Revista Brasileira de Geociências 12(1-3): 121-131, Mar.-Sel., 1982 - Silo Paulo

ARCHEAN AND EARLY PROTEROZOIC SEDIMENTATION STYLES lN THE KAAPVAAL PROVINCE, SOUTH AFRICA AND PILBARA BLOCK, AUSTRALlA

KENNETH A. ERIKSSON*

ABSTRACT Archean sedimenta lntercalared within the lower volcanic interval in the Barber­ ton Mountain Land and Pilbara Block accumulated in an anorogenic, shallow-oceanic environment distant from a continental influence. Overlying terrigenous sedimenta represent continental margin dcposits : in South África. tcrrigcnous influx across occanic crust was in responsc to crusta! short­ cniug and uplift of gncisscs lo thc south. Both steep and shclf-rimmed continental margine are recognized. ln South África, sedimentation was terminated by ocean closure through lhe north­ ward advance of the zone of crustaI shortening. The late-Archean Pongola Supergroup is inter­ preted as a response 10 this closure with sedimentation taking place in a cratonic shelf basin to the south. The lower Proterozoic witwatcrsrand and Transvaal Supergroups likcwise accumulated in cratonic shelf basins with sediment dcrived from lhe north. Terrigenous sediment was supplied by fluvial systems but lhe dominant mode of sedimentation in lhe pericratonic seas was tida I and took place during prolonged periods of submergence of the Kaapvaal Province. Offshelf basinal equivalents are nôt recognized for Ihese cratonic sequences. Intracratonic rift basins are repre­ sented by the Ventersdorp Supergroup and Soutpansberg Group and are also developed at the base of lhe Hamersley basin. A. third style of lower Proterozoic sedimentation is rêpresented by the chemical sedimentary unit in lhe Transvaal basin. A basal ramp margin developed after initial drowning of the Kaapvaal Province and evolved into a rimmed shelf margino ln the latter, plat­ form tidal flat and subtidal stromatolitic asserublages pass laterally into high-energy oolitic and stromatolitic plutform-edge facies. Basinal equivalents are present and consist ofnon-stromatolitic, ferruginous dolomite and limcstonc. mudstone, chert and finely-laminated iron-formation. The Hamersley Group is a lithologic equivalem of lhe basinal. fades. Platform fades are poorly-repre­ sented in Australia, and comprise lhe thin Carawine Dolomite in the eastern Pilbara Block.

INTROPUCTION Numerous faeies models eonsisting in the Limpopo Province (Fig. J ; Barton et ai., 1978), exisled ofrepetitive and ordered associations offaeies aredeveloped at the time of volcanism, no basement to the volcanics or for terrigenous and earbonate depositional systems (see for influence of a sialic provenance during volcanism has been example Walker, 1979). These models are constructed on the recognized. Volcanisrn took place in an anorogenic ocean basis of observations in the Phanerozoic rock reeord and floor environment distaOnt from any continental influence in Holoeene environments, drawing heavily on the princi­ (Lowe 1980. 19821. The overlying terrigenous sedimentary pie of uniforrnitarianism. ln the Precambrian a number of intervals reflcct uplift and weathering of a sialic provcnance these facies models are represented but not ali sequences and sedimcntation along a continental margin (Eriksson, can be understood by comparison with these models. 1982(/). ln South Afriea it can be demonstrated that terri­ ln this paper the Archean and early Proterozoic sedimen­ genous influx was in rcsponse to crustaI shortening and tary record ín South Africa and Western Australia is exa­ uplift ofthe Ancient Gneiss Complex (Jackson and Eriksson, mined (Fig. I). Various styles ofsedimentation are recogniz­ in preparation) which has a similar age to the Onverwacht ed in terms of basin type and depositionaJ environments. volcanies (Barton et ul., 1980). Thc terrigenous sedimentary Applicable facies models are emphasized and new models intervals are probably greater than 3.3 Ga although no developed which may be peculiar to the Precambrian be­ direct age determinations are available. cause ofdifferent tectonic, atmospheric and biologic condi­ tions existing prior to 1.80a. Sedimentation in a volcanic environment Sediments inter­ calated within the lower volcanic sequences in both areas are exclusivcly of intrabasinal origin (Lowe, 1980. 1982). Four sedimentary associations are recognized, namely py~ GREENSTONE BElT SEDIMENTATION General roclastic-volcaniclastic, orthochemical, biochemical and ter­ geology Greenstone belts in lhe Barberlon Mountain rigenous (Lowe, 1982). Land, South Africa, and Pilbara Block, Australia (Fig. I l, Pyroclastic debris accumulated mainly around felsic vol­ belong to the older (3.55-3.0 Gal generation of Archean canic centres as lahars and airfall tuffs .. Volcaniclastic sedi­ volcano-sedimentary successions. ln both areas lower vol­ ments were produced by traction reworking in terrestrial canie and upper terrigcnous .sedimentary intervals are and shallow-water environments (Photo I) and resedimen­ developed (Fig. 2). The volcanic intervals give maximum tation as shallow-water debris 1I0ws and turbidites. Fine­ ages of 3.55 Ga (Hamilton et ai., 1979, 1980) and contain -grained ash deposits are ofairfall origin or display evidence ultramafic-mafie and felsic components (Fig. 2). Although of shallow-water reworking. Within 5 to 30 m lhick deposi­ older sialic nuclei, such as lhe 3.8 Ga Sand River Gneisses tional sequences volcaniclastic sediments decrease in abun-

