The structural development of the Salina basin has been studied by means of a series of maps that represent the thickness of individual formations and of the sequences of formations in and adjacent to the basin.
The interpretation of structural movements from thickness maps is based on the following concept: if a sequence of rocks is deposited on an originally flat surface, and if this sequence of rocks is warped and folded before the development of a younger flat horizontal surface, the variations in thickness of the rocks between the two surfaces will reveal the amount and place of the deformation.
The deformation of the first surface may precede or follow the deposition of the overlying rocks or occur during the deposition of the sequence. The presence of a marked unconformity or disconformity within the sequence will not alter the overall deformation, which is the total amount of deformation between the development of the reference planes.
The accuracy with which the thickness maps record the structural movements is dependent upon the completeness with which the surfaces above and below the sequence approach base level whether by deposition or erosion. A depositional surface generally presents a nearly perfect horizontal datum plane. Most of the eroded surfaces of the area have little topographic relief, and thus resemble peneplains. The topographic relief of most of the surfaces utilized in preparing the thickness maps is negligible in comparison with the regional variations of thickness that are due to structural movement. The topographic relief of some surfaces, however, introduces erratic configuration of the thickness lines.
Valleys on imperfectly beveled surfaces are generally recognized by the thinning of beds of one sequence accompanied by a compensating greater thickness of the overlying beds. On the other hand, hills are revealed by local thickening of the lower sequence accompanied by a compensating thinning of the overlying formation. Minor structural and topographic features are generally not revealed by 50-foot thickness lines. In oil fields where many wells have been drilled, the local structural features are commonly expressed by thickness lines drawn at small intervals, and the thickness lines combined with detailed stratigraphic studies reveal in some cases local topographic features on the eroded surfaces (Lee and Payne, 1944, p. 105 and fig. 14).
Beveling of stratified rocks is recognized by the thinning and wedging out of successive units immediately below an unconformity. A progressive development of anticlinal features is indicated by the localized thinning of two or more consecutive units in the same area. In such an area, it is assumed that the surface at the locality was either progressively arched during deposition or that exposure and erosion caused thinning of the consecutive units in the same place. The latter, although not impossible, is unlikely. Similarly, progressive synclinal movement is indicated by the localized thickening of two or more consecutive units in the same locality.
Three principal periods of folding of distinctly different character are revealed by the thickness maps and two others are revealed by structure maps. The first period of folding affected the rocks lying between the Pre-Cambrian surface and the base of the St. Peter sandstone. The second affected the rocks between the St. Peter sandstone and the base of the Mississippian and possibly also the Kinderhookian rocks. The third, which produced the Nemaha anticline and the Salina basin, affected the Mississippian, Pennsylvanian, and Permian rocks. The third period of folding began in Mississippian time and continued with decreasing structural vigor through most of the Permian. The fourth occurred between Permian and Cretaceous times, and the fifth between Cretaceous time and the present.
The thickness of the rocks between the Pre-Cambrian surface and the base of the St. Peter sandstone is shown in Plate 1 by lines drawn at 100-foot contour intervals. A relatively level Pre-Cambrian surface is indicated by the wide distribution of the Bonneterre dolomite which is underlain in most areas by the Lamotte sandstone, but low topographic relief in Missouri, Iowa, and eastern Kansas is revealed by local variations in the thickness of the Lamotte and in its absence of the Bonneterre.
That hills of considerable height remained on the Pre-Cambrian surface is suggested by the record of a well in Vernon County, Missouri, where the Gunter sandstone member of the Van Buren formation directly overlies Pre-Cambrian rocks (McQueen, 1931, pl. 10). The relations of the Gunter to the Pre-Cambrian at that place, however, could have resulted from pre-Gunter and pre-Eminence uplift, exposure, and erosion.
The surface on which the St. Peter was deposited is known from outcrops in northern Missouri where it was trenched by shallow channels. Wells in that part of Missouri and in eastern Kansas have revealed local thicknesses of the St. Peter sandstone as great as 403 feet (Lee, Grohskopf, Reed, and Hershey, 1946, sheet 1). These thicknesses, when compared with the regular thickness of the sandstone, rarely less than 50 feet or more than 100 feet, indicate that the surface was probably affected by sink holes.
Although the surface on which the St. Peter was deposited beveled all the rock formations extending downward from the Jefferson City-Cotter sequence to Pre-Cambrian granite, it seems to have been reduced to a plain which had low topographic relief and local sink holes. That a subsiding basin, the Ozark basin, prior to St. Peter time extended from south-central Missouri northward into eastern Iowa is indicated by maps showing the thickness of the rocks between the top of the Roubidoux and the Pre-Cambrian in southern Missouri (Lee, 1943, fig. 10), and between the base of the St. Peter and the Pre-Cambrian in northern Missouri (Lee, Grohskopf, Reed, and Hershey, 1946, sheet 1). Also, at that time a broad beveled southward plunging arch, the Southeast Nebraska arch, extended from southeastern Nebraska southward across Kansas more or less parallel to the contemporaneous Ozark basin.
Cross section B-B' (Fig. 8 (as PDF), Pl. 1) across the southern extension of the Southeast Nebraska arch between wells 4B and 6 reveals the Roubidoux dolomite resting directly on the Pre-Cambrian as the result of beveling or overlap of pre-Roubidoux rocks. Also, the Roubidoux overlies Pre-Cambrian rocks in a broad ill-defined area extending roughly northeast from wells 2 and 3. In McPherson County a pre-Roubidoux syncline is suggested by a deep well (No. 4) in which Keroher and Kirby (1948) found some 360 feet of pre-Roubidoux sediments. The trend of this syncline toward the northeast, shown on Plate 1, is vaguely indicated by a well in sec. 9, T. 17 S., R. 3 W., in which Miss Leatherock identified 170 feet of Bonneterre dolomite. The syncline becomes less prominent toward the northeast but is indicated by the thickening of the Bonneterre dolomite between wells 29 and 30 of cross section A-A' (Pl. 1).
The interpretation of these structural features is based on the assumption that the sequence of pre-Roubidoux formations, as recognized in western Missouri, was deposited in eastern Kansas and was deformed and beveled before Roubidoux time. Possibly there were Cambrian hills that were not covered by sediments until Roubidoux time but this does not seem to be supported by the present relations of the Roubidoux to the Bonneterre, Eminence. Van Buren, and Gasconade formations in well 4B (Fig. 8 (as PDF)) in McPherson County.
The well in McPherson County (well 4B, cross section B-B', Sinclair No. 8 Morehouse, sec. 4, T. 21 S., R. 3 W.) penetrated a seeming abnormal thickness, 360 feet, of the Jefferson City and Cotter dolomites, which seems to be due to possible repetition by a pre-Pennsylvanian thrust fault (Bunte and Fortier, 1941, p. 114, fig. 5) whose trace passes near the well. Possibly the thickness of Jefferson City and Cotter dolomites has been increased as much as 150 feet by repetition along the fault. This seeming duplication has been taken into account in preparing cross section B-B' and the thickness map. The fault is believed not to have affected the thickness of the pre-Jefferson City formations.
