a paleohydrologic model for early proterozoic dolomitization and silicification abstract primary depositional features observed in the intertidal and supratidal facies of the 2 25 ga malmani dolomite including tepees and finely laminated domal stromatolites have equivalents in holocene salinas on the southern coast of australia the hydrological regime so important in the formation of these recent primary features is still active and so it has been possible to interpret the palaeohydrological significance of analogous features in the malmani dolomite the palaeoclimate of the malmani dolomite shoreline was probably humid to semi arid and the supratidal zone is interpreted as a zone of groundwater resurgence tepees indurated fenestral limestone sheets algal mats small domal stromatolites and intraclast breccias were forming on the supratidal flats in areas of periodically outcropping groundwaters in the shallow subsurface the mixing zone between these resurging continental groundwaters and the marine derived phreatic waters was probably a zone of secondary coarse grained dolomite precipitation and or chert nodule formation this groundwater controlled mixing zone model explains why such fabrics are restricted to porous and immediately adjacent units which at the time of diagenesis were below the regional water table in the upper supratidal zones above the regional water table no secondary dolomites or chert nodules could form nor could diagenetic fabrics form in the subtidal zone for there the stromatolitic mounds contained no palaeoaquifer

preliminary results from tree increment and playa sediment cores from the paroo north western new south wales australia abstract the paroo ecosystem of far north west new south wales is sensitive to enso the paroo is also currently subject to contention for water extraction aquat conserva mar freshwater ecosystems 8 1998 361 in which information about long term ecosystem dynamics may be useful the environmental history of the paroo has not been described despite the existence of potential records in saline lakes dunes and callitris trees geomorphic and pollen analyses are used to determine the duration and nature of the pollen record in both lake and dune deposits a 2 m core recovered from one unnamed lake informally named palaeolake and the slow accumulation of sediment ca 175 years cm 1 shows the record may cover 38 k years discontinuities in the lake sediments are expected to be less than the larger southern salinas and the gaps will be partly recorded in the dune materials in tandem the tree rings from two stands of callitris glaucophylla have been tested to determine the high resolution dendroclimatological record available ring width patterns were matched between trees using skeleton plots the temporal variability of these patterns and the uniform response of the trees indicates that ring width geometry is dependant upon climate preliminary results for organic material mineral magnetic susceptibility and stratigraphy of the playa and dune facies promise a good record of pleistocene climatic change the results show that the lake records give long but temporally discontinuous information whereas tree rings provide short and relatively continuous information together they record climate changes over the last few thousand years and the impact of european management on the vegetation and rates of sediment deposition

sedimentological characteristics and facies of the evaporite bearing kirmir formation neogene beypazari basin central anatolia turkey a thick sedimentary sequence comprising fluvial lacustrine and volcano sedimentary rocks is present in the neogene beypazari basin central anatolia these units display considerable lateral facies variation and interfinger with alkaline volcanic rocks along the north eastern margin of the basin the uppermost miocene kirmir formation contains rlumerous evaporite horizons the evaporite sequence is up to 250 m thick and may be divided into four lithofacies in ascending stratigraphical order these are 1 gypsiferous claystotie facies 2 thenardite glauberite facies 3 laminar gypsum facies and 4 crystalline gypsum facies these facies interfinger with one another laterally along a section from the margins to central parts of the basin the lithological and sedimentological features of the kirmir formation indicate fluvial saline playa mudflat hypersaline ephemeral playa lake and very shallow subaqueous playa lake depositional environments which probably were influenced by alternating semi arid and evaporative conditions

local scour downstream of an apron measurements have been made of the rate of scour downstream of a rigid apron due to a jet of water issuing through a sluice opening experiments are carried out for various sand sizes sluice openings efflux velocities and lengths of apron mean velocity profiles in rigid models simulating the shape of the scour hole are studied in detail they are found to exhibit similarity scales of velocity and length that are adopted bring together all the data concerning the decay of maximum velocity in the scour hole with respect to streamwise distance a simple semi empirical theory based on a characteristic mean velocity in the scour hole is proposed to predict the time rate of scour predicted results are compared with experimental data

oxygen and carbon isotope trends and sedimentological evolution of a meromictic and saline lacustrine system the holocene medicine lake basin north american great plains usa abstract the succession of sedimentary subfacies in the medicine lake basin south dakota usa documents numerous changes in total dissolved solids tds lake level and the nature and stability of meromixis during the holocene the lake formed about 10 6 ka b p when coarse clastics and massive fine grained muds were deposited in a well mixed freshwater system subsequently the lake became saline and meromictic depositing a finely laminated unit composed of variegated and gray muds organic facies gypsum and aragonite laminae phototrophic bacterial plate communities bpc at the chemocline were significant producers of pelagic organic matter relatively high lake levels at least 10 m higher than today but saline and meromictic conditions predominated through the early to mid holocene at the end of this phase lake level dropped and stable meromixis was disrupted sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities bmc spread over the lake replacing bpcs as the predominant microbial producers when lake level rose again massive and laminated units were deposited under less saline conditions another pronounced episode of low lake level and high salinity followed the most recent sediments are sapropels massive or banded muds and littoral sands the 18o record from authigenic aragonite shows more positive values during the arid mid holocene interval with an abrupt increase at the end of this period followed by generally more negative values during the cooler and wetter late holocene the small range of 18o values 3 suggests that the lake was well buffered against high frequency climatic cycles probably as a result of groundwater modulation we postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system either bpcs or bmcs exerted major controls on the carbon cycle and consequently on the development of isotopic trends linking 13c and 18o values more negative 13c values during the early and mid holocene are interpreted as a consequence of the predominance of 12c enriched microbial carbon from bpcs examples of both covariant and inverse covariant isotope trends between 13c and 18o values occur in the medicine lake record inverse covariant trends characterize long periods of stable meromixis when bpcs predominate over bmcs we hypothesize that well developed water stratification caused by greater influx of relatively 18o depleted freshwater induces higher productivity at the chemocline and consequently more positive 13c values in the precipitated carbonates unstable phases of meromixis or holomictic conditions are characterized by a predominance of bmcs or other biogenic producers over bpcs and a number of other factors combine to promote covariant trends the medicine lake record is consistent with the paleoclimatic interpretation for the northern great plains of a more arid early and middle holocene followed by an abrupt transition to a relatively cooler and wetter late holocene the sedimentary succesion in medicine lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late holocene climate punctuated by arid episodes

