geochemical constraints on the origin of calcite veins and associated limestone alteration ordovician viola group arbuckle mountains oklahoma u s a abstract isotopic and elemental analyses constrain the origins of two types of calcite veins and associated limestone alteration in the ordovician viola group arbuckle mountains oklahoma u s a calcite from small millimeter wide veins has similar 87 sr 86 sr ratios 0 70781 0 70792 and 13c values 1 4 to 1 6 pdb to host viola limestone 87 sr 86 sr 0 70779 0 70794 13c 2 2 to 1 7 pdb indicating that the fluids from which the vein calcite precipitated were chemically buffered by the host limestone the low 18o values 8 2 to 5 5 pdb of the vein calcite relative to the host limestone 5 0 to 3 1 pdb suggest that the vein calcite probably precipitated from 18o depleted meteoric water calcite from large veins 5 50 cm wide is characterized by radiogenic 87 sr 86 sr ratios 0 70887 0 70915 and low 13c values 8 0 to 2 5 pdb relative to host viola limestone such isotopic compositions demonstrate that the fluids responsible for the formation of the large veins were enriched in 87sr and depleted in 13c relative to the host limestone the 18o values 4 8 to 3 7 pdb of calcite from the large veins coupled with the presence of hot 50 90 c and saline fluid inclusions constrain the formation of the vein calcite to 18o enriched 18 o 2 smow fluids on this basis calcite from the large veins is believed to have precipitated from basin derived fluids adjacent to the large veins alteration zones up to 0 5 m wide have developed in host limestone relative to other limestones the altered limestones display radiogenic 87 sr 86 sr ratios 0 70792 0 70880 low 13c values 8 2 to 2 0 pdb and low sr concentrations the similar isotopic compositions of the altered limestone to those of the large veins suggest that alteration of limestones was also caused by basinal fluids

the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan a reply to m tagiri y hiroi and s banno

aragonite crystal fans in neoproterozoic cap carbonates a case study from brazil and implications for the post snowball earth coastal environment abstract neoproterozoic cap carbonates are characterized by anomalous sedimentary features including sea floor precipitates represented by aragonite pseudomorph cement crusts and crystal fans these features are found in restricted periods of geological history and their generation in the context of the neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation new sedimentary and geochemical results on well preserved neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the sete lagoas formation bambui group central brazil are reported the micrite settling facies shows the highest abundance of crystal fans arranged in centimeter scale layers interfingered with the micrite matrix this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters a numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pco2 in these conditions the mg ca ratio of the neoproterozoic seawater is calculated at ca 1 2 alkalinity oversaturation is monitored by a variable evaporation degree aragonite formation is kinetically favored and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pco2 hence alkalinity is higher these modeling conditions do not preclude the contribution of incomplete organic matter degradation to alkalinity as suggested by negative carbon isotope signatures in addition the large number of aragonite micrite pairs suggests a seasonal or paleoclimatic forcing other cases of abiotic aragonite precipitation through time are also briefly examined by comparison

facies distribution and retreat of middle triassic platform margin guizhou province south china the southern margin of the vast yangtze platform in central guizhou province china retreated during the anisian early middle triassic by shedding skeletal debris and boundstone blocks into the margin of the adjacent basin anisian platform deposits are shoaling up cycles that commonly terminated in subaerial exposure platform margin facies are obscured by massive dolomitization and mechanical erosion distal basin deposits are terrigenous mudstone and siltstone at the basin margin a wedge of mixed carbonate and terrigenous rocks consists of a thin bedded dolostone and limestone b lime breccia with thin bedded mudstone and c lime mudstone with breccia blocks within the breccias indicate that the shelf margin contained extensive boundstone formed by tubiphytes encrusting organisms and early marine cement interspersed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin the profile of the shelf margin from detailed mapping indicates 1 7 2 7 km of platform margin retreat during deposition of a basin margin wedge 250 m thick intertonguing of various basin margin facies reflects alternating minor episodes of advance and retreat of the margin near parallelism of the tongues suggests low relief at the platform margin an upward stratigraphic progression to more distal carbonate free terrigenous basin facies indicates a cessation of carbonate production on the platform owing to emergence during early anisian time retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick the largest observed dimensions a re entrant in the margin 7 km wide and 10 km deep could also reflect collapse retreat occurred along 175 km of the platform margin the lack of platform margin facies along this front suggests 3 7 km of retreat and a total area of 875 km2 the anisian platform margin retreat in guizhou is one of relatively few examples of platform margin retreat in the geological record

syndepositional cements associated with nannofossils in the marmolada massif evidences of microbially mediated primary marine cements middle triassic dolomites italy the marmolada platform is characterized by striking globose masses evinosponges arranged in concentric bands of fibrous calcite cements ranging in size from centimeters to several decimeters the lithogenetic importance of these peculiar cements has been recognized in many middle triassic buildups of the western tethys eds microanalyses revealed that these fibrous cements contain 1 3 mol of mg and detectable amounts of sr over 1000 ppm sometimes exceeding 10 000 ppm the strontium geochemical signature in neomorphic calcite could reflect the replacement of an aragonitic carbonate precursor the boundaries between the fibrous calcite bands are marked by alignments of microcrystalline aggregates of fluorapatite and or dolomite the presence of fluorapatite may indicate a depositional microenvironment eutrophic or rich in microbial communities epifluorescence analyses showed bright bands alternated to dark ones confirming that organic matter remains occur within the studied cements and could have played a significant role in supporting the widespread syndepositional cementation high magnification sem observations on bright epifluorescent bands showed the presence of widespread more or less spherical bodies ranging in size 100 300 nm these bodies could represent the relicts of nannobacterial cells

isotopic evidence for the paleoenvironmental evolution of the mesoproterozoic helena formation belt supergroup montana usa high resolution isotopic and geochemical data constrain long debated marine vs lacustrine depositional models for the helena formation a carbonate unit within the mesoproterozoic belt supergroup montana paired carbon and oxygen isotope and minor element analyses of microsamples from a diverse array of petrographic components effectively distinguish between primary and secondary isotopic signatures 13c values reflecting primary depositional conditions increase progressively from an average of 0 near the base of the helena formation to an average of 1 4 near the top over a stratigraphic thickness of 100 200 m because the base of this 1 6 shift coincides with a basin wide facies change marked by the appearance of stromatolitic units and an increased occurrence of features indicative of evaporative conditions and subaerial exposure the shift toward more positive 13c values is interpreted to reflect an increase in biologic productivity coupled with increasingly evaporative conditions in the eastern part of the belt basin 13c values from the lower part of the helena formation are similar to those from other marine carbonates of similar age and suggest that the belt basin was marine during deposition of the helena formation taken as a whole these relations suggest that helena sediments were deposited in a shallow marine basin that was increasingly isolated from the open mesoproterozoic ocean original 13o values have been retained only in syndepositional and early diagenetic intergranular cements in oolitic grainstones that occur in the upper part of the helena formation the most positive 13o values derived from these cements 6 1 pdb are up to 3 higher than previous estimates for the 13o value of mesoproterozoic marine calcite however because these phases are associated with facies deposited under restricted marine conditions interpretation of 13o values from these cements with regard to the composition of the open mesoproterozoic ocean awaits study of additional coeval marine carbonate successions

