rare earth element characteristics of paleoproterozoic cap carbonates pertaining to the sausar group central india implications for ocean paleoredox conditions abstract the paleoproterozoic era is marked by significant changes in the level of oxygen in the atmosphere and chemistry of the ocean and the presence of world wide glaciation geochemistry of chemogenic sediments provide clues to these changes the sausar group 2478 2250 ma of central india is one of the few places in the world where paleoproterozoic glacial unit overlained by a cap carbonate horizon has been identified major trace and rare earth element ree contents of paleoproterozoic cap carbonates of the sausar group have been analysed for their implications for ocean redox conditions during interglacial periods of the huronian glaciation the la la gd gd lusn lasn and y ho ratios of the cap carbonate reflect their pristine marine nature conspicuous ce anomaly ce ce up to 1 75 accompanied by the enrichment of fe mn zn and u provides insight into the redox state of the ocean at the time of deposition of the cap carbonate succession based on these observations we infer anoxic depositional conditions in the basin during the interglacial period of the paleoproterozoic glaciation before the great oxidation event goe and the level of oxygen in the atmosphere fluctuating between 10 5 and 10 2 of the present atmospheric level pal

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

influence of provenance and transport process on the geochemistry and radiogenic hf nd and sr isotopic composition of pleistocene glacial sediments minnesota usa abstract we develop a broad geochemical dataset from 50 samples of pleistocene glacial till matrix including three lacustrine samples representing four sediment provenances collected from across minnesota usa such a dataset is useful both in the context of regional studies of glaciation and in efforts to understand how provenance and glacial processes impact the geochemistry of sediment archives the principal sediment sources of the four provenances include materials from the archean proterozoic canadian shield the proterozoic midcontinent rift paleozoic carbonates and cenozoic and mesozoic clastic sediments we measured major element geochemistry in all till samples in both

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

iron isotopes constrain biogeochemical redox cycling of iron and manganese in a palaeoproterozoic stratified basin abstract the hotazel formation in the uppermost stratigraphic portion of the neoarchaean palaeoproterozoic transvaal supergroup of southern africa is an unusual sedimentary sequence of banded iron formation bif intercalated with three manganese rich layers as such it is a succession that holds great potential to offer a unique view of one of the most dramatic transitions in early earth history the switch to a full oxidative cycle in shallow oceans at ca 2 3 ga we present iron isotope results from bif and mn rich samples collected across the entire hotazel sequence with a view to constraining processes of biogeochemical redox cycling for both metals close to the transition from a reducing to an oxidizing ocean atmosphere system the recorded de coupling of fe and mn reduction during anaerobic organic carbon cycling in the hotazel strata suggests that manganese became an important electron acceptor in stratified marine environments of the palaeoproterozoic during periods of increased primary manganese precipitation relative to iron very low 57fe values registered across the entire hotazel sequence and especially in manganese rich samples 2 4 to 3 5 signify deposition of iron and manganese in a terminal stratified aqueous reservoir that was depleted in the heavy iron isotopes these isotopic signatures in conjunction with the unusual endowment of the hotazel sequence in manganese are interpreted to have evolved by rayleigh distillation processes during protracted deposition of mn poor bifs as preserved in the lower stratigraphic portion of the transvaal supergroup kuruman and griquatown bifs the unique end member geochemical and isotopic characteristics of the hotazel rocks may therefore constitute a potential link between the widespread deposition of bif during the neoarchaean and palaeoproterozoic and the postulated rise in atmospheric oxygen levels around 2 3 ga ago

