archean sedimentation
TRANSCRIPT
Alln. Rev. Earth Planet. Sci. 1980.8: 145-67 Copyright© 1980 by Annual Reviews Inc. All rights reserved
ARCHEAN SEDIMENTATION
Donald R. Lowe Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803
INTRODUCTION
xl0128
As a time unit, the Archean includes that interval between the formation of the earth, approximately 4.6 billion years (b.y.) ago, and the development of large areas of stable sialic crust about 2.6 to 2.4 b.y. ago. As a tectonic-stratigraphic unit, it is characterized by a distinctive bimodal association of granite-greenstone belts and high grade gneiss-granulite terranes. Although preserved terrestrial rocks represent only the last 1.3 to 1.2 b.y. of Archean time, they are of great importance as the major source of direct information on the origin and evolution of life, the development of the earth's crust and mantle, and the characteristics of the early ocean and atmosphere.
Relatively unaltered sedimentary rocks underlie large areas of many Archean shields. In a series of classic papers, Pettijohn (1934, 1943) demonstrated that these strata could be studied and interpreted in the same way as Phanerozoic sedimentary deposits. Subsequent stratigraphic and sedimentological work in the Canadian Shield has provided the backbone of information regarding its geologic and tectonic evolution (Goodwin 1973, 1977a, b, Goodwin & Ridler 1970, Turner & Walker 1973, Walker 1978, Walker & Pettijohn 1971).
The sedimentological histories of other shields have only recently been investigated. Although brief descriptions of sedimentary rocks have traditionally been part of areal investigations and have frequently accompanied volcanological, geochemical, and geochronological studies, the results of more systematic investigations in the South African (Eriksson 1978, 1979, Lowe & Knauth 1977, 1978, von Brunn & Hobday 1976), Western Australian (Barley et a11979, Glover & Groves 1978), and Indian (Naqvi 1977, Naqvi et al 1978) cratons have appeared largely within the last five years. Although these areas represent one of the most vital and dynamic periods of earth history and possess an immense potential for
145 0084-6597/80/0515-0145$01.00
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146 LOWE yielding fundamental information on the early earth through the application of modern stratigraphic and sedimentological techniques, they have until recently been bypassed in the course of sedimentary research. The recent spate of Archean studies, evidenced by the many new books on the subject (Glover & Groves 1978, McCall 1977, Salop 1977, Windley 1976, Windley & Naqvi 1978), includes some new sedimentological investigations. Hopefully these will provide a more comprehensive and systematic picture of Archean sedimentation, complementary to that emerging from geochemical, geochronological, and structural research.
The present review first attempts to synthesize a general picture of the lithology, stratigraphy, and sedimentology of Archean sedimentary rocks, principally in greenstone belts. The sedimentary evolution of these belts is then considered in terms of the constraints it places on interpretations of their tectonic development and the general evolution of the Archean crust.
DISTRIBUTION OF ARCHEAN SEDIMENTARY ROCKS
Most Archean sedimentary rocks are preserved within tectonic-stratigraphic units called greenstone belts. These belts are widely developed in Archean shield areas and are regarded by many as characteristic structural units of the Archean. They are best preserved and have been most completely studied in Canada, South Africa, Rhodesia, Western Australia, and India (Figure 1). Other greenstone belts in the Baltic Shield (GaaJ et a1 1978) or central and northern Africa (e.g. Williams 1978) are at least in part of Archean age but remain largely unknown. The general characteristics of Archean greenstone belts are discussed by Anhaeusser ( 1971a, b, 1973), Anhaeusser et al ( 1969), Engel ( 1968), Glikson ( 1970, 1976), Glikson & Lambert ( 1976), Goodwin ( 1973, 1977a), Goodwin & Ridler ( 1970), Turner & Walker ( 1973), Viljoen & Viljoen ( 1969a, b, c), and Walker (1978).
Greenstone belts typically occur as highly deformed volcanic and sedimentary keels, steeply infolded into surrounding, coeval to younger metamorphic and granitoid plutonic rocks. Adjacent to the plutons, the greenstone sequences commonly show narrow aureoles of higher grade metamorphism, but over large areas of most belts, the rocks are altered only to the lowest greenschist facies.
Anhaeusser et al ( 1969) have emphasized the remarkably similar rock association, stratigraphy, structure, metamorphism, and tectonic evolution of known Archean greenstone belts. The stratigraphic sections usually range between 10 and 25 kilometers thick and include distinct
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ARCHEAN SEDIMENTATION 147
Figure 1 Generalized distribution of the principal Archean shields containing relatively
unmetamorphosed greenstone belts discussed in this report.
volcanic and sedimentary sequences (Figure 2). In older belts, such as the Barberton Mountain Land (3.0 to 3.5 b.y.) and Pilbara Block (3.0 to 3.5 b.y.), there is usually a single thick volcanic sequence that is overlain by one main sedimentary succession (Hickman & Lipple 1975, Lipple 1975, Viljoen & Viljoen 1969a). In younger, generally post-3.0 b.y. old belts, such as those in thc Canadian and Yilgarn shields, there are commonly several volcanic cycles, each capped by a thick sedimentary unit (Glikson 1976, Goodwin & Ridler 1970, Henderson 1972, McGlynn & Henderson 1970, Turner & Walker 1973).