reef fabrics biotic crusts and syndepositional cements of the latemar reef margin middle triassic northern italy the latemar reef buildup of the central dolomites northern italy provides a rare opportunity to examine an in place middle triassic upper anisian to lower ladinian platform margin that is not strongly deformed or dolomitized the margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms across this transition the depositional profile relates directly to a consistent lateral facies pattern 1 restricted biota grainstone of the platform interior 2 tubiphytes rich boundstone and 3 diverse biota grainstone that grades into 4 foreslope breccia beds the boundstone and diverse biota grainstone facies comprise the platform margin the boundstone facies consists of a framework of small 10 cm skeletal remains 10 by volume with associated biotic crusts internal sediments and syndepositional cements crusts and cements constitute most of the rock volume and created the boundstone fabric biotic crusts exhibit gravity defying geometries and range from a light grey structure grumeleuse rind to dark grey micritic laminae both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits indicating a syndepositional origin the diverse biota grainstone facies primarily consists of skeletal peloidal grainstone with a diverse open marine biotic assemblage in contrast to the restricted biota grainstones of the platform interior that have a low diversity restricted marine biota metre scale hexacoral boundstone and centimetre scale sponge boundstone and microbial boundstone occur as isolated patches tens to hundreds of metres apart within the diverse biota grainstone facies the depositional profile facies zonation and biotic constituents all indicate that the latemar buildup had a shallow water reef margin in contrast to previous interpretations that these were upper slope reefs the syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave resistant reef fabric

manganese oxide mineralogy in an exhalative environment from the solomon islands the mineralogy and chemistry of oxidate facies sediments associated with a solomon island cupriferous pyrite deposit are described systematic variations in sediment composition are consistent with the gradual imposition of an oxidising environment on a pool of exhaled solution lying in a topographic depression on the sea floor manganese is first precipitated in the mn2 oxidation state as pyrochroite mn oh 2 with later seafloor weathering and diagenesis this oxidises initially to the mn3 intermediate minerals manganite mno oh or in the presence of iron hydroxides to groutite mno oh subsequently these oxidise further to mn4 as the mno2 dimorphs pyrolusite or ramsdellite respectively todorokite mno2 is indicative of more oxidising conditions and is found in the upper horizons of the manganiferous sediment it is invariably non stoichiometric in composition and its mn2 mn4 ratio is a direct function of the eh prevailing during its precipitation adsorption of na k ca2 and ba2 ions at the time of formation stabilises the todorokite structure and provides an explanation for its persistence with its mn2 ions in association with the completely oxidised mn4 mineral mno2 pyrolusite the enrichment of cu zn and pb in manganiferous sediments from both pelagic and exhalative environments is associated with the occurrence of goethite the enrichment of k ba and ca in manganiferous sediments from a similar range in environments is associated with the occurrence of todorokite both the mineralogy and trace element characteristics of a manganiferous sediment are thus a function of the environment of deposition rather than the ultimate source of the metals concerned the trace element contents of manganiferous floaters should therefore only be used as an indicator of related sulphide mineralisation in the presence of other favourable factors

sabkha facies and the preservation of a falling stage systems tract at the sauk ii iii supersequence boundary in the late cambrian eilean dubh formation nw scotland the durness group of northwest scotland represents a nearly one kilometer thick succession of mid cambrian to lower middle ordovician carbonates these rocks crop out in a narrow belt along the foreland of the caledonian orogen and in the moine thrust zone stretching some 170 km from loch eriboll southwestwards to the isle of skye the cambrian parts of the succession represent deposition on a paleo southeast facing low latitude passively subsiding continental margin situated at around 20 s carbonate deposition in scotland formed part of the extensive great american carbonate bank the sauk ii iii supersequence boundary occurs above an interval of thin meter scale shallowing upward peritidal cycles containing intraclast dolorudstones stromatolites tepees siliciclastic sand and surface karst evidence of former evaporites in the succession comprises pseudomorphs of nodular and bedded anhydrite crystal shaped calcite filled vugs representing growth of sulfate dissolution breccias and local halite casts these features represent sabkha facies arranged into cycles that constitute a preserved falling stage systems tract fsst the top of the fsst is marked by a pronounced karst surface of sandstone filled fissures the section preserves a rare snapshot of a seldom preserved environment on the cambrian laurentian margin its preservation during a time of falling relative sea level under slow tectonic subsidence is discussed

manganese in the s w pacific a brief review abstract manganese occurs as relatively small scale terrestrial deposits throughout the s w pacific and has been mined in vanuatu fiji new caledonia new zealand and papua new guinea listed in order of importance as producers in each case mining has now ceased to be economically profitable present information on s w pacific terrestrial manganese deposits is limited and dated and little work is being carried out no modern synthesis of these deposits or their mode of genesis exists data on the manganese distribution in soils are sparse in the marine environment substantial work has been carried out this review indicates the need for a comprehensive study of manganese ore deposits and manganese in soils in the s w pacific with a particular emphasis on comparison of results from different localities and settings

association of tepees and palaeokarst in the ladinian calcare rosso southern alps italy the ladinian calcare rosso of the southern alps provides a rare opportunity to examine the temporal relationships between tepees and palaeokarst this unit comprises peritidal strata pervasively deformed into tepees repeatedly capped by palaeokarst surfaces mantled by terra rossa palaeokarsts characterized by a regional distribution across the southern alps occur at the base and at the top of the unit local palaeokarsts confined to this part of the platform occur within the calcare rosso and strongly affected depositional facies tepee deformation ranges from simple antiformal structures peritidal tepees to composite breccias floating in synsedimentary cements and internal sediments senile tepees peritidal tepees commonly occur at the top of one peritidal cycle in association with subaerial exposure at the cycle top while senile tepees affect several peritidal cycles and are always capped by a palaeokarst surface cements and internal sediments form up to 80 of the total rock volume of senile tepees the paragenesis of senile tepees is extremely complex and records several superimposed episodes of dissolution cement precipitation fibrous cements laminated crusts mega rays and deposition of internal sediments marine sediment and terra rossa petrographical observations and stable isotope geochemistry indicate that cements associated with senile tepees precipitated in a coastal karstic environment under frequently changing conditions ranging from marine to meteoric and were altered soon after precipitation in the presence of either meteoric or mixed marine meteoric waters stable isotope data for the cements and the host rock show the influence of meteoric water average 18o 5 8 while strontium isotopes average 87sr 86sr 0 707891 indicate that cements were precipitated and altered in the presence of marine triassic waters field relationships sedimentological associations and paragenetic sequences document that formation of senile tepees was coeval with karsting senile tepees formed in a karst dominated environment in the presence of extensive meteoric water circulation in contrast to previous interpretations that tepees formed in arid environments under the influence of vadose diagenesis tepees initiated in a peritidal setting when subaerial exposure led to the formation of sheet cracks and up buckling of strata this porosity acted as a later conduit for either meteoric or mixed marine meteoric fluids when a karst system developed in association with prolonged subaerial exposure relative sea level variations inducing changes in the water table played a key role in exposing the peritidal cycles to marine mixed marine meteoric and meteoric diagenetic environments leading to the formation of senile tepees the formation and preservation in the stratigraphic record of vertically stacked senile tepees implies that they formed during an overall period of transgression punctuated by different orders of sea level variations which allowed formation and later freezing of the cave infills

p t evolution of abukuma metamorphic rocks in north east japan metamorphic evidence for oceanic crust obduction the abukuma metamorphic terrane of northeast japan is well known for the type terrains of andalusite sillimanite progressive metamorphism of miyashiro s classic studies 1958 1961 later however relict kyanite was found in the high grade pelitic gneisses kano kuroda 1968 and there have been arguments about whether the abukuma metamorphism was plurifacial or polymetamorphic earlier kyanite sillimanite type and later andalusitesiuimanite type since 1984 we have been studying the geology and petrology of the abukuma metamorphic rocks to reveal their p t t paths and the tectonic evolution of the terrane the purpose of this paper is to outline our new insight into the p t t paths of rocks and their correlation with deformation events of the terrane detailed geology mineralogy and petrology will be presented elsewhere in this paper mineral abbreviations of kretz 1983 are used to represent compositions and reactions of minerals geological and petrological setting the abukuma metamorphic terrane is one of the major geological units of north east japan which is bounded from south west japan by the tanakura tectonic line fig 1 the abukuma metamorphic rocks which are extensively intruded by the cretaceous plutonic rocks shibata uchiumi 1983 shibata tanaka 1987 are lithologically divided into two the gosaisyo series to the north east and the takanuki series to the south west fig 1 the gosaisyo series is composed mainly of metabasites and metacherts with subordinate amounts of meta ultrabasites metapelites and limestones having affinities with oceanic crust original structures and textures however are poorly preserved owing to intense deformation hiroi et al 1987 reported

the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan abstract the structures and microstructures of the takanuki and hitachi areas in the abukuma massif northeast japan are described in the takanuki area the basic gosaisho series thrusts the pelitic takanuki ones in a hp metamorphic context the nappe structure is afterwards refolded by a migmatitic dome the samegawa dome in a ht metamorphic context microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation geometric features suggest that the samegawa dome was emplaced by diapirism the role of the thrust surface as an instable interface promoting the doming is emphasized the hitachi metamorphic rocks composed of basic schist limestone and sandstone shist thrust the pelitic rocks of the western hitachi gneisses as for the takanuki area the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement owing to lithologic petrologic structural similitudes the nappe of the hitachi metamorphic rocks and that of the gosaisho series are unified into a unique nappe with a northward motion the emplacement occurred between late permian and late cretaceous likely in late jurassic the allochthonous units of the abukuma massif are correlated with the green schist nappe described in southwest japan since they are surrounded by the same zones namely the tanba zone and the kurosegawa kitakami one moreover both in southwest and northeast japan the emplacement of the green schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport that is eastward in southwest japan and northward in northeast japan owing to the late bending of the japanese islands the late jurassic nappe structure is obliquely overprinted by a ht metamorphism ryoke in southwest japan abukuma in northeast japan and afterwards cut by late faults as the median tectonic line or the tanakura fault giving rise to the present complexity

triassic monsoonal climate and its signature in ladinian carnian carbonate platforms southern alps italy abstract paleoclimatic general circulation models suggest the existence of a monsoonal climate during the permo triassic over wide parts of the megacontinent pangea and its adjacent oceans this paper discusses how ladinian carnian sedimentary successions outcropping in the southern alps record the signature of this climate sedimentological associations of tepees capped by terra rossa paleokarst braided fluvial sediments capped by caliche soils and evaporite beds alternating with clay rich delta deposits all indicate that net precipitation values changed substantially over short geological time scales early diagenetic features including episodes of dissolution in stratigraphies of meteoric calcite cements corrosion and hematitization of siliciclastic detritus prior to deposition early eu edral tectosilicate cementation and dolomitization from evaporation concentrated seawater record frequently changing paleohydrological conditions sedimentological and early diagenetic data recording highly variable seemingly conflicting paleoclimate information can best be attributed to fluctuations in net precipitation intensities controlled by monsoonal climate ranging from seasons to 106 yr d18o values in early meteoric cements 5 2 to 6 4 reflect the presence of strongly depleted meteoric waters which are not compatible with the southern alps paleolatitude or paleotopography in the triassic and can be directly related to precipitation intensities associated with a monsoonal climate strong precipitation could have also resulted in decreased surface water salinities and depleted d18o in surface waters d18o time series from marine rocks and early meteoric cements indicate a trend from middle triassic values generally deplet d with respect to the expected marine signature to less depleted late triassic values this is interpreted to represent an unusually wet episode high net precipitation transitionally grading in the late triassic into a relatively add period or low net precipitation despite the fact that several factors controlled the intensity of monsoonal precipitation the effects of its variations through time are evident at different time scales in the middle and upper triassic record of the southern alps

