The beds in and near Russell County lie essentially flat and undisturbed, showing only a slight regional inclination, which differs in amount and direction in different beds and is interrupted by gentle folds and small faults.
Throughout western Kansas the dip of the surface rocks is in general northward, and in Russell County, where it is north-northeastward, it averages about 7 feet to the mile. [Darton, N. H.; The structure of parts of the central Great Plains: U. S. Geol. Survey Bull. 691; Pl. I; 1918.] In the northern part of the county the regional dip per mile is about 12 feet, but in the southern part it is only 5 feet.
The northward dip is not perfectly smooth, for it is modified by numerous local variations. Throughout Russell County there are low domes, anticlines, terraces, structural noses, synclines and basins, so that the facetious remark that is occasionally made by geologists who have worked in the region, that a "ten-foot closure can be. found in every section," is not so gross an exaggeration as it might seem. (As is discussed under the heading, "Economic significance of method of origin," very few of these small folds are thought to be favorable for the accumulation of oil.) Faults, however, are remarkably uncommon in Russell County.
Structural contour map of the top of the Greenhorn limestone--The accompanying structure contour map of Russell County (Plate IV) has been constructed almost entirely by combining the horizontal locations of the different beds sketched with the elevations shown on the topographic maps of the Russell, Osborne, Plainville, Ellsworth, Beloit and Hays quadrangles. The structure contour lines thus derived obviously cannot be correctly placed in detail. Where the regional dip is less than 10 feet to the mile, an error of 25 feet in the elevation of a key bed might cause a horizontal misplacement of a contour line of nearly 3 miles, and such an error could result (1) from uncertainty as to the surface elevation of a point between contour lines on a topographic map having a 20-foot contour interval, (2) from an inaccurate location of the outcropping strata in hasty reconnaissance mapping, and (3) from slight errors in the topographic maps. However, an attempt was made to smooth out the contour lines by using average key-bed elevations for each square mile, and rejecting even these averages if they stood above or below the surrounding area and were not verified by additional information. This smoothing and the interval chosen for the contours--50 feet--make the structure map a fairly reliable representation of the broader structure of the county. Comparisons with detailed maps prepared by company and independent geologists indicate that the map is essentially correct in its major features, but that many irregularities of small horizontal extent are omitted.
Prominent northsoard-irendinq anticlines--The folds in Russell County and its vicinity appear to be divisible into two more or less distinct types-the prominent, northward-trending anticlines near the east and the west border of the county, and the smaller, less systematic folds in the remainder of the area. The persistent northward trend of some of the larger folds in Russell and adjoining counties is striking. These larger anticlines, such as the Fairport-Natoma fold and the more or less connected structurally high area in Lincoln County, near the Russell County line, are topographically rather prominent, due to the fact that they stand somewhat above the general structural level. The eastern one is not a long, simple anticline, however, for it consists of a series of elongate domes separated by structural saddles. At some places, as in an area extending northward from a point near Gorham to the northwest corner of the county, and in an area that extends northward from Wilson through the western part of Lincoln County, these anticlines are bounded on the west by a parallel but rather discontinuous syncline. An additional peculiarity of these anticlines is their parallelism to a very irregular, structurally high area west of the bounding syncline, as in the northeast corner of Ellis County and in the area extending from Dorrance through Lucas, in eastern Russell County.
The Fairport-Natoma anticline--The best developed example of the larger folds in the vicinity of Russell County is the Fairport-Natoma anticline, which is traceable in a line that runs uniformly 8° or 10° east of north for at least 20 miles along the northern half of the western border of Russell County. The dips on the west side are steeper, the rocks commonly descending at rates of 50 to 200 feet to the mile into a parallel syncline in which the beds are 100 feet or more below their elevation on the axis of the anticline. East of the axis the beds dip 15 to 40 feet to the mile. Minor domes and depressions occur on top of the anticlinal axis and in the syncline. It is in a dome on this anticline that oil and gas have been found in Russell County.
Lesser folds--The characteristics of the prominent anticlines are in contrast to the structural habit of the remainder of Russell County. The regional north-northeast dip in the wide area between the prominent anticlines is interrupted very irregularly by numerous domes, terraces and basins that are in general of small horizontal extent and that have slight structural relief or closure and no definite trends or approach to systematic arrangement. The crests of these lesser folds appear to fall into a general structural level in a way that is comparable with the accordance of summit levels in a maturely dissected peneplain topography. The synclinal and anticlinal axes are somewhat continuous over the county, but in general they are very sinuous. However, a few of these lesser structural features show alignments, though no constant structural trend is recognizable. A persistent, even if discontinuous, anticlinal axis extends northeastward from a region near Gorham to. one north of Russell, and possibly to the vicinity of Lucas. A syncline trends westward from an area near the northeast corner of the county to one north of Luray. A low anticline that trends slightly east of north extends from Milberger, in T. 15 S., R. 14 W., to an area a few miles southeast of Russell. An elongation of both anticlines and synclines west-northwestward is recognizable throughout much of the northwestern quarter of Russell County. A fairly sharp anticlinal nose extends northeastward from near the southwest corner of the county for about 10 miles, (See Plate IV.)
