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Forest City Basin

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Stratigraphy, continued

Rocks of Pennsylvanian Age

The Pennsylvanian rocks of northeastern Kansas and northwestern Missouri are well known, and the Missouri and Kansas Geological Surveys are in essential agreements as to the sequence and correlation of almost all of the minor units. Moore (1936) reexamined the Pennsylvanian rocks as a whole from the standpoint of cyclical deposition and paleontologic development, and as a result introduced many new formational names and redefined the limits of many of the older terms. This new classification was adopted for the State Geological Survey of Kansas.

The principal changes of the group and series units as adopted by Moore consist of the separation of the upper part of the earlier Missouri series as the Virgilian series, the redefinition of the Wabaunsee, Shawnee, and Douglas groups, the redefinition and restriction of the Lansing and Kansas City groups, and the segregation of the lower part of the original Kansas City group as the Bronson group. The differences in classification by the Missouri and Kansas Geological Surveys of the Pennsylvanian beds into series, groups, and formations have been shown by McQueen and Greene (1938, pl. 5). The series and groups as now used by the Geological Survey of Kansas for the Pennsylvanian rocks are listed below, in descending order.

Virgilian series
Wabaunsee group
Shawnee group
Douglas group
Missourian series
Pedee group
Lansing group
Kansas City group
Bronson group
Bourbon group
Desmoinesian series
Marmaton group
Cherokee shale

Because Moore ( 1936) has discussed in detail the classification, thickness, lithology, and stratigraphic relations of the Pennsylvanian rocks in northeastern Kansas, an extended discussion of these rocks in their surface manifestations is unnecessary in this report. Only a brief resume of the essential lithologic features of the Pennsylvanian rocks in outcrops of northeastern Kansas will be presented. Additional data bearing on the structure and stratigraphy derived from a study of these rocks in the subsurface are added, with special emphasis on features that may have a bearing on the search for oil and gas.

The distribution of the Pennsylvanian and Permian rocks in the area covered by this report, as well as in other parts of Kansas, is shown on the geologic map of Kansas published by the State Geological Survey in 1937. This map not only shows the outcrops of the groups described herein, but also the outcrops of the most prominent limestones. It is particularly useful in indicating to operators which beds occur at the surface in a given locality and hence the approximate depth necessary to drill to reach a chosen oil sand.

Desmoinesian Series

Cherokee shale

Outcrops of Cherokee shale extend in a broad belt from eastern Oklahoma through Cherokee county, Kansas, around the flanks of the Ozark dome into parts of northeastern Missouri. The deposits consist of several hundred feet of various kinds of shale, beds of sandstone, and infrequent thin beds of limestones and coal. The lower limit of the Cherokee shale in Kansas is the eroded surface of the Mississippian limestone. Its upper limit has been set at the base of the Fort Scott limestone, a conspicuous and persistent limestone at the bottom of the Marmaton group.

The Cherokee shale is argillaceous, micaceous, silty, and sandy with gradations of all types. The sandstones are mainly micaceous and not very porous; some of them, however, are relatively pure in places and have good but unpredictable porosity. The lack of porosity is due to several causes--among others, the secondary cementation of the grains and the original inclusion of clay. However, some of the sandstones, such as the Bartlesville, are elongate lenticular bodies of clean sand known as shoestring sands. The limestones are thin and inconspicuous. The characteristics of the Cherokee are its predominant shaly deposits, its impure sandstones, and the subordinate amounts of limestone and coal.

The Cherokee shale in the Forest City basin in northeastern Kansas differs little in general character from that in its outcrops in the Cherokee basin farther south. The re-elevation of the Nemaha anticline at the time of the formation of the Forest City basin produced an eastward-facing escarpment bounding the basin on the west. North of Geary County, the crest of this escarpment consisted mainly of granite and probably other Precambrian rocks. To the south, the crest of the escarpment was underlain mainly by rocks of Arbuckle age.

