Cambrian System Period

(from the Latin Cambria, the old name for Wales), the first system of the Paleozoic group, which corresponds to the first period of the Paleozoic era in the geological history of the earth. The Cambrian period began 570 million years ago, after the Riphean, and lasted 70 million years, up to the Ordovician.

The complex of rocks corresponding to the Cambrian system was identified by the English geologist A. Sedgwick in 1835 in Wales, where he established three subdivisions. The more precise subdivisions later defined by the American geologist C. Walcott, the English geologist C. Lapworth, and others led to the modern conception of the series of the Cambrian; these series were accepted by the Fourth International Geological Congress in 1888. The first studies of the Cambrian in Russia were conducted in the Baltic region and are associated with such names as A. Mickwitz and F. Schmidt. The identification of Cambrian deposits and the establishing of their extensive development in Siberia was due to the work of such scientists as E. Toll’ (end of the 19th and beginning of the 20th century), V. A. Obruchev, E. V. Lermontova, A. G. Vologdin, and P. S. Krasnopeeva (the 1920’s and 1930’s). In 1956 the first unified schema for the Cambrian in Siberia was recognized; among those participating in the work were F. G. Gurari, A. K. Bobrov, I. T. Zhuravleva, K. K. Zelenov, N. P. Lazarenko, N. V. Pokrovskaia, N. P. Suvorova, and N. E. Chernysheva. Systematic geological surveying conducted in the USSR has identified Cambrian deposits in many other regions of the country as well.

Subdivisions. The Cambrian system is divided into three series; it is the only system of the stratigraphic chart that does not have any generally recognized international stages, although there have been many attempts to divide it into stages (see Figure 1). In most of the regions of the world the division of the Cambrian into zones is just being worked out, and the existing version requires further specification. The question of drawing the boundary between the Cambrian and Precambrian was at issue up to the preceding decade. In most instances it was drawn along the unconformity at the base of the deposits containing Cambrian fossils. During the 1970’s the lower boundary of the Cambrian has been drawn along the base of the first zone containing a complex of fossil skeletal forms.

General characteristics. Deposits of the Cambrian system are very widespread and are known to exist on all continents. The most widely found are the marine deposits of the Lower Cambrian, which correspond to a time of extensive marine transgressions when much of the present continents was covered by warm seas with an abundant fauna. On the basis of facies analysis it is assumed that in the early Cambrian the water temperature in the Siberian seas did not drop below 25°C. A characteristic feature of the sedimentation of the early Cambrian was extensive development of red marine calcareous rock and the accumulation of thick salt beds. At this time migrations of fauna occurred as a result of which in regions that are far from each other (for example, Siberia and Australia) there are similar associations of fossil organisms. In the middle Cambrian a significant reduction of the sea basins occurred, which continued at the beginning of the late Cambrian. Evidently, in the middle and late Cambrian there occurred a more substantial climatic differentiation than in the early Cambrian; this differentiation led to the formation of biogeographic provinces. In the deposits of the late Cambrian authentic red lagoonal rock has been established for the first time.

The main tectonic structures arose as early as the Precambrian and continued to exist during the Cambrian. The structural elements of the platforms and geosynclines (syneclises, anticlises, synclinoria, and anticlinoria) that formed at the end of the Riphean and during the Cambrian, particularly the early Cambrian, preserved a similar configuration. Only in the middle Cambrian, as a result of the activation of tectonic movement in many regions (particularly in the folded areas of southern Siberia), did the structural plan change substantially. The intensification of tectonic movements led to a situation in which in many instances sections of the Middle and Upper Cambrian are much more fragmentary than those of the Lower Cambrian. In the geosynclinal areas, along with normal sedimentary rock, thick strata of effusive rock, most often of a basic composition, were formed. Intrusive rock is represented by a number of rocks of diverse composition, from ultrabasic to persilicic. On the plat-forms there are only small diabase bodies.

The organic world. During the Cambrian period, for the first time in the geological history of the earth, skeletal organisms appeared (at first Archaeocyatha, gastropods, radiolarians, sponges, and branchiopods and later trilobites, ostracods, coelenterates, and other organisms). By the end of the Cambrian virtually all types of the animal kingdom were represented. An abundance of fossils with a phosphate skeleton (Hiolithelmintida, Tommotia) is characteristic for the earliest periods (Tommot age).

Trilobites and Archaeocyatha are particularly characteristic for the seas of the early Cambrian; they are used to distinguish the various deposits of the early Cambrian. Trilobites from the superfamilies Olenelloidea, Eodiscidea, and Redlicheoidea predominate. Sponges, brachiopods, chiolites, gastropods, and worms are present in considerable numbers; coelenterates (hydroconozoans, stromatoporoids, scyphozoans, and protomedusans), monoplacophorans, bivalve mollusks, and very primitive cephalopods are encountered less frequently. Blue-green and red algae are numerous. Microphytoplankton (acritarchs), which are used to distinguish the early Cambrian terrigenous deposits of the Eastern European Platform, developed extensively. By the end of the early Cambrian the Archaeocyatha had become virtually extinct. At the end of the early Cambrian the number of articulate brachiopods constantly increased. In the late Cambrian tabulate corals and graptolites appeared. Trilobites (Dikelacephaloidea, Ptychoparioidea) continued to play a very important role.

Cambrian deposits in the USSR. Cambrian deposits on Soviet territory are very widely distributed, particularly on the Siberian Platform, in the Altai-Saian folded area, and on the Eastern European Platform. Moreover, they are found in the Urals, the Caucasus, Kazakhstan, Middle Asia, the Far East, the Kolyma basin, and folded regions of Chita Oblast, of the Buriat ASSR, and of Khabarovsk Krai and have also been discovered by drilling on the Western Siberian Plain.

On the Siberian Platform, Cambrian deposits are represented almost exclusively by a series of calcareous rock from 100 to 1,000 m thick. Most characteristic are red and black bituminous limestones, as well as various types of biogenic calcareous rock containing abundant remains of fauna.

On the Eastern European Platform, Cambrian deposits are found almost everywhere in its northern part and are also known from the western parts of Byelorussia and the Ukraine. Lower Cambrian deposits are represented by marine sandy-argillaceous rocks, often very weakly altered and containing scarce remains of fauna. The best known are the “blue clays” of the Baltic region. Deposits of the Middle Cambrian are represented by shallow-water, beach-type sands; authentic Upper Cambrian deposits have been identified only in a few places. The thicknesses of the Cambrian on the Eastern European Platform usually do not extend beyond the first few hundred meters of deposition. Cambrian deposits from folded areas are represented by a complex group of geosynclinal formations several thousand meters in thickness (alternating organogenic calcareous, volcanic, and terrigenous rock containing deposits of phosphorites, iron ores, and so forth).

Minerals. The largest deposits of phosphorites are associated with Cambrian deposits (Kazakhstan, Mongolian People’s Republic, People’s Republic of China). Deposits of oil (the Irkutsk amphitheater and the Baltic region, for example), lead (North Africa), manganese (the Kuznetsk Alatau), vanadium (Kazakhstan), and salts (the southwestern part of the Siberian Platform, India) have also been found. In many regions calcareous rock from the Cambrian is used as a raw material for cement as well as for the metallurgical industry, while marbles are used as a facing material (for example, in the Moscow Subway).