پرکامبرین

پرکامبرین

Precambrian

 

A major interval of geologic time between about 540 million years (Ma) and 3.8 billion years (Ga) ago, encompassing most of Earth history. The Earth probably formed around 4.6 Ga and was then subjected to a period of intense bombardment by meteorites so that there are few surviving rocks older than about 3.8 billion years. Ancient rocks are preserved exclusively in continental areas. All existing oceanic crust is younger than about 200 million years, for it is constantly being recycled by the processes of sea-floor spreading and subduction. The name Hadean has been proposed for the earliest turbulent part of Earth's history. Development of techniques for accurate determination of the ages of rocks and minerals that are billions of years old has revolutionized the understanding of the early history of the Earth.  See also: Dating methods; Geologic time scale; Geochronometry; Hadean; Rock age determination

Detailed sedimentological and geochemical investigations of Precambrian sedimentary rocks and the study of organic remains have facilitated understanding of conditions on the ancient Earth. Microorganisms are known to have been abundant in the early part of Earth history. The metabolic activities of such organisms played a critical role in the evolution of the atmosphere and oceans. There have been attempts to apply the concepts of plate tectonics to Precambrian rocks. These diverse lines of investigation have led to a great leap in understanding the early history of the planet.  See also: Plate tectonics

 

Subdivisions

 

Early attempts to subdivide the Precambrian were mainly based on geological characteristics such as rock structure and metamorphic grade. Meaningful subdivision of the Precambrian has become possible only with the development of sophisticated techniques for deciphering the ages of rocks. The Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences (IUGS) has suggested a subdivision based on specific time intervals (Fig. 1). Some have claimed that subdivisions of the Precambrian should correspond to specific evolutionary changes that can be observed in the rock record. Many of these changes occurred at widely different times in different parts of the planet so that “arbitrary” time subdivisions are somewhat artificial. To overcome some of these conceptual problems, a simple subdivision using geons, which are time units of 100 million years' duration, has been suggested (Fig. 1). The era names suggested by the IUGS have been widely accepted, whereas the smaller subdivisions (equivalent to periods in younger rocks) have not. Subdivision of the Precambrian has been contentious. Most agree on names for major subdivisions, but the concept of rigidly applicable global subdivisions based on arbitrary times poses philosophical problems for some.

 

 

Fig. 1  Major subdivisions of geologic time. The Precambrian comprises the Archaean and Proterozoic eons.

 

 

 

fig 1

 

 

 

 

Archean

 

Rocks of the Archean Eon (2.5–3.8 Ga) are preserved as scattered small “nuclei” in shield areas on various continents. The Canadian shield contains perhaps the biggest region of Archean rocks in the world, comprising the Superior province. Much of the Archean crust is typified by greenstone belts, which are elongate masses of volcanic and sedimentary rocks that are separated and intruded by greater areas of granitic rocks. The greenstones are generally slightly metamorphosed volcanic rocks, commonly extruded under water, as indicated by their characteristic pillow structures (Fig. 2). These structures develop when lava is extruded under water and small sac-like bodies form as the lava surface cools and they are expanded by pressure from lava within. Such structures are common in Archean greenstone assemblages in many parts of the world.  See also: Archean; Metamorphic rocks

 

 

Fig. 2  Neoproterozoic pillow lavas from Anglesey, North Wales.

 

 

 

fig 2

 

 

 

Ultramafic magnesium-rich lavas, known as komatiites, are considered to have formed at high temperatures (about 3000°F or 1650°C). These unusual lavas have been interpreted to mean that the interior of the Earth (mantle) was much hotter in the Archean than it is now. Geochemical and other investigations of lavas preserved in greenstone belts have led to the interpretation that they formed in a number of environments, analogous to present-day ocean floors, oceanic and continental volcanic arcs, and rifts.

Many of the associated sedimentary rocks are “immature” in nature, having formed by rapid erosion and deposition, from volcanic and, in some cases, older continental crustal sources. Such sedimentary rocks are commonly known as turbidites, formed as a result of gravity-controlled deposition in relatively deep water. Relatively rare “supermature” sediments formed in shallow-water environments as the result of extreme weathering. They are composed almost entirely of quartz and are known as orthoquartzites. In addition, iron-rich sediments formed by chemical precipitation from seawater (Fig. 3). These sediments indicate the existence of liquid water on the Archean Earth's surface and provide important clues concerning the nature of the ancient atmosphere and oceans.  See also: Quartz; Sedimentary rocks; Turbidite

 

 

Fig. 3  Banded iron formation (BIF) from the Archean of Finland. The light layers are silica-rich (chert), and the dark layers consist of iron oxides. The highly contorted nature of many such iron formations is due to early slump movements of these chemical sediments prior to complete consolidation.

 

 

 

fig 3

 

 

 

Many Archean shield areas also contain terranes composed of highly metamorphosed rocks (high-grade gneiss terranes). The origin of these gneissic terranes is controversial, but some may represent subduction zones located at ancient continental margins where sedimentary rocks were carried to great depths, metamorphosed, and intruded by igneous rocks. There has been a growing (although not universal) tendency to interpret Archean shield areas as the result of some form of plate tectonic activity, probably involving accretion of island arcs and collision of small continental nuclei. Theoretical considerations and the presence of magnesium-rich komatiitic lavas have led to the suggestion that the interior of the early Earth was much hotter than at present. The end of the Archean is marked in many places by amalgamation of large numbers of volcanic arcs and crustal fragments, which together formed the world's first large continental masses. This process appears to have occurred over a long period of time, beginning at about 3.0 Ga in South Africa and Australia and culminating at about 2.5 Ga in North America. The latter date has been adopted as the demarcation point between the Archean and the Proterozoic.  See also: Continental margin; Continents, evolution of; Igneous rocks; Lava

