a tonian age for the visingso group in sweden constrained by detrital zircon dating and biochronology implications for evolutionary events detrital zircon u pb ages from samples of the neoproterozoic visingso group sweden yield a maximum depositional age of 886 9 ma 2 a minimum depositional age is established biochronologically using organic walled and vase shaped microfossils present in the upper formation of the visingso group the upper formation correlates with the kwagunt formation of the 780 740 ma chuar group in arizona usa and the lower mount harper group yukon canada that is older than 740 ma mineralized scale microfossils of the type recorded from the upper fifteenmile group yukon canada where they occur in a narrow stratigraphic range and are younger than 788 ma are recognized for the first time outside laurentia the mineralized scale microfossils in the upper formation of the visingso group seem to have a wider stratigraphic range and are older than c 740 ma the inferred age range of mineralized scale microfossils is 788 740 ma this time interval coincides with the vase shaped microfossil range because both microfossil groups co occur the combined isotopic and biochronologic ages constrain the visingso group to between 886 and 740 ma thus tonian in age this is the first robust age determination for the visingso group which preserves a rich microfossil assemblage of worldwide distribution the organic and mineralized microorganisms preserved in the visingso group and coeval successions elsewhere document global evolutionary events of auto and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the neoproterozoic chronostratigraphic subdivision

acritarchs from the doushantuo formation at liujing section in songlin area of guizhou province south china implications for early middle ediacaran biostratigraphy abstract a taxonomically diverse and morphologically disparate microfossil assemblage is recovered from the upper doushantuo formation in the liujing section of songlin area guizhou province including acanthomorphic and sphaeromorphic acritarchs multicellular algae and filamentous cyanobacteria acritarch mengeosphaera membranifera sp nov is newly erected and genus cymatiosphaeroides and several species c forabilatus c kullingii bacatisphaera baokangensis are emended in the systematics the acanthomorphic species are dominated by cymatiosphaeroides forabilatus and mengeosphaera membranifera sp nov the liujing assemblage shares many species with other ediacaran coeval assemblages from south china australia siberia the east european platform india mongolia and svalbard and indicates a significant taxonomic similarity to the global age diagnostic assemblages new occurrence of those known taxa in the liujing succession documents the biodiversity of the ediacaran acritarchs in south china the liujing assemblage extends the palaeogeographic distribution of the ediacaran acritarchs and may be assigned to the tanarium conoideum cavaspina basiconica assemblage zone that was recognized in the yangtze gorges area of south china alternatively it could be partially correlated with the barren interval between t conoideum c basiconica assemblage zone and the overlying tanarium pycnacanthum ceratosphaeridium glaberosum assemblage zone

organic walled microfossils from the tonian gouhou formation huaibei region north china craton and their biostratigraphic implications abstract the meso and neoproterozoic huaibei group in the huaibei region north china craton ncc is emerging as a target for geobiological and tectonic studies due to its thick unmetamorphosed sediments with well preserved organic walled fossils however the lack of accurate age constraints for this sedimentary sequence dramatically hampers our ability to take full advantage of the rich geological and paleontological history recorded in the huaibei group particularly the depositional age of the uppermost unit of the huaibei group the gouhou formation is a controversial issue although it is widely accepted that there is an unconformity between the gouhou formation and the overlying early cambrian houjiashan formation the magnitude of this unconformity and hence the scale of the underlying tectonic event are unconstrained due to the uncertainty of the sedimentary age for the gouhou formation which has been variously interpreted as cambrian cryogenian ediacaran 720 ma to 541 ma and tonian 1000 ma to 720 ma to improve our knowledge about this sedimentary gap we carried out a biostratigraphic study of the gouhou formation our investigation using a low manipulation maceration technique revealed a diverse organic walled microfossil assemblage a total of 22 taxa have been revealed including a new species dictyosphaera tacita n sp importantly the co occurrence of trachyhystrichosphaera valeria and dictyosphaera in the gouhou formation suggests a likely tonian age the biostratigraphic data indicate an infra cambrian depositional gap of 200 myr between the tonian gouhou formation and cambrian houjiashan formation perhaps driven by a major tectonostratigraphic event

