ion probe determinations of the rare earth concentrations of individual meteoritic phosphate grains abstract a new ion probe method for quantitative measurements of the concentrations of all the ree down to the ppm level in 5 20 m spots is presented the first application of the method is the determination of ree abundances in meteoritic phosphates results are shown to be in good agreement with previous inaa and ion probe determinations the merrillites in the st severin amphoterite are richer in ree than the apatites the enrichment factors for various ree range from 2 3 to 14 2 in contradiction with the results of ebihara and honda 1 provided good standards for other mineral phases are found or implanted marker ion techniques are used the method should find a wide range of applications for the study of both terrestrial and extraterrestrial crystals at the microscopic level

the defect microstructure of oxidized mantle olivine from dish hill california the defect microstructure and oxidation state of fe in metasomatized olivine crystals from peridotite xenoliths from dish hill california have been studied by transmission electron microscopy tem and mdssbauer spectroscopy previous petrographic studies suggested that these composite xenoliths were metasomatized under upper mantle conditions mijssbauer data indicate that between i and 60 o of the fe in the olivine crystals is fe3 tem revealed the presence of layers 0 6 nm wide and parallel to 001 of olivine the individual layers do not offset the surrounding olivine periodicities and are interpreted to be laihunite an fe3 derivative of olivine rather than humite group minerals the layers are found in l strong association with dislocations with abundances decreasing rapidly with distance from the dislocation cores 2 at grain boundaries where layers of considerable length decrease in abundance with distance from the grain boundary and 3 in local regions where discontinuous layers possibly formed by homogeneous nucleation are evenly distributed throughout the olivine these patterns suggest hat the layers formed by alteration of olivine tem images from many areas revealed a very fine domain structure l0 to 20 nm in diameter possibly indicating short range order offe3 and vacancies some dislocations are associated with trails of submicrometer sized fluid inclusions that contain iron oxide crystals a few tens of nanometers wide these features may be healed fractures that provided pathways for oxidized fe bearing fluids the association between laihunite and dislocations probably reflects the role ofdislocations as rapid diffusion pathways and as sites for heterogeneous nucleation in olivine that is supersaturated with respect to fe3 the subsequent reactivity rheological and transport properties of olivine may be modified substantially by laihunite layers on dislocations and at grain boundaries

meso archaean and palaeo proterozoic sedimentary sequence stratigraphy of the kaapvaal craton abstract the kaapvaal craton hosts a number of precambrian sedimentary successions which were deposited between 3105 ma dominion group and 1700 ma waterberg group although younger precambrian sedimentary sequences outcrop within southern africa they are restricted either to the margins of the kaapvaal craton or are underlain by orogenic belts off the edge of the craton the basins considered in this work are those which host the witwatersrand and pongola ventersdorp transvaal and waterberg strata many of these basins can be considered to have formed as a response to reactivation along lineaments which had initially formed by accretion processes during the amalgamation of the craton during the mid archaean faulting along these lineaments controlled sedimentation either directly by controlling the basin margins or indirectly by controlling the sediment source areas other basins are likely to be more controlled by thermal affects associated with mantle plumes accommodation in all these basins may have been generated primarily by flexural tectonics in the case of the witwatersrand or by a combination of extensional and thermal subsidence in the case of the ventersdorp transvaal and waterberg wheeler diagrams are constructed to demonstrate stratigraphic relationships within these basins at the first and second order levels of cyclicity and can be used to demonstrate the development of accommodation space on the craton through the precambrian

the volcano sedimentary succession of the archean sodium group ventersdorp supergroup south africa volcanology sedimentology and geochemistry abstract the volcano sedimentary succession of the sodium group in the south western corner of the kaapvaal craton rests unconformably on basement granites and is usually correlated with the middle ventersdorp supergroup this neoarchean 2700 ma large igneous province lip has been interpreted as deposited in a tectonic escape basin scenario following the collision of the zimbabwe and kaapvaal cratons or alternatively in a mantle plume induced cratonic rift basin the basaltic komatiitic to andesitic dacitic volcanic sequence and the accommodated sedimentary deposits developed in a graben to half graben setting within a cratonic failed rift system the sodium group sedimentary facies and geochemistry of volcanic rocks correlate well with the ventersdorp supergroup the major and trace element and ree geochemistry correspond with previously published results crow and condie 1988 grobler et al 1989 and the sr and nd isotopy confirms crustal contamination of the sodium group magmatic rocks a preliminary weighted mean 207 pb 206 pb zircon age of 2739 39 ma for a tuff in the upper ongers river formation is reported for the lower part of the sodium group the ongers river t kuip and omdraaivlei formations reflect sedimentation and volcanism in a rift induced graben setting with fast sedimentation from horst flanks and high burial rates in the graben valleys and subsequently a less pronounced topography post graben environment accompanying explosive phreatomagmatic and effusive mafic to acidic bimodal volcanism with abundant pyroclastics delivered ample material for graben fill deposits repeated horst reactivation has episodically exposed the basement to renewed erosion sedimentation along graben flanks comprised alluvial fans mass flow deposits braided river and related modes of deposition in distal shallow lakes volcaniclastic suspension and current sedimentation low density turbidites and stromatolitic reefs were accumulated

the role of subduction induced subsidence in the evolution of the karoo basin abstract the stratigraphic record indicates that during the late carboniferous to early triassic south africa experienced an episode of large scale subsidence that resulted in the development of the karoo basin the southern margin of the region was contemporaneously dominated by an episode of compressional tectonics and there is evidence for subduction of the paleo pacific plate beneath the karoo basin we propose that the long wavelength component of the basin subsidence resulted from the deflection of the lithosphere due to mantle flow coupled to the adjacent subduction this was followed by a recovery phase during which the entire region was uplifted on cessation of subduction and subsequent loss of dynamic support for subsidence contributing to the present day high elevation topography of south africa numerical models of mantle convection are presented and show that a shallow dipping slab is able to reconcile the observed long wavelength subsidence of the basin a significant component of the n