Archean sedimentary rocks outside greenstone belts occur locally as platform deposits younger than greenstone belts or cratonization. The 3.0 b.y. old Pongola Supergroup (von Brunn & Hobday 1976) and slightly younger Dominion Reef strata of South Africa are indistinguishable from widespread Phanerozoic stable shelf carbonate-quartzite associations, and the auriferous 2.7 to 2.3 b.y. old Witwatersrand basin deposits (Pretorius 1975, Vos 1975) were deposited mainly by fluvial processes in an intracratonic basin. These types of stable shelf or cratonic basin deposits characterize many early Proterozoic sedimentary sequences and set them apart from the subjacent greenstone sedimentary assemblages.
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148 LOWE
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Figure 2 Representative greenstone belt stratigraphic sequences. All at same scale. A. Barberton Mountain Land, South Africa. Zwartkoppie Formation of Viljoen & Viljoen ( 1 969a, c) is considered to be a facies of the upper Kromberg Formation and lower Fig Tree Group. B. Eastern Pilbara Block, Western Australia (Hickman & Lipple 1 975). C. Sioux Lookout area, southern Superior Province, Canada (Turner & Walker 1 973). D. Belingwe greenstone belt, Rhodesia. Bvude Formation is mainly hornblende schist, amphibolite, and calc-silicate rock with minor ultramafic schists and felsites (Orpen 1978).
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ARCHEAN SEDIMENTATION 149
GREENSTONE BELT VOLCANIC SEQUENCE
The characteristic rocks of greenstone belts are volcanic in origin. They include three principal varieties: mafic and ultramafic komatiites, tholeiitic basalt, and felsic lavas and fragmental rocks. The mafic and ultramafic lavas, both komatiites and tholeiites, tend to form massive sections, commonly several kilometers thick, made up of a large number of individual flow units. In general, individual greenstone belts tend to show a trend from komatiitic lavas at the base of the volcanic sequence to more tholeiitic and felsic units toward the top. The common development of pillows and, in the ultramafic units, crystalline quench textures suggests rapid subaqueous extrusion and crystallization (Viljoen & Viljoen 1969b).
Felsi<; volcanic rocks are best developed in the upper part of the volcanic sequences. They may occur in units up to several kilometers thick or as relatively thin zones of tuff. Most of the felsic material was extruded in a fragmental form and subsequently redistributed by sedimentary processes. Many workers have emphasized the presence in greenstone belts of volcanic-sedimentary cycles consisting from the base upward of mafic or ultramafic volcanic rocks, felsic volcanics, and a chert cap (Viljoen & Viljoen 1969a,c, Anhaeusser 1971b).
Felsic Volcanic and Sedimentary Rocks
Felsic volcanic rocks are a characteristic fades of the volcanic portions of Archean greenstone belts. Such units are present in the Theespruit and Hooggenoeg Formations of the Onverwacht Group in the Barberton Mountain Land of South Africa (Figure 2) and in the Gindalbie and Gundockerta Formations in the Eastern Goldfields Province of the Yilgarn Block (Williams 1973) and include the Hokonui and Koodoovale Formations in the Rhodesian Belingwe Belt (Orpen 1978; Figure 2), the Duffer and Wyman Formations of the Warrawoona Group in the Pilbara Block (Hickman & Lipple 1975), the Jackson Lake Formation and similar units in the Slave Province (Henderson 1972, Lambert 1978), and many formations in the Superior Province (Goodwin & Ridler 1970, Dimroth 1977, Dimroth & Demarcke 1978). The volcanic rocks in these units are typically dacitic, rhyodacitic, or rhyolitic in composition, and contrast strongly with the associated tholeiitic and komatiitic flows. In the Canadian Shield, felsic formations commonly include andesitic and other calc-alkaline volcanic rocks (Baragar & Goodwin 1969) which are largely lacking in other greenstone belts.
Although individual felsic units may persist over wide areas, many are markedly lenticular and most exhibit major thickness and facies changes
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150 LOWE
(Barley et al 1979, Goodwin & Ridler 1970). The thickest parts of felsic formations commonly reach several hundred to several thousand meters and are characteristically composed of coarse, poorly stratified fragmental material and flow rock. The principal rock types include coarse breccia, tuff breccia, tuff, and coarse-grained volcaniclastic sedimentary units which collectively formed cones of loose autoclastic, pyroclastic, and epiclastic debris constructed around active vents. The abundance of tuffs and the local development of shallow-water and terrestrial volcaniclastic deposits (Barley et al 1979, Williams 1973) indicate that the tops of these cones were commonly above sea level (Dimroth 1977). The building of large vent complexes probably produced broad subaerial volcanic edifices of considerable relief. Ash flow tuffs, lahars, and oligomict conglomerate, formed by penecontemporaneous erosion of the vent cones, are locally important constituents of terrestrial and shallow-water portions of these sequences (Dimroth & Demarcke 1978, Lambert 1978, Williams 1973).
Most of the vent cones are composed in large part of subaqueously redeposited fragmental volcanic debris. Although the vent cones locally developed narrow fringing aprons of shallow-water volcaniclastic sand and gravel, most of the debris apparently moved directly into deeper water within massive subaqueous sediment gravity flows (Barley 1978, Dimroth 1977, Lambert 1978, Orpen 1978, Tasse et al 1978, Viljoen & Viljeon 1969c). The deposits of these flows are massive to crudely graded and resemble those described from Phanerozoic sequences (Fiske 1963). The subaqueous slopes upon which the flows originated and across which they moved were probably relatively steep, perhaps locally approaching 10 to 25 degrees, the common range of the angle of repose of loose, watersaturated debris. They mark the first appearance of significant slopes within the greenstone belt basins.
Away from the vent complexes, felsic units tend to thin and fine (Lambert 1978). Massive debris flow deposits grade laterally away from the vents into sequences that reflect deposition from turbidity currents of progressively lower energy (Williams 1973). The most distal deposits are fine airfall tuffs. A similar, vertical sequence, formed as a result of waning vent activity, characterizes the deposits marginal to some vent complexes. In the Barberton Mountain Land, the upper felsic volcanic cycle in the Komati Gorge section of the Hooggenoeg Formation shows massive debris flow deposits (Viljoen & Viljoen 1969c) grading upward into turbidites dominated by Bouma A and B divisions which are in turn succeeded by gray cherts formed by the silicification of turbidites showing mainly B, C, and 0 Bouma divisions (Lowe & Knauth 1977).
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ARCHEAN SEDIMENTATION 151 The erosion of these volcanic complexes during the waning stages of
felsic volcanism, accompanied by gradual subsidence, provided broad, low . relief platforms upon which were deposited a variety of shallowwater sediments under volcanically quiescent conditions. Over 500 m of banded gray and white chert in the Barberton Mountain Land, originally considered to be part of the upper Hooggenoeg Formation (Viljoen & Viljoen 1969c, Lowe & Knauth 1977) but now known to mark the lowest Kromberg Formation (Figure 2), was deposited on top of an eroded felsic volcanic platform. The chert includes desiccation features and silicified gypsum indicating shallow-water to subaerial, low-energy deposition of a carbonate-silica-evaporite sequence (Lowe & Knauth 1977).
Away from the vent complexes, thin felsic tuff units are also capped by chert layers. Lowe & Knauth (1977, 1978) have shown that many of these chert units in the Barberton Mountain Land formed by silicification of fine-grained vitric pyroclastic material. In some cases, these were airfall units but others clearly represent current-deposited volcaniclastic sediment. Most are relatively thin, less than 10 m thick, and are underlain by or grade laterally into un silicified felsic tuff or breccia. The chert ranges from light to dark gray or greenish in color and contains abundant microcrystalline chlorite and sericite impurities formed during devitrification and recrystallization of the volcanic ash. Hence, it is distinguished from chemical precipitates or silicified carbonate rock, which tend to lack intergrown phyllosilicate minerals.
In the Barberton Mountain Land this type of deposit is developed throughout the tholeiitic Hooggenoeg Formation and includes the wellknown Middle Marker at the base of the Formation (Figure 2). Many of these chert units show brecciation and shrinkage cracks, intraformational conglomerates, cross-laminations, and units of accretionary lapilli suggesting deposition under shallow-water to subaerial conditions (Lowe & Knauth 1977, 1978).
Although some felsic volcanic units reach several thousand kilometers, mbst are less than 2 km in thickness. The frequency with which the vent cones became subaerial suggests that either the original water depths were not great or that regional epeirogenic uplift, possibly related to intrusive activity, accompanied felsic volcanism.
.
N on- Volcanogenic Sedimentary Rocks
Within most volcanic sequences, sedimentary rocks other than those obviously of volcanogenic origin make up a minor proportion of the stratigraphic column. The most common interftow units are cherts but thin layers of carbonate and terrigenous clastic rocks occur locally.
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152 WWE
BANDED AND FERRUGINOUS CHERTS Perhaps the most widely distributed interflow sedimentary rocks are banded and/or ferruginous cherts, including banded gray and white chert, banded ferruginous chert, and a variety of iron-rich cherty rocks loosely referred to as iron-formation. Ferruginous rocks are largely absent in the Onverwacht Group, but banded gray and white cherts are well developed especially in the Hooggenoeg and Kromberg Formations (Figure 2). Thin ferruginous chert and iron-formation units, primarily of an oxide facies, are reported from many mafic and ultramafic volcanic sequences (Archibald et al 1978, Barley et al 1979, Beukes 1973, Condie & Harrison 1976, Horwitz & Smith 1978, Orpen 1978, Williams 1973, and many others).
These cherty layers are generally less than 20 m thick and commonly mark individual stratigraphic horizons over a wide region. In other cases they are lenticular and some single beds, when traced laterally, split into several units separated by volcanic flows (Beukes 1973). These minor interflow sedimentary units accumulated during breaks in the eruptive sequence. Some are accompanied by tuffaceous material indicating some subaerial volcanic activity, but many lack any associated detrital or pyroclastic material suggesting quiet sedimentation in areas far removed from subaerial vents or erodible source terranes. In some instances, these units show evidence of shallow-water to evaporitic depositional environments (Barley et a1 1979, Dunlop 1978).
Banded chert and ironstone units up to several hundred meters thick occur in many greenstone belts (Goodwin 1973, Goodwin & Shklanka 1967, Orpen 1978, Lipple 1975). Most of these thicker units include sulfide and carbonate as well as oxide iron-formation facies. They tend to occur at stratigraphic breaks between contrasting volcanic units and represent major breaks in the extrusive sequence rather than simple interflow deposits.
Most of these deposits are probably chemical precipitates, although the primary sediments may have included a wide variety of ferruginous and non-ferruginous silicate and carbonate materials. They are typically finely laminated; lack scour, cross-stratification, clastic sediments, and other features indicative of current deposition; and were apparently deposited in quiet water. Some controversy exists as to the interpretation of the various iron-formation facies, however (compare Goodwin 1973 and Walker 1978), and it is not yet possible to estimate specific water depths for these deposits.
