PRECAMBRIAN TIME

period of time that extends from a little more than 3.9 billion years ago, which is the approximate age of the oldest known rocks, to the beginning of the Cambrian Period, roughly 540 million years ago. The Precambrian era thus represents more than 80 percent of the whole of geologic time. interval of geologic time from 3.8? billion years ago, the age of the oldest known rocks, to 540 million years ago, the beginning of the Cambrian Period. This interval represents about 80 percent of the geologic record and thus provides important evidence of how the continents evolved through time. The Precambrian is divided into two eons, the Archean and the Proterozoic, with the time boundary between them at 2.5 billion years ago. Precambrian rocks are exposed typically in vast areas several hundreds or even thousands of kilometres across; examples of such include the Canadian, Baltic, Indian, and West Australian shields, as well as the Ukrainian shield. Because there are few definitive fossils in Precambrian rocks, dating is provided by the isotopic analysis of pairs of radioactive elements. In the early Precambrian, heat production by the breakdown of radioactive isotopes was several times higher than it is today. Just as it does today, however, the bulk of the heat must have escaped from the Earth's mantle through oceanic ridges in connection with very rapid seafloor spreading and consequent subduction by some form of primitive plate tectonics. (Subduction is the process in which one lithospheric plate descends beneath another at regions where the two plates converge.) The result of these processes was the formation of three distinctive Archean rock groups: (1) voluminous island arcs, called greenstone belts, such as those in western Australia, Zimbabwe, India, and southern Canada, where the rocks are the host of many economic mineral deposits (e.g., gold, chromium, nickel, copper, and zinc); (2) rare slabs of ophiolite-like ocean floor that were thrust over the arc rocks, as in the Barberton Mountains of South Africa and in the Yellowknife district of northwest Canada; and (3) abundant tonalites and granites that are chemically comparable to Mesozoic equivalents (i.e., those formed between about 245 and 66 million years ago) in the Andes. These tonalites and granites were overthrust by the greenstone belts and thus were pushed down into the deep continental crust, where they were highly deformed and metamorphosed to become granulites and gneisses. Uplifted representatives of these rocks can be seen today in western Greenland, northern Finland, and southern India. The Archean was a time of rapid crustal growth and thickening. This activity resulted in uplift and erosion, which in turn gave rise to deposition of clastic sediments in major sedimentary basins, such as the late Archean Witwatersrand basin in South Africa and the early Proterozoic Huronian basins near the Great Lakes of North America. The Archean-Proterozoic boundary marked an important turning point in continental evolution, for it shows evidence of the beginning of what might be termed "modern-style" plate tectonics. The Archean magmatic arcs had coalesced to form the first major continent or continents around which shelf-type sediments were subsequently deposited; also, ocean floor was subducted, giving rise to Andean-type granitic batholiths along active continental margins. Consequently, continents were able to drift and collide with one another to form the first linear Himalayan-type mountain belts. Examples of these early Proterozoic collisional orogens are the Wopmay, Wollaston, and Labrador belts in Canada, the Ketilidian in southwest Greenland, and Svecofennian in Finland. It is likely that the formation of so many collisional orogenic belts at this time gave rise to a supercontinent by about 1.5 billion years ago. The breakup of this enormous landmass resulted in the formation of individual continents and new oceanic crust, the subduction of which produced new collisional orogenic belts, as, for example, the Grenville, which extends along the eastern side of North America and which persisted from 1.5 to 1 billion years ago. The remnants of many island arcs 1.1 billion to 500 million years old have been found in Saudi Arabia and Egypt. Such island arcs resemble those in Indonesia today. By the end of the Precambrian a new supercontinent had probably formed. On and around it there existed many sedimentary basins (e.g., the Sinian in China) that were filled with conglomerates and sandstones derived through erosion of the surrounding mountainous collisional belts. Certain types of sediment bear testimony to the climatic conditions that prevailed during the Precambrian. Most important are the tillites (glacial sediments left behind by retreating glaciers), especially those deposited in North America and South Africa 2.3 billion years ago. The most extensive glaciation in Earth history occurred between 1 billion and 600 million years ago, when tillites were laid down in most continental areas. The Precambrian was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth began by the early Archean and that fossilized organisms became more and more abundant throughout Precambrian time. Both