sea level changes and the continental freeboard concept general principles and application to the precambrian abstract eustatic sea level changes reflect variation in ocean water or ocean basin volume or changes to the hypsometric curve which is in itself geographically and chronologically variable freeboard the elevation of a continent above mean sea level is closely related to changes in both sea level and this curve relative sea level change occurs due to tectonism sediment supply compaction and eustatic movements hydroisostatic compensation modifies first order sea level variation by about one third short term and localised change in sea level may be ascribed to waves and tides storm winds hurricanes tsunamis and catastrophic sediment slumps wide gently sloping shelves with more uniform circulation systems inferred for the precambrian may have enhanced tide and wave induced sea level changes salinity and water temperature mutually dependent affect eustatic and relative sea levels elevated water temperatures postulated for the early precambrian would have been offset partly by enhanced evaporation and increased salinity longer term changes in sea level regional or eustatic may result from orogenesis volcanism sediment supply compaction and loading isostasy and thermal mechanisms glacioeustatic changes compounded by related salinity variations moderated by glacioisostatic effects and modified by hydroisostasy reach ca 150 m isostatic rebound in interglacial episodes will produce rapid and marked sea level drops up to ca 250 m this is a localised effect and contrasts with the global enhancement of sea level during interglacials however evidence for glaciation in the geological record is limited also for most of the precambrian sea level oscillations in non glacial episodes may reflect flexure of tectonic plates change up to 100 m or changes to earth s geoid 250 m eustasy up to 350 m will result from change to ocean basin volume due to variation in mid ocean ridge volume and spreading rates also related to supercontinent amalgamation and breakup intra plate hot spots and oceanic plateaus will change eustatic sea level more slowly up to 100 m higher heat flow in the archaean and concomitant enhanced volcanism suggest that these were important influences on precambrian sea levels the exponential elevation age relationship observed for phanerozoic oceanic lithosphere may have been more significant in the precambrian as continental crust grew from ca 4 0 ga declining heat flow and concomitant deepening of the ocean basins maintained an approximately constant freeboard during the archaean although still hotly debated there is strong support for episodic growth models inferring rapid formation of continental crust near the archaean proterozoic boundary this would imply constant freeboard conditions after 2 5 ga as well the constant freeboard model however is one based on average global conditions freeboard was still subject to geographic and chronologic variation and these potential changes lie within the bounds of eustatic and relative sea level change continental crustal growth the freeboard concept and sea level changes are thus interdependent variables in precambrian geology

actinide chemistry in allende ca al rich inclusions the microscale distribution of u and th has been studied by fission track radiography in a set of ca al rich inclusions from the allende meteorite in the type b inclusions generally believed to have been melted the major phases melilite and fassaite are important actinide host phases with similar th u and roughly three times higher concentrations in melilite than fassaite significant enrichments are also found on grain boundaries and in some regions of alteration the distribution is not completely understood in terms of igneous partitioning and may also reflect incomplete assimilation of relict materials during melting on the rims of type b inclusions and throughout all other inclusions studied perovskite is the dominant actinide host phase however perovskite does not have a characteristic th u value measured ratios range from 1 4 to 12 both above and below the accepted solar system value of 3 8 overall neither alteration nor loss or gain of an actinide rich phase appears to have been an important th u fractionation mechanism volatility differences may be the dominant factor some results e g high th u for fluffy type a inclusions can be interpreted as resulting from incomplete incorporation of the more volatile u during a primary condensation stage but more complex interpretations are required for inclusions with low th u the th u and ree abundance patterns for the spinel plus perovskite rim suggest derivation of this rim by volatilization of interior material the time scale for this heating is required to be brief 10 4 sec but within this constraint spinel perovskite rim formation by several mechanisms deceleration heating passage through nebular hot spots differential sublimation or vaporization plus recondensation is possible

precise u pb titanite age constraints on the emplacement of the bushveld complex south africa the c 2 06 ga bushveld complex intrudes the transvaal supergroup south africa calcsilicate xenoliths within the mafic phase of the bushveld complex rustenburg layered suite or rls preserve calcsilicate xenoliths with high temperature c 1200 c mineralogies that were later metasomatized by hydrous retrograde c 600 700 c fluids whose timing has been unconstrained new u pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058 9 0 8 ma this suggests that retrogression was due to hydrothermal circulation associated with the cooling rls and not slightly later bushveld related granites the new data place a tight constraint on the minimum age of emplacement of the rls

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

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

microbial mat control on siliciclastic precambrian sequence stratigraphic architecture examples from india abstract an attempt has been made to understand within a cause and effect framework the sequence building pattern in proterozoic time that witnessed non uniformiteranean microbial mat growth and epeiric sea development marine or marginal marine successions in three neoproterozoic formations the sonia sandstone in western india and the sirbu shale and upper bhander sandstone in central india have been examined here for this purpose the strikingly common feature of all three formations is vertical stacking of highstand systems tracts without intervention of any significant transgressive deposits in one instance only the transgressive systems tract is represented by a thin granular transgressive lag and in all other cases the evidence of transgression is simply marked by the presence of marine flooding surfaces the absence of transgressive strata relates to the low sea floor gradients which facilitated rapid transgressions combined with a generally low sediment supply aggradation under normal regressive highstand conditions in spite of the low sediment supply was promoted by the prolific growth of microbial mats which reduced the effects of wave and current reworking by organic binding of clastic particles

laihunite and jarosite in the yamato 00 nakhlites alteration products on mars 1 nakhlites martian clinopyroxenites are believed to have experienced aqueous alteration on the martian surface on the basis of the presence of reddish brown iddingsite on the rim and along cracks of large olivine crystals amorphous to poorly crystalline phyllosilicates smectites and carbonates are major products of aqueous alteration of the yamato 00 nakhlites the yamato 000593 nakhlite and its pair yamato 000749 contain large olivine crystals having opaque black material intimately associated with iddingsite on their rims and along cracks synchrotron radiation x ray diffraction micro raman spectroscopy and transmission electron microscopy revealed that the opaque material is the nonstoichiometric olivine type mineral laihunite 0 42 mg2 fe2 0 79fe3 0 79si1 00o4 typically formed as a high temperature oxidation product under nonequilibrium conditions because laihunite has been decomposed into a mixture of magnetite and amorphous silicate near the fusion crust it is preterrestrial in origin this is the first observation of large olivine crystals in martian meteorites being replaced by laihunite although its presence in olivine in the mesostasis of the miller range 03346 nakhlite has been suggested by micro raman spectroscopy because laihunite in the nakhlites does not form nanometer sized intergrowth with iron oxides fluid assisted high temperature oxidation may have played an important role in transporting away free fe2 cations liberated from olivine during laihunite formation although the yamato 00 nakhlites also contain jarosite we could not confirm the presence of preterrestrial jarosite based only on its occurrences

controls on precambrian sea level change and sedimentary cyclicity abstract although uniformitarianism applies in a general sense to the controls on relative and global sea level change some influences thereon were more prominent in the precambrian short term base level change due to waves and tides may have been enhanced due to possibly more uniform circulation systems on wide low gradient precambrian shelves the lack of evidence for global glacial events in the precambrian record implies that intraplate stresses and cyclic changes to earth s geoid were more likely explanations for third order sea level change than glacio eustasy higher heat flow in the earlier precambrian may have led to more rapid tectonic plate formation transport and destruction along with an increased role for hot spots aseismic ridges and mantle plumes superplumes all of which may have influenced cyclic sedimentation within the ocean basins a weak cyclicity in the occurrence of plume events has an approximate duration comparable to that of first order supercontinental cycle sea level change second order cyclicity in the precambrian largely reflects the influences of thermal epeirogeny changes to mid ocean ridge volume as well as to ridge growth and decay rates and cratonic marginal downwarping concomitant with either sediment loading or extensional tectonism third order cycles of sea level change in the precambrian also reflected cyclic loading unloading within flexural foreland basin settings and filling deflation of magma chambers associated with island arc evolution the relatively limited number of studies of precambrian sequence stratigraphy allows some preliminary conclusions to be drawn on duration of the first three orders of cyclicity archaean greenstone basins appear to have had first and second order cycle durations analogous to phanerozoic equivalents supporting steady state tectonics throughout earth history in direct contrast however preserved basin fills from neoarchaean palaeoproterozoic cratonic terranes have first and second order cycles of considerably longer duration than phanerozoic examples supporting less evolved tectonism affecting cratonic plates it is possible that oceanic tectonic realms underwent more rapid and dynamic plate movements and arc generation whereas early continental cratonic plates offered more stable platforms and may have been subject to slower migration rates the wide range of controls on precambrian sea level change allied to their apparent variability in rates and periodicity through precambrian time supports the conclusion that each order of cyclicity is relative and must be defined within the stratigraphic context of each individual case study this underlines the importance of establishing a hierarchical order of cyclicity in sequence stratigraphic interpretations of precambrian basins based on the relative importance of sequences rather than their temporal duration

a case study of microbial mat related features in coastal epeiric sandstones from the paleoproterozoic pretoria group transvaal supergroup kaapvaal craton south africa the effect of preservation reflecting sequence stratigraphic models on the relationship between mat features and inferred paleoenvironment abstract the magaliesberg formation sandstones within the upper part of the pretoria group are interpreted as a second order highstand systems tract within an overall clastic epeiric marine setting where episodic braided fluvial systems fed sediment directly into a tidally dominated coastline setting mat related features are abundantly preserved within the inferred tidal flat and supratidal deposits the daspoort formation sandstones at a lower stratigraphic level are ascribed to a closely analogous paleoenvironmental setting yet have hardly any preserved mat features at all they are ascribed to a second order transgressive systems tract in a sequence stratigraphic framework during highstand the depositional systems are inferred to have aggraded thus preserving the mat related features formed in magaliesberg intertidal supratidal environments during second order transgression the formation of the wave tidal ravinement surface in the subtidal intertidal environments largely obliterated the features of the daspoort intertidal and supratidal environments leading to a paucity of mat related structures in the preserved daspoort succession this case study thus supports the application of mat related features to integrated paleoenvironmental and sequence stratigraphic analysis while emphasizing the role of preservation

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

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

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

experimental geochemistry of pu and sm and the thermodynamics of trace element partitioning the partitioning of pu and sm between diopside liquid and whitlockite liquid has been investigated experimentally to evaluate the geochemical coherence of pu and the light rees pu d sm d is 2 for both diopsidic pyroxene and whitlockite this small amount of fractionation would be decreased further if pu were compared to ce or nd our experimental results thus validate the suggestion that pu behaves as a lree during igneous processes in reducing environments our data and the data of ray et al 1983 indicate that temperature rather than melt composition is the most important control on elemental partitioning this is true even though we demonstrate that additions of only 1 2 wt of p2 o 5 to the diopside anorthite albite system change pu d cpx by a factor of two our data suggest that p 2o 5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides

kaapvaal craton changing first and second order controls on sea level from c 3 0 ga to 2 0 ga abstract well preserved supracrustal basin fills including the oldest large depository witwatersrand known makes the kaapvaal craton an ideal example for studying controls on first and second order stratigraphic cyclicity the duration of the three basins studied witwatersrand ventersdorp and transvaal from c 3 0 to c 2 0 ga is almost twice that of the entire phanerozoic period from which the paradigms of classical sequence stratigraphy have been derived amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through earth history this does not appear to be borne out by the precambrian record studied here instead a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first and second orders is preferred first order cycles within the c 3 0 2 0 ga period on kaapvaal are synonymous with the entire evolution of a supracrustal basin fill defined by a specific basin type second order cycles define basin fill packages of unique character which reflect large scale shifts in the accommodation sedimentation balance early to middle archaean evolution of the kaapvaal craton encompassed amalgamation of a mosaic of subdomains to produce an early south southeastern nucleus by c 3 1 ga accretion of composite terranes from both north and west followed from c 3 0 2 7 ga this was contemporaneous with evolution of the c 3 0 2 8 ga witwatersrand basin the high grade limpopo terrane was amalgamated with the kaapvaal craton between 2 95 and 2 69 ga although a younger age for this orogensis is also possible the witwatersrand first order depositional cycle was controlled largely by subduction related tectonic loading and had a duration of c 190 my the c 5 my first order cycle of the c 2 7 ga ventersdorp basin was essentially controlled by thermal uplift induced extensional subsidence and that of the c 650 my transvaal basin by extensional and subsequent thermal subsidence it is noteworthy in the three basins examined that first order cycles controlled by plate tectonics witwatersrand and transvaal basins were about two orders of magnitude longer relative to the ventersdorp plume tectonic cycle second order cycles also varied greatly from c 100 my to c 1 my within the witwatersrand and transvaal basins subsidence allowed extensive seaways to develop within which classical sequence stratigraphic systems tracts can be recognized reflecting different stages of shoreline shifts within the short lived ventersdorp continental basin system sedimentological criteria can be used to discriminate high and low accommodation systems tracts the transvaal first order cycle was terminated at c 2 0 ga by the bushveld complex plume event

re os isotope and pge constraints on the timing and origin of gold mineralisation in the witwatersrand basin abstract analyses of the re os isotope system and pge abundances have been used to investigate the timing and origin of mineralisation in the vaal reefs section of the witwatersrand basin the world s largest gold deposit consistent interelement pge fractionation in both organic and sulphide phases intimately associated with gold mineralisation suggests that the re os isotope system was fractionated during gold mobilisation the re os isotope data on organic separates intimately associated with the gold in this part of the basin indicate open system behaviour until 2 26 0 19 ga 2 of the order of 450 myr younger than the minimum age of sedimentation the overlying ventersdorp volcanics preserve a re os isochron age of 2 43 0 21 ga 250 ma younger than their u pb zircon emplacement age these data indicate substantial post sedimentary noble metal mobility within portions of the witwatersrand basin re os model age calculations on os rich fractions suggest that noble metals were derived from a 3 0 3 1 ga terrain of granite greenstone affinities significantly younger than the barberton greenstone belt these data support a hybrid model involving post depositional mobilisation of ancient probably detrital noble metals for the origin of mineralisation in this part of the witwatersrand basin such a model is applicable to other areas of gold mineralisation in the basin that display similar textural characteristics and geochemistry

understanding cratonic flood basalts the origin of continental flood basalts has been debated for decades these eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years although flood basalts are found in a variety of settings no locale is more puzzling than cratonic areas such as southern africa or the siberian craton where strong thick lithosphere is breached by these large basaltic outpourings conventionally flood basalts have been interpreted as melting events produced by one of two processes 1 elevated temperatures associated with mantle plumes and or 2 adiabatic decompression melting associated with lithospheric thinning in southern africa however there are severe problems with both of these mechanisms first the rifting circumstances of several wellknown basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep seated plume specifically rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts second the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains as an alternative to these models we propose a new two stage model that interprets cratonic flood basalts not as melting events but as short duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale reservoir creation existence stage i requires long term e g 1 ma supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm appropriate for the archean a slow thermal perturbation e g thermal blanketing or large scale mantle upwelling or a subduction related increase in volatile content the drainage event stage ii occurs in response to an abrupt stress change in the lithosphere which leads to the initiation and propagation of lithospheric dikes such a model accounts for the short eruption time of flood basalts the evidence for lithospheric control of the eruptions and the continued survival of cratonic lithosphere following these magmatic events the most notable consequence of this model is that it implies the existence of large reservoirs of magma comparable to the eruption volume of flood basalts that have been and are still likely present beneath stable continental lithopshere

