a conceptual model for glaciogenic reservoirs from landsystems to reservoir architecture abstract glaciogenic sediments are present in many hydrocarbon producing basins across the globe but their complex nature makes it difficult to characterise the reservoir quality sedimentary units despite this ordovician glacial deposits in north africa and carboniferous permian glaciogenic sequences in the middle east have been proven to host significant economical hydrocarbon accumulations additionally discoveries have been made in the shallow

the san venanzo maar and tuff ring umbria italy eruptive behaviour of a carbonatite melilitite volcano the late pleistocene san venanzo maar and nearby pian di celle tuff ring in the san venanzo area of umbria central italy appear to represent different aspects of an eruptive cycle accompanied by diatreme formation approximately 6x106 m3 of mostly lapillisized juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted the stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions the pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion roughly stratified lapilli tuff deposited by concentrated pyroclastic surge chaotic scoriaceous pyroclastic flow and inverse graded grain flow deposits the key feature of the pyroclastics is the presence of concentric shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits the rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite phonolite and calcitic melilitite pyroclasts juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds the san venanzo maar forming event is believed to have been trigered by fluid rich carbonatite phonolite magma the eruptive centre the moved to the pian di celle tuff ring where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area in both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of co2 which flowing through the diatremes produced the peculiar intrusive pyroclastic facies and triggered explosions

comment on palaeomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain by c mccabe and j e t channell in a recent paper mccabe and channell 1 report a late ordovician palaeomagnetic pole from the shelve inlier wales and argue for a iapetus ocean of 30 degrees latitudinal width between laurentia and southern britain in late ordovician times they further suggest that southern britain lay in high latitudes adjacent to the northern margin of gondwana at this time unlike mccabe and channell we contend a primary llanvirn mid ordovician remanence age for the shelve volcanics additionally several important points regarding the palaeomagnetism of southern britain may have been overlooked by mccabe and channell in particular re examination of a previous palaeomagnetic study of the shelve inlier 2 reveals a dual polarity remanence which passes a fold test at 95 confidence and indicates temperature southerly palaeolatitudes of 32s 8

evidence from early proterozoic anhydrite for sulphur isotopic partitioning in precambrian oceans relative to meteoritic sulphur sulphur in the contemporary exogenic cycle is marked by a depletion of 34s in biogenic sulphide that is balanced by excess 34s in sulphate of the oceans and evaporites1 2 the onset of this isotopic partitioning during the precambrian required sulphate in the ocean in sufficient concentration to permit significant isotopic fractionation during biogenic reduction3 4 and the evolution of sulphate reducing bacteria5 7 the conclusion that partitioning existed at 2 300 myr has been derived from strongly 34s depleted sedimentary sulphide of this age from south africa4 i report here on 34s enriched stratiform nodular anhydrite of similar age from canada which supports partitioning at this time strontium isotope data on these samples indicate that the anhydrite was derived from continental brines possibly in the inland portion of a continental sabkha this requires that the reported enrichment in 34s be considered a minimum for seawater sulphate at 2 300 myr

sulfur isotopic studies of archean slate and graywacke from northern minnesota evidence for the existence of sulfate reducing bacteria abstract sulfur isotopic studies of pyrite from metasediments in the 2 6 byr old deer lake green stone sequence minnesota have been conducted in order to evaluate the possible importance of sulfate reducing bacteria in sulfide formation pyrite occurs as ovules up to 2 cm in diameter within graphitic slates and as fine disseminations in metagraywacke units sem studies indicate the pyrite is framboidal in morphology 34 s values of pyrite from the deer lake sediments range from 2 3 to 11 1 with a peak at 2 isotopic data are consistent with either high temperature inorganic reduction of circulating seawater sulfate or low temperature bacterial reduction however the lack of sulfide bands or massive occurrences in the sediments the restriction of pyrite mineralization to the sediments and the absence of evidence for hot spring activity suggest that a diagenetic origin of pyrite is more feasible sulfide in such an environment would be produced principally by the action of sulfate reducing bacteria results of the study are in agreement with those of goodwin et al 1976 who suggest that dissimilatory sulfate reduction was operative in the archean ocean some 2 75 byr ago

appalachian piedmont landscapes from the permian to the holocene abstract between the potomac and susquehanna rivers and from the blue ridge to the fall zone landscapes of the piedmont are illustrated for times in the holocene late wisconsin early miocene early cretaceous late triassic and permian landscape evolution took place in tectonic settings marked by major plate collisions permian arching and rifting late triassic and development of the atlantic passive margin by sea floor spreading early cretaceous erosion proceeded concurrently with tectonic uplift and continued after cessation of major tectonic activity atlantic outer continental shelf sediments record three major erosional periods 1 late triassic early jurassic 2 late jurassic early cretaceous and 3 middle miocene holocene the middle miocene holocene pulse is related to neotectonic activity and major climatic fluctuations in the piedmont upland the holocene landscape is interpreted as an upland surface of low relief undergoing dissection major rivers and streams are incised into a landscape on which the landforms show a delicate adjustment to rock lithologies the fall zone has apparently evolved from a combination of warping faulting and differential erosion since late miocene the periglacial environment of the late wisconsin and earlier glacial epochs resulted in increased physical erosion and reduced chemical weathering even with lowered saprolitization rates geochemical modeling suggests that 80 m or more of saprolite may have formed since late miocene this volume of saprolite suggests major erosion of upland surfaces and seemingly contradicts available field evidence greatly subdued relief characterized the early miocene time near the end of a prolonged interval of tropical morphogenesis the ancestral susquehanna and potomac rivers occupied approximately their present locations in early cretaceous time local relief may have been as much as 900 m and a major axial river draining both the piedmont and appalachians flowed southeast past baltimore the late triassic landscape was influenced by rift basin development streams drained into a hydrologically closed basin no through flowing rivers seem to have been present a limestone escarpment along the blue ridge may have existed as a consequence of a semi arid climate the permian may have been a time of himalayan like mountains and mountain glaciers streams and glaciers generally flowed southwest and west

early permian rotliegendes palaeowinds of the north sea abstract the classical desert sediments of the early permian upper rotliegendes were deposited in a post variscan basin that extends from eastern england to the russo polish border which has been referred to as the southern permian basin these sediments comprise four main depositional facies fluvial wadi aeolian sabkha and lacustrine the lacustrine facies includes important bedded halites the southern permian basin is separated from the smaller northern permian basin by the fragmented mid north sea ringkobing fyn system of highs and beyond the grampian spur lies the even smaller moray firth basin these latter basins contain rock sequences similar to those of the southern basin with the exception that bedded halite has not been recognised in the lacustrine facies the rotliegendes of the southern basin apparently conforms to deposition in a northern hemisphere tradewind desert similar to the modern southern sahara apart from regional palaeomagnetic considerations and the occurrence of sediments of a non arid nature south of the variscan highlands the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding this is seen in both outcrop and core and derived from the continuous dip meter logs of many north sea wells by plotting the poles to the bedding attitudes on polar nets palaeowind directions have also been deduced from the dip meter logs of wells drilled in the northern and moray firth basins in general they indicate winds that blew in a direction opposed to those of the southern basin thus an area of high barometric pressure seems to have been located over the mid north sea high when the bedding attitudes in a well are roughly unidirectional an origin on a transverse dune is normally indicated with some wells the indicated palaeowind direction is regionally anomalous in many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune the indicated palaeowind is thus changed by up to 90 and then commonly fits the regional pattern these early permian desert basins were smaller and closer to the equator than the modern sahara their location in the centre of a continent between the caledonian and variscan mountain ranges suggests the analogy of a more tropical equivalent of the central asian takla makan desert

