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Precambrian time

 Precambrian time comprises that long period of geological history between the formation of the Earth, now estimated on the basis of meteorite analysis to have occurred 4.6 billion years ago, and the beginning of the Phanerozoic Eon, about 600 million years ago. The total duration thus is nearly 4 billion years, more than 80 percent of all geologic time.

The name stems from using the beginning of the Cambrian Period, the oldest major time unit of the Phanerozoic Eon, as the temporal reference. Prior to about 1965 the spelling pre-Cambrian prevailed, and this form still is preferred by some, notably those who favor the establishment of a still older time period within the Phanerozoic Eon. The term Cryptozoic, with essentially the same chronological definition as Precambrian but with connotations concerning life forms, has been in the literature for many years but appears now to be obsolescent.

Unlike the Phanerozoic Eon, which has an internationally accepted hierarchy of time units that include 3 eras, 12 periods, and more than 40 epochs, the structurally complex and sparsely fossiliferous Precambrian is entirely lacking in standard subdivisions. Most of the schemes that have been advanced are rudimentary in form and provincial in derivation and application. A typical example of a regional scheme is that used for the United States by the U.S. Geological Survey. In it the Precambrian is divided into two eons--the older Archean and the younger Proterozoic--separated by a boundary set at 2.5 billion years ago. The Proterozoic in turn is divided into three subunits: Precambrian X (2.5 billion to 1.6 billion years ago), Precambrian Y (1.6 billion to 0.8 billion years ago), and Precambrian Z (0.8 billion to 0.6 billion years ago). The terms Proterozoic and Archean, with definitions comparable to those given above, appear in many regional time scales, including those of Canada, the USSR, India, and Australia; so it now appears likely that this twofold subdivision will become the first element of an internationally accepted time scale for the Precambrian.

The lower limit of the Archean is essentially the age of the oldest known rocks, about 3.8 billion years ago. The unnamed earliest phase of Earth history, from the time of origin to 3.8 billion years ago, can only be inferred from evidence derived from meteorites and other solar bodies and from the inherited character of isotopic systems such as rubidium-strontium and lead-uranium.

THE ROCK RECORD

Crystalline rock such as granite gneiss and schist constitutes a significant percentage of the exposed Precambrian, particularly in the central part of continental regions, as in the Canadian Shield. Taken as a whole, however, virtually all rock types are present in the Precambrian in varying degrees of preservation, although chemical sediments, such as phosphorite and salt, and organic deposits, such as coal and petroleum, are notably sparse. Volcanic rocks are abundantly represented, often as greenstone belts that transversely cut the gneissic terranes of continental shields or lie at the margins of cratons. Tillite and associated glaciogene deposits, consolidated but otherwise entirely similar in character to the glacial deposits of recent times, are surprisingly well preserved in many parts of the world. Such deposits are of at least two ages: about 700 million years ago and about 2.4 billion years ago (see ICE AGES). One of the more distinctive sedimentary-rock types in the Precambrian is iron formation; although not entirely lacking in the Phanerozoic, this curious rock clearly was deposited in greatest abundance during the Precambrian, with a peak occurrence in stratigraphic sequences about 2 billion years ago.

Throughout the world, local sequences of Precambrian sedimentary rock as thick, complex, and extensive as any in younger eras are preserved, recording deposition in basins or troughs of the continental crust formed earlier. Those in Southern Africa are particularly well preserved and are named as follows: Swaziland Supergroup with an age of 3.5 to 3.3 billion years; Pongolo Group with an age of 3.2 to 2.9 billion years; Witwatersrand Supergroup with an age of 2.8 to 2.6 billion years; Transvaal Supergroup with an age of 2.5 to 2.0 billion years; and Waterberg Group with an age of 1.9 to 1.5 billion years. Each sequence is thousands of meters thick, and in most areas the rocks are only slightly metamorphosed; primary sedimentary features such as cross-bedding and ripple marks can generally be observed and measured, even in the oldest strata. In North America some of the thicker well-known sequences are the Huronian Supergroup with an age of 2.6 to 2.2 billion years; the Marquette Range Supergroup with an age of 2.1 to 1.9 billion years; and the Belt Supergroup with an age of 1.4 to 0.9 billion years.