centimetre-size stromatolites (sheetlike mats deposited by algae) and millimetre- to micrometre-size carbonaceous spheroids occur in well-preserved sediments as old as 3.5 billion years. Archean organisms were prokaryotes that could survive the high radiation levels in the early anoxygenic atmosphere that had no ozone screen, and they were succeeded in the Proterozoic by eukaryotes that used oxygen for their growth in the increasingly oxygenic atmosphere. Soft-bodied organisms without skeletons began to appear toward the end of the Precambrian; they were the forerunners of the metazoans (multicelled organisms whose cells are differentiated into tissues and organs) that proliferated throughout the Phanerozoic and eventually gave rise to primates and the human species. Brian Frederick Windley Additional reading L.D. Ayres et al., Evolution of Archean Supracrustal Sequences (1985), presents good overviews of worldwide occurrences. Kent C. Condie, Archean Greenstone Belts (1981), offers a detailed synthesis. D.R. Hunter (ed.), Precambrian of the Southern Hemisphere (1981), includes authoritative accounts of Australia, southern Africa, and South America. Alfred Krner and Reinhard Greiling (eds.), Precambrian Tectonics Illustrated (1984), reviews classic occurrences. S. Mahmood Naqvi and John J.W. Rogers, Precambrian Geology of India (1987), is a succinct review. Other surveys of Archean and Proterozoic stratigraphy and paleontology include those by E.G. Nisbet, The Young Earth: An Introduction to Archaean Geology (1987); and by J. William Schopf (ed.), Earth's Earliest Biosphere: Its Origin and Evolution (1983). Brian Frederick Windley Precambrian environment In this section the types of environment that may have existed during Precambrian time are considered. Several rock types, notably banded-iron formations, paleosols, and red beds, are very useful for deriving information about the conditions of the atmosphere, and tillites (indurated sedimentary rocks formed by the lithification of glacial till; see below) reveal what the climatic patterns were like during Precambrian glaciations. Paleogeography One of the most important factors controlling the nature of sediments deposited today is continental drift. This follows from the fact that the continents are distributed at different latitudes, and latitudinal position affects the temperature of oceanic waters along continental margins; in short, sedimentary deposition is climatically sensitive. At present, most carbonates and oxidized red soils are being deposited within 30 degrees of the equator, phosphorites within 45 degrees of it, and evaporites within 50 degrees. Most fossil carbonates, evaporites, phosphorites, and red beds of Phanerozoic age dating back to the Cambrian have a similar bimodal distribution with respect to their paleoequators. If the uniformitarian principle that the present is the key to the past is valid, then in the Precambrian such sediments would have likewise been controlled by the movement and geographic position of the continents. Thus it can be inferred that the stromatolite-bearing dolomites of the Riphean in the former Soviet Union were deposited in warm tropical waters. Even the 3.5-billion-year-old, extensive evaporites in the Pilbara region of northwestern Australia could not have been formed close to their paleopole. Today, phosphate sediments are deposited primarily along the western side of continents, where they receive upwelling, nutrient-rich currents as they move toward the equator. The major phosphorite deposits in the Proterozoic Aravalli belt of Rajasthan in northwestern India are associated with stromatolite-rich dolomites and were most likely deposited within the tropics on the western side of a continental mass. Precambrian life Precambrian rocks were long ago defined to predate the Cambrian and therefore to predate all life, although the term Proterozoic was later coined from the Greek for "early life." It is now known that Precambrian rocks do in fact contain the evidence of the very beginnings of life on Earth (and thus the record of its evolution for more than 3 billion years), of the explosion of life-forms without skeletons before the Cambrian, and even of the development of sexual reproduction on Earth. The first evidence of terrestrial life is found in the Early Archean sedimentary rocks of the greenstone-granite belts of Barberton in South Africa and of Warrawoona in the Pilbara block of Western Australia, which are both about 3.5 billion years old. There are two types of these early, simple, biological structures: microfossils and stromatolites. Microfossils and stromatolites The microfossils occur in cherts and shales and are of two varieties. One type consists of spherical carbonaceous aggregates, or spheroids, which may measure as much as 20 millimetres in diameter. These resemble algae and cysts of flagellates and are widely regarded as biogenic. The other variety of microfossils consists of carbonaceous filamentous threads, which are curving, hollow tubes up to 150 micrometres (0.006 inch) long. These tubes are most likely the fossil remains of filamentous organisms, and hundreds of them can be found in some rock layers. The 2.8-billion-year-old