grain fabric of experimental gravity flow deposits abstract grain fabric of deposits accumulated from a high density surge type gravity turbidity current in an experimental flume was measured vertical sequential change 0 2 mm interval in imbrication shows that a bed can be divided into lower middle upper and uppermost parts the lower part is characterized by both up current and down current imbrication with a wide range of angles dominant up current imbrication and rare down current imbrication characterize the middle part the imbrication angle of this part tends to be smaller than in the lower part the upper part is represented by intervals with up current imbrication nearly flat imbrication is dominant in the uppermost intervals statistically significant preferred orientation was observed from the lower middle and upper parts and it deviates up to 13 in both clockwise and anticlockwise directions from the current direction the lower and middle parts of the bed may correspond to the bouma a division judging from the wider range of imbrication angles and the presence of down current imbrication which have been reported from natural turbidite beds the upper part which is characterized by up current imbrication is interpreted to be the bouma b division the uppermost interval may coincide with the d division the episodically appearing down current imbrication in the lower and middle parts can probably be attributed to oscillation of an interface between a denser basal layer and superjacent low density layer of the turbidity current measurement of grain orientations in this and previous studies implies that at least 30 of deviation from the flow axis should be considered for paleoflow analyses based on grain fabrics

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

preliminary geology mineral chemistry and diamond results from the c29 30 candle lake volcanic complex saskatchewan canada abstract the c29 30 kimberlite is one of two diamondiferous kimberlites in the candle lake cluster located in east central saskatchewan canada approximately 70 km from the fort a la corne kimberlite field the kimberlites are hosted by a cretaceous sequence of marine mudstone and shale of the lower colorado group and underlying siltstone and sandstone of the mannville group this sequence overlies paleozoic carbonates that were deposited over the proterozoic crystalline basement based on the country rock stratigraphy and morphology of the body c29 30 is inferred to be cretaceous in age the elongated kimberlite body has a lateral extent of approximately 2 km with the long axis oriented in a south east to north west direction and an estimated surface expression of 75 3 ha the investigation of 47 drill cores suggests that this body is a single volcanic complex dominated by a single phase of volcaniclastic kimberlite that is characterised by absent to rare phlogopite within the groundmass of preserved juvenile clasts minor amounts of at least one other phase of kimberlite containing conspicuous groundmass phlogopite have also been documented the subsurface shape of c29 30 is complex and is interpreted to result from a combination of explosive volcanic activity that formed two craters from separate feeder vents the formation of the elongated trough is poorly understood it may have formed by a fissure style eruption or erosive processes related to the mass flow of material away from one of the craters or possible the collapse of an eruption column the c29 30 kimberlite is similar to bodies of the fort a la corne kimberlite field with respect to country rock setting pipe morphology and the dominant textural varieties present this contribution presents a preliminary geological model of c29 30 based on data obtained from the drilling programmes completed in 2006 2007 and 2008

sedimentary facies of the nova scotian upper and middle continental slope offshore eastern canada a detailed survey of the upper and middle nova scotian continental slope at 42 50 n and 63 30 w indicates a complex morphology dominated by mass movements on various scales and an immature turbidity current channel the range of sediment facies is diverse including hemipelagic and turbidite muds turbidite sands and gravelly sandy muds of debris flow origin deformed units interpreted as slump deposits are also observed several facies associations related to discrete morphological environments are recognized thick turbidite sand units with minor intervening mud beds are characteristic of the high relief uppermost slope and channel margin thinner turbidite sands deformed slump beds and various mud facies are associated with small scale hummocky mid slope topography sand beds are more abundant in the depressions than on intervening hummocks indicating the preferred transport paths of small turbidity currents at the lower end of the main turbidity current channel frequent turbidite sand beds with relatively minor mud beds are deposited on a depositional lobe in areas unaffected by mass movements alternating bioturbated mud and sandy muds make up the core sequences a local model of sedimentation is proposed for this area and illustrates that simple models of continental slope sedimentation only apply to a limited range of settings

pyroclastic kimberlite deposits from the victor northwest pipe ontario canada the transition from phreatomagmatic to magmatic explosivity magmas of all compositions including kimberlites may undergo both magmatic and phreatomagmatic fragmentation during emplacement in this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the victor northwest kimberlite northern ontario canada detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles each consisting of three repeated eruptive phases pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine grained poorly sorted deposits containing broken olivine crystals angular country rock fragments accretionary lapilli and variably vesicular irregular shaped juvenile pyroclasts all observations indicate these deposits formed by phreatomagmatism subsequent clastogenic coherent kimberlite deposits were formed as a result of hawaiian fire fountaining during the second phase of both cycles these phases were followed by mass wastin

discrimination of reworked pyroclastics from primary tephra fall tuffs a case study using kimberlites of fort a la corne saskatchewan canada reworked volcaniclastics are traditionally discriminated from primary tephra fall pyroclastics by an absence of features such as blanketing juvenile lapilli grain welding and poor sorting frequently these features are difficult to identify especially in small outcrops ancient or altered successions debris flows and surge deposits crystal rich volcaniclastics such as kimberlites have a large proportion of coarse euhedral crystals and abrasion leading to rounding can be recognised and classified with relative ease a petrographic method of discriminating reworked material has been devised based on the degree of grain roundness and is illustrated using volcaniclastic kimberlite from fort a la corne saskatchewan canada petrographic thin sections of samples at regular intervals throughout the borehole core and from a nearby crater facies kimberlite show that the percentages of rounded subrounded and euhedral grains define two distinct groupings the first group contains a higher percentage of euhedral grains and includes all the samples from the basal 4 8 m of the 14 1 m thick kimberlite section in borehole 004 and all of the crater facies tephra fall tuffs a second group contains more rounded subrounded and fragmental grains and includes all the data from the upper 6 3 m which are interpreted as reworked strata thus point counting concurs with hand sample interpretation and may be used as a verification tool in discriminating reworked pyroclastic sands from primary tephra fall tuffs