It might be supposed that the weathering of this granite ridge would yield a large amount of coarse-grained arkose, at least on the west side of the Forest City basin. The information from the few wells available indicates that only a relatively small quantity of coarse-grained arkosic sediments was deposited at the base of the Cherokee in the Forest City basin. In the Ladd et al. No. 1 Achten well, in sec. 12, T. 4 S., R. 14 E., in the deepest part of the basin only 12 to 15 miles from the escarpment, the lower 500 feet of the Cherokee consists almost exclusively of black and dark micaceous shale with only small amounts of fine sand and no arkose. The next overlying 150 feet, however, includes several beds of coarse-grained arkosic material and some gray chert probably derived from the ridge to the west. Even in the Arab No. I Ogle well, sec. 9, T. 1 N., R. 14 E., in Nebraska, less than 6 miles east of the buried escarpment, the lower 300 feet of the Cherokee consists almost exclusively of black shale, although the remainder of the section through the Marmaton and up to the base of the Hertha limestone consists mainly of akrose and coarse, poorly sorted red sand.

Farther out in the basin the lower part of the Cherokee is also dominantly shale, much of which is black and micaceous. The shale beds higher in the section are interstratified with micaceous impure sand, generally not very porous. The sandstones seem to be in part lenticular and in part to grade locally into bodies of pervious sandstone, ill-sorted nonmarine sandy shale, or shaly sand.

The Burgess sand, one of the well-known oil and gas sands in the Cherokee shale, is primarily a basal conglomerate. It is a sandstone consisting of coarse quartz sand and in some places includes chert fragments from the Mississippian limestones. It is composed of whatever detrital materials were available to the advancing Cherokee sea and accumulated in large measure in local depressions on the pre-Cherokee land surface. The Burgess sand has been observed in wells in Franklin County, but it is not common in the Forest City basin where the basal Cherokee beds are generally black shale. The Burgess sand should not be confused with the McLouth sand, to be described later, which in most wells is separated from the Mississippian by black shale.

Many unnamed sand bodies of only local distribution occur in the Cherokee shale. Some of these are sufficiently porous to contain water. Under favorable structural conditions many of them might serve as reservoirs for gas and oil.

The McLouth gas sand immediately overlies the Mississippian limestone in some wells, but in most wells it is separated from the Mississippian by black micaceous shale. In the McLouth field the top of the sand is about 450 feet below the Fort Scott limestone, and about 350 feet above the deepest part of the Forest City basin. It appears to have been deposited at or about the time that the Forest City and Cherokee basins were joined. This sand, which has been carefully studied during the development of the McLouth field, is really a sandy, apparently nonmarine zone, about 40 feet thick. The McLouth sand includes several beds of impure and poorly sorted sandstone of irregular thickness, some parts of which are porous. The varying porosity results in varying gas yields.

The Bartlesville and associated sands are lenticular shoestring sands that, as Bass (1936) has shown, are buried offshore sand bars. The Bartlesville shoestring sands have not been identified in the Forest City basin, and probably were not deposited there. Their base is 250 to 300 feet below the Fort Scott limestone, and they are, therefore, younger than the McLouth sand and were probably deposited after the union of the Forest City and Cherokee basins. The Bartlesville was deposited on the shores of a shallow embayment caused mainly by the gentle flexing of the Cherokee basin.

The Squirrel sand is a sandy zone in the upper part of the Cherokee shale. It is a well-known and widely distributed gas sand, but its thickness and yield are subject to abrupt variations. It is reported to lie 25 to 102 feet below the top of the Fort Scott limestone and to have a thickness of 10 to 80 feet. In strata whose deposition was locally so regular as the lower Pennsylvanian rocks of northeastern Kansas, such reported fluctuations in position are interpreted as indicating a sandy zone whose variations in porosity are due to the amount of clay and other impervious materials that were deposited with the sand. The Squirrel sand has not been studied as carefully as the McLouth sand. It is probably not a homogeneous entity like the Bartlesville shoestring sands but a sandy zone like the McLouth sand, which is dependent for production upon the development of porosity in one or several of its impure sandstone beds in anticlinal areas. The pay sand may lie at different depths in the sandy zone under the same anticline. The Squirrel sand is a valuable gas producer in Johnson, Leavenworth, Wyandotte, and other counties in eastern Kansas and northwestern Missouri. It received its name from drillers who early observed that it "hopped around" in relation to recognized datum beds like the Fort Scott limestone (Greene, 1933, p. 16).

The only named limestone in the Cherokee shale is the Ardmore limestone member, 90 to 110 feet below the top of the formation. The Ardmore member is commonly a thin bed 1 to 3 feet thick and is thus frequently not recorded in well logs. Some other locally developed limestone lenses occur in various parts of the Cherokee shale but they have not been named and in general they represent calcareous sandstones or shales rather than true limestone beds.