goldreefs (conglomerate beds with rich gold deposits) of the Witwatersrand Basin contain carbonaceous columnar microfossils up to seven millimetres long that resemble modern algae, fungi, and lichens. They probably extracted gold from the environment in much the way that modern fungi and lichens do. Stromatolites are, as previously explained, stratiform, domal, or columnar structures made of sheetlike mats precipitated by communities of microorganisms, particularly filamentous blue-green algae. The Early Archean examples form domes as tall as about 10 centimetres. Stromatolites occur in many of the world's greenstone-granite belts. In the 2.7-billion-year-old Steep Rock Lake belt in Ontario, Can., they reach three metres in height and diameter. Stromatolites continued to form all the way through the geologic record and today grow in warm intertidal waters, for example, at Shark Bay in Western Australia. They provide indisputable evidence that by 3.5 billion years ago life had begun on Earth by algal photosynthesis in complex, integrated biological communities. These Archean organisms were prokaryotes that were incapable of cell division. They were relatively resistant to ultraviolet radiation and were able to survive during the early history of the Earth when the atmosphere lacked an ozone layer to block out such radiant energy. The prokaryotes were predominant until about 1.4 billion years ago, when they were overtaken by the eukaryotes. The latter make use of oxygen in metabolism and for growth and thus developed profusely in the increasingly oxygenic atmosphere of the Middle Proterozoic. The eukaryotes were capable of cell division, which allowed DNA (deoxyribonucleic acid), the genetic coding material, to be passed on to succeeding generations. By Early Proterozoic time both microfossils and stromatolites had proliferated. The best-known occurrence of microorganisms is in the two-billion-year-old, stromatolite-bearing Gunflint iron formation in the Huronian Basin of southern Ontario. These microbial fossils include some 30 different types with spheroidal, filamentous, and sporelike forms up to about 20 micrometres across. Sixteen species in 14 genera have been classified so far. Microfossils of this kind are abundant, contain beautifully preserved organic matter, and are extremely similar to such present-day microorganisms as blue-green algae and microbacteria. There are comparable microfossils of the Early Proterozoic in Minnesota and Michigan in the United States, the Belcher Islands in Hudson Bay in Canada, southern Greenland, Western Australia, and northern China. These microbiota lived at the time of the transition from an anoxygenic to an oxygenic atmosphere. During the Late Proterozoic stromatolites reached their peak of development and became distributed worldwide. The first metazoa (multicelled organisms whose cells are differentiated into tissues and organs) also appeared at this time. The stromatolites diversified into complex, branching forms. From about 700 million years ago, however, they began to decline significantly in number. Possibly the newly arrived metazoa ate the stromatolitic algae, and their profuse growth destroyed the habitats of the latter. Precambrian rocks General occurrence and distribution Archean regions within Proterozoic cratons surrounded by Phanerozoic mobile belts. This 1/4 Precambrian rocks as a whole occur in a wide variety of shapes and sizes. There are extensive Archean regions, up to a few thousands of kilometres across, that may contain either greenstone-granite belts or granulite-gneiss belts or both and that are variously designated in different parts of the world as cratons, shields, provinces, or blocks. Some examples are the North Atlantic craton that includes northwestern Scotland, central Greenland, and Labrador; the Kaapvaal and Zimbabwean cratons in southern Africa; the Dharwar craton in India; the Aldan and Anabar shields in Siberia in Russia; the Baltic Shield that includes much of Sweden, Finland, and the Kola Peninsula of far northern Russia; the Superior and Slave provinces in Canada; and the Yilgarn and Pilbara blocks in Western Australia. There are linear belts, up to several thousand kilometres long, that are frequently though not exclusively of Proterozoic age, such as the Limpopo, Mozambique, and Damaran belts in Africa, the Labrador Trough in Canada, and the Eastern Ghats belt in India. Also, small relict areas, only about a few hundred kilometres across, exist within or against Phanerozoic orogenic belts, as, for instance, the Lofoten islands of Norway, the Lewisian Complex in northwestern Scotland, and the Adirondack Mountains in the northeastern United States. Some extensive areas of Precambrian rocks are still overlain by a blanket of Phanerozoic sediments, as under the European and Russian platforms and under the central United States; these are mostly known from borehole samples (see figure). Archean rock types Archean rocks occur in greenstone-granite belts that represent the upper crust, in granulite-gneiss belts that formed in the mid-lower crust, and in sedimentary basins, basic dikes, and layered complexes that were either deposited on or intruded into the first two types of belts.