Geological Survey Water-Supply Paper 1475-D Geology and Occurrence of Ground Water at Jewel Cave National Monument, South Dakota

Formations at the monument range in age from Precambrian to Pennsylvanian but the sequence is not complete. As elsewhere throughout the Black Hills area, rocks of Silurian and Devonian age are absent (Darton and Paige, 1925, p. 2). Ten feet of sandstone penetrated by the drilling of a test well directly below the Englewood limestone is tentatively identified as Ordovician in age.

The monument is on a broad plateau of sedimentary rocks that begins about 6 miles east of the cave and slopes gently westward into Wyoming. Because most of the sedimentary formations do not crop out within the area, a reconnaissance was made of their outcrops at the scarp face of the plateau. The surficial geology of the consolidated rocks in the monument area is shown on figure 24.

Figure 24.— Geologic map of the Jewel Cave National Monument area, South Dakota (modified after Darton and Paige, 1925). (click on image for an enlargement in a new window)

Beginning with the oldest of the rock units, the lithology and stratigraphic relations apparent at the outcrops are briefly described below.

PRECAMBRIAN ROCKS

The sedimentary rocks that crop out at the scarp face of the plateau are underlain by quartz-mica schist of Precambrian age. Just east of the escarpment the schist is cut by many dikes of quartz, amphibolite, and granite pegmatite. Presumably the Precambrian rocks that crop out at the foot of the escarpment are representative of those that underlie the monument area.

CAMBRIAN SYSTEM

DEADWOOD FORMATION

Lying unconformably upon the Precambrian rocks at the base of the escarpment is an estimated 90 to 100 feet of the Deadwood formation that is largely covered by talus from the overlying Pahasapa and Englewood limestones. The parts of the formation that could be observed indicate that the Deadwood consists chiefly of quartzitic sandstone and dense to finely crystalline dolomite.

MISSISSIPPIAN SYSTEM

ENGLEWOOD LIMESTONE

The contact between the Deadwood formation and the overlying Englewood limestone is covered by talus, thus, the presence of a thin intervening sandstone penetrated by the test well at the monument was not recognized.

Although the total thickness of the Englewood at the outcrop is estimated to be 50 feet, the lower part of the formation is generally covered by talus. The uppermost 8 feet of the formation was examined at several locations and was found to consist of highly fossiliferous, massive, finely crystalline, pink and buff limestone.

PAHASAPA LIMESTONE

The upper part of the Englewood limestone grades upward into the Pahasapa limestone through a zone of impure buff limestone that ranges from several inches to 2 feet in thickness. Due east of the monument the Pahasapa constitutes the crest of the scarp face of the plateau and the formation is widely exposed between the escarpment and the west boundary of the monument. Average altitude of the top of the formation within the monument is about 5,400 feet above mean sea level.

Because of differential erosion, the thickness of the Pahasapa differs considerably along the escarpment. Within the monument the exposed thickness of the Pahasapa ranges from about 70 to about 300 feet. Nearly the entire thickness of the formation is exposed in Hell Canyon half a mile north of the entrance to Jewel Cave.

Although the thickness of the Pahasapa at Jewel Cave was estimated to be slightly more than 300 feet, on the basis of measurements at the outcrop along the escarpment, the test well penetrated 432 feet of the formation. The discrepancy between the estimated and actual thickness probably is caused in part by a difference in the attitude of the formation at the cave and at the escarpment.

Where it was observed at outcrops, the Pahasapa is mostly fine grained and massive. Within the monument the upper part is generally more massive than the lower part and the top consists primarily of brown and light-gray chert. The lower part, exposed in Hell Canyon, consists of relatively thin bedded brown and buff limestone and dolomite with much brown earthy clay. In addition to the large caverns of Jewel Cave the formation contains many smaller cavities lined with calcite.

PENNSYLVANIAN SYSTEM

MINNELUSA FORMATION

The Minnelusa formation lies directly upon and obscures, the Pahasapa limestone on most of the prominent ridges in the monument area. On the east side of the monument most of the Minnelusa has been removed by erosion and only minor remnants are found in the immediate vicinity of Jewel Cave.

The total thickness of the Minnelusa at the outcrops east of the monument ranges from a few inches to about 100 feet. The formation thickens west of the cave and on the west side of the monument reaches a thickness of about 250 feet.

In general, the Minnelusa consists of interbedded limestone, dolomite, sandstone, and red shale. Within the monument it is composed chiefly of red shale and red-brown, yellow, or white, fine-grained sandstone.

RECENT SERIES

ALLUVIUM

The floors of the deeper canyons within the monument are covered with a deposit of rock fragments and soil and the slopes and summits of the higher ridges are overlain by thin deposits of soil. The rock fragments are chiefly Pahasapa limestone and a few scattered pieces of sandstone from the Minnelusa formation.

The maximum thickness of the alluvium is not known but it may be as much as 30 feet in Hell Canyon south of Jewel Cave. At the test well, the Pahasapa limestone is overlain by 2-1/2 feet of dark-red clayey soil containing many fragments of chert. The soil apparently is a product of residual weathering of the Pahasapa limestone combined with residual or slightly transported remains of the red shale of the Minnelusa formation.

FAULTING

A prominent fault crosses the monument about 1,500 feet north of the entrance to Jewel Cave. The fault begins in the Precambrian rocks to the east and extends westward to the west side of the monument, causing a pronounced offset in the escarpment. West of the monument the fault breaks into two diverging branches and disappears within a very short distance. Except for a gentle northward deflection along a distance of about half a mile, the fault trends nearly east-west across the monument.

U.S. Highway 16 follows the fault line from the escarpment to the east side of the monument and parallels cliffs of Pahasapa limestone that mark the edge of the upthrown fault block through most of this distance. South of the highway, along the downthrown block of the fault, the lower part of the Minnelusa formation lies opposite the upper part of the Pahasapa limestone. One mile west of the entrance to Jewel Cave the total displacement of the fault is about 120 feet (Darton and Paige, 1925, p. 18).

An interesting phenomenon observed during the reconnaissance of the surface geology was the scarcity of soil in Hell Canyon north of the fault. South of the fault the floor of Hell Canyon is covered with an estimated 6 to 10 feet of red clayey soil. North of the fault the alluvial fill is chiefly boulders and smaller fragments of limestone and little soil. It is not known if the soil has been removed by water moving down the valley and entering the fault or if limestone fragments are accumulating faster than soil can cover them.

Additional test drilling would be necessary to determine with certainty what effect the fault has on ground-water occurrence near Jewel Cave.

SPRINGS

In addition to the spring that furnishes the present water supply at Jewel Cave, three other springs, all outside the monument, were visited. Locations of the springs are shown on figure 25.

FIGURE 25.—Map of the Jewel Cave National Monument area, South Dakota, showing location of springs and test well. (click on image for an enlargement in a new window)

A spring in Hell Canyon about 1 mile north of the cave (SE1/4NW1/4NE1/4 sec. 35, T. 3 S., R. 2 E.) supplied enough water in the late 1930's for a Civilian Conservation Corps camp in the canyon. Development of the spring consists of a concrete box about 5 feet square and 4 feet deep in an excavation at a seep in the bottom of the canyon. When the site was visited in May 1959, only about 2 inches of water covered the floor of the concrete box and no water was observed entering the box through the intake pipe about 6 inches above the floor. Whether enough water is now available at the site to again furnish a supply is unknown.

A second spring (NW1/4SW1/4SE1/4 sec. 2, T. 4 S., R. 2 E.) about half a mile south of the entrance to Jewel Cave on the west side of Hell Canyon yielded about 1 gpm (gallon per minute) of potable water in May 1959. Visible development of the spring consists of a section of 1-1/4-inch pipe thrust into a seep on the side of the canyon. The pipe extends downhill to a stock tank about 20 feet east. Apparently much more water is escaping from the spring than issues from the pipe as the ground is waterlogged and soft in the floor of the canyon 150 feet below and 600 feet south of the spring.

The third spring (SW1/4SE1/4NW1/4 sec. 6, T. 4 S., R. 3 E.), locally known as Lithograph Spring, is in Lithograph Canyon about 1-3/4 miles east of the entrance to Jewel Cave. The spring was yielding about half a gallon per minute when the site was visited in May 1959. A 1-1/4-inch pipe extends from a seep in the bottom of the canyon to a stock tank about 30 feet west and 4 feet lower than the seep.

Because of their distances from the monument facilities and their small yields, the springs are not considered to be potential sources of water supply for the monument.