Prev Page--Historical Summary || Next Page--Economic Geology
Structural Geology
The structural geology of Mitchell and Osborne counties was studied by means of data obtained both in the field and from well logs. During the field investigation elevation readings were made with an altimeter on key horizons such as the top of the "post rock" in Mitchell County and the base of the Niobrara in Osborne County, and by means of these data a structure map of the surface formations was drawn (Plate X). As control points were fairly numerous and frequent temperature and pressure corrections were made for the altimeter readings, the writer believes this map illustrates the regional structure with fair accuracy. By calculating the elevations of different formations encountered in wells drilled in Mitchell and Osborne and surrounding counties J. W. Ockerman was able to draw subsurface contour maps illustrating the regional structure of the top of the Permian (Plate XI), the base of the Permian (Plate XII), and the base of the Pennsylvanian (Plate XIII). The number of wells was insufficient to give complete control in all parts of the two counties, so only a general picture of the subsurface structure is possible. Errors in correlation of the formations from one well to another would cause some errors in the contour maps, but the writer does not believe that these, if present at all, are of sufficient magnitude to change seriously the regional structure.
Surface Structure
The surface formations in north-central Kansas have a dip to the north averaging between 10 and 15 feet to the mile, but a remarkable exception occurs in Mitchell County due to the presence there of a structural basin (Plate X). This basin covers a large area in the center of the county, and the formations dip into it from all sides, especially from the southwest and northeast. Two anticlinal noses lie in the southeast part of the county and another one, which probably has some closure, lies in western Cloud County and extends over into northeastern Mitchell County. An exceptionally long, northward-plunging anticline occurs along the western side of Mitchell County and separates the basin to the east from a synclinal area to the west in eastern Osborne County.
The typical north dip in Osborne County is modified by the northern extension of the Fairport anticline which produces oil in northwestern Russell County. This anticline enters Osborne County near the southwestern corner and as it extends northward in bifurcates, one branch trending to the northeast through the town of Osborne and the other continuing only slightly east of north and entering Smith county a short distance west of Portis. As the two branches continue northward the western one increases in magnitude, while the eastern one decreases and dies out just south of the Smith county line. A small basin lies south and west of Bloomington. The southeastern township of Osborne County is structurally high and some closure probably is present there.
Some very pronounced domes lie in northern Lincoln and southern Mitchell counties. These are too local to appear on the regional structural map, but some of them have closures as great as 90 feet and dips up to 4 degrees. The most prominent domes lie east of Salt creek, in the northern part of T. 9 S., R. 8 W., and the southern part of T. 8 S., R. 8 W., and south of Salt creek, in T. 9 S., R. 7 W. Abnormal dips may be observed along most of the Greenhorn outcrop in the Salt and Rock creeks drainage. These frequently cause It stratum to dip beneath a tributary valley and then reappear again for a short distance farther up the valley. Most of the domes arc elliptical in outline, but a few were noted which were practically circular. Abnormal dips were also observed northwest of Beloit, in sections 20 and 29, T. 6 S., R. 7 W. Limestones in the Greenhorn formation had a pronounced southward dip in section 29 and a relatively steep westward dip in section 20. Only a few of the domes in Mitchell County have been tested with the drill, but disappointment has so far followed all attempts to find oil in such structures both here and in Lincoln County.
Subsurface Structure
The contour map drawn on top of the Permian (Plate XI) is very similar to the one constructed from surface data. Both show a pronounced anticlinal nose trending northeastward from the southwestern corner of Osborne County. No bifurcation of this structure is shown on the subsurface map, but that could be explained by lack of data due to absence of wells drilled along critical parts of the fold. The synclinal area in eastern Osborne County appears on both maps. The Tipton anticline is not apparent on the subsurface map, but this may be due to lack of control points in southwestern Mitchell County. The large structural basin can only be inferred on the subsurface map by the manner in which the contour lines diverge when approaching this area. South-central Mitchell County is high on all three of the subsurface maps.
The base of the Permian (Plate XII) dips westward in Osborne and western Mitchell County at a rate of about 7 feet to the mile. A northward trending anticlinal nose extends into southern Mitchell County from Lincoln County. A similar anticline extends into the southwestern corner of Osborne County. Although control points are scarce in central Mitchell County the dips appear to flatten out there.
The basal Pennsylvanian structure (Plate XIII) is dominated by a northward plunging syncline through central Osborne County and a northward plunging anticline in south-central Mitchell County. The lowest elevations are in the vicinity of Portis in north-central Osborne County while the highest are in the southwestern corner of the same county. The relatively steep rise to the southwest is caused by the presence of the Fairport anticline in northwestern Russell County. The elevation of the base of the Pennsylvanian is 200 feet higher in southwestern Osborne County than it is on the axis of the anticline in southern Mitchell County.
The lack of uniformity between the structure of the base of the Permian and that of the base of the Pennsylvanian is due to thickening of the Pennsylvanian sediments to the east and northeast.
Origin of Surface Structures
The well-developed domes occurring in southern Mitchell and northern Lincoln counties appear to be ideal places in which to drill. Yet several of the domes have been tested and no oil has been found in them. Furthermore, it appears that some of the wells which were best located in regard to surface structure actually found the Pennsylvanian formations at lower elevations than near-by wells. Such was the case in the Abercrombie and Gurley wells shown at the right side of the correlation chart (Plate IX). The Abercrombie well entered the Permian about 60 feet higher than the Gurley, but was 80 to 90 feet lower in the Pennsylvanian. The writer believes that the sharp domes so common in this section of Kansas are confined to the Cretaceous rocks and are due to inclined deposition of the Cretaceous sediments on the flanks of Permian hills. A more complete discussion of this problem is in preparation.
Prev Page--Historical Summary || Next Page--Economic Geology
Kansas Geological Survey, Geology
Comments to webadmin@kgs.ku.edu
The URL for this page is http://www.kgs.ku.edu/Publications/Bulletins/16/06_struct.html