u pb zircon age for a tuff in the campbell group griqualand west sequence south africa implications for early proterozoic rock accumulation rates an ion microprobe u pb age of 2552 11 ma has been obtained on zircon separated from a regional banded tuff horizon in the nauga formation beukes 1980b upper campbell group griqualand west sequence south africa this age permits time constraints to be placed on lithologically correlated units within the adjacent transvaal sequence and correlations to be made with the further removed hamersley group in australia calculated rock accumulation rates of 2 to 4 m m y for these strata of mainly shale and banded iron formation suggest that sedimentation rates were significantly slower in the late archean early proterozoic than is generally assumed

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

pore structure and fractal characteristics of organic rich shales a case study of the lower silurian longmaxi shales in the sichuan basin sw china abstract shales from the lower silurian longmaxi formation in the sichuan basin are among the most important shale gas reservoirs in china and have been investigated because of their great shale gas potential to understand the pore structure and fractal characteristics of the shales a series of experiments was conducted on core samples from the lower silurian longmaxi formation in the sichuan basin of china including x ray diffraction xrd total organic carbon toc content and vitrinite reflectance ro analysis field emission environmental scanning electron microscope fe esem observation and low pressure n 2 adsorption desorption experiments frenkel halsey hill fhh method was applied to calculate fractal dimensions in addition the pore genesis the relationships between composition and thermal maturity the pore structure parameters and the fractal dimensions are discussed fe esem observation results show that the longmaxi formation shales are dominated by organic matter om pores along with interparticle interp pores intraparticle intrap pores and fracture pores this study identified the fractal dimensions at relative pressures of 0 0 45 and 0 45 1 as d 1 and d 2 respectively d 1 ranged from 2 60 to 2 71 and d 2 ranged from 2 71 to 2 82 d 1 was typically smaller than d 2 indicating that the smaller pores in shales were more homogeneous than the larger ones the formation of these om pores is owing to kerogen deformation during the thermal maturation which results in a large number of nanopores the pore structure of the longmaxi formation shales is primarily controlled by toc content and thermal maturity toc content is a controlling factor on the fractal dimensions as it exhibited positive correlations with d 1 and d 2 fractal dimensions are useful for the characterization of the pore structures complexity of the longmaxi formation shales because d 1 and d 2 correlate well with pore structure parameters as they both increase with the increase of surface area and the decrease of average pore diameter

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

carbon and oxygen isotope systematics of a paleoproterozoic cap carbonate sequence from the sausar group central india abstract well preserved sedimentological features of the paleoproterozoic age sausar group of central india indicate glaciogenic origin for the diamictite in the lower part of the unit deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap carbonate followed by fine clastics with manganese ore analyses of 13 c and 18 o contents in the carbonate unit overlying the diamictite together with sr and ba contents and ree data indicate the preservation of primary geochemical signatures the average 13 c carb content ranges between 3 1 and 0 1 v pdb having peak negative 13 c excursions of up to 7 4 v pdb similar to paleoproterozoic and neoproterozoic cap carbonates elsewhere high sr and ba contents indicate the presence of aragonite and barite precursors similar to well studied neoproterozoic cap carbonates a single continuous carbonate unit shows 13 c excursions from 4 4 to 2 6 v pdb the lithological association and chemostratigraphy are comparable with the carbonate tillite association of the huronian supergroup of canada the snowy pass supergroup of the usa the transvaal supergroup of south africa and the turee creek group of australia

