deep marine arc apron deposits and syndepositional magmatism in the alisitos group at punta cono baja california mexico rocks exposed at punta cono include very fine grained to coarse grained tuffs lapilli tuffs and tuff breccias deposited in a deep marine environment syndepositional basaltic intrusive activity was common in one locality a hyaloclastite peperite complex formed slumped sections with fluidal basalt clasts derived from intrusions that entered the sediment pile from below are present elsewhere abundant soft sediment folds in fine grained laminated subaqueous fall out tuff suggest steep gradients these are cut by shallow channels filled with coarse grained tuff lapilli tuff and rare tuff breccia the combination of marine fossils extreme textural immaturity abundant slump features and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano recognition of magma wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments products of magma water sediment interactions at punta maria include 1 jigsaw puzzle hyaloclastite formed by non explosive hydroclastic fragmentation of magma upon contact with water and water bearing sediment 2 peperites produced by mixing of magma with sediment and 3 an unusual tuff breccia unit the result of non explosive mixing of wisps of lava with sediment during remobilization of an unconsolidated section low explosivity magma water sediment interactions are favoured by relatively high hydrostatic pressures in sub wave base settings

extensive felsic lavas and rheoignimbrites in the keweenawan midcontinent rift plateau volcanics minnesota petrographic and field recognition the 8 km thick sequence of plateau lavas north shore volcanic group in northeastern minnesota that was erupted during the formation of the 1 1 ga midcontinent rift system includes an unusually large proportion 10 25 of felsic flows rhyolites and icelandites individual flows are now recognized with estimated volumes up to 600 km3 and lateral extents up to 40 km some were probably originally much broader many are anomalously large compared to most silicic lava flows primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows in particular the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas and others as strongly rheomorphic tuffs it may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top where the only evidence poorly to strongly welded shards may exist of the pyroclastic origin of a large otherwise lava like flow interior textures of these crystalline flows provide evidence of high temperature eruption especially in the form of abundant paramorphs of magmatic tridymite in the groundmass commonly arranged in a pilotaxitic flow structure based on structures textures field relations and geochemistry we conclude that a all the icelandites and some rhyolites were emplaced as large moderately to very fluid lava flows that spread widely while crystallizing tridymite and that b some large rhyolites which are only sparsely phyric were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites the mode of emplacement of one of the largest aphyric flows remains enigmatic vent areas or characteristics are unknown the low apparent viscosity of the erupted magmas is probably attributable to compositional factors f content high fe2 as well as to high temperatures and eruption rates estimated temperatures of about 1000 1100 c are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting

borehole effects on downhole seismic measurements there are increasing interests in acquiring crosshole and vsp surveys at frequency up to 1 khz or more with the intention to resolve the fine details of structures and lithology between wells bregman et al 1989 harris 1988 tura 1991 however at frequencies on the order 1 khz the existence of the fluid filled borehole has a strong influence on the downhole measurements depending on the frequency and angle of incidence as well as the formation properties the measured displacements on the borehole wall or pressure at the center of borehole may be significantly different from those of the incident wave without taking proper care of this effect any imaging or inversion algorithm which utilizes both the amplitude and phase information of recorded energy will produce erroneous results

a comparison of miocene and archean rhyolite hyaloclastites evidence for a hot and fluid rhyolite lava abstract in this paper we compare the geology and petrography of miocene and archean submarine rhyolite hyaloclastites the hyaloclastites are sparsely 10 or less plagioclase quartz and pyroxene phyric the hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods the lava bodies consist of a holocrystalline core with microlitic texture grading outward into a flow layered rim zone and finally into obsidian the proportion of plagioclase and pyroxene microlites decreases outward some layers of the rim zone may be pumiceous vesicularity up to 50 vesicle size 1 mm or less but most of the lava has less than 5 vesicles one or a few cm long the obsidian shows perlitic fracture patterns the lava bodies grade through an in situ breccia into a hyaloclastite composed of angular obsidian granules and in many cases of fragments of lava lobes evidence for alteration at high temperature is as follows in the archean rhyolite hyaloclastites plagioclase microlites are overgrown by quartz albite spherulites furthermore parts of the miocene and archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite hyaloclastite cemented at high temperature locally shows columnar joints at low temperatures obsidian has been hydrated and or has been replaced by clay minerals zeolites chlorite or prehnite chess board albite and fibroradial prehnite in archean hyaloclastite is possibly a pseudomorph after zeolites the sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high near liquidus temperature furthermore pumice is present only locally in the flow layered rim zone and in fragments derived from that zone these features suggest that vesiculation was inhibited by the weight of the water column high temperature and possibly the volatile h 2 o content explain the relatively low viscosity and shear strength of the lava and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows

present day temperatures in northern scandinavia during the last glaciation scandinavia is generally considered to have been covered extensively with ice throughout marine isotope stages mis 4 2 between 75 and 10 ka here we present evidence for ice free warm conditions in the central area of the scandinavian glaciations during mis 3 our multiproxy data obtained from a lacustrine sequence in northern fin land reveal not only significant response in the northeastern sector of the scandinavian ice sheet to warming during the early part of mis 3 but also indicate rapid climate warming to present day temperatures in this ice free period new climate model simulations for interstadial conditions in mis 3 confirm the high mean july temperatures northeast of the scandinavian ice sheet in response to the high insolation values and the presence of the ice sheet during mis 3

imaging near borehole structure using directional acoustic wave measurement directional acoustic measurements made in the borehole are used for imaging a near borehole geological formation structure and determination of its orientation four component cross dipole data set measured in a deviated borehole in combination with the directionality of the compressional waves in the dipole data give the orientation of bed boundaries crossing the borehole the low frequency content 2 3 khz of the data allows for imaging the radial extent of the formation structure up to 15 m greatly enhancing the penetration depth as compared to that obtained using conventional monopole compressional wave data a combination monopole dipole arrangement of sources and receivers may also be used for imaging of bed boundaries

early mis 3 glacial lake evolution ice marginal retreat pattern and climate at sokli northeastern fennoscandia lake development at sokli northern finland is traced through the analysis of diatoms and other siliceous micro fossils in a 2 meter thick minerogenic laminated clay silt deposit dated to the early part of marine isotope stage mis 3 fluctuating water levels and changes in lake extent depicted by the siliceous micro fossil record together with lithology suggest that an important part of the sediment sequence was deposited in a glacial lake the proxy based glacial lake evolution is tested using a digital elevation model and geomorphologic evidence including eskers dated to the early mis 3 tulppio interstadial at sokli despite the apparent ice dammed nature of the lake the sediment is relatively rich in fossils and there are limited signs of re deposition of older fossil material the siliceous micro fossil record together with data from other proxies previously analysed in the same sediment samples provides a coherent picture of past environmental changes around the sokli site this is most probably due to the sheltered position of the coring site in a lake embayment quantitative climate reconstructions based on the diatom record show mean july air temperatures as high as present day values at sokli and the temperature ranges indicated by the diatom record are in agreement with temperature reconstructions based on chironomids the position of sokli in the northeastern portion of the central area of the scandinavian glaciations and the northern retreat pattern implies that an important part of eastern fennoscandia was deglaciated during the early mis 3 warming event

composition and origin of ash zones from marine isotope stages 3 and 2 in the north atlantic abstract the composition and distribution of four north atlantic ash zones from the faroe islands margin is presented including the widespread north atlantic ash zone ii naaz ii 52 53 ka bp and three mainly basaltic ash zones here referred to as the faroe marine ash zones fmaz i ca 15 4 14 c ka bp ii ca 23 14 c ka bp also referred to as the fugloyarbanki tephra and iii ca 33 14 c ka bp one core from the south east labrador sea was also analysed naaz ii was found in all investigated cores indicating very large eruption events in the early marine isotope stage mis 3 detailed investigations of naaz ii in high resolution cores show that two separate layers can be distinguished a lower mixed alkalic basalt and rhyolitic layer and an upper predominantly tholeiitic basalt tephra the main component of the basaltic fmaz ii was found in all cores from the faroe island margin and the labrador sea indicating that this tephra is the most widespread tephra from mis 2 in the north atlantic discovered so far we discuss the possibility to use these tephra layers for exact correlations between marine records and land sections such as the greenland ice cores and terrestrial records on the european mainland

in situ formation of welded tuff like textures in the carapace of a voluminous silicic lava flow owyhee county sw idaho the badlands rhyolite on the owyhee plateau of southwestern idaho can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes its well exposed near vent features and its response to pre existing topography however samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures including material which closely resembles the compressed welded glass shards which are characteristic of ignimbrites formation of these tuff like textures involved processes probably common to emplacement of most silicic lava flow units decompression upon extrusion causes inflation of pumice at the surface of the lava flow some of this pumice is subsequently comminuted producing loose bubble wall shards bits of pumice chips of dense glass and fragments of phenocrysts this debris sifts down around loose blocks and into open fractures deeper in the flow where it can be reheated compressed and annealed to varying degrees the end result is a dense vitrophyre layer beneath the true upper non welded carapace breccia which can be extremely texturally heterogeneous with areas of flow foliated lava occurring very near lava which in many aspects looks like welded ignimbrite complete with flattened pumices identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow foliated rock which otherwise appears to be lava the textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites at least at scales ranging from thin sections to outcrops voluminous silicic units with scattered fragmental textures but with otherwise lava like features are probably true effusive lava flows

new field and laboratory evidence for the origin of hyaloclastite flows on seamount summits new field observations with the submersible alvin and photographic evidence from a study of the summits of seamounts near the east pacific rise show that hyaloclastite deposits occur commonly hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m these new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies most of the hyaloclastite samples are of hydrovolcanic eruptive origin but a few show evidence of a predominantly sedimentary origin primarily from morphology we identify several vent areas from which hyaloclastite presumably erupted the surface appearance of the hyaloclastite deposits varies with distance to these vents leading us to propose a facies model for deep sea hyaloclastites on seamount summits hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding parallel alignment of platy shards they consist of blocky sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment the shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous we propose that the shards form mainly by cooling contraction granulation but cannot rule out the possibility of limited steam explosivity hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate promoting clastic dominated versus flow dominated eruptive behavior is the dominant control on hyaloclastite formation we propose that shard formation occurs during submarine lava fountaining gravitational instability of the resulting slurry of shards sea water and possibly steam causes gravity flow that carries the shards outward from the vent further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism eruption dynamics and deposition of deep sea hyaloclastites

late diagenetic changes in manganese ores of the upper proterozoic penganga group india sedimentary manganese oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive upper proterozoic marine sequence of the penganga group andhra pradesh india this sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed the manganeseoxide ores show unusual coexistence of the primary sedimentary minerals todorokite and birnessite with their diagenetic conversion products manganite braunite and bixbyite the mineralogical characters and chemical compositions of the important phases as well as their texture are discussed braunite occurs in two generations one earlier or contemporaneous with bixbyite and the other later than bixbyite the different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved the general observations suggest that bixbyite was also produced by conversion of todorokite birnessite pyrolusite and cryptomelane conventionally considered as diagenetic products of mn oxide hydroxide precursors deposited as primary sediments are here rarely present and then only as supergene derivatives this documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese oxide ores that abound in the precambrian

syndepositional and postdepositional features of the manganese ore deposits of the proterozoic penganga group adilabad district andhra pradesh india the proterozoic penganga group consisting of terrigenous and orthochemical sediments including a manganese orebody is well developed in the northwestern part of the adilabad district andhra pradesh the manganese orebody of unmetamorphosed and undeformed interbanded manganese oxide ore chert and minor calcareous shale has retained excellent syndepositional and postdepositional features both on the macro and microscales the primary depositional features include meso and microbands of manganese oxide and silica of different descriptions scour and fill structures and mn oxide micronodules spherical siliceous m sized structures and other features of biogenic origin have been observed diagenetic features such as fabric changes syneresis cracks concretionary pods and mn oxide nodules have been recorded they are accompanied by penecontemporaneous deformation structures such as pinch and swell structures gravity density features brecciation and folding and faulting of various kinds all these features suggest that the manganese orebody was formed in a shallow marine environment on a stable shelf possibly behind a barrier bar and subsequently underwent diagenetic reorganization and penecontemporaneous deformation when the sediments were still in a hydroplastic state

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

felsic fire fountaining beneath archean seas pyroclastic deposits of the 2730 ma hunter mine group quebec canada abstract the lapilli tuff breccias ltb 1 and ltb 2 of the archean hunter mine group in the south central part of the abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining intercalated sub wave base iron formations interstratified turbiditic tuffs the absence of wave induced sedimentary structures and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites support this contention a complete eruptive sequence shows a tripartite division into a massive breccia b stratified lapilli tuff and c turbiditic tuff lapilli tuff division the massive breccia division is characterized by clusters of isolated and compressed irregular shaped clasts inferred to be deposited directly from the hot magmatic lava fountain abundant vesicular pyroclasts with a vesicle content of up to 60 exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption the prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy rich high density current to the site under a self generated steam cupola ubiquitous subequant lapilli size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic the turbiditic tuff lapilli tuff division composed of pumice lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume this type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the archean and possibly during incipient arc and backarc formation in modern day settings

proterozoic microfossils from manganese orebody india the presence of carbonaceous matter and stromatolitic structures in manganese deposits of proterozoic 600 myr age1 3 provides indirect evidence for the activity of living organisms during their formation here i report the presence of microfossils from the proterozoic manganese orebody of the penganga group india these microfossils occur commonly in chert and are present within manganese oxide sediments the microfossils probably have affinities with the cyanobacteria and may have been planktonic both coccoid and filamentous forms are present the presence of these microfossils suggests a possible genetic relationship between microbiota and manganese deposition and provides further information about proterozoic ecology

the last interglacial glacial cycle in fennoscandia abstract the climate and glacier fluctuations during the last interglacial glacial cycle in fennoscandia seem to correspond roughly in time with the climate fluctuations recorded in france germany holland however the climate in fennoscandia was considerably colder and the coldest phases are recorded as periods with glacier fluctuations admittedly the exact age and amplitudes of some fluctuations are still much debated

facies associations and depositional environment of the proterozoic carbonate hosted microbanded manganese oxide ore deposit penganga group godavari rift basin india the late proterozoic penganga group pranhita godavari valley south india is a sequence of siliciclastic carbonate sediments that was deposited in an intracratonic rift basin the nonvolcanogenic sedimentary succession containing listric basin marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies the cratonic sediments of the basin are mature siliciclastics with clear imprints of tide dominated shallow subtidal deposition the siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment gravity flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part the limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale uniformly thick laterally persistent parallel sided and thin bedded limestone shale rhythmites and parted limestone rhythmites characterize the ribbon limestones except for widespread thin parallel lamination structures formed by waves tidal currents and storm induced currents are totally absent the limestone is brown and argillaceous in the basal part and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone the physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep water slope and basinal environment of carbonate deposition under continued transgression the bedded manganese oxide ore interstratified with chert simulating iron formation typically shows micron scale to millimeter scale thin parallel laminae of almost uniform thickness texture and composition the manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size todorokite and birnessite are the most abundant sedimentary manganese oxides they were locally transformed to braunite and bixbyite during diagenesis the centimeter thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz the sedimentary attributes of the ore chert sequence suggest deposition from suspension fallout in a low energy environment penecontemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor the sedimentology and stratigraphy of the penganga group suggest a sediment gravity flow influenced deep water base of slope basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting on the basis of sedimentological evidence and parallel studies done on iron formations a possible hydrothermal source of manganese is also considered this represents a significant departure from the prevalent views of high energy shallow water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits and thus it has important implications for genesis of manganese ores

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

fluidal clast breccia generated by submarine fire fountaining trooper creek formation queensland australia abstract a distinctive monomictic breccia composed of fluidal and blocky basaltic andesite clasts occurs in a cambro ordovician submarine volcanic succession in northern queensland australia associated with this fluvial clast buccia facies are coherent facies and coarse and fine breccia facies of the same composition the fluidal clast breccia facies is internally massive and 250 m thick varying only in the ratio of fluidal clasts to blocky clasts fluidal clasts range in size from 2 cm to 170 cm and have moderately to highly vesicular cores and thick up to 1 cm non vesicular formerly glassy rims blocky clasts are highly vesicular to non vesicular subaqueous fire fountain breccias are distinguished from subaerial fire fountain breccias by thick glassy margins on fluidal clasts the lack of welding and agglutination and the distinctive association of highly vesicular fluidal clasts with non vesicular angular blocky clasts recognition of submarine fire fountain breccias in volcanic successions constrains the eruption style proximity tens of metres to source and environment of deposition

a multi proxy study of holocene lake development lake settlement and vegetation history in central ireland stratigraphical investigations geomorphological mapping and diatom plant macrofossil and pollen analyses were undertaken in and around two lakes in central ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the holocene similar investigations of an adjacent mire reveal early holocene changes in lake level and area the palaeoecological data show high correlations related to variations in lake depth and area catchment vegetation type organic inputs and trophic status catchment scale deforestation is gradual and occurs through the bronze and the iron ages and the construction of a crannog in the early medieval period seventh century ad appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities in the early to middle holocene similarities in the proxy data appear to be climatically driven through changing lake levels and areal extent whereas the later holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes the advantages of combining multi proxy indicators of lake hydroecology with the vegetation record are illustrated

wavefield simulation of 3d borehole dipole radiation abstractreflections in acoustic borehole logging data can be used to image near borehole geologic structures using either monopole or dipole measurements the latter of which resolve azimuthal ambiguities dipole methods in particular can characterize small and subtle fractures in reservoir environments accurate tools for wavefield simulation are required to account for wave modes in and out of the borehole and for the receiver responses to these modes we have evaluated a 3d elastic staggered grid finite difference method for isotropic and anisotropic media in the context of the borehole acoustic imaging problem based on the convolutional perfectly matched layer and the multiaxial perfectly matched layer schemes a hybrid perfectly matched layer method was developed and implemented to reduce artificial boundary reflections the superiority of this hybrid perfectly matched layer beyond the convolutional perfectly matched layer and the multiaxial perfectly matched layer was proven based on numerical be

basinal autoclastic mass flow regime in the precambrian chanda limestone formation adilabad india abstract the precambrian chanda limestone formation in adilabad district andhra pradesh india transcribes carbonate mass flow deposition in an extensional basin within the godavari rift valley system in the lower part of the formation conglomerate beds which commonly interrupt the sequence of thin bedded lime mud turbidites register pulsatory reinvigoration of mass flow processes these gravelly mass flow products are significant because they indicate i deposition far below the wave base ii intrabasinal origin iii disassociation with any depositional slope like reef margins iv occurrence in discrete vertical packages v clear signals for sediment gravity flows and vi invariable facies successions the conglomerate bodies are extensive parallel to the depositional strike but restricted across the latter the main mode of accretion is dominantly vertical aggradation only the muddy turbidites spreading far and wide into the basin the course of temporal evolution of the mass flows are also interpreted from vertical facies transitions although every flow eventually gave rise to turbidity currents their initial states differred in three distinct ways which are correlated with gradient of the translational slope gradual decrease in declivity of the depositional substrate through time is interpreted

simulation of borehole sonic waveforms in dipping anisotropic and invaded formations a numerical simulation study has been made of borehole sonic measurements that examined shoulder bed anisotropy and mud filtrate invasion effects on frequency dispersion curves of flexural and stoneley waves numerical simulations were considered for a range of models for fast and slow formations computations are performed with a cartesian 3d finite difference time domain code simulations show that presence of transverse isotropy ti alters the dispersion of flexural and stoneley waves in slow formations the flexural wave becomes less dispersive when the shear modulus c44 governing wave propagation parallel to the ti symmetry axis is lower than the shear modulus c66 governing wave propagation normal to the ti symmetry axis conversely the flexural wave becomes more dispersive when c44 c66 dispersion decreases by as much as 30 at higher frequencies for the considered case where c44 c66 and decrease

proterozoic deep marine volcanic eruptive product obsidian glass in chanda limestone penganga group india its implications for basin evolution and metallogenesis abstract subaqueous effusive to low explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine grained volcaniclastic debris flow conglomerate and turbidite occurring in a deep marine sequence of thin and even bedded grey siliceous micritic limestone of the chanda limestone the siliceous limestone conformably encloses stratiform manganese deposit obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass oriented microlites and microphenocrysts of quartz and k feldspars euhedral crystals resorbed rounded and embayed crystals spherical vesicles microlites and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava the blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion gravitational instability of the resulting slurry of glass fragments and the ambient sea water sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition this study records maiden evidence of volcanism in the chanda limestone of the neoproterozoic penganga group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica the inferred tectono sedimentary setting facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal

the paleoproterozoic supracrustal kolhan group in singhbhum craton india and the indo african supercontinent abstract the 2 0 ga supracrustal kolhan group to the south of chaibasa in singhbhum craton eastern india is a shale dominated succession deposited in continental rift setting it begins with thin plane and cross bedded red and purple sandstones consisting of ferric oxide rich quartz arenite with beds lenses of conglomerates deposited in shallow ephemeral braided streams the thick and extensive shale deposited in lacustrine environment overlies the sandstone the shale succession consisting of thin and even bedded shale and silty shale contains lenticular bodies of finely laminated thin bedded limestones and manganese rich interval towards the basal part it lacks subaerial exposure and tidal features and at places exhibits small wave ripples on bed surfaces petrography geochemistry and cia values of kolhan siliciclastics suggest passive margin tectonic setting an intensely weathered low relief provenance dominantly composed of granitoid rocks and a warm and humid palaeoclimate based on similarities in age lithology petrography depositional environment and type of mineralization the kolhan group in singhbhum craton is correlated with manganese bearing wyllies poort formation of the soutpansberg group northeast kaapvaal craton south africa this correlation suggests the two cratonic blocks may formerly have been juxtaposed and represents the sundered fragments of an archean craton

imaging of near borehole structure using full waveform sonic data the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near borehole structural features these arrivals are used to form an image of the near borehole structural features in a manner similar to seismic migration possible uses of this technique include horizontal well logging structural dip and contour determination fault salt dome pinnacle reef and fracture zone imaging and eor steam flood monitoring since both the source and the receivers pass through structures that cross the borehole the downdip structure and the updip structure can be imaged separately the technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis separate matrices are formed for the updip and for the downdip raypaths the basic technique is illustrated with synthetic data generated to approximate the case of a sonic tool logging

shallow to deep water deposition in a cratonic basin an example from the proterozoic penganga group pranhita godavari valley india abstract the unmetamorphosed proterozoic succession dominated by deep water lithographic limestone and shale in the western flank of the pranhita godavari valley is designated as the penganga group the succession in different parts of the valley includes the pranhita sandstone 25 400 m the chanda limestone 300 m and the sat nala shale 2000 m in ascending order the pranhita sandstone and the chanda limestone reveal considerable variations in the character of the stratal packages and depositional settings from mancherial in the south to adilabad in the north the sat nala shale in both southern and northern outcrop belts is completely devoid of sand it is brown to purple in colour and resembles present day deep water mud deposits in the mancherial area the pranhita sandstone consists of 25 400 m thick conglomerate pebbly red arkose and quartzose sandstone succession of coastal alluvial fan to shallow shelf origin the chanda limestone is micritic and locally includes interbedded lenses of cross stratified quartzose sandstone in the lower part the depositional milieu varies from shallow shelf to below wave base outer ramp carbonate platform around adilabad the pranhita sandstone 25 m lacks the conglomerate pebbly arkose association at the base and comprises only quartzose sandstone and shale of shoreface to muddy shelf settings the chanda limestone is essentially micritic but in contrast to the mancherial area includes several interbedded intervals of slope related autoclastic debris flow limestone conglomerates and calciturbidites and represents deep outer ramp to slope and basinal settings a predominantly deep water micritic limestone and deep water shale succession suggests that the penganga basin evolved to a vast deep epicratonic sea connected to an open ocean the absence of a coastal alluvial fan association at the lower part of the pranhita sandstone and presence of a slope to basinal association in the chanda limestone in the northern outcrop belts around adilabad indicates a northward transition from coastal to deep shelf slope and basinal conditions and a northerly paleoslope of the basin

the dharwar craton and the assembly of peninsular india peninsular india can be separated into five discrete crustal areas the bhandara singhbhum and aravalli bundelkhand cratons the eastern ghats and a block in southern india containing the dharwar craton and adjoining granulite terrain these areas are separated by four major joins eastern ghats front godavari rift mahanadi rift sukinda thrust and narmada son lineament and associated thrusts all joins except the godavari rift are former orogenic belts where thrusts juxtapose rocks of different metamorphic grade these thrusts and the absence of clear correlation of suites among different cratons suggest that the indian shield formed by accretion of separate continental fragments epicontinental late archean early pro terozoic sediments throughout india however may indicate that the joins are intracontinental formed within a coherent block the term dravidian shield is used for the southernmost of the five crustal blocks including all of india south of the deccan basalts and godavari rift

new radiocarbon dates from finnish mammoths indicating large ice free areas in fennoscandia during the middle weichselian new radiocarbon dates from finnish subfossil mammoth material mammuthus sp transported by glacial ice range in age from ca 32000 to ca 22500 yr bp these results suggest that there was a larger ice free area in fennoscandia during the middle weichselian than previously assumed in addition two dates are also presented for bones found in clay with a different transport history copyright 1999 john wiley sons ltd

numerical simulation of radiation reflection and reception of elastic waves from a borehole dipole source abstracta recent advance in single well reflection imaging is the use of a dipole acoustic system in a borehole to radiate and receive elastic waves to and from a remote geologic reflector in formation this dipole acoustic imaging technology is evaluated by numerically simulating the radiation and reflection of the wavefield generated by the borehole dipole source and analyzing the receiving sensitivity of the dipole system to the incoming reflected waves the analyses show that a borehole dipole source can radiate a compressional wave p wave and two types of shear waves i e sv and sh waves into the formation the sh wave has wide radiation coverage and the best receiving sensitivity and is most suitable for dipole shear imaging in an acoustically slow formation the dipole generated p wave has strong receiving sensitivity and can also be used for reflection imaging an important feature of dipole imaging is its sensitivity to reflector azimuth which results from the directivity of the dipole s

chronology and paleoenvironment of marine isotope stage 3 from two high elevation speleothems austrian alps abstract a new high resolution stable isotope record from the alpine kleegruben cave 2165 m central alps is presented this record largely duplicates a previously reported speleothem record from this site high precision u series thermal ionization mass spectrometry dates constrain the growth history of this new sample 56 48 ka to the marine isotope stage mis 3 both stalagmites reveal highly similar variations in o isotopes that can be directly compared to o isotope variability associated with dansgaard oeschger interstadials recorded in the greenland ice cores greenland interstadials gis based on the new record we refine our previous age assignments of the gis during this time interval i e gis 15b at 55 7 ka gis 15a at 55 3 ka onset mid point of the prominent gis 14 at 54 5 yr and onset of gis 12 at 48 ka the overall uncertainties associated with these ages are 0 1 0 3 ka the timing of these dansgaard oeschger interstadials is within 0 2 ka of the newly proposed grip sfcp04 timescale the age data for both kleegruben stalagmites demonstrate that liquid water was at least seasonally present in the shallow sub surface allowing calcite precipitation to continue even during stadials of mis 3 given the present day low temperature 2 4 c we propose that these unusual speleothems formed in a karst system overlain by a warm based glacier

early holocene glacial lakes and ice marginal retreat pattern in labrador ungava canada abstract the existence of glacial lakes naskaupi mclean minto melezes and wapussakatoo during the deglaciation of labrador ungava canada was recognized at the end of 1950s however glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in labrador ungava reconstruction of the ice flow succession in labrador ungava has often primarily focused on till lineation systems these reconstructions often depict a late glacial ice dispersal center situated over central labrador ungava more recent studies on glacial geomorphology including meltwater features however suggest that north central labrador ungava exhibited cold based conditions at least during the latest deglaciation cold based conditions inhibit basal sliding and formation of landforms except for meltwater traces such as meltwater channels glacial lake shorelines and deltas this situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold based deglaciation evidence presented in this study such as glacial lake shorelines fossil deltas and spillway and drainage channels in north central labrador ungava indicates the existence of numerous previously unmapped glacial lakes the mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which in turn are used to constrain the late glacial ice margin retreat pattern over the inferred cold based areas of north central labrador ungava a total of 26 glacial lakes 65 sub stages existed during the deglaciation of labrador ungava these lakes were impounded along the southern margin of the shrinking ice sheet the required damming ice margins indicate that the last ice remnant of the laurentide ice sheet in labrador ungava was situated over the southern ungava bay and the adjacent southern shore