the auriferous late archaean sedimentation systems of south africa unique palaeo environmental conditions abstract studies of the auriferous witwatersrand placers and associated rocks have revealed that certain palaeo environments especially braided fluvial are particularly well represented in this part of the rock record however there is a paucity of lithofacies indicating certain other palaeo environments possible reasons for their scarceness or absence are suggested in this paper it is generally assumed that pre vegetational fluvial systems would have been characteristically braided because of the absence of land plants necessary to stabilise river banks a question arising is what kind of downstream changes such pre vegetational braided streams underwent a recent study of a braided stream placer revealed that the depositing system retained its braided character right down to the palaeo shoreline however gravel did not reach the palaeo beach as with many modern examples beach conglomerates are rare in the witwatersrand rocks the paucity of conglomeratic beach placers is ascribed to the low probability of gravel being transported across coastal plains because of the relatively low slopes or depths of rivers here the witwatersrand fluvial channels are generally considered to have had high width to depth ratios because of the absence of land vegetation to stabilise channel banks however two examples of deep relatively narrow scour features with predominantly fine grained fills occur near the base of the central rand group the low width to depth ratios of these scour features which probably represent palaeo valleys are ascribed to severe incision during a rapid sea level fall the auriferous fluvial systems of the witwatersrand are generally considered to have been entirely braided due to the lack of bank stability however the fluvial b placer of the welkom goldfield is confined to discrete channels their banks are thought to have been stable due to the cohesive nature of the lutite into which the gravelly streams incised in addition braiding sensu stricto was probably inhibited by initial incision and a low sediment supply no deposits of specifically deltaic sub environments have been found in the central rand group their absence is attributed to the following factors a the paucity absence of well defined palaeo river mouths b low concentrations of suspended sediment c intermittent sediment supply to the palaeo coastline and d reworking by tidal and longshore currents alluvial fan deposits are also apparently absent in the witwatersrand rocks the absence of fan deposits is attributed to the poor potential for development and preservation of fan deposits in the compressive tectonic setting proposed for the witwatersrand basin although ventifacts have been found in the witwatersrand rocks no aeolianites have been reported their apparent absence is probably due to a reworking in a predominantly humid climate and b transport of sand by dominant winds to areas unfavourable for the preservation of aeolianites

the behavior of actinides phosphorus and rare earth elements during chondrite metamorphism new data on the u pu and p distributions in less metamorphosed h chondrites type 3 5 coupled with literature results permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism preferential associations of phosphates with metals and or sulndes in all chondrites strongly indicate an initially siderophile or conceivably chalcophile character for p in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation this oxidation occurred prior to or during chondritic metal silicate fractionation uranium is initially concentrated in chondrule glass at 100 ppb levels with phosphates primarily merrillite in h 3 chondrites being essentially u free 20 ppb as chondrule glass devitrified during metamorphism u migrated into phosphates reaching 50 ppb in nadiabondi h 5 merrillite and 200 300 ppb in merrillite from equilibrated chondrites but froze out before total concentration in phosphates occurred relative 244 pu fission track densities in the outer 5 m of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in bremervorde h 3 give pu mesostasis pu merrillite 0 01 implying total concentration of pu in phosphates similarly no detectable pu 0 1 ppb was found in chondrule mesostasis in tieschitz and sharps whereas direct measurements of tracks in phosphates in h 3 chondrites are consistent with high 10 ppb pu concentrations thus a strong pu p correlation is indicated for ordinary chondrites there is variable pu u fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites therefore the pu u ratio in phosphates appears relatively useless for relative meteorite chronology literature data indicate that the ree are located in chondrules in unequilibrated chondrites most likely in glass thus there may also be strong pu nd fractionation within these meteorites like u the ree migrate into phosphates during metamorphism but unlike u appear to be quantitatively concentrated in phosphates in equilibrated chondrites thus relative ages based on pu nd may be possible for equilibrated chondrites but the same chronological conclusions are probably obtainable from pu concentrations in phosphates i e on the pu p ratio however pu p chronology is possible only for ordinary chondrites so there appears to be no universal reference element to cancel the effects of pu chemical fractionation in all meteorites available data are consistent with but certainly do not prove that variations in pu p represent age differences but if these age differences do not exist then it is conceivable that the solar system 244 pu 238 u ratio important for cosmochronology is still lower than the presently accepted value of 0 007

sequence stratigraphy of the precambrian rooihoogte timeball hill rift succession transvaal basin south africa third order sequence stratigraphic analysis is performed on the rooihoogte timeball hill second order rift succession of the paleoproterozoic transvaal basin south africa this provides a case study for systems tract and sequence development during a time of glacio eustatic fall when accommodation was generated by subsidence related to syn rift and post rift tectonic processes two third order depositional sequences have been identified separated by a basin wide subaerial unconformity the lower third order sequence includes the complete succession of lowstand transgressive and highstand systems tracts lst tst and hst whereas the upper third order sequence only preserves lowstand and transgressive systems tracts this indicates that the fall in base level associated with the upper second order boundary of the rooihoogte timeball hill sequence was of higher magnitude relative to the third order subaerial unconformity which is in agreement with the principles of boundary hierarchy based on the magnitude of base level changes the position of the lower boundary of the rooihoogte timeball hill second order sequence has been revised from the base of the chert breccias to the contact between the breccias and the overlying chert conglomerates this is because a major tilting event occurred between the deposition of the two facies which are genetically unrelated and which are separated by a subaerial unconformity the lithostratigraphic contact between the rooihoogte and timeball hill formations is interpreted as a diachronous transgressive surface of erosion in this interpretation the polo ground member of the rooihoogte formation may be coeval with the basal black shales of the timeball hill formation the two facies fluvial and marine respectively forming together a transgressive systems tract d 2002 elsevier science b v all rights reserved

the concentrations of the noble metals in southern african flood type basalts and morb implications for petrogenesis and magmatic sulphide exploration concentrations of the platinum group elements have been determined in several suites of southern african flood type basalts and mid ocean ridge basalt morb covering some 3 ga of geologic evolution and including the etendeka karoo soutpansberg machadodorp hekpoort ventersdorp and dominion magmas the magmas cover a compositional range from 3 7 to 18 7 mgo 26 720 ppm ni 16 250 ppm cu and 1 255 ppb total platinum group elements pge the younger basalts etendeka karoo tend to be depleted in pge relative to cu while most of the older basalts hekpoort machadodorp ventersdorp dominion show no pge depletion relative to cu further the younger basalts tend to have lower average pt pd ratios than the older basalts and the morbs have lower average pt pd than the continental basalts within the broad groupings of old and young basalts this may reflect 1 a decreasing degree of mantle melting through geologic time and 2 source heterogeneity in that the morbs are derived from predominantly asthenospheric mantle whereas the continental basalts also contain a lithospheric mantle component enriched in pt in addition to these factors some pge fractionation also occurred during differentiation of the magmas with pd showing incompatible behaviour and the other pge variably compatible behaviour the examined southern african flood type basalts and morb appear to offer limited prospects for magmatic sulfide ores largely because they show little evidence for significant chalcophile metal depletion that could be the result of sulphide extraction during ascent and crystallization

