mass occurrence of benthic coccoid cyanobacteria and their role in the production of neoarchean carbonates of south africa abstract the sparse archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post mortem silicification hitherto as an exception sedimentary carbonate rocks from the neoarchean nauga formation of south africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria chroococcales and pleurocapsales their remains visible under the scanning electron microscope sem after etching of polished rock samples comprise capsular envelopes mucilage sheaths and groups of cells mineralized by calcium carbonate with an admixture of al k mg fe silicates the capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the neoarchean microbiota described and their modern analogues the new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the neoarchean and offer a solution to the problem of the origin of archean carbonate platforms stromatolites and microbial reefs

locating volcano seismic signals in the presence of rough topography wave simulations on arenal volcano costa rica summary quantifying the scattering effects of pronounced volcano topography on the seismic wavefield is an important component in locating and interpreting volcano seismic sources in this study we perform seismic wave simulations to quantify the scattering generated by a 3 d digital elevation map and 1 d velocity model of arenal volcano costa rica full waveform synthetic seismograms were generated using a 3 d elastic lattice method including complex topography several different simulations were performed where the source location source type and topographic models were varied synthetic seismograms were calculated for 35 seismic arrays each one comprising nine stations at each array the slowness vector of wave propagation is estimated from the time delays between the sensors obtained using the cross spectral method results show that the backazimuth estimated for some arrays in particular those close to the source deviate from the true source position suggesting strong topographic effects in these regions the maximum of the probability density function obtained by crossing the backazimuths of the remaining arrays coincides exactly with the true source location we also compare our synthetic seismograms with array results from a physical field study the true and calculated location misfit depends largely on the topography but also on the number of antennas the distance from the source and the spatial resolution of the antennas the results show that this kind of study could be undertaken prior to the installation of seismic arrays to select the sites that minimize the topographic effects leading to improved source locations

a paleohydrologic model for early proterozoic dolomitization and silicification abstract primary depositional features observed in the intertidal and supratidal facies of the 2 25 ga malmani dolomite including tepees and finely laminated domal stromatolites have equivalents in holocene salinas on the southern coast of australia the hydrological regime so important in the formation of these recent primary features is still active and so it has been possible to interpret the palaeohydrological significance of analogous features in the malmani dolomite the palaeoclimate of the malmani dolomite shoreline was probably humid to semi arid and the supratidal zone is interpreted as a zone of groundwater resurgence tepees indurated fenestral limestone sheets algal mats small domal stromatolites and intraclast breccias were forming on the supratidal flats in areas of periodically outcropping groundwaters in the shallow subsurface the mixing zone between these resurging continental groundwaters and the marine derived phreatic waters was probably a zone of secondary coarse grained dolomite precipitation and or chert nodule formation this groundwater controlled mixing zone model explains why such fabrics are restricted to porous and immediately adjacent units which at the time of diagenesis were below the regional water table in the upper supratidal zones above the regional water table no secondary dolomites or chert nodules could form nor could diagenetic fabrics form in the subtidal zone for there the stromatolitic mounds contained no palaeoaquifer

carbonate platforms of passive extensional continental margins types characteristics and evolution abstract carbonate platforms of extensional margins may be grouped into several major categories homoclinal ramps have gentle slopes into deep water may have skeletal or ooid pellet sand shoal complexes that grade without break in slope into deep ramp nodular limestone and then into pelagic hemipelagic basin facies they generally lack significant slump and sediment gravity flow deposits in the deeper water facies distally steepened ramps differ from the above in having a marked increase in slope at the seaward edge of the deep ramp and abundant slumps slope breccias and turbidites however clasts of shallow platform margin facies are generally absent from breccias rimmed shelves have linear trends of shelf edge lime sands and reefs a marked increase in slope into deep water and foreslope and slope sands breccias with clasts of platform margin rocks and turbidites grading seaward into basin hemipelagic pelagic muds they may be divided into accretionary bypass and erosional margins isolated platforms are broad flat topped shallow platforms surrounded by deeper water few hundred meters to 4 km deep many are bypass margins but accretionary and erosional margins also occur finally incipiently to completely drowned or open platforms may develop by rapid submergence of ramps shelves or isolated platforms shallow platform margin facies are shifted landward and the earlier shallow water platform is covered with transgressive lags and deeper water blankets of hemipelagic or pelagic facies or open marine whole fossil wackestone the carbonate prisms commonly develop above a basal clastic phase ramps develop early and evolve into rimmed shelves however with drowning rimmed shelves may evolve up into ramps rimmed shelves may also develop into ramps where miogeoclinal faces prograde into filling marginal basins carbonate miogeocline deposition terminates when the shelf becomes a prograding clastic shelf or where it becomes part of a collisional orogen the various platform types may be recognized from continental margin sequences ranging from proterozoic to holocene in age

