(Sid Covington)

This section describes the rocks and unconsolidated deposits that appear on the digital geologic map of Mount Rushmore National Memorial, the environment in which those units were deposited, and the timing of geologic events that created the present landscape.

The Precambrian sediments and granitic rocks that are now exposed in Mount Rushmore National Memorial were once buried about 14 kilometers (9 mi) below the surface by younger sediments. Intense heat and pressure at these great depths changed the Precambrian sediments to metamorphic rocks such as schist. The metamorphic and igneous rock units arched upward during the Laramide Orogeny creating the Black Hills. The younger, overlying sediments were later eroded exposing the oval-shaped Precambrian core rimmed by Paleozoic and Mesozoic rocks (fig. 3).

Figure 3. Diagram of the Black Hills dome showing progressively younger sedimentary rock layers ringing the central Precambrian core. Mount Rushmore National Memorial lies within the central crystalline core; Jewel Cave National Monument and Wind Cave National Monument lie within Paleozoic limestone; and Devils Tower is a Tertiary feature. Modified from Strahler, 1960.

Precambrian History of the Black Hills Region

The Precambrian rocks exposed in the Black Hills are predominantly Early Proterozoic (1,600-2,500 Ma) metasedimentary units. Exposures show both northwesterly and northeasterly structural trends and a locally dominant northwest-trending metamorphic foliation. In the southern Black Hills, the Harney Peak Granite forms a dome that disrupts and modifies the local structure of the Proterozoic metamorphic rocks.

The sheer number of Precambrian units mapped for the Black Hills region illustrates the complexity of the Precambrian depositional and tectonic history. Understanding the evolution of the Black Hills during the Precambrian is limited due to the complex geology, pervasive metamorphism, and deformation, as well as a lack of precise age control for metasedimentary rocks in the core of the Black Hills. However, some age control is possible due to radiometric age dates that are available for igneous rocks in the Black Hills.

The following interpretations of Precambrian history are based on stratigraphic, depositional, tectonic, and chronologic studies of the Black Hills (Redden et al. 1990). Correlation with units in surrounding areas is difficult because the Black Hills are more than 300 kilometers (190 mi) from correlative outcrops in southern Wyoming and more than 1,100 kilometers (680 mi) from similar age rocks in Manitoba and Saskatchewan.

2,500 (Ma)
Archean basement rocks underlie the Black Hills. These basement rocks consist of metamorphosed fine-to coarse-grained epicontinental clastic rocks and granite. During deposition of these ancient sediments, Wyoming formed the southern margin of the landmass that later became the core of the North American craton.

2,500 Ma to 2,170 Ma
An extensional tectonic setting developed along the passive southern margin of the craton. Coarse-grained clastic rocks with local anomalous concentrations of uranium and minor amounts of chromite were deposited on the continental Archean basement. Pronounced changes in lithology, remnant sedimentary features, and faults that formed as the sediments were deposited indicate an alluvial fan depositional setting. Sediments deposited in the alluvial fan were eroded from a western source. Sediments become finer-grained towards the top and may preserve progressive deposition into a marine basin that had originally formed to the east.

2,170 to 1,980 Ma Deformation Event
Gabbro sills intruded into the Early Proterozoic strata and Archean basement about 2,170 Ma (Redden et al. 1990). Folding occurred along north-northwest.trending axes with subsequent erosion. These events indicate a change from an extensional tectonic setting to one of compression along the cratonic plate margin. The deformation occurred between 2,170 Ma and 1,980 Ma and may record early activity associated with the 1.9 to 1.7 billion-year-old Trans-Hudson Orogeny that resulted in more land being accreted to the Wyoming province (fig. 6). Sediments derived from the east were deposited in alluvial fans that connected to a marine shelf and tidal flat to the west and south. Folding caused by tectonic compression and increased sedimentation resulted in subsidence and the creation of the main Black Hills basin. Quartz-rich and feldspathic sediments deposited into the Black Hills basin eventually were metamorphosed to quartzite and mica schist.

At about 2,000 Ma, black shale was deposited in a deepening Black Hills basin. This deposition was accompanied by the submarine eruption of basalt. Density currents (turbidites) transported sediment from more shallow, shelf environments into deeper water and deposited these sediments over the basalt.

