Proterozoic

Home > Earth and the Environment > Ecology and Environmentalism > Environmental Studies

Proterozoic Late Precambrian (Cryptozoic) time is encompassed by the Proterozoic Eon, customarily held to span the interval 2.5 Ga to 590 Ma (2.5 × 10⁹−5.9 × 10⁸ years). Proterozoic rocks rest upon the Archaean and are succeeded by the Cambrian, but the boundaries are problematical. No rock standard has been identified formally as marking the base; at the top of the Proterozoic the Cambrian–Precambrian boundary is identified on the basis of fossils. In the past there have been several different divisions of the later Proterozoic into eras and periods, etc., based upon regional successions. Several of these, for example the Sinian Era, the Vendian Period, and the Ediacarian and the Varangian epochs, have received intensive study on account of the fossils they contain. So far their acceptance by the international community is not unanimous and their usage remains difficult. Generally accepted today are the Palaeoproterozoic (1.6–2.5 Ga), Mesoproterozoic (1.6–1.0 Ga), and Neoproterozoic (1.0 Ga–540 Ma) eras, respectively represented by approximately 20, 25, and 40 per cent of the Precambrian crust. The Subcommission on Precambrian Stratigraphy of the International Commission on Stratigraphy has given names to four periods within the Palaeoproterozoic and three to each of the other eras. The latest of these periods, the ‘Neoproterozoic III’, awaits a formal name and, at 650 Ma, follows on the Cryogenian, a period characterized by its glacial climate and sediments.

Proterozoic rocks occur in all the great Precambrian cratons of the world. Here they have been deeply eroded and then uplifted so that at outcrop they display features formed originally when deeply buried. Many of the terrains in which they lie exhibit plate-tectonic attributes, especially asymmetric linear orogenic belts, identified by tectonic style and geochronology. They include a wide variety of platformal clastic and carbonate sediments, and basinal clastics as well as numerous volcanics, banded ironstone formations, evaporites, and phosphorites. Their disposition was controlled by the stabilized Archaean continents and long-lasting sedimentary basins, and by cordilleran and collision types of orogeny. Major crustal rifting has also played a part in the evolution of large bodies of Proterozoic sedimentary and igneous rocks. Conspicuous igneous intrusive activity has resulted in gigantic dyke swarms, massive ultrabasic layered sheets, and large granitoid plutons.

Early Proterozoic rocks have been identified in all con-tinents, but the largest outcrops by far are those of the North American–Greenland–northern Europe (Baltic Shield) region. India and Antarctica have the smallest areas of Early Proterozoic rocks. All regions show a great variety of composition and grades of metamorphism, with amphibolite facies orthogneiss–migmatite subordinate to high-grade granulite terrains and low-grade foreland fold belts. The last-mentioned possess distinctive platform–shelf–slope–rise facies that formed on wide stable shelves. Orthoquartzite– carbonate–banded ironstone sequences are very common. Many fold belts include greenstone–turbidite assemblages and they contain the oldest known ophiolites, glaciogenic rocks, and continental red beds. As a result of photosynthesis by plants, the Earth's environment was at that time rich enough in oxygen to support aerobic (animal) organisms. The biomass was greatly enlarged to include diverse cyanobacteria, stromatolites, and many problematica (structures thought to be of organic origin but of uncertain affinity). The lithospheric plates making up the crust of the Earth were substantial in size and continuously in motion. Their continental parts were involved in a great deal of deformation, both while in passage and during collision with each other. Much underplating of the continents occurred as whole regions of continental crust were driven under their neighbours by subduction. A grand climax occurred at the end of the era, when, as if in response to a sudden surge of terrestrial heat through the crust, there was a global orogeny (1890–1850 Ma).

Mid-Proterozoic rocks occur in greatest mass in North America, with moderately extensive outcrops in South America, Australia, and Antarctica and relatively little elsewhere. Successions of rocks found in the Precambrian fold belts are largely of moderately metamorphosed supracrustal rocks and plutonic granites; those of the stable cratonic areas are of terrigenous sediments, ironstones and volcanics, and thick pelite–carbonate sequences. Continental rifting took place and was accompanied by immense basalt floods, and gabbroic–carbonatite intrusions. The orogenic belts display thick sequences of stacked thrust sheets, together with shear zones. By the Mid-Proterozoic biogenic photosynthetic activity had raised oxygen to about 4 per cent of present atmospheric level (pal) and the earliest eukaryotic organisms appeared. Plate-tectonic activity resulted in the development of a thick stable crust with extensional rifting in response to thermal plumes in the underlying mantle. There were some plate collisions with accretionary arcs adhering to the major continental masses. During the mid-part of the era large masses of rapakivi granites were generated, as were charnockites, mantle-derived anorthosites, and layered gabbros. At the end of the era (around 1.0 Ga) a very widespread major orogeny occurred.

Late Proterozoic rocks dominate the Precambrian geology of the Gondwanaland continents—Africa, South America, Antarctica, Australia, and India. Formations of this age are widespread throughout Eurasia, but are only minor components of the North American shield. They are comprised of extensive spreads of shallow-water marine platform carbonates and arenites, and red beds. Shallow depositional basins extended across enormous areas of the continents, which underwent rifting and the development of huge sediment-filled troughs (aulacogens). Continental red beds are common and there are local evaporites. Although banded ironstones of this age are widespread, they are not as thick as those deposited earlier. Enormous masses of Late Proterozoic glaciogenic deposits are found in all continents, but especially around the North Atlantic. They include many tilloids and associated rocks, indicating an intense and prolonged glacial period with many advances and retreats of the ice. This does not appear to have retarded the development of the biosphere with its ubiquitous stromatolites and widespread cyanobacteria. Towards the end of the era various groups of soft-bodied metazoa appeared, no doubt aided by the increasing level of atmospheric oxygen (12 per cent pal).

Towards the end of the era plate-tectonic orogenies occurred in which the old stable cratons were broken and rearranged to constitute a single supercontinent. In some of these movements ophiolitic slices were squeezed up from the oceanic crust and large masses of island-arc volcanics were accreted to the continental margins. This great land mass was vigorously eroded and peneplained before the onset of Palaeozoic time with its global rise in sea level.

D. L. Dineley

Bibliography

Goodwin, A. M. (1991) Precambrian geology: the dynamic evolution of the continental crust. Academic Press, London.
Nisbet, E. G. (1987) The young Earth: an introduction to Archaean geology. Allen and Unwin, Boston.
Plumb, K. A. (1991) New Precambrian time scale. Episodes, 14, 139–40.
Schopf, J. W. and and Klein, C. (1992) The Proterozoic biosphere. Cambridge University Press.

Proterozoic The eon of geological time extending from the end of the Archaean, about 2600 million years ago, to the start of the present eon (see Phanerozoic), about 570 million years ago. Life in the early Proterozoic was dominated by bacteria, which flourished in shallow seas and muds. They depended on a wide variety of metabolic strategies, including photosynthesis, which were crucial in determining the composition of the earth's atmosphere and oceans. The oldest eukaryotic fossils date from after the middle Proterozoic, about 1200 million years ago. These early protoctists are thought to have arisen through symbiotic associations of various prokaryotes (see endosymbiont theory), probably on several independent occasions. Towards the end of this eon comes the first fossil evidence of multicellular animal life, the so-called Ediacara fauna, named after a rocky outcrop in Australia but also found elsewhere. These rocks, dated to around 650 million years ago, reveal traces of soft-bodied fanlike or quiltlike creatures, perhaps unrelated to any modern forms, as well as animals resembling jellyfish and worms.

Proterozoic The eon of geologic time, 2500–542 Ma ago, between the Archaean and Phanerozoic Eons, and ending in the abundantly fossiliferous Ediacaran Period. It wa the final period of the former Precambrian

Proterozoic The most recent of the three subdivisions of the Precambrian, dated at about 2500–590 Ma ago. It culminated with the first abundantly fossiliferous period, the Ediacaran.

Proterozoic The most recent of the three subdivisions of the Precambrian, dated at about 2500–590 Ma ago.

Proterozoic The most recent (about 2500–575 Ma ago) of the three subdivisions of the Precambrian.