Drainage development and chert gravel deposits in Butler County, Kansas.

Abstract

Chert gravel in Butler County, Kansas can be divided into two distinctive types. Residual chert gravels have been weathered out in 'place on the dip slopes of the Flint Hills. These gravels are the source for the other type of chert gravels found in Butler County. The second type of chert gravel deposits found in this area are alluvial in nature. These alluvial chert gravels can be used to trace the past positions of streams in Butler County. Alluvial chert gravels are found almost exclusively to the north of east-to-west flowing tributaries in southern Butler County. Apparently these .streams are migrating laterally to the south as they cut downward. The simplest and most probable explanation for this southward migration and the resulting asymmetrical preservation of these alluvial chert gravels is a subtle southward tilting of the underlying bedrock. An apparent disparity exists between the pebble roundness value tests conducted: in Butler County and those done along other streams east of the Plint Hills. Bast of the Plint Hills roundness values tend to show a high degree of angularity at the source and a steady increase in roundness values with downstream distance. In Butler County the opposite trend exists. Chert east of the Plint Hills is weathered out on face slopes and is subject to initial mechanical breakup. West of the Plint Hills the chert weathers out on dip slopes by chemical means and is not subject to initial mechanical breakup. This helps to explain why the highest roundness values found in Butler county are associated with a residual deposit and the apparent trend to slight increases in angularity due to mechanical breakup with downstream distance. Some quartzite pebbles have been found within the alluvial chert gravels in Butler County. These quartzite pebbles probably originated from a source to the west namely Ogallala-type arkosic gravels in western Kansas. There are two distinct levels of deposits of alluvial chert gravels in Butler County. The upper level occupies a position at or near the top of hills forming the drainage divides between tributaries to the east of the Walnut River. The second level is located somewhat lower on the slopes and is referred to as a high-terrace gravel. Quartzite pebbles have been found only in the upper hill-top locations and it is assumed that the source for quartzite pebbles was not available at the time the younger high-terrace gravels were deposited.