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See also: Geologic Timescale (table)Geologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)
Cenozoic Quaternary Holocene 10,000 years ago to the present
Pleistocene 2 .
..... Click the link for more information.
Precambrian,name of a major division of geologic time (see Geologic TimescaleGeologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)
Cenozoic Quaternary Holocene 10,000 years ago to the present
Pleistocene 2 .
..... Click the link for more information. , table), from c.5 billion to 570 million years ago. It is now usually divided into the Archean and Proterozoic eons. Precambrian time includes 80% of the earth's history.
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
..... Click the link for more information. are mostly covered by rock systems of more recent origin, but where visible they commonly display evidence of having been altered by intense metamorphismmetamorphism,
in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains essentially in the
..... Click the link for more information. . Precambrian rocks often occur in shields, which are large areas of relatively low elevation that form parts of continental masses. One of the largest exposed areas of early Precambrian rocks is the Canadian Shield, where geologist Sir William LoganLogan, Sir William Edmond,
1798–1875, Canadian geologist. Educated in England, he managed (1831–38) coal mines and a copper smelter in Wales. In addition to making studies of clays underlying coal seams, he made extensive geological maps and sections.
..... Click the link for more information. did his pioneer work. It covers most of Greenland, extends over more than half of Canada, and reaches into the United States as the Superior Highlands and the Adirondack Mts.
The rocks of this region, and of the early Precambrian as a whole, are generally granitegranite,
coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present.
..... Click the link for more information. , schistschist
, metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. Schists are distinguished from the other foliated rocks, slates and gneisses, by the size of their mineral crystals; these are
..... Click the link for more information. , or gneissgneiss
, coarse-grained, imperfectly foliated, or layered, metamorphic rock. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist.
..... Click the link for more information. . The most notable formations are the Keewatin and Coutchiching of Minnesota and the adjoining part of Canada; the Grenville of Ontario, which, however, may be late Precambrian; and the widely distributed Laurentian. The Keewatin series of rocks is composed chiefly of metamorphosed lava, with some sediments; the Coutchiching series is chiefly of sedimentary gneisses and schists. The Grenville limestone, marble, gneiss, and quartzite are predominantly metamorphosed sediments; the Laurentian gneiss and granite are probably younger than the other series, having been forced up through the Grenville as igneous rock. After the appearance of the Laurentian, the Temiskaming, or Sudburian, sediments were deposited, and a second series of gneisses and granites, the Algoman, was formed.
Elsewhere in North America, early Precambrian rocks are exposed in the Grand Canyon of Arizona and in the Teton Range of Wyoming. Among the other shield areas composed of early Precambrian rocks are the Angara Shield in Siberia, the Australian Shield, the Baltic Shield in Europe, the Antarctic Shield, and the African Shield comprising most of the African continent. In South America, the Amazon River basin separates the Guiana and the Brazilian shields. Fossils have been reported from this era, but few have been found in strata universally acknowledged to be early Precambrian. Evidence such as bacteria and algallike spheroids, supports the belief that rudimentary life existed. During the early Precambrian, radioactive heat from the new planet may have been so great that little permanent crust could survive.
By the latter Precambrian, heat dissipated enough to allow the continental crust to form; crustal rifting, mountain building, and volcanic activity then dominated, as did sedimentation. The life of the late Precambrian is poorly represented by fossils, but a few invertebrates including creatures resembling jellyfish and worms have been discovered. The best evidence that there probably were numerous forms of life is the variety and complexity which suddenly appears in Cambrian fauna. Mineral deposits associated with Precambrian rocks have yielded most of the world's gold and nickel in addition to large quantities of copper, silver, radium, and uranium.
the most ancient strata of the earth’s crust and the time corresponding to their formation, which comprises approximately six-sevenths of the geological history of the earth.
For a long time the geological history of the Precambrian remained unknown, in spite of the fact that various traces of life had long since been found in Precambrian rocks. The paleontological method, which had proven effective in dividing the Phanerozoic (post-Precambrian) formations, was considered unacceptable for Precambrian rocks. In individual regions a purely local division of the Precambrian sections was established, since there was no method for their interregional correlation. It was only in the 1930’s that radio-metric methods came into use in studying Precambrian stratigraphy and geochronology, along with attempts to correlate Precambrian sedimentary strata in terms of the stromatolites (structures of ancient algae). According to modern data, the Precambrian covers the time from the origin of the oldest geological formation known to us, with an age of approximately 3.5 billion years, up to the beginning of the Cambrian (approximately 570 million years ago). Thus, the duration of the Precambrian was more than 2.9 billion years. The earlier period of the earth’s history (approximately 1 billion years) is as yet inaccessible to geological study.
Division. During the second half of the 19th century, on the basis of general historical geological data, the degree of metamorphism of the rock, and other features, a division of the Precambrian in North America into the Archean, or Archeozoic (established by J. Dana in 1872), and the Proterozoic (established by E. Emmons in 1888) was proposed. According to modern conceptions, the boundary between them coincides with the major era of folding and granitization that occurred 2.5-2.7 billion years ago. In the USSR the Proterozoic is usually divided into the Lower, Middle, and Upper (see Table 1). The Upper Proterozoic is often separated under the name “Riphean” (“Infracambrian,” or “Sinian”) and is divided into four parts in terms of the composition of organic remains. The upper subdivision of the Riphean which previously was considered part of the so-called transitional strata from the Proterozoic to the Paleozoic has been differentiated under the name “Vendian” (“Judomian,” “Eocambrian,” and others).
|Precambrian subdivisions||End of subdivisions (in millions of years)|
|Proterozoic PR||Upper (Riphean)||Vendian||570|
In the initial period of studying the Precambrian formations, instead of the term “Precambrian,” a number of other names were used which are now encountered only in the works of foreign geologists. These include the Protozoic of Sedgwick (1838), the type of which was the heavily metamorphosed rock from the Precambrian basement of the Scottish Highlands, Wales, and elsewhere. The sharp difference between the Riphean and the more ancient formations of the Precambrian (the relatively slight metamorphism of the rock composing the Riphean and the possibility of the paleontological dating of the included series) led to a situation in which it became more and more common to divide the Precambrian into the Lower (early) Precambrian, which encompassed the strata of rock from the Archean and Lower and Middle Proterozoic, and the Upper (late) Precambrian, which corresponded to the Riphean (from 1.6 billion years ago to the beginning of the Cambrian).
General characteristics. Precambrian deposits are encountered on all continents within the ancient platforms, including the Eastern European, Siberian (along with its ancient folded margin—the Stanovoi Range, the Patom Plateau, the northeastern part of the Vostochnyi Saian and the Enisei Ridge), Sino-Korean, Southern Chinese, and Indian, as well as the North American, South American, African, Australian, and Antarctic. Moreover, Precambrian formations make up the cores of uplifts, including median masses in the younger folded zones—the Baikalian, Caledonian, Hercynian (Variscian), and Alpine—that border the ancient platforms.
The geological formations of the early Precambrian, which have been metamorphosed to a significant degree, form the basement of the ancient platforms and emerge on the surface in the area of the platforms’ crystalline shields, including the Baltic, Aldanian and Canadian shields. Sometimes within the ancient consolidation cores (in North America, South Africa, and elsewhere) slightly altered sedimentary and volcanic rock can be observed belonging, according to radiological data, to the early Precambrian. At the same time, the ancient platforms include extensive areas (the Grenville zone of North America, the Mozambique zone of Africa, and others) containing deposits from the early Precambrian that were subjected to thermal action in later eras of Precambrian folding. Often the gneiss series in these zones which show a “rejuvenated” absolute age are without sufficient reason considered to be late Precambrian.
Formations of the Upper Precambrian most often comprise the lower part of the sedimentary cover of the ancient platforms and are represented by rock that is weakly altered. Often this rock contains glauconite and other minerals which decompose when there is even slight metamorphism.
The early Precambrian formations consist of gneisses, migmatites, various crystalline schists, and amphibolites and more rarely jaspilites, quartzites, and marbles. They form thick series that have been shaped into complex folds and pierced by large intrusions of varying age and composition (including granites, granodiorites, and gabbros). These series testify to the mobile (geosynclinal) development conditions that predominated on all continents in the early Precambrian. At the end of the early Precambrian slightly dislocated and slightly metamorphosed formations of the platform type appeared, pierced by intrusions of basic and granitic rock. The formations of the late Precambrian are closer in type to the Paleozoic and are represented by thick series of quartz sandstones and quartzites, clay shales and phyllites, various volcanic formations, and stromatolite dolomites and limestones and more rarely by series of interbeddings resembling flyscn. At the very end of the Precambrian, rock series appeared that are similar to molasses.
On the basis of interruptions and unconformities in the stratification of the rock and sharp changes in the degree of the metamorphism of the rock, it has been established that there were a number of eras of increased tectonomagmatic activity in the Precambrian.
Flora and fauna. Precambrian deposits lack skeletal remains of organisms, which are the basis for building the stratigraphic scale of the Phanerozoic, yet have many diverse traces of organic life. Among them are the products from the vital activities of blue-green algae (stromatolites) and bacteria (oncolites). The age of the most ancient deposits containing these products, according to radiological data, has been set at more than 2.5 billion years. The stromatolites and oncolites have been successfully used for comparing the sedimentary series of the Upper Proterozoic within individual regions and even for intercontinental correlation. Among the siliceous rock of the Early Precambrian unique filamentous algae have been found that have been so well preserved that details of the cellular structure of the organism can be observed. On many stratigraphic levels in the Precambrian formations, minute circular bodies (up to 50 microns in size) of algal origin have been found that were previously thought to be spores. They are known as acritarchs and “spheromorphids.” These formations have been noted in early Precambrian deposits, but they are particularly abundant in the late Precambrian, where attempts have been made to correlate the sections on the basis of the vertical distribution of these formations.
The animal kingdom of the Precambrian is significantly poorer than the plant kingdom. Individual indications of the presence of animal remains in Precambrian rock are actually objects that are apparently of inorganic origin (Aticocania Walcott,Telemar kites Dons,Eoioon Dawson,Brooksella Bassler) or are products of the leaching of stromatolites (Carelozoon Metzger). Many fossils of the Precambrian have not been completely identified (Udokania Leites) or do not have an accurate dating (Xenusian querswalde Pompecki). In the Middle Riphean of Turukhansk Raion—Krasnoiarsk Krai —ancient sabellidites are known; these are compared with modern Pogonophora. The animal kingdom of the Vendian is the richest and most diverse and has been studied in detail from the unique finds made in the Pound sandstones of Australia and the Nama series of southern Africa. Forms have been found there that are close to modern sea pens (Rangea and Gharnia), as well as ancient annelids (Spriggina), numerous medusoid forms (Ediacaria, Beltanella, and Protodipleurosonia), and forms of uncertain taxonomic affiliation (Parvancoria and Dickinsonia). Many medusoids were discovered in the Vendian of the Eastern European Platform. Vendia sokolovi Keller, with a clearly segmented body but not having the bilateral symmetry inherent to trilobites, was also found in the Vendian. All the listed forms have no solid skeletal parts, and the impressions of these forms can survive only in exceptional instances.
The Precambrian in the USSR. In the USSR early Precambrian deposits are very widely found. They have been best studied on the Baltic Shield and in Eastern Siberia. The early Precambrian formations of the Baltic Shield are represented by rock formations of Archean and Lower and Middle Proterozoic age. Archean-age rocks include the Kola series of mica and mica-garnet gneisses with ferruginous (ore) quartzites of the Kola Peninsula and the gneiss-schist White Sea series of Karelia. Both series have been complexly dislocated and pierced by intrusions of very ancient basic rock (amphibolites), granodiorites, and granites. The age of the Archean folded structures (Saamide and Belomoride) has been determined at 2.5-2.7 billion years. Lower Precambrian formations of Proterozoic age are represented by the Karelian formations of Karelia and the Kola Peninsula. The Svecofennian formations of southern Finland and Sweden are considered to be of the same age as the Karelian formations. The stabilization of the Baltic Shield started 1.9-2 billion years ago in the Karelide belt and ended 1.8-1.9 billion years ago in the Svecofennide belt. On the Ukrainian Shield the Archean formations include the Verkhovtsy-na-Konke series of amphibolites, schists, and ferruginous quartzites; the series of deeply metamorphosed gneisses of the Bug River area, Volyn’, the Dnieper region, and other regions; and the associated granitoids and charnockites. The Lower and Middle Proterozoic are most typically represented by the Saksagan’ metabasite series and the overlying iron-ore sandstone-schist Krivoi Rog series. There are similar series in the Voronezh Massif. According to drilling data from the basement of the Russian Plate, the formations of the early Precambrian do not differ from those of the Baltic Shield.
In Siberia Lower Precambrian formations have developed on the Siberian Platform (within the Anabar Massif and the Aldanian Shield) in its western, southern and southeastern rim (from the Enisei Ridge and the Altai to the eastern end of the Stanovoi Range), as well as in the Malyi Khingan, the Khanka Massif, along the northern coast of the Sea of Okhotsk, in the Kolyma Basin, and on the Chukchi and Taimyr peninsulas.
The most ancient formations of the Archean era in Siberia are represented by mica-garnet, cordierite-sillimanite, and pyroxene gneisses and schists and by granulites and marbles divided into several series which are as yet very difficult to interrelate. The Lower and Middle Proterozoic are represented by various gneisses (including mica, amphibole, and sillimanite gneisses), in part by marbles and quartzites (the Vostochnyi Saian, the Enisei Ridge, and elsewhere), and by less-metamorphosed sandy-aleurolite series (the Udokan Range, for example).
Deposits of the Upper Precambrian, which forms the cover of the platforms, are found in the Urals and Timan; they have been discovered in deep boreholes on the Russian Plate and are exposed in natural outcroppings along the banks of the Dnestr and the coast of the White Sea. They are represented particularly completely in the southern Urals, where three large series have been discovered in a section of the late Precambrian, with a total thickness of 15 km. The Urals section, which contains a number of horizons with stromatolites, and glauconite partings, was used by N. S. Shatskii as the type of the Riphean group. Within the Russian Plate the Riphean forms linearly elongated troughs (aulakogenes), and Vendian deposits are found on larger areas, underlying the Moscow Syneclise and other depressions. The Vendian deposits are represented chiefly by sandstones and aleurolites and contain beds of tillites deposited by an ancient glacier. On the Siberian Platform, the late Precambrian series are represented by quartz sandstones and quartzites on the bottom and by thick calcareous series above. All the subdivisions of the Riphean are designated by the composition of the stromatolites that they contain. In the troughs bordering the Siberian Platform, suites composed of calcareous rock alternate with series of sandstones and schists. In the upper levels of the section along the southern and western rims of the Siberian Platform, series with coarser detritus appear, and sometimes red rock. Many researchers view these series as orogenic formations, comparing them with the molasses of later geological periods.
Precambrian formations are also known in the folded systems of Middle Asia, Kazakhstan, and the Far East. Early Precambrian deposits as well as Riphean deposits have been found there. The Riphean deposits are composed of sedimentary and volcanic geosynclinal series, sometimes with beds of stromatolite limestones, which put the deposits in which they occur in the Middle or Upper Riphean and the Vendian. In some places underneath the Riphean series (the Kirghiz Range of the Tien-Shan, the Ulutau Range in Kazakhstan, the Malyi Khingan, and elsewhere), there are outcroppings of deeply metamorphosed deposits of the early Precambrian represented by gneisses, metamorphic schists, quartzites, and other rocks.
The most important paleogeographic features have been reconstructed in sufficient detail for the late Proterozoic. For the early Precambrian there are only fragmentary data. It has been established that in the late Proterozoic the platform massifs were uplifted and large areas of them were subjected to destruction. The products of this destruction filled all the basins within the platforms and were carried to the bordering troughs where unique formations composed of quartzite-sandstone formations were formed. The subsided portions of the platforms were covered by shallow seas in which calcareous rock with stromatolites was deposited. On a larger part of the platforms in the middle and late Riphean an arid climate predominated. In the Vendian the situation altered sharply. It became much colder, and extensive areas were covered by continential glaciation. As a result of the development of the Baikalian folding, uplifts occurred in the geosynclinal areas, and these uplifts provided a significant quantity of detrital material.
Minerals. The Precambrian is very rich in minerals. Enormous deposits of iron ores (ferruginous quartzites and jaspilites), aluminum raw materials (cyanite and sillimanite), and manganese ores are found in the Precambrian. The largest deposits of gold and uranium ores are associated with Precambrian conglomerates. Major deposits of copper, nickel, and cobalt ores are associated with basic and ultrabasic rock, and lead-zinc deposits are associated with calcareous rock. Precambrian pegmatites are the main source of mica (muscovite), ceramic raw materials, and rare metals. The most ancient oil deposits (Irkutsk Oblast, RSFSR) are associated with the uppermost Precambrian formations.
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B. M. KELLER and K. O. KRATTS