early holocene regressive spit platform and nearshore sedimentation on a glaciofluvial complex during the yoldia sea and the ancylus lake phases of the baltic basin sw finland abstract the depositional history of the virttaankangas plain a fan like enlargement of the large sakylanharju glaciofluvial ridge in sw finland is examined the sakylanharju ridge has been interpreted as having formed as an interlobate esker and the morphology of the virttaankangas plain as representing predominantly subaqueous glaciofluvial to glaciolacustrine deposition this investigation uses new sedimentologic outcrop data with extensive subsurface drilling gpr surveys seismic soundings and gravimetric measurement data provided by groundwater investigations to describe the stratigraphy morphology and depositional history of the plain in contrast to earlier interpretations the results emphasise the role of littoral processes in the origin of the virttaankangas plain the plain is interpreted as having originated during the development of a progradational sequence of spit platform and superimposed nearshore sediments deposited during the glacio isostatic forced regression of the yoldia sea and ancylus lake phases of the baltic basin 9900 9000 14 c bp documented depositional history supports an interval of retarded regression at the beginning of the ancylus lake phase i 9500 9200 14 c bp in the sakylanharju area

environmental controls on the late holocene carbonate sedimentation of a karstic lake in the middle atlas mountains lake afourgagh morocco the lake afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation afourgagh is a small shallow permanent lake located in the middle atlas mountains in morocco in a karstic context it is fed by ground waters that are relatively enriched in mg resulting from the leaching of the jurassic dolomitic bedrock of the catchment this eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high mg ca ratio the maximum extension of the holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies a littoral crust palaeosols palustrine silts and charophyte tufas which reflect different environments from the shoreline toward the deeper water in the more distal parts the charophyte tufas display a well expressed lamination punctuated by the development of microstromatolites on algae thalli the mineralogical composition of the carbonates is linked to the facies while the charophyte tufas are characterized by a relatively high content in aragonite in addition to low mg calcite the littoral crust is mainly composed of magnesite this pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers calcium carbonate precipitation on algae thalli both bioinduced and microbially mediated progressively induces an increase in the mg ca ratio of the lake water while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline producing magnesite during early diagenesis this effect is characteristic of two episodes part of the roman warm period and the beginning of the dark age cold period the carbonate mineralogy of the different depositional sequences at afourgagh indicates lake level and water chemistry fluctuations under a climatic influence therefore among other regional records the lake afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes

microbially mediated carbonates in the holocene deposits from sarlieve a small ancient lake of the french massif central testify to the evolution of a restricted environment both the mineralogy and facies of lacustrine bio induced carbonates are controlled largely by hydrological factors that are highly dependent upon climatic influence as such they are useful tools in characterizing ancient lake environments in this way the study of the sedimentary record from the small ancient sarlieve lake limagne massif central france aims to reconstruct the hydrological evolution during the holocene using petrographical mineralogical and geochemical analyses the fine grained marls mainly calcitic display numerous layers rich in pristine ca dolomite with small amounts of aragonite which are clearly autochthonous as these minerals are rather unusual in the temperate climatic context of western europe the question arises about their forming conditions and therefore that of the lacustrine environment ca dolomite prevails at the base of the sequence as a massive dolomicrite layer and in the middle part it builds up most of the numerous laminae closely associated with organic matter scanning electron microscope observations reveal the abundance of tiny crystals tens to hundreds of nanometres mainly organized as microspheres looking like cocci or bacilli such a facies is interpreted as resulting from the fossilization of benthic microbial communities by dolomite precipitation following organic matter consumption and extracellular polymeric substance degradation these microbial dolomites were precipitated in a saline environment as a consequence of excess evaporation from the system as is also suggested by their positive 18o values the facies sequence expresses the following evolution i saline pan i e endorheic stage with a perennial lowstand in lake level boreal to early atlantic periods ii large fluctuations in lake level with sporadic freshening of the system atlantic iii open lake stage sub boreal and iv anthropogenic drainage sub atlantic

a stable isotope study of microbial dolomite formation in the coorong region south australia abstract the coorong region of south australia is a classic location for the study of modern dolomite formation a number of ephemeral lakes here contain dolomite whilst other lakes in close proximity contain aragonite with minor magnesite our previous field and experimental work has revealed that sulphate reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area we here use a combination of oxygen carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments we report isotopic data from four dolomite and three aragonite containing lakes 34 s data for dissolved lake and porewater sulphates reveal 34 s enrichment of residual sulphate in all seven lakes consistent with bulk closed system microbial fractionation by sulphate reducing bacteria calculations using temperature dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes a comparison of 18 o data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt like saturated mud horizon coincident with the zone of sulphate reduction surprisingly 13 c data from the carbonates do not show the characteristic wide range of values often associated with organogenic carbonates but instead show that these bacterially mediated precipitates incorporate carbon primarily from the inorganic lakewater reservoir partially diluted by an organic component a coorong type microbial model of dolomite formation may be particularly applicable to for example precambrian sedimentary dolomites where benthic microbial communities which are today restricted to extreme environments dominated the shallow marine ecosystem prior to the emergence of metazoa

the stratigraphic and geochemical record of playa cycles monitoring a pangaean monsoon like system triassic middle keuper s germany abstract the combination of high resolution stratigraphy with high resolution geochemistry allowed monitoring the cyclicity of a playa system in two dimensions time and space the playa cycles of the middle upper triassic steinmergel keuper located in southern germany were studied both in terms of lateral facies shift variations and long term cyclic patterns the lateral variation was expressed by investigating a time equivalent transect that spanned the kieselsandstein horizon within a 130 km distance from marginal to mid playa position whereas the long term cyclic pattern was investigated in core malschenberg where a time record of ca 5 106 years was encountered mudstone dolomite cycles with dolomites representing the wet and mudstones the dry phases reflect cyclic playa lake level changes indicating varying intensities of monsoon like circulation moisture transport from the tethys ocean into the german basin is implied by 18o depletion trends of isochronous dolomite beds from the margin to mid playa position suggesting dominantly rayleigh like circulation patterns distinct covariance of 18o 13c is used as indicator of lake level stands the k al curve with inverse covariation compared to 18o reflects fluctuating weathering conditions on the fan and the vindelician high a hierarchic cycle stacking pattern composed of metre scale to tens of metre cycles is interpreted to be a result of changing pangaean monsoon like intensities in the milankovitch frequency band spectral analysis shows that the power spectra are dominated by precession and eccentricity

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

pedogenic phreatic carbonates on a middle devonian givetian terrigenous alluvial deltaic plain gilwood member watt mountain formation northcentral alberta canada in the muskeg trough of northcentral alberta the gilwood member contains widespread carbonate deposits that formed within terrigenous mudstone and sandstone hosts stratigraphic depositional and petrographic relationships indicate that these carbonates represent calcretes and dolocretes calcretes observed best with cathodoluminescence display microcrystalline alpha fabrics circumgranular cracks root networks displacive growth fabrics elongate channel voids and rare coloform growths with flower spar similarly dolocretes have microcrystalline alpha fabrics brecciation gradational contacts with host mudstones extensive layered nodular horizons and are associated with anhydrite and pyrite 13c values range between 7 to 1 and 6 to 3 for calcretes and dolocretes respectively oxygen isotopes are more variable and differ with host lithologies 18o of calcretes ranges between 11 to 8 for sandstones and 8 to 3 for mudstones whereas 18o of dolocretes ranges between 3 to 1 for marine mudstones and 6 to 2 for pedogenic mudstones regional mapping indicates that calcretes thicken towards the deepest parts of the muskeg trough widespread dolocretes extend beyond the eastern and western limits of muskeg trough and are useful marker intervals for regional correlations dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcretized channel sandstones calcrete isotopic values are interpreted as indicative of carbonate precipitation from waters with meteoric water input however the higher 18o values in dolocretes are indicative of a contribution from an isotopically heavier source such as seawater stratigraphically calcretes are most common along the western and northern edges of muskeg trough thus calcrete accumulation was further controlled by meteoric water in flow from the highland to the west and sluggish groundwater flow in muskeg trough in contrast regionally widespread dolocrete horizons appear to have formed from mixing of fresh waters derived from the highland to the west and seawaters introduced from the east regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out flow from the permeable channel sandstones which resulted in calcretization in channel proximal mudstones and dolomitization in channel distal mudstones