MAJOR EVENTS

The initial accumulation of cosmic material to form the Earth and the separation of the planet into an inner molten metallic core and an outer silicate mantle belong more to the realm of cosmology than to geology, but the development of the outermost shells--the crust, the hydrosphere, and the atmosphere--is a geologic process that has been active throughout Earth history. The specific features of these outer shells, therefore, have been evolving in the past and will continue to evolve in the future. Nevertheless, the essential character of the Earth's crust, oceans, and atmosphere--other than geographic distribution, which was profoundly dependent on the breakup of supercontinents about 200 million years ago--was established by the end of the Precambrian.

Crustal Evolution

At the present time, oceanic crust is being generated by the extrusion of basalt along mid-oceanic rifts and is being laterally transferred by seafloor spreading. Whether these processes were operating during early Precambrian time is questionable, but certainly some elements of what is now known as plate tectonics were effective as far back as 2 billion years ago.

In contrast to oceanic crust, continental crust (often referred to as sial, a term derived from silicon and aluminum) evolves mainly by a process of partial melting in the lower crust and upper mantle and progressive transfer of lighter elements to higher levels. Early in Earth history, the sialic masses thus formed were relatively small and thin. In most parts of the world these islandlike masses coalesced to form larger cratons at various times during an epoch of structural disturbance 2.9 to 2.5 billion years ago. Thus by the end of the Archean Eon the essential character of most continental areas was established.

In North America this epoch of crustal disturbance is referred to as the Kenoran, or Algoman, orogeny. In later Precambrian time the North American craton, like others in the world, was modified by the development of transcurrent troughs and "mobile zones," and by marginal accretion such as exemplified by the formation of continental-type crust about 1.7 billion years ago in the present area of the southwestern United States.

Evolution of the Atmosphere and the Oceans

Little or no direct evidence is available as to the nature of the primordial atmosphere and hydrosphere that was produced by the expulsion of volcanic gases during the earliest phase of Earth history.

Observations on the oldest known rocks, however, indicate deposition of sediments in an aqueous environment and show that the deposited materials do not differ radically from those of younger eras. Bodies of water existed, therefore, at least 3.8 billion years ago, and the atmosphere was probably dominated then, as at present, by nitrogen, carbon dioxide, water vapor, and oxygen.

The amounts and relative proportions of these gases doubtless have undergone strong changes with time. Accumulations of uraninite and pyrite fragments in rock with an age of more than 2.5 billion years, such as that found in the Witwatersrand of South Africa, indicate that the then-prevailing atmosphere was devoid of or deficient in oxygen and was perhaps higher in carbon dioxide. Deposition of the oxide-rich iron formations of the world (such as Labrador and the Mesabi Range) at about 2.0 billion years, however, suggests that a rapid buildup in oxygen occurred in the interval from 2.5 to 2.0 billion years ago, possibly as a result of photosynthesis by newly evolved organisms.

Evidence of Life

Current research is showing that biologic activity extends, far back into Precambrian time. What appear to be fossilized algae or bacteria are found in South African rocks with an age in excess of 3 billion years. The existence of microbiota in these old rocks is further indicated by the presence of stromatolites, which are domal calcareous structures, in part of organic origin. Stromatolites become more common in carbonate rock of older Proterozoic age, such as the Transvaal Supergroup of South Africa; in strata of younger Proterozoic (Riphean) age in the USSR they are sufficiently abundant and diverse to furnish a basis for time-stratigraphic classification.

Microfossils are particularly well preserved in certain dark cherts, notably in the Gunflint Formation of Ontario, with an age of about 2.0 billion years. Nucleated forms (eucaryotes) occur in younger Precambrian strata, as in the 850-million-year-old Bitter Springs chert of Australia. A few shelly fossils of tubular-shaped organisms appear in the rocks of the late Precambrian age (700-570 million years ago)--Cloudina, found in Namibia, and Sinotubulites, found in southern China. Both had shells composed of calcium carbonate, and they are considered to be precursors of the much more variegate shell fauna of the Cambrian age.

Much controversy has surrounded the edicaran fauna (first discovered in 1947 in the Ediacara Hills of South Australia), which also arose in the late Precambrian age. Once widely thought to be ancestors of certain jellyfishes, worms, and soft corals, they are now believed by many paleontologists to be a distinct form of life that became extinct at the onset of the Cambrian age. Characterized by flat, leaflike, sometimes quilted bodies, the water-dwelling Ediacaran fauna may have carried out metabolic functions directly through their outer skin (instead of developing complex inner organs as other multicellular fauna have done).

Harold L. James

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