seismic stratigraphic framework and depositional history for cretaceous and cenozoic contourite depositional systems of the mozambique channel sw indian ocean this study describes previously unrecognized contourite depositional systems cdss in the mozambique channel which constrain palaeoceanographic models for this area the stratigraphic stacking patterns record nine seismic units su1 to su9 separated by eight major discontinuities a to h oldest to youngest key seismic markers in cds evolutionary history occur during aptian albian 122 ma late cenomanian 94 ma early 38 2 36 2 ma and late 25 23 ma oligocene and early middle miocene 17 15 ma epochs these record onset 122 to 94 ma growth 94 to 25 23 ma maintenance 25 23 to 17 15 ma and burial 17 15 ma to the actual time stages for cdss cdss first develop during the onset stage which coincides with the opening and deepening of the african southern ocean gateway at 122 and 100 ma respectively the growth stage beginning in the late cenomanian 94 ma correlates with the opening and deepening of the equatorial atlantic gateway during the growth stage two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early 38 2 36 2 ma and late 25 23 ma oligocene these in turn coincide with the onset and local enhancement of antarctic water masses cds growth continued until the early middle miocene during the maintenance stage 17 15 ma most cds growth ceased at the end of the maintenance stage circulation of the north atlantic water mass into the southern hemisphere led to a deepening of antarctic water masses in the area

molybdenum isotopes in late archean carbonate rocks implications for early earth oxygenation abstract we present carbonate mo isotope data of the 2 64 2 5 ga old ghaap group transvaal supergroup south africa to contribute to the debate on early earth oxygenation mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non skeletal carbonates closely monitor the ambient seawater 98 95 mo composition the record presented here is characterized by three phases of moderately to strongly fractionated 98 95 mo with values of up to 1 6 interrupted by two shifts towards detrital levels this near continuous presence of heavy 98 95 mo suggests a quasi permanent availability of free atmospheric o 2 because fractionation relative to the continental background signature occurs exclusively when mo is present as dissolved molybdate ions moo 4 2 we compare our carbonate data to a previously published black shale mo record on samples from the same drill core the general interpretation of both records is identical a rise in o 2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by 150 ma however in the ghaap group carbonates fractionated mo is found at a time when the coeval black shales still indicate detrital sedimentation a combination of detritus admixture and or redox controlled mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale and heavier carbonate 98 95 mo the apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions redox vs non redox dependent such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers thus by means of the combined archives the conspicuous 98 95 mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic anoxic fluctuations simultaneously the carbonates provide a better approximation of the long term 98 95 mo seawater evolution this study introduces the mo isotopic composition of carbonates as a promising new tool for paleo redox studies in particular when combined with black shales

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

digital reconstruction and stratigraphic evolution of a microbial dominated isolated carbonate platform terminal proterozoic nama group namibia abstract a terminal proterozoic ca 550 ma isolated carbonate platform developed downdip on a carbonate ramp of the nama group of southern namibia the stratigraphic evolution of the platform was reconstructed from an extensive dataset that was compiled by using digital surveying technologies and traditional outcrop observations digital field geology techniques enabled rapid and accurate mapping over the 10 km outcrop belt with spatial and geological information captured at scales ranging from centimeters to kilometers furthermore these digital geology techniques made the reconstruction and interpretation of the development of the carbonate platform considerably more efficient and quantitative the carbonate platform deposits encompass three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long term accommodation increase aggradation and progradation during the first cycle produced a flat topped sheet like platform deposits of the first cycle show a coarsening upward motif grading from dominantly columnar stromatolitic thrombolites and interbedded mudstone grainstone at the base to dominantly massive and cross bedded dolostones toward the top the second cycle features aggradation and is typified by the formation of a distinct margin and development of a bucket shaped geometry the margin contained thrombolite mounds and domes whereas columnar stromatolitic thrombolites dominated the platform interior the third and last cycle records a gradual deepening trend with initial aggradation and formation of well bedded thin deposits in the interior and microbial mounds at the margins while the interior drowned the platform margins kept up with rising sea level and complex pinnacle reefs formed which contained fused and coalesced thrombolite mounds flanked by bioclastic grainstones and collapse breccias the top of the unit consists of a set of isolated large thrombolite mounds flanked by shale and is interpreted to indicate the final stage of the carbonate platform before drowning the overall geometric evolution and spatial scale of this platform from flat topped to bucket shaped with elevated margins is recorded in many proterozoic and phanerozoic isolated carbonate platforms this study clearly illustrates that accommodation changes are a fundamental control on the development of carbonate platforms regardless of whether they are formed from microbial or metazoan components or are of precambrian or phanerozoic age significantly the architectural elements and spatial dimensions are comparable throughout earth history

