the concentration of uranium in freshwater vadose and phreatic cements in a holocene ooid cay a method of identifying ancient water tables abstract the concentration of uranium has been measured using the fission track technique in vadose and freshwater phreatic cements from south joulters cay in the bahamas these analyses reveal dramatic differences in the u concentration of vadose 0 28 0 11 ppm and freshwater phreatic 1 80 75 ppm cements this distinctive change in concentration is proposed to result from 1 variations in the co2 of the waters of the phreatic and vadose environments and 2 selective exclusion of u during the precipitation of calcite the pco2 is important because it controls the ph and hence the concentration of co32 ions in the pore fluids as well as the amount of dissolution of the u rich aragonitic precursors for example an increas in pco2 will lower the ph and decrease the concentration of co32 present in the porewaters as the u present as the uranyl ion uo22 is mainly complexed with co32 a decrease in the concentration of co32 will not only alter the speciation of u but also increase the uo2 co3 22 co32 ratio of the fluid and hence the concentration of u in the precipitated carbonate cements the second mechanism for explaining the increase of u in phreatic cements relies on the low distribution coefficient for the incorporation of u into low mg calcite lmc the precipitation of lmc therefore leads to an increase in the concentration of u in the residual solution as fluids do not remain in the vadose zone but rather accumulate in the freshwater phreatic portion of the lens phreatic waters become relatively enriched in u compared to vadose fluids the clear difference in the u concentrations of cements in these two zones may enable the u concentration of cements to be used as an indicator of paleo water tables in ancient rocks even where other geochemical and petrographic evidences are equivocal

precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

comments and reply on precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites comment

meteoric stabilization and preservation of limestone within the late proterozoic beck spring dolomite of eastern california petrographic and geochemical study of limestone silicified carbonate and dolostone indicates that meteoric diagenetic processes and paleogeographic position played important roles in the stabilization and preservation of limestone within the pervasively dolomitized beck spring dolomite the authors conclude that beck spring marine cement allochems precipitated as aragonite and high magnesium calcite in the vicinity of saratoga spring these metastable mineralogies were dissolved eroded stabilized to calcite and replaced by silica through interaction with meteoric waters early stabilization of limestone components along the margin of the amargosa basin apparently decreased susceptibility to subsequent dolomitization

petrology petrochemistry and stromatolites of the middle to late proterozoic beck spring dolomite eastern mojave desert california the beck spring dolomite is the medial unit of the middle to late proterozoic pahrump group the oldest sequence of sedimentary rocks in eastern california stratigraphic sections of the beck sprin

enigmatic carbonates of the ombombo subgroup otavi fold belt namibia a prelude to extreme cryogenian anoxia abstract the ombombo subgroup of the otavi fold belt kaokoveld namibia preserves a succession of clastic and carbonate sediments with unusual sedimentary features the stratigraphy of these units is discussed here in detail for the first time since their initial definition with particular emphasis on the sedimentology of carbonate units early neoproterozoic shales of the beesvlakte formation equivalent to the zambian katangan copperbelt s lower roan formation host evaporitic lithologies and minor copper mineralisation the overlying dolomitic 760 ma devede formation contains carbonate platformal lithologies which are in many ways dissimilar to phanerozoic shallow water carbonates this includes unusual curl breccias sheet cavities carbonate shrubs and tepee carbonate lithologies which contain large quantities of fibrous cements curl breccias are defined here as distinctive curled intraclasts of laminated dolomite that often have shrinkage cracks in their margins and are cemented by fibrous dolomite cements fibrous cements take on two forms an early length fast fascicular optic dolomite and a later length slow phase with unit extinction the presence of overlying internal sediments the fibrous habit of these first generation cements as well as their preserved cathodoluminescent and optical character suggests that these cements originally precipitated as calcite and dolomite marine cements respectively after this initial marine calcite precipitation all components of devede formation carbonates have been mimetically dolomitised preserving original depositional fabrics combined with the presence of marine dolomite cements this style of dolomitisation is suggested to be syn sedimentary similar to that of some cryogenian dolomites suggesting unusual ocean conditions during the early neoproterozoic in particular the presence of dolomite marine cements which have been linked to ocean anoxia and high seawater mg ca conditions suggests that the onset of marine anoxia in the neoproterozoic may have occurred during the growth of the devede formation carbonate platforms prior to the sturtian glaciation this implies that glaciation may not be the sole cause for the development of marine anoxia during the neoproterozoic it is possible that marine anoxia in this southern african ocean basin may have contributed to a build up of metals in seawater perhaps pre enriching basinal fluids for large scale stratabound copper mineralisation

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

seismic velocity ratio in the crust and uppermost mantle in southeast wellington province new zealand the regional variation of the seismic velocity ratio over a 200 km long traverse has been studied by means of microearthquake surveys the wadati plot method is used with a minimum of four p and s arrivals for each of 49 earthquakes the area as a whole is found to be characterized by a value of 1 74 1 76 for earthquakes of depth 12 40 km except in a 50 km long section near wellington where is low at 1 60 this low has been attributed to the fault zones in the region a small change of is observed between the upper crust 5 km and lower crust 12 km but there is no change of between the lower crust and uppermost mantle

the genesis of dedolomites a discusion based on reactive transport modeling abstract dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of ca rich water this process has been described in the literature as either increasing preserving or decreasing porosity of the initial dolostone according to textures observed in prades ne spain dedolomitization took place due to two simultaneous processes non pseudomorphic replacement of dolomite with calcite at the wall rock of fractures and pseudomorphic replacement in the bulk rock a multicomponent reactive transport model is used to analyze the two dedolomitization processes observed diffusion of ions from the fractures was the driving force for non pseudomorphic replacement in the wall rock the dissolution of dolomite and precipitation of calcite took place in accord with a 1 2 stoichiometry as a consequence of the mineral molar volume the porosity was sealed and replacement limited to a rim on both sides of the fractures according to calculations the development of a rim took less than 500 years advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock due to the absence of empty volume for expansion replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation according to calculations pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters as the infiltrating water was also undersaturated with respect to calcite one stage of calcite dissolution postdated dedolomitization this process was responsible for the widening of open fractures where the dissolution front followed the dedolomitization front into the dolomite matrix intensive calcite dissolution was also developed following the advective flux from the top of the bed downwards drusy calcite is observed to precipitate in the pore spaces which consist of intergrains fractures and dissolution cavities this process is not predicted in detail by reactive transport calculations although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization

analysis of strong phases other than p and s from a microearthquake survey in the wellington region new zealand a few subcrustal earthquakes located by means of a microearthquake survey in the wellington region at the indian and pacific plate boundary exhibit strong phases between first p and first s arrivals these phases have been identified as having been converted from s to p sp or p to s ps at the mohorovicic discontinuities of the indian and pacific plates examples of these phases are shown here two events are selected and simple models are assumed to draw ray paths of these phases as possible explanations of mode conversion

a seismic structure study in the kaoping area southwestern taiwan the difference between s wave and s to p wave conversion s p phase arrival times is enhanced with rectilinear motion detector filtering to describe alluvial sediment thickness in the kaohsiung pingtung kaoping plains area a more complete understanding of the underground structures of the kaoping area is provided in this paper and explains why the surrounding regions in taiwan experience more earthquakes than the kaoping area data are based on seismic activity recorded by the portable array for numerical data acquisition panda for the period from 1995 to 1997 the difference between s wave and s p phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts p wave travel time residuals high frequency attenuation parameters kappa and quality factor q p q s and coda waves confirm this result we also determined the orientation of the chaochou fault using the first motion of p wave arrivals to the east of the chaochou fault stress trends southeast northwest while to the west it trends northeast southwest the change in stress trends east and west of chaochou fault suggests the presence of a highly fluid accretionary wedge in the kaoping area the chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the kaoping plains we demonstrate that these features are the reason there are relatively few earthquakes in the kaoping area the presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest kaoping coasts the peikung high acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region online material figures illustrating calculations of kappa q c p and s wave spectra q p and q s from ground motion data

dolomite formation and biogeochemical cycles in the phanerozoic summary based on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation the abundance and distribution of dolomite throughout the phanerozoic remains an enigma sometimes referred to as the dolomite problem comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it the recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective a biogeochemical approach to the dolomite problem reveals a plausible connection between phanerozoic geochemical cycles and dolomite formation in particular periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans lowered oxygen levels would have fostered a more active community of anaerobic microbes including sulphate reducing bacteria which in turn could have led to more extensive dolomitization of marine carbonates

