INTRODUCTION
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. This
point of view is not entirely valid, because of widespread seismic activity and
recent fault movements (Madole et al. 1991). In eastern Kansas, considerable erosion
of the landscape has taken place since Miocene time. Alluvial deposits of
chert gravel of presumed Miocene, Pliocene, and
Pleistocene ages are widespread
and abundant on drainage divides, isolated hill tops, and high terraces throughout
eastern Kansas. High-terrace chert gravels mostly are associated with existing stream
valleys. In contrast, hill-top 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 hill tops contain quartzite and other exotic pebbles derived from the
High Plains and/or Rocky Mountains to the west.
DESCRIPTION OF STUDY REGION
The area of investigation includes portions of four physiographic regions--Flint
Hills, Osage Cuestas, Chautauqua Hills, and McPherson-Wellington-Arkansas Lowlands.
The study region is centered in east-central Kansas, including Chase, Butler, Lyon,
Greenwood, Osage, Coffey, Woodson, Wilson, Anderson, Allen, and Neosho counties,
as well as portions of surrounding counties. This region includes parts of several local
drainage basins: Neosho, Cottonwood, Walnut, Verdigris, Fall, lower Arkansas, Smoky Hill
and Marais des Cygnes. The latter two are within the greater Missouri basin; the others
are portions of the greater Arkansas basin.
Physiographic map of Kansas
Kansas counties
Kansas drainage basins
Rivers of study region
Geologic map of Kansas
The Arkansas drainage divide marks a major boundary in the geomorphology of central Kansas. To the west, the McPherson-Wellington-Arkansas Lowlands form a nearly flat plain that slopes imperceptibly toward the south. It is underlain by a variable thickness of unconsolidated Neogene and Quaternary sediments of arkosic composition. These sediments were derived from the west and deposited on a wide alluvial plain. The upper surface of the lowland is little modified by subsequent erosion. However, its eastern and northern margins are locally truncated by headward erosion of tributaries within the Walnut, Cottonwood, and Smoky Hill drainage systems.
The Osage Cuestas and Chautuaqua Hills are generally lower in elevation and relief compared to the Flint Hills. Elevations are mostly in the 250 m to 350 m range. Upper Pennsylvanian shale and poorly consolidated sandstone underlie most of these regions. Thick limestone units are more resistant to erosion and support conspicuous escarpments, where local relief may reach 50 m. Elevations exceeding 360 m are found on the Missouri-Arkansas drainage divide in central Anderson County. Chert is relatively scarce within upper Pennsylvanian bedrock throughout eastern Kansas.
Eastward from the Flint Hills, chert gravel deposits attain higher and higher relative
positions in the local topography. This trend culminates in Anderson County, where
chert gravel is preserved on the Missouri-Arkansas drainage divide, up to 75 m above
adjacent floodplains. Frye (1955) concluded that these highest gravels mark an early
river system that crossed in an easterly direction what is now a major drainage divide
and joined with streams in the Ozark region of Missouri. In an earlier paper, I used
the name Old Osage River to refer to this supposed through-drainage route to the
east (Aber 1985).
The ages of the chert gravel deposits can be estimated only on the basis of topographic
positions above modern floodplains and degree of soil development. The
deposits consist of insoluble siliceous minerals; all soluble components have been
removed by prolonged weathering. Thus, appropriate fossils or materials suitable for
dating are not preserved. Most geologists have agreed upon Neogene--Miocene and/or
Pliocene--age for upland chert gravels, and they are classified as Tertiary on state
and county geologic maps (Aber et al. 1994). Frye (1955) considered that the
oldest chert gravels date from the early Tertiary (Paleogene). I previously designated
upland chert gravels within the Walnut drainage basin as the Leon Gravel, a
lithostratigraphic unit of formation rank (Aber 1992). However, similar chert gravel
deposits in other basins have not received any formal stratigraphic recognition.
