Cheyenne Bottoms Geomorphology

Cheyenne Bottoms Geomorphology

Mike Sedlacek
November 28, 2009
ES 546: Field Geomorphology
Dr. James Aber, Instructor

Introduction Origin of the Cheyenne Bottoms
What is the true origin? What is the true origin Part II
Climate changes and alluvium deposits References

Field Geomorphology Homepage

Figure 1: An aerial view of Cheyenne Bottoms looking southeast. Note Blood Creek on the right side, and the marsh ponds in the upper right corner. This picture was taken from the ES-546 Blog.


Introduction

Cheyenne Bottoms is a nationally known wetland located near Great Bend (Barton County), Kansas. To date, scientists are still unsure of the origin of this unique geologic feature. Cheyenne Bottoms is a closed, undrained, circular basin, and is approximately 64 square miles in size (Collins, 1985). This basin is enclosed by a high ridge of terraced, Cretaceous sandstone on the north, south, and west sides, known as the Dakota Sandstone, which rises over 100 feet above the valley floor (Zimerman, 1990). On the east and southeast sides, low ridges of alluvial sand dune deposits of up to 40 feet above the basin floor are present (Bayne, 1977). Figure 1 (above) shows a relief map of Cheyenne Bottoms. Note the distinctive basin feature, as well as the sandstone ridge.

To better understand the geomorphology of this anomaly, historical geology of the area needs to be discussed. Presently, the beds in Central Kansas dip slightly to the west (Collins, 1985). However, the basin of Cheyenne Bottoms currently slopes eastward, that is to say that the elevation of the basin is higher in the west (Latta, 1950). Keep in mind that sloping beds don't necessarily determine surface topography--however, many variables have played significant roles in the geomorphological development of the Cheyenne Bottoms (Zimmerman, 1990). The history of the area essentially starts in the Precambrian, when the "basement" of igneous rocks was formed. Fast-forwarding to 500 million years ago (Ma),the Cheyenne Bottoms area went through countless cycles of sea regression and transgression on the continent up until approximately Permain time; around 250 Ma.

During the Mississippian, compressional forces arched bedrock in the Cheyenne Bottoms vicinity to create a NW-SE axis anticline (Bayne, 1977). Not too long later, during the early Pennsylvanian, an event known as the Central Kansas Uplift ocurred. This event elevated and upwarped beds in central Kansas. By 250 Ma, the sea had transgressed away from Kansas, and for the next 100 Ma (Triassic-Jurassic), the area remained dry (Zimmerman, 1990).

By 65 Ma (Cretaceous), the Laramide Orogeny was in full swing (Zimmerman, 1990). At the same time, the Cheyenne Bottoms area was part of the brackish waters of a marine embayment. During the Miocene, the uplift of the Colorado Front Range drastically changed the climate of the Plains via the rain shadow effect--from a savanna to a drier grassland. During the Pleistocene, harsh northwest winds blew sediment in a south/southeast direction. In addition, at the same time, the Cheyenne Bottoms was part of the Smoky Hill drainage basin--multiple streams flowed through the basin from the north, and exited in the south (Zimmerman, 1990). During the Holocene, the climate began to warm up, and the land surface began to dry out (Stanley, 2004). Dunes of sand blew around and began to stabilize around mid-Holocene.

Figure 2: A relief map of Cheyenne Bottoms, Kansas, showing the topographical variations at Cheyenne Bottoms. Notice the high topography of the basin rim (in red), and the homogeneous nature of the basin, colored in green. Map taken from Aber and Pavri (2004).

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Origin of the Cheyenne Bottoms

As mentioned by Latta (1950), there are four main hypotheses that help explain the origin of Cheyenne Bottoms; they are: 1.) fluvial action via stream piracy, 2.) subsidence caused by dissolution of subsurface evaporite deposits, 3.) wind scour by Pleistocene winds, and 4.) tectonic movement.

Fluvial Action

It has been hypothesized by Bayne (1977) that during the Pleistocene, the Smoky Hill watershed captured many of the streams that drain into Cheyenne Bottoms today (Zimmerman, 1990). Later in the Pleistocene, faulting north of Cheyenne Bottoms caused the Smoky Hill watershed to lose the Cheyenne Bottoms streams. Dunes of sand currently separate the Arkansas and Smoky Hill basins. It turns out that streams that now drain into the Cheyenne Bottoms show evidence of a fluvial transition (Latta, 1950).

Blood Creek, which flows northwest to southeast in the Cheyenne Bottoms, has thinner alluvial deposits upstream (Latta, 1950). This helps reinforce the concept that the Cretaceous cliffs were already present when Blood Creek changed its upstream velocity. These deposits, which can be as thick as 120 feet, generally consist of poorly sorted silt and clay, which is not typical for an alluvial deposit (Latta, 1950). However, deltaic deposits have been found in the northwest part of basin, which shows that gradients must have been sufficiently low in order to deposit the sediments (Aber). Regardless of gradient changes during the Pleistocene, these streams deposited layers of alternating sediments over the last 100,000 years (Aber and Aber, 2009). Since the basin has graded upward over the course of the Pleistocene, fluvial action was probably not the main agent of basin subsidence.

Subsidence by Salt Dissolution

Thus far, the general idea of evaporite dissolution has taken hold as one of the better hypotheses for the origin of the Cheyenne Bottoms. The salt beds; the Hutchinson Salt Member of the Wellington Formation; were laid down before the Cretaceous Dakota Sandstone during Permian time (Aber and Aber, 2009). The Hutchinson Salt Member is believed to be dissolving. Evidence of dissolution comes from areas east of Cheyenne Bottoms, where the Hutchinson Member is both shallower (surface wise) and thicker (Bayne, 1977). Subsidence and sinkhole formation has been documented in places such as the city of Hutchinson, Kansas, as a direct result of evaporite mining (Bayne, 1977). The ultimate question is "can this be extrapolated to apply to Cheyenne Bottoms, which is to the west?"

Bayne (1977) believes that for a number of reasons, surface subsidence is not occurring at Cheyenne Bottoms. According to Walters (1978), the Hutchinson Salt Member is the only evaporite deposit in the area that is capable of supporting large-scale dissolution--enough to lower the basin 100 feet. Bayne (1977) argues that even though the the Hutchinson is deep enough to cause mass dissolution, the fact that all beds below the Greenhorn Formation are deformed in the same manner does not support the hypothesis that dissolution did indeed occur. In Figure 3 (a topographic map of the Hutchinson Member), a dome feature is present in the lower right quadrant of the map. This dome feature disappers from topographic maps before alluvial sediments are deposited during the Pleistocene. In addition, a syncline that is present from the present-day surface down, becomes far more pronounced in the Hutchinson (Bayne, 1977). However, Aber and Aber (2009) have found evidence that subsidence has been aided by salt solution and creep.

Pleistocene Wind Scour

Wind scour and transport during the windy Pleistocene is another factor that may have caused the basin formation at Cheyenne Bottoms. According to Bayne (1977), it is no coincidence that sand dunes as deep as 120 feet are found on the south-southeast corners of the basin. During the windy and cold periods of the Pleistocene, winds howled out of the northwest (Latta, 1950). This is why Bayne (1977) believes that the northwest corner of the basin bluff is more eroded than the other sections.

The northwest winds also help explain why there are sand dunes on the south-southeast corners of the basin; the northwest winds would have deposited sediment to the southeast if there was a northwest wind (Latta, 1950). It is also believed that a wet/dry cycle was repeated countless times during the past 100 thousand (100 Ka) years (Zimmerman, 1990). A wetter environment persisted more often between 100 and 80 Ka, in which a series of blue, grey silts, sands and clays were deposited in a water-logged environment. Around 50 Ka, the climate started to become dryer, and soil formation began in the basin at Cheyenne Bottoms (Zimmerman, 1990).

At the tail end of the Pleistocene, around 15 Ka, conditions continued to dry--thick accumulations of loess, some of which are over fifteen meters thick, began to form in the basin--especially the southeast side. From the early Holocene to present day, alternating wet/dry sedimentation patterns have shown up in alluvial deposit samples (Zimmerman, 1990). Many believe that it was the strong winds of the Pleistocene that swooped down into the Cheyenne Bottoms through the semi-open northwest corner, where sediment became entrained by the wind, causing deposited on the south/southeast side of Cheyenne Bottoms. Massive sand dunes by the Arkansas River tend to support this hypothesis, but the source area for the sand in the dunes is still unknown (Zimmerman, 1990).

Tectonic Movement

It is thought that during Pennsylvanian time, a northwest-trending fault system caused uplift and faulting throughout a 5,700 square mile area in central Kansas (Merriam, 1963). It is believed that Cheyenne Bottoms may actually be the junction of intersecting faults, which is causing the basin of Cheyenne Bottoms to lower, while the Cretaceous walls rise (Collins, 1985). Bayne (1977) believes that tectonism has (and currently does) play an important role in the development of the basin. In figures 2-6, Bayne shows select topographic maps of the Cheyenne Bottoms from the Precambrian through present day. Some interesting features show up when comparing each of the five maps; most notably the effects of the Central Kansas Uplift. Each successive map shows a basin feature in the area of grids R 12 W and T 18 S (Bayne, 1977).

In addition, the properties of an anticline and syncline can be observed in the units that were subjected to the Central Kansas Uplift, with the syncline becoming more pronounced at and below the Hutchinson Member (Bayne, 1977). Aber and Aber (2009) believe that the formation of the basin at Cheyenne Bottoms is due to structural subsidence. To complicate matters, layers of alluvium and sand, tens of feet deep, have been deposited on top of the Mesozoic beds, hiding much of the relief found below. Much progress has been gained, though, by using drill cores to interpret paleo-environments through time (Bayne, 1977).

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What is the true origin?

To date, no simple answer has been found that clearly explains the origin of the Cheyenne Bottoms (Aber). Of all of the hypotheses, each seems logical in some shape or form. From the fluvial argument, it seems as though when the Smoky Hill divide was breached by some sort of tectonic movement (Zimmerman, 1990), streams like Blood Creek and Deception Creek lost mass levels of discharge, and therefore, sediment. Erosion could very well have occurred within the basin, but the question as to "how did the whole basin become lowered" comes to mind. It is possible that fluvial erosion mixed with eolian erosion, caused subsidence of the basin, and allowed the formation of the southeast wall, which is eolian sediment (Latta, 1950). Surface subsidence via salt dissolution is a logical hypothesis, and there is much evidence supporting this hypothesis, including salt dissolution by creep (Aber and Aber, 2009), and salt dissolution within the Hutchinson Member (Walters, 1978). Topographic maps from Bayne (1977) show uneven terrain between the top and the bottom of the Hutchinson Member (see Figures 4 and 5), but no evidence of mass dissolution is present between those two maps.

After analyzing elevation variations in Figures 4 and 5, the differences in elevation were negligible, and the thickness of the basin (found in grids R 12 W and T 18 S) appears to be slightly thicker than most outlying areas (Bayne, 1977); I would attribute that to the closed basin itself. If it was a true basin when the Hutchinson Member was deposited, more evaporite deposits likely settled in the basin than in the higher elevation areas. Overall, thickness calculations of the Hutchinson Member as a whole averaged around 260 feet in depth, with a few places being thicker or thinner by 10-20 feet. Much like Bayne (1977), I would have to say that the Hutchinson Member isn't large enough to singly create such a large and deep basin. In addition, Bayne points out that the Hutchinson Member was subjected to folding during the Central Kansas Uplift. If this is true, there should be notable differences in elevation if the salt had indeed dissolved after being folded because of the tilted surfaces.

Thus far, there is no evidence that (mass-scale) dissolution has ocurred (Bayne, 1977) because of the similar topographic features of the Hutchinson Member throughout the basin. The enclosed basin can be noted in the topography of every bed down to the Precambrian basement, where the topography is more subdued; but yet, still resembles a basin (Bayne, 1977). Hylton and Merriam (1985) found that the Stone Corral Formation; found just above the Hutchinson Member; is an amalgamation of lagoonal deposits containing dolomite, mud deposits, and shale. It is very possible that the shale within the Stone Corral Formation has acted as a buffer to evaporite dissolution, but salt creep still could have occurred. Since dolomite is more durable than limestone, it is more difficult to dissolve dolomite because of the attached magnesium cation (Hylton and Merriam, 1985). If it is true that the dolomite in the Stone Corral Formation is resistant, then any dissolution below that formation would likely have caused karst features, vice bed failure. No karst features have been noted to date according to Bayne's drill cores.

Current Topography

Figure 3: A topographic map of the current topography at Cheyenne Bottoms Bayne (1977).

The Hutchinson Salt Member (top surface).

Figure 4: A topographic map of the top of the Hutchinson Member (evaporite deposits) of the Wellington Formation (Bayne, 1977).

The Hutchinson Member (bottom surface)

Figure 5: A topographic map of the bottom of the Hutchinson Member. Note the presence of an enclosed depression at grids T 18 S and R 12 W. Also note the elevated hill at 1160 feet (T 19 S and R 12 W)--this circle of 1160 feet is a salt dome (Bayne, 1977).

The Winfield Limestone

Figure 6: A topographic map of the Winfield Limestone, which was deposited before the Hutchinson Member. Note the characteristic basin in grids T 18 S and R 12 W (Bayne, 1977).

The Precambrian basement

Figure 7: A topographic map of the Precambrian basement. Note how a natural basin appears in the lower right-hand corner of the map, while higher elevations are found on the left-hand side. Grids R 12 W and T 18 S are where the basin features have been found on every other map throughout geologic time--including present-day (Bayne, 1977).

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What is the true origin Part II

The hypothesis that tectonic movement formed the Cheyenne Bottoms has caused as much debate as the salt dissolution hypothesis (Bayne, 1977). Bayne argued that structural subsidence was the most logical argument for the basin formation. Evidence does support Bayne's hypothesis, such as the regional tilting that ocurred during the Central Kansas Uplift. The Cheyenne Bottoms area is known to have been faulted in the past--Bayne (1977) believes the basin area may be where multiple faults intersect--thereby causing a large area to structurally sink. Merriam (1963) located a northwest trending uplift in Central Kansas, which includes Cheyenne Bottoms. To put icing on the cake, Merriam points out the fact that Cheyenne Bottoms contains anticline and syncline features. We know that tectonics has played at least some role in deforming the beds beneath Cheyenne Bottoms, but to what extent, no one knows for sure.

Drill cores have shown various formations and tectonic activity in the past, but that information still doesn't fully explain whether the folding and faulting formed the basin. Bayne (1977) went a litle further to investigate topographic maps for the major formations at Cheyenne Bottoms. Every bed below the Greenhorn Limestone had been subjected to folding during the Central Kansas Uplift. All folded beds project similar features--especially two noteworthy features (the basin feature, and the syncline feature). See Figures 3-7 for a comparison of those topographic features. Surprisingly, above the Greenhorn Limestone, those two prominent features can be noted with a careful eye. Even above tens of meters of alluvium and sand deposits, those two features can still be located (Bayne, 1977). This evidence tends to support the hypothesis that whatever caused the Cheyenne Bottoms to become lower than the surrounding Cretaceous bedrock, lowered the basin in tact without destroying any structural features (Bayne, 1977).

Knowing this information, it could be extrapolated that since those features remain noticeable on the surface, the origin of the Cheyenne Bottoms might have something to do with the precambrian basement (Bayne, 1977). I believe that many of the surface features were formed as a result of the topography of both the Precambrian basement, and also the folding during the Central Kansas Uplift. No evidence of wide-scale tectonics has been shown to occur (Aber). In addition, if salt dissolution did occur, sub-surface voids (karst) would have formed, and drill cores would have showed the voids. In addition, if karst did not occur, the dissolution of salt would have caused the beds immediately above the Hutchinson Member to collapse and fill the void, causing fractures, and/or small rock fragments to be present. No records showing a disruption in the folding sequence has been found (Bayne, 1977). The concept of salt creep (Aber and Aber, 2009) is a far more logical argument. If the Hutchinson Member had been subjected to creep, sub-surface movement would have ocurred without creating voids or fractures.

When analyzing all of the data, no one hypothesis can fully support all variables involved. Though I do believe that the Precambrian surface topography does play an important role in the basin topography, the other mentioned erosional agents (wind and water), as well as tectonics and salt creep all appear to have interacted throughout the Quaternary (Aber). Wind would have been a major component of erosion during the Pleistocene, when strong winds blew out of the northwest (Latta, 1950). Sand dunes of Quaternary age are found on the southeast perimeter of the Cheyenne Bottoms. The northwest winds likely deposited the sediment at an obstruction on the southeast side. Tectonics too, could have been the initial factor causing a basin to form (since tectonism was thought to have occurred during Carboniferous time) (Bayne, 1977). Any slight depression caused by minor tectonism could have been further eroded by wind and water, especially if there was an opening in the northwest wall of the Dakota Sandstone. In essence, a domino effect of erosion is possible in this scenario.

The Central Kansas Uplift
Figure 8: A diagram representing the area of Kansas that was subjected to the Central Kansas Uplift. This uplift caused regional tiliting to the west (Baars, Watney, Steeples, and Brostven, 1989). Diagram taken from
(Baars, Watney, Steeples, and Brostven, 1989).

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Climate changes and alluvium deposits

The alluvium deposits at Cheyenne Bottoms are strictly Pleistocene in age. Cheyenne Bottoms underlain by these deposits, which include unconsolidated clay, silt, sand, and gravel deposits that range in age from Pleistocene to present-day (Aber). The thickness of these deposits range from less than 20 feet near the margins of the Bottoms to more than 100 feet in the deepest part of an old buried stream channel (Bayne, 1977). Over the last 100,000 years, alternating deposits of lacustrine, fluvial, and eoilian sediments were deposited in the Cheyenne Bottoms (Aber). Between 24,000-11,000 years ago (Woodfordian time), an extensive unconformity formed at the Cheyenne Bottoms (Fredlund, 1995). If it is true that the Cheyenne Bottoms is simply an extension of the Precambrian topography, then the Woodfordian unconformity must have weathered and eroded sediment at a uniform scale throughout the basin. Since the Cheyenne Bottoms is 64 square miles in size, it is fully possible that the Woodfordian unconformity eroded uniformly throughout the basin. Maps from Bayne (1977) help solidify this hypothesis.

During the Holocene, climate began to fluctuate on a regular basis, causing alternating deposits of lacustrine, fluvial, and eolian sediments (Aber). During the early Holocene, dune formation began to occur, and stability of dunes started to occur during the mid-Holocene (Aber). Records of sediment deposits from Bayne (1977) show that during the Quaternary, fine to medium-sized sand deposits formed over the Cretaceous beds (Bayne, 1977). Silt and clay beds of various colors (lithologies) are found above the sand deposits, which represents a lacustrine environment of high water and a milder climate (Bayne, 1977). Above the lacustrine sediments, alluvial deposits can be found in most areas. By the mid-Holocene, the climate shifted once more toward a dryer evironment (Fredlund, 1995). Fluvial deposits covered most of the Cheyenne Bottoms to depths of a few meters during this time (Bayne, 1977).

These variations in sediment deposition and lithology represent changing climates during the Holocene (Fredlund, 1995). One thing is for sure though--deposition has ocurred far more often during the Quaternary than erosion. Climate shifts continue into present-day, where Glantz (2003) believes that a Woodfordian-like time of dryness and climatic warming could occur again in the near future.

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References

Website created by Mike Sedlacek: November 28, 2009

Emporia State University homepage

ESU Earth Science Department homepage

For more information on Cheyenne Bottoms, visit this popular Cheyenne Bottoms website.
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