upper proterozoic and lower cambrian rocks of the sheeprock mountains utah regional correlation and significance apparently conformable upper proterozoic and lower cambrian miogeoclinal rocks in the sheeprock mountains utah attain a maximum thickness of at least 7 200 m the sequence begins at the base with phyllite quartzite glaciomarine diamictite and shale deposited near the northern edge of a subsiding basin these rocks are assigned to the sheeprock group 2 700 4 300 m overlying quartzitic rocks 1 950 4 000 m are correlated with specific formations of the brigham group huntsville sequence revision of earlier accounts of the stratigraphy in the sheeprock mountains is suggested by the recognition of low angle faults that attenuate the stratigraphic section stratigraphic relations in the sheeprock mountains bear on regional correlation the probable presence in the deep creek range of two diamictite units separated by quartzite is reaffirmed this sequence is grossly similar to that of the sheeprock mountains it is suggested that the caddy canyon quartzite brigham group inter fingers to the south and west of the sheeprock area with siltstone shale and some limestone possibly no rocks exposed in the san francisco mountains and canyon range are older than the caddy canyon quartzite the mccoy creek group of western utah and eastern nevada is probably for the most part equivalent to the caddy canyon quartzite the osceola argillite unit g mccoy creek group may be equivalent to the inkom formation and it perhaps records a marine transgression that temporarily reduced the clastic supply the correlation of the mutual formation of the platform sequence in the wasatch range to a lithologically similar unit in the miogeocline to the west remains the simplest interpretation although the platform mutual may be older than the unit of the same name in the miogeocline

preliminary paleomagnetic poles and correlation of the proterozoic uinta mountain group utah and colorado abstract stable paleomagnetic directions have been obtained from seven sites spanning much of the 7 km thick middle proterozoic uinta mountain group the characteristic magnetization is carried partly by detrital and secondary specularite and partly by hematite pigment preliminary paleomagnetic poles for six of the seven sites are closely grouped and their mean pole is 0 6 n 157 2 e n 6 95 5 4 the pole for the seventh and stratigraphically highest site lies to the west 4 6 n 140 1 e and n 8 95 15 9 on the basis of the poles revisions to correlations of the uinta mountain group with other sequences are proposed the uinta mountain group appears to correlate with the chuar group and perhaps the upper member of the underlying nankoweap formation and post dates the unkar group of the grand canyon supergroup in addition the upper part of the uinta mountain group appears to correlate with the little dal group of the northwest territories canada the uinta mountain group appears to entirely post date the belt supergroup of montana and idaho correlation of the uinta mountain group with the chuar and little dal groups is supported by the presence of the fossil chuaria and by apparently overlapping isotopic ages a maximum age range for the uinta mountain group extrapolated from the grand canyon supergroup and the little dal group extends from 770 m y

an experimental study of sulfidation reactions involving andradite and hedenbergite three sulfur buffering sulfide silicate reactions which limit the stabilities of the skarn silicates andradite garnet and hedenbergite pyroxene under sulfidizing conditions were studied hydrothermally at a pressure of 2 kb over the temperature range 700 degrees to 350 degrees c reaction 1 andradite s 2 wollastonite anhydrite quartz pyrite or pyrrhotite reaction 2 andradite magnetite s 2 anhydrite quartz pyrite or pyrrhotite andreaction 3 hedenbergite s 2 andradite quartz pyrrhotite the equilibrium value of the sulfur fugacity f sub s 2 for each reaction assemblage at any given temperature was calculated from the measured composition of a coexisting sulfur fugacity indicator pyrrhotite or sphalerite as appropriate the log f sub s 2 data were adequate for the determination of the log f sub s 2 1 t relations for each of the andradite sulfidation reactions owing to excessive scatter in the data for the hedenbergite sulfidation reaction however the log f sub s 2 1 t relation for this reaction derived from the log f sub o 2 data of burton et al 1982 is preferred to the one developed by gamble 1976 1978 the log f sub s 2 1 t relations for the three sulfidation reactions were combined with equilibrium data available in the literature for other reactions in the model system ca si fe o s and used to construct quantitative isothermal isobaric log f sub s 2 log f sub o 2 phase diagrams applicable to the formation of ca si fe skarns the quantitatively derived phase relations were found to be consistent in all major respects with the schematic phase relations deduced by burt 1972a and b the influence of diopside component on the stability of hedenbergitic pyroxene was also studied diopside was found to enlarge the stability field of hedenbergitic pyroxene increasing the temperature at which pyroxene becomes stable with pyrite from 288 degrees c for end member hedenbergite burton et al 1982 to 395 degrees c for 50 mole percent diopside diopside was found to be less effective than the johannsenite component in stabilizing hedenbergitic pyroxene

a new paleomagnetic pole for the neoproterozoic uinta mountain supergroup central rocky mountain states usa abstract hematite cemented strata of the neoproterozoic age 800 750 ma uinta mountain group umg provide a new well defined mid neoproterozoic paleomagnetic pole for laurentia thermal demagnetization results and rock magnetic experiments reveal the presence of two discrete magnetizations in umg rocks both magnetizations are carried by hematite the dominant characteristic remanent magnetization chrm is an east directed or antipode shallow positive to negative inclination magnetization that is typically unblocked over a narrow range of high laboratory temperatures between 660 and 680 c the second magnetization exclusively of normal polarity is north to northeast directed and has a moderate to steep positive inclination and probably represents a recent or modern day viscous remanent magnetization the response of umg samples to thermal and chemical demagnetization comparison with previously obtained data from umg rocks in the area results from rock magnetic experiments and the inferred low latitude of laurentia during umg deposition in the neoproterozoic are all consistent with the interpretation that the west or east directed shallow magnetization is a near primary remanence the presence of dual polarities of the chrm with the west directed remanence inferred to be of normal polarity and the east directed of reverse polarity suggests that magnetization acquisition in the umg rocks took place over a sufficient amount of time to average secular variation of the geomagnetic field the mean paleomagnetic pole derived from the inferred primary magnetizations from umg strata 0 8 n 161 3 e 95 4 6 n 9 sampling localities consisting of 79 sites is fully consistent with recently reported results from rocks of mid neoproterozoic age for southwest laurentia which define a tight counterclockwise loop located in the south pacific in the apparent polar wander path for laurentia during the neoproterozoic

1 3 billion years of acritarch history an empirical morphospace approach abstract acritarchs are a group of organic walled vesicular microfossils interpreted as protists and are among the first eukaryotes preserved in the fossil record taxonomic inconsistencies amongst acritarch workers have made it difficult to address the evolutionary history of this group through more traditional methods i e biodiversity through species counts we have constructed an empirical morphospace to examine the first 1 3 billion years of acritarch evolution we show that protist morphologic evolution is broadly correlated with major environmental and biologic revolutions in earth history such as late neoproterozoic global glaciations the first appearance of the ediacaran metazoans and the cambrian explosion our results also show that protist morphologic expansion precedes their taxonomic diversification this pattern similar to that seen in phanerozoic animal clades suggests that early morphospace saturation and convergence are common occurrences in eukaryote macroevolution in addition our data do not support a monotonic increase in maximum diameter of acritarch vesicles through the proterozoic instead maximum vesicle diameter appears to fluctuate in the proterozoic before decreasing significantly in the early cambrian

infilling history of a neoproterozoic intracratonic basin nd isotope provenance studies of the uinta mountain group western united states abstract neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the neoproterozoic uinta mountain group umg in north eastern utah and northwestern colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the proterozoic geologic history of the southern margin of the archean wyoming province in the east lobe of the uinta mountains coarse grained components of the jesse ewing canyon formation the oldest recognized formation of the umg have measured nd values of 23 to 28 and were most probably derived from archean basement of the wyoming province to the north fine grained sediment in the southern portions of the jesse ewing canyon formation and shales and quartz arenites from overlying undifferentiated umg all contain at least a component of detritus derived from proterozoic crust in the west lobe the oldest members of the umg the red castle lake gilbert peak and moosehorn lake formations and arkosic arenite comprising the northern portions of the overlying mount watson formation were also derived exclusively from archean sources however as in the east lobe the more mature sandstones that dominate the southern sections of the mount watson formation have isotopic compositions that require a significant proterozoic crust derived detrital component our data support a model in which sedimentary rocks with an archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the umg was deposited a source for these sediments within the archean wyoming province is consistent with the high light rare earth element lree and th abundances of finegrained low nd sedimentary rocks given that late archean granitic rocks within the wyoming province have similar trace element characteristics sediments with proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis the proterozoic detrital component in these sediments originated either through direct erosion of the colorado province in the neoproterozoic or through erosion of mixed archean and proterozoic crust located to the east of the umg basin that originally formed during the paleoproterozoic accretion of the colorado province to the southern margin of the wyoming craton

megafossils from the mesoproterozoic rohtas formation the vindhyan supergroup katni area central india abstract three megafossils are described from the upper part of the mesoproterozoic rohtas formation semri group lower vindhyan generally they occur on the bedding surfaces of shale and limestone the megafossils are grouped as carbonaceous discs filamentous forms and dubiofossils chuaria circularis is assigned to the carbonaceous discs filamentous forms are represented by grypania spiralis and form a dubiofossils are referred to as moulds of a grypania like form

microbiota from the neoproterozoic liulaobei formation in the huainan region northern anhui china abundant and well preserved acritarchs have been obtained from shales of the neoproterozoic liulaobei formation in the huainan region northern anhui province china this microfossil assemblage consisting of 9 genera 11 species and an undetermined form is dominated by sphaeromorphic acritarchs as well as filamentous cyanobacteria with the exception of a few new forms e g pololeptus rugosa yin c y comb nov p biacris yin c y comb nov and the undetermined taxon most components of this microbiota are well known from neoproterozoic deposits elsewhere in the world an integrational assessment of this microbiota in conjunction with the accompanying occurrence of the chuaria tawuia megafossil assemblage as well as radiometric datings tends to suggest that the liulaobei formation might be bracketed within a late riphean time range of 850 700 ma providing an useful reference for estimating the age of the overlying jiuliqiao formation which yields carbonaceous worm like fossils probably of a metazoan origin

aspects of problematic acid resistant organic walled microfossils acritarchs in the upper proterozoic of the north atlantic region abstract proterozoic acritarchs are a heterogeneous group of acid resistant organic walled microfossils of obscure biological affinities the acritarch record is regarded as strongly baised by a combination of mechanisms such as the character of the original ecosystems depositional conditions diagenetic history and thermal and tectonic events three distinctive acritarch assemblages are recognized in upper proterozoic sequences in scandinavia and greenland they can be compared with upper proterozoic upper riphean and vendian assemblages previously reported from the russian and siberian platforms china and elsewhere substantial taxonomic changes characterize these assemblages and are usually connected with the existence of major unconformities in the investigated rock sequences the acritarch assemblages can be traced over vast areas and are considered as constituting a promising tool for the biostratigraphic correlation of the upper proterozoic and lower cambrian

microbiotas from the late proterozoic chuar group northern arizona and uinta mountain group utah and their chronostratigraphic implications abstract samples of shales from the dominantly clastic upper proterozoic chuar and uinta mountain groups in northern arizona and utah yielded relatively abundant assemblages of acritarchs these are taxonomically varied and are interpreted as consisting of openwater cosmopolitan eukaryotic plankton the assemblages are compared with late proterozoic late riphean and early vendian c 800 700 ma sequences in the southern urals of the ussr the russian platform northern and southern scandinavia svalbard and east and north west greenland in all instances both the assemblages and discrete form taxa predate the varangerian vendian glacial event of the north atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western north america one new acritarch species vandalosphaeridium walcottii is erected and the partly megascopic species chuaria circularis is emended

organic walled microfossils from the late mesoproterozoic to early neoproterozoic lower shaler supergroup arctic canada diversity and biostratigraphic significance abstract the diversification of acritarchs organic walled vesicular microfossils of unknown affinity filamentous and multicellular microorganisms happened during a time of profound environmental biological and ecological change the mesoproterozoic to neoproterozoic transition is a key interval notably for eukaryotic organisms new assemblages of organic walled microfossils from the ca 1230 ma to 900 ma lower shaler supergroup of arctic canada record an impressive diversity including macroscopic compressions of chuaria circularis and numerous unambiguous eukaryotic taxa the index taxon trachyhystrichosphaera aimika co occurs with eukaryotic taxa hitherto reported only from earlier dictyosphaera gigantosphaeridium satka favosa sensu javaux and knoll 2016 or later microlepidopalla time intervals five new taxa comprising the spheroidal acritarchs with inner wall sculpture nunatsiaquus cryptotorus n gen n sp and daedalosphaera digitisigna n gen n sp the acanthomorphs spiny acritarchs with regularly distributed processes herisphaera arbovela n gen n sp and herisphaera triangula n sp and the process bearing multicellular ourasphaira giraldae n gen n sp are reported along with three unnamed forms collectively these remarkable microfossils 63 taxa including 25 eukaryotic forms demonstrate the greatest diversity of eukaryotes ever recorded for this time interval

an assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china abstract macroalgae play a crucial ecological and biogeochemical role in modern marine ecosystems although eukaryotic algae have a deep history within the precambrian most pre cryogenian alga fossils are microscopic and macroalga fossils are poorly documented during this period here we report a well preserved assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china this assemblage consists of seventeen distinct morphotaxa including eleven macroalgae showing remarkable taxonomic diversity that is more diverse than many other tonian macrofossil assemblages all over the world three taxa from the shiwangzhuang assemblage show evidence of simple multicellularity with two of them consisting of giant cells and comparable to giant celled green algae and seven taxa with diagnostic transverse annulations are likely coenocytic algae the shiwangzhuang assemblage indicates that some eukaryotic clades had achieved macroscopic growth through simple multicellularity or coenocytism paving the way either ecologically or phylogenetically for the eventual appearance of complex multicellularity in addition there are some biostratigraphically significant genera including the chuaria tawuia and sinosabellidites protoarenicola pararenicola assemblages indicating a tonian age for the shiwangzhuang formation this study also suggests that the tonian diversity of multicellular or coenocytic macroalgae is probably underestimated pending for a systematic and thorough investigation of tonian macroscopic carbonaceous compressions macroalgae may have an initial ecological expansion at least locally if not globally in the tonian period

ultrastructure of chuaria walcott vidal and ford acritarcha from the late proterozoic pendjari formation benin and burkina faso west africa abstract mega acritarchs previously referred to as chuaria circularis walcott from the late proterozoic pendjari formation of benin and burkina faso west africa are reinvestigated both optical microscopy of thin sections and scanning electron microscope study of these fossils in situ on the bedding surface of shales have revealed a considerable thickness and a multilamellar structure of the acid resistant wall these two fundamental features had not been recognized in the only previous study of the topotype chuaria circularis from the chuar group arizona usa using scanning electron microscopy however a third feature a canal like structure which had been reported on topotype specimens by transmission electron microscopy has just been recorded in the present material too using the same technique

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

microfossils from the lower mesoproterozoic kaltasy formation east european platform abstract basinal shales of the lower mesoproterozoic kaltasy formation sampled from three boreholes drilled into the southeastern east european platform russia contain abundant and moderately well preserved microfossils 34 distinct entities have been identified most assigned to simple sphaeromorphic or small filamentous taxa found widely and characterized by long stratigraphic ranges ornamented microfossils found in coastal successions of other lower mesoproterozoic basins are absent but large filamentous microfossils interpreted as possible benthic photosynthetic eukaryotes are recorded drawing comparisons to relatively deep water shales in siberia in overall aspect the kaltasy microfossils are consistent with other broadly coeval assemblages but they highlight the importance of environment as well as age in determining the distributions of remains that record the early diversification of marine eukaryotes rectia magna is described as a new species

early neoproterozoic sequence stratigraphy and acritarch biostratigraphy eastern officer basin south australia abstract the early neoproterozoic torrensian latest tonian 800 750 ma succession in the central and eastern officer basin was deposited in an intra cratonic rift basin the outcropping pindyin sandstone consists of basal lowstand conglomerate and upward fining transgressive sandstone and is conformably overlain by marine highstand shale of the alinya formation in the giles 1 well the pindyin sandstone contains aeolian to fluvial deposits and is overlain by peritidal and sabkha sediments of the alinya formation seismic interpretation suggests that the succession is thin on the murnaroo platform and thickens toward the munyarai trough the two sedimentary provinces are separated by the ungoolya hinge which probably marks the southeastern margin of the fraser musgrave fold belt highstand organic rich shale in the alinya formation is probably an important petroleum source rock acritarchs occur in both chert and shale in the alinya formation and their distribution was probably controlled by sedimentary environments similar assemblages from outcrop and subsurface provide a reliable evidence for regional correlation the assemblages in giles 1 are dominated by benthic coccoids and contain many distinct spinose acritarchs which are particularly abundant in the subtidal grey shale whereas mainly phytoplankton is present in the chert the assemblage represented by trachyhystrichosphaera vidalii t aimica cymatiosphaeroides kullingii can be used for global correlation in the early neoproterozoic the abundant microfossils from the alinya formation and equivalent beds around the world probably indicate a microorganism explosion at 750 800 ma prior to the neoproterozoic glaciations thirty seven acritarchs spheroids and seven cyanobacterial filaments are briefly described in this study including 5 new species comasphaeridium tonium eoentophysalis gilesis goniosphaeridium alinyum gorgonisphaeridium pindyium and gorgonisphaeridium torrensium and a newly combined species archaeoellipsoides karatavicus comb nov

patterns of phytoplankton radiation across the precambrian cambrian boundary late proterozoic and early phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity their radiation patterns around the proterozoic cambrian boundary interval are becoming increasingly well known for numerous sequences in the east european and siberian platforms china baltoscandia and north america transgressive sequences in the east european platform and northern baltoscandia comprise alternating sandstones mudstones and claystones their sedimentology is well documented and microfossils acritarchs chitinous fossils sabelliditids shelly fossils and trace fossils are well represented rocks immediately overlying the transitional beds have yielded lower cambrian trilobite faunas that are utilized for regional correlations continuous successions of planktic microorganisms consisting of prokaryotic photoautotrophic cyanobacteria and algal protists are known from rocks comprising the transitional vendian cambrian sequence in the east european platform the rock sequence attributed to the lower cambrian platysolenites antiquissimus zone has yielded acritarchs of 9cambrian type9 abruptly replacing a morphologically simple proterozoic type microbiota recorded in beds of the upper proterozoic sabellidites vendotaenia zone the radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late proterozoicarly cambrian shelf and is parallelled by the nearly contemporaneous emergence of cambrian shelly fossils and trace fossils along with new acritarch taxa the diagnostic acritarch assemblage of the platysolenites antiquissimus zone consists of widely distributed species occurring also in the lowermost cambrian deposits of the siberian platform baltoscandia greenland north america svalbard and scotland it is suggested that along with shelly faunas trace fossils and geochemical data the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the precambrian cambrian boundary it is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the precambrian cambrian boundary

tectonic setting and provenance of the neoproterozoic uinta mountain and big cottonwood groups northern utah constraints from geochemistry nd isotopes and detrital modes abstract the neoproterozoic uinta mountain group was deposited in an east trending intracratonic rift bounded on the north by an active fault system and opening into a shallow sea on the west where the big cottonwood group was deposited in an estuary although this rift may have been associated with the early stages in the breakup of rodinia it was not an aulacogen geochemical nd isotope and detrital mode studies indicate that uinta mountain group sediments were derived from mixed archean and paleoproterozoic sources with the former dominating big cottonwood group sediments appear to have been derived predominantly from paleoproterozoic sources the archean sediment source is the wyoming craton and source rocks comprised dominantly granites enriched in th u y zr hf and ree the relative abundance of enriched granite implied by sedimentary rocks of the uinta mountain group indicates that the wyoming craton is anomalous compared to other archean cratons cia values and a cn k relationships in shales of the uinta mountain and big cottonwood groups indicate high degrees of weathering of sources probably in subtropical to tropical climates supporting a near equatorial location for southwestern laurentia at about 800 ma differences in the nd isotopic composition between the big cottonwood group and neoproterozoic sedimentary rocks in western utah and northeast nevada suggest a northwest striking uplift in northwest utah possibly ancestral to the paleozoic toole uinta arch

stratigraphic and ecologic implications of late precambrian microfossils from utah abundant and well preserved microfossils have been discovered in black shales and siltstones of the glaciogenic upper precambrian mineral fork forma tion ncar salt lake city utah the rocks are interpreted to have been deposited close to an ice margin in a shallow restricted marine embayment during glacial retreat several distinct morphotypes are discernible but following moorman 1974 we interpret most of these as stages in the life cycle of a single planktonic endosporulating alga bavlillellll jllveolllta shepeicva vidal the low taxonomic diversity of this assemblage coupled with the large population size and almost complete dominance by bavlillella suggests an ecologically stressed environment the source of this stress was probably the melting glacier on the basis of stratigraphic position and lithological correlation with a radio metrically dated sequence in vashington the mineral fork formation has been considered to be about 800 m y old however most well dated occurrences of bavlillella are in rocks of vendian 650 570 m y age the presence of bavlillella javeolata in mineral ork strata raises significant questions concerning both the stratigraphic range of this presumed vendian index fossil and the timing of late precambrian glaciation in the l orth american conlillera

problematic chuaria from the bhima basin south india abstract the bhima seriess of karnataka state south india occupies an area of 4200 km 2 in the gulburga district and is bounded by latitude 17 14 00 and longitude 77 04 00 the present paper deals with an abundant occurrence of fossil chuaria circularis in the lower bhima shales much confusion prevails regarding the nomenclature of this fossil which had been reported elsewhere under different names the type material chuaria circularis has been augmented by bhima material to provide nomenclatural stability and a sound basis for correlating different late proterozoic formations

high resolution δ13c stratigraphy of the chuar group ca 770 742 ma grand canyon implications for mid neoproterozoic climate change a high resolution c isotope record based on 13 c org from organic rich shales and 13 c carb from dolomites in the ca 770 742 ma chuar group provides important new data for evaluating the signifi cance of large magnitude c isotope anomalies in neoproterozoic climate change three successive large magnitude isotopic excursions 8 15 are interpreted to represent primary seawater values based on a series of diagenetic tests and they are not associated with evidence of signifi cant long term 10 6 10 7 m y sea level change nor glaciomarine deposits intrabasinal correlation of 13 c org values suggests that most chuar shales record primary values and is consistent with previously reported h c ratios of 0 49 indicating that chuar shales experienced minimal thermal alteration although some chuar dolomites reveal early diagenetic alteration their 13 c dol values typically fall near those of coeval least altered dolomites or organic rich shales relative to dolomite values the chuar carbon record is interpreted to refl ect predominantly primary organic carbon 13 c values and contains suf