Valley Erosion Since Pliocene "Algal Limestone" Deposition in Central Kansas

Abstract

Major stream valleys of central Kansas have been cut to depths of more than 200 feet below the top of the Pliocene Ogallala formation. The top of the Ogallala is marked by discontinuous thin deposits of "Algal limestone" which is believed to have been deposited over an extensive fluvial plain, approximately at the level of present ridges, prior to erosion of the present valleys. Recently this "Algal limestone" has been observed at 12 localities atop the Saline-Smoky Hill Divide in Russell, Lincoln, and Ellsworth counties, east of any previously described exposures. It here rests directly on Cretaceous shale and chalk. The geographic and topographic position of these exposures suggests that the "Algal limestone" was formed in lakes which occupied consequent depressions and abandoned channel segments on a piedmont alluvial plain. This interpretation furnishes a stratigraphic datum for measuring erosion in post-Ogallala time. After deposition of the "Algal limestone," major streams of this area cut through more than 150 feet of bedrock prior to the Wilson Valley fill of possible Kansan age. Since about Kansan time the Saline River has eroded more than 150 feet of bedrock plus a probable 60 feet of Pleistocene fill, and the Smoky Hill River has eroded 75 feet of bedrock plus a probable 60 feet of valley fill.