towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

evidence of a tectonically driven sequence succession in the middle ordovician taconic foredeep the late mohawkian through early cincinnatian ordovician trenton group carbonates and coeval siliciclastic succession of the central mohawk valley new york state were deposited in a foreland basin during the taconic orogeny development of this facies mosaic records changes in stacking patterns that took place during episodic basin deepening and simultaneous carbonate ramp shallowing along a ramp to basin transect located on the distal slope of the foreland basin differential rates of subsidence across the region produced accommodation rates that increased by an order of magnitude from west to east high rates of siliciclastic sediment supply and even higher subsidence rates resulted in a thick deep water succession in the eastern part of the study region the coeval trenton group succession in the west formed under conditions of moderate sediment supply and lower subsidence rate resulting in a net shallowing upward succession these relationships suggest that active margin tectonism was the proximate cause of the mohawk valley facies architecture

geometry spatial arrangement and origin of carbonate grain dominated scour fill and event bed deposits late jurassic jubaila formation and arab d member saudi arabia

an integrated stratinomic model for the genesis and concentration of small shelly fossil style phosphatic microsteinkerns in not so exceptional conditions abstract carbonate fluorapatite filled millimeter scale microsteinkerns are a key element of the small shelly fossil ssf fauna generally associated with the late proterozoic cambrian the purported disappearance of these fossils and this style of phosphatic preservation during the cambrian stage 2 botomian extinction has been linked to changes in ocean chemistry and or changes to the physical environment due to the evolution of metazoans as well to the changing role of metazoans in the marine environment through a qualitative meta analysis of existing literature we document a broader temporal distribution for ssf style preservation than previously recognized we examine these occurrences for common patterns of depositional conditions and we offer a conceptual model for both the origin of the phosphatic microsteinkerns and their eventual concentration in certain sediments based on known biogeochemical diagenetic processes of authigenic calcium fluorapatite precipitation phosphogenesis and subsequent concentration of phosphatic sedimentary particles this polycyclic phosphogenic condensation ppc model makes predictions as to how and where this form of preservation might occur in cambrian and non cambrian marine strata alike

modeling carbonate sequence development without relative sea level oscillations standard conceptual models propose that stratal patterns in carbonate depositional sequences are controlled predominantly by relative sea level oscillations even during greenhouse periods when amplitudes of relative sea level oscillations were small however numerical experiments with a two dimensional forward model of carbonate systems suggest that variations in sediment transport rate and carbonate productivity are capable of producing similar parasequence stacking patterns without relative sea level oscillations this result suggests two alternative end member models one driven by relative sea level change the other driven by changes in transport rate and productivity controlled ultimately by climatic factors many carbonate systems may represent some combination of these end member possibilities

facies stratigraphy and evolution of a middle ediacaran carbonate ramp khufai formation sultanate of oman the khufai formation is the oldest carbonate platform of the cryogenian to lowermost cambrian huqf supergroup a stratigraphic characterization of this unit includes detailed facies descriptions a sequence stratigraphic interpretation and evaluation of lateral heterogeneity and overall ramp evolution the khufai formation comprises one and one half depositional sequences with a maximum flooding interval near the base of the formation and a sequence boundary within the upper peritidal facies most of the deposition occurred during highstand progradation of a carbonate ramp facies tracts include outer ramp and midramp mudstones and wackestones ramp crest grainstone shoal deposits and extensive inner ramp microbially dominated peritidal deposits outcrops in the oman mountains are deep water deposits including turbiditic grainstone and wackestone interbedded with siliciclastic rich siltstone and crinkly laminite facies patterns and parasequence composition are variable both laterally across the outcrop area and vertically through time because of a combination of ramp morphology siliciclastic supply and possible syndepositional faulting the lithostratigraphic boundary between the khufai formation and the overlying shuram formation is gradational and represents significant flooding of the carbonate platform the stratigraphic characterization presented here along with the identification of key facies and diagenetic features will help further future exploration and production of hydrocarbons from the khufai formation

tsunamis in a stormy sea middle cambrian inner shelf limestones of western argentina abstract coarse grained strata deposited on continental shelves and epeiric seas in some tens of meters water depth are conventionally ascribed simply to storms of varying intensity which interrupted the settling out of suspended mud surprisingly the possibility of tsunamis is virtually never evaluated even though they might indeed should have been common in many regions portions of the middle cambrian la laja formation of the argentine precordillera formed in such a submerged epeiric platform or shelf they comprise a suite of three limestone facies variably argillaceous burrowed mudstone bioclastic grainstone and intraclastic conglomerate composed of pieces of mudstone including cemented haloes around burrows sporadically developed eocrinoid meadows and trilobite populations are recorded respectively by disarticulated ossicles and usually broken sclerites the latter indicate that generally exoskeletons became fragmented by repeated bottom turbulence hence the mudstones and grainstones show that before stabilization by calcite cementation which began beneath the sediment water interface and away from burrow margins physical reworking of the surface sediment by storms was so frequent that it did not leave a specific signature of individual events by contrast scattered lenticular beds of intraclastic conglomerate were due to anomalous brief episodes of deep scour and strong oscillatory flow from powerful wave action these conglomerates then escaped subsequent reworking because they are abruptly interbedded and encased within uniform mudstone and there are no stratigraphic patterns allochthonous material or unconformities to indicate sea level fluctuation the events that created the conglomerates were more likely tsunamis not storms they could have been unleashed by faulting elsewhere on the shelf or rifting beyond the shelf margin to the present day west to southwest those originating in the rift basin may have been diffracted by marginal shoals and attenuated by the shallowness of the platform which caused the waves to break some distance offshore in any case the study area was not affected by backwash recognition of tsunami laid beds urges a reexamination of the standard tempestite paradigm with its use of coarse grained strata as a simple measure of storm frequency and intensity