'" Department of Geological Sciences, Virgínia Polytechnic Instítute and State Uníversity, Blacksburg, Virgínia 24061, USA 122 Revista Brasileira de Oeocíêncuu, Volume 12 (1-3), 1982

// :::>",'" -zOu "'­~> «o Na: Oa. ~

Barberton Mountain Lond

YILGARN BLOCK \ ... \ \ \ \ sAL-DANIAN;------PRO o o 300km ! ! !

l:ti/iM Folded cratonic cover ~ Cratonic bosement l~l Cratonic cover .' ':,'. (outcrop/ interred ) D Precratonic

Figure 1 - Localltv ntap o] file Koapvoal Provínce ond Pilhara Bíock showíng dtstrtbntton of file eartv Proterozaic sedirnentarv basins (adapted [mm Bvnon, 1976," Gee, 1979 .. Tankard et al., 1982). P = Pongola .. W = Wítwatersrand:V = Ventersdorp; T= Transvaal; GW = Griqualand West,' WB = Waterberg; S = Soutpansberg; M = Matsap; H =. Homerstey: A = Ashhurton (J = Johannesburg; P = Perthí

BARBERTON PILBARA dance upwards and give way lo orthochemical and biochem­ ical sediments. Orthochemical fades include evaporites, limestone and chert whereas stromatolites, carbonaceous laminations and detrital carbonaceous granules indicate bio­ genic processes. Associated desiccation cracks and rip-up algal mats (Photo 2) indicate periodic exposure. Uitramafic and mafic volcaniclastic sandstones and rnudstones in the Pilbara Block probably formed by reworking of hyaloclas­ tites. These volcaniclastic sediments are arranged in two ------to five thick upward-fining and upward-shoaling sequences capped by evaporites and stromatolites (Dunlop and Buick, 1980; Groves et al., 1981). Terrigenous sediments are con­ o vvvvvvvvvv vvvvvvvvvv fined to rare felsic volcanic, fuchsitic, black chert and jas­ ...... li ., vvvvvvvvvv per clasts associated with the proximal fades of volcani­ vvvvvvvvvv c1astic sequences. Iron-formations are rarely developed ~~~~~~~~~0 3.5 Ga vvvvvvvvvv within the volcanic sequences, ~ ? The sedimentological studies in the Barberton Moüntain Land and Pilbara Block indicate shallow-water conditions 1"'.':::,':::;.:'1 Shallow Marine Terrigenous Sedimenf s for the duration of the volcanism (Lowe, 1982). The first G Fluvial Terrigenous Sediments evidence of deepening is found in the uppermost chert ho­ rizons in the Barberton Mountain Land. Supporting evi­ E=:=31 Submarine Fon Temqenous Sediments dence for shallow water depths is the size and abundance • Iron - Formation of vesicles in the uitramafic-mafic volcanics in the Pilbara which indicate water depths of less than 100 m (Dunlop E::J Felsic Volconics with intercoloted votccnicrcst!c and Buick, 1980). ~ Mofic - Ultromofic } and orthochemicol sediments The ultramafic and mafic volcanism produced flat, sub­ ~ Volconics aqueous shields whereas local low relief volcanic cones Figure 2 - Generalízed stratígraphy of the Archean geology tn lhe developed around felsic centres. The preserved record of Barberton Mountaín Land and Pilhara Block (after Anhaeusser, the early Archean oceans indicates high-standing volcanic 1973.. Bariey el aI., 1979) platforms. It is feasible that Archean oceans at ca 3.5 Ga RevistaBrasileira de Geocténctae, Volume 12 (I~3). 1982 123

Terrigenous continental marglnsedimentatlon Influx of extrabasinal terrigenous sediment to the ocean basin was coincident with deepening of the ocean in both areas (Eriks­ son, 1980; 1982a). Reference has already been made to the fact that terrigenous supply in South Afriea was in response to uplift of the Ancient Gneiss Terrain in Swaziland. The crustal shortening involved northerly vergence of fold and thrust structures and it is likely that deepening of the ocean basin resulted from tectonie loading of sialic thrust sheets onto ocean crust. Subsidence may have been enhanced by cooling ofocean crust related to waning volcanism (Jackson and Eriksson, in preparation). The cause ofbasin deepening in the Pilbara is not understood. Terrigenous sedimentation in both areas is characterized by an abrupt braided alluvial to submarine fan transition which is considered to re/lect an initial steep continental margin with no sign.ficant shelf (Eriksson, I982a). ln the Pilbara Block the braided alluvial sediments occur as prox­ imal time equivalents ofthe submarine fan deposits whereas in the Barberton Mountain Land these two depositional systems are arranged in a progradational sequence separated by a fan delta complex (Fig. 2). Within the braided alluvial facies belt in both areas depo­ sitional sequences resembling the Scott, Donjek and Platte facies models of Miall (1978) are widely developed (see Photo 3; Eriksson, 1978; in press). The submarine fan de­ posits can likewise be interpreted with reference to the exist­ ing facies models ofMutti and Ricci Lucchi (1972)and Wal­ ker (1978). Two submarine fan facies associations are rec­ ognized in the Pilbara Block (Eriksson, 1982b).:ln the con­ glomerate sandstone-shale facies association conglomeratic upper fan channel and related levee deposits (Photo 4) pass basinwards into graded and stratified pebbly sandstones PhOIO J - Cross-bedded volcantclastíc sedíments from lhe basal volcanic unit tn lhe Pílbara Block and sandstones (Photo 5) of braided suprafan origino The coarseness ofsediment supplied to the midfan is considered to have promoted braiding. The second facies association comprises sandstone-shale in which incised midfan channel­ -fill deposits developed as a result of the finer grain size. Both midfan channel types pass basin-wards into upward­ -coarsening basin plain to lobe sequences. Iron-formations associated with the submarine fan deposits are interpreted as analogues ofHolocene pelagic sediments (Eriksson, 1981). These orthochemical sediments occur at the base of the sub­ marine fan package (Fig. 2), within basin plain and inter-