Cross section A-A' (Pl. 1) reveals beveling of the Bonneterre dolomite on the flank of the Southeast Nebraska arch and overlap of the Roubidoux formation onto the Bonneterre, and suggests that the Bonneterre and the Roubidoux were removed from the crest of the arch immediately prior to St. Peter time. Although structural movement was more or less continuous throughout Arbuckle deposition, there seems to have been two periods of especially active deformation in eastern Kansas. The first period of special activity preceded the deposition of the Roubidoux which was deposited upon the beveled surface of the older rocks (Fig. 8 (as PDF)) which had been arched on axes trending slightly east of north. The second period preceded the deposition of the St. Peter sandstone. This movement is especially well revealed on the crest of the Southeast Nebraska arch (cross section A-A', Pl. 1) which shows the overlap of the St. Peter on Pre-Cambrian rocks as well as the overlap of the Roubidoux on the Bonneterre.
After St. Peter time a broad area in southeastern Nebraska and northeastern Kansas known as the North Kansas basin which had previously been a positive area (the Southeast Nebraska arch) began a long period of differential subsidence. Also at this time the Ozark region of Missouri rose and the Chautauqua arch and the Central Kansas uplift began their upward movement (Lee, 1943; Lee, Grohskopf, Reed, and Hershey, 1946).
The deformation in post-St. Peter time was intermittent and occurred both during periods of sedimentation and during periods of emergence. In some areas the re-elevated surface was eroded without deformation; in others, surfaces of low relief beveled strata previously warped and re-elevated.
Deformation of Rocks of Simpson Age
Rocks of Simpson age in the Salina basin consist of the St. Peter sandstone below and the Platteville formation above (Leatherock, 1945). The St. Peter sandstone lies unconformably on the earlier Ordovician and Upper Cambrian rocks, and unconformably also below the Platteville. Local variations in the thickness of the St. Peter owing to erosional relief equal or exceed its regional variations in some localities. In consequence, no conclusions concerning structural movements that were developed during its deposition can be drawn from its thickness.
The important unconformity between the St. Peter sandstone and the Platteville is expressed by the absence in Kansas of several thick formations cropping out in this interval in southeastern Missouri. In northeastern Kansas and in the Salina basin the contact between the two formations seems to have been unusually even over a broad area. The basal dolomite member of the Platteville rests on the upper member of the St. Peter in some places and on the middle member in others (Leatherock, 1945, p. 13) irrespective of the thickness of the dolomite member. The dissected surface of the St. Peter was reduced to an exceptionally perfect plain, presumably by tidal and wave action of the Platteville sea. The fact that different parts of the St. Peter underlie the base of the Platteville in different areas suggests that there was local deformation during the hiatus between the St. Peter and Platteville. The data are, however, inadequate to permit an analysis of these movements.
The Platteville is too thin and too irregular in thickness for structural deformation to be revealed by a thickness map. It is unconformable with the overlying Kimmswick as well as with the underlying St. Peter. Toward the south and southwest, the Kimmswick overlaps beyond the Platteville and is in contact with the St. Peter (well 1, cross section A-A', Pl. 2; well 25, cross section A-A', Pl. 4). The minor irregularities of the upper contact of the Platteville are probably the result of low topographic relief of the Platteville surface. The formation, as shown in the cross section of Plate 4, thickens toward the center of the North Kansas basin and the initial subsidence responsible for this thickening was contemporaneous with the deposition of the Platteville or took place during the hiatus between Platteville and Kimmswick times. The thickness map and the cross section (Pl. 2) reveal that subsidence of the North Kansas basin and the contemporaneous elevation of the Chautauqua arch and the Central Kansas uplift began before the deposition of the Kimmswick. A suggestion that the deformation was initiated even before the end of Platteville time is offered by cross section A-A' of Plate 4.
Deformation of the Kimmswick-Maquoketa Sequence
The three zones of the Kimmswick limestone form a conformable sequence, and each thickens more or less regularly into the North Kansas basin as shown in cross section A-A' (Pl. 2). The wedging out of the first zone toward the south and the overlap of the second zone upon the Platteville and locally upon the St. Peter, suggest that the pre-Kimmswick surface was gently warped prior to the deposition of the first zone and that this zone was deposited only in the deeper parts of the basin. With continued subsidence the second zone overlapped beyond the previously unsubmerged margin of the basin.
The thickening of the second zone toward the north implies subsidence in this direction contemporaneous with its deposition, but it may be due wholly to subsidence and beveling during the hiatus between the Kimmswick and the overlying Maquoketa shale, for the Maquoketa overlaps from the third zone onto the second zone (Pl. 2) on the flank of the Central Kansas uplift. The deformation of the third zone probably occurred both during deposition and during the pre-Maquoketa hiatus.
Deformation of the Maquoketa-Silurian Sequence
Plate 4 shows the thickness of the Simpson, Kimmswick, Maquoketa, and Silurian rocks. Because the upper and lower surfaces of this sequence are known to have been exceptionally smooth and level (Lee, Grohskopf, Reed, and Hershey, 1946, sheet 2), its thickness, which increases regularly into the North Kansas basin, is regarded as a reliable measure of the character of the structural movements that occurred between pre-St. Peter and pre-Devonian beveling.
In order to determine if either the upper or lower surface of the Maquoketa was smooth enough to be used for the preparation of thickness maps that signify structural movement, Plates 2 and 3 were drawn for comparison with Plate 4. Plate 2 shows the combined thicknesses of the Kimmswick dolomite and the Maquoketa shale and Plate 3 shows the combined thicknesses of the Maquoketa shale and Silurian dolomite. Although the thickness of the Kimmswick and Maquoketa increases toward the center of the North Kansas basin, the configuration of the thickness lines (Pl. 2) deviates widely from the pattern of the lines of Plate 4. It is inferred that the significance of the thickness lines of Plate 2 in determining deformation is materially modified by the unconformity at the top of the Maquoketa shale, and that Plate 2 thus does not portray the details of structural movement although a general subsidence of the North Kansas basin is indicated.
The thickness lines of Plate 3, although not as closely spaced, conform approximately to the thickness lines of Plate 4. This similarity implies that the deformation shown by the thickness lines of Plate 3 adequately represents the increment of folding from the base of the Maquoketa to the pre-Devonian surface, and that the base of the Maquoketa, although having some relief, was a relatively more level surface than its top.
The cross section of Plate 3 shows that the thickness of the first zone of the Silurian tends to increase where the Maquoketa is thin and to decrease where the Maquoketa is thick. The areal geology of Plate 4 shows that pre-Silurian erosion cut deep enough to remove the Maquoketa shale in parts of Smith and Osborne counties. Some erosional relief at the base of the Maquoketa as well as at the top is suggested in well 24 of cross section A-A' (Pl. 3), where the Kimmswick thickens at the expense of the Maquoketa.
Because of the pronounced unconformity at the base of the Silurian and the regularity in thickness of its second zone, the cross section of Plate 3 has been correlated on the base of this zone. The cross section shows that northeasterly subsidence occurred during the deposition of the third zone and that little or none occurred during the deposition of the fourth zone.
At the end of Silurian time the whole region was elevated above sea, probably by a series of differential vertical and deformational movements. During the ensuing erosion the surface was reduced to a nearly flat plain although in some areas there were low hills (Pl. 5; and Lee, Grohskopf,, Reed, and Hershey, 1946, sheets 2 and 3, Chariton County, Mo.). All the rocks down to the Lower Ordovician dolomites were beveled on the flanks of the Chautauqua arch and the Central Kansas uplift (Pl. 4). On the line of the cross section of Plate 3 pre-Devonian erosion beveled the Silurian from the fifth zone to the fourth zone, but on the cross section of Plate 4 the Devonian overlaps onto the Maquoketa shale also. Cross section A-A' of Plate 3 shows that the subsidence of the North Kansas basin was not less than 400 feet between the beginning of Maquoketa time and the beveling of the pre-Devonian surface.
Overall Deformation Between the Base of the St. Peter and the Base of the Devonian
Plate 4, which shows the thickness of rocks from the base of the St. Peter sandstone to the base of the Devonian rocks, indicates the subsidence of the North Kansas basin up to the time of pre-Devonian beveling. On the northern flank of the Chautauqua arch, which was developed during the deposition of this sequence, pre-Devonian erosion beveled rocks ranging in age from Silurian to Jefferson City (Lee, Grohskopf, Reed, and Hershey, 1946, sheet 2).
The thickness maps and cross sections of Plates 2, 3, and 4 reveal that there was no perceptible deformation in northeastern Kansas during the deposition of the St. Peter sandstone but that gradual subsidence of the North Kansas basin occurred during Platteville, Kimmswick, and Silurian times and continued intermittently until the beveling of the pre-Devonian surface. The pre-Devonian subsidence of the North Kansas basin was about 800 feet.
Deformation of the Devonian Rocks
After Devonian time and probably in part during the Devonian, the North Kansas basin was again warped downward. Subsequently the whole region was eroded to a plain that rivaled the pre-Devonian surface in its smoothness.
Plate 5 shows the thickness of the Devonian rocks and also the pre-Chattanooga areal geology. Inliers of Silurian rocks shown on the map in Dickinson County indicate that the pre-Devonian surface possesses some topographic relief in that area.
The Devonian rocks are thickest in the central part of the North Kansas basin in southeastern Nebraska. Toward the margin of the basin they were beveled and completely removed from the major areas of uplift where older rocks also were beveled. The post-Devonian plain is well developed in most parts of eastern Kansas and in adjoining states, but deep valleys were cut in its surface toward the west. In northern Rice County a broad open valley trending northward in Reno County joins another trending westward across Marion and McPherson counties. Thence, the drainage seems to have flowed northwest. In Marion and McPherson counties, the valley cut through the Devonian, Silurian, and Maquoketa rocks and exposed the upper beds of the Kimmswick. An outlier of Devonian rocks was left south of the valley in Harvey and parts of adjoining counties as shown on Plate 5.
The thickness of the Devonian is significant of structural movement only where the original beveled surface lies beneath the Chattanooga and outside the pre-Chattanooga valley described above. In the area of control (Pl. 5) as thus limited the increase in the thickness of the Devonian rocks conforms to the pattern of thickness lines of Plate 4 and indicates continued subsidence of the North Kansas basin. The subsidence of the base of the Devonian in the North Kansas basin was at least 600 feet. The pre-Chattanooga areal geology shown on Plate 5 reveals that during Devonian time or at its close the Chautauqua arch became separated from the Central Kansas uplift.
Figure 9 shows a west-east section across the Devonian outlier on line B-B' of Plate 5. It reveals pre-Chattanooga arching trending somewhat vaguely toward the north in T. 22 S., R. 2 W. in Harvey County. This fold is more clearly revealed by the thickness contours of Plate 7. Folding more or less parallel to the subsequently folded Nemaha anticline is indicated in the Forest City basin by similar thinning of the Devonian on the eastern border of Plate 5 and by Lee, Grohskopf, Reed, and Hershey (1946, sheet 3). These belts of thinning suggest that there was minor structural activity parallel to the Nemaha anticline before Chattanooga time, but there is no evidence that initial movements occurred on the anticline itself at that time.
Fig. 9--Stratigraphic cross section across Reno and Harvey Counties on line B-B' of Plate 5 showing anticline of probable pre-Chattanooga age. The anticline is not revealed on the Devonian thickness map on account of the pre-Chattanooga topography. It is clearly shown, however, by the thickness lines of Plate 7 which reveal also the trend of the fold.
Deformation of Chattanooga and Boice Shales
Plate 6 shows the combined thickness of the Chattanooga and Boice shales. The thickness of this shale sequence in the Salina basin is more expressive of the topography of the pre-Chattanooga surface than of the structural developments of the period. The pre-Chattanooga valley, which is revealed by the meanderings of the thickness lines in McPherson and Marion counties and in the adjoining counties to the south, was eroded on the beveled surface of the Devonian rocks. The wedging out of the shales on the western and northern margins of the shale area is, however, the result of beveling at the close of Mississippian time and is thus not related to structural movements that occurred between post-Devonian planation and the deposition of the Mississippian limestones.
Outside the pre-Chattanooga valley and the areas of westward and northward thinning of the shales, there is a gradual thickening toward the northeast, in the direction of the deeper part of the North Kansas basin. This thickening is more significant when taken in connection with broader areas including the Forest City basin (Lee, 1940, pl. 4; Lee, Grohskopf, Reed, and Hershey, 1946). The thickness lines of Plate 6 are dotted across areas from which the shales were removed by post-Mississippian erosion. The dotted lines extending northward from Republic and Washington counties, Kansas, are intended to suggest the probable original thickness of the shales in that part of the area.
Overall Deformation Between the Base of the St. Peter Sandstone and the Base of the Mississippian Limestones
Plate 7 shows the thickness of rocks between the base of the St. Peter sandstone and the top of the Chattanooga-Boice shale sequence and it reveals the deformation that took place during the corresponding time interval. The extent of this deformation represents the total of the separate increments of deformation indicated in Plates 2 to 6. The post-Devonian anticlinal arch in Harvey County, revealed by cross section B-B' of Figure 9, is clearly shown on Plate 7 as is its northerly trend approximately parallel to the Nemaha anticline. Plate 7 reveals also that the subsidence of the North Kansas basin was at least 1,400 feet below the flank of the Chautauqua arch and more than 1,000 feet below the pre-Mississippian flank of the Central Kansas uplift. The total downwarping of the North Kansas basin below the crests of these structural features was considerably greater and may have reached 2,000 feet.
Deformation During Mississippian Time
As pointed out in the discussion of the stratigraphy of the Chattanooga-Boice shale sequence, the surface at its upper contact was exceptionally level. The first deposits upon this surface were Kinderhookian limestones. In southeastern Kansas they consist of the Chouteau and Sedalia limestones both of which thicken northward from a featheredge at the Kansas-Oklahoma border to 245 feet in central Iowa where the Kinderhookian Hampton formation occupies a position corresponding to the Chouteau and Sedalia (Laudon, 1931).
The gradual northward thickening of the Kinderhookian formations from southeastern Kansas toward the north as shown in cross section A of Figure 4, reveals that during Kinderhookian time and at its close central Iowa was subsiding and southeastern Kansas was stationary or being slightly elevated. At the end of Kinderhookian time the region was raised above sea level and during a period of relative stability the formations were beveled as is illustrated on the cross-section in Figure 4. With the advent of Osagian time, the beveled surface seems to have been tilted southward, for the earliest Osagian deposit, the St. Joe limestone, wedges out northward. The Osagian formations, the Reeds Spring, Burlington, and Keokuk limestones, and the Meramecian "Warsaw," Spergen, and St. Louis limestones overlap in succession northward on Kinderhookian rocks, as shown diagrammatically in cross section B of Figure 4. The differential tilting of the surface toward the south, which began before St. Joe time and must have been more or less continuous during the long period of overlap, kept the Kinderhookian rocks of Iowa above sea level. Although the surface was probably seldom raised much above sea level some erosion of the Kinderhookian rocks of Iowa must have occurred during Osagian time. Figure 4 shows the structural relations of the Mississippian rocks east of the Nemaha anticline which was already undergoing initial deformation in early Mississippian time.
Figure 6 shows the relation of Kinderhookian and younger Mississippian formations west of the Nemaha anticline in the Salina basin. The wells are correlated on the contact between the Reeds Spring and Burlington limestones. It will be noted that only the upper member of the Sedalia is represented in this area. The absence of the lower member of the Sedalia and the Chouteau limestone is the result of pre-Chouteau elevation of the area west of the Nemaha axis which confined these deposits to a subsiding structural basin to the east.
West of the Nemaha anticline the Kinderhookian rocks thicken appreciably toward the north and wedge out toward the south where outliers of upper Sedalia rocks lie unconformably below the Gilmore City (Fig. 5) or below the St. Joe limestone as shown in wells 10 and 12 of the cross section of Figure 6. Figure 6 shows also that the Osagian formations thicken toward the south and unconformably overlap the Kinderhookian rocks toward the north. These phenomena reveal subsidence toward the north during Kinderhookian time and beveling at the end of Kinderhookian time, followed by a gradual rising of the surface toward the north during Osagian and Meramecian time in the same sequence of movements as east of the Nemaha anticline.
The division of the basin in which the early Osagian rocks were being deposited by a northerly trending belt of land too high to be covered until Burlington time reveals that initial movements of the Nemaha anticline occurred prior to or during the deposition of the St. Joe and Reeds Spring (Fig. 5B) . The greater thickness of the Mississippian limestones below the Short Creek oolite member of the Keokuk (Lee, 1940, p. 63, pl. 6, cross section E-F') east of the anticline than west of it reveals movement on the trend of the Nemaha axis before the deposition of the Short Creek oolite. There were thus at least two types of deformation going on contemporaneously during Mississippian time: (1) initial movement of the Nemaha anticline, the maximum development of which occurred at the end of Mississippian deposition; and (2) the progressive tilting of the region from south to north during Kinderhookian time and later tilting in the opposite direction. A third type of deformation that produced folds trending to the northwest is recognized in the post-Mississippian development of the Salina basin, but no evidence has been discovered that this type of deformation was active during the deposition of Mississippian rocks. Early Mississippian movements of this type seem probable inasmuch as regional folds trending in this direction had been active since St. Peter time and continued into the Pennsylvanian.
Deformation at the End of Mississippian Time
Plate 8 shows the thickness of the Mississippian rocks and the areal geology at the beginning of Pennsylvanian deposition. The principal structural movements at the end of Mississippian time included the formation of the Nemaha anticline, the Forest City basin, and the Salina basin.
The deformation of the Mississippian limestones was probably brought about by differential movements during which erosion of the gradually emerging surface kept pace with its elevation. In conformity with this concept, it is assumed that the rocks were beveled by submarine or subareal erosion as fast as they were elevated, that the surface was rarely much above sea level, and that the rocks were essentially beveled during the whole period of deformation as well as the end.
Before the invasion of the Pennsylvanian sea the beveled surface was re-elevated, deformed again, and subjected to relatively brief erosion. A broad valley was cut on the beveled surface of the Mississippian limestones in the Forest City basin and in consequence the expression of structure by thickness lines in that area is modified by topographic relief (Lee, Grohskopf, Reed, and Hershey, 1946, sheets 5 and 6). This valley is revealed by a local thickening of the Pennsylvanian rocks accompanied by a corresponding thinning of the upper Mississippian formations at the same locality. In the Salina basin local shallow channels or possibly sink holes have been noted. Errors in interpretation of structure due to such topographic irregularities in the Salina basin have been eliminated in some degree by including with the Mississippian sequence the cherty conglomerate at the base of the Pennsylvanian, for the top of the conglomerate, a residual debris, was more nearly level than the eroded surface of the Mississippian.
The Nemaha anticline is the most striking of the new structural features produced in eastern Kansas by post-Mississippian folding. It extends with varying structural relief from near Omaha, Nebraska, southwestward beyond Oklahoma City, Oklahoma. Throughout its length the eastern limb of the anticline is notably steeper than the western limb. The northern end of the anticline was finally raised so high that the erosional beveling in northeastern Kansas exposed Pre-Cambrian rocks on the crest and truncated the overlying rocks in parallel bands on its flanks. Toward the south, where the structural relief decreases, the Mississippian limestones were only partly eroded from its crest except at places of exceptional deformation.
The Central Kansas uplift, which is outlined on the areal map of Plate 8 by the beveled outcrops of Mississippian and older rocks, was vigorously rejuvenated at the end of Mississippian time, but the Chautauqua arch, which showed only slight movement at the close of Chattanooga time, remained quiescent, although feeble secondary movements parallel to its axis are shown locally by 50-foot thickness lines (Lee, 1939, pl. 1).
Many geologists have assumed that the northeasterly trending Forest City basin and the northwesterly trending Salina basin are the result of the intersection of the North Kansas basin by the Nemaha anticline. This concept is difficult to accept in the light of the relations shown by the thickness contours of the two basins and their relation to the Nemaha anticline. The Nemaha anticline strikes across the eastern side of the North Kansas basin (compare Pls. 7 and 8) and neither the Salina basin nor the Forest City basin bears a close relation to the North Kansas basin.
The Forest City basin parallels the Nemaha anticline and lies high on the southeast margin of the North Kansas basin between the Nemaha anticline and the northwestern flank of the contemporaneously re-elevated Ozark uplift (Lee, Grohskopf, Reed, and Hershey, 1946, sheet 5). The only feature common to the Forest City basin and to the North Kansas basin is their position on the northwestern flank of the Ozark uplift.
At the end of post-Mississippian beveling, 350 feet of Mississippian limestones survived in the deepest part of the Salina structural basin and 450 feet in the Forest City basin. These thicknesses do not, however, represent the total amount of deformation.
The Salina basin strikes northwest, parallel to the northeastern flank of the Central Kansas uplift. It lies between this structural feature and a new broad contemporaneously developed area of uplift in southeastern Nebraska trending northwest from the northern end of the Nemaha anticline across the deepest part of the old North Kansas basin. The only feature common to the Salina basin and to the North Kansas basin is their position on the northeast flank of the Central Kansas uplift. It seems probable that a Salina basin would have been formed even had there been no North Kansas basin.
The Salina basin is bounded on the southeast by the Nemaha anticline but where the syncline which forms the Salina basin intercepts the anticline in southeastern Chase County the continuity of the Nemaha axis is broken. The Salina basin syncline continues weakly to the southeast and fades out in central Greenwood County (Lee, 1939, Pl. 1). The Central Kansas uplift confines the Salina basin on its southwestern side but between the southeastern end of the uplift and the Nemaha anticline the Salina basin is confined by a broad low arch between the 250-foot thickness lines (Pl. 8). This arch trends northwestward across central McPherson County, and is alined with the flank of the Central Kansas uplift.
The folds that trend northeast parallel to the Nemaha anticline and those that trend northwest paralleling the Central Kansas uplift although they intersect nearly at right angles seem to have developed contemporaneously, for the interruption of the Nemaha anticline, at its intersection by the Salina basin syncline in Chase and Marion Counties, continued to develop in early Pennsylvanian time (Pl. 9).
Northeast-trending folds were formed both to the east and west of the axis of the Nemaha anticline. The constricted area between the Central Kansas uplift and the Nemaha anticline contains more known northeasterly trending anticlines than any other area in Kansas and most of them yield oil and gas. The Voshell anticline is the longest and most prominent of the anticlines in the constricted area but it is cut off toward the north by the Salina basin. A northeasterly trending post-Mississippian thrust fault in pre-Pennsylvanian rocks was mapped in the subsurface by Bunte and Fortier (1941) on the west side of the Voshell anticline.
The prominent Abilene anticline on the northeast side of the Salina basin is recognized in the surface rocks in Riley County and extends southward into Dickinson County. It resembles the Nemaha anticline in that the beds dip steeply on its southeastern side and very gently to the northwest. Not many subsurface data are available on the Abilene anticline and the thickness lines have been drawn to conform with the scanty data and with the structure of the surface formations. The Abilene anticline is interrupted on the south by the Salina basin syncline.
The anticline on the southwestern limb of the Salina basin on which the Olsson pool (T. 16 S., R. 3 W.) lies, extends farther northeast into the Salina basin than the other anticlines. Available data from a single well, Northern Ordnance Inc. No. 1 Warner, sec. 10, T. 15 S., R. 3 W., suggest that this anticline may cross the Salina basin to its northeastern side.
Northwest-trending folds paralleling the Salina basin and the Central Kansas uplift are not clearly revealed by the Mississippian thickness lines of Plate 8. In Chautauqua and Elk counties there is some indication of northwesterly trending folds (Lee, 1939, pl. 1). The exposure of Cambrian and Ordovician rocks in Ellsworth County on the areal geology map (Pl. 8) suggests northwesterly trending folds on the flank of and paralleling the Salina basin.
Deformation During Pennsylvanian and Permian Time
The structural movements that occurred during Pennsylvanian and Permian time are based on the thickness maps of four sequences of rocks. The upper and lower surfaces of each sequence were chosen at beds that were (1) originally essentially flat and horizontal, (2) commonly reported in drillers logs with reasonable accuracy, and (3) spaced at more or less regular intervals. The thickness of each sequence ranges from 750 to 1,000 feet although there are wide variations in each sequence due to contemporaneous structural movements.
The five datum planes used are (1) the top of the Pennsylvanian basal cherty conglomerate, (2) the base of the Hertha limestone at the base of the Kansas City group, (3) the top of the Topeka limestone at the top of the Shawnee group, (4) the base of the Florence limestone member at the bottom of the Barneston limestone of the Chase group of the Permian, and (5) the top of the Stone Corral dolomite of the Sumner group of the Permian.
The use of the top of the cherty conglomerate at the base of the Pennsylvanian results in some confusion where the Pennsylvanian overlaps upon the surface of pre-Mississippian rocks on the Central Kansas uplift. In such areas an extremely irregular line of zero thickness results. A part of the irregularity is due to erosional relief and to the occurrence of karst topography described by Walters (1946, pp. 690-699). A part is due to the fact that where the basal conglomerate is represented by noncherty clastic deposits it has been included with the Pennsylvanian instead of with the Mississippian sequence. The irregular configuration of the zero thickness line as shown on the Central Kansas uplift cannot therefore be accepted as representing local structural movements although in some places it seems to do so.
Deformation Between the Top of the Pennsylvanian Basal Conglomerate and the Hertha Limestone
Plate 9 shows the thickness of the pre-Hertha Pennsylvanian rocks above the Pennsylvanian basal conglomerate. The structural movements revealed by the thickness of the Mississippian limestone (Pl. 8) were generally revived during the deposition of the lower Pennsylvanian rocks but there were some modifications in their character, particularly on the Nemaha anticline.
Before the advance of the Pennsylvanian sea into Kansas, topographic basins developed east and west of the Nemaha anticline whose crest became a low barrier separating the basins. The Forest City basin to the east was deeper than the Salina basin to the west and was invaded and received Pennsylvanian deposits before the Salina basin. Differential movements kept the crest of the Nemaha anticline above sea level until Marmaton time near the southern border of the Salina basin area and until middle Kansas City time in southeastern Nebraska. During much of this period a long narrow peninsula extended southward from Nebraska into the Pennsylvanian sea but the surface was probably too low to warrant its designation as a ridge except in a structural sense.
In the deepest part of the Salina basin 400 feet of Pennsylvanian rocks had been deposited but in the deepest part of the Forest City basin they were 1,050 feet thick. The difference in the thickness of the deposits is the measure of the vertical displacement of the Forest City basin below the Salina basin.
Except for such outstanding local structures as the Burns dome and the Elmdale and Eldorado anticlines, arching on the axis of the Nemaha anticline was too low to be expressed by 50-foot contours. The renewed activity during early Pennsylvanian time increased the displacement, partly by faulting on its east side, and tilted the upraised block toward the west. Plate 9 shows a belt of thinning west of the axis of folding extending south from western Riley County to northwestern Butler County. This belt of thinning is not a structural feature but the result of the westward migration of the divide between the Salina and Forest City basins during the long period of exposure when the crest of the anticline was kept above sea level by differential upward movements.
The Central Kansas uplift also continued to develop and in Hertha time a considerable part of its area was land.
The thickness lines of Plate 9 show the renewed development of the Salina basin along the same trend as indicated by the thickness of the Mississippian limestones. Plate 8 shows that the thickest section of Mississippian limestones lies in Saline County but Plate 9 shows that the thickest section of pre-Hertha Pennsylvanian rocks is in or near Jewell County and that the position of maximum deformation had thus moved some 60 miles northwest.
The divide in McPherson County that separated the Salina basin from the deeper structural basin toward the south on Plate 8 is only faintly shown by the thickness lines of Plate 9. The Salina basin interrupts the Nemaha anticline at the same place as at the end of Mississippian time (Pl. 8). The perpetuation of this feature probably indicates the contemporaneous development of the two intersecting structural features throughout a long period of time. Structural movement during pre-Hertha Pennsylvanian time lowered the surface of the Mississippian limestones in the Salina basin about 450 feet below the crest of the Central Kansas uplift.
Secondary northeasterly trending folds such as the Voshell anticline were less active than during the period ended by post-Mississippian beveling and are revealed only locally by 50-foot thickness lines. The data are inadequate to determine the activity of the Abilene anticline although some movement probably occurred. Secondary northwesterly trending folds are not revealed by 50-foot thickness lines.
Deformation Between the Hertha Limestone and the Topeka Limestone
Plate 10 shows the thickness of the Pennsylvanian rocks between the base of the Hertha limestone and the top of the Topeka limestone. The deformation indicated by the thickness of this sequence is similar to that revealed in Plates 8 and 9 but was of declining intensity. At this time downward displacement on the east limb of the Nemaha anticline was only about 300 feet below the crest near the Nebraska-Kansas border and less than 100 feet near the southern border of Plate 10. Continued development of the Burns dome is revealed by a 50-foot thickness line encircling its crest but only minor anticlinal movements, so far as known,, occurred at other places.
Deformation of the Salina basin declined and the structural basin of earlier times became a structural embayment. The low structural divide in McPherson County which originally cut off the Salina basin from the subsiding area to the south had no expression during this period and probably became inactive even before the deposition of the Hertha limestone.
The thickness lines on the Central Kansas uplift form a broad bulge and reveal a decline of structural activity. The structural relief between the crest of the uplift and the deeper part of the Salina basin directly opposite was less than 150 feet.
The Voshell anticline is only faintly expressed by 50-foot thickness lines and only in certain areas. Minor movements of less than 50 feet, however, were general along the trend. The available wells along the Abilene anticline suggest Some activity of this structural feature.
Deformation Between the Topeka Limestone and the Barneston Limestone
Plate 11 shows the thickness of Pennsylvanian and Permian rocks between the Topeka limestone of Pennsylvanian age and the Barneston limestone of Permian age. The sequence transgresses the boundary between the Pennsylvanian and Permian Systems. A low angular unconformity is believed to separate these systems and it would have been desirable to divide the sequence at or near the contact. Unfortunately a surface of considerable relief occurs at the contact and for several hundred feet both above and below the contact the limestone formations are so thin and so infrequently identified in well logs that no suitable datum beds are available for dividing the sequence near the Permian-Pennsylvanian contact. The deformation indicated by the thickness map, therefore, includes movements of both late Pennsylvanian and early Permian age.
The thickness map of this sequence is in most respects similar to Plate 10 but less deformation is revealed in spite of the fact that the average thickness of the sequence is greater. Structural activity in the Salina basin again declined. The re-entrant shows a lower overall structural gradient toward the southeast than during the older sequence although in some areas the gradient is steeper. The arching of the Central Kansas uplift had nearly ceased and the thickness lines show it as a southeasterly plunging arch on a southeastwardly dipping monocline.
The deformation of the Voshell anticline is shown only at the northern end where a 50-foot contour touches the anticline. Some rejuvenation, however, is revealed at other places along the anticline by thinning of less than the contour interval. Very little control is available on the Abilene anticline but deformation is suggested vaguely by a few wells and by abrupt deformation of more than 50 feet in surface formations on its east side a short interval above the top of this sequence.
Most of the area in which the entire sequence survives lies west of the Nemaha anticline. The thickness lines therefore do not reveal the activity of this structural feature except toward the south. The sequence is 55 feet thinner on the crest than on the flanks of the Burns dome and is 40 feet thinner on the crest than on the flanks of the north end of the Eldorado anticline. It is probable that minor movements occurred at this time at other points along the crest of the Nemaha anticline. No abrupt thickening indicating a structural escarpment is evident immediately east of either the Burns dome or the Eldorado anticline but subsidence, up to 40 feet, is shown by irregular thickening of the interval in a number of wells in the adjacent synclinal area to the east.
Deformation Between the Barneston Limestone and the Stone Corral Dolomite
Plate 12 shows the thickness of Permian rocks between the base of the Florence limestone member of the Barneston limestone and the top of the Stone Corral dolomite. The Stone Corral was eroded from a large part of the Salina basin before the deposition of the Cretaceous rocks and in consequence the full thickness of this sequence can be mapped only in the western part of the Salina basin. The thickness lines reveal a regional tilt only a little east of south. They show no movement of the Central Kansas uplift. Most of the Salina basin is outside the area of control but the thickness lines in Osborne, Mitchell, and Lincoln counties do not show the re-entrant that characterized the later stages of the development of the Salina basin as shown on Plate 11.
The cessation of structural activity on the Central Kansas uplift brought to an end a long period of anticlinal movement. Its almost continuous development from St. Peter to early Permian time is recorded by thinning on its flank or crest of nearly every mappable unit from Kimmswick to Barneston. The 50-foot thickness lines reveal no structural activity of the Central Kansas uplift after Barneston time although minor movements on local northwesterly trending anticlines probably continued through the Cretaceous. The Salina basin was first revealed by the thickening of the Mississippian limestones, continued its development with declining vigor into early Permian time and, like the Central Kansas uplift, became inactive after Barneston time.
A structural bench, trending from Rush and Ellis to Lincoln and Ellsworth counties, breaks the regular southward increase in thickness of the Florence-Stone Corral sequence. The widening of the intervals between the thickness contours in this area occurs where the Hutchinson salt member is thickest and is the expression of the downwarping that accompanied the development of the salt basin. The thickest salt marking the greatest subsidence of the salt basin overlies the crest of the formerly active Central Kansas uplift (Fig. 7).
The southward thickening of the sequence below the Stone Corral dolomite is a change from the previously dominant southeasterly thickening of the older sequences. It suggests that the regional center of the synclinal movements may have moved westward from the Ouachita basin.
During Pennsylvanian and early Permian time Kansas formed a part of a structural province which comprised Illinois, Kansas, Missouri, Nebraska, Iowa, and parts of Oklahoma and Arkansas and which was dominated by the Ouachita basin of west-central Arkansas and southeastern Oklahoma. This basin extended from east to west and was flanked on the south by contemporaneously rising land. Miser (1934, p. 979) reports that 18,000 to 20,000 feet of clastic Pennsylvanian rocks of Cherokee and earlier age were deposited in the Ouachita basin. Compared with these deposit, the maximum thickness of Cherokee rocks of approximately 800 feet in the Forest City basin and 200 feet in the Salina basin seem insignificant.
The Nemaha anticline and the Central Kansas and Ozark uplifts were structural features inherited from pre-Pennsylvanian time but they continued to develop in Pennsylvanian time. The Forest City basin and the Cherokee basin together represent an arm of the Ouachita basin which extended northward between the Ozark uplift and the Nemaha anticline. In a broad sense, particularly after Hertha time, the Salina basin, lying between the Central Kansas uplift and the Nemaha anticline, was also an arm of the Ouachita basin.
Attention has been called in the section on stratigraphy to the decrease in the thickness of shale and clastic deposits toward the north and west and the relative regularity of the thicknesses of the limestone formations. All the Pennsylvanian series and groups in eastern Kansas thicken toward the Ouachita basin except where their thickness is modified by local deformation and intercyclical erosion. Most of the thickening occurs in the shale formations and in the shale members of the formations in which limestone predominates. Table 18 shows the comparative thickness of limestones and clastic deposits in the wells shown in Figure 10.
Fig. 10--Map showing locations of wells feferred to in Table 18 and their relation to Pennsylvanian structural features.
Formations that are composed predominantly of limestone can generally be recognized in sample logs and in some drillers logs but the detailed thickness of interbedded shale cannot be determined with accuracy. The thicknesses of shale and limestone shown in Table 18 were compiled by scaling the electric logs of the respective wells. In order to test the accuracy of the data, some of the electric logs were scaled several times and the measurements compared. The several measurements were found to differ by less than 10 percent.
It will be noted that in general the thicknesses of the clastic beds in Table 18 decrease with the distance from the Ouachita basin and that the aggregate thickness of the limestones of each sequence is singularly constant in the different areas. Some of the deviations from regularity in the thickness of the limestones are caused by the difficulty of scaling the many thin limestones of the electric logs with consistent accuracy. Most of the differences in the thicknesses of the limestones and shales are due to the removal of some of the limestone beds during intercyclical erosion and their replacement by shale during the next cycle of deposition as well as to local structural movement which caused the deposition of greater or less thicknesses of shale in the areas affected.
Table 18--Comparative thickness, in feet, of limestone and clastic beds in groups of the Upper Pennsylvanian and Lower Permian rocks in wells shown on Figure 10.
|Map no.||Well and location||County||Lansing and
Council Grove and
Chase groups to base
of Barneston limestone
|Total, base of
Kansas City to base
of Barneston limestone
|1||E. S. Adkins No. 1
Dater, sec. 14,
T. 24 S., R. 8 E.
|2||Veeder Supply Co.
No. 1 Borth, sec. 29,
T. 19 S., R. 2 W.
|3||Lion Oil Co. No. 2
Murray, sec. 18,
T. 17 S., R. 10 W.
|4||Stanolind Oil & Gas
Co., No. 1 Boxberger,
sec. 10. T. 14 S.,
R. 14 W.
|5||Harbar Drilling Co.
No. 1 Coddlington,
sec. 4, T. 10 S,
R. 20 W.
|2||Veeder Supply Co.
No. 1 Borth, sec. 29,
T. 19 S., R. 2 W.
|6||E. S. Adkins No. 1
Weis, sec. 32,
T. 14 S., R. 2 W.
|7||D. W. McLaughlin
No. 1 Gravenstine,
sec. 21, T. 8 S.,
R. 6 E.
|a The Wyandotte and other Kansas City limestones were replaced by shale in this area.
b Thinning of limestones is due in part to overlap of Kansas City upon the pre-Pennsylvanian
surface and consequent nondeposition of lower Kansas City limestones.
c Thickening of shales is due to deposition in subsiding Salina basin.
In well 1 of Table 18 the Wyandotte and other limestones of the Kansas City group were either eroded during intercyclical erosion and replaced by shale or graded into shales southeastward toward the source of sediments. In this area the thickness of limestone was thus decreased and the shale increased. The abnormal thickness of the shale in well 6, especially in the Douglas group, is probably due to the location of this well in the center of the differentially subsiding Salina basin syncline. In wells 4 and 5, the thinning of the limestones in the Lansing and Kansas City groups is due to overlap and nondeposition of the basal Kansas City rocks on the exposed surface of the Central Kansas uplift.
From the data in Table 18, as well as in the discussion of the Pennsylvanian stratigraphy, it seems probable that the limestones were deposited during quiescent periods of the cyclothems in a broad belt beyond the reach of clastic sediments, and that in this belt they were deposited with little variation in thickness except toward land areas where they grade into or interfinger with clastics. The shales and sandstones, however, were deposited during periods of differential subsidence. They filled the subsiding basin with material that was worn from contemporaneously rising marginal land areas and distributed by tides and currents. The differential tilting of the border regions toward the Ouachita basin was the outstanding structural development in the midcontinent region during Pennsylvanian and Permian times. The Nemaha anticline and the contemporary structures that seem so prominent in eastern Kansas were scarcely more than ripples on the monoclinal dip into the Ouachita basin.
After Permian time a broad syncline developed in southwestern Kansas, and the whole region was raised, roughly beveled, and covered by rocks of Cretaceous age. The details of deformation during the hiatus between the Permian and the Cretaceous are now only partly known.
Late as well as early Permian rocks were laid down in the Salina basin, in eastern Kansas, and perhaps even in the Mississippi Valley, but they were completely removed from the region east of the Salina basin before Cretaceous time. Triassic rocks, 20 feet thick, have been tentatively identified on lithologic grounds in the subsurface of southwestern Kansas. They unconformably overlie the Permian and wedge out toward the east. Upper Jurassic rocks are represented in western Kansas by the Morrison formation which rests unconformably on the Triassic or the Permian and is unconformably overlain by Cretaceous rocks. Neither the Triassic nor the Jurassic rocks now extend into the Salina basin where the Cretaceous rocks overlap from the Morrison onto the Permian.
The post-Permian and pre-Cretaceous structural movements as they affected western Kansas and the Salina basin are shown by the generalized structure maps of Figure 11. Figure 11C shows the present attitude of the Stone Corral dolomite by 250-foot structure contours. Figure 11B shows the present attitude of the top of the Dakota formation (after Bass, 1926a, p. 85) by 250-foot structure contours. Figure 11A, which represents the structure of the Stone Corral dolomite before the deformation of the Dakota, shows a broad southwesterly plunging syncline whose axis lay some 40 to 50 miles west of the present syncline as shown in Figure 11C. The regional dip of the Stone Corral in the Salina basin was about 10 feet per mile toward the southwest.
The post-Dakota regional dip (Fig. 11B) is roughly 7 feet per mile, slightly east of north in Ellis County, and about 7 feet per mile toward the northwest in Republic County. The post-Dakota deformation tilted the syncline in western Kansas toward the north, shifted the position of the lowest part of the structural basin toward the east, and changed the dip of the Stone Corral and older Permian and Pennsylvanian rocks in the Salina basin from southwest to a somewhat variable dip of 6 to 10 feet per mile toward the northwest.
Fig. 11--Generalized contour maps showing by 250-foot contours. A, the structure of the Stone Corral dolomite at the end of Dakota time; B, the present structure of the top of the Dakota formation; C, the present structure of the Stone Corral dolomite. B shows structure arbitrarily referred to a datum 1,250 feet above sea level. For altitudes referred to sea level add 1,250 feet to each contour. Structure of A shows relative elevations only.
The present northwesterly dip of the Permian and Pennsylvanian rocks in the Salina basin and eastern Kansas is thus not the result of a single pre-Cretaceous structural movement but a composite of at least two separate and different structural components. The first, itself perhaps a composite movement, occurred mainly after Permian time and before Cretaceous time (Fig. 11A), and the second occurred mainly after Cretaceous time (Fig. 11B). The final structural movements raised Cretaceous rocks of Gulfian age to about 1,400 feet above sea level in the Salina basin.
Details of the post-Cretaceous structural history of the Salina basin are imperfectly recorded in Kansas by a series of terrestrial deposits of Tertiary and Quaternary age laid down on the eroded surface of the older rocks and also by a series of dissected alluvial benches along the major streams.
Plates 13 and 14 show the present attitude of the rocks along the lines X-X' and Y-Y' of the thickness maps. The exaggerated vertical scale of these cross sections shows distorted dips. The true rate of dip in feet per mile is shown by the insert diagrams.
As has been shown in the preceding pages, the present attitude of the rocks of the Salina basin is the result of structural movement at several different times. The folding which was thus brought about in the main by minor increments of deformation is expressed by the thickness of certain stratigraphic sequences. Each new structural movement modified the previous structure by warping and tilting the rocks in the same or different directions. The closure of some anticlines of low relief was greatly reduced and in some cases the position of the crest in the surface rocks was shifted by later regional tilting (Lee and Payne, 1944, p. 70, fig. 12, p. 79). When the original dips of an anticline are less than a subsequently imposed regional dip, the anticline may be reduced to a structural nose. In a somewhat similar way, an anticline in an upper sequence of rocks may overlie an unconformable sequence of rocks whose regional dip is too great to show closure (Lee, 1943, pp. 128-133); this is the explanation of the so-called "loss of closure" in drilling into deeper rocks.
The movements of fluids and gas in the rocks toward structural and stratigraphic traps must have been facilitated by the numerous structural adjustments by which both the local and regional structure were developed. These structural movements brought about many periods of exposure and erosion and it is probable that with each re-elevation of the rocks above sea level the connate water escaped or was redistributed and that migration of nascent oil and gas was materially affected by the erosion of the rocks as well as by the intermittent changes of elevation. Thus any consideration of the time and manner of the accumulation of oil and gas and their subsequent adjustment in the positions in which they are now found must take into account the geologic history of the rocks in which they are trapped.
The accompanying maps show the areal distribution of the formations at different periods and indicate the areas from which well-known productive zones have been eroded and will thus not be found in drilling. The maps and the cross sections show also the belts of overlap and beveling along which conditions are favorable for stratigraphic traps if the beveled edges of the rocks are porous and structurally closed.
The structural deformation that occurred prior to St. Peter time was not accompanied, so far as known, by the accumulation of oil and gas. However, so few wells have been drilled through the pre-St. Peter sedimentary sequence that any local structures are completely unknown, and the major structural features are revealed only in the most general way. Any local anticlines that may be present in the pre-St. Peter rocks may not correspond in location or character to the structural features of the younger rocks. The pre-St. Peter sedimentary rocks are productive of oil and gas on the Central Kansas uplift and in some places on the Chautauqua arch in Montgomery County and adjoining areas in southeastern Kansas. In these areas the anticlines in which the oil is found trend west and northwest, at right angles to the structural axes of known pre-St. Peter folds and they may have been initiated between St. Peter and Mississippian time, when a broad regional anticline extended west, from the Ozarks as the Chautauqua arch and continued northwest as the Central Kansas uplift.
Local structural features parallel to the Chautauqua arch are not known, however, to have been developed until the end of Mississippian time when productive anticlines trending parallel to this fold were formed in Montgomery and adjacent counties (Lee, 1939, pl. 1). These local anticlines were folded after regional movement of the Chautauqua arch had ceased. They are not strongly developed and, although there is no direct evidence, they are believed to represent the rejuvenation at the end of Mississippian time of folds originally initiated during the development of the Chautauqua arch.
The Central Kansas uplift itself began its upward movement shortly after St. Peter time, reached its period of maximum deformation before Hertha time, and continued to develop with declining intensity through late Pennsylvanian and early Permian time. Most of the productive anticlines on the uplift also trend northwest. The date of the initial movements of the anticlines cannot now be determined because the area was stripped of pre-Pennsylvanian and post-Lower Ordovician rocks by successive periods of exposure and erosion. It is probable, however, that such folds were being developed at the end of Mississippian time contemporaneously with the Salina basin.
During the development of the Salina basin, the Nemaha anticline and other structural features trending east of north were initiated and continued to develop contemporaneously with the northwestward trending folds. Northeasterly trending folds are prominently revealed on the thickness map of the Mississippian but only the most prominent anticlines formed after Hertha time are revealed by 50-foot thickness lines. It is significant that to the present most of the oil from anticlines paralleling the Nemaha axis has been discovered in the constricted area between the Central Kansas uplift and the Nemaha anticline.
The few wells in northern Lincoln County suggest anticlinal conditions there. The interpretation of the thickness of the Mississippian rocks for that area seems to indicate a northwesterly trending anticline but the thickness lines of the pre-Hertha Pennsylvanian rocks in the same area suggest a northerly trending anticline.
The central and northern part of the Salina basin have not been adequately tested. Only a few wells have been drilled on the Abilene anticline. The occurrence of small amounts of low gravity oil in one well on the part of the anticline in Clay County (sec. 21, T. 9 S., R. 21 E.) shows that oil occurs on the northeastern side of the Salina basin. The offsets of this well, however, were all dry. In an effort to determine the subsurface structure of this anticline it was recontoured, eliminating post-Permian regional tilting. The structure as thus restored revealed a considerable shift in the position of the low crests on the axis of the present anticline. The crest upon which the small Clay County well was drilled was thus shifted about three-quarters of a mile toward the northwest and the crest of the steeper subsurface structure thus does not occur beneath the crest in the surface rocks. The recontouring of this area therefore suggests that some of the wells drilled on the Abilene anticline have not been well located to test the deeper rocks.
Kansas Geological Survey, Geology
Placed on web Dec. 28, 2007; originally published Nov. 1948.
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