neoproterozoic glacial rainout intervals observations and implications stratigraphically discrete glacigenic dropstone intervals have been identified within six separate neoproterozoic glaciomarine successions the ca 720 ma chuos formation southwestern congo craton and surprise diamictite southwestern laurentia the ca 600 ma ghaub formation congo craton blasskrans formation northwestern kalahari craton and wildrose diamictite southwestern laurentia and the post 595 ma lower southern highland group northeast laurentia these dropstone intervals are interstratified with hemipelagic dropstone free lithologies that record periods devoid of glacial rainout such episodic deposition requires at a most rudimentary level temporally compatible environmental fluctuations to generate melting of ice the ghaub diamictite is particularly revealing in this regard given that according to the snowball earth hypothesis it had to be deposited either during glacial maxima totally frozen seas or melt back geologically instantaneous owing to its low latitude setting and association with carbonates that record highly depleted 1 3 c values the consistency of facies features shared by all six glaciomarine successions leads us to speculate that the temporally discrete glacial rainout events including diamictites deposited in low latitudes record a dynamic glacial environment and that such conditions were characteristic of neoproterozoic glaciations in general these observations indicate that neoproterozoic seas were not totally frozen and that the hydrologic cycle was functioning

an upper dalradian microfossil reassessed an acritarch from the highest dalradian macduff slate of northeast scotland is confirmed as veryhachium cf lairdii its presence implies that the macduff slate is ordovician this conclusion is compatible with the view that there is stratigraphical and structural continuity between the dalradian and lower palaeozoic rocks in the vicinity of the highland border e g the lower cambrian keltie water grit formation but runs counter to other lines of evidence notably biotite cooling ages which suggest that the dalradian had undergone metamorphism by the late cambrian further investigations are needed to reproduce and confirm the biostrati graphical evidence for the age of the macduff slate

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

testing the climatic significance of dropstones an example from southeast spain dropstones are clasts which pose a hydrodynamic paradox or which have an exotic lithology with uncertain provenance dropstones are usually associated with glacial processes and have recently been used to substantiate the presence of cool climatic phases in the geological record the aim of this paper is to demonstrate using an example from southeast spain that dropstones can be unreliable palaeoclimatic indicators and that greater care is required in their interpretation

87sr 86sr chemostratigraphy of neoproterozoic dalradian carbonates below the port askaig glaciogenic formation scotland abstract the neoproterozoic cambrian dalradian supergroup of scotland contains three glaciogenic sedimentary units the port askaig formation the stralinchy reelan formation and the loch na cille boulder bed in ascending order although the idea that relatively cold climates occurred during the latest proterozoic is widely accepted the cause of the onset of glaciation is still controversial we use new carbonate 87 sr 86 sr data to better constrain this environmental change some dalradian limestones have high strontium contents 1000 ppm low manganese contents and mn sr ratios 87 sr 86 sr values of paleoseawater in the neoproterozoic the 87 sr 86 sr values of limestones of the islay formation ca 0 7067 sampled on the island of islay are similar to the values of previous studies however the 87 sr 86 sr values of limestones just below the port askaig formation ca 0 7064 collected on the island of garbh eileach have the lowest values recorded in the dalradian supergroup comparing these 87 sr 86 sr values with coeval 87 sr 86 sr values of other regions in the world such low 87 sr 86 sr i87 sr 86 sr ratio from the tonian through the cryogenian and ediacaran to the earliest cambrian the results show that the 87 sr 86 sr ratio of seawater decreased just before the older cryogenian glaciation that it increased by two steps in the early and middle ediacaran and that there were at least two abrupt positive excursions in the middle ediacaran and at the precambrian cambrian boundary the decline of the 87 sr 86 sr ratio of seawater just before the older cryogenian sturtian glaciation suggests that enhancement of weathering related to break up of the rodinia supercontinent and continental flood basalt volcanism was a likely driver for generating global cooling for the snowball earth conditions godderis et al 2003 donnadieu et al 2004

