James S. Aber
Earth Science Department
Emporia State University
Emporia, Kansas 66801

Table of Contents
Geology Introduction
Bedrock stratigraphy
Lower Permian cyclothems
Structural features
Mapping exercises



The Flint Hills form a prominent erosional massif that stands well above lower plains to the east and west. The focus for excursions and field exercises is the central Flint Hills terrain in Chase (CS), Butler (BU), Lyon (LY) and surrounding counties--Figure 1. This region represents a classic example of "layer-cake stratigraphy" at its best. The bedrock strata are revealed as a result of pervasive stream erosion, which has dissected deep valleys that cross the Flint Hills in all directions. The Flint Hills includes the largest region of native tallgrass prairie remaining in North America, and so the surficial geology and geomorphology are readily visible in the landscape, on air photos, and in satellite images.

See Flint Hills upland physiographic region.
Image obtained from the Kansas Geological Survey.

The eastern margin of the Flint Hills is marked by a major escarpment that is especially prominent in northwestern Greenwood, southeastern Chase and eastern Butler counties. Maximum elevations exceed 500 m, relief is locally up to 100 m, and stream valleys are deeply entrenched. The Walnut, Verdigris, Cottonwood, and Fall drainage basins meet at divides on the Flint Hills crest in this region--Figure 1. From their eastern crest, the Flint Hills slope gently westward, down the regional bedrock dip, to the western limits of the Walnut and Cottonwood drainage basins.

The Flint Hills are underlain by lower Permian limestone, shale and evaporites. This bedrock generally dips gently toward the west or northwest. Local variations in bedrock dip are found over the crest of the buried Nemaha uplift. Erosion of interbedded shale and limestone strata has resulted in landscapes with steep east-facing escarpments separated by gentle west-sloping cuestas--Figure 2. Thick cherty limestone units weather to produce residual chert lag deposits that are highly resistant to chemical breakdown. Such residual chert is responsible for maintaining high topographic relief and gives the Flint Hills their name. Unconsolidated sediments are common, especially within river valleys and on some upland areas. Soils are developed in residual (weathered) bedrock material, alluvial deposits, and loess sediment.

Oil pump operating in the Shinn NE oil field, southeastern Butler County. Photo date 9/89, © J.S. Aber.

Butler County became an important petroleum production and refining center early in the 20th century following the initial discovery of gas near Augusta (1906) and later discovery of the giant El Dorado oil field in 1915. As a result of this economic development, detailed surface and subsurface geological mapping was carried out in the El Dorado vicinity by Fath (1921). Fath's maps demonstrate the remarkable structural complexity of surficial bedrock in the area. Other portions of the Flint Hills also display structural deformation of surficial bedrock, in connection with recurrent movements of the buried Nemaha Uplift and Humboldt Fault zone.

Bedrock Stratigraphy

The formal bedrock stratigraphy was defined and described by Zeller (1968). The Council Grove Group is found in eastern Chase County, where it crops out in the lower portion of the Flint Hills escarpment--Figure 3, and the Chase Group forms most of the Flint Hills crest.

Bedrock stratigraphy of the Flint Hills region, east-central Kansas. The Admire Group is considered upper Pennsylvanian; the Council Grove and Chase groups are lower Permian (Baars et al. 1994). Chart adapted from Kansas Geological Survey (1997).

The outcrop patterns of bedrock units along the Flint Hills escarpment are extremely intricate--Figure 4, and many small erosional outliers of resistant bedrock units are present along the crest of the escarpment. Resistant limestone beds form stone lines on the hill sides, which makes mapping of the bedrock stratigraphy relatively easy.

Stone line formed by the Crouse Limestone in the vicinity of the "cattle pens" exit on the Kansas Turnpike (I-35), southeastern Chase County. Photo date 9/95, © J.S. Aber.

