lithological discrimination based on statistical analysis of multi sensor drill core logging data in the matagami vms district quebec canada in mineral exploration traditional core logging is largely based on the visual inspection of drilled cores a process which is often subjective and non reproducible however a number of physical chemical and mineralogical properties of rocks can now be measured at high spatial resolution on drill cores the resulting large multiparameter databases can help geologists to quantitatively discriminate between lithologies study hydrothermal alteration and potentially vector towards mineralization multivariate statistical methods are important tools to assist geologists in interpreting such large databases we present an application of model based cluster analysis in the matagami base metal mining district more specifically to improve lithological discrimination at the zinc rich bracemac and mcleod volcanogenic massive sulfide vms deposits the model based cluster analysis method is able to efficiently discriminate different geological units encountered even in cases where two units are visually similar or in the presence of strong hydrothermal alteration

an isotopic model for basal freeze on associated with subglacial upward flow of pore water 1 subglacial freezing in polar glaciers can have a significant dynamical effect recent studies have shown that freezing of pore water flowing upward through subglacial fine grained sediments at the freezing interface and progression of this freezing front downward are responsible for fast ice flow stoppage in ice streams the upward pore water flow leads to the formation of debris bearing basal ice layers a model for stable isotope composition both in d and 18o is developed for predicting the isotopic composition of the ice segregated by such a mechanism the development of this isotopic model for water films present along the grains of the subglacial sediment predicts the absence of apparent fractionation for the ice formed this prediction is tested against two east antarctic outlet glaciers by studying the d 18o relationships in the basal ice layers of these glaciers

phanerozoic analogues for carbonaceous matter in witwatersrand ore deposits thoriferous bitumen nodules in quartz pebble conglomerates and pebbly sandstones of phanerozoic age in the british isles are analogues for the uraniferous bitumen nodules flyspeck in the archean witwatersrand gold uranium deposits they are comparable in morphology relationship to bedding planes replacive behavior toward quartz mineralogy fragmentation and displacement of inclusions in each case the quartz rich sedimentary host rocks represent regionally important transgressive deposits the contrast in dominant radioelement thorium or uranium probably reflects differences in mobility of the elements as a function of variable eh conditions the phanerozoic thoriferous nodules occur only in pale low eh beds which lack uranium deposits but in the archean witwatersrand deposits fractionation of uranium and thorium may not have been as marked in the phanerozoic examples there is good evidence for migration of hydrocarbons through the host rocks and the bitumen nodules can confidently be attributed a hydrocarbon origin by analogy this supports proponents of a hydrocarbon origin for the witwatersrand carbon rather than a microbial origin

the diagenetic and economic significance of composite grains of monazite and hydrocarbon in western australian arenites composite grains consisting of detrital monazite and solid hydrocarbon ranging up to 0 5 mm in diameter have been found in phanerozoic sedimentary rocks in three western australian basins the most common form is a monazite core surrounded by a hydrocarbon envelope the envelopes which have conchoidal fracture and radial cracks apparently congealed when hydrocarbons were polymerized and crosslinked by irradiation from the monazite some composite bodies contain fractured monazite grains within solid hydrocarbon and a few consist of a microbreccia of hydrocarbon and disseminated monazite fragments the breakup of the monazite was probably a diagenetic chemical process that began when crosslinked hydrocarbons within cracks and open cleavages swelled as they were infiltrated by fluid hydrocarbons of low molecular weight five microprobe analyses of the monazite reveal the following ranges in percent composition la 2 o 3 10 88 13 66 ce 2 o 3 25 58 28 78 pr 2 o 3 3 03 5 38 nd 2 o 3 9 15 12 11 tho 2 7 0 11 49 u 3 o 8 0 1 44 2v extends from 12 5 15 5 in the northern perth basin the composite bodies which contain cores of monazite derived from local gneissic basement rocks formed after the growth of siderite cement but before extensive silicification in the central perth basin the composite bodies seem to have formed after the growth of kaolinite and before silicification textures associated with the bodies allow them to be slotted into the diagenetic sequence and they therefore show when the oil was present analysis of the bodies has indicated a contribution of oil from the lower triassic kockatea shale but some may have grown from more than one generation of petroleum given the ubiquity of detrital monazite similar bodies are likely to be useful diagenetic and economic indicators elsewhere

diagenesis of low mobility elements ti rees th and solid bitumen envelopes in permian kennedy group sandstone western australia the permian kennedy group carnarvon basin western australia is a shallow marine shelf deposit of sandstone and interlaminated siltstone the original sand was apparently arkosic but was altered by dissolution of feldspar and by other diagenetic processes into a kaolinitic sideritic silicified quartz arenite after lithification hydrocarbons moved through the sandstone leaving microscopic solid bitumen envelopes that had formed around radioactive detrital minerals when irradiation polymerized crosslinked and locally solidified the oil most radioactive cores are monazite a few are zircon and xenotime some monazite grains contain minute elongate randomly oriented perforations that almost certainly represent fission tracks and are filled with bitumen the diagenetic sequence can be divided into three phases phase 1 the earliest is an episode of complex diagenesis phase 2 covers the time when pores were filled with hydrocarbons and the solid bitumen envelopes formed but other diagenetic reactions were inhibited phase 3 begins with the entry of formation waters phase 1 authigenic minerals comprise anatase anhydrite brookite florencite thorite halite muscovite pyrite quartz possible rutile siderite cryptocrystalline silica sphalerite and xenotime rare thorite thsio 4 locally replaces monazite siderite crystals contain nine zones the main variations being in fe and mg and contain solution cavities that may have formed in phase 1 two minerals not commonly reported as authigenic are florencite ce la al phosphate mainly as minute pseudocubic euhedra replacing feldspar and mica and xenotime ypo 4 as outgrowths on detrital zircon grains phase 2 diagenesis includes formation of the solid bitumen envelopes and phase 3 includes authigenesis of quartz kaolinite and zoned calcite and perhaps solution of some florencite the solid bitumen envelopes surrounding the radioactive accessory minerals enclose rare minute authigenic halite and anhydrite crystals whose absence elsewhere suggests that the envelopes preserved them from solution during phase 3 titanium dioxide probably from ilmenite or titano magnetite or both partly replaces feldspar and mica some aspects of the diagenesis are particularly significant 1 solid bitumen envelopes not only indicate the former presence of hydrocarbons but also enable the complex diagenetic sequence to be divided into three well defined phases 2 the envelopes can trap and preserve minerals apparently unable to resist solution outside the envelopes i e minerals that would otherwise have disappeared 3 the zoned siderite calcite and florencite indicate changes in pore water composition in the first and third diagenetic phases 4 ti and some ree were mobile at least on the scale of a thin section during early diagenesis

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

tectonic evolution of the late archaean to early proterozoic mount bruce megasequence set western australia the mount bruce megasequence set formerly the mount bruce supergroup was deposited on the pilbara craton during the late archaean and early proterozoic it comprises two megasequences the chichester range megasequence and the overlying hamersley range megasequence each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of phanerozoic style tectonics the basal nullagine supersequence which comprises mostly subaerial basalt and terrigenous sedimentary rock was deposited during an episode of wnw ese directed crustal extension that was initiated at about 2770 ma the overlying mount jope supersequence which comprises mostly basalt and tuff was deposited during the subsequent development of a wnw ese trending rift before about 2690 ma mudrock carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 ma marra mamba supersequence package were deposited on the ensuing divergent craton margin an unconformity or condensed succession covering a time span of as much as 130 m y separates the top of the chichester range megasequence from the hamersley range megasequence during which time an oceanic island arc is interpreted to have collided with the southern pilbara craton margin subsequently the subducting oceanic plate flipped from southwest dipping to northeast dipping and convergence resulted in the formation of a marginal subduction related orogen with associated arc and backarc volcanism iron formation and distal arc derived tuffs of the brockman supersequence package were deposited on a siliciclastic starved platform in a neutral continental backarc setting at around 2470 ma the overlying circa 2440 ma woongarra supersequence was also deposited in a backarc continental setting and comprises iron formation felsic tuffs and probably coeval mafic rocks the latter two rocks types were derived from a backarc magmatic belt shortening of the southern pilbara craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the turee creek supersequence were deposited and ultimately resulted in a continent continent collision the mount bruce megasequence set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen

organic geochemistry of the 9 6 km bertha rogers no 1 well oklahoma abstract organic geochemical analyses of fine grained rocks from the 9 590 km bertha rogers no 1 well have been carried out total organic carbon soxhlet extraction and silica gel chromatography c15 saturated and aromatic hydrocarbon gas chromatography and mass spectrometry pyrolysis kerogen analysis x ray diffraction and visual kerogen analysis rocks ranged in age from permian to ordovician the well has an estimated bottom hole temperature of 225 c some data from this study are inconsistent with conventional theories concerning the generation and thermal destruction of hydrocarbons for example appreciable amounts of c15 gas condensate like hydrocarbons are present in very old rocks currently at temperatures where current theory predicts that only methane and graphite should remain also substantial amounts of pyrolyzable c15 hydrocarbons remain on the kerogen in these deeply buried paleozoic rocks this suggests at least in somes cases that temperatures much higher than those predicted by current theory are required for generation and thermal destruction of hydrocarbons the data from this well also suggest that original composition of organic matter and environment of deposition may have a much stronger influence on the organic geochemical characteristics of fine grained sediments than has previously been ascribed to them the results from this well from other deep hot wells in which temperatures exceed 200 c and from laboratory experiments suggest that some of the basic concepts of the generation and maturation of petroleum hydrocarbons may be in error and perhaps should be reexamined

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

hydrocarbon shows and petroleum source rocks in sediments as old as 1 7 109 years we report the discovery of indigenous live oil in 1 4 gyr old rocks in the mcarthur basin of northern australia previously reported occurrences of indigenous precambrian oil are 1 gyr old1 5 potential petroleum source rocks in the mcarthur basin range in age from 1 4 to 1 7 gyr and were deposited in marine and lacustrine environments in parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons they span the period corresponding to the appearance of eukaryotic organisms and because of their low degree of thermal alteration they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers the hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin these results show that exploration of previously ignored mid proterozoic sediments may lead to the discovery of new reserves of oil

upb zircon geochronology of archaean felsic units in the marble bar region pilbara craton western australia abstract archaean supracrustal rocks that are well exposed in the marble bar region pilbara craton have been assigned to the warrawoona group and younger sequences the predominantly volcanic warrawoona group previously dated at 3300 to 3500 ma is largely basaltic with locally intercalated thick felsic volcanic units the supracrustal rocks have been folded and are distributed around the margins of large roughly circular to ovoid granitic gneissic batholiths a u pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area especially the big stubby north pole barite and miralga creek zn pb cu au deposits in support of efforts to improve archaean lead isotope models the results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area the new conventional zircon u pb data indicate that most of the warrawoona group from the duffer formation upwards was deposited over a period of 20 ma from 3470 ma to 3450 ma the age of the duffer formation is closely constrained by results of 3471 5 ma and 3465 3 ma and the age of the upper salgash subgroup appears to be established at 3454 1 ma the panorama formation at the southern margin of the north pole dome is 3458 1 9 ma unless the duffer formation was deposited over a period of at least 10 ma this casts doubt on a previously suggested panorama duffer correlation the age of the lower part of the north pole succession is greater than 3458 ma consistent with the correlation of the basal greenstones at north pole with the talga talga subgroup the north pole chert barite unit which is well known for its preservation of early archean stromatolites microfossils and evaporite sediments is clearly older than 3458 ma the 3325 ma age now established for the wyman formation shows that this unit must be excluded from the warrawoona group the age of the talga talga subgroup remains uncertain a felsic schist in the mount ada basalt which has been dated at 3449 3 ma is interpreted to be a sill probably related to the 3450 ma granitoid bodies an age 3724 ma for a zircon xenocryst in the panorama formation is the oldest obtained so far for zircon from the pilbara granite greenstone terrane and indicates the existence of pre pilbara supergroup sialic basement ages of 3465 3 ma and 3449 2 ma applied to previous pb isotope data for the big stubby and miralga creek deposits respectively in combination with comparable data for other syngenetic archaean sulphide deposits yield a general single stage pb evolution model with the parameters t0 4560 ma a0 9 0818 b0 9 9002 and c0 29 343 this gives model ages in accord with known constraints for many archean deposits a model lead age of 3490 ma for the north pole deposit which is older than 3457 ma may be too great but attempts to directly date this deposit have not yet succeeded there are striking parallels between the warrawoona group ages reported here and those recently obtained for the barberton region kaapvaal craton south africa the talga talga subgroup and lower onverwacht tjakastad subgroup sequences have approximately equivalent constraints and an age for the upper salgash subgroup agrees closely with those for the hoogenoeg formation of the upper onverwacht group geluk subgroup

archean oil evidence for extensive hydrocarbon generation and migration 2 5 3 5 ga archean sedimentary rocks from the pilbara craton australia contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite thorite and monazite grains the nodules are preserved in sandstones at several stratigraphic levels in the fortescue group 2 75 ga and lalla rookh formation 3 0 ga both nonmarine successions and in deltaic sediments of the mosquito creek formation 3 25 ga the most ancient evidence comes from the warrawoona group 3 46 ga where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification bituminous relics of petroleum are also commonly preserved in shallow marine sandstones of the black reef formation 2 59 ga and the witwatersrand supergroup 2 85 ga from the kaapvaal craton south africa along with subeconomic methane accumulations in all cases the petroleum was apparently sourced from archean shales generated during the archean and migrated before the late archean or early early proterozoic metamorphism occluded fluid pathways given this widespread and abundant evidence for hydrocarbon generation and migration in archean depositional basins it seems that primordial bacterial biomass producing labile type i kerogen was often buried in sufficient quantities to successfully generate and expel petroleum depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration and probably to the accumulation of petroleum thus the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in archean basins are the subsequent destructive effects of deformation and metamorphism which causes trap breaching imperfect sealing or thermal obliteration however there are ancient stable cratons where such disruption may not have occurred and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in archean rocks

the sichuan basin southwest china a late proterozoic sinian petroleum province abstract the sichuan basin in southwest china is a large intracratonic sedimentary basin which first subsided in the late proterozoic sinian the basin has had a long complex subsidence history that has continued through to the present day tectonic subsidence curves indicate that in its early history the basin developed as a complicated extensional basin whose subsidence and tectonic history was polyphase much of the subsidence was driven by pulses of extension followed by thermal relaxation of the lithosphere the sichuan basin is an important petroleum base for the people s republic of china because of the discovery and development of over 60 gas fields and 10 oil fields one of the fields is the large weiyuan gas field which is considered to be both sourced and reservoired in late proterozoic sedimentary rocks these rocks are the late sinian carbonates of the dengying formation which form part of a thick platform yangtze platform succession that was deposited from the sinian to the silurian the weiyuan gas field occurs in a large anticlinal structure 53 26 km 850 km2 of which 200 km2 is prospective for gas up until 1988 91 wells had been drilled in the field and 40 were in production production is approximately 800 000 m3 day 28 3 million cubic feet day with total production to 1987 being 13 6 billion m3 480 billion cubic feet total reserves in the weiyuan gas field are estimated at 30 40 billon m3 1060 1410 billion cubic feet the sinian and earliest cambrian succession in the sichuan basin is highly prospective for the discovery of additional reserves of natural gas

an early proterozoic three stage rift system kaapvaal craton south africa abstract a three stage model that constitutes a fully evolved rift system is proposed for a number of superimposed early proterozoic basins on the kaapvaal craton based on interpreted depositional systems and syndepositional tectonics this model consists of three superimposed basins 1 1 a pre graben stage that influenced the dominion group witwatersrand supergroup and klipriviersberg group of the ventersdorp supergroup 2 2 graben development that superimposed the platberg group bothaville and allanridge formations of the ventersdorp supergroup 3 3 post graben thermal subsidence that superimposed the black reef quartzite formation and the chuniespoort ghaap group of the transvaal supergroup oblique collision of the kaapvaal craton and zimbabwe craton is interpreted to have generated stresses that initiated the three stage system at different time intervals compressional deviatoric stress appears to have replaced tensional stress and kept the system in operation tectonically the system is analogous to the baikal rift system and the pannonian basin system while the general configuration of the superimposed basins is comparable to the north sea basin or the north china basin

volcanic sedimentary and tectonostratigraphic environments of the 3 46 ga warrawoona megasequence a review abstract the warrawoona megasequence comprises a 3 46 ga assemblage of tholeiitic and calc alkaline volcanic rocks interlayered with cherty sedimentary rocks deposition occurred in a range of shallow to deeper water environments ranging from the shoreline to distal deposits in sediment starved basins volcanism provided the dominant or only source of clastic sediment sedimentary rocks also contain evaporites probable stromatolites other evidence for the existence of microbial life and possible evidence of meteorite impact the tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic arc or near arc settings the compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc alkaline suites developed above subduction zones the calc alkaline suite has a complex petrogenesis which probably involves mantle derived basaltic melts intermediate to silicic melts derived from subducted mafic crust magma mixing and fractional crystallization intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the warrawoona megasequence contains examples of early archaean volcanic arc and near arc assemblages

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

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

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

precambrian vein pyrobitumen evidence for petroleum generation and migration 2 ga ago abstract isotopic optical reflectance and chemical analyses of large pure samples of lower proterozoic vein pyrobitumens anthraxolite and thucholite of older reports collected from five widely separated localities in the great lakes region of the united states and canada suggest that in the lower proterozoic oil was generated from kerogen and migrated in much the same way as during the phanerozoic furthermore maturation and organic metamorphism followed the same successive stages that mark the generation and destruction of phanerozoic petroleum kerogens derived from algae and bacteria were deposited with subaqueous muds that ultimately became black shales or slates some of the kerogen however yielded petroleum which migrated into fractures and subsequently became pyrobitumen the 13c values average 32 6 pdb this value compares favorably with those from phanerozoic petroleum and asphaltites and is consistent with the proposition that the carbon isotopes were fractionated by biological activity reflectance values of the pyrobitumens range from 0 9 for elliot lake ontario to 9 5 for iron river michigan and h c ratios range from 0 6 for elliot lake to 0 1 for sudbury ontario when compared with the stages of petroleum maturation these values indicate maturation which ranges from the catagenesis stage through the metagenesis stage of petroleum generation and metamorphism