numerical forward modeling of carbonate platform dynamics an evaluation of complexity and completeness in carbonate strata abstract it is clear that carbonate strata are complex and incomplete but the nature and origins of the complexity and the origin and distribution of hiatuses remain poorly understood a three dimensional numerical forward model of carbonate platform systems has been used to investigate how variations in complexity and completeness arise in carbonate strata the model includes depth dependent carbonate production occurring as a stochastic mosaic of producing elements across a topographic surface it also includes depth dependent sediment erosion transport and deposition governed by a regional transport direction that can vary stochastically through time and by wave refraction that causes local variations in transport direction combined mosaic production and variable sediment transport create a complex distribution of prograding islands generating self organizing shallowing upward parasequences of variable thickness and variable lateral extent hiatuses occur throughout the parasequences not just at the base and top generated by nondeposition due to mosaic production and by erosional scour this erosion is triggered by variations in water depth and fetch distance created by complex island morphologies preserved strata represent between 15 and 60 of total elapsed time measured at a 2 ky resolution and much of the preserved thickness represents punctuated deposition with depositional events spanning only a few thousand years stratigraphic completeness depends on subsidence rate and sediment transport rate so that completeness generally increases with increasing subsidence rate and decreasing transport rate but shows a nonlinear relationship with subsidence rate this suggests that reliable estimates of completeness of ancient carbonate strata require more than knowledge of subsidence rate and sea level change particularly because the real situation is likely to be considerably more complex than represented in this model the stochastic production mosaic and stochastic variations in regional transport direction create complex strata whereas the self organizing island progradation process creates simpler ordered strata a combination of the two processes creates an intermediate level of complexity producing strata consisting of laterally impersistent parasequences with numerous erosion surfaces and complex stacking patterns and complex thickness distributions this has important implications for parasequence correlation in outcrop and the subsurface

spatial patterns of sediment accumulation on a holocene carbonate tidal flat northwest andros island bahamas abstract to characterize spatial patterns of sedimentation and analyze the morphology of part of the modern tidal flats of northwest andros island in the bahamas this study integrated remote sensing geographic information systems gis and carbonate sedimentology the fundamental data are a landsat tm image that has been classified to create a thematic map of eight subfacies interpreted to represent a distinct tidal flat subenvironment such as adjacent marine exposed levee beach ridge pond and algal marsh spatial statistics of the thematic map characterize the patterns of sediment accumulation quantitative analysis highlights several interesting results concerning subfacies character and distribution 1 of the eight mapped subfacies low algal marsh is most widespread representing 27 5 of the total area whereas exposed levee beach ridge is the least widespread accounting for 10 of the area 2 the patches of different subfacies have different shape complexities with low algal marsh high algal marsh and mangrove ponds being the least complex and exposed levee beach ridge being the most complex 3 markov chain analysis suggests that lateral transitions between different subfacies are highly ordered and 4 frequency distribution of subfacies patch area and lacunarity gap size distribution data exhibit power law relationships over several orders of magnitude consistent with fractal characteristics the fractal nature of patch size and gaps between facies illustrate that on this tidal flat neither the size nor the spatial distribution of subfacies has a characteristic scale this statistical behavior is consistent with the presence of self organization or emergence of pattern in the absence of a template or external forcing the statistical self organization on the tidal flat is the cumulative expression of local processes but it becomes apparent only through analysis of the whole system these results are inconsistent with models suggesting that tidal flats include a migrating complex of randomly distributed randomly sized subenvironments ancient successions that include random patterns may reflect the more pronounced influence of forces external to the sedimentary system instead of an absence of those forces