CARBONACEOUS CHERT Some interflow chert units in the Theespruit, Hooggenoeg, and Kromberg Formations of the Barberton Mountain Land and in the TaIga Taiga Subgroup in the Pilbara Block contain
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ARCHEAN SEDIMENTATION 153
abundant carbonaceous particles that have yielded microstructures interpreted by many as ancient life forms (Engel et al 1968). These cherts contain a minor amount of sericite but are otherwise relatively free of non-quartzose silicate minerals. Those in the Kromberg Formation include granular and stromatolitic carbonaceous material and are interbedded with iron-rich dolomite showing evidence of shallow-water to supratidal deposition (Lowe & Knauth 1977). In the Pilbara Block, similar cherts are associated with barite deposits thought to represent replaced gypsum deposited in shallow-water to evaporite environments (Dunlop 1978).
CARBONATE ROCK Carbonate rocks are trace components of greenstone belt sequences, but are widely distributed, particularly in post-3.0 b.y. old belts. They generally occur at major breaks within the volcanic sequence, such as the Manjeri Formation on the Rhodesian craton (Wilson et al 1978), or at the contact between the volcanic and sedimentary greenstone belt sequences, such as in the Slave Province of Canada (Henderson 1975) and parts of the Rhodesian craton (Wilson et aI1978). Most of the carbonate is calcite, dolomite, ferruginous dolomite, or ankerite. These units are particularly important because they contain the oldest known stromatolites (Bickle et al 1975, Henderson 1975, Schopf et al 1971, Wilson et al 1978) and because they have provided important geochemical information on the composition of the Archean oceans and atmosphere (Schidlowski et a11975, Veizer 1976).
Primary carbonate sediments may have been much more widely distributed than present occurrences would indicate. Many relatively pure chert layers include disseminated carbonate grains and may represent silicified carbonate or evaporite units (Dunlop 1978, Lowe & Knauth 1977). A large amount of very early diagenetic marine carbonate also occurs in carbonated tuff and volcaniclastic units (Lowe & Knauth 1979).
TERRIGENOUS CLASTIC ROCKS Terrigenous clastic sediments within the volcanic sequences show an irregular distribution. The most ancient preserved volcanic sequences, including the Onverwacht and Warrawoona Groups and the true greenstone belts of the Indian Shield (Naqvi et al 1978), virtually lack non-volcanogenic epiclastic rocks. The Onverwacht Group contains essentially no shale and the only sandstone is composed of locally derived felsic volcaniclastic material. No debris or other evidence has been found to indicate a continental, plutonic, metamorphic, or older sedimentary provfmance (Lowe & Knauth 1977). The Warrawoona Group contains only a few thin beds of epiclastic sediment (Lipple 1975) and these appear to be of volcanic derivation.
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154 LOWE
Younger, post-3.0 b.y. old greenstone belts commonly include a small but important component of non-volcanogenic clastic detritus within the volcanic sUIte. Shale, argillite, mudstone, quartzose sandstone, and conglomerate containing clasts of older plutonic or metamorphic rocks have been described as interflow sediments from the Yilgarn Block (Williams 1973), the Bulawayan Group on the Rhodesian Craton (Harrison 1970, Orpen 1978, Wilson et al 1978), and from many formations in the Canadian Shield (Donaldson. & Jackson 1965, Turner & Walker 1973).
BARITE Stratiform barite occurs in the Warrawoona Group in the Pilbara Block (Dunlop 1978) as interflow sediment associated with chert. It has been interpreted as including both primary barite sediment and diagenetic replacement after gypsum (Dunlop 1978, Barley 1978). Deposition apparently took place in shallow water. Barite in the Barberton Mountain Land is developed mainly in the Fig Tree Group and, 10caIIy, in the Onverwacht Group (Heinrichs & Reimer 1977).
GREENSTONE BELT SEDIMENTARY SEQUENCE
The upper formations of most greenstone belts consist predominantly of sedimentary rocks, commonly 1000 to 10,000 m thick. In contrast to sedimentary units in the subjacent volcanic sequences, these have been the object of numerous sedimentological studies, particularly in the Canadian Shield where their study dates back to the classic work of Pettijohn ( 1943).
These sedimentary sequences are made up largely of terrigenous detritus including shale, mudstone, siltstone, sandstone, and conglomerate. Many include a significant volcaniclastic component, and volcanic units, commonly of felsic, alkaline, or calc-alkaline type, are locally interbedded with the sedimentary rocks (Anhaeusser 1971b). The coarser detritus typically includes quartz, microcline, plagioclase, chert, and rock fragments indicating a provenance composed of uplifted parts of the subjacent volcanic and sedimentary sequences and a variety of plutonic and metamorphic rocks. The high Ni and Cr content of associated shales indicates the weathering of ultramafic source rocks (Naqvi 1976).
The sedimentary environments represented are generally like those in modern orogenic belts. Donaldson & Jackson ( 1965), Goodwin & Ridler ( 1970), Goodwin & Shklanka ( 1967), Henderson ( 1972), Ojakangas ( 1972), Pettijohn ( 1943), Turner & Walker ( 1973), Walker ( 1978), and Walker & Pettijohn ( 1971) have provided detailed discussions of the sedimentary portions of the Archean belts in the Canadian Shield. Their studies demonstrate the existence of a deep- to shallow-water clastic association
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ARCHEAN SEDIMENTATION 155
dominated by thick turbiditic submarine fan or flysch sequences juxtaposed with subaerial alluvial-fan facies (Hyde & Walker 1977, Walker 1978). There seems to be a notable scarcity of sha'llow-water clastic shelf and shoreline sequences.