eruption processes and facies architecture of the orion central kimberlite volcanic complex fort a la corne saskatchewan kimberlite mass flow deposits in a sedimentary basin abstract the fort a la corne diamondiferous kimberlite field consists of at least 70 bodies of volcaniclastic kimberlite hosted within a contemporaneous non volcanic sedimentary succession this study of the three dimensional stratigraphy and facies architecture across the orion central kimberlite volcanic complex highlights the variations in upper and extra vent processes the sedimentary succession consists of continental to marginal marine quartz sandstones and mudstones overlain by marginal to deep marine dark mudstone and muddy sandstones and siltstones relatively thin conformable volcaniclastic kimberlite packages are interbedded throughout the host rock stratigraphy extremely thick up to at least 211 m thick discordant to concordant volcaniclastic packages series infill at least three elongate northwest trending craters 145a 145b and 219 craters and contain laterally equivalent conformable extra crater deposits bound by marine mudstones indicative of a prevailing dominantly marine environment the volcaniclastic deposits within the 145a and 145b craters respectively are separated by a considerable hiatus whereas the deposit infilling the 219 crater was formed around the same time as 145b crater deposit multiple depositional units of massive to stratified olivine rich sand to pebble sized volcaniclastic facies infill craters and were emplaced by megaturbidite pulses fed by crystal rich eruption fountains which interacted with the crater relief stacked normally graded thick to very thick bedded matrix supported olivine rich facies characterized by brief depositional breaks between some beds represent syn to post eruptive turbidite pulses associated with the early eruptive event in the 145a crater thin layers of light grey kimberlitic mudstone underlie or occur near the base of and above the main volcaniclastic packages associated with the 145b and 219 eruptions crater infilling volcaniclastic deposits were later reworked by storm induced currents into thin to medium graded moderately sorted fine to coarse olivine rich sandstone with intercalated discontinuous muddy laminae laminated to cross laminated olivine rich silt to sand sized volcaniclastic facies constitutes a small volume volcaniclastic turbidite which marks the last preserved kimberlite emplacement event at orion central

a climatic model for southwestern amazonia in last glacial times abstract multidisciplinary data indicate that southwestern amazonia was a savanna environment during last glacial times interpretation of biogeographical and geological records as compared with the present climatic conditions permit us to postulate a dry season more pronounced and prolonged than the present one dry northerly winds vinculated with southern bemisphere trade winds dominated during the dry season and the southerly cold air masses locally termed friagem or surazo would have been more frequent and intensive than today

alteration styles in the orion central volcanic complex fort a la corne kimberlite field saskatchewan and their effects on primary volcaniclastic textures implications for facies mapping and diamond exploration diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes which are generally determined from volcaniclastic facies characteristics however the effects and distribution of secondary alteration styles which usually mask primary kimberlite facies have been given little attention at the fort a la corne kimberlite field saskatchewan complex overprinting alteration textures and mineral phases are recognized this study documents the characteristics overprinting relationships and spatial distribution of alteration styles within a crater filling volcaniclastic deposit body 219 of the orion central volcanic complex data were acquired from macroscopic drill core observations microscopic petrography sem imaging eds element mapping and xrd analyses olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite late stage serpentine has migrated into the interstitial medium bulk rock alteration textures include 1 fine grained crystal rich domains which are alteration pseudoclasts and or relict juvenile pyroclasts 2 a heterogeneous interstitial medium varying from early stage disseminated calcite rich serpentine to amorphous serpentine and crystalline calcite and 3 overprinting serpentine carbonate and magnetite veins and serpentine nodules alteration styles defined by unique alteration characteristics are spatially zoned through the body 219 volcaniclastic deposit and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit recognition of alteration processes their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies and by consequence the distribution of diamond grades

a new approach to kimberlite facies terminology using a revised general approach to the nomenclature of all volcanic rocks and deposits descriptive to genetic although kimberlite pipes bodies are usually the remains of volcanic vents in vent deposits and subvolcanic intrusions the terminology used for kimberlite rocks has largely developed independently of that used in mainstream volcanology existing kimberlite terminology is not descriptive and includes terms that are rarely used used differently and even not used at all in mainstream volcanology in addition kimberlite bodies are altered to varying degrees making application of genetic terminology difficult because original components and depositional textures are commonly masked by alteration this paper recommends an approach to the terminology for kimberlite rocks that is consistent with usage for other volcanic successions in modern terrains the eruption and emplacement origins of deposits can often be readily deduced but this is often not the case for old variably altered and deformed rock successions a staged approach is required whereby descriptive terminology is developed first followed by application of genetic terminology once all features including the effects of alteration on original texture and depositional features together with contact relationships and setting have been evaluated because many volcanic successions consist of both primary volcanic deposits as well as volcanic sediments terminology must account for both possibilities

the crater facies kimberlite system of tokapal bastar district chhattisgarh india abstract discovery of diamondiferous kimberlites in the mainpur kimberlite field raipur district chhattisgarh in central india encouraged investigation of similar bodies in other parts of the bastar craton the earlier known tokapal ultramafic intrusive body located beyond the 19 km milestone in tokapal village along the jagdalpur geedam road was reinterpreted as crater facies kimberlite its stratigraphic position in the meso neoproterozoic intracratonic sedimentary indravati basin makes it one of the oldest preserved crater facies kimberlite systems ground and limited subsurface data dug tube wells and exploratory boreholes have outlined an extensive surface area 550 ha of the kimberlite the morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic epiclastic rocks that are surrounded by a 2 km wide spread of pyroclastic rocks lapilli tuff tuff ash beds and volcaniclastic breccia drill holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the kanger and jagdalpur formations and also form sills within the sedimentary sequence of the indravati basin the lapilli tuffs are commonly well stratified and display slumping base surges and lava flows occur in the southern part of the tokapal system the geochemistry and petrology of the rock correspond to average group i kimberlite with a moderate degree of contamination however the exposed rock is intensely weathered and altered with strong leaching of mobile elements ba rb sr layers of vesicular fine grained glassy material represent kimberlitic lava flows tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc serpentine carbonate matrix with locally abundant spinel and sphene garnet has not been observed and phlogopite is very rare very limited microdiamond testing two 18 kg samples proved negative however the composition of chromite grains indicate crystallization in the diamond stability field

pre emergent construction of a lacustrine basaltic volcano pahvant butte utah usa the subaqueous phases of an eruption initiated approximately 85 m beneath the surface of pleistocene lake bonneville produced a broad mound of tephra a variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level at the base of the volcano and very near inferred vent sites are fines poor well bedded broadly scoured beds of sideromelane tephra having local very low angle cross stratification m1 lithofacies these beds grade upward into lithofacies m3 which shows progressively better developed dunes and cross stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption intercalated lithofacies are weakly bedded tuff and breccia m2 and nearly structureless units with coarse basal layers above strongly erosional contacts m4 the former combines products of deposition from direct fall and moderate concentration sediment gravity flows and the latter from progressively aggrading high concentration sediment gravity flows early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous unsteady dilute density currents which produced the m1 lithofacies near the vent dunes and crossbeds which are better developed upward in m3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion generated pressure waves and eruption jets impinged upon and occasionally breached the surface intermingling of a tephra emplaced after brief transport by tephra jets within a gaseous milieu and b laterally flowing tephra formed lithofacies m2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water exclusion zones above a vent m4 comprises mass flow deposits formed by disruption and remobilization of mound tephra intermittent explosive magma water interactions occurred from the outset of the pahvant eruption with condensation entrainment of water and lateral flow marking the transformation from eruptive to sedimentary processes leading to deposition of the mound lithofacies

a practical guide to terminology for kimberlite facies a systematic progression from descriptive to genetic including a pocket guide kimberlite terminology remains problematic because both descriptive and genetic terms are mixed together in most existing terminology schemes in addition many terms used in existing kimberlite terminology schemes are not used in mainstream volcanology even though kimberlite bodies are commonly the remains of kimberlite volcanic vents and edifices we build on our own recently published approach to kimberlite facies terminology involving a systematic progression from descriptive to genetic the scheme can be used for both coherent kimberlite i e kimberlite that was emplaced without undergoing any fragmentation processes and therefore preserving coherent igneous textures and fragmental kimberlites the approach involves documentation of components textures and assessing the degree and effects of alteration on both components and original emplacement textures this allows a purely descriptive composite component textural and compositional petrological rock or deposit name to be constructed first free of any biases about emplacement setting and processes then important facies features such as depositional structures contact relationships and setting are assessed leading to a composite descriptive and genetic name for the facies or rock unit that summarises key descriptive characteristics emplacement processes and setting flow charts summarising the key steps in developing a progressive descriptive to genetic terminology are provided for both coherent and fragmental facies deposits rock units these can be copied and used in the field or in conjunction with field e g drill core observations and petrographic data because the approach depends heavily on field scale observations characteristics and process interpretations only the first descriptive part is appropriate where only petrographic observations are being made where field scale observations are available the progression from developing descriptive to interpretative terminology can be used especially where some petrographic data also becomes available

geology of the mwadui kimberlite shinyanga district tanzania abstract the mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in fill albeit without the tuff ring the geology of mwadui can be subdivided into five geological units viz the primary pyroclastic kimberlite pk re sedimented volcaniclastic kimberlite deposits rvk granite breccias subdivided into two units the turbidite deposits and the yellow shales listed in approximate order of formation the pk can be further subdivided into two units lithic rich ash and lapilli tuffs which dominate the succession and lithic poor juvenile rich ash and lapilli tuffs the lower crater is well bedded down to at least 684 m from present surface extent of current drill data the bedding is defined by the presence of juvenile rich lapilli tuffs vs lithic rich lapilli tuffs and the systematic variation in granite content and clast size within much of the lithic rich lapilli tuffs four distinct types of bedding have been identified in the pyroclastic deposits diffuse zones characterised by increased granite abundance and size and upward fining units represent the dominant types throughout the deposit lateral heterogeneity was observed in addition to the vertical changes suggesting that the eruption was quite heterogeneous or that more than one vent may have been present the continuous nature of the bedding in the pyroclastic material and the lack of ash partings suggest deposition from a high concentration ejecta sustained eruption column at times e g the massive very diffusely stratified deposits the paucity of tractional bed forms suggest near vertical particle trajectories i e a clear air fall component but the poorly sorted matrix supported nature of the deposits suggest that pyroclastic flow and or surge processes may also have been active during the eruption available diamond sampling data were examined and correlated with the geology data derive from the old 120 37 m 200 61 m 300 92 m and 1200 ft 366 m levels pits sunk during historical mining operations drill logs as well as more recent bench mapping correlating macro diamond sample data and geology shows a clear relationship between diamond grade and lithology localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post eruptive sedimentary in fill processes clear distinction can be drawn between upper re sedimented and lower pyroclastic crater deposits at mwadui both from a geological and diamond grade perspective finally an emplacement model for the mwadui kimberlite is proposed geological evidence suggests that little or no sedimentary cover existed at the time of emplacement the nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous but that the eruption column may have been heterogeneous both petrologically as well as geometrically volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re sedimented volcaniclastic material derived from the tuff ring the mwadui kimberlite exhibits marked similarities compared to the orapa kimberlite in botswana

sedimentologic and stratigraphic constraints on emplacement of the star kimberlite east central saskatchewan abstract diamond bearing kimberlites in the fort a la corne region east central saskatchewan consist primarily of extra crater pyroclastic deposits which are interstratified with lower cretaceous albian and cenomanian marine marginal marine and continental sediments approximately 70 individual kimberlite occurrences have been documented the star kimberlite occurring at the southeastern end of the main fort a la corne trend has been identified as being of economic interest and is characterized by an excellent drill core database integration of multi disciplinary data sets has helped to refine and resolve models for emplacement of the star kimberlite detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex micropaleontologic and biostratigraphic analysis of selected sedimentary rocks and u pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at star the oldest kimberlite in the star body erupted during deposition of the predominantly continental strata of the lower mannville group cantuar formation kimberlites within the cantuar formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate successive eruptive events occurred contemporaneous with deposition of the marginal marine upper mannville group pense formation kimberlites within the pense formation consist primarily of terrestrial airfall deposits fine to medium grained cross stratified kimberlitic olivine dominated sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression the location of the source feeder vents of the cantuar and pense kimberlite deposits has not been identified the youngest and volumetrically most significant eruptive events associated with the star kimberlite occur within the predominantly marine lower colorado group joli fou and viking formations kimberlite beds which occur at several horizons within these units consist of subaerial and marine fall deposits the latter commonly exhibiting evidence of wave reworking black shale encased resedimented kimberlite beds likely deposited as subaqueous debris flows and turbidites are particularly common in the lower colorado group during its multi eruptive history the star kimberlite body is interpreted to have evolved from a feeder vent and overlying positive relief tephra ring into a tephra cone initial early joli fou volcanism resulted in formation of a feeder vent 200 m diameter and tephra ring subsequent eruptions dominated by subaerial deposits partly infilled the crater and constructed a tephra cone a late joli fou eruption formed a small 70 m diameter feeder pipe slightly offset to the nw of the early joli fou feeder vent deposits from this event further infilled the crater and were deposited on top of early joli fou kimberlite proximal to the vent and sediments of the joli fou formation distal to the vent the shape of the tephra cone was modified during multiple marine transgression and regression cycles coeval with deposition of the lower colorado group resulting in wave reworked kimberlite sand along the fringes of the cone and kimberlitic event deposits tempestites turbidites debris flows in more distal settings

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

development of foliation and fold interference patterns produced by sedimentary processes penecontemporaneous folds lineations foliation and fold interference patterns are found in part of the pigeon point formation a turbidite sequence occurring along the central california coast these structures are associated with slumped sheets and they formed while the rocks were largely unlithified the foliation which is commonly roughly axial planar to folds is clearly shown in the hinges of slump folds and is defined by preferred alignment of inequant grains and silt sand domains the latter having formed from disrupted bedding laminae models that attempt to explain the genesis of penecontemporaneous foliation involve simple load compaction superimposed on pre existing slump folds model 1 or rotation of grains facilitated by slump folding model 2 evidence from this study indicates that a modified version of model 2 is commonly active it is suggested that a critical content of interstitial water is necessary whereby sufficient water pressure can build to decrease grain cohesion and induce liquefaction of layers further compaction in the hinge areas of slump folds then expels water during slumping such that preferred alignment of silt sand domains and inequant grains is facilitated all three of the basic fold interference patterns normally attributed to hard rock folding occur in these slumped sheets and along with foliation and related structures strongly resemble features generated by tectonic movement these structures can persist into the higher metamorphic grades where they may be misidentified as evidence for early repeated folding in this case the structural history of that part of the orogen may be reported as more complicated than it actually is

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

fragmentation in kimberlite products and intensity of explosive eruption the explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes shapes and textures however all pyroclastic deposits of kimberlite comprise two main types of pyroclasts 1 pyroclasts of kimberlite with or without enclosed olivine crystals and 2 olivine crystals which lack coatings of kimberlite here we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption 1 olivine crystals break during kimberlite eruption and 2 kimberlite melt can be efficiently separated from crystals during eruption these ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at diavik nt olivines are expected to break because of rapid pressure changes during the explosive eruption disruption of kimberlite magma and pyroclast production is driven by ductile deformation processes rather than by brittle fragmentation the extent to which melt separates from olivine crystals to produce kimberlite free crystals is a direct consequence of the relative proportions of gas melt and crystals lastly the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions a fragmentation index is proposed for kimberlite eruption based on a crystal size distributions of olivine and on b ratios of selvage free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals

a shallow marine volcaniclastic facies model an example from sedimentary rocks bounding the subaqueously welded ordovician garth tuff north wales u k abstract volcaniclastic sedimentary rocks bounding the ordovician garth tuff in north wales were deposited in a shallow marine setting adjacent to a magmatic arc the volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz angular euhedral feldspar and volcanic rock fragments these grains are of a pyroclastic origin but have been reworked to various degrees sedimentary rock fragments rounded quartz muscovite biotite iron rich chlorite and various clay minerals occur in lesser amounts the sedimentary rocks can be divided into a proximal offshore to foreshore facies association poffa and an offshore to lower shoreface facies association olsfa the sand dominated poffa exposed near capel curig is characterized by large wave ripples hummocky cross stratification co sets of trough cross beds reactivation surfaces plane beds and graded turbidite layers in suspension deposited mudstone water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross stratification and by facies association vary from a few meters to around 30 m 45 50 m theoretical maximum in a 45 m thick stratigraphic section beneath the tuff eight kilometers to the southeast the garth tuff is bounded by a thick sequence of suspension deposited laminated mudstone with thin graded beds of siltstone and fine grained sandstone this facies association olsfa represents the introduction of material by distal turbidites and by pelagic sedimentation the olsfa was deposited in water depths well below storm wave base possibly in excess of 200 m no evidence of storm waves or surface generated currents occur until over 40 m above the tuff this study documents marine sedimentary rocks deposited in water depths ranging from foreshore to offshore as the bounding facies of the garth tuff it is thus reasonable to conclude that the garth tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study

the submarine eruption and erosion of surtla surtsey iceland abstract surtla is the site of a short lived submarine vent which built basaltic elastic deposits almost to sea level in 1963 early in the eruption of surtsey since then wave and current activity have eroded the volcanic pile such that in july 1981 its top was a fairly level plateau 45 m below sea level and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules this winnowed layer was underlain by a nonreworked poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles steam explosions and quench brecciation during the eruption the explosion violence and associated comminution increased as the pile built up to shallower water depths it is argued that at times of continuous effusion a cupola of steam was situated over the vent as indicated by scoriaceous spatter which shows agglutination and bread crust features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption

mid jurassic volcanic structures in the outer moray firth basin uk 3d seismic mapping in the southern part of quadrant 15 in the outer moray firth basin reveals an array of circular to oval downthrown blocks bound by steep ring faults these lows affect triassic and older strata are approximately 1 5 2 km in diameter and 0 5 1 km deep the footwall crests of the structures now lie at a depth of c 2 5 km several alternative causal mechanisms for these circular fault blocks are examined including salt dissolution strike slip pull apart and impact cratering but in terms of geological and geometrical criteria the most likely mechanism appears to be small calderas or diatreme conduit collapse seismic data indicates that the circular structures cut triassic permian and devono carboniferous strata and are infilled with pre upper oxfordian sediments therefore they are broadly synchronous with mid jurassic volcanism local well control demonstrates the presence of several thousand feet of basalt tuffs and lavas the model proposed here to account for the circular structures consists of an initial phase of diatreme maar cratering as magmas associated with the rattray volcanics member intruded shallow aquifers in the pentland formation fluviodeltaic rocks these maars enlarged and deepened via conduit wall collapse supplemented by downsag faulting due to magma withdrawal the post volcanic evolution of the structures included 1 differential erosion creating accommodation space which may have been infilled with oxfordian reservoir sands and 2 local reactivation by underlying basement faults in the kimmeridgian

1 92ga kimberlitic rocks from kimozero nw russia their geochemistry tectonic setting and unusual field occurrence abstract the kimozero body is inferred to represent the oldest known diamond bearing kimberlite it was emplaced into a basaltic sequence as a gently dipping sheet like unit comprising several rock varieties with textures indicating a supracrustal origin minor subvolcanic bodies are represented by several narrow dykes and diatremes which formed during at least two intrusive phases the kimberlitic rocks have been exposed to extensive interactions with hydrous and co 2 rich fluids which led to extreme changes in the chemical compositions of the rocks making it difficult to identify their origin geochemical data indicate a within plate anorogenic tectonic setting of the kimozero magmatic body trace element compositions show that the rocks have a clear geochemical affinity to orangeite micaceous group ii kimberlite and olivine lamproite nd and sr isotope compositions of the kimozero rocks show that they were derived from a mantle source resembling bulk silicate earth new zircon u pb isotope data indicate emplacement age of the kimberlite at 1 92 ga suggesting that the rocks were associated with a late stage of paleoproterozoic rifting of the karelian craton related to the formation and growth of the svecofennian ocean

geology and evaluation strategy of the star and orion south kimberlites fort a la corne canada abstract diamond bearing kimberlites in the fort a la corne field east central saskatchewan consist primarily of volcaniclastic deposits which are interstratified with lower cretaceous continental marginal marine and marine sediments seventy kimberlite occurrences have been drill confirmed with the majority of those occurring in the fort a la corne main trend the star 352 ha and orion south 358 ha kimberlite complexes are located at the south eastern extremity of the main trend and contain some of the world s best examples of preserved primary volcaniclastic kimberlite deposits the advanced exploration programs on both the star and orion south complexes offer a rare opportunity to examine a wide variety of in situ volcaniclastic kimberlite deposits both kimberlite complexes are composed of multiple eruptive units that were deposited episodically over 4 to 6 million years separated by periods of volcanic quiescence marked by reworking of kimberlite and siliciclastic sedimentation detailed logging of core from 100 m spacing grid drilling has revealed well defined tuff ring tuff cone crater and extra crater deposits evaluation of these large tonnage lower grade coarse diamond size frequency kimberlite complexes requires a methodical and multi branched exploration strategy utilizing 1 grid pattern core drilling to gain an understanding of the three dimensional architecture of the kimberlite complexes 2 underground bulk sampling to provide appreciable carats for accurate estimates of diamond grade and price and 3 grid pattern mini bulk sampling via large diameter drilling to obtain an understanding of the diamond grade across the breadth of each kimberlite

particle fabric in a small type 2 ignimbrite flow unit laacher see germany and implications for emplacement dynamics ignimbrites of the 13 ka upper laacher see tephra were deposited from small highly concentrated moderately fluidized pyroclastic flows their unconsolidated nature and the prominence of accidental devonian slate fragments make these ignimbrites ideal for clast fabric studies the upper flow unit of ignimbrite m14 has characteristics typical of a type 2 ignimbrite layer 2a and the lower part of layer 2b of the flow unit have strong upstream inclined a p fabrics a p means long particle axes parallel to flow direction only clasts with a b axial ratios of 2 5 or greater preserve good a p fabrics whereas the a b planes of flat fragments dip upstream irrespective of axial ratio the a axis fabric becomes weaker flatter and more girdle like in the upper half of layer 2b at one locality the a axis fabric appears to rotate 40 up through the flow unit suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base the existence of similarly strong a p fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non shearing material layer 2b on a thin shear layer layer 2a and that the entire flow freezes en masse to form the deposit the data suggest that if the flow froze en masse it was shearing pervasively through at least half its thickness another possibility is that the flow unit aggraded progressively from the base up and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition

an ignimbrite veneer deposit the trail marker of a pyroclastic flow abstract the term ignimbrite veneer deposit ivd is proposed for a new kind of pyroclastic deposit which is found associated with and passes laterally into taupo ignimbrite of valley pond type in new zealand it forms a thin layer mantling the landscape over 15 000 km 2 and is regarded as the deposit from the trailing tail of a pyroclastic flow where a relaxation of shear stress favoured the deposition of the basal part of the flow the ivd differs little in grain size from the associated ignimbrite but it shows a crude internal stratification attributed to the deposition of a succession of layers one after the passage of each pulse of the pyroclastic flow it locally contains laterally discontinuous lenses of coarse pumice lee side lenses on the far vent side of topographic obstacles in nearvent exposures the taupo ivd shows lensoid and cross stratified bed forms even where it stands on a planar surface attributed to deposition from a flow travelling at an exceedingly high velocity an ivd can be distinguished from a poorly sorted pyroclastic fall deposit because the beds in it show more rapid lateral variations in thickness it may show a low angle cross stratification and it contains carbonised wood from trees not in the position of growth from the deposit of a wet base surge because it lacks vesicles and strong antidune like structures and contains carbonised vegetation and from a hot and dry pyroclastic surge deposit because it possesses a high content of pumice and fines the significance of an ivd is that it records the passage of a pyroclastic flow where the flow itself has moved farther on

volcaniclastic sedimentation in a shallow water marginal basin the early miocene koura formation sw japan abstract the early miocene koura formation sw japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics and constitutes part of the volcanic zone of sw japan of that time reworked volcaniclastic sequences are thick and mostly coarse grained and based on conventional facies concepts are interpreted as shoreface to beach deposits turbidites and debris flow deposits thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash flow emplacement or in some cases both andesite pyroclastic facies include surge and fall deposits facies relationships together with the fossil record indicate deposition in a shallow lacustrine environment the deposition rate of the koura formation was probably high enough to rapidly fill the shallow lake but sedimentation was balanced by rapid subsidence of the lake similar to lakes present within the modern volcanic zones of taupo new zealand and hohi japan the koura formation grades upward into marine deposits and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back arc or intra arc rifting

pebble orientation on large experimental debris flow deposits abstract replicable pronounced orientation of discoid pebbles 8 mm embedded on surfaces of large 10 m3 experimental debris flow deposits reveals that strongly aligned imbricate fabric can develop rapidly over short distances in mass flows pebble long axes aligned subparallel to deposit margins as well as subparallel to margins of surge waves arrested within the deposits pebble alignment exhibited modes both parallel to a p the primary flow direction intermediate axes dipped preferentially inward from surge wave margins b i orientation repetitive development of margin parallel imbricate fabric distributed across deposit surfaces provides compelling evidence that deposits formed dominantly through progressive incremental accretion rather than through simple en masse emplacement pronounced fabric along deposit and arrested surge wave margins reflects significant grain interaction along flow margins this sedimentological evidence for significant marginal grain interaction complements theoretical analyses iverson 1997 and other experimental data major 1996 iverson 1997 that indicate that resistance along flow margins is an important factor affecting debris flow deposition the fabric on the experimental deposits demonstrates that debris flows can develop strongly imbricate particle orientation that mimics fabric developed during fluvial deposition particle shape and local stress fields appear to have more control over fabric development than does general depositional process other criteria in addition to particle orientation are needed to discriminate mass flow from fluvial gravel deposits and to unravel depositional history

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

shallow water volcaniclastic deposition in the early archean panorama formation warrawoona group eastern pilbara block western australia abstract the panorama formation of the 3 3 3 5 ga warrawoona group in the eastern pilbara block western australia is a regional unit composed largely of silicified felsic volcaniclastic sandstone and tuff deposited at the close of a major period of early archean felsic volcanism it is divided into eight lithofacies a volcaniclastic tuff and tuff breccia b chert clast conglomerate c well bedded tuff and tuff breccia d cross stratified volcaniclastic sandstone e laminated and cross laminated siltstone f banded chert g silicified evaporite and h felsic lava wave formed features occur in lithofacies b c d and e the eight lithofacies occur in four facies associations 1 sequences of banded chert with lesser amounts of tuff representing clastic starved probable basinal environments 2 sequences of siltstone with variable amounts of tuff representing lower shoreface and prodelta environments 3 sequences of cross stratified sandstone with one or more other facies representing upper shoreface fan delta front and braided stream environments and 4 sequences of turbiditic and ash fall tuff representing subaqueous settings under the direct influence of pyroclastic volcanism in the north pole dome the panorama formation is developed directly over basalt and consists of shoaling upward sequences from base to top of associations 1 2 and 3 interpreted as sedimentation in prograding fan delta complexes flanked laterally by shoreface environments in other areas associations 3 and 4 are developed over thick sections of felsic lava and breccia of the duffer formation felsic volcanism controlled sediment composition sediment size and the rate of sediment supply to surrounding epiclastic systems deposition took place under regionally developed shallow water to subaerial conditions in which deep water distal facies were not well developed despite the abundance of wave influenced shallow water environments evidence for tides is rare probably because deposition took place under regionally microtidal marine conditions

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

geology and diamond distribution of the 140 141 kimberlite fort a la corne central saskatchewan canada abstract the cretaceous age fort a la corne falc kimberlite province comprises at least 70 bodies which were emplaced near the edge of the western canadian interior seaway during cycles of marine transgression and regression many of the bodies were formed during a marine regression by a two stage process firstly the excavation of shallow but wide craters and then subsequent infilling by xenolith poor crater facies subaerial primary pyroclastic kimberlite the bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin but high volume pyroclastic kimberlite deposits each body is composed of contrasting types of kimberlite reflecting different volcanic histories and therefore are considered separately the 140 141 kimberlite is the largest delineated body in the province estimated to have an areal extent below glacial quaternary sediments in excess of 200 ha the infilling of the 140 141 crater is complex resulting from multiple phases of kimberlite the central part of the infill is dominated by several contrasting phases of kimberlite one of these phases is a primary pyroclastic airfall mega graded bed up to 130 m in thickness the constituents grade in size from very fine to coarse macrocrystic kimberlite through to a basal breccia the mega graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe a second different phase of kimberlite is interpreted as representing a younger nested crater within the mega graded bed centrally located thicker intersections 450 m of this younger kimberlite may indicate a vent for the kimberlite crater the thickness of the mega graded bed increases with proximity to the younger kimberlite in the study area macrodiamond minibulk sample grades from the mega graded bed have been obtained from nine large diameter drill holes located within the northwest part of the body from an area of 20 ha which represents approximately 10 of the currently modeled kimberlite outline diamond grade increases with depth within the mega graded bed and also increases within the same unit towards the centrally positioned younger kimberlite macrodiamond sample grades vary from low at the top of the mega graded bed to considerably higher grades near the base total sample grade per drill hole varies from moderate near the vent feature to lower grades 200 300 m from the vent feature macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega graded bed there is a strong correlation between depth and increased stones per tonne and a similar correlation between stones per tonne and proximity to the vent feature the data supports the emplacement model of the mega graded bed and in turn this information is useful in understanding the macrodiamond distribution within this bed

emplacement and reworking of cretaceous diamond bearing crater facies kimberlite of central saskatchewan canada in central saskatchewan canada kimberlites were emplaced into cretaceous marine and nonmarine clastic sediments core recovered from one drill hole that intersects kimberlite smeaton fac uk core 169 8 was selected for an integrated study involving sedimentology volcanology mineralogy geochemistry palynology micropaleontology organic petrology and radiometric age determination only crater facies kimberlite has been observed there is no indication of the locations of feeder dikes four varieties of kimberlite occur all originating from subaerial volcanism 1 fluvial reworked kimberlite 2 diamondiferous kimberlite lapillistone air fall deposits 3 kimberlite olivine crystal tuff air fall deposits and 4 diamondiferous marine wave reworked kimberlite within the multiple primary eruptive phases of the kimberlite air fall deposits the volcanic style changed upward with time from violent strombolian to more explosive volcanism the bulk of the volcanism formed conformable air fall deposits on terrestrial sediments of the cantuar formation resulting in the development of positive relief tephra cones subsequent marine transgression associated with the westgate formation partially beveled the top of the cone the kimberlite air fall deposits contain microdiamonds 5 to 25 m in diameter the maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits although originally derived from magma at high temperatures did not thermally affect entrained surficially derived clasts or the country rock during emplacement the chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile at least two episodes of kimberlite volcanism occurred in the middle and late albian paleontologically assigned a u pb perovskite radiometric age of 101 1 2 2 ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post middle albian and pre late albian to late albian

growth of phreatomagmatic explosion craters a model inferred from suoana crater in miyakejima volcano japan abstract a subvertical cross section of a maar diatreme volcano is exposed in the wall of the a d 2000 caldera on miyakejima volcano the maar suoana is one of the lateral vents of this volcano and it was inferred to be formed in the 7th century the subvertical wall of the a d 2000 caldera truncated the suoana maar crater at its center revealing the near comlete cross section of this small maar diatreme volcano exposed in the cross section are a 400 m wide maar crater an associated tuff ring with a maximum thickness of 20 m a diatreme extending vertically to a depth of about 220 m from the floor of the maar crater and a feeder dike connected to the base of the diatreme the depth of the diatreme structure is about 260 m from the original ground surface the outline of the diatreme resembles an upward opening funnel with an almost vertical wall below 560 m asl and an upward flaring wall above 560 m asl coarse grained volcanic breccia fills the diatreme the deposits of which can be divided into 6 units based on lithological and structural characteristics the upper half of the diatreme is filled with landslide deposits mainly derived from the surrounding crater wall the bottom of the diatreme is occupied by massive explosion breccia some coherent blocks were detached from the wall of the diatreme and preserved in the diatreme fill the y shaped cross sectional geometry of the suoana diatreme is the result of a combination of an underground subsidence in the lower part of the diatreme and the surface landslide in its upper part the inwardly inclined stratification of pyroclastic rock units and development of many small faults in the diatreme filling breccia indicate successive collapse and deformation of these materials within the diatreme during the eruption the upper part of the diatreme was formed by the landslides of the crater wall induced by the subsidence of the crater floor discharge of tephra from the bottom of the diatreme caused infill subsidence which induced sliding of the inner wall of the crater as a result the topographic diameter of the crater became much larger than that of the diatreme itself the tephra ring surrounding the crater consists mainly of pyroclastic fall deposits

experiments on deaerating granular flows and implications for pyroclastic flow mobility 1 granular flows were generated by the release of beds of particles in various fluidized states which then deaerated in a horizontal channel we describe characteristics of the flows and their deposits morphological similarities between deposits in experiments and in the field suggest that pyroclastic flow deposits form from a fluidized mixture the experiments show that slightly expanded fluidized flows are more mobile than non fluidized flows of equivalent volume and material composition they travel to a fixed distance from their source which depends only weakly on their initial degree of fluidization flows of fine particles 100 m deaerate slowly and are highly mobile pyroclastic flows commonly have large amounts of fine ash which may have a controlling influence on their high mobility

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions

eruption of kimberlite magmas physical volcanology geomorphology and age of the youngest kimberlitic volcanoes known on earth the upper pleistocene holocene igwisi hills volcanoes tanzania the igwisi hills volcanoes ihv tanzania are unique and important in preserving extra crater lavas and pyroclastic edifices they provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes cosmogenic 3he dating of olivine crystals from ihv lavas and palaeomagnetic analyses indicates that they are upper pleistocene to holocene in age this makes them the youngest known kimberlite bodies on earth by 30 ma and may indicate a new phase of kimberlite volcanism on the tanzania craton geological mapping global positioning system surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices craters and lavas the volcanoes stand 102 to 106 pa s for kimberlite attributed to degassing and in vent cooling each volcano is inferred to be the result of a small volume short lived days to weeks monogenetic eruption the eruptive processes of each igwisi volcano were broadly similar and developed through three phases 1 fallout of lithic bearing pyroclastic rocks during explosive excavation of craters and conduits 2 fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent and 3 effusion of degassed viscous magma as lava flows these processes are similar to those observed for other small volume monogenetic eruptions e g of basaltic magma

submarine crystal tuffs their origin using a lower devonian example from southeastern australia summary the submarine crystal rich volcaniclastics of the lower devonian merrions tuff contain concentrations of angular to euhedral volcanic quartz plagioclase and orthoclase lithic fragments largely altered vitriclasts are minor components and the average matrix content is 37 6 associated dacite andesite and rhyodacitic lavas have average groundmass contents of 64 2 rare shards in the graded pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions the sedimentology of the volcaniclastics suggests subsequent redeposition by cold state mass flow processes the volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption the high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed so concentrating the crystal fraction the crystal rich character of the crystal tuffs is not simply due to explosive eruption several primary and secondary factors processes could have interacted to ultimately produce the crystal rich character these being i eruption of highly crystallized magmas 65 phenocrysts ii concentration of crystals in primary eruption columns iii concentration of crystals in any resulting pyroclastic flows fine vitric ashes being elutriated out and being carried away in accompanying trailing ash clouds iv concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean and v concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine subaerial settings

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites

lithic breccias in intermediate volume phonolitic ignimbrites tenerife canary islands constraints on pyroclastic flow depositional processes abstract lithic occurrences within quaternary phonolitic ignimbrites on tenerife canary islands fall into three broad groupings 1 massive ignimbrite with large floating lithics 2 basal lithic rich bedforms and 3 matrix to clast supported lithic breccias within ignimbrite massive ignimbrite divisions commonly contain large outsized either solitary or scattered lithics which are mostly monomictic and derived locally from substrate erosion by pyroclastic density currents low and high aspect ratio lithic rich bedforms occur at the base of several ignimbrites most bedforms are asymmetric with steeper lee sides and they are buried by stratified and or massive ignimbrite internally most bedforms are massive some however exhibit an upstream inclined stratification indicating upcurrent migration of the bedform which is interpreted to record antidune like flow regimes locally derived basalt clasts are important constituents of some of these bedforms which have also been sourced through substrate erosion matrix to clast supported pebble to boulder sized lithic breccias occur at various heights within massive ignimbrite lithic breccias at or near the top of most ignimbrites contain vent derived lithics and record a late flux of lithic material from the vent during a sustained pyroclastic flow event extensive boulder sized breccias at the top of two ignimbrites are interpreted to record the onset of caldera collapse late in the pyroclastic flow forming eruptions some breccias contain boulders that are oriented with long axes transverse to flow indicating late stage tractional transport thin bedded matrix supported lithic breccias or lithic trains and stratified ignimbrite are commonly developed adjacent to or immediately above topographic highs these facies laterally merge into massive ignimbrite infilling topographic lows massive ignimbrite is interpreted to be deposited from a high particle concentration hindered settling zone inverse grading and long axis parallel to bedding imbrication of lithics within the thin bedded breccia facies are interpreted to record deposition under modified grain flow conditions a variety of depositional processes are thus recorded and the relationship between lithic breccias ignimbrite facies and topography indicates that there were spatial variations in depositional mechanisms at any one instant the vertical sequence of bedform stratified ignimbrite massive ignimbrite characterises the lower part of several ignimbrites and is interpreted to record increasing rates of deposition from a sustained current multilayered lithic breccias at various heights within massive ignimbrite are a form of clast compositional zonation and reflect temporal changes in supply flow competence and capacity