Riphean


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Riphean

 

(from the Latin Riphaei Montes [Riphean Mountains], the name sometimes used by ancient geographers for the Ural Mountains), a major stratigraphic subdivision corresponding to the Late Precambrian; it is represented by a complex of slightly altered rocks with geological ages between 1,650 and 570 million years; it is followed by the Cambrian. The Riphean was identified by the Soviet geologist N. S. Shatskii in the Southern Urals in 1945. It corresponds approximately to the Upper Proterozoic. In general characteristics it corresponds to such earlier-identified subdivisions as the Algonkian (American geologist C. D. Walcott, 1889), the Sinian System (American geologist A. Grabau, 1922), and the Hyperborean (Finnish geologist J. Sederholm, 1932). Since 1950 the Riphean has been studied by the Soviet geologists E. P. Bruns, M. I. Garan’, B. M. Keller, I. N. Krylov, V. V. Menner, A. I. Olli, M. A. Semikhatov, B. S. Sokolov, V. V. Khomentovskii, and N. M. Chumakov. The first major summary of the Riphean, Stratigraphy of the USSR: Upper Precambrian, was published in 1963.

Subdivisions. Four subdivisions are identified within the Riphean, corresponding approximately to systems: Lower (1,650-1,400 million years), Middle (1,400–1,100), Upper (1,100-680), and Vend (sensu lato, 680-570). The last subdivision is often called the Terminal Riphean or Vendian. The Soviet geologist B. S. Sokolov places the Vendian in a narrow zone corresponding to the Volynia-Valdai beds of the Eastern European Platform. This Vendian (sensu stricto) correlates with the upper part of the Terminal Riphean. Each of these four subdivisions contains a characteristic complex of structures of blue-green algae—stromatolites that are the basis of the delineation and correlation of Riphean deposits—and other organic remains.

General description. The marine and continental deposits of the Riphean are widely distributed on all continents. During the Riphean, the platforms occupied a smaller area than in the Paleozoic, especially in the southern hemisphere. They were flat land on which pre-Riphean rocks composed of gneisses and granites were eroded over large areas. The products of their destruction accumulated in troughs where layers of quartz and arkose sandstones were deposited (Eastern European, North American, and other platforms). In addition to these sandstones, clay strata, carbonate rocks with stromatolites, and, in some cases, flysch (for example, in the Tien Shan area) were deposited in the external troughs of geosynclinal systems. The products of underwater volcanic activity, primarily basal rocks, accumulated in the internal troughs that developed on ocean crusts. Riphean geosynclines surrounded the North American, Eastern European, Siberian, Australian, and Indian platforms and divided the South American and African platforms into blocks. At the end of the Riphean, in connection with the epoch of the Baikal fold, molasses were deposited over broad areas (the Urals, Enisei Ridge, the Vostochnyi Saian, the Baikal Region, the Scandinavian Mountains, and the Rocky Mountains). The climate during the Riphean was arid, which promoted the accumulation of red beds and, in rare cases, evaporites (the Southern Urals and South Australia). However, in the upper half of the Upper Riphean and in the Terminal Riphean, glacial deposits are found in many countries. In the Upper Riphean they gravitate toward the southern hemisphere (Equatorial Africa and Australia); in the Terminal Riphean, large areas of Europe and North America were covered by the vast Laplandian glaciation. Glacial deposits (tillites) of this age are known in the Urals, Tien Shan, the Eastern European Platform (Byelorussia), Scandinavia (Norway), Greenland, and the Rocky Mountains.

Organic world. Riphean deposits contain the presumed remains of algae and their structures. Riphean carbonate rocks contain numerous stromatolites, among which the genera Kussiella, Baicalia, Conophyton, Gymnosolen, Boxonia, and Linella are especially typical. Microphytolites in the form of various organic concretions related to carbonate rocks (Vesicularites, Osagia) are important in delineating Riphean subdivisions. Acritarchs, tiny bodies of plant origin 20-50 micrometers in size, are found in Riphean deposits; they were formerly believed to be spores and are of some stratigraphic significance. The largest (2-3 mm) were described as the subspecies Chuaria Wimani Brotzen. Certain types of invertebrates that flourished in the Early Paleozoic first appeared in the Riphean. Remains of multicellular animals without hard skeletons have been found in the upper half of the Riphean. The oldest appear to be the Upper Riphean Charnia and Charnodiscus from the Charnwood Forest Series in Great Britain. Abundant Terminal Riphean fauna has been discovered in the Ediacara region in South Australia, where approximately 1,600 examples have been found. Of these, the Australian geologists M. Glaessner and M. Wade have described approximately 30 species. More than 60 percent of this collection are coelenterates. Especially numerous are medusoid forms (Beltanella, Medusinites, and Cyclomedusa) and unconfirmed forms that are close to modern sea pens (Rangea and Arborea). Annelids are well represented (five species of polychaete worms of the genera Spriggina and Dickinsonia). Additionally, there are arthropods (Precambridium) and animals of uncertain taxonomic position (Tribrachidium). Representatives of the Ediacara fauna are found at numerous points on the earth, including North and

Figure 1. Chart of Riphean stratigraphy
SystemGeochronological divisions (millions of years)USSRChinaIndiaNorwayCanadaAustralia
Southen Urals (stratotype)Eastern Siberia
SeriesSuperseries
*Deposits unknown †Corresponds to the Vendian of the Eastern European Platform
Lower
Cambrian
570*RipheanRiphean*RipheanRipheanRiphean
*
Terminal Riphean680Asha†YudomaUpper SinianVariagian†WindermereAdeiaide
Karatau
Upper
Riphean
1,100UiMiddle SinianEsmarkian
Maiskii
Middle Riphean1,400YurmatauAimLower SinianVindhyanJotnianBeltian
Lower Riphean1,600-1,650Mashak
Burzuan
Uchur*Carpentarian (Upper)

South America, Africa, and Newfoundland. The richest collections of multicellular Riphean animals in the USSR have been gathered in the Dnestr River region and on Letnii Bereg of the White Sea (Onega Peninsula), where medusoids (Cyclomedusa plana), Pteridinium Dickinsonia, and other forms are known. In wells on the Eastern European Platform of approximately the same horizons, the unique Vendia sokolovi and Beltanelliformis brunsae have been found. There have been isolated finds in various regions of Siberia.

Riphean deposits in the USSR. In a typical cross section of the Southern Urals, the Riphean is composed of cyclically constructed series of rocks. The lower sections comprise quartzites and sandstones, and the upper sections are chiefly carbonate rocks with stromatolites, shales, and, in some places, iron ores; the uppermost part, or Asha Series, consists mainly of sandstones and aleurolites.

On the Eastern European Platform, Riphean deposits have been found by deep boreholes. Their complete cross sections gravitate toward certain linearly elongated troughs (Orsha and Pachelma), where terrigenous strata of the Middle and Upper Riphean with subordinate pockets of carbonate rock are developed. Thick carbonate strata of the Lower and Middle (?) Riphean are known in the Cisurals. Rocks of the Terminal Riphean (Vendian) have been uncovered by boreholes over broad areas and outcrop at the surface in the Dnestr River region and on Letnii Bereg and Zimnii Bereg of the White Sea. On the Siberian Platform, Riphean deposits make up vast areas in the Uchur-Maiskii region, the Anabar Massif, and the Olenek upland; all the subdivisions of the Riphean are identified in these areas. In the geosynclinal troughs that frame the Siberian Platform (the Yudoma-Maiskii [Iudoma-Maiskii] region, the Enisei Ridge, and the Baikal Region) the thickness of the rocks increases, and terrigenous formations appear, consisting of shale formations at the lower levels of the Riphean cross section and the red orogenic formations in the Upper and Terminal Riphean. In the mountain structures of Western Siberia, Riphean deposits are represented by thick strata of dark, platy limestones. Geosynclinal Riphean facies represented by terrigenous and volcanic strata are developed in Kazakhstan.

Minerals. Riphean deposits contain iron ores (Southern Urals, Enisei Ridge, and Tien Shan) and phosphorites (Western Siberia and the Baikal Region). In some places lead-zinc deposits are associated with Riphean carbonate strata (Gorevskoe on the Angara River). In equatorial Africa there are also rich deposits of copper, cobalt, and uranium ores in Riphean rocks. Terminal Riphean deposits on the Siberian Platform coincide with gas condensate deposits (Markovskoe in Irkutsk Oblast). Promising petroleum shows have also been noted in the Valdai Series of the Eastern European Platform.

REFERENCES

Shatskii, N. S. Ocherki tektoniki Volgo-Ural’skoi neftenosnoi oblasti i smezhnoi chasti zapadnogo sklona luzhnogo Urala. Moscow, 1945. (Materialy k poznaniiu geologicheskogo stroeniia SSSR: Nov. seriia, fasc. 2 [6].)
Shatskii, N. S. “Printsipy stratigrafii pozdnego dokembriia i ob”em rifeiskoi gruppy.” In Stratigrafiia pozdnego dokembriia i kembriia. Moscow, 1960.
Keller, B. M., and M. A. Semikhatov. “Opornye razrezy rifeia materikov.” In Stratigrafiia: Paleontologiia: 1967. Moscow, 1968. (Itogi nauki: Seriia Geologiia.)
Stille, H. Assintskaia tektonika v geologicheskom like Zemli. Moscow, 1968. (Translated from German.)
Sokolov, B. S. “Vendskii etap v istorii Zemli.” In Paleontologiia. Moscow, 1972.
Sokolov, B. S. “Problema granitsy dokembriia i kembriia.” Geologiia i geofizika, 1974, no. 2.
Vendomii i ego regional’nye podrazdeleniia. Moscow, 1974. (Itogi nauki i tekhniki: Seriia Stratigrafiia i paleontologiia, fasc. 5.)

B. M. KELLER