isolated stationary carbonate platforms the middle triassic ladinian of the marmolada area dolomites italy as a result of a phase of extensional tectonics in the western tethyan region a horst and graben topography formed during the middle triassic ladinian in northern italy horsts were sites of shallow water carbonate sedimentation while pelagic and volcaniclastic sediments were deposited in the grabens two carbonate platforms approximately 500 m thick can be distinguished in the marmolada area of the dolomites the marmolada platform proper which covered an area of 6 km2 and the costabella platform which extended for about 12 km in a nw se direction and was about 3 km across the facies of these isolated platforms reflect the influence of storms from the sw windward platform margins were characterized by a marine sand belt of skeletal and aggregate grainstones with a dominant platform directed cross stratification the central portions of the platforms were occupied by supratidal sand cays which are made up of storm washovers leeward parts of the platforms are composed of shallow subtidal sand flat deposits laterally discontinuous reefs chiefly composed of various calcareous algae are developed at the outer margins of the platforms along windward margins reefs may form a belt several hundred metres wide along leeward margins their width is commonly reduced to some tens of metres foreslope talus breccias surround the platforms clinoform bedding showing basinward dips of 30 40 is typical of this facies belt which is approximately 2 km wide basinal sediments only some tens of metres thick are radiolarian micrites abundant sediment gravity flow deposits expand the basinal sequence at the toes of windward margins and were probably triggered by storm return flows synsedimentary faults striking both nne ssw and nw se separate the bedded platform limestones from flank deposits and reefs they account for the stationary nature of the platforms neptunian dykes show preferred nne ssw and e w trends sinistral displacements are associated with nw se trending faults depressions in the basins filled with red turbiditic pelagic sediments show n s trends and are probably compressional in origin the structural pattern may have resulted from oblique nw se oriented extension of the e w trending middle triassic graben zone of the dolomites in the ladinian of the dolomites the stationary platform type can be distinguished from a retrograding type whereas continuously prograding platforms apparently did not develop

hydrothermal diagenesis of paleozoic seamount carbonate components we present geochemical results of paleozoic carbonate components brachiopods biogenic low mg calcite blmc crinoids biogenic high mg calcite bhmc and whole rock admixture of aragonite and calcite wrac from open ocean permo carboniferous seamounts of japan situated in the tropical mid panthalassic ocean the diagenetic history of these and other carbonate allochems was determined by visual optical inspection of their external morphology and luminescence by evaluation of their microstructures and by assessment of their trace element and stable isotope distributions the degree and trends of the diagenetic alteration tests were then reconciled with their concomitant sr isotope values and compared to those of the established seawater sr isotope curve the altered carbonate material from three of the four time slices support the novel concept of diagenesis in the presence of marine hydrothermal fluids leading to less radiogenic sr isotope values in addition sr isotope values of unaltered blmc from three of the four time slices do not concur with the values for the concurrent time of the established seawater 87sr 86sr curve an adjustment of their stratigraphic position is untenable and unsatisfactory in light of their biostratigraphic assignment which suggests that sr isotope diagenesis is much more complex and complicated than is espoused in the literature consequently the concept of less radiogenic sr isotope values in diagenetic marine carbonates has profound implications for the veracity of the established seawater 87sr 86sr profile and curve and it needs to be re evaluated in light of the findings

tepees in peritidal carbonates origin via earthquake induced deformation with example from the middle cambrian of western canada abstract tepee structures are common in many ancient carbonate tidal flat successions and because analogues are known from tropical coastal areas they are well established as evidence for synsedimentary lithification of subaerially exposed carbonate sediment existing explanations involving desiccation thermal expansion and contraction fluctuating groundwater pore pressure or diagenetic force of crystallization do not adequately account for the style and degree of deformation seen in some examples such as those in the middle cambrian cathedral formation of the southern rocky mountains western canada these tepees provide evidence for essentially instantaneous events causing brittle failure of decimetre thick crusts of ooidal and pisoidal grainstone followed by their dilation rupturing separation tilting and shingling they can be explained as breccias generated by sporadic episodes of strong ground motion from earthquakes that emanated from a syndepositionally active fault system nearby variably directed tensile and compressive stresses broke crusts shifted them upwards and shuffled them laterally subsequent microbial encrustation and vigorous flushing by seawater in the resulting shelter cavities led to rapid stabilization by cementation recognition of the role of seismically induced deformation in explaining some tepee structures furnishes a proxy for synsedimentary tectonism and adds another member to the family of seismites which reflect a range of rheological properties of unlithified and lithifying sediments

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

palaeokarst influenced depositional and diagenetic patterns in upper permian carbonates and evaporites karstryggen area central east greenland the karstryggen area of eastern greenland represents the western edge of sedimentation in the jameson land basin an arm of the northern zechstein seaway upper permian strata of this area were deposited as two major sequences the first marine incursion transgressed largely peneplaned lower permian strata and deposited thin paralic conglomerates sandstones and shales the huledal formation followed by a thick package of carbonates and evaporites the karstryggen formation although the karstryggen formation represents the transgressive maximum of this sequence it contains only marginal or restricted marine strata including micritic stromatolitic and peloidal carbonates and thick but localized bedded gypsum deposits these lithofacies indicate that relatively arid climates prevailed in this basin as in most of the zechstein region a major regression associated with a change to a more humid climate terminated karstryggen sedimentation pre existing evaporites and carbonates underwent diagenetic alteration including widespread calcitization and dissolution of gypsum more importantly topographic relief in excess of 120 m was generated by fluvial drainage systems and karstic sinkholes a second marine incursion accompanied by a return to a semi arid climate drowned this high relief topography producing a complex sequence of strata the wegener halvo formation in which sedimentation was greatly influenced by the rugged underlying terrain marine cemented algal molluscan grainstones draped pre existing palaeotopography during the initial stages of flooding continued drowning led to differential sedimentation on highs and in lows oolitic and bryozoan brachiopod grainstones formed as shoals on the crests of most prominences whereas shales conglomeratic debris flows evaporites or oolitic turbidites were deposited in the lows more restricted sedimentation took place in the westernmost areas which lay closest to the mainland shoreline and were situated to the west of a palaeotopographic ridge there oolitic stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions three subcycles mark smaller scale relative changes of sea level that occurred during deposition of the wegener halvo formation they are delimited by regional surfaces with moderate relief 5 20 m developed during subaerial exposure widespread diagenetic changes including leaching of aragonitic grains dissolution collapse brecciation of evaporites and meteoric calcite cementation occurred in association with these smaller scale sequence boundaries again reflecting climatic oscillations relative sea level fluctuations coupled with regional climate changes played a dominant role in determining both depositional and diagenetic relations in these strata these features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern zechstein basin and barents shelf and may have economic significance for the localization of hydrocarbons

geochemistry of preserved permian aragonitic cements in the tepees of the guadalupe mountains west texas and new mexico u s a abstract tepees are common depositional structures within the shelfal capitanian stage yates and tansill carbonate strata of the permian reef complex of the guadalupe mountains as part of the tepees botryoids of radiating aragonite crystals formed essentially contemporaneously with the host pisolitic strata although most of the cement in the tepees is composed of diagenetic low magnesium calcite x ray diffraction analyses indicate that aragonite constitutes as much as 27 within centimeter size patches back scatter electron microprobe analyses demonstrate a highly irregular pattern of alteration of partly preserved aragonite crystals these patches with the preserved aragonitic fibers have undetectable cathodoluminescence sr concentrations that range up to 3 900 ppm and 87sr 86sr ratios from 0 70689 to 0 70695 which are in agreement with worldwide marine values for the capitanian stage mass balance calculations show that in some samples the diagenetic low mg calcite associated with the preserved aragonite has over 2 300 ppm sr thus the diagenesis to calcite occurred under very low water to rock ratios the least altered of the aragonitic cements have 13c values as high as 7 8 v pdb equivalent to 6 v pdb for calcite and 18o values as low as 5 5 v pdb equivalent to 6 2 v pdb for calcite the high 13c values are consistent with worldwide marine values whereas the very low 18o values probably represent precipitation in hot equatorial shallow marine to tidal flat environments contrary to most diagenetic pathways displayed by marine carbonates increasing alteration has generally resulted in higher 18o values in these relict permian aragonites these analyses demonstrate conclusively that paleozoic aragonitic cements are still preserved in some strata

coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification draken formation upper riphean svalbard the draken formation 120 250 m of northeast spitsbergen svalbard forms part of a thick upper proterozoic carbonate platform succession it consists predominantly of intraformational dolomitic conglomerates with excellent textural preservation six main lithofacies were recognized in the field quartz sandstones stromatolitic mats conglomerates with silicified intraclasts dolostone conglomerates with desiccated mudrocks oolitic pisolitic grainstones and fenestral dolostones a series of five main gradational biofacies were recognized from silicified and rare calcified microfossils biofacies 1 represents low energy subtidal benthos erect filaments and plankton acritarchs and vase shaped microfossils whereas biofacies 2 to 5 are microbial mat assemblages with filamentous mat builders and associated dwellers and washed in plankton ranging from basal intertidal to high intertidal supratidal colour values a measure of the lightness of the colour shade of sawn rock samples were quantified using a munsell chart and exhibit a pronounced variation means of major groups varying from 4 0 to 5 95 across the spectrum of subtidal to supratidal sediments as inferred from other criteria the lightening in progressively more exposed sediments is related to lowering of organic carbon contents probably mainly by oxidation six types of early cement have been recognized calcite microspar type 1 is common as a subtidal cement in many proterozoic formations whereas types 2 subtidal isopachous fringes 3 subtidal hardground dolomicrite and 4 intertidal meniscus dolomicrite are very similar to phanerozoic examples except for their dolomitic mineralogy types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena they characterize the fenestral lithofacies and compare with modern supratidal cements consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching dolomitization silicification were all significant syndepositional processes altering the original metastable carbonates the data set provides evidence for a spectrum of peritidal environments including ooid shoals protected subtidal tidal sandflats and protected carbonate mudflats different sections show a preponderance of particular facies the coastal lithofacies continuum was completely dolomitized unlike offshore to ooid shoal facies of adjacent formations dolomitization thus bears a relationship to depositional bathymetry although hydrodynamics clearly have a role the potential importance of whiting precipitation in raising mg ca in marginal marine environments is also stressed

comments on the paper the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan by m faure f lalevee y gusokujima j t iiyama and j p cadet commentaire sur l article de m faure f lalevee y gusokujima j t iiyama et j p cadet paru dans earth planet sci letters nld 1986 77 384 398

tepee shaped agglutinated microbialites an example from a famennian carbonate platform on the lennard shelf northern canning basin western australia distinctive metre scale antiformal structures are well developed in a famennian carbonate platform in the chedda cliffs area of the lennard shelf reef complexes the structures are distinguished by chevron shaped crests and thickened cores and contain abundant non skeletal allochems ooids pisoids peloids and intraclasts of silt to pebble size and variably developed laminations and fenestrae the internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment the facies and stratigraphic context of the mounds support a shallow subtidal transitional backreef to reef flat setting however alone these mounds do not provide unequivocal environmental information other large antiformal structures in famennian platforms on the lennard shelf previously described as tepee structures show morphological similarities to the chedda cliffs mounds which suggests that these other structures may also be microbial mounds the presence of microbial mounds in platform successions further highlights the importance of microbial communities in the lennard shelf reef complexes

proterozoic ministromatolites with radial fibrous fabric small digitate stromatolites with diameters in the range of 0 2 20 mm ministromatolites are common in early and middle proterozoic carbonate sequences and extend stratigraphically from the archaean to the holocene an occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial fibrous fabric and microscopically crinkled microcrenate lamination is described from the 1 9 gyr old belcher supergroup mcleary formation in southeastern hudson bay canada the structures which can be considered to be a variety of tufa are unusually well preserved because of early diagenetic silicification columnar types are referable to pseudogymnosolen asperia and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern canada where the fabric is normally preserved by a secondary mosaic of dolomite the textural evidence of angulate cross sections and rectilinear divergent patterns indicates that the radial fibrous fabric represents primary or very early diagenetic precipitation and that pseudogymnosolenids with mosaic dolomite originally also had radial fibrous structure the precipitation may have been within or on microbial mats

paleomagnetism of the middle cretaceous iritono granite in the abukuma region northeast japan abstract we have studied the paleomagnetism of the middle cretaceous iritono granite of the abukuma massif in northeast japan together with 40 ar 39 ar dating paleomagnetic samples were collected from ten sites of the iritono granite 102 ma 40 ar 39 ar age and two sites of its associated gabbroic dikes the samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses most of natural remanent magnetizations show mixtures of two components 1 h component high coercivity b c 50 90 mt or high blocking temperature t b 350 560 c component and 2 l component relatively low b c or low t b component h component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination i 29 9 d 311 0 95 2 7 n 12 this direction is different from the geocentric axial dipole field at the present latitude i 56 5 and the typical direction of the cenozoic remagnetization in northeast japan since rock magnetic properties indicate that the h component of the iritono granite is carried mainly by magnetite inclusions in plagioclase this component probably retains a primary one thus the shallow inclination indicates that the abukuma massif was located at a low latitude 16 1 1 6 n about 100 ma and then drifted northward by about 20 in latitude the northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast japan due to miocene opening of the japan sea according to this model the low pressure and high temperature low p high t metamorphism of the abukuma massif which has been well known as a typical location would have not occurred in the present location on the other hand the l component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination i 42 8 d 311 5 95 3 3 n 9 since this direction is intermediate between the h component and early cenozoic remagnetization in northeast japan some thermal event would have occurred at lower temperature than pyrrhotite curie point 320 c during the middle cretaceous to early cenozoic time to have resulted in partial remagnetization