petrography and geochemistry of sandstones interbedded with the rooiberg felsite group transvaal sequence south africa implications for provenance and tectonic setting abstract the 2150 ma rooiberg felsite group transvaal sequence contains thin sandstone interbeds within its 3500 5000 m thick volcanic succession whereas uppermost feldspathic arenites and wackes are thought to represent reworked felsitic material quartz arenites and subordinate lithic sandstones present throughout most of the rooiberg sequence comprise mainly reworked sedimentary detritus probably belonging to the underlying pretoria group sandy braided river systems probably transported clastic material into the basin from relatively stable source areas subject to intense chemical weathering geochemistry and sandstone petrography indicate that mixing of pretoria group detritus with sediment derived from erosion of felsitic material occurred within the basin during late rooiberg ti es the inferred hiatuses in volcanism represented by the predominant siliceous sedimentary interbeds appear to have been of relatively short duration and occur throughout much of the rooiberg stratigraphy upper arkosic sandstones indicate longer breaks in volcanism as rooiberg eruptions came to an end the sandstones provide evidence compatible with an impact origin for the rooiberg felsite group and for its successor the bushveld complex

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

the conformable emplacement of the bushveld mafic rocks along a regional unconformity in the transvaal succession of south africa abstract a previously unrecognized regional unconformity occurs in the proterozoic transvaal succession of the kaapvaal province of southern africa the predominantly marine and siliciclastic rocks of the lower two thirds of the pretoria group are unconformably overlain by a terrestrial and largely volcanic sequence the most voluminous portion of which is the rooiberg felsite if the bushveld complex is mentally subtracted from map patterns the rooiberg felsite also locally overlies pre pretoria sequences and the archean crystalline basement the bushveld complex is not lopolithic as commonly supposed instead regionally condordant emplacement of the complex along the unconformity explains both the seemingly discordant footwall features and the lack of pretoria and older strata other than a few xenoliths in the hanging wall the lack of any geochemical evidence for major crustal contamination of bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation the bushveld complex the unconformity and the enclosing strata are synformally folded making the bushveld appear to be lopolithic due to this folding and multiple periods of erosion the original extents of the bushveld complex the unconformity and the enclosing strata are now much reduced other smaller mafic intrusions along or immediately below the same regional unconformity probably are bushveld related

palaeomagnetism of the upper zone of the bushveld complex abstract palaeomagnetic data were acquired from eight sampling sites situated in the upper zone of the bushveld complex transvaal south africa specimens were subjected to alternating field and thermal demagnetization a mean magnetization direction yielding a palaeomagnetic pole at 16 1 s 148 5 w was found fold tests indicate that the upper zone had acquired its remanent magnetization with the igneous layering in its present position with this new data on the layered sequence it is possible to divide the mafic portion into three magnetic polarity zones namely mpza mpzb and mpzc

some evidence for the base metal potential of the pretoria group stratigraphic targets tectonic setting and ree patterns the timeball hill and silverton formations of the 2 1 2 3 ga pretoria group have regional lithological associations which are thought to have been favourable for the genesis of stratiform sulphide deposits the observed association of carboniferous and pyritic black shales tuffaceous material stromatolitic carbonates and inferred turbidity current deposits is common in stratiform sulphide deposits of the sedimentary exhalative group massive sulphides in the silverton formation are compatible with a syngenetic brine discharge probably related to deep fracture systems the basal shales of the timeball hill formation are significantly enriched in base metals and ba interlayered tuff beds at this stratigraphic level have pge contents of up to 1 g t the ree geochemistry of pretoria group sedimentary rocks supports hydrothermal activity as an important factor in both stratigraphic units

sedimentary petrography of the early proterozoic pretoria group transvaal sequence south africa implications for tectonic setting abstract sandstone petrography geochemistry and petrotectonic assemblages of the predominantly clastic sedimentary rocks of the early proterozoic pretoria group transvaal sequence point to relatively stable cratonic conditions at the beginning of sedimentation interrupted by minor rifting events basement uplift and a second period of rifting occurred towards the end of pretoria group deposition which was followed by the intrusion of mafic sill swarms and the emplacement of the bushveld complex in the kaapvaal craton at about 2050 ma the latter indicating increased extensional tectonism and incipient continental rifting an overall intracratonic lacustrine tectonic setting for the pretoria group is supported by periods of subaerial volcanic activity and palaeosol formation rapid sedimentary facies changes significant arkosic sandstones the presence of non glacial varves and a highly variable mudrock geochemistry

structure of the bushveld complex from resistivity measurements knowledge of the deep structure of the arc like bodies constituting the bushveld complex south africa is essential not only to understand the emplacement history of this huge layered intrusion but also for success in the exploration for the platiniferous merensky reef and chromitite layers occurring near its base despite many studies the structure of the complex has remained obscure here we describe new insights into the structure and additional geophysical boundary conditions based on a geoelectrical study this has provided a geoelectrical stratigraphy according to which a conductive layer below the granitic rocks of both the eastern and western compartments of the complex is correlated with sedimentary rocks of the transvaal sequence over large areas rather than with the mafic sequence as conventionally assumed a new feature of this model is that the mafic sequence in both compartments occurs as segments of an inward dipping sheet transgressing the layering of the sedimentary rocks into which it intruded the geoelectrical model supports earlier gravity models in that mafic rocks are absent from the central part of the complex but shows that here the bushveld granite directly overlies archaean basement rather than transvaal sequence rocks as previously accepted

the 2 7 2 0 ga volcano sedimentary record of africa india and australia evidence for global and local changes in sea level and continental freeboard abstract the 2 7 2 0 ga volcano sedimentary records of the african indian and australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas concomitant with lowered freeboard carbonate banded iron formation bif platforms characterised the first such event at ca 2 6 2 4 ga africa and australia to 2 7 ga india these earlier globally enhanced sea levels are ascribed to increased mid ocean ridge activity possibly related to breakup of a postulated late archaean southern supercontinent alternatively a transition from global scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the late archaean nelson 1998 earth planet sci lett 158 109 119 both explanations of increased mid ocean ridge activity are compatible with significant early to middle archaean crustal growth armstrong 1981 phil trans r soc london a 301 443 472 with the emergent high freeboard cratons being subjected to aggressive weathering and erosion enhanced continental crustal growth near the archaean proterozoic boundary mclennan and taylor 1982 j geol 90 347 361 related to the development of significant island arc complexes would have resulted in common lowered freeboard enhanced sea level conditions at the passive margins of the southern cratons the diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and or the thermal state of the cratons from ca 2 4 2 2 ga cratons that make up the present day continents of india africa and australia had relatively high continental freeboard and lowered sea levels glacigenic deposits are preserved on the kaapvaal africa singhbhum india and pilbara australia cratons the second broadly defined drowning event at ca 2 15 ga was probably due to post glacial climatic amelioration freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned in india carbonates were more prominent than the siliciclastic sediments including prominent black shales seen in africa and australia

volcanism of the palaeoproterozoic bushveld large igneous province the rooiberg group kaapvaal craton south africa abstract the volcanic rocks of the rooiberg group represent the uppermost unit in the palaeoproterozoic transvaal supergroup and form one of the largest provinces of silicic volcanic rocks in the world although stratigraphically associated with the transvaal basin fill the rooiberg group is petrogenetically linked with the larger bushveld magmatic event for which emplacement was preceded by the extrusion of the vast rooiberg lava flows in the northern part of the kaapvaal craton like many silicic dominated large igneous provinces lips which are increasingly recognised in the rock record the rooiberg group volcanics are intracontinental subaerial and are dominated by voluminous lava flows originally the rooiberg group is inferred to have covered an area of more than 200 000 km2 of which after erosion an area of 50 000 67 000 km2 remains the lava flows form a stratigraphic succession up to 6 km thick and are divided into four formations in ascending order dullstroom damwal kwaggasnek and schrikkloof due to a scarcity of reliable geochronological data the temporal span of the rooiberg group is poorly understood the rooiberg group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 c minor explosive eruptions are represented by pyroclastic rocks and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes the rocks are essentially undeformed and have not been buried so that their original textures are well preserved the bushveld complex and the associated rooiberg group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume this thorough review of the geochronology physical volcanology and geochemistry of the rooiberg group enables construction of a geodynamic model

