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Geologic Structure
Structural Features and Relation to OCcurrence of Oil and Gas
The regional dip of the rocks in Cowley County is westward and averages about 25 feet per mile, as in most of the eastern half of Kansas. This westward dip or tilting, which in part accounts for the parallel belts of outcrop of the rock units, is modified by local structures in which the beds are almost horizontal, or even dip eastward. Many of the important oil fields in the county have been developed on these structural features--anticlines, synclines, anticlinal noses, domes, and basins--which are generally of small areal extent. Even the major structures, which may extend many miles beyond the borders of the county, seem to be composed of groups of smaller features that are aligned in one general trend.
The most pronounced surface structural anomaly in the county is the Dexter Anticline, which trends due north from a point near Otto to Dexter and thence northeast. The east flank of this fold is steeper than the west flank; the rocks dip eastward more than 100 feet in about half a mile eastward from the axis of the anticline, but they dip westward only about 100 feet in a mile west of the axis. This anticline is divided into a series of local "highs" by transverse sags or saddles. The structure in this part of the county is evident where inliers caused by structural movement are exposed by subsequent erosion in sec. 24, T. 34 S., R. 6 E., and sec. 7 and 18, T. 34 S., R. 7 E. (Pl. 1).
The Winfield Anticline, whose axis trends northeast-southwest about 2 miles east of Winfield is a prominent structural feature that is evident in the surface rocks, although the dips along the flanks of this structure are even more gentle than those on the Dexter Anticline.
The third prominent structure in Cowley County crosses T. 30 S., R. 3 E., in the northwestern part of the county. This structure is a part of the Nemaha Anticline, which trends from Nemaha County in northeast Kansas southwest into Oklahoma. In Cowley County this structure is not so apparent on the surface as is the Dexter Anticline, because the surface rocks over it are principally soft shale. Thin limestone beds within the shale indicate some tilting of the strata, and the Winfield Limestone crops out west of where it would under normal structural conditions.
Many smaller structural features, some of which have a surface relief of less than 10 feet, lie between and roughly parallel with these anticlines. Structural relief in the subsurface is more pronounced than on the surface, and the trace of the axis on the surface may not be directly over the trace of the axis in the subsurface. Plate 5 shows the structure at the top of the Douglas Group, including the three large and many of the small structural features. Contours of the top of the Douglas Group show more structural relief than do contours of the surface rocks, but they more nearly reflect surface conditions than would contours on any deeper strata. Many producing oil and gas wells have been drilled on the three large anticlines and on the small structures.
Relation of Structure to Occurrence of Ground Water
The availability of ground water in an area underlain by alternating layers of shale and limestone is closely related to the structure of the area. Water entering an aquifer moves downdip and collects in synclines or basins. Wells drilled in these structural depressions generally yield more water and are more dependable in periods of drought than nearby wells situated higher structurally. Porosity and permeability are increased by fractures and joints over the sharper flexures in the anticlines and synclines. Water enters the aquifer more readily through these fractures, and movement within the aquifer is aided by the joints and fractures. The greater recharge rate and transmissibility increase the quantity of water available from wells in the structurally low areas. Wells in the synclinal or basin areas east of both the Dexter and the Winfield Anticlines yield more water than nearby wells outside these low areas. More water is recorded in logs of oil wells drilled in structurally low areas than in structurally high areas.
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Kansas Geological Survey, Geology
Placed on web May 21, 2009; originally published August 1962.
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