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

differential weathering in ultramafic rocks of new caledonia the role of infiltration instability abstract ultramafic rocks of new caledonia present extensive pseudokarstic features undulations of the base of the weathering profile fractures enlarged by dissolution dolines possible influence of the infiltrative instability on differential weathering of ultramafic rocks in tropical climate is investigated here starting from the tiebaghi case for which a groundwater model is available infiltration instability is governed by non dimensional numbers i e the damkohler and peclet numbers and by the permeability contrasts between the bedrock and the different layers of the weathering profile these numbers are firstly estimated for the tiebaghi case and secondly a sensitivity study of the weathering style is achieved through numerical modeling computations are firstly carried out without the impervious laterite cover and then the effect of this cover is considered the characteristic time constant for weathering of peridotites is directly estimated at around 7 yrs from the thickness of the weathering front and the fluid velocity the results show that a permeability contrast of at least 100 between the most pervious layer in the weathering front and bedrock and a diffusivity of chemical species lower than 2 5 10 8 m 2 s are required to allow instability the tiebaghi case lies at the limit of instability and dissolution fingers develop in a time lapse of 1 ma however this requires the infiltration of nearly 800 mm of rainfall yearly which is not the case presently the development of dissolution fingers is promoted by any initial pervious zone such as fracture concentration around a faulted zone and the characteristic time for instability development drops to 0 1 0 3 ma extension of these results to other peridotite massifs in the tropical zone submitted to different temperatures and rainfalls is finally discussed

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

microbial primary dolomite from a norian carbonate platform northern calabria southern italy the origin of fine grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the norian dolomia principale of northern calabria southern italy microbial carbonates there consist of stromatolites thrombolites and aphanitic dolomites high relief thrombolites and stromatolites characterize sub tidal facies and low relief and planar stromatolites with local oncoids typify the inter supratidal facies skeletal remains are very rare in the latter whereas a relatively rich biota of skeletal cyanophycea red algae and foraminifera is present in the sub tidal facies some 75 of the succession consists of fabric preserving dolomite especially within the microbial facies whereas the rest is composed of coarse dolomite with little fabric preservation three end members of dolomite replacement fabric are distinguished type 1 and type 2 fabric retentive with crystal size 5 and 5 60 m respectively and type 3 fabric destructive with larger crystals from 60 to several hundred microns in addition there are dolomite cements precipitated in the central parts of primary cavities during later diagenesis microbialite textures in stromatolites are generally composed of thin dark micritic laminae of type 1 dolomite alternating with thicker lighter coloured laminae of the coarser type 2 dolomite thrombolites are composed of dark micritic clotted fabrics with peloids composed of type 1 dolomite surrounded by coarser type 2 dolomite marine fibrous cement crusts are also present now composed of type 2 dolomite scanning electron microscope observations of the organic rich micritic laminae and clots of the inter supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains these probably derived from the fossilization of micron sized coccoid bacteria and spheroidal ovoidal nanometre scale dwarf type bacterial forms furthermore there are traces of degraded organic matter probably also of bacterial origin the microbial dolomites were precipitated in a hypersaline environment most likely through evaporative dolomitization as suggested by the excess ca in the dolomites the small crystal size and the positive 18o values the occurrence of fossilized bacteria and organic matter in the fabric preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn sedimentary dolomite

microseismicity and tectonics at the indian pacific plate boundary south east wellington province new zealand summary the seismicity structure and tectonics of part of the north island plate boundary have been studied by means of a microearthquake traverse in the south eastern part of wellington province the short term microseismicity observed in this survey is similar to the long term macroseismicity about 200 microearthquakes have been located in a four week survey microearthquakes occur mostly within the depth range 20 40 km defining a band of activity which is fairly continuous and uniform along the strike direction of the subducted pacific plate a break in seismic activity is observed below 40 km depth however in vertical sections along the direction of the dip of the subducted plate systematic differences in microearthquake activity are observed between the supposedly locked and unlocked regions of the subducted plate the sections show further how the foci deepen progressively towards the north west focal mechanisms of 115 well located microearthquakes are examined in relation to the structure and tectonics of the region the mechanisms are related to the plate interface geometry in a manner consistent with that found for other regions of plate convergence microearthquakes in the overriding plate are associated with thrust fault mechanisms while those in the subducted plate exhibit normal faulting the stress pattern is also consistent with the tectonics of the region the t axes of the normal fault mechanisms are parallel to the dip of the subducted plate whereas the p axes of the thrust fault mechanisms are nearly in the e w direction this is consistent with the suggestion that the plates are locked in the coastal part of the region in addition small but systematic differences in the thickness and dip of the benioff zone are observed in the sw the zone is about 5 km thinner and 6 less steep than in the ne on the landward side there is a bend in the benioff zone at 25 30 km depth the dip of the zone sharply increases below this depth where it enters the region of negative gravity anomaly

development of a positive eu anomaly during diagenesis diagenetic remobilization of eu is possible under conditions of reduction to eu2 at low oxidation potential and near neutral solution producing pore waters relatively enriched in eu2 as distinct from its trivalent ree neighbours eu is reprecipitated in the 3 state in environments of high oxidation potential these processes have produced positive eu anomalies in highly oxidized pleistocene muds of the amazon deep sea fan similar occurrences should be rare but where found they do provide valuable information on local diagenetic processes and conditions

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

factors controlling evolution of karst conduits in sandy limestone and calcareous sandstone turnov area czech republic abstract karst potential is rarely studied in calcareous sandstones and sandy limestones the minimum content of carbonate necessary for the development of conduits is unknown and the mechanisms of the origins of conduits in these rocks are poorly understood this article focuses upon these questions in an important regional aquifer samples of limestones and sandstones were taken from outcrops in vertical profiles and were subjected to a rapid dissolution test with 10 hcl the degree of disintegration of the samples after dissolution of the carbonate was observed in order to estimate which portions of the rock enabled formation of karst conduits carbonate content was measured by calcimetry distribution of calcite cement and other sources of cohesion were studied by scanning electron microscopy tracer tests in the conduits were evaluated calcite is the dominant carbonate mineral and main cementing agent in the jizera fm aquifer mean content 51 however calcite content was found to be a weak predictor of karst potential most of the samples disintegrated completely if the calcite content exceeded 55 on the other hand some rocks with a calcite content up to 70 did not disintegrate due to microcrystalline amorphous silica which bound the quartz grains together and or formed a foam like supporting structure after in situ calcite dissolution the rock in karst prone zones is turned to residuum which consequently is washed out by fast flow 0 01 0 2 m s based on tracer tests microscopic study of cementing agents of the siliciclastic carbonate sediments and dissolution tests are essential for understanding the mechanisms of conduit origin in ghost rock karst