concept and styles of reciprocal stratigraphies western canada foreland system the late cretaceous palaeocene western canada foreland system provides a record of high frequency cycles of reciprocal flexural tectonics superimposed on longer term changing dynamic subsidence initial dominance of dynamic subsidence during the deposition of the bearpaw marine succession resulted in continual subsidence with differential rates across the flexural hinge line subsequent dominance of flexural tectonics resulted in opposite base level changes between the proximal and distal reaches of the foreland system during the deposition of the post bearpaw nonmarine succession in both cases the contrasting base level changes generated out of phase stratigraphic sequences which defines the concept of reciprocal stratigraphies two styles of reciprocal stratigraphies have been identified in relation to the pattern of base level changes across the foreland system the bearpaw style consists of a conformable succession of correlative transgressive and regressive systems tracts suggesting continuous basin wide sedimentation with the rates within the range of variation of the rates of base level rise the post bearpaw style shows sequences correlative to age equivalent sequence boundaries related to coeval rising and falling base level respectively the succession of bearpaw and post bearpaw sequences corresponds to a cycle of marine to nonmarine foreland sedimentation controlled by changing ratios between dynamic subsidence and flexural tectonics the amount of sediment supply derived from the orogen to the foreland system may also reflect the rates of dynamic subsidence as a decrease in dynamic loading may lead to accelerated denudation of the sediment source areas

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

bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 c small particles 200 m in diameter from the hydrous carbonaceous chondrites orgueil ci murchison cm2 and tagish lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry the three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures and thus the brief heating up to 1000 c drastically changed the mineralogy changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400 600 c serpentine and saponite decomposition to amorphous phases at 600 and 700 c respectively decomposition of mg fe carbonate at 600 c recrystallization of secondary olivine and fe oxide or metal at 700 800 c and recrystallization of secondary low ca pyroxene at 800 c the ambient atmospheric pressures controlled species of secondary fe phase taenite at pressures lower than 10 2 torr magnesiowstite from 10 3 to 10 1 torr and magnetite from 10 2 to 1 torr the abundance of secondary low ca pyroxene increases in the order of murchison orgueil and tagish lake and the order corresponds to saponite abundance in samples prior to heating mineralogy of the three unmelted micrometeorites f96ci024 kw740052 and kw740054 were investigated in detail in order to estimate heating conditions the results showed that they might have come from different parental objects carbonaterich tagish lake type carbonate poor tagish lake or ci type and cm type respectively and experienced different peak temperatures 600 700 and 800 900 c respectively at 60 80 km altitude upon atmospheric entry

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

why was flood volcanism on submerged continental platforms so common in the precambrian abstract many archean and proterozoic greenstone belts contain volcanic rocks that erupted subaqueously but interacted with continental crust on their way to the surface this points to emplacement in a tectonic setting that was rare to absent throughout the phanerozoic the eruption of flood basalts onto submerged continental platforms to explain the situation requires that sea levels were high during periods of flood volcanism perhaps because the ocean basins were partially filled by extensive active and high standing oceanic ridges or by oceanic plateaus the common preservation of crust contaminated submarine basalts even in the oldest supracrustal sequences may indicate that continental crust was voluminous even at an early stage in earth history

pre transvaal wrench tectonics along the northern margin of the witwatersrand basin south africa abstract the pre transvaal tectonic history of the northern margin of the witwatersrand basin is re evaluated in terms of wrench fault tectonics three major east northeast trending left lateral wrench fault systems are recognized the sugarbush rietfontein and kromdraai systems they are characterized by pull apart basins e g bezuidenhout valley and kromdraai grabens narrow slivers of displaced stratigraphy e g langermanskop rietfontein block and marginal convex upward thrusts changes in fault orientation result in areas of compression passing laterally into areas of extension over short distances splays terminate individual fault systems regions of wrench fault overlap are characterized by fold domains e g west rand syncline east rand basin with folds characteristically trending nw se the left lateral strike slip system appears to be of craton wide scale and probably extends at least as far as the barberton greenstone belt the wrench system appears to have come into existence during witwatersrand times but experienced its major movement during middle ventersdorp times the left lateral strike slip system is believed to be related to rifting along the ventersdorp trough and to right lateral faulting along the southwestern margin of the witwatersrand basin these tectonics occurred prior to a post transvaal period of folding and thrusting related to the emplacement of the vredefort dome the latter is responsible for the penetrative regional cleavage and bedding sub parallel faults throughout the area concerned the structural model proposed here has implications for the existence of possible outliers of mineralized upper witwatersrand stratigraphy

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

archean foreland basin tectonics in the witwatersrand south africa the witwatersrand basin of south africa is the best known of archean sedimentary basins and contains some of the largest gold reserves in the world sediments in the basin include a lower flysch type sequence and an upper molassic facies both of which contain abundant silicic volcanic detritus the strata are thicker and more proximal on the northwestern side of the basin which is at least locally bound by thrust faults these features indicate that the witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an andean type arc remarkably similar phanerozoic basins may be found in the southern andes we suggest that the continental collision between the kaapvaal and zimbabwe cratons at about 2 7 ga caused further subsidence and deposition in the witwatersrand basin regional uplift during this later phase of development placed the basin on the cratonward edge of a collision related plateau now represented by the limpopo province similarities are seen between this phase of witwatersrand basin evolution and that of active basins north of the tibetan plateau e g the tarim and tsaidam basins the geologic evidence does not appear so compatible with earlier suggestions that the witwatersrand strata were deposited in a rift or half graben

testing for eustatic sea level control in the precambrian sedimentary record abstract stratigraphic successions are deposited under the influence of three primary controls which tend to vary systematically through time these controls are tectonism basin subsidence and uplift sea level change and sediment supply the development of sequence stratigraphy has enabled the introduction of systematic quantitative methods for the analysis of these controls the use of the sequence approach requires the establishment of a regional sequence framework and detailed chronostratigraphic control working with the phanerozoic record this approach has revealed a variety of sequence types reflecting long term tectonostratigraphic processes caused by continental rifting continental collisions and cratonic dynamic topography high frequency thrust loading cycles in foreland basins and high frequency cycles caused by orbital forcing glacioeustasy 104 5 year periodicity and eustasy driven by changing rates of sea floor spreading 107 8 year periodicity have emerged as two significant global processes documenting the sequence record in the precambrian has proved difficult owing to fragmentary preservation and structural deformation methods of dating are at present not precise enough to permit reliable regional correlation or testing of high frequency sequence generating mechanisms some studies of precambrian sequence architecture have been achieved but these indicate tectonic processes acting at a much slower rate than during the phanerozoic further analysis of this problem is required

archean plate tectonics what can be learned from continental geology some basic questions about archean plate tectonics can be addressed by examining accretionary archean margins in particular fault zones with significant strike slip components on the canadian shield 1 were the oceanic plates typically rigid like modern plates yes significant lateral viscosity contrasts in the lithosphere between plates and plate boundaries are required for major strike slip faults to exist conversely strike slip faults are a kinematic consequence of rigid plates 2 did large oceanic plates exist in the archean probably first the length and offset of the longest preserved segments of archean faults are similar to modern examples such as in alaska less directly the duration of a period with a consistent sense of strike slip at a point on the continental side of an accretionary margin should be related to the time that a typical oceanic plate remains outboard of the margin this time varies proportionally with size of typical ocean plates and inversely with their velocity the

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

sequence stratigraphy of the vaal reef facies associations in the witwatersrand foredeep south africa abstract facies analysis and sequence stratigraphic methods resolve the correlation and relative chronology of vaal reef facies associations at the great noligwa and kopanang mines in the klerksdorp goldfield witwatersrand basin four depositional sequences separated by major erosional surfaces build the vaal reef package lowstand transgressive and highstand systems tracts hst are identified based on stratal stacking patterns and sedimentological features the core of the vaal reef deposit is represented by a transgressive systems tract tst which is built by shoreface proximal facies at great noligwa and inner shelf distal facies at kopanang the proximal tst includes the g v bosch reef and the overlying mb4 quartzites these facies are correlative to the stilfontein conglomerates and quartzites respectively at kopanang the overlying upper vaal reef is part of a lowstand systems tract lst it is only preserved at kopanang with a thickness that equals the difference in thickness between the proximal and distal transgressive facies this suggests that the upper vaal is an erosional remnant that accumulated on a gently dipping topographic profile traditionally the mb4 quartzite has been considered the hanging wall relative to the vaal reef we infer that these quartzites are merely associated with the g v bosch conglomerate and interpret that they are coeval with the stilfontein quartzites and older relative to the upper vaal facies from a genetic viewpoint the true hanging wall to the vaal reef begins with the zandpan gravel lag that extends across the entire study area paleocurrent analysis of shoreface and fluvial systems indicate a curved shoreline and a northern elevated source area the coastline underwent north south transgressive and regressive shifts during vaal reef time

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

sole marks and combined flow storm event beds in the brixton formation of the siliciclastic archean witwatersrand supergroup south africa abstract sole marks and associated storm event beds are described for the first time from the middle archean witwatersrand supergroup the sole marks are excellently preserved and include a variety of marks formed by combined flow turnabout curvilinear and low angle intersecting groove marks bipolar prod marks recurved flute marks and shallow scour hollows the sole marks are intimately associated with other sedimentary structures formed by combined flow or oscillatory flow such as wave ripple marks undulatory parallel lamination hummocky cross stratification and hummock and swale structures the storm event beds with sole marks were deposited in a transitional shelf setting and grade upwards into swaly and trough cross bedded orthoquartzite deposited on the inner shelf interbedded storm event beds display a variety of sole mark assemblages reflecting widely varying flow conditions in the storm generated bottom boundary layer oscillatory currents were mainly oriented perpendicular to paleoshore and were combined in some cases with offshore directed storm surge relaxation currents some sole mark assemblages specifically those with turnabout and low angle intersecting groove marks contain evidence for the presence of shore oblique or shore parallel geostrophic currents the storm event deposits in the witwatersrand supergroup are thus compared with models derived from odern storm deposition

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

iodine xenon studies of allende inclusions eggs and the pink angel the iodine xenon systems of six allende inclusions five eggs and the pink angel appear to have been altered by non nebular secondary processes evidence for this includes temperature ordered variations in the initial i isotopic composition within several objects with older apparent i xe ages associated with higher extraction temperatures and the absence of primitive i xe ages the span of apparent ages seen in allende objects 10 ma or more is probably too long to reflect any nebular process so at least some alteration probably occurred on the parent body the range in initial 244pu238u ratios for the eggs 3 14 10 3 includes the current best estimates of the bulk solar system value 4 7 10 3 for egg 3 the puu ratio varies by a factor of two between extractions probably the result of fractionation of pu from u among different phases

small is beautiful the analysis of nanogram sized astromaterials the capability of modern methods to characterize ultra small samples is well established from analysis of interplanetary dust particles idps interstellar grains recovered from meteorites and other materials requiring ultra sensitive analytical capabilities powerful analytical techniques are available that require under favorable circumstances single particles of only a few nanograms for entire suites of fairly comprehensive characterizations a returned sample of 1000 particles with total mass of just 1 g permits comprehensive quantitative geochemical measurements that are impractical to carry out in situ by flight instruments the main goal of this paper is to describe the state of the art in microanalysis of astromaterials given that we can analyze fantastically small quantities of asteroids and comets etc we have to ask ourselves how representative are microscopic samples of bodies that measure a few to many kilometers across with the galileo flybys of gaspra and ida it is now recognized that even very small airless bodies have indeed developed a particulate regolith acquiring a sample of the bulk regolith a simple sampling strategy provides two critical pieces of information about the body regolith samples are excellent bulk samples because they normally contain all the key components of the local environment albeit in particulate form furthermore because this fine fraction dominates remote measurements regolith samples also provide information about surface alteration processes and are a key link to remote sensing of other bodies studies indicate that a statistically significant number of nanogram sized particles should be able to characterize the regolith of a primitive asteroid although the presence of larger components e g chondrules calcium aluminum rich inclusions large crystal fragments etc within even primitive meteorites e g murchison points out the limitations of using data obtained from nanogram sized samples to characterize entire primitive asteroids however the most important asteroidal geological processes have left their mark on the matrix because this is the finest grained portion and therefore most sensitive to chemical and physical changes thus the following information can be learned from this fine grain size fraction alone 1 mineral paragenesis 2 regolith processes 3 bulk composition 4 conditions of thermal and aqueous alteration if any 5 relationships to planets comets meteorites via isotopic analyses including o 6 abundance of water and hydrated material 7 abundance of organics 8 history of volatile mobility 9 presence and origin of presolar and or interstellar material most of this information can be obtained even from dust samples from bodies for which nanogram sized samples are not truly representative future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano to microgram sized samples even further this highlights a key advantage of sample returns that the most advanced analysis techniques can always be applied in the laboratory and that well preserved samples are available for future investigations

