Chert Gravel and Neogene Drainage in East-central Kansas 29 Chert Gravel and Neogene Drainage in East-central Kansas

Eastern Kansas has an active geomorphic regime dominated by stream erosion, which is strongly influenced by bedrock structure. Alluvial deposits of chert gravel of presumed Neogene age are widespread and abundant on hill tops and high terraces throughout the region. Many of these gravels contain quartzite and other exotic pebbles derived from western sources. Geographic information system (GIS) techniques are used to document and analyze the spatial distribution of upland chert gravel deposits. On this basis, Neogene drainage routes may be recognized for the ancestral Arkansas, ancestral Verdigris, ancestral Neosho, and ancestral Marais des Cygnes. These drainages bore little resemblance in position or direction to the modern rivers. Stream captures, valley entrenchment, and wholesale inversion of topography led to the modern drainage systems. During entrenchment, streams have shifted systematically southward and eastward, possibly as a result of long-term and continuing crustal tilting (neotectonism) down toward the Gulf of Mexico. Eastern Kansas south of the limit of glaciation is part of the Osage Plains of the central United States. This region is commonly portrayed as a tectonically stable terrain of low relief, in which near-peneplain conditions prevail. Widespread seismic activity and recent fault movements (Madole et al., 1991), however, suggest that the region may not be as stable as once thought. In eastern Kansas, considerable erosion of the landscape has taken place since Miocene time. Alluvial deposits of chert gravel of pre-sumed Miocene, Pliocene, and Pleistocene ages are widespread and abundant on drainage divides, isolated hilltops, and high terraces throughout eastern Kansas (fig. 1). High-terrace chert gravels mostly are associated with existing stream valleys. In contrast, hilltop chert gravels in many places bear no obvious relationships to any river systems of today. Many, but not all, of these gravels on high terraces and hilltops contain quartzite and other exotic pebbles derived from the High Plains or Rocky Mountains to the west