hydrogeological characterization and modelling of weathered karst aquifers applicability to dewatering operations in limestone quarries limestone aquifers represent an important part of groundwater resources in the world they are usually considered as fractured karstic rock formations however a different type called weathered or ghost rock karst aquifer has recently been highlighted in this particular type of aquifer the karst is not expressed as open conduits but consists in zones made of a residual alterite resulting from the in situ partial dissolution of limestone these particular structures are crucial in some areas because they actually govern groundwater flow and transport in the aquifer they have however never been studied from a hydrogeological perspective in this study we present a methodology to characterize and model such aquifers results show that the ghost rock features are usually organized into networks and that their hydraulic properties are variable within each feature according to the limestone weathering intensity measurements in a specific aquifer in belgium are presented the modelling approach derived from the common equivalent porous medium approach enables to spatially distribute the variable hydraulic properties within the features from the hard fresh rock to the alterite in finite difference grids the method allows creating models including thousands of features and respecting correct connectivity relations the approach is considered at different scales with varying grid cells sizes results show that the induced error increases significantly when the size of the grid cells exceeds the width of the main features controlling flow the approach is applied on a case study consisting in dewatering operations in an exploited quarry located in a weathered karst aquifer presented results provide new insights regarding characterization and modelling of this kind of aquifers and highlight their importance in hydrological studies

pn velocity study using a temporary seismographic network in the shillong plateau northeast india abstract seismic p wave velocity pn of the uppermost mantle beneath the shillong plateau has been studied using a temporary seismographic network refracted pn arrivals of shallow distant earthquakes m 3 0 recorded at pairs of stations lying within 2 azimuthal differences with respect to a given epicenter are used in this study since the method is based on relative travel time differences the measurement is independent of origin time and location errors the average pn velocity is found to be 8 57 0 02 km sec which is higher than the average mantle velocity 8 13 km sec under other areas of the indian subcontinent the observed high pn velocity beneath the plateau may be attributed to the presence of high density material

continental paleoclimates from δd and δ18o of secondary silica in paleokarst chert lags approximate continental paleotemperatures can be inferred from oxygen and hydrogen isotopic analyses of authigenic silica that precipitated within chert lags on paleokarst surfaces data for two proterozoic examples suggest continental weathering temperatures that ranged from 30 to 43 c quantitative results appear possible for rocks at least as old as 1 1 ga

characterizing carbonate reservoir fracturing from borehole data a case study of the visean in northern belgium abstract fractured carbonate rocks are widely used as hydrocarbon or geothermal reservoirs to provide a better understanding of the distribution and characteristics of such fractures a general workflow will be presented for the characterization of fracturing from borehole data by exploration and statistical analyses of integrated datasets in a case study of visean limestones in a belgian borehole both cores and geophysical well logs were used to investigate which factors control the characteristics of partially open veins which contribute to permeability relationships between multiple variables were tested statistically lithology geochemistry and geophysical well log values were taken into account as well as quantified veins vugs and stylolites from cores although natural joint frequencies appeared hard to quantify from the available data partially open vein characteristics could be well quantified the results show that differential compaction controlled the development of fractures fracturing occurred preferentially in massive reefal buildup boundstones in contrast to layered wacke to grainstones layer parallel slip along bedding surfaces could also have reduced fracture development in the latter limestones frequencies of cemented veins and partially open veins are positively correlated which suggests that the partially open veins result from either re opening by dissolution or re opening due to a later fracturation phase in summary this multi source study provides a workflow for fracture characterization from boreholes as well as insights into the factors controlling the distribution and characteristics of partially open veins which enhance reservoir permeability

geochronology of the proterozoic basement of southwesternmost north america and the origin and evolution of the mojave crustal province the proterozoic baldwin gneiss in the central transverse ranges of southern california a part of the mojave crustal province is composed of quartzofeldspathic gneiss and schist augen and granitic gneiss trondhjemite gneiss and minor quartzite amphibolite metagabbro and metapyroxenite sensitive high resolution ion microprobe shrimp data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 12 and 1675 19 ma adjacent to or through thinned archean crust high u th rims on zircons in most samples suggest an early metamorphic event at 1741 ma but peak amphibolite facies metamorphism and penetrative west vergent deformation occurred after 1675 ma the baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a proterozoic shelf slope sequence joshua tree terrane we hypothesize that emplacement of this regional allochthon occurred during a late early or middle proterozoic arc continent collision along the western margin of laurentia

origins of microbial microstructures in the neoproterozoic beck spring dolomite variations in microbial community and timing of lithification abstract microbialite microstructures form at the scale of microbial processes and can preserve evidence of growth degradation and lithification in microbial communities understanding how these processes are preserved in microstructures will provide insights into microbial ecosystems and how microbial communities have evolved through time microbialites in the neoproterozoic beck spring dolomite southern california u s a contain microstructures that record variations in the morphology of microbial communities as well as the timing of degradation versus lithification in the communities these processes are represented by five end member microbial microstructures 1 distinct laminated microstructure 2 diffuse laminated microstructure 3 distinct clotted microstructure 4 diffuse and regular clotted microstructure and 5 diffuse and irregular clotted microstructure these microstructures are distinguished by fabrics defined by organic inclusions in distinct microstructures organic inclusions sharply define microbial features such as laminae that are laterally continuous for millimeters or 50 200 micron diameter microdots the contacts between distinct microstructures are always sharp in diffuse microstructures zones of organic inclusions lack distinct boundaries neighboring laminae and clots grade into each other within tens of microns and boundaries between microbial structures and encasing cements are indistinct diffuse microstructures grade into each other and into distinct microstructures in a systematic way that suggests that differences are due to variations in the timing of mineralization relative to growth and degradation of microbial communities sharply defined textures in distinct microstructures suggest that lithification started during growth of the microbial community and the close spatial association but lack of intergrading between distinct laminated and distinct clotted microstructures suggests that they formed from two distinct microbial communities that lived in the same environment by contrast diffuse microstructures formed where lithification occurred after the onset of decay and degradation of the primary growth structure the continuum from distinct microbial textures through more diffuse and irregular textures documents variations in the timing of lithification relative to growth and decay of the microbial community thus where petrographic preservation is extremely good such as in the beck spring dolomite sub millimeter scale microbial microstructures record a gradation between growth and degraded microbial structures depending on the relative timing of lithification and heterotrophic decay

geochemistry of post extinction microbialites as a powerful tool to assess the oxygenation of shallow marine water in the immediate aftermath of the end permian mass extinction rapid and profound changes in earth surface environments and biota across the permian triassic boundary are well known and relate to the end permian mass extinction event this major crisis is demonstrated by abrupt facies change and the development of microbialite carbonates on the shallow marine shelves around palaeo tethys and western panthalassa microbialites have been described from a range of sites in end permian and basal triassic marine sedimentary rocks immediately following the end permian mass extinction here we present geochemical data primarily focused on microbialites our geochemical analysis shows that u v mo and ree ce anomaly may be used as robust redox proxies so that the microbialites record the chemistry of the ancient ambient sea water among the three trace metals reputed to be reliable redox proxies one v is correlated here with terrigenous supply the other two elements u and mo do not show any significant authigenic enrichment thereby indicating that oxic conditions prevailed during the growth of microbialites ree profiles show a prominent negative ce anomaly also showing that the shallow marine waters were oxic our geochemical data are consistent with the presence of some benthic organisms ostracods scattered microgastropods microbrachiopods and foraminifers in shallow marine waters that survived the mass extinction event

