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Geology of Subsurface Rock Units
Subsurface rocks in Cowley County were not studied in detail; the description of the rocks is therefore generalized. The rock units are discussed in general by system or a combination of systems. Although the rocks of each system are divided into small units in the Kansas classification, some of these smaller units have not been identified in most well logs, but many "key horizons" can be readily identified in logs of most wells in Cowley County.
Precambrian Rocks
Rocks of Precambrian age do not crop out in Kansas. Logs of deep wells in the county and adjacent areas indicate that this basement complex of crystalline rocks is composed chiefly of granite and schist, but it also may include quartzite, quartz porphyry, diabase, and other types of igneous or metamorphic rocks. A few wells in Cowley County have reached the Precambrian rocks. These wells indicate that the surface of the Precambrian dips in a westerly direction from an altitude of about 2,400 feet below sea level in the eastern part of the county to about 3,800 feet below sea level in the southwestern part. In the northwestern part of the county there is a reversal of dip on the Precambrian surface at the east flank of the Nemaha Anticline. Here the Precambrian surface rises sharply, and locally may be only 2,000 feet below sea level. Locally, detrital material eroded from adjacent areas mantles the Precambrian surface in this area.
Cambrian and Ordovician Systems
Deep wells in Cowley County have penetrated a thick series of strata composed of limestone, sandy limestone, dolomite, sandstone, and minor amounts of shale. These rocks, of Cambrian and Ordovician age, lie on the weathered Precambrian surface and are overlain by rocks of Mississippian age. This group of rocks has been divided, in descending order, into the St. Peter Sandstone, Cotter and Jefferson City Dolomites, Roubidoux Formation, Gasconade Dolomite, and Van Buren Formation of Ordovician age, and the Bonneterre Dolomite and Lamotte Sandstone of Cambrian age.
The St. Peter Sandstone is present only in western Cowley County, and the Bonneterre Dolomite and Lamotte Sandstone are present only in eastern Cowley County. The other units underlie the entire county, but there probably has been some thinning of the upper unit by pre-Mississippian erosion. The rocks between the St. Peter and Lamotte Sandstones are included in the Arbuckle Group in the Kansas classification and are commonly called the "Arbuckle lime" or "Siliceous lime" by oil men.
The St. Peter Sandstone is assigned to the Simpson Group in the Kansas classification. It is probably not more than 50 feet thick in the county.
The thickness of the Arbuckle Group ranges from about 650 feet in northeast Cowley County to about 1,000 feet in southwest Cowley County. Oil and gas are produced from the St. Peter Sandstone and from porous zones in the upper part of the Arbuckle Group. All these rock units locally yield large quantities of mineralized water, which has been used as injection water in secondary recovery of oil. These rocks also are widely used for disposal of brine produced with petroleum.
Mississippian System
Rocks above the post-St. Peter disconformity and below the sub-Pennsylvanian unconformity are included in the Mississippian System in this report, although the lower division, the Chattanooga Shale, in part may be of Devonian age. The Chattanooga consists of black carbonaceous fissile shale, locally somewhat sandy in the basal part, and commonly ranges in thickness from about 50 feet to as much as 200 feet, but it may be absent over some structurally high areas. Structural movement may have elevated the strata one or more times preceding or during deposition of younger Mississippian rocks, allowing erosion to bevel or even remove Chattanooga rocks. The Chattanooga underlies all of Cowley County except an area near Dexter and another in the northeastern part of the county (Bass, 1929, p. 30).
The upper division of the Mississippian System in Cowley County consists of light-gray to dark-gray cherty limestone containing some shale locally. These deposits range in thickness from about 225 feet to as much as 400 feet and collectively are called the "Mississippi lime" by the oil industry. Four rock divisions of formational rank are recognized in the county: the St. Joe Limestone, the Reeds Spring Formation, and the Keokuk Limestone of the Osagian Series, and the Cowley Formation of the Meramecian Series. The Reeds Spring and St. Joe rocks underlie all of the county except small areas where they have been removed by erosion. The Keokuk Limestone is present only in northern Cowley County. The Cowley Formation is the thickest of the Mississippian rock units in the county. This unit was deposited in an erosional basin extending southward from northern Cowley County, where the formation abuts but does not overlap the Keokuk Limestone. The Cowley Formation generally overlies Reeds Springs or St. Joe rocks, but in some places it rests on Chattanooga or older rocks.
The Mississippian rocks thin considerably over structurally high areas, owing to pre-Pennsylvanian erosion there, and are productive of oil and gas from the upper, porous part of the strata. Locally, large yields of strongly mineralized water are obtained from these rocks. This water is used to some extent as injection water in the secondary recovery of oil.
Pennsylvanian System--Middle Pennsylvanian Series
Interbedded limestone, shale, and sandstone having an aggregate thickness ranging from about 2,000 to 2,400 feet compose the Pennsylvanian rock section in Cowley County. Only the upper part of the Middle Pennsylvanian Series and the Upper Pennsylvanian Series of the Pennsylvanian System are represented in the county. The Middle and Upper Pennsylvanian Series have been divided into groups, formations, and members on the outcrop, and although many of these units may be recognized in the subsurface, others are not so easily recognized; hence these rocks are discussed below by groups.
Cherokee Group
The Cherokee Group includes the strata between the Mississippian System at the base and the Fort Scott Limestone at the top. The Cherokee unconformably overlies the Mississippian rocks and in Cowley County is composed of gray and dark-gray shale and small amounts of sandstone. The rocks range in thickness from about 130 feet in northwest Cowley County to about 300 feet in south and southeast Cowley County. The Cherokee rocks thin over areas where the Mississippian rocks are structurally high. Discontinuous sandstone bodies, generally much greater in length than in width, are present at several different horizons in the Cherokee group and seem to be channel or beach deposits. They are important reservoirs for the accumulation of oil and gas.
In some places the basal Cherokee deposits are composed of eroded fragments derived from the "Mississippi lime" and consisting of rounded pebbles and sand grains of chert and small amounts of other resistant rocks. These deposits are present only locally but are commonest along the flank of the Nemaha Anticline. Oil and gas are produced from some of these deposits, which are called "Mississippi chat" by the oil industry. The sandstones and "chat" yield mineralized water in parts of the area. Some of the water obtained in the production of oil is reinjected in secondary recovery operations.
Marmaton Group
The rocks between the top of the Cherokee Group and the disconformity at the base of the Hepler Sandstone are included in the Marmaton Group. This group constitutes the upper part of the Middle Pennsylvanian Series in Kansas. In Cowley County these rocks are composed of limestone, shale, and sandstone and have a thickness ranging from about 200 to 250 feet. Elsewhere in eastern Kansas these sandstones are important producers of oil and gas, but they are not productive in Cowley County. The Marmaton rocks yield relatively small quantities of strongly mineralized water in Cowley County.
Pennsylvanian System--Upper Pennsylvanian Series
The major disconformity at the top of the Marmaton Group marks the boundary between the Middle Pennsylvanian Series and the Upper Pennsylvanian Series.
Pleasanton Group
The rocks between the disconformity at the top of the Marmaton Group and the base of the Hertha Limestone, which marks the base of the Kansas City Group, are included in the Pleasanton Group. In Cowley County this group ranges in thickness from about 50 to 100 feet and is composed chiefly of clastic sediments. The Checkerboard Limestone is the only important limestone in the group and it overlies the lowest formation of the group, the Hepler Sandstone. Pleasanton rocks are not productive of oil or gas in Cowley County and yield only small quantities of strongly mineralized water.
Kansas City Group
Rocks from the base of the Hertha Limestone to the base of the Plattsburg Limestone compose the Kansas City Group. These rocks have been divided into the Bronson, Linn, and Zarah Subgroups. In Cowley County these units have a total thickness of about 450 feet. The Bronson Subgroup forms the lower third of the Kansas City Group. This unit contains more limestone than the two upper units, although the individual limestone and shale beds are thinner. The Linn and Zarah Subgroups are composed principally of clastic materials, but the Iola Limestone, at the top of the Linn Subgroup, is persistent and serves as a good marker bed in subsurface mapping. In northern Cowley County the upper part of the Kansas City Group is composed chiefly of gray limestone. The limestone units locally produce oil and gas. Sandstone in the Linn Subgroup, called the "Layton sand" by the oil industry, and correlative with the Cottage Grove Sandstone Member of the Chanute Shale, is an important reservoir for accumulation of oil and gas. Water contained in the sandstone of the Kansas City Group is strongly mineralized but is used as injection water in some secondary recovery projects.
Lansing Group
The Lansing Group includes three formations which, in ascending order, are the Plattsburg Limestone, Vilas Shale, and Stanton Limestone. In the subsurface in Cowley County this sequence of rocks contains less limestone than on the outcrop to the east of Cowley County. This difference is due partly to facies change, as the shales thicken westward and the Plattsburg Limestone becomes shaly, and partly to cutting out of the Stanton and the overlying Weston Shale of the Pedee Group by erosion early in Douglas time. Where the limestone formations in the Lansing are not present, separation of the Lansing from the underlying Kansas City Group is difficult. The lower boundary of the Lansing Group has been placed at the base of the Plattsburg Limestone for some time, but the upper boundary has in the past been placed as high as the top of the Tonganoxie Sandstone of the Douglas Group ("Stalnaker sand" of the oil industry, Winchell, 1957), thus, as in the past, including it and all of the Pedee Group in the Lansing Group. In the present report, the beds from the base of the Plattsburg to the unconformity at the base of the Douglas Group have been included with the Lansing. Under this grouping, the Weston Shale of the Pedee Group is included with the Lansing.
The thickness of the Lansing Group ranges from about 60 feet in western Cowley County to about 180 feet in eastern Cowley County, where the Weston Shale is present. Here the thickness of the Stanton Limestone, Vilas Shale, and Plattsburg Limestone is about 100 feet. Oil and gas have been produced in Cowley County from sandstone bodies in the shale units of the Lansing. Water in these units is strongly mineralized.
Douglas Group
The Douglas Group includes the strata below the base of the Oread Limestone of the Shawnee Group and above the unconformity at the base of the Tonganoxie Sandstone Member of the Stranger. The rocks of the Douglas Group are composed chiefly of shale and sandstone but include small amounts of limestone, and have a thickness ranging from 300 to 350 feet. The Douglas Group is divided into two formations: the Stranger Formation in the lower part and the Lawrence Shale in the upper part. The Stranger Formation is composed of shale and much sandstone and, locally, two thin limestones.
In Cowley County the Tonganoxie Sandstone Member at the base of the Stranger seems to lie in an erosional basin and to be a sheet deposit rather than a channel fill. Locally the Weston Shale of the Pedee Group and the Stanton Limestone of the Lansing Group have been cut out, leaving the Tonganoxie Sandstone Member in contact with middle or lower Lansing rocks. The thickness of the Tonganoxie ranges from 0 in northeast Cowley County to about 125 feet in the north-central and southern parts of the county. Generally the Tonganoxie yields moderately large quantities of strongly mineralized water, and oil and gas are produced from the sandstone in favorable structural conditions. Some water from the Tonganoxie is used as injection water in secondary recovery projects. The Lawrence Shale consists of shale, sandy shale, and sandstone. The Ireland Sandstone Member of the Lawrence Shale is a channel sandstone, which rests unconformably on rocks of Stranger age, the division between the Lawrence Shale and the Stranger Formation being placed at this contact.
Shawnee Group
The Shawnee Group is composed of four limestone and three shale units. In ascending order these are the Oread Limestone, Kanwaka Shale, Lecompton Limestone, Tecumseh Shale, Deer Creek Limestone, Calhoun Shale, and Topeka Limestone. On the outcrop in eastern Cowley County the limestones in this group are thicker than the, limestones in the overlying Wabaunsee Group or those in the underlying Douglas Group, and are the most conspicuous lithologic unit. In Cowley County the Shawnee Group is 450 to 500 feet thick and contains less limestone than in the outcrop area. Sandstone is present in most of the thick shales, and oil and gas have been produced from these sandstone bodies. Water contained in these rock units is strongly mineralized.
Wabaunsee Group
In Cowley County the lower part of the Wabaunsee Group is confined to the subsurface, but inasmuch as the upper part crops out, the entire group is discussed under outcropping rock units.
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Kansas Geological Survey, Geology
Placed on web May 21, 2009; originally published August 1962.
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