Although the structural features in the county are divisible into the two types described, an almost perfect gradation between them may be found. In places the northward-trending axes appear to merge for miles into the homoclinal slope, as one of them seems to do south of Gorham, and the trend of one of the most persistent of the lesser folds, the one extending northward through townships 15 and 14 S., R. 14 W., is almost identical with that of the Fairport-Natoma anticline. The differences in the characteristics and the areal distribution of the two types, however, are so striking as to justify the division made, especially as these differences seem to indicate different conditions of origin. Furthermore, oil and gas have been found in paying quantities on one of the northwardtrending anticlines, but neither has been found (June, 1924) on the less prominent, unsystematically arranged folds.
Faults--Almost all of the faults that have been found in the vicinity of Russell County are in the Niobrara formation. Three parallel faults found in the Fairport member of the Carlile shale in the NW sec. 29, T. 11 S., R. 16 W., in Ellis County, 5 miles west of Russell County, strike S. 10° W., and are each downthrown a few feet on the western side. The fissures along these fault planes, which dip westward at an angle of less than 45°, are filled with veins of crystalline slickensided calcite several inches thick. The strata between these faults dip rather steeply. Farther west, in the area of outcrop of the Smoky Hill member of the Niobrara formation, faults that are similarly marked by veins of calcite are locally common and the associated beds show dips that are unusual in their direction and steepness. The faults trend in many directions. Sufficient observations were not made to justify any generalization regarding their alignment or their most persistent course. The clean exposures of brittle rock beds in the Carlile and Greenhorn formations indicate strongly that faults are not equally common east of the Niobrara outcrops. Possibly the boundary of the Niobrara outcrop chances to coincide closely with the margin of a separate structural province, but it seems much more likely that the faults are restricted essentially to the Niobrara rocks and that the lower formations have yielded to the fault-forming stresses by folding, not by breaking.
Attitude of Rocks Not Exposed
Logs of wells drilled in and near Russell County show that the deeper beds lie nearly flat, although these deeper beds, like those at the surface, show gentle regional inclinations, which differ in amount and direction because of progressive changes in the thickness of some of the beds.
Thinning of shallower rook units northeastward--The units of rock below the top of the Dakota sandstone, recorded in well logs chiefly as sand, red rock and shale, representing the Dakota and possibly some rocks of Lower Cretaceous age, decrease in thickness eastward across Russell County from more than 400 feet near the western border of the county to 300 feet or less near its eastern boundary. The "Red Beds," which underlie the Cretaceous strata unconformably, are cut into more deeply to the east and northeast, so that over 300 feet more of this series remains uneroded near the southwest corner of the county than in its northeastern part. These two units together decrease 600 to 700 feet in thickness from eastcentral Rush county northeastward to northwestern Lincoln County, a distance of 60 miles. The combined effect of this thinning of Mesozoic and greater erosion of Paleozoic strata eastward is to shift the regional dip in Russell County from north-northeastward in the Cretaceous rocks to westward or northwestward in the strata below the top of the Permian "Red Beds."
Map of the elevations of the base of the salt series--The accompanying map (Plate V) shows the elevation above sea level of the horizon usually recorded in logs as the base of the salt in those wells near Russell County that have been drilled to this depth. The elevations given for the mouths of wells are taken chiefly from topographic maps and are therefore only approximately correct. The logs of a number of wells in and near Russell County show a thin bed of salt from 25 to 50 feet below the thick, more or less unbroken salt unit, and in the logs of a few wells this lower bed or series of beds has apparently been included in a thick series recorded as entirely salt. For example, this appears to have been done in the M. M. Valerius Oil and Gas Company's Phillips No. 1 well, in sec. 3, T. 13 S., R. 13 W., and the base of the salt as recorded for this well is therefore probably too low. The wide, almost perfectly flat structural terrace or nose indicated in Russell County and in areas farther west presents a notable contrast to the relatively steeply inclined, westward dipping series of beds in western Lincoln and Ellsworth counties. [3. Wells drilled since this report was written indicate some errors in the location of the contour lines in Lincoln and Rice counties. The necessary changes are so small and the information is still so very incomplete that a revision of Plate V at this time seems unnecessary.]
Plate V--Attitude of base of salt series of Wellington formation in Russell and adjacent counties.
The regional inclination of the strata below the top of the "Red Beds" is essentially parallel to that of the base of the salt series, though a gradual westward thinning of 150 to 200 feet in a 1,500-foot section of the beds below the base of the salt makes the westward dip somewhat gentler in the lower beds. [Wells in and near Russell County that have been drilled recently indicate the presence of an unconformity 100 to 400 feet below the oil-bearing horizon. These wells indicate that from the producing field the early Paleozoic rocks slope gradually eastward about 10 or 15 feet to the mile to the eastern part of the county, and that they fall on an average of about 30 feet to the mile north ward along the axis of the Fairport-Natoma anticline to the north line of ihe county, about 15 feet to the mile southward to the southwest corner of Russell County, and perhaps more than 100 feet to the mile westward for three miles.] The records of wells that are near together, however, show significant differences in the thickness and structure of the buried rocks.