storm deposits and storm generated coarse carbonate breccias on a pelagic outer shelf south east basin france uppermost jurassic limestones of the south east basin france are organized into four facies associations that were deposited in four distinct zones 1 peritidal lagoonal limestones 2 bioclastic and reefal limestones 3 pelagic lime mudstones 4 lime mudstones calcarenites coarse breccias calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave induced combined flow in subzone 4a both vertical and lateral transitions from lime mudstone calcarenite to breccia indicate in situ brecciation under wave cyclic loading breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor deposits in subzone 4a record relatively long periods 400 kyr of sedimentation below wave base alternating with periods of deposition under wave induced currents and periods of in situ deformation in this zone storm waves were attenuated by wave sediment interaction and wave energy was absorbed by the deformation of soft sediment with reference to present day wave attenuation water depths in this zone ranged between 50 and 80 m landwards of the attenuation zone in zone 3 storm waves were reduced to fair weather wave heights storm wave base was not horizontal and became shallower landwards as a consequence water depth and wave energy were not linearly related on a small area of the seaward edge of subzone 4a cobbles were removed by traction currents and redeposited in subzone 4b there they formed a 100 m thick wedge which prograded over 3 km and was built up by the stacking of 5 to 20 m thick cross stratified sets of coarse breccia this wedge records the transport and redeposition of cobbles by a high velocity unidirectional component of a combined flow the increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel like structure of the downwelling in the gulf formed by the basin in more distal subzone 4c the hydrodynamic effect of wave induced currents was quasi permanent and brecciation by wave sediment interaction occurred only episodically this indicates that seawards of the attenuation zone hydrodynamic storm wave base was deeper than mechanical storm wave base uppermost jurassic carbonates were deposited and soft sediment deformed on a hurricane dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration located seawards of a zone of sedimentation below wave base

poisson processes of carbonate accumulation on paleozoic and holocene platforms abstract cambro ordovician carbonate lithofacies units in the elbrook and conococheague formations exposed at wytheville virginia as well as those in many other phanerozoic peritidal sequences exhibit exponential thickness frequency distributions that is occurrence frequency decreases exponentially with linear increase in unit thickness such distributions are characteristic of waiting times between independent poisson events this relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions abundances of different lithologies both as net stratigraphic thickness and as number of occurrences also decreases exponentially among successively rare sediment types with each lithology being about 60 as plentiful as the next more abundant rock type relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all wytheville units of 0 48 m a relation indicating that thickness distribution is independent of facies type similar relations are apparent for the horizontal extent of carbonate sediment bodies from the holocene florida bahama platform areal extents of individual facies units lithotopes are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this holocene surface lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies with each being about 70 as extensive the next more abundant sediment type relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2 2 103 km2 for all florida bahamas lithotopes we consider several numerical models of stochastic carbonate accumulation although not demonstrably unique scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits such simulations of poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers to the as yet unknown degree that peritidal lithofacies area and thickness are correlated data from paleozoic and holocene platforms suggest that carbonate units should exhibit length height ratios of approximately 105 given the decimeter scale over which facies are designated in most paleozoic peritidal successions these relations predict mean lateral extents on the order of several tens of kilometers a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in paleozoic carbonate sequences

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

quasi planar laminated sandstone beds of the lower cretaceous bootlegger member north central montana evidence of combined flow sedimentation abstract lower shoreface to shallow shelf strata of the bootlegger member of the lower cretaceous blackleaf formation are characterized by interbedded sandstone and mudstone sandstone beds are characterized by a peculiar planar lamination showing a subtle although perceptible undulation spacing to height ratios of the undulation are generally 100 or more typically the undulation shows no evidence of lateral accretion but only vertical aggradation and as a result most beds consist of a single laminaset aspects of quasi planar laminated beds indicate single event storm sedimentation and paleocurrent data indicate offshore sediment transport by its sedimentary characteristics and its similarity with a bed configuration generated in un experimental wave duct quasi planar lamination is produced by high energy combined flows this style of stratification should be common in the shallow marine stratigraphic record and its recognition should aid in interpreting high energy combined flow depositional events

reconstruction of a 3700 ma transgressive marine environment from isua greenland sedimentology stratigraphy and geochemical signatures abstract the rare preservation of a stratigraphy in the northwest of the eoarchean isua supracrustal belt greenland demonstrates the world s earliest recognised marine transgression stratigraphically lowest is the 3710 ma solvang volcanic formation of picrites basalts and basaltic andesites with arc like geochemical signatures close to its erosional top this formation s volcanic rocks show increase in k2o with depletion of na2o depletion in sr with no alteration in eu abundance and 18ovsmow increase to 16 interpreted as subaerial weathering at 3700 ma the sedimentary snowpatch formation was deposited unconformably on the solvang volcanic formation locally the unconformity is marked by the laterally discontinuous conglomerate member dominated by rounded quartz clasts but also with clasts of basalt and andesite up to 10 cm across layered chert and ultramafic rocks the conglomerate member carries sparse detrital zircons the youngest of which are 3710 ma the conglomerates are succeeded by the sandstone and dolostone member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross bedding with scoured layers tempestite breccia beds and very rare stromatolite horizons the sandstone and dolostone member is succeeded by the 3695 ma bif member the snowpatch formation quartz dolomite sandstones and interlayered stromatolites have seawater like ree y trace element patterns variably modified by a negligible to significant influx of a terrigenous component trace element signatures such as ti v ratios and normalising ree y data to a 3710 ma weathered volcanic rock rather than post archean average shale suggest this component was derived from a juvenile arc source resembling the basement solvang volcanic formation the detailed field and geochemical evidence such as positive correlation between fe mg and pr yb in the snowpatch formation dolostones and bif banded iron formation indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation during a marine transgression over a weathered 3710 ma volcanic arc basement followed by bif deposition in deeper water thus by 3700 ma shallow water exposed settings had already been established which were the site of stromatolite development

carbonate platform dominated by peloidal sands lower ordovician la silla formation of the eastern precordillera san juan argentina the lower ordovician la silla formation of the precordillera of west central argentina is part of the west facing early palaeozoic tropical carbonate platform succession that comprises the core of the cuyania terrane up to 360 m thick it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km la silla formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert it accumulated on a more or less uniformly subsiding passive margin copious precipitation of microcrystalline calcite probably influenced by microbial activity to varying degrees led to the generation of peloids ooids and aggregates of these grains as well as small amounts of lime mud intraclasts stromatolites and thrombolites rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites bioturbation is present locally the array of carbonate rock types is grouped into eight recurring lithofacies in order of decreasing abundance i peloidal grainstone ii laminated dolostone iii intraclastic rudstone iv microbial laminite v peloidal packstone vi ooidal grainstone vii thrombolite boundstone and viii mudstone these facies represent sediments that formed solely in a shallow subtidal marine environment with no evidence of restricted conditions hypersalinity or subaerial exposure no events of eustatic sea level change are recorded by far the dominant facies is grainstone composed of well sorted fine sand sized peloids and peloidal aggregates in homogeneous tabular to gently undulating medium to thick beds cross lamination is scarce clusters of sub metre sized microbial patch reefs developed sporadically the shallow platform is envisaged to have been covered by extensive peloidal sand flats and low relief banks and little lime mud was generated the setting was probably microtidal and may not have been affected by strong trade winds it was washed by frequent relatively gentle wave action but without experiencing powerful storms in the middle member anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift faulting seaward of the platform margin the remarkably uniform peloidal grainstone composition over a broad area shows that given the appropriate combination of climate environmental and ecological factors large portions of some early palaeozoic platforms were dominated by grainy sediment and remained under well agitated conditions within fair weather wave base without distinct lateral facies differentiation or tidal flat aggradation

carbonate deposition and hydrocarbon reservoir development at the precambrian cambrian boundary the ara group in south oman abstract the ediacaran early cambrian ara group in the south oman salt basin consists of six evaporite carbonate cycles a0 a1 to a6 that record tectono eustatic sea level changes the a4 cycle developed at the precambrian cambrian boundary 542 ma it forms a single depositional sequence with evaporites deposited in a lowstand systems tract lst overlying carbonates represent an open marine shallowing upward ramp succession that developed in transgressive tst and highstand systems tracts hst the low energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high the tst facies include sulfates evaporite carbonate laminites and organic rich carbonate laminites that record initial flooding deepening of the basin and establishment of an outer ramp depositional environment carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity subsequent hst facies comprise mostly fine grained clastic carbonates and stromatolites that formed in middle and inner ramp settings these facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp sediment was redistributed in suspension by muddy turbidity currents muddy debris flows storm and shallow water currents during the late hst carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying lst evaporite unit a combination of strong tectonic subsidence and transient flooding caused significant shallow water evaporite deposition to occur not only down dip but also on top of the former carbonate platform where several hundreds of meters of evaporites accumulated the transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit the presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties distribution of organic material and carbonate mud can be linked to specific environmental conditions low physical and biogenic disturbance of sediment and stratified water mass and the sequence stratigraphic position low flux of fine grained carbonate during tst in contrast diagenetic evaporite formation has largely degraded reservoir quality in porous shallow water facies near the top of the a4c

modern shoreface and inner shelf storm deposits off the east texas coast gulf of mexico abstract the modern shoreface and inner shelf sediments off the east texas coast on the gulf of mexico were examined using sediment cores and high resolution seismic data low sediment input and low effective accommodation space resulted in the overall thinness of the modern marine sediment cover in the region erosion on the inner shelf during historic 100 yr times produced patches of sand in a predominantly mud dominated environment storm beds are scarce on the inner shelf the shoreface succession off galveston island is characterized by offshore directed crosscutting storm channels of varying sizes the existence of these channels and the relatively high frequency of storms and hurricanes that have affected the region during historical imes are inconsistent with the paucity of storm deposits on the inner shelf the offshore directed channelized flows may have been prevented from going beyond the shoreface by strong along shelf storm currents that impinged on the shoreface in contrast to the storm record on the east texas shelf the central texas shelf shows a greater frequency of and more laterally persistent storm sand beds due to greater effective accommodation space and higher sediment supply there

combined flow origin of edgewise intraclast conglomerates sellick hill formation lower cambrian south australia unusually thick coarse grained edgewise intraclast conglomerates occur at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the lower cambrian sellick hill formation south australia the intraclast beds are flat based and laterally discontinuous forming bar like structures that must have exhibited bathymetric relief of as much as 1 m the internal fabrics of these beds are variable thinner beds are dominated by flat lying intraclasts thicker beds contain both chaotic randomly oriented steeply inclined intraclasts and clusters of fan shaped vertically stacked edgewise intraclasts the sellick hill formation intraclast conglomerates are inferred to have been formed by intense storm generated combined flows on a broad subtidal carbonate ramp superimposition of wave induced oscillatory motions on geostrophic bottom flows during large storms generates short lived but exceptionally high instantaneous shear stresses in the bottom boundary layer entrainment of the relatively large intraclasts occurs through sliding rather than pivoting edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclasts during passage of waves and the chaotic nature of collisions between intraclasts moving within the boundary layer collisions between intraclasts impart a rotating moment causing intraclasts to tip up during maximum fluid shear stress lodgement or packing of clasts in vertical or steeply inclined positions occurs within scours where intraclasts can wedge between other vertically inclined clasts or where intraclasts are pinned in steep orientations by collisions with shallowly inclined intraclasts differential erosional resistance of the intraclast deposits probably led to the development of sharp lateral changes in thickness the sellick hill formation intraclast conglomerates record erosion and reworking of subtidal subfairweather wave base environments by exceptionally intense and presumably rare storm flows the intraclast horizons represent a substantial loss in stratigraphic resolution due to widespread erosion of the ramp