the architecture eruptive history and evolution of the table rock complex oregon from a surtseyan to an energetic maar eruption abstract the table rock complex trc pliocene pleistocene first documented and described by heiken heiken g h 1971 tuff rings examples from the fort rock christmas lake valley basin south central oregon j geophy res 76 5615 5626 is a large and well exposed mafic phreatomagmatic complex in the fort rock christmas lake valley basin south central oregon it spans an area of approximately 40 km2 and consists of a large tuff cone in the south trc1 and a large tuff ring in the northeast trc2 at least seven additional smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions the first period of activity trc1 initiated with a surtseyan style eruption through a 60 70 m deep lake the trc1 deposits are dominated by multiple 1 2 m thick fining upward sequences of massive to diffusely stratified lapilli tuff with intermittent zones of reverse grading followed by a finely laminated cap of fine grained sediment the massive deposits are interpreted as the result of eruption fed subaqueous turbidity current deposits whereas the finely laminated cap likely resulted from fallout of suspended fine grained material through a water column other common features are erosive channel scour and fill deposits massive tuff breccias and abundant soft sediment deformation due to rapid sediment loading subaerial trc1 deposits are exposed only proximal to the edifice and consist of cross stratified base surge deposits the eruption built a large tuff cone above the lake surface ending with an effusive stage which produced a lava lake in the crater 365 m above the lake floor a significant repose period occurred between the trc1 and trc2 eruptions evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences the trc2 eruption was the last and most energetic in the complex general edifice morphology and a high percentage of accidental material suggest eruption through saturated trc1 deposits and or playa lake sediments trc2 deposits are dominated by three dimensional dune features with wavelengths 200 500 m perpendicular to the flow and 20 200 m parallel to the direction of flow depending on distance from source large u shaped channels 10 32 m deep run up features over obstacles tens of meters high and a large 13 m chute and pool feature are also identified the trc2 deposits are interpreted as the products of multiple erosive highly inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions

generation of volcanic ash by basaltic volcanism 1 the recent eruptions of eyjafjallajokull and grimsvotn volcanoes in iceland demonstrate the importance of a better understanding of processes leading to the formation of volcanic ash specifically of fine volcanic ash that poses a threat to air traffic continuous deformation and brittle type experiments were carried out to better constrain these processes the studies on short time continuous deformation behavior of basaltic melt showed viscoelastic properties deviating from hydrodynamic newtonian models by more than 5 orders of magnitude high temperature deformation experiments on basaltic rock samples revealed an increase of elastic strengths as approaching the melting regime also pointing to a very complex behavior at the solid ductile boundary understanding magma fragmentation from the liquid side is a challenge but meanwhile we propose a pragmatic solution a thermodynamic model based on fracture mechanics this model is in agreement with experiments and observations that show that fine volcanic ash is produced by brittle type fragmentation of magma a critical material property was defined characterizing the conditions for brittle fragmentation the fracture surface energy density which represents the critical fragmentation energy short term fracture experiments using silicate glass have been performed to investigate the formation of ash sized particles by brittle failure and to extract this critical physical property which was found to range between 40 and 130 j m2 this value is in good agreement to fragmentation energies determined from experiments using remelted volcanic rocks now there is a tool to define critical conditions for the production of volcanic ash of a specific magma type

character genesis and tectonic setting of igneous rocks in the sistan suture zone eastern iran abstract igneous rocks in the sistan suture zone have characteristics that can be correlated with important tectonic events a late cretaceous ocean basin is recorded by ophiolites now exposed in numerous melange zones subduction beneath the afghan block is indicated by late cretaceous paleocene calc alkaline volcanics collision of the lut block with the subduction complex in the middle eocene produced widespread deformation and was followed by the emplacement of late eocene early oligocene calc alkaline granitic batholiths that probably formed by widespread anatexis of marine sediments a dominantly oligocene magmatic event is represented by widespread alkaline volcanics and minor intrusions that appear to be related to major transcurrent faults miocene calc alkaline activity was limited to sporadic volcanism in the north and minor intermediate intrusions farther south these units are largely underformed and not related to any major faults the youngest magmatic event is recorded by late miocene pliocene mafic flows that are weakly alkaline clearly related to right lateral faults and probably were derived from a deep crustal or upper mantle source

lithofacies association and stratigraphy of the paredao volcano trindade island brazil abstract a stratigraphic framework can be constructed through lithofacies description and interpretation the application of this method in volcanic systems is a fundamental tool for understanding their evolution and eruptive dynamics and for establishing hazard assessments in active volcanic areas this work presents and discusses a detailed stratigraphy of the paredao volcano allowing the understanding of the lateral and vertical distribution of volcanic facies and the 3d geometry of the deposits generating a model for the volcanism and discussing the erupting mechanisms the paredao volcano is a scoria cone located in the southeast of trindade island and represents the youngest subaerial volcanic episode pleistocene recorded in brazil its stratigraphic organization indicates nephelinitic flows alternate with lapillistones and lapilli tuffs of same composition at the base and a pyroclastic cone with nearly 200 m high above the sea level through lithofacies description and association 4 stratigraphic columns were constructed in scale 1 50 three of them at the tartarugas beach trv 01 trv 02 trv 03 and the other one at the pyroclastic cone trv 04 the petrographic characterization of facies was carried through conventional optical microscopy the stratigraphic analysis resulted in the distinction of seven lithofacies divided into two coherent and five volcaniclastic one autoclastic and four pyroclastic the coherent lithofacies are of massive npm and vesicular porphyritic nephelinite npv texturally the lavas are holocrystalline and vesicular composed by olivine phenocrysts in a very fine matrix of clinopyroxene nepheline zeolite and titanomagnetite at times with phlogopite the autoclastic lithofacies of nephelinitic breccia nb is composed of fragments 2 mm 15 cm of vesicular nephelinite and the pyroclastic ones are composed mainly of lapillitic scoria fragments with subordinate bombs and volcanic ash the pyroclastic lithofacies are of massive lapillistone lm and lapilli breccia lbm planar stratified lapilli tuff ltp and reverse grading lapillistone lrg the lithofacies associations characterize a a rubbly pahoehoe and pahoehoe flow types distal and proximal pyroclastic deposits the interpretation of the vertical and lateral facies distribution leads to the conclusion that the volcanism occurred with an initial phreatomagmatic stage succeeded by a strombolian stage pyroclastic with primary fragments domain and a final hawaiian stage lava flows domain

descriptive nomenclature and classification of pyroclastic deposits and fragments the iugs subcommission on the systematics of igneous rocks herein presents its recommendations on the nomenclature and classification of pyroclastic and mixed pyroclastic epiclastic deposits using descriptive mainly granulometric criteria the recommendations are the result of an international inquiry by means of questionnaires during the last four years

precambrian sulfur isotopes and a possible role for sulfite in the evolution of biological sulfate reduction this paper examines how and when the biochemical process of dissimilatory sulfate reduction evolved during sulfate reduction by modern sulfate reducing bacteria the sulfide produced is enriched in 32s compared to the starting sulfate this phenomenon is used to interpret the origin of sulfide in both modern and ancient environments the isotopic compositions of precambrian sulfide and sulfate minerals suggests that until ca 2 0 ga before present b p sulfide was the major s component in sediments and the hydrosphere and was formed mainly by volcanogenic processes after ca 2 0 ga b p when free oxygen was maintained in the atmosphere by the biological photolysis of water sulfate became the major sulfur component of the seas and was the main sulfur source for both biological and abiological sulfide formation dissimilatory sulfate reduction then became widespread the extent of 34s enrichment of a minor proportion of archaean and lower proterozoic volcanogenic sulfides indicates their formation by reduction of an oxidised sulfur source this source was not sulfate which formed the archaean deposits 34s so42 2 2 but most probably so2 so32 or hso3 introduced into the hydrosphere in exhalations in which h2s so2 and where isotopic equilibria favour 34s enrichment in the more oxidised sulfur compound it is suggested that 1 1 in the anoxic environments of the primitive earth sulfite was the most readily available and persistent s source to evolving procaryotes 2 2 the biochemical evolution of sulfite reduction occurred early among procaryotes and was the initial event leading to the evolution of dissimilatory sulfate reduction 3 3 dissimilatory and assimilatory sulfate reduction evolved when sulfate became the predominant sulfur compound available to the evolving procaryotes around 2 0 ga b p

thermal expansion on volcanic tuff rocks used as building stones examples from mexico three cities in the center of mexico declared as cultural heritage were erected mainly of volcanic tuffs as main construction element many of the historic buildings of these cities show significant damage and deterioration one of the causes of these damages can be attributed to a phenomenon poorly studied in volcanic tuffs the thermal expansion to understand the response of volcanic tuffs to thermal expansion as well as to know their thermal expansion coefficient thermal expansion test was performed on 12 representative tuffs of these localities in the same way and to know which of their petrographic petrophysical mineralogical and geochemical properties influence this phenomenon several laboratory studies were carried out to determine which of these plays an important role in the expansion the results of our tests showed that volcanic tuffs rocks have very varied thermal expansion values ranging from 0 7 mm m at their maximum expansion to even contraction in the order of 0 5 mm m equally varied were the thermal expansion coefficient values ranging from 10 1 to 7 2 10 6 k the most dominant behavior type is the anisotropic no residual strain behavior in general the behavior of the tuffs in practically all the tests carried out showed a very important heterogeneity in the rocks however in spite of the non homogeneous behavior of the tuffs three main factors could be identified that play an important role in the thermal expansion of the studied rocks the chemical composition e g acid volcanic tuff rocks rhyolitic composition had greater expansions while the more basic rocks basaltic composition expanded less the textural and fabrics homogeneity e g the most homogeneous rocks texturally have higher expansion values and the degree of crystallinity and e g the tuff rocks with the largest amount of glass showed smaller thermal expansions than the tuffs composed of microcrystals

petrological and stable isotope studies of carbonate and sulfide minerals from the gunflint formation ontario evidence for the origin of early proterozoic iron formation abstract the 1 9 ga gunflint formation in the thunder bay district of ontario principally comprises iron formation overlain by an iron poor limestone dolostone member the upper limestone member rocks are virtually unmetamorphosed retaining primary textures except locally around diabase sills the gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron formation and to study the changes from iron rich to iron poor sedimentation of the carbonate minerals in iron rich units siderite formed at or near the sediment water interface with ankerite and calcite forming later during diagenesis and during metamorphism around sills heaviest 13c values of siderite and of dolomite in the upper limestone member are consistently near 0 throughout the stratigraphic section indicating that marine bicarbonate was the source of carbon a spread of values between 0 and 7 for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis whereas 13c values more negative than 7 are the result of metamorphism the heaviest 18o values from unmetamorphosed rocks 5 3 pdb for siderite 6 1 for ankerite and 6 7 for calcite are considered to represent the original marine composition lighter values to 17 represent isotopic exchange reactions with pore fluids at higher temperatures and or isotopic exchange with 18o depleted meteoric water low s c ratios and a relatively narrow range of positive 34s values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean locally higher sulfide contents and a wider range of 34s values were the product of the introduction of sulfur after deposition it is proposed that the gunflint formation was deposited in a stratified water column with anoxic bottom water the depositional basin had restricted communication with the open ocean and was affected by distal volcanism hydrothermal activity associated with the volcanism provided a large source of dissolved iron and possibly silica which helped buffer o2 and sulphate in water to low levels low concentrations of sulphate limited the generation of h2s that would otherwise have restricted the solubility of iron during periods of increased hydrothermal activity the anoxic oxic water boundary moved upwards permitting transport of iron to the shallow shelf where it was precipitated as siderite iron hydroxides iron silicates or pyrite depending on physico chemical conditions the transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary

stable isotope geochemistry of early protezozoic carbonate concretions in the animikie group of the lake superior region evidence for anaerobic bacterial processes abstract thirteen carbonate concretions from the rove thomson and gunflint formations of the early proterozoic 2 0 ga animikie group in the lake superior region were analyzed for variations in carbon and oxygen isotopes the isotope data of ferroan dolomite concretions from the rove formation yield trends comparable to the isotope systematics displayed by authigenic carbonate in modern organic rich marine burial environments the isotope systematics of the ferroan dolomite concretions suggest that carbonate initially precipitated in a zone of bacterial methane production and later at greater burial depths in a zone of thermal decarboxylation recrystallization of calcite concretions in the rove and thomson formations has homogenized the oxygen isotopic composition however because the carbon system was apparently rock dominated the primary 13c signateres have been preserved calcite concretions from the rove formation display a strong correlation between 13c weight percent carbonate and position in the concretion the data suggest that these concretions formed progressively during burial in the zone of bacterial sulfate reduction sulfur isotopic analysis of pyrite disseminated in the concretions yield high positive 34s values isotopic data for the pyrite and the host concretion are consistent with models that suggest that high positive 34s values of early proterozic pyrite result from bacterial reduction of a limited supply of seawater sulfate the isotopic composition of concretions in the gunflint formation suggest early formation in a system open with respect to seawater bicarbonate the detailed isotopic study of precambrian carbonate concretions is a useful approach for investigating precambrian organic matter diagenesis and the evolution of bacteria

neogene and quaternary ignimbrites in the area of arequipa southern peru stratigraphical and petrological correlations abstract this study focuses on the correlation of four newly identified rhyolitic ignimbrites located around arequipa which were previously described as a single ignimbrite sheet 1 the rio chili ignimbrite ca 13 33 ma crops out in the rio chili canyon and consists of a cooling unit of non welded to partially welded massive crystal rich lapilli tuff this ignimbrite represents the base of the neogene ignimbrite succession in the arequipa area and may be stratigraphically equivalent to the 14 13 ma huaylillas ignimbrites in southern peru 2 the second voluminous 20 km3 and widespread la joya ignimbrite sheet ca 4 87 ma filled the arequipa depression its probable source is now buried underneath the chachani volcanic complex the la joya ignimbrite covers both the arequipa batholith toward the rio vitor piedmont and also part of the altiplano to the east and north of the chachani complex 3 the arequipa airport ignimbrite aai sheet ca 1 6 ma 18 km3 the third and latest infill of the arequipa basin consists of a lower white columnar jointed massive lapilli tuff indurated by vapor phase recrystallization and is overlain by an upper pink lithic rich non welded massive lapilli tuff the source of this ignimbrite is also buried underneath the chachani complex as indicated by anisotropy of magnetic susceptibility measurements and component lithology 4 the yura non welded pumice flow deposits ca 1 02 ma 1 5 km3 are restricted to the northern and western flanks of the chachani complex and may correlate with tuffs of the capillune formation on the altiplano all ignimbrites are high k calc alkaline to alkaline rhyolites which contain variable amounts of plagioclase biotite quartz sanidine and fe ti oxides the modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites specific geochemical fingerprints such as sr y rb ba cs ree and sr isotopes were effective discriminator elements magmatic differentiation crustal residence time and contamination and source melting processes are discussed in order to explain compositional variations the older la joya ignimbrite and rio chili ignimbrite reflect initial wetter crustal melts closer to granite composition whereas with time the younger arequipa airport ignimbrite and yura tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting the voluminous 40 km3 neogene ignimbrites of the arequipa area were produced by three major explosive volcanic pulses in the central volcanic zone of southern peru whose genetic relation with tectonic uplift and crustal thickening of the andean cordillera are still a matter of debate

paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain abstract in order to shed light on the early paleozoic paleogeography of iapetus bordering continents samples for paleomagnetic study were collected from middle ordovician llanvirn volcanic rocks of the shelve inlier welsh borderland the folding within the inlier is constrained to late ordovician ashgill time thus offering a fold test with tight age brackets after stepwise thermal demagnetization a pre folding characteristic magnetization component was isolated from 95 specimens the mean of 11 site mean directions for the characteristic magnetization gives d i 116 4 67 9 k 89 4 95 4 9 and the paleomagnetic pole lies at 26 6 n 36 1 e p 6 9 m 8 2 the fold test is positive at the 99 confidence level indicating that the magnetization was acquired during the middle or late ordovician we infer from geological considerations that this magnetization is not a primary thermoremanence but was acquired during late ordovician time as a result of a thermochemical disturbance of that age our results indicate that southern britain was near 51 south latitude during the later ordovician since middle to late ordovician paleolatitudes for gondwana and laurentia are well determined our result can be used to test various scenarios for the closure of iapetus we conclude that 1 the iapetus ocean in britain still had a latitudinal width of some 30 at this time thus rendering a postulated ordovician closure unlikely 2 southern britain avalon and hercynian europe all occupied high latitudes during the later ordovician consistent with a paleoposition near the northern margin of gondwana and 3 the acadian orogeny in britain and north america could have been the result of the collision of gondwana and laurentia during early devonian time

a note on the age and pyroxene chemistry of the igneous rocks of the shelve inlier welsh borderland geological survey electron microprobe analyses of clinopyroxenes and isotopic age determinations have revealed that the intrusive dolerites and basic intermediate lavas of the shelve ordovician inlier are part of a co magmatic suite which show transitional affinities between tholeiites and alkali basalts the ages of most of the intrusives are shown to be mid ordovician

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

triassic rifts and facies patterns in western and central europe the triassic evolution of western and central europe was governed by regional crustal extension that induced the subsidence of a complex network of grabens and troughs these formed an integral part of the arctic north atlantic and the tethys central atlantic gulf of mexico rift and rift wrench systems which in the course of the mesozoic and cenozoic developed into major break up axes of pangea

pyroclast tracking velocimetry a particle tracking velocimetry based tool for the study of strombolian explosive eruptions image based techniques enable high resolution observation of the pyroclasts ejected during strombolian explosions and drawing inferences on the dynamics of volcanic activity however data extraction from high resolution videos is time consuming and operator dependent while automatic analysis is often challenging due to the highly variable quality of images collected in the field here we present a new set of algorithms to automatically analyze image sequences of explosive eruptions the pyroclast tracking velocimetry pytv toolbox first a significant preprocessing is used to remove the image background and to detect the pyroclasts then pyroclast tracking is achieved with a new particle tracking velocimetry algorithm featuring an original predictor of velocity based on the optical flow equation finally postprocessing corrects the systematic errors of measurements four high speed videos of strombolian explosions from yasur and stromboli volcanoes representing various observation conditions have been used to test the efficiency of the pytv against manual analysis in all cases 106 pyroclasts have been successfully detected and tracked by pytv with a precision of 1 m s for the velocity and 20 for the size of the pyroclast on each video more than 1000 tracks are several meters long enabling us to study pyroclast properties and trajectories compared to manual tracking 3 to 100 times more pyroclasts are analyzed pytv by providing time constrained information links physical properties and motion of individual pyroclasts it is a powerful tool for the study of explosive volcanic activity as well as an ideal complement for other geological and geophysical volcano observation systems

development of the late paleozoic glaciations of the south american gondwana in western argentina abstract the late paleozoic ice age in the south american segment of gondwana consisted of several glaciations which can be grouped into two main glacial periods the oldest of them occurred in late early to early late carboniferous time and the younger in early permian time paleontological evidence in the western argentine precordillera supports a late visean age for the beginning and a probable early westphalian age for the end of the carboniferous glacial period in this span of time subsidence and deposition of glacial debris took place in the western mobile belt and in patagonia but no sediments of that age were deposited in the eastern portion of the south american gondwana where subsidence started later a carboniferous glacial sequence is remarkably well preserved in the san eduardo group on the western flank of the precordillera where at least three distinct glaciations are recognized this sequence can be taken as a reference section for the carboniferous glacial deposits shorter than the former and probably restricted to the asselian epoch the permian glacial period is better known the evidence for it is well developed in eastern south american gondwana although a relic is also present in the precordillera the recognition of two glacial periods within the late paleozoic ice age is supported by the existence of a long lasting interglacial which seemingly began in the late westphalian and lasted until the end of the carboniferous period this major interglacial period is inferred from the total absence of glacial features in the upper carboniferous sequences in spite of the existence of a mountainous belt parallel to the pacific during the carboniferous period the presence of marine beds and glacial drift well to the east of these ranges suggests that it was not altitude that was the main factor in determining the formation of glaciers but latitude plus a global change in temperature it is argued that the ice sheets extended greatly during the carboniferous glaciations and reached neighboring areas of the paganzo basin in central argentina

petrographic and diagenetic signatures of climatic change in peri and postglacial karoo sediments of sw tanzania abstract the basal idusi formation of the karoo succession in sw tanzania represents deposits of the final late palaeozoic deglaciation event it reflects the change from a cold to a temperate more humid climate the lower unit of the idusi formation lisimba member consists of periglacial deposits with glaciolacustrine diamictites sandstones dropstone bearing mudstones and varvites as well as fluvial and deltaic conglomerates and sandstones the upper unit lilangu member is dominated by organic rich siltstones and sandstones which were accumulated in an eutrophic postglacial lake the petrography of the idusi formation reveals an arkosic composition indicating provenance from cratonic basement rocks the lisimba member is characterized by low mineralogical maturity with a high proportion of garnet fine grained matrix particles in the lisimba member authigenic formation of chlorite corrensite and laumontite at a moderate stage of burial was due to alkaline pore fluid conditions with high ionic strength caused by restricted percolation in the matrix rich sediments and their low mineralogical maturity the postglacial sedimentary rocks are characterized by early diagenetic kaolinite siderite pyrite and barite this paragenesis formed under the influence of acidic meteoric derived pore water and the decay of organic matter such early diagenetic conditions are frequently met in humid continental environments

antique stone quarries in turkey a case study on tuffs in the temple of apollon smintheus all types of stones have been used as building stones depending on their durability visual harmony with the intended construction and availability in the hellenistic period as in other periods tuff was preferred as a building stone due to its convenience it was easy to extract transport and use for building in the present study three ancient quarries that were the possible tuff sources for the apollon smintheus temple in canakkale are investigated by comparing the durability properties of stones in the temple and in the quarries these properties are determined using physical and physico mechanical tests comparing fresh and artificially weathered samples microstructural and elemental correlations were found using optical microscopy stereomicroscopy x ray diffraction scanning electron microscopy methylene blue adsorption and x ray fluorescence analyses the results indicate that temple tuffs and two of three quarries have similar geological engineering and microstructural properties with the strong claim that those two quarries could have been the source of building stone for the temple gold open access this article is published under the terms of the cc by 3 0 license historically only high status stone blocks were transported for longer distances whereas local quarries were used for larger constructions gomezheras fort gonzalez 2004 torok p ikryl 2010 since local stones do not entail high transportation costs they can be extracted whenever is suitable bell 1990 what matters most is that the rock must not include closely spaced joints cracks or other planes of weakness in large blocks extracted by stone quarrying rapp 2002 in hellenistic architecture all types of stones marble gneiss limestone sandstone granite andesite tuff breccias serpentinite porphyry and trachyte were used among all other stones tuffs were easy to get carry and process and were therefore relatively cheap and could be used in any part of a building on condition that it was sheltered from the weather tucci 2015 in turkey tuffs that have low unit weight high to very high porosity and poor to good durability properties topal doyuran 1997 1998 topal 2002 topal sozmen 2003 yavuz 2012 celik et al 2014 are seen in antique structures as well as in historic monuments of the seljuk ottoman period that have survived to the present day unremarkably antique quarries can be found with the masons marks and shear ledges of dimension stones however it is not possible today to observe the tuff quarries as they appeared in ancient times because tuffs deteriorate over time when exposed to weathering and or because of the urban construction mass p ikryl 2006 p ikryl torok 2010 in the case of lack of knowledge of the original quarry the most contiguous geological resources are examined through mineralogical chemical and physical analyses and in situ observation and testing to find replacement stones that are compatible in terms of characteristics and appearance this approach may yield misapplication of stones the purpose of this study is to determine the provenance of the tuffs used in the temple of apollon smintheus architecture dating from the hellenistic period by comparison of physical mechanical chemical and mineralogical features to that end samples taken from the temple and samples taken from three potential quarry regions were compared using several analytical techniques to from p ikryl r torok a gomez heras m miskovsky k theodoridou m eds sustainable use of traditional geomaterials in construction practice geological society london special publications 416 http doi org 10 1144 sp416 17 2015 the author s published by the geological society of london publishing disclaimer www geolsoc org uk pub ethics determine the source quarry for the temple tuffs the results of the analyses may serve for surface deterioration studies and durability assessment as well site description the apollon smintheus temple is situated on the southwest corner of the biga peninsula troas fig 1 location map of the study area fig 2 temple of apollon smintheus d ergenc et al

finding eruptive mechanisms through pyroclasts from the current eruption 1984 present at nevado del ruiz volcanic complex colombia abstract the dominant fragmentation mechanisms in the magma involved in a volcanic eruption can be understood from the compositional and textural analysis of the fragments emitted as pyroclasts individual pyroclasts can be juvenile accessory and accidental and detailed analyses of their relative proportions as well as the juvenile clast features can shed light into the fragmentation processes involved in the eruption despite the multiple hundreds eruptive pulses since the nevado del ruiz re activation in 1984 following a quiescence of nearly 100 years such analyses have not been carried out for the ash emissions preceding the recently emplaced plug here four deposits related to the recent emissions were analysed from their pyroclasts the samples correspond to eruptive pulses from the years 1985 1986 1988 and 2012 their characterisation allowed proposing differences between likely juvenile fragments i e representative of involved magma and accessory fragments i e pre existing co magmatic the results indicate that alteration can be used as a criterion to distinguish them although this could not account for syn eruptive hydrothermal alteration of juvenile glass or for the preservation of accessory glass emplaced in glacial conditions this criterion could serve as a foundation to direct fresh glass fragment analyses following this criterion the fresh glass fragments which are assumed to be juvenile fragments are highly vesicular they exhibit spongy and fibrous textures that are related to the predominant vesicle shape their predominance suggests a magmatic fragmentation mechanism while blocky shaped poorly vesicular cracked glass shards suggest a phreato magmatic origin specifically november of 1985 and january of 1986 glassy ashes are primarily highly vesicular suggesting magmatic explosive events by contrast samples from the march of 1988 and may of 2012 events are primarily characterised by low fresh glass content they also exhibit quenched surfaces indicating that the magma water interaction acted as an important factor for triggering the eruptive pulses real time componentry petrography and glass characterisation in fine ash could significantly complement traditional geophysical monitoring and importantly improve the hazard assessments on the civil aviation during the ongoing nevado del ruiz activity

nature and classification of waterlain glaciogenic sediments exemplified by pleistocene late paleozoic and late precambrian deposits this study of waterlain glaciogenic sediments is designed to present both a review and new information on glaciogenic subaquatic deposits of differing age in a number of localities in north and south america and south africa the late paleozoic glaciogenic deposits of the parana basin in brazil and the karoo basin of south africa are singled out for special attention as they show a reasonably complete lateral sequence of terrestrial to off shore glaciogenic sedimentation although the environment of subaquatic glaciogenic sedimentation varies from one area to the next certain common elements are found which can be used to develop a generalized model for both glaciomarine and glaciolacustrine sedimentation for descriptive purposes the model is divided into two broad categories a shelf facies and a basinal facies the shelf facies is marked by massive diamicton ite which may be 200 m or more in thickness and which is frequently overlain by a complex of clastic sediments consisting primarily of gravity and fluid flows the basinal facies is marked by products of subaquatic slumps and more distal turbidites and glaciomarine sediments new terminology is introduced the massive diamicton ite which is diagnostic of the shelf facies probably represents deposition from the base of active ice in a subaquatic environment and is termed undermelt diamicton ite the gravity and fluid flows which are usually found overlying undermelt diamicton ite and in the basinal facies are subdivided into six categories glaciogenic subaquatic outwash glaciogenic suspension flow glaciogenic chaotic debris flow glaciogenic subaquatic debris flow glaciogenic slurry flow and glaciogenic turbidity flow the relative abundance of undermelt diamicton ite and the various types of gravity and fluid flows can be used to define inner shelf outer shelf inner basin and outer basin facies of glaciomarine sedimentation

permian to recent volcanism in northern sumatra indonesia a preliminary study of its distribution chemistry and peculiarities sumatra has been a volcanic arc above an ne dipping subduction zone since at least the late permian the principal volcanic episodes in sumatra n of the equator have been in the late permian late mesozoic palaeogene miocene and quaternary late permian volcanic rocks of limited extent are altered porphyritic basic lavas interstratified with limestones and phyllites late mesozoic volcanic rocks widely distributed along and w of the major transcurrent sumatra fault system sfs which axially bisects sumatra include ophiolite related spilites andesites and basalts possiblepalaeogene volcanic rocks include an altered basalt pile with associated dyke swarm in the extreme nw intruded by an early miocene 19 my dioritic stock and variable pyroxene rich basic lavas and agglomerates ranging from alkali basaltic to absarokitic in the extreme sw miocene volcanic rocks widely distributed especially w of the sfs and cropping out extensively along the w coast include calc alkaline to high k calc alkaline basalts andesites and dacites quaternary volcanoes 3 active 14 dormant or extinct are irregularly distributed both along and across the arc thus they lie fore arc of the sfs near the equator but well back arc farther north the largest concentration of centres around lake toba includes the 2000 km3 pleistocene rhyolitic toba tuffs quaternary volcanics are mainly calc alkaline andesites dacites and rhyolites with few basalts they seem less variable but on the whole more acid than the tertiary

the late palaeozoic gondwana glaciation the glacial deposits of gondwana are today spread over one whole hemisphere on the re assembled gondwana continents they still occupy an area exceeding that of the pleistocene glaciation the onset of the glaciation in the different areas has not yet been dated satisfactorily the earliest lower and middle carboniferous glacial beds occur in the andean belt and in eastern australia where high mountains may have existed at that time during stephanian sakmarian time when the pole wandered over antarctica large ice sheets reached sealevel in all the major depositories palaeotopographical reconstructions allow the conclusion that some of the ice centres were supported by uplands which reached altitudes of up to 1 500 m above sealevel depending on their palaeotopographical positions the glacigenic sediments exhibit the full facies range to be expected between glaciated uplands and glaciomarine environments there are indications that ice may have flowed from africa into the parana basin and from antarctica into the great karoo basin and into australia at least 12 advances and retreats have been recognized in the parana basin closer to the ice centres the record is less complete the retreats were probably more of an interstadial than an interglacial nature there is some evidence that the final deglaciation proceeded in stages from south america over africa to antarctica the ice centres were fed by moist air from the pacific ocean and the tethys

a study of the correlation between electrical resistivity and matric suction for unsaturated ash fall pyroclastic soils in the campania region southern italy in the territory of the campania region southern italy critical rainfall events periodically trigger dangerous fast slope movements involving ashy and pyroclastic soils originated by the explosive phases of the mt somma vesuvius volcano and deposited along the surrounding mountain ranges in this paper an integration of engineering geological and geophysical measurements is presented to characterize unsaturated pyroclastic samples collected in a test area on the sarno mountains salerno and avellino provinces campania region the laboratory analyses were aimed at defining both soil water retention and electrical resistivity curves versus water content from the matching of the experimental data a direct relationship between electrical resistivity and matric suction is retrieved for the investigated soil horizons typical of an ash fall pyroclastic succession the obtained relation turns out to be helpful in characterizing soils up to close saturation which is a critical condition for the trigger of slope failure in such a regime the water content and the matric suction have small variations while electrical resistivity variations can be appreciated in a larger range of values for this reason besides suction measurements on very small soil volumes through classical tensiometers our analyses suggest the direct monitoring of in situ electrical resistivity values as an effective tool to recognise the hydrological state of larger and more representative soil volumes and to improve early warning of dangerous slope movements

mechanism of formation of volcanic bombs insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models volcanic bombs and achneliths are a special type of pyroclastic fragments formed by mildly explosive volcanic eruptions models explaining the general shapes of those particles can be divided in two broad categories the most popular envisages the acquisition of shapes of volcanic bombs as the result of the rush of air acting on a fluid clot during flight and it includes many variants the less commonly quoted model envisages their shapes as the result of forces acting at the moment of ejection of liquid from the magma pool in the conduit experiencing an almost negligible modification through its travel through air quantitative evidence supporting either of those two models is limited in this work i explore the extent to which the anisotropy of magnetic susceptibility ams might be useful in the study of mechanisms of formation of volcanic bombs by comparing measurements made on two spindle and two bread crusted bombs the results of this pilot study reveal that the degree of anisotropy of spindle bombs is larger and their principal susceptibility axes are better clustered than on bread crusted bombs also the orientation of the principal susceptibility axes is consistent with two specific models one of the in flight variants and the general ejection model consequently the reported ams measurements albeit limited in number indicate that it is reasonable to focus attention on only two specific models to explain the acquisition of the shapes of volcanic bombs based on a parallel theoretical assessment of analytical models a third alternative is outlined envisaging volcanic bomb formation as a two stage process that involves the bursting of large m gas bubbles on the surface of a magma pond the new model advanced here is also consistent with the reported ams results and constitutes a working hypothesis that should be tested by future studies richer in data fortunately since this work also establishes that ams can be used to determine magnetic fabric in small size samples the possibility to expand this study to a larger collection of bombs is granted

hawaiian and strombolian style monogenetic volcanism in the extra andean domain of central west argentina abstract in the provinces of mendoza and la pampa central western argentina retroarc volcanism was active from the miocene through historic times there are approximately 40 basaltic eruptive centers between 36 s and 37 30 s to the east of 68 40 w these centers were studied in order to define their volcanological evolution the vents are constituted of seven lithofacies i e a 1 weakly welded lapilli and bomb beds a 2 agglutinated spatter and bomb beds a 3 agglutinated spatter beds b ash beds c lava flows d conduit fill and e dikes three eruptive episodes phases were identified the first one generated agglutinated spatter and bomb beds lithofacies a 2 which constitute the internal part of the edifices these are followed by weakly welded lapilli and bomb beds lithofacies a 1 and agglutinated spatter beds lithofacies a 3 the final episode includes lava flows lithofacies c minor agglutinated spatter deposits lithofacies a 3 and isolated lapilli and bomb clasts we postulate hawaiian style fountaining activity as responsible for the generation of agglutinated spatter and bombs beds and variations between strombolian and hawaiian activity for the formation of successions of weakly welded lapilli and bomb beds and agglutinated spatter beds the vent features and the homogeneity of the erupted magma suggest that the studied volcanoes were monogenetic

the 2011 expedition to the el gygytgyn impact structure northeast russia toward a new geological map for the crater area el gygytgyn is a 3 6 ma 18 km diameter impact crater formed in an approximately 88 ma old volcanic target in northeast siberia the structure has been the subject of a recent icdp drilling project in parallel to those efforts a russian german expedition was undertaken in summer 2011 to investigate the permafrost soil lake terraces and the volcanic rocks of the southern and eastern crater rim this provided the unique opportunity for mapping and sampling of the volcanic target rocks around a large part of this complex impact structure samples from 43 outcrops were collected and analyzed petrographically and geochemically the results were combined with earlier mapping outcomes to create a new geological map of this impact structure and its immediate environs at the scale of 1 50 000 compositions of our rock suites are compared with the lithologies of the 2009 icdp drill core the ignimbrite described as lower bedrock in the icdp drill core shows petrographically and chemically strong similarities to the rhyolitic and rhyodacitic ignimbrites observed on surface the suevite sequence exposed in the icdp drill core is a mixture of all observed target rocks at their respective proportions in the area in contrast to previous studies the calculated average target composition of el gygytgyn takes the contribution of the basic target rocks into consideration mafic and intermediate rocks approximately 7 5 and felsic rocks approximately 92 5

the palaeogeographic evolution of southern britain during early palaeozoic times a reconciliation of palaeomagnetic and biogeographic evidence abstract palaeomagnetic biogeographic and palaeoclimatic evidence together suggest that in the early ordovician southern britain lay off the west african margin of gondwana at around 60 s the combination of these disciplines reduces latitude uncertainties to around 5 to 10 degrees rifting of avalonia from gondwana was followed during most of the ordovician by northward drift as avalonia and baltica converged with subduction of tornquist sea crust both continents had similar moderate northward drift rates 3 6 cm yr so their convergence probably had a considerable longitudinal component silurian and early devonian data indicate that northward drift of avalonia and baltica continued after their collision and that southern britain reached a latitude of around 20 s by c 400 ma

magma and sediment ii problems of interpreting palaeovolcanics buried in the stratigraphic column president9s anniversary address 1982 the preservation of volcanoes never more than partial involves a complex interaction of processes and these operate over periods of time which are often undervalued from a review of the relative rates of these processes it seems that short lived structures such as surtseyan ash rings may be preserved albeit in modified form around their whole circumference for larger composite volcanoes however growth and remoulding by activity is balanced by penecontemporaneous and later erosion in such a way that the central and most of the proximal zones are likely to be removed before they can be buried either by transported volcaniclastic or regional epiclastic sediments even the preservation of distal zones may be asymmetrical a one sided hinge line subsidence sedimentation model is therefore suggested to explain the elongate lens shaped aspects of the many extrusive piles known to lie approximately concordantly within stratigraphic sequences formed in both continental and littoral environments

glacial events of the past and their geological significance abstract traces of glaciations are unevenly distributed in the stratigraphic column and exhibit rhythmicity of several ranks this rhythmicity makes it possible to establish the corresponding hierarchy of climatic events in descending order glacio and thermo eras glacial and interglacial periods and epochs as well as smaller events which appear to be compared to the most important climatic fluctuation during the pleistocene the direct factors causing initiation and termination of glaciations were climatic fluctuations the palaeolatitudinal position of the continents controlled a degree of asymmetry and perhaps a certain diachronism of glaciations in different hemispheres during the glacial periods in high and middle latitudes of the earth the glacial and cold temperate climates either were predominant or alternated the thermo eras and at least some interglacial periods were characterized by warm temperate and warm climates at high and middle latitudes glaciations have been numerous and important geological events they affected all outer spheres of the earth from asthenosphere to atmosphere they apparently stimulated the biological evolution too the study of ancient glaciations is important from both the scientific and economical points of view

development of n w europe s southern permian gas basin summary the southern permian basin contains over 4 1 10 12 m 3 of recoverable non associated gas most of which is found in early permian rotliegend sandstones the most important source of this gas is carboniferous coal deposited in a proto tethys foredeep basin where the coals were overlain by rotliegend desert sands and by late permian zechstein halite the scene was set for creating important reservoirs for gas deformation of the reservoir and seal to create a trap and the generation and migration of carboniferous gas resulted from the structural history of the area which was shaped in part by underlying crustal blocks and zones of crustal weakness much of this history was related to events most clearly expressed beyond the limits of n w europe in the north atlantic and tethys 1 e w tension which gave rise to the oblique slip induced subsidence of many sub basins within the southern permian basin caused the consecutive creation of the viking central graben system and rockall trough and ended with the crustal spreading of the atlantic ocean and 2 the early mesozoic opening of tethys its cretaceous closure and ensuing alpine orogeny which induced inversion in many areas of the southern permian basin that are now gas bearing

a revised palaeozoic apparent polar wander path for southern britain eastern avalonia summary a revised apparent polar wander path apwp is presented for palaeozoic southern britain the new path based on a structural reinterpretation of existing data combined with new data from wales differs significantly from previous estimates in the following ways 1 the 1 ocus of the ordovician path segment is extended by approximately 25 degrees of arc 2 revised time calibration of the apwp suggests more ordovician apw than previously calculated and 3 the new path implies middle ordovician separation across the british sector of iapetus to be greater than previous palaeomagnetic estimates based on apwp analyses c 3300 km northern britain c 133 southern britain c 45s

the palaeogeography of part of southwestern gondwana during the permo carboniferous glaciation abstract the elevation of the base of the glacial deposits areas where glaciated bedrock is overlapped by post glacial sediments palaeoice flow directions thickness of the glacial sequence and the lithology lithofacies and clast composition were used as integrated parameters to reconstruct the permo carboniferous glaciated land surface over southern africa the palaeography is depicted by pre glaciation westphalian maximum glaciation end of carboniferous and post glaciation sakmarian maps the pre glaciation palaeogeography was dominated by a fluvially dissected highland surrounded in the south and southwest by flat lying lower palaeozoic marine sediments this surrounding platform in part covered by shallow seas and in the south and southwest bounded by a possible island arc system was the main depocentre for the glacial debris the palaeogeography during maximum glaciation consisted of a platform region karoo basin overdeepened troughs e g kalahari basin highlands windhoek cargonian and southern with extensive glacial valleys incised into these highlands and a shallow palaeo pacific sea arm reaching into the interior the alpine type southern highlands formed by late subduction from the south the post glaciation palaeogeography was controlled by isostatic crustal movements ratio of crustal depression by the weight of the ice and sediments to post glacial rebound and sea level rises large areas including major valleys were inundated by shallow seas irati and whitehill seas and fjords so that the land surfaces represent remnants including islands of the former highlands local high rebound rates caused erosion of the glacial sediments and the deposition of fluvial and deltaic beds in fjord heads and embayments favourable conditions for the formation of fossil fuels and base mineral and metal deposits occured in parts of the inundatedbasins

variability of ash deposits at piton de la fournaise la reunion island insights into fragmentation processes at basaltic shield volcanoes 25 it is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic 26 shield volcanoes at piton de la fournaise la reunion island france eruptive activity occurs mostly within the 27 non populated enclos fouque caldera and generally does not pose any risk to the population however historical 28 observations recent monitoring data and field work on tephra deposits suggest that some eruptions have produced 29 unusual and unexpected explosive phases a comprehensive sampling of tephra from major historical and recent 30 eruptions on this volcano allowed us to perform systematic componentry grain size chemical and morphological 31 analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions this 32 integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation 33 efficiencies and multiple associated mechanisms primary ductile and partially brittle fragmentation of various 34 intensity of juvenile magma emitted during hawaiian fountaining or mild strombolian explosions were identified 35 as the most common fragmentation mechanisms in particular during the 2014 2018 dominantly effusive eruptive 36 sequence in parallel we distinguished more efficient short lived fragmentation events related to i plug 37 pressurization and brittle fragmentation enhanced by syn eruptive crystallization ii magma water interactions 38 iii rare phreatic explosions and iv secondary fragmentation producing fine ash during caldera collapse we 39 conclude that textural geochemical and morphological analysis make it possible to identify and characterize the 40 variability in eruptive processes with the grain size and the componentry of the ash particles being probably the 41 most important parameters to quantify both the efficiency and the nature of the fragmentation 42 43

ordovician volcanism in the welsh borderland basaltic andesitic and rhyodacitic rhyolitic volcanism was widespread during ordovician time in the welsh basin new chemical data are presented for llanvirn to caradoc lavas and tuffs from the welsh borderland which during ordovician time formed the southeastern margin of the welsh basin in view of the observed chemical alteration immobile elements are used in the interpretation of the original geochemical character the data indicate that the llanvirn stapeley volcanic group of the shelve inlier was a bimodal basalt basaltic andesite rhyodacite rhyolite association the basalts have trace element contents of tholeiitic associations with a subduction related character the caradoc whittery and hagley volcanic groups of the shelve inlier comprise lavas and tuffs of calc alkaline andesite blocks sampled from the breidden hills show that these were also derived from a calc alkaline volcano associated caradoc pumice and ash flow deposits from the breidden hills are probably of altered calc alkaline rhyodacite rhyolite composition the sibdon carwood basalt flow the only known example of ordovician volcanism east of the pontesford linley and church stretton lineaments has transitional tholeiitic to alkaline character with trace element contents influenced by subduction related processes the overall tholeiitic to calc alkaline nature of the magmatism is consistent with the view that during llanvirn to caradoc time the welsh basin was an ensialic marginal basin

a carbon oxygen hydrogen and sulfur isotopic study of the gold mineralization at loulo mali abstract the isotopic compositions of oxygen hydrogen carbon and sulfur have been determined on silicate carbonate and sulfide samples from auriferous tourmalinized sandstone its footwall and hanging wall of birimian age 2 1 ga at loulo in mali the sandstones have undergone two phases of deformation the samples were taken from three different orebodies loulo 0 loulo 3 and p64 for loulo 0 18o values for vein quartz are consistently between 16 1 and 16 6 smow for oxygen with one exception at 11 6 the 18o values for detrital quartz grains from the tourmalinized sandstone are also 16 1 tourmaline has a high 18 o of 13 9 the 18o values for total rocks are also relatively homogeneous at 13 2 to 15 7 the lowest values 13 2 and 14 being those for the footwall samples dolomites from loulo 0 and p64 have 18o values between 15 1 and 19 5 smow for oxygen and 13c values of 13 5 to 2 0 pdb with the exception of one sample from the hanging wall which is of sedimentary origin 13 c 0 5 18 o 22 2 the sampled dolomites are in veinlets or veins or disseminated in rocks and were deposited by hydrothermal fluids the 34s values of samples from the three orebodies range between 6 0 and 12 9 cdt they fall into different groups according to location but pre and post schistosity sulfides within each group are indistinguishable the results are interpreted as indicating the existence of two hydrothermal systems enriched in 18o at different times one of these gave rise to tourmalinization silicification and dolomitization of the sandstone and the deposition of sulfides the high d values of the tourmalines in agreement with petrological data indicate that the fluid of this system was of marine origin the two phases of folding intervened between the activity of this hydrothermal system and the initiation of the second system which gave rise to the stockwork this fluid rich in 18o was possibly of metamorphic origin with a low water rock ratio it remobilized the pre schistosity sulfides concentrating them into post schistosity veins the partly remobilized silica and carbonates were deposited under temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone the vertical extent of this hydrothermal system is as yet poorly known it is very extensive horizontally and strongly pervasive in the tourmalinized sandstone but in the vertical sense appears to be confined to fractures in the footwall

iron and sulfur in the pre biologic ocean tentative geochemical cycles for the pre biologic earth are developed by comparing the relative fluxes of oxygen dissolved iron and sulfide to the atmosphere and ocean the flux of iron is found to exceed both the oxygen and the sulfide fluxes because of the insolubility of iron oxides and sulfides the implication is that dissolved iron was fairly abundant and that oxygen and sulfide were rare in the atmosphere and ocean sulfate produced by the oxidation of volcanogenic sulfur gases was the most abundant sulfur species in the ocean but its concentration was low by modern standards because of the absence of the river borne flux of dissolved sulfate produced by oxidative weathering of the continents these findings are consistent with the geologic record of the isotopic composition of sedimentary sulfates and sulfides except in restricted environments the sulfur metabolism of the earliest organisms probably involved oxidized sulfur species not sulfide

reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants waipiata volcanic field south island new zealand the miocene waipiata volcanic field new zealand is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension the field includes remnants of at least 55 volcanoes in an area of 5000 km2 vent filling deposits comprising predominantly lava e g plugs necks lava flows or dykes often associated with thin basal phreatomagmatic pyroclastic deposits were classified as type 1 vents and are inferred to be the remnants of scoria cones vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and or maar volcanoes type 3 vent complexes are groups of closely spaced or overlapping vents with voluminous preserved lava flows they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products pyroclastic rocks of most of the waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater followed by strombolian style eruptions aligned and clustered vents are accommodated to structural features of the regional basement rock otago schist

two new ordovician volcanic centres in the shelve inlier powys wales summary as a direct result of new work in the shelve ordovician inlier two new important extrusive andesitic members of a revised stapeley volcanic formation are described these members were originally regarded as extrusive however they have for over half a century been considered intrusive and a brief literature review attempts to account for this anomaly a palaeogeographic reconstruction suggests that the members represent the remains of two llanvirn volcanic islands which following subsidence were overlapped and buried by later llanvirn sedimentation this reinterpretation considerably increases the volume of andesitic material known to have been erupted in llanvirn times in this part of the welsh borderland

palaeomagnetism of late ordovician igneous intrusions from the northern welsh borderlands implications to motion of eastern avalonia and regional rotations the palaeomagnetism of a late ordovician dolerite suite widely distributed in the northern welsh borderlands has been investigated to evaluate i regional rotations associated with later deformations and ii lower palaeozoic latitudes of eastern avalonia only local effects of acadian and variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling related magnetization magnetic properties in the breidden dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence d i 333 1 71 6 6 sites 95 10 4 predates late ordovician folding dolerites elsewhere in the breidden hills inlier record the same reversal of remanence and the combined mean d i 314 3 72 8 is rotated clockwise from the mean remanence d i 291 5 59 5 21 sites 95 6 0 in a similar dolerite suite within the adjoining shelve inlier which is also of dual polarity and predates late ordovician folding the difference in magnetic declinations is similar to the rotation of late ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation possibly about a constraining bend in the severn valley fault system the moel y golfa andesite of caradocian soudleyan age has a remanence d i 294 3 50 4 13 samples 95 2 8 which study of an adjoining volcanic conglomerate shows to be of primary cooling related origin the equivalent palaeolatitude is 31 s in early caradocian times a similar estimate is derived from a probable diagenetic overprint in the contemporaneous castle hill conglomerate at montgomery the mynydd bryn andesite in the berwyn inlier of probable similar origin and age to moel y golfa has a magnetization of opposite polarity but similar inclination d i 239 1 47 4 14 samples 86 5 5 declination differences between these volcanic outcrops are a possible signature of later block rotations between the lower palaeozoic inliers and identify the importance of terrane definition in analysis of lower palaeozoic palaeomagnetic results from the caledonides a dolerite sheet complex at hendre in the berwyn hills has a younger remanence d i 182 3 3 0 20 samples 95 7 8 probably of mid carboniferous age the collective upper ordovician results define a rapid movement of the avalonian plate probably spanning no more than 10 ma from 30 s in early caradocian times to 57 s prior to folding of the dolerites in late caradocian to early ashgill times this motion into higher southerly palaeolatitudes marks a reversal of the motion of avalonia into more shallow southerly palaeolatitudes between arenig and llanvirn times and shows that ordovician closure of the iapetus ocean was more complex than recognized hitherto