Exotic pebbles of quartzite were noted in upland chert gravels by some early
investigators, who thought the exotics had been washed into the region by glacial melt
water. Conversely, some later geologists overlooked the existence of exotic pebbles or
discounted their importance. According to Frye and Leonard (1952, p. 184):
Soil series, as depicted in county soil survey reports, proved to be excellent indicators
for both residual and alluvial chert gravel deposits (e.g. Penner et al. 1975, Swanson and
Googins 1977, Neill 1981). The Florence series is developed
in residual chert weathered from
limestones on Flint Hills uplands. Chert in these soils has not been transported by
streams, although some mass movement may have taken place on hill slopes. The Olpe series is formed on alluvial chert gravel on hill tops
and high terraces. Chert in these soils was transported by streams and deposited in channels
as various kinds of bars, dunes, and riffles. Chert gravel may be interbedded with finer
alluvial sediments.
In nearly all cases, field observation verified that alluvial chert gravel is
present as depicted on soil survey reports. Very few instances were found in which
Olpe soil proved to be residual chert, rather than alluvial deposits. In a few
cases, thin deposits of alluvial chert were discovered that are not identified as
Olpe soil on the county reports. Other than these few exceptions, the Olpe soil is
a regionally reliable indicator for alluvial chert gravel deposits in the uplands of
eastern Kansas.
Exotic pebbles are present in trace amounts (< 1%) in most
gravel deposits. The exotics include, in relative order of abundance: quartzite,
quartzose sandstone, dark flint, and a single piece of weathered granite. Some petrified
wood may also be exotic. Most exotics are pebbles, but a very few are
cobbles, up to 9 cm long. They are usually well rounded and polished. Quartzite
pebbles display typical metamorphic features: schistose or gneissic fabrics,
undulatory extinction of quartz grains, corroded or sutured grain boundaries, and
veins of biotite or epidote. Quartzite pebbles fall into three general color groups
(Aber 1985).
The lithology of exotics matches closely with those found in arkosic alluvium of the
McPherson-Wellington-Arkansas Lowlands as well as with basal Cretaceous conglomerate
of the Smoky Hills region (Aber 1985)--see physiographic
regions. Exotic pebbles are trace constituents of upland gravel deposits in most
portions of the study region. However, exotics have not been discovered along the Marais
des Cygnes valley in southern Osage and northeastern Lyon counties. Nor have exotics been
found along the South Fork Cottonwood valley in central Chase County. Exotic pebbles are
seemingly most abundant in gravels of the lower Walnut basin and on the Missouri-Arkansas
divide in Anderson County. In many cases, exotic pebbles tend to be locally more abundant
in higher (= older) gravel deposits.
PREVIOUS INVESTIGATIONS
Chert gravel deposits in uplands of eastern Kansas were recognized early and
investigated in different locations. Early ideas for the genesis of upland chert
gravel included outwash drift of glacial origin and residual accumulations on a
peneplain (Aber 1985, 1988). Studies since the 1950s have emphasized the alluvial
genesis of chert gravel that was transported from the Flint Hills and deposited in
ancient stream channels (O'Conner 1953). Subsequent valley entrenchment and drainage
diversions have left the old alluvial gravels situated on high terraces and hill
tops.
This interpretation was repeated in other reconstructions of Kansas drainage
development. Seevers and Jungmann (1963) suggested, in contrast, that Neogene
drainage from central Kansas did cross the Flint Hills and transported quartzose
materials derived from Cretaceous source strata into eastern Kansas.METHODS OF INVESTIGATION
Various techniques were utilized to document and analyze the distribution and
composition of upland chert gravel deposits in the region of study. Detailed mapping
of surficial geology was carried out in Butler County (Aber 1991), and reconnaissance
observations were made throughout the study region (Aber 1988). Gravel riffles were examined in modern stream channels (Byerley 1995).
Conventional field observations were
supplemented by laboratory analysis of chert gravel characteristics. Several hundred pebbles
of quartzite, quartzose sandstone, and other exotics have been collected, and these were
classified according to rock type and color. Thin sections of representative specimens
were cut and examined with a petrographic microscope.
Field and laboratoryGIS and remote sensing
Current investigations have emphasized applications of geographic information
systems (GIS) and remote sensing for depicting and analyzing the spatial distribution
of chert gravel in the landscape. GIS and image processing were carried
out using IDRISI software. Two kinds of GIS databases were compiled.