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

mid crustal granulite facies metamorphism in the central kaapvaal craton the bushveld complex connection abstract the numerous greenstone remnants which occur as inclusions in the archaean gneissic granitoid basement exposed in the core of the vredefort dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three stage metamorphic history 1 in metapelites migmatites and refractory restites were produced in a mid crustal 0 5 gpa m1 anatectic event at temperatures in excess of 900 c in the restites 2 in the migmatites the peak metamorphic assemblages are variably overprinted by a high grade retrograde reaction which involved crystallizing anatectic melts m2 3 both the peak metamorphic assemblages and the retrogression textures are cross cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the vredefort dome and 4 the peak metamorphic assemblages and the m2 retrogression textures are overprinted by new generations of remarkably fine grained post shock high grade phases which crystallised at a pressure some 0 25 gpa lower than that of both the peak metamorphic conditions and the retrogression m3 these data indicate an anticlockwise p t evolution where mid crustal anatexis most probably resulted from the intraplating of bushveld complex related ultramafic magmas at 2 06 ga a period of approximately isobaric cooling and retrogression was followed by the vredefort catastrophe which resulted from the impact of a large meteorite into the terrane at 2 02 ga this shock event halted the natural mid crustal metamorphic progression prior to the terrane cooling below 650 c and resulted in the rapid exhumation of the terrane by some 9 km in the process it provided a window into the deep levels of the central kaapvaal craton not seen elsewhere in the region

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

strontium isotope evidence for preserved density stratification in the main zone of the bushveld complex south africa the bushveld complex formed from two isotopically distinct magmas magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates rocks with ratios of 0 7065 lie below the lower 2 2 km of the main zone ratio 0 7085 whereas those with ratios of 0 7073 lie above it this unit represents a liquid layer intruded into the density stratified magma pile near the base of the chamber the merensky reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals

petrography and geochemistry of a 2000 2200 ma old hematitic paleo alteration profile on ongeluk basalt of the transvaal supergroup griqualand west south africa abstract a 4 m thick paleo alteration profile thought to represent one of the oldest known ferruginized hematitic paleoweathering zones is described from a depth of 720 m in a drill core on the farm drakenstein 263 some 20 km northwest of hotazel in griqualand west south africa the paleo alteration profile is developed on 2240 57 ma ongeluk basalt along the angular unconformity that separates transvaal strata from 2070 95 ma basal red beds of the olifantshoek group basaltic ongeluk lava was subjected to greenschist facies metamorphism represented by a mineral assemblage of fe pumpellyite chlorite albite and quartz before paleosol development the subsequent paleo alteration is characterized by concentration of fe2o3 in the form of hematite iron rich chlorite and abundant sericite the abundance of sericite results in a large excess of k2o relative to the amount of al2o3 that could have been derived from weathering of the parent basalt the origin of the excess potassium is problematical and could have been derived from basin water groundwater or metamorphic fluids field relationships favour introduction from basin water during deposition of overlying olifantshoek red beds elements such as sodium calcium and magnesium were leached from the alteration profile whereas ree became concentrated within it the most interesting aspect of the ree distribution is the preferred concentration of the lighter lanthanides la and nd in the middle part of the alteration profile relative to heavy ree and the development of a marked negative ce anomaly in the paleo weathered material hydrothermal vein alteration took place after deposition of the overlying olifantshoek red beds

voluminous acid volcanism in the busheveld complex a review of the rooiberg felsite the 2 1 b y old rooiberg felsite roofs and is intruded by the mafic layered rocks and granites of the bushveld complex the felsite unit which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300 000 km3 is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types volcanic rocks of more mafic composition occur towards the base of the sequence the rooiberg episode essentially terminated the volcanic activity in the transvaal basin and heralded the emplacement of the bushveld complex

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

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

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

distribution and association of selected trace elements at the rosebery deposit tasmania studies of selected elements in the ores and wall rocks at the rosebery deposit indicate the presence of three elemental suites 1 elements concentrated in the upper portions of the orebody and in the adjacent wall rocks pb zn au ag cd and sb 2 elements concentrated in the lower portions of the orebody and adjacent wall rocks cu bi as and fe and 3 elements dispersed well outside zones of strong mineralization tl and hg these groupings are the result of different physicochemical conditions in the ore forming environment group 1 elements were deposited under lower temperature conditions approximately 250 degrees c in response to either a decrease in temperature or an increase in f sub o 2 group 2 elements deposited at higher temperatures approximately 300 degrees c in response to a decrease in temperature or an increase in ph tl was deposited either by oxidation of h 2 s or by adsorption onto clay minerals in the wall rocks

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

strontium isotopic evidence against magma addition in the upper zone of the bushveld complex abstract sr isotopic data from the main and upper zones of the bushveld complex show that the evolution of the upper zone started with a large influx of magma close to the level of the pyroxenite marker a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites whole rock samples which span the complete stratigraphic succession ca 2100 m above this layer fall on a single rb sr isochron 2066 58ma and hence have a common initial ratio of 0 7073 1 this ratio is significantly lower than those of the main zone ca 0 7085 below the level of the pyroxenite marker the entire upper zone crystallized from a mixed magma which was thoroughly blended before crystallization this magma had an isotopic ratio intermediate between that of the main zone and the added magma which had an initial ratio of ca 0 7067 further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma which is considered improbable hence the layering and mineralogical diversity of the upper zone was produced by internal processes and not produced by magma influxes during crystallization the lithological compositional and isotopic changes at the pyroxenite marker and the petrological coherence of all rocks above this horizon support the placing of the upper zone boundary at this point in the stratigraphy

geochemical evolution of the rooiberg silicic lavas in the loskop dam area southeastern bushveld the rooiberg felsite is the voluminous volcanic precursor to the plutonic episodes of bushveld magmatism in the area north of the loskop dam a 3 520 m thickness of felsite is dominated by flows of dacitic and rhyodacitic lava occasional silicic andesites are developed near the base whereas true rhyolites become overwhelmingly dominant toward the top of the sequence nine major stratigraphic units of lava are present these are usually separated by and sometimes include volumetrically minor pyroclastic and sedimentary deposits whole rock chemical data on 165 lava samples embracing the entire stratigraphy suggest that nonuniform alteration processes have been most effective in the upper units nevertheless field petrographic and chemical criteria enable identification of three felsite magma types a high mg type mgo 1 7 containing low zr and y contents is present only within the lowermost 1 400 m of section a low mg type mgo 2 asymp 1 p 2 o 5 asymp 0 35 type which only occurs in the lower part of the succession the high mg lavas exhibit a systematic eruptive progression from more to less evolved compositions apparently reflecting continuous tapping of successively deeper levels of a normally zoned magma chamber during the early volcanism the low mg lavas were erupted intermittently with the high mg types probably from a separate and much larger chamber pronounced differentiation of this magma is only evident in the youngest volcanic units where several element profiles display conspicuous steplike variations across the prominent ash flow horizons the steps reflect upward enrichments in nb zr and y and corresponding incremental depletions in ti p and sc such trends imply the existence of a discontinuous but reversed compositional zonation in the evolved low mg chamber the high fe ti p lavas were also erupted from the low mg chamber this may indicate the development of an early type of chemical stratification that was markedly different from the compositional layering in the evolved system

near field modeling of the 1964 alaska tsunami the role of splay faults and horizontal displacements near field observations of tsunami waves generated by the mw9 2 1964 alaska earthquake reveal a complex relationship between coseismic slip and the tsunami wavefield in the source area the documen