The Florence Limestone forms the resistant cap along much of the escarpment. However, in eastern Chase County the Florence has been removed by erosion, and the Wreford Limestone forms the escarpment crest. Residual chert from the Florence and Wreford often drapes over underlying shale and limestone formations. The Chase Group is the surficial bedrock for most of Butler and western Chase counties west of the Flint Hills escarpment. Although the landscape is generally less rugged, intricate outcrop patterns have resulted from stream dissection, and outliers of resistant bedrock units form small hills--Figure 5.

Lower Permian Cyclothems

Cyclothems are systematic repetitions of bedrock lithology and fossils in the stratigraphic column. The upper Pennsylvanian cyclothems of eastern Kansas have been world famous since the work by Moore (1964). No less impressive are lower Permian cyclothems of the Flint Hills region. Cyclothem strata were laid down as a result of repeated marine transgressions and regressions of a very shallow continental shelf and emergent coastal lowland. The cyclothem model includes six units, from the top down--Figure 6.

6. Limestone: tan to gray, platy to massive, fossiliferous limestone, chalky limestone, or dolomitic limestone; cross bedding, oolites, and ripple marks common; locally cherty toward base with algal bed toward top; diverse fossils. Examples: Morrill, Funston, Schroyer, Fort Riley, Cresswell, Herington. Environment: offshore regressive sea to shallow, high-salinity, lagoon.

5. Shale: gray or green, fissile to platy, calcareous, fossiliferous shale or shaly limestone; diverse fossils including trilobites. Examples: Florena, Havensville. Environment: farthest offshore sea of maximum transgression.

4. Cherty limestone: tan or light gray, platy to massive, fossiliferous limestone or chalky limestone; scarce to abundant nodules or beds of chert; locally may display algal banding or cross bedding; diverse fossils including many echinoids and fusulinids. Examples: Neva, Cottonwood, Crouse, Threemile, Florence, Stovall. Environment: far offshore, shallow sea of near-maximum transgression.

Florence Limestone exposed along K-96 highway in southeastern Butler County. Light gray chert beds and nodules are conspicuous within the limestone. Scale pole marked in feet. Photo date 10/89, © J.S. Aber.

3. Shale: gray, tan or green, platy, fossiliferous, calcareous shale and shaly limestone; abundant brachiopod and mollusk fossils. Examples: upper Eskridge, upper Easly Creek, upper Speiser, upper Wymore, upper Gage. Environment: shallow, normal salinity, transgressive sea.

2. Limestone: one or more, thin, gray or tan, chalky or coquina, fossiliferous limestones; abundant brachiopods and mollusks. Examples: middle Eskridge, middle Speiser. Environment: shallow to estuarine, low-salinity sea.

Students examine the Eskridge Shale in the spillway at Lake Kahola, KS. The fall is formed by the "middle" limestone within the Eskridge Shale. Blocks of the Cottonwood Limestone rest on top of the fall. Photo date 10/81, © J.S. Aber.

1. Shale: platy, somewhat calcareous, maroon or red shale above green and black shale; generally few fossils, black shale has inarticulate brachiopods, maroon shale lacks marine fossils. Examples: lower Eskridge, lower Speiser, lower Wymore, lower Gage. Environment: lagoon or estuarine (green or black) to emergent (maroon) tidal flat or sabhka.

Students examine the red shale zone below the middle limestone bed in the Eskridge Shale in the spillway at Lake Kahola, KS. Photo date 10/89, © J.S. Aber.

Major cyclothems are 50-100 feet (15-30 m) thick and include thick, cherty limestone units--Figure 6. The Speiser/Wreford and Blue Springs/Barneston cyclothems are typical examples. Minor cyclothems are usually only 25-40 feet (8-12 m) thick, and thin limestones of variable lithology are typical. Good examples include the Eskridge/Beattie, Easly Creek/Crouse, Blue Rapids/Funston, Wymore/Kinney, Gage/Winfield, and Odell/Nolands cyclothems. Water depth probably did not much exceed wave base during deposition of minor cyclothems. The absence of black shale from the core position (unit 5) of major cyclothems also suggests that deep water conditions were not achieved during major marine transgressions. The total range of relative sea-level change between exposed sabhka stage (unit 1) and maximum transgression (unit 5) was probably 100 feet (30 m) or less (McCrone 1964).

The bedrock stratigraphy has remarkable lateral consistency--Figure 7; unit thickness varies little, and only a few noticeable facies changes are present. Slight thinning of bedrock strata occurs in the central portion of Butler County over the crest of the Nemaha Uplift. Many limestone units show evidence of deposition in very shallow water across this area. Typical sedimentary structures include cross bedding, ripple marks, oolites, and algal laminations. Such features are present in limestone units from the Cottonwood up through the Fort Riley. This region is part of what Imbrie et al. (1964) named the Greenwood Shoal of the early Permian sea.

Cottonwood Limestone cross-laminated, algal facies indicative of shallow water of the Greenwood Shoal, eastern Butler County. Photo date 7/90, © J.S. Aber.
Morrill Limestone ripple marks indicative of shallow water of the Greenwood Shoal, eastern Butler County. Photo date 7/89, © J.S. Aber.
Ft. Riley Limestone cross-bedded, oolitic facies indicative of shallow water of the Greenwood Shoal, central Butler County. Photo date 7/90, © J.S. Aber.

Lower Permian cyclothems are similar to Upper Pennsylvanian cyclothems of eastern Kansas (Klein and Willard 1989); each cyclothem includes a triad of limestone-shale-limestone (units 4, 5 and 6) and represents mainly marine sedimentation. Lithologies of individual units within Lower Permian cyclothems differ considerably, however, from Upper Pennsylvanian cyclothems. Red beds, cherty limestone, dolomite, and evaporites (subsurface) are common in Lower Permian strata; whereas coal, sandstone, and black shale are scarce to absent. The lithologic differences between Upper Pennsylvanian and Lower Permian cyclothems are thought to reflect a combination of factors during early Permian time (Aber 1991).

Red and maroon colored strata of the Gage Shale, central Butler County. The small fold is likely the result of collapse following subsurface solution of evaporites. The red beds represent an arid, terrestrial sedimentary environment, Photo date 1/91, © J.S. Aber.
Herington Limestone, northwestern Butler County. This rock is dolomitic, chalky limestone with numerous quartz concretions. It indicates arid conditions of shallow marine sedimentation. Photo date 6/90, © J.S. Aber.

Many explanations for Pennsylvanian and Permian cyclothems of the midcontinent have been proposed. These explanations generally fall into three categories--crustal movements, glacio-eustatic sea-level changes, and delta-lobe shifting. The Lower Permian cyclothems of the Flint Hills do not contain features associated with deltaic sedimentation (i.e. channel sandstone), and the remarkable lateral consistency of units argues against significant local crustal movements during the early Permian (Moore 1964). The most reasonable explanation for repeated marine transgressions and regressions is glacioeustasy related to glaciation in Gondwana (Aber 1991).

Structural Features

Bedrock dip is normally westward or northwestward at 20 to 40 feet per mile (4-8 m/km), but this varies in many areas. Northward, southward, and eastward dips as steep as 60 to 80 feet per mile (11-15 m/km) are commonly developed in association with folds. Dip angle may locally be as great as 10° in these situations. In some other cases, bedrock dips less than 20 feet per mile (< 4 m/km) over broad areas.

Fold structures--anticlines and synclines--are present in surficial bedrock in many parts of Butler and Chase counties--Figure 8 and Table 1 (below). In most cases, anticlines and synclines occur in closely matched pairs, but this is not true in all situations. Basement control of structures in surficial bedrock of the midcontinent was recognized long ago by Fath (1921, p. 149), who stated that these structures are "in large part the surface expression of deep-seated readjustments along ancient faults or lines of weakness present in the pre-Cambrian basement rocks of the region."