the cyclone winifred storm bed central great barrier reef shelf australia abstract shelf sediments were collected immediately before 8 20 january 1986 immediatly after 9 10 february 1986 and one year after 27 february 1987 cyclone winifred crossed the central great barrier reef shelf on 1 february 1986 the storm produced a normally graded mixed terrigenous carbonate storm bed more than 11 cm thick covering an area at least 1 200 km2 and extending more than 30 km offshore to water depths greater than 40 m the storm layer encompasses two lithologically distinct shelf parallel facies belts the nearshore 0 25 m depth storm bed is 311 cm thick thins offshore and grades upward from medium sand to silty clay offshore 25 43 m depth the storm bed thickens to more than 11 cm and consists of normally graded well sorted relict quartz and skeletal gravelly sands capped by a thin mud veneer the cross shelf distributions of grain size mineralogy carbonate content and carbon isotope ratios were very similar before and after winifred suggesting the storm bed did not result from offshore directed storm currents but rather in situ resuspension and settling of the shelf sand with shoreward transport of mud in contrast to storm sedimentation models which predict thinning and fining offshore winifred created a layer that becomes thicker and coarser grained in deeper water in response to cross shelf substrate changes after one year the winifred storm layer was well preserved nearshore 30 m water depth below fairweather wave base preservation may not be a matter of insulating the layer from physical reworking below fairweather wave base but of burying it where rates of sediment accumulation outpace bioturbation the effects of storms similar in size to winifred may be well concealed in ancient shelf sequences subject to rapid biological remixing of storm layers

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

premixing of magma and water in mfci experiments experimental studies have been performed to evaluate pre explosive water melt mixes with respect to explosive volcanic molten fuel coolant interaction mfci i e phreatomagmatic explosion remolten ultrabasic volcanic rock was used as a magma simulant measurement of the explosion intensity was used to determine optimal premixing conditions a well defined optimal range was found for the hydrodynamic mixing energy differential flow speed of 4 2 m s as well as for the water melt mass ratio 0 03 to 0 04 under experimental conditions the mass flux of water had a minor influence on the explosion intensity additionally transparent mixing experiments with silicon oil and inked water were carried out they indicate a direct dependence of the pre explosive water melt interface area on the explosion intensity the experimental results show that the contact conditions of water and melt required for explosive mfci may easily be established in natural volcanic systems thus explosive mfci is a probable mechanism of explosive volcanism

did anoxia terminate ediacaran benthic communities evidence from early diagenesis abstract the ediacaran oceanic redox landscape was heterogeneous where many basins had a shallow and highly dynamic chemocline above anoxic ferruginous or euxinic or low oxygen manganous waters seawater mmg ca ratio was also high promoting early diagenetic dolomitisation how the benthos responded to these conditions is fundamental to understanding their ecological dynamics here we utilise redox sensitive elements in early marine carbonate cements to investigate possible water column redox controls on the distribution and growth of the oldest metazoan communities skeletal communities in the zaris sub basin of the nama group namibia 550 547 ma grew in shallow waters where fine grained carbonate sediment often shows evidence of early dolomitisation mid ramp cloudina reefs are composed of open highly porous structures that formed multiple successive assemblages each assemblage is terminated by thin reef cements show a paragenetic sequence from synsedimentary to early marine cement and final burial which we infer were precipitated under dynamic redox conditions first acicular pseudomorphed aragonite cement formed under oxic conditions low fe and mn next the presence of iron rich dolomicrite sediment often associated with a recrystallised ferroan dolomite crust suggests that originally aragonitic or calcitic sediment and a high mg precursor cement were preferentially dolomitised dolomitisation may have been enhanced either via upwelling of deeper water anoxic ferruginous seawater or by later fluid remobilisation from adjacent shales a following mn rich calcite cement is inferred to be early marine due to its inclusion rich fibrous form and well preserved cl zonation the final blocky cement precipitated under oxic conditions probably during shallow burial the cements likely record a general shallow to deeper water transect from oxic shallow waters to low oxygen manganous waters then to oxic shallow burial conditions we hypothesize that cloudina reef communities were terminated by episodic sediment incursions during short term transgressive cycles possibly accompanied by upwelled anoxic ferruginous and dolomitising waters although the timing for this is poorly constrained more generally such incursions may have terminated ediacaran benthic communities that grew close to the chemocline

