The Jewel Cave SW quadrangle is on the southwest side of the broad domal Black Hills uplift. The sedimentary rocks in the area generally dip from 2° to 4° SW. Structural features superimposed upon this regional dip are: the east-northeast-trending Dewey fault, two northwest-trending anticlines, contortion of bedding and other minor structures resulting from subsidence, and minor structures in the Permian and Triassic rocks that have probably resulted from sliding of the rocks parallel to the dip.
The most conspicuous structural feature in the quadrangle is the rather sinuous Dewey fault that extends across the center of the quadrangle; it has a trend of about N. 75° E. The fault is known to extend at least 6 miles to the southwest (Brobst, 1961, p. 51), but its northern extension is unknown. The fault is exposed at only one place, the gully. in the SW1/4NE1/4 sec. 10, T. 6 S., R. 1 E., and here it seems to be vertical. Elsewhere the fault can be recognized by the steep dips in the beds along it and by the omission of strata on the south side. The rocks on the south side of the fault have apparently been displaced downward. The amount of displacement varies, but the maximum displacement is near the west side of the quadrangle. The movement has been accomplished by separation along the fault and by bending of strata adjacent to the fault. The cross sections of figure 49 illustrate the apparent displacement. Along section BB' the total stratigraphic displacement, including that caused by bending, is about 440 feet; and along section DD' the total stratigraphic displacement is about 250 feet. Because of the rather sinuous trend of the fault, it seems most likely that the major component of movement was parallel to the dip of the fault.
Two broad, gentle anticlines are present in the quadrangle. The southernmost anticline extends from sec. 2, T. 7 S., R. 2 E. in the Edgemont NE quadrangle southeast of the Jewel Cave SW quadrangle to sec. 34, T. 5 S., R. 1 E. in the Jewel Cave SW quadranglea total distance of about 10 miles. Throughout this distance the trend of the axis of the fold, which changes from about N. 50° W. at the south to about N. 40° W. at the north, is very, nearly parallel to the regional strike of the formations. The northernmost of the two anticlines extends for only 3 miles between Hell Canyon and Tepee Canyon. The trend of the axis is about N. 45° W., roughly parallel to the regional strike. The dips on the limbs of these folds are small; they range from about 6° to 13°. At several places the northeast-dipping limbs are somewhat steeper than the southwest limbs.
There are several areas of closure on the folds. On the northern fold the closure is about 110 feet. The axis of the southern fold, although nearly horizontal, undulates gently and produces long areas of closure. Southeast of the Jewel Cave SW quadrangle, in the Edgemont NE quadrangle, oil has been produced from the lower part of the Minnelusa Formation on the closure that marks the south limit of the fold. The center of sec. 7, T. 6 S., R. 2 E. is the crest of an area of inferred closure of about 30 feet on the base of the Sundance Formation. In this area rocks of the upper part of the Spearfish Formation crop out, but are very poorly exposed; thus the delineation of the shape of this part of the structure is uncertain. Just north of the Dewey fault there is a closure of at least 120 feet on the top of the Minnekahta Limestone; the closure is due largely to drag along the fault. The casing of an abandoned well (listed by the Conservation Division, U.S. Geol. Survey, as "L. Gokel, Pass Creek well, drilled 1935, abandoned Jan. 1936, T.D. 1035") is visible on the crest of this part of the structure.
In the Jewel Cave SW quadrangle many minor structural features were caused by the subsidence of overlying units when large volumes of soluble sediments were dissolved, principally from the Minnelusa Formation. N. H. Darton mentioned the presence of breccias in the Minnelusa at several localities in the Black Hills, but he did not give any interpretation of them. In 1930 F. H. Brady4 described the transition from an area containing about 150 feet of gypsum to an area of brecciation in the northern part of the Black Hills, but he did not explain the origin of the breccias, and he interpreted the termination of the gypsum beds as a primary depositional feature. Breccias in the Minnelusa in the southern part of the Black Hills were ascribed to tectonic forces by P. M. Work.5 The breccias, which are present in the type section of the Minnelusa along Rapid Creek, were interpreted by D. C. Boardman6 as having originated by leaching of anhydrite from the formation. J. P. Gries (1952, p. 71) expressed the view that:
In a recent publication, R. L. Bates (1955) describes breccias both in the Black Hills and in the Hartville uplift in Wyoming. He interprets these breccias as being the result of solution of anhydrite "penecontemporaneous" with their deposition in Permian time.