physicochemical environments for the formation of quartz free manganese oxide ores from the early proterozoic hotazel formation kalahari manganese field south africa physicochemical environments for the formation of quartz free manganese oxide ores from the hotazel formation of the kalahari manganese field northern cape province south africa are discussed quantitatively on the basis of the phase relations in the system mn fe si o and qualitatively in the system ca mn fe si o the phase relations involve major oxide minerals of the ores namely braunite braunite ii hausmannite bixbyite jacobsite and hematite and were constructed under quartz deficient conditions using existing and estimated thermody namic data and mineral assemblages from the ores standard enthalpy delta hm 0 sub f 298 and gibbs free energy delta g 0 sub f 298 values of braunite are estimated to be 1 011 9 or 5 and 937 3 or 5 kcal mole respectively on the basis of published experimental data and predicted entropy and heat capacity the low grade diagenetic to very low grade metamorphic mamatwan type ores consisting mainly of braunite hausmannite jacobsite hematite kutnohorite and calcite are altered by hydrothermal solutions into the high grade high temperature wessels type ores consisting of braunite ii bixbyite hausmannite hematite calcite and andradite with trace amounts of braunite the peak temperature of formation of the high grade ores appears not to have exceeded about 300 degrees to 400 degrees c the ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite jacobsite hematite to that defined by the assemblage bixbyite hausmannite with increasing temperature under hydrothermal quartz deficient conditions the stability of braunite is enlarged to higher temperatures with increasing silica activity a sub sio sub 2 aq because braunite is much less abundant in the high grade wessels type ore than in the low grade mamatwan type ore the maximum a sub sio sub 2 aq in the solutions could be estimated at 10 super 4 to 10 super 3 at 300 degrees to 400 degrees c from the stable assemblage bixbyite hausmannite braunite kutnohorite and to some extent calcite was a major source of ca reacting with silica released from braunite to form braunite ii and andradite

structural history of the southwestern corner of the kaapvaal craton and the adjacent namaqua realm new observations and a reappraisal abstract the rocks along the southwestern margin of the kaapvaal craton were deformed up to 7 times during the early to middle proterozoic the oldest deformation d 1 is recorded in the n s trending uitkoms cataclasite of pre makganyene age 2 24 ga on the craton and interpreted as a bedding parallel thrust it is assumed to be a branch rising towards the surface from a blind sole thrust that initiated early n s trending f folds above it d 2 is represented by mainly n s but also ne sw and nw se trending imbricates and recumbent fold zones ranging in size from small gravity slumps to large tectonic decollements in asbesheuwel bif and the koegas subgroup and is younger than d 1 or equals d 1 in age these age these structures pre date the westerberg dyke sheet intrusion d 3 south verging folds and thrusts are the oldest post matsap deformations just less than 2 07 1 88 ga d 4 are upright to east vergent and n s trending folds deforming all previous structures d 4 post dates the westerberg dyke sheet and probably reactivates n s folds above the earlier sole thrust during renewed e w compression d 5 producing the main nw trending namaqua structures is only very feebly developed in the kheis terrain and absent from the cratonic areas overlain by olifantshoek and older strata i e ne e and se of the marydale high very gentle d 6 e w to ene wsw folds produce culminations and depressions in all nw trending older structures during d 7 the nw se trending doornberg lineament an oblique left lateral wrench and smaller n trending faults such as the westerberg fault developed these and similar but right lateral faults are the last movements along the rim of the craton and occurred around 1 0 ga multiple folding and thrusting with riebeckite mobilization happened prior to namaqua events and resulted inter alia in discernable duplication and thickening of the transvaal supergroup along the southwestern margin of the kaapvaal craton and at least some 130 km into the craton interior this complicates stratigraphic correlation as well as true thickness estimates of bif units in griqualand west and affects the model for the environmental evolution of the ghaap group a structural model of thin skin decoupling at the base of the transvaal supergroup and starting in the middle early proterozoic is proposed

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene