Geological Survey Bulletin 1063—G Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota

PERMIAN ROCKS

MINNELUSA FORMATION

The name Minnelusa Sandstone was given by Winchell (1875, p. 38 and 65) to rocks on the west side of the Black Hills in the valley of Cold Spring Creek. The limits of the formation were redefined by Darton (1901, p. 510) as the top of the Pahasapa Limestone of Mississippian age and the base of the red mudstone of the Opeche Formation. Darton and Paige (1925, p. 8) designated Rapid Creek on the east side of the Black Hills near Rapid City as the type locality; a detailed description of the formation at this locality is given by Boardman.¹

---

¹Boardman, D. C., 1942, Minnelusa formation in Rapid Canyon area, Black Hills, South Dakota: Iowa Univ. M.S. thesis.

---

The Minnelusa Formation in the southern Black Hills has been divided into six units (C. G. Bowles, oral communication, 1961), but only unit 1 crops out within the Jewel Cave SW quadrangle.

The Minnelusa Formation was first thought to be of Pennsylvanian age, but lithologic correlations with rocks of eastern Wyoming (Agatston, 1954; Bates, 1955; McCauley, 1956; Foster, 1958; and Maughan, written communication, 1960) indicate that the top of the Pennsylvanian System should be placed at the base of a persistent bed of red mudstone informally called "the red marker" (pl. 21). The red marker is the base of unit 1 of the Minnelusa.

Foraminifers collected from the USGS 2, Pass Creek core test (pl. 21) confirm the Early Permian age of unit 1 of the Minnelusa Formation in the Black Hills. The following fossils collected from about 150 feet below the top of the formation were identified by L. G. Henbest:

Osagia? sp. or girvanellid accretions
Textulariidae
Globivalvulina sp.
Tetrataxis sp.
Primitive Lagenidae?
Spandelinoides? sp.
Geinitzina postcarbonaria Spandel, 1901
Spandelina ? sp. and other supposed Lagenidae
Echinoid spines having complex cribriform structure

He states that the G. postcarbonaria and related primitive Lagenidae? indicate Wolfcamp or Leonard age. The seemingly Permian forms of Globivalvulina, Tetrataxis, and echinoid spines support that determination.

The lower part of the Minnelusa (units 2-6), which is composed predominantly of limestone, dolomite, shale, and sandstone, does not crop out within the Jewel Cave SW quadrangle. The lithologic data for this part of the formation (pl. 21) were obtained from surface exposures along Hell Canyon in the adjoining Jewel Cave and Jewel Cave NW quadrangles, and from subsurface cores (USGS 1, Hell Canyon core test in the NW1/4 sec. 1, T. 6 S., R. 1 E.).

In the subsurface, unit 1 of the Minnelusa Formation (pl. 21) is characterized by thick beds of anhydrite that alternate with beds of sandstone and dolomite; but in most surface exposures the anhydrite has been dissolved, and the sandstone and dolomite are brecciated and altered.

The upper 120 feet of unit 1, which consists of unbrecciated sandstone, dolomite, and gypsum, is exposed in lower Hell Canyon (sec. 31, T. 5 S., R. 2 E.) (see pl. 21). Farther up Hell Canyon, in the northeast corner of the quadrangle, the same part of the formation is exposed, but the upper two gypsum beds have been dissolved, and the sandstone and carbonate rocks are brecciated and contorted.

The Minnelusa Formation in the Jewel Cave SW quadrangle is about 1,000 feet thick where anhydrite or gypsum is present, but only about 870 feet thick where the anhydrite has been removed by leaching.

PETROGRAPHY OF THE UPPER PART OF THE MINNELUSA FORMATION

The USGS 2, Pass Creek core test, in the NW1/4 sec. 1, T. 6 S., R. 1 E., penetrated the upper 430 feet of the Minnelusa Formation (pl. 21). In this drill hole, 237 feet of anhydrite are interbedded with dolomite, anhydrite- or gypsum-cemented sandstone, and minor mudstone.

SANDSTONE

The sandstone in this well is uniformly fine grained to very fine grained, well sorted, and has subangular to subrounded grains. Most of the units seem to be thinly and horizontally laminated, but the thickest unit, which is at the top of the formation (pl. 21), is strongly crossbedded in the outcrop. The crossbeds are planar and about 6 feet in length.

The color of the sandstone varies—the upper two beds, which total 70 feet in thickness, are red to yellow, the lower beds are dominantly light gray to dark gray. In several beds the top of the sandstone is mottled purplish red, and the base is gray. Some beds are irregularly spotted and mottled with red, yellow, or purple in streaks paralleling the stratification. Gray sandstone changes to red near fractures.

The detrital constituents of the sandstone are more than 95 percent quartz, less than 2 percent feldspars (plagioclase, microcline, and untwinned potassium feldspar), and chert. The accessory minerals are dominantly well rounded grains of zircon, tourmaline, and leucoxene.

The cement of the two red sandstone beds near the top of the formation is fibrous to coarsely crystalline gypsum. Some patches of coarsely crystalline gypsum surrounding several sand grains contain small rounded to rectangular crystals of anhydrite.

The cement of the gray to mottled sandstone is predominantly coarsely crystalline anhydrite that poikilitically includes sand grains.

Many crystals of carbonate are distributed through the anhydrite and gypsum cement. The carbonate is dolomite in most places, but in the upper red sandstone some of the carbonate is calcite. Where the carbonate is dolomite, it occurs as either very small isolated rhombs or as aggregates of rhombs floating in the sulfate. Where the carbonate is calcite, it occurs as rather large crystals that include several sand grains or partially replaced quartz grains.

Pyrite occurs with anhydrite as interstitial filling in several of the gray sandstone beds. The relationship between pyrite, iron oxide, gypsum, and anhydrite is of particular interest in the sandstone bed that occurs between 578 and 594 feet below the top of the drill hole. The lower part of this bed is light gray to dark gray and contains interstitial pyrite and anhydrite; the upper part is mottled pink to dark purple, contains no pyrite, and is cemented largely by gypsum. This relation indicates that oxidation has altered pyrite to iron oxide, inducing the pink to purple color, and that anhydrite has been converted to gypsum.

Authigenic iron oxide occurs in the upper two red sandstone beds but was not observed. in lower beds. The oxide occurs as crystals in the interstices of the sandstone. These crystals commonly are euhedral and, although very small, they appear to be flat rhombohedrons having striated faces.

DOLOMITE

All the beds of dolomite consist entirely or predominantly of the mineral dolomite, which is extremely fine grained. The color varies from dark gray to grayish red. Some specimens are noticeably mottled and have concentrations of red or yellow iron oxide around small shell fragments or around small vugs. Stratification is apparent only at the contact with other units where there is frequently a gradational boundary showing fine planar or contorted laminations.

ANHYDRITE

The anhydrite is light gray to dark gray and consists of many small rectangular crystals that have a tendency to be elongate parallel to the stratification. This fine-grained euhedral texture is useful in distinguishing primary anhydrite from the replacement anhydrite discussed below. Detrital impurities are very rare. Rather large lath-shaped crystals of quartz were observed in several slides. These crystals are too large to be detrital, and many contain small inclusions of anhydrite. They frequently form aggregates in which the individual laths radiate from a common center.

Anhydrite has been partially converted to gypsum throughout the entire depth of the core. Hydration is patchy, however, and is most common adjacent to sandstone or dolomite beds and along fractures. Some of the anhydrite has a fetid odor when freshly broken.

REPLACEMENT OF DOLOMITE BY ANHYDRITE

Replacement of dolomite by anhydrite is rare. However, in several slides there are conspicuous patches of coarsely crystalline anhydrite that differ markedly in texture from the typical fine-grained primary sulfate. Because many of these patches contain relict outlines of fossils, they are believed to have been caused by replacement of the dolomite by anhydrite.

FLUORITE

Small crystals of fluorite were found in two specimens in the Minnelusa and also in a similar rock type in the Minnekahta Limestone. In the Minnelusa the two fluorite occurrences are similar.

In the core of USGS 2, Pass Creek, the contact between dolomite and anhydrite at the depth of 484 feet is very irregular. Nodular to lenticular masses of anhydrite, which have the typical fine-grained rectangular texture of primary anhydrite, occur with masses of fine-grained dolomite. The dolomite masses contain small clots of coarsely crystalline anhydrite, which appear to be filling small vugs. The vugs are rimmed with red iron oxide, and within many are small cubes of fluorite. The fluorite is colorless, except for irregular purple patches.

The highest dolomite in the core of USGS 2, Pass Creek contains abundant patches of anhydrite replacing fossils and the dolomite groundmass. In the adjacent surface outcrops in Hell Canyon, this bed contains fluorite within patches of gypsum that have the same textural appearance as the replacement anhydrite in the core.

LEACHING or ANHYDRITE AND ALTERATION OF ASSOCIATED ROCKS

Throughout the Black Hills the anhydrite beds in the upper part of the Minnelusa Formation have been leached in a zone near the outcrop. Only two places are known to the writer where gypsum is preserved at the surface. One locality northeast of Sundance, Wyo., has been described by Brady.² The other locality is in the Jewel Cave SW quadrangle in sec. 31, T. 5 S., R. 2 E. Leaching of anhydrite from correlative beds in the Hartville uplift of eastern Wyoming has been described by Bates (1955).

---

²Brady, F. H., 1930, Some problems of the Minnelusa Formation near Beulah, Wyoming: Iowa Univ. M.S. thesis.

---

The removal of several hundred feet of rock from the lithologic column has resulted in the formation of many subsidence structures, which are described in the section on "Structure." In the zone from which anhydrite beds have been removed, the remaining rocks have been altered to an important degree. The alterations recognized are extensive oxidation, the replacement of anhydrite and dolomite cement by calcite, and the alteration of dolomite to dolomitic limestone.

OXIDATION

Most of the sandstone units penetrated in the USGS 2, Pass Creek core test are gray. However, the upper two beds in the core, and most of the brecciated beds in the outcrop, are shades of yellow and red. Pyrite, though not abundant, is a common constituent of sandstone in the core, but has not been observed in surface exposures. Authigenic crystalline iron oxide was observed in all surface sandstone examined and in the two top sandstone beds in the core.

The dolomites in the USGS 2, Pass Creek core are dominantly gray to grayish red, but in the surface outcrops many of the carbonate rocks are markedly red and contain abundant spots and dendrites of manganese oxide. Surface carbonate units that are largely gray are oxidized to reds and yellows in vuggy zones and along fractures.

It is probable that rock that has undergone the initial stages of oxidation is present in the core from USGS 2, Pass Creek. The sandstone between 578 and 594 feet from the top of the core is oxidized in the upper part, and the anhydrite cement has been altered to gypsum. Many of the other units in this core, both dolomite and sandstone, have patchy to mottled iron oxide near small vugs and along fractures. The dominant gypsum cement in the upper two red sandstone beds contains both anhydrite remnants and authigenic iron oxide crystals. It seems likely that these sandstone beds were altered from the typical gray of underlying sandstone at the same time the anhydrite cement was altered to gypsum.

REPLACEMENT OF ANHYDRITE AND DOLOMITE CEMENT BY CALCITE AND IRON OXIDE

In the USGS 2, Pass Creek core the sandstone is completely cemented by anhydrite or gypsum containing rhombs of dolomite. In the outcrop all the sandstone is friable, and the dominant cements are calcite and iron oxide. The calcite cement is present as coarse crystals that include several sand grains.

ALTERATION OF DOLOMITE TO DOLOMITIC LIMESTONE

The carbonate rocks in the core of the USGS 2, Pass Creek are dominantly very fine grained dolomite. In the outcrop the carbonate rocks are calcitic dolomite breccia, calcitic dolomite, and coarse-grained limestone that contains less than 1 percent dolomite (pl. 21). These mixed carbonate rocks are the product of replacement of dolomite by calcite.

OPECHE FORMATION

The Opeche Formation was named and defined by Darton (1901, p. 513). The type locality of the formation is on Battle Creek on the east side of the Black Hills.

The formation crops out in Tepee, Schenk, and Hell Canyons in the northeastern part of the quadrangle, where the thickness ranges from 75 to 115 feet. Part of the variation in thickness is believed to be due to minor flexures in the overlying Minnekahta Limestone that are not reflected concentrically in the base of the Opeche. At two locations the thickness of the formation was measured beneath the trough of a syncline in the Minnekahta and compared with the thickness beneath the crest of the adjacent anticline as follows:

The Opeche Formation consists predominantly of red mudstone. The upper 15 to 20 feet are a distinctively purplish red in contrast to the reddish brown of the rest of the formation. Two gypsum beds 5 to 7 feet thick occur—one near the middle, the other very near the base of the formation. The lower bed was reached at a depth of 150 feet in the USGS 2, Pass Creek core test; at this depth it consists of more than 50 percent anhydrite.

Northeast of sec. 29 in Hell Canyon the gypsum beds are present only as isolated patches. The gypsum in the intervening areas has probably been leached away.

In the extreme northeast corner of the quadrangle, a wedge of sandstone is at the base of the formation. This wedge consists of beds of pink to yellow fine-grained sandstone that range from 2 to 20 feet in thickness and are separated by thin beds of red siltstone. At the north boundary of the quadrangle the thickness of this wedge is about 40 feet.

No fossils have been found in the Opeche Formation, but it is under lain and overlain by rocks of Permian age.

MINNEKAHTA LIMESTONE

The Minnekahta Limestone was named and defined by Darton (1901, p. 513); it is a medium-gray to light-red, thinly bedded limestone that ranges from 35 to 50 feet in thickness. The type locality of the formation is the area around Minnekahta, S. Dak.

The Minnekahta Limestone in the Jewel Cave SW quadrangle can be divided into four recognizable units. For convenience of description, these units are numbered from bottom to top.

Unit 1, which is about 4 feet thick, consists of alternating thin beds of pure limestone and thin beds of silty or argillaceous limestone. Individual strata are thinly lenticular and do not extend laterally more than a few feet. Small pelecypod shells and casts of gastropods are very common. This unit is less resistant than the overlying unit, and seldom is exposed.

Unit 2, which is about 15 feet thick, consists of layers of light-gray limestone alternating with thin layers of reddish-brown limestone; the color variation parallels the bedding. Etched polished surfaces indicate that the thin red layers contain a higher proportion of insoluble material, and in thin sections this insoluble material is seen to be predominantly clay size.

Unit 2 consists of a mosaic of anhedral crystals of calcite whose sizes range from 0.02 to 0.15 mm; the smaller sizes are the more abundant. Quartz grains are not abundant, but in some specimens they make up about 5 percent of certain laminae.

