaded the area and marine deposits accumulated across all of northwestern Kansas during Pennsylvanian time. During the latter part of the Paleozoic, marine conditions were less prevalent and at times sediments accumulated on the surface of the land. Thus marine and nonmarine deposits occur alternately throughout rocks representing upper Pennsylvanian and Permian time. Evaporites and nonmarine sediments became more prevalent throughout Permian time, indicating an intermittent but progressive withdrawal of the sea.
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.