The subsidence structures in the Jewel Cave SW quadrangle include: collapse breccias in the Minnelusa, residual limestone-dolomite breccias in the Spearfish, undulations and normal faults in the formations overlying the Minnelusa, and breccia pipes that extend upward from the Minnelusa at least as high in the stratigraphic section as the Lakota Formation.
COLLAPSE BRECCIAS IN THE MINNELUSA FORMATION
The strata of the upper part of the Minnelusa Formation in the northeast corner of the quadrangle are extensively fractured, brecciated, and contorted. The highest bed in the Minnelusa is only moderately deformed, as are the overlying formations. The thick sandstone at the top of the Minnelusa (pl. 21) changes downward from almost undeformed rock to highly fractured rock and finally to breccia in which the fragments have been rotated and mixed to a minor extent with fragments of the underlying unit. Below this sandstone is about 300 feet of highly deformed rocks. Individual strata of limestone, sandstone, or siltstone are moderately to intensely brecciated and markedly contorted. The former positions of anhydrite beds can be identified in most places because weakly brecciated beds are overlain by beds that are intensely brecciated at the base and that grade upward into less intensely brecciated rock.
The contortion of strata is made apparent by the alternation of thin resistant carbonate rock units and nonresistant sandstone units. The carbonate beds maintain a relatively uniform thickness, but the intervening sandstone varies appreciably in thickness. Beds of sandstone may range in thickness from 2 to 16 feet in a horizontal distance of about 30 feet. This variation in thickness must be the result of the poorly cemented sandstone's having been squeezed into areas from which anhydrite was being removed.
In several places, intrusive masses of rock resembling conglomerate occur along the contact between beds. This conglomeratic-looking rock probably resulted from the partial milling of breccia fragments of sandstone as it was squeezed into its present position. In addition to being brecciated and undulatory, thick beds of sandstone are broken by many small vertical or steep gravity faults.
Many of the above-described features are shown in figure 50, which was photographed in Hell Canyon northeast of the mapped area. As shown in this picture, the rocks at the base of the cliff are brecciated and fractured only to a minor degree. These rocks occupy a position below that part of the Minnelusa that in the subsurface contains abundant anhydrite. It can be inferred that below this horizon only thin beds of anhydrite were present, because only a few thin units are brecciated.
All the anhydrite has probably been leached from the Minnelusa Formation in the northeast corner of the Jewel Cave SW quadrangle. The downdip limit of complete leaching as located on plate 20 is based on several facts: The exposures of the Minnelusa in Hell Canyon, in the NE1/4 sec. 31, T. 5 S., R. 2 E., contain gypsum beds and are not brecciated. The boundary of extensive solution must lie updip from these exposures. There is a noticeable concentration of breccia pipes in the northeast corner of the quadrangle, and the bounding line was placed downdip from this concentration.
A gentle structural depression extending more than 3,000 feet along the east side of sec. 9, T. 6 S., R. 2 E. (pl. 20) contains a large collapse pipe. In this depression the base of the Hulett Sandstone Member of the Sundance may be downwarped as much as 100 feet (fig. 52). It seems probable that the depression overlies an isolated area from which anhydrite was dissolved from the Minnelusa Formation.
BRECCIAS IN THE SPEARFISH FORMATION
The G2 and G3 gypsum beds in the lower half of the Spearfish Formation are absent in the northeast corner of the Jewel Cave SW quadrangle. North of the boundary line shown on plate 20 the equivalent positions of the gypsum beds are occupied by 3- to 10-foot-thick beds that consist of fragments of dolomite and limestone in a matrix of red siltstone. The transition from gypsum to dolomite, as observed in two places, occurs within a horizontal distance of about 300 feet.
UNDULATIONS AND NORMAL FAULTS IN THE MINNEKAHTA LIMESTONE
Subsidence structures are particularly noticeable in the Minnekahta Limestone, which is the first resistant unit above the Minnelusa. Throughout much of the northeast corner of the quadrangle the Minnekahta Limestone crops out on dip slopes. The area of extensive outcrop overlies the breccia zone of the Minnelusa in the northeast and the unbrecciated Minnelusa in the central part of the quadrangle. A difference in the character of the outcrop is progressively evident toward the areas of extensive subsidence. The Minnekahta Limestone is distinctly undulatory where it overlies breccia in the Minnelusa. Irregular domes alternating with bowl-shaped depressions are common. The size of the individual basins or domes is variable; the largest are about 500 to 1,000 feet in diameter and have amplitudes of a few tens of feet. Commonly, small domes or basins are superimposed on larger ones. The structure contours in the northeast corner of the map (pl. 20) are based largely on elevations on top of the Minnekahta. No attempt was made to smooth or generalize the contours when they were converted to the base of the Hulett, because it was desired to illustrate the subsidence deformation. The shape of the contours in secs. 17, 18, and 20, T. 5 S., R. 2 E., are considered to be typical of the areas underlain by extensive collapse. The contours in secs. 25, T. 5 S., R. 1 E., and 30, and 31, T. 5 S., R. 2 E., along the anticline, are based on the top of the Minnekahta Limestone also, and are considerably smoother. This area is believed to be underlain by unbrecciated Minnelusa.
On the walls of Hell Canyon in secs. 16, 17, 20, 21, and 29, T. 5 S., R. 2 E., the undulations can be seen in cross section. On these cliffs many small faults are exposed, but none of the faults can be traced away from the canyon walls.
Breccia pipes are vertical, nearly cylindrical bodies within which the rock has subsided more than the surrounding rock. They are most abundant in the northeast corner of the quadrangle, but a few pipes have been found as much as 4 miles downdip from the area of complete leaching. The locations of all known pipes are shown on plate 20.
The appearance of the breccia pipes varies. Those that penetrate the upper part of the Minnelusa Formation are generally small; they range from about 10 to 100 feet in diameter and are filled with a breccia consisting of limestone or calcareous sandstone fragments set in a matrix of calcareous sandstone.
Breccia pipes that penetrate the Minnekahta Limestone range in size from a diameter of about 40 feet (fig. 51) to a maximum observed diameter of about 300 feet. The Minnekahta Limestone within the pipes is characteristically intensely brecciated. The amount of downward movement of the Minnekahta could be estimated in two pipes; in one the displacement exceeds 80 feet, and in the other the displacement is about 100 feet. In several places coalescing groups of pipes suggest that joint sets, striking about N. 80° E. and N. 50° W., have localized the solution of underlying rock.
Several breccia pipes having diameters of about 50 feet penetrate the Spearfish Formation. These pipes are filled with fragments of dolomite from the overlying gypsum beds of the Spearfish set in a matrix of calcareous red siltstone. Because of the calcite cement, some of these pipes are more resistant than the surrounding siltstone and stand 15 to 20 feet above the ground level.
The largest pipe within the quadrangle penetrates the base of the Sundance Formation in the northeast corner of sec. 9, T. 6 S., R. 2 E. Where the pipe penetrates the Canyon Springs Sandstone Member, it is approximately 450 feet in diameter. The vertical displacement at this level is about 70 feet (fig. 52). The Hulett Sandstone Member in the center of the pipe is not brecciated, but dips gently inward toward the center of the pipe.
Several breccia pipes are known to penetrate as high as the lower part of the Lakota Formation. One of these pipes is in the Edgemont NE quadrangle, and has a vertical displacement of about 60 feet (G. B. Gott, oral communication, 1958). Another is in the NE1/4 sec. 24, T. 6 S., R. 1 E., in the Jewel Cave SW quadrangle. This pipe is marked only by a conical downwarp of the strata in the base of the Lakota. Subsidence in this pipe probably did not extend much higher into the Lakota.
ORIGIN OF SUBSIDENCE STRUCTURES
All the structures described are clearly related to the removal of soluble rocks from underlying formations. The logs of many wells that penetrate the Minnelusa Formation around the periphery of the Black. Hills have been published by the Geological Survey of South Dakota (Baker, 1947, 1948, 1951). All but two wells contained an hydrite. The anhydrite, which is present in many thin beds, is concentrated in the upper half of the Minnelusa Formation. A few beds less than 5 feet thick are in the lower half of the formation. The total thickness of anhydrite in the upper half of the formation ranges from about 100 feet north of the Black Hills to about 300 feet at the southern end of the Black Hills.
Most of the described collapse features are due to the solution of anhydrite from the upper part of the Minnelusa. The brecciated parts of the Minnelusa in the outcrop are at the same stratigraphic position as the major anhydrite beds in the subsurface. Only minor breccias or subsidence structures occur below the middle of the Minnelusa, and these are due to solution of a few thin beds of anhydrite that occur in the lower part of the Minnelusa.
Subsidence structures above the middle part of the Spearfish Formation may be due to, or augmented by, leaching of about 60 feet of anhydrite from the lower part of the Spearfish Formation.
TIME OF LEACHING
The leaching of anhydrite from the Minnelusa Formation must have occurred at some time after the Early Cretaceous, because breccia pipes penetrate strata as young as Early Cretaceous. From Early Cretaceous time until uplift of the region in Late Cretaceous and early Cenozoic time, anhydrite-bearing sediments were buried beneath more than 3,000 feet of nearly horizontal sediments. The sedimentary cover of the center of the Black Hills had been tilted and eroded down to the Precambrian rocks by the Oligocene Epoch, and probably earlier, and thus artesian circulation may have-been established in the anhydrite-bearing units early in the Cenozoic Era. The climate of this region during the early Cenozoic is believed to have been more humid than it was during the later part of the period (Brown, 1952, p. 91), and solution as extensive as that described would be facilitated by a humid climate. Solution features produced under the semiarid climate, which exists today, are very minor in the gypsum beds of the Spearfish Formation. Thus it seems probable that the major part of the leaching was accomplished in the early part of the Cenozoic, although solution on a limited scale may have continued to the present time.
In addition to the subsidence features previously described, a group of structural features occurs in the Minnekahta Limestone and Spearfish Formation that are believed to have been produced by slippage of the formations downdip. This group includes small thrust faults and related minor folds, dolomite breccias, pull apart structures, small tight folds in thin gypsum beds, and cross folds.
Description of structural features.Thrust faults of small displacement and associated minor folds occur throughout the outcrop area of the Minnekahta Limestone. The observed displacements on the faults range from several inches to about 3 feet (fig. 53). The faults may be either smooth planes having slickensides parallel to the dip of the faults, or they may be narrow brecciated zones. Where the fault displacement has taken place through a zone instead of along a single surface, the thin strata of the Minnekahta have been rotated around an axis parallel to the strike of the fault. Intense rotation has produced a brecciated zone; less intense rotation has produced asymmetric minor folds. In the latter, the steeply dipping limb of the fold is parallel to the thrust fault zone. Some of the faults cut entirely across the Minnekahta; others are restricted to the Minnekahta and die out both upward and downward within the formation.
The strike of the thrust faults and the axes of the associated minor folds have a preferred orientation of about N. 45° W., which is almost parallel to the direction of strike of the formations in the vicinity (fig. 54). The thrust faults dip about 45° either to the northeast or to the southwest.
A varying thickness of thinly laminated dolomite commonly occurs in the Spearfish Formation at the base of gypsum bed G2 (fig. 44). In many exposures the dolomite is extensively brecciated. At some places small northwest-striking thrust faults extend from the brecciated dolomite upward into the overlying gypsum.
The structure shown on the right side of figure 55 is termed a "pull-apart" structure by the writer. The basal bed of the G3 gypsum in the Spearfish Formation has been pulled apart a distance of about 40 feet, and the underlying siltstone and thin gypsum beds have been forced upward into its former position. The third dimension of this structure could not be observed. Smaller examples of pull-apart structures in thin gypsum beds are shown in figure 56.
At many localities in the Spearfish Formation the thin beds of gypsum between beds G2 and G3 are tightly folded. The folds range from small symmetrical folds, having amplitudes of about 1 foot, to overturned and recumbent folds like those shown in figure 56.
Several sharp anticlines and synclines that trend at nearly right angles to the regional strike occur in the Minnekahta Limestone in the northeast quarter of the quadrangle. An anticline that trends about N. 30° E. begins in the SE1/4 sec. 28, T. 5 S., R. 2 E., and extends for at least 1 mile to the northeast (pl. 20). The Spearfish Formation is flexed at the southern end of this fold. A sharp V-shaped syncline extends from the middle of sec. 31, T. 5 S., R. 2 E., to the SE1/4 sec. 36, T. 5 S., R. 1 E. It is paralleled on the north side of Hell Canyon by several smaller synclines that have similar shapes.
The G3 gypsum bed in the Spearfish has been folded to form an arcuate syncline-anticline pair in sec. 26, T. 5 S., R. 1 E. (pl. 20). Near the center, and at the southern boundary of sec. 26, several reverse faults too small to be mapped occur just east of the synclinal trough (left side of fig. 55). The syncline exposed in the eastern part of sec. 34, T. 5 S., R. 1 E., may be a continuation of the folds in sec. 26.