early diversification of ordovician chitinozoans on baltica new data from the jagala waterfall section northern estonia abstract chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the tremadocian and diversified during the rest of the great ordovician biodiversification event to become extinct in the devonian the early diversification of chitinozoans on the baltica palaeocontinent has remained poorly known due to preservation and collecting bias in this study we document the distribution of chitinozoans through the upper tremadocian to lower darriwilian strata in the jagala waterfall section from northern estonia in order to better understand the regional diversity patterns and biogeographic links fifty samples were studied from the 7 5 m succession of sandstones marls and carbonates revealing 47 chitinozoan species and 11 genera the lowermost productive samples attributed to the late tremadocian represent the earliest rich chitinozoan fauna from baltica combined with previous reports this shows a diversity peak in the tremadocian with balanced total diversity reaching 17 possibly followed by a decline in the floian and then a rather gradual increase through the dapingian and early darriwilian the new diversity curve shows more than three times higher values for the tremadocian and dapingian than indicated in previous studies and thus less abrupt radiation in the darriwilian the jagala section provided also new data on the regional biozonal species conochitina cucumis and cyathochitina regnelli which characterize the middle and upper volkhov and lower kunda regional stages respectively several other biostratigraphically significant taxa typical of other regions were recovered from the jagala section including conochitina decipiens and desmochitina bulla a new short ranging species laufeldochitina toilaensis sp nov is introduced having potential to become an index species for middle dapingian strata the early darriwilian assemblage from jagala includes rhabdochitina sp a with up to 2 7 mm long vesicle making it the largest chitinozoan ever reported

paleostress evolution of the northern andes eastern cordillera of colombia implications on plate kinematics of the south caribbean region 1 new tectonic field data and paleostress determinations in the eastern cordillera of colombia enable us to identify stress regimes that prevailed in the northern andes since late cretaceous times until present day these regimes were characterized by an e w to wsw ene active contraction from late cretaceous to late paleocene this direction subsequently changed to nw se and finally became wnw ese during the andean tectonic phase the characteristics and relative chronology of these regimes were deduced from field structures and stratigraphic criteria using stress inversion of fault slip data sets to evaluate our model we used the rotation poles of the south america caribbean and nazca plates relative to north america and paleogeographic reconstructions of the caribbean region to derive a kinematic model of the northern andes assuming a general correspondence between the regional convergence directions and the regional compression we built a theoretical model of the regional stress regimes based on kinematics of the northwestern south america since late cretaceous times this model reveals a major variation at the end of the paleocene when relative divergence shifted to convergence between the americas this is in agreement with our tectonic model based on independent tectonic field data inversion in the eastern cordillera this regional kinematic and tectonic change which affected the caribbean at the end of the paleocene was responsible for and consistent with the major change in stress patterns and structural style in the eastern cordillera it is also coeval with the development of a major regional unconformity in colombia

growth dynamics of neoproterozoic calcimicrobial reefs mackenzie mountains northwest canada abstract neoproterozoic buildups of the little dal group grew in a deep water epicratonic basin these kilometer scale reefs display aggradational and progradational geometries comparable to those described from phanerozoic reefs four phases of reef growth correspond to four regional shale to carbonate packages in laterally equivalent off reef strata the lower shaly part of each package is interpreted to reflect transgressive to highstand deposition the upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea level lowstand reefs nucleated at the beginning of the first major transgressive event reefs typically aggraded during transgressive intervals but could also prograde or contract likely depending on the rate of relative sea level rise versus reef growth rate reefs prograded during regressions probably owing to reduction of accommodation space they shed talus at lowstand in response to increased erosion and or progradation over unstable substrates reef growth ended with return of shallow water conditions the overall architecture of aggradation and progradation demonstrates that the neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied ecologically complex and faunally diverse buildups of the phanerozoi

amplified hazard of small volume monogenetic eruptions due to environmental controls orakei basin auckland volcanic field new zealand orakei maar and tuff ring in the auckland volcanic field is an example of a basaltic volcano in which the style and impacts of the eruption of a small volume of magma were modulated by a fine balance between magma flux and groundwater availability these conditions were optimised by the pre 85 ka eruption being hosted in a zone of fractured and variably permeable plio pleistocene mudstones and sandstones orakei maar represents an end member in the spectrum of short lived basaltic volcanoes where substrate conditions rather than the magmatic volatile content was the dominant factor controlling explosivity and eruption styles the eruption excavated a crater 80 m deep that was subsequently filled by slumped crater wall material followed by lacustrine and marine sediments the explosion crater may have been less than 800 m in diameter but wall collapse and wave erosion has left a 1 000 m diameter roughly circular basin a tuff ring around part of the maar comprises dominantly base surge deposits along with subordinate fall units grain size texture and shape characteristics indicate a strong influence of magma water and magma mud interactions that controlled explosivity throughout the eruption but also an ongoing secondary role of magmatic gas driven expansion and fragmentation the tuff contains 70 of material recycled from the underlying plio pliestocene sediments which is strongly predominant in the 2 fraction the magmatic clasts are evolved alkali basalt consistent with the eruption of a very small batch of magma the environmental impact of this eruption was disproportionally large when considering the low volume of magma involved dre 0 003 km3 hence this eruption exemplifies one of the worst case scenarios for an eruption within the densely populated auckland city destroying an area of 3 km2 by crater formation and base surge impact an equivalent scenario for the same magma conditions without groundwater interaction would yield a scoria spatter cone with a diameter of 400 550 m destroying less than a tenth of the area affected by the orakei event

understanding spatio temporal barrier dynamics through the use of multiple shoreline proxies abstract at the coast risk arises where and when static human developments are situated within dynamic surroundings barrier islands are often sites of heightened coastal risk since they frequently support substantial human populations and undergo extensive morphological change owing to their low lying form and persistence in energetic hydrodynamic and meteorological conditions using the mixed sand gravel barrier of blakeney point this study argues that to avoid an only partial understanding of coastal zone processes it is necessary to make use of multiple shoreline proxies capturing processes operating both at different timescales and different cross shore positions here five shoreline proxies were extracted from three data sources shoreline error was quantified and compared to observed shoreline change rates to establish proxy specific appropriate timescales for shoreline change analysis the map derived mean high water line at blakeney point revealed landward retreat of 0 61 m a 1 over the past 130 years with a shift from drift towards swash alignment of the barrier since 1981 over the past 24 years the high water line ridge line and vegetation line reveal proxy specific response to management regime change the termination of barrier reprofiling of the eastern section of the barrier has resulted in increased sediment release to the downdrift barrier terminus buffering retreat there at the expense of the updrift section the vegetation line represents an effective proxy for storm driven overwash with maximum shoreline retreat during surge events of 172 m illustrating a strong event driven component to barrier morphodynamics by comparison to the other proxies the lidar light detection and ranging derived mean high water line offers relatively limited insights into barrier dynamics emphasising the importance of multi proxy approaches in the face of technological advance we demonstrate the continued importance of critical attention towards the dependencies that exist between shoreline proxy selection and the processes that can be observed as a result

self organizing models of shallow water carbonate accumulation abstract the stratigraphic record of shallow water carbonate accumulation is remarkably complex the combination of a wide variety of facies elements the great abundances of individual elements that constitute even relatively short stratigraphic sections and the possibilities of both high and low frequency organization of those facies into hierarchical stratigraphic units has made shallow water carbonates one of the most closely studied and hotly debated of all sedimentary systems carbonate stratigraphy has until now been analyzed largely through a combination of field observations and simple one and two dimensional forward models of deposition herein we demonstrate the use of a cellular model of carbonate accumulation that displays emergent self organizational behavior the utility of such a model resides in its ability to generate stratigraphic patterns both vertically and spatially that are mathematically and geometrically similar to those encountered in natural systems importantly these cellular models reproduce significant components of natural stratigraphic complexity through the use of a single simple numerical algorithm it is the application of that linking algorithm across a cellular grid that produces emergent stratigraphic organization specifically simulations consisting of two modeled lithologies produce vertical records of facies composition that exhibit thickness frequency distributions similar to the negative exponential forms reported from numerous stratigraphic sections likewise area frequency distributions across simulated depositional surfaces are similar to those found to describe facies mosaics buildups patch reefs mud banks on modern carbonate platforms as such this stochastically driven cellular model produces successful simulations of shallow water carbonate stratigraphy and strongly indicates that the application of this and similar models potentially provides both a novel and an important avenue of investigation into the complex and dynamic carbonate depositional environment

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

long term controls on continental scale bedrock river terrace deposition from integrated clast and heavy mineral assemblage analysis an example from the lower orange river namibia abstract establishing relationships between the long term landscape evolution of drainage basins and the fill of sedimentary basins benefits from analysis of bedrock river terrace deposits these fragmented detrital archives help to constrain changes in river system character and provenance during sediment transfer from continents source to oceans sink thick diamondiferous gravel terrace deposits along the lower orange river southern namibia provide a rare opportunity to investigate controls on the incision history of a continental scale bedrock river clast assemblage and heavy mineral data from seven localities permit detailed characterisation of the lower orange river gravel terrace deposits two distinct fining upward gravel terrace deposits are recognised primarily based on mapped stratigraphic relationships cross cutting relationships and strath and terrace top elevations and secondarily on the proportion of exotic clasts referred to as proto orange river deposits and meso orange river deposits the older early to middle miocene proto orange river gravels are thick up to 50 m and characterised by a dominance of karoo supergroup shale and sandstone clasts whereas the younger plio pleistocene meso orange river gravels 6 23 m thick are characterised by more banded iron formation clasts mapping of the downstepping terraces indicates that the proto gravels were deposited by a higher sinuosity river and are strongly discordant to the modern orange river course whereas the meso deposits were deposited by a lower sinuosity river the heavy minerals present in both units comprise magnetite garnet amphibole epidote and ilmenite with rare titanite and zircon grains the concentration of amphibole epidote in the heavy minerals fraction increases from the proto to the meso deposits the decrease in incision depths recorded by deposit thicknesses above strath terraces and the differences in clast character size and roundness and type between the two units are ascribed to a more powerful river system during proto orange river time rather than reworking of older deposits changes in provenance or climatic variations in addition from proto to meso orange river times there was an increase in the proportion of sediments supplied from local bedrock sources including amphibole epidote in the heavy mineral assemblages derived from the namaqua metamorphic complex this integrated study demonstrates that clast assemblages are not a proxy for the character of the matrix and vice versa because they are influenced by the interplay of different controls therefore an integrated approach is needed to improve prediction of placer mineral deposits in river gravels and their distribution in coeval deposits downstream

superheavy pyrite δ34spyr δ34scas in the terminal proterozoic nama group southern namibia a consequence of low seawater sulfate at the dawn of animal life sulfur isotope analysis 34 s of well preserved carbonates spanning an 10 ma interval of the terminal proterozoic nama group reveals that disseminated pyrite is consistently enriched in 34 s relative to coeval seawater sulfate as preserved in carbonate associated sulfate cas this observation is not consistent with the current paradigm for interpreting the geologic record of sulfur isotopes which assumes that pyrite 34 s 34 s pyr will be equal to or less than co occurring cas 34 s 34 s cas due to the kinetic isotope effect of bacterial sulfate reduction bsr that favors the lighter isotope of sulfur 32 s during sulfur oxygen bond breakage although the precise mechanism of pyrite sulfur isotope enrichment is debatable our combined observations of extremely 34 s enriched pyrite low bulk rock concentrations of sulfur from cas and pyrite and high frequency fluctuations in 34 s cas and 34 s pyr throughout the nama group carbonates point to very low concentrations of sulfate in portions of the terminal proterozoic ocean the additional occurrence of 34 s enriched pyrite in contemporaneous terminal proterozoic sections from poland and canada reveal that low seawater sulfate may have been widespread in the oceans at this time however the absence of such extremely 34s enriched pyrite from well preserved coeval carbonate sections in oman suggests that such conditions were not globally uniform low geographically varied concentrations of marine sulfate in terminal proterozoic time are consistent with elevated geographically varied concentrations of reactive nonpyritized iron in marine shales recently reported for this interval suggesting that high dissolved fe ii and low o 2 persisted in a range of marine facies as late as 543 ma tens of millions of years after the origin of animals yet prior to their major diversification in early cambrian time

post eruptive development of the ukinrek east maar since its eruption in 1977 a d in the periglacial area of south west alaska maar eruptions form small initially steep walled basins that contain important archives for the climatic and palaeoenvironmental history in continental areas the two ukinrek maars in south western alaska erupted between 30 march and 9 april 1977 and are the best documented maars that have erupted in historical time this study presents a preliminary analysis of geomorphology hydrology magnetic susceptibility geochemistry and sedimentology data of a field study in august 2004 these results photographs and topographic surveys are combined for reconstructing the post eruptive evolution of ukinrek east maar within less than 30 years the initially polygonal shape of the crater with nearly vertical crater walls has developed into an almost elliptic form with slopes of 35 inclination on debris fans between a few escarpments the water table and the crater floor have risen significantly and the crater diameter depth ratio increased from 3 4 to 5 7 whereas the average height of the crater rim remained almost constant the main sub aerial resedimentation process is formation of rock falls rock slumps and scree resulting in debris flows and turbidites within the lake that is ice covered throughout about half of the year distal lake sediments consist of laminated minerogenic clayey sandy silts that document frequent turbidity currents from the linear sedimentation rate of only ca 5 mm year 1 in 17 cm long cores it may be concluded that the largest portion of the crater sediments formed within the first few months of the maar history however this has to be confirmed by future studies

pyroclastic deposits of san diego maar central colombia an example of a silicic magma related monogenetic eruption in a hard substrate

geomorphic evolution of a storm dominated carbonate ramp c 549 ma nama group namibia the well exposed hoogland member c 549 ma of the northern nama group kuibis subgroup namibia represents a storm dominated carbonate ramp developed in a foreland basin of terminal proterozoic age the ramp displays facies gradients involving updip grainstones which pass downdip into broad spatially extensive tracts of microbial laminites and finely laminated mudstones deposited above and below storm wave base trough cross bedded coarse grainstones are shown to transit downdip into finer grained calcarenites irregular microbial laminites and mottled laminites siliciclastic siltstones and shales were deposited further downdip platform growth was terminated through smothering by orogen derived siliciclastic deposits ramp morphology was controlled by several different processes which acted across many orders of magnitude millimetres to kilometres including in situ growth of mats and reefs scouring by wave produced currents and transport and infilling of coarse grained carbonates and fine grained carbonates and clastics at the smallest scale roughening of the sea floor through heterogeneous trapping and binding by microbial mats was balanced by smoothing of the sea floor through accumulation of loose sediment to fill the topographic lows within the upward propagating mat at the next scale up parasequence development involved roughening of the sea floor through shoal growth and grainstone progradation balanced by sea floor smoothing through shale infilling of resulting downdip accommodation as well as the metre scale topographic depressions within the mosaic of shoal water facies at even larger sequence platform scales roughening of the sea floor occurred through aggradation and progradation of thick carbonates balanced by infilling of the foreland basin with orogen derived siliciclastic sediments at all scales a net balance was achieved between sea floor roughening and sea floor smoothing to maintain a more or less constant ramp profile