A limestone bed which occurs 20 feet above the McLouth sand in the McLouth field appears to have a wide distribution. Although highly argillaceous, it is crinoidal and probably includes other marine fossils. Because it is thin and black to chocolate-colored and resembles the adjacent shales, it is easily overlooked in the examination of samples. This limestone is a good datum bed, however, and in the McLouth field is underlain by a thin bed of white siltstone that is more easily seen than the limestone. Traces of coal, apparently from between these datum beds, have been found in the cuttings of some wells.

On the structurally high areas of the McLouth field, which lies on a structural arch, this limestone is 620 feet below the base of the Hertha limestone. Near the deepest part of the basin, in the Ladd et al. No. 1 Achten well, in sec. 12, T. 4 S., R. 14 E., a crinoidal dark calcareous shale, believed to represent the same limestone, occurs 735 feet below the Hertha limestone. An impure dark fossiliferous limestone that seems to be the same bed is reported by McQueen and Greene (1938, p. 174) in the Forest City well in Holt County, Missouri. This marine argillaceous limestone has been noted in many other wells. It is believed to indicate a brief invasion of marine waters into the Forest City basin at about the time the Bourbon arch, which separated the Forest City and Cherokee basins, was submerged. On the assumption that the thin limestone was deposited upon an essentially flat surface, it is presumed that the variations in the interval from this bed to the base of the Hertha limestone represent structural warping of the bed during the corresponding time interval and that the depth of the Forest City basin was gradually increased 50 to 75 feet by warping during the interval.

The thickness of the Cherokee shale is variable. Its exact thickness is difficult to determine because in many places in the Forest City basin the Fort Scott limestone, which immediately overlies the Cherokee, is not well developed and in some wells is neither logged nor sampled. The Cherokee shale is thickest in the Ladd et al. No. 1 Achten well, in sec. 12, T. 4 S., R. 14 E., where it is 805 feet thick. It is thinnest on the divide between the Forest City and Cherokee basins, where Bass (1936, pl. 1) reports only 350 to 400 feet of Cherokee shale. On the northerly trending arch upon which the McLouth field is located the Cherokee has a minimum thickness of 467 feet. The thickness of the Cherokee shale is dependent upon three factors: the original structural configuration of the Forest City basin, the topographic relief of the surface upon which it was deposited, and the deformation of the basin that took place during its deposition.

The Cherokee shale is thicker in southeastern Kansas and in northeastern Kansas than elsewhere in the state. It is by means of this thickening of the Cherokee rocks in their respective areas that the Cherokee basin of southeastern Kansas and eastern Oklahoma and the Forest City basin of northeastern Kansas and parts of adjoining states are known. The Cherokee basin was the arm of a deep structural basin on the north flank of the Ouachita mountains of Oklahoma and Arkansas and from it extended northward on the west side of the Ozark dome into southeastern Kansas. A basin in Illinois, known as the Illinois basin, was somewhat similar to the Cherokee basin and flanked the Ozark region on the east.

The broad low structural divide (the Bourbon arch) of Mississippian rocks that separated the Forest City and Cherokee basins trends vaguely toward the northwest across Bourbon, Alien, and Coffey counties, Kansas (Lee, 1939, pl. 2). The Forest City basin was separated from the Illinois basin in northeastern Missouri by a similar arch of Mississippian rocks, the Northeast Missouri arch, trending northward from the Ozark dome. These two divides were barriers confining the Forest City basin, and each is now overlain by Cherokee shale of about the same thickness. The Cherokee shale in the center of the basin is about 400 feet thicker than on the divides. This thickness represents the approximate structural relief of the basin at the time of deposition of the Fort Scott limestone. After the flooding of the divides, the Forest City basin ended its brief existence as a separate unit and became merely an extension of the Cherokee basin.

Rocks of Morrowan age were deposited in parts of Arkansas and Oklahoma but are not known to have reached Kansas. They are, however, separated from correlatives of the Cherokee rocks by an unconformity.

The Cherokee sea advanced from Oklahoma into Kansas, but the deposits did not entirely bury the re-elevated Nemaha anticline except in southern Kansas. Cherokee deposits beginning with a basal conglomerate encroached on the anticline from the east, but most of the re-elevated crest remained an elongate island or peninsula until Marmaton time and some places on its northern crest were not covered until Bronson time. No marked unconformity separates the Cherokee shale from the Marmaton rocks.