Origin of the minor deformational structures.Possible mechanisms of formation of the minor structural features are irregular subsidence due to leaching of anhydrite, deformation due to expansion when anhydrite was converted to gypsum, plastic flowage of beds of gypsum, and deformation as a result of sliding of large masses of rock.
In the northeast corner of the quadrangle all the anhydrite originally present in the Minnelusa, Opeche, and Spearfish Formations has been leached out, and the approximate southwest limit of complete leaching is believed to extend across Hell Canyon near the SW corner, sec. 29, T. 5 S., R. 2 E. All the minor structures are southwest of the area of complete leaching. The thrust faults and minor folds in the Minnekahta Limestone also occur where the Minnekahta overlies the leached area, and they probably formed before the leaching of an hydrite took place. The writer believes that the only minor structures described that could be related to solution of underlying rock are the long southwest-plunging synclines in the Minnekahta Limestone and the arcuate syncline in the Spearfish Formation. The long synclines could be the reflection of subsidence directly over elongate solution chambers. This possibility is rejected, however, because breccia pipes were not observed along the trends of the synclines and because the parallelism between the trend of the synclines and the sharp anticline in the Minnekahta Limestone (which could not be due to subsidence) suggests a common origin.
The conversion of anhydrite to gypsum could result in expansion of the rocks and produce the minor deformational features. In USGS 2, Pass Creek core test (pl. 21), the sulfate beds of the Opeche Formation, which were penetrated at a depth of about 150 feet, are gypsum. However, underlying beds of sulfate in the Minnelusa Formation, are anhydrite. Where the Minnekahta Limestone is exposed on the surface and where stream valleys do not cut into the Minnelusa Formation, sulfate beds in the Minnelusa are probably still anhydrite; consequently the sulfate beds must be dismissed from the present considerations. The writer does not believe that the expansion of the two 5-foot beds of sulfate in the Opeche Formation, which are 60 feet below the Minnekahta or that expansion of sulfate beds in the Spearfish, which are 100 feet above the Minnekahta, could have had any appreciable deforming effect upon this limestone unit. Those structures in which the gypsum beds themselves have been deformed are the ones most likely to have been produced by recrystallization, if this mechanism is effective. It does not seem possible that the intense folding and segmentation of thin gypsum beds, such as that shown in figure 56, could be the result of expansion of anhydrite. The large pull-apart structure shown in figure 55 implies the extension of the gypsum bed rather than the compression of it; the relation of this extension to volume increase during recrystallization is extremely difficult to visualize.
Many examples of plastic flowage of salt and gypsum have been described in the literature, and De Sitter (1956, p. 79) lists gypsum as one of the most incompetent rock types. Thus the structures in the gypsum beds may be due to plastic flowage caused by differential compaction or incipient folding. It is probable that these beds were converted to gypsum only after erosion had removed all but a few hundred feet of the overlying strata, and it is also probable that the structures shown by these beds were developed during uplift of the Black Hills in Late Cretaceous. The writer's impression is that when the beds were deformed, the sulfate units were more competent than the enclosing siltstone units. The sulfate beds fractured, and they were segmented and separated in much the same fashion as thin competent units caught up in a mass of plastically deforming shale.
The uniform preferred orientation of the small thrust faults and minor folds in the Minnekahta Limestone indicates that this unit was deformed by compressive stress directed parallel to the bedding in a northeast-southwest direction. The compressive stress was probably the small component of the weight of the overlying column of rocks that, following the tilting of the rocks away from the center of the Black Hills, acted parallel to the dip. As the angle of tilt increased, the component of gravity probably became great enough to cause shortening of the rocks parallel to the direction of the dip.
This line of reasoning indicates that the siltstone in the Opeche and lower part of the Spearfish were particularly incompetent parts of the stratigraphic section at the time of uplift of the Black Hills, and that the overlying rocks may have slid relatively more downdip than the underlying units. Within the incompetent units, minor structures were produced as a result of the sliding. The southwest-trending anticlines and synclines in the Minnekahta Limestone are nearly parallel to the postulated direction of sliding. These folds may have been produced as a result of adjacent masses of rock sliding in slightly different directions. The pull-apart structure in the G3 gypsum bed could also have been produced by adjacent rock sliding that tended to stretch the bed. The many small folds in the thin gypsum beds and the dolomite breccias would have formed as a result of the plastic deformation of the incompetent zone during sliding. The angle of dip (sliding gradient) in the Jewel Cave SW quadrangle is about 2°; it becomes somewhat steeper to the southwest.
Last Updated: 28-Nov-2007