secular changes in the amount and texture of dolomite the amount of dolomite in cenozoic carbonates is lower than throughout most of the paleozoic a difference that could be due to changes in either chemical conditions or physical conditions that lead to extensive dolomitization the abundance of mimetic texturally retentive dolomites in the cenozoic suggests that chemical conditions were unusually favorable for dolomite formation and therefore it must be physical conditions that explain the paucity of cenozoic dolomites one reasonable explanation for the paucity of cenozoic dolomites is that glacio eustatic sea level fluctuations reduced the amount of time carbonate sediments were in contact with dolomitizing solutions

extreme ocean anoxia during the late cryogenian recorded in reefal carbonates of southern australia abstract the neoproterozoic was a time of great change in the earth s surface and marine environments including extensive climate variability the widespread oxygenation of the oceans and the accompanying rise of animal life however the timing of ocean oxygenation remains uncertain particularly in regard to cryogenian seas which were disrupted by large periods of global glaciation interglacial cryogenian reef complexes in the northern adelaide fold belt of australia contain abundant primary marine dolomite cements these cements have well preserved textural and growth zonation indicating they preserve their original marine chemistry and can be used as geochemical proxies for late cryogenian paleooceanography analysis of marine cements from peritidal nearshore facies shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo depth while nearshore cements have low fe contents and commonly contain iron oxide inclusions the shallow and deep cements have very high fe concentrations chalcophile elements cu cd pb zn etc are most abundant in the nearshore cements whereas rare earth elements are found in highest concentrations in the deeper facies cements rare earth element profiles are unusual with shallow and deep facies having convex profiles with negligible ce ce anomalies and positive eu eu anomalies being constrained by sedimentology this carbonate geochemistry provides a window into interglacial cryogenian ocean chemistry and structure the marine cements reveal pronounced chemical stratification in this late cryogenian ocean a thin veneer of oxic surface waters existed at the ocean surface in peritidal facies with increasingly anoxic and fe rich seawater at depth the distribution of strongly chalcophile elements like cd and cu across the chemocline suggests that although ferruginous deeper anoxic waters probably contained some dissolved sulphide these conditions describe a ferro sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late precambrian a return to archean like ocean conditions at this time suggests large scale disruption of the ocean system during the neoproterozoic

an updated global review of solutional weathering processes and forms in quartz sandstones and quartzites abstract quartz is considered one of the less soluble minerals of the earth s crust and thus hardly affected by chemical weathering despite this for more than forty years it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz rich lithologies is common and shares several similarities to the well known karstic ones in carbonate rocks in the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world and the geochemical processes involved these studies have clearly shown that solutional weathering is a fundamental process acting through intergranular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion this process has been described in the concepts of arenization and phantomization and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term karst and its application to these peculiar lithologies nonetheless the process is complicated by several factors related both to environmental conditions water chemistry and availability as well as to the compositional and textural characters of the lithology presence of clays iron hydroxides carbonate cement etc all these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution in this review the state of knowledge on the relevant chemical processes weathering mechanisms and speleogenesisk involved in the surface and underground karstification and clear examples of quartz solution and solutional landforms from different world locations are outlined and discussed

synsedimentary diagenesis in a cryogenian reef complex ubiquitous marine dolomite precipitation abstract analysis of the synsedimentary diagenetic phases from a pervasively dolomitised cryogenian 650 ma reef complex south australia reveals a fundamentally different style of marine diagenesis to that of phanerozoic carbonates textural evidence from dolomitised and undolomitised lithologies of the oodnaminta reef complex indicates that depositional components were precipitated initially as calcium carbonate aragonite and calcite dissolution textures and preserved fabrics in ooids indicate an originally aragonitic mineralogy early fibrous marine cements that have well preserved fibrous crystallographic fabrics with a length fast optical character are likely to have precipitated as high magnesium calcite however following this very early phase of diagenesis the dominant marine process occurring in these reefs was dolomite precipitation this involved the dolomitisation of earlier depositional and diagenetic phases as well as the precipitation of fibrous dolomite cements these fibrous marine cements include the newly described radial slow fascicular slow radiaxial slow and rhombic dolomite cements the cements have optical properties chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates dolomite precipitation during marine diagenesis in this reef complex suggests that the oceans of the cryogenian were chemically different to those of the phanerozoic abundant dolomite formation appears to be linked to anoxic magnesium rich seawater during this time marine dolomite precipitation under these conditions may explain the abundance of dolomite in the precambrian sedimentary record

the effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern cascadia accretionary margin analytical challenges in obtaining high quality measurements of rare earth elements rees from small pore fluid volumes have limited the application of rees as deep fluid geochemical tracers using a recently developed analytical technique we analyzed rees from pore fluids collected from sites u1325 and u1329 drilled on the northern cascadia margin during the integrated ocean drilling program iodp expedition 311 to investigate the ree behavior during diagenesis and their utility as tracers of deep fluid migration these sites were selected because they represent contrasting settings on an accretionary margin a ponded basin at the toe of the margin and the landward tofino basin near the shelf s edge ree concentrations of pore fluid in the methanogenic zone at sites u1325 and u1329 correlate positively with concentrations of dissolved organic carbon doc and alkalinity fractionations across the ree series are driven by preferential complexation of the heavy rees simultaneous enrichment of diagenetic indicators doc and alkalinity and of rees in particular the heavy elements ho to lu suggests that the heavy rees are released during particulate organic carbon poc degradation and are subsequently chelated by doc ree concentrations are greater at site u1325 a site where shorter residence times of poc in sulfate bearing redox zones may enhance ree burial efficiency within sulfidic and methanogenic sediment zones where ree release ensues cross plots of la concentrations versus cl li and sr delineate a distinct field for the deep fluids z 75 mbsf at site u1329 and indicate the presence of a fluid not observed at the other sites drilled on the cascadia margin changes in ree patterns the presence of a positive eu anomaly and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous crescent terrane located east of the drilled transect

reflected and mode converted seismic waves within the shallow aleutian subduction zone southern kenai peninsula alaska pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern kenai peninsula alaska are identified as reflected p and s and converted s to p phases originating within four depth ranges in the upper few kilometers of the cook inlet tertiary basin at midcrustal depths within the overthrust north american plate at about 35 km depth near the top of the wadati benioff seismic zone in proximity to the inferred interplate megathrust and at about 5 10 km below the megathrust in the subducted pacific plate beneath the landward margin of the accreted chugach terrane the mid upper plate mup discontinuity dips as steeply as 20 degree 30 degree to the west northwest over the depth range 12 18 km at shallower depths it flattens and possibly arches with the crest at about 10 km depth similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the chugach terrane the extensive reflectors may have a common origin possibly caused by the presence of underplated rocks relative amplitudes and polarities of the secondary phases originating at mup depths provide few constraints on the nature of the discontinuity it appears that the mupmore discontinuity is seismically inactive and does not represent a brittle ductile transition zone within the upper plate the two converted s to p phases generated near the top of the subducted plate could indicate a low velocity zone associated with subducted oceanic crust less