PhOIO 2 - Ripped-up algal mats from basal volconic uni! in lhe Pilbara Block (field of vrew is 13 mm wides were universally shallow and bouyant because of higher geothermal gradients and more numerous, shorter ridge systems. Smaller volumes of sialic crust at 3.5 Ga would also have resulted in shallower oceans (Lowe, 1982). Sedimentation in early Archean oceans was unlike that which is taking place in Holocene oceans distant from con­ tinents, Pelagic sedimentation was subordinate; instead, Photo j - Cíast-supported congíomerate and íntercaíated horízon• traction reworking and shallow-water orthochemical and taliy-stratífied sandstone from proximal, Scdtt-type braidedalluvial biochemical processes predominated. sediments in lhe upper sedimentary unit in lhe Pilbara Block 124 Revista Brasileira de Geocilncios, Volume 12 (l~3). 1982

channel mudstones and capping graded turbidite beds, Iron­ ment was still supplied to the ocean basin to thc north by -formations aceumulated ín any deepwater environment alluvial systems but instead of resedimentalion into deep starved of terrigenous influxo water, reworking took place in shallow rnarineenvironrnents (Eriksson, 1979). A deltaic assemblage of faeies displays evidence of deposition ln a macrotidal envíronment anal­ ogous to the Ord delta in Western Australia (Coleman, 1976) or·the Elbe delta in Gerrnany (Gadow and Reineck , 1969). Recognizable subenvironments are tidal channels and tidal flats of thc delta plain, tidal sand shoal and delta front. Longshore reworking of riverborne sediment from the delta led to the dcvelopment of barrier islands and back-barrier tidal flats under micro-to-mesotidal conditions, AIl of the shallow marine environments recognized in the Barberton Mountain Land have analogues in the Holocene and can be compared with existing facies models. Thc envisaged coastline experimented overall micro- to mesotidal ranges with macrotidal conditions devcloped locally in ernbayments. Sedimentation in both areas was probably terminated by ocean basin closure. ln the Barberton Mountain Land this was achieved by the northerly advance of the zone of crusta! Photo 4 - Horizontallv lamiruued síltstone muimudstone with starv­ shortening and probable obduction of thc ocean crust and ed rtpples constitutínglevee deposits (o subrnaríne upperjan channol ín til!' Pííbara Block mantle ofcontinental rnargin sedirnents (Jackson and Eriks­ son, in preparation).

CRATONIC SEDIMENTARY BASINS General geology Cratonic sedimentary basin on the Kaapvaal Province developed during the time period 3.0 to 1.8 Ga and comprise the late Archean Pongola Supergroup and the early Proterozoie Witwatersrand, Ventersdorp. Trans­ vaal-Griqualand West and Watcrberg-Matsap-Soutpans­ berg basins (Figs. I and 3). The Hamersley basin in Western Australia (Figs, I and 4) which occupies the southern half of the Pilbara Block is probably a time equivalent of the Transvaal basin (Button, 1976). ln contrast to the southerly source terrain which provided the terrigenous sediment to the Archean continental margin in South Africa, sediments in the eratonic basins were deriv­ ed almost exclusively from the north. The cratonic basins developed in response (o the progressive northward migra­ tion of basin axes; the Pongola Supergroup is confined to the southeasterly margin of the provinee whereas the young­ er basins are present further towards the north (Fig. I). Migration ofbasin axes has been related to a complementary northward migration of loci of granite emplacement (Hun­ ter, 1974). 1t is also noticeable that the front ofcrustal short­ ening and overthrusting, which commenced at ca. 3.40a in Swaziland, also youngs to the north. Both intraeratonic rift and eratonie shelfbasins are recog­ nizable in South Africa and Australia. Conglomerates and arkoses in the Ventersdorp Supergroup and Soutpansberg Photo 5 - Graded pebbty sandstones and sandstones from submarine Group (Fig. 3) and locally at the base ofthe Hamersley basin braided suprafan in the Pilbara Block, Note flame structure at lhe (Fig. 4) are intracratonic rift fills, Quartz arenites and mud­ base 01 overlying bed . stones as well as arkoses and conglomerares in the Pongola, Witwatersrand and Transvaal Supergroups, the Waterberg Group (Fig. 3) and the terrigenous lower division of the ln the Barberton Mountain Land the steep margin was Hamersley basin (Fig. 4) are interpreted as cratonic shelf succeeded by the development ofa continental shelfon which deposits. The thick chemical sedimentary units in the Trans­ an assemblage of shallow-rnarine faeies accumulated. The vaal Supergroup and Hamersley basin as well as thinner development of the continental shelf was probably in res­ chemical sedimentary horizons within shelf mudstones in p,onse to the earlier outbuilding of the braided alluvial-sub­ the Pongola, Witwatersrand and Transvaal Supergroups are marine fan wedge possibly associated with a rise in sea levei similarly ofcratonic shelforigino A fault control on sedimen­ related to further basin subsidence (Eriksson, 1980). Sedi- tation in the cratonic shelf basins is not apparent, in the Revista Brasileira de Geocténctas, Volume 12 (1~3). 1982 125

mANSVAAL SLPERGROJP vo ••, Mofic Inlrusives ~ Volconics /i.%;w 21 Go 00 Quartz Arenites O Shole - Siltstone O Arkose OOs [illJ Conglomerole WITWATERSRAt\ID SUPER(Rü..!P Iron ~ Formotion i\)t>G0l11 r~. ~._~~ 0f-_---' •~ corboncte '~ Q.SÊ~ [;:] Chert Go Q L-- 287 .",' Bcsemen! WATERBERG E:iJ QF4 GROUP .-.- Unconformities ~ ~.~••~.~•.~.~.. 2.2 Go Of== I 2 2 O 1===1 . Go SOUTPANSBERG VENTERSOORP GROUP SPERGR()JP

' ~}?2:\' 2.3Go . .. .;·:.:~,<;I.c~.'-':'

" '" .::·)~~i 3000 O -- ..

O 2000 Go I.SGo ·~(\m-iii; 2.8 Go 1000 O J:: o m + + + + + + + + + + + + 1tl+ + + + + 3.1 Go + + 2.7 Go + + 2.0Go + + 2.0Go + ++++++ + + + + + + + + + + + + + + + + +

Figure 3 - Generalized stratígraphíc columns for lhe ear!y Protevozoic sedimentar}' basíns on lhe Kaapvaa/ Province, South Africa tadapted from Tankard et al., /982) NE SW Duck Creek Shelf Ashburton Tçough

§ Carbonate

+ Chert + GJ + + Arkose + + D + + + + + + 6D'""'l~.. ' Conglomerote (Fe - pebble •.;) + + + + + + + + + + + Ivvvlvvv Volcanics ~ Homersley Basin (± 2.5 Go ) ~ Turbidites Approx. Strotigraphic Scale ~ Mudstone O 4 8 KM ~ I I , iii Iron Formation - Transport Directions ""'- Unconformities [z;;] Bosement Figure 4 - Generalized stratígraphv ofthe Hamerstey Basín and Ashburton Trough on lhe southern margin oflhe Pilhara B/ock tafter Horwirz ; /980; Goode, /980, Compston ct al.. /98}) 126 Revista Brasileira de Geoaíêndas, Volume 12 (1-3), 1982

Kaapvaal Province any fault bounded margins must have siding basin. An analogous situation exists on the 'Holocene been to the north ofthe present outcrop limits oflhe basins. Kosi River in India (Tankard et ai., 1982). ln the Witwatersrand Supergroup the braided alluvial Alluvlal daposltional systems Alluvial sediments are of both sedimentary sequences consist ofupward-fining, 30 to 600 m­ alluvial fan and braided alluvial origino Alluvial fan se­ -thick genetic packages separated by lo" angle unconformi­ quences are extensively developed in the inlracratonic rift ties (Tankard et ai., 1982). Within the packages represen­ Ventersdorp Supergroup (Fig. 3) and arealso present in basal tatives of one or more of the Scott, Donjek and Platte Hamersley basin (Fig. 4). ln the Ventersdorp Supergroup facies model are -present ; these can also be shown to grade alluvial fan sediments are known only from the subsurface into onc another down the paleoslope. Braided alluvial and in the Welkom goldfield consist of coarse scree and sediments 10 the other basins can also be interpreted with debris flow conglomerates upwards of 1,000 m thick (Buck, reference to existing facies models. ln most cases the braid­ 1980). Sediment was derived from adjacent horsts of basal ed alluvial systems took the form of fan deltas, feeding Ventersdorp, Witwalersrand and Archean basement rock sediment directly to the shallow cratonic seas. The Copper types, Associated playa lake sediments consist of mudstone, River fan delta in Alaska (Galloway, 1976) is envisaged as marl and stromatolitic carbonates. Alluvial fan deposits are a Holocene analogue. also locally developed at the base of lhe Ventersdorp Super­ Whereas the thick braided alluvial sequences irnply rapid group southwest of Johnannesburg (Fig. I) where block aggradation, the unconformities in the Witwatersrand Super­ faulting took place in pre-Ventersdorp time. Both debris group are degradational features produced by epeirogenic 1I0wand sheet 1I00d facies have been recognized in the thin uplift of the depositional interface above base leveI. Button Venterspost placer which unconformably overlies Witwa­ and Adams (1981) have proposed that the Au-U placers tersrand sediments (Krapez, 1980). The basal Hamersley require such degradational conditions to formo Under such alluvial fan deposits contain chert clasts varying from a few conditions fines are removed from the erosional surface to centimeters to 5 m in diameter (Photo 6). The clasts are concentrate heavy minerais and pebbles, It is significant in supported in a mudstone matrix indicating debris 1I0w dep-' this respect that most of the Wilwatersrand placers are pre­ osition. Near Nullagine in the Pilbara a number of over­ sent along unconformities and that arenítes associated with lapping debris 1I0w lobes are recognizable and grade up­ the placers are texturally and mineralogically more mature wards into horizontally-stratified pebbly sandstones of than arenites within the genetic packages (Minter, 1978). probable sheetflood origino The Venterspost placer also rests on an unconformity but with a significant angular discordance. ln addition to the alluvial fan facies mentioned previously the placer con­ tains braided alluvial deposits with significant concentrations ofheavy minerais. Regionàlly the placer varies from Oto 4 m thick and is considered to represent the residue of sediment in transit in a degrading system well above base leveI. The placer is preserved only due to fossilization at the time of volcanic outpouring (Fig. 3; Krapez, 1980; Tankard et ai., 1982).

Terrlgenous shelf pepositional systems The majority of the quartz arenites and mudstones in the Pongola, Witwa­ -tersrand and Transvaal Supergroups (Fig. 3) are interpreted as macrotidal shelf to tidal flat deposits (Von Brunn and Hobday, 1976; Button and Vos, 1977; Watchorn, 1980; Eriksson et ai., 1981). Exceptions are, firstly the basal quartz arenite in the Witwatersrand Supergroup (Fig. 3) in which stacked and often truncaled, upward-fining sequences display features characteristic of tidal inlet deposition (Camden­ Photo 6 - Matrix-supported congíomerate from afluviaf [an deposií -Smith, 1980; Eriksson et ai., 1981). Secondly, the lower­ at the base 01 the Hamersley basin most quartz arenite and mudstones in the Transvaal Su­ pergroup (Fig. 3) are arranged in an upward-coarsening Braided alluvial conglomerates and arkoses constitute a depositional sequence and the quartz arenites isopach as large proportion of the Witwatersrand Supergroup (Tan­ lobate bodies (Button, 1973a). These criteria are diagnostic kard el ai., 1982) and most of the Waterberg, Matsap and of deltaic sedimentation under conditions of high wave Soulpansberg Groups (Fig. I, Eriksson and Vos, 1979; energy. Barker, 1979). Similar sediments are present in the Pongola Whereas the above two depositional environments have Supergroup (Watchorn, 1980), Ventersdorp Supergroup well-documented modern analogues and established fa­ (Buck, 1980; Krapez, 1980) and lower and upper terrige­ cies models this is not the case for the more abundant noús units of the Transvaal Supergroup (Button, 1973a; shallow shelf deposits on the Kaapvaal Province (Fig. I I. Button and Vos, 1977). The lowermost arkose in the Ha­ These sediments accumulated in seas with dimensions rnersley basin is also largely of braided alluvial origino Sedi­ upwards ofO.5 x lO· square kilometres. Sedimentation took mentation took place mainly in aggradational systems at place during prolonged periods of submergence of the or below base leveI. The thickness (up lo 5 km) and lateral Kaapvaal Province between 3.0 and 2.1 Ga. Tidal shelf persistence (thousands of square kilometres) of lhe braided deposits in the Transvaal Supergroup commence with the alluvial sediments is considered lo be related to continuai quartz arenite at the base of the thick orthochemical unit channel switching on a humid region fan in a rapidly sub- (Fig. 3). Paleocurrent patterns (Button, 1973a) indicate elo- RevistaBrasileira de Oeocténctas, Volume 12 (l~3). 1982 127

ckwise tidal circulation in the cratonic sea. A possible Holo­ cene analogue is lhe Nerth Sea amphidromic system but with dimensions considerably less than envisaged for the Transvaal sea. More typically, shelf-tidal flat associations are developed as upward-coarsening, progradational se­ quences Dr as northward-coarsening, timeequivalent facies. Distal orthochemical sediments pass into offshoreshelf'mud­ stone which contains increasing proportions Df siltstone and sandstone towards quartz arenites. ln the Transvaal basin oolitic ironstone lenses within the rnudstones imply periodic shoaling of lhe shelf (Button, 1973a). The thick quartz arenites are interpreted as shallow subtidal shoals analogous to those existing 00 the northwest Australian shelf(Button and Vos, 1977). Longshore paleocurrent modes are common in lhe quartz arenites in the Transvaal basin (Buuon. 1973a) and are probably again related lo ciockwise circulation of tidal waves in the cratonic seas (Tankard et al., 1982). The uppermost, shallow-water parts of the quartz arenite units typically contam onshore-offshore herringbone Photo 8 - First-order ripples wíth superimposed transverse forms, cross-bedding IPhoto 7) related to tidal currents oriented also showíng water Ievelmarks, from tidal flat deposits ín lhe lower perpendicular to the shoreline. Wítwatersrand Supergroup

The evidence of strong tidal currents in lhe Pongola, Witwatersrand and Transvaal basins implies that these de­ positories were open to an ocean sea, and had the form of epicratonic or pericratonic seas. Macrotidal conditions are considered to -have developed as a result of amplification of lhe tida I wave across the wide stable cratonic shelf repre­ scnted by lhe Kaapvaal Province (Eriksson et al.. 19~11. Pa­ leocurrent and fades trends as wcll as isopach datasuggest that the open ocean lay to lhe south. an arca now occupied by the high-grade metamorphic Natal and Namaqua Prov­ inces (Fig. I). The suggestion of an open ocean to the south inces is supported by the presence of a northward-obducted ophiolite suite in Natal aiong the southern margin of the Kaapvaal Province (Matthews, 1972; Dix, 1981). Obduction took place at some stage after 2.0 Ga. Photo 7 - Herringbone cross-bedding from upper part of shallow subtídal shoal in the Iower Witwatersrand Supergroup Non-terrigenous platforrn-to-basin transition Chem icaI se­ diments in the Transvaal-Griqualand West Supergroups (Figs. 3 and 5) and Hamersley basin (Fig. 4) record platform to basin transitions. Subordinate terrigenous sedimenls are The shelf sandstones may be overlain by fluvial sand­ present mainly at the base ofand tuffaceous sediments within stones of fan delta origin or tidal flat quartz arenites and lhe chemical intervals. The chemical stratigraphic unit in mudstones. The former developed in response to high se­ the Transvaal-Griqualand West Supergroups has been stud­ diment influxo Transgressive reworking, upon abandonrnent ied in detail (see for example Button, 1973h; Eriksson and of fluvial lobes, 100 to the development of mature quartz Truswell, 1974; Eriksson et ai., 1976; Beukes, 1978, 1980) arenites structured by herringbone cross-beds (Button, 1979) and the platform to basin transition is relatively well-under­ or swash lamination (Vos and Eriksson, 1977). Tidal flats stood. ln the Hamersley basin only parts of the transitional developed either lateral to active fluvial lobes or under model are developed due lo limited preservation and young­ conditions of reduced fluvial influx, Lower-tidal flat de­ er cover. For this reason the Transvaal-Griqualand West posits consist ofquartz arenites with herringbone cross-beds occurrence will be discussed first and lhe Hamersley basin containing numerous mudstone drapes. Flaser, wavy and related to the model. lenticular bedding constitute the rnidflat, and mudstones with occasional desiccation structures the upper-flat depos­ TRANSVAAL BASIN The following discussion will fo­ its. Stromatolites are locally present within the tidal flat cus on the basal, dominantly terrigenous sedimentary inter­ sequences. The above structures indicate the diagnostic tidal vai, resting either on basernent or overlying the protobasinal flat processes of current reversals, alternating bedload and phase of the Transvaal Supergroup, and the overlying thick suspension sedimentation and periodic exposure. Associat­ carbonates of the Campbellrand Subgroup and Malrnani ed superimposed ripples inciuding flat-topped, double­ Dolomite (Fig. 5). This stratigraphic interval comprises a -crusted and transverse forms (Photo 8), as well as water- genetic package of strata, the stratigraphic evolution of -level marks and dewatering pits, indicate ebb runoff. which can be viewed in terms of established passive margin 128 Revisto Brasileiro de Geociênciae, Volume 12 (1-3), 1982

models (Read, 1982). Recognizable is ahomoclinal ramp ships in these facies involve recrystallization and silicification which evolves into a rimmed shelf followed by formation ofprimary fine-grained dolomites and are attributed to mix­ of a drowned platform. ing between resurging continental waters and marine-de­ The ramp margin is represented by the basal terrigenous rived phreatic waters (Eriksson and Warren, submitted). unit in which quartz arenites oftidal shelf and tidal t1atorigin Associated tepee structures are ascribed to groundwater pass southwards into platform-margin oolitic carbonates outt1ow. Deeper subtidal stromatolites are from 5 to 20 m followed by basinal mudstones (Fig. 5). Associated with wide and up to 40 m long (Photo 10). ln contrast to the rapid platform drowning and transgression, basinal mudstone facies changes in the recrystallized dolomite and chert inter­ followed by limestone with ferruginous dolomite blanketed vais these stromatolitic dolomites display considerable ver­ the platform quartz arenites (Fig. 5). tical and lateral persistence (Button, 1973h; Beukes. 1978). A temporally more persistent rimmed shelf margin evolv­ Well-preserved lamination in the large stromatolitic mounds ed from the drowned ramp. Platform, platform-edge, slope is taken to indicate a primary origin for the dolomite, Inter­ and basinal environments are recognizable, Platform facies stratification of the tidal flat-shallow subtidal and deeper consist of tidal t1at and shallow subtidal recrystallized stro­ subtidal stratigraphic units can be related to intermittent matolitic dolomite with chert, and deeper subtidal, fine­ transgressions resulting in partial or, in one case complete -grained, iron-poor stromatolitic dolomite (Fig. 5). ln the platform drowning (Fig. 5). former, t1at algalaminites with small-scale digitate stroma­ Platform-edge facies are confined to lhe Campbellrand tolites (Photo 9) display evidence ofdesiccation and rework­ Subgroup and consist of linear belts of lirnestone and fer­ ing. This assemblage of structures is taken to indicate ruginous dolomite containing oolites, oncolites and colum­ upper intertidal to supratidal conditions (Eriksson and Trus­ nar stromatolites (Photos 11 and 12; Beukes, 1978). Plat­ well, 1974; Eriksson, 1977). Larger stromatolitic domes with form-edge stabilization was achieved by algal binding in a associated beach rosettes and wave-rippled sands developed high-energy shelf-edge environment; blue-green algae were further down lhe paleoslopc. Complex diagenetic relation- lhe only reef builders in lhe Preeambrian. Compared to lhe

Piloto 9 - Small-scale digita te stromatolites from tidal flat [acíes Piloto 10 - Largc-scute, subtídal stromcuoiitic mounds jrom fine­ ín the Malmani Dolomite (field of \'ielV Ú' 13 mm lI'ide) -grained, iron-poor dolomite unit in the Malmani Dolomite

.:..:..:J B~\e"'eol .~' =:J VOiCOO,C\ ~ .-. ' ';~~ O MOlol1 Te"'Qeoou~ Soodslone , - 400 SCALES ", D MudslOM ;::J Che,l' Smle E1'eccoo c'vcczcaasssssarn o eo 100 km

Figure 5 - Genemlized cross-section (~f rife chemícai sedimentar)' w1Í1 in lhe 1'raI1S1'Oa/ anel Griqualand West Supergroups, aiso showing rela­ tionship lo underlvíng 011(/ overlving stratlgraphic units (odaptedfrom Button, 1973h,' Eríksson ct al., 1976: Beukes, 1978). Line ofsection shown on inset Revista Brasileira de Geoctências, Volume 12 (l-ã), 1982 129

Pho II - Photo micrograph ofplatform-edge oolitvs and oncoíitcs Piloto 13 - Basinal [erruginous doíomites and prato íron-formatíons FOIII lhe Cumpbeíírand SuhKI'OUP (fie/d o] \'iell' is 13 111111 Iride) from the Compbellrand Subgroup

Final drowning of the carbonato platforrn resulted in blanketing by lirnestoncs and ferruginous dolomites and associatcd basinal facies (Fig. 5). The basinal deposits extend for upwards of 1,000 km across thc platform. The lack of stromatolites in these uppermost carbonatos of the Carnp­ bellrand Subgroup and Malmani Dolomite indicare submer­ gence below the euphotic zone. A similar platform-drowning event is recognizable in the Cambro-Ordovician stratigraphy of lhe Appalachians (Read, 1982). Overlying iron-forrrrarions in the Transvaal and Griqua­ land West Supergroups (Fig. 5) largely mimic the deposition­ ai environments of lhe carbonates although stabilized plat­ form margins are not developed. Instead lhe margin had more of a ramp morphology with finely-laminared basinal iron-formations shoaling upwards into clastic-texturcd, plat­ form iron-formations and finally into restricted lacustrine Photo 12 - Platform-edge columnar stromatolítes from the Carnp­ facies. Basinal iron-formations contain a large volcanic ash bellrand Subgroup component and reach a maximum development in the south whereas c1astic-textured iron-forrnations are more abundant over lhe pre-existing carbonato platform (Beukes, 1978). The transition from carbonates to iron-forrnarions is considered by Beukes (1978) to be related to a ehange in climate from warmer to colder conditions. ramp margin relatively steep slopes fronted the rimmed shelf, favoring lhe development of foreslope breccias (Beukes, HAMERSLEY BASIN By cornparison wirh the recons­ 1978). lrueted paleogeography of the chemical sedimentary unit Non-stromatolitic limestone and ferruginous dolomitc, in the Transvaal basin, a number of inferences can be made carbonaceous mudstone and sporadic chcrt horizons tcrrned regarding the depositional cnvironments of the chernical proto iron-formation (photo 13; Button, 1976) comprise sediments in the Hamersley basin (Fig. 4). ln general terms lhe basinal facies. The carbonates are predominantly fine it can be stated that preferential preservation of basinal grained and of probable pelagic origino Locally clastic-tex­ facies has occurred in the Hamersley basin whereas in the tured, resedimented platform carbonates are present in the Transvaal basin platform facies are predominant. Plarform form of thin, Bouma-like turbidite horizons (Beukes, 1978). sedirnentation in the Hamersley basin probably took place Graded siliciclastic horizons are also found; these are pro­ across much of the Pilbara Block (Fig. I) but these faoics bably airfall tuffs. Cherts are of either replacement or pri­ are preserved only aiong the eastern margin of the block mary origino Deeper basinal deposits contain less resediment­ as lhe Carawine Dolomite (Button. 1976; Goodc, 1981 I.This ed carbonate horizons but more abundant carbonaceous stratigraphic interval is analogous in ali respeets to the tidal and pyritic mudstones, The presence of ankeritized lime­ flat-shallow subtidal, reerystallized dolomite and ehert units stone, primary chert and carbonaceous and pyritic mudstone in lhe Transvaal basin (Fig. 5). ln particular, lhe Carawine implies that lhe basinal waters were midly acidic and reduc­ Dolomite is rich in stromatolites and chert and is rarely ing. This contrasts with lhe shelf waters which were more ankeritic or calcitic, Shallow-water structures include cross alkaline except in the diagenetic environment. bedding, wave ripples and intraclastic breccias(Goode, 1980). 130 Revista Brasileirade Geocíêncías, Volume 12 (1-3), 1982

Chemical sediments and associated mudstones in the main lower Ordovician platforrn-to-basin sedimentation in the Hamersley basin on the southern margin of the Pilbara Appalachians (Pfeil and Read, 1980) which displays many Bloek (Fig. 4) are identical to the basinal facies ofthe Camp­ features in common with the Transvaal and Hamersley basin bellrand Subgroup (Fig. 5). lron-formations are finely-lami­ facies transitions, can be shown to have taken place exclu­ nated (Trendall and Bloekley, 1970). Interbedded carbon­ sively on continental crust (Harris et ai., 1981). ates are calcitic and ankeritic and devoid of stromatolites. Resedimented clastic-textured carbonate horizons and grad­ CONCLUSIONS A number of distinct sedimentation ed siliciclastic horizons are presento Chert or proto iron­ styles have been recognized in the Archean and early Pro te­ -formation is common and mudstone, often carbonaceous rozoic stratigraphic record of the Kaapvaal Province and and tuffaceous, is widespread. Pilbara Block. Archean sedimentation occurred initially in a High-energy platform margin and slope facies have not shallow volcanic environrnent distant frorn a continental been recognized to date in the Hamersley basin. This is con­ influcnce and thereafter on the margins of srnal! cratonic sidered to be a function of preservation and it is possible nuclei. Proterozoic sedirnentation took place exclusively that such :platform to basin transitional facies may be de­ in a cratonic setting. Recognizable environrnents are intra­ veloped in the eastern extent of the main Hamersley basin cratonic rift, cratonic shelf or platform, platform margin (Fig. 1). and cratonic basinal. The latter probably occurred towards the margin of cratons but not on oceanic crust. A number of early Precambrian sedimentation styles are cornpatible with existing fades models, notably tenigcnous DISCUSSION A number of the depositional environ­ alluvial fan-Iacustrine, braided alluvial, deltaic, barrier ments recognized in the Transvaal and Hamersley basins beach, tidal flat and submarine fan, as well as non-terrige­ have modern analogues and established facies models. Sta­ nous platform margin, slope and base ofslope environments. bilization of platform margins, formation of slope breccias Exceptions are terrigenous platform environrnents for which and gravity flow resedimentation from platform to basin Holocene analogues do not exist and facies models have not have occurred throughout geologic time, the only change been developed. Likewise sedimentation in Archean volcan­ being in the buildup forming organisms (Heckel, 1972). ic environments and non-terrigenous basinal environrnents Areally-extensive shallow platform seas sueh as covcred appear to be largely peculiar to the early Precambrian due the Kaapvaal Province at 2.3 Ga are not present in the Ho­ respectively, to differences in depth and composition of the locene and no facies models exist for epeiric seas of this hydrosphere. type. Analogues of the basinal mudstones are widespread in the geological record but widespread basinal inorganic Acknowledgements The summary of Archean sedi­ cherts, iron-forrnations and ankeritic carbonates are not mentation in a volcanic environment is based primarily on known from the Phanerozoic record or frorn the Holocene. the work of John Ounlop and Don Lowe. Other ideas pre­ The presence of these sediments in the Transvaal and Ha­ sented in this paper have drawn heavily on lhe work of rnersley basins can be understood if early-Precambrian ba­ colleagues. I have benefitted considerably from discussions sinal watcrs were firstly anoxygenic thereby enhancing the and field trips with these individuais. Amongst those whom solubility of iron, and secondly, in the absence of silica-sc­ I wish to acknowledge are Mike Bickle, Nic Beukes, Andrew creting organisms, considerably enriched in Si02 . Neither Button, Martin Jackson, Bryan Krapez, Lawrence Minter, postulate is unlikely (see for example Orever, 1974; Oehler, Fred Read, John Truswell, and Richard Vos. Research over 1976). the past ten years has been supported both financially and The recognition of basinal environments in the Transvaal logistically by the University of the Witwatersrand, the and Hamersley basins does not imply abyssal depths. Sedi­ Council for Scientific and Industrial Rcsearch. the Universi­ mentation certainly took place below the euphotic zone and ty of Western Australia and the Oivision of Earth Sciences from Photo 3 this may have been at depths in excess of5oo m. National Seience Foundation Grant .EAR7842307. I also Likewise, the existence of basinal facies does nOJ neces­ wish to thank Jean Perdue for typing, Sharon Chiang and sarily imply the presence of oceanic crust. Cambrian and Martin Eiss for drafting and L1yn Sharp for photography.

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WALKER, R.G. - 1978 - Deep-water saodslone faeies and ancieni sub­ GADOW, S. and REINECK, H.E. -- 1969 ~ Ablandiger sand transport marine fans: models for exploration for stratigraphie traps. Amer. bei sturmfluten. Senckenbergíana Marit. I: 63-78. Assoe. Petrol. Geo{ogi.l"t.\" BII/I. 62: 932-966. GALLOWAY, W.E. - 1976 - Sediments and stratigraphic framework of WALKER, R.G. (cd.) - 1979 - Faeies models. Geoscience Canada, Re· the Copper River fan·delta, Alaska. J. Sedimento Pelrol, 46: 726-737. print Series. GEE, R.D. - 1979 - Structure and tectonie style ofthe Western Australian shield. Tecwnophys. 58: 327-369. Recebido em IS de agosto de 1982