dolomitized calcrete in the middle devonian winnipegosis carbonate mounds subsurface of south central saskatchewan canada dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the middle devonian winnipegosis mud mounds in the subsurface of south central saskatchewan individual calcrete profiles are composed of 2 to 5 horizons including in descending order laminar crust massive pisolitic breccia chalky and transitional horizons the calcrete profiles are interpreted to be pedogenic in origin formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds this interpretation is based on the occurrence of an orderly set of well differentiated horizons disconformities vadose pisoids micritic stringers circumgranular cracks linkage coatings and gradational contacts between the calcrete and underlying host carbonate rocks lack of root related structures such as rhizoconcretion and alveolar septal fabric in the winnipegosis calcrete suggests a limited influence of macrophytes on the formation of calcrete the occurrence of up to three discrete pedogenic calcrete profiles in a single winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the elk point basin of saskatchewan during middle devonian time d 2004 elsevier b v all rights reserved

rapid growth rates of syndepositional marine aragonite cements in steep marginal slope deposits bahamas and belize abstract growth rates of marine botryoidal aragonite cements from steep 35 45 marginal slope deposits in the bahamas and belize have been determined by accelerator mass spectrometer radiocarbon dating of samples taken at the base and top of individual botryoids the pore filling cements which range from approximately 11 000 13 000 years old grew at average rates of 8 10 mm 100 yr with maximum rates 25 mm 100 yr radiocarbon dating of coexisting skeletal components indicates that cementation was syndepositional microsampling transects across individual botryoids for stable isotope analyses show little variation in 13c and 18o supporting the conclusion that cementation was extremely rapid although the cements show a progressive depletion in isotopic composition of approximately 1 13c and 2 18o from 13 ka to 11 ka the average variation 1 within individual por filling cements ranging in size from 2 mm to 32 mm bottom to top was 0 11 13c and 0 14 18o results of this study provide the first quantitative data on growth rates of marine carbonate cements in a marginal slope environment the data indicate that marginal slope deposits may lithify within several tens of years and suggest that geologically instantaneous cementation may be critical in stabilizing steep carbonate slope deposits at or above angles of repose

a sedimentological and stable isotopic study of travertines and associated sediments within upper triassic lacustrine limestones south wales uk the upper triassic in south wales is composed of up to 100 m of red dolomitic mudstones of the mercia mudstone group which overlie and are laterally equivalent to basin margin coarse clastic deposits in the sully island and dinas powys areas a series of carbonate deposits was laid down within small basins adjacent to the main bristol channel basin the rocks consist of dolomites containing replaced evaporites overlain by perilittoral freshwater limestones the limestones are fenestral intrasparites and contain abundant pedogenic and stromatolitic horizons as well as locally developed travertines evidence for vadose diagenesis within the limestones is common the travertines consist of sheets of fibrous calcite flowstone associated with pisoids and floe calcite most of the travertines consist of single sheets several millimetres in thickness although mounds up to 1 m in height and 5 m in diameter are also present the topmost metre of the limestones which has been dolomitized is deformed into tepee and megapolygonal structures the carbon and oxygen stable isotopic composition of the limestones suggests that they were precipitated in low salinity waters successive samples from individual bands of flowstone show a covariance of 18c and 13c which is consistent with the mixing of resurgent groundwaters with pools of more evolved waters at the surface the sedimentological and geochemical evidence suggests that the limestones were deposited in a small enclosed basin fed by upwelling meteoric groundwaters in an environment distinct from that in which the laterally equivalent gypsiferous red mudstones were formed

facies architecture of an isolated carbonate platform tracing the cycles of the latemar middle triassic northern italy the 720 m thick succession of the middle triassic latemar massif dolomites italy was used to reconstruct the lagoonal facies architecture of a small atoll like carbonate platform facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations which reflect a syndepositional palaeorelief based on tracing of lagoonal flooding surfaces the metre scale shallowing upward cycles are interpreted to be of allocyclic origin short term sea level changes led to subaerial exposure of wide parts of the marginal zone resulting in the development of a tepee belt of varying width occasional emergence of the entire lagoon produced lagoon wide decimetre thick red exposure horizons the supratidal tepee belt in the backreef area represented the zone of maximum elevation which circumscribed the sub to peritidal lagoonal interior during most of the platform s development this tepee rim the subtidal reef and a sub to peritidal transition zone in between stabilized the platform margin the asymmetric width of facies belts within individual metre scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present day south and west the overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit overlain by three tepee rich intervals separated by tepee poor units composed of subtidal to peritidal facies this stacking pattern reflects two third order sequences during the late anisian to early middle ladinian

brachiopods as indicators of original isotopic compositions in some paleozoic limestones the original stable isotopic composition of paleozoic marine carbonate rocks is important to establish a baseline for evaluating diagenetic alteration and providing a reliable data base for modeling exogenic geochemical cycles of the elements we have compared the 18 o and 13 c values in the nonluminescent portions of brachiopods with similar published data of whole brachiopods other whole fossils and estimates of original isotopic composition of marine cements the oxygen and carbon isotopic records for each of these paleozoic components show generally good internal consistency significant discrepancies however were noted between results on the different components isotopic and trace element data presented in this study suggest that permo carboniferous whole brachiopods are little altered chemically nonetheless a more reliable but not fool proof approach for obtaining pristine isotopic values is microsampling of least altered regions based on cathodoluminescence in conjunction with staining techniques and elemental analyses the carbon and oxygen isotopic compositions of all components decrease in 18 o and 13 c in geologically older material neither temporal trend is monotonic however oxygen isotopic results suggest an important positive shift in 18 o of about 2 during the early carboniferous whereas the carbon isotope trend shows a major positive shift 2 3 during the mid carboniferous

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

reply to the comment on first records of syn diagenetic non tectonic folding in quaternary thermogene travertines caused by hydrothermal incremental veining by billi et alii abstract in our previous paper billi et al 2017 using field geological observations u th dating and stable isotope analyses we studied two deposits of pleistocene thermogene travertines from tuscany in central italy we concluded our study 1 warning that the common stratigraphic concept of travertine being a sedimentary succession with age younging from bottom to top is not always correct 2 demonstrating that caco3 mineralization and veins can develop within the travertines after their formation with this syn diagenetic process being able to modify the continuous bottom up age evolution and 3 showing that this post depositional mineralization veining process can not only modify the temporal succession but also deform and change the initial depositional travertine structure and its petrophysical properties these conclusions could potentially make the interpretation of a travertine series more difficult than commonly thought alcicek et alii questioned our conclusions claiming that the travertine structures that we observed in tuscany and interpreted as post depositional features should have been interpreted in analogy to similar structures from travertines elsewhere as primary structures although we recognize as already thoroughly stated in billi et al 2017 that the travertine depositional post depositional processes generally require further studies we reaffirm the validity of our original interpretation at least for the structures analyzed in our previous paper we therefore counter all criticisms by alcicek et alii and conclude by indicating the way forward to further explore the depositional and post depositional processes of thermogene travertines

microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation mesoproterozoic gaoyuzhuang formation china abstract silicified stratiform stromatolites of the mesoproterozoic gaoyuzhuang formation 1 4 1 5 ga china contain well preserved microfossils the cherts also harbor varied synsedimentary precipitates and void filling cements replaced by early diagenetic silica minerals these precipitates disclose microenvironments characterized by supersaturated solutions in protected shallow depressions within an intertidal setting the precipitates provided surfaces for selective microbial settlement and rigid sediment matrix for microbial growth together with early silicification the frequent precipitation events contributed to preservation of microfossils by reducing sediment compaction and shearing fossiliferous cherts display fine flat and wavy lamination characterized by an alternation of highly silicified thin organic rich layers with thick sediment rich layers organic rich layers are dominated either by coccoid or by filamentous microfossils whereas sediment rich layers contain abundant synsedimentary precipitates within which the microfossils are preserved in their growth position four dominant microfossils coccostratus dispergens n gen et sp eoentophysalis belcherensis eoschizothrix composita and siphonophycus inornatum occur contiguously through several tens of laminae and are identified as main frame building biological components of gaoyuzhuang stromatolites community composition microbial density distribution orientation and developmental patterns of the frame building microfossils are closely correlated with the changing depositional events of gaoyuzhuang cherts contrasting conditions of sedimentary kinetics with those of sedimentary stasis each assemblage of frame building microfossils responded to sedimentation with different mechanisms to escape burial high sedimentation rates correlate with scattered colonies of coccoids and with loose webs of predominantly upright filaments low sedimentation rates correlate with dense laterally connected colonies of coccoids and with a change in filament orientation from vertical to horizontal in multi trichomous microfossil eoschizothrix composita low sedimentation rates are also accompanied with an increase in number of trichomes per filament the observed morphological variability of the frame building microfossils is explained by microbial development reproduction and behavior by interactions between sedimentological and biological controls

sea level changes versus hydrothermal diagenesis origin of triassic carbonate platform cycles in the dolomites italy abstract the triassic platform carbonate cycles of the dolomites best developed and studied at the latemar have been considered key examples for the documentation of eustatic sea level changes the cycles appear to reflect a low energy lagoonal environment which is however difficult to reconcile with the small platforms lacking marginal energy barriers in a tropical stormy climate an alternative interpretation to eustatically controlled cyclicity and repeated subaerial exposure is provided by diagenesis in a hydrothermal field elevated heat flow 0 2 ma before the onset of the mafic igneous activity triggered the circulation of diagenetic fluids with a composition similar to seawater by intermittent opening of fractures through some platforms this caused tepee formation by precipitation of displacive cement pendant cement and pisolite formation and favoured cyanobacterial mat formation and their early dolomitization the abundance of cycles correlates with that of the somewhat younger volcanic dikes crosscutting some platforms diffuse venting or seeping in the upper unconsolidated sediment layers explains the stratiform occurrence of the diagenetic features and alternation with normal marine deposition in deep but photic water at the wave base

salina margin tepees pisoliths and aragonite cements lake macleod western australia their significance in interpreting ancient analogs tepee structures banded aragonite cements and pisoliths are currently forming in lake macleod a carbonate evaporite salina in western australia although lake macleod is separated from the indian ocean by a barrier it lies 3 4 m below sea level which promotes the seepage of seawater through the barrier and its discharge from vents and seepage mounds around the margin of the salina discharging waters have precipitated and diagenetically altered carbonate sediments within these seepage mounds to form tepee structures of lithified protodolomite overlying cavities that are lined with banded aragonite cement and floored by both cement and pisoliths significant variations in 18o 5 1 pdb and 13c 5 5 pdb of the aragonite cements were documented and are thought to record shifts in the isotopic composition of the water brought about by the effects of evaporation influx of meteoric water and oxidation of organic water carbon 14 dating of cements indicates that cementation began about 3400 b p and has proceeded at a rate of about 0 2 to 0 4 mm 100 yr the highest rate occurring during evaporative episodes by analogy with lake macleod and other australian salinas peritidal tepee structures and associated diagenetic carbonates in the permian capitan reef complex may owe their origin to speleanlike diagenesis operative in a marine groundwater discharge zone

transgressive oversized radial ooid facies in the late jurassic adriatic platform interior low energy precipitates from highly supersaturated hypersaline waters dark gray oolitic units characterized by oversized ooids with primary radial calcite fabrics occur in the interior of the late jurassic late tithonian adriatic platform a large mesozoic tethyan isolated platform in croatia they differ from open marine platform margin ooid grainstones in their dark color cerebroid outlines broken and recoated grains abundant inclusions highly restricted biota and lack of cross stratifithey have been interpreted as being of vadose origin vadoids at tops of upward shallowing parasequences however detailed sections show that most oolitic units occur at bases of precessional parasequences overlying erosional surfaces on fenestral carbonates the oolitic units are similar to quiet water ooids that form today in low energy settings they developed in an arid climate during initial transgression of supratidal fl ats along lowenergy shores seaward of tidal fl ats and along the margins of restricted lagoons and intertidal ponds superimposed fenestral fabrics meniscus micrite cements and grain breakage occurred as they aggraded to high tide level and were subjected to wetting and drying thermal expansion and contraction and wind transport they migrated landward with transgression forming extensive sheets and were overlain by subtidal lagoonal facies that shallow up into fenestral carbonates these distinctive facies may have been overlooked in the geological record or their geological distribution requires juxtaposition of calcite seas high calcite supersaturation states arid climate and presence of fl at topped carbonate platforms in a greenhouse world

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

reef margin collapse gully formation and filling within the permian capitan reef carlsbad caverns new mexico usa an area of reef margin collapse gully formation and gully fill sedimentation has been identified and mapped within left hand tunnel carlsbad caverns it demonstrates that the capitan reef did not at all times form an unbroken border to the delaware basin geopetally arranged sediments within cavities from sponge algal framestones of the reef show that the in situ reef today has a 10 basinwards structural dip similar dips in adjacent back reef sediments previously considered depositional probably also have a structural origin reoriented geopetal structures have also allowed the identification of a 200 m wide 25 m deep gully within the reef which has been filled by large some 15 m randomly orientated and in places overturned blocks and boulders surrounded by finer reef rubble breccias and grainstones block supply continued throughout gully filling implying that spalling of reef blocks was a longer term process and was not a by product of the formation of the gully gully initiation was probably the result of a reef front collapse with a continued instability of the gully bordering reef facies demonstrated by their incipient brecciation and by faults containing synsedimentary fills gully filling probably occurred during reef growth and younger reef has prograded over the gully fill blocks contain truncated former aragonite botryoidal cements indicating early aragonite growth within the in situ reef in contrast former high magnesian calcite rind cements post date sedimentation within the gully the morphology of cavern passages is controlled by reef facies variation with narrower passages cut into the in situ reef and wider passages within the gully fill gully fills may also constitute more permeable zones in the subsurface

link of laminated crusts associated with pisolites to sea level change a case study from the ladinian middle triassic longtou formation guizhou china abstract the nature depositional or diagenetic of laminated crusts associated with pisolites is still in debate which hampers the interpretation of sea level change on carbonate platforms the ladinian yangtze platform margin formed by the longtou formation contains abundant laminated crusts associated with pisolites providing an opportunity to decode their formation in response to sea level variations there individual laminae are formed of isopachous radiaxial fibrous calcite rfc crystals the rfc displays strong undulose extinction and orange red or dull red cathodoluminescence the pisolites vadoids coexisting with the laminated crusts are characterized with inverse graded bedding stalactitic cements and red luminescence or non luminescence the laminated crusts and associated pisolites have 18o values ranging from 10 56 to 3 83 and from 7 12 to 6 33 respectively which are more negative than that of the ladinian normal seawater they also share similar rare earth element and yttrium ree y distribution patterns with enriched heavy rare earth element hree enriched y element true negative ce anomalies and y ho ratios between 34 and 51 detailed petrographic and geochemical analyses indicate that the laminated crusts and associated vadoids formed in caves in response to alternating meteoric water and seawater with the fluctuating water table resulting in their mixed deposition the preservation of laminated crusts and vadoids in the longtou formation implies that they formed during a period of overall sea level transgression which was punctuated by sea level regression and allowed formation of cave fillings our results suggest that laminated crusts present in ancient carbonate successions may indicate the positions of subaerial exposure related to sea level regressions

deev jahi model of the permian triassic boundary mass extinction a case for gas hydrates as the main cause of biological crisis on earth the smoking gun revealing the secrets of the end permian mass mortality is a unique 1 2 m thick layer consisting of 5 20 cm long crystals of calcite that occurs precisely at the permian triassic boundary ptb in iran armenia turkey and china this layer is interpreted as synsedimentary abiotic seafloor cement indicative of precipitation from a highly carbonate supersaturated seawater its 13 c composition 13 c 0 pdb is 4 to 5 pdb lower than the typical upper permian values 4 to 5 pdb suggesting the involvement of massive amounts of gas hydrate ch 4 13 c 60 pdb the temporal coincidence of the cement layer with the ptb suggests that the process that promoted seafloor cementation was also responsible for the biological crisis a cementation model is developed based on accumulation dissociation cycle of gas hydrates which also explains the mass extinction at the ptb the upper permian accumulation period of gas hydrates ended abruptly adjacent to the ptb and the dissociation event began releasing 3 2 to 4 7 10 18 g ch 4 into the ocean oxidation of ch 4 in the water column created a seawater that was charged with co 2 an oceanic acid bath and had lower than normal o 2 content but not anoxic this oceanic acid bath first dissolved suspended fine grained carbonate particles and small calcareous organisms followed by extensive dissolution of platform carbonates raising ca 2 and hco 3 concentrations of seawater when the release of ch 4 declined the acid bath ocean became a soda ocean precipitating massive amount of seafloor cements observed globally at the ptb the study suggests that prior to cement precipitation the ptb ocean was charged with co 2 warm had low oxygen high ca 2 and high hco 3 concentrations these conditions collectively created stressful conditions causing the marine mass mortality the leakage of ch 4 to the atmosphere produced a super hot climate resulting in the biological devastation on land the proposed kill mechanism is developed on the basis of the physical clue the cement layer left behind by the killing process the change in ocean chemistry the accumulation dissociation cycles of gas hydrates also explain the 13 c pattern of marine carbonates and the periodicity of mass extinction events during the phanerozoic accumulation periods were long 5 to 20 my providing favorable conditions for ecosystem development pardeess phase the dissociation events were short and catastrophic 10 to 500 ky causing low oxygenation super hot climate and biological devastation doozakh phase it appears that most mass extinctions of the phanerozoic have been related to the internal working of the earth system during the phanerozoic methane has played the role of the deev jahi a female demon in persian mythology whose task is to attack the earth every so often to kill life on land and in sea

hydrothermal manganese deposits in island arcs and related to subduction processes a possible model for genesis abstract submarine hydrothermal manganese deposits have recently been recovered from a number of island arc settings such as the tonga kermadec ridge ogasawa arc trench bonin arc mariana arc and bismarck archipelago it has also been shown that large volumes of fluid are released during dewatering of subducting sediments the amount of water subducted globally 1 5 10 15 g yr 1 is about two orders of magnitude less than that advected at ocean spreading centres hydrothermal manganese deposits may be precipitated from these waters the distribution of terrestrial manganese deposits around the circum pacific belt and elsewhere suggests that some of these deposits may be of hydrothermal origin related to subduction processes at present most of these terrestrial deposits are inadequately described to permit the establishment of the precise tectonic setting at the time of deposition of the manganese deposits and therefore an unambiguous assessment of this hypothesis

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations