comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

basic elements of maar crater deposits in the hopi buttes volcanic field northeastern arizona usa maar craters are volcanically produced but they are preserved as sedimentary deposits that reflect 1 the depositional environment into which these volcanoes erupted and 2 post eruptive sedimentary environments in the surrounding area study of maar crater deposits in the hopi buttes volcanic field arizona show that volcanoes there erupted into a broad playa and their craters were subsequently filled by complex sedimentary packages of reworked eruption ejecta laminated lacustrine carbonates and clastic sediment including reworked tuff from outside the craters data from the hopi buttes deposits are used to formulate a model for crater sedimentation in playa settings applicable to interpretation of other maar crater deposits

reconstructing paleoenvironment eruption mechanism and paleomorphology of the pliocene pula maar hungary abstract pula maar is a partially eroded pliocene maar diatreme volcano part of the mio pliocene bakony balaton highland volcanic field the surficial remnant of the maar diatreme volcano consists of 1 a distinct depression with a thick post eruptive lacustrine alginite sediment infill interbedded with coarse grained volcaniclastic sediments 2 a narrow marginal zone inside the depression consisting of primary pyroclastic rock units that are interpreted to be partly collapsed and subsided blocks of entire sections from the tephra ring formerly surrounding the maar crater depression and 3 coarse grained volcaniclastic debris flow deposits closely associated with the collapsed primary pyroclastic rock units in the marginal zone the presence of coherent lava rocks below the crater fill units their distribution pattern and their association with scoriaceous beds indicate that after the maar diatreme forming phreatomagmatic explosive activity small 100 m scale scoria and or spatter cones erupted in the maar crater these cones are the likely source of the lava flows that partially filled the maar crater basin the widespread dm to m thick basaltic sand and or silt units at the base of the post eruptive crater filling sedimentary succession are interpreted to be reworked volcaniclastic material from the intra maar scoria spatter cones as well as from the tephra ring based on comparative analyses of 53 core descriptions this study reveals that the original maar crater basin was larger than previously suggested the deep level of the maar crater is reconstructed to be a northeast southwest elongated depression currently forming a c 50 m deep basin geomorphological considerations suggest that most of the phreatomagmatic pyroclastic rocks are composed of base surge and tephra fall deposits around the deep maar depression these allochthonous rock units form a 50 400 m wide zone of proximal tuff ring sequences the formation of this zone is inferred to be a result of a combination of syn eruptive subsidence due to mass deficit in the rigid triassic dolomite basement caused by the phreatomagmatic explosions as well as post eruptive subsidence of the crater and diatreme filling successions due to diagenetic compaction the facies in the centre of the maar lake is a soft laminated alginite mainly botryococcus colonies diatom frustles calcium carbonate crystals clay minerals in the section exposed in the pula open cast mine a single turbiditic layer is present this layer originated in a landslide which possibly could have been caused by either syn eruptive earthquake and or a sudden post eruptive subsidence event of the diatreme fill

specific gravity characteristics of recent volcaniclastic sediment implications for sorting and grain size analysis specific gravity of water saturated surface dried individual grains and bulk sieved samples of pumice lapilli ash and slightly vesiculated rock fragments produced by recent eruptions at mount st helens washington as well as the composition of sediment derived from this material indicate two major departures in the character of volcaniclastics from more familiar quartzo feldspathic sediment these are 1 a three to five fold variation in the specific gravity of volumetrically important detrital grains as opposed to a less than 5 variation in 99 of the volume of average quartzo feldspathic sandstone and 2 significant variation in specific gravity that is inversely related to grain size within clast populations of the same composition this latter observation is attributed to the larger volume of vesicles particularly non interconnected vesicles with increasing grain size sorting in volcaniclastic sediment therefore is not only a function of depositional process environment and post de

green river laminites does the playa lake model really invalidate the stratified lake model proponents of the playa lake model have proposed deposition of most of the green river formation microlaminated carbonates including oil shales in lakes that were not perennially stratified meromictic however there is a variety of evidence favoring a meromictic depositional environment 1 close similarity of much of the lamination to varves in modern meromictic lakes 2 evidence that hydrologic events favoring development of meromixis chemical stratification occurred prior to deposition of major accumulations of oil shale 3 mutually exclusive distribution of fossil nekton especially fish and normal lacustrine benthos including mollusks and 4 analogy with a quaternary playa that became a meromictic lake following increased inflow the playa lake model is untenable for the typical fish bearing kerogen rich microlaminated sediments these laminites were probably deposited in a large ectogenic meromictic lake a chemically stratified lake that formed when increased fresh water inflow drowned a saline playa complex

cenozoic deformation and mesozoic abandoned spreading centre in the tagus abyssal plain west of portugal results of a multichannel seismic survey the seismic stratigraphy of the sedimentary section that fills in the tagus abyssal plain is specified by means of new multichannel seismic data six sedimentary sequence units are identified these ranging from late jurassic to recent have been dated by extrapolation of deep sea drilling project dsdp and ocean drilling program odp drilling results and from sampling by dredging and submersible surveys based on this stratigraphic calibration and other geophysical data a geodynamic evolution can be established for the tagus abyssal plain faulting and folding in these sedimentary units clearly indicate that both the portuguese margin and the adjacent oceanic crust were deformed by a miocene compressive event an oxfordian kimmeridgian rifting in the lusitanian basin and on the portuguese margin may herald a sea floor spreading in the tagus abyssal plain the continent ocean boundary is marked by landward dipping reflectors similar to those observed along the canadian margin the oceanic structure of

interdune deposits of the namib sand sea abstract the namib sand sea of southwestern africa is dominated by large north south trending linear dunes interdune areas however comprise about 50 of the area of the sand sea and may consist of exposures of the pre dune surface or sediments deposited between the dunes four major types of interdune deposits are identified and described 1 1 coarse poorly sorted aeolian sands 2 2 localized calcareous lacustrine deposits 3 3 silty deposits of ephemeral rivers that have flooded into the sand sea margins 4 4 sabkhas and salt marshes near the coast the preservation potential of these sediments is poor except where they are buried and protected by dunes or stabilised by high groundwater levels in coastal areas

miocene phreatomagmatic volcanism at tihany pannonian basin hungary abstract a late miocene 7 56 ma maar volcanic complex tihany maar volcanic complex tmvc is preserved in the pannonian basin and is part of the bakony balaton highland volcanic field base surge and fallout deposits were formed around maars by phreatomagmatic explosions caused by interactions between water saturated sediments and alkali basalt magma carrying peridotite lherzolite xenoliths as well as pyroxene and olivine megacrysts subsequently nested maars functioned as a sediment trap where deposition built up gilbert type delta sequences at the onset of eruption magma began to interact with a moderate amount of groundwater in the water saturated sand as eruption continued phreatomagmatic blasts excavated downward into limestones providing access to abundant karst water and deeper to sandstones and schist both providing large amount of fracture filling water at the surface this wet eruption led to the emplacement of massive tuff breccias by fall surge mudflow and gravity flow deposition the nature of the tmvc maar eruptions and their deposits appears to depend on the hydrological condition of the karst and or fracture filling aquifer which varies seasonally with rainfall and spring runoff the west and east maar volcanoes of tmvc are interpreted to represent low water input from the karst and or fracture filling aquifer summer vent whereas the east maar is interpreted to have formed when abundant karst and or fracture filling water was available spring vent

eruptive and depositional history of a pliocene tuff ring that developed in a fluvio lacustrine basin kissomlyo volcano western hungary abstract kissomlyo volcano is a pliocene erosion remnant of an alkaline basaltic tuff ring belonging to the little hungarian plain volcanic field late miocene shallow subaqueous fluvio lacustrine sand and mud units underlie sub horizontally bedded lapilli tuff and tuff beds with an erosional contact the pyroclastic units a sequence up to 20 m thick constitute a semi circular mound with gentle

phreatomagmatic vs magmatic eruptive styles in maar diatremes a case study at twin peaks hopi buttes volcanic field navajo nation arizona the hopi buttes volcanic field hbvf is located on the colorado plateau northern arizona in this miocene volcanic field the erosion level increases southward allowing the study of maar diatreme volcanoes from top posteruptive crater infill and ejecta ring to bottom lower diatreme the twin peaks volcanic complex consists mostly of two hills north peak and south peak with thick lavas at their summits and pyroclastic rocks underneath in the hbvf such volcanic remnants have received little scientific attention so far despite their relative abundance our field observations allow us to interpret the north and south peaks as remnants of two maar diatreme volcanoes which evolved into lava lakes filling the craters within the complex we distinguish four volcanic units from unit 1 at the bottom to unit 4 at the top on the basis of the field description of the deposits and the componentry measurements we suggest that unit 1 is phreatomagmatic unit 2 is phreato strombolian with mixed phreatomagmatic and strombolian characteristics unit 3a is phreato hawaiian with mixed phreatomagmatic and hawaiian characteristics unit 3b is hawaiian formed by lava fountains and unit 4 consists of lava lakes filling the maar craters there is therefore a progressive evolution from a purely phreatomagmatic eruptive style which excavated the craters and diatremes and partly filled them to magmatic explosive to nonexplosive eruptive styles which filled the maar craters up to the pre eruptive surface we discuss traditional criteria used to distinguish phreatomagmatic from magmatic eruptive styles in ultramafic to mafic maar diatreme volcanoes

preservation potential of ultrabasic volcanic sand in an arid intracontinental setting will the hopi buttes maar diatreme field be preserved in the rock record abstract petrographic study of modern sand samples collected near hopi buttes volcanic field northeastern arizona underscores the low preservation potential of mafic minerals and ultrabasic volcanic detritus even in an arid intraplate tectonic setting the miocene pliocene hopi buttes volcanic field has been interpreted as a maar diatreme and scoria cone field in which phreatomagmatic eruptions took place in a playa lake basin the tephra necks dikes flows and maar crater fill are now sources of sediment found in local ephemeral streams once the volcanic field is eroded away the main record of its existence will be as a provenance component in the sand it is now shedding the locally derived volcanic components include monocrystalline pyroxene glass shards scoria and a narrow range of ultrabasic volcanic rock fragments thin section point counts 450 grains on four sieved sand fractions from each of 7 sites downstream 0 2 4 8 12 21 and 31 km from one margin of the volcanic field provided the following results 1 the proportion of volcanogenic to total sand grains decreases from an initial value of about 40 at a rate of about 0 8 per kilometer downstream from the source rocks over the first 30 km of stream length 2 dilution by sand eroded from other local source rocks rather than physical grain size reduction or chemical dissolution drives the diminution of volcanogenic grain proportions detritus sourced from the hopi buttes volcanic field has a very low preservation recognition potential despite the arid climate and stable tectonic setting far removed from a plate margin it follows that maar diatreme volcanism is generally underrepresented in the sedimentary record

pin stripe lamination a distinctive feature of modern and ancient eolian sediments abstract pin stripe laminations are a distinctive feature of modern and ancient eolian sediments in sets of eolian ripple or translatent strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples in sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits the textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination the pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones the cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post depositional history of the rock pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems they may prove useful in the recognition of ancient eolian sediments

iberian plate kinematics a jumping plate boundary between eurasia and africa the rotation of iberia and its relation to the formation of the pyrenees has been difficult to decipher because of the lack of detailed sea floor spreading data although several models have been proposed1 7 here we use detailed aeromagnetic measurements from the sea floor offshore of the grand banks of newfoundland to show that iberia moved as part of the african plate from late cretaceous to mid eocene time with a plate boundary extending westward from the bay of biscay when motion along this boundary ceased a boundary linking extension in the king s trough to compression along the pyrenees came into existence finally since the late oligocene iberia has been part of the eurasian plate with the boundary between eurasia and africa situated along the azores gibraltar fracture zone

peperite as a record of early stage phreatomagmatic fragmentation processes an example from the hopi buttes volcanic field navajo nation arizona usa abstract peperite is the product of near surface magma wet sediment mingling some peperites are inferred to record frozen coarse mixing stage fuel coolant interactions fcis and hence can be used in attempts to determine specific parameters related to phreatomagmatic fragmentation such as water magma mass ratios confining pressures and vent region hydrology two lamprophyric dikes in the miocene pliocene hopi buttes volcanic field arizona contain ellipsoidal regions of dilation buds associated with peperite these peperite rimmed zones occur within identifiable stratigraphic horizons and at known depths beneath the elevation of the local pre eruptive surface the presence of both fluidal and blocky juvenile clasts combined with physical aspects of the sediment lamprophyre contacts suggest that coarse mixing stage fci was a potential fragmentation mechanism within these systems confining pressures calculated for sites of single buds range from 0 6 to 1 0 mpa average inferred water magma ratios within peperite bodies at the time they formed vary from 0 09 to 0 16 consistent with experimental values for water melt mixing ratios necessary for explosive heat transfer a number of factors may have inhibited further explosive activity in these peperite forming mixing events including anisotropic saturation of the host sediment low confining pressures and water melt ratios and damped explosive interactions resulting from increased density viscosity and surface tension of sediment water coolants nevertheless these deposits record localized thermohydraulic interactions between magma and wet sediment and provide valuable information about the geometry and mixing ratios present during peperite formation

syneruptive and intereruptive lithofacies in lacustrine environments the cretaceous beolkeum member wido island korea abstract the cretaceous beolkeum member was deposited in a lacustrine environment affected by explosive volcanism and a number of syneruptive lithofacies are intercalated with lacustrine mudstones the syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features which can be grouped into four syneruptive lithofacies assemblages sla 1 to sla 4 from bottom to top primary syneruptive lithofacies are represented by welded massive lapilli tuff in sla 1 and a normally graded lapilli tuff in sla 2 in sla 1 sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water resulting in the formation of a welded texture and columnar joints in sla 2 unsteady pyroclastic density currents rapidly disintegrated mixed with ambient water and transformed into high density turbidity currents depositing thick normally graded lapilli tuff resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions when volcaniclastic sediments were continuously supplied into the lake by debris flows a volcaniclastic fan can be formed sla 1 resulting in a coarsening upward trend and progradational geometry in case of relatively small amounts of volcaniclastic sediment supply turbidity currents would be a main depositional process depositing a series of normally graded tuff on the primary syneruptive lithofacies sla 2 showing a fining upward trend in sla 3 and sla 4 there are only a few cm thick normally graded tuff reflecting minor volcaniclastic sediment supply however overlying normally graded sandstones in sla 3 showing a coarsening upward trend and a thick normally graded sandstone in sla 4 suggest favorable conditions for the generation of the sediment gravity flows probably due to an increase in volcaniclastic sediment supply after eruptions

diachronous evolution of late jurassic cretaceous continental rifting in the northeast atlantic west iberian margin 1 regional 2 d seismic reflection profiles outcrop and borehole data are used to characterize the evolution of deep offshore sedimentary basins in southwest iberia alentejo basin the interpreted data indicate the bulk of late jurassic earliest cretaceous subsidence occurred in the present day continental slope area as shown by 1 significant thickening of synrift strata basinward from a slope bounding fault system sfs west of which the total thickness of sediment can reach more than 9 0 km and 2 relatively thin mesozoic strata east of the sfs where thickening of synrift units against principal faults is limited five principal regressive events and their basal unconformities reflect tectonic uplift and relative emersion in proximal basins which were located on the rift shoulder to subsiding tilt blocks west of the sfs these regressive events are correlated with major rift related events occurring on the deeper margin direct comparisons with the peniche basin of northwest iberia reveal that significant portions of the iberian lower plate margin were uplifted and eroded during the last stages of continental rifting this process was repeated at different times and in different areas as the locus of rifting and continental breakup migrated northward as a result two distinct rift axes are recognized in west iberia a first axis extending from the porto basin to the alentejo basin and a second axis located on the outer proximal margin north of 38 30n in addition the sfs delimited 1 prograding deposits of cretaceous paleogene age and 2 late cenozoic deposits draping the modern continental slope these latter facts demonstrate that on lower plate passive margins the relative position of the continental slope is established during the final rifting episode s preceding continental breakup

pliocene subaqueous fans and gilbert type deltas in maar crater lakes hopi buttes navajo nation arizona usa maar craters of the mio pliocene hopi buttes volcanic field of arizona formed within a broad playa system and accumulated a variety of lacustrine sedimentary deposits many craters initially held isolated groundwater fed lakes ephemeral streams crossing the playa entered some of the lake filled craters and built coarse grained gilbert type deltas and subaqueous fans along the margins of these craters the small coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits however the local production of coarse grained volcanic sediment low gradients in the local stream catchment steep subaqueous relief and the small size of the receiving basins resulted in a unique combination of features cone shaped subaqueous fans initially formed at the mouths of incised feeder streams the fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density modified grain flows gravelly gilbert type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream they are concave lakeward mimicking the underlying crater wall topography complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds they suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes bed specific carbonate alteration is common and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate saturated lakewaters during between flood periods of high net evaporation

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

dissected paleotopography and base level changes in a triassic fluvial sequence at one stratigraphic level in petrified forest national park in arizona fluvial mudrocks of the chinle formation are truncated by areally extensive deep scours that are filled dominantly by pedogenically modified mudrocks color differences between paleosols developed on scoured and scour fill deposits facilitate recognition of the dissected interval the mudstone filled scour system records a period of channel incision and widespread gullying caused by lowered base level although it is difficult to document subsidence rates in the chinle basin the tectonic setting suggests that the enormous scour system could be the result of fluctuations in regional thermal activity during late triassic time similar scour and fill should occur in other fluvial sequences however because they both truncate and are filled by mudrocks identification can be difficult recognition of them is important because they are indicators of base level fluctuations and thus are useful in unraveling the depositional history of a basin

depositional architecture of a maar pitted playa sedimentation in the hopi buttes volcanic field northeastern arizona u s a abstract primary and reworked tephra in the mio pliocene bidahochi formation of northeastern arizona was produced by and partly preserved in maar volcanoes and related hydrovolcanic structures the temporal and geometric relationships among lithofacies of the bidahochi formation illustrate the dynamic interplay between volcanism and sedimentation in an essentially closed sedimentary system maar volcanism in the low relief basinal environments of the bidahochi appears to have resulted in little net aggradation both because the total amount of magma erupted was relatively modest and because the large maar volcano craters produced areas of lowered base level within the basin that formed sediment sinks for much of the newly erupted tephra the volcanically eroded maar craters commonly contain deposits of lakes multiple stream channels and eolian dunes they formed an integral part of the basin s overall depositional environment and affected sedimentation in the playa lake stream systems beyond the vents both during and following the major eruptive period

sedimentary characteristics and interactions among volcanic terrigenous and marine processes in the late permian kuishan member eastern block of the north china craton abstract understanding the variability of depositional environment and depositional processes of the late permian kuishan member north china craton is crucial for unraveling the evolution of sedimentary basin and epicontinental sea in shandong tectonic domain the kuishan member in the eastern block of the north china craton formed in association with general regression of an epicontinental sea is composed of abundant volcanic deposits and nearshore sedimentary deposits three lithofacies assemblages together with eleven lithofacies are identified in the study area including volcanic deposits of the turbidity lithofacies assemblage epiclastic deposits of the nearshore lithofacies assemblage and mixed volcaniclastic and epiclastic sedimentary deposits of the terrestrial lithofacies assemblage these alternating volcanic epiclastic lithofacies assemblages illustrate the depositional environments induced by volcanic activities and nearshore wave influence syn eruptive lithofacies in the shallow water area were formed by high density turbidity currents and ash fall into water introduced by rising sea levels and high rainfall due to volcanic eruptions to the north whereas thin bedded shallow marine silty mud developed in the distal area inter eruptive lithofacies were represented by tuffaceous mudstone deposited in an exposed and oxidized setting due to falling sea level whereas epiclastic nearshore deposits accumulated by wave or current flow regimes paleocurrent measurements show that the predominant regression direction changes from uniform southwest to divergent southwest and southeast during volcanic inter eruptive periods the kuishan succession therefore can be regarded as a volcano related sequence in which the input of volcaniclastic and epiclastic deposits were significantly modulated by controls of volcanism with frequently changing coastline position of the epicontinental sea and alternating depositional environments

the influence of complex intra and extra vent processes on facies characteristics of the koala kimberlite nwt canada volcanology sedimentology and intrusive processes the koala kimberlite northwest territories canada is a small pipe like body that was emplaced into the archean koala granodiorite batholith and the overlying cretaceous to tertiary sediments at 53 ma koala is predominantly in filled by a series of six distinct clastic deposits the lowermost of which has been intruded by a late stage coherent kimberlite body the clastic facies are easily distinguished from each other by variations in texture and in the abundance and distribution of the dominant components from facies analysis we infer that the pipe was initially partially filled by a massive poorly sorted matrix supported olivine rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe forming eruption this unit is overlain by a granodiorite cobble boulder breccia and a massive poorly sorted mud rich pebbly sandstone these deposits represent post eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim the crater then filled with water within which 20 m of non kimberlitic wood rich silty sand accumulated representing up to 47 000 years of quiescence the upper two units in the koala pipe are both olivine rich and show distinct grain size grading these units are interpreted to have been deposited sub aqueously from pyroclastic flows sourced from one or more other kimberlite volcanoes the uppermost units in the koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes

cerro xalapaxco an unusual tuff cone with multiple explosion craters in central mexico puebla abstract the xalapaxco tuff cone is located on the northeast flank of la malinche stratovolcano in central mexico an unusually large number 10 of explosion craters concentrated on the central and on the uphill side of the cone expose alternating beds of stratified surge deposits and massive fall deposits the morphology of the cone and the characteristics of its deposits point to the involvement of significant quantities of groundwater during its eruption the phreatomagmatic eruptions which led to the cone s formation pierced an alluvial fan whose source is a glacially carved canyon near the summit of la malinche volcano the large canyon was cut during repeated glacial episodes the last of which ended ca 8500 years ago the present alluvial fan mostly consists of reworked glacio fluviatile andesite dacite material from la malinche rising magma encountered substantial amounts of groundwater within the limestone basement and in overlying intercalated pyroclastic and glacio fluviatile deposits of the alluvial fan short lived phreatomagmatic eruptions produced surge and airfall deposits xenoliths found in the cone beds are composed of dacite and andesite clasts limestone chert and rare ignimbrite fragments no juvenile material could be unequivocally identified but is represented most probably by porphyritic dacite similar in texture and composition to la malinche lavas the multiple craters were formed as a response to changes in water and magma supply during the short lived eruption hence the locations where ideal magma water ratios existed to fuel phreatomagmatic explosions shifted in time and space analysis of diameter depth ratios of the craters indicates that the activity shifted from the center of the cone to its periphery in the west due to the configuration of the hydrographic environment more groundwater flowing from la malinche was available from the fan on the uphill side than below the cone at later stages of the eruption the apparently anomalous position of the tuff cone on the slopes of a stratovolcano in a presently dry environment can be explained by more humid climatic conditions prevailing at the time of eruption

transition zone between the upper diatreme and lower diatreme origin and significance at round butte hopi buttes volcanic field navajo nation arizona round butte is a small but complex miocene diatreme that crops out 190 m below the pre eruptive surface in the southeastern part of the hopi buttes volcanic field erosional remnants consist of a diatreme 170 190 m in diameter of which the central 130 150 m is well exposed in a massif featuring 20 30 m high sub vertical cliffs and a 50 cm thick basanite dike field mapping allowed us to define three main groups of pyroclastic rocks in the diatreme undisturbed beds disturbed beds and non bedded rocks pyroclastic rocks range in grain size from coarse tuff to tuff breccia and in componentry from juvenile rich to lithic rich with a dominance of heterolithic lapilli tuffs rocks from the undisturbed bedded pyroclastic group are present above an unconformity found all around the massif whereas the disturbed bedded and the non bedded pyroclastic groups are always found below it this unconformity was previously understood as the contact between the upper and the lower diatremes the undisturbed beds above the unconformity indeed compose the upper diatreme but the assemblage of non bedded rocks invasive columns and disturbed beds residual columns below it is not typical of the lower diatreme instead they represent a transition zone between the upper and lower diatremes such a transition zone also occurs in other diatremes it is important genetically and we propose to add it to the general model of maar diatreme volcanoes

primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of jeju island korea jeju island is a quaternary shield volcano built upon the yellow sea continental shelf off the korean peninsula decades of borehole drilling reveals that the shield forming lavas of the island are underlain by extensive hydrovolcanic deposits the seoguipo formation which are about 100 m thick and show diverse depositional features this study provides criteria for distinguishing between hydrovolcanic deposits formed by primary pyroclastic and secondary resedimentation processes in subaerial and submarine settings based on the observations of several selected cores from the formation five facies associations are identified including i primary hydrovolcanic deposits formed by pyroclastic surges and co surge fallouts in tuff rings facies association phtr ii primary hydrovolcanic deposits formed by surtseyan fallout and related pyroclastic transport processes in tuff cones facies association phtc iii secondary hydrovolcanic deposits formed by debris flows hyperconcentrated flood flows sheet floods and rill flows in subaerial settings facies association rhae iv secondary hydrovolcanic deposits formed in submarine settings under the influence of waves tides and occasional mass flows facies association rhmar and v non volcaniclastic and fine grained deposits formed in nearshore to offshore settings facies association nvmar the primary hydrovolcanic facies associations phtr and phtc are distinguished from one another on the basis of distinct lithofacies characteristics and vertical sequence profiles these facies differ from the secondary hydrovolcanic and non volcaniclastic facies associations rhae rhmar and nvmar because of their distinctive sedimentary structures textures and compositions the depositional processes and settings of some massive and crudely stratified volcaniclastic deposits which occur in many facies associations could not be discriminated unambiguously even with microscopic observations nevertheless these facies associations could generally be distinguished because they occur typically in packets or sequences several metres to tens of metres thick and bounded by distinct stratigraphic discontinuities and comprise generally distinct sets of lithofacies the overall characteristics of the seoguipo formation suggest that it is composed of numerous superposed phreatomagmatic volcanoes intercalated with marine or non marine volcaniclastic or non volcaniclastic deposits widespread and continual hydrovolcanic activity together with volcaniclastic sedimentation is inferred to have persisted for more than a million years in jeju island under the influence of fluctuating quaternary sea levels before effusion of the shield forming lavas extensive distribution of hydrovolcanic deposits in the subsurface of jeju island highlights that there can be significant differences in the eruption style growth history and internal structure between shelfal shield volcanoes and oceanic island volcanoes

paleomagnetic study of the messejana plasencia dyke portugal and spain a lower jurassic paleopole for the iberian plate abstract the only iberian lower jurassic paleomagnetic pole come from the central atlantic magmatic province related messejana plasencia dyke but the age and origin of its remanence have been a matter of discussion with the aim of solving this uncertainty and to go further into a better understanding of its emplacement and other possible tectonic features a systematic paleomagnetic investigation of 40 sites 625 specimens distributed all along the 530 km of the messejana plasencia dyke has been carried out rock magnetic experiments indicate psd low ti titanomagnetite and magnetite as the minerals carrying the nrm the samples were mostly thermally demagnetized most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites where samples with reversed polarities have been observed the paleomagnetic pole derived from a total of 35 valid sites is representative of the whole structure of the dyke and statistically well defined with values of pla 70 4 n plo 237 6 e k 47 9 and a 95 3 5 paleomagnetic data indicates that i there is no evidence of a cretaceous remagnetization in the dyke as it was suggested ii most of the dyke had a brief emplacement time furthermore two dyke intrusion events separated in time from it by at least 10 000 y have been detected iii the high grouping of the vgps directions suggests no important tectonic perturbations of the whole structure of the dyke since its intrusion time iv the pole derived from this study is a good quality lower jurassic paleopole for the iberian plate and v the messejana plasencia dyke paleopole for the iberian plate is also in agreement with quality selected european and north american lower jurassic paleopoles and the magnetic anomalies data sets that are available for rotate them to iberia

extensional tectonics in the jeanne d arc basin offshore newfoundland implications for the timing of break up between grand banks and iberia

rift related magmatism of the central atlantic magmatic province in algarve southern portugal abstract lower jurassic volcanic sedimentary successions of the algarve basin 198 ma are associated with the central atlantic magmatic province camp that was one aspect of pangean intracontinental rifting volcanic products of the algarve camp include subaerial lava flows pyroclastic deposits and peperites and contemporaneous sedimentation is dominated by mudstones and conglomerates often containing volcanic fragments intercalated in the volcanic sequence these lithological characteristics and associations are compatible with a facies model typical of continental basaltic magmatism volcanic rocks are fractionated low ti tholeiites enriched in large ion lithophile elements relative to high field strength elements the compositional spectrum e g ba nb zr nb is consistent with derivation of primary magmas by partial melting of heterogeneous sources initially continental lithospheric mantle progressively replaced by an asthenospheric source fractional crystallization processes occurred in magma chambers located at depths close to the crust mantle boundary from density and geobarometric constraints reflecting magma underplating during the initial stages of continental rifting sporadic occurrence of crustal xenoliths widespread evidence for basement carbonate assimilation as well as sr 87sr 86sr 0 0 7050 and oxygen 18o 7 40 isotope ratios that are higher than those typical of continental lithospheric mantle indicate that crustal contamination also contributed to the observed geochemical variations the carnian norian 228 0 to 203 6 ma or older age estimated for the basal silves sandstones suggests that the inception of rift related sedimentation preceded volcanism by 20 to 30ma these results lead us to propose a passive rifting model for the algarve sector of camp magmatism the orientation of which was conditioned by strain localization along pre existing lithospheric mechanical anisotropies

subterranean fragmentation of magma during conduit initiation and evolution in the shallow plumbing system of the small volume jagged rocks volcanoes hopi buttes volcanic field arizona usa monogenetic volcanoes have limited magma supply and lack long lived sustained magma plumbing systems they erupt once often from multiple vents and sometimes over several years and are rarely or never re activated eruptive behavior is very sensitive to physical processes e g volatile exsolution magma water interaction occurring in the later stages of magma ascent at shallow crustal depths 1 km which yield a spectrum of eruptive styles including weak to moderate explosive activity violent phreatomagmatism and lava effusion jagged rocks complex in the late miocene hopi buttes volcanic field arizona usa exposes the frozen remnants of the feeding systems for one or a few monogenetic volcanoes it provides information on how a shallow magmatic plumbing system evolved within a stable non marine sedimentary basin and the processes by which magma flowing through dikes fragmented and conduits were formed we have identified three main types of fragmental deposits 1 buds which emerge from dikes 2 pyroclastic massifs and 3 diatremes these represent three different styles and intensities of shallow depth magma fragmentation they may develop successively and at different sites during the evolution of a monogenetic volcano the deposits consist of a mixture of pyroclasts with varying degrees of welding and country rock debris in various proportions pyroclasts are commonly welded together but also reveal in places features consistent with phreatomagmatism such as blocky shapes dense groundmasses and composite clasts loaded and cored the extent of fragmentation and the formation of subterranean open space controlled the nature of the particles and the architecture and geometry of these conduit structures and their deposits

a pleistocene pyroclastic poor maar from central anatolia turkey influence of a local fault on a phreatomagmatic eruption in central anatolia the late pleistocene narkoy maar formed on a fault zone a 5 m thick pyroclastic succession erupted from a crater 900 m in diameter the presence of gas fumeroles hot springs and hydrothermal alteration zones indicates that the area is geothermally active the products of the maar are large blocks and lapilli to coarse ash planar bedding and cross stratification are well developed within coarse ash beds the maar deposits were generated by a single period of phreatomagmatic explosion with two episodes the first episode produced wet eruption clouds while the second one was relatively dry in nature the estimate of the volume of pyroclastic ejecta compared with the dimensions of the maar demonstrates a material deficiency which is attributed to a large volume of gas emanated during the phreatomagmatic explosion along a fault presently cross cutting the maar in addition a relatively small volume of magma intruded along the fault and mixed with meteoric water in the fault zone to produce the phreatomagmatic eruption the existence of a hydrothermal system in the fault zone is important in affecting the eruption style and evolution of narkoy maar

40ar 39ar geochronology of neogene phreatomagmatic volcanism in the western pannonian basin hungary abstract neogene alkaline basaltic volcanic fields in the western pannonian basin hungary including the bakony balaton highland and the little hungarian plain volcanic fields are the erosional remnants of clusters of small volume possibly monogenetic volcanoes moderately to strongly eroded maars tuff rings scoria cones and associated lava flows span an age range of ca 6 myr as previously determined by the k ar method high resolution 40 ar 39 ar plateau ages on 18 samples have been obtained to determine the age range for the western pannonian basin neogene intracontinental volcanic province the new 40 ar 39 ar age determinations confirm the previously obtained k ar ages in the sense that no systematic biases were found between the two data sets however our study also serves to illustrate the inherent advantages of the 40 ar 39 ar technique greater analytical precision and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field periods of increased activity with multiple eruptions occurred at ca 7 95 ma 4 10 ma 3 80 ma and 3 00 ma these new results more precisely date remnants of lava lakes or flows that define geomorphological marker horizons for which the age is significant for interpreting the erosion history of the landscape the results also demonstrate that during short periods of more intense activity not only were new centers formed but pre existing centers were rejuvenated

sedimentology and evaporite genesis in a holocene continental sabkha playa basin bristol dry lake california bristol dry lake a 155 km2 continental sabkha playa basin in the mojave desert of south eastern california is filled with at least 300 m of interbedded terrigenous clastics gypsum anhydrite and halite evaporite facies conform approximately to a bull s eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite transects through bristol dry lake from the alluvial fan to the centre of the playa reveal 1 crudely bedded alluvial fan clastics interfingering with 2 playa margin sand flat and wadi sand and silt followed by 3 gypsum anhydrite chaotic mud halite and clay of the saline mud flat and 4 salt pan halite beds terrigenous clastics were deposited in bristol dry lake by sheetflow and by suspension settling from ponded floodwater some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin thin nodular like beds of anhydrite and several types of gypsum occur across most of the playa giant hopper shaped halite cubes are suspended in saline mud flat facies suggesting that they grew displacively in brine soaked sediment just below the surface thick beds 4 m of halite in the playa centre may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods intense evaporation and brine level lowering and capillary discharge of brines the stratigraphy in the playa centre is cyclic an ideal cycle consists of 1 chaotic mud halite at the base overlain by 2 green to red clay with abundant giant hoppers and at the top 3 red clay gypsum and anhydrite with flaser to wavy bedded sand and silt this type of cycle probably records a gradual progradation of mud flat facies over salt pans bristol dry lake sediments are nearly identical to some of the permian evaporites of the permian basin region u s a and they can serve as modern analogues for ancient sabkha facies analysis

volcanism sedimentation interaction in the campo de calatrava volcanic field spain a magnetostratigraphic and geochronological study this work focuses on the influence of cenozoic volcanism of the campo de calatrava volcanic field on the sedimentation of two small continental basins in spain argamasilla and calzada moral basins the volcanism in this area was mainly monogenetic according to the small volume volcanic edifices of scoria cones that were generated and the occurrence of tuff rings and maars a sedimentological analysis of the volcaniclastic deposits led to the identification of facies close to the vents low density dilute pyroclastic surges secondary volcanic deposits and typical maar deposits whole rock k ar dating together with palaeomagnetic constraints yielded an age of 3 11 3 22 ma for the onset of maar formation the deposition finished in the late gauss early matuyana using both techniques and previous paleontological data allowed it to be inferred that the maar formation and the re sedimentation stage that occurred in argamasilla and calzada moral basins were roughly coeval the occurrence of syn eruption volcaniclastic deposits with small thicknesses that were separated by longer inter eruption periods where fluvial and lacustrine sedimentation was prevalent together with the presence of small volume volcanic edifices indicated that there were short periods of volcanic activity in this area the volcanic activity was strongly controlled by previous basement faults that favoured magma feeding and the faults also controlled the location of volcanoes themselves the occurrence of the volcanoes in the continental basins led to the creation of shallow lakes that were related to the maar formation and the modification of sedimentological intra basinal features specifically valley slope and sediment load

desert stream sequences in the sinai peninsula abstract desert sedimentary sequences of late quaternary age mainly subrecent were studied in several stream segments of wadi el arish northern sinai egypt morphologically these stream systems can be divided into three types confined open and terminal floodplains the confined and open floodplain sequences consist of repeated flood originated sedimentation units each around 50 cm thick an idealized sequence begins with mud clasts or either sand or pebble bearing plane beds reflecting the upper flow regime during peak flow stage of the flood these are overlain by cross bedded sands and climbing ripples lower intensities of the flow result in sequences beginning with cross stratified sands cracked mud layers represent the final desiccation stage of the flood in comparison with th confined floodplain units the open up to thousands of meters wide floodplain units are thinner 20 50 cm the horizontal strata are less developed and the mud layers are thicker the terminal floodplain which in the modern examples is a flat a few kilometers wide is the area where the remaining flood energy dissipates and therefore the resultant sedimentation units axe very thin usually less than 20 cm and consist of either finely laminated massive mud or cross laminated fine sand the wadi systems studied are in close geographic relationship with eolian dune fields nevertheless the wadi sequences consist of waterlaid units only

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

structure and evolution of the galicia interior basin atlantic western iberian continental margin abstract an industrial multichannel seismic reflection data set is presented on the galicia interior basin g i b a sedimentary basin that has been poorly studied until now seismic stratigraphic units defined in other areas of the western iberian continental margin as well as some deeper reflections can also be identified within this basin although they show some particular characteristics the g i b is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs western banks the structural framework of the area consists of nnw sse trending normal faults cross cut by ne sw transfer faults the inception of the g i b probably ocurred during triassic times within the late jurassic early cretaceous rifting phase it probably experienced several extensional episodes the most intense one being during valanginian times evidence for compressional tectonic activity in this basin during cenozoic times has also been found when comparing the g i b with the adjacent deep galicia margin a gap in the timing of the main extensional events as well as some differences in the structural styles have been found in a wider regional context an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western iberian margin is consistent with a northward propagating rift for the late jurassic early cretaceous phase

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

the 1982 eruptions of el chichon volcano mexico 3 physical properties of pyroclastic surges two major pyroclastic surges generated during the 4 april 1982 eruption of el chichon devastated an area of 153 km2 with a quasi radial distribution around the volcano the hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation field evidence indicates voidage fraction of 0 99 in the surge cloud with extensive entrainment of air thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud the dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10 20 characterized by stoss side erosion and lee side deposition of 1 10 cm reversely graded laminae a systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals resulting in loss of at least 10 25 of the cloud mass due to winnowing out of fines during surge emplacement greatest fractionation from the 1 0 0 0 grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud extensive pumice rounding indicates abrasion during bed load transport flow of pyroclastic debris in the turbulent surge cloud was by combination of bed load and suspended load transport the surges are viewed as expanding pyroclastic gravity flows which entrain and mix with air during transport the balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence which persisted to the distal edge of the surge zone

unravelling the magmatic system beneath a monogenetic volcanic complex jagged rocks complex hopi buttes az usa the jagged rocks complex is the eroded remnant of the plumbing systems of closely spaced monogenetic alkaline volcanic centres in the southern hopi buttes volcanic field az usa it contains different clinopyroxene populations with distinctive textures and geochemical patterns in the northwestern part of the complex which exposes the best developed system of conduits most of the clinopyroxenes consist of large to medium sized resorbed cores overgrown by euhedral rims type 1 small moderately resorbed greenish cores with the same overgrown rims type 2 and phlogopite as an accessory phase by contrast in the southern part of the complex the majority of clinopyroxenes are euhedral with oscillatory zonation type 3 and are accompanied by minor euhedral olivine the differences between these mineral assemblages indicate a composite history of crystallization and magmatic evolution for the two parts of the complex governed by different mechanisms and ascent patterns from a single source at 50 km depth 16 kbar the northwest system preserves a high pressure assemblage that cooled rapidly from near liquidus conditions suggesting direct ascent from the source to the surface at high to moderate transport rates average 1 25 m s by contrast the southern system represents magma that advanced upward at much lower overall ascent rates stalling at times to form small volume mid crustal storage zones e g sills or a network of sheeted intrusions this allowed the re equilibration of the magma at lower pressure 30 km 8 kbar and led to nucleation and growth of euhedral clinopyroxene and olivine phenocrysts

40ar 39ar geochronology of the neogene quaternary harrat al madinah intercontinental volcanic field saudi arabia implications for duration and migration of volcanic activity abstract new 40 ar 39 ar ages based on incremental heating techniques for groundmass separates of 25 samples are presented for the harrat al madinah volcanic field part of harrat rahat in the north western part of the arabian plate this area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 ad field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions in addition to local more explosive eruptions the lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite benmoreite and trachyte that occur mainly as domes tuff cones and occasionally as lava flows previous k ar dating shows that the harrat al madinah lava flows and associated domes comprise seven units spanning an age range of ca 1 7 ma recent the new 40 ar 39 ar age determinations confirm to a great extent the previously obtained k ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units however the 40 ar 39 ar plateau ages show that volcanism in this area began in the neogene 10 ma and continued to recent with the most voluminous eruptions occurring in the quaternary neogene volcanism occurred in at least three pulses around 10 5 and 2 ma whereas quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1 90 ma recent thus the whole duration of volcanic activity in the harrat al madinah 10 ma recent appears much longer than that previously identified the longevity of volcanism in the same part of the moving arabian plate and absence of evidence for uni directional migration of volcanic activity indicate that there is no fixed plume beneath this region the nnw trending distribution of the volcanic vents is parallel to the red sea and suggests their origin is related to periodic extensional episodes along the reactivated red sea fault system

lava sediment interaction in desert settings are all peperite like textures the result of magma water interaction abstract this study reports on lava sediment interaction in dry depositional settings focusing on the early cretaceous volcano sedimentary sequence in the huab basin nw namibia as a detailed example here an active aeolian sand sea erg system was progressively engulfed by lavas of the etendeka flood basalt province 133 ma bp this volcanic flooding resulted in 1 the unprecedented preservation of large parts of the active dune system 2 the development of a variety of sediment interlayers and 3 a record of excellent examples of the dynamic interaction between lava and aeolian sand the lava sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava sediment contacts at small scales peperite like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands such interaction could involve lava cascading down the front of dune faces or active aa style flow fronts bulldozing into and oversteepening dune faces causing rapid influx of sand into the active flow front resulting breccia horizons have been found up to 5 m thick locally the aeolian sand is baked to a quartzite by the hot lava in contrast on other surfaces the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction in places wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with however examples from arid environments demonstrate that peperite forming processes are diverse and may not always require water therefore it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two this leaves the presence or lack of water as an issue for interpreting the processes by which the peperite formed

margin segmentation prior to continental break up a seismic stratigraphic record of multiphased rifting in the north atlantic southwest iberia a dense grid of multichannel 2d seismic profiles tied to borehole dredge and outcrop data are used to analyze the multiphased rifting structural architecture and rift locus migration across the southwest iberian margin in the study area three distinct sectors show different structural evolutions from the late triassic to the late jurassic earliest cretaceous the three structural sectors are distinguished by 1 the presence of incipient tilt blocks on the inner proximal margin which denotes limited syn rift subsidence 2 developed tilt blocks on the outer proximal margin 3 evidence of significant fault related subsidence on the outer proximal margin during the middle jurassic followed by an oxfordian tithonian berriasian rift phase leading to seafloor spreading and 4 marked crustal stretching on the distal margin where highly rotated tilt blocks overlain by thick late triassic to late jurassic units are observed this work demonstrates that significant subsidence occurred in southwest iberia several millions of years prior to the latest jurassic earliest cretaceous extensional episode leading to continental breakup the magnitude of early subsidence episodes approaches that of the last subsidence pulse preceding continental breakup across the southwest iberian margin the observed structural sectors differ from each other in terms of the age of rift climax of syn rift strata we interpret the multiple extensional pulses recorded in southwest iberia as resulting not only from continental rifting between iberia and newfoundland but also between nova scotia and morocco thus it is considered that pre breakup units in the deep offshore basins of iberia comprise multiple rift related sequences whose distribution and relative thickness depends on local subsidence rates on the diachronous northward migration of rifting and on the relative crustal stretching experienced by individual sub basins

stokes surfaces and the effects of near surface groundwater table on aeolian deposition abstract stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near surface water tables stokes 1968 a water table forms a downward limit of scour through the cohesion of damp or wet sand near water table and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand air interface study of modern analogues reveals that stokes surfaces exist in a variety of depositional settings including a coastal offshore prograding sand sea jafurah saudi arabia a coastal onshore prograding sand sea guerrero negro mexico and a continental sand sea white sands new mexico usa these modern analogues indicate that our concept of stokes surfaces must be broadened to include the following i modern analogues for stokes surfaces described here cover areas on the order of 25 km2 these may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas ii stokes surfaces occupy a continuum in scale from local to extensive and erosional surfaces of different magnitude may be stacked closely in the sediments iii stokes surfaces although erosional in nature are commonly associated with deposits both above and below the stokes bounding surface which plainly reveal the influence of a near surface groundwater control on wind sedimentation moreover the erosional relief of the bounding surface itself as well as other features reveals the influence of a groundwater table iv stokes surfaces may be diachronous representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface v stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments including interdunes tidal flats lagoons beaches lakes and non aeolian sabkhas finally modern examples from different depositional settings suggest that while most stokes surfaces have many features in common such as erosional ridges due to early cementation there are some features which may with further study be revealed to be distinctive of an individual depositional setting

stratigraphic sedimentologic and petrologic record of late miocene subsidence of the central oregon high cascades pliocene to holocene volcanics of the central oregon high cascades largely confined to an intra arc graben bury older volcanic centers early high cascade volcanism is recorded by late miocene volcanics and sediments of the deschutes formation that accumulated in a fluvial basin east of the cascades aggradation in the adjacent fluvial basin occurred episodically when pyroclastic debris choked streams deschutes basin aggradation commenced near 7 4 ma and was synchronous with extension influenced cascade magmatism dominated by basalts and basaltic andesites initial subsidence of the cascades occurred at about 5 4 ma in response to plate margin processes structural isolation of the basin from ignimbrites and volcanism induced sedimentation resulted in widespread development of superimposed paleosols in the basin subsequent mafic lavas erupted near the eastern margin of the initial depression were truncated by east stepping down to the west faults at about 5 3 ma

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

dikes sills and stress regime evolution during emplacement of the jagged rocks complex hopi buttes volcanic field navajo nation usa abstract the dikes and related intrusions formed below small volcanoes in volcanic fields are remnants of the simplest volcanic plumbing systems their geometry is controlled by interaction of magma driven cracks with country rock and reveals regional structural and stress patterns at the time of their emplacement the shallow stress field however may change during the time an intrusion complex is emplaced in response to addition or removal of magma and country rock during associated surface eruptions the jagged rocks complex in the miocene hopi buttes volcanic field navajo nation arizona is exposed 300 350 m below the pre eruptive surface it comprises a group of generally nw se striking dikes punctuated in places by buds a saucer like intrusion larger pyroclastic massifs and a diatreme we made measurements of 13 dikes divided into 172 segments with thicknesses from 8 to 122 cm mean 43 cm and lengths of 60 to 780 m several sills and inclined sheets in places are thicker than dikes having mean thicknesses of 48 cm and 73 cm respectively dikes typically show straight parallel and en echelon patterns while sills and inclined sheets are curved the northwestern dikes differ from the rest in containing large mafic crystals and are inferred to have been emplaced after the others we find that the strike of the overall complex dikes and other sheets elongate massifs and aligned sub cylindrical bodies reflects a crystalline basement control that is evident throughout hopi buttes over the period of the complex s emplacement local stress patterns were not stable we infer that excavation of deep maar craters and perhaps the construction of a scoria cone at the surface modified the local stress patterns to favor emplacement of sills and en echelon dikes later in the complex s evolution in contrast to simple straight dikes as the complex first formed

monogenetic volcanoes fed by interconnected dikes and sills in the hopi buttes volcanic field navajo nation usa although monogenetic volcanic fields pose hazards to major cities worldwide their shallow magma feeders 500 m depth are rarely exposed and therefore poorly understood here we investigate exposures of dikes and sills in the hopi buttes volcanic field arizona to shed light on the nature of its magma feeder system shallow exposures reveal a transition zone between intrusion and eruption within 350 m of the syn eruptive surface using a combination of field and satellite based observations we have identified three types of shallow magma systems 1 dike dominated 2 sill dominated and 3 interconnected dike sill networks analysis of vent alignments using the pyroclastic massifs and other eruptive centers e g maar diatremes shows a nw se trend parallel to that of dikes in the region we therefore infer that dikes fed many of the eruptions dikes are also observed in places transforming to transgressive ramping sills estimates of the observable volume of dikes maximum volume of 1 90 106 m3 and sills minimum volume of 8 47 105 m3 in this study reveal that sills at hopi buttes make up at least 30 of the shallow intruded volume 2 75 106 m3 total within 350 m of the paeosurface we have also identified saucer shaped sills which are not traditionally associated with monogenetic volcanic fields our study demonstrates that shallow feeders in monogenetic fields can form geometrically complex networks particularly those intruding poorly consolidated sedimentary rocks we conclude that the hopi buttes eruptions were primarily fed by nw se striking dikes however saucer shaped sills also played an important role in modulating eruptions by transporting magma toward and away from eruptive conduits sill development could have been accompanied by surface uplifts on the order of decimeters we infer that the characteristic feeder systems described here for the hopi buttes may underlie monogenetic fields elsewhere particularly where magma intersects shallow and often weak sedimentary rocks results from this study support growing evidence of the important role of shallow sills in active monogenetic fields

evolution and hydrological conditions of a maar volcano atexcac crater eastern mexico abstract the atexcac maar is located in the central part of the serdan oriental lacustrine playa basin in the eastern mexican volcanic belt it is part of a dispersed and isolated monogenetic field consisting of maar volcanoes basaltic cinder cones and rhyolitic domes atexac is a maar volcano excavated into pyroclastic deposits basaltic lava flows and the flanks of a cinder cone cluster which itself was built on a topographic high consisting of limestone it has an ene trending elliptical shape with beds mostly unconsolidated deposits that dip outward at 16 22 the atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water temporal migration of the locus of the explosion and periodic injection of new magma were important controls on the evolution of the maar crater variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions producing different eruptive conditions with fluctuating water magma interactions deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli this could suggest differential degrees of fragmentation due to inherited previously acquired fragmentation and or relative distance to the locus of explosions initial short lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma drier explosions progressed downward and or laterally northward sampling subsurface rock types particularly intrusive limestone and andesitic zones as well as localized altered zones n ne caused by repetitive injection of basaltic magma a final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation we infer the aquifer was formed by fractured rocks predominantly andesitic lava flows and limestone rocks andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area these local hydrogeological conditions contrast with those at nearby maar volcanoes where the water for the magma water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer producing tuff rings with a much lower profile than atexcac

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

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

from unthinned continent to ocean the deep structure of the west iberia passive continental margin at 38 n abstract the crustal and lithospheric mantle structure at the south segment of the west iberian margin was investigated along a 370 km long seismic transect the transect goes from unthinned continental crust onshore to oceanic crust crossing the ocean continent transition oct zone the wide angle data set includes recordings from 6 obss and 2 inland seismic stations kinematic and dynamic modeling provided a 2d velocity model that proved to be consistent with the modeled free air anomaly data the interpretation of coincident multi channel near vertical and wide angle reflection data sets allowed the identification of four main crustal domains i continental east of 9 4 w ii continental thinning 9 4 w 9 7 w iii transitional 9 7 w 10 5 w and iv oceanic west of 10 5 w in the continental domain the complete crustal section of slightly thinned continental crust is present the upper ucc 5 1 6 0 km s and the lower continental crust lcc 6 9 7 2 km s are seismically reflective and have intermediate to low p wave velocity gradients the middle continental crust mcc 6 35 6 45 km s is generally unreflective with low velocity gradient the main thinning of the continental crust occurs in the thinning domain by attenuation of the ucc and the lcc major thinning of the mcc starts to the west of the lcc pinchout point where it rests directly upon the mantle in the thinning domain the moho slope is at least 13 and the continental crust thickness decreases seaward from 22 to 11 km over a 35 km distance stretched by a factor of 1 5 to 3 in the oceanic domain a two layer high gradient igneous crust 5 3 6 0 km s 6 5 7 4 km s was modeled the intra crustal interface correlates with prominent mid basement 10 15 km long reflections in the multi channel seismic profile strong secondary reflected pmp phases require a first order discontinuity at the moho the sedimentary cover can be as thick as 5 km and the igneous crustal thickness varies from 4 to 11 km in the west where the profile reaches the madeira tore rise in the transitional domain the crust has a complex structure that varies both horizontally and vertically beneath the continental slope it includes exhumed continental crust 6 15 6 45 km s strong diffractions were modeled to originate at the lower interface of this layer the western segment of this transitional domain is highly reflective at all levels probably due to dykes and sills according to the high apparent susceptibility and density modeled at this location sub moho mantle velocity is found to be 8 0 km s but velocities smaller than 8 0 km s confined to short segments are not excluded by the data strong p wave wide angle reflections are modeled to originate at depth of 20 km within the lithospheric mantle under the eastern segment of the oceanic domain or even deeper at the transitional domain suggesting a layered structure for the lithospheric mantle both interface depths and velocities of the continental section are in good agreement to the conjugate newfoundland margin a 40 km wide oct having a geophysical signature distinct from the oct to the north favors a two pulse continental breakup

a small alluvial crevasse splay abstract a crevasse splay deposited on the floodplain of the clarence river in northeastern new south wales australia by a small ephemeral tributary provides a guide to the geometry and sedimentary structures of crevasse splays in general and to the processes affecting their final form the splay consists of a main lobe of sand with smaller sand fingers that end in slipfaces around its edges water seeping out of the channel reworks part of the splay into a gently thinning sheet small scale facies models derived from different parts of this splay explain the origins of solitary planar cross sets and gently inclined bedding in some ancient crevasse splay sandstones

shallow seated explosions in the construction of the motukorea tuff ring auckland new zealand evidence from lithic and sedimentary characteristics at least 52 eruption centres are scattered within the 360 km2 auckland volcanic field avf motukorea now an island in the waitemata harbour is one of 39 avf volcanoes that experienced a phreatomagmatic explosive phase before a magmatic phase the volcano erupted through a 200 300 m thick consolidated mudstone sandstone sequence of the miocene waitemata group which overlies the waipapa terrane greywacke basement detailed field descriptions of the sedimentary characteristics of the early phreatomagmatic deposits were carried out along with examination of lithics the ejecta ring deposit comprises 55 to 60 vol lithics of which waitemata group fragments constitute approximately 90 vol whereas 10 vol are waipapa fragments suggesting a dominance of shallow fragmentation the sedimentary characteristics of the stratigraphic sequence at motukorea suggest a dominance of wet surges at the beginning of the eruption with progression into drier sequences upwards this is reflected in increasing inter bedded juvenile pyroclast dominated fall deposits up sequence these characteristics are attributed to the changing hydrogeological conditions within the diatreme and the host rocks these findings shed light on the eruption dynamics of phreatomagmatic eruptions through consolidated rocks in the avf and enable the depiction of a scenario of future eruptions within the field in similar substrates

proximal braidplain deposits in the middle devonian malbaie formation of eastern gaspe quebec canada the middle devonian malbaie formation of eastern gaspe canada comprises sharply alternating conglomerate and sandstone units their petrography suggests derivation from the same source but palaeocurrents indicate different dispersal systems for the sand and gravel the principal conglomerate facies is horizontally stratified with well developed imbrication characteristic of deposition on a high energy proximal braidplain minor cross stratified conglomerate shows well defined size sorting attributed to avalanching down foresets and sorting within minor bedforms on bar tops the sandstone units mainly comprise erosion surfaces overlain by mudstone intraclasts alternating with lineated low angle to horizontally stratified sandstone or trough cross stratified sandstone the sandstone units were deposited on a proximal braidplain with highly variable discharge but the absence of calcrete indicates that dry periods were not prolonged the malbaie is the coarsest uppermost formation of a devonian clastic wedge formed during the acadian orogeny the uniformity of facies association and palaeocurrents particularly in conglomerate units indicates derivation from a broad upwarp to the south this implies that the acadian uplands resulted from straight compression in contrast to the localized uplifts formed by strike slip faulting during the carboniferous

evidence for the overflow origin of the grand canyon abstract the colorado river s course through grand canyon and across the topographic high imposed by the kaibab plateau represents one of the most well known transverse drainages only four processes allow a river to flow across a topographic obstruction antecedence superimposition piracy and overflow since powell first described the canyon in 1875 various authors have repeatedly proposed variations of all four mechanisms to explain the formation of the grand canyon and this paper does not review or try to refute this prior scholarship this paper s purpose rests in providing a detailed evaluation of evidence associated with overflow upstream across and downstream of the east kaibab monocline paleodivide upstream of the paleodivide we re interpreted the bidahochi formation the older units of this formation represent a fluctuating system between playa and lacustrine deposition fed by multiple localized tributaries entering a topographically barred basin the younger bidahochi units signify the abrupt arrival of an additional drainage with a distinctive geochemical signature into the basin roughly 6 8 ma supported by three lines of evidence 1 an order of magnitude increase in depositional rates from 3 m ma in the older units to 50 m ma in the younger units that buried the paleo landscape at least locally up to an elevation of at least 2250 m 7380 ft 2 new evidence of an elevated 87sr 86sr ratio in the younger units when compared to the older units and the modern colorado river and 3 fossil evidence of the existence of large freshwater lakes and rivers in the bidahochi basin during deposition of the younger units newly re evaluated evidence at the east kaibab paleodivide focuses on a southern bend of the colorado river s path across the kaibab plateau as well as the confluence of the colorado and the little colorado rivers at the junction of the kaibab plateau and cedar ridge both lines of evidence at the paleodivide focus on the impact of topography on grand canyon incision and both lines of evidence are consistent only with an overflow process evidence downstream of the paleodivide includes the abrupt arrival of river water with an elevated 87sr 86sr ratio followed by colorado river sediment into a series of successively lower downstream basins that integrated via overflow new 10be 9be geochemistry data support the relative timing of deposition of the bidahochi formation followed soon after by deposition of the bouse formation downstream of the grand canyon we conclude that newly presented and re evaluated available evidence supports an overflow process for the colorado river s transverse section across the kaibab plateau ultimately resulting in incision of the grand canyon

phreatomagmatic eruptions through unconsolidated coastal plain sequences maungataketake auckland volcanic field new zealand abstract maungataketake is a monogenetic basaltic volcano formed at 85 89 ka in the southern part of the auckland volcanic field avf new zealand it comprises a basal 1100 m diameter tuff ring with a central scoria spatter cone and lava flows the tuff ring was formed under hydrogeological and geographic conditions very similar to the present the tuff records numerous density stratified wet base surges that radiated outward up to 1 km decelerating rapidly and becoming less turbulent with distance the pyroclastic units dominantly comprise fine grained expelled grains from various sedimentary deposits beneath the volcano mixed with a minor component of juvenile pyroclasts 35 vol subtle lateral changes relate to deceleration with distance and vertical transformations are minor pointing to stable explosion depths and conditions with gradual transitions between units and no evidence for eruptive pauses this volcano formed within and on 60 m thick plio pleistocene poorly consolidated highly permeable shelly sands and silts kaawa formation capped by near impermeable water saturated muds tauranga group these sediments rest on moderately consolidated miocene aged permeable turbiditic sandstones and siltstones waitemata group magma water fuelled thermohydraulic explosions remained in the shallow sedimentary layers excavating fine grained sediments without brittle fragmentation required on the whole the resulting cool wet pyroclastic density currents were of low energy the unconsolidated shallow sediments deformed to accommodate rapidly rising magma leading to development of complex sill like bodies and a range of magma water contact conditions at any time the weak saturated sediments were also readily liquefied to provide an enduring supply of water and fine sediment to the explosion loci changes in magma flux and or subsequent stabilisation of the conduit area by a lava ring barrier led to ensuing strombolian and fire fountaining eruption phases future eruptions in littoral environments around auckland are likely to be of this type producing base surges that rapidly decrease in energy over short runout distances 1 km

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

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

william river an outstanding example of channel widening and braiding caused by bed load addition the lower william river in northwestern saskatchewan canada presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load a relatively narrow and deep single channel stream as it flows northward to lake athabasca the river picks up a 40 fold increase of bed load over a 27 km reach as it encounters a large dune field just south of the lake as a result of the large infusion of eolian sand the channel develops a thoroughly braided pattern while undergoing a 5 fold increase in width and a 10 fold increase in width depth ratio

sheet flow deposits of graded beds and mudstones on an alluvial sandflat playa system upper triassic blomidon redbeds st mary s bay nova scotia the 400 m of blomidon redbeds accumulated in a semi arid rift valley in the subtropics at st mary s bay these redbeds are 64 sandy mudstone playa mudflats 25 graded beds sandflats at the toes of alluvial fans 10 fissile claystone playa lakes and 1 channel sandstone stream channels flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north east as sheet flows on sandflats and playa mudflats deceleration of the sheet flows deposited graded beds 2 83 cm in thickness on the sandflats and thin layers of mud on the playas nine sequences consisting of arrangements of six lithologies compose 90 of the graded beds in order of decreasing abundance these are ripple cross laminated siltstone horizontally laminated mudstone fining upward ripple cross laminated siltstone ripple cross laminated sandstone horizontally laminated mudstone cross bedded sandstone horizontally laminated mudstone ripple cross laminated sandstone ripple cross laminated siltstone rippledrift cross laminated siltstone horizontally laminated mudstone fining upward ripple drift cross laminated siltstone cross bedded sandstone ripple cross laminated siltstone and cross bedded sandstone ripple cross laminated siltstone horizontally laminated mudstone the sheet flows perhaps up to 1 m in depth had a high concentration of suspended load deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration there are 32 thinning and fining up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone the cycles range in thickness from 1 3 to 13 3 m averaging 4 6 m each cycle is initiated by avulsion to a new active channel network on a fan gradual abandonment of the channel network produces the thinning and fining up cycle the cycles are grouped in three 60 70 m fining up megacycles upwards within each megacycle the packages of sandy mudstone compose a progressively larger proportion of the cycles each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor subsequent erosion gradually lowered the relief to yield a fining up megacycle