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

a 3 800 million year isotopic record of life from carbon in sedimentary rocks an increased ratio of 12c to 13c an indicator of the principal carbon fixing reaction of photosynthesis is found in sedimentary organic matter dating back to almost four thousand million years ago a sign of prolific microbial life not long after the earth s formation partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

pixe and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2 2ga ongeluk formation south africa implications for ancient seawater salinity abstract the ongeluk formation mainly consists of submarine volcanics pillow lavas and sheet flows composed of basaltic andesites that are between underlying and overlying glaciogenic deposits i e the makganyene diamictite and a dropstone layer at the base of the hotazel formation respectively the stratigraphic position of the ongeluk formation indicates that the ongeluk volcanism occurred during a period of global glaciation the ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids the hydrothermal quartz contains many primary type 1 and secondary type 2 liquid vapor fluid inclusions as well as inclusions that are randomly distributed without a trace of secondary healed cracks type 3 all these fluid inclusions types were individually analyzed with microthermometry and particle induced x ray emission pixe methods the results show that type 1 fluid inclusions are highly saline whereas type 2 fluid inclusions are relatively less saline type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of type 1 and type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types among the various fluid inclusion salinities of fluid inclusions a wide range in na ca values was identified in the high salinity fluid inclusions i e type 1 and a subset of type 3 which are thought to represent subseafloor fluids circulated by the ongeluk submarine volcanism the wide range in na ca values can be explained by a mixing process between na rich and ca rich fluids associated with albitization of the host basaltic andesites in the albitization process na is removed from the fluids and fixed by the host rocks by contrast ca is released from the host rocks into the fluids pixe analysis also showed two distinct trends i e vertical and horizontal on variation diagram of ca cl versus cu cl and positive correlations between mn cl cu cl zn cl and pb cl these data are best explained by the presence of other mixing processes between 1 a na rich ca and cu poor fluid 2 a ca rich na and cu poor fluid and 3 a na and cu rich ca poor fluid the cu mn zn and pb rich fluid likely represents a high temperature hydrothermal fluid from a deep reaction zone in the ongeluk subseafloor hydrothermal system by contrast the 1 na and k rich ca and cu poor endmember for the high salinity primary inclusions is considered to represent the composition of 2 2 ga ongeluk seawater we propose the estimation of 2 2 ga ongeluk seawater composition is 2237 mmol kg of na 200 1000 mmol kg of k

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

atmospheric circulation associated with persistent generalized frosts in central southern south america abstract this paper describes the large scale atmospheric circulation associated with persistent generalized frosts gfs at least 75 of the stations report frosts in the east central region of argentina known as the wet pampa the gf events are grouped according to their persistence and ncep ncar reanalysis data are used to create daily composites of mass and wind field anomalies during the 1961 90 winters the gfs are caused by an anticyclonic anomaly that enters south america generating southerly wind anomalies and cold air advection that are strengthened by the meridional layout of a cyclonic anomaly over the south atlantic ocean in the case of the more persistent events the wind anomaly grows during the previous days and becomes quasi stationary also the study identifies at 250 hpa a double train of eastward moving rossby waves along the subtropical and subpolar latitudes respectively of the southern hemisphere the layout of both wave trains favors the development of an intense southerly wi

correlation of the stratigraphic cover of the pilbara and kaapvaal cratons recording the lead up to paleoproterozoic icehouse and the goe abstract the turee creek basin of the pilbara craton is critical for timing onset of the early paleoproterozoic icehouse and the goe we establish correlation to the koegas postmasburg supersequence of the kaapvaal craton based on a common compressional basin with the pilbara succession deposited near the orogen and the kaapvaal succession near the flexural arch whereas turee creek cyclicity was controlled by tectonic subsidence glacioeustatic influence is discernible in the koegas postmasburg supersequence key features of the turee creek basin are the mid kungarra formation glaciogenic meteorite bore member unconformity bound sequences above the kungarra formation and the basaltic bubbawalyee formation conformably above quartzite 3 on the kaapvaal craton the equivalent glaciogenic unit makganyene formation overlying the sedimentary koegas subgroup and underlying the basaltic ongeluk formation is unconformity bound we correlate the sub ongeluk unconformity with the sub quartzite 3 unconformity and propose tectonostratigraphic equivalence of the ongeluk and bubbawalyee formations there are no equivalents of the post ongeluk hotazel and mooidraai formations above the bubbawalyee formation on the pilbara craton which is significant because the hotazel formation provides the earliest evidence for the goe consequently rather than spanning the goe the turee creek basin provides a chemostratigraphy during its eve all carbonate sediments deposited before and after the first global glaciation meteorite bore makganyene yield carbon isotope values close to 0 establishing that the first carbon isotope excursion occurred long after closure of the turee creek koegas postmasburg basin and onset of the goe it therefore appears that the carbon cycle responded to rather than triggered the goe