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

fe3 distribution in oxidized olivine a synchrotron micro xanes study synchrotron micro xanes spectroscopy smx is used to examine the amount and distribution of fe super 2 and fe super 3 in five samples of fayalite previously studied by mossbauer spectroscopy rockport fayalite is homogeneous and the fe in it is completely reduced olivine from qianan is almost completely oxidized and probably contains finely intergrown silica laihunite and hematite with hematite predominating pantelleria olivine has an oxidized rim around a reduced core suggesting either a post crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth the texture of olivine from the mourne mountains suggests exsolution from a fayalite laihunite solid solution based on the substitution of three fe super 3 atoms for two fe super 2 and one vacancy that was stable at high temperature laihunite from the type locality lai he village shows 1 3 mu m mottling between light and dark areas in backscatter images but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk fe super 3 sigma fe of such a combination would not correspond to the smx value of 67 fe super 3 rather the texture observed in backscatter is interpreted to represent alternating areas of magnetite rich and laihunite rich compositions with a similar i e 67 fe super 3 sigma fe extent of oxidation

microdistribution of boron particle track images from samples of a caledonian igneous complex 7li recoil tracks in lexanr formed as a product of irradiation with thermal neutrons give an image of in sifu microdistribution of boron in dioritic to granitic rocks of the caledonian ballachulish zoned complex and in a carbonate rock from its contact aureole the images exhibit a polyphase incorporation of boron into primary and secondary minerals the boron microdistribution is inhomogeneous isomorphous incorporation of boron into pyroxene and plagioclase during the main stage of crystallization cannot be established because the boron distribution is blurred by secondary alteration in contrast to the intracrystalline feature the crystal interfaces are enriched in boron increased concentrations are also disclosed by overgrowths of primary crystals exsolution lamellae in alkali feldspar and alteration rims of clinopyroxene the highest amounts of boron in a siliceous dolomite marble of the contact aureole are found in the alteration products of olivine

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

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

high temperature ash flow wet sediment interaction in the makwassie formation ventersdorp supergroup south africa abstract the late archaean ventersdorp supergroup in south africa constitutes a supracrustal succession consisting of mafic lava at the base that is overlain by a clastic wedge deposit known as the kameeldoorns formation which formed in grabens that were the result of extensional tectonics overlying these units is an intermediate to felsic ash flow deposit the goedgenoeg and makwassie formations that are now represented by feldspar and quartz feldspar porphyries at t kuip in the northern cape province spherical structures or clasts varying greatly in size occur at the contact of the makwassie formation with the underlying sedimentary kameeldoorns formation the larger clasts have cavities of different shapes and are regarded to have developed as a result of the deposition of a high temperature ash flow onto water saturated sediments

the 190pt 186os decay system applied to dating platinum group element mineralization of the bushveld complex south africa abstract we evaluate the 190pt 186os decay system measured by laser ablation mc icpms as a useful geochronometric tool for direct dating of platinum group minerals pgm in large mafic ultra mafic intrusions by analysing mineralised rocks from the geochronologically well constrained bushveld complex s africa mixed pgm laurite cooperite laurite platarsite sperrylite and pt fe alloy from the merensky reef yield a pt os isochron age of 1995 50 ma 2 n 48 mswd 1 16 initial 186os 188os 0 119819 0 000006 while this is 59 myr younger than the u pb zircon age for the merensky reef scoates and friedman 2008 it is consistent with recent re os ages for the platreef considering the younger age recorded by both re os and pt os systems we propose that the pt os isochron age records a late low temperature hydrothermal event affecting pgm and sulphides in the platreef and merensky reef pt rich phases may also yield single grain 190pt 186os model ages since initial 186os 188os can be well constrained a pt os model age of 2024 ma 101 ma was calculated for a merensky cooperite a pt fe alloy from the onverwacht pipe and a tweefontein hill sperrylite give pt os model ages of 2113 106 ma and 2042 102 ma respectively a pt os isochron derived from all bushveld units gives an age of 2012 47 ma 2 186os 188osi 0 119818 0 000006 with an mswd of 1 19 hence pge mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the pt os chronometer nonetheless the potential for the pt os system in dating events in pge mineralised systems is evident and the agreement of single grain model ages in high pt os phases with the pt os and re os isochron ages indicates their usefulness in the geochronology of these systems

mafic dikes displacing witwatersrand gold reefs evidence against metamorphic hydrothermal ore formation the process responsible for the gold mineralization in the witwatersrand basin south africa remains controversial the debate is between a detrital placer enrichment in laterally extensive conglomerate layers and hydrothermal introduction by metamorphic fluids long after sedimentation while textural and geochronological data may be ambiguous a first order geological observation severely delimits the mechanism and timing of gold emplacement the individual gold reefs are cut and displaced by numerous mafic dikes which based on geochemical data presented here are feeders to flood basalts immediately overlying the witwatersrand the displacements preclude fracture controlled fluid focusing over tens of kilometers along a single conglomerate horizon during or after regional metamorphism instead basin scale gold enrichment occurred during sedimentation or early diagenesis well before local redistribution by metamorphic fluids

mantle plume uplift in the sedimentary record origin of kilometre deep canyons within late neoproterozoic successions south australia in south australia canyons 200 250 km long are incised to depths of c 1 5 km in late neoproterozoic after c 590 ma marine deposits of the adelaide fold belt and canyons 90 km long and 700 m deep occur in correlative marine strata of the officer basin 1000 km to the nw in the adelaide fold belt the isotopic signature of limestone veneering canyon walls canyon sinuosity and sedimentary structures in canyon fill suggest that the canyons were cut subaerially and filled by shallow water marine deposits during coastal onlap eustacy cannot account for the kilometre scale change in sea level required for subaerial canyon erosion and chemostratigraphy and the lack of evaporites at that stratigraphic level in southern australia argue against a messinian style drawdown in each region uplift and crustal extension preceding or coeval with canyon incision are indicated by a regional unconformity turbidite or mass flow deposition and extensional faults these features and canyon incision may be explained by regional uplift above a rising mantle plume over a distance of 1000 km in southern australia flood basalt volcanism in the officer basin tentatively dated at 563 40 ma may have followed this uplift mantle plume uplift is expressed in the sedimentary record by emergent trends in facies regional thinning regional unconformities turbidites gravity slides normal faults and incised canyons and palaeovalleys

martian dunite nwa 2737 integrated spectroscopic analyses of brown olivine received 5 october 2007 revised 13 december 2007 accepted 6 march 2008 published 18 june 2008 1 a second martian meteorite has been identified that is composed primarily of heavily shocked dunite northwest africa nwa 2737 this meteorite has several similarities to the chassigny dunite cumulate but the olivine is more mg rich and most notably is very dark and visually brown carefully coordinated analyses of nwa 2737 whole rock and olivine separates were undertaken using visible and near infrared reflectance

normal faulting block tilting and decollement in a stretched crust extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas if we except continental and oceanic rifts stretched portions of the crust exhibit complex faulting patterns at upper levels with high angle and low angle normal faults very little is known concerning the geometry of structures and physical processes at depth it is therefore difficult to extrapolate surface observations to the crustal scale five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed two natural examples the west armorican atlantic margin and eldorado mountains basin and range province are presented they are characterized by an association of high angle normal faults tilted blocks and low angle normal faults field and geometrical arguments are given to demonstrate that low angle faults are controlled by decollement surfaces along preexisting interfaces geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults in such cases the amount of stretching cannot be related to bulk thinning of the crust

flexural partitioning of the late archaean witwatersrand foreland system south africa abstract the witwatersrand basin is part of a retroarc foreland system formed during the late archaean in response to crustal thickening along the northern margin of the kaapvaal craton this foreland system may be partitioned into foredeep forebulge and back bulge flexural provinces the preserved sedimentary fill occupies the region of the flexural foredeep and displays a typical transition from underfilled deep and shallow marine to overfilled fluvial and alluvial fan phases loading in the witwatersrand thrust belt took place in relation to the oblique convergence of two compressional stress fields as a result the foredeep is restricted along the strike to a distance of the witwatersrand foreland system formed on a newly cratonized continental lithosphere which gave the basin a small flexural wavelength i e 490 km from the center of orogenic loading to the axis of the back bulge relative to most phanerozoic counterparts the forebulge unconformity may be associated with the lack of dynamic loading which is probably a common feature for all precambrian retro foreland systems due to the lower rates of subduction associated with the early stage of plate tectonic processes

