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Forest City Basin

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Stratigraphy, continued

Rocks of Mississippian Age

The Mississippian rocks are exposed at the surface in wide areas in the Mississippi valley in Missouri and Illinois and extend westward hundreds of miles in the subsurface of Kansas and adjoining states.

The Mississippian limestones represented in northeastern Kansas consist of the following formations, listed in descending order.

Meramecian series
Ste. Genevieve limestone
St. Louis limestone
Spergen limestone
Warsaw limestone
Osagian series
Keokuk and Burlington limestones (undifferentiated)
Kinderhookian series
Gilmore City limestone
Chouteau limestone
Rocks of Devonian or Mississippian age
Chattanooga shale

Kinderhookian Series

Chouteau limestone

The outcrops of Chouteau limestone nearest to northeastern Kansas are in Pettis County, Missouri, on the southeastern side of the Forest City basin. This county includes the type locality of the Chouteau named by Swallow in 1855. Swallow recognized upper and lower members of the Chouteau. Moore (1928, p. 149) restricted the name Chouteau to the lower member because he found the upper member, which he termed Sedalia, to be lithologically and faunally different from the lower member. He correlated the upper member of the outcrops, which is a cherty dolomite, with some part of the Fern Glen limestone (of early Osagian age).

The Chouteau restricted, or lower Chouteau, consists of light bluish-gray to dark-drab compact earthy to silty impure limestone banded with dull-gray chert nodules 1 to 4 inches thick. In the outcrops its thickness ranges from 20 to 55 feet. The correlatives of the lower Chouteau in southeastern Kansas and southwestern Missouri are the Northview shale and the underlying relatively pure semigranular Compton limestone.

Three members of the Chouteau are recognized in the subsurface of northeastern Kansas: a relatively pure semigranular limestone at the base that corresponds to the Compton limestone of southeastern Kansas, an impure very cherty sucrose gray dolomite in the middle that corresponds to the Northview shale, and a brown to buff sucrose dolomite containing only small amounts of chert at the top. The uppermost member may correspond to one of the lithologically similar formations of the upper part of the Hampton formation of Iowa, or to the original upper Chouteau of Swallow.

The Compton limestone maintains its identity at the base of the Chouteau, but its upper limit is transitional and in areas of the Chouteau limestone is not sharply separable. The Compton is 25 to 40 feet thick in most wells in northeastern Kansas but locally, as in part of Franklin County, it is less than 10 feet thick and it wedges out near the Oklahoma border.

The middle or cherty member of the Chouteau limestone is generally impure and grades southward into the Northview shale. The amount of chert is somewhat exaggerated in the samples of wells drilled by standard tools. The chert is commonly 50 to 95 percent of the volume but the same zones represented by rotary samples have a maximum of only 25 percent chert. This member of the Chouteau varies much in thickness. It is 10 to 20 feet thick in the belt bordering the Nemaha anticline on the east, but in counties in Kansas bordering the Missouri river its thickness increases to 75 or 80 feet. There is considerable variety in the character of the chert. Some is dull gray and mealy in texture and some breaks with conchoidal fractures and shows a stippled pattern. Most of the chert, however, has unique characteristics and is easily identified.

The uppermost member of the Chouteau is buff to brown sucrose dolomite. It has very small amounts of insoluble residue and in many localities is noncherty to sparsely cherty. Even in wells where it includes 5 to 10 percent chert its contrasts sharply with the underlying very cherty middle member. This member occupies the same position as the Sedalia limestone of the outcrops in Pettis County, Missouri, but the Sedalia is reported to be conspicuously cherty and is of Fern Glen (Osagian) age. This member was described in an earlier report (Lee, 1940, p. 37) as equivalent to the Sedalia limestone with which it has been correlated in northwestern Missouri. The rocks of this member, however, are conspicuous because of their lack of chert and are overlain in the subsurface by the Gilmore City limestone of Kinderhookian age. These considerations seem to prevent this member from being correlated with the Sedalia limestone which is accepted as of Osagian age in northeastern Missouri. The uppermost member of the Chouteau is more regular in thickness than the other members of the Chouteau and is more widely distributed toward the west. It is the oldest Mississippian limestone west of the Nemaha anticline where it averages about 10 feet in thickness but ultimately it wedges out toward the west upon the red, weathered surface of the Chattanooga shale. Toward the east its thickness increases to 30 feet near the Missouri river. East of the Nemaha anticline it overlies the middle member of the Chouteau which is absent west of the anticline.

The combined thickness of the three members of the Chouteau is 53 feet just east of the Nemaha anticline, but it increases to 141 feet in Johnson County (Lee, 1940, cross section D-C, pl. 6). The Chouteau becomes thinner again toward the outcrops in Missouri where its upper member appears in the stratigraphic position of the Sedalia.

The absence of the Hannibal shale and the Louisiana limestone between the Chouteau and Chattanooga formations in northeastern Kansas indicates unconformable relations between these formations. Unconformable relations occur also at the top of the Chouteau limestone, for there are many places where the uppermost member of the Chouteau is thin or absent. In some wells the upper member is replaced by or is overlain by the Gilmore City formation implying an unconformity between the Chouteau and the Gilmore City. In other wells the Chouteau is overlain unconformably by rocks of undifferentiated Burlington and Keokuk limestones in contact with either the upper or middle member of the Chouteau.

Gilmore City limestone

The only known outcrops of the Gilmore City limestone are in north-central Iowa on the northeastern margin of the Forest City basin. The fauna of these rocks was studied by Laudon (1933) who reported them to be of late Kinderhookian age. The limestones are characterized lithologically by oolitic beds in parts of the formation. According to Laudon, the Gilmore City limestone thickens into a basin trending toward the southwest in southeastern Iowa. The distribution of the limestone suggests that its area of deposition was related to deformation of the North Kansas basin. Its maximum, thickness, as reported in the subsurface by Laudon, exceeds 250 feet.

Limestone above the Chouteau in northeastern Kansas, which is believed to represent the Gilmore City, has been described by Lee (1940, p. 40). It is semigranular, ordinarily slightly buff or yellowish-gray, noncherty or only slightly cherty, and includes many crinoid stems and shell fragments. The matrix is opaque and so soft that most of the cuttings are lost in washing. Oolites in the limestone are irregular in size and form. Some of the oolites are black or have black cores.

The maximum known thickness of the Gilmore City limestone in north-central Kansas is 76 feet in Cloud County. It is 45 feet thick in southern Jackson County. A recent well, the Duffens et al. No. 1 Stanley, in sec. 3, T. 14 S., R. 21 E., revealed the presence of 15 feet of Gilmore City limestone in east-central Douglas County which extends the known area of deposition about 45 miles to the southeast.

Considerable new information concerning the stratigraphic relations of the Gilmore City limestone has been secured since the publication of the report on the subsurface Mississippian rocks of Kansas (Lee, 1940). The theories expressed therein regarding the relations of the Gilmore City and the underlying rocks have been confirmed by more recent drilling. Several wells now show oolitic Gilmore City limestone overlying the Chouteau, and also in several wells the Gilmore City has been found replacing the eroded top member of the Chouteau in the position of the Sedalia limestone. The Gilmore City is overlain by limestone of undifferentiated Burlington and Keokuk age. In some wells the Gilmore City has been eroded and its position is occupied by the Burlington-Keokuk sequence. The Duffens et al. No. 1 Stanley well revealed the Gilmore City limestone replacing the eroded top member of the Chouteau. Similar relations occur in the McCain No. 1 Doane well, in sec. 34, T, 12 S., R. 22 E., but the Dunn No. I Morgan well, in sec. 6, T. 13 S., R. 22 E., 3 miles distant, showed the Burlington-Keokuk, sequence resting on middle Chouteau, both the Gilmore City and the top member of the Chouteau being absent.

A normal sequence of the Gilmore City limestone overlying the uppermost Chouteau member occurs in McLaughlin Sons No. 1 Thorpe well, in sec. 27, T. 10 S., R. 20 E., Continental et al. No. 1 Berridge well, in sec. 8, T. 9 S., R. 17 E., and Bird and Sheedy No. 1 O'Neill well in sec. 34, T. 8 S., R. 19 E. From these relations, it is evident that the Gilmore City is unconformable upon the underlying Chouteau and also that it is separated from the overlying undifferentiated Burlington and Keokuk by an unconformity.

Osagian Series

Rocks of the Fern Glen age, including the St. Joe limestone and the Reeds Spring limestone which crop out in southwestern Missouri, were not generally deposited in northeastern Kansas, although thin overlapping wedges of rocks of upper Fern Glen age may be present beneath the Burlington in some places in the southern part of the synclinal belt east of the Nemaha anticline.

Burlington and Keokuk limestones (undifferentiated)

The Burlington and Keokuk limestones are widely distributed throughout the interior of North America at the surface and in the subsurface. Their type localities are in southeastern Iowa, where these formations consist of cherty limestone and dolomite. They are distinguishable lithologically and faunally at the type locality, but the lithologic characteristics are not persistent enough to serve as a safe guide in differentiating the Burlington and Keokuk at distant points. They have not as yet been satisfactorily identified in the subsurface, although some local lithologic criteria seem to furnish basis for differentiation.

The Burlington-Keokuk sequence in the subsurface of northeastern Kansas consists of two dissimilar members. The lower part of the Burlington-Keokuk sequence is a very cherty sucrose dolomite. Some of the samples consist almost entirely of chert, but there has been some concentration of chert and loss of dolomite in drilling. The chert is opaque or semiopaque and gray or white. Much of it breaks with a smooth fracture but some is dull and breaks with a rough surface. Drusy quartz and splintered fragments of quartz crystals are common constituents of the insoluble residues. The lower 10 to 30 feet in Douglas, Franklin, and Johnson counties consists of limestone nearly free from chert. This may represent the northern edge of rocks of Fern Glen age which are thick in southeastern Kansas but wedge out toward the north, or the change to limestone may be merely another example, as yet unexplained, of local transition from limestone to dolomite or dolomite to limestone that is common in some formations of the Mississippian.

The top of the lower members is porous and most wells fill with water from this contact zone. In the McLouth pool, wells drilled to this porous zone yield oil on the crest of the anticline. An unconformity is suspected at this contact. Some wells find the top of this zone to consist of nearly 100 percent chert. Other wells reach the zone of maximum chert content lower in the formation. The lower member of the Burlington-Keokuk sequence has a thickness of 50 to 90 feet, a variation, if it were local, that might be regarded as an indication of unconformity between the upper and lower members.

The upper member is composed predominantly of semigranular limestone with chert amounting in some samples to 50 percent of the volume. The chert is somewhat irregularly distributed both vertically and horizontally. Some noncherty zones can be traced short distances. The limestones are fossiliferous and crinoidal. A few widely disseminated grains of glauconite are found in some samples. The matrix of some zones, 5 to 15 feet thick, consists of sucrose dolomite cementing fragments of broken fossils. Such fragments are subordinate to the dolomite in some wells or may be entirely absent. These dolomite zones, like the zones free of chert, are persistent in some localities and locally serve as convenient reference markers.

Some of the chert breaks with a conchoidal fracture and some with rough fracture. The chert may be stippled or grainy, opaque or semiopaque, particularly near the top of the formation where it contrasts with the opaque microfossiliferous chert of the overlying Warsaw limestone. In some areas, notably in the McLouth pool, the lower part of the upper member contains a large amount of microfossiliferous and spicular chert which in some respects is similar to that in the Warsaw limestone. It is, however, distinguishable from the Warsaw chert which is opaque by the semitranslucent to vitreous character of the microfossils. This type of chert is sparsely present near the base of the upper member of the undifferentiated Burlington and Keokuk in the Kasper No. 1 James well, in sec. 8, T. 13 S., R. 25 E., but in the McLouth field and adjacent areas it occurs throughout the middle and lower parts of this member. It disappears toward the west and has not been observed in the residues of samples of wells from other parts of Kansas.

Some tripolitic chert occurs in the upper part of the Burlington-Keokuk sequence but it is less common than it is in the rocks of southeastern Kansas which are tentatively correlated with the Keokuk. Some irregularly distributed drusy quartz in sparse amount has been noted in the insoluble residues.

In one well, the Duffens et al. No. 1 Stanley, in sec. 3, T. 14 S., R. 21 E., a bed of noncherty oolitic and semigranular limestone lies a few feet below the top of the Burlington-Keokuk unit. This bed is 284 feet above the top of the Chattanooga shale and may represent the Short Creek oolite noted in the upper part of the Keokuk in a number of wells in southeastern and central Kansas (Lee, 1940, p. 61) where the interval down to the Chattanooga east of the Nemaha anticline is 250 feet.

The top of the undifferentiated Burlington and Keokuk limestones in the McLouth field has been placed at the top of a bed of sucrose dolomite, 5 to 20 feet thick, immediately underlying the Warsaw limestone with its characteristic microfossiliferous chert. The insoluble residues of this dolomite consist of spongy crumbs of interlocking spicules accompanied by only minor amounts of chert. This spicular sponge is of a type common in residues of the Spergen dolomite above the Warsaw limestone. Possibly this bed of dolomite should be excluded from the Burlington-Keokuk sequence and considered as the basal member of the Warsaw limestone. Its inclusion in the Burlington-Keokuk sequence is arbitrary, but provides a sharp contact at the base of the Warsaw. The dolomite bed has not been recognized outside the McLouth field.

The thickness of the upper member of the Burlington-Keokuk sequence varies from 55 to 90 feet. Its greatest thickness is in Jefferson, Douglas, and Johnson counties. Toward the west this member is cut off by post-Mississippian erosion.

The combined thickness of the undifferentiated Burlington and Keokuk limestones where they are overlain by Warsaw limestone varies from 135 to 170 feet. The thickness decreases irregularly toward the north. The upper part of the Mississippian rocks was beveled by post-Mississippian peneplanation toward the Nemaha anticline, and on its flank the upper part of the Burlington-Keokuk sequence along with all the younger Mississippian limestones was eroded (fig. 16).

The Burlington-Keokuk unit is underlain unconformably by the Gilmore City limestone or the Chouteau limestone. West of the Nemaha ridge the Burlington-Keokuk overlies the Chattanooga shale in some places. The St. Joe and Reeds Spring limestones which normally intervene between the Burlington-Keokuk unit and the Chouteau in southeastern Kansas are generally absent in northern Kansas where they are overlapped toward the north by the Burlington and Keokuk limestones. The rocks of Burlington and Keokuk age are unconformably overlain by rocks referred to the Warsaw in southeastern Kansas. They overlie the Burlington and Keokuk also in northeastern Kansas but in this area the unconformable relations are not obvious.

Meramecian Series

Warsaw limestone

The Warsaw limestone at its type locality near Warsaw, Illinois, across the Mississippi river from northeastern Missouri, consists of fossiliferous earthy limestone and blue marl. In the subsurface it loses its characteristic lithology and consists of cherty limestone and dolomite. In southwestern Missouri, the Warsaw has been identified by the Missouri Geological Survey in the Carthage quarries and in other outcrops. This limestone is characterized by a somewhat unique chert enclosing the silicified remains of a great number of microfossils and spicules. It has been traced in the cuttings of wells into the Forest City basin.

The rocks in northeastern Kansas which correspond to the Warsaw of southwestern Missouri and southeastern Kansas consist of semigranular cherty limestone in most wells. In Douglas County and western Johnson County, however, the Warsaw consists chiefly of cherty sucrose dolomite in some places interstratified with semigranular limestone or with broken shell fragments in a dolomite matrix.

The chert is opaque and crowded with masses of microfossils whether the rock itself is limestone or dolomite. Some of the beds contain chert with masses of coarser but still microscopic fossils among which minute crinoid joints and minute fragments of broken shells are recognizable. In the McLouth area the Warsaw overlies a zone of sucrose dolomite, 5 to 20 feet thick, with spongy residues of spicules. This dolomite zone is not present in other areas where the contact is determined by the change in the character of the chert from microfossiliferous to massive or grainy and stippled.

The Warsaw has a thickness in northeastern Kansas of 30 to 40 feet, although it is thinner in some wells and has an abnormal thickness of 75 feet in the Kasper No. 1 James well, in sec. 8, T. 13 S., R. 25 E.

The Warsaw is unconformable above the Burlington-Keokuk sequence but it appears to be conformable beneath the overlying silty dolomite called Spergen in this report. The interval from to top of the Warsaw to the top of the porous dolomite that yields oil in the undifferentiated Burlington and Keokuk ranges from 125 to 128 feet in the McLouth gas field. Elsewhere the interval is generally greater, amounting to 140 to 160 feet in some areas and up to 220 feet in the Kasper James well where the Warsaw is abnormally thick. Like other intervals in the Mississippian this interval thins toward the north, and it is only 29 feet thick in sec. 31, T. 3 S., R. 20 E. Nevertheless, the top of the Warsaw is locally a reasonably reliable datum bed from which to measure the interval to the oil-producing zone of porous dolomite.

Spergen limestone

This limestone, named for outcrops at Spergen Hill in Indiana, is widely distributed in southern Indiana, Illinois, and adjoining states. It is the Bedford limestone of Indiana where it is known also as the Salem limestone. It is reported by Ulrich (1905, pp. 28-30) to consist generally of light to dark oolitic limestone with interbedded limestone of various kinds and thin seams of yellowish shale. Weller and St. Clair (1928, p. 204) describe as follows the varied lithologic features of the Spergen in eastern Missouri and Iowa:

In the north it is for the most part an impure limestone in places arenaceous and very commonly magnesian. To the south it becomes more and more a pure limestone until the impure magnesian beds have almost or quite disappeared in Ste. Genevieve County.

A zone of somewhat variable rocks overlying the Warsaw in northeastern Kansas is believed to represent the Spergen limestone. These rocks contain only small amounts of chert or none at all in contrast to the Warsaw rocks which are conspicuously cherty. In some wells, as the Kasper No. 1 James well in Johnson county, the entire section of 50 feet consists of semigranular limestone.

In most of the area of northeastern Kansas where the Spergen limestone has been preserved it consists chiefly of silty buff to gray, finely sucrose dolomite. In a few wells the dolomite has a greenish cast and contains enough silt to remain slightly coherent after the solution of the dolomite. The silty nature of the dolomite is ordinarily not apparent, however, without treating the dolomite with acid. The dolomite in some wells is interstratified with semigranular limestone.

Some of the Spergen samples of both limestone and dolomite have small amounts of chert which resembles the microfossiliferous chert of the Warsaw. Insoluble residues of some samples show molds of microfossils in the chert, the chert having replaced the matrix but not the calcareous material of the organisms. Another not infrequent characteristic of the residues of the Spergen limestone is the occurrence of white, pink, or salmon-colored chalcedonic chert occasionally recognizable as secondary crusts. This material may be the expression of post-Spergen exposure and weathering, for the same type of chert is occasionally encountered in the Warsaw limestone where the Spergen limestone has been eroded. Spongy masses of siliceous spicules are common in the insoluble residues.

The foraminifer Endothyra baileyi is an important fossil in the outcrops of the Spergen. This fossil has been noted not only in the limestone phase of the Spergen in the Kasper No. 1 James well but also in limestone interstratified with silty dolomite in the McCain No. 1 Doane well, in sec. 34, T. 12 S., R. 22 E., and in many of the wells in the McLouth field.

The thickness of the Spergen limestone in northeastern Kansas is fairly constant at about 35 feet although variations in thickness occur. It is 50 feet thick in the Kasper No. 1 James well and 47 feet thick in the Plymouth and Barnholdt No. 1 Elliot well, in sec. 31, T. 3 S., R. 20 E. In the McLaughlin No. 1 Frazier well, in sec. 22, T. 9 S., R. 18 E., where the Spergen is overlain by the St. Louis limestone, it is less than 15 feet thick. It is only 29 feet thick in the Cohen No. 1 Decker well, in sec. 12, T. 8 S., R. 20 E.

The Spergen limestone overlies the Warsaw limestone conformably so far as known. It may be unconformable below the St. Louis, but the variations in thickness encountered in certain areas may be the expression of transitional deposition from the Spergen to the St. Louis.

St. Louis limestone

The type locality of the St. Louis limestone is within the city of St. Louis. The outcrops nearest to northeastern Kansas are in east-central Iowa and in southwestern Missouri. In southwestern Missouri the St. Louis limestone has been preserved in a faulted block in Dade county (Clark, 1937, p. 8), where lithographic limestone is interstratified with fossiliferous limestone. Some beds are oolitic. The St. Louis limestone in Ste. Genevieve County in southeastern Missouri is a compact dense limestone of lithographic to sublithographic texture. Locally some beds are dolomitic.

The St. Louis limestone underlies a relatively small area of northeastern Kansas in counties near or bordering the Missouri river. It consists of lithographic to sublithographic limestone interstratified with semigranular limestone. In some localities it includes beds of oolitic limestone.

Pre-Pennsylvanian erosion removed the top of the St. Louis and the overlying Mississippian rocks in most areas of northeastern Kansas. The St. Louis limestone where it is overlain by the Ste. Genevieve limestone has been drilled in only two wells in northeastern Kansas. In one of these, the Plymouth and Barnholdt No. 1 Elliot well, in sec. 31, T. 3 S., R. 20 E., its thickness is 44 feet. In the other, the Wakefield et al. No. 1 National Life, in sec. 17, T. 6 S., R. 20 E., the thickness is 33 feet. Greater thicknesses of the St. Louis, however, have been noted in several wells in which the Ste. Genevieve has not been recognized. In the Kasper No. 1 James well, the St. Louis is 50 feet thick and in the Cohen No. 1 Decker well, in sec. 12, T. 8 S., R. 20 E., it is 49 feet thick. In both these wells the St. Louis is overlain unconformably by Pennsylvanian rocks.

The St. Louis limestone was once more widely distributed than at present. In northeastern Kansas it survived post-Mississippian erosion only in the post-Mississippian syncline along Missouri river northwest of Kansas City, as revealed by the Mississippian thickness map (fig. 16). In this area it is generally unconformable below Pennsylvanian rocks, but in the deepest part of the syncline the St. Louis is overlain by Ste. Genevieve limestone. Weller and St. Clair (1928, p. 214) observed slight unconformity in southeastern Missouri between the Ste. Genevieve and the St. Louis, but it is impossible to confirm this relation in the subsurface in Kansas. The presence of traces of sand in the overlying Ste, Genevieve limestone and the variable thickness of the St. Louis limestone suggest that the formations are unconformable. The St. Louis is probably conformable upon the underlying Spergen.

Ste. Genevieve limestone

Outcrops of the Ste. Genevieve limestone are found in eastern Missouri, southern Illinois, and in parts of adjoining states. This formation was named for Ste. Genevieve, Missouri, where outcrops of the formation were first described. It is reported in the subsurface from such widely separated areas as Scott County in western Kansas and central Illinois. Post-Mississippian erosion removed the Ste. Genevieve from much of the intervening region. It has survived locally only where synclinal structural conditions have favored its preservation. In Ste. Genevieve County, Missouri (Weller and St. Clair, 1928, p. 217), the Ste. Genevieve limestone exhibits considerable diversity. The lower 60 feet of the formation consists mainly of cross-bedded oolitic and sandy limestones locally interbedded with crystalline limestone. The oolite and the sand grains, however, are thinly disseminated in most of the beds. The lower part is free of chert but the upper parts of the formation, which consist of evenly bedded limestones, have a a great deal of chert in discontinuous lentils.

The Ste. Genevieve limestone has been identified in only two wells in northeastern Kansas--the Plymouth and Barnholdt No. 1 Elliot and the Wakefield et al. No. 1 National Life, whose locations have already been given. The limestone is gray to white in color and finely granular to mealy in texture. It includes 10 to 20 percent of fine grains of broken angular quartz sand but no chert. In the Plymouth and Barnholdt well a thin band of slightly oolitic sandy limestone occurs at the base. In the Wakefield et al. No. 1 National Life well, the limestone is overlain by about 20 feet of coarse subangular calcareous sand that is probably of Ste. Genevieve age but may be of Chesterian or basal Cherokee age. Only a part of the Ste. Genevieve is preserved in northeastern Kansas. In the Elliot well the Ste. Genevieve is 30 feet thick; in the National Life well it is 28 feet thick.

Inasmuch as unconformable relations have been observed between the Ste. Genevieve limestone and the St. Louis limestone at the outcrops in Ste. Genevieve County, Missouri, similar relations probably exist in the subsurface in northeastern Kansas, although the evidence is inconclusive. Great areas of the Ste. Genevieve were undoubtedly stripped from the surface during post-Mississippian erosion and the small surviving remnants were covered by Cherokee shale, the oldest of the Pennsylvanian rocks of Kansas.

Post-Mississippian Cave Deposits

Deposits closely associated with the younger Mississippian rocks but almost certainly of post-Mississippian age have been encountered in a number of wells in and near the McLouth gas field. These deposits consist chiefly of slightly silty and micaceous black shale. Some samples are distinctly silty and include a few grains of sand. Some samples enclose fine particles of green shale thinly disseminated in the black shale. Some of the black shale displays rough irregularly shaped microscopic white siliceous aggregates cementing the constituents of the shale. Much of the shale is well laminated but clastic deposits with grains of older shale are common and silt, sand, and even particles of weathered chert occur locally.

These deposits were noted in the Jackson et al. No. 1 Fevurly well, in sec. 1, T. 9 S., R. 20 E., where they were at first attributed to cavings from lower Cherokee shale which they closely resemble. It was later learned that casing had been set in the Mississippian limestone and that neither the shale with green grains nor that with the siliceous cement could be found in cuttings from above the Mississippian. Similar cuttings were later found in samples from several other wells in the McLouth field and in parts of Leavenworth and Johnson counties. In some cases where the Cherokee had not been cased off these deposits were accompanied by unquestionable lower Pennsylvanian cavings.

The shales under consideration have been found in all parts of the Mississippian down to the top of the lower member of the undifferentiated Burlington and Keokuk limestones but not below. They do not have a sedimentary relation to the Mississippian limestone nor to each other, and in nearby wells they may not occur in the same part of the Mississippian. They cannot, therefore, be considered as deposits interstratified with the Mississippian limestones. The theory that they might represent gouge in shattered zones connected with faulting was considered. These shales are especially abundant in the Young No. 1 McLeod well, in the NW SW sec. 4, T. 10 S., R. 20 E., which did in fact drill through a fault with a shortening of the Mississippian section by about 60 feet. No shortening or lengthening of stratigraphic intervals, however, has been shown in any of the other wells in which these shales have been recovered. The shales have been found not only at various stratigraphic depths, but in some wells the deposit is repeated at greater depths. Many wells, however, find the shale at the porous dolomite zone in the middle of the Burlington-Keokuk sequence. The writer believes that these erratically occurring black shales were deposited in solution openings and caves in the upper part of the Mississippian and that they occur in the faulted well because the faulting favored the formation of solution openings and caverns. The source of the clastic materials is uncertain, but some of the constituents have not been found in the Pennsylvanian. Porous zones more than 100 feet below the top of the Mississippian limestone have been noted in Reno and McPherson counties (Lee, 1939, p. 40). In Jefferson county the average depth of the lowest black shale body is about 160 feet below the top of the Mississippian.

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Kansas Geological Survey, Forest City Basin
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Web version July 2005. Original publication date Dec. 1943.