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

raindrop sorting induced by vertical drafts in convective clouds evidence of raindrop sorting induced by a convective updraft is presented using a vertically pointing 94 ghz doppler radar 3 2 mm and capitalizing on the resonant nature of the backscattering cross section as a function of the raindrop size mie scattering the vertical air motions to an accuracy of 0 1 ms 1 and the shape of the raindrop size distribution are retrieved from the doppler spectra the interaction of vertical drafts and raindrops is documented for the first time by high resolution radar data the updraft structure clearly causes horizontal and vertical sorting of the raindrops in the updraft core small raindrops d 1 7 mm that have terminal velocities less than the updraft velocities 6 7 ms 1 and a clear absence of drops 3 mm are observed towards the updraft periphery a gradual increase in the raindrop sizes is documented where large raindrops d 3 mm are observed the observations demonstrate the importance of updrafts in distributing the raindrops in space

shrimp u pb zircon ages for the palaeoproterozoic kuruman iron formation northern cape province south africa evidence for simultaneous bif deposition on kaapvaal and pilbara cratons abstract stilpnomelane rich tuffaceous mudrocks are an important component of the palaeoproterozoic kuruman and griquatown iron formations in the northern cape province they document volcanic activity during the deposition of these iron formations and the zircon populations that they contain record syn depositional volcanism resedimentation and post depositional isotopic disturbance the trace element signatures of tuffaceous samples suggest that although zircons are felsic volcanogenic in origin mixing of felsic ash with basaltic ash and or muds in the water column was also likely in addition these rocks record minor terrigenous input and some samples indicate mixing with iron rich clays oxides and smectites moreover they reflect a shift from mafic volcanism e g mafic tuffs in the underlying gamohaan formation towards more felsic volcanism that is a trend also recorded for tuffaceous rocks in the brockman iron formation the best estimate of the depositional age for the top of the riries member and hence the kuruman iron formation is 2460 ma where most of the zircon populations were derived from volcanism coeval with bif deposition ages obtained in this study that are older than the 2465 5 ma age for the lower riries member probably represent resedimented pyroclastic deposits i e give eruptive ages from earlier eruptions that contain an unimodal zircon population and as such only provide maximum depositional ages although a compacted sedimentation rate csr for the entire kuruman iron formation is 3 4 m per million years a csr of about 22 m per million years is deduced for the riries member suggesting that there are cryptic gaps within the kuruman stratigraphic record the latter is comparable to the csr of 33 m per million years calculated for hamersley province bifs ages of the tuffaceous horizons in the kuruman iron formation are comparable to those from the brockman iron formation of the hamersley province the 2460 5 ma age for the top of the riries member corresponds to the 2459 3 ma age for the base of the joffre member and the 2463 5 ma whaleback shale member of the brockman iron formation although ca 2468 2478 ma ages reflect detrital or syn depositional zircon populations they are comparable to the 2470 4 ma s13 and the 2479 3 ma s9 macrobands of the dales gorge member brockman iron formation moreover the zircon age spectra and pb loss histories of both the kuruman and brockman iron formation tuffaceous sediments are strikingly similar without doubt the kuruman and brockman iron formations were deposited synchronously whether they were a response to global magmatic events or were deposited in the same basin are questions still being debated