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

discrete metamorphic events in the limpopo belt southern africa implications for the application of p t paths in complex metamorphic terrains essentially all models for the tectonic evolution of the high grade limpopo belt of southern africa invoke a single orogenic event at 2 7 ga involving the collision of the zimbabwe and kaapvaal cratons with a distinct central zone by implication rocks within the belt must have followed parallel pressure temperature time p t t paths the main reasons for this situation are the belief that syn to postdeformational granitic magmatism in the belt was coeval and the fact that p t paths are similar new geochronological data indicate however that this magmatism occurred at distinct times in different parts of the belt over a span of at least 70 m y in addition the data indicate that high grade metamorphism and associated tectonism in the central and southern marginal zones occurred at about 2 0 and 3 15 ga respectively therefore p t t paths of these zones were independent until their juxtaposition more recently than 2 0 ga

precambrian sequence stratigraphy abstract sequence stratigraphy studies the change in depositional trends in response to the interplay of accommodation and sediment supply from the scale of individual depositional systems to entire sedimentary basin fills as accommodation is controlled by allogenic mechanisms that operate at basinal to global scales the change in depositional trends is commonly synchronized among all environments established within a basin thus providing the basis for the definition of systems tracts and the development of models of facies predictability all classical sequence stratigraphic models assume the presence of an interior seaway within the basin under analysis and are centered around the direction and types of shoreline shifts which control the timing of all systems tracts and sequence stratigraphic surfaces in overfilled basins dominated by nonmarine sedimentation the definition of systems tracts is based on changes in fluvial accommodation as inferred from the shifting balance between the various fluvial architectural elements the method of sequence stratigraphy requires the application of the same set of core principles irrespective of the age of strata under analysis from precambrian to phanerozoic the study of precambrian basins is often hampered by poorer stratal preservation and by a general lack of time control however where sedimentary facies are well preserved the lack of time control may be partially compensated by a good knowledge of facies architecture and relationships as well as by paleocurrent data the latter are particularly important to understand the stratigraphic record of tectonically active basins where abrupt shifts in paleoflow directions allow one to infer tectonic events and map the corresponding sequence bounding unconformities arguably the most important contribution of precambrian research to sequence stratigraphy is the better understanding of the mechanisms controlling stratigraphic cyclicity in the rock record and hence of the criteria that should be employed in a system of sequence stratigraphic hierarchy there is increasing evidence that the tectonic regimes which controlled the formation and evolution of sedimentary basins in the more distant geological past were much more erratic in terms of origin and rates than formerly inferred solely from the study of the phanerozoic record in this context time is largely irrelevant as a parameter in the classification of stratigraphic sequences and it is rather the stratigraphic record of changes in the tectonic setting that provides the key criteria for the basic subdivision of the rock record into basin fill successions separated by first order sequence boundaries these first order basin fill successions are in turn subdivided into second and lower order sequences that result from shifts in the balance between accommodation and sedimentation at various scales of observation irrespective of the time span between two same order consecutive events sequences identified in any particular basin are not expected to correlate to other first and lower order sequences of other basins that may be similar in age but may have different timing and duration

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

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

in situ trace element microanalysis we define in situ trace element microanalysis as the measurement of concentrations of trace elements 1000 ppm in individual mineral phases in polished sections analogous to electron microprobe analysis for major elements it is our opinion that such measurements are important and can provide a new dimension to petrology and geochemistry this article is both a review of work already done and a summary of potential advances in the future the advantages of in situ analyses as opposed to methods involving physical and or chemical mineral separations are that the trace element data can be interpreted in a petrographic context and that ambiguities associated with the purity of the mineral separates can be avoided with data on individual grains comparisons of intergrain and intragrain zoning variations between major and trace elements can be made and the importance of inclusions can be assessed within the spatial resolution of the microanalysis technique

sequence stratigraphic framework and application to the precambrian abstract sequence stratigraphy highlights stratal stacking patterns and changes thereof within a time frame each stratal stacking pattern defines a particular genetic type of deposit with a unique geometry and sediment dispersal pattern within the basin common genetic types of deposit are referred to as forced regressive lowstand normal regressive transgressive and highstand normal regressive these genetic units are the basic building blocks of the sequence stratigraphic framework at any scale of observation and are bounded by sequence stratigraphic surfaces the recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply which correspond to sequences in the rock record depending on the scale of observation sequences and sequence stratigraphic surfaces may be ascribed to different hierarchical orders the concept of accommodation which defines the space available for sediments to fill is central to sequence stratigraphy changes in accommodation are in part controlled by regional to local tectonism and therefore are location specific the construction of accommodation curves is based on wheeler diagrams the preserved thickness of sequences and the paleodepositional environment accommodation curves may or may not correlate between different sedimentary basins or even between different sub basins of the same sedimentary basin depending on the interplay of local versus global controls on sedimentation the offset between the accommodation curves that characterize different depozones tends to increase for cycles of increasingly lower rank while the workflow and principles of sequence stratigraphy remain the same irrespective of the age of strata under analysis the differences and similarities between the phanerozoic and the precambrian rock record provide significant clues to improving our approach to the application of the sequence stratigraphic method

evidence of tidal processes from the lower part of the witwatersrand supergroup south africa abstract a 1600 m succession of quartz arenites and associated shaley deposits comprising the hospital hill subgroup at the base of the witwatersrand supergroup is considered to have been deposited largely under the influence of tidal processes facies analysis indicates that deposition occurred in the following environments 1 marine shalf 2 shallow subtidal to intertidal 3 intertidal flat and 4 tidal inlet the presence of strong tidal currents implies that the witwatersrand basin was open to an ocean basin at least during the early stages of its evolution palaeocurrent trends and isopach data suggest that this probably lay to the southwest an area now occupied by the high grade natal namaqua metamorphic belt the contrast between the supermature quartz arenites of the hospital hill subgroup and the overlying gold bearing immature subgreywackes feldspathic quartzites and conglomerates of fluvial origin is believed to be a function of tidal reworking of sediments