 

Proterozoic

 

The Proterozoic Eon extends from 2.5 Ga until 540 Ma, the beginning of the Cambrian Period and Phanerozoic Eon. Proterozoic successions include new kinds of sedimentary rocks, display proliferation of primitive life forms such as stromatolites, and contain the first remains of complex organisms, including metazoans (the Ediacaran fauna). Sedimentary rocks of the Proterozoic Eon contain evidence of gradual oxidation of the atmosphere. Abundant and widespread chemical deposits known as banded iron formations (BIF) make their appearance in Paleoproterozoic sedimentary basins.  See also: Banded iron formation; Proterozoic

 

Paleoproterozoic

 

In most Proterozoic depositional basins there is a higher proportion of sedimentary than volcanic rocks. The world's first widespread glacial deposits are preserved in Paleoproterozoic successions. The existence of glacial conditions so long ago (2.3 Ga) is indicated by the presence of unusual conglomerates (diamictites or tillites) with large rock fragments set in an abundant fine-grained matrix (Fig. 4a). These peculiar conglomerates consist of a variety of rock fragments set in an abundant finer-grained matrix. Such conglomerates may form as a result of deposition of unsorted sediments from glaciers, in which case they are called tillites. Additional evidence of glaciation includes glacially scratched (striated) rock surfaces, striated and faceted rock fragments and “dropstones” (rock fragments that are interpreted as having been released, by melting, from floating icebergs into finely bedded sediments; Fig. 4b). Possibly contemporaneous Paleoproterozoic glacial rocks are known from five continents.  See also: Depositional systems and environments; Glacial geology

 

 

Fig. 4  Glacial sedimentary rocks. (a) Diamictites forming part of the Paleoproterozoic Gowganda Formation (2.3 Ga) on the north shore of Lake Huron, Ontario, Canada. (b) Dropstones from the Gowganda Formation. Such large rock fragments in otherwise fine-grained bedded sedimentary rocks are thought to have been “vertically” emplaced from melting icebergs.

 

 

 

fig 4

 

 

 

Paleoproterozoic sedimentary rocks contain some of the world's greatest accumulations of valuable minerals, such as the uranium deposits of the Elliot Lake region in Ontario, Canada, and the great banded iron formations of the Lake Superior region. The uranium deposits are believed to have resulted from deposition of mineral grains in ancient river systems. Because uranium minerals (and associated pyrite) were not oxidized, it has been inferred that the Earth's atmosphere had a very low free oxygen content. By contrast, the somewhat younger (2.0 Ga) banded iron formations are thought to indicate an oxygen-rich hydrosphere/atmosphere system.

The Paleoproterozoic also contains abundant carbonate rocks. Some of these contain spectacular organo-sedimentary structures called stromatolites (Fig. 5). The organo-sedimentary nature of these structures is confirmed by the preservation of microscopic primitive prokaryotic cells of photosynthetic cyanobacteria. These photosynthetic microorganisms trapped, or in some cases precipitated, carbonates from seawater, resulting in the construction of stromatolitic mounds of highly varied morphology. The metabolic activity of the microorganisms responsible for building these structures contributed to oxygenation of the atmosphere. Identical structures occur in some areas of the modern ocean, such as shallow-water saline environments of Shark Bay in Western Australia. Widespread development of stable continental shelves at the beginning of the Proterozoic (in contrast to the unstable, dominantly volcanic settings of much of the Archean) may have favored stromatolitic growth.  See also: Stromatolite

 

 

Fig. 5  Stromatolites in the upper part of the Paleoproterozoic Snowy Pass Supergroup in southeastern Wyoming, United States.

 

 

 

fig 5

 

 

 

Two theories have emerged to explain oxygenation of the atmosphere. Increased photosynthetic activity, due to proliferation of stromatolites, is one. A second theory suggests that oxygen produced by marine photosynthetic microorganisms was consumed by reducing reactions related to widespread hydrothermal circulation of seawater at mid-ocean ridges and in other oceanic volcanic systems. Although there was significant production of photosynthetic oxygen, little was released to the atmosphere. This idea is supported by the preservation of different proportions of carbon isotopes (12C and 13C) in ancient organic carbon and carbonate rocks. Removal of 12C during photosynthesis results in seawater that is enriched in the heavier isotope. The carbon-isotopic composition of seawater may, under certain conditions, be preserved in carbonate rocks. Differences in the ratios of carbon isotopes of such carbonates and of organic carbon have been detected in many Archean sedimentary rock successions. These data suggest that photosynthetic activity was fairly widespread even during the early history of the Earth, but that release of significant amounts of free oxygen to the atmosphere was delayed until the tectonic regime changed from being ocean-dominated at the end of the Archean.  See also: Atmosphere, evolution of; Cyanobacteria; Mid-Oceanic Ridge; Photosynthesis; Seawater

Another aspect of sedimentary geology that supports an increase in free oxygen during the Paleoproterozoic Era is the appearance of redbeds, sedimentary rocks that contain finely disseminated hematite. Many sedimentary rocks of this kind formed in subaerial or shallow-water settings where they were in close contact with the atmosphere, which must have contained at least some free oxygen, to impart the characteristic red or purple coloration of the highly oxidized iron.  See also: Redbeds

Another indication of atmospheric change is the preservation of widespread banded iron formations at around 2.0 Ga. The major problem with these economically important and enigmatic chemical sedimentary rocks is that iron is virtually insoluble in present-day rivers and oceans. Under reducing conditions, such as those postulated for the Archean, the solubility of iron would have been greatly increased. The development of widespread banded iron formations in relatively shallow depositional settings in the Paleoproterozoic is th

/ 0 نظر / 15 بازدید