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Sedimentary evidence for an archaean shallow-water volcanic-sedimentary facies, Eastern Pilbara Block, Western Australia
74
Earth and Planetary Science Letters, 43 (1979) 74-84 O Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [5]
SEDIMENTARY EVIDENCE F O R AN ARCHAEAN SHALLOW-WATER VOLCANIC-SEDIMENTARY FACIES, EASTERN PILBARA BLOCK, WESTERN A U S T R A L I A M.E. BARLEY, J.S.R. DUNLOP, J.E. GLOVER and D.I. GROVES Department of Geology, University of Western Australia, Nedlands, W.A. 6009 (Australia)
Received September 13, 1978 Revised version received December 30, 1978
Sedimentological studies of the dominantly volcanic, ca. 3.5 b.y. Warrawoona Group, eastern Pilbara Block, Western Australia, indicate widespread shallow-water deposition. Many cherty metasediments within the ultramafic-mafic sequence represent silicified carbonate mud, sand, breccia and conglomerate, and show cross-lamination, ripple marks, scour-and-fill structures, and evidence of reworking. At North Pole, some cherty metasediments appear to be silicified and baritized gypsiferous evaporites, and contain microfossils. Felsic volcaniclastic rocks include pyroclastic deposits, cross-laminated tuffaceous metasediments and conglomerate. Subaerial volcanism apparently increased as deposition proceeded. The depositional basin was large, volcanically active and apparently shallow with subdued marginal relief. Felsic volcanoes formed topographic highs within the basin from which sheets of volcanically derived sediments interfingered with ultramafic-mafic volcanics. The Onverwacht Group of the Barberton Mountain Land, South Africa, is of similar age to the Warrawoona Group and probably represents a similar environment, but other greenstone belts may have formed in contrasting basins, possibly under differing tectonic regimes.
1. Introduction Recent interest in Archaean geology has caused much speculation about the tectonic setting of granitoid-greenstone terrains. Geochemical studies of Archaean metavolcanics have been extensively used to define tectonic environments [ 1 - 4 ] , although interpretations are equivocal [5]. In contrast, there have been few attempts to constrain tectonic models b y analysis of Archaean sedimentary environments and basins. In this paper, we use intercalated metasedimentary rocks to infer the depositional environment of the dominantly metavolcanic, 3 . 5 - 3 . 4 b.y. old Warrawoona Group of the eastern Pilbara Block, Western Australia. The extensive shallow-water environments envisaged constrain models for the evolution of the Warrawoona Group. We conclude that the evolutionary pattern of sedimentary environments within other greenstone belts warrants closer attention, as meta-
sediments may indicate environmental differences between greenstone belts that are not otherwise evident.
2. Sedimentology and Archaean tectonic environments Metasedimentary rocks, with or without banded iron formations, are generally found in three main associations in greenstone belts. (1) Thick sequences of epiclastic metasediments, either developed as major (several km thick) terminal associations (e.g. Barberton, Pilbara, Yilgarn) or as important intravolcanic associations representing major hiatuses in volcanism (e.g. Cheshire Formation, Rhodesia [6] and Abraham and Minnitaki Groups, Sioux Lookout, Canada [7]). (2) Volcaniclastic metasediments associated with laterally extensive felsic metavolcanic sheets or with
75
more isolated volcanic centres. These are generally poorly documented. (3) Thin, but laterally extensive, cherty metasediments intercalated with ultramafic-mafic metavolcanic sequences. Detailed sedimentological studies of this association are rare, with one notable exception [8]. However, it is this association which is most important in understanding the early tectonic environment of greenstone belts as it, together with the structures of the metavolcanic rocks, best illustrates the depositional environment of the volcanic sequences that most commonly initiate their development. In the Barberton Mountain Land and eastern Pilbara Block, two of the best-documented greenstone sequences older than 3 b.y., these sedimentary associations appear to be related broadly to lithostratigraphic position. Fig. 1 is a composite lithostratigraphic succession of the Archaean sequence of the eastern Pilbara interpreted from our mapping. A stratigraphic section drawn at A, embracing all rock types in the sequence, agrees with the generalized sections of other workers in the area [9,10], and is remarkably like the hypothetical greenstone succession of the Barberton Mountain Land [11]. Lateral
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Fig. 2. Simplified geological map of part of the eastern Pilbara Block, modified from Hickman and Lipple [ 12] and Hickman [10] showing areas and localities referred to in the text.
with intervening greenstone synclinoria. The greenstone sequence can be divided into a lower, predominantly metavolcanic Warrawoona Group, and overlying metasedimentary rocks of the Gorge Creek Group [9]. The Warrawoona Group (Fig. 1) consists of a tholeiitic and komatiitic lava sequence (Talga Talga and Salgash Subgroups) and chemically distinct felsic metavolcanics of calc-alkaline affinity (Duffer and Wyman Formations). Greenstone sequences have been metamorphosed to varying grades, but metamorphism was largely unaccompanied by high strain, so that original structures and textures are commonly well preserved. U-Pb ages for zircons and galena-Pb
ages indicate that part of the Warrawoona Group may be as old as 3.5 b.y. [ 1 3 - 1 5 ] . The emplacement of the domal granitoid batholiths and arching of the greenstone sequence is thought to post-date at least the early development of the greenstone sequence [10], so that the present exposures are not limited to shallow basin margins, and cross-sections through many parts of the basins are revealed. Sedimentological data for the Warrawoona Group presented here result from studies in the McPhee Dome and Kelly Greenstone Belt (vicinity of Copper Hills) by Barley, and the North Pole Dome by Dunlop (Fig. 2). Emphasis has been placed throughout on
77 sections containing the best-preserved sedimentary sequences. The cherty metasediments of North Pole and the McPhee Dome-Kelly Greenstone Belt have many common features which are described below, with particular reference to North Pole. Major differences are the importance of evaporitic features at North Pole and the greater terrigenous component in the other areas. In all areas, thick (>5 m) cherty metasediments directly overlie ultramafic and mafic meta-volcanic rocks. In the McPhee Dome and Kelly Greenstone Belt similar cherty metasediments overlie felsic volcaniclastic metasediments, but at North Pole felsic metavolcanics are absent (Fig. 1).
4. Cherty metasediments in ultramafic-mafic metavolcanic sequences
Stratiform cherty metasediments are extensive in many greenstone terrains, and have received special attention in this study because their well-preserved depositional structures and textures are invaluable palaeoenvironmental indicators. Before discussing the Warrawoona rocks, a resum6 of the possible origins of chert is warranted. Chert may form by primary, inorganic precipitation, by local redistribution of silica perhaps initially precipitated by organisms, by replacement of preexisting rock (commonly limestone) with silica from an external source, or by leaching of sodium and subsequent dehydration of sodium silicate gel magadiites). Early chertification of ancient limestone assists in interpreting the environment of deposition because it protects primary features from destruction by diagenesis and metamorphism. However, chertification of magadiite is accompanied by a ca. 25% decrease in volume which may superimpose features such as desiccation polygons on primary structures [16]. The cherty metasediments throughout the area consist of black carbonaceous and pyritic units, banded grey and white units and minor graphitic and pyritic shales. The black cherty metasediments consist of massive and finely laminated (0.5-1 cm) units with disseminated carbon and pyrite outlining a palimpsest clotted or granular fabric (0.05-5 mm) within a free-grained (>0.05 mm) quartz cement. At
North Pole they are fossiliferous [17]. They are similar to units described by Lowe and Knauth [8] from the Barberton Mountain Land. Some grey and white cherty metasediments consist of planelaminated units of aphanitic fine-grained ('>0.05 ram) quartz and are of equivocal origin, although interbedded sediments commonly have structures and textures indicative of shallow-water deposition. Beds with fine-grained precursors generally show a fine (ca. 0.5 mm) planar lamination. Parallel or cross-laminations (Fig. 3) are distinguished by a slight colour and grain-size difference, with the base of individual laminae being slightly coarser grained than the top. Cross-laminae have low angle sets (ca. 10°) with low (1:20) amplitude to wavelength ratios. Ripple marks developed along some bedding planes in the cherty metasediments are slightly asymmetrical with low amplitude to wavelength (<1 : 10) ratios. Some flame structures are developed. Finely laminated cherty metasediments grade both laterally and vertically into silicified coarse-grained arenitic metasediments with graded bedding, crossbedding and scour-and-fill structures. Lenses of arenitic metasediment (Fig. 3) appear in the finely laminated cherty metasediment, and finely laminated clasts are common within the arenite. Clasts were derived from partly or totally lithifed mud. Thin beds of ooid-like grainstone are locally developed at North Pole. Silicified vesiculated clasts and rare discrete quartz grains, some of which contain dendritic ruffle, are also present. Small quantities of detrital zircon, pyrite and spinel are common. There are also small lenticular (ca. 50 cm thick) edgewise conglomerate beds containing thin imbricate tabular clasts of fnely laminated chert (Fig. 4). Fabrics in this sediment type range from clast support to matrix support, and imbrication ranges from subperpendicular to parallel to bedding. Clasts commonly show a micritoidal, possibly oncolitic, overgrowth and at North Pole some contain microfossils [17]. This type of cherty metasediment closely resembles intraclast breccia described by Logan [ 18] from high-energy supratidal deposits. Infraformational pebble and cobble conglomerate horizons are developed in some cherty metasedimentary units and consist of clasts texturally identical to the underlying rock, indicating shallow-water reworking of previously lithified sediments. At North Pole,
78
Fig. 3. Cherty metasediment from North Pole area showing fine cross laminae in fine-grained sediments (adjacent to I), scour-andfill structure (2), coarse-grained,crossqaminated lenses (3) and rosettes of silica pseudomorphs after sulphates (4). Length of field of view of 25 cm.
polymictic conglomerates containing clasts of texturally different silicified sediment types and sulphate clasts are common, and indicate high-energy erosion of surrounding units. It must be emphasized here that not all cherts show evidence of shallow-water deposition. The extensive and distinctive red, white and black banded Marble Bar Chert [9], for example, shows no unequivocal structures or textures indicating either shallow or deep water origin, and other planelaminated cherts are equally problematical. In addition to the widely distributed features described above, some important lithologies, textures and structures are restricted to the lowest unit in the sequence of cherty metasediments around the North Pole Dome (Fig. 2). This unit consists of bedded barite and fossiliferous chert, with minor volcaniclastic components [ 10,17,19]. It ranges in thickness between 10 and 15 m, and extends about 25 km around the south and east of the Dome. In several localities, small (ca. 5 mm) silica crystallites, whose interfacial angles measured by universal stage are typical of gypsum, are contained within finely laminated cherty metasediments. These silica
pseudomorphs after gypsum have a random relationship to the laminae, and some contain ghosts of laminae indicating diagenetic origin. Structures within bedded barite closely resemble the "cavoli" and "grass-like" structures, interpreted as having grown in shallow, standing water by Richter-Bernburg [20] from Messinian selenite deposits of the Sicilian Basin, and the "cauliflower structures" within barite from the Barberton Mountain Land [21]. North Pole barite typically occurs as radiating crystal groups sub-perpendicular to the bedding and radiating upwards. Tops of the crystal clusters show evidence of erosion and fine silicified metasediments are draped over and fill interstices between barite crystals (Fig. 5). Universal-stage measurements of interfacial angles show that at least some of the barite pseudomorphs gypsum. Sulphur isotope investigations of the deposits [22,23] are consistent with evaporative sulphate precipitated from waters recharged from a large reservoir of isotopically homogeneous sulphate ions. To summarize, a significant number of cherty metasediments studied within a large, well-exposed area of the Warrawoona Group represent silicified
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Fig. 4. Edgewise conglomerate from North Pole. Finely laminated, silicified fossiliferous clasts, some with possible oncolitic overgrowth, are present in a fine-grained, siliceous matrix. Width of field of view is 2 cm.
mud, sand, breccia and conglomerate with features including fine laminated bedding, graded bedding, cross-bedding, scour-and-fill structures, evidence of reworking, imbricate structures and ripple marks. Silica and barite pseudomorphs after gypsum, which accord with evaporitic conditions, are restricted to the North Pole area. Some individual structures are equivocal but, together, they indicate deposition above wave base in a variety o f shallow-water to subaerial environments. The initial composition of the sediments seems to have been carbonate. Evidence includes the presence of rhombic voids and minor carbonate in chert from a number of localities, and the occurrence at North
Fig. 5. Barite pseudomorphs after gypsum overlain by finely laminated cherty metasediment, North Pole. Crystal tops show evidence of erosion while the cherty metasediment fills crystal interstices and drapes over crystals. Width of field of view is 10 cm. Pole of ooid-like grains, possible oncoliths, and apparent gypsum palimpsests resembling the gypsum of modern carbonate rocks.
5. Volcaniclastic metasediments in felsic metavolcanic sequences The Archaean felsic metavolcanic sequences in the eastern Pilbara are characterized by metamorphosed pyroclastics and volcanic detritus, with minor metamorphosed flows and associated subvolcanic intrusives. The Duffer Formation (Fig. 1) represents the earliest widespread phase of pyroclastic volcanism in the Warrawoona Group, perhaps marking the establishment o f subaerial vents. The metavolcanics in the Duffer Formation are dominantly andestitic or dacitic, whereas those o f the younger Wyman Forma-
80 tion are dominantly rhyolitic. Many felsic metavolcanic units are laterally continuous for more than 20 km and, south of Copper Hills, where the Wyman Formation directly overlies older felsic metavolcanics, their combined thickness reaches about 5 km (Fig. 2). Thick (>2 m) tuff breccia units within the Duffer Formation are composed of poorly sorted angular to subangular lithic and tuffaceous fragments up to 20 cm in diameter in a lapilli- and ash-sized matrix (Fig. 6). The matrix is commonly recrystallized and evidence for the former existence of glass shards and pumice is rare. Poor sorting, absence of bedding and presence of large angular blocks suggest that these units were deposited by mass flowage. A variety of finer-grained units associated with the tuff breccias suggests deposition in a number of environments. Sections examined contain minor interbedded lapilli- and ash-tuffs exhibiting variable degrees of sorting. In some sections normal to reverse graded bedding is developed in plane laminated units, and is associated in some places with minor smallscale oblique laminations and slump structures. This association of sedimentary structures suggests deposition from turbidity currents. In these sections, it is most likely that coarse pyroclastic units are the
result of subaqueous pyroclastic flows, using the term in an intbrmal sense [24]. Individual units may have either been the direct result of volcanic explosions or subsequent mud flows. Well-developed turbidite sequences of doubly graded sequences of tuff beds [25] indicative of deposition or eruption in deep water are not developed in the McPhee Dome and Kelly Greenstone Belt. In other sections, particularly in tlae Kelly Greenstone Belt to the north of Copper Hills (Fig. 2), coarse pyroclastic units are interbedded with tuffaceous metasediments. Mediumgrained metasediments display both planar lamination and abundant, low angle ( 8 - 1 0 ° ) tabular and trough-shaped crosslaminations (Fig. 7). Co-sets range in thickness from 5 to 20 cm. Scour and channel-fill structures are common, as are thin (<5 cm) layers of fine-grained tuffaceous metasediment and horizons of mud flakes. Reworking of previously lithified mud layers is observed in coarser clastic units. Metaconglomerate containing well-rounded pebbles and boulders eroded from coarse pyroclastic units is commonly interbedded with other tuffaceous metasediments (Fig. 8). Bedding in most of the area studied is remarkably uniform and coarse pyroclastic units form laterally
Fig. 6. Poorly sorted, felsic fragmental unit (lahar) containing coarse clasts, Duffer Formation, Kelly Greenstone Belt.
81 indicate reworking of pre-existing pyroclastic deposits in shallow-water high-energy environments. In the Wyman Formation, near Copper Hills, a metamorphosed vitric welded tuff showing excellent textural preservation of welded shards and pumice fragments is exposed. Reworked volcaniclastic metasediments are common, and metamorphosed conglomerate containing boulders of black subvolcanic porphyry common at Copper Hills also occurs. Epiclastic metasediments, containing material derived from underlying volcanics, represent a common lithology in the overlying Gorge Creek Group (Fig.
1). 6. Deposition of the Warrawoona Group Fig. 7. Trough-shaped cross-laminations in medium-grained subaqueous tuffaceous metasediment, Duffer Formation, Kelly Greenstone Belt. extensive sheets, suggesting a more subdued topography than typical of most felsic volcanic terrains. Graded turbidity deposists suggest subaqueous deposition, and a variety of tuffaceous metasediments
Fig. 8. Conglomerate horizon with well-rounded, locally derived felsic clasts overlying medium-grained subaqueous tuffaceous metasediment, Duffer Formation, Kelly Greenstone Belt. Scale is in centimetres.
Comparison of the metasediments of the Warrawoona Group with present-day, deep-water marine sediments is difficult: what were the equivalents of the organic oozes which, with siliceous clays, characterize the abyssal plains [26], and are distributed asymmetrically with terrigenous turbidites and volcaniclastic sediments in trenches [27] and marginal basins [28] ? Cherty rocks, whose origins are not always clear, are found in many Mesozoic and Tertiary deep-sea sequences, but do not show the textures and structures developed in the cherty metasediments of the Warrawoona Group. There is no convincing evidence for deep-sea deposition in the Warrawoona Group. The strongest evidence for the environment of deposition is the occurrence of several features which point to shallow-water or, more rarely, evaporitic origin. Evidence for shallowwater deposition occurs throughout the sequence. Cherty metasediments and metamorphosed felsic volcaniclastics, apparently of shallow-water to subaerial origin, are continuous for tens of kilometres in well-documented localities. The lateral extent of thin (<20 m) shallow-water metasedimentary horizons, conformable with both underlying and overlying metavolcanics, indicates deposition in an area of gentle topography and minimal orogenic uplift. The landscape during deposition of the Warrawoona Group was probably fairly flat, with the vents of felsic volcanoes forming local topographic highs within the basin. This type of volcanic landscape could have been produced by a series of ultramafic-
82 mafic fissure eruptions, or by a gently sloping shield volcano with lavas interfingering with the debris and pyroclastic products of a more elevated felsic volcano or volcanoes. A significant amount of the cherty metasediments within ultramafic-mafic metavolcanic sequences was initially deposited in extensive shallow basins as carbonate and evaporite, with relatively little detritus from surrounding volcanic areas. Sediments around penecontemporaneous felsic volcanic centres were locally derived. They were deposited in shallow water, forming extensive sheets over surrounding areas of subdued relief and interfingering with the ultramafic-mafic metavolcanics and cherty metasediments.
7. Significance to Archaean tectonic interpretations Sediments within the Warrawoona Group suggest a model for initial greenstone belt development involving deposition in a shallow volcanically active basin surrounded by a subdued topography. Sources of coarse detritus were restricted to emergent felsic volcanoes within the basin. This sedimentary environment may be similar to that envisaged for some large Proterozoic basins by Trendall [29], and contrasts markedly with that of modern oceans and marginal basins. Sedimentological studies of the Onverwacht Group [8], of similar age to the Warrawoona Group, also reveal a shallow-water origin for a significant proportion of the sediments within the mainly volcanic pile, and it is notable that this area contains significant stratiform barite [21], similar to that of North Pole. Shallow-water environments are also represented within volcanic sequences in younger greenstone belts [30,31 ], but available data indicate that they are not dominant as in the Warrawoona and Onverwacht Groups, and that bedded sulphates are absent. Plane-laminated iron formations or sulphidic shales, showing none of the structural and textural characteristics of the Warrawoona cherts, appear to be the dominant interflow sediment type. Most previous Archaean sedimentological studies have been carried out upon dominantly clastic metasedimentary sequences, interpreted to have been deposited in deep unagitated "geosynclinal" basins in which marginal fluvial, alluvial fan or submarine fan deposits were commonly associated with turbidites
[7, 32-34]. Such sediments become a major part of the eastern Pilbara and Barberton Mountain Land greenstone sequences only during the latter part of their development [8,12,35]. The rapid transition from volcanism to clastic sedimentation in these sequences, combined with the contrast in sediment type between sequences, probably represents a major change in tectonic regime, with shallow stable basins giving way to deeper, rapidly sinking basins. The occurrence of detritus from granitoids, in addition to detritus from underlying metavolcanics in the clastic metasedimentary sequences [35,36], suggests that these sediments were, in part, eroded from granitoids, possibly after early diapiric uprise [37]. However, further detailed studies of the Gorge Creek Group are required to test this model. There are, as yet, insufficient data on the metasedimentary rocks within the predominantly metavolcanic sequences of most Archaean greenstone belts to describe, or even compare precisely, the nature of early depositional basins or trends in basinal evolution. Further sedimentological data are also required to resolve the nature of the transitions from dominant eruption to dominant sedimentation common to many belts. Such sedimentological data will be more useful than geochemical comparisons between volcanic rocks and modern analogues. Meanwhile, our data, and those from Onverwacht Group, indicate that significant proportions of these ancient greenstone sequences formed in extensive shallow basins. The tectonic mechanism for their formation remains unclear.
Acknowledgements This study was supported by the University of Western Australia, and Commonwealth Postgraduate Scholarship (J.S.R.D) and University Postgraduate Studentship (M.E.B.) awards. Field support was given by I.D. Martin and Alcoa of Australia (W.A.) Ltd., and Dresser Minerals International. The work was discussed with B. Bolton and A.H. Hickman. M.J. Bickle and R. Buick criticized the manuscript.
References 1 A.J.R. White, P. Jakes and D.M. Christie, Composition of greenstones and the hypothesis of sea-floor spreading in
83 the Archaean, Geol. Soc. Aust. Spec. Publ. 3 (1971) 47. 2 B.-M. Jahn, C.Y. Shih and V.R. Murthy, Trace element geochemistry of Archean volcanic rocks, Geochim. Cosmochim. Acta 38 (1974) 611. 3 J. Tarney, I.W.D. Dalziel and M.J. de Wit, Marginal basin "Rocas Verdes" complex from S. Chile: a model for Archaean greenstone belt formation, in: The Early History of the Earth, B.F. Windley, ed. (Wiley, London, 1976) 131. 4 K.C. Condie and N.M. ttarrison, Geochemistry of the Archean Bulawayan Group, Midlands Greenstone Belt, Rhodesia, Precamb. Res. 3 (1976) 253. 5 R.K. O'Nions and R.J. Pankhurst, Early Archaean rocks and geochemical evolution of the earth's crust, Earth Planet. Sci. Lett. 38 (1978) 211. 6 J.F. Wilson, M.J. Bickle, C.J. Hawkesworth, A. Martin, E.G. Nisbet and J.L. Orpen, Granite-greenstone terrains of the Rhodesian Archaean craton, Nature 271 (1978) 23. 7 C.C. Turner and R.G. Walker, Sedimentology, stratigraphy, and crustal evolution of the Archean greenstone belt near Sioux Lookout, Ontario, Can. J. Earth Sci. 10 (1973) 817. 8 D.R. Lowe and L.P. Knauth, Sedimentology of the Onverwacht Group (3.4 billion years) Transvaal, South Africa, and its bearing on the characteristics and evolution of the early Earth, J. Geol. 85 (1977) 699. 9 S.L. Lipple, Definitions of new and revised stratigraphic units of the eastern Pilbara region, Geol. Surv. West. Aust. Annu. Rep. 1974 (1975) 58. 10 A.H. Hickman, Precambrian structural geology of part of the Pilbara region, Geol. Surv. West. Aust. Annu. Rep. 1974 (1975) 68. 11 C.R. Anhaeusser, Cyclic volcanicity and sedimentation in the evolutionary development of Archaean greenstone belts of shield areas, Geol. Soc. Aust. Spec. Publ. 3 (1971) 57. 12 A.H. Hickman and S.L. Lipple, Explanatory notes on the Marble Bar 1:250,000 Geological Sheet, Western Australia, West. Aust. Geol. Surv. Rec. 1974/20 (1975). 13 R.T. Pidgeon, 3450-m.y.-old volcanics in the Archaean layered greenstone succession of the Pilbara Block, Western Australia, Earth Planet. Sci. Lett. 37 (1978) 421. 14 D.F. Sangster and W.A. Brook, Primitive lead in an Australian Zn-Pb-Ba deposit, Nature 270 (1977) 423. 15 J.R. Richards, Lead isotopes and ages of galenas from the Pilbara region, Western Australia, J. Geol. Soc. Aust. 24 (1977) 465. 16 H.P. Eugster, Inorganic bedded cherts from the Magadi area, Kenya, Contrib. Mineral. Petrol. 22 (1969) 1. 17 J.S.R. Dunlop, M.D. Muir, V.A. Milne and D.I. Groves. A new microfossil assemblage from the Archaean of Western Australia, Nature 274 (1978) 676. 18 B.W. Logan, Inventory of diagenesis in Holocene-Recent carbonate sediments, Shark Bay, Western Australia, in: Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia, B.W. Logan et
19
20
21
22
23
24 25
26
27
28
29 30
31
32
33
34
al., eds (American Association of Petroleum Geologists, Tulsa, Okla., 1974) 195. J.S.R. Dunlop, Shallow-water sedimentation at North P01e, Pilbara, Western Australia, Publ. Geol. Dep. Extension Serv., Univ. West. Aust. 2 (1978) 30. G. Richter-Bernburg, Facies and paleogeography of the Messinian evaporites in Sicily, in: Messinian Events in the Mediterranean, W. Drouger, ed. (North-Holland, Amsterdam, 1973) 141. T.K. Heinrichs and T.O. Reimer, A sedimentary barite deposit from the Archean Fig Tree Group of the Barberton Mountain Land (South Africa), Econ. Geol. 72 (1977) 1426, I.B. Lambert, Sulphur-isotope investigations of Archaean mineralization and some implications concerning geobiochemical evolution. Publ. Geol. Dep. Extension Serv., Univ. West. Aust. 2 (1978) 45. I.B. Lambert, T.H. Donnelly, J.S.R. Dunlop, and D.I. Groves, Stable isotope studies of early Archaean evaporitic sulphates at North Pole, Western Australia and possible equivalents at Barberton, South Africa, Nature (in press). R.S. Fiske, Subaqueous pyroclastic flows in the Okanapecosh Formation, Geol. Soc. Am. Bull. 74 (1963) 391. R.S. Fiske and T. Matsuda, Submarine equivalents of ash flows in the Takiwa Formation, Japan, Am. J. Sci. 262 (1964) 76. P.J. Cook, J.J. Veevers, J.R. Hiertzler and P.J. Cameron, The sediments of the Argo Abyssal Plain and adjacent areas, northeast Indian Ocean, Aust. Bur. Miner. Resour. J. Geol. Geophys. 3 (1978) 113. W.R. Dickinson, Reconstruction of past arc-trench systems from petrotectonic assemblages in the island arcs of the Western Pacific, in: The Western Pacific, Islands Arcs, Marginal Seas, Geochemistry, P,J. Coleman, ed. (University of Western Australia Press, Nedlands, W.A. 1973) 569. D.E. Karig and G.F. Moore, Tectonically controlled sedimentation in marginal basins, Earth Planet. Sci. Lett. 26 (1975) 233. A.F. Trendall, Revolution in Earth history, J. Geol. Soc. Aust. 19 (1972) 287. M.J. Bickle, A. Martin and E.G. Nisbet, Basaltic and peridotitic komatiites and stromatolites above a basal unconformity in the Belingwe greenstone belt, Rhodesia, Earth Planet. Sci. Lett. 27 (1975) 155. J.B. Henderson, Archean stromatolites in the Northern Slave Province, Northwest Territories, Canada, Can. J. Earth Sci. 12 (1975) 1619. R.G. Walker and F.J. Pettijohn, Archean sedimentation: analysis of the Minnitake Basin, norhtwestern Ontario, Canada, Geol. Soc. Am. Bull. 82 (1971) 2099. J.B. Henderson, Sedimentology of Archean turbidites at Yellowknife, Northwest Territories, Can. J. Earth Sci. 9 (1972) 882. R.S. Hyde, Archean sedimentary environments: implications regarding paleotectonic settings of greenstone belts
84 and the development of continents, Geol. Soc. Am. Annu. Meet., Abstr. Progr. (1976) 933. 35 K.A. Eriksson, Alluvial and destructive beach facies in the Archaean Moodies Group of the Barberton Mountain Land, Econ. Geol. Res. Unit Inf. Circ. 115 (1978). 36 M.J. Fitton, R.C. Horwitz and G. Sylvester, Stratigraphy
of the Early Precambrian in the West Pilbara, Western Australia, C.S.I.R.O. Miner. Res. Lab. Rep. FP 11 (1975), 24 pp. 37 B.E. Gorman, T.H. Pearce and T.C. Birkett, On the structure of Archean greenstone belts, Precamb. Res. 6 (1978) 23.
Earth and Planetary Science Letters, 43 (1979) 74-84 O Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [5]
SEDIMENTARY EVIDENCE F O R AN ARCHAEAN SHALLOW-WATER VOLCANIC-SEDIMENTARY FACIES, EASTERN PILBARA BLOCK, WESTERN A U S T R A L I A M.E. BARLEY, J.S.R. DUNLOP, J.E. GLOVER and D.I. GROVES Department of Geology, University of Western Australia, Nedlands, W.A. 6009 (Australia)
Received September 13, 1978 Revised version received December 30, 1978
Sedimentological studies of the dominantly volcanic, ca. 3.5 b.y. Warrawoona Group, eastern Pilbara Block, Western Australia, indicate widespread shallow-water deposition. Many cherty metasediments within the ultramafic-mafic sequence represent silicified carbonate mud, sand, breccia and conglomerate, and show cross-lamination, ripple marks, scour-and-fill structures, and evidence of reworking. At North Pole, some cherty metasediments appear to be silicified and baritized gypsiferous evaporites, and contain microfossils. Felsic volcaniclastic rocks include pyroclastic deposits, cross-laminated tuffaceous metasediments and conglomerate. Subaerial volcanism apparently increased as deposition proceeded. The depositional basin was large, volcanically active and apparently shallow with subdued marginal relief. Felsic volcanoes formed topographic highs within the basin from which sheets of volcanically derived sediments interfingered with ultramafic-mafic volcanics. The Onverwacht Group of the Barberton Mountain Land, South Africa, is of similar age to the Warrawoona Group and probably represents a similar environment, but other greenstone belts may have formed in contrasting basins, possibly under differing tectonic regimes.
1. Introduction Recent interest in Archaean geology has caused much speculation about the tectonic setting of granitoid-greenstone terrains. Geochemical studies of Archaean metavolcanics have been extensively used to define tectonic environments [ 1 - 4 ] , although interpretations are equivocal [5]. In contrast, there have been few attempts to constrain tectonic models b y analysis of Archaean sedimentary environments and basins. In this paper, we use intercalated metasedimentary rocks to infer the depositional environment of the dominantly metavolcanic, 3 . 5 - 3 . 4 b.y. old Warrawoona Group of the eastern Pilbara Block, Western Australia. The extensive shallow-water environments envisaged constrain models for the evolution of the Warrawoona Group. We conclude that the evolutionary pattern of sedimentary environments within other greenstone belts warrants closer attention, as meta-
sediments may indicate environmental differences between greenstone belts that are not otherwise evident.
2. Sedimentology and Archaean tectonic environments Metasedimentary rocks, with or without banded iron formations, are generally found in three main associations in greenstone belts. (1) Thick sequences of epiclastic metasediments, either developed as major (several km thick) terminal associations (e.g. Barberton, Pilbara, Yilgarn) or as important intravolcanic associations representing major hiatuses in volcanism (e.g. Cheshire Formation, Rhodesia [6] and Abraham and Minnitaki Groups, Sioux Lookout, Canada [7]). (2) Volcaniclastic metasediments associated with laterally extensive felsic metavolcanic sheets or with
75
more isolated volcanic centres. These are generally poorly documented. (3) Thin, but laterally extensive, cherty metasediments intercalated with ultramafic-mafic metavolcanic sequences. Detailed sedimentological studies of this association are rare, with one notable exception [8]. However, it is this association which is most important in understanding the early tectonic environment of greenstone belts as it, together with the structures of the metavolcanic rocks, best illustrates the depositional environment of the volcanic sequences that most commonly initiate their development. In the Barberton Mountain Land and eastern Pilbara Block, two of the best-documented greenstone sequences older than 3 b.y., these sedimentary associations appear to be related broadly to lithostratigraphic position. Fig. 1 is a composite lithostratigraphic succession of the Archaean sequence of the eastern Pilbara interpreted from our mapping. A stratigraphic section drawn at A, embracing all rock types in the sequence, agrees with the generalized sections of other workers in the area [9,10], and is remarkably like the hypothetical greenstone succession of the Barberton Mountain Land [11]. Lateral
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with intervening greenstone synclinoria. The greenstone sequence can be divided into a lower, predominantly metavolcanic Warrawoona Group, and overlying metasedimentary rocks of the Gorge Creek Group [9]. The Warrawoona Group (Fig. 1) consists of a tholeiitic and komatiitic lava sequence (Talga Talga and Salgash Subgroups) and chemically distinct felsic metavolcanics of calc-alkaline affinity (Duffer and Wyman Formations). Greenstone sequences have been metamorphosed to varying grades, but metamorphism was largely unaccompanied by high strain, so that original structures and textures are commonly well preserved. U-Pb ages for zircons and galena-Pb
ages indicate that part of the Warrawoona Group may be as old as 3.5 b.y. [ 1 3 - 1 5 ] . The emplacement of the domal granitoid batholiths and arching of the greenstone sequence is thought to post-date at least the early development of the greenstone sequence [10], so that the present exposures are not limited to shallow basin margins, and cross-sections through many parts of the basins are revealed. Sedimentological data for the Warrawoona Group presented here result from studies in the McPhee Dome and Kelly Greenstone Belt (vicinity of Copper Hills) by Barley, and the North Pole Dome by Dunlop (Fig. 2). Emphasis has been placed throughout on
77 sections containing the best-preserved sedimentary sequences. The cherty metasediments of North Pole and the McPhee Dome-Kelly Greenstone Belt have many common features which are described below, with particular reference to North Pole. Major differences are the importance of evaporitic features at North Pole and the greater terrigenous component in the other areas. In all areas, thick (>5 m) cherty metasediments directly overlie ultramafic and mafic meta-volcanic rocks. In the McPhee Dome and Kelly Greenstone Belt similar cherty metasediments overlie felsic volcaniclastic metasediments, but at North Pole felsic metavolcanics are absent (Fig. 1).
4. Cherty metasediments in ultramafic-mafic metavolcanic sequences
Stratiform cherty metasediments are extensive in many greenstone terrains, and have received special attention in this study because their well-preserved depositional structures and textures are invaluable palaeoenvironmental indicators. Before discussing the Warrawoona rocks, a resum6 of the possible origins of chert is warranted. Chert may form by primary, inorganic precipitation, by local redistribution of silica perhaps initially precipitated by organisms, by replacement of preexisting rock (commonly limestone) with silica from an external source, or by leaching of sodium and subsequent dehydration of sodium silicate gel magadiites). Early chertification of ancient limestone assists in interpreting the environment of deposition because it protects primary features from destruction by diagenesis and metamorphism. However, chertification of magadiite is accompanied by a ca. 25% decrease in volume which may superimpose features such as desiccation polygons on primary structures [16]. The cherty metasediments throughout the area consist of black carbonaceous and pyritic units, banded grey and white units and minor graphitic and pyritic shales. The black cherty metasediments consist of massive and finely laminated (0.5-1 cm) units with disseminated carbon and pyrite outlining a palimpsest clotted or granular fabric (0.05-5 mm) within a free-grained (>0.05 mm) quartz cement. At
North Pole they are fossiliferous [17]. They are similar to units described by Lowe and Knauth [8] from the Barberton Mountain Land. Some grey and white cherty metasediments consist of planelaminated units of aphanitic fine-grained ('>0.05 ram) quartz and are of equivocal origin, although interbedded sediments commonly have structures and textures indicative of shallow-water deposition. Beds with fine-grained precursors generally show a fine (ca. 0.5 mm) planar lamination. Parallel or cross-laminations (Fig. 3) are distinguished by a slight colour and grain-size difference, with the base of individual laminae being slightly coarser grained than the top. Cross-laminae have low angle sets (ca. 10°) with low (1:20) amplitude to wavelength ratios. Ripple marks developed along some bedding planes in the cherty metasediments are slightly asymmetrical with low amplitude to wavelength (<1 : 10) ratios. Some flame structures are developed. Finely laminated cherty metasediments grade both laterally and vertically into silicified coarse-grained arenitic metasediments with graded bedding, crossbedding and scour-and-fill structures. Lenses of arenitic metasediment (Fig. 3) appear in the finely laminated cherty metasediment, and finely laminated clasts are common within the arenite. Clasts were derived from partly or totally lithifed mud. Thin beds of ooid-like grainstone are locally developed at North Pole. Silicified vesiculated clasts and rare discrete quartz grains, some of which contain dendritic ruffle, are also present. Small quantities of detrital zircon, pyrite and spinel are common. There are also small lenticular (ca. 50 cm thick) edgewise conglomerate beds containing thin imbricate tabular clasts of fnely laminated chert (Fig. 4). Fabrics in this sediment type range from clast support to matrix support, and imbrication ranges from subperpendicular to parallel to bedding. Clasts commonly show a micritoidal, possibly oncolitic, overgrowth and at North Pole some contain microfossils [17]. This type of cherty metasediment closely resembles intraclast breccia described by Logan [ 18] from high-energy supratidal deposits. Infraformational pebble and cobble conglomerate horizons are developed in some cherty metasedimentary units and consist of clasts texturally identical to the underlying rock, indicating shallow-water reworking of previously lithified sediments. At North Pole,
78
Fig. 3. Cherty metasediment from North Pole area showing fine cross laminae in fine-grained sediments (adjacent to I), scour-andfill structure (2), coarse-grained,crossqaminated lenses (3) and rosettes of silica pseudomorphs after sulphates (4). Length of field of view of 25 cm.
polymictic conglomerates containing clasts of texturally different silicified sediment types and sulphate clasts are common, and indicate high-energy erosion of surrounding units. It must be emphasized here that not all cherts show evidence of shallow-water deposition. The extensive and distinctive red, white and black banded Marble Bar Chert [9], for example, shows no unequivocal structures or textures indicating either shallow or deep water origin, and other planelaminated cherts are equally problematical. In addition to the widely distributed features described above, some important lithologies, textures and structures are restricted to the lowest unit in the sequence of cherty metasediments around the North Pole Dome (Fig. 2). This unit consists of bedded barite and fossiliferous chert, with minor volcaniclastic components [ 10,17,19]. It ranges in thickness between 10 and 15 m, and extends about 25 km around the south and east of the Dome. In several localities, small (ca. 5 mm) silica crystallites, whose interfacial angles measured by universal stage are typical of gypsum, are contained within finely laminated cherty metasediments. These silica
pseudomorphs after gypsum have a random relationship to the laminae, and some contain ghosts of laminae indicating diagenetic origin. Structures within bedded barite closely resemble the "cavoli" and "grass-like" structures, interpreted as having grown in shallow, standing water by Richter-Bernburg [20] from Messinian selenite deposits of the Sicilian Basin, and the "cauliflower structures" within barite from the Barberton Mountain Land [21]. North Pole barite typically occurs as radiating crystal groups sub-perpendicular to the bedding and radiating upwards. Tops of the crystal clusters show evidence of erosion and fine silicified metasediments are draped over and fill interstices between barite crystals (Fig. 5). Universal-stage measurements of interfacial angles show that at least some of the barite pseudomorphs gypsum. Sulphur isotope investigations of the deposits [22,23] are consistent with evaporative sulphate precipitated from waters recharged from a large reservoir of isotopically homogeneous sulphate ions. To summarize, a significant number of cherty metasediments studied within a large, well-exposed area of the Warrawoona Group represent silicified
79
Fig. 4. Edgewise conglomerate from North Pole. Finely laminated, silicified fossiliferous clasts, some with possible oncolitic overgrowth, are present in a fine-grained, siliceous matrix. Width of field of view is 2 cm.
mud, sand, breccia and conglomerate with features including fine laminated bedding, graded bedding, cross-bedding, scour-and-fill structures, evidence of reworking, imbricate structures and ripple marks. Silica and barite pseudomorphs after gypsum, which accord with evaporitic conditions, are restricted to the North Pole area. Some individual structures are equivocal but, together, they indicate deposition above wave base in a variety o f shallow-water to subaerial environments. The initial composition of the sediments seems to have been carbonate. Evidence includes the presence of rhombic voids and minor carbonate in chert from a number of localities, and the occurrence at North
Fig. 5. Barite pseudomorphs after gypsum overlain by finely laminated cherty metasediment, North Pole. Crystal tops show evidence of erosion while the cherty metasediment fills crystal interstices and drapes over crystals. Width of field of view is 10 cm. Pole of ooid-like grains, possible oncoliths, and apparent gypsum palimpsests resembling the gypsum of modern carbonate rocks.
5. Volcaniclastic metasediments in felsic metavolcanic sequences The Archaean felsic metavolcanic sequences in the eastern Pilbara are characterized by metamorphosed pyroclastics and volcanic detritus, with minor metamorphosed flows and associated subvolcanic intrusives. The Duffer Formation (Fig. 1) represents the earliest widespread phase of pyroclastic volcanism in the Warrawoona Group, perhaps marking the establishment o f subaerial vents. The metavolcanics in the Duffer Formation are dominantly andestitic or dacitic, whereas those o f the younger Wyman Forma-
80 tion are dominantly rhyolitic. Many felsic metavolcanic units are laterally continuous for more than 20 km and, south of Copper Hills, where the Wyman Formation directly overlies older felsic metavolcanics, their combined thickness reaches about 5 km (Fig. 2). Thick (>2 m) tuff breccia units within the Duffer Formation are composed of poorly sorted angular to subangular lithic and tuffaceous fragments up to 20 cm in diameter in a lapilli- and ash-sized matrix (Fig. 6). The matrix is commonly recrystallized and evidence for the former existence of glass shards and pumice is rare. Poor sorting, absence of bedding and presence of large angular blocks suggest that these units were deposited by mass flowage. A variety of finer-grained units associated with the tuff breccias suggests deposition in a number of environments. Sections examined contain minor interbedded lapilli- and ash-tuffs exhibiting variable degrees of sorting. In some sections normal to reverse graded bedding is developed in plane laminated units, and is associated in some places with minor smallscale oblique laminations and slump structures. This association of sedimentary structures suggests deposition from turbidity currents. In these sections, it is most likely that coarse pyroclastic units are the
result of subaqueous pyroclastic flows, using the term in an intbrmal sense [24]. Individual units may have either been the direct result of volcanic explosions or subsequent mud flows. Well-developed turbidite sequences of doubly graded sequences of tuff beds [25] indicative of deposition or eruption in deep water are not developed in the McPhee Dome and Kelly Greenstone Belt. In other sections, particularly in tlae Kelly Greenstone Belt to the north of Copper Hills (Fig. 2), coarse pyroclastic units are interbedded with tuffaceous metasediments. Mediumgrained metasediments display both planar lamination and abundant, low angle ( 8 - 1 0 ° ) tabular and trough-shaped crosslaminations (Fig. 7). Co-sets range in thickness from 5 to 20 cm. Scour and channel-fill structures are common, as are thin (<5 cm) layers of fine-grained tuffaceous metasediment and horizons of mud flakes. Reworking of previously lithified mud layers is observed in coarser clastic units. Metaconglomerate containing well-rounded pebbles and boulders eroded from coarse pyroclastic units is commonly interbedded with other tuffaceous metasediments (Fig. 8). Bedding in most of the area studied is remarkably uniform and coarse pyroclastic units form laterally
Fig. 6. Poorly sorted, felsic fragmental unit (lahar) containing coarse clasts, Duffer Formation, Kelly Greenstone Belt.
81 indicate reworking of pre-existing pyroclastic deposits in shallow-water high-energy environments. In the Wyman Formation, near Copper Hills, a metamorphosed vitric welded tuff showing excellent textural preservation of welded shards and pumice fragments is exposed. Reworked volcaniclastic metasediments are common, and metamorphosed conglomerate containing boulders of black subvolcanic porphyry common at Copper Hills also occurs. Epiclastic metasediments, containing material derived from underlying volcanics, represent a common lithology in the overlying Gorge Creek Group (Fig.
1). 6. Deposition of the Warrawoona Group Fig. 7. Trough-shaped cross-laminations in medium-grained subaqueous tuffaceous metasediment, Duffer Formation, Kelly Greenstone Belt. extensive sheets, suggesting a more subdued topography than typical of most felsic volcanic terrains. Graded turbidity deposists suggest subaqueous deposition, and a variety of tuffaceous metasediments
Fig. 8. Conglomerate horizon with well-rounded, locally derived felsic clasts overlying medium-grained subaqueous tuffaceous metasediment, Duffer Formation, Kelly Greenstone Belt. Scale is in centimetres.
Comparison of the metasediments of the Warrawoona Group with present-day, deep-water marine sediments is difficult: what were the equivalents of the organic oozes which, with siliceous clays, characterize the abyssal plains [26], and are distributed asymmetrically with terrigenous turbidites and volcaniclastic sediments in trenches [27] and marginal basins [28] ? Cherty rocks, whose origins are not always clear, are found in many Mesozoic and Tertiary deep-sea sequences, but do not show the textures and structures developed in the cherty metasediments of the Warrawoona Group. There is no convincing evidence for deep-sea deposition in the Warrawoona Group. The strongest evidence for the environment of deposition is the occurrence of several features which point to shallow-water or, more rarely, evaporitic origin. Evidence for shallowwater deposition occurs throughout the sequence. Cherty metasediments and metamorphosed felsic volcaniclastics, apparently of shallow-water to subaerial origin, are continuous for tens of kilometres in well-documented localities. The lateral extent of thin (<20 m) shallow-water metasedimentary horizons, conformable with both underlying and overlying metavolcanics, indicates deposition in an area of gentle topography and minimal orogenic uplift. The landscape during deposition of the Warrawoona Group was probably fairly flat, with the vents of felsic volcanoes forming local topographic highs within the basin. This type of volcanic landscape could have been produced by a series of ultramafic-
82 mafic fissure eruptions, or by a gently sloping shield volcano with lavas interfingering with the debris and pyroclastic products of a more elevated felsic volcano or volcanoes. A significant amount of the cherty metasediments within ultramafic-mafic metavolcanic sequences was initially deposited in extensive shallow basins as carbonate and evaporite, with relatively little detritus from surrounding volcanic areas. Sediments around penecontemporaneous felsic volcanic centres were locally derived. They were deposited in shallow water, forming extensive sheets over surrounding areas of subdued relief and interfingering with the ultramafic-mafic metavolcanics and cherty metasediments.
7. Significance to Archaean tectonic interpretations Sediments within the Warrawoona Group suggest a model for initial greenstone belt development involving deposition in a shallow volcanically active basin surrounded by a subdued topography. Sources of coarse detritus were restricted to emergent felsic volcanoes within the basin. This sedimentary environment may be similar to that envisaged for some large Proterozoic basins by Trendall [29], and contrasts markedly with that of modern oceans and marginal basins. Sedimentological studies of the Onverwacht Group [8], of similar age to the Warrawoona Group, also reveal a shallow-water origin for a significant proportion of the sediments within the mainly volcanic pile, and it is notable that this area contains significant stratiform barite [21], similar to that of North Pole. Shallow-water environments are also represented within volcanic sequences in younger greenstone belts [30,31 ], but available data indicate that they are not dominant as in the Warrawoona and Onverwacht Groups, and that bedded sulphates are absent. Plane-laminated iron formations or sulphidic shales, showing none of the structural and textural characteristics of the Warrawoona cherts, appear to be the dominant interflow sediment type. Most previous Archaean sedimentological studies have been carried out upon dominantly clastic metasedimentary sequences, interpreted to have been deposited in deep unagitated "geosynclinal" basins in which marginal fluvial, alluvial fan or submarine fan deposits were commonly associated with turbidites
[7, 32-34]. Such sediments become a major part of the eastern Pilbara and Barberton Mountain Land greenstone sequences only during the latter part of their development [8,12,35]. The rapid transition from volcanism to clastic sedimentation in these sequences, combined with the contrast in sediment type between sequences, probably represents a major change in tectonic regime, with shallow stable basins giving way to deeper, rapidly sinking basins. The occurrence of detritus from granitoids, in addition to detritus from underlying metavolcanics in the clastic metasedimentary sequences [35,36], suggests that these sediments were, in part, eroded from granitoids, possibly after early diapiric uprise [37]. However, further detailed studies of the Gorge Creek Group are required to test this model. There are, as yet, insufficient data on the metasedimentary rocks within the predominantly metavolcanic sequences of most Archaean greenstone belts to describe, or even compare precisely, the nature of early depositional basins or trends in basinal evolution. Further sedimentological data are also required to resolve the nature of the transitions from dominant eruption to dominant sedimentation common to many belts. Such sedimentological data will be more useful than geochemical comparisons between volcanic rocks and modern analogues. Meanwhile, our data, and those from Onverwacht Group, indicate that significant proportions of these ancient greenstone sequences formed in extensive shallow basins. The tectonic mechanism for their formation remains unclear.
Acknowledgements This study was supported by the University of Western Australia, and Commonwealth Postgraduate Scholarship (J.S.R.D) and University Postgraduate Studentship (M.E.B.) awards. Field support was given by I.D. Martin and Alcoa of Australia (W.A.) Ltd., and Dresser Minerals International. The work was discussed with B. Bolton and A.H. Hickman. M.J. Bickle and R. Buick criticized the manuscript.
References 1 A.J.R. White, P. Jakes and D.M. Christie, Composition of greenstones and the hypothesis of sea-floor spreading in
83 the Archaean, Geol. Soc. Aust. Spec. Publ. 3 (1971) 47. 2 B.-M. Jahn, C.Y. Shih and V.R. Murthy, Trace element geochemistry of Archean volcanic rocks, Geochim. Cosmochim. Acta 38 (1974) 611. 3 J. Tarney, I.W.D. Dalziel and M.J. de Wit, Marginal basin "Rocas Verdes" complex from S. Chile: a model for Archaean greenstone belt formation, in: The Early History of the Earth, B.F. Windley, ed. (Wiley, London, 1976) 131. 4 K.C. Condie and N.M. ttarrison, Geochemistry of the Archean Bulawayan Group, Midlands Greenstone Belt, Rhodesia, Precamb. Res. 3 (1976) 253. 5 R.K. O'Nions and R.J. Pankhurst, Early Archaean rocks and geochemical evolution of the earth's crust, Earth Planet. Sci. Lett. 38 (1978) 211. 6 J.F. Wilson, M.J. Bickle, C.J. Hawkesworth, A. Martin, E.G. Nisbet and J.L. Orpen, Granite-greenstone terrains of the Rhodesian Archaean craton, Nature 271 (1978) 23. 7 C.C. Turner and R.G. Walker, Sedimentology, stratigraphy, and crustal evolution of the Archean greenstone belt near Sioux Lookout, Ontario, Can. J. Earth Sci. 10 (1973) 817. 8 D.R. Lowe and L.P. Knauth, Sedimentology of the Onverwacht Group (3.4 billion years) Transvaal, South Africa, and its bearing on the characteristics and evolution of the early Earth, J. Geol. 85 (1977) 699. 9 S.L. Lipple, Definitions of new and revised stratigraphic units of the eastern Pilbara region, Geol. Surv. West. Aust. Annu. Rep. 1974 (1975) 58. 10 A.H. Hickman, Precambrian structural geology of part of the Pilbara region, Geol. Surv. West. Aust. Annu. Rep. 1974 (1975) 68. 11 C.R. Anhaeusser, Cyclic volcanicity and sedimentation in the evolutionary development of Archaean greenstone belts of shield areas, Geol. Soc. Aust. Spec. Publ. 3 (1971) 57. 12 A.H. Hickman and S.L. Lipple, Explanatory notes on the Marble Bar 1:250,000 Geological Sheet, Western Australia, West. Aust. Geol. Surv. Rec. 1974/20 (1975). 13 R.T. Pidgeon, 3450-m.y.-old volcanics in the Archaean layered greenstone succession of the Pilbara Block, Western Australia, Earth Planet. Sci. Lett. 37 (1978) 421. 14 D.F. Sangster and W.A. Brook, Primitive lead in an Australian Zn-Pb-Ba deposit, Nature 270 (1977) 423. 15 J.R. Richards, Lead isotopes and ages of galenas from the Pilbara region, Western Australia, J. Geol. Soc. Aust. 24 (1977) 465. 16 H.P. Eugster, Inorganic bedded cherts from the Magadi area, Kenya, Contrib. Mineral. Petrol. 22 (1969) 1. 17 J.S.R. Dunlop, M.D. Muir, V.A. Milne and D.I. Groves. A new microfossil assemblage from the Archaean of Western Australia, Nature 274 (1978) 676. 18 B.W. Logan, Inventory of diagenesis in Holocene-Recent carbonate sediments, Shark Bay, Western Australia, in: Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia, B.W. Logan et
19
20
21
22
23
24 25
26
27
28
29 30
31
32
33
34
al., eds (American Association of Petroleum Geologists, Tulsa, Okla., 1974) 195. J.S.R. Dunlop, Shallow-water sedimentation at North P01e, Pilbara, Western Australia, Publ. Geol. Dep. Extension Serv., Univ. West. Aust. 2 (1978) 30. G. Richter-Bernburg, Facies and paleogeography of the Messinian evaporites in Sicily, in: Messinian Events in the Mediterranean, W. Drouger, ed. (North-Holland, Amsterdam, 1973) 141. T.K. Heinrichs and T.O. Reimer, A sedimentary barite deposit from the Archean Fig Tree Group of the Barberton Mountain Land (South Africa), Econ. Geol. 72 (1977) 1426, I.B. Lambert, Sulphur-isotope investigations of Archaean mineralization and some implications concerning geobiochemical evolution. Publ. Geol. Dep. Extension Serv., Univ. West. Aust. 2 (1978) 45. I.B. Lambert, T.H. Donnelly, J.S.R. Dunlop, and D.I. Groves, Stable isotope studies of early Archaean evaporitic sulphates at North Pole, Western Australia and possible equivalents at Barberton, South Africa, Nature (in press). R.S. Fiske, Subaqueous pyroclastic flows in the Okanapecosh Formation, Geol. Soc. Am. Bull. 74 (1963) 391. R.S. Fiske and T. Matsuda, Submarine equivalents of ash flows in the Takiwa Formation, Japan, Am. J. Sci. 262 (1964) 76. P.J. Cook, J.J. Veevers, J.R. Hiertzler and P.J. Cameron, The sediments of the Argo Abyssal Plain and adjacent areas, northeast Indian Ocean, Aust. Bur. Miner. Resour. J. Geol. Geophys. 3 (1978) 113. W.R. Dickinson, Reconstruction of past arc-trench systems from petrotectonic assemblages in the island arcs of the Western Pacific, in: The Western Pacific, Islands Arcs, Marginal Seas, Geochemistry, P,J. Coleman, ed. (University of Western Australia Press, Nedlands, W.A. 1973) 569. D.E. Karig and G.F. Moore, Tectonically controlled sedimentation in marginal basins, Earth Planet. Sci. Lett. 26 (1975) 233. A.F. Trendall, Revolution in Earth history, J. Geol. Soc. Aust. 19 (1972) 287. M.J. Bickle, A. Martin and E.G. Nisbet, Basaltic and peridotitic komatiites and stromatolites above a basal unconformity in the Belingwe greenstone belt, Rhodesia, Earth Planet. Sci. Lett. 27 (1975) 155. J.B. Henderson, Archean stromatolites in the Northern Slave Province, Northwest Territories, Canada, Can. J. Earth Sci. 12 (1975) 1619. R.G. Walker and F.J. Pettijohn, Archean sedimentation: analysis of the Minnitake Basin, norhtwestern Ontario, Canada, Geol. Soc. Am. Bull. 82 (1971) 2099. J.B. Henderson, Sedimentology of Archean turbidites at Yellowknife, Northwest Territories, Can. J. Earth Sci. 9 (1972) 882. R.S. Hyde, Archean sedimentary environments: implications regarding paleotectonic settings of greenstone belts
84 and the development of continents, Geol. Soc. Am. Annu. Meet., Abstr. Progr. (1976) 933. 35 K.A. Eriksson, Alluvial and destructive beach facies in the Archaean Moodies Group of the Barberton Mountain Land, Econ. Geol. Res. Unit Inf. Circ. 115 (1978). 36 M.J. Fitton, R.C. Horwitz and G. Sylvester, Stratigraphy
of the Early Precambrian in the West Pilbara, Western Australia, C.S.I.R.O. Miner. Res. Lab. Rep. FP 11 (1975), 24 pp. 37 B.E. Gorman, T.H. Pearce and T.C. Birkett, On the structure of Archean greenstone belts, Precamb. Res. 6 (1978) 23.