Increasing steepness of dip with depth below surface--The logs of the first four wells drilled in secs. 8 and 17, T. 12 S., R. 15 W., on the Fairport-Natoma anticline, show a progressive divergence of the buried strata that results in steeper dips on the successively lower beds. The westernmost of these four wells, the Stearns and Streeter Company's Ed Oswald No. 1, in the SW SW of sec. 8, starts at a point that is about 15 feet higher structurally than the Valerius Oil and Gas Company's Jesse Muncell No. 1 well, in the NW NE of sec. 17, less than half a mile to the east. The top of the 40- to 50-foot bed of anhydrite encountered at a depth of about 900 feet, however, is about 32 feet lower structurally in the southeast than in the west well. The top of a 20- to 30-foot series of fed rock or red shale that was found at depths ranging from 1,624 to 1,659 feet in these four wells shows an eastward dip that makes the bed about 49 feet lower in the Valerius company's well than in the Stearns and Streeter well. The top of a series of beds recorded as red rock or red shale at depths of 2,057 to 2,116 feet is about 73 feet lower structurally in the southeastern than in the western well. The bed of rock 4 to 10 feet thick, recorded as sand, lime or sandy shale, and as yielding water or showings of oil, occurring from 2,590 to 2,658 feet in these wells, is about 82 feet lower in the Valerius than in the Stearns-Streeter well. That is to say, a divergence of about 65 feet occurs rather uniformly through a stratigraphic interval of 2,600 feet in a horizontal distance of less than half a mile. The resulting increase in the steepness of dip with increase of depth is a feature' that is common in the oil fields of the Midcontinent region.
By W. W. Rubey
The origin of the structure of an isolated oil field cannot be determined, nor even satisfactorily discussed, until a great deal of drilling has been done in the region; that is, no final statements can be made about the origin of its structure until a field has passed its period of maximum productivity. However, many facts bearing on the manner in which the deformation occurred are known early in the history of a region, and additional inferences may be made by comparison of the region with near-by, similar and better-known fields. Since a knowledge of the origin of the structure of a field is an extremely valuable guide to the prospecting of it for oil, it is desirable to bring together these scattered bits of evidence and to formulate whatever theories seem justified before complete information is available. Theories so formulated, even if erroneous, are valuable in that they point out critical observations that can be made later.
The origin of the structural features in Russell County is probably closely related to the origin of similar features elsewhere in the Midcontinent oil field, for both geographically and in the structure of its surface rocks, Russell County seems to be a part of that field. As sufficient drilling has not been done to determine the structure of the buried rocks in Russell County, it is helpful in discussing the method of formation of the folds there to make use of the more complete data that have been gathered and partially analyzed in other parts of the Midcontinent field.
The widespread presence in the Midcontinent oil field of minor folds at great distances from areas of orogenic disturbance has given rise to much speculation regarding the origin of these folds. Simple horizontal compression has been generally considered inadequate to form them, because of the incompetency of the soft shale beds, in which the folds are usually developed, to transmit thrusts for these great distances. The explanations of Midcontinent structure that seem most plausible may be mentioned briefly, and their possible application to the structure of Russell County considered.
Conformity of Strata to the Shape of Buried Land Forms
Blackwelder, [Blackwelder, Eliot; The origin of the central Kansas oil domes: Am. Assoc. Petroleum Geologists Bull., vol. 4, pp. 89-94; 1920.] Mehl, [Mehl, M. G.; The influence of the differential compression of sediments on the attitude of bedded rocks: Science, new ser., vol. 51, p. 520; 1920 (abstract).] and more recently, Powers, [Powers, Sidney; Reflected buried hills and their importance in petroleum geology: Econ. Geology, vol. 17, pp. 233-259; 1922.] have developed a theory that surface folds may originate by the deposition and consolidation of sediments over an old, eroded, irregular surface. The idea common to the three authors cited is that some deposits, notably those of mud, are much more compactible than others. Thus, according to the theory proposed, beds of mud deposited around a hill of hard rock--granite, limestone or sandstone--would be more greatly reduced in volume by the pressure of beds subsequently deposited upon them than would the harder rocks, and the beds above the hill, therefore, would form an anticline or dome that would conform in shape with the buried mass of hard rock. Satisfactory data on the amount of cubic compressibility of mudstones are apparently lacking, and quantitative tests of the adequacy of the process outlined, though highly desirable, cannot readily be made. However, the theory merits consideration, for it accounts for features that are not readily explainable otherwise, and as deeper wells are being sunk in many oil fields from Texas to Kansas, evidence is accumulating to strengthen it.
If this process produced anticlines over large buried hills of hard rock, it should likewise produce, although less conspicuously, a reflection of the entire buried topography, for few hills are truly isolated, and in some Iand surfaces, valleys that lie below the general level cause greater local relief than do hills and ridges. It would therefore appear that structural features caused by the unequal settling of sediments over an irregular surface should show in a broad way, though of course with gentler slopes, the configuration of the old surface. If the old surface had been more or less reduced to slopes, broadly sinuous anticlinal axes-the old divides-from which more or less elongated structural noses extend, should be definitely recognizable if sufficiently large areas are studied. Stream valleys should appear as intricately curving and repeatedly branching but gentle synclinal axes. Domes, generally more or less elongate, would commonly be found on the anticlinal folds over the junctions of the primary, secondary and tertiary stream divides, but, if no other factor had been operative in the formation of the structure, basins or "closed" synclines should be very rare. Wells drilled in areas where the structure was so formed would find the deeper beds dipping more and more steeply and more nearly in conformity with' the buried land surface, but the form or structure of the beds beneath the unconformity would have little relation to that above.
Effect of Buried Lenses of Sandstone
A modification of the theory of settling, held by Gardescu and Johnson, [Gardescu, I. I.; and Johnson, R. H.; The effect of stratigraphic variation on folding: Am. Assoc. Petroleum Geologists Bull., vol. 5, pp. 481-483; 1921.] and others, [Monnett, V. E.; Possible origin of some of the structures of the Midcontinent oil field: Econ. Geol., vol. 17, pp, 194-200, 1922.] is that lenses of some less compactible rock, such as sandstone, would, by the greater consolidation of the surrounding shale, be reflected as anticlinal ridges in the structure of the overlying beds. Interfingering of sandstone and shale is commonly observed in the younger Paleozoic sediments of the Midcontinent oil field at their outcrops, and well logs show that similar lensing occurs in beds not exposed.
The shape and arrangement of folds formed by the compacting of sediments over lenses of sandstone should be governed by that of the lenses: (1) Lenses made by stream deposition would be elongate, generally curving, occasionally branched or braided, and would vary in thickness because of changes in the course or rapidity of flow of the stream. The structure developed over such a body of sand, according to this theory, should be a sinuous, elongate, but more or less discontinuous anticline from which but few subsidiary axes (the tributaries) extend. Because of variations in thickness, small, elongate domes and basins should be superimposed upon the anticlinal axis. If no other factors were at work, the ate a on both sides of the anticlinal axis should be structurally flat. (2) Sandstone lenses formed in the sea by offshore bars would be extremely elongate, very straight (except for occasional sharp offsets and terminal curves), and, if restricted to one horizon, relatively narrow. Anticlines formed over such beds of sandstone should be nearly unbroken for miles and should have few irregularities along the crest line. (3) A lens of sandstone deposited at a shore line would be elongate in a more or less irregular line, and whether derived from streams or caused by local weathering, would become gradually and unequally thinner seaward and along the shore from its source. Structure resulting from the consolidation of beds over such a body of sand should show a gentle terrace, merging into a flat area on one sidethe seaward-and possibly to structure characteristic of that formed over a buried land surface on the other. Few irregularities should occur on the very gentle, broad fold formed on the lens of sandstone of this origin. (4) The relatively extensive deposits of sand formed by tidal or current sweeps would not be likely to result in folds sufficiently acute to be discernible in surface structure. (5) Windblown sandstones accumulated on land would be so rarely preserved as thick detached masses that lenses thus formed may be disregarded in the present discussion.
An important characteristic of any folds that might be formed over lenses is that the structure below the horizon of the lenses would be relatively flat; no corresponding anticline would be found deeper.
Structure in Surface Rocks Formed by Movement Along Deep-Seated Faults
Fath has called attention to the regional alignment of en ichelon faults, of folds, and of the buried "granite ridge" of eastern Kansas, and has pointed out that recurrent horizontal and vertical movements along buried fault planes or shear zones in more competent rocks would account for these features. [Fath, A. E.; The origin of the faults, anticlines and buried "granite ridge" of the northern part of the Midcontinent oil and gas field: U. S. Geol. Survey Prof. Paper 128; 1921.] Fath's explanation of the en echelon faults, which is the key to his interpretation of the other features, follows R. T. Chamberlin's conception of the origin of asimilar series of oblique faults in south-central Montana. [Chamberlain, R. T.; A peculiar belt of oblique faulting: Jour. Geology, vol. 27, pp. 602-613; 1919.] Briefly, it is that horizontal differential movement will produce cracks at angles of about 45° to the direction of the movement. Fath's major contribution to the idea was his recognition that the deeply buried, more competent rocks would transmit a horizontal thrust for great distances and that planes or zones of weakness in these rocks would serve to localize the effect of the differential movement and form a series of parallel faults. The folds he interpreted as the effect of slumping of higher soft beds over minor fault planes, that, starting just above the more competent rocks, did not reach the surface. Subsequent investigations in Osage county, Oklahoma, and elsewhere have further verified the facts demonstrated by him and strengthened the validity of his major premises. [By members of the U, S. Geological Survey, the writer among others.]
Fath also carried his conception of zones of weakness in deeper rocks to the parallel "granite ridge" of Kansas, pointing out that the great length of this ridge--more than 170 miles--and its straightness could more plausibly be explained as a result of faulting or slipping along zones of weakness in pre-Pennsylvanian rocks than of folding or of purely erosional development of the mountains before younger deposits buried them. Moore, in a summary of the known facts regarding the "granite ridge" of central Kansas, which he calls the Nemaha mountains, concludes that the igneous rocks could not be intrusive, but that the younger beds must overlie the crystalline rocks unconformably. [Moore, R. C.; Geologic history of the crystalline rocks of Kansas: Am, Assoc. Petroleum Geologists Bull., vol. 2, pp. 98-113; 1918.] The evidence indicates that this unconformity is probably post-Mississippian and pre-Pennsylvanian. According to these interpretations, the ridge was formed by the faulting of early Paleozoic beds of limestone, which were deeply eroded before the Pennsylvanian sediments were laid down upon them. The major lines of weakness postulated by Fath might possibly have been formed at the time of this faulting, but the great extent of the ridge and its great distance from areas of pronounced orogenic activity in Mississippian time make it appear more probable that such, lines were first blocked out during a time of great widespread disturbance--perhaps pre-Cambrian--and that regional stresses at the close of the Mississippian caused the early Paleozoic rocks to break along these old lines.
The series of faults in northern Oklahoma, along which slight movements have obviously occurred since the Pennsylvanian strata were deposited, led Fath to suppose that the folds at the surface over the "granite ridge" were formed by repeated movements along the major zones of weakness. By postulating these recurrent later movements and local unconformities, he seeks also to explain the progressively greater steepness of the beds of rock on the anticlines with increase of depth.
More recent drilling in Kansas, Oklahoma and Missouri indicates that other "granite ridges" lie parallel to the Nemaha mountains and the oblique fault belts in northeastern Oklahoma. Wells drilled near the Missouri-Kansas line have found igneous rocks above their expected depth from Miami, Okla., north-northeastward to a point less than 75 miles south of Kansas City, Mo., and scattered wells drilled farther east in Missouri have also reported granite at unexpectedly high elevations. In northeastern Oklahoma granite has been reported in a number of wells that are so located as to suggest several roughly parallel ridges extending (1) from Muskogee to Vinita, (2) through Rogers' and Nowata counties, (3) from a point south of Tulsa past Bartlesville to a well near lola, Kan., and (4) from the Cushing oil field to Hominy. Other wells drilled into granite east and west of the Nemaha mountains in Kansas suggest the existence of other parallel ridges.
R. C. Moore, in a forthcoming paper, considers the relation of the "granite ridge" to the structure of the overlying rocks and calls attention to certain structural features of the outcropping beds that seem to mark the position of the buried ridge. He points out that the parallel line of similar structural features in the outcropping beds from a point southwest of Eureka, Kan., nearly to Frankfort, Okla., may also overlie a buried ridge.
The scattered granite wells, combined with the evidence of the line of steeply dipping surface rocks and the belts of en echelon faults, indicate more than a half-dozen long parallel ridges or deepseated faults in eastern Kansas, northeastern Oklahoma and western Missouri.
Several bits of evidence indicate that long and straight northwardtrending lines of deformation exist west of eastern Kansas in many parts of the Great Plains area. Rich, [Rich, John L.; A probable buried mountain range of early Permian age east of the present Rocky Mountains in New Mexico and Colorado: Bull. Am. Assoc. Petroleum Geologists, vol. 5, pp. 605-608; 1921.] calling attention to the narrow northward-trending area of pre-Cambrian outcrops south of Pedernal mountain and east of the southern prolongation of the Front Range in central New Mexico, to a few wells in northeastern New Mexico that were reported to have struck granite, and to small outcrops of gneissoid granite in southeastern Colorado, suggests that the large northeast-trending anticline in northeast New Mexico and southeast Colorado may overlie the northward buried continuation of the mountain range exposed in the vicinity of Pedernal mountain. [Darton, N. H.; Geologic structure of parts of New Mexico: U. S. Geol. Surv. Bull. 726, fig. 16; 1922. Darton, N. H.; The structure of parts of the central Great Plains: U. S. Geol. Survey Bull. 691, pl. 1; 1918.]
In a discussion of a paper by W. J. Miller, entitled, "Pre-Cambrian Folding in North America," Miss Margaret Fuller states that a recent detailed field study of the pre-Cambrian geology of the Front Range in Colorado discloses the presence of numerous batholiths, which have arched and metamorphosed the sediments in a long northward-trending belt, and suggests that the recent arching took place along the old pre-Cambrian line of diastrophism. [Bull. Geol. Soc. America, vol. 34, No.4, pp. 700-701; 1923.] In eastern Wyoming, north of the Laramie mountains, the eastern margin of the Hartville uplift, the Old Woman anticline, and a steep monocline on the west side of the Black Hills, are prominent structural features trending slightly east of north that are apparently aligned. [Darton, N. H.; The structure of parts of the central Great Plains : U. S. Geol. Survey Bull. 691, pl. I; 1918. Darton, N. H.; U. S. Geol. Survey Geologic Atlas, Newcastle Folio (No. 107), fig. 3; 1904.] The pre-Cambrian rocks exposed in the Black Hills in South Dakota, are reported to have a cleavage that trends northward except where it has been destroyed by later intrusions. [Van Hise, C. R.; The pre-Cambrian rocks of the Black Hills: Geol. Soc. America Bull., vol. 1, pp. 209, 231-233, 241; 1890.]
It is noteworthy in this connection that the broad northward-trending anticline mapped by Darton [Darton, N. H.; The structure of parts of the central Great Plains; U. S. Geol. Survey Bull. 691, pl. I; 1918.] in western Kansas may extend northward instead of turning northwestward as shown, for Darton admits there is little evidence for connecting the' fold on the Republican river near the southern border of Nebraska with the Chadron anticline in the northwestern part of the state. [Darton, N. H.; The structure of parts of the central Great Plains; U. S. Geol. Survey Bull. 691, p. 12; 1918.]
