Precambrian

 

PRECAMBRIAN ROCK EXPOSURES

Large areas of Precambrian cratons are exposed in what are called shields in Australia, the central part of Canada, western (Fig. 1) and central Africa (Fig. 2), the Baltic region (Fig. 3), and major parts of China, Brazil (Fig. 4) and Argentina, and India. Many small outcrops of Precambrian rocks are exposed as basement rocks in other parts of the world such as Korea (Fig. 5), Peru (Fig. 6), Mt. Sinai (Fig. 7), and southern Israel at Timnah. Outcrops in the United States are found in places like northern Michigan, the bottom of the Grand Canyon, the Colorado Rocky Mountains (Fig. 8 & Fig. 9), New Mexico (Fig. 10), and the Transverse Ranges of southern California (Fig. 11). Some of the first cratons to form at about 3.3 Ga were the Kaapvaal and Zimbabwe (Fig. 12) cratons in southern Africa and the Pilbara and Yilgarn cratons in Australia. Some of the other three dozen cratons include: the Slave and Superior cratons in Canada, three in Greenland, and cratons in Scandinavia and the Antarctic.

After the cratons formed, they eroded to produce sedimentary deposits between them and some collided due to early plate tectonic motion. The collisions produced volcanic activity, granitic intrusions, and metamorphism of the sediments that often resulted in green rocks. These granite-greenstone mobile fold belts are common between various cratons exposed on the continents. Some volcanic rocks intruded under water forming pillow structures, e.g., in Russia (Fig. 13), South Africa (Fig. 14), and Michigan (Fig. 15). Other volcanic rock resulted from mantle melting at especially high temperatures yielding the uniquely high-magnesium komatiite, named after the Komati River (Fig. 16) in South Africa; one good example of komatiite comes from western Russia (Fig. 17). Granitic magma was intruded underground to form chambers as in Namibia (Fig. 18), to break up the host rock as in eastern South Africa (Fig. 19), and to mix with host rock as near Cape Town (Fig. 20). Igneous, magmatic activity associated with craton movement produced large dikes of low silica rocks such as the Great Dyke in Zimbabwe and layered igneous intrusions such as the Skaergaard in Greenland and the Bushveld Complex in South Africa that are important sources of platinum group elements. Metamorphosed sediments include sandstone, limestone, and shale, with turbidites in South Africa (Fig. 21) as one example. Other examples of fold belts include: the several 0.9-0.5 Ga Pan-African fold belts in Zambia (Fig. 22) between the Congo and Zimbabwe cratons that are a major source of copper, the 3.5-3.2 Ga Barberton granite-greenstone complex in South Africa between the Zimbabwe and Kaapvaal cratons that displays greenstone rock (Fig. 23), and the 3.8 Ga Isua Greenstone Belt in Greenland that is one of the oldest.

Numerous impact structures are found in the Precambrian such as the large Vredefort Dome (Fig. 24) in South Africa formed at 2 Ga and the Sudbury structure in Ontario, Canada formed by a 10-15 km bolide at 1.8 Ga.

In many places an unconformity exists between the Precambrian rocks and the fossil-bearing Cambrian sediments above them, e.g., in Colorado Springs (Fig. 25). However, in a few places the unconformity in the sedimentary record may be lacking, e.g., in Morocco (Schmitt, 1978) and Newfoundland (Hutchinson, 1962; Narbonne et al., 1987; Brasier et al., 1994).

The formation of Earth's oxygen atmosphere has been a subject of discussion for many years. The standard interpretation suggests that near the end of the Archean at about 2 Ga, single-celled photosynthetic organisms developed that could turn carbon dioxide into oxygen, forming about 1% of the Earth's atmosphere. This is called the Great Oxidation Event. Iron was then oxidized, resulting in banded iron formations (BIFs) that now make up 90% of the Earth's iron reserves, e.g., in Minnesota and Michigan (Fig. 26) and in Finland/Russia (Fig. 27).

The most prominent evidence for life in the Precambrian are stromatolites apparently formed from microbial mats. These concentric geologic structures are interpreted to result from single-celled organisms growing in sheets that catch sedimentary particles upon which the next mat of bacteria grows. Modern day examples of these are found in Shark Bay, Australia. Examples from the Precambrian are found in the bottom of the Grand Canyon, in the Upper Peninsula of Michigan (Fig. 28), in southern Africa, and many other places.

REFERENCES

PICTURES -- CLICK ON ANY PICTURE FOR A LARGER VIEW

 

FIGURE 1a. [N07.167º × E03.342º] The 0.7-0.5 Ga granitic Olumo Rock, Abeokuta, Nigeria. (Schlüter, 2008, p.198)

 

 

 

 

FIGURE 1b. [N07.167º × E03.342º] In this second picture, the aligned feldspar crystals suggest direction of magma flow.

 

 

 

 

FIGURE 2a. [N00.48705º × E32.61037º] These Paleoproterozoic (2.5-1.6 Ga) metamorphic rocks are part of the Buganda-Toro System of the Ruwenzori Fold Belt. (Schlüter 2008, p.263)

 

 

 

 

FIGURE 2b. [N00.48705º × E32.61037º] In this second picture, banded granitic gneiss containing pink potassium feldspar minerals located north of Kampala, Uganda.

 

 

 

 

FIGURE 3a. [N65.878º × E34.848º] Metamorphic banded and folded 2.7 Ga gneiss on Stolbikha Island in the White Sea off northwestern Russia. (Peltonen et al., 2008, p.40, stop 4.1)

 

 

 

 

FIGURE 3b. [N65.87815º × E34.84832º] In this second picture, the gneiss is shown to contain eclogites - metamorphic rocks formed at high temperatures (750ºC) and pressures (14 kilobars, or about 40 km depth) consisting of red garnets and green omphacite (pyroxene) minerals.

 

 

 

 

FIGURE 4. [S23.4138º × W47.2604º] A roadcut displaying 0.58 Ga Itu Granite near Itu City, Brazil. The A-type (anorogenic) granite was emplaced in the middle of a tectonic plate rather than at the edge. (Janasi, 2009)

 

 

 

 

FIGURE 5. [N37.61810º × W127.49168º] Precambrian banded metamorphic gneiss of the Gyeonggi massif near Gapyeong, Korea with an estimated age of 2-3 Ga. (Jwa, 2005)

 

 

 

 

FIGURE 6. [S14.706º × W75.590º] Canyon in Precambrian gneiss, south of Ocucaje near Ica, Peru. Our rubidium-strontium data suggest that these rocks may have an isotope age of 2.5 Ga.

 

 

 

 

FIGURE 7. [N28.57º × E33.95º] Mt Sinai (traditional) granite with an age of approximately 1 Ga.

 

 

 

 

FIGURE 8. [N38.58º × W107.72º] Gray 1.7 Ga metamorphic gneiss and schist at Painted Wall of the Black Canyon of the Gunnison, Colorado. The rocks formed during the collision of an ancient volcanic island arc with the southern part of the Wyoming craton. The lighter-colored Vernal Mesa granitic (quartz monzonite) dikes were intruded into cracks at about 1.4 Ga. Slow cooling allowed large crystals of pink potassium feldspar to form. (Matthews et al., 2003, p.53)

 

 

 

 

FIGURE 9. [N40.287º × W105.544º] Long's Peak in Rocky Mountain National Park is one of Colorado's highest peaks at an elevation of 14,256 feet. It is made up of 1.4 Ga Silver Plume granite which in other places yields gold and silver. (Lovering and Tweto, 1953)

 

 

 

 

FIGURE 10a. [N35.1º × W106.7º] Sandia Mountain near Albuquerque, New Mexico.

 

 

 

 

FIGURE 10b. [N35.2153º × W106.4883º] In this second picture, the 1.4 Ga Sandia granite shows pink potassium feldspar that can be used for radiometric dating.

 

 

 

 

FIGURE 11. [N34.0962º × W116.9535º] Metamorphic 1.7 Ga Baldwin banded gneiss near Forest Falls in the San Bernardino Mountains of southern California (Barth et al., 2000). Note pencil for scale.

 

 

 

 

FIGURE 12a. [S20.4937º × E28.5137º] Precambrian (2.6 Ga) granitic rock of the Zimbabwe craton as seen from Cecil Rhodes grave in Matopos National Park near Bulawayo, Zimbabwe. (Fountain, 1982)

 

 

 

 

FIGURE 12b. [S20.49374º × E28.51369º] In this second picture, the aligned crystals indicate the direction of magma flow before cooling.

 

 

 

 

FIGURE 13. [N64.65488º × E30.50327º] Basaltic pillow lava of the 2.8 Ga greenstone belt in the Karelian craton near Kostomuksha, Russia close to its border with central Finland. The ellipsoidal structures are interpreted as the result of rapid cooling of volcanic lava intruded under water. (Peltonen et al., 2008, p.98, stop 6.12)

 

 

 

 

FIGURE 14. [S26.02898º × E30.99065º] Pillow basalt of the 3.3 Ga Onverwacht group at the base of the Barberton Greenstone Belt, east of Johannesburg, South Africa. (Moyen et al., 2007, p.26, stop 6)

 

 

 

 

FIGURE 15. [N46.5459º × W87.4662º] Pillow lava structures (with surficial glacial striations) in metamorphosed volcanic basalts of the approximately 2.7 Ga Mona Schist greenstone belt west of Marquette in the Upper Peninsula of Michigan.

 

 

 

 

FIGURE 16. [S26.0359º × E30.9969º] The Komati River in the Barberton area east of Johannesburg, South Africa. This is the location where komatiite gets its name. (Moyen et al., 2007, p.25-26, stop 0)

 

 

 

 

FIGURE 17. [N64.7º × E30.5º] Komatiite lava of the 2.8 Ga greenstone belt close to the location for Fig. 13. The common and distinctive spinifex texture displayed here consists of bladed crystals of the mineral olivine. It is the result of rapid crystallization of liquid lava at the margin of a flow. Komatiite is an ultramafic mantle-derived volcanic rock which means that its composition is close to that of the early earth and has differentiated little from that. It is generally restricted to rocks of Archean age when the mantle may have been as much as 500ºC hotter due to residual heat from planetary accretion and a greater abundance of radioactive elements. The higher temperatures resulted in greater partial melting of the mantle giving a different geochemical signature in the extracted lava. (Peltonen et al., 2008, p.98, stop 6.11)

 

 

 

 

FIGURE 18. [S22.6758º × E14.9125º] A 0.5 Ga granitic (pink) intrusion into metamorphic (gray) gneiss of the 0.8-0.5 Ga Damara belt in the Khan riverbed east of Swakopmund, Namibia. The Damara belt sediments were deposited in between rifting, then convergence, of the Congo and Kalahri cratons after which they were metamorphosed and intruded by magma. Vertical dimension is about 100 m. (Kisters et al., 2007, p.34, stop 3.5)

 

 

 

 

FIGURE 19. [S26.037º × E30.802º] Pink magma of the 3.45 Ga Theespruit granitic pluton intruded into the gray metamorphic rocks of the Onverwacht group at the base of the Barberton Greenstone Belt, east of Johannesburg, South Africa. The magma brecciates (breaks up) the metamorphic rocks. (Moyen et al., 2007, p.31, stop 2.4)

 

 

 

 

FIGURE 20. [S33.922º × E18.377º] The Cape Granite Suite (pink) intruded the 0.8-0.6 Ga Malmesbury metamorphosed sandstones and slates (black) at about 0.63 Ga as seen at the Sea Point contact, Capetown, South Africa. The white spots are feldspar crystals. After cooling and uplift, the granite and metamorphic rocks were eroded to a peneplain on which the Ordovician Cape Supergroup was deposited that now forms Table Mountain. The Sea Point contact was made famous by Charles Darwin's observations published in 1844. For more information, see University of Cape Town geology website.

 

 

 

 

FIGURE 21. [S26.024º × E30.988º] Turbidites of the 3.3 Ga Onverwacht group at the base of the Barberton Greenstone Belt, east of Johannesburg, South Africa. The alternating course- and fine-grained sediments in the turbidite are considered to be deposited rapidly under water. (Moyen et al., 2007, p.25, stop 2)

 

 

 

 

FIGURE 22a. [S15.938º × E28.387º] Metamorphosed shales and sandstones appearing as schist and quartzite in a roadcut of the Munali Hills southeast of Lusaka, Zambia. This 1.1 Ga Nega Formation is part of the Zambezi fold belt between the Congo and Zimbabwe Cratons. (Johnson, 2007)

 

 

 

 

FIGURE 22b. [S15.938º × E28.387º] In this second picture, the quartz boudinage implies high pressures that squeezed and pinched the sedimentary layers.

 

 

 

 

FIGURE 23. [S26.1819º × E30.5381º] Greenstone (weathered red) of the 3.2 Ga Fig Tree Group of the Barberton Greenstone Belt, east of Johannesburg, South Africa. The sausage-shaped structures or boudinage are due to pinching of the layers during high pressure metamorphism. (Moyen et al., 2007, p.56, stop 5.4.c)

 

 

 

 

FIGURE 24. [S26º × E28º] Fracture filled with pseudotachylite breccia in a slab of 3.3 Ga Vredefort granite in one of the pillars of the Johannesburg airport, South Africa. The fracture is interpreted to be the result of a catastrophic meteor impact that brecciated the granite, briefly fluidizing part of the rock before solidifying to the pseudotachylite black matrix. The 2 Ga meteor impact is the oldest preserved such structure on Earth. The 10-15 km large meteor formed a crater 250-300 km wide and 5 km deep causing a magnitude 14 seismic event. The present-day Vredefort Dome 120 km southeast of Johannesburg is a semicircular range of hills about 80 km in diameter that upturned some 20 km of rock layers. (McCarthy and Rubidge, 2005, p.132)

 

 

 

 

FIGURE 25. [N38.8688º × W104.9246º] The unconformable contact between the 1.1 Ga Pike's Peak granite below and the horizontal Cambrian Sawatch Quartzite above is located near Colorado Springs, Colorado.

 

 

 

 

FIGURE 26. [N46.4868º × W87.6545º] A banded iron formation (BIF) of the Negaunee Iron Formation located at Jasper Knob, Ishpeming in the Marquette District of the Upper Peninsula of Michigan. The 2 Ga BIF consists of folded, alternating bands of gray specular hematite (Fe2O3) and red, iron-stained cherty quartzite (SiO2) often called jasperite. The layering has variously been interpreted to result from alternating cycles of oxic (oxygen present) and anoxic (oxygen absent) conditions in the oceans, where iron was oxidized (to Fe3+) and precipitated to form a red layer and then not oxidized (remaining as Fe2+) to give a gray layer, respectively. One of several theories suggests that the first photosynthesizing bacteria produced the oxygen which oxidized the iron. BIFs are often mined for iron ore.

 

 

 

 

FIGURE 27. [N64.6767º × E30.6559º] Kostomuksha iron deposit is part of the 2.8 Ga greenstone belt close to the location for Fig. 13. This deposit consists of volcanic-sedimentary rock and shales. (Peltonen et al., 2008, p.96, stop 6.7)

 

 

 

 

FIGURE 28. [N46.5051º × W87.3646º] Stromatolites in the 2.2 Ga Kona Dolomite (a magnesium limestone) on US 41 across from the Michigan Welcome Center near Marquette in the Upper Peninsula of Michigan. The domed layering is interpreted to result from sticky mats of cyanobacteria trapping sedimentary particles in a seasonal pattern or as the result of periodic flooding.