carbonate recrystallization in shallow marine environments a widespread diagenetic process forming micritized grains abstract integrated morphologic petrographic and mineralogic analyses of sediment grains of the miliolid foraminifer archaias and the green alga halimeda from shallow marine environments in florida the bahamas and belize document pervasive syndepositional recrystallization in these grains scanning electron microscopy shows that original skeletal rods and needles recrystallize on the sea floor to a variety of equant micritic fabrics in thin section this textural alteration corresponds to progressive micritization with loss of the familiar golden brown color exhibited by fresh miliolids and halimeda and gradual development of gray cryptocrystalline carbonate x ray diffraction and electron microprobe studies show that this textural alteration may be accompanied by mineralogical changes from mg calcite to aragonite or from aragonite to mg calcite our findings support petrographic studies that reported syndepositional recrystallization in a wide variety of carbonate grains about thirty years ago but which have been largely ignored together these studies indicate that carbonate recrystallization is a widespread process of early diagenesis extensive recrystallization in shallow tropical seas challenges basic principles regarding the setting and timing of textural and mineralogic alteration of metastable carbonate grains in addition recognition of recrystallization as an important process of micritization contradicts prevailing theories that micritization results solely by infilling of microborings finally our findings raise fundamental questions concerning the nature of carbonate crystal structure mechanisms of crystal growth and driving forces for recrystallization answering these questions will require sedimentologists to step beyond traditional bounds into fields such as biomineralization and materials science

stromatolites near the permian triassic boundary in chongyang hubei province south china a geobiological window into palaeo oceanic fluctuations following the end permian extinction abstract stromatolites just after the end permian extinction are exceptionally well developed in the chongyang area of hubei province south china the stromatolites include endolithic coccoidal microbes and exhibit three growth forms columnar layered pseudo columnar and columnar each located in different stratigraphic horizons the columnar type is the most conspicuous and provides key information on the mode of their construction in this paper we describe the mega to microscopic features of these stromatolites together with interpretations of their construction the columnar type typically comprises alternations of lighter and darker laminae with intercalations of dense micritic laminae the lighter laminae are composed of fine to coarse calcite cement and fine dolomite whereas the darker laminae are composed of micrite and microspar endolithic coccoids are sporadic in these laminae in contrast the dense micritic laminae consist of micrite with abundant endolithic coccoids accentuating the laminae of stromatolites the columnar stromatolites were formed under deteriorating conditions after the end permian mass extinction as follows 1 the alternating development of lighter and darker laminae due to the colonization of different microbial communities in lower energy shallow to deep subtidal settings 2 carbonate precipitation within each microbial community through microbial metabolic activity and degradation of extracellular polymeric substances eps leading to the alternations of lighter and darker laminae 3 episodic interruption of the growth of the lighter and darker laminae by endolithic coccoidal activity and 4 high levels of endolithic activity accompanied by the precipitation of carbonate within boreholes to produce the dense micritic laminae in contrast the columnar layered and pseudo columnar types comprise alternations of lighter and darker laminae composed mainly of fine to coarse calcite cement fine dolomite micrite peloids and microspar with sporadic endolithic coccoids the dense micritic laminae are absent or rare in these stromatolites these types were essentially produced by the repetition of processes 1 and 2 documented in the columnar type the finely alternating laminae of these stromatolites are therefore the product of the repeated initiation and cessation of growth of relevant microbial communities in response to changing environmental conditions the stromatolites were formed immediately after the end permian extinction and persisted intermittently throughout the early triassic the chongyang stromatolites are undoubtedly some of the best developed stromatolites near the permian triassic boundary and retain a high resolution record of microbial successions and palaeo oceanic perturbations immediately following the end permian extinction

calcimicrobial stromatolites at the permian triassic boundary in a western tethyan section bukk mountains hungary basal triassic carbonates of the gerennavar formation bukk mountains northern hungary were studied the succession was deposited in an open marine deep ramp environment of the western tethys and can be divided into three parts on the basis of the dominant microfacies the lowermost 0 5 m thick interval is made up by thin bedded and bioturbated mudstones which contain ostracods and earlandia along with relict skeletal fragments the successive 8 m thick deposits consist of planar stromatolites with well preserved microfabrics the most characteristic microfabric elements are sphere clusters in a dense micrite groundmass bushy aggregates of micrite clots bundles of prostrate micrite threads and peloids the formation of the spheres and the surrounding dense micrite is attributed to calcification of a coccoid cyanobacteria dominated mat community the tufted micrite clot clusters crudely resemble angusticellularia a filamentous microbial origin is likely for the thread bundles the peloids are interpreted either as reworked pieces of calcified mat or rolled microbial clumps the reduced preservation of microfabric components in the upper unit refers to less favourable condition for mat development which is probable due to deepening of the depositional environment the overlying mudstone unit is very poor in fossils the preservation of microbial microfabrics in the stromatolites indicates that early lithification by carbonate precipitation was widespread and intense even in open marine environments following the end permian boundary events this supports the assumption that significant changes took place in geochemistry of seawater during the late permian early triassic time interval d 2006 elsevier b v all rights reserved

texture of microbial sediments revealed by cryo scanning electron microscopy abstract textures of modern lacustrine stromatolites on kiritimati line islands central pacific ocean and of buried layers in the stromatolitic carbonate sediments from french polynesian atoll lakes kopara have been studied using cryo scanning electron microscopy sem equipped with a freeze drying sample preparation device this study confirms that microscopic three dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria and from other extracellular polymer secretions are common components of microbial sediments of which they may form the framework in addition to this role in sediment cohesion and formation of microstructure the organic framework appears to be involved in carbonate precipitation with n the stromatolites through chemical nucleating steric and hydrodynamical controls the role of the dead organic constituents of the stromatolites is not only confirmed in micrometer size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments and to the formation of peloids that evolve into spherulites bacterial including nannobacterial carbonate bodies and carbonate impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied all these carbonate precipitation processes may cooperate in lithification of a given sediment

lower triassic peritidal cyclic limestone an example of anachronistic carbonate facies from the great bank of guizhou nanpanjiang basin guizhou province south china abstract most shallow marine sections described from the lower triassic formed in mixed carbonate siliciclastic systems shallow marine limestones from an isolated carbonate platform the great bank of guizhou in the nanpanjiang basin south china provide a unique opportunity to examine a pure peritidal carbonate depositional system the platform had a low relief bank profile in the early triassic with oolite shoals at the margin shallow subtidal and peritidal deposits in the interior and pelagics debris flow deposits and turbidites on gentle basin margin slopes interior strata are 265 m thick beginning in the lowermost triassic with renalcis biostromes followed by lime mudstone oolite and cyclic peritidal limestone the cyclic limestone is olenekian in age 164 m thick and contains up to 83 cycles parasequences parasequences are 0 2 7 4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by renalcis reef mounds or biostromes and capped by intertidal flaser bedded ribbon rock the skeletal packstone and reef mounds contain a low diversity biota of cyanobacteria echinoderms bivalves gastropods lingulid brachiopods spirorbids and ostracodes ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae scour surfaces ripple cross lamination lime mud drapes and minor prism cracks parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness defining a third order sequence boundary and several 4th order parasequence sets that correlate between interior sections parasequence stacking patterns similar to those of lower palaeozoic green house sequences and different from ice house sequences the lower triassic platform interior parasequences represent an anachronistic facies in that they are strikingly similar to the common renalcis mound ribbon rock parasequences of the lower palaeozoic and different from other permo triassic parasequences e g capitan shelf parasequences or loferites facies similarities such as renalcis mounds and flaser bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end permian extinction similarities in parasequence stacking patterns reflect low amplitude high frequency sea level fluctuations resulting from green house conditions common to the early palaeozoic and early triassic

microbialite and microstratigraphy the origin of encrustations in the middle and upper capitan formation guadalupe mountains texas and new mexico usa abstract the permian capitan reef of the guadalupe mountains was bound lithified and preserved by microbially precipitated micrite a microbial origin or association is based on the distribution texture fossil content and order of microstratigraphic succession in samples from the middle capitan formation micrite layers about 1 cm thick encrusted the top sides and undersides of in situ organisms and also piled above organisms in thick 0 5 to 3 cm accumulations with high angles of repose up to 90 primary encrustations commonly consist of thin homogeneous subparallel layers of archaeolithoporella interpreted to be a red alga that probably lived in dimly lit cavities secondary encrustations 0 5 30 mm thick contain 10 50 m diameter peloids and fossil cyanophytes and are usually bound by thin layers of micrite secondary encrustations of gravity defying peloidal micrites are interpreted as having precipitated within mucilaginous microbial aggregations probably in dark cavities the secondary microbial micrites were followed by cavity filling aragonitic botryoids thin 1 mm layers of clotted micrite were deposited on the first botryoids probably by microbial biofilms in contrast to samples from the middle capitan formation samples from the upper capitan formation taken near the transition from reef to outer shelf are composed of smaller 1 2 mm framework organisms and are bound by layers of thinner 1 mm primary encrustations in the upper capitan formation samples dense clotted micrite forms secondary encrustations 1 mm thick commonly underneath primary framework secondary encrustations are succeeded by more abundant tertiary encrustations 1 4 mm thick of sparse clotted micrite with vague dendritic structures the secondary and tertiary encrustations are interpreted as microbial the samples from the upper capitan formation were deposited in the shallowest part of the reef 1 to 10 m deep and most of the samples from the middle capitan were deposited in deep water 10 140 m the difference in the encrustations and in the overall scale of the two fig 1 map of the study area observations were made on samples from each of the canyons shown but samples from mckittrick and walnut canyons are the focus of this study end page 956 fabrics is probably related to differences in environmental energy during deposition

early ordovician reef construction in anhui province south china a geobiological transition from microbial to metazoan dominant reefs the lower ordovician records a transition from microbial to metazoan dominant reefs immediately prior to the great ordovician biodiversification event the hunghuayuan formation late tremadocian early floian of the jianxin section in anhui province south china includes well preserved reefs which consist of a combination of microbial boundstones and microbial lithistid sponge receptaculitid boundstones the microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of girvanella rich layers and thick micrite layers in which the filamentous calcimicrobe girvanella and bioclasts occur sporadically microbial biofilms including the filamentous microbes may have trapped and bound lime mud and or precipitated micrite during their growth and metabolic activities these activities contemporaneously contributed to substrate stabilisation encrustation and the construction of microbialite frameworks stromatolites in contrast the microbial lithistid sponge receptaculitid boundstones are characterised by more micritic constituents than skeletal reef building constituents thin and or domal crusts produced by peloid and dense micrite together with various proportions of girvanella occur upon bioclastic sediments and reef building skeletal organisms these micritic crusts with girvanella therefore played roles as binders and stabilisers and as consolidators by encrusting reef building skeletal organisms in the boundstones it is remarkable that the degradation of lithistid sponges due to microbial decay produced micrite in the sponge bodies as a result lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors these lower ordovician reefs in which microbialites e g stromatolites dominate but coexist with metazoans e g sponges were widespread at the time they provide excellent examples for understanding reefal palaeoecology during the transition from microbe dominant reefs to reefs enriched in metazoans

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

experimental study of potential wellbore cement carbonation by various phases of carbon dioxide during geologic carbon sequestration hydrated portland cement was reacted with co2 in supercritical gaseous and aqueous phases to understand the potential cement alteration processes along the length of a wellbore extending from a deep co2 storage reservoir to the shallow subsurface during geologic carbon sequestration the 3 d x ray microtomography xmt images showed that the cement alteration was significantly more extensive with co2 saturated synthetic groundwater than dry or wet supercritical co2 at high p 10 mpa t 50 c conditions scanning electron microscopy with energy dispersive spectroscopy sem eds analysis also exhibited a systematic ca depletion and c enrichment in cement matrix exposed to co2 saturated groundwater integrated xmt xrd and sem eds analyses identified the formation of an extensive carbonated zone filled with caco3 s as well as a porous degradation front and an outermost silica rich zone in cement after exposure to co2 saturated groundwater cement alteration by co2 saturated groundwater for 2 8 months overall decreased the porosity from 31 to 22 and the permeability by an order of magnitude cement alteration by dry or wet supercritical co2 was slow and minor compared to co2 saturated groundwater a thin single carbonation zone was formed in cement after exposure to wet supercritical co2 for 8 months or dry supercritical co2 for 15 months an extensive calcite coating was formed on the outside surface of a cement sample after exposure to wet gaseous co2 for 1 3 months the chemical physical characterization of hydrated portland cement after exposure to various phases of co2 indicates that the extent of cement carbonation can be significantly heterogeneous depending on the co2 phase present in the wellbore environment both experimental and geochemical modeling results suggest that wellbore cement exposure to supercritical gaseous and aqueous phases of co2 during geologic c sequestration is unlikely to damage the wellbore integrity because cement alteration by all phases of co2 is dominated by carbonation reactions this is consistent with previous field studies of wellbore cement with extensive carbonation after exposure to co2 for three decades however xmt imaging indicates that preferential cement alteration by supercritical co2 or co2 saturated groundwater can occur along the cement steel or cement rock interfaces this highlights the importance of further investigation of cement degradation along the interfaces of wellbore materials to ensure permanent geologic carbon storage published by elsevier ltd

upper lower triassic stromatolite from anhui south china geobiologic features and paleoenvironmental implications abstract widespread stromatolites and other microbialite deposits characterize lower triassic marine successions worldwide this study documents a stromatolite deposit 1 1 m thick from the upper spathian lower triassic of the susong area south china the stromatolite comprises distinct laminated domes in the basal part and columns at the upper part dark laminae are loosely spaced and interlayered with thicker light colored laminae diffusive dark colored laminae are composed of peloidal micrite that grade locally into microclotted structures and yield copies of bacteria clump like and coccoid like spheroid aggregates the former are characterized by cloudy micrite nuclei rimmed by coarse grained euhedral sparry calcite crystals while the latter are comprised of solid calcite crystal nuclei coated with rather thin micrite envelopes the cloudy micrite nuclei resemble bacteria clump like structures observed in present day travertine both the coccoid like spheroids and bacteria clump like structures are surrounded by coarse grained euhedral calcite crystals suggesting a similar accretion mechanism both spherical structures therefore could be crucial in the accretion of the susong stromatolite the laminated microclotted structures are interpreted as the result of variation in timing of lithification relative to the timing of the decay of microbes micro analysis also unravels the common occurrence of authigenic micro quartz crystals in association with fe bearing illite clay minerals in the stromatolite columns their coalescing nature with each other together with the associated pyrite grains strongly support the formation of micro quartz crystals from microbial reduction of an fe bearing smectite precursor by sulfate reducing bacteria a comparison of the susong stromatolite with its counterparts from the upper lower triassic strata in dajiang south china reveals many similarities in stromatolite microstructures suggesting that a harsh euxinic anoxic environment resulting in the bloom of sulfate reducing bacteria most likely extended into the latest spathian in south china

onset development and cessation of basal early triassic microbialites betm in the nanpanjiang pull apart basin south china block abstract new investigations in the nanpanjiang basin indicate that the onset of the iconic microbialites associated with the paleozoic mesozoic boundary was early triassic in age bathymetry water agitation oxygenation light penetration and clastic load are shown to have exerted a direct control on the growth of microbialites carbonate supersaturation is also required for the deposition of the microbialites bathymetric control is further corroborated by the inheritance of the topography of a latest permian pull apart basin into early triassic times with a distribution of basal early triassic microbialites betm restricted to uplifted blocks and the accumulation of carbonaceous black shales in adjacent troughs the geographically most extensive nanpanjiang betm bloomed on a large nw se trending uplifted block exceeding 12 000 km 2 luolou platform centered on northwestern guangxi post triassic displacements along the youjiang fault obscure the paleogeographic relation of betm exposed west of this fault triassic foraminifers occur in the basal most betm episode which is locally bracketed by high energy grainstones made of reworked permian foraminifers therefore the permian triassic boundary ptb is within the unconformity that separates the late permian heshan fm from the basal most betm where accommodation space was sufficient up to five event surfaces are associated with the unconformity microfacies analysis supports chemical dissolution but did not reveal evidence for subaerial erosion although intercalated grainstone made of permian foraminifers indicate reworking chemical dissolution and mechanical erosion both conceivably contributed to the genesis of the unconformity the upward shift from tabulated to domical microbial build ups is accompanied by accumulation of coquinoid lenses between domes which indicates deepening of the luolou platform betm the main drowning resulting from both regional tectonic subsidence and a global sea level rise led to the cessation of the betm that were buried under predominant fine siliciclastics any concomitant change in sea water chemistry appears unlikely

wellbore cement fracture evolution at the cement basalt caprock interface during geologic carbon sequestration abstract composite portland cement basalt caprock cores with fractures as well as neat portland cement columns were prepared to understand the geochemical and geomechanical effects on the integrity of wellbores with defects during geologic carbon sequestration the samples were reacted with co 2 saturated groundwater at 50 c and 10 mpa for 3 months under static conditions while one cement basalt core was subjected to mechanical stress at 2 7 mpa before the co 2 reaction micro xrd and sem eds data collected along the cement basalt interface after 3 month reaction with co 2 saturated groundwater indicate that carbonation of cement matrix was extensive with the precipitation of calcite aragonite and vaterite whereas the alteration of basalt caprock was minor x ray microtomography xmt provided three dimensional 3 d visualization of the opening and interconnection of cement fractures due to mechanical stress computational fluid dynamics cfd modeling further revealed that this stress led to the increase in fluid flow and hence permeability after the co 2 reaction xmt images displayed that calcium carbonate precipitation occurred extensively within the fractures in the cement matrix but only partially along the fracture located at the cement basalt interface the 3 d visualization and cfd modeling also showed that the precipitation of calcium carbonate within the cement fractures after the co 2 reaction resulted in the disconnection of cement fractures and permeability decrease the permeability calculated based on cfd modeling was in agreement with the experimentally determined permeability this study demonstrates that xmt imaging coupled with cfd modeling represent a powerful tool to visualize and quantify fracture evolution and permeability change in geologic materials and to predict their behavior during geologic carbon sequestration or hydraulic fracturing for shale gas production and enhanced geothermal systems

biostratigraphy and sedimentology of upper permian and lower triassic strata at masore western slovenia abstract upper permian and lowermost triassic strata of the masore section in western slovenia have been restudied by means of bio and lithostratigraphy this section is mainly characterized by a carbonate succession of the bellerophon formation deposited in a shallow marine ramp environment that was located in the western part of the paleotethys the permian triassic boundary ptb transition is marked by laminated microbialites stromatolites interpreted to reflect a deeper ramp environment the conodont elements recovered enabled the recognition of the hindeodus praeparvus zone of the uppermost permian upper changhsingian in the bellerophon formation gondolellids are documented in the ptb transitional interval with microbial microfacies whereas the isarcicella isarcica zone lower griesbachian lower triassic is recognized in the tectonically separated werfen formation just above the microbial microfacies part of the section the lowermost part of the microbialites is characterized by late permian species of foraminifers indicating that at least this part of the section is still upper permian

microfacies diagenesis and hydrocarbon potential of the neoproterozoic cap carbonate of the southern amazon craton abstract the low latitude marinoan glaciation 635 ma record in the southern amazon craton has been one of the most important sites with strong evidence for the neoproterozoic snowball earth hypothesis in central brazil the 50 m thick carbonate succession of the post marinoan cap carbonate exposed in the tangara da serra region central brazil represents a complementary section of the classical occurrence of the puga cap carbonate in mirassol d oeste region that overlies glaciogenic deposits marking the well preserved cryogenian ediacaran boundary in south america the 8 m thick cap dolostone that represents the mirassol d oeste formation consists of pinkish microcrystalline peloidal dolomudstone exhibiting even parallel to low angle lamination locally tubestone and megaripple bedded dolopackstone the cap dolostone is succeeded upsection by the cap cementstone limestone basal guia formation comprising laminated lime mudstone rich in silt sized terrigenous grains quartz and subordinately bituminous shale dolomitic marls intraformational breccias and abundant calcite crystal fans and crusts crystal fans occur in isolated and decimetre scale arrays and are considered aragonite pseudomorphs primary precipitation of dolomite crystal fans and micrite and the formation of tubestone are related to depositional processes the cap dolostone was affected by the neomorphism of the dolomicrite development of vug and intercrystalline porosity euhedral dolomite precipitation synsedimentary fracturing calcite and quartz precipitation chemical compaction fe oxide substitution and precipitation the diagenetic processes of the cap cementstone limestone were pyrite precipitation calcite cementation neomorphism of micrite and crystal fan partial crystal fan dissolution dolomitization chemical compaction fracturing ferrous dolomite and calcite precipitation pyrite pseudomorph and micrite replacement with fe oxide and replacement of rhombohedral dolomite by mn oxides hydrocarbons are rare and its migration represents the last diagenetic event being found 1 in fractures and interpeloidal vug and intercrystalline pores 2 with low permeability and connectivity of the cap dolostone and 2 in intercrystalline pores fractures and rare pores in crystal fans of the cap cementstone limestone during the post marinoan sea level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm action the progressive transgression led the implantation of a deep anoxic and caco3 oversaturated aragonitic platform the similarities of cap carbonate microfacies among the post marinoan succession worldwide reflects an extraordinary and non recurring global precipitation event in geological history in contrast besides the majority of cap carbonate have the same textural and compositional framework the diagenetic history seems to be regional and completely dependent on the tectonic thermal and burial history of the depositional basins where they were deposited

geochemical and geomechanical effects of scco2 and associated impurities on physical and petrophysical properties of permotriassic sandstones germany an experimental approach effects of impurities sox nox and o2 from the extraction processes of the power plants in the scco2 fluids are studied with an experimental approach combining geochemical alteration in high p medium t autoclaves compressional tests in a heated triaxial testing device as well as geochemical and mineralogical analyses before and after the influences of distinct scco2 fluids on 140 70 mm sized samples of reservoir sandstones from the north german basin with carbonate cementation silicate cementation and a combined silicate carbonate cementation on the background of variable lithological compositions the attempt was made to normalize the scattering geomechanical data by the pure co2 behavior measurement data to achieve a direct impression of any differences due to the impurities changes in the chemical composition of the saline fluids were systematically measured during the scco2 alteration experiments in the autoclave systems at distinct pt conditions the development of the silica concentrations and of the ph value in the fluid solution during alteration showed a clear dependency on the fluid composition through systematical analysis of changes on the mineral surfaces using sem microscopy images before and after scco2 impurities alteration experiments or triaxial compression tests corrosion effects were detected but no differences which could be directly related to differences in the fluid composition could be identified in all the types of sandstones visible or measurable changes on mineral surfaces and pore fluid composition respectively take place as an interaction of rock with fluids of scco2 with and without impurities changing deformability and compressive strength of the rocks are the most apparent consequences due to these compositional changes it is supposed that the chemical fluid differences are responsible for these changes also a specific distribution of the different fluids in the ambient pore space is indicated with even undetected consequences for the fluid influence on stress transmission and distribution within the grain structure all work was performed within the project co2 purity for capture and storage cooral supported by the german federal ministry for economic affairs and energy

sem imaging of bacteria and nannobacteria in carbonate sediments and rocks abstract bacteria and nannobacteria 0 1 m dwarfed forms are abundant in both non etched and gently etched travertines deposited by hot springs in viterbo italy they are also plainly visible in bahama ooids and hardgrounds and in great salt lake aragonite ooids and cements in addition nannobacteria are found in paleozoic and mesozoic rocks because of their extraordinary abundance in some specimens they probably play a prominent role in catalyzing the precipitation of carbonate materials

an upper turonian fine grained shallow marine stromatolite bed from the munecas formation northern iberian ranges spain abstract a fine grained stromatolite bed laterally continuous on the kilometer scale and with small synoptic relief crops out in the munecas formation in the northern iberian ranges the munecas fm was deposited during the late turonian on a shallow water platform in the upper cretaceous intracratonic iberian basin the stromatolite bed has a tabular to domed biostromal macrostructure its internal mesostructure consists of planar wavy to hemispherical stromatoids that display a broad spectrum of microstructures including dense micrite bahamite peloids peloidal to clotted microfabrics irregular micritic wall tubes which are suggestive of algae and filamentous microframeworks which are suggestive of filamentous cyanobacteria various stromatolite growth stages have been linked to the dominance of different accretion processes the accretion of the entire fine grained stromatolite involves a complex mosaic of processes trapping and binding of quartz silt grains and bahamites which form the agglutinated parts of some laminae and microbially induced precipitation which forms spongiostromic and micritic laminae tubiform and filamentous microframeworks resembling porostromatate or skeletal stromatolitic growth were also recognized laser icp ms measurements of al si mg mn sr s and fe were analyzed to detect the influence of siliciclastic inputs and major trends during stromatolite accretion carbon and oxygen isotopic compositions from the stromatolite and associated facies were used to identify possible microbial signatures these data describes a unique and well preserved example of a shallow marine upper turonian fine grained stromatolite

upper devonian frasnian stromatactis bearing mud mounds western alberta canada reef framework dominated by peloidal microcrystalline calcite

controls on carbonate platform architecture and reef recovery across the palaeozoic to mesozoic transition a high resolution analysis of the great bank of guizhou

facies changes and diagenetic processes across the permian triassic boundary event horizon great bank of guizhou south china a controversy of erosion and dissolution the permian triassic boundary interval in shallow shelf seas of south china shows upper permian limestones overlain by lowermost triassic microbialites global sea level rose across the permian triassic boundary but an irregular top permian erosion surface across a 10 km north south transect of the great bank of guizhou contains evidence of sea level fluctuation the surface represents the event horizon of mass extinction below the biostratigraphic permian triassic boundary defined by first appearance datum of conodont hindeodus parvus an upper permian foraminiferal grainstone beneath this surface contains geopetal sediments etched grains and pendent and meniscus cements interpreted here as vadose however these latter diagenetic processes occurred before the event horizon and were followed by erosion of the final permian surface this erosion cuts previous fabrics but lacks evidence of weathering or bioerosion a few centimetres below is an earlier grainstone that was also eroded but lacks proof of sub aerial processes samples therefore reveal one or possibly two small scale relative sea level changes before the triassic transgression in this area and these may relate to local tectonics the final permian surface is subject to at least four interpretations i sub aerial physical erosion and dissolution by carbon dioxide enriched fresh water or carbon dioxide enriched mixed water prior to triassic transgression ii sub aerial physical erosion overprinted by dissolution related to carbon dioxide enriched sea water in the early triassic transgression iii submarine dissolution affected by acidified sea water due to rapid increase in volcanically derived carbon dioxide and oxidized methane released from marine clathrates iv submarine dissolution due to acid anoxic waters rising across the continental shelf unrelated to atmospheric carbon dioxide or oxidized methane field and petrographic evidence suggests that i is the simplest option and it is possible that ii and iii occurred but none are proved option iv is unlikely given the evidence and modelling of supersaturation of upwelled waters with respect to bicarbonate

controls on facies architecture of a large triassic carbonate platform the great bank of guizhou nanpanjiang basin south china abstract the great bank of guizhou gbg is an exceptionally well exposed isolated triassic platform in the nanpanjiang basin of south china the platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth evolution and demise the gbg formed near the southern margin of the yangtze platform during a deepening event that expanded the nanpanjiang basin and drowned the region surrounding the gbg in the latest permian initial accumulation of the gbg began in the latest permian with small reef mounds and open marine skeletal packstones following the end permian extinction cyanobacterial boundstones grew over the bank top during the early triassic the gbg developed a low relief bank profile with mobile oolite shoals at the margin shallow subtidal and peritidal deposits in the interior and gentle slopes dominated by pelagics debris flow deposits and turbidites at the basin margin in the middle triassic anisian the gbg developed a progressively steepening profile rimmed with massive tubiphytes reefs the platform was flat topped with tidal flat deposits across the interior basin margin deposition was dominated by turbidites and debris flow deposits but eventually shifted to avalanche and rock fall deposits as the slopes steepened to the angle of repose in the middle triassic ladinian an erosional escarpment up to 1700 m high developed at the margin platform margin strata are bedded packstones similar to interior strata whereas breccias at the basin margin contain coral boundstone clasts suggesting erosion of reefs from the escarpment a restricted subtidal lagoon formed in the interior producing an atoll like morphology later a flat topped profile was restored as tidal flats spread across the interior in the beginning of the late triassic deepening contributed to termination of the gbg before siliciclastic turbidites and shales were deposited over the platform in contrast with the well known platforms of the dolomites of northern italy the gbg contains abundant muddy carbonates and a progressively steepening bank to reef rimmed and escarpment architecture the italian platforms contain little mud and have angle of repose pinnacle geometries the gbg s larger size increased mud production and protected it from extensive winnowing of mud which in turn resulted in off bank shedding of muddy sediments that were stable on relatively gentle basin margin slopes which progressively steepened and ultimately led to avalanche deposits and a high relief erosional escarpment in contrast the lesser mud content of the dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins which in turn produced their angle of repose geometries

nonskeletal peloidal precipitates in upper triassic reefs yukon territory canada abstract peloidal sediments and crusts form up to 75 percent of framework limestones in upper triassic reefs in the yukon territory the peloids are spherical to irregular in shape 20 250 m in size and are commonly surrounded by one or more rims of sparry calcite the peloidal sediments occur in confined cavities and in open spaces between reef framebuilders the peloidal crusts generally coat calcareous sponges and tabulozoans internal evidence suggests that the peloids are not detrital grains or fecal pellets and comparisons with mg calcite peloids and peloidal crusts in modern environments suggest that they are precipitates possibly resulting from bacterial activity

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

painting altamira cave shell tools for ochre processing in the upper palaeolithic in northern iberia abstract much of our knowledge of the symbolic world of upper palaeolithic hunter gatherers is based on the study of the graphic representations found in western european caves however to date few studies have been conducted on rock art apart from chronological and stylistic characterisation altamira cave northern iberia is characterised by an outstanding rock art ensemble whose representations cover practically the whole upper palaeolithic the site is equally important for the rich upper palaeolithic deposits in the cave entrance which contain large shell assemblages traditionally the presence of shells in hunter fisher gatherer settlements has been interpreted as part of the diet and or the symbolic world through the creation of ornaments of these groups regardless of their possible use as an instrument in this paper we utilise use wear methodology chemical analysis and analytical experimentation to verify the initial hypothesis that shells in the archaeological deposits of altamira were used to obtain the ochre powder utilised to produce the magnificent and diverse rock art ensemble in the cave the results provide new information on the process of obtaining pigments for the realisation of paintings and confirm that the use of shells to obtain ochre was a systematic activity throughout the whole study period finally our conclusions support the explanatory model that highlights the role played by marine resources for upper palaeolithic human populations

the fabrics and origins of peloids immediately after the end permian extinction guizhou province south china abstract peloids are major contributors to modern and ancient limestones although their origins are not yet comprehensively understood upper permian to lower triassic carbonate successions are well preserved at the bangeng area in guizhou province south china where microbialites thrombolites coccoidal microbes and peloids are preserved together these peloids are classified into three types peloid a b c that typically occur together along with coccoidal microbes and restricted bioclasts in a variety of microenvironments peloid a is most common and exhibits close relationship with coccoidal microbes peloid a1 has spherical grain and a size range typical for coccoidal microbes 10 60 m in diameter whereas peloid a2 is marked by larger spherical and subrounded grains with diffuse margins ranging from 70 to 200 m in diameter peloid a1 is in part produced by the complete micritic filling in coccoidal microbes by their metabolic activities whereas peloid a2 is related to the calcification of a colony of coccoidal microbes by their metabolic activities or simply to the aggregation of individual peloids and coccoidal microbes peloids of this type might also originate from other carbonate precipitation processes peloid b is derived merely by micrite filling in ostracodes and gastropods skeletons whereas peloid c is of micritized bioclast origin these peloidal origins do not explain those of all environments and ages peloids are polygenetic in origin irrespective of the presence of coccoidal microbes however it is emphasized that the peloids formed after the end permian extinction are dominated by microbes related ones those kinds of peloids might have occurred repeatedly and predominantly especially in severely deteriorated environments such as those after the end permian mass extinction

factors controlling carbonate platform asymmetry preliminary results from the great bank of guizhou an isolated permian triassic platform in the nanpanjiang basin south china abstract a well exposed isolated carbonate platform the great bank of guizhou in the nanpanjiang basin of south china developed from the latest permian to the earliest late triassic platform strata are dissected by a faulted syncline exposing a complete cross section through the interior margins and flanks enabling a detailed assessment of depositional controls previous studies portrayed the platform as having a relatively symmetrical architecture even though much of the former work was focused on the platform interior and northern margin basin transition our research reveals five aspects of the southern margin facies and stratigraphy that are significantly different from those of the northern margin 1 subaerial exposure and unconformity developed on top of the upper permian sponge boundstone and in the overlying lower triassic strata 2 permian and triassic clasts chaotically admixed within early triassic breccias 3 lower triassic strata remarkably thinner on the southern margin 4 a much narrower tubiphytes reef facies preserved along the southern margin in the middle triassic and 5 large scallop shaped reentrants at the southern margin evident in satellite images three end member models may explain the asymmetry 1 antecedent topography of the underlying upper permian reef rimmed margin coupled with eustatic sea level fluctuation 2 differential tectonic uplift and 3 large scale submarine collapse of the platform margin subaerial exposure and admixing of permian and triassic clasts observed at yungan section is best explained by the tectonic uplift model however the submarine collapse model also explains several of the observations if it is associated with uplift s or sea level fall s submarine collapse is supported by large concave erosional reentrants scallops visible in satellite images taken together our observations suggest that a combination of tectonic uplift and margin collapse contributed to platform asymmetry further work promises to further constrain the details and timing of processes that contributed to the asymmetry

a microbialite carbonate crust at the permian triassic boundary in south china and its palaeoenvironmental significance a 1 m thick carbonate crust layered and commonly domal caps crinoidal limestones on reef complexes of the top permian changxing formation in the huaying mountains eastern sichuan china the crust s stratigraphic level lies at a sharp change in facies and is overlain by poorly fossiliferous laminated micrites and shales of the lower triassic feixianguan formation the crust therefore appears to coincide with the end permian extinction event although the dating of the strata is currently imprecise the crust is composed mostly of digitate carbonate locally thrombolitic with remnant lobate fabric and resembles microbialites but is mostly recrystallised and a microbial origin is unconfirmed it is enclosed in micrite with pyrite crystals ostracode and other shell debris the crust is absent from interreef areas previous interpretations of karstification and calcrete formation are not upheld and the facies were deposited under water overlying sediments are low energy with abundant ferroan calcite and pyrite reflecting anoxia associated with rising sea level various interpretations of the crust are possible 1 it was organic and microbia grew preferentially on topographic highs of reef tops 2 microbia were a disaster biota in the absence of grazers 3 microbia took advantage of favourable conditions for calcification associated with a rapidly rising environmental co2 content during the late permian 4 the crust was an inorganic precipitate associated with co2 rich water microbial crusts are rare after the cambrian but whether this crust is organic or not its presence in these strata reinforces the view that oceanic atmospheric conditions in the permian triassic boundary interval were unusual

the use of lithic artefacts for making rock art engravings observation and analysis of use wear traces in experimental tools through optical microscopy and sem in this paper we present the results of observations made via optical microscopy and sem of microwear traces on experimental lithic tools used in the replication of rock art engravings the overall aim of this experimental programme is to generate diagnostic microscopic criteria that could subsequently be used a in the identification of archaeological artefacts that have been involved in the production of rock art engravings and b in the inference of the engraving techniques in which they have been used the methods of replication of engraved figures and lines and the observation of the microwear traces on the experimental artefacts used are presented the complementary use of stereomicroscope metallographic microscope with bright field dark field and laser illumination and sem is also discussed the resulting microwear traces observed on the tools used in the experiment are characterized and are linked to the two different engraving techniques with which the rock art replication has been carried out

stone pipe making tools in ancient north america abstract several types of smoking pipes have been manufactured and used throughout later prehistoric and historic times around the world although substantial information exists on the styles of these pipes little is known about their methods of manufacture or the types of sites on which they were made this study focuses on testing a postulated model of pipe manufacture through the experimental replication of an artifact type known as the florence windom pipe made of soft red argillite pipestone and the use wear analysis of chipped stone artifacts representative of stone tools found in association with these pipes results of analyses showed that certain tool types such as reamers drills tabular tools scrapers and gouges were integral parts of the manufacture process other types such as punches and awls proved not to have been employed in this activity while our model was applied to a regionally specific pipe style and raw material we suggest that our methods and approach can easily be used generally and more broadly in the analyses of similar archaeological assemblages worldwide

identifying geochemical reactions on wellbore cement caprock interface under sequestration conditions the integrity of wells which are key components for co2 sequestration depends mainly on the seal between the wellbore cement and the geologic formation to identify the reaction products that may alter the cement caprock interface batch experiments and computer modelling were conducted and analysed over time the dissolution and precipitation of minerals alters the physical properties of the interface including its tightness one main objective of the simulation was thus to analyse the evolution of the porosity of cement and caprock over time the alteration of the cement caprock interface was identified as a complex problem and differentiated depending on rock type the characteristic feature of a cement shale contact zone is the occurrence of a highly carbonated compacted layer within the shale which in turn causes cement shale detachment in the case of a cement anhydrite interface the most important reaction is severe anhydrite dissolution secondary calcite precipitation takes place in deeper parts of the rock the cement rock contact zone is prone to rapid mineral dissolution which contributes to increased porosity and may alter the well integrity comparison of computer simulations with autoclave experiments enabled the adjustment of unknown parameters this enhances the knowledge of these particular assemblages overall a good match was obtained between experiments and simulations which enhances confidence in using models to predict longer term evolution

late ediacaran boxonia bearing stromatolites from the gobi altay western mongolia abstract a couplet comprising a boxonia bearing stromatolite unit and phosphatic layers occurs widely at the base of the zuun arts formation in the zavkhan terrane of gobi altay province western mongolia the stromatolite unit is late ediacaran in age and forms bioherms of several kilometres in lateral extent the stromatolites consist of two parts the lower columnar boxonia stromatolites ca 7 m thick change abruptly into the upper domed stromatolites ca 4 5 m thick in the bayan gol gorge the columnar stromatolites are made up of columnar structures 2 6 cm in diameter formed by the accumulation of upward convex laminae and partly protrude laterally to form bridges between neighbouring columns in contrast the domed stromatolites 30 60 cm in width are composed of accumulations of low convex laminae both types of stromatolite are characterized by alternating darker and lighter laminae which consist of peloids two types of micritic clot homogeneous lime mud and spar filled fenestral fabrics these stromatolites are inferred to have been deposited in a subtidal setting below the wave base with no evidence of sediment displacement by strong currents the stratigraphic transition from columnar to domed growth forms reflects deepening of the basin the change in column diameter observed within the columnar stromatolites might reflect fluctuations in microbial activity the appearance of inter columnar bridges in the columnar stromatolites might indicate more active colonization of microbial mats which expanded into the inter columnar areas neither type of stromatolite includes microbial remains such as filamentous or coccoidal cells however the various micritic components of the stromatolites clots peloids and homogeneous lime mud originated from in situ precipitation through microbial activity and reflected differences in the timing and intensity of microbial calcification and degradation these stromatolites including the variety of micritic components may provide clues to the variations in microbial metabolic activity and degradation processes that were associated with stromatolite formation

micritized skeletal grains in northern belize lagoon a major source of mg calcite mud abstract lime mud in northern belize lagoon is composed predominantly of silt size cryptocrystalline grains and clay size clumps both of which consist of anhedral equant crystals less than 0 5 m in size this mud is 70 90 mg calcite containing 9 10 mole mgco3 detailed petrographic morphologic and mineralogic analyses suggest that the cryptocrystalline grains are micritized skeletal grains and that the mud formed by abrasion and bioerosion of these altered grains micritization of skeletal grains occurred primarily by recrystallization of original skeletal carbonate laths prisms and blades of mg calcite aragonite and calcite to equant crystals of micritic mg calcite with additional alteration by micritic infilling of microborings and skeletal cavities because of d fferences between crystal morphologies and mineralogies of original skeletal components and their micritized counterparts mud produced by diminution of micritized grains is distinct from that produced by skeletal breakdown the abundance of cryptocrystalline grains in the bahamas and other shallow carbonate environments suggests that micritized grains are potentially an important source of lime mud that has not previously been identified

wellbore cement degradation in contact zone with formation rock in the work the risk of co2 migration in deep wells caused by integrity loss on cement rock interface and thow wellbore integrity correlated with the formation rock lithology were determined 19 composed samples of rock and wellbore cement were exposed to co2 saturated brine in the autoclave reactor under the formation conditions 50 c and 10 mpa mineralogical and textural changes in the cement rock interface in the case of selected rocks sandstones shale limestones dolomites and anhydrites were characterised the performed examination indicates that both cement and formation rocks react with co2 saturated brine under the experimental conditions the cement alteration is characterised by carbonation process in the outer rim but it is enhanced on the interface with formation rock it was stated that the performance of the cement rock interface is essentially dependent on the rock lithology including mineral composition and rock structure some minerals are very easily dissolving e g anhydrite gypsum calcite and feldspars what is contributing to an increase in the porosity and permeability in cement rock contact zone primary dissolution of certain minerals in the first stages of the experiment results in the secondary precipitation after the last stage of reaction contributing to a secondary reduction of pore space

use wear analysis of obsidian artifacts from later stone age shell midden sites on the red sea coast of eritrea with experimental results this paper presents results of use wear study on lithic artifacts from two later stone age sites gelalo and misse on the red sea coast of eritrea the sites produced large quantities of lithic artifacts in association with mollusk shells and ostrich eggshell beads but it is unclear if all the stone tools were required for bead and mollusk shell processing the study involved recording of microfracture damage traces in order to infer the use material and the manner in which the artifacts were used a large percentage of the analyzed samples from gelalo and misse preserve wear patterns suggestive of human use the diagnostic wear types include 1 dense step snap crushing and hinge fractures typically confined on the working edges and 2 feather scars organized in a scalar manner visible on the ventral and dorsal surfaces of the active parts the observed damage patterns suggest cutting and engraving medium to hard materials the evidence is incomplete for more generalization about the specific activities carried out at the sites a brief experimental study involving ostrich eggshell drilling oak twig sawing and bark scraping meat slicing and mollusk shell sawing and drilling was carried out to aid interpretation of wear features observed on the archaeological specimens wear traces produced by sawing mollusk shell and oak wood showed close affinity to those observed on the archaeological specimens the study contributes important information about early holocene site use on the red sea coast of eritrea the close association of used lithic artifacts symbolic objects beads and broken shell remains indicates that the sites were habitation areas

a permian triassic boundary microbialite deposit from the eastern yangtze platform jiangxi province south china geobiologic features ecosystem composition and redox conditions abstract a permian triassic p tr boundary microbialite ptb microbialite deposit occurs near xiushui on the southern margin of the eastern yangtze platform south china this 2 m thick microbialite overlies uppermost permian bioclastic limestone with a 5 cm thick oolite like grainstone layer at the contact this ptb microbialite consists of lower thrombolite 1 5 m thick and an upper dendrolite 0 5 m thick the thrombolite interval is characterized by a clotted texture of reddish sparitic patches embedded in a gray micritic matrix whereas the dendrolite texture consists of brownish sparitic patches embedded within a yellow micritic matrix abundant calcified microbes occur in both the thrombolite and the dendrolite including 1 columnar and clustered fabrics which are especially rich in the thrombolite assigned as products of gakhumella or related microorganisms and 2 microspheroids 40 60 m in diameter with different interior structures these calcimicrobes are considered to have played an important role in constructing the xiushui ptb microbialite the demise of the microbialite was likely due to a sea level fall during the early griesbachian lowermost triassic which resulted in a lowering of the wave base thereby destroying the microbialite frameworks and facilitating the deposition of winnowed skeletal grainstone metazoan community structures within the microbialite changed sharply across the end permian extinction horizon which is placed at a sharp negative shift in 13 c carb quantitative analyses show that the xiushui microbialite community is similar to that of chongyang and cili microbialites this is probably because all of these microbialites grew along the margins of the lower yangtze region during the p tr transition however unlike other ptb microbialites the xiushui ecosystem contains fairly abundant metazoans which are dominated by ostracods with minor occurrences of foraminifers microgastropods and microconchids both the ostracod assemblage and the pyrite framboid analyses indicate an upper dysoxic zone to a well oxygenated condition for the xiushui microbialite this implies that some ptb microbialite ecosystems developed in conditions that were not always harsh instead some were hospitable for some metazoans to survive immediately after the end permian crisis considering that the redox conditions indicated by ptb microbialites are quite variable we infer that oxygen levels probably were not the crucial factor affecting the growth of the ptb microbialites

early triassic calcimicrobial mounds and biostromes of the nanpanjiang basin south china stanley s model 1988 proposed that a permiantype reef fauna survived the early triassic reefless period in yet undiscovered refuges in the ancestral pacific ocean and then reappeared as the builders of middle triassic reefs flugel 1994 in contrast suggested that the middle triassic reefbuilding fauna are not holdovers from the permian but bear only a superficial resemblance to permian fossils recent publications reflect a consensus that the early triassic lacked reefs and reef mounds although some authors acknowledged the possibility that reef like structures may have existed in poorly explored areas such as south china and russia senowbari daryan et al 1993 flugel 1994 it is shown herein that calcimicrobial framework mounds and biostromes existed in the early triassic in the nanpanjiang basin of south china organic framework in this paper refers to a three dimensional biologically constructed network of calcified fossils that forms a rigid mass and supports an interconnected network of internal cavities that were open during accumulation although the calcimicrobial frameworks discussed herein are much smaller than frameworks of modern metazoan reefs the term framestone is more appropriate for these fabrics than other reef rock terms such as bindstone or bafflestone embry and klovan 1971 these calcimicrobial mounds and biostromes are important as the first organic framestones documented from the early triassic

characterization of the use wear and residues resulting from limestone working experimental approach to the parietal art of la vina rock shelter la manzaneda asturias spain abstract the palaeolithic rock engravings that are located along the nalon river basin in northern spain central area of asturias have been studied from various perspectives morphology depth style manual range but no use wear studies on the stone tools used to produce such engravings have ever been undertaken this paper aims to explore a new approach to this type of incisions based on use wear analysis of experimental lithic tools used to engrave limestone blocks and slabs our results show that the use wear traces generated by engraving limestone are well developed and can be defined with specific criteria the principal objective of this study was to provide the first experimental reference collection of use wear resulting from engraving limestone using flint and quartzite experimental tools to compare with the traces that appear on tools in the archaeological record in contexts with parietal and portable art and more specifically to add a new approach to the multidisciplinary study of the la vina rock shelter

marine peloids a product of bacterially induced precipitation of calcite high magnesian calcite peloids are a common constituent of cemented marine carbonate accumulations they are abundant and well developed within both isolated microcavities and surface crusts the nuclei of marine peloids are composed of fossil bacterial clumps encased within anhedral submicron to micron sized high magnesian calcite crystals the marine peloids are similar to silt sized particles found in travertine deposits that also are composed of fossil bacterial clumps and their surrounding calcite coronas these similarities include size 20 60 mu m a cloudy brown nucleus and rims of clear euhedral crystals in addition previous studies have established the presence of fatty acids of bacterial origin within peloidal deposits and enriched stable isotopic signatures have been interpreted as recording organic fractionation concomitant with calcite precipitation these characteristics all indicate that the nuclei of many peloids originate as calcite precipitates within and around clumps of bacteria and that this precipitation was induced by the vital activity of bacterial colonies

calcified algae as sediment contributors to early paleozoic limestones evidence from deep water sediments of the cow head group western newfoundland abstract calcified algae are important elements of deep water carbonate sediments in the cow head group large algal boundstone boulders in debris flow conglomerates consist of girvanella sheets and arborescent clusters of epiphyton silt and sand sized particles in fine grained thin bedded slope limestones include abundant girvanella as single tubules rafts of intertwined tubules oncolites and intraclasts integration of petrographic information from the boulders and fine grained limestones indicates that many micritic peloids of silt to very fine sand size the single most abundant particle type in the cow head group are a combination of comminuted girvanella tubules and broken epiphyton rods much of the structure grumeleuse in intraclasts is also diagenetically modified calcified algae these findings suggest that many peloids and intraclasts in early to middle paleozoic carbonates in both shallow and deep water may have been derived from calcified algae calcified algae may have been just as important in producing carbonate sediment during this time as benthic calcified algae are in the modern ocean the difficulty in recognizing the algal contribution in most ancient sediments is probably caused by the simple microfabric of the resultant grains filling and obliteration of tubules with micrite cement or micritization

peloidal fabrics in upper jurassic reefal limestones weald basin southern england abstract coraliferous limestones from the corallian group upper jurassic of subsurface of the weald basin southern england record the localized development of superimposed patch reefs and banks the reefal limestones exhibit a variety of peloidal fabrics in particular well sorted peloids 10 60 m in diameter with a well developed euhedral rim of fibrous sparry calcite around a micritic anhedral nucleus they occur in confined intraskeletal pores corals molluscs and borings growth cavities between framebuilders and as stromatolitic crusts the peloids in interskeletal cavities and as crusts resemble microbial precipitates and merge into spongiostromate fabrics the distribution of these peloidal fabrics seems to be facies related and is best developed in zones which had relatively low framework growth rates low sedimentation rates and moderate energy levels the peloids are identical in form and occurrence to those in some ancient and quaternary reefs the association of the peloids with stromatolitic coatings and spongiostromate fabrics leads us to interpret the peloids as microbial precipitates either by direct precipitation or by induction the abundance of microbial carbonate in these reefal limestones places them in an intermediate category between frame built reefs and microbial mud mounds

evolution of early triassic outer platform paleoenvironments in the nanpanjiang basin south china and their significance for the biotic recovery detailed microfacies and paleoenvironmental analyses were conducted through the early triassic interval of the outer platform ammonoid rich series of the nanpanjiang basin luolou formation guangxi province south china extensive investigations on outcrops and on thin sections reveal that the widely reported well diversified latest permian fauna dominated by abundant corals calcareous algae calcareous sponges crinoids brachiopods gastropods ostracods and foraminifera is abruptly replaced by decimated early triassic communities essentially composed of low diversity bivalves microgastropods ostracods and rare foraminifera along with this drastic change in faunal assemblages the recolonization of devastated habitats by calcimicrobial frameworks is observed in the aftermath of the end permian mass extinction calcimicrobialites of griesbachian age are followed by mixed ammonoid and conodont rich carbonate siliciclastic series consisting of dark suboxic laminated mudstones intercalated with organic rich shales of dienerian to late smithian age the carbonate siliciclastic series are interrupted by a brief carbonate episode during the early middle smithian massive carbonate production resumes only from the early spathian onward and persists throughout the spathian the initial deposition of both carbonate episodes is coeval with i re establishment of well oxygenated conditions ii a marked early smithian to extreme early spathian ammonoid conodont diversification iii a significant increase in the skeletal grain diversity and abundance and iv peak carbon isotope values a short interval of black organic rich shales precedes the onset of these two distinct carbonate events which are both marked by a positive carbonate carbon isotope excursion known from other tethyan marine localities the comparison between the early triassic stratigraphic evolution of tethyan outer platform paleoenvironments and platform basin margin settings of the nanpanjiang basin suggests a causal connection between large fluctuations of the global carbon cycle climate shifts sea level changes oxygen availability in the oceans and the biological rediversification in the wake of the end permian biotic crisis

paleoenvironmental reconstruction of a downslope accretion history from coralgal coralline sponge rubble to mud mound deposits eocene ainsa basin spain abstract in the lutetian intraslope ainsa sub basin small sub spherical carbonate mud mounds occur associated with hemipelagic marls and mixed gravity flow deposits the studied mud mounds consist of a mixture of allochthonous parautochthonous and autochthonous components that show evidences of reworking bioerosion and accretion by different fossil assemblages at different growth stages the crusts of microbial lithistid sponges played an important role stabilizing the rubble of coralgal coralline sponges and formed low relief small benthic patches in a dominant marly soft slope environment these accidental hard substrates turned into suitable initiation nucleation sites for automicrite production dense and peloidal automicrites on which the small mud mounds dominated by opportunistic epi and infaunal heterozoan assemblages grew a detailed microfacies mapping and paleoenvironmental analysis reveals a multi episodic downslope accretion history starred by demosponges coralline and lithistid sponges agariciid corals calcareous red algae putative microbial benthic communities and diverse sclerobionts from the upper slope to the middle slope the analyzed mud mound microfacies are compared with similar fossil assemblages and growth fabrics described in many fossil mud mounds and with recent deep water fore reefs and cave environments