authigenic trioctahedral smectites controlling ph alkalinity silica and magnesium concentrations in alkaline lakes abstract the mineralogical composition of salts and the chemical composition of brines of lake yoa ounianga el kebir northern chad both reveal the neoformation of stevensite a trioctahedral smectite accompanied by aragonite the coprecipitation of these two minerals clearly controls the evolution of ph alkalinity and ca mg and silica concentrations in alkaline brines the geochemical evolution characterized by a strong depletion of ca and mg in solution and an increase in alkalinity and ph is confirmed as the dominant process governing the formation of soda alkaline lakes

characterization of hyperalkaline fluids produced by low temperature serpentinization of mantle peridotites in the oman and ligurian ophiolites a regional survey of alkaline springs in oman and ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite the first order correlation between the na and k and cl concentrations points to fluid compositions only partly due to evaporation the scatter around the evaporation line implies that na and cl may not be conservative during the alteration of the ultramafic rocks mg is almost entirely depleted at ph 10 5 as a result of serpentine formation within the ultramafic body and of brucite and minor hydrotalcite precipitation at the springs ca accumulates in the high ph fluids and is consumed by ca carbonate formation at the springs by mixing with river waters or by the co2 supply from the atmosphere thermodynamic calculations show that brucite saturation is reached at ph values around 10 5 which triggers major changes in the water composition the waters evolve from a quartz saturated low ph continental environment to a brucite dominated high ph serpentinizing system at low temperature the highest water salinities are found in springs located along the basal thrust plane of the ophiolite the highest al concentrations are found in some springs located on the crustal side of the mantle crust boundary this poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations key points new temperature and composition of the alkaline waters brucite formation at ph 10 5 triggers major chemical and mineralogical changes relationship between the water compositions and the hydrologic pathways

paleoclimate of the past 4000 years at lake turkana kenya based on the isotopic composition of authigenic calcite abstract authigenic calcite was analyzed for 13 c and 18 o in a 12 m long core from lake turkana that spans the last 4200 yr the 18 o record is interpreted to reflect lake level history and indicates high stands prior to 4000 yr b p from 1800 to 2000 yr b p and within the last century this coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for lake turkana based on 14 c dates of exposed shoreline deposits the 13 c curve roughly parallels the 18 o curve and may reflect the influence of the omo river plume both of the isotope records are strongly cyclic with periodicites common to both at 80 120 150 and 225 yr other somewhat longer periodicites are nearly equal in the two records and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same turkana core

assessing the extent of carbonate deposition in early rift settings select lacustrine and marine depositional settings show a spectrum of styles of carbonate deposition and illustrate the types of carbonates with an emphasis on microbialites and tufa to be expected in early rift settings early rift lake examples examined in this review article are all from east africa lakes turkana bogoria natron and magadi manyara and tanganyika other lake examples include four from the western united states great salt lake and high lake level lake bonneville mono lake and high lake level russell lake pyramid lake and high lake level lake lahontan and searles lake and two from australia lakes clifton and thetis marine basin examples are the hamelin pool part of shark bay from australia marginal marine and the red sea marine rift landsat images and digital elevation models for each example are used to delineate present and past lake basin margins based on published lake level elevations and for some examples the shorelines representing different lake levels can be compared to evaluate how changes in size shape and lake configuration might have impacted carbonate development the early rift lakes show a range of characteristics to be expected in lacustrine settings during the earliest stages of continental extension and rifting whereas the red sea shows well advanced rifting with marine incursion and reef skeletal sand development collectively the lacustrine examples show a wide range of sizes with several of them being large enough that they could produce carbonate deposits of potential economic interest three of the areas great salt lake and high lake level lake bonneville pyramid lake and high lake level lake lahontan and the red sea are exceedingly complex in that they illustrate a large degree of potential depositional facies heterogeneity because of their size irregular pattern and connectivity of subbasins within the overall basin outline

seismic characteristics and distribution of hydrothermal vent complexes in the cretaceous offshore rift section of the campos basin offshore brazil abstract hydrothermal vents are structures that occur close to volcanically active areas their occurrence can have considerable impacts on the evaluation of the oil reserves of a basin in the campos basin eastern brazilian continental margin no hydrothermal vents have been previously recognized and related with magmatic events on seismic sections this work aims to map the hydrothemal vents ducts and associated intrusive bodies observed in the rifte section with the use of 2d seismic data and a well with lithologic information the occurrence of ten hydrothermal vents and seven ducts were mapped and the interval with volcanic intrusive rocks was characterized the identification of intrusive rocks in seismic data is given by reflectors with high amplitude and disk or half moon geometry with abrupt endings the vent geometries shows dome and eye shaped types and the ducts are fault related downward tapering cone type this study highlights new geological elements that can influence the petroleum systems of the campos basin such as the identification of hydrothermal vents in seismic sections and the correlation with regional tectono magmatic events

hot spring systems geobiology abiotic and biotic influences on travertine formation at mammoth hot springs yellowstone national park usa multiple abiotic and biotic factors combine in nature to influence the formation of calcium carbonate limestone deposits systems geobiology studies of how micro organisms respond to or sometimes even control the coupled effects of environmental change and mineralization will permit more accurate interpretation of the fossil record of ancient microbial life mammoth hot springs in yellowstone national park usa serves as a natural laboratory for tracking how the dynamic interplay of physical chemical and biological factors come together to form hot spring limestone called travertine systematic downstream correlations occur at mammoth hot springs between travertine deposition geomorphology crystalline structure and geochemistry microbial communities mat morphology pigmentation and phylogenetic and metabolic diversity and spring water conditions temperature ph geochemistry and flow field based microscale and mesoscale experimentation indicates that microbes directly influence travertine growth rate and crystalline structure at the macroscale time lapse field photography and numerical modelling suggest that travertine terrace geomorphology is influenced strongly by hydrology heat dispersion and geochemistry these results from mammoth hot springs allow establishment of a conceptual framework across broad spatial and temporal scales in which to track how multiple geological and biological factors combine to control caco3 crystal precipitation and the resulting formation of travertine deposits

paleo co2 variation trends and the cretaceous greenhouse climate the cretaceous was one of the most remarkable periods in geological history with a greenhouse climate and several important geological events reconstructions of atmospheric co2 using proxies are crucial for understanding the cretaceous greenhouse in this paper we summarize the major approaches for reconstructing co2 based on paleobotanical or geochemical data and synthesize the co2 variations throughout the cretaceous the results show that atmospheric co2 levels remained relatively high throughout the cretaceous but were lower in the early cretaceous highest in the mid cretaceous and gradually declined during the late cretaceous however this overall trend was interrupted by several rapid changes associated with ocean anoxic events oaes and the end cretaceous catastrophic event new data on paleo co2 levels from paleobotanical and paleosol evidences support not only the overall trends indicated by geochemical models but provide more precise records of the short term fluctuations related to brief episodes of climate change temporal resolution within the long quiet magnetic period in the middle cretaceous is one of the obstacles preventing us from a more comprehensive understanding of the co2 climate linkage but new paleo co2 determinations and climatic data from stratigraphic sections of sediments intercalated with datable volcanic rocks will allow a better understanding of the relationships between fluctuations of atmospheric co2 climate change and geological events

sublacustrine precipitation of hydrothermal silica in rift lakes evidence from lake baringo central kenya rift valley abstract many lakes in volcanic regions are fed by hot springs that in some basins can contribute a large percentage of the annual recharge especially during times of aridity it is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record hot spring deposits travertine sinter provide the most tangible evidence for thermal recharge to lakes although subaerial spring deposits have been widely studied lacustrine thermal spring deposits especially sublacustrine siliceous sinters remain poorly known detailed field petrographic and scanning electron microscope sem studies have been made of fossil sublacustrine sinter exposed at soro hot springs along the northeastern shoreline of ol kokwe a volcanic island in lake baringo kenya modern hot springs at soro which discharge na hco3 cl waters from a deep reservoir 180 c have thin 1 10 mm friable microbial silica crusts around their subaerial vents but thicker 1 cm sinter deposits are not forming the fossil sinter which is present as intergranular cements and crusts in littoral conglomerates and sandstones is composed mainly of opaline silica opal a three types of fossil sinter are recognized 1 massive structureless silica which fills intergranular pores and forms crusts up to 5 cm thick 2 pore lining silica some of which is isopachous and 3 laminated silica crusts which formed mainly on the upper surfaces of detrital particles all three types contain well preserved diatoms including lacustrine planktonic forms microbial remains mainly filamentous and coccoid bacteria including cyanobacteria and extracellular polymeric gels are locally abundant in the opaline silica together with detrital clays and thin laminae composed of authigenic chlorite most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water silica precipitated upon rapid cooling of thermal 90 c waters at and just below their interface with the overlying cooler 25 c lake waters microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids some of the silica originally may have been soft and partly gelatinous planktonic diatoms and detrital clay rained down then became incorporated in the amorphous silica following a fall in lake level the opal a lithified and partly altered to cristobalite opal c and chalcedony the lowest fossil sinters were later encrusted by calcite stromatolites with calcite and quartz forming late pore filling cements the age of the sublacustrine sinters is unknown but some of the deposits could date back to the late pleistocene similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring lake bogoria such rocks may provide evidence for deep hot fluid recharge to lakes when encountered in the geological record

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

are spherulitic lacustrine carbonates an expression of large scale mineral carbonation a case study from the east kirkton limestone scotland lacustrine carbonate deposits with spherulitic facies are poorly understood but are key to understanding the economically important pre salt mesozoic strata of the south atlantic a major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite dominated deposits which are known in outcrop stratigraphy and petrography suggest one of the best analogue systems is found in the carboniferous of scotland the east kirkton limestone here we propose a hydrogeochemical model that explains why the caco3 sio2 mg si al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock demonstrating links between igneous bedrock chemistry lake and spring water chemistry and mineral precipitation this model has implications for studies of lacustrine sediments in rift basins of all ages using empirical and theoretical approaches we analyze the relationship between metal mobilization from sub surface volcaniclastic rocks and the potential for precipitation of carbonate minerals various mg bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting this suite of minerals is most likely formed by in gassing of co2 to a carbon limited alkaline spring water consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater we suggest that an analogous system to that at east kirkton caused development of the pre salt spherulitic carbonate deposits

the hydrochemical evolution of alkaline volcanic lakes a model to understand the south atlantic pre salt mineral assemblages abstract understand the origin of the exotic strontium rich calcite magnesium rich clay silica deposits formed during the early cretaceous in the south atlantic pre salt lakes is challenging our ability to comprehend the chemical evolution of alkaline volcanic lakes here we present a new hydrochemical model based on an open basin concept thermodynamic equilibrium and chemical data from lake baringo east african rift to shed light on the mechanisms facilitating carbonate clay silica precipitation this model explores the effects that the leakage to adjacent waterbodies the combination of lake evaporation and lake recharge with river hydrothermal and marine waters and the bathymetric effects of the partial co2 pressure have in the stationary precipitation of pre salt lacustrine mineral assemblages sediment thickness calculations suggest that the facies cyclicity in the pre salt and elsewhere are likely reflecting temporal fluctuations in the type of waters sourcing the lake and in the degree of leakage to aquifers over multiple evaporative freshening cycles a plausible link between enhanced strontium uptake into pre salt calcite allochems and ca2 co32 lake water stoichiometries allows to infer different scenarios for strontium rich calcite formation with or without magnesium clays our conceptual model allows to quantify the role that hydrology groundwater charge discharge to aquifers evaporation exert on carbonate clay silica precipitation providing with a refined framework to understand the basin scale chemical sedimentation in alkaline lakes furthermore this model represents a robust template upon which critically test the superimposed kinetic factors likely attached to the genesis of specific mineral factories such as those encountered in the south atlantic pre salt alkaline lakes

is small scale convection responsible for the formation of thick igneous crust along volcanic passive margins 1 boutilier and keen 1999 showed that small scale convection may produce the observed thick igneous crust along volcanic passive margins provided that a non linear rheology is adopted and the viscosity of the upper mantle is increased suddenly after 132 km of continental crust extension the sudden viscosity increase is justified by the melt induced dewatering of the upper mantle rocks through this comparative study it is shown that inclusion of the pressure of the newly formed igneous crust into the melting model and or adopting a more realistic viscosity law significantly reduces the thickness of igneous crust produced by boutilier and keen s models furthermore it is shown that melt migration would narrow the width of the thick igneous crust results of this study show that small scale convection may not play a significant role in the formation of the observed thick igneous crust along volcanic passive margins

challenges of anticipating the 2011 tohoku earthquake and tsunami using coastal geology 1 can the magnitude of a giant earthquake be estimated from paleoseismological data alone attempts to estimate the size of the jogan earthquake of ad 869 whose tsunami affected much of the same coast as the 2011 tohoku tsunami offers an excellent opportunity to address this question which is fundamental to assessing earthquake and tsunami hazards at subduction zones between 2004 and 2010 examining stratigraphy at 399 locations beneath paddy fields along 180 km of coast mainly south of sendai we learned that a tsunami deposit associated with the ad 869 jogan earthquake had run inland at least 1 5 km across multiple coastal lowlands and that one of the lowlands had subsided during the jogan earthquake and an earlier earthquake as well radiocarbon ages just below above sand deposits left by the pre jogan tsunamis suggested recurrence intervals in the range of 500 to 800 years modeling inundation and subsidence we estimated size of the jogan earthquake as moment magnitude 8 4 or larger and a fault rupture area 200 km long we did not consider a longer rupture like the one in 2011 because coastal landform and absence of a volcanic ash layer make any jogan layer difficult to identify along the sanriku coast still sendai tsunami geology might have reduced casualties by improving evacuation maps and informing public awareness campaigns

human evolution in a variable environment the amplifier lakes of eastern africa abstract the development of the cenozoic east african rift system ears profoundly re shaped the landscape and significantly increased the amplitude of short term environmental response to climate variation in particular the development of amplifier lakes in rift basins after three million years ago significantly contributed to the exceptional sensitivity of east africa to climate change compared to elsewhere on the african continent amplifier lakes are characterized by tectonically formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area such amplifier lakes respond rapidly to moderate precessional forced climate shifts and as they do so apply dramatic environmental pressure to the biosphere rift basins when either extremely dry or lake filled form important barriers for migration mixing and competition of different populations of animals and hominins amplifier lakes link long term high amplitude tectonic processes and short term environmental fluctuations east africa may have become the place where early humans evolved as a consequence of this strong link between different time scales

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

mineralogical assemblages forming at hyperalkaline warm springs hosted on ultramafic rocks a case study of oman and ligurian ophiolites 1 we report on the mineralogical assemblages found in the hyperalkaline springs hosted on liguria and oman ophiolites based on exhaustive x ray diffraction and scanning electron microprobe analyses in liguria hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric co2 uptake and neutralization of the hyperalkaline waters no brucite and portlandite minerals are observed the discharge of alkaline waters in oman ophiolite forms white orange precipitates calcium carbonate minerals calcite and or aragonite are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in liguria this process is observed as a thin surface crust made of rhombohedral calcite morphological features of aragonite vary from needle bouquet dumbbell spheroidal like habitus according to the origin of carbon temperature and ionic composition of the hyperalkaline springs and the biochemical and organic compounds brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo moho the varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation the layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros finally the dominant mineralogical associations we found in oman ca bearing carbonates and brucite in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the lost city hydrothermal site in a totally marine environment

diagenetic pathways linked to labile mg clays in lacustrine carbonate reservoirs a model for the origin of secondary porosity in the cretaceous pre salt barra velha formation offshore brazil

the chemical and hydrological evolution of an ancient potash forming evaporite basin as constrained by mineral sequence fluid inclusion composition and numerical simulation the chemical evolution of the brine in a potash evaporite basin has been investigated by x ray microanalysis of frozen primary inclusions trapped in halite a computer program based on thermodynamic equilibrium and mass balance principles has been used to simulate evaporation paths the comparison between the results of calculations the observed mineralogy and mineral sequence and the solute content in fluid inclusions has placed constraints on the hydrological evolution of the basin the upper eocene basin of navarra southern pyrenees spain began as a marine basin evolving from a moderate to a high degree of restriction depositing first a basai anhydrite horizon and then a thick sequence of massive halite an additional inflow of cacl2 in the basin during seawater evaporation is proposed as the process responsible for the sulfate depletion required for sylvite instead of mg sulfates to form mixing of seawater with continental waters bacterial sulfate reduction and in situ dolomitization are discarded the basin subsequently closed to the sea and evolved with decreasing volume alternating bands of clays halite sylvite and then clays halite carnallite were deposited under the influence of seasonal continental recharge before reaching total desiccation the residual brine was diluted by continental water the basin then evolved under an endoreic regime where continental recharge and the recycling of previously formed halite led to deposition of alternating beds of clays and halite

palaeohydrology of the lower cretaceous pre salt lacustrine system from rift to post rift phase santos basin brazil abstract large oil accumulation discoveries in the last decade in the pre salt rock succession off the brazilian coast aroused renewed interest in carbonate reservoirs prompting several studies aiming at unravelling the conditions of formation of these rocks despite that there are still many gaps in the knowledge concerning environmental conditions of the deposition of the pre salt carbonates of santos basin located at the southeastern brazilian margin in this study we present and discuss a new suite of geochemical and biostratigraphic data of the lacustrine rift to post rift succession broadly correlated with itapema and barra velha formations of the santos basin whose deposition occurred in the aptian the integration of the petrographic biostratigraphic and geochemical data serves the ultimate purpose of the study which is to apply the new radiogenic strontium data as a proxy for the reconstruction of the evolution of the lake hydrology associated with the deposition of the carbonates of the itapema and barra velha formations these data are integrated into numerical models whose results bring new ideas on the general hydrologic conditions of the lake system and on its evolution through time the mass balance calculations indicate that two end members alone can account as potential sources of sr to the palaeolake a mafic and a felsic component respectively the latter likely contributed with significantly higher fluxes roughly one to two and up to four orders of magnitude higher than the former end member for most of the depositional history in order to generate the 87sr 86sr trends observed in the carbonate rocks finally the results obtained from the integration of the observations with the numerical models point to decreasing fluxes of water with time as contributed from both sources suggesting that the formation of the deposits became very likely constrained to progressively more evaporitic and alkaline lake waters these results also open up a new window into the environmental conditions of the interior gondwana continent immediately prior to the definitive opening of the south atlantic ocean

geochemical evolution of closed basin lakes general model and application to lakes qinghai and turkana in contrast to most previous models for the evolution of closed basin lakes we present an integrated model which considers various water budget patterns clay regradation so4 reduction and subbottom leakage in addition to the classical equilibrium approach of mineral precipitation the model was applied to lakes qinghai and turkana which significantly differ in the lithologies of their drainage areas but are representative of the carbonate rich sedimentary rock province of the tibet qinghai plateau and the silicate rock province of eastern africa both lakes are now topographically closed but to some degree hydrologically open subbottom leakage major results of the mode calculations show that the lithology controls the ultimate brine which is of na k cl type for qinghai lake and of na hco3 cl type for lake turkana so4 reduction delays the onset of sulfatic mineral precipitation and favours the formation of na carbonates such as trona at the expense of calcite clay mineral regradation plays an important role before the saturation of sulfatic or chlorine minerals is reached in particular magnesite formation may be in competition with mg bearing clay minerals finally simulations with various hydrological scenarios have shown that the modern hydrochemistry of both lakes cannot be reproduced by simply evaporating inflow water but reflects long term accumulation and evolution of solutes by continuous inflow over several thousand years the diversity of lake water composition within a uniform lithological province can thus be largely ascribed to varying hydrological conditions

a depositional model for spherulitic carbonates associated with alkaline volcanic lakes the south atlantic aptian pre salt reservoirs are formed by a combination of spherulitic carbonates and mg rich clays accumulated in volcanic alkaline lake settings with exotic chemistries so far outcrop analogues characterised by metre thick successions deposited in lacustrine scenarios are elusive so disentangling the genesis of spherulitic carbonates represents a major scientific challenge with business impact in particular the controls on spatial distribution and the environment of spherulitic facies formation remain poorly constrained little studied and hotly debated to shed light on this conundrum a spherulitic carbonate rich alkaline volcanic lacustrine succession has been analysed at outcrop scale the carboniferous east kirkton limestone scotland despite clays being very scarce and limited to layers of amorphous mg si minerals a diverse array of spherulitic calcitic components were formed including coated grains crusts and build ups this setting enables the mechanisms of spherulitic calcite development and the patterns of sediment accumulation to be explored in a geobiological and hydrochemical scenario similar to the pre salt subsurface occurrences but divorced from clay influence the integration of logs borehole data outcrop photomosaics and petrographic observations collectively allowed the reconstruction of a depositional model for the east kirkton lacustrine succession in this model calcite spherule nucleation took place at the sediment water interface in the littoral zone driven by the co occurrence of 1 high alkalinity 2 ca mg rich hydrochemistry and 3 microbial derived colloidal exopolymeric substances these environmental conditions permitted the coeval development of spherulitic cementstone build ups and spherulitic grainstone packstone within the wave agitated zone and the accumulation of floatstones and laminites of spherulitic grains in deeper lake regions by means of downslope reworking this model is consistent with the previously documented microbial bloom occurrences and highlights the need to better understand the complex microbe solution interactions before any reliable facies model is envisaged

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

growing spherulitic calcite grains in saline hyperalkaline lakes experimental evaluation of the effects of mg clays and organic acids the origin of spherical radial calcite bodies spherulites in sublacustrine hyperalkaline and saline systems is unclear and therefore their palaeoenvironmental significance as allochems is disputed here we experimentally investigate two hypotheses concerning the origin of spherulites the first is that spherulites precipitate from solutions super saturated with respect to magnesium silicate clays such as stevensite the second is that spherulite precipitation happens in the presence of dissolved organic acid molecules in both cases experiments were performed under sterile conditions using large batches of a synthetic and cell free solution replicating waters found in hyperalkaline saline lakes such as mono lake california our experimental results show that a highly alkaline and highly saline solution supersaturated with respect to calcite control solution will precipitate euhedral to subhedral rhombic and trigonal bladed calcite crystals the same solution supersaturated with respect to stevensite precipitates sheet like stevensite crystals rather than a gel and calcite precipitation is reduced by 50 compared to the control solution producing a mixture of patchy prismatic subhedral to euhedral and minor needle like calcite crystals enhanced magnesium concentration in solution is the likely the cause of decreased volumes of calcite precipitation as this raised equilibrium ion activity ratio in the solution on the other hand when alginic acid was present then the result was widespread development of micron size calcium carbonate spherulite bodies with further growth time but falling supersaturation these spherules fused into botryoidal topped crusts made of micron size fibro radial calcite crystals we conclude that the simplest tested mechanism to deposit significant spherical radial calcite bodies is to begin with a strongly supersaturated solution that contains specific but environmentally common organic acids furthermore we found that this morphology is not a universal consequence of having organic acids dissolved in the solution but rather spherulite development requires specific binding behaviour finally we found that the location of calcite precipitation was altered from the air water interface to the surface of the glassware when organic acids were present implying that attached calcite precipitates reflect precipitation via metal organic intermediaries rather than direct forcing via gas exchange

