The Stratigraphy and Structural Development of the Forest City Basin in Kansas
Authors: Lee Wallace 
Published: January 2025

This report embodies the results of a study of the stratigraphy and structural history of that part of the Forest City basin lying in northeastern Kansas. It is based on the microscopic examination of samples from wells and the correlation and interpretation of the lithologic units recognized in the area. Well samples of rocks ranging in age from Precambrian to Permian have been examined and identified. They show many similarities to the nearest outcrops of rocks of the same age, but certain differences occur.
The Precambrian rocks penetrated in drilling in northeastern Kansas consist of red granite, but red quartzite is reported in adjoining areas. The overlying rocks of Upper Cambrian age in Kansas consist of the Lamotte sandstone, Bonneterre dolomite, and Eminence dolomite. The Lamotte sandstone is present in most places in the subsurface of northeastern Kansas where it has a thickness of 5 to 30 feet. The Bonneterre dolomite which conformably overlies the Lamotte is a non-cherty argillaceous dolomite. It has a maximum known thickness of 91 feet in Douglas County and thins toward the northwest. The Eminence dolomite is unconformable above the Bonneterre. It is a light-colored crystalline dolomite with considerable gray and blue vitreous chert characterized by insoluble residues of dolocastic chert. It is 175 feet thick in Douglas County but wedges out toward the northwest.
Arbuckle rocks of Ordovician age in northeastern Kansas are represented, in ascending sequence, by the undifferentiated Van Buren formation and Gasconade dolomite, the Roubidoux formation, and the undifferentiated Jefferson City and Cotter dolomites. These formations are separated from each other by minor unconformities.
The undifferentiated Gasconade dolomite and Van Buren formations unconformably overlie the Eminence dolomite. The Gasconade and the Van Buren have a thickness of 206 feet in Douglas County, but thin rapidly toward the northwest. The Roubidoux formation consists of alternating sandstone and dolomite. It has a maximum thickness of 175 feet. It is beveled by the St. Peter sandstone in Shawnee County. The undifferentiated Jefferson City and Cotter dolomites consist of cherty finely crystalline dolomite, in part argillaceous, which is 106 feet thick in Douglas County. They are beveled by the St. Peter sandstone and do not extend much farther toward the northwest in Kansas.
The St. Peter sandstone is unconformable above the undifferentiated Jefferson City and Cotter and bevels all the older Ordovician and Upper Cambrian formations. In Washington and Marshall counties it overlies granite. The St. Peter has an average thickness of about 55 feet but greater thicknesses are known from some wells,
Ordovician rocks younger than the St. Peter in northeastern Kansas consist, in ascending sequence, of a possible correlative of the Plattin limestone, the Decorah shale, the Kimmswick limestone, and the Maquoketa shale. The stratigraphic relations of these formations to each other are obscure in the subsurface. The supposed Plattin limestone, which is unconformable on the St. Peter, occurs only in the deeper part of the North Kansas basin. It consists of non-cherty lithographic limestone and sucrose dolomite only 15 to 20 feet thick. The Decorah shale immediately overlies the Plattin where this formation is present. In most places it is unconformable on the St. Peter. It consists of sandy dolomite and sandy clay 10 to 50 feet or more thick. The Kimmswick limestone in most places consists of very cherty dolomite but is composed of limestone in the area nearest the Ozark region. Its thickness increases toward the north from 95 to 235 feet. Thin deposits of noncherty limestone and dolomite overlying very cherty phases of the Kimmswick may be correlatives of the Fernvale limestone. The Maquoketa shale in northeastern Kansas is in part dolomitic and cherty. It has an average thickness of 75 feet where not reduced in thickness or entirely removed by pre-Devonian erosion.
Silurian rocks crop out in an area northwest of Jefferson County. They consist almost entirely of coarsely crystalline dolomite with only small amounts of insoluble residues. The following zones, in ascending order, are recognized in the Silurian; a zone with dolomitic oolites, a zone which yields residues of opaque white chert, a zone characterized by the sparse occurrence of forminifera in the small silty insoluble residues, and a zone which yields residues of drusy quartz. One or more of these zones may be absent. The Silurian rocks were beveled by pre-Devonian erosion and are absent in most of Jefferson County and to the southeast. Their thickness increases to at least 150 feet toward the northwest.
Devonian rocks are represented by lithographic limestone and coarsely sucrose dolomite. The lithographic limestone is dominant in the area bordering the Ozark uplift but interfingers with and grades laterally into dolomite toward the northwest. A sandy limestone marks the base of the Devonian in most wells, but toward the north and west the absence of sand makes the contact with the Silurian difficult to determine. The Devonian was deposited upon an eroded and beveled surface and overlaps all the older rocks from Silurian to upper Arbuckle. Devonian rocks also were eroded and beveled prior to the deposition of the Chattanooga shale and they thus increase in thickness from a few feet in Miami County to at least 250 feet to the north and west.
The Chattanooga shale overlies the post-Devonian surface of erosion and is in contact with all the rocks exposed on this surface from Devonian to the upper part of the Arbuckle. The Chattanooga shale consists of gray and greenish-gray silty and micaceous shale, parts of which are dolomitic. The basal beds are generally darker and contain abundant spores. The thickness increases toward the northwest from 50 feet to more than 250 feet.
All the limestone formations of the Mississippian in Kansas from the Chouteau limestone to the Ste. Genevieve limestone except the correlatives of the Fern Glen are represented in northeastern Kansas. The development of the Nemaha anticline and subsequent erosion removed the Mississippian from the crest of this fold. As a result, the Mississippian is thin on the eastern flank of the fold, and toward the east progressively younger formations underlie the Pennsylvanian. Along Missouri River between Kansas and Missouri, the Mississippian limestones, including the Ste. Genevieve at the top, have a thickness of more than 370 feet.
A normal section of Pennsylvanian and Permian rocks overlies the Mississippian east of the Nemaha anticline. The subsidence of the surface east of this structure and the deformation of the post-Mississippian erosion surface formed the Forest City basin. The Cherokee shale, the lowest division of the Pennsylvanian in Kansas, is exceptionally thick in this basin. However, the Cherokee shale was not deposited upon the elevated crest of the Nemaha fold, which was not completely submerged until Bronson time when the thick early Pennsylvanian deposits finally overlapped upon and buried the escarpment.
Of the many unconformities recognized in the Paleozoic rocks, the most important, from a structural point of view, are at the base of the St. Peter sandstone, the base of the Devonian, the base of the Chattanooga, and the top of the Mississippian. The erosion that is represented by each of these unconformities was preceded by important structural movements, and each epoch of erosion resulted in almost complete base-leveling.
Three periods of structural deformation have been recognized. Data secured from the deep wells of the area reveal that during Upper Cambrian and in pre-St. Peter time (Ordovician) the central Ozark area was sinking slowly and that the surface in southeastern Nebraska and northeastern Kansas was rising slowly. These movements were attended by numerous advances and retreats of the sea. Several hundred feet of limestone was eroded during some of the emergent phases. The deposition of the St. Peter sandstone followed one of the emergent phases in which all the earlier rocks that had survived were again warped, beveled, and peneplaned. After the deposition of the St. Peter, the pattern of structural deformation changed markedly, and before the end of Maquoketa time the Ozark area, which had been subsiding, began to rise, and southeastern Nebraska, which had been rising, began to subside. The initial movements of the Ozark uplift, the Chautauqua arch, and the North Kansas basin thus began in the late Ordovician before the end of Richmond time.
The North Kansas basin continued to be the dominant structural feature of northeastern Kansas from the end of Maquoketa time to the end of Chattanooga time. Local deformation continued in the same pattern, even through the Kinderhookian epoch, but by that time the first movements along the line of the Nemaha anticline were also being felt.
Development of the Nemaha anticline introduced a new and revolutionary structural trend. Structural movements that produced this anticline and other parallel strongly directional folds trending northeast culminated after Mississippian deposition. The Nemaha anticline bisected the North Kansas basin and formed the Salina basin on the west and an unnamed structural basin along Missouri river on the east in which Ste. Genevieve limestone is preserved, but this movement did not immediately form the Forest City basin. These folded rocks were peneplaned. Renewed movement along the Nemaha anticline resulted in the re-elevation of the region west of the anticline. The line of re-elevation was sharply marked and was probably accompanied in most places by faulting that led to the formation of an eastward-facing escarpment. The erosion surface east of the escarpment was flexed downward forming the Forest City basin. The Forest City basin was separated from an arm of the Cherokee basin of Oklahoma, which was formed at the same time, by a low, broad divide (the Bourbon arch) trending northwest from Bourbon county.
In middle Cherokee time these two basins were joined by the filling of the Forest City basin and by the flooding of the divide. Thenceforth, the Forest City basin was merely a northern extension of the Cherokee basin. Until lower Permian time and possibly even later, the Forest City basin region continued to be dominated structurally by subsiding movements east of the Nemaha anticline and by mild flexing in the Forest City-Cherokee basin syncline. Erosion has destroyed the stratigraphic record of Permian deformation in most of the basin areas.
The western dip given to the surface rocks of Kansas and adjoining states occurred mainly prior to the deposition of the Cretaceous rocks of western Kansas, although there has been some deformation and great elevation of the whole region above sea level since that time.
The widely different changes in regional dip in the structural history of the region have resulted in a lack of parallelism of the strata in different parts of the stratigraphic column. The effect of surface anticlines of low structural relief on tilted datum beds in different parts of the section is illustrated by cross-sectional diagrams.
Most of the anticlines that have been mapped in the surface rocks of northeastern Kansas are of low structural relief. In some places the regional dip has reduced an original low anticline to a structural nose in the surface rocks. Where it has been possible to study the structure in oil and gas fields, the structural relief has been found to increase in depth at a rate more than adequate to compensate for the regional dip.
Local structural features trending northeast or located on axes of folding trending in that direction were initiated after the deposition of the Mississippian rocks and continued active during Pennsylvanian time. In consequence, such anticlines become steeper in depth down to the Mississippian rocks. Structural features initiated prior to the Mississippian and presumably trending toward the northwest may be expected to increase in relief down to the St. Peter sandstone.
Many unconformities occur in the columnar section, but where limestone was deposited upon a porous weathered zone of exposed limestone the porosity seems to have been destroyed by cementation during the deposition of the overlying calcareous sediments. Such porous zones where they were covered by noncalcareous deposits have remained open and constitute most of the limestone reservoirs for oil and gas in eastern Kansas
In the pre-Pennsylvanian rocks, the wedging out of sandstone beds caused by erosional beveling seems not to form favorable stratigraphic traps for oil and gas accumulation unless some structural deformation is present that will prevent the escape of oil up dip through the weathered zone of adjacent exposed limestone beds. Many productive stratigraphic traps, however, occur in the lower Pennsylvanian shales in which there are porous bodies of lenticular sandstone, shoestring sands, and sand sealed up dip by variations in porosity.