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Geohydrology of Brown County

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Structural Development

Much of the following discussion on structural development is based on the report, Stratigraphy and Structural Development of the Forest City Basin by Lee (1943).

Many unconformities are recognized in the Paleozoic rocks in northeastern Kansas. The most important of these are at the base of the Middle Ordovician St. Peter Sandstone, the base of the Devonian, the base of the Upper Devonian arid Lower Mississippian, the Chattanooga Shale, and the top of the Mississippian. The erosion represented by each of these unconformities was preceded by important structural movement, and each period of erosion resulted in almost complete base leveling.

Prior to deposition of the St. Peter, northeastern Kansas had been an area of slow general uplift. During this period, many hundreds of feet of rock were deposited and, in turn, removed over minor flexures. It was on this base-leveled surface that the St. Peter Sandstone was deposited. At the end of the deposition of the St. Peter Sandstone, the pattern of structural movement changed, and the area began a general subsidence, marking the beginning, in Late Ordovician time, of the North Kansas basin. This basin was the dominant structural feature in northeastern Kansas until the end of Chattanooga time, although local deformations occurred in the area throughout this period.

The first movement along the Nemaha Anticline occurred during Early Mississippian time, resulting in folding of Lower Mississippian rocks over the anticline and the development of local parallel structures. This folding was slow and probably continued throughout the deposition of Mississippian rocks. The Nemaha Anticline bisected the North Kansas basin, forming the Salina basin on the west and an unnamed basin on the east. The folded rocks were then eroded to near base level. This was followed by renewed movement along the Nemaha Anticline which caused the re-elevation of the area west of the Anticline and the downwarping of the area east of the Anticline, forming the Forest City basin.

The Forest City basin was separated from the Cherokee basin of Oklahoma and southern Kansas by the Bourbon Arch, through Bourbon, Allen, and Coffey counties. Early Cherokee deposits in both the Forest City and Cherokee basins are abnormally thick and differ in lithology to the extent that the sandstones commonly found in the Cherokee basin are largely absent in the Forest City basin. In middle Cherokee time, the Bourbon Arch was topped, and the Forest City basin became an extension of the larger Cherokee basin.

Minor flexing of the area continued during Pennsylvanian and Permian time, although the Permian record is largely lost through later erosion. The westward dip of the surface rocks in the area occurred principally prior to Cretaceous time, but there has been some deformation and great elevation of the entire area since that time.

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Kansas Geological Survey, Geology
Placed on web May 29, 2009; originally published May 1967.
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