upb geochronologic constraints on deposition of the campbellrand subgroup transvaal supergroup south africa abstract a volcanic ash bed that crops out 40 50 m below the contact between the campbellrand subgroup and the kuruman iron formation transvaal supergroup south africa yields an u pb zircon age of 2521 3 ma this new age demonstrates that the campbellrand malmani carbonate platform was deposited during late archean time and is the oldest extensively preserved carbonate platform known in addition the age constrains deposition of the kuruman griquatown and penge iron formations to latest archean to earliest paleoproterozoic time although a previously reported pb pb age of 2557 49 ma for dolomites at the base of the campbellrand subgroup is broadly consistent with this u pb zircon age new pb isotopic data for carbonate cements and massive sulfides lie along the same pb pb array as the dolomites this relationship implies that the isochron is the result of post depositional resetting of the u pb system in dolomite by fluids responsible for sulfide mineralization and that the age which corresponds to its slope has no geological significance

thrust zones and shear zones of the margin of the namaqua and kheis mobile belts southern africa abstract the prieska area of the cape province of south africa lies across the junction between two major proterozoic low angle shear thrust zones the first zone marks the margin to the kheis mobile belt and carried proterozoic schists and quartzitic sediments the matsap together with archaean basement to the se over a foreland of proterozoic transvaal sediments the second re thrust the matsap rocks to the ne but also covered them with high grade metamorphic rocks of the namaqua belt later dextral strike slip shear zone movements and large back folds have enabled the se verging thrust zones to be traced well over 100 km on to the foreland thus giving a minimum estimate of displacement both sets of shears are considered to represent a type subduction zones some considerable distance from any oceanic suture

an expanded shale δ98mo record permits recurrent shallow marine oxygenation during the neoarchean abstract multiple attempts have been made using the ancient shale record to track the molybdenum isotope composition 98mo of seawater during the final two hundred million years of the archean eon 2 7 to 2 5 billion years ago or ga this seawater 98mo value is important because it should have scaled with levels of ocean oxygenation during the runup to the great oxidation event goe unfortunately however it is difficult to tell if the majority of the existing late archean shale 98mo record tracks an ancient seawater value here we further attempt to track pre goe seawater 98mo using an expanded and well characterized shale sample set from western australia jeerinah wittenoom and mt sylvia formations and south africa nauga and klein naute formations most importantly and in contrast to most previous mo isotope studies of similarly aged shales local redox conditions for our shales have been independently constrained using the iron fe speciation proxy and bottom water mo contents at the time of deposition have been qualitatively estimated using mo toc ratios local redox conditions and bottom water mo availability are important parameters because transfer of the seawater 98mo to sediments today is shown to be dependent on these conditions according to our updated sedimentary 98mo record seawater 98mo commonly exceeded 1 0 between 2 69 ga and 2 50 ga in order to drive such a heavy seawater 98mo there must have been a marine sink with a strong preference for lighter mass mo isotopes frequently present over this timeframe the operation of some anaerobic processes in late archean marine settings could theoretically explain the heavier seawater 98mo such processes are known to promote the preferential retention of lighter mass mo isotopes in marine sediments today e g interactions between mo and organic matter or the formation of thio complexes alternatively or in addition adsorption of lighter mass mo isotopes to fe and mn oxide minerals formed in oxygenated marine environments can explain the heavier seawater 98mo a compilation of previous work suggests that oxygenated shallow marine environments were fairly common during the late archean and thus mo adsorption to fe mn oxides formed in these settings probably played an important role in driving heavy seawater 98mo over this timeframe

transvaal supergroup carbonates implications for paleoproterozoic δ18o and δ13c records abstract the paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean atmosphere system in this study carbonates from the transvaal group the silverton lucknow rooinekke and particularly the duitschland formations were analyzed for isotopes of carbon oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution although the trace element data showed variable alteration of the carbonate samples selected samples with a lesser degree of alteration yielded interpretable results the best measured 87sr 86sr isotope value of 0 7031 at 2 35 ga ago fits well with the published curve for precambrian seawater confirming that paleoproterozoic budget of sr was dominated by input from continental sources during the paleoproterozoic times carbonates with 13c values of about 10 are known to have been present worldwide and they have been studied also during this research the observation that in the duitschland formation such high isotope values and normal marine values were both present within a single formation indeed within its single beds suggested that the large spread of 13c values reflects local or regional rather than global phenomena the oxygen isotope data show a large spread of values from 3 to 16 while their corresponding 13c signals are almost unchanged over this entire range with the lucknow and silverton formations clustering around 10 and the rooinekke formation around 2 the duitschland formation has a bimodal distribution with both 13c populations present the last formation also contains samples with 18o as low as 20 particularly in the vicinity of the bushveld intrusive body these highly 18o depleted samples show homogenization of 13c values towards 5 modelling suggests that the trends could have resulted from post depositional alteration by basinal fluids at near surface to about 50 c temperatures in a rock buffered system the duitschland formation samples particularly at duitschland farm the locality closest to the bushveld complex were then further altered at temperatures up to about 80 c in response to this intrusion

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

possible indicators of microbial mat deposits in shales and sandstones examples from the mid proterozoic belt supergroup montana u s a abstract it has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the proterozoic however domination of mat morphology by depositional processes post depositional compaction and poor potential for cellular preservation of mat building organisms make their positive identification a formidable challenge within terrigenous clastics of the mid proterozoic belt supergroup a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces among these are a domal buildups resembling stromatolites in carbonates b cohesive behaviour of laminae during soft sediment deformation erosion and transport c wavy crinkly character of laminae d bed surfaces with pustular wrinkled appearance e rippled patches on otherwise smooth surfaces f laminae with mica enrichment and or randomly oriented micas g irregular curved wrinkled impressions on bedding planes h uparched laminae near mud cracks resembling growth ridges of polygonal stromatolites and i lamina specific distribution of certain early diagenetic minerals dolomite ferroan carbonates pyrite although in none of the described examples can it irrefutably be proven that they are microbial mat deposits the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other proterozoic sequences

precambrian si isotope mass balance weathering and the significance of the authigenic clay silica sink abstract the geological cycles of carbon and silicon are linked via the silicate weathering feedback but growing understanding of reverse weathering the formation of authigenic clay minerals via back reaction of silica with cations indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed recent carbon cycle modeling has suggested that enhanced reverse weathering during precambrian time could have played an important role in regulating climate the topology of the precambrian silica cycle including the size of the authigenic clay sink and its influence on the carbon cycle is challenging to reconstruct due to inherent preservation biases in the rock record here we used an alternative approach using si isotope values to invert for the magnitudes of precambrian silica fluxes by assuming that the silica cycle must be in mass balance with the bulk silicate earth over geological time scales with respect to si isotopes to estimate uncertainties associated with these flux reconstructions we used monte carlo simulations based on the natural variation exhibited in si isotope ratio data from the precambrian sedimentary record n 2118 spanning 3 8 ga to 525 ma and modern authigenic clays n 123 the perspective of si isotope mass balance shows that the previously observed increase in chert 30si values across precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature results yielded a secular increase in the relative contribution of the authigenic clay sink across most of precambrian time this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater this result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust this approach also highlights key areas where improving si isotope records could further enhance our constraints on the precambrian silica cycle and its relationship with the carbon cycle and climate stability

giant subtidal stromatolites forming in normal salinity waters we report here the discovery of giant lithified subtidal columnar stromatolites 2 m high growing in 7 8 m of clear oceanic water in current swept channels between the exuma islands on the eastern bahama bank they grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples the discovery is important to geologists and biologists because similar organo sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the precambrian stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1 2 until the discovery of small lithified subtidal columnar stromatolites in the bahamas3 the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of hamelin pool at shark bay western australia4 7 shark bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height the shark bay stromatolites have strongly influenced geological interpretation by analogy many ancient stromatolites have been considered to have grown in intertidal and or hypersaline conditions8 although hypersalinity was not a necessity for growth during the precambrian because grazing metazoan life had not then evolved

correlating multiple neoarchean paleoproterozoic impact spherule layers between south africa and western australia abstract well studied successions in the griqualand west basin south africa and the hamersley basin western australia that both straddle the archean proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities each succession hosts a minimum of three impact spherule layers implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years the occurrences of the youngest spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact close similarities in stratigraphic setting age and petrographic characteristics of the spherules as well as the restriction of distinctive irregular melt particles to only these layers suggest the oldest layers in the two basins were likewise formed by a single impact in contrast the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the spherules suggesting they are products of different impact events these results suggest that using impact spherule layers to establish a global network of high resolution stratigraphic markers for early precambrian successions may be an achievable goal but careful comparisons between potentially correlative layers will be necessary to achieve it

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

filamentous microfossils in the early proterozoic transvaal supergroup their morphology significance and paleoenvironmental setting abstract well preserved filamentous microfossils siphonophycus transvaalensis n sp are described here from the carbonate campbellrand subgroup to iron formation kuruman iron formation transition of the transvaal supergroup south africa estimated to be 2 5 2 3 ga years old the microfossils occur in petrographic thin sections of a core sample of carbonate chert they are preserved by permineralization in both chert and in sparry ferroan dolomite stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence campbellrand to the overlying iron rich sediments of the kuruman griquatown iron formations the average 13 c value of the kerogen in the sample is about 36 9 the microfossils are filamentous unbranched tubular to somewhat flattened 15 25 m in diameter and a few to many hundred microns in length they exhibit a coriaceous finely granular external surface texture resulting from the presence of adhering randomly distributed fine mineralic carbonate needles in salient morphological characteristics they are comparable to the tubular originally polysaccharide encompassing sheaths of extant oscillatoriacean cyanobacteria in comparison with previously described precambrian microfossils these fossil filaments are unusual because of their preservation in sparry carbonate in addition to chert their relatively large diameter and their coating by adhering precipitated carbonate needles they appear to be among the oldest assured microfossils now known from proterozoic age sediments the microfossils are interwoven occurring in subparallel aggregates that form a stromatolitic mat like fabric they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a deep shelf environment at the front distal margin of the campbellrand carbonate platform the water depth for this environment at the break in slope between deep shelf and euxinic basin is estimated to have been of the order of 40 45 m paleomagnetic data support our interpretation that the micro organisms inhabited a warm water marine environment probably at low latitudes

feedbacks between flow sediment motion and microbial growth on sand bars initiate and shape elongated stromatolite mounds abstract elongated stromatolites are often used as indicators of current direction and shoreline orientation especially in paleoenvironmental reconstructions however mechanisms that create shore parallel m scale elongated stromatolite mounds in carbonate sand are not well understood we propose that this geometry is initiated by microbial growth on the parts of sand bars that experience low wave induced bed shear stresses we test this idea by growing microbial mats on carbonate sand bars in a laboratory wave tank cyanobacterial mats grow on the bar runnels where sediment motion is negligible but are absent from the bar ridges where the waves generate migrating ripples when microbially promoted lithification reinforces and preserves this initial pattern elongated stromatolites should initiate in the runnels of sand bars with long wavelengths 5 100 m and small width to wavelength ratios 0 3 these dimensions are consistent with modern shore parallel stromatolites in hamelin pool western australia and with patterns of microbial colonization in other sandy sediments this model of elongated stromatolite mounds can inform paleoenvironmental reconstructions by clarifying and quantifying feedbacks among waves sediment transport and microbial growth

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

sedimentology of the kuruman and griquatown iron formations transvaal supergroup griqualand west south africa abstract iron formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow water carbonate platform at the time of deposition of the campbellrand carbonate sequence a subsequent transgression resulted in the deposition of open shelf iron formation on top of the campbellrand carbonate platform progradational sedimentation coupled with shoaling followed and an iron formation sequence represented by the kuruman and griquatown iron formations was deposited this sequence consists from the base upwards of stacked open shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units toe of slope greenalite siderite rhythmites platform slope greenalite siderite rhythmites with grainflow bands platform edge sideritic orthochemical and allochemical iron formations epeiric sea orthochemical and allochemical sideritic hematitic and greenalitic iron formations supratidal disclutites and lacustrine banded greenalite lutite landwards the lacustrine felutites were followed by deltaic chloritic claystone siltstone and quartz wacke of the koegas subgroup autochthonous ferhythmites of the kuruman iron formation reach a maximum development in a basin near prieska whereas orthochemical and allochemical units are more abundant on the kaapvaal craton iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in eh and ph in the depository and may be related to biological activity chert mesobanding in the iron formations is essentially of an early diagenetic origin

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

u pb geochronology and paleomagnetism of the westerberg sill suite kaapvaal craton support for a coherent kaapvaal pilbara block vaalbara into the paleoproterozoic precise geochronology combined with paleomagnetism on mafic intrusions provides first order information for paleoreconstruction of crustal blocks revealing the history of supercontinental formation and break up these techniques are used here to further constrain the apparent polar wander path of the kaapvaal craton across the neoarchean paleoproterozoic boundary u pb baddeleyite ages of 2441 6 ma and 2426 1 ma for a suite of mafic sills located on the western kaapvaal craton in south africa herein named the westerberg sill suite manifests a new event of magmatism within the kaapvaal craton of southern africa these ages fall within a ca 450 myr temporal gap in the paleomagnetic record between 2 66 and 2 22 ga on the craton our older westerberg suite age is broadly coeval with the woongarra magmatic event on the pilbara craton in western australia in addition the westerberg suite on the kaapvaal craton intrudes a remarkably similar archean proterozoic sedimentary succession to that on the pilbara craton supporting a stratigraphic correlation between kaapvaal and pilbara i e vaalbara the broadly coeval westerberg woongarra igneous event may represent a large igneous province the paleomagnetic results are more ambiguous with several different possibilities existing a virtual geomagnetic pole obtained from four sites on the westerberg sills is 18 9 degrees n 285 0 degrees e a 95 14 1 degrees k 43 4 sample based vgp n 34 16 8 degrees n 2879 9 degrees e dp 4 4 degrees dm 7 7 degrees if primary i e 2441 2426 ma it would provide a further magmatic event within a large temporal gap in the kaapvaal craton s paleoproterozoic apparent polar wander path it would suggest a relatively stationary kaapvaal craton between 2 44 ga and 2 22 ga and ca 35 degrees of latitudinal drift of the craton between ca 2 66 ga and 2 44 ga this is not observed for the pilbara craton suggesting breakup of vaalbara before ca 2 44 ga however it is likely that the woongarra paleopole represents a magnetic overprint acquired during the ophtalmian or capricorn orogeny invalidating a paleomagnetic comparison with the westerberg sill suite alternatively our westerberg virtual geographic pole manifests a 2 22 ga magnetic overprint related to ongeluk volcanism the similarity between ongeluk and westerberg paleopoles however may also infer magmatic connections if both are primary directions despite the apparent 200 million year age this difference c 2015 elsevier b v all rights reserved less

3700 ma pre metamorphic dolomite formed by microbial mediation in the isua supracrustal belt w greenland simple evidence for early life chemical meta sedimentary rocks in the amphibolite facies 3700 ma isua supracrustal belt w greenland are mostly strongly deformed so there is only a small chance of the survival of features such as stromatolites or microfossils that would be direct proof of a 3700 ma biosphere therefore the search for evidence of 3700 ma life in isua rocks has focused on chemical signatures particularly c isotopes the new approach presented here is based on whole rock chemistry rather than isotopic signatures isua chemical sedimentary rocks have ca mg fe bulk compositions that coincide with ferroan dolomite siderite fe oxide mixtures most have low al2o3 tio2 contents 0 5 and 0 05 wt respectively showing minimal contamination from terriginous materials identical seawater like ree y shale normalised trace element signatures with la ce eu and y positive anomalies are found in magnetite rich banded iron formation bif such as the geochemical standard if g dolomite rich rocks and quartz carbonate calcsilicate rocks additionally from a rare small area of low deformation in isua there are 3700 ma pillow lava interstices consisting of quartz tremolite calcite derived from pre metamorphic dolomite silica thus the dolomite in the chemical sediments and the pillow interstice was part of the pre metamorphic assemblage and was deposited from seawater and or low temperature groundwater as shown by the ree y chemistry therefore at least some isua carbonate rocks are sedimentary or diagenetic in origin rather than being formed by metasomatism at 600 500

geochemical and mineralogic effects of contact metamorphism on banded iron formation an example from the transvaal basin south africa intrusion of a diabase sill into the paleoproterozoic kuruman iron formation transvaal basin south africa makes it possible to distinguish between effects of contact and regional metamorphism on mineralogic elemental and isotopic compositions of banded iron formation bif lithofacies temperatures within the aureole of contact metamorphism likely did not significantly exceed 450 c as suggested by the presence of early diagenetic carbonates greenalite and stilpnomelane formed prior to intrusion in chert rich facies as close as 4 m from the sill in sediments 32 m away from the intrusion a temperature of 280 c is indicated by the magnitude of 18o fractionation between successive chert and siderite microbands in unmetamorphosed equivalents similar isotopic analyses on whole rock samples yield temperature estimates between 150 and 230 c in comparison with samples of similar lithofacies from the underlying campbellrand kuruman transition zone carbonates from within the metamorphic aureole are depleted in 13c and 18o the abundance and c isotopic composition of total organic c toc in whole rock samples is lower and more enriched in 13c than those in samples outside of the zone of contact metamorphism within 5 m of the diabase sill the abundance of fe increases dramatically and the oxidation state of the sediments fluctuates considerably more than in sediments outside of the metamorphic aureole isotopic and mineralogic systematics in the zone of contact metamorphism in the kuruman iron formation are similar to those in siliceous dolomites and limestones altered by similar thermal processes as the most distinct isotopic and elemental variations occur within 10 m of the diabase sill it is suggested that volatilization and fluid infiltration associated with the intrusion were the main processes of alteration consideration of the effects of metamorphism both contact and regional as well as early diagenesis on the mineralogy and isotopic chemistry of bif sediments may be a key to our understanding of primary mineralogies and oxidation state of these enigmatic deposits

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

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

lithofacies control on multiple sulfur isotope records and neoarchean sulfur cycles abstract triple sulfur isotope ratios 32 s 33 s 34 s were measured for 141 bulk rock samples from two agouron scientific drill cores gkp01 and gkf01 that recovered neoarchean successions of the transvaal supergroup south africa these two deep time cores are correlated to each other with 14 tie lines using volcanic and impact spherule layers in a sequence stratigraphic framework allowing us to evaluate both lithofacies and temporal controls over multiple sulfur isotope systematics the ca 2 5 ga giga annum before present basinal klein naute formation and the ca 2 6 ga peritidal boomplaas and vryburg formations yield an array of data characterized by 33 s 1 4 34 s these linear trends are found in both shallow water and deepwater facies but are characteristic to rocks with high iron content suggesting these may reflect isotopic compositions of aqueous sulfide elemental sulfur reservoirs in the neoarchean oceans data that deviate from this linear array are interpreted as resulting from additional inputs of sulfide from microbial sulfate reduction with or without contribution from sulfur disproportionation sulfate derived sulfur evolved to be either enriched or depleted in 34 s depending on the local depositional environment for example the reivilo formation in core gkf01 is characterized by abundant microbialite textures and shows an isotopic signature of closed system sulfate reduction rapid cementation of these carbonate fabrics may have attenuated the supply of sulfate to pore waters resulting in the progressive 34 s enrichments during bacterial sulfate reduction below the sediment water interface in contrast signatures of open system sulfate reduction are associated with slope facies dominated by granular dolostones preserved in the upper nauga formations in gkf01 and gkp01 the two different sulfur isotope patterns interpreted to reflect closed versus open system sulfate reduction are both found in the lower reivilo formation in both cores this lateral variation of isotope signals documents that observed 34 s shifts can be controlled locally and may not have temporal significance this study demonstrates critical importance of recovering sulfur isotope data from stratigraphically correlated drill cores to evaluate both geographical and temporal shifts of sulfur cycles and their links to the great oxidation event at the end of the archean eon

an overview of the geology of the transvaal supergroup dolomites south africa in the neoarchaean intracratonic basin of the kaapvaal craton between approximately 2640 ma and 2516 ma two successive stromatolitic carbonate platforms developed deposition started with the schmidtsdrif subgroup which is probably oldest in the southwestern part of the basin and which contains stromatolitic carbonates siliciclastic sediments and minor lava flows subsequently the nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the transvaal supergroup epeiric sea around 2550 ma ago this transgression led to the development of a carbonate platform in the areas of the preserved transvaal and griqualand west basins which persisted for 30 50 ma during this time shales were deposited over the nauga formation carbonates in the southwestern portion of the epeiric sea a subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical iron rich silica precipitates of the banded iron formations bif over the entire basin carbonate precipitation in the archaean was largely due to chemical and lesser biogenic processes with stromatolites and ocean water composition playing an important role the stromatolitic carbonates in the preserved griqualand west and transvaal basins are subdivided into several formations based on the depositional facies reflected by stromatolite morphology and on intraformational unconformities interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins thorough dolomitisation of most formations took place at different post depositional stages but mainly during early diagenesis partial silicification was the result of diagenetic and weathering processes karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater such periods occurred locally at the time of carbonate and bif deposition main karstification however probably took place during an erosional period between approximately 2430 ma and 2320 ma

implications for neoarchaean ocean chemistry from primary carbonate mineralogy of the campbellrand malmani platform south africa the precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform building process during deposition of the 2560 2520 ma campbellrand malmani carbonate platform south africa aragonite fans and fibrous coatings are common in unrestricted shallow subtidal to intertidal facies they are also present in restricted facies but are absent from deep subtidal facies decimetre thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies the proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0 to 65 depending on the facies subtidal facies commonly contain 20 35 in situ precipitated carbonate demonstrating that neoarchaean sea water was supersaturated with respect to aragonite carbonate crystal growth rates were rapid compared with sediment influx rates and the dynamics of carbonate precipitation were different from those in younger carbonate platforms the abundance of aragonite pseudomorphs suggests that sea water ph was neutral to alkaline whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans

neoarchaean c 2 58 ga halite casts implications for palaeoceanic chemistry possibly the most extensive and best preserved archaean halite casts yet discovered occur in the c 2 58 ga upper black reef and basal oaktree formations transvaal supergroup in mpumalanga province south africa halite casts are isolated on bedding planes range in size from c 1 mm to 20 mm and have cubic dumbbell and triangular shapes as well as hopper like pyramidal hollows on cube faces some of the casts display distinct hopper shapes characteristic of halite crystals the halite cast bearing pavements are developed within silicified mudstone interbedded with siltstone or stromatolitic dolomite associated sedimentary structures pointing to subaerial exposure include adhesion ripples and warts desiccation and prism cracks rill marks and tepee structures halite cast bearing beds are interpreted as supratidal flat or sabkha deposits the presence of isolated casts and hopper shaped crystals suggests that halite resulted from displacive growth within the sediment from supersaturated residual brines after mudstone deposition absence of any indication of the former presence of gypsum or anhydrite supports previous contentions that the neoarchaean ocean was deficient in sulphate or contained a high bicarbonate to calcium ratio such that with progressive evaporation most calcium was consumed before the gypsum stability field was reached the association of halite and carbonate in the upper black reef and basal oaktree formations constrains the palaeolatitude of the transvaal basin at 2 58 ga to subequatorial 10 30

neoarchean carbonate associated sulfate records positive δ s anomalies mass independent fractionation of sulfur isotopes reported as 33s recorded in archean sedimentary rocks helps to constrain the composition of earth s early atmosphere and the timing of the rise of oxygen 2 4 billion years ago although current hypotheses predict uniformly negative 33 s for archean seawater sulfate this remains untested through the vast majority of archean time we applied x ray absorption spectroscopy to investigate the low sulfate content of particularly well preserved neoarchean carbonates and mass spectrometry to measure their 33 s signatures we report unexpected large widespread positive 33 s values from stratigraphic sections capturing over 70 million years and diverse depositional environments combined with the pyrite record these results show that sulfate does not carry the expected negative 33 s from sulfur mass independent fractionation in the neoarchean atmosphere

evolution of early proterozoic passive margin carbonate platform rocknest formation wopmay orogen northwest territories canada abstract the superbly exposed rocknest formation wopmay orogen northwest territories canada is an early proterozoic 1 89 ga carbonate platform that formed along the eastern passive margin of slave craton archean the palinspastically restored shelf sequence is an eastward thinning prism 0 1 100 m thick extending for over 220 km parallel to strike and over 200 km perpendicular to strike it contains from west to east slope outer shelf shoal complex and inner shelf facies slope facies are carbonate rhythmites megabreccias and platy rhythmite breccias outer shelf facies include stromatolitic reefal boundstone and backreef ooid intraclast grainstore shoal complex facies are shallowing upward cycles of tufa cryptalgalaminite laminated dolosiltite lutite overlain by tepees b eccias and pisolite these formed a paleotopographic high restricting an eastern lagoon from western ocean inner shelf facies are dominantly asymmetric shallowing upward cycles of lagoonal mixed carbonates and siliciclastics that grade into intertidal stromatolitic dolomites cyclic facies pass eastward into noncyclic siliciclastics of central and eastern lagoon the rocknest lagoon spanned the width of slave craton and was bounded by a western carbonate shoreline shoal complex and by a tectonically active eastern siliciclastic shoreline the rocknest carbonate shelf developed on an underlying siliciclastic shelf an initial ramp rapidly evolved into a progradational accretionary rim and shoal complex followed by vertical aggradation of the rim concurrently the shoal complex underwent numerous west to east progradational events to produce many shallowing upward cycles in the shelf interior cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf probably caused by fourth order eustatic sea level oscillations the shelf was terminally drowned and overlain by deep water synorogenic siliciclastics during attempted subduction of the margin carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity suggesting emergence of the shelf prior to drowning

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

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

the carbonates of the transvaal and griqualand west sequences of the kaapvaal craton with special reference to the lime acres limestone deposit the campbell group carbonates were deposited in a basin transgressing from southwest to the east and to northeast at the southwestern margin of the kaapvaal craton in griqualand west mainly tidal flat carbonates and shallow marine deposits are found to be followed by subtidal carbonates marking the slow transgression north of the griquatown fault where the center of the griqualand west sub basin developed continuous subsidence matched by stromatolitic carbonate growth led to accumulation of the thickest carbonate pile 2000 m there the largest limestone deposits of south africa are located the deposit at lime acres and mining operations of the ppc lime ltd are described and discussed in respect to diagenetic processes that permitted limestone preservation

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago