archean paleoproterozoic transition the indian perspective abstract india has perhaps the richest paleoarchean to paleoproterozoic crustal components on earth miller et al 2018 notwithstanding this much of the indian subcontinent remains unexplored compared to other ancient cratons in north america australia brazil and africa there are five archean cratonic nuclei in india the aravalli bundelkhand singhbhum bastar and the dharwar with well preserved proterozoic supracrustal sequences this paper critically reviews the paleoarchean to paleoproterozoic supracrustal record of the indian subcontinent with special emphasis on the archean paleoproterozoic transition which is generally placed at 2 5 ga based on the emplacement age of the great dyke of zimbabwe in general the archean paleoproterozoic transition in indian cratonic blocks is associated with high continental freeboard condition unlike north america south africa and western australia the indian cratonic blocks except bastar are devoid of paleoproterozoic glacial diamictite mazumder et al 2015 however similar to the pongola supergroup of south africa there are evidences of neoarchean glaciation in dharwar ojakangas et al 2014 in contrast to the global scenario of extensive development of bif across the archean paleoproterozoic transition the indian paleoproterozoic successions are devoid of banded iron formation bif except the 1 85 ga old bif in bundelkhand in contrast the majority of the indian bifs are of paleoarchean or neoarchean age thus the geodynamic and supracrustal data collected from five indian archean cratonic blocks do not correspond uniquely and precisely to a global change in tectonic style onset of neoarchean global magmatism and rifting or lithostratigraphic and environmental changes suggested across the archean paleoproterozoic transition van kranendonk et al 2012 therefore a revision and redefinition of the archean proterozoic boundary is outstanding this article is part of a special issue entitled archean earth processes edited by rajat mazumder and patrick eriksson

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

quantifying the oldest tidal record the 3 2 ga moodies group barberton greenstone belt south africa the 3 2 ga moodies group in the barberton greenstone belt south africa contains the oldest preserved record of tides the tidal record is preserved in a tidal sand wave deposit in the lower moodies group as bundles of sandstone foresets separated by mudstone drapes detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal fortnightly and monthly tidal periodicities thick thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood tidal currents respectively cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap spring neap tidal cyclicity alternating thicker and thinner neap spring neap cycles are comparable to anomalistic perigean apogean tidal signatures this quantitative record of tides in the middle archean moodies group represents by 2 2 b y the oldest such documentation tidal cyclicity recognized in the moodies sand wave deposit is comparable to that recorded in modern tidal settings and identified in the carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

late archaean tidalites from western margin of chitradurga greenstone belt southern india abstract tidalites are preserved within a metavolcano sedimentary succession of the late archaean bababudan group dharwar supergroup along the western boundary of the nnw sse trending chitradurga greenstone belt west dharwar craton southern india these may represent the oldest record of tidal processes in peninsular india millimetre thick sand mud alternations bidirectional cross strata mudstone draped sandy foresets reactivation surfaces indicating time velocity asymmetry sigmoidal cross strata and mudstone draped flaser lenticular bedding are displayed by the sandstone mudstone heterolithic facies in the upper part of the bababudan succession together implying a tidal depositional system thick thin pairs of rhythmic foreset bundles correspond to neap spring tidal cycles within a semidiurnal tidal system development of the studied coastal sediments suggests formation of stable platform along the western margin of the late archaean chitradurga greenstone belt

quartzite development in early palaeozoic nearshore marine environments quartzites are especially characteristic of proterozoic and cambro ordovician shallow marine strata whereas equivalent age fluvial deposits are commonly arkosic the absence of land vegetation in the pre silurian influenced weathering processes and styles of fluvial deposition it may also have had an impact on shallow marine sedimentation two field studies from the english channel region are presented to investigate the processes leading to quartzite formation on alderney nearshore marine and fluvial facies occur interbedded on a metre scale and are interpreted to represent deposition on the lower reaches of an alluvial plain and in beach and upper shoreface environments the marine and fluvial sandstones display marked differences in textural and mineralogical maturity pointing to a process of sediment maturation by the destruction of feldspar and labile grains at the shoreline at erquy fully mature marine quartzites occur bounded above and below by alluvial deposits via sharp or erosional surfaces and are interpreted to represent high energy storm and tidally influenced lower shoreface and inner shelf deposits a model for quartzite development is proposed where under a cool climate frequent storms in un vegetated tectonically rejuvenated uplands provided an abundance of arkosic sand to fluvial basins and clastic shorelines the model proposes that the marine basins were subject to high wave energies frequent storm events and tidal currents these were conditions conducive to transforming arkosic sand to quartz rich sand by the attrition of feldspar at the shoreline and in the shallow marine environment on sediment burial further feldspar destruction occurred during diagenesis the proposed model highlights the potential for a step change in sediment maturity to occur at the shoreline in early palaeozoic depositional systems tracts

geological constraints on the precambrian history of earth s rotation and the moon s orbit over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin i e stacked thin beds or laminae usually of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided information on earth s paleorotation and the evolving lunar orbit for precambrian time before 540 ma depositional environments of tidal rhythmites range from estuarine to tidal delta with a wave protected distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long detailed rhythmite records the potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range or maximum tidal height and consequent current speed hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness the validity of the findings can be investigated by testing the primary observed values for internal self consistency through application of the laws of celestial mechanics paleotidal and paleorotational values provided by late neoproterozoic 620 ma tidal rhythmites in south australia are validated by these tests and indicate 13 1 0 1 synodic lunar months yr 400 7 solar days yr a length of day of 21 9 0 4 h and a relative earth moon distance a a0 of 0 965 0 005 the mean rate of lunar recession since that time is 2 17 0 31 cm yr which is little more than half the present rate of lunar recession of 3 82 0 07 cm yr obtained by lunar laser ranging the late neoproterozoic data militate against significant overall change in earth s moment of inertia and radius at least since 620 ma cyclicity displayed by paleoproterozoic 2450 ma banded iron formation in western australia may record tidal influences on the discharge and or dispersal of submarine hydrothermal plumes and suggests 14 5 0 5 synodic months yr and a a0 0 906 0 029 the combined rhythmite data give a mean rate of lunar recession of 1 24 0 71 cm yr during most of the proterozoic 2450 620 ma suggesting that a close approach of the moon did not occur during earlier time concentrated study of precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early earth moon system and may permit the lunar orbit to be traced back to near the time of the moon s origin

timing and nature of multiple 3700 3600 ma tectonic events in intrusive rocks north of the isua greenstone belt southern west greenland one of the oldest well preserved sequences of tectonic events on earth is recorded in intrusive rocks that lie north of the isua greenstone belt southern west greenland detailed mapping and u pb dating of zircon and titanite build upon previous work to better define the timing and nature of the 3700 3600 ma tectonic history voluminous 3698 2 ma tonalite was converted into gneiss during a deformation event d 1 that predated intrusion of 3659 2 ma diorite dikes the dikes and the earliest gneissosity s 1 are most abundant in a zone of low regional deformation d 2 located 8 km north of the isua greenstone belt f 2 folding produced the dominant gneissosity s 2 during injection of meter wide granite and pegmatite sheets along s 2 at ca 3650 ma d 3 shearing transposed s 2 into a narrow mylonite zone at the contact between the intrusive rocks and the isua greenstone belt motion on this contact must have occurred in the early archean mylonites are cut by dolerite dikes that are correlated with nearby middle archean dikes titanite from the low d 2 strain zone yielded concordant u pb dates of 3606 3 ma indicating that these rocks were little affected by late archean metamorphism in contrast significant late archean heating of rocks adjacent to the isua greenstone belt is shown by titanite that lost considerable pb and or was largely recrystallized at 2915 32 ma late archean strain was also restricted to rocks adjacent to the belt this sequence of overprinting deformation and metamorphism is similar to that in younger orogenic belts

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

record of tide wave influence on the coal bearing permian barakar formation raniganj basin india abstract the coal bearing barakar formation early permian of the raniganj gondwana basin was previously interpreted as continental fluvial sediments with little evidence for tide wave influences the present contribution details the sedimentary attributes of marginal marine tidal and wave activities preserved within an overall braided meandering fluvial depositional setting from the barakar formation tide dominated sedimentary features include i tidal bundle sequences showing sandstone foresets with mud drapes ii periodic variation in foreset thickness iii lateral and vertical accretion of cross strata bundles with a downcurrent change to sigmoidal cross strata iv mutually opposite cross strata sets separated by sub horizontal plane laminated sandstone mudstone and v alternate sand and mud dominated rhythmites draping of the tidal stratifications by wavy laminated muddy siltstone suggests immediate invasion by low energy silt mud depositing waves that helped preserve the tidal features during subsequent reworking open marine storm wave activities are evident from hummocky cross stratification and various types of wave ripples preserved separately within coarse to fine grained sandstone in the middle upper part of the succession recognition of tide storm influenced marine sedimentation significantly changes the existing paleogeographic model to a fluvial estuarine depositional environment

the chitradurga greenstone succession in south india and evolution of the late archaean basin the chitradurga greenstone succession of south india comprises a thick pile 10 km of late archaean volcanic flows and terrigenous clastic sediments metamorphosed from greenschist to low grade amphilobite facies an older near shore sedimentary sequence of cratonic affiliation and an off shore bimodal volcanic sequence were deposited contemporaneously on a gneissic basement the volcanics are metasomatically altered and major minor and trace element data fail to discriminate the metavolcanics in terms of modern plate settings a younger turbidite sequence of coarser elastics covered the older deposits without any apparent tectonic or erosional break all the rocks of the succession display evidence of similar deformation prior to invasion by younger granites 2 5 ga in a late syn kinematic phase this suggests that initially a simple flat lying downwarp in a continental crust served as the passive receptacle of the platform type sediments and also witnessed volcanism along extensional faulting this phase of the basin was not associated with any compressive deformation subsidence of the chitradurga basin by the denser volcanics and uplift in the gneissic borderlands provided the infrastructure for subsequent development of the younger turbidite sequence covering the still virtually undeformed older deposits a compressive orogeny accompanied by granitic intrusion 2 5ga in a late kinematic phase ultimately deformed and uplifted the basin fill during the declining phase of basinal activity there is no evidence in the belt to suggest that the plate tectonic wilson cycle processes pending a terminal orogeny were operative during evaluation of the chitradurga basin

the akilia controversy field structural and geochronological evidence questions interpretations of 3 8 ga life in sw greenland abstract field relations structure and geochronology demonstrate that controversial rocks on akilia sw greenland cannot host evidence of earth9s oldest life mafic ultramafic gneiss that contains a layer of quartz amphibole pyroxene gneiss a few metres thick with purported biogenic graphite is not cross cut by the protoliths of 3 82 3 65 ga tonalitic gneiss the contact between these gneisses is tectonic so their relative ages are unknown multiple episodes of intrusion and deformation in the tonalitic gneiss preceded the earliest deformation fabrics and structures seen in the mafic ultramafic gneiss although previously interpreted as a volcano sedimentary stratigraphy the mafic ultramafic gneiss displays no evidence of such an origin and could be partly or wholly derived from igneous intrusions existing geochronology indicates that the mafic ultramafic and qtz am px gneisses have ages c 2 68 ga during late archaean high grade events that complicate any protolith interpretation u pb dating of apatite from a variety of rocks on akilia shows that this mineral crystallized or possibly recrystallized at c 1 75 ga and it thus provides no indication of an early archaean age for any associated graphite regardless of whether or not the latter is biogenic supplementary material ion microprobe u pb analytical methods and isotope data tables are available at http www geolsoc org uk sup18333

archean sedimentation on dharwar craton india and its implications abstract the dharwar craton is the largest archean cratonic nuclei of india and is made up of western and eastern blocks separated by the 2 5 ga closepet granite batholith the western dharwar craton consists mostly of 3 3 3 0 ga ttg the peninsular gneissic complex older mesoarchean greenstone sargur type and younger neorchean greenstone the dharwar supergroup comprising of lower bababudan and upper chitradurga groups in contrast the eastern dharwar craton is mostly made of granitoid gneisses and felsic volcanics with some neorchean supracrustal enclaves sandur ramgiri kolar kadri schist belts recently it has been suggested that the dharwar cratonic block is formed by assembly of western central and eastern micro blocks separated by shear zones jayananda et al 2018 the greenstone successions are present in all three dharwar micro blocks however the proportion of greenstones is much higher and the quality of preservation is much better in the western micro block the mesoarchean sargur group hosts barite deposit and consists of fuchsite quartzites carbonates metapelites banded iron formations and metamorphosed basalts and komatiites researchers have postulated a platformal orthoquartzite carbonate pelite association for the sargur group some researchers examined the barite deposits of the sargur group and inferred a subaerial volcanism during the sargur sedimentation the neoarchean bababudan group comprises largely terrestrial to shallow marine sedimentary successions glacial deposits bif volcanic and volcaniclastics rocks in contrast the overlying chitradurga group comprises conglomerates greywackes phyllites carbonates bifs cherts shales associated with a variety of volcanic rocks ranging from pillowed basalts to rhyolites and indicates relatively deep water sedimentation in a subsiding basin in an active continental margin setting stromatolites are abundant in the chitradurga group as also in other neoarchean supracrustal successions of the dharwar craton in this review we have critically synthesized published sedimentological data from both older as well as younger greenstone successions of the dharwar craton and have presented the archean sedimentation history of the dharwar craton we have highlighted the research gaps that researchers may find helpful

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

apogean perigean signals encoded in tidal flats at the fluvio estuarine transition of glacier creek turnagain arm alaska implications for ancient tidal rhythmites turnagain arm is a macrotidal fjord style estuary glacier creek is a small glacially fed stream which enters the estuary tangentially near girdwood alaska trenches and daily sedimentation measurements were made in a mudflat along the fluvio estuarine transition of glacier creek during several summers since 2003 each year the flats appear to erode during the winter and then accrete vertically in the spring and summer in each of the years studied tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap spring tidal cycles in 2004 bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats five laminae bundles alternated between thicker and thinner bundles corresponding to the perigean high spring and apogean low spring tides well preserved apogean perigean cycles have rarely been documented in modern tidal flat sediments at this location vertical accretion of tidal rhythmites with well developed neap spring cyclicity is possible because of the near complete removal of the flat from the previous year which creates accommodation space for vertical accretion without significant reworking macrotidal conditions no reworking by infaunal invertebrates protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation the position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles in the summer of 2004 apogee and perigee were closely aligned with the new and full moons resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick thin spring cycles encoding the apogean and perigean tidal cycle the apogean perigean signal was not observed in subsequent years

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

lower cretaceous tides revealed by cross bedding with mud drapes the sand dominated folkestone beds widely exposed around the weald of south east england are of upper aptian to lower albian lower cretaceous age and range in thickness up to 84 m refs 1 3 their content of ammonites bivalves and sponges indicates a marine origin while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land one striking sedimentary facies is formed by medium to coarse grained cross bedded sands in which several foreset and or bottomset surfaces support mud drapes although it was suggested4 that this facies may have been formed by the storm induced movement of sand waves i now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides and that the mud drapes are formed at slack water a model of this process has been described elsewhere5 and i now describe measurements of a section which supports my earlier conjecture6that the separation of the mud drapes is systematically related to the phase of the spring neap tidal cycle at the time of deposition although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime

implications of lunar orbital periodicity from the chaibasa tidal rhythmite india of late paleoproterozoic age harmonic analysis of the sandstone foreset laminae thickness series from the chaibasa tidal rhythmite india clearly shows that a normal semidiurnal tidal system with synodic month of 32 lunar days was in effect during the late paleoproterozoic 2100 1600 ma the minimum number of solar days in a lunar sidereal month was 31 published quantitative tidal rhythmite data of others in combination with data derived from the chaibasa tidal rhythmite indicate long term stability of the lunar orbit and progressive increase in the earth moon distance during the proterozoic

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

high resolution sequence stratigraphy of middle eocene hazad member jambusar broach block cambay basin india abstract the middle eocene hazad member of the ankleswar formation in the cambay basin is characterized by 13 sandstone units gs 0 to gs 12 representing progradational pulses within an overall fining up 2nd order transgressive systems tract tst the overall facies architecture of the hazad member implies deposition in a tide affected marginal marine deltaic environment high resolution sequence stratigraphic analysis based on core and electrolog data allows to subdivide the 2nd order tst into three 3rd order systems tracts namely i basal shaly transgressive systems tract tst followed by ii a highstand systems tract hst incorporating four intervening parasequence sets gs 0 to gs 2 gs 3 gs 4 to gs 6 and gs 7 to gs 9 separated by 4th order flooding surfaces and iii a transgressive systems tract tst at the top consisting of parasequence sets gs 10 to gs 12 capped by the kanwa shale that includes the maximum flooding surface mfs the amalgamated sands of the 3rd order sequences found landward split basinward to form six coarsening up 4th order cycles parasequence sets which are further subdivided into twenty one 5th order cycles parasequences separated by minor events of marine flooding the depositional model and paleogeographic maps using 3rd 4th and 5th order cycles reveal a minimal sand supply at the beginning of 3rd order hst and a significant increase in fluvial sand supply thereafter which resulted in progradation leading to constructive delta building till gs 9 the topmost transgressive parasequence sets constituting gs 10 to gs 12 mark landward retreat due to a paucity of sand supply and or a rise in relative sea level which resulted in back stepping of the paleoshoreline and development of estuaries over the older deltaic system the model helps to explain the extent of short term transgressive regressive cycles improving elucidation of the hydrocarbon bearing sand units of the cambay basin in future exploration

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

hydrothermal metasomatic and tectono metamorphic processes in the isua supracrustal belt west greenland a multi isotopic investigation of their effects on the earth s oldest oceanic crustal sequence abstract despite superimposed metamorphic overprinting and metasomatic alterations primary volcanic features remain preserved in low strain domains of mafic volcanic sequences in the western isua supracrustal belt isb west greenland these basaltic successions represent the hitherto oldest known fragments of oceanic crust on earth early archean metasomatic fluids rich in light rare earth elements lree th u pb ba and alkalies invaded the supracrustal package and distinctively altered the basaltic sequences field relationships source characteristics traced by pb isotopes and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the isb between 3 81 and 3 74 ga ago subsequent early archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks allanite occurs as the most characteristic and critical secondary metasomatic metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration even in domains of low strain because of its high concentration of lree th and u this secondary mineral accounts for much of the disturbances recorded by the sm nd and th u pb isotope systematics of the pillowed metabasalts the supracrustal sequences were tectono metamorphically affected to varying degrees during a late archean 2 6 to 2 8 ga old event also recognized in the adjacent gneiss terranes of the isuakasia area the degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite hosted parent daughter elements to the overall whole rock mass budget of the respective isotope systems although low strained initially only weakly metasomatized pillow basalts remained more or less closed with respect to the u pb and rb sr systems since 3 74 ga the sm nd system appears to have been partially opened on a whole rock scale during the late archean event this diversified behavior of the whole rock isotope systems with respect to late archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late archean recrystallization processes with associated renewed fluid flow in reactivated zones of high strain where primary metasomatic alteration is most prominently developed late archean partial resetting also of the u pb isotope system on a whole rock scale occurred this is consistent with an apparent late archean age of kyanite which initially crystallized during the early archean metamorphism its age is controlled by the u pb systematics of allanite inclusions which have exchanged their isotopic properties during the tectono metamorphic event that overprinted the oceanic crustal sequence at isua more than 1000 myr later these results underline the need for care in the interpretation of whole rock geochemical data from polymetamorphic rocks in general and from the isua oceanic crustal sequences in particular to constrain isotopic models of early earth s evolution likewise this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from isua to infer models for geotectonic settings relevant for their formation

modelling of cyclic tidal rhythmites carboniferous of indiana and kansas precambrian of utah usa as a basis for reconstruction of intertidal positioning and palaeotidal regimes analyses are presented for a variety of ancient cyclic tidal rhythmites which exhibit well developed neap spring tidal periods many such rhythmites were formed within the upper intertidal zone and exhibit truncated cycles that contain relatively few discrete lamina per neap spring cycle in such cases it can be difficult to determine if the originated palaeotidal system was diurnal or semidiurnal based on sedimentological controls observed in modern analogues the development of cyclic tidal rhythmites can be modelled by use of predicted tidal height information the modelling used a 19 year long series of tidal heights which were subsequently used to approximate tidal velocities and rhythmite sedimentation modelling was based on a range of diurnal to semidiurnal regimes and comparisons were performed at a number of levels within the intertidal zone this modelling produced a long series of simulated laminae thickness series which could be cross correlated with ancient laminae thickness series measured directly from ancient rhythmites this approach involved 1 6 106 comparisons of each ancient rhythmite series to the series simulated from the predictive tidal height information the high correlations derived for such comparisons indicate that reasonable approximations to the palaeotidal systems can be made in some cases it is possible to determine the diurnal or semidiurnal nature of the originating palaeotidal system

u pb geochronology of 3810 3630 ma granitoid rocks south of the isua greenstone belt southern west greenland abstract the oldest well preserved granitoid rocks on earth envelope the isua greenstone belt southern west greenland the timing and nature of geological events that affected granitoid rocks south of the isua belt southern gneisses are refined with new mapping and u pb geochronology these results and previous work allow for a detailed comparison between the southern gneisses and similar granitoid rocks north of the arcuate isua belt northern gneisses zircons from tonalitic gneiss and meta tonalite that yielded ages of 3810 3795 ma are interpreted as having grown during igneous crystallisation of their protoliths based on the consistency of ages within and between samples and the oscillatory internal zoning that is typical of magmatic zircons two dated tonalitic rocks are particularly important because they occur as concordant layers within supracrustal rocks amphibolite with deformed pillow lava structures in the southwestern part of the isua belt and quartz rich gneiss likely derived from a chemical sediment that forms an enclave in the southern gneisses although these layers lack cross cutting intrusive relationships they are interpreted as having intruded as sheets into 3800 ma supracrustal rocks most tonalite was converted into gneiss before intrusion of diorite dated at 3658 3 1 2 ma and deformed again during or shortly after intrusion of 3650 3630 ma granitic rocks whereas a few small lenticular zones of meta tonalite escaped nearly all strain the timing of amphibolite facies metamorphism may be recorded by zircon and titanite growth in amphibolitic gneiss at 3632 3620 ma other thermal events were recorded by zircon growth at 3550 and 2660 ma and titanite growth or pb loss at 2650 and 2600 ma these new data and interpretations suggest that the southern gneisses underwent many geological events along with the northern gneisses including 3658 ma deformation 3658 ma diorite intrusion and 3655 3630 ma crystallisation of granitic rocks during significant metamorphism and deformation the major differences are that most tonalite in the southern gneisses is 100 my older than the northern tonalite and the intensity of the neoarchaean tectonothermal activity increased southward any large scale juxtaposition of the southern gneisses against the northern gneisses across the isua belt likely occurred before intrusion of the 3658 ma diorite

tectonostratigraphy of the late archean dharwar supergroup dharwar craton india defining a tectonic history from spatially linked but temporally distinct intracontinental and arc related basins abstract the dharwar supergroup comprises the unconformity bound bababudan and chitradurga groups the bababudan group which is best preserved in the western dharwar craton records a divergent margin comprising a basalt dominant intracontinental rift sequence a shale bif drift sequence and a sandstone shale thermal subsidence sequence the rift stage evolved from 2765 ma to 2720 ma whereas the succession was folded uplifted and eroded during development of a convergent continental margin from 2680 ma ocean opening was to the east or southeast the chitradurga group records a two stage back arc basin behind an east facing continental arc stage i evolved between 2609 ma and 2582 ma and stage ii between 2582 ma and 2540 ma the two stages are separated by a subaerial unconformity stage i comprises siliciclastic fluvial shallow marine and deep marine sedimentary rocks and bimodal volcanic rocks stage i is best preserved in the western dharwar craton but there are equivalents in the eastern dharwar craton stage ii in the western dharwar craton comprises deep marine turbidites of siliciclastic and volcaniclastic provenance and basaltic and felsic volcanic rocks volcanic stage ii sequences in the eastern dharwar craton define an arc adjacent position tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time particularly for stage ii sequences a remnant of a

late precambrian tidal rhythmites in south australia and the history of the earth s rotation sedimentary rhythmites of siltstone and fine sandstone from late precambrian c 650 800 ma glaciogenic formations in south australia are interpreted as distal ebb tidal deposits that record variability in the velocity and range of palaeo ebb tides variations in lamina thickness encode a full spectrum of palaeotidal cycles including semidiurnal diurnal fortnightly and monthly tidal cycles as well as the lunar apsides perigee and nodal cycles a half yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni solar conjunction and lunar declination the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation the data allow determination of the earth9s palaeorotation and the past dynamics of the earth moon system with an accuracy previously unattainable for the precambrian the late precambrian c 650 ma year contained 13 1 0 5 lunar months and c 400 20 days and the late precambrian lunar month c 30 5 1 5 days these values suggest an average equivalent phase lag near 3 since late precambrian time rather than the present value of 6 the period of 19 5 0 5 years determined for the lunar nodal cycle c 650 ma ago indicates a lunar distance 96 9 1 7 of the present distance the low rate of lunar recession since late precambrian time revealed by the rhythmite data militates against a close approach of the moon during the proterozoic precambrian sedimentary rhythmites may hold a key to the early history of the earth9s rotation

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s