The raster database for Olpe and Olpe-complex soils was created with gravel
elevation as the value for each cell. This grid system has many shortcomings for
accurate cartographic work, but it is convenient to use, and for the purpose of this
particular database small locational anomalies are of little significance. The key
attribute for this database is actual elevation of chert gravel, not accurate areal limits.
County metric topographic maps (1:100,000) were used for determining the grid and
transfering data from soil survey reports. The selection criteria for entering an
Olpe cell into the database were as follows.
A detailed GIS database was created for a six-county area in the middle
and lower Neosho basin (Byerley 1995). Geographic data were entered by
manual digitizing in vector format from 7½-minute topographic
quadrangle maps. The vector database contains all main rivers and their
principal tributaries, reservoirs, county boundaries, and selected cities,
as well as geomorphic distribution of alluvial soils and chert gravel.
Other kinds of GIS databases were utilized, including various digital
elevation models (DEM), and Landsat multispectral scanner (MSS) images
from the 1988 growing season (Aber et al. 1997).GRAVEL COMPOSITION
High-terrace and hill-top gravel deposits are comprised almost entirely of crudely bedded chert derived from lower
Permian bedrock of the Flint Hills. Milky quartz and siliceous fossils are derived
also from the same bedrock sources as chert. These components typically make up 98%
to > 99% of the small-pebble (4-8 mm) fraction of high-terrace and hill-top gravels
in eastern Kansas. Other locally derived components include sandstone, shale, petrified
wood, and iron concretions, which may comprise up to 2% of gravel deposits, especially in
vicinity of the Chautauqua Hills.
Yellow, orange and light brown 5 Y, 5 YR, 10 YR about 60%
Pink, red and red-purple 5 R, 10 R, 5 RP about 30%
Purple, dark brown, and gray 5 P, 5 Y, 5YR, N about 10%
REGIONAL DISTRIBUTION OF UPLAND CHERT GRAVEL
Regional distribution of high-terrace and hill-top gravel deposits is depicted according
to actual elevation on the raster database--see chert map.
This database overestimates the actual area of upland gravels, as individual gravel cells
may represent full coverage to as little as 10 acres of Olpe soil. Nonetheless, the database
provides for a regional overview of gravel distribution and elevation. Highest gravel
deposits are located in headwaters of the Walnut, Verdigris, Fall, and Cottonwood
drainage basins at elevations > 400 m. These gravels are preserved near the crest of the
Flint Hills escarpment. Lowest gravels are found near the eastern and southeastern margins
of the study region at elevations < 300 m.Walnut drainage basin
Chert gravel deposits are abundant on high terraces and hill tops in southern
Butler and northernmost Cowley counties--see study region.
Gravel deposits are associated with the Little Walnut River, Hickory Creek, and Rock Creek
valleys, as well as the lower Walnut River valley. These streams have entrenched their
headwaters into or below the Florence Limestone, which is the primary source of residual
chert. Streams in western and northern Butler County either have not yet entrenched or
are just beginning to erode into the Florence Limestone. This explains the general lack of
older chert alluvium in northern and western portions of the Walnut drainage basin.
Upland chert gravel deposits are formally named the Leon Gravel after the city of Leon (Aber 1992). A stratotype is designated in NE¼ Sec. 35, T27S, R5E. As defined, the Leon Gravel is equivalent to the Olpe or Olpe-Norge soil map units within the Walnut drainage basin of Butler and Cowley counties. The Leon Gravel is usually 1-2 m thick and rests on Permian bedrock. It can be informally separated in some places into upper and lower members on the basis of topographic position in relation to adjacent modern valleys.
Chert gravel is nearly continuous along the northern side of the main Cottonwood River valley, and gravel is also abundant along the western side of the north-trending South Fork Cottonwood River valley. Similar patterns are seen for smaller tributary valleys; gravel is preserved north of west-east valleys and west of north-south valleys.
Exotic pebbles are relatively common in gravels associated with the main Cottonwood River valley, but exotics have not been found in gravels along the South Fork Cottonwood River valley. Exotics are scattered in thin (non-cherty) soils on limestone hill tops in eastern Marion County. These exotic pebbles match in type and color those found in chert gravels to the east, but many of the Marion exotics bear glossy polish and facets typical of ventifacts. They appear to represent a residue of formerly extensive, non-cherty gravel deposits (Aber 1988).
In northeastern Greenwood County, the Verdigris River turns toward the south-southeast. Along most of this stretch, chert gravels bearing exotics are found west of the valley, and gravel occurs east of the valley in a few places. Near the junction with the Fall River, chert gravel caps the drainage divide between the two basins. Exotics are also found as isolated pebbles resting on bedrock terrain across north-central Greenwood County.
INTERPRETATION OF ANCIENT DRAINAGE
The oldest drainage routes are represented by gravels that are highest in the local
landscape and are unrelated to modern river systems. Distribution of exotic pebbles
is another important factor for reconstructing ancient west-to-east through-drainage
routes. Four oldest drainage routes may be recognized on these bases, from south to
north (Aber et al. 1995)--see chert map and study region.
Reconstruction of oldest drainage
The eastern edge of the McPherson-Wellington-Arkansas Lowlands marks the divide between the Neosho/Cottonwood and Walnut basins to the east and the lower Arkansas drainage to the west--see drainage basins. This edge has retreated westward, due to steeper gradients and more aggressive erosion in headwaters of the eastern systems. At one time, the Arkansas alluvial plain must have sloped gently eastward. Isolated exotic pebbles on hill tops demonstrate the former extent of the alluvial plain. In like manner, the terrain east of the Flint Hills must have been considerably higher than today. For example, highest gravels of the ancestral Verdigris are preserved on the drainage divide in northeastern Greenwood County, at 390 m elevation, 80 m above the modern Verdigris floodplain.
Gravels on the Missouri-Arkansas divide in Anderson County were earlier interpreted as an eastern extension of gravel trends along the Cottonwood and Neosho valleys (Fyre 1955; Aber 1985). However, the regional slope of these gravels to the southwest argues strongly against this point of view. These gravels apparently represent a river flowing from the northeast. Origin of the headwaters for this stream and sources for exotics are uncertain. The gravels are situated up to 70 m above regional floodplains.
At the time of deposition, chert gravels occupied the lowest topographic positions--stream channels--in the surrounding landscape. Preservation of exotic-bearing gravels in drainage-divide positions demonstrates a wholesale inversion of topography in eastern Kansas. Considerable erosion has taken place, such that former low points now occupy the highest positions in the local landscape. The minimum magnitude of vertical erosion can be estimated from the elevations of highest chert gravels in relation to present stream-valley floodplains in each drainage basin.
| Drainage Basin / Erosion | Drainage Basin / Erosion| Walnut / 80 m
| Verdigris / 80 m | Marais des Cygnes / 70 m
| Neosho / 50 m | Fall / 50 m
| Cottonwood / 40 m
| |
|---|
This pattern of valley asymmetry has been noted before and several explanations considered (Aber 1985), including Coriolis force, unequal input of sediment from tributary streams, and crustal tilting. Of these possibilities, it seems that slow, continuing crustal warping downward to the south and east toward the Gulf of Mexico is the only viable explanation on a regional basis (Aber 1990). Local exceptions may result from differential movements in basement structures. However, crustal tilting and local structural movements remain impossible to verify through independent evidence. If such crustal tilting has occurred, it would have the effect of increasing gradients for streams that drain toward the south or southeast. Those streams, thus, would have an erosive advantage during dissection of the landscape. This may explain the predominance of drainage captures by streams flowing toward the south or southeast. It could also explain why northwest-trending valleys are so prominent in the modern landscape.
CONCLUSIONS
ACKNOWLEDGEMENTS
I have benefitted from discussions with many colleagues, especially P. Berendsen, L.L.
Brady, J.W. Harbaugh, P.L. Johnston, D.F. Merriam and F.W. Wilson. The presence of
exotic pebbles in chert gravel was first shown to me by W. Dort, Jr. Many students
have contributed to the GIS compilation of data for this report: R.D. Byerley, M. Husain,
R. Krueger, T. Peterson, L.L. Rand, R.O. Sleezer and N.H. Wilkins. M.J. Guccione
and C.W. Martin reviewed an earlier version of this article and offered many valuable
suggestions for improvement. I offer my thanks to all these individuals. This research
was supported by grants from Kansas Geological Survey, Kansas Dept. of Wildlife and Parks,
NASA, and Emporia State University.LITERATURE