microbialites of the neoproterozoic beck spring dolomite southern california ancient microbialites reflect interactions between microbial communities and environmental conditions however evaluating the relative roles of microbial community processes and environmental influences on microbialite morphology and internal fabric in the rock record can be challenging the neoproterozoic beck spring dolomite preserves diverse microbialites and thus provides an opportunity to explore the factors that influenced microbialite development locally stromatolitic thrombolitic and composite microbialites are abundant in subtidal to upper intertidal carbonates in the beck spring dolomite thrombolitic and composite microbialites have not been recognized previously in this unit but compose much of the newly defined thrombolitic member stratigraphic relationships demonstrate that these three types of microbialites formed in close spatial and temporal association in subtidal to intertidal environments the relative proportions and distributions of stromatolitic and thrombolitic microbialites vary with depositional environment stromatolitic microbialites dominate in deeper intertidal to subtidal facies whereas thrombolitic textures are more abundant in upper intertidal facies composite microbialites composed of intermingled clotted and laminated textures formed in all environments but are most abundant in intertidal facies the broad environmental distribution of stromatolitic thrombolitic and composite microbialites and the intermingling of textures suggest that laminated and clotted textures reflect diverse microbial community morphologies rather than environmental variations furthermore the ca 750 ma age of thrombolitic microbialites in the beck spring dolomite requires that they formed without the influence of calcimicrobes or metazoans colonizing and grazing the microbial mat surface thus these thrombolites provide further evidence that the biostratigraphic distribution of thrombolites cannot be uniquely attributed to evolution of calcifying and grazing organisms in the earliest cambrian and that older microbial communities were capable of producing clotted textures

ghost rock karstification of devonian limestone flooring the athabasca oil sands in western canada abstract this study interprets a new style of ghost rock karstification at a site in northeast alberta western canada the karstic area of the upper devonian paleotopography was localized at the western end of the bitumount trough a 50 km long collapse structure that resulted from removal of a 120 m thick interval of salt beds of the middle devonian prairie evaporite only 200 m below ghost rock karstification developed in massive light grey limestone beds that alternate with chlorite rich argillaceous intervals of the moberly member devonian waterways formation these weathered rock profiles expanded latterly and were deepened by slowly moving hydraulic gradient driven phreatic groundwater descending along clusters of closely spaced 2 10 cm wide joints that cross cut the fracture shattered limestone beds the uppermost beds of greyish green chlorite rich argillaceous limestone were decalcified as the flows along the joint clusters carried insoluble residues consisting of quartz silt mixed with illite chlorite clays into the substrate these chlorite rich insoluble residues accumulated as replacement fabrics within decalcified porous zones of the altered limestone in the substrate as the joint opening plugged with insoluble residues within the deepened decalcification weathered rock intervals the alterite zones expanded laterally to form vertical pod shaped structures these 5 20 m in diameter and 10 20 m deep pseudo sinkhole zones consisting of alterite retained ghost rock traces of the original limestone strata this area was subsequently covered by the lower cretaceous athabasca oil sands

marine cements reveal the structure of an anoxic ferruginous neoproterozoic ocean neoproterozoic oceans provided the setting for the rise of animals yet little is known of their chemical composition marine carbonates from the cryogenian oodnaminta reef complex south australia reveal the chemical structure of a neoproterozoic ocean pseudo depth profiles from shallow to deep water reef facies have been constructed from geochemical and sedimentological analysis of marine cements evidence suggests that under a peritidal oxic anoxic chemocline the water column was largely anoxic strongly ferruginous and had a chemistry profoundly different from that of modern seawater these geochemical data suggest early archaean like conditions for this late cryogenian ocean posing problems for metazoan evolution in extremely anoxic conditions supplementary material detailed methods and data tables are available at www geolsoc org uk sup18764

a global transition to ferruginous conditions in the early neoproterozoic oceans eukaryotic life expanded during the proterozoic eon1 2 5 to 0 542 billion years ago against a background of fluctuating ocean chemistry2 3 4 after about 1 8 billion years ago the global ocean is thought to have been characterized by oxygenated surface waters with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron containing ferruginous deeper waters5 6 7 the spatial extent of sulphidic waters probably varied through time5 6 but this surface to deep redox structure is suggested to have persisted until the first neoproterozoic glaciation about 717 million years ago8 9 10 11 here we report an analysis of ocean redox conditions throughout the proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops we find a global transition from sulphidic to ferruginous mid depth waters in the earliest neoproterozoic coincident with the amalgamation of the supercontinent rodinia at low latitudes we suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on rodinia we propose that this transition essentially detoxified ocean margin settings allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago

earthquakes as collapse precursors at the han sur lesse cave in the belgian ardennes abstract collapse activation is an ongoing process in the evolution of karstic networks related to the weakening of cave vaults because collapses are infrequent few have been directly observed making it challenging to evaluate the role of external processes in their initiation and triggering here we study the two most recent collapses in the dome chamber of the han sur lesse cave belgian ardenne that occurred on or shortly after 3rd december 1828 and between the 13th and 14th of march 1984 because of the low probability that the two earthquakes that generated the strongest ground motions in han sur lesse since 1800 on 23rd february 1828 mw 5 1 in central belgium and 8th november 1983 mw 4 8 in liege occurred by coincidence less than one year before these collapses we suggest that the collapses are related to these earthquakes we argue that the earthquakes accelerated the cave vault instability leading to the collapses by the action of other factors weakening the host rock in particular the 1828 collapse was likely triggered by a smaller mw 4 2 nearby earthquake the 1984 collapse followed two months of heavy rainfall that would have increased water infiltration and pressure in the rock mass favoring destabilization of the cave ceiling lamina counting of a stalagmite growing on the 1828 debris dates the collapse at 1826 9 ce demonstrating the possibility of dating previous collapses with a few years of uncertainty furthermore our study opens new perspectives for studying collapses and their chronology both in the han sur lesse cave and in other karstic networks we suggest that earthquake activity could play a stronger role than previously thought in initiating cave collapses

chaotic breccia zones on the pembroke peninsula south wales evidence for collapse into voids along dilational faults chaotic breccias and megabreccias locally called gash breccias hosted within the pembroke limestone group visean mississippian lower carboniferous of southwest wales are re mapped along with spatially related crackle and mosaic breccias of thirteen studied megabreccia bodies seven lie along steep nnw or nne striking strike slip faults originating during north south variscan late carboniferous shortening though reactivated during later extension four bodies are conformable with e w striking steeply dipping bedding and two have irregular or indeterminate margins the bedding parallel zones are interpreted as the dilational tips of listric normal faults and the cross strike faults as transtensional transfer zones sub horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults most fissures have geometrically matched margins produced by this dilational faulting and only locally have the indented margins indicating solutional processes the most likely age for the main fissure extension and fill is late triassic based on analogous dated fills at the eastern end of the bristol channel basin the pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones

mid neoproterozoic biostratigraphy and isotope stratigraphy in australia abstract palynomorph and stromatolite assemblages from mid neoproterozoic strata of australia appear highly significant when collated with results obtained from isotope stratigraphy and allow correlation across the centralian superbasin and adelaide rift complex the majority of palynomorphs from the lower mid neoproterozoic 830 ma have limited biostratigraphic value although a few apparently have restricted ranges carbonates at this stratigraphic level contain the distinctive stromatolite acaciella australica and other taxa of the acaciella australica stromatolite assemblage these carbonates have 13ccarb values of between 4 and 0 and associated anhydrite has 87sr 86sr ratios of 0 7069 these depleted 13ccarb values are immediately preceded by values of 6 1 in the western officer and amadeus basins of the centralian superbasin similar c and sr isotopic features are seen in the similarly aged upper shaler supergroup of canada upper mid neoproterozoic 760 ma carbonates contain the distinctive stromatolite baicalia burra and other stromatolites of the baicalia burra stromatolite assemblage siliciclastic horizons in the same interval contain the acritarch cerebrosphaera buickii and have 13ccarb values of between 0 and 8 the highest mid neoproterozoic 13ccarb values of between 6 7 and 8 1 occur within the stratigraphic range of c buickii in australia in the western officer basin and adelaide rift complex the mid neoproterozoic 13ccarb peaks are preceded by values of ca 0 the apparently restricted range of c buickii in australia suggests the need for reinterpretation of the stratigraphic position of its occurrence in the svanbergfjellet formation akademikerbreen group spitsbergen which was previously considered a post sturtian glacial deposit

origin of dolomitization on the barbwire terrace canning basin western australia a thick areally extensive subsurface sequence of upper devonian carbonates occurs on the barbwire terrace in the canning basin of western australia it is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized slightly deeper water mark have remained as limestones the major regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite up to 600 m with no primary calcite a small volume of evaporitic supratidal dolomite is present at one location this dolomite is derived from highly saline fluids developed in an arid supratidal environment replacement dolomite of the regional dolomite type has a xenotopic form with undulose extinction and irregular crystal boundaries in addition saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite this suggests the regional dolomite type was precipitated at slightly elevated temperatures dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution geochemically the synsedimentary supratidal and regional dolomite types are quite distinctive supratidal dolomites have 6 o values which are significantly higher 6180 2 to i pdb than the regional dolomite type 6l80 9 to 2 pdb assuming the lowest 6 o values for the sabkha dolomite represent replacement in marine waters the oxygen isotopic composition for upper devonian canning basin marine dolomite would be around 6l80 2 pdb the petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin however sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the barbwire terrace therefore it is suggested that dolomitization may have taken place in a near surface environment with a major recrystallization event superimposed during burial diagenesis

cerium anomaly variations in ediacaran earliest cambrian carbonates from the yangtze gorges area south china implications for oxygenation of coeval shallow seawater abstract the late neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean however apart from the emergence of animal fossils little supporting evidence has been found for the rise of oxygen in shallow seawater here we present ce anomaly data a redox proxy with a higher reduction potential than many other redox proxies for carbonates from well preserved marine successions the doushantuo formation dengying formation and lower yanjiahe formation of the ediacaran period and the upper yanjiahe formation of early cambrian on the neoproterozoic yangtze platform in the yangtze gorges area in order to constrain the redox evolution of the shallow marine environment calculated ce anomalies were screened using the following criteria al 36 in acetic acid leached carbonate to target samples that preserve primary seawater ree features and ce anomalies the samples satisfying this screening show seawater like ree distribution patterns in leached carbonates and may have recorded ce anomalies of the seawater from which the carbonates precipitated these data show that the doushantuo formation has ce ce values between 0 92 and 0 71 the dengying formation between 0 90 and 0 40 and the yanjiahe formation between 0 52 and 0 72 the ce ce values of the doushantuo formation suggest that shallow waters during the doushantuo stage 635 551 ma were anoxic to suboxic from the bottom to the top of the dengying formation ce ce values decrease systematically which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the dengying stages from 551 ma to ediacaran cambrian boundary these results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the ediacaran period

rare earth elements in pore waters of marine sediments abstract the rare earth elements rees were measured in pore waters of the upper 25 cm of sediment from one site off peru and three sites on the california margin the pore water ree concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern from these analyses and from comparison to other chemical species measured dissolved fe mn ba oxygen nitrate phosphate it is suggested that pore water rees can be grouped into three categories those that are from an fe source those that are from a poc source and cerium oxide rees from the fe source appear where anoxia is reached they have a distinctive middle ree mree enriched pattern the concentrations in this source are so elevated that they dominate ree trends in the fe oxide reduction zone the net result of flux from the poc source is relative enrichment of heavy rees hrees over light rees lrees reflecting remineralizing poc and complexation with doc a common linear ree pattern seen in both oxic and anoxic sediments is associated with this poc source as well as a hree enriched pattern that is seen in surficial sediments at the peru site overall the pore water results indicate that mn oxides are not an important carrier of rees in the oceans a ree biogeochemical model is presented which attempts to reconcile ree behavior in the water and sediment columns of the oceans the model proposes that poc fe oxide and ce oxide sources can explain the ree concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water the model is also consistent with our observation that the ce anomaly of pore water does not exceed unity under any redox condition

plio quaternary landscape evolution in the uplifted ardennes new insights from 26al 10be data from cave deposited alluvium meuse catchment e belgium abstract despite a wealth of recent studies dealing with the evolution of the drainage network in the uplifted ardennes massif e belgium especially from the middle pleistocene onwards the ardennian landscape evolution and long term incision rates in the meuse catchment remain poorly documented over the whole plio quaternary alluvium filled multilevel cave systems represent a relevant setting to unravel the late cenozoic history of regional river incision we present here a dataset of 26al 10be concentration data obtained from fifteen pebble samples washed into the chawresse system one of the largest multi level cave systems of belgium which developed in devonian limestones of the lower ourthe valley the main ardennian tributary of the meuse the sample collection spans an elevation difference higher than 120 m and their depleted 26al 10be ratios yield burial ages ranging from 0 25 to 3 28 ma after critical assessment of our dataset for intra karstic reworking issues the most striking outcome of the obtained burial ages is the acceleration by a factor five of the incision rates from 30 to 150 m ma during the first half of the middle pleistocene integrating this incision peak and our pre burial denudation rates we then revisit the existing framework of plio quaternary denudation and river incision in the ardennian meuse catchment whilst our 26al 10be concentration data shed new light on the temporal and spatial variability of the local river and hillslope system response to coupled tectonic and climatic forcings it simultaneously highlights sampling issues and the need for further chronological data

a model for the distribution of trace elements between calcite and dolomite abstract a crystal growth model is proposed which allows ions of a trace element to enter the ca and mg sites of dolomite in proportion to the size of the ions relative to that of ca and mg ions and which assigns equal portions of the trace element to the ca site of dolomite and the ca site of associated calcite the model produces calcite dolomite distribution coefficients of 0 79 for mn and 0 43 for fe which may be compared with 0 85 and 0 28 as observed in marble and a distribution coefficient of 2 0 for sr and ba which may be compared with observed values of 2 3 for sr and 1 8 for ba

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

experimental and natural mimetic dolomitization of aragonite ooids abstract to better understand fabrics in dolomitized carbonates aragonitic ooids were replaced with dolomite in laboratory experiments recent ooids from cat cay were reacted in a solution of 0 5 m mgcl2 and 0 4 m cacl2 in teflon lined stainless steel bombs at 200 c and equilibrium vapor pressure x ray diffraction and petrographic study were used to characterize compositional and textural changes during dolomitization reaction progressed from aragonite through unordered ca mg carbonate to ordered dolomite within 180 hr reaction products are texturally identical irrespective of composition and order we believe that near stoichiometric compositions and ordering were achieved by microscale dissolution and reaction of intermediate phases dolomitization occurred by dissolution of aragonite concurrent with precipitation of rhombohedral ca mg carbonate dolomitization occurs first at ooid surfaces where microsucrosic crystals precipitated and coalesced to form a rind with time reaction preferentially proceeded into discrete zones of ooid microporosity where dolomite successively replaced algal borings outer ooid cortices and inner ooid cortices precipitation and coalescence of dolomite crystals in cortical laminae closely mimics original ooid structure comparison of experimental fabrics with calcitized partially dolomitized and completely dolomitized aragonite ooids of the late proterozoic beck spring dolomite indicates that mimetic concentric fabric originates by aragonite dissolution and early dolomite precipitation within porous cortical laminae importantly these experimental and natural replacement fabrics show that fine scale dolomite ooid fabrics are not indicative of primary dolomite precipitation isotopic compositions of beck spring mimetic dolomite ooids are enriched relative to other dolomitized metastable marine components including acicular cement aragonite carbonate mud aragonite rich and columnar cement magnesian calcite these data suggest that 1 initial reaction of aragonite ooid cortices occurred early in the diagenetic history in marine dominated fluid and 2 selective reaction of aragonite ooid cortices was promoted by a combination of high solubility high cortical porosity and high reactant surface area relative to other metastable components accordingly textural and chemical data gathered from mimetic concentric dolomite ooids may be useful indicators of precursor aragonite and early dolomitization events

the nature and significance of rift related near surface fissure fill networks in fractured carbonates below regional unconformities fissure fill networks are a widely recognized but relatively little described near surface phenomenon supplementary material appendix a is available at https doi org 10 6084 m9 figshare c 5023103

numerical modeling of the evolution of arcades and rock pillars abstract arcades i e lenticular and other specifically shaped hollows controlled by discontinuities have recently been recognized as a weathering form typical for sandstones weathered quartzites granites or tuffs they are produced by accelerated weathering and erosion in stress shadows related to the redistribution of gravity induced stress along planar discontinuities in the rock these forms occur worldwide in various settings inland humid arid and coastal the origin of arcades has been demonstrated via physical experiments and supported by a relatively simplistic numerical modeling however details on their shaping and the evolution of related forms have not been explained we performed an advanced numerical modeling to produce various shapes of arcades and rock pillars during the erosion of rock masses dissected by discontinuities we demonstrate that the erosion model in which erosion takes place when the maximum principal stress is below a certain critical value can adequately describe the formation of arcades in the modeling we set higher critical values for stresses at discontinuities than in a homogeneous material representing a rock mass to represent the higher tendency for disintegration of the discontinuity material which was weakened by the discontinuity formation processes by applying various discontinuity geometries and values of critical stresses we were able to reproduce the formation of various arcade shapes and complex three dimensional clusters of arcade cavities with rock pillars discontinuities and stress controlled erosion weathering are the only necessary conditions for arcade formation

trends in oolite dolomitization across the neoproterozoic cambrian boundary a case study from death valley california abstract changes in dolomite fabric within the neoproterozoic cambrian stratigraphic succession in death valley california preserve evidence for a first order biogeochemical change in early diagenetic environments through the neoproterozoic cambrian transition oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time two key generalizations can be made 1 late neoproterozoic dolomitization in death valley occurred very early in the paragenetic sequence was volumetrically significant and preserved fine detail of the original aragonite or calcite precursor and 2 early cambrian dolomitization in death valley occurred late in the paragenetic sequence with burial was commonly incomplete and obliterated details of the calcite or aragonite precursor early diagenetic dolomitization of any kind was absent the dolomitization fabrics in death valley parallel the presumed pattern of seawater mg ca with respect to mimetic dolomitization in the paleozoic low mg ca little no mimetic dolomitization and cenozoic higher mg ca more common mimetic dolomitization however the volume of neoproterozoic mimetic dolomitization is immense compared to the cenozoic examples we speculate that a fundamental shift in the early diagenetic environment occurred in late neoproterozoic time perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment capping microbial mats an oscillatory trend in the fabric and volume of dolomite through the phanerozoic has been suggested but the termination of volumetrically significant mimetic dolomitization across the neoproterozoic cambrian transition a critical time in earth history for many reasons dwarfs the oscillatory trends noted in the phanerozoic

polarization filtering for automatic picking of seismic data and improved converted phase detection summary data adaptive polarization filtering is used to improve the detection of converted seismic phases both direct waves and mode converted ps and sp arrivals may be more easily picked on the filtered records an autopicking routine is applied that cuts the polarization filtered traces according to the modelled traveltime of each phase through an initial structure use of forward modelled source receiver times reduces the likelihood of an automatic pick being incorrectly made on spurious spikes in the polarization filtered trace it is therefore a realistic way of automatically picking multiphase data sets or more generally linearly polarized phases where low signal to noise ratios may be encountered the method is suitable for any three component seismic data and is here applied to local earthquakes recorded in north island new zealand intermediate energy is observed between the direct p and s arrivals due to phase conversion at the interface between the indo australian and subducting pacific plates the amplitudes of these converted arrivals are often too low for them to be identified above the p wave coda but polarization filtering of the records enables the yield of converted phase picks to be greatly increased

stable isotope variations in the neoproterozoic beck spring dolomite and mesoproterozoic mescal limestone paleokarst implications for life on land in the precambrian proterozoic karst events lowered 13c values by as much as 11 for the 800 ma beck spring dolomite california and as much as 8 5 for the 1 1 ga mescal limestone central arizona relative to the originally deposited carbonate the 13c changes are attributed to input of 13c depleted organic co2 derived from photosynthetic organisms that colonized the ancient land surface the large isotopic shift and its presence at two separate localities suggest that proterozoic karst surfaces were colonized by significant photosynthetic communities with phytomasses possibly approaching those of today

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

carbonate dissolution in a modern mixing zone the potential for dissolution of calcite and aragonite and precipitation of dolomite by the mixing of fresh and saline ground waters has been demonstrated theoretically1 3 although geologists have used these processes in the explanation of the diagenesis of recent4 7 and ancient8 12 carbonates there has been only limited investigation of the geochemical processes of modern mixing zones13 14 here we report the results of chemical and rock profiling through a modern mixing zone on andros island in the bahamas our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite and corresponding pervasive dissolution of the carbonate rocks at this level undersaturation is significantly enhanced by bacterial processes which are also responsible for precipitation of an iron rich crust despite chemical supersaturation precipitation of dolomite was not observed thereby casting some doubt on the mixing zone model of dolomitization

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

cover collapse sinkholes in the tournaisis area southern belgium abstract in the tournaisis area southern belgium cover collapse sinkholes have been reported since the beginning of this century in this area the relatively flat lying carboniferous limestone is overlain by an unconformable mesocenozoic cover recent research has shown that these sinkholes developed from reactivated paleokarsts the intensive pumping for domestic and industrial water supply combined with the dewatering due to deep limestone quarries operations resulted in the lowering of groundwater levels this triggered the reactivation of paleokarstic systems and thus the occurrence of cover collapse sinkholes these sudden and unexpected collapses have already had major impacts in the region in 1977 about thirty sinkholes opened up along the escaut river downstream from the city of tournai as a consequence the groundwater was polluted by an extensive loss of river water into the carboniferous aquifer more recently a house and industrial facilities were severely damaged by such cover collapse sinkholes in this context a geoharzard map would be a valuable tool as far as regional development is concerned this points at the importance to develop methods to locate and characterize hidden paleokarsts a study site has been chosen in an area where sinkholes have been opening up for about ten years a geophysical survey of the site which included electrical resistance mapping of the area has shown weakness lineaments these lineaments are parallel to a main fault affecting the region

the process of ghost rock karstification and its role in the formation of cave systems abstract this paper presents an extensive review of the process of ghost rock karstification and highlights its role in the formation of cave systems the process integrates chemical weathering and mechanical erosion and extends a number of existing theories pertaining to continental landscape development it is a two stage process that differs in many respects from the traditional single stage process of karstification by total removal the first stage is characterised by chemical dissolution and removal of the soluble species it requires low hydrodynamic energy and creates a ghost rock feature filled with residual alterite the second stage is characterised by mechanical erosion of the undissolved particles it requires high hydrodynamic energy and it is only then that open galleries are created the transition from the first stage to the second is driven by the amount of energy within the thermodynamic system the process is illustrated by detailed field observations and the results of the laboratory analyses of samples taken from the karstotype area around soignies in southern belgium thereafter a series of case studies provide a synthesis of field observations and laboratory analyses from across western europe these studies come from geologically distinct parts of belgium france italy and the united kingdom the process of ghost rock karstification challenges a number of axioms associated with the process of karstification by total removal on the basis of the evidence presented it is argued that it is no longer acceptable to use karst morphologies as a basis with which to infer specific karstogenetic processes and it is no longer necessary for a karst system to relate to base level as ghost rock karstification proceeds along transmissive pathways in the rock there is also some evidence to suggest that ghost rock karstification may be superseded by karstification by total removal and vice versa according to the amount of energy within the thermodynamic system the proposed chemical weathering and subsequent mechanical erosion of limestone suggest that the development of karst terrain is related far more closely to the geomorphological development of aluminosilicate and siliceous terrains than is generally supposed it is now necessary to reconsider the origin of many karst systems in light of the outlined process of ghost rock karstification

