depositional processes and stratigraphy of fluvially dominated lacustrine deltas mississippi delta plain abstract a geomorphic and stratigraphic investigation of lacustrine delta deposits in the atchafalaya basin louisiana revealed that lacustrine delta formation is rapid and cyclic in nature during the holocene thin coarse grained regionally extensive fluvially dominated lacustrine deltas filled interdistributary basins in the transition zone between the alluvial valley and the marine delta plain of the mississippi river one delta lake fausse pointe delta prograded 6 5 km partially filling the lake and covering more than 29 km2 in area in twelve years depositional processes in lake fausse pointe ranged from suspension settling of mud and organic matter during low sediment input to traction deposition of sand during floods hyperpycnal flow conditions set up by the introdu tion of sediment laden river water into the freshwater lake induced underflows that scoured the lake bottom and deposited upward coarsening lobes the low relief of the mississippi basin a constant large volume of fine grained sediment supply and prevalent fluvial processes have formed fluvially dominated lacustrine deltas that differ sedimentologically and stratigraphically from gilbert type and brackish water lacustrine deltas parallel laminated prodelta mud rippled to cross laminated delta front silty sand and very finegrained to medium grained distributary mouth bar sand are characteristic of rapidly deposited sediment whereas rooted and burrowed sediments signify periods of minimal deposition in a fluvially dominated setting delta geometry and sediment distribution patterns are controlled by river mouth processes basin shape and bathymetry thickest sand accumulations occur in linear dip elongate distributary m uth bar and natural levee lobes that are separated laterally by mud filled channels and interdistributary troughs lacustrine delta deposition is driven by basin subsidence regional downwarping and sediment compaction and the concomitant development of distributary channels in the interdistributary basins numerous lacustrine deltaic wedges each bounded by rooted backswamp clay are preserved multilaterally in atchafalaya basin sediments although lacustrine deltaic deposits constitute much of the basin s sedimentary fill each individual delta sequence records depositional events of only 100 years duration these deltaic deposits represent basin filling aggradational episodes that preceded the formation of marine delta complexes such as the maringouin teche la fourche and atchafalaya deltas of the mississippi river

sea level changes in a back arc foreland transition the late carboniferous permian karoo basin of south africa abstract the late carboniferous to early permian succession in the karoo basin consists in ascending stratigraphic order of diamictites mudrocks greywackes and immature sandstones the diamictites are interpreted as glacigene deposits laid down by a marine ice sheet on a shelf post glacial suspension settling of predominantly mud occurred on the basin floor during open marine and later brackish conditions basin floor fan deposits accumulated in the foredeep whereas deltaic mud silt and sand finally infilled the basin glacigene sedimentation occurred in a back arc basin during the late carboniferous and early permian the post glacial succession accumulated in a foreland basin in which the rising orogenic belt was the predominant sediment source relative sea levels following the major namurian regression apparently remained low during the extensive gondwana glaciation marine ice sheets are highly sensitive to sea level fluctuations and minor sea level rises probably climatically controlled were recorded as interglacials when parts of the marine ice sheet collapsed final collapse of the marine ice sheet was associated with a major transgression post glacial mudrock deposition was characterised by relatively high sea levels except for the period of basin floor fan deposition when there was a sea level lowstand final infilling of the basin by prograding deltas was accompanied by a major regression the late carboniferous to early permian sea level fluctuations in the karoo basin show correspondence with global eustatic cycles reflecting low tectonic interference with sea level patterns however late permian sea levels were predominantly tectonically controlled as a result of foreland basin evolution possible global eustatic events complemented apparent sea level changes within the basin during this period

paleo to mesoarchean ttg accretion and continental growth in the western dharwar craton southern india constraints from shrimp u pb zircon geochronology whole rock geochemistry and nd sr isotopes a multidisciplinary study involving field petrographic shrimp u pb zircon titanite ages whole rock geochemical and nd sr isotope data is presented for the peninsular gneisses and associated plutons forming core of the dharwar craton two major periods 3350 3280 ma and 3230 3200 ma of crustal growth through ttg accretion sub contemporaneous with greenstone volcanism are documented elemental and nd sr isotope data suggest that the ttgs originated by low and high pressure melting of heterogeneous mafic sources thickened island arc or oceanic plateau crust at different depths among the early accreted ttgs magmatic protoliths of low al gneisses formed by low pressure 10 12 kbar melting of a depleted mafic source at shallow levels island arc type crust with plagioclase amphibole ilmenite in the residue whilst high al gneisses were probably produced by high pressure melting 14 18 kbar of a chondritic or a less depleted mafic source with garnet amphibole pyroxene ilmenite in the residue base of island arc crust or thickened oceanic plateau the 3230 3200 ma trondhjemitic plutons marking the second accretion event derived by higher pressure 14 16 kbar melting of a mafic source island arc type crust with garnet plagiocalse hornblende ilmenite with a minor involvement of previously accreted ttgs this plutonism was coeval with a major phase of crustal scale diapiric overturn that had a major imprint on crustal reworking a later magmatic episode at ca 3100 ma produced granitic plutons derived from crustal melting and leucocratic gneisses derived from a depleted mafic source in the lower crust documentation of depleted shallow and deep mantle reservoirs during 3350 3280 ma magmatism implies massive differentiation of the mantle during an earlier episode of crust formation around 3800 600 ma as substantiated by nd model ages and published pb isotope data as well as u pb detrital zircon ages with in situ hf isotope data

bison and deer bone mineral densities comparisons and implications for the interpretation of archaeological faunas abstract volume bone mineral densities are derived for modern bison skeletal parts using dual energy x ray densitometry these density measurements are compared to those published by lyman 1984 journal of anthropological archaeology3 259 299 for small cervids differences that exist can be explained in terms of body size and patterns of locomotion density data then are compared to skeletal part frequencies from two great plains bison assemblages bison bone mineral densities provide a more appropriate model than do cervid densities for examining variability in large bovid bone assemblages and point to the need for constructing similar data bases for other taxa

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

comments on the paper reconnaissance of upper jurassic morrison formation ichnofossils rocky mountain region usa paleoenvironmental stratigraphic and paleoclimatic significance of terrestrial and freshwater ichnocoenoses by stephen t hasiotis

pleistocene seasonal temperature variations recorded in the d18o of bison priscus teeth abstract oxygen isotope analysis of phosphate in tooth enamel of mammals 18 o p constitutes a valuable method to reconstruct past air temperatures in continental environments the method is based on interdependent relationships between the 18 o of apatite phosphate body fluids environmental waters and air temperatures continuous tooth growth and absence of enamel remodelling in bovid teeth ensures a reliable record of the intra annual variability of air temperature through an incremental 18 o analysis from apex to cervix this method has been applied to bison priscus dental remains of the late middle pleistocene from the fossiliferous layer of a cave at coudoulous i in south western france layer 4 the stacked oxygen isotope signal obtained by combining 9 bison teeth shows sinusoidal variations 15 0 to 19 1 v smow of seasonal origin over 2 5 yr the corresponding computed mat of 9 3 c is about 4 c lower than at present seasons appear more contrasted in coudoulous i during layer 4 deposition with summers as warm as present ones 19 3 c and significantly colder winters about 0 3 c compared to 6 1 c at present

rates and mechanisms of mesoarchean magmatic arc construction eastern kaapvaal craton swaziland the mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth we summarize the first detailed study of a well preserved mesoarchean crustal section from the oldest portion of the kaapvaal craton southern africa which records the complex interactions between deformation and magmatism during craton assembly ca 3 3 3 2 ga we use chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims u pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic usutu magmatic suite intruded into the ca 3 66 3 45 ga ancient gneiss complex over a period of 16 ma ca 3236 3220 ma as discrete pulses of magma with variable intrusive styles usutu rocks retain magmatic fabrics that preserve a history of nw se regional shortening consistent with synchronous deformation recorded in the adjacent barberton greenstone belt u pb zircon dates of ca 3 28 3 23 ga and apatite cooling dates from the nhlangano gneiss se of the ancient gneiss complex reveal that they represent an older largely undocumented period of crustal growth and modification synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca 3 28 to 3 22 ga geochemistry of the usutu suite reveals differences in magma sources from north to south which when combined with nd isotopic signatures are consistent with their production in a subduction zone the combination of 1 ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for archean rocks

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

zircon geochronology and hf isotopes of the dwalile supracrustal suite ancient gneiss complex swaziland insights into the diversity of palaeoarchaean source rocks depositional and metamorphic ages abstract we report the results of combined u pb and lu hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano sedimentary rocks of the dwalile supracrustal suite dss from the ca 3 66 to 3 20 ga ancient gneiss complex agc in swaziland the results indicate that the dss is made up by palaeo and mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the neoarchaean the oldest dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca 3 46 ga and are derived from the same magma source as the tsawela gneisses based on the combined u pb hf isotope data two groups of metasedimentary rocks can be distinguished an older dwalile group i that is similar in age to the hooggenoeg and kromberg formations 3 46 3 40 ga but with a different source area a younger dwalile group ii is coeval with the fig tree and possibly moodies groups the u pb hf isotope data and petrological observations demonstrate that the dss was affected by three stages of metamorphic overprint an event at 3 23 ga pervasive amphibolite facies metamorphism at 3 15 ga and low p granulite facies metamorphism at 2 99 ga metamorphic zircon in the amphibolite facies rocks resulted from new growth in aqueous fluids at 3 15 ga whereas in the granulite facies rocks new zircon was formed in the presence of melt the 3 23 ga event coincides with the last significant metamorphic overprint documented in the barberton greenstone belt bgb whereas as two younger events are restricted to the agc the data support the model suggesting amalgamation between the agc and bgb terranes at ca 3 23 ga in addition they require crustal thickening in the agc terrane at ca 3 15 ga causing barrovian type metamorphism in the dss and granulite facies metamorphism in the luboya kubuta terrane low p granulite facies metamorphism at ca 2 99 ga is most likely related to crustal extension during the onset of pongola basin rifting causing mantle upwelling mafic magmatism and heat transfer from the mantle into the thinned crust

taphonomic analysis in lacustrine environments two different contexts for triassic lake paleofloras from western gondwana argentina abstract during the earliest triassic several rift basins developed along the western margin of gondwana associated with the pre breakup of pangea they were filled by exclusively non marine sediments including alluvial fluvial and lacustrine deposits in the ischigualasto villa union basin the lacustrine deltaic succession is placed in the los rastros formation and consists of several coarsening upward cycles of black shale siltstone and sandstone the paleontologic content of the succession includes abundant floral remains related to the dicroidium type flora invertebrates conchostracans insects and vertebrates fishes a temnospondyl amphibian ichnites at the cerro puntudo area in the cuyana basin the lacustrine succession forms the upper part of the cerro puntudo formation and consists of limestone stromatolitic limestone mudstone sandstone and tuff the paleontologic content includes scarce floral remains and rhizoliths invertebrates are represented exclusively by traces associated with ichnofacies of skolithos and scoyenia and vertebrates by a fragment of the pelvic girdle of a basal arcosaur the taphonomic analysis performed in the two triassic lacustrine successions allows recognition of two different taphonomic histories for the plant remains the los rastros lake preserved both autochthonous originated in the littoral zone and allochthonous originated in the upstream fluvial system elements the offshore lacustrine area was dominated by autochthonous well preserved elements and allochthonous plant debris and wood which formed time averaged accumulations the delta deposits are characterized by allochthonous elements with varied preservational conditions usually showing evidence of mechanical degradation and accumulation within a short time autochthonous and allochthonous material were preserved in the los rastros lake by means of anoxic conditions in the offshore lacustrine area and high sedimentation rates in the delta in contrast the cerro puntudo lake preserved only autochthonous elements originated in the littoral zone including rhizoliths and foliar material which formed autochthonous and parautochthonous accumulations at the littoral zone in spite of aerobic conditions this was the result of tuffaceous material that enhanced preservation thus these very different lacustrine environmental contexts are showing different preservational modes the fossil assemblages allowed the reconstruction of the original communities from this part of gondwana thus the los rastros lake margins were characterized by shrubs and small trees of ginkgoales and corystospermales and herbaceous members of the sphenophyta the sphenophytes were also the dominant floral component along the river margins whereas the corystospermales cycadales pteridophyta and conifers formed the woodland upstream probably related to the floodplains of a trunk fluvial system the littoral zone of the cerro puntudo lake was dominated mainly by herbaceous sphenophytes and lycopsids these fossil assemblages characterize the paleoflora associated with lacustrine systems a fuller understanding of the processes that generate these assemblages is essential for comparisons with other continental paleobotanical records in the middle triassic of gondwana e g australia south africa

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

trace fossil suites and composite ichnofabrics from meandering fluvial systems the oligocene lower freshwater molasse of switzerland abstract the oligocene lower freshwater molasse of switzerland is host to spectacular outcrops of fluvial deposits in distal areas of the swiss molasse basin deposits of a north eastward draining meandering system are exposed combined sedimentologic and ichnologic observations in the chli sunnhalde and roggwil quarries allow an improved understanding of animal substrate interactions in deposits of meandering fluvial systems strata include meander belt sandstone levee sandstone and mudstone crevasse channel and splay sandstone floodplain mudstone and sandstone and paleosols while the meander belt and the more proximal crevasse channel deposits are typically unbioturbated the more distal deposits show a clear biogenic overprint with distal crevasse channel crevasse splay and floodplain deposits being intensely bioturbated simple grazing and locomotion trails included in the cochlichnus anguineus helminthoidichnites tenuis suite were formed subaqueously in ponded areas of the floodplain meniscate burrows taenidium isp suite and root trace fossils occur along the margin of these floodplain water bodies and at the top of crevasse splay deposits some of the floodplain crevasse splay and crevasse channel deposits are pervasively bioturbated by crayfish burrows camborygma litonomos suite crevasse splay deposits locally host escape trace fossils where cross cutting relationships are apparent a colonization sequence can be established as follows the cochlichnus anguineus helminthoidichnites tenuis suite is cross cut by the taenidium isp suite which in turn is cross cut by the camborygma litonomos suite and root trace fossils composite ichnofabrics are common in these meandering river deposits recording progressive lowering of the water table in overbank subenvironments frequency of depositional events was a first order control on degree of bioturbation position of the water table and degree of substrate consolidation exerted considerable influence over the types of trace fossils produced distinct bioturbation phases may result in the formation of different trace fossil suites representative of disparate ichnofacies preserved on a single bedding plane this study further supports the notion that meandering fluvial systems are characterized by trace fossil suites belonging to the scoyenia camborygma and or mermia ichnofacies

correlation between the oxygen isotope ratio of north american bison teeth and local waters implication for paleoclimatic reconstructions abstract the oxygen isotope ratios of tooth enamel carbonate from 64 north american bison bison bison from eleven locations were measured the mean enamel oxygen isotope ratios for bison populations ranged from 28 3 to 15 9 smow and correlated well with the annual mean oxygen isotope ratios of local surface waters and precipitation the standard deviation of oxygen isotope values among different individuals within each bison population averaged 1 0 and ranged from 0 7 to 1 4 the variability of enamel oxygen isotope ratios was not significantly different among different populations and did not correlate with changes in temperature precipitation or relative humidity these results demonstrate that the average oxygen isotope values of bison tooth enamel can be used for use as a quantitative proxy for reconstructing the values of surface waters and therefore may provide valuable paleoclimatic information this study provides a baseline comparison for analyses of the oxygen isotope ratios of bison and other large herbivores from across north america

hafnium isotope record of the ancient gneiss complex swaziland southern africa evidence for archaean crust mantle formation and crust reworking between 3 66 and 2 73 ga abstract combined u pb and lu hf isotope analyses of zircons from 16 tonalite trondjemite granodiorite ttg gneiss and granite samples from swaziland reveal that the oldest rocks of the ancient gneiss complex in southern africa formed by reworking of early archaean or perhaps late hadean crust at 3 66 ga and that new crust was extracted from a depleted mantle source during palaeoarchaean events between 3 54 and 3 32 ga this interpretation is supported by ehf t of 1 6 2 0 obtained from 3 66 ga ttg gneisses corresponding to hafnium model ages between 3 77 0 18 ga for a presumed hadean early archaean chondritic mantle and 4 08 0 18 ga for a presumed hadean depleted mantle reservoir with the first model age being the most likely in the light of recent data from worldwide sources furthermore it is reflected by superchondritic ehf t up to 2 2 2 0 for ttgs formed at 3 54 3 45 and 3 32 ga the new datasets additionally show that the palaeoarchaean crust formed between 3 54 and 3 32 ga was intensely reworked afterwards without significant addition of depleted mantle derived material during orogenic and intracratonic melting processes at 3 23 3 1 and 2 7 ga this is well reflected by an array of decreasing ehf t from 2 2 to 7 2 between 3 3 and 2 7 ga which can be forced by 176 lu 177 hf of 0 0113 which is similar to that of present day average continental crust and might result from lower crust zircon fractionation during archaean crust reworking supplementary material results of in situ u pb and lu hf isotope zircon analyses and concordia diagrams are available at www geolsoc org uk sup18465

a long and winding road towards a palethnograpic interpretation of the middle gravettian site of la picardie indre et loire france abstract after nine years of excavation 1998 2008 the site of la picardie has delivered a significant lithic collection more than 13 000 artefacts coordinated chronologically related to the raysse burin phase of the gravettian second phase of middle gravettian ca 25 23 ky bp uncal the study of the lithic material has provided several important results including the chronological attribution of the site the description of a new type of retouched bladelet and the reconstruction of the flint knapping process for blade and bladelet production on site the palethnographic study and interpretation of the site have however not been discussed to date in spite of some taphonomic issues the coalescence of different studies has allowed us to further our understanding of the site notably via the functional analysis of stone tools the evaluation of different skill levels in flint knapping the vertical and horizontal distributions of artefacts and tools and finally the study of the different organised structures made of limestones blocks using several examples we will describe in a step by step manner the various elements that helped us to rebuild the probable function s of the site and propose a first palethnographic assessment

formation of archean 3600 2500 ma continental crust in the dharwar craton southern india abstract the generation preservation and destruction of continental crust on earth is of wide interest in understanding the formation of continents cratons and supercontinents as well as related mineral deposits in this contribution we integrate the available field petrographic geochronologic elemental nd hf pb isotope data for greenstones ttg gneisses sanukitoids and anatectic granites from the dharwar craton southern india this review allows us to evaluate the accretionary processes of juvenile crust mechanisms of continental growth and secular evolution of geodynamic processes through the 3600 2500 ma window hence providing important insights into building of continents in the early earth the dharwar craton formed by assembly of micro blocks with independent thermal records and accretionary histories the craton can be divided into three crustal blocks western central and eastern separated by major shear zones the western block contains some of the oldest basement rocks with two generations of volcano sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic ttgs abundant younger transitional ttgs remnants of ancient high grade supracrustal rocks linear volcanic dominated greenstone belts voluminous calc alkaline granitoids of sanukitoid affinity and anatectic granites in contrast the eastern block comprises younger transitional ttgs abundant diatexites thin volcanic sedimentary greenstone belts and calc alkaline plutons published geochronologic data show five major periods of felsic crust formation at ca 3450 3300 ma 3230 3150 ma 3000 2960 ma 2700 2600 ma and 2560 2520 ma which are sub contemporaneous with the episodes of greenstone volcanism u pb ages of inherited zircons in ttgs as well as detrital zircons together with nd pb hf isotope data reveal continental records of 3800 3600 ma the u pb zircon data suggest at least four major reworking events during ca 3200 ma 3000 ma 2620 2600 ma and 2530 2500 ma corresponding to lower crustal melting and spatially linked high grade metamorphic events the ttgs are sub divided into the older 3450 3000 ma ttgs and the younger 2700 2600 ma transitional ttgs the older ttgs can be further sub divided into low al and high al groups elemental and isotopic data suggest that the low al type formed by melting of oceanic island arc crust within plagioclase stability field in contrast the elemental and isotopic features for the high al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels 55 65 km with variable garnet and ilmenite in residue the transitional ttgs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub arc mantle with minor contamination of ancient crustal components the geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs finally anatectic granites were produced by reworking of crustal sources with different histories in the light of the data reviewed in this contribution we propose the following scenario for the tectonic evolution of the dharwar craton during 3450 3000 ma ttgs sources oceanic arc crust formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in ttg magmas on the contrary by 2700 ma the depth of slab melting increased melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc mantle generated the magmatic precursor to transitional ttgs finally at ca 2600 2560 ma eventual breakoff of down going slab caused mantle upwelling which induced low degree 10 15 melting of overlying enriched mantle at different depths thereby generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism reworking and final cratonization the crustal accretion patterns in the dharwar craton share similarities with those in other archean cratons such as the bundelkhand craton in central india pilbara yilgarn craton in western australia southern africa swaziland and limpopo belt north china craton tanzania craton antongil craton ne madagascar

trace fossils from carboniferous floodplain deposits in western argentina implications for ichnofacies models of continental environments abstract floodplain deposits from the carboniferous tupe formation of western argentina contain a low diversity ichnofauna of invertebrates and plants the assemblage consists of archaeonassa fossulata didymaulichnus lyelli helminthoidichnites tenuis palaeophycus tubularis planolites isp and root traces this ichnofauna is characterized by the dominance of very simple forms superficial or very shallow trace fossils combination of locomotion grazing and dwelling structures and production by arthropods and vermiform organisms primary sedimentary fabrics are disturbed only by plant trace fossils while animal trace fossils are mostly restricted to bedding surfaces resulting in the virtual absence of bioturbation the poorly defined morphology of the ichnofossils and the absence of backfilled striated trace fossils and structures indicative of subaerial exposure suggest subaqueous production the envisaged depositional environment is a freshwater body developed in a floodplain and overfilled by overbank deposits a review of floodplain ichnofaunas documented from the stratigraphic record indicates that two recurrent assemblages may be recognized the first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils moderate to high diversity of vertebrate structures and common presence of meniscate backfilled structures bilobate trace fossils with scratch marks arthropod trackways and tetrapod trackways it is regarded as an example of the scoyenia ichnofacies developed in desiccated floodplains the second type of floodplain assemblage illustrated by the example documented in this paper is characterized by grazing trails locomotion trails and dwelling burrows low to rarely moderate ichnodiversity simple trace fossils and superficial to very shallow structures although formed in floodplain environments also this type of assemblage is remarkably different from the scoyenia ichnofacies it is here regarded as an example of the mermia ichnofacies basic features of these ichnofaunas reflect subaqueous production of the assemblage the lower ichnodiversity of these floodplain assemblages in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of floodplain water bodies our study supports the view that ichnofacies are not indicators of sedimentary environments but reflect sets of environmental factors