The Fig Tree Group in South Africa (Figure 2)is mainly a deep-water turbiditic graywacke-mudstone assemblage in northern outcrops (Anhaeusser et a1 1969, Condie et al 1970, Reimer 1975) but, southward, includes oxide-facies banded iron-formation, volcaniclastic units, bedded barite, and shale-graywacke units (Reimer 1967, Heinrichs & Reimer 1977). This latter association reflects deposition in an area of volcanic and tectonic instability under mainly quiet subaqueous conditions but with local shallow-water deposits around tectonic highs or emergent volcanic cones. The clastic sediments include both locally derived volcaniclastic debris deposited around active vents and material derived by the erosion of larger tectonic uplifts (Condie et aI 1970). The Moodies Group conformably overlies the Fig Tree Group in northern areas (Eriksson 1977, 1978, 1979), but steps unconformably across Fig Tree rocks to the south where it rests on units as old as the lower Kromberg Formation (Eriksson 1978). Detailed sedimentological stlldies of the Moodies Group (Eriksson 1977, 1978, 1979) have shown that it is composed largely of quartzose sandstone, orthoquartzite, and shale deposited in a variety of coastal environments including deltaic plain; tidal flats" .channels, and shoals; barrier island; and shallow marine shelf.
The sedimentary sequences on other cratons are less well known than those in Canada and South Africa. The Shamvaian Group in Rhodesia �nCludes several formations, apparently separated by unconformities (Stowe 1971), which show an alternation of argillaceous units and sandstone deposited in shallow water (Stowe 1971, Wilson et al 1978). Studies of thick sedimentary :sequences at the top of the Dharwar Group in India, termed'\geosynclinal" accumulations by Naqvi ( 1976), suggest deposition in alluvial, shallow-marine, and deep-sea environments .(Naqvi 1977, Radhakrishna & Vasudev 1977). In the Pilbara Block of Western Australia, the pre-3.0 b.y. old Gorge Creek Group includes a succession of quartzite, feldspathic:sandstone, and banded iron-formation showing some evidence of shallow-water sedimentation (Hickman & Li�l'ile 1975, Lipple 1975). A younger post-3.0 b.y. Archean sedimentary group is apparently present as well, possibly including .turbidites, but little is known of its stratigraphy and sedimentology ,(Fitton et al 1975). Studies by Dunbar & McCall ( 1971) indicate the occurrence of strati:graphically complex deep-sea sequences in parts of the Yilgarn Block of Western Australia, whereas other areas include shallow-water to terrestrial deposits (Marston 1978).
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THE SEDIMENTARY EVOLUTION OF
GREENSTONE BELTS
Greenstone belt sedimentary rocks display a number of well-defined secular trends reflecting the magmatic and tectonic development of the belts. In the lower part of the volcanic sequence, the character of sedimentation is fully controlled by the style and cyclicity of volcanism. The rapid submarine eruption of low-viscosity ultramafic and mafic flows constructed massive shield volcanoes of low relief, but produced little pyroclastic or auto clastic debris that could have fed local sedimentary systems. The paucity of interflow sediment must reflect the short time intervals between flows. The few sediments that did accumulate between eruptions are represented largely by banded and ferruginous cherts probably of chemical origin. There is little evidence for the existence of subaerial exposures subject to weathering and erosion. Sedimentation probably took place in quiet, extremely low-relief subaqueous to subaerial clastic-starved environments.
The first appearance of clastic detritus generally coindded with the initiation of felsic volcanism. The silicic magmas were probably of higher viscosity and contained more volatiles than the more mafic lavas, and explosive vesiculation and auto brecciation during eruption resulted in the construction of relatively steep vent cones composed largely offragmental debris. These cones, which freque�tly became subaerial, were important sources of loose sediment and their tops, flanks, and the surrounding sea floor provided a range of sedimentary environments, including terrestrial, shallow-marine, slope, and submarine-fan, for its accumulation. The absence or extreme paucity of debris derived from mafic portions of the sequence and from non-volcanic sources indicates that, although magmatically active, most greenstone belts were essentially an orogenic during their volcanic stages.
There is no evidence that tectonic uplifts systematically accompanied the formation or volcanic development of greenstone belts. Although thin sedimentary quartzite and polymictic conglomerate layers occur within the volcanic sequences in some belts (Donaldson & Jackson 1965, Wilson 1964, Wilson et al 1978), radiometric dating of granitic and gneissic terranes around some belts indicates the existence of local areas of older sialic rocks (Wilson 1973, Wilson et al 1978), and local contacts occur where parts of the volcanic sequences rest unconformably on older sialic basement (Baragar & McGlynn 1976, Bickle et aI 1975), there is generally no evidence within the greenstone belts themselves that associated older sialic blocks influenced their formation or early evolution, or served as
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ARCHEAN SEDIMENT A nON 157
major sediment sources. These older basement terranes were evidently of low relief and tectonically inactive until the later stages of greenstone belt development.
The transition into the sedimentary sequences marks a profound change in the tectonic and magmatic histories of greenstone belts. These rocks document the waning and ultimate cessation of volcanism, both the rapid effusion of mafic flows and the dominance of felsic pyroclastic volcanism as the main source of sedimentary debris. The decline in volcanic activity is clearly accompanied by major tectonic uplift of the marginal parts of the greenstone belts and of plutons that have intruded them. The abundance of quartz, micro cline, and granitic detritus documents the first appearance of sialic basement as a major source of sedimentary debris. In greenstone belts that seem to have been associated with older basement blocks, such as those in the Canadian Shield, many of these orogenic uplifts appear to have been composed largely of older basement rock rather than subjacent portions of the greenstone belt itself.