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

shear wave velocity of the sedimentary basin in the upper mississippi embayment using s to p converted waves from mid october 1989 to august 1992 40 three component panda portable array for numerical data acquisition stations were deployed in the central new madrid seismic zone three component digital seismograms recorded by the panda stations in the region are characterized by 1 the very weak direct s arrivals on the vertical component which can be identified unambiguously from the two horizontal components and 2 at least two prominent secondary arrivals between the direct p and s arrivals one sp dominant on the vertical component and another ps with smaller amplitude on the two horizontal components travel time differences between the sp and s and between the p and ps are the same for different earthquakes recorded at the same station but are different at different stations even for the same event polarization analyses of three component seismograms and travel time measurements confirm the interpretation that these two secondary arrivals are the p to s ps and s to p sp converted waves that occur at the bottom of the sedimentary basin beneath each station since abundant well log data are available in the upper mississippi embayment the thickness of the sediments beneath each seismic station can be estimated travel time differences between the direct and the converted waves can be used to calculate average shear wave velocity for the sediments beneath each station the estimated shear wave velocities of the sediments beneath panda stations vary from 0 45 to 0 67 km sec the higher shear wave velocity associated with thicker sediments can be interpreted as a consequence of increasing compaction of unconsolidated sediments due to increasing overburden

structural and lithological guidance on speleogenesis in quartz sandstone evidence of the arenisation process a detailed petrographic structural and morphometric investigation of different types of caves carved in the quartz sandstones of the tepui table mountains in venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves such as funnel shaped pillars pendants and floor bumps samples of fresh and weathered quartz sandstone of the mataui formation roraima supergroup were characterised through wds dispersive x ray chemical analyses picnometer measurements edax analyses sem and thin section microscopy in all the caves two compositionally different strata were identified almost pure quartz sandstones with content of silica over 95 and high primary porosity around 4 and phyllosilicate rich quartz sandstone with contents of aluminium over 10 and low primary porosity lower than 0 5 phyllosilicates are mainly pyrophyllite and kaolinite sem images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development depending on the alteration state of the samples grain boundary dissolution increases the rock porosity and gradually releases the quartz grains suggesting that arenisation is a widespread and effective weathering process in these caves the primary porosity and the degree of fracturing of the quartz sandstone beds are the main factors controlling the intensity and distribution of the arenisation process weathering along iron hydroxide or silt layers which represent inception horizons or a strata bounded fracture network predisposes the formation of horizontal caves in specific stratigraphic positions the loose sands produced by arenisation are removed by piping processes gradually creating anastomosing open fracture systems and forming braided mazes geometric networks or main conduit patterns depending on the local lithological and structural guidance on the weathering process this study demonstrates that all the typical morphologies documented in these quartz sandstone caves can be explained as a result of arenisation which is guided by layers with particular petrographic characteristics primary porosity content of phyllosilicates and iron hydroxides and different degrees of fracturing strata bounded fractures or continuous dilational joints

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

mass balance implies holocene development of a low relief karst patterned landscape abstract we constructed mass balances of both calcium and phosphorus for two watersheds in big cypress national preserve in southwest florida usa to evaluate the time scales over which its striking landscape pattern developed this low relief carbonate landscape is dotted with evenly spaced evenly sized shallow surface depressions that annually fill with surface water and thus support wetland ecosystems e g cypress domes embedded in a pine dominated upland matrix with exposed bedrock local and landscape scale feedbacks between hydrology ecological dynamics and limestone dissolution are hypothesized to explain this karst dissolution patterning this hypothesis requires the region to be wet enough to initiate surface water storage which constrains landscape formation to interglacial periods the time scale therefore would be relatively recent if creation of the observed pattern occurred in the current interglacial period i e holocene and older time scales could reflect inherited patterns from previous inter glacial periods or from other processes of abiotic karstification we determined phosphorus stocks across four landscape compartments and estimated the limestone void space i e wetland depression volume across the landscape to represent cumulative calcium export we calculated fluxes in e g atmospheric deposition and out i e solute export of the landscape to determine landscape denudation rates through mass balance comparing stocks and annual fluxes yielded independent estimates of landscape age from the calcium and phosphorus budgets our mass balance results indicate that the landscape began to develop in the early mid holocene 12 000 5000 ybp radiocarbon dating estimates implied similar rates of dissolution 1 m per 3000 3500 years and were in agreement with holocene origin this supports the hypothesis that ecohydrologic feedbacks between hydrology and vegetation occurring during the present interglacial period are sufficient to shape this landscape into the patterns we see today and more broadly suggests the potential importance of biota in the development of macro scale karst features

scsp observed on north island new zealand implications for subducting plate structure seismic phase conversions provide important constraints on the layered nature of subduction zone structures recordings from digital stations in north island new zealand have been examined for converted scs to p scsp arrivals from deep 150 km tonga kermadec earthquakes to image layering in the underlying hikurangi subduction zone consistent p wave energy prior to scs has been identified from stations in eastern and southern north island where the subducted plate interface is at a depth of between 15 and 30 km two scs precursors are observed ray tracing indicates that the initial precursor scsp1 corresponds to conversion from the base of an 11 14 km thick subducting pacific crust the second precursor is interpreted as a conversion from the top of the subducting plate the amplitude ratio scsp1 scs increases from 0 10 to 0 19 from northern to southern north island this is within the range expected from a simple first order velocity discontinuity at an oceanic moho a 1 2 km thick layer of low velocity sediment at the top of the subducting plate is required to explain the remaining scsp waveform our results imply that the abnormally thick hikurangi chatham plateau has been subducting beneath new zealand for at least 2 9 myr thus explaining the high uplift rates observed across eastern north island

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

evidence for primary aragonite precipitation lower proterozoic 1 9 ga rocknest dolomite wopmay orogen northwest canada abundant precipitation of aragonite is suggested by well preserved fabrics in dolostones of the lower proterozoic 1 9 ga rocknest formation fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans and cement crusts in cryptalgal tufas aragonite was common on the 1 9 ga rocknest shelf as it is on modern tropical shelves a search for similar marine cement fabrics in other precambrian carbonates should help resolve whether aragonite was an important precipitate from precambrian tropical seas implying an ocean carbonate chemistry similar to today or if aragonite precipitation was restricted to rare times in the past calcite and even dolomite being the main carbonates precipitated as recently suggested by some authors

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today

life on land in the precambrian microfossils have been discovered in cavity fill and replacement silica that occurs between chert breccia clasts in 1200 million year old paleokarst at the top of the mescal limestone central arizona and in 800 million year old paleokarst at the top of the beck spring dolomite southeastern california microbial communities on precambrian g550 million years ago land may have been extensive enough to affect weathering erosion sedimentation and geochemical processes