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Subsurface Rocks
Stratigraphy and Structure
Osage County lies in the southwestern part of the Forest City basin. Subsurface conditions along a north-south line through the central part of the county are shown on Figure 2. Major rock units are differentiated.
The surface rocks generally dip west by northwest, but the pre-Chattanooga subsurface units dip more northerly into the North Kansas basin.
A brief discussion of the stratigraphy of the major rock divisions present in the subsurface of Osage County is given in the following paragraphs.
Pennsylvanian Rocks
Pennsylvanian rocks in Osage County have a maximum thickness of about 2,250 feet. The west by northwest dipping Pennsylvanian rocks do not have a constant thickness due to slight lateral changes in the thickness of the units, and because some have been removed from the eastern part of the county by erosion.
The Wabaunsee rocks are well exposed in western and northwestern Osage County. The thickness of Wabaunsee rocks in the county is about 500 feet. The group of rocks is composed of thin fusulinid-bearing limestone and clayey and sandy shale.
The Shawnee group consists of less than 350 feet of clayey and sandy shales ranging up to about 80 feet in thickness, and limestones generally less than 20 feet thick. The Douglas-Pedee group is composed mostly of clastic material and discontinuous thin beds of limestone and shale. They have a maximum thickness of 275 feet. The Lansing-Kansas City group comprises predominantly limestones. The combined groups have a total average thickness of about 375 feet. The Pleasanton shale averages about 130 feet in thickness in the county. The Marmaton group, about 140 feet thick, is characterized by limestones separated by thin shales. The "Cherokee" rocks are made up of about 500 feet of shales with some thin discontinuous limestones and sandstones.
Mississippian Rocks
The undifferentiated Mississippian rocks in Osage County consist of an uninterrupted sequence of characteristically cherty limestones. They range from 250 to 400 feet in thickness (Lee and others, 1946, sheet 5). The Chattanooga black shale ranges in thickness from 50 to nearby 150 feet. The greater thickness is in the northwestern part of the county.
Pre-Chattanooga Rocks
The Chattanooga shale overlaps the upper part of the "Hunton" limestone, the Maquoketa shale and the beveled beds of the Viola limestone in northwestern Osage County. Along the line of the cross section (Fig. 2) the "Hunton" limestone ranges from a featheredge to about 35 feet. Silurian rocks are believed to be absent in the county (Lee, 1943, fig. 13). Some Maquoketa shale (uppermost Ordovician) is present in the northern part of the county (wells 1 and 2, Fig. 2). The Viola limestone ranges from 50 to 100 feet in thickness along the line of the cross section. The Simpson rocks consist of sandstones which range in thickness from 10 to 50 feet. The interval between the top of Pre-Cambrian rocks and the top of the St. Peter sandstone ranges from about 400 feet in northwestern Osage County to almost 800 feet in the southeastern corner (Lee and others, 1946, sheet 1).
Keroher and Kirby (1948) indicate that (1) the Jefferson City-Cotter rocks in Osage County range from 100 feet in the northwest corner to more than 250 feet in the southern part, (2) the Roubidoux rocks have an average thickness of 100 feet, (3) the Van Buren-Gasconade strata range in thickness from less than 50 to more than 100 feet from west to east across the county, (4) the Eminence dolomite is absent in southwestern Osage County and ranges from a featheredge to more than 50 feet in thickness northeastward, (5) the Bonneterre dolomite, more than 50 feet thick, lies on Pre-Cambrian rocks throughout the county.
Oil and Gas Exploration
No commercial quantities of oil or gas have been reported from Osage County. The Pomona gas pool (now inactive), located in Franklin County, just east of the eastern Osage County line, produced from the "Squirrel sand" in the upper part of the "Cherokee" group. Gas wells have been reported in sec. 16, T. 16 S., R. 17 E. and in sec. 5, T. 17 S., R. 17 E.; however, production has not been substantiated.
The Geological Survey has records of 64 exploratory tests in Osage County (Pl. 2). Although the tests geographically extend throughout most of the county, few were drilled deeper than the top of Mississippian strata. Because of this, the possibility of commercial production from Osage County should not be precluded.
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Kansas Geological Survey, Geology
Web version April 2002. Original publication date May 1955.
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