dolomite formation in the shallow seas of the alpine triassic the alpine triassic units of switzerland northern italy and western austria offer an extensive geological archive in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context recent studies clearly demonstrate that large amounts of the alpine triassic dolomites are late diagenetic or hydrothermal nevertheless as part of multiple generations of diagenetic overprint some generations of fine crystalline ca rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments in this review three cases of alpine triassic dolomites are discussed where dolomite rocks may have formed during or soon after sedimentation the sedimentary facies indicate contrasting palaeoenvironmental conditions and hence document three different possible processes of dolomite formation i in the dolomite mountains northern italy dolomite beds of the partly isolated middle triassic anisian ladinian latemar platform are confined to the very top of shallowing upward lagoonal facies cycles ii dolomite beds of the san giorgio basin southern switzerland an intra platform basin that opened during the anisian ladinian transition are associated with organic carbon rich shales which were deposited in a deeper water environment under anoxic conditions iii in the entirely dolomitized platform facies of the dolomia principale hauptdolomit formation a very early generation of fine crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles this platform extended over thousands of square kilometres along the tethys margin during the late triassic carnian and norian and large amounts of carbonate were deposited under hypersaline sabkha like conditions representing three distinct depositional environments these three different triassic systems show features in common with several dolomitization models developed from the study of modern dolomite forming environments for example the sabkha model the evaporative lagoon lake model the organogenic model and the microbial model although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation dolomite forming processes during the triassic were apparently quite different from the modern world in terms of distribution and scale recent developments in stable isotope geochemistry and high resolution geochemical probing offer the possibility to make better reconstructions of triassic palaeoceanographic conditions and suggest a non actualistic approach to better understand dolomite formation during the triassic

stable isotope study of carbonate sediments from the coorong area south australia the mineralogy and isotope geochemistry of carbonate minerals in the coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water the different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral in addition the dolomite was not formed by simple solid state cation exchange the occurrence of two types of dolomite was shown by isotope analysis and sem observations the dolomite which is isotopically light 13c 1 to 2 0 18o 3 to 5 0 and of fine grain size 0 5 m probably precipitated under the influence of evaporitically modified continental water coarser grained dolomite up to 4 m is isotopically heavier 13c 3 to 4 0 18o 5 to 6 0 contains mg in excess of ca and was formed in or close to equilibrium with atmospheric co2 probably by the dolomitization of aragonite

sedimentology mineralogy and isotopic analysis of pellet lake coorong region south australia gravity cores of holocene sediments from a shallow ephemeral lake in the coorong region pellet lake southeastern coastal australia show a mineral assemblage and sequence particular to its hydrology the mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core i e organic rich aragonite to magnesite hydromagnesite aragonite followed by a relative decrease in salinity i e magnesite aragonite hydrornagnesite to aragonite hydromagnesite in the upper portions of the core this sequence is capped by 0 4 m of micritic dolomite and minor amounts of hydromagnesite with the relative abundance of dolomite increasing upwards three stratigraphically and spatially distinct dolomite units upper lower and margin are recognized using stable carbon and oxygen isotope data unit cell calculations and mgc03 mole per cent data of the dolomite detailed x ray diffraction xrd analyses of samples with more than 80 dolomite shows that the dolomite is ordered average unit cell parameters calculated from the xrd patterns indicate that the upper dolomite unit has crystal lattices expanded in the c direction c 16 09 a relative to ideal dolomite c 16 02 a and contracted in the a direction a 4 796 a relative to ideal dolomite a 4 812 a the mol fraction of mgc03 in the upper dolomite shows up to 4 0 f 2 0 mole per cent excess mg in the dolomite crystal lattice calculated from xrd this unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated mg ca ratios transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution the modulated microstructure found in calcium rich dolomite is completely lacking dolomite ordering reflections are present in electron diffraction patterns but are weak stable oxygen and carbon isotope values of the upper dolomite are tightly grouped ave 6l80 x 7 55 6i3c 4 10 yet show three upward lightening oxygen cycles the oxygen cycles correlate with three upward decreases in the calculated mg content of the dolomite zone these cycles may indicate the increased importance of rain water dilution of the brine at times when the water in the lake was at its shallowest levels analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit the lower dolomite is close to stoichiometric has less evaporitic stable isotope values than the upper dolomite and has only a slightly expanded c axis the margin dolomite is ca rich has a more homogeneous microstructure and has expanded a and c axes the abundance of relatively soluble mg bearing phases such as hydromagnesite and magnesite may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment this process may leave a finely laminated dolomicrite deposit which retains little if any evidence of evaporite minerals

origin of fluids and anhydrite precipitation in the sediment hosted grimsey hydrothermal field north of iceland the sediment hosted grimsey hydrothermal field is situated in the tjornes fracture zone tfz which represents the transition from northern iceland to the southern kolbeinsey ridge the tfz is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area hydrothermal activity occurs in the grimsey field in a 300 m 1000 m large area at a water depth of 400 m active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field clear metal depleted up to 250 c hydrothermal fluids are venting from the active chimneys anhydrite samples collected from the grimsey field average 21 6 wt ca 1475 ppm sr and 3 47 wt mg the average molar sr ca ratio is 3 3 10 3 sulfur isotopes of anhydrite have typical seawater values of 22 0 7 34s indicating a seawater source for so42 strontium isotopic ratios average 0 70662 0 00033 suggesting the precipitation of anhydrite from a hydrothermal fluid seawater mixture the endmember of the venting hydrothermal fluids calculated on a mg zero basis contains 59 8 mol kg sr 13 2 mmol kg ca and a 87sr 86sr ratio of 0 70634 the average sr ca partition coefficient between the hydrothermal fluids and anhydrite of about 0 67 implies precipitation from a non evolved fluid a model for fluid evolution in the grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater before and during mixing seawater is heated to 200 250 c which causes anhydrite precipitation and probably the formation of an anhydrite rich zone beneath the seafloor

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

the database of individual seismogenic sources diss version 3 summarizing 20 years of research on italy s earthquake geology abstract this paper describes the main characteristics the evolution and the structure of the database of individual seismogenic sources diss and particularly of its release of early 2007 the database contains the results of the investigations of the active tectonics in italy during the past 20 years the first two sections of this paper document the recent evolution in mapping and archiving italian active fault data in relation to important achievements in the understanding of italian tectonics some of which were spurred by significant earthquakes the central sections describe the current structure of the database the reasons for its assumptions and data categories its current contents its evolution through several years of improvements the last section describes how the current contents of the database correspond with the existing strain and stress data available from focal mechanism borehole breakout and gps data for the whole of italy the database supplies a fresh and unified view of active and seismogenic processes in italy by building on basic physical constraints concerning rates of crustal deformation on the continuity of deformation belts and on the spatial relationships between adjacent faults both at the surface and at depth

petrology of two tertiary oil shale deposits from queensland australia sediments from two queensland tertiary lacustrine oil shale deposits the lowmead and duaringa basins have been petrologically characterized from standard x ray powder diffraction analyses backscattered electron microscopy and electron microprobe data the results presented illustrate the potential of backscattered electron imagery for the study of diagenesis in fine grained organic rich sediments the lowmead formation consists of lamosites claystones and brown coals deposited in freshwater lacustrine and fringing peat swamp environements their dominant constituents are kaolinite and quartz with opaline silica and siderite important as eogenetic phases despite its similar geological setting the duaringa formation contains a more varied lithological assemblage with less organic matter being preserved freshwater lacustrine claystones and lamosites are interbedded with carbonate sediments deposited during periods of saline alkaline conditions in the lake within these carbonate sediments major growth of eogenetic mineral phases occurred principally dolomite and analcite as in the lowmead basin siderite is an important eogenetic phase in the duaringa formation two types of sideritic ironstone band occur throughout both basins sphaerosiderites within claystone coal sediments and fine grained bands of siderite within lamosites the bulk mineralogy of the siliciclastic detritus shows no significant trends with depth within either basin

search for signs of ancient life on mars expectations from hydromagnesite microbialites salda lake turkey the 9white rock9 constituting a portion of what may be a lacustrine sedimentary sequence near the margin of a crater c 90 km across in sabaea terra mars measures 18 15 km 180 540 m high it is re interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an ancient evaporating crater lake were life to have emerged on mars as seems feasible then the 9white rock9 might be expected to comprise a complex of stromatolitic mounds salda golu lake in turkey is taken as an analogue this enclosed lake is nearly surrounded and underlain by partially serpentinized harzburgite hydromagnesite stromatolites microbialites up to 7 m high coalesce to form a group of small islands 200 m across the microbialites are seen to be growing near the mouth of the usually dry salda river in the southwestern sector smaller developments of hydromagnesite encircle the lake and image processing of satellite data reveals a second extensive zone beneath the lake surface over a delta in the southeast individual columns a few centimetres high constitute bulbous mounds which are about 2 m in diameter these columns terminate in domes a centimetre or so across the domes are often annulated and are covered with a green biofilm a few millimetres thick comprised of cyanobactrial filaments the columns consist of alternating fine and coarse hydromagnesite layers differentiated on a millimetric scale the coarser layers near the surface still contain traces of the biofilm fossil microbialites were also discovered in the friable hydromagnesite cliffs shoreward of the main developments though the structures of the individual microbes have not survived instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosity annular structures on their upper surfaces can be seen in places the intervening and overlying material also comprised of hydromagnesite is a semi lithified mud bulbous megascopic structures separated by finer grained magnesium carbonate mudstone within strata in the 9white rock9 would be strong evidence of a photosynthetic microbial genesis another deposit of white rock on the western margin of juventae chasma could have a similar origin

the role of mixing zone dolomitization in sandstone cementation evidence from the triassic buntsandstein the iberian range spain abstract petrographic and geochemical data suggest that dolomite cement in shallow marine sandstones of the triassic buntsandstein in the ateca horst of the iberian range spain has precipitated in the zone of mixing between phreatic meteoric and marine waters dolomite or rhombohedric crystals consisting of concentric bands of alternating syntaxial dolomite and calcite have filled isolated voids 350 12 mm in diameter subsequently limpid dolomite followed by sparry calcite precipitated in void center as the latest cements the carbonates have low 13 c pdb 9 5 to 6 0 and 18 o pdb 8 1 to 3 2 values and are enriched in radiogenic strontium 87 sr 86 sr 0 710112 to 0 720293 indicating a dominantly meteoric component in the porewaters the overall sequence of carbonate cementation suggests precipitation from porewaters that evolved in the mixing zone with successively increased meteoric influence and finally to fully meteoric probably due to a major sea level fall

saline lake carbonates within an upper jurassic lower cretaceous continental red bed sequence in the atacama region of northern chile the codocedo limestone member is a thin but laterally persistent lacustrine sequence within the red beds of the upper jurassic lower cretaceous quebrada monardes formation in the atacama region of northern chile the thick succession of clastic terrigenous sediments of the quebrada monardes formation was deposited in an arid to semi arid environment sedimentary facies are indicative of deposition of aeolian dunes alluvial fans and braided streams playa lake mudflats and saline lakes and coastal lagoons the strata accumulated in a n s elongated extensional back arc basin on the landward side of an active volcanic arc the 3 m thick codocedo limestone member marks striking facies changes within the quebrada monardes formation it is underlain by a thick sequence of conglomerates and sandstones deposited on alluvial fans the limestone itself is characterized by evaporite minerals and laterally continuous laminations indicative of deposition by vertical accretion in a perennial saline lake the overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa lake mudflats the limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession the lake was possibly formed quite suddenly for example by damming of the basin by a lava flow sedimentation in the perennial lake was predominantly cyclical seasonal planktonic algal blooms produced millimetre scale laminations interbedded with these laminites are centimetre scale beds of evaporitic gypsum anhydrite and minor halite the evaporite minerals have been largely replaced by calcite chalcedony and quartz the centimetre scale cycles may have resulted from periodic freshwater input into the lake after a period of about 3000 yr the lake dried up to be replaced by extensive playa lake mudflats the codocedo limestone member possibly formed a plane of detachment during an early tertiary phase of e w directed regional compression the limestones and evaporites were folded and extensively brecciated this deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement by calcite and quartz

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

shallow water evaporitic environments and their source rock potential in the major evaporitic environments on the world s surface today most organic matter accumulates in shallow subaqueous to seasonally subaerially exposed algal mat sediments given the present depositional setting and the most likely flushing by oxidizing water it is unlikely this organic matter could be preserved to form source rocks however if we place such evaporite deposition into a geologic context then there are times in the past when source rocks could have formed in shallow water settings such settings were characterized by hydrological conditions which allowed the retention of hypersaline anoxic pore waters to depths where the organic material was buried deeply enough to generate hydrocarbons when deep basin shallow water evaporite successions were laid down in basins such as the mediterranean in the late miocene the michigan basin in the silurian and other large saline giants then conditions were right for the formation of source rocks within shallow water and salt flat evaporitic environments the evaporites in these saline giants were deposited under conditions of relatively shallow water 50 m the basin never appears to have dried out but water levels changed quickly nearly equal 10 000 years from shallow to deep continual water saturation coupled with saline pore fluids prevented the inflow of fresh oxidizing groundwater into the basin center of shallow water organic rich evaporites immature hydrocarbons derived from such rocks today drip from the 5 5 m y evaporites of sicily in active salt and sulfur mines organic rich sediments could also be preserved to generate hydrocarbons in rapidly subsiding rift basins in such basins rapid burial has prevented the entrance of fresher oxygenated waters and the associated degradation and destruction of the organic matter the early continental riff stage generates the source rocks the ephemeral streams wadis and dune fields generate the reservoirs and the subsequent evaporite stage seals the reservoir

origin of dolomite and its spatial and chronological distribution a new insight abstract one of the oldest problems in sedimentology is the origin of dolomite dolomite camg co3 2 is one of the most common sedimentary carbonate minerals its density is 2 866 and calcite s density is 2 710 the absolute abundance of dolostones as well as the dolostone limestone ratio increase with geologic age changes in seawater magnesium concentration were invoked to explain these observations during the last 30 years for both scientific and economic reasons many attempts were made to explain the scarcity of recent dolomites carbonate rocks composed mainly of dolomite generally have high porosities and are important reservoir rocks for oil dolomite is the thermodynamically stable carbonate phase in seawater its relative scarcity in recent marine carbonate sediments therefore cannot be explained simply by the thermodynamic properties of dolomite dolomite formation appears to be inhibited by seawater most sedimentologists assumed that the formation of dolomite is mainly controlled by the dissolved magnesium calcium mg2 ca2 ratio in seawater the molar mg2 ca2 ratio in seawater is 5 3 it was therefore assumed that dolomite formation requires a still higher ratio this explanation however had to be abandoned when primarily during the last 2 to 3 years dolomite was observed as an important constituent of many modern marine organic rich sediments for example in th california borderlands gulf of california the japan trench cariaco basin off the coast of venezuela and the solar lake in israel in the guaymas basin for example dolomite actively forms from pore fluids with mg2 ca2 ratios of 1 to 2 similar dolomite formed in the miocene monterey formation either as penecontemporaneous or early diagenetic nodules or as laminated dolomite the dgr18o values of these dolomites range from 6 to 7 pmil pdb and their dgr13c values range from 30 to 20 pmil experiments conducted recently in our laboratory have shown that the important condition of dolomite formation is not high mg2 ca2 ratios but low dissolved sulfate so4 4 content which inhibits dolomite formation seawater contains 28 mmso42 dolomitization of calcite is already inhibited at sulfate concentrations of approximately 5 to 7 of seawater value aragonite dolomitization however though strongly retarded at these low dissolved so42 concentration values is inhibited at sulfate concentrations of approximately 50 of seawater s value we have also shown that dolomitization of caco3 proceeds through protodolomite a calcium rich disordered dolomite which would transform t dolomite if equilibrium were established dissolved so42 also retards the rates of protodolomite transformation to dolomite and dedolomitization sulfate ions adsorbed to the surfaces of calcite dolomite or aragonite may affect the kinetics of phase transformations in the carbonate system in a sulfate depleted environment sulfate is rapidly desorbed at 15 to 35 c 59 to 95 f the process is completed in 1 to 2 days thus favorable sites for dolomite formation in the marine environment are those where dissolved so42 concentrations are low the most effective processes of sulfate removal from or its dilution in marine pore fluids are microbial reduction in organic rich sediments and mixing of seawater with large amounts of fresh water this happens for example in continental margin sediments with high rates of deposition as in the gulf of california or in deeper water sediments associated with black shales the presence of much dissolved so42 in seawater also explains the almost absence of dolomite in apparently highly favorable environments for example in open marine carbonates the relative paucity of dolomite rich carbonate rocks dolostones formed during the last 100 to 120 million years as compared with their abundance in older sediments in particular in the precambrian and early paleozoic could be explained on the basis of changes in the depositional environments and in the carbonate mineralogy of the primary calcareous sediments these differences do not necessarily reflect changes in seawater magnesium concentrations end of article last page 2156