green river laminites does the playa lake model really invalidate the stratified lake model proponents of the playa lake model have proposed deposition of most of the green river formation microlaminated carbonates including oil shales in lakes that were not perennially stratified meromictic however there is a variety of evidence favoring a meromictic depositional environment 1 close similarity of much of the lamination to varves in modern meromictic lakes 2 evidence that hydrologic events favoring development of meromixis chemical stratification occurred prior to deposition of major accumulations of oil shale 3 mutually exclusive distribution of fossil nekton especially fish and normal lacustrine benthos including mollusks and 4 analogy with a quaternary playa that became a meromictic lake following increased inflow the playa lake model is untenable for the typical fish bearing kerogen rich microlaminated sediments these laminites were probably deposited in a large ectogenic meromictic lake a chemically stratified lake that formed when increased fresh water inflow drowned a saline playa complex

depositional environments in an arid closed basin and their implications for oil and gas exploration the lower permian fengcheng formation in the junggar basin china the fengcheng formation is a nonmarine carbonate dominated succession that formed under arid climatic conditions in a hydrologically closed basin two transects and two seismic profiles were examined the characteristics and environmental significance of different lithofacies were studied and a model of depositional environment divisions was proposed the sedimentary model involved an alkaline lake in which the depositional environments consisted of a shallow saline lake margin slope saline lake center and steep lake margin from northeast to southwest the perennial central salty lake was located in the southwestern part of the study area whereas there were widespread low gradient lake margins in the northeast east southeast and southern parts of the study area lake level fluctuations had a major influence on the shallow saline lake system and complicated the depositional environments in these areas the deposits are derived from bedrock reworking volcanic eruptions and authigenic minerals that precipitated from brine during the hypersaline phase fine grained terrigenous clastic sediments volcanic ashes and dusts and authigenic minerals mixed in the depocenter concentration center of the brine pool which was covered by high salinity brines and the depositional environment was anoxic as a result of salinity based brine stratification a thick sodium carbonate succession occurred in the depocenter of the ancient mahu lake where bedded sodium carbonate alternated with fine grained organic rich tuff or tuffaceous hydrocarbon source rocks microorganisms bloomed in the alkaline high salinity brine and the organic matter was well preserved which is similar to those modern alkaline saline lakes in eastern africa and western north america thus the permian fengcheng formation contains source rocks that formed in an alkaline saline lake

large scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows large scale 20 m to 7 km wavelength bedforms are common on the seafloor yet there is a lack of consensus on how they form and thus what to call them we conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments the data form three distinct groups 1 small scale 20 300 m wavelength sediment waves with mixed relief made of medium sand to cobble sized sediment that form in confined settings which we call small sediment waves 2 large scale 300 7000 m wavelength sediment waves with mixed relief made of fine grained sediment that form in relatively unconfined settings which we call large sediment waves and 3 large scale fully enclosed depressions in the seafloor which we call scours there is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact this data gap probably results from the environments in which sediment waves form being either confined e g channel or canyon or unconfined e g open slope bedform migration direction is available for 36 of the data and includes small and large scale sediment waves of these examples all are shown to migrate up current up current migration is indicative of supercritical flow thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves therefore we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size the migration direction for scours remains unknown scours may form from similar processes to small and large sediment waves or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre existing defects in the seafloor this novel statistical analysis of a global database shows that up current migrating bedforms associated with supercritical flow are unusually widespread and are recognised at two distinct scales

onset of submarine debris flow deposition far from original giant landslide submarine landslides can generate sediment laden flows whose scale is impressive individual flow deposits have been mapped that extend for 1 500 km offshore from northwest africa these are the longest run out sediment density flow deposits yet documented on earth this contribution analyses one of these deposits which contains ten times the mass of sediment transported annually by all of the world s rivers understanding how this type of submarine flow evolves is a significant problem because they are extremely difficult to monitor directly previous work has shown how progressive disintegration of landslide blocks can generate debris flow the deposit of which extends downslope from the original landslide we provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide encased within deposits of a more dilute flow type called turbidity current very little sediment was deposited across the intervening large expanse of sea floor where the flow was locally very erosive sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea floor gradient from 0 05 degrees to 0 01 degrees this debris flow was probably generated by flow transformation from the decelerating turbidity current the alternative is that non channelized debris flow left almost no trace of its passage across one hundred kilometres of flat 0 2 degrees to 0 05 degrees sea floor our work shows that initially well mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean

experimental study of shortite na2ca2 co3 3 formation and application to the burial history of the wilkins peak member green river basin wyoming usa abstract the temperature at which shortite na2ca2 co3 3 forms from pirssonite na2ca co3 2 2h2o and calcite using pure synthetic phases in the system na2co3 caco3 h2o has been experimentally determined at 1 atm pressure shortite forms via the reaction na2ca co3 2 2h2o caco3 na2ca2 co3 3 2h2o above 55 2 c this equilibrium temperature is lower than determined previously 90 25 c by bradley and eugster 1969 the solution in equilibrium with synthetic shortite pirssonite and calcite approximates a binary h2o na2co3 brine with 1 1 m na2co3 10 6 wt na2co3 the equilibrium temperature is lowered to 52 2 c with 5 m nacl added to the system which shows that this reaction is weakly dependent on the activity of h2o a h 2 o this study suggests that shortite does not occur in surficial alkaline saline environments because temperatures are too low shortite is abundant in the wilkins peak member wpm of the eocene green river formation green river basin wyoming usa 78 million tons km2 where it occurs as diagenetic displacive crystals fracture fills and pseudomorphous replacements of a precursor na ca carbonate the large area over which shortite occurs in the wpm indicates that saline pore fluids once existed in the buried lacustrine sediments and thus at times large na co3 rich saline alkaline lakes or laterally extensive saline groundwaters existed in the green river basin during wpm time the thermal stability of shortite together with vitrinite reflectance data and inferred regional geothermal gradients establish that the shortite bearing intervals of the wpm were buried to maximum depths of 1000 m in the green river basin and since experienced 800 m of erosional exhumation