Anhydrite is a common accessory mineral in this unit, but does not exceed 5 percent by volume. The former existence of anhydrite is indicated in the outcrop by many small vugs. In fresh samples, the anhydrite is in flattened ellipsoidal masses, as much as 0.5 inch in diameter, around which the stratification in the carbonate is deflected.

Zones of ellipsoidal calcite concretions occur at several localities in unit 2. These concretions range from about 6 inches to 1 foot in diameter. Stratification passes through the concretions, and some beds contain abundant small pelecypod fossils both outside and within the concretions. Somewhat similar structures in the Minnekahta Limestone in the northern Black Hills have been described by Knaack³, and were considered by him to be algal structures (Rhodophyceae). Polished slabs from the concretions in the Jewel Cave SW quadrangle show no structures that could be interpreted by the writer as algal. Many strata within some of the concretions are crenulated on a very fine scale, but crenulations of this type occur outside the concretions as well. Examination under the binocular microscope shows that the crenulation has resulted from compaction around many small pelecypod shells. The origin of these concretions is not known.

---

³Knaack, E. L., 1936, Origin of certain structures of the Minnekahta formation in the Whitewood region, northern Black Hills, South Dakota: Iowa Univ. M.S. thesis.

---

Unit 3 is 3 to 5 feet thick and consists of thinly laminated, dark-reddish-gray limestone containing many red-siltstone laminae. Pelecypod shell fragments are abundant.

Unit 4 is about 15 feet thick and is a thinly stratified dominantly reddish gray limestone, which is frequently mottled with irregular splotches of medium gray color. The mottled parts of the rock commonly have a fetid odor when broken. The mottling is probably due to the small amount of organic matter that maintains the iron in the reduced valence. Irregular patches of anhydrite were observed in several specimens, and may make up as much as 10 percent of the rock.

Throughout the center of the quadrangle the Minnekahta is overlain by basal gypsum beds of the Spearfish Formation. Here, the upper 2 feet of the formation is a thinly laminated pink to red anhydritic limestone. On the outcrop, most of the anhydrite has been leached out; the leaching produced a honeycomb texture. In fresh rock the anhydrite occurs as large crystals that commonly have a rectangular or rhombic cross section. In several places the lamination in the limestone is deflected around these crystals. Associated with the anhydrite are large irregular to subhedral crystals of calcite that contain inclusions of anhydrite. Subhedral grains of fluorite, which are 0.1 to 0.25 mm in diameter, occur sparingly along certain laminae. Some of the fluorite grains are surrounded by fine-grained calcite, some are entirely within anhydrite crystals, and some are partly within anhydrite and partly within coarse calcite. No age relation of the fluorite to the other constituents of the rock could be seen.

The only dolomite observed in the Minnekahta is a 1-foot-thick bed of white to light-gray rock that occurs near the top of the formation in sec. 29, T. 5 S., R. 2 E.

Very near the top of the formation in the NW1/4NW1/4 sec. 1, T. 6 S., R. 1 E., are structures that may be algal heads. These structures resemble flat, inverted saucers and range from 2 to 4 feet in diameter. Although good cross sections of the structures were not visible, the doming of the limestone laminae apparently is limited to a thickness of about 4 inches.

Fossil remains of pelecypods and gastropods are very common throughout the formation. Most fossil remains are small internal casts that are hard to identify. Published data concerning the identification of these fossils are summarized below:

Dr. John Chronic of the University of Colorado (written communications, 1958) examined material collected by the writer from the lower part of the formation and reported the following forms to be present:

Gastropods:
   Bullimorpha cf. B. meeki Sayre
   Sphaerodoma cf. F. fusiformis (Hall)

Pelecypods:
   Nuculana cf. N. obesa White
   Pleurophorus cf. P. albequus Beede
   Schizodus cf. S. wheeleri Swallow

He stated: "In my opinion the Minnekahta samples are definitely Permian. In comparison with known Triassic faunas of the Rocky Mountain states, these are quite different, environmentally as well as faunally. The earliest Triassic faunas of Wyoming are quite specialized, but are not at all like that of the Minnekahta."

Fragments of a fish found in the lower part of the formation were examined by Dr. D. H. Dunkle, U.S. National Museum. He reported (written communication, 1957): "The only known group to which it might possibly belong is the amphicentrids, or Amphicenridae, a group of chondrostean fishes which parallel the platysomids in body habit and stratigraphic distribution (Mississippian to Upper Permian)."

PERMIAN AND TRIASSIC ROCKS

SPEARFISH FORMATION

The Spearfish Formation was defined by Darton (1901, p. 516) as the red shale and gypsum beds underlain by the Minnekahta Limestone and overlain by the gray sandstone and shale of the Sundance Formation of Jurassic age. The type locality is near Spearfish, S. Dak.

In the Jewel Cave SW quadrangle, as in the rest of the Black Hills, the nonresistant nature of the Spearfish has resulted in the development of a broad grass-covered valley at the position of the outcrop. Thus the formation generally lacks good exposure, and its stratigraphic thickness is difficult to measure.

The Spearfish Formation consists predominantly of siltstone, sandstone, and gypsum, and contains a minor amount of carbonate rocks. The variations of lithology within the formation are shown on figure 44.

FIGURE 44.—Lithology of the Spearfish Formation. (click image for a PDF version)

In the Jewel Cave SW quadrangle it was possible to map the distribution of three layers of the Spearfish Formation, consisting principally, of gypsum. In adjacent areas, however, it was possible only to subdivide the formation into lower, middle, and upper units. For the sake of continuity, the Spearfish in this report has been divided into lower, middle, and upper units, and the lower and middle units have been further subdivided into separate subunits consisting principally of gypsum or siltstone (pl. 20; fig. 44).

The red siltstone and minor, very fine grained red sandstone in the lower, middle, and basal upper units are almost structureless. Thin laminations are common in the sandstone in the top of the upper unit, and some beds are ripple laminated. The siltstone and sandstone consist of about 65 percent quartz, 2 percent feldspar, 3 percent chert grains, 2 percent white mica, 9 percent: detrital dolomite grains, 15 percent matrix, and 4 percent calcite cement. Rounded grains of ilmenite are common, and in some specimens they make up almost 2 percent of the volume. The matrix consists of iron oxide, carbonate, mica, and clay. Authigenic overgrowths are scarce, except on the detrital dolomite grains, which frequently have euhedral rhombic overgrowths. The calcite cement occurs in scattered patches of coarse crystals that enclose many detrital grains. The red pigment of the rock is concentrated in the matrix and occurs as thin coatings on the detrital grains. The detrital dolomite grains are irregularly coated with pigment on the original rounded margins, but the secondary rhombic overgrowths are free of pigment.

The G₁ gypsum bed in the lower unit (fig. 44) rests directly on the Minnekahta Limestone, but the bed occurs only in the center of the quadrangle (pl. 20). The G₂ and G₃ gypsum beds in the middle unit are continuous along the outcrop between the Jewel Cave SW quadrangle and Hot Springs, S. Dak., a distance of about 28 miles, but in the northeast corner of the quadrangle these two gypsum beds have been leached out, and their stratigraphic positions are marked by thin beds of dolomite-siltstone breccia. All the gypsum beds are virtually free of detrital material. Most thin sections of gypsum contain small, rounded to rectangular remnants of anhydrite, and a small percentage of small dolomite rhombs.

Carbonate rocks were found in the lower and middle units. A thin bed of carbonate and gypsum occurs in the lower unit about 65 to 85 feet above the Minnekahta. At most places where this bed was observed, the carbonate was dolomite; in the SW1/4 sec. 33, T. 5 S., R. 2 E., it consists of rather coarse-grained limestone containing abundant unidentifiable fossil fragments. A 5- to 15-foot-thick bed of thinly laminated dolomite occurs at the base of the middle unit about 100 feet above the Minnekahta. Locally, a thin dolomite bed about 6 inches thick occurs at the base of the G₃ gypsum bed.

No identifiable fossils have been found in the Spearfish in the Black Hills. The age of the formation must be inferred from regional correlation with the red-bed sequence in Wyoming and with the fossiliferous marine units in Idaho and adjacent areas. The latest published correlations are by Thomas (1940) and Condra and others (1940). These authors believe that the Minnekahta Limestone is well below the Permian-Triassic boundary. Condra has placed the top of the Permian at the top of the part of the Spearfish that contains thin limestone and gypsum beds (near the base of the upper unit). Thus the age of the Spearfish Formation is probably Late Permian and Triassic.

JURASSIC ROCKS

SUNDANCE FORMATION

The Sundance Formation was named and defined by Darton (1899). He (in Darton and O'Harra, 1909, p. 3) referred to the area north of Sundance, Wyo., as the type locality. Darton recognized several persistent lithologic units within the formation but did not name them. Imlay (1947) studied the formation throughout the Black Hills, subdivided it into five members, and suggested that a well-exposed area 1 mile north-northwest of Spearfish, S. Dak., be used as a standard reference.

The total thickness of the Sundance Formation in the Jewel Cave SW quadrangle is about 360 feet.

CANYON SPRINGS SANDSTONE MEMBER

The Canyon Springs Sandstone Member at the base of the Sundance is 35 feet thick in the southeast corner of the quadrangle, but it thins to the northwest. North of the Dewey fault, along the east side of the Elk Mountains, the member is from 0 to 7 feet thick. The contact with the underlying Spearfish Formation is sharp and generally conformable. Just north of the Dewey fault in sec. 10, however, the contact is a rolling surface having perhaps 20 feet of relief. Black or brown pitted and polished chert pebbles as much as 2 inches in diameter occur just above the basal contact. Where the Canyon Springs is fairly thick it generally consists of two parts. The lower part consists of about 15 feet of massive pink to brown crossbedded sandstone. The crossbeds are troughlike and form a festoon pattern suggestive of eolian deposition. The sandstone is predominantly fine grained, but many of the cross laminae contain about 15 percent of well-rounded medium to coarse grains floating in the finer grained sand matrix. The upper part of the Canyon Springs ranges from 5 to 15 feet thick and is slabby and horizontally stratified in contrast to the massive crossbedded lower part. Individual strata may have either ripple lamination or thin, low-angle cross lamination.

Both the upper and lower parts of the member are similar mineralogically and texturally. The finer grained parts consist of subrounded grains of quartz and minor amounts of chert, feldspar, limestone fragments, and shell material. The coarser grained part is well rounded and consists of quartz, quartzite fragments, chert, fine-grained limestone fragments, and carbonate shell fragments. Calcite cement is abundant. Fragments of limestone and shell fragments are more abundant in the upper, horizontally stratified part. It is likely that the horizontally stratified upper part represents the moderate reworking of the crossbedded lower part at the time of the transgression of the Sundance sea.

STOCKADE BEAVER SHALE MEMBER

The thickness of the Stockade Beaver Shale Member ranges from 45 to 60 feet. It is a dark-gray thinly bedded shale that contains a few very thin beds of calcareous sandstone and limonitic nodules. At one locality, geodes containing calcite and barite were observed. The contact with the underlying Canyon Springs is sharp, and the contact with the overlying Hulett Sandstone Member is gradational through a few feet.

HULETT SANDSTONE MEMBER

The Hulett Sandstone Member consists of about 55 feet of thin-bedded sandstone. The contact with the underlying and overlying members is gradational, but can be placed within a few feet. The sandstone is fine grained and well sorted, and consists predominantly of quartz and minor amounts of feldspar, chert, glauconite, and detrital calcite grains. Coarse fossil fragments, calcite cement, and oscillation ripple marks are common. Many thin strata of gray shale occur between the sandstone layers.

LAK MEMBER

The Lak Member consists of 70 feet of fine-grained red sandstone and a persistent 3-foot bed of light-gray sandstone about 15 feet below the top of the member. Stratification is very poor; it is represented in the lower part by irregular contorted laminae of siltstone and in the upper part by very thin horizontal laminae marked by concentrations of opaque minerals.

Two point-counts of about 350 grains each show that the Lak Member consists of 60 to 80 percent quartz, 2 percent feldspar, 1 percent chert grains, 7 to 13 percent detrital dolomite grains, 0 to 25 percent matrix, and 1 to 3 percent calcite cement. The matrix is probably largely very fine grained dolomite and quartz. The calcite cement occurs as scattered patches of coarse crystals enclosing the detrital grains. Red pigment forms thin rims on the detrital grains and is disseminated through the matrix. The detrital dolomite grains have rounded iron oxide-coated cores and clear rhombic overgrowths. In texture and mineralogy the 3-foot-thick light-gray bed is similar to the rest of the unit, except that there is very little matrix.

REDWATER SHALE MEMBER

The Redwater Shale Member is about 150 feet thick and consists of greenish-gray soft fissile shale and minor amounts of sandstone near the base and at the top. Stratification is very pronounced; most strata are thin, ranging from a fraction of an inch to several feet in thickness. Glauconite is an abundant constituent of the sandstone.

The contact of the Redwater with the red beds of the Lak is fairly sharp, but it is locally gradational through a few feet. The contact with the overlying Morrison Formation is sharp and is marked by the abrupt change from thinly laminated interbedded shale and sandstone to blocky calcareous mudstone.

REGIONAL CORRELATION

About 10 miles north of the Jewel Cave SW quadrangle, the Gypsum Spring Formation of Middle Jurassic age rests disconformably upon the Spearfish Formation. Southward the Gypsum Spring has been so beveled that in the area discussed the Sundance Formation of Late Jurassic age rests upon the Spearfish. Thus the regional disconformity at the base of the Sundance in the southern Black Hills represents part of the Triassic and all of Early and Middle Jurassic.

Imlay (1947) and Peterson (1954) have correlated the Sundance Formation with the Rierdon and Swift Formations in Montana on the basis of similar lithology and fossils. Peterson (1954) has suggested the following modifications: that the formation be raised to the status of a group (Sundance Group); that the names Rierdon and Swift be extended into Wyoming; that the Canyon Springs, Stockade Beaver, Hulett, and Lak Members be included in the Rierdon Formation; and that the Redwater Shale Member be called the Swift Formation. This nomenclature has not been used by geologists in this area.

ENVIRONMENT OF DEPOSITION

The Sundance Formation apparently was deposited in marine water in a shelf area, which extended northwestward between the Williston basin in North Dakota and the Twin Creek trough in southwestern Wyoming. There is evidence that the Sundance sea became shallow during the close of the time of deposition of sediments equivalent, to the Rierdon Formation (Peterson, 1954). The deposition of red beds equivalent to the Lak Member occurred over much of southern Wyoming and western South Dakota during this period of regression. Peterson stated (1954, p. 502):

The relatively uniform nature and the well bedded character of the "Sundance red" or Lak red beds suggest that they were probably deposited under marine conditions, and their association locally with gypsum suggests restricted (possibly lagoonal) conditions.

He thought that the source of the red sediment was probably the late Paleozoic and early Mesozoic red-beds of the region, and that the red color was preserved because the depositional environment was non reducing. Peterson also stated (1954, p. 503):

* * * Most of the Swift and Rierdon shales may have been originally laid down as red sediments which were subsequently changed to their characteristic grey and green colors through the reducing action of decaying organic matter. * * *

Glauconite is common in the units that overlie and underlie the Lak Member. It is believed that glauconite forms under slightly reducing conditions in water of normal marine salinity and probably in the presence of organic matter (Pettijohn, 1957, p. 503). Peterson's concepts of the depositional environments are consistent with those implied by the presence of glauconite. When the marine environment was abnormally saline (that is, abundant dolomite precipitated) and organic matter did not accumulate, red strata were deposited. When the salinity was normal and the environment slightly reducing (as indicated by the presence of glauconite), the red color of the sediment was destroyed.

MORRISON FORMATION

The sequence of light-colored claystones of Late Jurassic age that overlie the Sundance Formation and underlie the Lower Cretaceous sandy rocks were formerly called the Beulah Clays by Jenney (1899), who studied these rocks in the Hay Creek coal field north of Beulah, Wyo. He recognized that these beds included the Atlantosaurus beds on the east side of the Black Hills, and that the Atlantosaurus beds were in turn similar in lithology and fauna to rocks along the Front Range in Colorado. The name Atlantosaurus beds was abandoned and the name Morrison Formation was extended by Darton (1904) into the Black Hills region from its type locality at Morrison, Colo. This name has replaced the original term Beulah Clays in the Black Hills. The formation crops out in the southwestern, northern, and eastern parts of the Black Hills, but it is absent in the southeastern part where the Unkpapa Sandstone of Late Jurassic age overlies the Sundance Formation.

The Morrison Formation in the Jewel Cave SW quadrangle consists predominantly of gray to grayish-green blocky-weathering non-carbonaceous mudstone, thin beds of lithographic clayey limestone, and, less commonly, thin beds of sandstone. Stratification within individual beds is indiscernible, except in the sandstone that is ripple laminated or cross stratified. It is impossible to determine accurately the lateral extent of the sandstone or limestone beds because the formation is generally poorly exposed. However, these beds probably do not persist laterally for more than a few thousand feet. Clayey limestone and lithographic limestone are most common in the lower half of the formation. In the NW1/4 sec. 14, T. 6 S., R. 1 E., a local channel filled with 10 feet of crossbedded clayey sandstone occurs about 10 feet above the base of the formation. All other sandstone beds are less than 2 feet thick and have lateral extents of at least 1,000 feet.

The thickness of the Morrison Formation in the quadrangle ranges from about 60 to 120 feet. The poor exposure of the formation does not permit the construction of a detailed isopach map, but it is possible to determine the general trend of thickness variation. Along the north-facing scarp in secs. 16 and 17, T. 6 S., R. 2 E., the formation is 60 feet thick. In the NE1/4NW1/4 sec. 28, T. 6 S., R. 2 E., where the formation was penetrated by a diamond-drill hole, the thickness is 80 feet. About one-half mile southwest of this drill hole the formation is 110 feet thick. In the NE1/4 sec. 19, T. 6 S., R. 2 E., the base is not exposed, but the total thickness must be greater than 90 feet. Just south of the Dewey fault the total thickness is 120 feet. It seems very likely that lines of equal thickness trend about N. 50° W., and that the formation thins to the northeast.

At many places in the Black Hills the upper part of the Morrison Formation and the lower part of the Lakota Formation are gradational, and the selection of a formation boundary is difficult. In the Jewel Cave SW quadrangle there is a sharp and persistent change in lithology that has been used as the contact. Below the contact the Morrison rocks are principally blocky noncarbonaceous claystone; above the contact the rocks of the Lakota are either laminated, highly carbonaceous siltstone or thick channel sandstone.

Several types of invertebrate fossils, which were collected in the extreme southeast corner of the quadrangle, were identified by personnel of the U.S. Geological Survey, and are listed below.

Identified by John B. Reeside, Jr.:

Amplovalvata scabrida (Meek and Hayden)
Lioplacodes? sp.
Unio cf. U. nucalis (Meek and Hayden)
Cyzicus ("Estheria") sp. undet.
Gyraulus veternus (Meek and Hayden)

Identified by Raymond E. Peck:

Class, Charophyta:
   Order, Charales:
      Family, Characeae:
         Aclistochara latisulcata (Peck)
         Latochara latitruncata (Peck)
         Praechara voluta (Peck)
         Stellatochara obovata (Peck)
         Aclistochara complanata (Peck)
         Sphaerichara verticillate (Peck)

Identified by I. G. Sohn:

Ostracodes:
   Theriosynoscum sp.
   "Metacypris" sp.
   Darwinula sp.

Both Peek and Reeside state that these forms are fresh-water types and generally imply ponded water.

The Morrison Formation throughout the western interior of the United States is a nonmarine formation consisting of channel deposits, flood-plain deposits, and pond or lake deposits. In the vicinity of the Jewel Cave SW quadrangle, the absence of appreciable sandstone filling ancient channels and the preponderance of clay, clayey limestone, and limestone suggest that no major streams crossed this area, but rather that the area was one of poor drainage and abundant ponds.

CRETACEOUS ROCKS

INYAN KARA GROUP

Sandstone units now included in the Inyan Kara Group of the Black Hills region have been subjected to a long sequence of nomenclatural changes since they were first described by Hayden in 1858. The classification used in this paper is the one that was defined by Karl M. Waagé (1959), and by E. V. Post and Henry Bell, 3d (1961).

The Inyan Kara Group consists of sandstone and mudstone that overlie the Morrison Formation and underlie the black marine Skull Creek Shale. The group is subdivided into two formations—the Lakota Formation at the base, and the Fall River Formation at the top. In some areas of the southern Black Hills, the Lakota Formation can be separated into three members—a basal unit called the Chilson Member, a medial limestone called the Minnewaste Limestone Member, and a top unit called the Fuson Member. The Minnewaste Limestone Member does not occur in the Jewel Cave SW quadrangle.

The local correlation and extent of the various lithologic units with in the Inyan Kara are a matter of some importance because of their implications to the regional stratigraphy and because of the occurrence of uranium deposits in the group at several localities in the Black Hills. For these reasons, each of the major lithologic units has been mapped (see pl. 20). Their relationships in the Jewel Cave SW quadrangle are shown schematically in plate 22.

LAKOTA FORMATION

The Lakota Formation is about 300 feet thick in the southeastern part of the Jewel Cave SW quadrangle, and it thins gradually to the west, where it is about 200 feet thick along Pass Creek. The major rock types are thinly laminated, highly carbonaceous siltstone; very fine to coarse grained and occasionally conglomeratic sandstone; and blocky-weathering gray, green, yellow, or red sandy mudstone.

CHILSON MEMBER

Basal carbonaceous siltstone.—Throughout most of the quadrangle the basal part of the Lakota consists mainly of thinly laminated, highly carbonaceous siltstone. The carbonaceous matter is present both as disseminations of microscopic particles producing black mudstone and as fairly large plant fragments. In many places the siltstone, which has weathered to large silver-gray sheets, is termed "paper shale." Very fine grained cross-stratified or ripple-laminated sandstone is present as beds ranging from a fraction of an inch to 5 feet in thickness. Flat ellipsoidal limestone concretions as much as 18 inches in diameter occur in the siltstone. These concretions consist of light-gray lithographic limestone and contain many small fossil shell fragments. The thickness of the carbonaceous siltstone unit ranges from 0 to about 30 feet. Part of this variation of thickness is due to erosion prior to the deposition of the overlying channel sandstone.

At most places the contact between the basal carbonaceous siltstone and the overlying S₁ sandstone is a disconformity caused by fluvial erosion. The best field example of this relation is in the NE1/4 sec. 28, T. 6 S., R. 2 E.; it is shown on figure 45. The contact relations between these two units are not everywhere as simple as the example in figure 45 would indicate. Along the common boundary between secs. 18 and 19, T. 6 S., R. 2 E., a well-exposed outcrop provides evidence that, at least locally, the S₁ sandstone is interbedded with the carbonaceous siltstone. The relations at this outcrop are shown diagrammatically in figure 46.

FIGURE 45.—Geologic map of part of secs. 28 and 29, T. 6 S., R. 2 E., showing the truncation of basal siltstone of the Lakota Formation by the overlying S1 channel sandstone.

FIGURE 46.—Diagrammatic cross section along the boundary of secs. 18 and 19, T. 6 S., R. 2 E. showing interfingering of the S1 channel sandstone and carbonaceous siltstone.

S₁ sandstone.—The S₁ sandstone is principally a complex of anastomosing channel sandstone that is interbedded with and that grades laterally into gray mudstone. Good exposures of the marginal features of the individual channels occur at several localities. At such places the channel sandstone either thins abruptly as a result of having been deposited in erosional channels in the underlying sediments or interfingers with the adjacent mudstone. The anastomosing nature of the S₁ sandstone is best exhibited in the southeast corner of the quadrangle. In this area of good exposure, individual channels can be traced for only a few hundred feet before their identity is lost because they merge with adjacent or overlying channels. Individual channels were not seen along the southwest and west sides of the quadrangle. Here the unit consists of a sheet of sandstone of relatively uniform thickness, which in a few areas interfingers with laminated, carbonaceous mudstone.

Mudstone.—The mudstone, which is interbedded with the S₁ sandstone, is predominantly light gray to medium gray, is thinly laminated, and contains thin layers of sandstone. Carbonaceous material is common, but it is not as abundant as in the basal siltstone.

Sandstone and mudstone.—At a few places, such as the NW1/4 sec. 17, T. 6 S., R. 2. E., thinly stratified sandstone and abundant interbedded mudstone occupies the entire interval of the S₁ sandstone. These places represent interstream areas that received predominantly very fine grained material during this stage of deposition. In such areas the contact of the thin sandstone and mudstone is gradational with the underlying and overlying units.

FUSON MEMBER

In some parts of the southern Black Hills the Minnewaste Limestone Member marks the boundary between the Chilson Member and the Fuson Member of the Lakota Formation. In this quadrangle the Minnewaste is missing. R. W. Schnabel (1963) believes that isolated bodies of limestone above the S₁ sandstone in the adjoining Burdock quadrangle are correlative with the Minnewaste Limestone Member. For this reason the top of the S₁ channel complex has been shown as the top of the Chilson Member in the Jewel Cave SW quadrangle, but this contact is gradational in many places.

Mudstone.—Between the top of the S₁ sandstone complex and the base of the Fall River Formation are several beds of mudstone that are separated by sandstone beds (the individual sandstone beds are described in the following paragraphs). The mudstone beds have several common features that set them apart from the underlying gray mudstone: (a) the almost complete absence of carbonaceous matter, (b) the lack of lamination (most are blocky weathering), and (c) the occurrence of green, yellow, red, and variegated colors. In addition, the mudstone that occurs between the S₁ sandstone and the S₃ sandstone is commonly slightly calcareous and contains abundant calcareous concretions as much as 5 feet in diameter. Beds of limestone have also been observed in this unit. Both the concretions and the limestone beds consist of aggregates of small spheres of calcite that are about 0.5 to 1.0 mm in diameter and that have a radial-fibrous structure. The concretions probably formed at the time the enclosing mudstone was being deposited rather than by later replacement, because the interstices between the calcite spheres are either empty or filled with chalcedony.

At many places the mudstone contains abundant sand-size quartz grains and may grade laterally into clayey sandstone. Where sand-size material is abundant, the mudstone is unstratified and very poorly sorted. Within a thin section the areas that consist of predominantly clay-size material are irregularly interspersed with patches containing as much as 50 percent sand.

Within the mudstone interval between the top of the S₁ complex and the base of the S₃ sandstone, there are a few small sharply defined sandstone bodies that clearly fill small channels.

S₃ sandstone.—The S₃ sandstone, which is generally between 20 and 30 feet thick, is a sheetlike body of thin- to thick-bedded cross-stratified gray sandstone that is dominantly fine grained but that is locally coarse grained or conglomeratic. At its upper and lower boundaries the S₃ sandstone is irregularly interbedded with sandy mudstone, and in places it contains thin interbeds of mudstone. Very commonly the upper 1 to 2 feet of sandstone is cemented with calcite.

In the southern part of the Jewel Cave SW quadrangle the S₃ sandstone occurs about 200 feet above the base of the Lakota Formation, but the base of sandstone S₃ truncates the underlying mudstone and the S₁ sandstone northward from the center of sec. 4, T. 6 S., R. 1 E. To the north in the Dewey, Clifton, and Fanny Peak quadrangles, the S₃ sandstone seems to fill a generally northeast- to north-trending channel in the lower part of the Lakota Formation (Brobst, 1961, p. 34).

White massive sandstone.—A distinctive massive noncarbonaceous white sandstone lies immediately above the S₃ sandstone in the southern part of the quadrangle; it ranges in thickness from 0 to about 30 feet, is well sorted and fine to very fine grained, and grades both laterally and vertically into sandy mudstone. On the east side of Pass Creek, in secs. 14 and 23, T. 6 S., R. 1 E., the sandstone contains large well-rounded, highly polished pebbles as much as 2 inches in diameter. On the west side of Pass Creek, similar pebbles were observed in the upper part of the S₃ sandstone.

Dolomite.—.Approximately 5 feet of gray to pinkish-gray massive dolomite caps an isolated hill in the NW1/4 sec. 17, T. 6 S., R. 2 E. The dolomite rests on about 10 feet of coarse-grained cross-stratified sandstone, which is mapped as S₃ sandstone. The dolomite consists of a mosaic of anhedral dolomite grains less than 0.03 mm in diameter, and 30 to 60 percent by volume of very fine grained quartz.

S₄ sandstone.— The S₄ sandstone fills a large channel that was cut from very near the top of the Lakota down into the upper part of the S₃ sandstone. The channel enters the quadrangle from the south in the NE1/4 sec. 29, T. 6 S., R. 2 E., and trends about N. 30° E. The channel is about 1 mile wide and approximately 70 feet deep at its center. Although only about 1-1/2 miles of the channel remain in the Jewel Cave quadrangle, the S₄ sandstone extends for about 20 miles to the southeast.

A thin bed of sandstone at the top of the Lakota Formation, which is believed to be correlative with the S₄ sandstone, crops out along the west side of Pass Creek south of the Dewey fault.

COMPOSITION OF SANDSTONE OF THE LAKOTA FORMATION

Twenty-two thin sections selected to represent the various sandstone units were examined. Most of these sections were stained with sodium cobalt nitrite to determine the presence of potassium feldspar. The volumes of the various types of detrital constituents were estimated visually.

The composition of sandstone from all units is very similar. In the fine-grained rocks the dominant detrital constituent is quartz, which makes up about 85 to 95 percent. Chert grains are the next most abundant material, and make up from 1 to 7 percent. The total feldspar content is about 1 to 10 percent and probably averages about 3 percent; plagioclase is a very minor constituent. The chert and quartz content varies considerably with change in grain size. Medium- to coarse-grained sandstone may contain as much as 60 percent of well-rounded chert grains. Many of these grains are clearly fragments of silicified limestone, because they contain minor remnants of calcite and silicified shell fragments. Much of the chert is white, and on weathered surfaces some of the grains are rather soft and may be mistaken for fragments of claystone.

Among the accessory minerals, zircon, ilmenite, and leucoxene are most abundant. Yellow-brown tourmaline is very common; garnet and rutile are consistently present in minor amounts; and traces of amphibole, apatite, muscovite, and monazite were observed.

The sandstone is only moderately well cemented. The dominant cementing material is quartz, which is present as oriented overgrowths. Calcite is a cement only in localized concretionlike masses or in irregular patches a few feet across. Iron oxide is locally an important cement and is probably a late addition resulting. from the near-surface oxidation of other iron-bearing constituents.

ENVIRONMENT OF DEPOSITION OF THE LAKOTA FORMATION

Clearly recognizable stream channels are present at all horizons within the formation. That the general direction of streamflow was to the north or northwest is indicated by the dominant trends of channel axes and by the directions of the dip of cross strata (fig. 47). Sandstone strata that have been described as sheetlike bodies have an observable lateral extent of less than 3 miles normal to the dominant direction of streamflow, and they probably represent broad, braided, or meandering stream courses.

FIGURE 47.—Plot showing the direction of dip of 115 cross strata in the S1 sandstone.

Laminated sandy carbonaceous mudstone that underlies or interfingers with the channel sandstone is interpreted as flood-plain and swamp deposits of the interstream areas. Nonlaminated, noncarbonaceous, blocky-weathering mudstone that contains abundant, large syngenetic calcite concretions makes up much of the upper part of the formation. Associated rock types are carbonate rock and massive unstratified sandstone; the former is dolomite in the Jewel Cave quadrangle and limestone (Minnewaste Limestone Member) in adjacent areas to the southeast. These sediments were probably deposited in interstream areas, possibly largely in ponds and lakes.

Chert grains containing silicified fossil fragments are very common, particularly in the coarse-grained sandstone, and indicate that an appreciable amount of the Lakota sediment was derived from preexisting sedimentary formations. The streams that deposited much of the Lakota sediment entered the area from the east and southeast, and in these directions the basal sandstone of the Cretaceous gradually transgresses lower and lower in the section until, in eastern South Dakota, it rests on Precambrian rocks. The Pahasapa Limestone of Early Mississippian age and the Minnelusa Formation of Pennsylvanian and Permian ages, which the Lakota streams must have crossed contain abundant limestone units from which the chert was probably derived.

FALL RIVER FORMATION

The Fall River Formation, which crops out only along the south side of the quadrangle (see pl. 20), is about 110 to 130 feet thick; it consists of a lower unit of laminated siltstone, a middle unit composed of thick medium-grained brown-weathering sandstone, interbeds of claystone, and very fine grained sandstone, and an upper unit of varicolored mudstone and thinly bedded sandstone.

LOWER UNIT

Carbonaceous siltstone.—In the southwest corner of the quadrangle the lower unit of the Fall River is composed of thinly bedded carbonaceous siltstone that contains many thin lenticular strata of sandstone. The siltstone ranges in thickness from 0 to about 25 feet. This unit contrasts markedly with the blocky-weathering, noncarbonaceous mudstone of the upper part of the Lakota.

MIDDLE UNIT

S₅ sandstone.—The S₅ sandstone is a fine- to medium-grained well-sorted sandstone forming an elongate body trending northwestward. The sandstone ranges in thickness from about 15 to 90 feet; it is about 1-1/2 to 2 miles wide in a northeast-southwest direction and has been traced for 25 miles along the southwest rim of the Black Hills (G. B. Gott, oral communication, 1958).

Only the southwest side of the body is present in the Jewel Cave SW quadrangle, the northwest side having been removed by erosion. The southwest margin of the S₅ sandstone is exposed along Pass Creek in secs. 22, 23, and 26, T. 6 S., R. 1 E. Here, the base of the sandstone seems to be an erosion surface that cuts across the underlying siltstone at a gentle angle. One 15-foot-thick sandstone, which overlies the carbonaceous siltstone in sec. 26, T. 6 S., R. 1 E., has been shown on the geologic map as continuous with S₅. This thin sandstone seems to have been truncated by the S₅ sandstone and is probably older.

The S₅ sandstone body is not flat on top; on the contrary, the interval between the base of the Fall River and the top of the S₅ sandstone increases from about 40 feet at the southwest margin to about 100 feet near the center of the body (pl. 22). The contact of this upper surface with the fine-grained marginal sediments is not well exposed, but it seems to be transitional. A zone. of thin-bedded sandstone and interbedded mudstone representing this transition zone was mapped in secs. 22, 23, 25, and 26, T. 6 S., R. 1 E. (pl. 20).

The southwest margin of the body trends about N. 45° W. in the Jewel Cave SW quadrangle, but the dominant dip direction of cross beds is to the west (fig. 48). Generally, the crossbedding consists of planar sets ranging from 1 to 2 feet thick separated by thin horizontally stratified sandstone beds. Very long cross strata, which extend across the full thickness of the S₅ sandstone (about 30 feet), occur near the margin of the body and are separated by thin partings of mudstone. The elongate form, the scoured lower surface of the sandstone body, and the short planar crossbeds suggest that the S₅ sandstone was deposited in a river channel; however, the anomalous features of crossbedding and the convex shape of the upper surface suggest the possibility that the S₅ may have been deposited in a distributary channel.

FIGURE 48.—Plot showing the direction of dip of 160 cross strata in the S5 sandstone.

Mudstone and sandstone at the southwest margin of the S₅ sandstone.—Thin beds of sandstone and mudstone make up the middle unit of the Fall River Formation in secs. 22 and 23, T. 6 S., R. 1 E. These thin beds in part are truncated by the S₅ sandstone and in part grade laterally into, the S₅ sandstone.

UPPER UNIT

A thin bed of yellow to gray mudstone overlain by thinly laminated siltstone and shale forms the upper unit of the Fall River Formation. These beds crop out only in the southwestern part of the quadrangle. They overlie the S₅ sandstone, where that sandstone is present, and where the S₅ sandstone is absent, they overlie the thin beds of sandstone and mudstone that make up the middle unit of the Fall River.

A composite section of the entire Fall River Formation, measured along the sides of Pass Creek in secs. 22 and 26 is given below.

Composite section of the Fall River Formation in the NW1/4 sec. 26, T. 6 S., R. 1 E.
[Subunits 4—S shown on pl. 20 as Kfmm]

COMPOSITION OF SANDSTONE OF THE FALL RIVER FORMATION

Ten thin sections representing the various sandstone units of the Fall River were selected for study. Several of these sections were stained with sodium cobalt nitrite to determine the presence of potassium feldspar, and the volumes of the various types of detrital constituents were estimated visually.

The fine-grained sandstone marginal to sandstone S₅ is uniform in composition. Quartz grains make up about 95 percent of all samples, chert grains make up only about 1 to 3 percent, and feldspar (primarily potassium) is less than 1 percent. White mica is common and may make up as much as 3 percent of the rock. The fine- to medium-grained samples from sandstone S₅ are similar to each other in composition. Quartz is dominant, feldspar (primarily potassium) occurs from trace amounts to about 3 percent, chert is less than 2 percent of the rock, and mica occurs only in traces.

Ilmenite, leucoxene, tourmaline, garnet, and zircon are abundant accessory minerals; rutile is common; amphibole and apatite were observed.

The sandstone of the Fall River Formation is moderately well cemented in most places. The dominant cementing material is quartz, which is present as oriented overgrowths on quartz. Calcite was observed in only a few very thin brown beds immediately below the Skull Creek Shale. Iron oxide is locally an important cement and probably results from the near-surface oxidation of other iron-bearing constituents. Limonite pseudomorphs after pyrite, which are very common near the margins of the S₅ sandstone in Pass Creek, suggest that the brown color of the unit is due to the oxidation of disseminated pyrite.

In the N1/2 sec. 29, T. 6 S., R. 2 E., near the top of Pilger Mountain, parts of the S₅ sandstone are very extensively silicified. The secondary silica is present as clear oriented quartz overgrowths on the well-rounded detrital quartz grains. The S₅ sandstone in this area also contains abundant iron oxide, which imparts to the rock a purplish-brown to dark-brown color. Examination in thin section shows that the iron oxide was deposited before the secondary quartz, because the oxide is present only as coatings around the original rounded quartz grains, and the quartz overgrowths cover the oxide rims. The original extent of silicified sandstone is unknown for the S₅ sandstone has been eroded from most of the adjacent area. However, a small hill in the south-central part of sec. 18, T. 6 S., R. 2 E., which is about 2 miles northwest of Pilger Mountain, is covered with float blocks of silicified S₅ sandstone. These two areas are only a few hundred feet from the crest of a prominent northwest-trending anticline. A third area of intensely silicified S₅ sandstone has been mapped in the southern Black Hills (V. R. Wilmarth and Henry Bell, 3d, oral communication, 1958) along the crest of the Chilson anticline south and west of Parker Peak. These areas of silicification, localized on structurally high areas, may have been places of major ingress of ground water into the Fall River at a time when a less dissected erosion surface truncated the Lakota and Fall River hogback. Little is known about the Cenozoic history of the southern Black Hills, but it seems to the writer that the areas of silicified sandstone may represent remnants of a pre-White River (Oligocene) pediment.

ENVIRONMENT OF DEPOSITION OF THE FALL RIVER FORMATION

Waagé (1959, p. 63), as. a result of a study of the features of the Inyan Kara and related rocks throughout the Black Hills, characterized the environment of the Fall River as follows:

The type of bedding prominent in the Fall River and the abundance of soft-bodied animals, indicated by the numerous burrows and castings, are indicative of marginal marine, probably tidal-flat, environment. Associated carbonaceous beds, dominantly shaly local phases, and massive sandstone, probably were formed in coastal swamp, estuarine and deltaic environments respectively. Ordinarily marine fossils might be expected at least in the tidal flat and estuarine phases. Washings of a few Fall River shale samples revealed worn mollusk shell fragments and some fragments of what may have been arenaceous Foraminifera, but no undoubted marine fossils.

From a study of the limited outcrops of the formation in the Jewel Cave SW quadrangle this writer cannot make any additions to this general picture.

SKULL CREEK SHALE AND MOWRY SHALE

The Skull Creek Shale was defined as a member of the Graneros Shale by Collier (1922, p. 79). The type locality is Skull Creek, southeast of Osage, Wyo. The Mowry Shale was defined as a part of the Benton Formation by Darton (1904). The type locality is in the east flank of the Bighorn Mountains on Mowrie Creek northeast of Buffalo, Wyo. Both of these members were raised to formation rank in this area by Reeside (1944).

The Skull Creek Shale and Mowry Shale crop out only in the extreme southwest corner of the quadrangle, and exposures are poor. The contact of the Skull Creek with the Fall River seems to be transitional with several feet of silty black fissile shale alternating with thin sandstone. The top of the Skull Creek is exposed only on the north side of a small gravel-capped hill in the NW1/4 sec. 27, T. 6 S., R. 1 E. The lower 15 feet of this exposure consists of black fissile shale in which no fish scales were found and which contained several beds 1/4 to 4 inches thick of light-gray bentonite. The upper part of the exposure consists of black fissile shale that weathers to silvery gray and that contains abundant fish scales. The part bearing fish scales was mapped as Mowry Shale.

Northwest of the Jewel Cave SW quadrangle a unit of sandstone and carbonaceous shale called the Newcastle Sandstone lies between the Skull Creek and the Mowry Shales. In the Jewel Cave SW quadrangle, the interval that would be occupied by the Newcastle is very poorly exposed. The only rock that might be placed in this interval is a mass of rubbly calcite-cemented sandstone, which forms the top of a very gentle hill in the NW1/4 sec. 27, T. 6 S., R. 1 E., about 500 feet from the west section line.

QUATERNARY DEPOSITS

Small remnants of gravel-covered terraces that occur along Pass Creek, Hell Canyon, and Schenk Canyon mark the former levels of these streams. Remnants of gravel terraces also cap hills at altitudes of 4,460 to 4,620 feet in the northern part of the quadrangle between Hell Canyon and Schenk Canyon. Darton (in Darton and Paige, 1925, p. 15) believed that this high-level gravel was deposited by a stream of probable Pleistocene age that drained southeastward along the Spearfish Valley toward Minnekahta, S. Dak. This stream would antedate the present drainage, which extends from the Precambrian southwestward through the Paleozoic and Mesozoic rocks. In the Jewel Cave SW quadrangle the high-level gravel consists of boulders and pebbles of the Minnekahta Limestone, chert, coarse-grained hematitic and silicified sandstone and conglomerate, and gray to yellow limestone. No Precambrian rocks were found.

Patches of landslide debris are particularly abundant on the steep hogback slopes that face into Spearfish Valley. Much of the debris is from the Inyan Kara rocks. In many of the slide areas, large masses of rocks have remained intact while sliding downhill and have been rotated so that they dip steeply toward the source of the slide. The landslide areas in the quadrangle are timbered, and no fresh slide scars were observed.

Alluvium fills the bottom of most of the major streams, except where the streams pass over resistant rock and are narrow. Some of the small valleys in the hogback composed of Inyan Kara also contain alluvial fill. In most places the alluvium has been partly eroded so that the present intermittent streams flow in narrow steep-walled gullies.