stratigraphy and sedimentology of the permian of the falkland islands lithostratigraphic and palaeoenvironmental links with south africa comparison of the facies ichnology and palaeocurrent patterns of the permian of the falkland islands and the dwyka ecca and beaufort part groups in south africa supports the hypothesis that the falkland islands lay east of south africa in a rotated position prior to continental break up key sections in the falklands nw lafonia and the eastern cape ecca pass would have lain about 200 km apart in permian time in the absence of accurate chonostratigraphic and biostratigraphic information from the permian of the falkland islands emphasis is placed on lithostratigraphic comparison the permian successions in both areas pass from diamictite through basin floor sediments including organic rich shale rhythmites and turbidites mud dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments points of close lithostratigraphic comparison are 1 the diamictites have identical appearance facies and compatible palaeocurrents 2 glacial sections are overlain by organic rich black shales 3 k bentonites occur in basin floor successions 4 presence of basin floor turbidite sandstone units with interbedded rhythmite units and a distinctive umfolozia undichna ichnofauna 5 coarsening up delta front sequences are overlain by fining up channel sandstone facies 6 there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc the permian successions of the two areas form parts of the fill of the eastern end of the main karoo basin and were subject to the same influences of tectonic development sediment derivation eustatic variation and climatic change

in situ u pb geochronology and hf isotope analyses of the rayner complex east antarctica in situ zircon u pb and hf isotopic analysis via laser ablation microprobe inductively coupled plasma mass spectrometer lam icpms of samples from kemp and macrobertson lands east antarctica suggests that the kemp land terrane evolved separately from the rest of the rayner complex prior to the ca 940 ma rayner structural episode several archaean metamorphic events in rocks from western kemp land can be correlated with events previously reported for the adjacent napier complex recently reported ca 1 600 ma isotopic disturbance in rocks from the oygarden group may be correlated with a charnockitic intrusion in the stillwell hills before ca 1 550 ma despite being separated by some 200 km t hf dm ages indicate felsic orthogneiss from rippon point the oygarden group havstein island and the stillwell hills share a ca 3 660 3 560 ma source that is indistinguishable from that previously reported for parts of the napier complex more recent additions to this crust include proterozoic charnockite in the stillwell hills and the vicinity of mawson station these plutons have distinct 176hf 177hf ratios and formed via the melting of crust generated at ca 2 150 2 550 ma and ca 1 790 1 870 ma respectively

the last stages of the avalonian cadomian arc in nw iberian massif isotopic and igneous record for a long lived peri gondwanan magmatic arc abstract the upper allochthonous units of nw iberian massif contain an extensive cambrian magmatism c 500 ma covering felsic to mafic compositions the magmatic activity generated large massifs of granitoids and gabbros with calc alkaline and tholeiitic compositions respectively petrological and geochemical features of these massifs are characteristic of volcanic arc the plutons intruded siliciclastic sedimentary series deposited in the periphery of the west africa craton u pb hf isotopic compositions of detrital zircon in the siliciclastic host series indicate continental arc activity between c 750 ma and c 500 ma it was characterized by a large variety of isotopic sources including from very old continental input even archean to the addition of a significant amount of juvenile mafic material these isotopic sources experienced an extensive mixing that explains the composition and isotopic features ehft from 50 until 15 of the represented cambrian plutons the cambrian igneous rocks of the upper units of nw iberia are related to the latest activity of the avalonian cadomian arc from the middle cambrian arc activity in the periphery of gondwana was replaced by pronounced extension associated with the development of continental rifting which finally led to separation of the microcontinent avalonia subsequent drifting of avalonia to the north caused progressive opening one of the main paleozoic ocean the rheic ocean

successive midcrustal high grade metamorphic events provide insight into mid archean mountain building along the se margin of the proto kaapvaal craton in this study we investigate the evolution of continental crust at the paleoarchean to mesoarchean boundary by documenting the deposition deep burial and partial melting of metasedimentary gneisses along the se margin of the proto kaapvaal craton successive high grade metamorphic events recorded by the gneisses between ca 3 23 and 3 07 ga not only coincided with the timing of subduction accretion in the adjacent barberton greenstone belt at ca 3 23 3 22 ga but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere mineral equilibria modeling demonstrates that the granulites evolved along pressure temperature p t paths similar to those documented for metamorphism in modern collisional orogens high temperature deformation at ca 3 11 3 07 ga during uplift of the terrane was coaxial with the main ne sw trending structural grain of the barberton greenstone belt and reflects the regional nw se shortening and ne sw orogen parallel extension exhibited by the younger potassic granites we reconcile these features with mesoarchean terrane assembly in barberton via two nw dipping subduction zones the trace of the first is represented by the main terrane boundary within the barberton greenstone belt the second resulted in accretion burial and high temperature metamorphism along the se margin of the kaapvaal craton with the granulites providing valuable insight into the mid to lower crustal response to what appears to have been a protracted accretionary orogenic event

ice marginal environment and ecosystem prior to initial advance of the late palaeozoic ice sheet in the mount butters area of the central transantarctic mountains antarctica basal rocks of the upper carboniferous to lower permian pagoda formation at mount butters provide an unusual view of periglacial conditions in the central transantarctic mountains region prior to the initial advance of the gondwanide ice sheet these rocks were deposited on a high relief unconformity that developed on granite deposition within relief on the unconformity possibly in the lee of a granite buttress protected the rocks from erosion during subsequent overriding by the ice sheet the succession reflects deposition in a glacial fed to ice contact lake that contained a freshwater crustacean fauna centimetre to decimetre scale basal layers include breccia and coarse grained sandstone the occurrence of breccia resting on weathered granite suggests sedimentation as scree and as mass flow deposits overlying decimetre to metre scale stratified diamictites interbedded with metre scale coarsening upward successions of siltstone to cross laminated sandstone suggest lacustrine deposition by suspension settling rain out of ice rafted debris and deltaic progradation thin zones with abundant conchostracans and or with prolific trace fossils in addition to less common remains of other crustaceans attest to the presence of a low diversity benthic fauna conchostracans are concentrated in a series of thin beds that reflect moderately lengthy perhaps seasonal periods of free flowing water patchy vertical and lateral distribution of intense bioturbation and profuse trace fossils probably reflect repeated colonization events during times of favourable environmental conditions massive diamictite overlies the basal rocks and indicates that the ice marginal lake was subsequently overridden by the late palaeozoic ice sheet occurrences of lodgement till glacitectonite and deformation till suggest deposition from temperate or warm based ice whereas underlying lacustrine and deltaic deposits along with a crustacean and trace fossil fauna suggest temperate periglacial conditions previous studies have stressed that upper palaeozoic glacigenic deposits in antarctica and in gondwanaland record deglaciation events in contrast rocks at mt butters provide an unusual glimpse into an ice margin lake and its fauna just prior to ice sheet advance

the shorebird ichnofacies an example from the miocene of southern spain the late miocene sorbas member of the sorbas basin almeria province southeast spain contains an extensive avian ichnofauna preserved in lagoonal marls three distinctive avian ichnotaxa can be identified antarctichnus fuenzalidae covacevich and lamperein 1970 iranipeda millumi n ichnosp and roepichnus grahami n ichnogen n ichnosp in common with many other cenozoic avian ichnofaunas these traces are associated with shorebirds including plovers storks ducks and or gulls respectively associated mammalian tracks include possible cat and artiodactyl footprints the avian tracks are abundant and show a range of behavioural aspects in common with other recorded examples of cretaceous recent shorebird tracks these include both solitary and group activities consistent with their postulated avian tracemakers

tetrapod tracks in a marginal lacustrine setting middle triassic argentina taphonomy and significance abstract fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting therefore in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track bearing beds during imprinting the analyzed section corresponds to thick middle triassic lacustrine deltaic deposits of the ischichuca los rastros formation ischigualasto villa union basin that crops out at the quebrada de ischichuca in northwestern argentina part of the track bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall cross cutting relationships observed among ripple marks the footprints and invertebrate traces of a softground suite of the scoyenia ichnofacies suggest an omission surface measured trackway orientations in the sandstones are perpendicular to the paleo shoreline with the animals coming and going along the exposed top of the bars probably for drinking laterally the distal delta front deposits interfinger with track bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment trackway orientations in the wackestones are in contrast consistent with the animals moving nearly parallel to the lake border probably along a preferred route evidences of a relative high groundwater table at the time of imprinting in the track bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces nevertheless temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones moreover footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the ischichuca los rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution footprints can also provide valuable information about locomotion dynamics and trackmaker behavior thus the sideways deformation observed in the studied footprints attributed to basal archosaurs and putative basal dinosaurs can be related to an outward rotation of the foot during the step cycle a condition that might allied to the development of the parasagittal posture in archosauria besides the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts the most important group of herviborous animals in the early mesozoic terrestrial ecosystems throughout gondwana

elements of the archean thermal history and apparent polar wander of the eastern kaapvaal craton swaziland from single grain dating and paleomagnetism abstract single grains of zircon hornblende biotite and feldspar have been dated to define the thermal history of the archean mbabane pluton swaziland coincident207pb 206pb zircon and40ar 39ar hornblende ages suggest that the pluton underwent rapid cooling to about 450 c at the time of emplacement at 2690 ma because the mbabane pluton is one of a suite of granites which represent the last major archean intrusive event in the eastern kaapvaal craton this time of emplacement marks the end of cratonization much younger biotite and feldspar ages indicate argon loss perhaps due to several low temperature events related to dike intrusion at 2000 2300 ma from such a thermal history the age of magnetization of a paleomagnetic pole polelatitude 19 7 n polelongitude 105 7 e a95 9 4 k 67 obtained from 5 sites can be determined based on the unblocking temperature of the magnetic carrier and the consistent direction seen in several sites it appears that the hornblende age and coincidently the207pb 206pb zircon age most accurately represents this age of magnetization thus the mbabane pluton yields a precisely dated archean 2687 6 ma 2 hornblende plateau age paleomagnetic pole which is different than other archean poles such as that from the nearby usushwana complex 2875 40ma although the two intrusions are separated by less than 12 km and there is no evidence of relative structural rotation between them the paleomagnetic poles from the two bodies are 65 apart this implies that there was significant apparent polar wander during the late archean

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

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

applications of ichnology in lacustrine sequence stratigraphy potential and limitations abstract in comparison with their marine counterparts lacustrine ichnofaunas have not been extensively used in sequence stratigraphy application of trace fossils in continental sequence stratigraphy cannot be based simply on the extrapolation of marine sequence stratigraphy marine substrate controlled ichnofacies in particular the firmground glossifungites ichnofacies develop in stable and cohesive substrates reflecting erosive exhumation of the sediment in contrast in lacustrine sequences substrate controlled ichnofossils only rarely indicate erosional exhumation because they are commonly related to desiccation of water bodies the tripartite division of overfilled balanced fill and underfilled lakes provides a comprehensive framework to evaluate the potential and limitations of lacustrine ichnofaunas in sequence stratigraphy based on a number of case studies spanning the three lake basin types a model of trace fossil distribution in overfilled balanced fill and underfilled basins is proposed overfilled lake basins contain well developed softground trace fossils mermia and skolithos ichnofacies and the softground suite of the scoyenia ichnofacies that are useful to delineate parasequences and parasequence sets fluvial discharge into overfilled lakes usually generates density currents that oxygenate lake bottoms allowing the establishment of epifaunal and infaunal communities additionally these are freshwater lakes where no stress due to hypersalinity occurs leading to the development of a relatively diverse benthos land plant derived organic matter is the prime source of nutrients favoring the development of a deposit feeding benthic fauna in permanently subaqueous low energy zones firmground suites are rare because such large lakes usually do not experience desiccation abundant firmground trace fossil suites of the scoyenia ichnofacies occur in balanced fill lakes but softground assemblages are usually depauperate lowstand deposits contain abundant and widespread ichnofaunas commonly meniscate trace fossils and arthropod trackways of the scoyenia ichnofacies lake hydrology is closed during lowstands and salinity usually increases imposing a stress factor on the lake biota and therefore softground ichnofaunas are depauperate ichnofaunas from turbidite systems in balanced fill lakes are less abundant and diverse than those from overfilled lacustrine turbidites freshwater conditions are common during transgression but dysaerobic conditions may prevail imparting a stress factor on the benthic biota in hydrologically closed lakes the scoyenia ichnofacies is widespread but the mermia ichnofacies is commonly absent or impoverished the scoyenia ichnofacies is associated with lowstand desiccated substrates in underfilled lakes the density of arthropod trackways may be high forming tracked omission surfaces some of these omission surfaces may represent sequence boundaries expressed by co planar surfaces of lowstand and subsequent flooding rapid changes in depositional conditions reflecting desiccation during vertical aggradation can lead to the formation of composite ichnofabrics that reflect successive bioturbation events

highly depleted hadean mantle reservoirs in the sources of early archean arc like rocks isua supracrustal belt southern west greenland abstract growing evidence from the accessible geological record reveals that crust mantle differentiation on earth started as early as 4 4 ga in order to assess the extent of early archean mantle depletion we obtained 176lu 176hf 147sm 143nd and high field strength element hfse concentration data for the least altered well characterized boninite like metabasalts and associated metasedimentary rocks from the isua supracrustal belt southern west greenland the metasediments exhibit initial ehf 3720 values from 0 7 to 1 5 and initial end 3720 values from 1 6 to 2 1 initial ehf 3720 values of the least altered boninite like metabasalts span a range from 3 5 to 12 9 and initial end 3720 values from 0 3 to 3 2 these initial hf isotope ratios display coherent trends with sio2 al2o3 tio2 and other relatively immobile elements indicating contamination via assimilation of enriched components most likely sediments derived from the earliest crust in the region this model is also consistent with previously reported initial os 3720 values for some of the samples in addition to the positive ehf 3720 values the least disturbed samples exhibit positive end 3720 values and a co variation of ehf 3720 and e d 3720 values based on these observations it is argued that the most depleted samples with initial ehf 3720 values of up to 12 9 and high 176lu 177hf of 0 05 to 0 09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field high precision high field strength element hfse data obtained for the isua samples confirm the contamination trend even the most primitive samples display negative nb ta anomalies and elevated nb ta indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite collectively the data for boninite like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from hf isotope data for hadean zircons and combined 142nd 143nd isotope data for early archean rocks

sm nd age and isotopic systematics of the bimodal suite ancient gneiss complex swaziland studies of the development and stabilization of the archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units greenstone belts and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust at the heart of this interest are the questions of the processes responsible for crust formation in the archaean and whether or not the various units of an archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials one area where this controversy has been particularly pronounced is the archaean crustal section of south west africa1 6 the oldest rocks in the kaapvaal craton consist of the onverwacht group of mafic to ultramafic metavolcanics of the barberton greenstone belt and a grey gneiss complex termed the ancient gneiss complex agc of swaziland we report here the results of a whole rock sm nd isotopic study of the agc and the implications these data may have for crustal evolution in the kaapvaal craton

marine event beds and recolonization surfaces as revealed by trace fossil analysis turbidites and tempestites exhibit characteristic distributions of trace fossils to gain maximum advantage of the sedimentological and palaeoecological significance of these traces an attempt should be made to relate each trace to the associated colonization surface this approach allows traces initiated at the normal top of an event bed to be distinguished from traces that subsequently passed into the bed from a higher colonization level or were initiated on the eroded surface of the bed