Folds formed largely by readjustments along buried tectonic lines would probably possess some distinctive characteristics. Simple vertical movements would produce faults in the strata immediately above the buried lines, but soft shale beds would cause small faults of this type to be dissipated upward and expressed only as steep dips. Simply vertical movements would produce no anticlines at the surface; it would form only a monocline-a narrow line along which the beds dip steeply in one direction-with flat-lying beds on both sides. If a narrow block between two fracture lines were uplifted, the surface result would probably be a broad, flat anticline with abrupt and steeply dipping limbs. Vertical movement, accompanied by a tilting of the blocks between the major fault zones, however, might produce anticlines and synclines with dips much steeper on one limb than on the other. Horizontal movement along the tectonic lines would produce minor faults, many of which would not reach the surface, and above them folds might be formed in the manner that Fath suggests. These folds, like others formed along buried lines, would have one limb steeper than the other; they would be elongated at angles of about 45° to the buried line of weakness, and should occur en echelon. Pressure folds, likewise running at angles of 45° to the buried line, but lying at right angles to the commoner faults and folds produced by tension and slumping, might also be produced, but they would probably be smaller and more inconspicuous in the higher beds. Finally, none of the structural features thus produced would necessarily be long and continuous, for the amount and direction of movement along the buried tectonic lines, as Fath points out, might vary greatly within rather short distances.
Folds Formed by Leaching of Salt Beds
Leaching of easily soluble beds might cause slumping of the overlying beds on a scale sufficiently large to form folds. This mode of origin cannot be ascribed to the folds in most of the Midcontinent oil fields, for no easily soluble beds are found there in sufficient thickness to make the effects of this leaching significant. In western Kansas, however, a persistent series of beds of salt, from 200 to 400 feet thick, underlies the surface rocks. The high solubility of salt and the saltiness of most of the ground waters in that region suggest that the thinning of this series of beds by partial solution might there produce features of the structure. Basins that have sunk several feet within historic time, in Meade county, Kansas, have been thus accounted for by some writers. [Coffey, G. N., Rice, T. D., and others; Reconnaissance soil survey of western Kansas: U. S. Dept. Agric., Bur. Soils, p, 10; 1912.] Unsaturated water capable of dissolving the salt out of these beds would presumably come from near-lying aquifers or would seep down from the surface. It would probably find access to the salt beds only in faulted zones and at places where porous beds lay in contact with them.
Logs of wells in central Kansas show only slight variations in the thickness of the salt beds, whether the wells are or are not on anticlines--a fact that tends to weaken the theory. This process is not regarded as impossible, but it is probably not of great importance nor of wide applicability in western Kansas. The structural features developed by this. means should be wide flats, interrupted locally--or possibly in zones along faults--by basins or closed synclines.
Application of Principles to Structure of Russell County
The regional dip of the rocks in Russell County may be the result, of broad deformation or tilting, but it is believed that the folds superimposed upon the regional homocline must have been caused by processes other than horizontal compression.
Gentle, unsystematic folds--The irregularity of trend of the axes of the folds throughout the greater part of Russell County suggests a mode of origin other than by horizontal compression, localization along buried tectonic lines, or consolidation over long lenses of sandstone formed as offshore bars, all of which methods of origin would probably form long, straight folds. It does not seem that the larger, at least, of these unsystematic folds can be explained as due to the compacting of sediments over local lenses of sandstone or the leaching out of salt beds. Rather, the sinuous and branching character of the gentle anticlines and synclines suggests that they may be largely due to unequal consolidation of the beds laid down on an old land surface.
According to this explanation, the structurally flat areas in the southern part of the county would be the surface reflection of a fairly smooth buried surface, on which the valleys and ridges ran in general northeastward or northward, The more steeply dipping beds in the northern part of the county would have derived their inclination from a more intricately dissected surface, on which most of the stream valleys and interstream divides extended eastward. The direction or directions of slope of these two hypothetical surfaces is unknown. Similar studies of adjacent counties may show whether the valleys enlarge to the northeast or southwest, or in both directions.
Basins or closed synclines, such as those in the northeastern part of Russell County and many smaller irregularities of structure not shown by the generalized contour lines of Plate V, seem to demand another explanation, however. Sandstone lenses that lie above and are elongated across buried valleys might produce in the overlying: rocks anticlinal folds that cross the "valley" synclines and thus give rise to closed basins on their "upstream" sides. The thicker parts of the Rocktown channel sandstone member of the Dakota sandstone, though perhaps never deeply buried, coincides in position with structurally high areas in central and east-central Russell County in such a way as to suggest that they may have modified somewhat the structure that was produced chiefly by other agencies. Sandstone lenses in the more deeply buried Pennsylvanian sediments would produce similar and perhaps more acute effects. Slumping over dissolved salt beds might also form some basins.
Prominent, northward-trending anticlines--The foregoing theory of origin may apply to the unsystematic gentle folds that are widely scattered over most of Russell County, but the prominent northward-trending structural features call for a different explanation. The linear trend and the length of these folds suggest that they may have been caused by horizontal compression of the rocks or consolidation over deposits made by offshore bars, or that they are in some way related to buried zones of weakness, such as those considered by Fath. [Fath, A. E.; The origin of the faults, anticlines and buried "granite ridge" of the northern part of the Midcontinent oil and gas field: U. S. Geol. Survey Prof. Paper 128, pp. 75-84; 1921.]
It seems to the writer that horizontal compression will not satisfactorily account for such structural features as the Fairport-Natoma anticline, for it is hundreds of miles from areas of pronounced folding, and there are no series of folds of equal or greater size in the areas intervening. The structural relief across the Fairport-Natoma anticline, amounting to 100 feet or more, makes it difficult to accredit the offshore bar theory highly. In the absence of exact data it may not be unjustified to assume that, in a section of 4,000 feet of strata, shale would be compacted to 80 per cent of its original volume, and that under similar pressure sandstone would remain relatively uncompacted. To form anticlines and synclines having a difference in elevation between crests and troughs of 100 feet, a sandstone lens would need to be 400 feet thick--an improbable thickness for such a migratory and shallow-water shore form as an offshore bar.
The most probable cause of the Fairport-Natoma anticline may be found in a relation to a buried tectonic line. The prominent folds in Russell County trend in a direction that is nearly identical with that of the en echelon fault series of northeastern Oklahoma, the "granite ridge" or Nemaha mountains, more than 100 miles to the east in Kansas, and other possibly related major structural features mentioned under the heading, "Structure in surface rocks formed by movement along deep-seated faults."
Raymond C. Moore, in an unpublished study of the detailed structure of the surface beds overlying the Nemaha mountains, has found that the domes and short anticlinal folds that lie near the crest of the ridge are parallel to it, and that gentle, variable westward dips and a continuous line of steep eastward dips mark the position of the ridge. Except that the Fairport-Natoma anticline is steepest on its west instead of its east side, its structure is similar to that of the surface beds overlying the "granite ridge."
Fath discussed two principal types of relation between the deeper lines of weakness and the structure of the surface rocks-slumping of the higher beds over the minor faults produced in intermediate strata by horizontal movement along the buried lines, and conformity of the surface beds to vertical displacements along the deep-lying zones of weakness themselves. The first process should produce a series of short folds elongated at angles of about 45° to the lines of weakness in the deeper beds. This type of structure is not that of the Fairport-Natoma anticline, though it is suggestively similar to that of the northward-trending line of domes and basins in western Lincoln County near the Russell County border, which may have been so formed in part. Slumping of the surface beds over a deep-lying zone of weakness would more probably cause a long, straight anticline such as the Fairport-Natoma anticline in the higher beds, but in order to produce this anticlinal structure the fault would need to be paralleled by other near-by faults, for if isolated it would be likely to produce only a narrow monocline at the surface. Gentle tilting of the blocks between several such fault lines would form long anticlines and synclines in the higher beds; one limb, overlying the fault, dipping gently, and the other, overlying the tilted surface, relatively steeply--a form most nearly agreeing with that of the Fairport-Natoma anticline.
However, the explanation cannot be as simple as this. The east limb of the Fairport-Natoma anticline dips from 25 to 40 feet to the mile in the surface rocks, and is steeper below, but the average inclination of the beds from the crest of this anticline eastward for 30 miles to the next hypothetical large fault zone is only 10 feet to the mile; that is, the eastward dip of the surface rocks on the anticline is at least twice as great, and in the deeper beds more than ten times as great, as that which would be caused by merely regional tilting between two fault lines. This steeper dip may be due to a bending upward of the upthrown edge of the block between the faults, or to local irregularities either on the surface of the tilted block or in the beds above.
The upthrown edge of the block would probably be bent upward only if the region were under horizontal compression-a condition that seems improbable. Local irregularities in the beds above the old surface of the tilted block that might account for the steeper dip would be either a long, straight lens of sandstone lying exactly along the upthrown edge of the block, or a narrow area in which the salt beds had been partially dissolved parallel to and east of the edge of the block. Of these two possibilities, an area of leached salt beds east of the edge of the block seems the more plausible, but neither explains these steeper dips as satisfactorily as would local irregularities on the buried surface of the tilted block. A more or less curving synclinal axis east of the anticline (Plate V) is the complement of this steeper eastward dip on the side of the anticline, and such a syncline may be interpreted as the surface reflection of a buried stream valley.
This interpretation, which is in 'harmony with the explanation offered for the gentle unsystematic folds in Russell County, suggests three aiternative explanations of the origin of the Fairport-Natoma anticline: (1) that a land surface on which valleys and ridges had been developed and which was buried beneath younger sediments was at some later time faulted; (2) that a land surface was faulted after valleys and ridges had been developed but before the overlying beds were deposited; or (3) that a land surface was faulted and afterwards valleys and ridges were' developed upon it. In the first alternative the valleys would show no relation to the fault zone and erosion would not have had an opportunity to modify the upthrown edge of the buried block. In the third alternative those valleys near the fault zone would tend to be elongated either parallel or at right angles to it, and the buried fault scarp would have been cut into by erosion. Long-continued erosion after the faulting would make the second alternative essentially the same as the third.
The synclines in Russell County and its vicinity appear to follow the directions streams would take on a land surface that had been previously faulted and slightly tilted. The synclines immediately west of the two prominent north-trending folds occupy positions where consequent valleys would be expected to develop--at the foot of fault scarps--and the curving syncline east of the Fairport-Natoma anticline suggests a valley so situated as to drain the hypothetical fault scarp ridge. As would be expected under this interpretation, the synclines and anticlines in the remainder of the county show no close relation to the assumed fault scarp.
No direct evidence is available regarding the possibility that erosion modified the upthrown edge of the buried block that is assumed to have formed the Fairport-Natoma anticline, but analogy with the Nemaha mountains in eastern Kansas, upon which erosion seems to have acted before Pennsylvanian deposition, likewise suggests that in Russell County streams modified an old land surface that had been previously faulted.
The interpretation that seems most consistent with the facts is that the Fairport-Natoma anticline was formed, not by movement along a buried fault, but chiefly by consolidation of sediments over a buried fault scarp.
Probable origin of structure of Russell County--Summarizing the foregoing assumptions and conclusions, the structure in the Russell County region may possibly be explained as follows:
Block faulting or differential movements between blocks of rigid pre-Cambrian rocks capped by Ordovician or Mississippian sediments took place along preexistent fault lines or sheer zones ,and exposed to erosion a surface of gentle slopes and steep fault scarps. The displacement along the faults varied from place to place, ranging from several hundred feet, as in the northwest corner of Russell County, to nearly nothing, as south of Gorham. Streams flowing on this land surface cut deeper the original depressions at the base of the fault scarps, and probably shifted backward, but did not eradicate the cliffs. Meanwhile streams developed upon and carved their channels into the tilted uplands, leaving the divides near the top of the fault scarps the most prominent features of the topography.
The sea later encroached on this old land surface and covered it with sediment. After the deepest and most irregular depressions in the sea bottom had been filled, the layers of sediments deposited conformed more or less closely to the form of the old buried surface. Later the deposits were shifted by waves and currents until the bottom of the sea became nearly horizontal; that is, a greater thickness of sediments was deposited around than upon the former hills and ridges. Occasional partial withdrawals of the sea may have permitted erosion of the sediments on the higher areas over the buried hills. With the increase in the thickness of the sediments, the lower mud deposits were continuously squeezed to a smaller volume, but the old hard rocks below the plane of unconformity underwent little or no corresponding compression. The result was nearly the same as if buried hills and ridges had been thrust upward into the overlying strata, the higher hills and ridges in the harder rocks serving to localize the bending and consequently the slight stretching and thinning of the overlying strata. Minor faults and fractures. produced at the fault scarp by this settling died out in the overlying beds. Thus the deposition of thicker sediments around the higher areas on the buried surface, and the stretching, and in some places local erosion, of the overlying strata above these higher areas, might combine to account for the uniformly increasing dip of the beds downward toward the plane of unconformity.
If this theory of origin is correct, granite mayor may not be encountered by wells drilled deeper on these anticlines, but hard limestone, sandstone, igneous or metamorphic rock will probably be found at much higher elevations under the crests of the folds than in the corresponding synclines. [Since this report was written granite or arkose was encountered at a depth of 3,628 feet in the Producers and Refiners Oil Corporation Haise No. 1 well, NW SE sec. 24, T. 12 S., R. 15 W., Russell County, six miles east of the Fairport-Natoma anticline.]
Economic Significance of Method of Origin
If the foregoing conclusions are. even approximately correct, some inferences may be made that have a direct economic bearing. The anticlinal folds formed over buried hills and ridges should become progressively steeper with increase of depth until the plane of unconformity is reached, and the effectiveness of the folds as traps for the accumulation of oil and gas should likewise increase downward until this plane is reached. The folds formed over buried scarps should develop steeper dips and sharper anticlines with increase of depth than would those formed over gentler-sloping ridges and hills. Accumulation of oil and gas in commercial quantities is more probable in an anticlinal trap immediately over a buried scarp than in the higher beds or in the smaller folds formed over gentler hills, because in differential compacting, local deformation of the sediments is likely and the faults and fractures formed increase the porosity and number of avenues for the migration of oil and escape of gas. The plane of the preexistent major fault itself, if the unconformity lay above Paleozoic sediments, might be a path of upward migration of older oil and gas, and minor movements along it after burial would develop other stresses and fractures that might be favorable to the origin and accumulation of petroleum. Thus the chance of finding oil in the Fairport-Natoma anticline probably increases with depth, at least until the plane of the unconformity is reached. [Wells drilled recently suggest that an unconformity separates early and late Paleozoic sediments in Russell County and that this unconformity in the producing field is probably about 100 feet below the oil-bearing horizon. Beds yielding water or small amounts of oil or gas have been found in the 100 feet below the producing horizon in wells throughout the county, and it is possible that the lower beds may be found to yield oil on the Fairport-Natoma anticline.]
Structural features developed by the consolidation of sediments about sandstone lenses that lay near the surface, and by slumping. at places over dissolved salt beds, would be unfavorable to the accumulation of oil and gas in all but the shallower sands. Basins or "closed" synclines, unless in part the result of deeply buried lenses of sandstone, would be a criterion of these unfavorable types of structure.
It is therefore concluded that the prominent anticlines that show steep dips and are developed more or less continuously along the persistent northward-trending lines of deformation in western Kansas are the most promising places for tests for oil and gas. The recognition of such a fold, however, by no means assures the discovery of oil, for the almost unpredictable factors--an adequate source of petroleum, suitable paths of migration, porous reservoir rock, and an impervious cover--must be combined to produce an oil field. Folds that lie along but at an angle to lines of deformation, and that are steeper on one limb than on the other, should be almost equally favorable. The gentler folds of no well-defined trend that occur at varying distances from the northward-trending lines are much less likely to yield oil. Other things being equal, the larger and the steeper-dipping of these gentler anticlines should be the more favorable for tests. Least promising of all the structural features in the Russell County region are the anticlines far from the regions of more intense deformation, which are inferred by their association with structural basins, by observations of outcrops, or by well logs, to be the result of compacting of sediments over shallow-lying sandstone lenses or of slumping of the strata into places from which beds of salt have been dissolved.
Kansas Geological Survey, Geology
Placed on web March 17, 2014; originally published 1925.
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