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Geology of Mitchell and Osborne Counties

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Historical Summary

By J. W. Ockerman and K. K. Landes

Pre-Cambrian rocks have been reached by only one well in Mitchell and Osborne counties, but it is possible to reconstruct a picture of the pre-Cambrian landscape from records of wells in adjacent counties. The region at that time was probably a broad undulating plain underlain by granite and other crystalline rocks. Erosion of the pre-Cambrian surface was brought to an end in late Cambrian or early Ordovician times by submergence beneath the sea. Thick limestone beds, subsequently altered to dolomite, and sand were deposited. Shale deposits were laid down next and these in turn were covered by thick limestones. Sedimentation during the Ordovician period was apparently interrupted several times, as some of the Ordovician formations are missing. Deposition continued on into the Devonian period, during which time more limestones were deposited. Then the land rose above the sea and erosion began. In the interval between the recession of the Devonian sea and the advance of the Mississippian sea marked orogenic movements took place. The Nemaha mountains and the Barton arch were pushed up and the Salina basin was formed between them. Mitchell and Osborne counties lie in the western part of Salina basin. Another long period of erosion followed, and a great thickness of Devonian and Ordovician rocks were eroded away. The region was probably reduced to a low plain of moderate relief by the time the Mississippian sea finally covered the area.

Two types of sediments were deposited in the Mississippian sea--a gray to black shale at the base and thick cherty limestone at the top. The silicification of the Mississippian limestone was widespread, for the limestone shows an abundance of chert wherever found.

Uplift and folding occurred again at the end of Mississippian time, and the regions of the Barton arch and Nemaha mountains were warped up as before. The Salina basin remained as a trough between the two uplifted areas. With the emergence of the land, erosional agencies began to remove the Mississippian, and in places it was entirely removed and the older formations eroded in part. Once more the area was reduced to a low plain of moderate relief, which was inundated by the Pennsylvanian sea advancing from the southeast.

The Pennsylvanian was predominantly a period of shale and limestone deposition. The presence of unconformities between the various Pennsylvanian formations suggests that the sea oscillated back and forth on its western border. Deposition continued apparently undisturbed from Pennsylvanian into Permian, as no unconformity has been found between them. The periods are separated by assuming that no thick red beds were deposited during the Pennsylvanian. The first deposits of the Permian were gray shale, anhydrite, gypsum and salt, with a few limestone beds. The salt series thins to the north and east and is not found in northeastern Osborne County or in the northern part of Mitchell County, either because it was not deposited there or because it has been eroded away. Later in the Permian thick deposits of nonmarine red beds were laid down.

Uplift of the continent caused the withdrawal of the Permian sea. A long period of erosion followed, during which time an unknown thickness of Permian sediments was removed. Just prior to the Cretaceous (Comanchean) submergence, diastrophism elevated the eroded Permian surface in north-central Kansas and caused a rejuvenation of the streams draining the area. The outcropping shales were eroded into hills and valleys of moderate relief. Before erosion could base level this area it was submerged once more and Cretaceous deposition began. Shales and sandstones and a few limestones were deposited over the hills and valleys. The earliest sea was probably marine (Comanchean), but later in the Cretaceous this sea became land-locked and fresh-water sediments of Dakota age were deposited.

Further submergence permitted the marine waters to once more cover the land and the Graneros shales and the limestones and limy shales of the Greenhorn formation were deposited. There followed a time when the sea received large amounts of clay and silt from the streams emptying into it, and the Carlile shale was deposited. By Niobrara time the sea had once more become very limy, so the sediments deposited were highly calcareous. Before the close of the Niobrara the land began to emerge and the sea to withdraw toward the west. A long period followed during which no sedimentation occurred in Mitchell and Osborne counties, but as the area was probably at no great elevation above the sea the amount of material eroded was of small magnitude.

At the close of the Cretaceous earth movements to the west caused the formation of the Rocky Mountains and the complete withdrawal of marine waters. During the succeeding period, the Tertiary, streams running eastward from the Rockies carried a heavy load of sediment which was deposited over thousands of square miles, forming the Great Plains. A veneer of Tertiary sediment probably originally extended across both Osborne and Mitchell counties.

The Tertiary rivers finally diminished in size and deposition ceased. The streams draining Mitchell and Osborne counties began to deepen and widen their valleys so that most of the Tertiary and some of the Cretaceous rocks were removed. Some of the streams shifted their channels, leaving sand and gravel deposits where the old" channels once were. The major streams finally reached the downward limit to which they could erode and commenced to deposit alluvium in the form of flood plains in the valley bottoms. ' Where a stream built a flood plain at a level that was but a temporary limit and then later deepened its valley a terrace was formed. This process of erosion and deposition still continues.

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Kansas Geological Survey, Geology
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