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

glacially influenced sedimentation in the late neoproterozoic mechum river formation blue ridge province virginia the late neoproterozoic ca 700 730 ma mechum river formation is exposed as a structural inlier within grenvillian basement in the blue ridge province of central virginia united states the southern portion of the mechum river belt preserves evidence of glaciomarine sedimentation at the margin and basinward of grounding line fans subaqueous glacial till coarse grained rhythmites and dropstones record the incursion of ice into the basin marine deposition is evidenced by persistent laminations cyclopsams in rhythmically bedded diamictite and possibly by tidally pumped distal turbidites in laminated mudstones glaciomarine sedimentation in the mechum river formation may have been coeval with sturtian rapitan glaciations in northwestern laurentia and australia and provides further evidence for a globally cold climate during the late neoproterozoic

high precision u pb monazite geochronology of the c 806 ma grampian shear zone and the implications for the evolution of the central highlands of scotland two pegmatites and mylonitic rocks from the grampian shear zone yield u pb monazite ages of 806 3 ma 808 9 11 ma and 804 12 13 respectively these confirm that shearing and pegmatite crystallization was essentially synchronous lower intercepts of c 440 ma indicate disturbance during ordovician reworking these new data question the concept of a single latest proterozoic early palaeozoic orogeny in the grampian terrane and the significance of the great glen fault

short paper the depositional age of the dalradian supergroup u pb and sm nd isotopic studies of the tayvallich volcanics scotland zircons from a keratophyre associated with the tayvallich volcanics in the dalradian rocks of the sw scottish highlands have been dated by u pb methods yielding an age of 595 4 ma this age indicates that most or all of the dalradian is precambrian in age and that dalradian sedimentation may have lasted for about 200 million years the age also constrains the time interval between cessation of dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the grampian orogeny sm nd isotopic data for the tayvallich volcanins and related metadolerite sills yield initial ond values of 2 to 4 which are thought to reffect the effects of melting of lithospheric mantle

characteristics and origin of the oak dam east breccia hosted iron oxide cu u au deposit olympic dam region gawler craton south australia oak dam east is a large 560 mt of 41 56 wt fe cu au u bearing massive fe oxide deposit in the northeastern gawler craton south australia it is one of several unmined deposits in the same province as the giant olympic dam deposit and it lies beneath more than 500 m of younger cover rocks the oak dam east breccia complex formed in a near surface environment and was characterized by an early history of skarnlike albite calc silicate magnetite formation from hypersaline 400 500 c fluids large scale brecciation was an early feature and the resulting complex was progressively filled in and replaced by 1 hematite and goethite hematite stage i 2 magnetite apatite quartz actinolite magnetite stage 3 hematite goethite quartz chalcedony pyrite chlorite monazite hematite stage ii and 4 chalcopyrite pitchblende illite hydromus covite florencite carrolite cu u au mineralization stage hematite stage ii assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent nacl equiv that were boiling at 170 to 190 c and had 18o values more negative than 3 7 to 1 7 per mil interpreted as meteoric water the upper 200 m of the breccia complex contains spectacular colloform hematite void fillings interpreted to have replaced hydrothermal goethite the cu u mineralized zones resemble olympic dam ore in terms of geometries being grossly subhorizontal 10 to 70 m thick and crosscutting with respect to the host breccias the mineralization is zoned vertically from pyrite 34s 6 to 0 at depth to shallower chalcopyrite 34s 14 to 7 high grade pitchblende occurs centrally within the chalcopyrite zone e g 10 m of 0 46 cu and 3 700 ppm u3o8 straddling a boundary between k mica illite alteration and underlying fe mg chlorite alteration the deposit is assigned to the 1570 to 1595 ma iron oxide cu au u event in the eastern gawler craton on the basis of its similarity to other cu au u deposits in the province an la icpms pb isotope age for monazite in hematite stage ii was calculated to be 1455 20 ma and possibly as young as 1370 54 ma to explain all isotopic data this is an age of isotopic resetting our preferred model is that cu u mineralization developed within a stratified hydrothermal brine complex analogous to the salton sea geothermal system copper fe and s in the deep brine was sourced from an evolving deep reservoir with local compositional controls on sulfur isotopes including cooling boiling and replacement of previous pyrite chalcopyrite u veins and disseminations formed as a result of mixing with overlying steam heated u rich saline ground water mixing induced cooling and dilution only occurred episodically following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex by contrast high grade uo2 formation occurred as deep upflow waned via reaction of the upper brine with hematite stage ii fe mg chlorite along an interface that descended with time this resulted in an increase of u grades through time caused by leaching and reprecipitation

the late ordovician early silurian glacial period abstract late ordovician to early silurian tillites and evidence of extensive glacial erosion were first discovered in the western sahara in the late 1950 s although the concept of a glacial period at this time had been around since the late 19th century further evidence of glaciation is now available from west africa south africa the arabian peninsula eastern north america west and central southe america and europe several glacial episodes can be distinguished the earliest evidence local mountain glaciation appears to be of caradoc age the major episode may have begun in caradoc time and lasted much of ashgill time when a major ice sheet covered at least north central and west africa which was then centered near the south pole however independent palaeontological and sea level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 ma this conflict has yet to be resolved the latest evidence of glaciation is wenlock in age but is confined to south america which by then was replacing africa over the south polar region however this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice sheet dimensions the lack of land in the north polar region probably explains the absence of tillites from the then northern hemisphere land masses the total area affected by glaciation in early palaeozoic time in north africa alone is at least 6 106 km2 combined with other glaciated areas glaciation may have been on the scale of that of present day antarctica

iceberg drop dump and grounding structures from pleistocene glacio lacustrine sediments scotland abstract dropstone dump and grounding structures resulting from the melt out of debris contained in floating and grounded icebergs are described from a pleistocene ice marginal glacio lacustrine sequence in scotland dropstones display bending penetration rucking and complete rupture of stratum occurring beneath them and onlap above them with the degree of deformation varying as a function of the size shape and axial disposition of the clast and of the sediment type into which they fell dump structures are conical mounds of gravel or diamict formed by the break up and overturning of dirt laden icebergs and the consequent release of large quantities of debris to the lake floor grounding structures are caused by the grounding of icebergs the down warping of underlying lake floor se iment and the subsequent in situ melt out of contained debris to form isolated troughs of diamict

87sr 86sr chemostratigraphy of neoproterozoic dalradian limestones of scotland and ireland constraints on depositional ages and time scales new calcite 87 sr 86 sr data for 47 limestones from the metamorphosed and deformed neoproterozoic cambrian dalradian supergroup of scotland and ireland are used to identify secular trends in seawater 87 sr 86 sr through the dalradian succession and to constrain its depositional age dalradian limestones commonly have sr 1000 ppm indicating primary aragonite and marine diagenesis low mn mn sr 18 o and trace element data indicate that many 87 sr 86 sr ratios are unaltered since diagenesis despite greenschist to amphibolite facies metamorphism consistent with the documented behaviour of sr and o during metamorphic fluid rock interaction thus the 87 sr 86 sr data are interpreted largely to reflect 87 sr 86 sr of coeval seawater currently available data show that neoproterozoic seawater 87 sr 86 sr rose from c 0 7052 at 850 900 ma to c 0 7085 or higher in the latest neoproterozoic temporal changes at c 800 ma and c 600 ma bracket the range in 87 sr 86 sr values of calcite in grampian appin and lowest argyll group c 0 7064 0 7072 and middle and uppermost argyll group c 0 7082 0 7095 limestones consistent with a rise in seawater 87 sr 86 sr around 600 ma 87 sr 86 sr data are consistent with the sedimentary affinity of the islay subgroup with the underlying appin group and with a possible time interval between deposition of islay and easdale subgroup rocks they indicate that the dalradian as a whole is younger than c 800 ma

global c cycle perturbations recorded in marbles a record of neoproterozoic earth history within the dalradian succession of the shetland islands scotland abstract the shetland islands in scotland contain a c 12 km thick mostly siliciclastic neoproterozoic succession that experienced deformation and greenschist to amphibolite facies metamorphism during the ordovician silurian caledonian orogeny correlating these rocks with neoproterozoic successions elsewhere has remained problematic however four marble units punctuate the succession these are in stratigraphic order the weisdale whiteness girlsta and laxfirth limestones two of which record hallmark c isotopic excursions diagnostic of neoproterozoic time the whiteness limestone is lithologically similar to the 635 ma marinoan post glacial cap carbonate sequence and displays a 13 c carbonate profile in which values decline from a range of 0 to 2 to a nadir of 5 and then rise towards and oscillate around 0 the girlsta limestone is a 700 900 m thick marble unit with values of 9 to 11 and is interpreted as recording the late neoproterozoic shuram wonoka event these correlations permit linking the shetland succession to neoproterozoic sections elsewhere and also show that rift related rocks in shetland post date the shuram wonoka event post c 600 ma pre c 550 ma thereby supporting the concept of a diachronous phase of extensional tectonism from c 700 ma in east central laurentia to near the close of the proterozoic in northeastern laurentia prior to the opening of the iapetus ocean

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

two late proterozoic glaciations mackenzie mountains northwestern canada glacial influence on deposition of the upper proterozoic sayunei and shezal formations rapitan group of northwestern canada is well established diamictites of the stelfox member of the ice brook formation 1 km higher in the section and resting on platformal carbonate and siliciclastic strata of the keele formation are here interpreted as glaciomarine the glacial interpretation is based on the presence of dropstones till pellets extremely angular quartz grains intercalations of laminated mudrocks and sandstone and rare striated stones the stelfox glacial deposits are unique in that they lie above the lowest occurrence of faunas of ediacaran type the establishment of two episodes of late proterozoic glaciation in northwestern canada as opposed to the previously recognized single episode enhances the possibility that these two glaciations correspond to the two recognized in parts of australia and africa

neoproterozoic environmental change recorded in the port askaig formation scotland climatic vs tectonic controls abstract the snowball earth hypothesis suggests that the neoproterozoic was characterized by several prolonged and severe global glaciations followed by very rapid climate change to hot house conditions the neoproterozoic port askaig formation of scotland consists of a thick succession of diamictite sandstone conglomerate and mudstone sedimentological and stratigraphic analysis of port askaig deposits exposed on the garvellach islands was carried out to establish the nature of neoproterozoic palaeoenvironmental change preserved in this thick succession particular emphasis was placed on identifying and distinguishing between climatic and tectonic controls on sedimentation port askaig formation diamictite units are attributed to deposition by sediment gravity flow processes or rainout of fine grained sediment and ice rafted debris in a glacially influenced marine setting associated facies record various depositional processes ranging from sediment gravity flows conglomerate massive sandstone and laminated mudstone to deposition under other unidirectional currents cross bedded and horizontally laminated sandstone the port askaig formation is also characterized by abundant soft sediment deformation features that occur at discrete intervals and are interpreted to record episodic seismic activity stratigraphic analysis of the port askaig formation on the garvellach islands reveals three phases of deposition phase i was dominated by sediment gravity flow processes and sedimentation was primarily tectonically controlled phase ii was a transitional phase characterized by continued tectonic instability an increased supply of sand to the basin and the preservation of current generated facies in the third and final phase of deposition the interbedded units of sandstone and diamictite are interpreted to reflect development of large sandy bedforms and ice margin fluctuations in a tectonically stable marine setting sedimentological and stratigraphic analysis of the port askaig formation demonstrates that tectonic activity had a significant influence on development of the lowermost parts of the succession climatic influences on sedimentation are difficult to identify during such phases of tectonic activity but are more easily discerned during episodes of tectonic quiescence e g phase iii of the port askaig formation the thick succession of diamictite interbedded with current deposited sandstone preserved within the port askaig formation is not consistent with deep freeze conditions proposed by the snowball earth hypothesis

late vendian cambrian glacially influenced deep water sedimentation macduff slate formation dalradian ne scotland the deep water metasedimentary rocks of the late dalradian macduff slate formation preserve a record of distal glaciomarine sedimentation in the form of ice rafted dropstones and related bedding deformation structures glaciogenic debris flow diamictites and an allochthonous slump unit the input of ice rafted debris was cyclical and probably related to periodic disintegration of the marine margin of an ice sheet downslope resedimentation processes typical of glaciated continental margins were responsible for the diamictites and slump deposits which were probably derived from the adjacent outer shelf and upper slope in common with pleistocene analogues off nw britain the uppermost diamictite bed is an integral part of the deep marine succession in contrast to the previously proposed lodgement till hypothesis

the neoproterozoic dalradian supergroup of scotland an alternative hypothesis the dalradian supergroup is interpreted traditionally as recording c 300 m y of episodic neoproterozoic rifting however lower dalradian pre easdale subgroup facies architecture is incompatible with rift basin fill and no unambiguous neoproterozoic extensional structures are present in those rocks consequently no objective evidence exists to infer that dalradian sedimentation was initiated during extensional tectonism that combined with the accumulating data for contractile deformation in scotland at c 870 800 ma the knoydartian orogeny permits the proposal of an alternative tectonostratigraphic evolution for the dalradian i propose that dalradian basin genesis was initiated as a foredeep in response to knoydartian orogenesis the coarsening and shallowing upward 6 8 km thick grampian group lower lochaber subgroup succession arguably represents a flysch to molasse knoydartian foredeep overlain by a moderately stable post orogenic shelfal sequence recorded by the relatively uniform thinner c 4 km and compositionally more mature rocks of the upper lochaber through islay subgroups lithospheric scale extensional tectonism and rifting did not occur until the late neoproterozoic as marked by the laterally variable volcanic and igneous bearing easdale subgroup and was followed by the late neoproterozoic early palaeozoic iapetan rift to drift transition through the southern highland group

two from donegal neoproterozoic glacial episodes on the northeast margin of laurentia two glacial episodes the sturtian ca 720 ma and marinoan varangerian ca 590 ma characterize many neoproterozoic successions worldwide in contrast northeast laurentia is typically considered as containing only the latter in scotland and ireland this is represented by the port askaig tillite here we present evidence ice rafted debris for a second younger neoproterozoic glacial event in the dalradian southern highland group of donegal ireland these deposits overlie strata correlative to ca 595 ma rocks in scotland making this heretofore unrecognized glacial episode a marinoan varangerian equivalent in that the port askaig occurs 3 5 km stratigraphically below the southern highland group rocks it is most likely sturtian not marinoan varangerian in age thus the dalradian succession in the irish caledonides contains strata representing both the sturtian and marinoan varanger glaciations

earth s magnetic field in the seventeenth century catalogs of old magnetic measurements contemporary publications and ship s logs have been researched to compile a data set of geomagnetic observations of declination and inclination in the interval 1601 a d to 1694 a d this search produced 2766 measurements of declination with quite good global coverage and 37 of inclination all inclinations were in the northern hemisphere and most in europe a literature search revealed many archeomagnetic and paleomagnetic measurements of field components for the seventeenth century 40 of these were selected and used in the modeling procedure intensity measurements were not found sufficiently reliable to allow any conclusions to be made about the overall strength of the geomagnetic field and declinations were too few to be of use this data set was analyzed to produce a field model for the seventeenth century the model was denoted epoch 1647 a d although no corrections were made for secular variation the model exhibits many of the stationary features of the recent geomagnetic field and indicates absence of secular variation from the pacific and a westward drift rate of about 0 25 per year in areas of the core mantle boundary beneath the south atlantic and indian oceans between 1647 a d and 1715 a d the date of bloxham s 1986 earliest field model this is likely to remain the earliest field model because only three inclination measurements have been found in the sixteenth century the data set will be used in the future to form a time dependent model for the historical period

two diamictites two cap carbonates two δ13c excursions two rifts the neoproterozoic kingston peak formation death valley california stratigraphic mapping of the neoproterozoic glaciogenic kingston peak formation death valley california provides evidence for two temporally discrete extensional deformation episodes these episodes are bracketed by the sourdough limestone and noonday dolomite the facies characteristics and 13 c data ranging between 2 15 and 2 56 and 1 88 and 4 86 respectively of which make them equivalent to sturtian and varangian age cap carbonates respectively this constrains the two extensional episodes along the southwestern margin of laurentia to ca 700 ma and ca 600 ma these observations and data show that the field evidence for mid neoproterozoic breakup and the predictions from tectonic subsidence curves for a latest neoproterozoic breakup are both correct thus neoproterozoic plate reconstructions must account for two discrete rift episodes separated by 100 m y or more confining rifting to within the kingston peak formation thereby places the younger proterozoic rocks of the southwestern great basin in the rift to drift tectonic phase

extensional geometries as a result of regional scale thrusting tectonic slides of the dunlewy nw donegal area ireland abstract the synmetamorphic ductile dislocations known in the british caledonian literature as tectonic slides pose a classical structural problem that is despite being associated with synchronous contractional folds and cleavages the low angle dislocations have the effect in many celebrated cases of juxtaposing younger over older rocks a geometry normally associated with extensional rather than contractional deformation recent models have attempted to demonstrate that this is the result of thrust reactivation of original sedimentary extensional growth faults the appin group dalradian metasediments of the complex and small dunlewy area of nw donegal ireland contain the following geometric elements a an early strike swing related stratigraphic facies change b a major inter deformational dolerite sheet c major regional recumbent folds and slides d major structures related to the 400 ma sinistral main donegal granite shear zone this solution to the structural geometry reveals that the early mid crustal 11 km depth d 2 ardsbeg dunlewy slide is a thrust to the northwest its hangingwall contains rocks two thirds of which are younger than the rocks of the footwall together with major recumbent folds coeval with the underlying thrust which face downwards into the thrust in the direction of transport rather than thrust reactivation of an original extensional growth fault we find that both stratigraphic and structural constraints are satisfied by a double thrusting model with fault bend folding onto an upper ramp of an earlier formed but penecontemporaneous and kinematically linked major fold pair this solution to the geology also allows us to recognize that the regional pre granite structure of the dalradian of nw donegal is a series of major d 2 synmetamorphic thrust bounded nappes possibly involving up to 250 km of northwesterly overthrusting

chemostratigraphy of neoproterozoic cap carbonates from the volta basin west africa abstract the neoproterozoic era includes some of the most extreme ice ages in earth history the exact number of glaciations is unknown although there were at least two events of global reach and possibly an equal number of lesser advances neoproterozoic glacial deposits in west africa have proven particularly difficult to correlate with better constrained successions elsewhere in most west african successions only a single glaciogenic deposit is present generally within a stratigraphic triad of tillite carbonate and bedded chert the age of these deposits and their synchroneity across the craton remain controversial we report isotopic data from carbonates that cap tillites in the volta basin sud banboli group in the three sections measured 13 c pdb begins at 2 to 3 and drops to 5 to 8 correlated with a drop in both 18 o and mg ca this pattern both the drop in 13 c and its correlation with 18 o and lithology has been observed in neoproterozoic cap carbonates on other cratons suggesting that it reflects primary depositional variations in addition the strong positive correlation among lithology 13 c and 18 o suggests that lithological variation may have exerted a principal control on isotopic variation implying that the dolomite formed in isotopic equilibrium with seawater as a primary precipitate or very early diagenetic replacement this would require low levels of sulfate estimated to be 1 mm or less consistent with available s isotopic data the presence of barite crusts in association with some cap carbonates and the conditions of seawater chemistry hypothesized in the aftermath of a snowball earth the stratigraphic pattern of carbon isotope variation in the volta basin cap carbonates suggests that underlying tillites are likely marinoan 650 600 ma in age this interpretation is supported by distinctive lithological characters observed in volta basin cap carbonates and accepted marinoan correlatives tillites elsewhere on the craton have been assigned latest proterozoic or early cambrian ages either the basis for these proposals requires reexamination or west african tillites record more than one glacial event

chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in earth history the neoproterozoic shuram formation nafun group oman abstract oman records in its neoproterozoic nafun group huqf supergroup an essentially complete carbonate rich ediacaran succession the nafun group overlies the presumed marinoan rift related fiq member ca 635 ma and ends just below the precambrian cambrian boundary 542 ma tectonic subsidence caused by thermal contraction following fiq aged rifting allowed the preservation of about 1 km of postrift stratigraphy with no major stratigraphic breaks the nafun group above the marinoan cap carbonate hadash formation represents two siliciclastic to carbonate grand cycles both initiated by significant transgressions these cycles comprise the masirah bay khufai formations and the shuram buah formations the khufai shuram boundary is associated with the start of a major carbon isotope perturbation the uppermost ramp carbonates of the khufai formation record a smooth decrease in 13c from about 4 to values around 0 followed by two descending steps across which values plunge to a nadir of 12 in the overlying red siltstones and shales interbedded with thin limestones of the shuram formation this fall in isotopic values is temporally rapid and coincident in both shallow and deep water sections in the time span of a single parasequence the 13c nadir is then followed by 50 million years of monotonic recovery the shuram shift represents the largest 13c inorganic carbon negative excursion in earth history although the snowball earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation the non glaciated context of the shuram formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non unique although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic the long term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate

giant cross beds in the neoproterozoic port askaig formation scotland implications for snowball earth abstract the neoproterozoic port askaig formation is a thick succession of interbedded diamictite conglomerate sandstone and mudstone that records glacially influenced marine sedimentation in the extensional dalradian basin giant cross beds individual units up to 11 m thick are observed in sandstone at the base of member iii of the port askaig formation the cross bedded sandstones are closely associated with other sandstone diamictite and conglomerate facies and are characterized by both planar and trough shaped foreset laminae with an average maximum dip angle of 14 the cross bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south the sandstone is medium grained well sorted and quartzitic the giant cross beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the port askaig formation the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine ice free depositional conditions the sedimentology of the cross bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal with periods of both ice free and ice influenced conditions the environmental conditions recorded in these port askaig deposits appear inconsistent with deep freeze or rapid meltback associated with a snowball earth

paleomagnetic evidence for low latitude glaciation during deposition of the neoproterozoic rapitan group mackenzie mountains n w t canada the rift related rapitan group of the mackenzie mountains of northwestern canada acquired several magnetizations due to pulses of hydrothermal activity the first pulse attributed to initiation of rapitan rifting produced a widespread overprint p2 that may be reflected in the basal mount berg formation two later pulses produced overprints similar to components found in an earlier study development of iron formation and hematite pigment in the overlying sayunei formation is attributed to the second pulse represented by a paleopole n 10 sites 334 e 01 s p m 4 9 that coincides with poles of the franklin igneous events of northern canada the franklin episode suggested on geological grounds to be coeval with sayunei deposition dates the sayunei at ca 725 ma this relation implies that rifting in mackenzie mountains could be related to rifting in northern canada a third pulse reflected by a pole at 007 e 16 n n 6 sites p m 6 12 is attributed to final rifting during dep