Two depositional settings have been proposed for the Black Hills area during this time. One setting is an intracontinental basin. Intracontinental basins form within the interior of the continent and away from plate boundaries. The other is a back-arc basin. Back-arc basins are located landward of a volcanic arc that forms above a subduction zone. The general composition of the turbidites, however, favors the interpretation of an intracontinental basin depositional environment (Redden et al. 1990).

While a marine basin formed in the Black Hills region, the area to the east was undergoing tectonic uplift and deformation as well as volcanism. Water depths increased from east to west and north to south. Source areas to the east supplied the sands that later became quartzites and silts and muds that later would be metamorphosed to mica schist.

1,980 Ma to 1,900 Ma Depositional Break
A depositional hiatus lasting about 90 my separates the turbidite deposits and basalt flows of the initial basin development and deepening from younger overlying conglomerates, debris flows, and scattered volcanic rocks. In some areas, the older rocks were folded and overturned prior to deposition of conglomerate and debris flow deposits. This suggests that a tectonic event may have occurred prior to the depositional break. Any deformation during this time is poorly understood. Although speculative, the deformation may be attributed to east-west closure of the Black Hills basin along part of what would become the axis of the Trans-Hudson Orogeny (Redden et al. 1990).

1,900 to 1,710 Ma Deposition and Deformation Events
A thick, widespread shale unit in the central Black Hills was deposited between 1,880 and 1,710 Ma. The main Black Hills basin continued to evolve as an intracontinental or back-arc basin from about 1,900 Ma to 1,800 Ma.

The Archean basement and overlying Early Proterozoic strata were highly deformed during the Trans-Hudson Orogeny that occurred approximately 1.7 to 1.9 billion years ago (Hoffman 1989; Redden et al. 1990). This deformation event resulted from the collision of the Superior craton of eastern Canada with the Hearne craton in northern Saskatchewan and the Wyoming craton of the western United States. The stratified rocks and Archean basement were folded along north-northwest trending axes and regionally metamorphosed. The orogeny included extensive folding and thrust faulting, metamorphism, and granitic intrusion along a belt that extended from Hudson Bay west through Saskatchewan and then south through the western portions of the Dakotas and Nebraska.

Rubidium-strontium (Rb-Sr) age dates suggest that the metamorphism may have occurred about 1,840 Ma. Igneous activity associated with the Trans-Hudson Orogeny primarily occurred from about 1,910 Ma to 1,840 Ma.

1,710 Ma: Harney Peak Granite
The Harney Peak Granite was likely derived from melting of the Archean crust and minor amounts of Early Proterozoic rocks that overlie the Archean basement. The intrusion of magma that created granite plutonic domes also deformed the previously folded structures. The 1,710 million-year-old Harney Peak Granite seems too young to be related to the 1,910-1,840 Ma main phase of igneous activity associated with the Trans-Hudson Orogeny (Redden et al. 1990). Tectonic activity in the Central Plains Orogeny to the south is the event more likely associated with the generation of the Harney Peak Granite.

During the Central Plains Orogeny, a series of island arcs were progressively added to the southern margin of North America from about 1,780 Ma to 1,610 Ma (Carlson and Treves 2001). The Central Plains Province (fig. 6) consists of metamorphic and granitic rocks in a belt more than 1000 kilometers (620 miles) long and at least 500 kilometers (310 miles) wide that truncates the rocks of the Trans-Hudson orogen (Sims and Peterman 1986).

Middle Proterozoic Uplift
Metamorphosed volcanic tuff (metatuff) and metamorphosed greywacke (metagreywacke) record a metamorphic event that 0ccurred between 1,600 Ma and 1,400 Ma (Redden et al. 1990). A northeast-trending metamorphic foliation that is found in the rocks of the Black Hills developed during this time. This foliation may have been part of a much larger structural regime, but the regional extent of this Middle Proterozoic tectonic activity is unknown.

Figure 6. Location of the Black Hills Uplift with regard to Precambrian terranes, northern Great Plains region. Modified from Lisenbee and DeWitt (1993).

Paleozoic History of the Black Hills Region

The Paleozoic strata that rim the core of the Black Hills record several episodes of marine deposition followed by subaerial exposure and erosion. Evidence of shallow marine depositional environments of a transgressive sea is preserved by the Cambrian and Ordovician sandstone, glauconitic shale, limestone, and dolomite units (Driscoll et al. 2002). Sea level rose in the latest Cambrian and flooded almost the entire North American continent, leaving a strip of land or a series of islands exposed along what is known as the Transcontinental Arch). This arch is an upland that stretched from northern Minnesota southwestward across South Dakota, northwestern Nebraska, Colorado and northwestern New Mexico. Before the end of the Early Ordovician, the sea receded from the craton. Throughout the Devonian, erosion stripped much of the Cambrian and Ordovician strata from the area. Silurian age rocks are not present in the Black Hills area because of either erosion or nondeposition.

From the Late Devonian to the Middle Mississippian, the western margin of North America was being deformed as the North American lithospheric plate collided with oceanic crust to the west (Johnson et al. 1991). This collision caused another west to east transgression of the sea onto the craton. In some intracratonic basins such as the Williston Basin in North Dakota and Montana, organic-rich marine shale accumulated in deep subtidal, dysaerobic (low oxygen) environments. These black shales are primary petroleum source rocks in these basins today. In the Black Hills, Devonian and Mississippian marine limestones overlie Devonian shales that were deposited in shallower, subtidal marine environments. Deposited in aerobic environments, the organic matter was biodegraded so that these shales did not become rich petroleum source rocks.

The Mississippian-age Madison (also called Pahasapa) Limestone records deposition in a warm, shallow-marine environment. When the sea regressed again in the Late Mississippian, this limestone was exposed at or near the land surface. Extensive erosion and elaborate karst (solution) features developed in the Madison Limestone at this time.

During the Pennsylvanian and Permian, the South American plate collided with the southern margin of North America, generating the forces responsible for the Ancestral Rockies in Colorado. The Pennsylvanian-Permian Minnelusa Formation unconformably overlies the Madison Limestone and is a coastal deposit with dune structures at the top of the formation that may preserve beach sediments (Driscoll et al. 2002). In general, the Pennsylvanian and Permian strata in the Black Hills record episodic transgressive and regressive cycles of the sea into the Black Hills basin. Marine sandstone, shale, siltstone, and limestones are overlain by evaporite deposits (anhydrite) and terrigenous red clastics.

Mesozoic History of the Black Hills Region

By the Early Triassic, the major landmasses had come together to form a supercontinent called Pangaea. The red shale, siltstone, and evaporite deposits in the Spearfish Formation preserve a record of subaerial deposition in a continental setting. By the end of the Triassic, Pangaea was breaking apart and the relatively high velocities of plate movements caused a eustatic (worldwide) sea level rise in the Jurassic.

To the west during the Jurassic, the Farallon plate was subducted beneath the North American plate as the early phases of the Sierra Nevada batholithic intrusions and related volcanic eruptions occurred (Brenner 1983). As relative sea level rose, the sea encroached southward from the Arctic and into the Western Interior of North American. Between the Early and Late Jurassic, the Black Hills area moved from 15o north latitude to about 35o north latitude as the North American continent drifted northward (Kocurek and Dott 1983; Brenner 1983). The abundance of red beds, evaporites, and shallow water carbonates suggest that Jurassic paleoclimate in this area was generally warm and dry. The shale, glauconitic sandstone, and limestone of the Sundance Formation in the Black Hills preserve at least two transgressive .regressive cycles, including the last and most extensive transgression of the Jurassic. Extending as far south as present-day New Mexico, this broad epeiric sea covered much of the Western Interior of North America. The sea covered an area bordered by rising mountains to the west as the Sevier Orogeny began to deform the west coast and on the east by the stable mid-continent craton. The widely distributed Morrison Formation records continental environments of deposition during the subsequent marine regression in the uppermost Jurassic.

Although the subduction zone along the western margin of North America was far to the west of the Black Hills region, compressive forces caused by the collision were felt far inland. Several processes acted in concert to change the landscape of the Western Interior. As layers of rock were thrust up into mountains along the western margin, the land east of the concentrated mass responded by bending, folding, and flexing downward into an expanding foreland basin. In addition, as the mountains were thrust above sea level, weathering and erosion produced a vast amount of cobbles, pebbles, sand, silt and clay, which were deposited into this down-flexing and expanding Western Interior basin. The sediments added more weight to the basin and resulting in further subsidence.

As the mountains rose in the west and the roughly north-south trending Western Interior basin subsided, the Gulf of Mexico separating North and South America rifted open in the south, and marine water flowed into the basin. At the same time, marine water began transgressing from the Arctic region in the north. The Early Cretaceous sandstones, shales, minor carbonates and coal of the Inyan Kara Group were deposited in fluvial, floodplain, and marsh environments, but these depositional patterns changed in the Upper Cretaceous when four main transgressions and regressions of shallow seas encroached into the area (Driscoll et al. 2002).

The multidirectional transgressions of the Late Cretaceous produced the most extensive interior seaway ever to cover the North American craton. The Western Interior Seaway extended from today’s Gulf of Mexico to the Arctic Ocean, a distance of about 4,800 kilometers (3,000 mi) (Kauffman, 1977). During periods of maximum transgression, the width of the basin was 1,600 kilometers (1,000 mi). The basin opened into the Gulf of Mexico to the south and the Arctic Ocean to the north (Kauffman, 1977). The Skull Creek Shale and younger Cretaceous shale and sandstone beds in the Black Hills record cycles of relative sea level rise and fall of this epeiric seaway.

Geologic Map of Mount Rushmore N Mem (May 2008)

Cretaceous – Tertiary Laramide Orogeny

As the Cretaceous neared an end, the Western Interior Seaway regressed across the area of the future Black Hills uplift. The seas gradually receded, forming a sedimentary sequence that thinned upward. The offshore marine Pierre Shale was overlain by nearshore marine sediments of the Fox Hills Sandstone (not exposed in the Black Hills), which transition upward into the fluvio-deltaic Lance Formation and Hell Creek Formation (not exposed in the Black Hills). The Lance Formation and equivalent Hell Creek Formation were deposited in the last 3 million years of the Cretaceous. Streams flowed eastward from the east side of the Powder River Basin and across the area that would become the Black Hills. Not until the Paleocene would sedimentation in the Powder River Basin indicate an uplift to the east. By the Eocene, a radial drainage pattern had developed around the core of the Black Hills uplift (Lisenbee and DeWitt 1993).

The combination of interaction of the subducted Farallon plate and northward movement of the Colorado Plateau geologic province initiated the Late Cretaceous to Early Tertiary Laramide Orogeny (Lisenbee and DeWitt 1993). The orogeny was marked by a pronounced eastward shift in deformation throughout Utah and into Colorado. Laramide thrust faults cut deeply into the earth’s crust, forcing ancient plutonic and metamorphic basement rocks to the surface. These thrust faults have steeply dipping fault planes at the surface that curve and flatten out in Precambrian basement crystalline rock at depths up to 9 kilometers (30,000 ft or 5.7 miles) below sea level (Gries, 1983; Erslev, 1993).

The Laramide Orogeny formed the modern Rocky Mountains, crystalline -cored arches bounded by thrust faults and separated by deep sediment-filled basins. The north-south trending, doubly-plunging Black Hills anticline is the easternmost expression of this orogeny (fig. 12). The initial phase of uplift in the region occurred in the Paleocene about 62 Ma. Igneous activity accompanied deformation. Stocks, dikes, sills, and laccoliths intruded across the northern Black Hills uplift during this initial tectonism (Lisenbee and DeWitt 1993). Devils Tower exemplifies this Paleocene magmatic event.

With uplift, erosion quickly produced sediments that were shed into the Powder River Basin and the Williston Basin. Erosion rates during the Paleocene averaged 10 to 13 centimeters (4 to 5 in) per 1,000 years (Lisenbee and DeWitt 1993).

A second phase of deformation related to the Laramide Orogeny affected the Black Hills region in earliest Eocene time (approximately 56 Ma). A second magmatic pulse followed at about 39 Ma. The plutons lie along a zone that offsets the boundary between the Wyoming Archean province and the Early Proterozoic Trans-Hudson province. This zone, with associated plutons, can be traced northward into Montana.

By the Late Eocene, the Laramide Orogeny was over. Erosion carved valleys into the uplift, creating the topographic form of the present Black Hills. These valleys were then filled by the post-tectonic White River Group.

The Black Hills uplift is bounded on the west by north-and northwest-trending monoclines, chiefly west facing. Smaller scale monoclines are common across the uplift where Paleozoic strata are exposed (Lisenbee and DeWitt 1993). Monoclines are inferred to overlie unexposed thrust faults formed during regional horizontal compression. Overlapping(en echelon) folds suggest an additional strike-slip component to faulting. Ancient Precambrian margins, structures, and fabrics involving the Wyoming Archean province and the Early Proterozoic Trans-Hudson province may have influenced Laramide folding, faulting, and magmatic activity (Lisenbee and DeWitt 1993).

Figure 12. Major tectonic elements and locations of Late Cretaceous to Early Eocene sedimentary, igneous, and metamorphic rocks associated with the Laramide Orogeny. The Black Hills uplift is the easternmost expression of the Laramide Orogeny. The basin west of the Black Hills uplift is the Powder River Basin, a prolific hydrocarbon-producing basin. Modified from Miller and others, 1992.

Quaternary History of the Black Hills Region

Both the Cheyenne River that drains the southern part of the Black Hills and the Belle Fourche River that drains the northern part cut across north trending structures. These east-west flowing rivers incised their channels at such a rate that they were able to maintain their original course as erosion exposed the Black Hills uplift. The subradial drainage pattern in the Black Hills developed as many tributaries also incised their channels into bedrock. These tributaries are discordant to major structures and perpendicular to the bedding of the Paleozoic and Mesozoic strata that surround the uplift (Wayne et al. 1991).

The Black Hills region was unglaciated during the Pleistocene so that many Pleistocene deposits are preserved along creeks in the Black Hills. Five terrace deposits have been identified in the Black Hills (Wayne et al. 1991). The oldest and topographically highest terrace deposit is the Mountain Meadow erosional surface. The gravel beneath the surface is dominated by Precambrian siliceous clasts. A camel ankle bone from Gigantocamelus was found in the Mountain Meadow gravel just south of Rapid City.

The Rapid terrace is composed of locally derived clasts. The terrace lies 55 meters (180 ft) above Rapid Creek, near Rapid City, and can be traced along the eastern flank of the Black Hills uplift. The Rapid terrace has been correlated with the Hot Springs mammoth site, a natural elephant trap formed by a karst depression containing a warm spring. A mammoth bone from this site yielded a radiocarbon date of 26,075 ± 880 years before present (ybp) (Wayne et al. 1991).

Ages for the Sturgis, Bear Butte, and Farmingdale terraces have not been determined. The Sturgis terrace contains well cemented clasts of Permian Minnekahta Limestone and is at least 30 meters (100 ft) thick along Fall River.

The Bear Butte terrace and lowermost Farmingdale terrace were identified below the Sturgis terrace along Bear Butte Creek. The Bear Butte terrace is a major terrace along the Belle Fourche River and along Bear Butte Creek as far upstream as Bear Butte. Farmingdale terrace extends along Rapid Creek from Farmingdale, South Dakota, east to the Cheyenne River.

Three hypotheses have been proposed for the origin of the Rapid and younger terraces (Wayne et al. 1991). These include:

• damming by glacial ice along the Missouri River
• local tectonism
• rejuvenated uplift of the Black Hills starting about 4.5 Ma.

Today, the Black Hills experiences an overall continental climate. Low precipitation, hot summers, cold winters, and extreme variations in both precipitation and temperatures impact erosion rates, vegetation growth, and sediment transport in the various drainage basins (Carter et al. 2002). Lithologic heterogeneity of the and surface water quality. Alluvial gravel, sand, silt, and Paleozoic and Mesozoic sedimentary strata, fracturing of mud continue to be deposited by perennial and Precambrian rocks, and differential weathering ephemeral streams while colluvium and talus slopes contribute to a complex groundwater system in the area. continue to be eroded and their sediment transported Diverse geologic conditions also influence stream flow down slope.

           Text from Mount Rushmore National Memorial Geologic Resource Evaluation Report, June 2008

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