The formation of high-standing source areas was accompanied by the development of adjacent subsiding sedimentary basins. The alternation of deep-sea flysch units and shallow-marine to terrestrial deposits within these basins indicates successive periods of rapid basin formation and filling. The common association of feldspathic and quartzitic sedimentary rocks indicates periods of tectonic uplift and oJ;ogeny followed by intervals of quiescence and maturation. No single tectonic or sedimentary trend characterizes all sedimentary sequences, but the overall direction seems to be toward the formation of substantial areas of subaerially exposed stable sialic crust. The termination of the greenstone belt cycle, which is not fully reflected in the sedimentary record, involves intrusion, deformation, and cratonization of the belts themselves.
THE TECTONIC EVOLUTION OF GREENSTONE BELTS
The development of the concept of plate tectonics has made possible an actualistic interpretation of sedimentary basins and basin deposits. Modern studies of sedimentary sequences have demonstrated that sedimentary deposits provide an important record of basin setting and the tectonic, magmatic, and metamorphic evolution of adjacent source terranes (Reading 1972, Dickinson 1974a, b).
Tectonic studies of Archean greenstone belts have focused, to a large degree, on the answers to two fundamental questions: (a) are greenstone belts primarily ensialic or ensimatic features, and (b) can the origin and evolution of greenstone belts be explained in terms of plate tectonics?
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Although sedimentary studies cannot provide uniq!le answers to these· questions, they can place significant constraints on the answers that are ultimately offered and accepted. In the following discussion, we explore some of these constraints.
Tectonic Setting of Greenstone Belts
A relatively thick sialic crust exerts a profound influence on the tectonics. of suprajacent basins and, hence, on basin sedimentation, particularly if there are local variations in its thickness or if it is involved in, rifting or compression. A relatively thin sialic crust, however, might behave little differently than a thin layer of low-density sedimentary or volcanic rock within the greenstone sequence, may have little or no effect on local tectonics, could be radiometrically destroyed by metamorphism and plutonism, and, hence, may be essentially undetectable. Greenstone belts developed on such a crust would be effectively en sima tic.
It is evident that major enclaves of sial existed at least 3.5-3.8 billion years ago. These. include the Amitsoq gneiss and Isua supracrustals in Greenland (McGregor 1973, Moorbath et al 1972), the Ancient Gneiss Complex of Swaziland (Hunter 1974), and the Rhodesian nucleus (Hawkesworth et al 1975, Moorbath et al 1977, Wilson 1973, Wilson et al 1978). Such thick masses of continental crust should have had clearly discernable effects on adjacent or suprajacent sedimentary basins. These effects seem to be visible in some greenstone belts but< absent in others.
The oldest well-preserved belts, those active generally between 3.5 and 3.1 b.y. and cratonized by 3.0 b.y., including the Barberton Mountain Land (Anhaeusser 1973), belts in the Pilbara Block (Compston & Arriens 1968), and possibly the true greenstone belts in the Dharwar terrane (Naqvi 1976, 1977, Naqvi et al 1978) contrast significantly with. younger greenstone sequences. The former tend to include major amounts of ultramafic komatiitic lavas, essentially lack calc-alkaline volcanic rocks (Hallberg 1972), and may show a more or less progressive evolution from the ultramafic through the tholeiitic to the sedimentary stages . . The sedimentary rocks in the volcanic sequence are composed either of iatrabasinal felsic volcanic debris or orthochemical sediments. There is an almost complete absence of non-volcanogenic detritus, including clay,. quartz, feldspar, and identifiable plutonic, metam'0l1phic, and sedimentary grains. T.he paucity of shale is particularly noteworthy inasmuch as many of the sedimentary units were deposited under lowenergy subaqueous conditions. During their volcanic development, there could not have been any large land areas of either l@w Oli high relief in or adjacent to these greenstone basins.
Archean, Proterozoic, and Phanerozoic basins kllG)\v,n: to have developed
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ARCHEAN SEDIMENTATION 159
on or adjacent to continental crtisFalmost inevitably contain debris derived from it, especially quartz and clay. Stable shelf and associated offshelf deep-sea deposits are characterized throughout geologic time by significant quartz and clay components, and clastic units deposited in association with tectonically active areas of continental crust include micro cline, plagioclase, and basement-derived rock fragments. If sialic crust existed locally around or beneath these pre-3.0 b.y. old greenstone belts, it must have been so thin as to be effectively absent. The sedimentary deposits indicate that the basins were effectively ensimatic and anorogenic.
The direct influence of older sialic blocks on these belts was negligible, but a somewhat different situation characterizes the pre-3.0 b.y. Sebakwian greenstone belts in Rhodesia. Although sparsely distributed and generally heavily altered, younger Sebakwian sequences contain small but significant amounts of basement-derived plutonic and metamorphic detritus within the volcanic sequence (Stowe 1971, Wilson 1973, Wilson et al 1978). The presence of quartzitic beds (Wilson 1964) and locally abundant interllow shaly units clearly indicates the exposure and weathering of older rocks, almost certainly the pre-3.5 b.y. old Rhodesian basement nucleus with which these belts are closely associated.
Post-3.0 b.y. old greenstone belts tend to show a pattern of development that also reflects the influence of older basement rocks. Those in the eastern Yilgarn Block and the Bulawayan belts of Rhodesia were generally intruded by plutonic rocks and stabilized about 2.7-2.6 b.y. ago with the main volcanism and sedimentation occurring between 2.9 and 2.6b.y. (Compston & Arriens 1968, Hawkesworth et a11975, Moorbath et aI 1977, Oversby 1975). These belts exhibit several well-developed volcanic-sedimentary cycles and include, in the volcanic sequences, minor but noticeable amounts of shale, quartzose sandstone, and, locally, conglomerate containing pebbles of granitoid and gneissic rocks (Harrison 1970) indicating presence of locally exposed older basement. The Rhodesian basement nucleus probably provided craton-derived debris to the Btilawayan belts, and a pre-3.0b.y. old Pilbara-like basement may have existed in or adjacent to the Yilgarn Block (Oversby 1975, Horwitz & Smith 1978).
Canadian greenstone belts in the Slave and Superior Provinces were stabilized during the Kenoran Orogeny about 2.6-2.5 b.y. ago (Stockwell 1968). Greenstone belt volcanism and sedimentation generally occurred in the interval between 2.9 and 2.6 b.y. ago and clearly reflect the influence of surrounding and underlying (?) sialic basement (Goodwin 1977a,b, Goodwin & Ridler 1970, Goodwin & Shklanka 1967, Walker 1978). Major vokanic-sedimentary cycles are well developed and are commonly repeated several times within individual belts. Ultramafic lavas are apparently absent, and calc-alkaline volcanics are abundant (Goodwin
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1977a, Goodwin & Ridler 1970). Thick units of shale and non-volcanogenic basement-derived sandstone and conglomerate occur, commonly at several levels throughout the volcanic sequence (Baragar & McGlynn 1976, Donaldson & Jackson 1965, Goodwin & Shklanka 1967, Salop 1977, Turner & Walker 1973). These sedimentary units document the existence of major, regionally extensive outcrops of older gneissic, granitic, and metamorphosed supracrustal sequences (Salop 1977) and provide an unambiguous model for greenstone belts developing on or adjacent to sialic crust.
The significance of the secular trend suggested by these observations is unclear. The essential termination of greenstone belt formation after the Archean suggests that some fundamental change in tectonic style occurred about 2.5 b.y. ago, perhaps related to the evolution of large areas of continental crust. If this crust developed progressively or episodically over a considerable part of the late Archean, then the probability of younger greenstone belts developing under the influence of sialic blocks would also increase with time. Such a trend may be reflected in the contrasts between older and younger preserved belts.
Although information is sparse in many areas, these comparisons suggest that there may be significant differences in greenstone belt sediments, volcanism, and general patterns of evolution and that these differences may reflect differences in the location of the greenstone belts relative to older sialic basement blocks. Greenstone belts developed on or adjacent to sialic basement contain unambiguous evidence of its presence. This evidence may include aspects of the stratigraphic and volcanic sequence, but will certainly include detritus derived from the basement by erosion.
Tectonic Evolution of Greenstone Belts
The formation and evolution of greenstone belts have been described in terms of virtually every known plate tectonic setting and many hypothetical tectonic styles. Early models generally depicted their development in non-plate-tectonic terms, commonly involving the vertical gravitational foundering of large mafic volcanic edifices constructed on an ensialic (Macgregor 195 1) or ensimatic (Glikson 1971) crust. More recent hypotheses have tended to describe greenstone belts in terms of plate-tectonic regimes, including island arcs (Anhaeusser 1973, Engel 1968, Tarney et aI1976), rifts or spreading centers (Condie & Hunter 1976), and hot spots (Fyfe 1978). Once again, although information provided by sedimentary studies cannot yet resolve these differences in interpretation, it can place important constraints on proposed models.
The an orogenic character of greenstone belt volcanic development makes it difficult to interpret greenstone belt origins and early evolution
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ARCHEAN SEDIMENTATION 161
in terms of rifting (e.g. Archibald et al 1978, Condie & Hunter 1976, Glikson & Lambert 1976). Modern rifts, whether ensimatic or en sialic, involve crustal block faulting and differential uplift, and there is little reason to suspect that substantially different conditions would have characterized Archean rifts. Phanerozoic sequences deposited within or adjacent to areas of rifting show basal detrital units composed of debris eroded from the uplifted rift blocks (Dickinson 1974b). The abundance in Archean greenstone belt volcanic sequences of rocks deposited in shallow-water environments indicates that block faulting or differential uplift would have formed subaerial exposures subject to weathering and erosion. The presence of interflow chemical deposits indicates that there were pauses in volcanism during which detrital sediments derived from such uplifts could have accumulated had any been produced. The paucity of non-volcanogenic detrital rocks within greenstone belt volcanic sequences, especially those outside of the Canadian Shield, indicates that greenstone belt formation did not involve major differential uplift and strongly suggests that these volcanic sequences did not fill fault-bounded rift depressions.
Propositions that greenstone belts developed in association with convergent plate junctions, particularly in a backarc or marginal sea setting (Tarney et al 1976), are somewhat more difficult to evaluate sedimentologically because both are dominated by volcanic and volcaniclastic rocks. Several characteristics of modern backarc and forearc basins are noteworthy when considering such models for Archean belts. Phanerozoic arc-related sequences almost always show a strong polarity, best characterized by classical eugeosyncline-miogeosyncline couples representing proximal- and distal-arc environments respectively. Although much work must be done before the facies of Archean greenstone belts are well understood, there is no evidence that their volcanic development was characterized by strong polarity. Felsic vent complexes seem to occur almost randomly, although careful paleocurrent, grain size, and isopach studies may suggest a more regular distribution. Felsic volcanic complexes also appear to be local intrabelt features rather than major zones defining greenstone belt margins. Other than felsic detritus, there is in the volcanic portions of greenstone belts an absence of volcaniclastic debris that could have been derived from associated volcanic arcs. Modern forearc and backarc deposits typically include abundant detritus derived by erosion of the arc itself. If volcanic arcs were associated with Archean greenstone belts, there is no evidence that they were reflected topographically or sedimentologically.
The overall sedimentological character of greenstone belt volcanic sequences does not coincide well with those of modern divergent or
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convergent plate junctions. Their anorogenic development is more like that of modern intraplate settings.
Although the sedimentary portions of greenstone belts. include thick sections of detrital sediments, less is known of the sedimentology and tectonic implications of these units, in some respects, than of those in the volcanic sequence. It is apparent that these detrital formations reflect orogeny, but the nature of these orogenic movements is not clear. In the Barberton Mountain Land, which shows little evidence of an associated older basement, much of the debris was derived from penecontemporaneous volcanism, syngenetic plutons, and uplifted parts of the greenstone belt itself. Some Canadian sedimentary units, deposited in basins that reflect the influence of older basement throughout their volcanic development, are made up largely of basement-derived debris and contain little volcanic or greenstone belt detritus. In some belts, it has been suggested that deposition took place in linear, fault-bounded basins (Marston 1978, Rutland 1976); whereas in others the sedimentary sequences more closely resemble those along modern continental margins (Eriksson 1978, 1979). It may be that the tectonic environments represented by these orogens are as diverse as those represented by Phanerozoic systems.
Perhaps the most popular model for the orogenic phase of greenstone belt evolution is that of a backarc basin or marginal sea (Anhaeusser 1973, Condie & Harrison 1976, Engel 1968, Tarney et al 1976). This popularity lies in large part in the apparent insufficiency of other models and in part in the fact that volcanic arcs represent the principal modern sites where large-scale volcanism is accompanied and succeeded by plutonism and orogeny. Sedimentologically, this model has yet to be evaluated although information presently available suggests that there may be problems of polarity, volcanic versus tectonic sources, and an overall paucity of arcderived volcanic detritus. A tectonic setting like that of modern volcanic arcs may have existed only during the volcanic-sedimentary transition in many greenstone belts.
SUMMARY
Although modern sedimentology and sedimentary petrography have proven to be powerful tools in the interpretation of Phanerozoic tectonics, their potential has not yet been realized in the study of Archean sequences. This review has attempted to summarize the sedimentary characteristics of Archean greenstone belts and to consider their implications regarding both the immediate environments and processes of deposition and the overall tectonic settings and development of greenstone belts. The paucity of suitable data severely hampers such an exercise at this time, but a
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ARCHEAN 'SEDIMENTATION 163 number of studies beginning or already under way should add much factual data to our knowledge of greenstone belt sedimentary rocks.
The lower volcanic portion of greenstone belts is 'characterized by two main types of sedimentary deposits : (a) chemical sediments, dominated by a variety of cherts and iron-formation, .and (h) units of felsic volcanic rocks and volcaniclastic sediments. Thiclc mafic and ultramafic flow sequences generally contain thin interflow units of chemical origin deposited in subaqueous, shallow-mar.ine environments on broad shield volcanoes of exceedingly low relief. Felsic volcanic rocks are often developed in thick fragmental associations representing vent cones. These cones were commboly subaerial and provided abundant loose pyroclastic debris, slopes over which it was moved, and a variety of environments in which it was deposited. These environments include submarine-fan, slope, shallow-shelf, and terrestrial. Very little debris reached the basins from extrabasinal or tectonic sources and, in spite of 'intense volcanic a.ctivity, greenstone belts were probably anorogenic during this stage of their development.
The sedimentary sequence or sequences in greenstone belts show depoSition of mixed volcanic and basement-derived detritus under orogenic conditions. Documented environments of aep0sition include deep-sea fan, shallow-shelf, deltaic, and alluvial, 'and generally point to the close association of topographically 'high source areas and adjacent deep basins of deposition. Uplifted sialic basement was subject �to rapid erosion, but eventual tectonic stabilization and denudati0n often led to its yielding abundant qua:rtzitic detritus.
Comparisons of preserved greenstone belts in southern Africa, Australia, India, and Canada suggest that those which lay near blocks of older sialic basement show .slightly different sedimentary, volcanic, .and stratigraphic histories than those developed in completely ensimatic settings.
The initial volcanic history of greenstone belts cannot be interpreted in terms of orogenic events. Rifting or compression involving the development of topographic relief was absent, even though sialic blocks lay adjacent to many belts. SedimentaJ:y sequences deposited during' the orogenic phase of greenstone belt development are grossly similar to1those of many modern orogens, particularly to those deposited adjacent to modern island arcs. This similarity arises 'because :island arcs .represent the only modern terranes where volcanism is succeeded by large-scale plutonism, uplift, and exposure to yield a sedimentary sequence reflecting this progression -of events. Archean greenstone belts may have evolved in settings like modern convergent plate jUh<ltions, they may have evdlved in intraplate settings such as over intra crustal fractures or mantle plumes, or they may have evolved at uniquely Archean sites of lVolcanism and plutonism.
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ACKNOWLEDGMENTS The author's research on the sedimentology of Archean cherts in South Africa and Western Australia has been supported by the National Science Foundation under Grants EAR76-06095 and EAR78-01642, respectively.
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