Table 1. Surficial anticlines and synclines of Butler County, Kansas. See Fig. 8 for locations.
Structure Associated Oil
Field or River
1. Blankinship Anticline Blankinship, Young.
2. Beaumont Anticline Ferrel, Ferrel South, Shinn,
Shinn NE, Wehrman.
3. Grouse Syncline Grouse Creek.
4. Latham Syncline none.
5. Brant Anticline Brant-Sensenbaugh.
6. Eagle Syncline Eagle Creek.
7. Cole Syncline Cole Creek.
8. Burns Dome oil field.
9. DeGraff Anticline none.
10. Walnut Syncline Walnut River and West
Branch Walnut River.
11. Dunkle Syncline unnamed stream.
12. El Dorado Anticline El Dorado.
13. Hammond-Fowler Syncline none.
14. Pierce Anticline Pierce.
15. Shumway Anticline El Dorado.
16. Augusta North Anticline Augusta North.
17. Augusta Syncline Walnut River.
18. Augusta South Anticline Augusta.
19. Gordon Syncline none.
20. Douglass Anticline oil field.
21. Salter Syncline none.
22. Rose Hill Anticline oil field.
23. Seltzer Syncline Seltzer Spring.

The crest and trough of an anticline/syncline pair are generally parallel to each other, with the syncline located from ½ to 2 miles east of the anticline. Between the anticline and syncline, bedrock dips toward the east. The fold pair may extend for several miles in curved northerly, northeasterly, or easterly trends. The two longest folds in Butler County are both synclines: Walnut Syncline (10) and Gordon Syncline (19).

Surficial folds of Butler County are located above major uplifts in the basement surface. Folds in the western portion of the county are associated with the Nemaha Uplift, which is bounded on its eastern side by the buried Humboldt Fault--Figure 8. The basement surface is offset by several 100 feet to more than 1000 feet (300 m) along the fault zone (Bickford et al. 1979). Local culminations on the Nemaha Uplift correspond to the Burns, El Dorado, and Augusta-Douglass regions of surficial folding. Surficial folds in the southeastern portion of Butler County are located above a more subtle uplift in the basement surface.

Joints are ubiquitous in bedrock of the Flint Hills. Joint measurements show that at most sites two sets of joints are present--Figure 9. Some sites have three joint sets, a few sites have only one set of visible joints. The two dominant joint sets (1 and 2) are complementary (around 90° angle) and are found at most sites. Joint set 1 is widely present throughout eastern Kansas, at nearly all sites. It parallels the Fredonia tectonic zone and the Silver City-Rose Dome tectonic zone, which trend about 50° (Berendsen and Blair 1991). Joints sets 2 and 3 are related to well-known basement structures. Joint set 2 is parallel to numerous northwest-trending basement faults found throughout eastern Kansas--the Neosho and Fall River trends. Joint set 3 corresponds to the buried Humboldt Fault zone.

Joints in Ft. Riley Limestone at the Augusta city lake, Butler County. This joint set trends 325°. Photo date 11/90, © J.S. Aber.
Thrust fault exposed in roadcut along U.S. highway 54 west of Augusta, Kansas. The band of shale marks the thrust within the Ft. Riley Limestone. Scale pole marked in feet. Photo date 2/91, © J.S. Aber.

Faults are relatively rare in surficial rocks of Chase and Butler counties. Both normal and thrust faults are present in a few localities in association with anticlines. This pattern suggests that faulting occurred in connection with layer-parallel folding of brittle rocks--Figure 10. For example, a thrust fault is exposed along U.S. highway 54-77 on the western flank of the Augusta North anticline (see fig. 8). The fault cuts through the Ft. Riley Formation, which here strikes/dips 160°/10°SW. The thrust fault strikes/dips 170°/20°SW and has an estimated throw of 20 feet. This site displays the greatest amount of structural disturbance seen at the surface anywhere in Butler County.

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Field Geology syllabus.
J.S. Aber © 2008.