evaporitic carbonates in the pre salt of santos basin genesis and tectonic implications abstract since the first discoveries of oil accumulations in the carbonate reservoirs of the lower cretaceous barra velha formation of the santos basin their genesis has been the subject of intense discussion different depositional models have been proposed with some authors advocating a microbial origin while others an abiotic origin as travertines or travertine like rocks this search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative an evaporative model supported by our petrographical and geochemical data evaporitic sediment patterns in endorheic basins in a depositional framework of brine drawdown are dominated by ephemeral salt pan hydrology bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment water interface salt grows diagenetically within saline and mud layers beneath the dry pan surface finally laminated sediments are deposited we observe a similar cyclic pattern in the pre salt core we described beds of shrub like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites the model we propose attempts to contribute solving the puzzle of pre salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the santos campos basin were deposited as the initial transitional stage of a closed sub sea level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts carnallite sylvite tachyhydrite basinwide evaporites are singular events in the geological record they form when a giant sub sea level basin becomes a sump of continental marine and hydrothermal fluids producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones the novelty in this paper is that we apply to pre salt carbonate deposition the sedimentology hydrology and geochemistry of ancient evaporitic basins with this concept in mind we propose a geochemical model for the unusual pre salt carbonate precipitation of the santos basin regarding the walvis rio grande volcanic high as a topographic barrier during the aptian we suggest that the main sources of calcium to the barra velha paleolake were hydrothermal brines mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier hyperextension of the lithosphere with a shallow moho increased geothermal gradients to levels favorable for the generation of hydrothermal cacl2 brines a process facilitated by the already elevated ca ion and low sulfate concentration of the cretaceous ocean these cacl2 rich fluids would pass through the volcanic barrier into a sub sea level paleolake in the rift basin the high salinity paleolake would become saturated in calcium bicarbonate carbonate as internally drained nahco3 bearing alkaline waters formed by physical chemical weathering of penecontemporaneous basaltic rocks mix with the cacl2 bearing hot brines percolating through the barrier producing a hybrid brine 2nahco3 cacl2 ca hco3 2 2nacl such a depositional system integrating the geochemical depositional controls of sag phase carbonates with those of the south atlantic evaporites has important implications for unravelling the genesis of these exotic deposits

frequent sediment density flows during 2006 to 2015 triggered by competing seismic and weather events observations from subsea cable breaks off southern taiwan at least 17 subsea telecommunications cables cross the gaoping canyon and manila trench system in the strait of luzon between taiwan and the philippines there cable breaks record rapid 5 16 ms 1 long run out 300 km sediment density flows triggered by earthquakes and typhoons four major cable breaking events have occurred in the last decade in 2006 the pingtung ml 7 0 earthquakes formed up to 3 individual flows some of which ran out for up to 460 km in 2009 typhoon morakot generated two sediment flows the first was triggered by hyperpycnal river discharge whereas a second flow formed 3 days later possibly due to failure of recently deposited flood sediment in upper gaoping canyon a flow in 2010 formed during a swarm of ml 3 5 earthquakes that followed the ml 6 4 jiashian earthquake finally typhoon soudelor of 2015 caused the gaoping river to form a hyperpycnal plume that failed to break cables at 2600 m depth but broke at least 6 cables in deeper water the 2006 pingtung earthquakes produced the largest sediment flow s that produced 22 cable breaks the other events were less destructive with each causing 6 9 breaks sources of the sediment flows varied with the triggering mechanism those associated with hyperpycnal discharges probably began near the mouth of the gaoping river in contrast earthquake triggered landslides and subsequent sediment density flows formed anywhere on the adjacent continental margin flow speeds generally declined with increased run out lower seabed slope and increased channel width most variability 7 20 ms 1 was observed over the 0 4 1 0 slopes of gaoping canyon but speeds were mainly restricted to 5 8 ms 1 in the 0 3 sloping manila trench the 2006 2015 period of frequent cable damaging flows is the first for the past two decades or longer in the absence of evidence for increased seismic or anthropogenic activity this disruptive period may reflect increased typhoon intensity and fluvial sediment discharge such typhoon related flows may complicate interpretation of palaeo seismic records derived from turbidites in such settings

how coriolis forces can limit the spatial extent of sediment deposition of a large scale turbidity current abstract the rotation rate of the earth controls the deposition scale of turbidity currents when their flow transit time is comparable to one day using laboratory experiments it is shown that the maximum length scale l of deposition is set by the scale where the rossby number of the current is equal to one the dimensionless rossby number is defined as ro u fl where u is a depth averaged velocity of the turbidity current and f is the coriolis parameter in these new laboratory experiments the coriolis parameter is varied by changing the platform rotation rate and there is good agreement between experimental observations and the prediction that the extent of the deposition scales as l 4 1 4 g v f 1 4 where g is the reduced gravity and v the initial volume of the sediment laden material this scaling implies that the depositional patterns will vary with latitude and that at higher latitudes one would expect smaller thicker deposits using this scaling implies that the 200 400 km spatial extent of the turbidite arising during the 1929 grand banks earthquake had ro 1 and so coriolis forces controlled the length scale of the turbidite deposition

sediment waves on the south china sea slope off southwestern taiwan implications for the intrusion of the northern pacific deep water into the south china sea abstract using an integrated multi beam bathymetry high resolution seismic profile piston core and ams 14 c dating data set the current study identified two sediment wave fields fields 1 and 2 on the south china sea slope off southwestern taiwan field 1 is located in the lower slope and sediment waves within it are overall oriented perpendicular to the direction of down slope gravity flows and canyon axis geometries morphology and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up slope migration field 2 in contrast is located more basinward on the continental rise instead of having asymmetrical morphology and discontinuous reflections as observed in field 1 the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another suggesting that vertical aggradation is more active and predominant than up slope migration three sediment wave evolution stages stage 1 through stage 3 are identified in both field 1 and field 2 during stage 1 the sediment waves are built upon a regional unconformity that separates the underlying mass transport complexes from the overlying sediment waves in both of these two fields there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3 even though up slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages the integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down slope gravity flows the net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and or turbidity currents the interactions of these currents with bottom contour currents induced by the intrusion of the northern pacific deep water into the south china sea and preexisting wavy topography in the lower slope result in the up slope migrating sediment waves in field 1 and the contourites as observed from cores ts01 and ts02 further basinward on the continental rise turbidity currents are waned and diluted whereas along slope bottom contour currents are vigorous and most likely dominate over the diluted turbidity currents resulting in the vertically aggraded sediment waves in field 2 the results from this study also provide the further evidence for the intrusion of the northern pacific deep water into the south china sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in south china sea at least since quaternary

seismic stratigraphy and tectonic structure from a composite multi channel seismic profile across the entire dangerous grounds south china sea abstract we interpret a more than 500 km long composite multi channel seismic reflection profile across the entire dangerous grounds the south china sea five tectonostratigraphic units are determined together with seven sequence boundaries detailed analysis of extensional features based on measurements of fault heaves revealed two major phases of extension separated by a distinct unconformity which likely correspond with the beginning of sea floor spreading in the south china sea early extension occurred during continental rifting late cretaceous early oligocene and resulted in formation of half grabens and rotated blocks controlled by a deeply rooted detachment system extension continued in our study area during the drifting phase of the east subbasin of the south china sea late oligocene early miocene until the subsequent opening of the southwest subbasin at about 25 ma but its intensity decreased markedly thereafter deeply rooted detachment systems evolved possibly during this second phase of extension at the continent ocean transition area from a coincidence of the lowermost reflections and moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust mantle boundary it is suggested that the dangerous grounds reflect a magma poor rift system at the initial stage of mantle unroofing we suggest that a widespread carbonate platform developed across the dangerous grounds concurrent with the period of seafloor spreading in the southwest subbasin of the south china sea fault related stretching factors f along the line show discrepancies with the whole crustal stretching factors c the continental crust of the dangerous grounds may have experienced nonuniform thinning

origin of salts and alkali carbonates in the udachnaya east kimberlite insights from petrography of kimberlite phases and their carbonate and evaporite xenoliths abstract the udachnaya east kimberlite is characterized by the presence of chlorides sulfates and alkali carbonates this highly atypical mineralogy underpinned a model for an anhydrous alkali rich primary kimberlite melt despite the absence of petrographic studies providing textural context to the exotic minerals the present work documents the petrography of the udachnaya east kimberlite in order to address this problem the pipe comprises two varieties of fort a la corne type pyroclastic kimberlite olivine rich and magmaclast rich and coherent kimberlite these kimberlites entrain xenoliths of limestones altered shales and siltstones halite dominated rocks dolomites and coarse calcite rocks the distinct varieties of the udachnaya east kimberlite carry different populations of crustal xenoliths which partially control the mineralogy of the host kimberlite in magmaclast rich pyroclastic kimberlite where halite is absent from the crustal xenoliths it is not observed in the interclast matrix or within the magmaclasts halite occurs in the interclast matrix of olivine rich pyroclastic kimberlite where halite xenoliths are common large 30 cm halite xenoliths are uniquely restricted to the coherent kimberlite and show a strong reaction with it the halite xenoliths are sourced from depths of 1500 to 630 m where carbonate beds host multiple karst cavities filled with halite and gypsum and occasional sedimentary evaporites the style of secondary mineralization at udachnaya depends on whether the kimberlite is coherent or pyroclastic shortite pirssonite and other alkali carbonates replacing calcite and possibly serpentine are abundant only in porous pyroclastic kimberlites of both types and in their shale siltstone xenoliths the lower porosity of the coherent kimberlite prevented the interaction of kimberlite with na brines serpentinization localized around halite xenoliths started at temperatures above 500 c as indicated by its association with high temperature iowaite the model of the dry na and cl rich primary kimberlite melt is invalidated on the basis of 1 the restriction of exotic salt minerals to certain kimberlite types and xenoliths and 2 the absence of halite rich melt inclusions in olivine of coherent kimberlite

spreading direction in the central south china sea recent work1 2 indicates that the south china sea is an atlantictype marginal basin of late tertiary age magnetic anomalies in the eastern part of the sea are consistent with seafloor spreading directed approximately north south1 2 we present here a new morphostructural study based on coupled seabeam mapping and single channel seismic reflection profiling which reveals dominant normal fault scarps striking n50 e between 113 and 119 e longitude near the axis of this basin such a structural fabric implies a nw se spreading direction at least in the 150 200 km wide axial region of the south china sea and places new constraints on geodynamic models for the formation of this basin in the tectonic and palaeogeographic framework of south east asia and the south west pacific

gaylussite formation at mono lake california abstract the salinity of mono lake has steadily increased since 1941 from 50 to about 90 due to diversion of tributary streams this increase has resulted in the newly discovered precipitation of gaylussite na2ca co3 2 5h2o chemical modeling of the lake water using pitzer equations suggests that gaylussite has been forming year round since about 1970 when the salinity first exceeded 80 and that it was earlier forming intermittently at lower salinities in the winter shortly after diversion began breaking down incongruently to aragonite during summers lake water appears to remain at a constant 9 fold supersaturation with aragonite at all salinities perhaps buffered by monohydrocalcite which appears to be just at saturation for all salinities other saline lakes also appear to be buffered by monohydrocalcite

geochemical characteristics and origin of sodium carbonates in a closed alkaline basin the lower permian fengcheng formation in the mahu sag northwestern junggar basin china abstract the lower permian fengcheng formation in the mahu sag of northwestern china is well known for its alkaline lacustrine deposits however previous research is scarce due to this formation s notable burial depth up to 5000 m and limited oil and gas exploration using xrd xrf micro xrf and c o isotope analyses we determined 1 the stratigraphic trends in mineral compositions 2 the cyclic depositional processes and 3 paleoclimate variations the petrologic and geochemical data elucidate depositional and diagenetic processes in the mixed siliciclastic volcaniclastic and chemical strata of the fengcheng formation sodium carbonate minerals in the studied core are dominated by wegscheiderite na2co3 3nahco3 and trona na2co3 nahco3 2h2o nahcolite nahco3 and a mg na carbonate mineral northupite na2co3 mgco3 nacl also occur although their abundance is obviously lower than that of wegscheiderite and trona these substances grew as bottom growth crystals from lakes overlying tuffs the volumetrically important ca na carbonate mineral shortite na2co3 2caco3 and some northupite precipitated diagenetically from shallow groundwater brines in unlithified dark colored siliciclastic sediment and volcanic ash shortite mainly precipitated from the ca na co3 rich shallow groundwater whereas northupite precipitated from the mg na co3 cl rich shallow groundwater sodium carbonates and tuffs fine grained tuffaceous rocks and siliciclastic rocks formed repetitive centimeter and meter scale cycles that are shown as dark colored lithological layers during lake expansion and contraction stages or during volcanic eruptions the brine composition which controlled the precipitation of sodium carbonates was influenced strongly by volcanism and volcanism related hydrothermal activity changes in climate and volcanic eruptions controlled the deposition of alternating beds of sodium carbonates and dark colored lithologies changes in climate caused the expansion and contraction stages in the lakes during relatively humid climate stages diluted water entered the basin and resupplied the lake water with calcium and magnesium during flooding events sandy to silty beds were deposited by turbidity currents during this stage during arid climate stages bedded sodium carbonates grew in the lakes and their co occurrence suggests high atmospheric pco2 and high temperatures the ashes that are provided by volcanic eruptions during arid climate stages can also stop the precipitation of sodium carbonates

an alkaline lake in the late paleozoic ice age lpia a review and new insights into paleoenvironment and petroleum geology abstract the ca 300 ma fengcheng formation in the junggar basin of nw china is reported to develop a rare alkaline lake soda lake in this study based on an expanded literature survey and new insights from geological and geochemical data the development of this ancient alkaline lake resulted from a convergence of favorable paleotectonic paleogeographic and paleoclimatic conditions a detailed characterization of the alkaline lake includes evidence for a ph of 9 0 indicated by 15n data average 18 4 the presence of alkali minerals such as trona and shortite globular bacteria like fossils low contents of clay minerals organic and inorganic geochemistry indicating high salinity strongly reducing conditions and hydrothermal activity and a complete sedimentary evolution sequence of an archetypal alkaline lake the studied lake deposit is presented as the first such deposit worldwide to generate a large oil reserve thus allowing scientific insights to be gained into the controls on hydrocarbon generation in alkaline lakes the alkaline lacustrine source rocks have high contents of organic matter favorable organic matter type abundant oil and poor natural gas multiple phases of hydrocarbon generation and high hydrocarbon volumes the genesis and development of this alkaline lake were controlled jointly by evaporation and volcanic hydrothermal activity however some aspects of this type of lake remain unclear and future work should explore the integrated evolution of biological climatic petroleum geology systems with respect to alkaline lakes and their associated deposits and their coupling research with worldwide event then namely glacial episode iii of the late paleozoic gondwanan glaciation our data suggest that the fengcheng formation is a well preserved archive of late paleozoic ice age lpia geology and geochemistry around 300 ma

determination of the solubility products of sodium carbonate minerals and an application to trona deposition in lake magadi kenya the ion interaction model of pitzer 1973 is very effective in deriving stability relationships at high concentrations for the system na cl hco3 co3 oh h2o the solubility products of the main sodium carbonates have been calculated from solubility data between 5 and 50 c the stability diagram in log pco2 temperature coordinates and the invariant points deduced from the newly determined data are in good agreement with the most recent measurements these results are used to calculate the activities of the major dissolved species in lake magadi brines kenya the thermodynamic treatment confirms the main conclusions reached earlier by eugster 1970 1980 mainly from field observations trona precipitation occurs at equilibrium while natron is likely to form when the temperature decreases below 25 c after the salt deposition the co2 supply from the atmosphere is too slow to allow equilibrium between the atmosphere and the brines in the next stages of evaporative concentration thermonatrite and halite precipitate the deposition of the latter salts along with the observed hco 3 depletion suggest that fractional crystallization is likely to control trona deposition

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

zeolitization of volcaniclastic sediments the role of temperature and ph abstract this paper addresses the question of why some volcanic ashes are zeolitized and others are not the ph of interstitial water is the most important control on zeolitization surface water and groundwater in volcanic areas is acidic but zeolitization requires neutral to alkaline conditions therefore acidic interstitial water must be removed or neutralized before zeolitization can take place conditions favoring zeolitization are deposition or redeposition under water or reheating of ash after acidic interstitial water has been removed or influx of slightly alkaline bicarbonate bearing groundwater from overlying or neighboring limestones

studies of quaternary saline lakes iii mineral chemical and isotopic evidence of salt solution and crystallization processes in owens lake california 1969 1971 abstract as a consequence of the 1969 1970 flooding of normally dry owens lake a 2 4 m deep lake formed and 20 of the 2 m thick salt bed dissolved in it its desiccation began august 1969 and salts started crystallizing september 1970 ending august 1971 mineralogic brine composition and stable isotope data plus field observations showed that while the evolving brine composition established the general crystallization timetable and range of primary and secondary mineral assemblages it was the daily monthly and seasonal temperature changes that controlled the details of timing and mineralogy during this depositional process deuterium analyses of lake brine interstitial brine and hydrated saline phases helped confirm the sequence of mineral crystallizations and transformations and they documented the sources and temperatures of waters involved in the reactions salts first crystallized as floating rafts on the lake surface natron and mirabilite salts whose solubilities decrease greatly with lowering temperatures crystallized late at night in winter when surface water temperatures reached their minima trona nahcolite burkeite and halite salts with solubilities less sensitive to temperature crystallized during the afternoon in summer when surface salinities reached their maxima however different temperatures were generally associated with crystallization at the surface and accumulation on the lake floor because short term temperature changes were transmitted to surface and bottom waters at different rates consequently even when solubilities were exceeded at the surface salts were preserved or not as a function of bottom water temperatures halite a nearly temperature insensitive salt was always preserved monitoring the lake brine chemistry and mineralogy of the accumulating salts shows 1 an estimated 0 9 10 6 tons of co 2 was released to the atmosphere or consumed by the lake s biomass prior to most salt crystallization 2 after deposition some salts reacted in situ to form other minerals in less than one month and all salts except halite decomposed or recrystallized at least once in response to seasons 3 warming in early 1971 caused solution of all the mirabilite and some of the natron deposited a few months earlier a deepening of the lake though the lake surface lowered and an increase in dissolved solids 4 phase and solubility index data suggest that at the close of desiccation na 2 co 3 7h 2 o never reported as a mineral could have been the next phase to crystallize

climatically induced lake level changes at lake van turkey during the pleistocene holocene transition sediment core k10 from lake van eastern turkey provides a continuous varve record back to 14 570 calendar years b p before present 1950 the longest unbroken and non floating lake varve sequence yet described the underlying sediment is unvarved and hard changes in the aragonite calcite ratio the presence of protodolomite and magnesite in certain profile sections the annual record of the sedimentation rate the water content of the sediment the concentrations of organic carbon and opal and the texture of the sediments from this core provide a record of the lake level history the new chronology enabled us to redate the old pollen profile van zeist and woldring 1978a b and to establish an accurate timescale for the reconstructed lake level change carbon 14 dates show that the highest lake terrace corresponds to high lake level at around 19 000 years b p during the last glacial 70 m above its present level before 15 000 years b p the lake must have been completely dry marking a reduction of lake level by 500 m in maximum 4000 years beginning at 14 600 years b p and ending at 12 040 years b p the lake level recovered by 250 m to fall again during the next 1400 years by 10 600 years b p the lake began to rise and reached following another regression between 9000 and 8100 years b p the holocene highstand by about 7500 years b p dropping to today s level at about 3000 years b p

a genetic model for na carbonate mineral precipitation in the miocene beypazari trona deposit ankara province turkey abstract the miocene beypazari trona deposit in central anatolia turkey with estimated reserves of 250 million of tons of soda ash sodium carbonate is the second largest na carbonate deposit in the world petrographic observations of sodium carbonate evaporites from basin center cores show that the evaporites and associated minerals underwent significant early diagenetic mineral transformations in the beypazari basin trona pirssonite and nahcolite are the major evaporite minerals with subordinate bradleyite shortite and thenardite pirssonite occurs in organic rich muds oil shales pirssonite is not a primary mineral but rather appears to have formed diagenetically from displacive gaylussite trona precipitated subaqueously in lake waters and as interstitial phase in playa muds nahcolite is a later diagenetic mineral replacing both trona and pirssonite beds evaporite minerals are closely related to pyrite in oil shales and are also associated with diagenetic silicates mg rich smectites zeolites k feldspar searlesite and idiomorphic quartz formed by reaction of alkaline brines with pyroclastic deposits the proposed sedimentary model involves an alkaline lake in which water depth fluctuated from deep perennial meromictic stratified stages to shallow ephemeral playa lake stages lake margin clastic deposits are absent in the center of the basin wherein na carbonate minerals were formed suggesting the prevalence of a groundwater regime during drawdown events

high resolution stratigraphy of an underfilled lake basin wilkins peak member eocene green river formation wyoming u s a abstract lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic tectonic or geomorphic processes this unique behavior of lacustrine basins can provide a high resolution record of geologic processes within the continental setting far from the globally averaged record of marine strata the wilkins peak member of the eocene green river formation in wyoming usa is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions these lacustrine cycles consist of up to four successive facies associations littoral profundal sublittoral palustrine and salt pan because they comprise disparate facies that may never have been simultaneously deposited in the basin wilkins peak member cycles are non waltherian successions that do not readily equate to any established sequence stratigraphic unit the completeness of the stratigraphic record in the wilkins peak member varies continuously across the basin at least 126 cycles are present in the erda white mountain 1 core near the basin depocenter whereas only about one third as many are recognizable 53 km north nearer to the basin margin cycle boundaries terminate northward by gradual amalgamation into palustrine facies reflecting the interplay between varying lake levels and a south dipping deposition gradient evidence for complete desiccation and hiatuses is commonplace even near the basin center a truly complete record may not be present anywhere based on recent 40ar 39ar geochronology of tuffs the average cycle duration in the white mountain 1 core is approximately 10 000 years however the true average duration is shorter due to the presence of lacunae and the time required for deposition of the thinnest cycles may have been less than 1000 years no external driving mechanism is presently known for eocene cycles of such short duration raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape evaporite deposition corresponded closely in time and space to maximum differential subsidence suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control differential accommodation was most pronounced during deposition of the lower wilkins peak member and became progressively less so after that time based on structural and stratigraphic observations a period of increased tectonic uplift commenced concurrent with deposition of the lower wilkins peak member

spatial and temporal variability in vegetation and coal facies as reflected by organic petrological and geochemical data in the middle miocene cayirhan coal field turkey abstract coal channel samples of middle miocene age are collected from the first tv and second tb seams from underground mines and from exploration boreholes within the cayirhan coal field in the beypazari basin turkey they are investigated in order to detect spatial and temporal changes in maceral and molecular composition of coal and to relate them to changes in vegetation and depositional environment the mean random reflectance values of ulminite 0 40 rr indicate a lignite to subbituminous c coal in rank maceral composition and biomarker ratios of the samples from both seams at the cayirhan deposit argue for coal formation in a limno telmatic environment under dysoxic to anoxic conditions alkaline surface waters of changing ph values and a high and unstable water level at the palaeomire are evidenced by moderate to high gelification index gi and ground water influence gwi values as well as high sulphur contents variations in tissue preservation index tpi and vegetation index vi values point to minor variations in the palaeovegetation during peat formation herbaceous plants dominated in both tb and tv palaeomires low tpi and vi values whereas in the surroundings arboreal vegetation were predominant the decreasing trends in pristane phytane ratios and carbon preference index cpi values towards the ne are suggested to reflect oxygen deficient conditions during peat formation due to a higher ground water level in this part of the basin the occurrence of c 29 diasterenes in low abundances provides evidence for periods of lower ph in the mire the borehole samples from the ne show slightly enhanced contributions of n alkanes from algal and microbial sources the terpenoid hydrocarbons present in the lignite argue for a major contribution of angiosperms to peat formation and slightly enhanced proportions of gymnosperms in the tv palaeomire in the ne based on the high concentrations of lupane type triterpenoids in the coal seams from the underground mine a higher density of betulaceae in the arboreal vegetation in the sw is indicated

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

structureless coarse tail graded bouma ta formed by internal hydraulic jump of the turbidity current abstract turbidity currents are commonly super critical if they travel on slopes steeper than 0 001 hence upon deceleration such flows must pass a hydraulic jump it is therefore surprising that sedimentary features related to the hydraulic jump are so poorly known and not clearly related to the bouma sequence to study this missing link a stability diagram of bed forms generated by decelerating initially super critical turbidity currents is presented here to sketch the position of hydraulic jump deposits relative to super and sub critical bed forms observed in turbidites the axes of the diagram are formed by the densiometric froude number the grain size and the particle fall out rate experimental evidence shows that the hydraulic jump is hardly manifested in the deposit if turbidity currents pass the jump as a low concentration single phase suspension flow only those turbidity currents that develop two phase suspension flow with a super critical flowing traction carpet cause vigorous erosion and liquefaction of the substrate at the jump locality to produce rip up of the substrate rip up clasts and flame structures which are trapped in structureless coarse tail graded deposits characteristic of the classic bouma ta unit the location of the jump is sensitive to density velocity variations of the traction carpet and can shift rapidly up and down the slope this would explain why structureless coarse tail graded deposits such as bouma ta develop sheet like

miocene alluvial fan alkaline playa lignite trona bearing deposits from an inverted basin in anatolia sedimentology and tectonic controls on deposition abstract during the middle through late miocene over 1200 m of clastics carbonates and evaporites accumulated in the depositional basin situated in northwestern ankara province of central anatolia detailed stratigraphic and sedimentological analyses of the exposed sequence exhibits three major facies 1 lower alluvial 2 upper alluvial and 3 lacustrine the lower alluvial facies consists primarily of volcaniclastic debris flow boulder and channel conglomerate sandstone siltstone mudstone and two lignite horizons these rock assemblages were deposited in alluvial fans and a braided river system the lower and upper lignite seams were laid down in swamps and lake margin mud flat environments associated with this river system the upper alluvial facies is characterized by red conglomerate fine grained and cross bedded sandstone mudstone and calcareous mudstone units the dominant depositional environments are interpreted to be alluvial fan braided river and mud flat the base of the lacustrine deposits interfingers with the lower alluvial facies and consists of dolomitic claystone bituminous shale trona and intraformational conglomerate lithofacies the sedimentary features of the dolomitic claystone and trona lithofacies suggest deposition in a playa lake type environment the bituminous shale is a transgressive deposit laid down in a shallow but expanded lake environment sedimentological and structural characteristics of the intraformational conglomerate lithofacies indicate synsedimentary tectonism intrabasinal erosion redeposition and sediment selection toward the deposition centre of the lake environment the green claystone lithofacies has three units a the claystone bedded chert analcitized tuff alternation and siliceous carbonate lithofacies of the lacustrine system deposited in a perennial lake environment b montmorillonitic olive green claystone containing mud cracks ripple mark and desiccation cracks filled by gypsum deposited in a saline or brackish ephemeral lake c gypsiferous claystone in a saline mud flat and thick gypsum beds including thin glauberite and tenardite lenses laid down in an ephemeral saline lake the green claystone and evaporite lithofacies interfinger with fine grained rocks of the upper alluvial facies and also with miocene pyroclastic volcanic rocks during the late middle miocene two northeast trending depressions isolated by basement rocks were filled rapidly by sediments of the lower alluvial facies extensional normal faults were developed at the northern and southern basin margins and controlled deposition of the facies during the miocene the basin sediments were folded and faulted as a result of neotectonic compressional stresses associated with movements on the north anatolian fault

the role of excess co2 in the formation of trona deposits abstract the prevailing theory for the formation of trona na3 co3 hco3 2 h2o relies on evaporative concentration of na hco 3 water produced by silicate hydrolysis of volcanic rock or volcaniclastic sediments given the abundance of closed drainage basins dominated by volcanics it is puzzling that there are so few trona deposits and present day lakes that would yield dominantly na co3 minerals upon evaporation groundwater in the san bernardino basin southeastern arizona usa and northeastern sonora mexico would yield mainly na co3 minerals upon evaporation but waters in the surrounding basins would not analysis of the chemical evolution of this groundwater shows that the critical difference from the surrounding basins is not lithology but the injection of magmatic co2 many major deposits of trona and na co3 type lakes appear to have had excess co2 input either from magmatic sources or from the decay of organic matter it is proposed that along with the presence of volcanics addition of excess co2 is an important pre condition for the formation of trona deposits

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

zeolites in closed hydrologic systems zeolites were first described from vugs and fissures in basaltic flows in the mid eighteenth century cronstedt 1756 and they are now known to be widespread in a variety of geological environments more zeolite species occur in vesicles and fractures of basaltic rocks than in any other geologic setting however zeolites from sedimentary rocks in particular represent the most important occurrences both in terms of aerial extent of deposits and in terms of the abundance of certain zeolite species the introduction of new analytical techniques in the latter part of the 20th century provided a significant stimulus to zeolite research these techniques allowed identification of large quantities of zeolites in widespread deposits of sedimentary rocks and in particular in sediments from saline alkaline lakes of the western united states sheppard and gude 1968 1969a 1973 based on geological and hydrologic environments zeolite occurrences in sedimentary rocks can be classified in the following framework a saline alkaline lakes b alkaline soils and land surfaces c deep sea sediments d open hydrologic systems e as products of hydrothermal alteration and f burial diagenetic or low metamorphic environments from a broader perspective zeolite deposits can be classified into two main groups namely closed hydrologic systems and open hydrologic systems the work of sheppard and gude 1968 1969a 1973 and later surdam 1977 discussing not only the mineralogy of the deposits but also the geology hydrology and chemistry of the depositional basins represent milestones in the study of closed hydrologic system formation of zeolites surdam 1977 considered closed hydrologic basins in two different tectonic settings a block faulted regions in arid and semiarid regions and b trough valleys associated with rifting examples of the first setting are the closed lakes of the basin and range province of the western united states whereas the

influence of seabed slope and coriolis effects on the development of sandbanks near headlands the mechanism responsible for asymmetric sediment accretion around an idealized symmetrical headland is clearly the coriolis force what is not evident however is how the coriolis force manifests itself in the headland dynamics to promote asymmetric depositional patterns in this study patterns of scour and accretion topographic and advective vorticity bed load and suspended load transport pathways around a theoretical coastal headland are examined previous studies have emphasized the contribution of the coriolis force to a secondary flow mechanism tidal stirring this investigation provides evidence to suggest that secondary flow dynamics are not significant in sandbank generation associated with headlands it further demonstrates the direct contribution of the coriolis force in reinforcing inertial terms near headland scour and asymmetric accretion examination of the vorticity balance throughout the simulations reveals that high levels of slope induced topographic vorticity act in direct opposition to advective vorticity near a headland tip it is also demonstrated that the coriolis force manifests itself in the production of vorticity this has been used in the investigation as a means of detecting the sensitivity of the flow to the coriolis force in regimes where advective and topographic vorticity production is very similar flat bed cases it is found that bed evolution is more sensitive to the coriolis force in regimes where topographic vorticity clearly dominates the vorticity balance in seabed slope case the influence of the coriolis force becomes less important

asymmetrical inversely graded upstream migrating cyclic steps in marine settings late miocene early pliocene fish creek vallecito basin southern california abstract cyclic steps are ubiquitous in modern sedimentary environments yet their recognition remains sparse in the rock record here we interpret three sets of undulating backsets 1 to 3 recognized in the late miocene early pliocene latrania formation in the anza borrego desert the fish creek vallecito basin southern california usa as the first cm to dm scale outcrop record of cyclic steps based on asymmetrical cross sections upstream migration and inversely graded laminae upstream migration and asymmetrical cross sections of backsets and concomitant backset laminae are attributed to supercritical to subcritical flow transitions through weak hydraulic jumps which are composed of i thin tens of centimetres and slower reported as flow velocities of 0 45 to 1 45 m s 1 with mean value of 0 89 m s 1 subcritical represented by internal froude numbers fr of 0 67 to 0 99 with mean value of fr 0 84 turbidity currents on the stoss sides and ii thin tens of centimetres and faster reported as of 0 99 to 4 03 m s 1 with mean value of 2 24 m s 1 supercritical represented by fr of 1 84 to 3 07 with mean value of fr 2 42 turbidity flows on the lee sides the inversely graded laminae in the troughs of backsets are 2 to 5 cm thick and consist of two discrete divisions i 1 to 2 cm thick lower finer grained divisions made up of parallel laminated siltstones overlain by very fine to fine grained sandstones and ii 2 to 3 cm thick upper divisions composed of medium to coarse grained sandstones with sporadic occurrence of subrounded pebbles these inversely graded laminae are related to stratified collisional and or frictional traction carpets under conditions of high fall out rates due to the poor preservation potential of cyclic steps the rock record of cyclic steps is generally considered to be rare the present outcrop based study presents a detailed analysis of sedimentary facies growth patterns and flow dynamics of marine cyclic steps thereby contributing to better understand architectural styles and lithological properties of cyclic steps in the geological record

large scale scours formed by supercritical turbidity currents along the full length of a submarine canyon northeast south china sea abstract high resolution multibeam bathymetric and seismic data enables a detailed morphological investigation of a submarine canyon west penghu canyon on the northeastern south china sea margin where twenty three 23 scours are observed along the canyon thalweg these scours form narrow topographic depressions in plan view and show asymmetrical morphologies in cross section the identified scours can be further divided into two groups types a and b based on their sizes and relative locations they are separated by a slope break at a water depth of 2850 m type a scours s1 s18 occur upslope from the slope break whereas type b scours s19 s23 lie downslope from this same break the scours are interpreted as net erosional cyclic steps associated with turbidity currents flowing through the west penghu canyon the currents that form type a scours reflect higher v q and el compared to the currents forming type b scours a change in slope gradient and loss of lateral confinement are proposed to control the change from type a to type b scours furthermore coriolis force influences the flow direction of turbidity currents leading to the preferential development and larger incision depths of scours towards the southwestern flank of the west penghu canyon our results contribute to a better understanding on the origin of scours in submarine canyons across the world

the green river salt mystery what was the source of the hyperalkaline lake waters abstract bicarbonate rich source waters were needed to form the largest sodium carbonate evaporite deposits in the geologic record the early and middle eocene green river trona nahco 3 na 2 co 3 2h 2 o in the bridger basin wyoming and nahcolite nahco 3 in the piceance creek basin colorado large modern and pleistocene trona deposits are associated with magmatic activity and na hco 3 rich hydrothermal inflow waters either within the depositional basin lake magadi kenya or at great distances searles lake california no evidence exists for magmatic sources of co 2 near the green river formation several regional volcanic centers were active 300 km or more to the north but drainage reconstructions show that waters from these areas did not discharge into the green river formation lakes during evaporite deposition alternatively na hco 3 rich waters could have drained northwestward from the colorado mineral belt to the bridger basin via the proposed aspen river a river originating in the colorado mineral belt sawatch uplift could also have provided source waters to the piceance creek basin field evidence however has not yet documented these flow paths and specific eocene volcanic centers and hydrothermal source areas have yet to be identified other explanations for the elevated alkalinities needed to form thick sodium carbonate evaporites include accelerated silicate mineral weathering rates during a period of high atmospheric p co 2 and eocene warmth amplified chemical weathering may explain the cluster of sodium carbonate evaporites in the usa and china that are eocene in age another possible source of alkalinity to the green river lakes is fault controlled upward migration of a deep sedimentary source of co 2 the large amount of organic matter preserved in the evaporitic wilkins peak and parachute creek members raises the possibility that decay of organic matter in green river lakes could have added sufficient co 2 and alkalinity to produce waters capable of precipitating trona and nahcolite although such degradation of organic matter has not created hyperalkalinity in modern lakes

occurrence of zeolites in sedimentary rocks an overview zeolites have been known since the mid 1750s but prior to the early 1950s most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks particularly basaltic lava flows indeed most of the large attractive zeolite specimens in museum collections were obtained from lavas in recent years zeolites have been recognized as important rock forming constituents in low grade metamorphic rocks and in a variety of sedimentary rocks most zeolites in sedimentary rocks are finely crystalline that is they occur as microscopic or submicroscopic crystals and they are therefore of little appeal to mineral collectors however deposits of this type are voluminous and have great geologic significance and economic potential zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks and they form in sedimentary rocks of diverse lithology age and depositional environment about twenty species of zeolites have been reported from sedimentary rocks but only eight zeolites commonly make up the major part of zeolitic rocks these are analcime chabazite clinoptilolite erionite heulandite laumontite mordenite and phillipsite this chapter will consider chiefly the zeolites in sedimentary rocks with emphasis on volcaniclastic deposits which contain the largest concentrations of zeolites the occurrence of zeolites in lava flows is mentioned only briefly journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the mineralogy and geology of natural zeolites was first published in 1977 as volume 4 of the mineralogical society of america s reviews in mineralogy the present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals a wide variety of zeolites has been identified in sedimentary deposits with the most common being clinoptilolite analcime heulandite laumontite and phillipsite less abundant zeolites include chabazite erionite mordenite natrolite