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

three dimensional stratigraphic sedimentological forward modeling of an aptian carbonate reservoir deposited during the sag stage in the santos basin brazil abstract stratigraphic sedimentological forward modeling is a useful technique to test the role of parameters that may act in the depositional processes of sedimentary rocks the technique was applied to the santos basin brazil using the software program dionisos to test possible depositional scenarios for an aptian section of a lacustrine carbonate platform developed during the late sag stage uppermost pre salt section of the basin different parameters were tested in a set of numerical simulations with the aim of reproducing the carbonate facies spatial distribution and the overall stratigraphic stacking pattern of the platform as observed in the available well data several models were simulated and four of them are discussed in this paper based on the available descriptive data from 10 wells model 3 is considered to produce the best fit to explain the development of the 100 m thick aptian carbonate platform occurrences of grainstones and stromatolites as proximal and intermediate facies respectively and the overall carbonate stacking pattern thus based on 3d stratigraphic sedimentological forward modeling the best set of environmental conditions able to explain the pattern of carbonate deposition observed in the study area are 1 a slow carbonate depositional rate 0 08 mm y over an interval of 2 4 my which is indicative of a larger microbial contribution and 2 lake level oscillations which were essentially induced by arid climatic conditions these results also enable new perspectives for the application of stratigraphic sedimentological forward modeling as a predictive tool for hydrocarbon exploration in carbonates and for the development of depositional models of other sag carbonate platform settings

presalt stratigraphy and depositional systems in the kwanza basin offshore angola the lower cretaceous presalt section in the kwanza basin contains an excellent petroleum system that includes synrift strata barremian overlain by a sag interval aptian and capped by the loeme salt the upper synrift is generally limestone with widespread mollusk packstones and grainstones coquinas deposited in a fresh to moderately saline alkaline lake the sag interval is characterized by carbonate platforms and silica rich isolated buildups formed in highly evaporated highly alkaline lakes shrubby dendritic microbially influenced boundstones and intraclast spherulite grainstones accumulated in shallow water on platform tops microbial cherts were deposited as organic buildups on large isolated structural highs basinward west of platforms and they apparently formed at low temperatures in very alkaline lake water shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability wackestones and packstones with calcitic grains mainly spherulites in dolomite or argillaceous dolomite were deposited in slightly deeper low energy sag environments in addition clays especially stevensite precipitated out of the silica rich highly alkaline lake waters during sag deposition calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms spherulites probably precipitated just below the sediment water interface spherulites and shrubby calcites are commonly recrystallized spherulites floating in stevensite probably formed in deeper lacustrine environments organic rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals and they are likely the source of most hydrocarbons in this system these interpretations are supported by seismic core petrographic and stable isotope data

lacustrine carbonate reservoirs from early cretaceous rift lakes of western gondwana pre salt coquinas of brazil and west africa abstract barremian aptian lower cretaceous pre salt lacustrine carbonates host large petroleum reservoirs in south atlantic rift basins yet remain poorly studied in part due to the scarcity of lacustrine carbonate reservoirs throughout the geological record this review provides an integrated discussion of the current state of knowledge on the coquina play having been deposited in extensive rift lakes during the early stages of the opening of the south atlantic thick successions of coquina are recorded from the santos to sergipe alagoas basins and the kwanza to gabon basins on the brazilian and west african continental margins respectively coquinas exhibit similarities in depositional processes across the south atlantic however have distinct diagenetic histories and stratigraphic stacking patterns having been deposited in lakes with different geologies climates and water chemistries west african coquinas exhibit depositional characteristics that are indicative of deposition on distally steepened ramps and rimmed platforms and accordingly facies are somewhat different to those in brazilian basins which were deposited on low angle ramps as petroleum exploration continues into ultra deep waters on the brazilian and west african margins accurate facies models will be important for the predictability of carbonate reservoir intervals to this end onshore outcrop in the sergipe alagoas basin has revealed that the molluscan faunas have marine origins suggesting that facies patterns may be more complex than previously considered it is hoped that this review will provide the impetus for further research on the coquinas especially given the presence of poorly studied easily accessible onshore outcrops