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

the crustal evolution of south america from a zircon hf isotope perspective hf isotope data of 1100 detrital zircon grains from the palaeozoic south central andean gondwana margin record the complete crustal evolution of south america which was the predominant source the oldest grains with crustal residence ages of 3 8 4 0ga are consistent with complete recycling of existing continental crust around 4ga we confirm three major archaean palaeoproterozoic transamazonian and late mesoproterozoic to early neoproterozoic crust addition phases as well as six igneous phases during proterozoic to palaeozoic time involving mixing of juvenile and crustally reworked material a late mesoproterozoic to early neoproterozoic grenville age igneous belt can be postulated along the palaeo margin of south america this belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages crustal reworking likely dominated over juvenile addition during the palaeozoic era and proterozoic and archaean zircon was mainly crustally reworked from the eroding thickened ordovician famatinian arc

contrasting geochemical patterns in the 3 7 3 8 ga pillow basalt cores and rims isua greenstone belt southwest greenland implications for postmagmatic alteration processes abstract pillow basalts from the early archean 3 7 to 3 8 ga isua greenstone belt west greenland are characterized by well preserved rims and concentric core structures the pillow rims and cores have different mineral assemblages and chemical and isotopic compositions the rims have systematically higher contents of fe2o3 mgo mno k2o rb ba ga y and transition metals than the cores in contrast the cores possess higher concentrations of sio2 na2o p2o5 sr pb u nb and the light rare earth elements rees than the rims these compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration variations of many major and trace element concentrations between the rims and cores of the isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration al2o3 tio2 th zr and the heavy rees display similar values in both rims and cores suggesting that these elements were relatively immobile during postemplacement alteration in addition the rims and cores have distinctive sm nd and rb sr isotopic compositions in that the rims are characterized by higher 143nd 144nd and 87sr 86sr ratios than the cores the pillow basalts yield 2569 170 ma and 1604 170 ma errorchron ages on 143nd 144nd vs 147sm 144nd and 87sr 86sr vs 87rb 86sr diagrams respectively the sm nd errorchron age may correspond within errors to a late archean tectonothermal metamorphic event recorded in the region the sm nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of nd relative to sm during late archean metamorphism although the rb sr errorchron age overlaps with the timing of an early to mid proterozoic tectonothermal metamorphic event recorded in the region because of a considerably large mean square of weighted deviates value and scatter in 86sr 87sr and 87rb 86sr ratios this age may not have a precise geological significance the 1 6 ga rb sr errorchron is likely to have resulted from the loss of radiogenic 87sr collectively the sm nd and rb sr data obtained from the 3 7 3 8 ga isua pillow basalt rims and cores are consistent with disturbances of the sm nd and rb sr systems by tectonothermal metamorphic events long after their eruption in contrast to the sm nd and rb sr systems the lu hf system appears to be largely undisturbed by metamorphism five core samples and three rim samples yield a 3935 350 ma age within error of the approximate age of eruption 3 7 to 3 8 ga two rim samples that have gained lu give an age of 1707 140 ma within error of the rb sr errorchron age initial 176hf 177hf ratios of the undisturbed samples at 3 75 ga lie within 1 e unit of the chondritic value suggesting no long term depletion in the mantle source of the basalts

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

u pb lu hf and ree in zircon from 3 2 to 2 6 ga archean gneisses of the repulse bay block melville peninsula nunavut abstract combined in situ u pb and lu hf isotope and ree analyses of zircons from the tonalite trondhjemite granodiorite ttg type gneisses and granitoids within the repulse bay block rbb of melville peninsula nunavut reveal that the block is composed of ca 2 86 ga and 2 73 2 71 ga granitoids that formed by reworking of a 3 25 3 10 ga basaltic crust the interpretation is supported by initial hf values for ca 2 86 ga and ca 2 73 2 71 ga gneisses of 2 7 to 0 7 and 4 8 to 2 3 respectively and the preservation of high uranium xenocrystic cores at 3 15 0 05 ga neoarchean plutonism also occurred at ca 2 69 2 66 ga and is characterized by two pyroxene bearing charnockite enderbite and hornblende biotite granitoid intrusions that yield initial hf values that range from 7 1 to 1 0 demonstrating that they were derived from juvenile magmas with significant assimilation of the existing crust the pyroxene and non pyroxene bearing granitoids also contain elevated lree in zircon a feature that is interpreted to reflect metasomatism of the source this study also demonstrates that in the rbb the results of lu hf isotopes in zircon combined with in situ u pb geochronology from inherited zircon cores differ from the results of whole rock sm nd isotopes

palaeoenvironmental framework of the glacial postglacial transition late paleozoic in the paganzo calingasta basin southern south america and the great karoo kalahari basin southern africa ichnological implications sedimentological palaeontological and geological data from the glacial to postglacial transition in the late paleozoic successions of the paganzo calingasta basin pc in southern south america and the great karoo kalahari basin gkk in southern africa are analysed revised and reinterpreted a brackish depositional setting is inferred for main areas previously considered to be nonmarine based upon ichnological interpretations three stratigraphic intervals have been defined based on changes in sedimentary facies and trace fossils association the glacial interval gi early postglacial interval epi and late postglacial interval lpi the gi and epi contain a dominance of arthropod trackways fish trails with and subordinate grazing and feeding traces the epi in the pc basin comprises both nonmarine and brackish marine ichnocoenoses without significant differences in ichnological composition trace fossils are preserved in underflow and turbidite beds of deltaic deposits opportunistic grazing traces constitute a post event ichnocoenosis while a pre event ichnocoenosis is preserved at the base of turbidite beds in the gkk basin ichnofossils were documented in turbidite fans the lpi in the gkk basin contains the first evidence of shallow water deltaic infauna and subordinate grazing traces conversely in the pc basin the infauna is lacking the gi and particularly the epi ichnofauna resemble the nonmarine mermia ichnofacies but they occur in large and long lived stable marine basins which received enormous inputs of melt water that markedly reduced the salinity they are not ichnologically comparable with brackish marginal marine environments that are affected by frequent sometimes diurnal fluctuations in physical parameters such as salinity and temperature the lpi in the gkk basin constitutes a typical record of the classical marginal marine ichnofaunas but in the pc basin the ichnofacies assignment is problematic grazing traces of the mermia ichnofacies appear in middle estuarine deposits where both palynomorphs and the presence of tidal features suggest brackishness the paganzo basin ichnology therefore shows some difficulties in differentiating nonmarine from subtle brackish environments without the full integration of sedimentological information ichnocoenoses dominated by trackways of arthropods are particularly inappropriate to distinguish between these types of depositional settings

trace fossils in coal bearing sequences in the past decade trace fossils have been recorded extensively from coal bearing sediments differing widely in facies age and location westphalian or stephanian 9coal measures9 in britain europe and eastern canada contain an ichnofauna produced by invertebrates and or vertebrates in upper delta plain sediments this contrasts with the marine related lower delta plain ichnofaunas known from pennsylvanian rocks of the united states and permian gondwana 9coal measures9 of south africa deltaic complexes of middle jurassic age in the north sea basin and upper cretaceous age in north america contain marine trace fossils and dinosaur footprints in coastal coal bearing facies these case histories illustrate the importance of trace fossils both in facies analysis of coal bearing sequences and in recording the presence of animals rarely known as body fossils in such clastic sediments

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

paleolimnology evolution in rift basins the ischigualasto villa union basin central western argentina during the triassic abstract extensional basins offer a high preservation potential for sedimentary and fossil records nonmarine rift basins contain diverse environments including fluvial and lacustrine lake deposits have long been recognized as significant archives of the paleoenvironmental record the ischigualasto villa union basin represents the only nearly continuous continental triassic record with up to 6000 m of alluvial fluvial and lacustrine deposits the deep lacustrine interval is known as the los rastros formation and consists of several lacustrine deltaic cycles a detailed facies analysis allowed the characterization of five facies associations and two types of cycles 1 a shoreline dominated progradational type i cycle containing facies association a offshore lacustrine deposits and facies association b marginal palustrine deposits along the shoreline of the lake or facies association c lacustrine siliciclastic coastal deposits and 2 a delta dominated progradational type ii cycle characterized by facies association a offshore lacustrine and prodelta deposits facies association d delta front deposits and facies association e delta plain deposits the sedimentary evolution of the chanares los rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts lowstand transgressive and highstand system tract biotic abiotic and sedimentologic data allowed the interpretation of the limnologic dynamics of the los rastros paleolake as a deep freshwater lake within a half graben exhibiting meromixis and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions the successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of bohacs et al to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin

trace fossil analysis of lacustrine facies and basins abstract two ichnofacies are typical of lacustrine depositional systems the scoyenia ichnofacies characterizes transitional terrestrial nonmarine aquatic substrates periodically inundated or desiccated and therefore is commonly present in lake margin facies the mermia ichnofacies is associated with well oxygenated permanent subaqueous fine grained substrates of hydrologically open perennial lakes bathymetric zonations within the mermia ichnofacies are complicated by the wide variability of lacustrine systems detected proximal distal trends are useful within particular lake basins but commonly difficult to extrapolate to other lakes other potential ichnofacies include the typically marine skolithos ichnofacies for high energy zones of lakes and substrate controlled still unnamed ichnofacies associated to lake margin deposits trace fossils are useful for sedimentologic analysis of event beds lacustrine turbidites are characterized by low diversity suites reflecting colonization by opportunistic organisms after the turbidite event underflow current beds record animal activity contemporaneous with nearly continuous sedimentation ichnologic studies may also help to distinguish between marine and lacustrine turbidites deep marine turbidites host the nereites ichnofacies that consists of high diversity of ornate grazing traces and graphoglyptids recording highly specialized feeding strategies developed to solve the problem of the scarcity of food in the deep sea deep lacustrine environments contain the mermia ichnofacies which is dominated by unspecialized grazing and feeding traces probably related to the abundance and accessibility of food in lacustrine systems the lower diversity of lacustrine ichnofaunas in comparison with deep sea assemblages more likely reflects lower species diversity as a consequence of less stable conditions increase of depth and extent of bioturbation through geologic time produced a clear signature in the ichnofabric record of lacustrine facies paleozoic lacustrine ichnofaunas are typically dominated by surface trails with little associated bioturbation during the mesozoic bioturbation depth was higher in lake margin facies than in fully lacustrine deposits while significant degrees of bioturbation were attained in lake margin facies during the triassic major biogenic disruption of primary bedding in subaqueous lacustrine deposits did not occur until the cretaceous

crustal geodynamics from the archaean bundelkhand craton india constraints from zircon u pb hf isotope studies a comprehensive study based on u pb and hf isotope analyses of zircons from gneisses has been conducted along the western part babina area of the e w trending bundelkhand tectonic zone in the central part of the archaean bundelkhand craton 207 pb 206 pb zircon ages and hf isotopic data indicate the existence of a felsic crust at 3 59 ga followed by a second tectonothermal event at 3 44 ga leading to calc alkaline magmatism and subsequent crustal growth the study hence suggests that crust formation in the bundelkhand craton occurred in a similar time frame to that recorded from the singhbhum and bastar cratons of the north indian shield

hardly used habitats dearth and distribution of burrowing in paleozoic and mesozoic stream and lake deposits limited use of substrate ecospace by a meager infauna in streams and lakes is indicated by analysis of more than 10 000 observations of bioturbation in permian through jurassic freshwater deposits in contrast marine substrates have been inhabited and bioturbated since the cambrian colonization of freshwater substrates differed fundamentally from that of marine substrates bioturbation enhanced flux of dissolved materials across the sediment water interface was not operating until after the paleozoic in freshwater ecosystems

trace fossils and rhizoliths as a tool for sedimentological and palaeoenvironmental analysis of ancient continental evaporite successions abstract recognition of evaporite formations from continental tertiary basins of spain provides evidence that trace fossils including rhizoliths can be abundant in some saline lake systems and their study helps in palaeoenvironmental interpretation of ancient continental evaporite sequences six main types of trace fossils have been distinguished and include 1 networks of small rhizoliths 2 large rhizoliths 3 tangle patterned small burrows 4 isolated large burrows 5 l shaped traces and 6 vertebrate tracks rhizoliths were related to both marginal areas of hypersaline lakes and lakes of moderately high saline waters in these settings pedoturbation resulted from colonization by grasses and bushes of distinct lake subenvironments the activity of burrowing invertebrate faunas was especially intense in lakes of moderately concentrated brines from which gypsum was the main evaporite mineral deposited a specific gypsum lithofacies bioturbated gypsum deposits forming thick massive beds has a widespread occurrence in many of the basins tangle patterned small burrows and minor isolated large burrows constitute the typical trace fossil types within the gypsum the traces are interpreted as having been caused by burrowing insect larvae probably chironomids coleopterans and annelids the behaviour of these organisms in recent lake environments yields information about the salinity range of lake waters from which gypsum precipitated concentration values averaging 100 150 g l may be thus deduced though some organisms involved in the formation of the traces can tolerate higher salinities the combined analysis of lithofacies and trace fossils from the lacustrine evaporite sequences contributes to the study of distinct saline lake subenvironments as well as changes in the sedimentary evolution of the lake systems consequently trace fossils can provide valuable insight for palaeoenvironmental analysis of at least some evaporite formations that accumulated in continental settings

facies sequences and trace fossils in lacustrine fan delta deposits hornelen basin m devonian western norway abstract study of a lacustrine fan delta succession in a marginal tract of hornelen basin shows that occasional lakes were infilled by marginal accretion and progradation of the adjacent fluvial system the lake deposits are dominantly thin graded rhythmites and thicker massive fine sandstones both of great lateral extent and formed by fine sediment gravity flows wave generated ripple lamination is also very common whereas current especially climbing ripple lamination occurs increasingly towards the southern edge of the lake much of the lacustrine sequence is contorted convoluted or deformed to a lesser degree trough cross stratified sandstones and pebbly sandstones characterised the lowsinuosity stream systems which fed the lakes from the south in a narrow transitional zone interpreted in terms of high sinuosity distributary channels and shoreline there are significant amounts of planar cross strata as well as ripple and plane parallel lamination eight distinct types of trace fossils chiefly arthropod tracks trails and burrows scoyenia ichnofacies are present in the fine sediments mudflats pools and low bars of this latter zone behaviour patterns are dominantly locomotion and resting traces possibly with some surface feeding but an absence of infaunal feeding or dwelling structures the repeated progradation and retreat of the sandy fan delta system into the lakes generated mudstone bounded sedimentary bodies these are mainly alluvial coarsegrained upward coarsening and 5 25 m thick in the proximal areas they are mainly lacustrine fine grained symmetrical coarsest in the middle or uniform and generally less than 5 m thick distally in origin these bodies probably represent northward crevassing out of the main westward flowing axial fluvial system and were possibly triggered by repeated base level changes caused by lowering of the basin floor against the northern fault margin the trace fossils described here for the first time in norway are also known from several other devonian alluvial sequences although the closest comparable ichnofauna occurs in ephemeral lacustrine deposits in south scotland the type of arthropod that made the trace fossils cannot be identified at present

c and n stable isotope measurements on eurasian fossil mammals 40 000 to 10 000 years bp herbivore physiologies and palaeoenvironmental reconstruction abstract seventy seven specimens of fossil upper pleistocene early holocene woolly mammoth reindeer deer and bison from 17 different locations in eurasia have been measured for the nitrogen and carbon stable isotope composition of their collagen and for the carbon stable isotope composition of bioapatite the aim was 1 to check the existence of differences in n and c isotope ratios between and within different herbivore species mainly between mammoth and reindeer 2 to discuss these data in terms of animal physiology temporal and spatial distribution of animals and effects of climatic changes and 3 to obtain further information about the environmental changes that occurred in that area during the period considered a geographical variation from south west to north east in the 15n and or 13c values of collagen and bioapatite for all the herbivore species is apparent the most negative 13c values and the most positive 15n values being located in the north eastern area this was probably due to a different temperature and water stress on animals and plants and to a different plant availability the differences in 13c 12c and 15n 14n isotope ratios among species are mainly related to different food intake and different metabolic pathways it seems also that the diet of mammoths from north eastern siberia was higher in protein content compared with that of mammoths from the russian plain from this study it is clear that there was a rapid and important change in the amount of precipitation soon after the last glacial maximum the rainfall conditions around 40 000 and 30 000 years bp were probably similar to those existing around 10 000 years bp though lower than those existing after the glacial maximum

u th pb and lu hf systematics of zircon from ttg s leucosomes meta anorthosites and quartzites of the limpopo belt south africa constraints for the formation recycling and metamorphism of palaeoarchaean crust abstract results of more than 400 in situ u th pb and lu hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the limpopo belt musina area formed during the palaeoarchaean between 3 40 and 3 27 ga experienced a first anatexis at 3 24 3 07 ga m1 and were overlain by sediments mainly between 3 24 and 3 0 ga subsequently the infra and supracrustal rocks together underwent two high grade metamorphic overprints a first during the neoarchaean at 2 65 ga m2 and a second at 2 025 ga m3 hafnium isotope data additionally reveal that the palaeoarchaean magmatic rocks result from substantial recycling of much older crust this is well reflected by nearly chondritic hf int obtained from primary magmatic zircon domains of four sand river ttg gneiss samples hf 3 27 3 28 ga 1 3 to 1 8 from leucosomes within the sand river gneisses hf 3 40 ga 2 0 hf 3 28 ga 1 5 and from two meta anorthosite samples of the messina layered intrusion hf 3 35 ga 0 1 1 4 crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples these data reflect a general increase of the hf int between 3 65 and 3 2 ga from ca 4 0 to 0 0 indicating that an eo to palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the limpopo belt the present study also demonstrates that hf isotope data provide a powerful tool which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events from such affected by a pseudomorphic alteration even if the investigated zircon grain underwent multiple alteration processes

jurassic early cretaceous paleogeography and paleoenvironments of the north eastern margin of gondwana insights from the carpentaria basin australia abstract although jurassic early cretaceous sedimentary systems were extensively developed on northeastern gondwana deciphering their paleogeography has been complicated by poor exposure and the lack of a robust chronostratigraphic framework the southeastern margin of the carpentaria basin northeastern australia is one of the few regions where these sedimentary systems are extensively exposed employing a combination of facies analysis and new data from paleontology and detrital zircon geochronology we present a temporally and environmentally refined paleogeographic framework for this region a late jurassic southeasterly directed marine incursion invaded northeastern gondwana extending inland across the carpentaria basin as demonstrated by a thin 30 m marine influenced fluvio estuarine stratigraphic succession capped by a sequence bounding 30 myr paraconformity the depositional hiatus marked the late jurassic early cretaceous uplift of the euroka arch with loss of sedimentary and fluvial connectivity between the carpentaria basin and adjoining eromanga basin subsequent deposition by low accommodation fluvial systems resulted in a thin fluviatile depositional package developing during the early cretaceous paleocurrent and provenance data indicate that the middle to late jurassic c 170 160 ma fluvial systems predating the paraconformity extended from the eromanga basin to the south across the southeastern carpentaria basin transporting sediment from distal sources in the lachlan orogen of southeastern australia fluvial systems of the southeastern carpentaria basin post dating the paraconformity and euroka arch uplift show a provenance shift to easterly sources in the mossman orogen and kennedy igneous association previously unrecognised jurassic early cretaceous igneous activity provided a persistent source of sediment to the southeastern carpentaria basin succession due to reworking of air fall tuff from an active magmatic arc located on the continental margin of northeastern gondwana

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

les bossats ormesson paris basin france a new early gravettian bison processing camp abstract in the paris basin the gravettian techno complex is documented by a limited number of sites and radiocarbon dates are available for only three locations arcy sur cure grande grotte and grotte du renne yonne la pente des brosses montigny sur loing seine et marne and chamvres yonne this period of human prehistory is poorly known in the region the recently discovered open air site of les bossats at ormesson seine et marne includes an early gravettian occupation with well preserved faunal remains this allows for radiocarbon dating and provides information on the ecological context that was previously inaccessible located on the southern limit of the maximum extension of loess deposits ormesson also provides a well developed pleistocene sedimentary sequence the peculiar geomorphological context of the gravettian occupation opens new avenues of investigation regarding taphonomy and the possible anthropogenic intra site spatial organisation here we present the primary results of excavations that began in 2009 the strategic location of the site the richness of the lithic industry the presence of two well preserved hearths and associated maintenance features near which were found both perforated and non perforated shells as well as the few elements of bone technology all suggest that the site was occupied for a relatively long period of time flint nodules are primarily obtained from local sources and were used to produce blades and bladelets the tool kit is however dominated by blades likely used in butchering activities as well as armature and burin production the occupants of ormesson seem to have killed and butchered bison near site and imported the most nutritious parts portions of reindeer and horse were also imported the gravettian occupation at ormesson thus appears to have been a medium to short term multi purpose campsite likely occupied for the time it took to process imported bison carcasses

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability

inner shelf to lower shoreface hummocky sandstone bodies with evidence for geostrophic influenced combined flow lower cretaceous west greenland abstract hummocky sandstone bodies were deposited on the margin of an early cretaceous rift basin in an overall progradational inner shelf to lower shoreface unit excellent outcrops of this unit permitted a detailed study of sedimentary structures and paleocurrents in relation to sandstone geometries the sandstone bodies are lensoid and 0 5 4 m thick with lateral extensions from a few meters to more than 30 m laterally they pinch into thin sandstone sheets that continue for at least 130 m along the entire outcrop or swell within a distance of 20 30 m to form another sandstone lens the lower boundaries are sharp with local erosional relief up to 1 m hummocky cross stratification wave ripple cross lamination and low angle cross stratification are closely interbedded within the sandstone aminated mudstone beds 10 60 cm thick separate the sandstone bodies and are generally continuous over the entire length of the outcrop sedimentary structures in the sandstone bodies were produced by high energy wave generated oscillatory currents and unidirectional geostrophic currents vertical stratification successions from individual storm events reflect deposition during both increasing storm current velocity succeeded by waning storm deposits and more typically waning storm facies successions deposition of the unusual vertical successions was probably controlled by an episodically high sediment supply the local presence of the hummocky sandstone bodies on a regional scale was controlled by a paleogeographic setting where the bathymetric contours locally caused geostrophic cu rents to decelerate