depositional age of the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa and implications for low level re os geochronology and paleoproterozoic global correlations abstract we present new re os isotope data from the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa to further evaluate low level re os geochronology of organic rich sedimentary rocks and improve global correlations of paleoproterozoic sedimentary rocks the klipputs member contains re 0 3 0 8 ppb total os 34 177 ppt and common os 192 os 10 66 ppt abundances that overlap upper crustal abundances of 0 2 2 ppb 30 50 ppt and 12 21 ppt respectively five samples yield a model 1 re os date of 2479 20 ma 22 ma if the 187 re decay constant uncertainty is included 2 mean square of weighted deviates mswd 0 01 using the cr vi h 2 so 4 digestion protocol dissolution of the same sample powders using inverse aqua regia yields a model 3 re os date of 2862 190 ma 2 mswd 148 comparison of re os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital os whereas the cr vi h 2 so 4 medium did not the re os date of 2479 22 ma for the klipputs member is consistent with published geochronological ages from underlying and overlying stratigraphic units suggesting the cr vi h 2 so 4 protocol yields accurate re os depositional ages from ors containing very low levels of hydrogenous re and hydrogenous radiogenic os hence there is potential for widespread application of low level re os ors geochronology including sedimentary successions deposited before the great oxidation event the initial 187 os 188 os 0 12 0 01 from the klipputs member cr vi h 2 so 4 regression indicates that dissolved os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and or cosmic dust rather than oxidative weathering of the radiogenic upper crust in light of the re os depositional age for the klipputs member we propose that the stratigraphic units in the upper hamersley group above the brockman iron formation western australia are younger than the ghaap group diamictite from the doradale formation in the ghaap group is constrained to be older than 2479 22 ma and thus may not be correlative with glacial deposits assigned to the oldest paleoproterozoic glaciation

sedimentological evaluation of pb zn exploration potential of the precambrian griquatown fault zone in the northern cape province south africa the griquatown fault zone gfz is a major target for pb zn exploration in south africa the sedimentary structural and thermal history of the fault zone are evaluated the fault zone experienced a synsedimentary period of activity between 2550 and 2500 ma and a major post postmasburg group less than 2223 ma episode of mainly vertical movements possible source rocks for generation of metalliferous brines are abundant along the southwestern margin of the kaapvaal craton and shales southwest of the gfz are time correlative to a thick peritidal stromatolitic carbonate sequence in the northeast fluids driven by compaction and orogenic pressure migrated across the gfz via the carbonates towards the east metamorphic overprint south of the gfz based on illite crystallinity and fluid inclusions north of the fault zone are above the oil window metamorphic peaks south of the fault zone are at 1750 ma and 1213 ma and the intensity of metamorphic overprint decreases from west to east because of high temperatures of metamorphic and orogenic overprint and possible remobilisation of fluids mississippi valley type mvt ore deposits are unlikely to be found within the griquatown fault zone but are expected rather to the northeast of it higher temperature remobilised vein related deposits could however occur in the gfz itself

u pb detrital zircon geochronology of the turee creek group hamersley basin western australia timing and correlation of the paleoproterozoic glaciations the early paleoproterozoic turee creek group tcg hamersley basin western australia records a continuous siliciclastic dominated sedimentary sequence across the great oxidation event goe and contains at least two glacial horizons showing many similarities with other paleoproterozoic glaciogenic deposits from north america huronian supergroup and south africa transvaal supergroup the correlation between these glacial events is uncertain however because of a lack of geochronological constraints contradictory stratigraphic correlations among di ff erent cratons and the occurrence of numerous sedimentary discontinuities in the transvaal and huronian successions in this study we performed in situ u pb dating of detrital zircons from drill core and surface samples of the tcg the underlying boolgeeda iron formation of the hamersley group and the overlying beasley river quartzite of the lower wyloo group over 1500 zircons analyzed by laser ablation coupled to icp ms yielded major age peaks spanning the archean eon at ca 2 45 ga 2 54 ga 2 68 ga and minor ones at 2 81 ga 2 96 ga and 3 14 ga a younger age of 2340 22 ma obtained at the base of the main glacial deposit of the meteorite bore member represents a maximum age for this glacial event a meter scale level of diamictite has previously been described in the boolgeeda formation at the boundary ridge locality located about 50 km to the east of the meteorite bore member succession and a similar horizon was newly identi fi ed in drill core samples from the hardey syncline area the youngest detrital zircons have an age of ca 2 45 ga in these dia mictites supporting the hypothesis that these horizons represent a di ff erent and older glacial event than the meteorite bore member considering that the tcg and underlying boolgeeda iron formation are in sedimentary continuity these results allow establishing a correlation with the other glaciogenic deposits recorded in the north america and southern africa basins and reconstructing the sequence of redox and climate changes at tending the early paleoproterozoic

the geology and geochemistry of the palaeoproterozoic makganyene diamictite abstract the palaeoproterozoic earth experienced a global glacial event at 2400 ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans the transvaal supergroup in the griqualand west basin south africa hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes the focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic makganyene formation in order to constrain its palaeoenvironmental settings the makganyene formation forms the base of the postmasburg group and has been regarded as resting on an erosive regional unconformity throughout the northern cape province systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal the makganyene formation is in fact conformable with underlying formations of the koegas subgroup in the deep southern prieska basin and rests on an unconformity only on the shallow ghaap platform to the north east the makganyene formation displays lateral facies changes that reflect the palaeogeography of the study area and the advance and retreat of ice sheets shelves geochemical investigations of glacial strata of the makganyene formation demonstrate that underlying banded iron formations of the transvaal supergroup acted as the main clastic source for the diamictite detritus geographic variations in bulk composition of the diamictites correlate well with field observations and show that sorting processes were controlled largely by the morphology of the palaeobasin carbon isotope results emphasize the transitional nature of the makganyene formation in terms of the environmental conditions that resulted in widespread global glaciation in the palaeoproterozoic on the basis of the above geological evidence it is proposed that the transvaal supergroup in the northern cape province represents a continuous depositional event that lasted approximately 250 ma and hence provides a unique opportunity for assessing the transitional changes experienced by the palaeoproterozoic earth

from bif to red beds sedimentology and sequence stratigraphy of the paleoproterozoic koegas subgroup south africa abstract the 2 4 ga old koegas subgroup of the transvaal supergroup contains siliciclastics and iron formations it occupies a transitional stratigraphic position between the major iron formations on the kaapvaal craton and strata with evidence for oxygenation of the environment koegas siliciclastics reflect a new phase of uplift and erosion on the craton after a period of chemical deposition paleocurrent data indicate transport to the west and northwest away from the craton however no precise source area can as yet be defined rocks were deposited in foreshore to offshore environments of a prograding delta or submarine fan system immature arkosic wackes and subarkoses occur in proximal parts and pass to terrigenous mudstones and chemical iron formations in offshore environments with low detrital input lateral facies transitions and the hierarchical stratigraphic stacking pattern of the koegas subgroup allow a sequence stratigraphic interpretation in which regressive coarse siliciclastics prograded repeatedly over transgressive units of chemical iron formations metals in the transgressive units have an intrabasinal hydrothermal source similar to underlying iron formations localized iron stained detrital units resemble red beds they reflect additional diagenetic fe remobilization from deeper water iron formations or possibly a continental fe source the second possibility would imply the presence of some free oxygen in the environment

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

carbonate petrography kerogen distribution and carbon and oxygen isotope variations in an early proterozoic transition from limestone to iron formation deposition transvaal supergroup south africa the transition zone comprises campbellrand microbialaminated replacing cryptalgalaminate limestone and shale with minor dolomite conformably overlain by the kuruman iron formation of which the basal part is characterized by siderite rich microbanded iron formation with minor magnetite and some hematite containing units the iron formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones the sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees c and pressures of 2 kbars carbonate minerals in the different rock types are represented by primary micritic precipitates now recrystallized to microsparite early precompactional sparry cements and concretions deep burial limpid euhedral sparites and spar cements precipitated from metamorphic fluids in close contact with diabase sills paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them kerogen occurs as pigmentation in carbonate crystals as reworked organic detritus in clastic textured carbonate units and as segregations of kerogen pigment around late diagenetic carbonate crystals locally kerogen may also be replaced by carbonate spar carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies integration of sedimentary petrographic geochemical and isotopic results makes it possible to distinguish between depositional early diagenetic deep burial and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence major conclusions are that deep burial thermal decarboxylation led to 13c depletion in euhedral ferroan sparites and 13c enrichment in kerogen organic carbon metamorphic sparites are most depleted in 13c carbonates in oxide rich iron formations are more depleted in 13c than those in siderite rich iron formation whereas the kerogens in oxide banded iron formations bif are more enriched this implies that the siderite rich iron formations were not derived from oxide rich iron formation through reduction of ferric iron by organic matter organic matter oxidation by ferric iron did however decrease the abundance of kerogen in oxide rich iron formation and led to the formation of isotopically very light sparry carbonates siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13c composition with the siderites depleted in 13c by 4 6 per mil on average relative to calcmicrosparite this means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13c by about 9 per mil relative to that from which the limestones precipitated this implies an ocean system stratified with regard to total carbonate with the deeper water from which siderite rich iron formation formed depleted in 13c iron formations were deposited in areas of very low organic matter supply depletion of 13c may therefore derive not from degradation of organic matter but from hydrothermal activity a conclusion which is supported by 18o composition of the carbonate minerals and trace element and rare earth element ree compositions of the iron formations

the late archean to mesoproterozoic major unconformity bounded units of the kaapvaal province of southern africa reply abstract the late archean to mesoproterozoic 3 1 1 4 ga sedimentary and volcanic successions in the kaapvaal province of southern africa previously have been subdivided into numerous lithostratigraphic units that generally were believed to have been deposited in local or regional basins however these lithostratigraphic units are remnants of thirteen unconformity bounded sequences or units 10 3 m thick with composite strike lengths of 250 1100 km and with durations of 10 7 to 10 8 yr all of these units rest somewhere on the crystalline basement the stratotypes but not the full geographic extents of these sequences from the base upward are the dominion group west rand group of the witwatersrand supergroup central rand group of the witwatersrand supergroup klipriviersberg group of the ventersdorp supergroup platberg group of the ventersdorp supergroup pniel group of the ventersdorp supergroup black reef quartzite formation and chuniespoort group of the transvaal supergroup pretoria group of the transvaal supergroup and the interval represented by the dullstroom formation rooiberg group and the loskop formation three or more major unconformity bounded units occur in younger proterozoic rocks in the kaapvaal province but are not definitive of it two occur in the soutpansberg group and one is most of the waterberg group some of these unconformity bounded units are remarkably similar to phanerozoic cratonic and miogeoclinal sequences they are the natural building blocks for reconstructing archean and proterozoic depositional basins plate tectonic events continents and a provincial time scale

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

pb o and c isotopes in silicified mooidraai dolomite transvaal supergroup south africa implications for the composition of paleoproterozoic seawater and dating the increase of oxygen in the precambrian atmosphere abstract the mooidraai dolomite formation is a unit of marine sedimentary carbonates in the upper transvaal supergroup which recorded significant changes in the composition of the earth s atmosphere and ocean previously available data had suggested that this dolomite was about 2 2 ga old and showed 13ccarb values around 0 8 pdb and hence was an exception to the positive excursion of 13ccarb values observed worldwide in marine carbonates deposited between 2 25 and 2 05 ga ago we studied the pb o c isotope systematics of drill core samples from a highly silicified and aluminosilicate free sub unit of the mooidraai dolomite formation that yields well preserved micritic dolomite grains selective leaching of the silicified dolomite revealed a significant pb isotopic contrast between the carbonate and the quartz fraction the former shows only a narrow range of 208pb 204pb 35 35 35 58 which does not correlate with 206pb 204pb indicating the absence of th derived detrital pb the carbonate bound pb defines an isochron that corresponds to an age of 2394 26 ma 2 mswd 1 8 n 22 which we interpret as the diagenetic age of the mooidraai dolomite the 13ccarb values range from 0 51 to 0 64 pdb and confirm the previous data the 18ocarb values range from 2 08 to 0 18 pdb and are amongst the heaviest reported yet from early precambrian marine sedimentary carbonates the quartz fraction is considerably lighter than the carbonate fraction and shows 18oqtz values between 9 22 and 6 73 pdb 21 40 and 23 97 smow together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 c this indicates that the intense silicification was post depositional comparison of 18ocarb values of silicified and non silicified mooidraai samples suggests that the post depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite the heavy 18ocarb values suggest that 2 4 ga ago the oxygen isotopic composition of seawater differed only little from that of today the new pb pb carbonate age for the mooidraai dolomite indicates that the upper transvaal supergroup is about 200 my older than previously thought this explains the normal carbon isotopic composition of the mooidraai dolomite and suggests that the pronounced increase of the oxygen content in the precambrian earth s atmosphere that is seen in strata underneath the mooidraai dolomite occurred before 2 4 ga ago on this background it appears unlikely that the positive excursion of the carbon isotopic ratios of 2 25 2 05 ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen

discovery of the huronian glaciation event in china evidence from glacigenic diamictites in the hutuo group in wutai shan abstract paleoproterozoic glacigenic diamictites have provided key evidence for the great oxidation event goe at 2 3 ga associated with the widespread huronian glaciation event hge here we report for the first time paleoproterozoic glacigenic diamictite from the sijizhuang formation of the hutuo group in wutai shan china formed during the late stage of hge the sijizhuang formation comprises diamictite sandstone and siltstone mudstone facies conforming to a glaciomarine association the diamictites show weak sorting and roundness and carry a few striated and faceted boulders typical dropstones and associated structures are present in the sandstone and siltstone mudstone facies diamictite facies which characterizes the lower portion of the sijizhuang formation displays an upward decrease with a concomitant increase in the proportions of sandstone and siltstone mudstone facies the lithofacies associations of diamictite with sandstone and siltstone mudstone indicate a frequently changing hydrodynamic environment the petrochemical compositions of different lithofacies show that the clasts were multi sourced and generated by limited weathering in keeping with a cold climate available isotope age data constrain the glacigenic deposition of the sijizhuang formation to between 2 40 and 2 13 ga and most likely from 2 2 to 2 1 ga this estimate compares well with the 2 4 2 1 ga age of the widespread hge but the sijizhuang diamictite appears to be younger than the last glacial unit gowganda formation in the type area of the hge in canada and suggests a locally protracted glacial event

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

facies dependent δ13c variation from a cryogenian platform margin south australia evidence for stratified neoproterozoic oceans abstract analysis of a cryogenian interglacial platform margin in the adelaide geosyncline reveals a strong carbonate 13 c facies relationship detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 in time equivalent strata shallow water back reef facies have 13 c values up to 8 2 while equivalent basinal sediments have 13 c values between 3 and 0 allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy 13 c values 4 9 and are surrounded by basinal calcareous shales with light 13 c values ave 0 8 the regional and stratigraphic consistency of these 13 c trends suggests a primary marine origin we interpret this 13 c facies correlation to be the result of ocean stratification stagnation that significantly reduced the rate of deep ocean ventilation and produced a large deep water organically derived carbon reservoir we suggest that stratification may have been a persistent feature of the global ocean throughout much of neoproterozoic time ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era including large scale 13 c variation extreme climatic fluctuations and the presence of cap carbonates a highly variable climatic regime would be expected with the development of a large deep water carbon reservoir small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface ocean and atmospheric reservoirs leading to intense greenhouse or icehouse conditions respectively

stratigraphic and structural setting of the griqualand west and the olifantshoek sequences at black rock beeshoek and rooinekke mines griqualand west south africa the black rock to rooinekke area has been influenced by the kheis and namaqua orogenies thrusting associated with the kheis orogeny is well documented throughout the mining area thrusting at rooinekke resulted in the griqualand west sequence rocks overlying parts of the olifantshoek sequence at black rock lithologies of the voelwater subgroup have been thrusted onto the olifantshoek sequence a prominent n s trending graben structure has been observed in the rooinekke beeshoek and black rock mining areas the thrusts are disrupted by these n s faults at rooinekke supergene replacement and enrichment of the iron ore is associated with pre olifantshoek n s faulting the namaqua orogeny may also have played a part in upgrading the existing ore bodies under discussion

the sequence stratigraphic concept and the precambrian rock record an example from the 2 7 2 1 ga transvaal supergroup kaapvaal craton abstract sequence stratigraphic concepts are applied to the 2 7 2 1 ga transvaal supergroup which constitutes the sedimentary floor to the bushveld igneous complex the transvaal succeeds the foreland fill of the witwatersrand supergroup and in its lowermost portions is partly synchronous with the ventersdorp supergroup lavas that followed upon the witwatersrand sediments in the stratigraphic record the unconformable contacts at the base and the top of the transvaal supergroup mark significant changes in the overall tectonic setting which qualifies them as first order sequence boundaries sedimentation within the transvaal basin was controlled by cycles of extensional and or thermal subsidence resulting in the accumulation of sequences separated by stages of base level fall generating sequence bounding subaerial unconformities the transvaal first order sequence is subdivided into five second order unconformity bounded depositional sequences i e the protobasinal black reef chuniespoort rooihoogte timeball hill and boshoek houtenbek sequences the span of time of these second order sequences averaging 130 ma is much longer than the duration of the second order phanerozoic cycles which is explained by the less evolved and thus slower plate tectonic processes that dominated the late archaean early proterozoic times each transvaal second order sequence preserves a variable number of systems tracts mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities in a complete succession a sequence would include a basal lowstand systems tract lst followed by transgressive tst highstand hst and falling stage fsst systems tracts the protobasinal and rooihoogte timeball hill sequences only preserve the lst and tst due to the strong post depositional erosion the black reef sequence is composed of lst tst and hst the chuniespoort sequence preserves the tst hst and fsst but lacks the lst due to ravinement erosion the boshoek houtenbek sequence develops the complete succession of systems tracts common features may be emphasized between the same types of systems tract developed within different sequences the lst includes high energy coarse prograding facies such as alluvial fans and fan delta systems that fill the irregularities of the pre existing topography and lead to the peneplanation of the depositional areas the tst may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments the hst marks the normal regression of the previous marine environments which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework the fsst would normally include only basinal facies age equivalent with the correlative subaerial unconformities in our case study the stages of base level fall resulted invariably in fully nonmarine environments within the transvaal area generally leading to the development of subaerial unconformities in particular circumstances the nonmarine environment may still accumulate and preserve fluvial alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base levels the interpretations of base level rise and fall resulting from the application of the sequence stratigraphic concept to the 2 7 2 1 ga transvaal supergroup enable estimation of sea level changes during deposition of this succession with an understanding of local tectonic conditions on the kaapvaal craton and of coeval sedimentation elsewhere in africa eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the transvaal supergroup

petrography and geochemistry of the paleoproterozoic hotazel iron formation kalahari manganese field south africa implications for precambrian manganese metallogenesis new data are presented concerning the petrography and geochemistry of the paleoproterozoic hotazel iron formation of the kalahari manganese field south africa mineralogically and texturally the hotazel iron formation is very similar to paleoproterozoic banded iron formations of the superior type comprising laminae of chert iron oxides magnetite hematite iron silicates greenalite minnesotaitc stilpnomelane riebeckite fe mica carbonates calcite ankerite minor siderite and pyrite the major chemical constituents of the hotazel rocks are sio 2 fe oxides cao and mgo whereas mno al 2 o 3 na 2 o and k 2 o contents are negligible below 1 wt elements of detrital derivation ti zr rb and transition metals such as zn cu ni co and v are present in low concentrations on the order of a few tens of parts per million stratigraphically higher the hotazel banded iron formation shows a conspicuous increase in cao contents that is coupled with a subtle decrease in bulk rock fe number ratios fe super 2 fe super 2 fe super 3 implying a gradual transition to a more oxidizing environment shale normalized rare earth element ree patterns are similar to those of older banded iron formations of the transvaal supergroup exhibiting light ree depletion and negative ce anomalies comparable to modern shallow level seawater occasional weak positive eu anomalies in iron silicate rich rocks indicate minor admixtures of a hydrothermal component below the chemocline the similarities between the hotazel iron formation and most superior type banded iron formations the lack of transition metal enrichments and the absence of strongly positive eu anomalies in the hotazel rocks challenge volcanogenic exhalative models proposed previously for the genesis of the interbedded kalahari mn deposits