Marmaton group

The Marmaton group includes the rocks from the top of the Cherokee shale to the unconformity at the base of the Missourian series. These rocks consist of thick and thin limestone beds alternating with shale, and include some thin coal beds and thick channel sands. The details of Marmaton stratigraphy in Kansas have been discussed at length by Jewett (1941).

The Fort Scott limestone is important as the lowest formation of the Marmaton group. Its base is used as a datum horizon in determining the thickness of the Cherokee shale. In the outcrops it is made up of three members: a limestone bed, 5 to 16 feet thick, at the base broken locally by limy shale; a middle member, 7 to 12 feet thick, of dark shale and black fissile shale including a thin lenticular limestone and a local bed of coal; and an upper bed of limestone, averaging about 16 feet in thickness (Jewett, 1941, pp. 302-309). The total thickness averages about 30 feet in the outcrops of southeastern Kansas where limestone constitutes about two-thirds of the formation.

The Fort Scott limestone is overlain by the Labette shale. This formation is of importance because locally it includes thick lenticular bodies of sandstone, known as the Peru sand, which have yielded gas in several counties in eastern Kansas and northwestern Missouri. Some of these sandstones seen in outcrops are channel deposits and locally replace eroded parts of the Fort Scott limestone. The Labette shale in the Kansas outcrops is 40 to 80 feet thick (Jewett, 1941, p. 309). The Pawnee limestone, as described by Jewett, is a formation about 30 feet thick which consists of two limestone members and two shale members. The lower shale member includes a thin sheet of limestone which thickens into northeastern Oklahoma.

The upper formations of the Marmaton group include the Bandera shale and overlying limestones alternating with shale. The Bandera shale, like the Labette shale, includes deposits of channel sandstone in some places. These sands yield oil and gas in Linn and neighboring counties. It is possible that some of the gas sands of eastern Kansas which have been called Peru sand are in the Bandera shale instead of the Labette. Several of the limestone beds in the upper part of the Marmaton group are thin. All of the Marmaton formations and most of the members have been traced great distances along the outcrops.

The limestone units of the Marmaton group are well developed in outcrops in southeastern Kansas, where they constitute about 20 percent of the group. In the subsurface of northeastern Kansas, particularly north of Kansas River, these limestones become thinner and correlation with the limestones in the outcrops becomes increasingly difficult. In areas where the basal limestone (the Fort Scott limestone) is thin, it is not recorded in some logs. In some wells it does not even appear in the samples; hence it may be absent in considerable areas. The same difficulty exists in regard to the higher limestones of the group. The Marmaton is represented in nearly all logs by a group of thin limestones, but there is doubt as to their specific identification.

Limestones of Marmaton age are not well developed in the Arab No. 1 Ogle well, in sec. 9, T. 1 N., R. 14 E., Nebraska. Because this well is only 5 to 6 miles east of the escarpment bounding the basin on the west, it is presumed that the accumulation of clastic materials from the bordering land area prevented the normal development of limestones during Marmaton time.

The Peru sand, which occurs in the upper part of the Labette shale between the Fort Scott and Pawnee limestones, has yielded gas in counties from Wyandotte to Linn and in neighboring counties both in Kansas and Missouri, Some gas and oil have come from a sand in the Bandera shale above the Pawnee limestone. This sand is known as the Wayside sand.

From the available information it is difficult or impossible to give a definite thickness for the Marmaton in the subsurface of northeastern Kansas. Its top is an eroded surface and its base is obscure in places. Its thickness probably does not exceed 150 feet and is less in some wells. Jewett reports the thickness of the Marmaton as about 250 feet in Kansas outcrops. The Marmaton is probably represented partly by arkose in some places near the Nemaha anticline, as in the Arab No. 1 Ogle well. On this anticline in Riley and Pottawatomie counties the Marmaton group is less than 100 feet thick and its basal beds are probably not represented.

The Marmaton group overlies the Cherokee shale without notable disconformity. Where the Cherokee failed to cover the escarpment formed by the re-elevation of the Nemaha anticline, the Marmaton overlapped the Cherokee shale and covered the crest on which it unconformably overlies Mississippian rocks toward the south end of the Nemaha escarpment and Devonian and even older rocks toward the north. An important unconformity marks the top of the Marmaton group. The period of exposure and erosion was long enough to effect an important faunal break before the deposition of the overlying Missourian series, but the Lenapah limestone near the top of the Marmaton in Kansas was only locally eroded. The Warrensburg channel deposits of Missouri are reported by Jewett (1941, p. 294) to be of Labette age. This means that an unconformity within the Marmaton is locally more conspicuous though less important than that at its top. The local relief at the top of the Marmaton, as reported by Jewett, is small and regionally is of the order of 50 feet.

Missourian Series

The Missourian series, as redefined by Moore (1936), consists of a basal group of sandy shale (the Bourbon group), three groups of rocks that are composed principally of limestone (Bronson, Kansas City, and Lansing groups), and a shale group (the Pedee group) at the top.

Bourbon group

The Bourbon rocks include sandstone, sandy shale, and dark-blue and dark shale, and in addition some dark-blue flaggy limestones in the upper part (Jewett, 1940) in outcrops in Linn and Bourbon counties. The basal formation of the Bourbon group is the Hepler sandstone, 3 to 20 feet thick, which, according to Jewett, is not to be correlated with the Warrensburg sandstone. Other sandstones of local development occur in the middle and upper parts of the formation.

The Bourbon rocks in the subsurface do not differ much from the outcrops. A sharp boundary between the Marmaton shale and the Bourbon shale is not recognizable in well cuttings except where the Hepler sandstone is well enough developed to be identified. This sandstone, which is 10 to 20 feet thick near Lawrence, has been noted also in many wells in the McLouth field and as far north as the Lebsack and Wamoff No. 1 Maduska well in sec. 36, T. 8 S., R. 20 E.

The unconformable relations at the base of the Bourbon group give it an irregular thickness. Jewett reports a thickness of 150 to 190 feet in outcrops in Linn County. The fact that the uppermost limestone members of the Marmaton are identified with some misgivings in well and sample logs adds to the difficulty of determining the exact thickness of the Bourbon. The interval from the top of the Bourbon shale to the uppermost recognized limestone of the Marmaton, although not the true thickness of the group in all wells, gives a rough measure of its thickness. This interval is fairly regular and as far north as the McLouth pool is 100 to 150 feet. It is thinner farther north. On the Nemaha ridge in Pottawatomie and Riley counties, where its overlies the Marmaton, the Bourbon is only 15 to 20 feet thick. This suggests continued or renewed activity of the Nemaha anticline during Bourbon times.

The relations of the top of the Bourbon to the overlying Hertha limestone (of Bronson age) are obscure in the subsurface. Although no marked unconformity occurs at this contact, it is worthy of note that a variety of different types of rocks underlies the Hertha limestone. The Hertha generally overlies shale, but in some wells the underlying rocks are sandstone, dolomite, or impure limestone. The thinning of the Bourbon interval toward the north and the variety of rocks underlying the Hertha suggests that there may have been a slight southerly tilting of the region accompanied by some erosion prior to the deposition of the Hertha. Such an hypothesis is consistent with the structural movements indicated by the thickness of the combined Bronson, Kansas City, and Lansing groups shown in figure 17B and in the thinning of the Bourbon over the Nemaha anticline.

Bronson, Kansas City, and Lansing groups

These three groups consist of alternating beds of limestone and shale, with limestone predominating. The several limestone and shale sequences present an interesting sequence of cyclical sedimentation in which certain types of sediments succeeded each other with extraordinary fidelity in response to the rhythmic advances and retreats of the sea. The Bronson group was originally included in the Kansas City group, from which it was separated in Kansas by Moore (1936, pp. 75-77). At the same time the boundary between the restricted Kansas City group and the Lansing was raised and a regrouping of some of the limestone members was made.

The lowest formation of the Bronson group is the Hertha limestone which consists of two limestone members, the Sniabar limestone above and the Critzer limestone below (Jewett, 1932, p. 99), separated by a thin shale bed. The base of the Hertha is an important datum bed because it is easily recognized in most wells.

A local impure limestone in the top of the Bourbon has been mistakenly included in the Hertha in the logs of some wells. The examination of samples has resulted in corrections of as much as 15 or 20 feet in a few logs to exclude these pre-Hertha deposits. The Hertha is overlain in ascending sequence by the thin gray Ladore shale, the Middle Creek limestone, the Hushpuckney shale, and the Bethany Falls limestone, the last three of which are members constituting the Swope limestone. The Hushpuckney shale is a black and fissile shale which is easily recognized 10 to 20 feet above the base of the Hertha limestone. There is, however, another black shale about the middle of the Bronson which is called the Stark shale. The Winterset limestone member of the Dennis limestone at the top of the Bronson group is characterized by the occurrence of small amounts of dark to black chert near its top.

The redefined Kansas City group extends from the base of the Fontana shale to the top of the Bonner Springs shale with frequent cyclical alternation of other shales and limestone beds. The Lansing group, as restricted by Moore (1936), extends from the base of the Plattsburg limestone immediately overlying the Bonner Springs shale to the top of the Stanton limestone, of which the topmost member is the Little Kaw limestone. In some outcrops the deep erosion that marks the base of the Virgilian series has cut so deeply that the entire Pedee shale group as well as the upper beds of the Stanton have been eroded.

In the subsurface the Bronson, Kansas City, and Lansing groups of the Missourian series closely resemble their features as revealed in outcrops. There is considerable diversity in the thickness of individual limestone and shale members in different parts of the area, and even in adjacent wells, because of the repeated unconformities between the cycles of deposition. In some places certain beds were reduced in thickness or entirely removed by erosion leaving a surface of low relief upon which the succeeding beds were deposited. In some instances the irregularities of the low topography were leveled off by the initial deposits of the succeeding cycle; in others, the first sediments failed to fill the inequalities of the surface left by erosion so that flatness of the sea bottom was not restored until several thin beds of shale and limestone had been deposited. In consequence, as is well shown by comparison of carefully kept logs of nearby wells, there is a lack of parallelism between individual beds. The aggregate thickness of the three groups, however, is in most places singularly uniform, although there is considerable variation in the thickness of individual formations and members.

A map showing the combined thickness of the Bronson, Kansas City, and Lansing groups is presented in figure 17B. The thickness ranges from 300 to 400 feet, being greatest toward the south. Some allowance has been made for the Little Kaw limestone where it appears to have been eroded from the top of the Stanton. Corrections of 15 to 20 feet for this reason have been made in the logs of some wells.

No very striking unconformity marks the contact of the Bronson with the Bourbon. If disconformity exists, it is probably no greater than many cyclical unconformities within the Bronson. The Lansing is essentially conformable beneath the overlying Pedee shale group, but in many places the entire Pedee group and a part of the Stanton limestone at the top of the Lansing group were eroded before the deposition of the Virgilian rocks.

Pedee group

The strata overlying the Stanton limestone up to the disconformable rocks of the Virgilian series constitute the Pedee group. These rocks consist of the Weston shale and the Iatan limestone which survives only locally near the top of the Pedee group. The Iatan crops out north of Leavenworth, but toward the south and in the Kansas River valley the Iatan limestone and part or all of the Weston shale were eroded in pre-Virgilian time.

No effort has been made to distinguish the Weston shale and the Iatan limestone in the logs from the shale and limestone of the Douglas group at the base of the Virgilian series because better datum beds occur both above the Douglas and below the Pedee. A well-defined sandstone of irregular occurrence that may represent the base of the Virgilian series is present in some wells in the shale overlying the Stanton limestone. Moore (1936, pp. 140, 141), reports a maximum thickness of 140 feet for the Weston shale and 22 feet for the Iatan in outcrops.

The Pedee group overlies the Stanton limestone without pronounced disconformity. An unconformity with pronounced relief separates it from the overlying Virgilian series. The extent of this relief, not less than 200 feet, is indicated by the erosion of the entire thickness (about 160 feet) of Pedee rocks and the upper members of the Stanton limestone which has been eroded locally to a depth of 10 feet or more.

Virgilian Series

Douglas group

This group is the lowest division of the Virgilian series (Moore, 1936, p. 145). It consists of two parts, the Stranger formation below and the Lawrence shale above. The Stranger formation, of which the Tonganoxie sandstone is the basal member, was deposited upon the dissected surface of the Missourian series. The Stranger formation grades upward without marked interruption of sedimentation into sandy shale. A thin coal (Sibley) and a thin widely distributed limestone (Westphalia) occur in the upper part of the Stranger formation. The Haskell limestone near the top of the formation is only 2 or 3 feet thick, but it was deposited over a very broad area. The Robbins shale overlies the Haskell limestone. It was locally eroded before deposition of the Lawrence shale.

The Lawrence shale was deposited after a period of exposure and erosion that left the surface of the Stranger formation deeply channeled. The Robbins shale and the Haskell and Westphalia limestones were cut out in some places. The channels were filled with the Ireland sand which grades up into the lower part of the sandy Lawrence shale. A thin limestone and several thin beds of coal are known in the upper part. The Douglas group crops out in northeastern Kansas in a belt extending southwest from Leavenworth through Lawrence.

No effort has been made to distinguish the members of this group in the subsurface. The intervals, complicated as they are by unconformities, are variable. The Haskell limestone appears to be present in most wells, but it may be confused with the Westphalia and both may be misidentified with the Iatan which, where present, occurs at about the same interval above the Stanton limestone. The Haskell limestone, although widely deposited, is not a very good datum bed because its stratigraphic intervals below the Oread and above the Lansing are not constant. It appears to have been deposited upon an imperfectly leveled surface. The sandstones of the Douglas group are particularly variable inasmuch as they were deposited upon a deeply eroded and locally channeled surface. The sandstone bodies in the Lawrence shale grade laterally into sandy shale and have no lithologic continuity.

There is no regularity in the thickness of the Douglas group. The interval from the top of the Stanton to the base of the overlying Oread limestone shown in figure 18A is a rough measure of its thickness, which ranges from less than 100 feet to more than 300 feet. The thinning is toward the north and toward the axis of the Nemaha anticline.

The Douglas group is included in the Virgilian series because of the pronounced unconformity already mentioned at the base of that group. No pronounced irregularity has been noted between the Douglas group and the overlying Oread limestone at the base of the Shawnee group. The beds are assumed to be essentially conformable, although cyclical exposure probably occurred at or near the contact.

Shawnee group

The Shawnee group consists of four prominent limestone formations (Oread limestone, Lecompton limestone, Deer Creek limestone, and Topeka limestone, named in ascending order) separated by prominent shale formations (the Kanwaka shale above the Oread, the Tecumseh shale above the Lecompton, and the Calhoun shale above the Deer Creek). This group crops out in a belt extending southwest from eastern Doniphan County through central Coffey County. The limestones form fairly well-developed eastward-facing escarpments. No great change takes place in the character of this group of beds in the subsurface except that toward the north and west the shale formations between the limestones become thinner so that there is a considerable thinning of the group as a whole.

The Shawnee group thins northward from about 350 feet in the deeper parts of the basin in Douglas County to less than 250 feet west of the Nemaha anticline, as shown by the thickness map (fig. 18B).

No pronounced unconformities occur at either the bottom or the top of the Shawnee group, although there is no reason to doubt that there were frequent periods of cyclical emergence at the beginning and at the end as well as during the deposition of the Shawnee group.

Wabaunsee group

The Wabaunsee group includes the youngest rocks of Pennsylvanian age. It consists of shales, sandy shales, and thin beds of sandstone interstratified with a number of limestone beds. Although the limestones are neither very hard nor very thick, they are more conspicuous in the outcrops than might be expected because of their relatively lesser resistance to erosion and the monotony of the associated shales. The most prominent of these limestones, named in ascending order, are Howard limestone, Burlingame limestone, Emporia limestone, Tarkio limestone, and Dover limestone.

The Wabaunsee group crops out in a narrow belt extending from eastern Brown County southwest through western Coffey County. It is exposed also on the crest of the Nemaha anticline north of Kansas river where the younger rocks, originally present above it, have been stripped away by erosion.

No great part of the area under discussion is underlain by the Wabaunsee group. It occupies a narrow belt of the Nemaha anticline that has been avoided, almost superstitiously by oil operators, because of its synclinal structure. There are in consequence few wells that penetrate the Wabaunsee rocks in this area.

Not enough wells have been drilled through the Wabaunsee to determine the variations of its thickness in this area. Inasmuch as its upper surface was deeply eroded before the deposition of the overlying rocks, it is certain that these variations will be great and unpredictable. Moore (1936, p. 201) reports its thickness to be reduced from 500 feet to a possible 375 feet where it is most deeply channeled. Like other Pennsylvanian groups in northeastern Kansas, the Wabaunsee group thins toward the north.

The Wabaunsee group is essentially conformable above the Topeka limestone at the top of the Shawnee group. Its top is the unconformable surface between the Pennsylvanian and Permian with a topographic relief of at least 80 feet and possibly 125 feet.

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