influence of cohesion on river bed scour in the wake region of piers abstractstudies on scour in the wake region of piers in cohesive sediments containing gravel were not reported in literature to the best of our knowledge results from the laboratory experiments based study on the process of wake scour are presented here the sediment used for the experiment were of two types 1 fine gravel mixed with clay in proportions varying from 20 to 60 by weight and 2 fine gravel and fine sand in equal proportion by weight mixed with clay in proportions varying from 20 to 60 by weight the experiments revealed that the process of scour as well as depth shape and geometry of scour hole developed in such cohesive sediment were significantly different from that of cohesionless sediments proportion of clay fraction and unconfined compressive strength of cohesive sediment mixture were found to be the most significant variables controlling the depth of scour in the wake of piers on the basis of dimensional considerations relationships have been proposed for the estimation

depositional setting of the upper jurassic hith anhydrite of the arabian gulf an analog to holocene evaporites of the united arab emirates and lake macleod of western australia the upper jurassic hith anhydrite is a major hydrocarbon seal in the arabian gulf region outcrops core samples from the subsurface and the literature indicate that the hith formation is composed mainly of anhydrite in most locations where a section of the hith formation has been measured this unit contains less than 20 carbonate much of which is in the form of thin laminations this lack of carbonate locally thick layers of salt and the predominance of anhydrite favor a playa for the setting in which this sediment was accumulated in fact much of the hith has the sedimentary characteristics of the holocene lake macleod playa of western australia which is dominated by layers of gypsum and halite what little carbonate that occurs is found in layers at the base o the section locally the hith appears to have accumulated in a sabkha setting particularly toward central abu dhabi where it pinches out into shallow water and peritidal carbonate this sabkha setting is indicated by the interbedded relationship of the hith anhydrites with these carbonates and the local predominance of horizontally flattened nodules and enterolithic layers of anhydrite these latter features match some of the characteristic fabrics found in the holocene coastal sabkhas of the united arab emirates as with the local occurrences in the hith the holocene sabkhas are dominated by carbonates and are divisible into a series of lateral facies belts these are also expressed as equivalent vertical layers traced from seaward to landward or from the base of the vertical sequence upward these facies are characterized by 1 algal mat 2 a layer of a gypsum crystal mush 3 active anhydrite replacement of gypsum 4 anhydrite with no gypsum mush and 5 recycled eolianite and storm washover sediments

earthquake induced resedimentation in the badenian middle miocene gypsum of southern poland in the badenian middle miocene basin of the carpathian foreland of southern poland gypsum breccias occur associated with laminated gypsum deposits these breccias consist of large clasts of gypsum carbonates marls and clay chips of variable size embedded in a gypsarenitic matrix constituent gypsum grains and clasts commonly appear to be mechanically abraded and chemically corroded crystals or fragments of selenitic laminated and alabastrine gypsum gypsorudites are commonly accompanied by laminated gypsarenites and gypsolutites which show graded bedding a vertical sequence of graded gypsum beds showing bouma sequences may be recognized in borehole sections microfolding is common within the folded laminated gypsum and is closely associated with expressions of extensional strain both are accompanied by pervasive microfaulting suggesting a semi coherent downslope mass movement the stratiform geometry of the breccias together with the intensity of slumping relatively independent of the palaeoslope suggest earthquake shocks as the initial main cause gypsum deposits form a constant laterally extensive sequence of different lithofacies the occurrence of the same lithologies and shallowing up cycles over a wide area reflects thrusting of the carpathians over the carpathian foredeep local tectonism has also played a significant role the tectonic framework favoured activation of dip slip faults promoting shallow focus earthquakes these in turn resulted in the resedimentation of gypsum by slumps debris flows and turbidites a similar basinward resedimentation of clastic material by gravity flows initiated by fault induced earthquakes could be of great importance in the foreland geological setting and may explain some phenomena observed in other evaporite formations from different geological settings especially of rift type

structure chemistry and origins of gypsum crusts in southern tunisia and the central namib desert gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0 10m to 5 0 m thick containing more than 15 by weight gypsum caso4 2h2o and at least 5 0 by weight more gypsum than the underlying bedrock the deposits are often but not invariably consolidated owing to cementation by gypsum the crusts are found in many of the world s deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year using structural fabric and textural criteria three main types of crust may be distinguished 1 bedded crusts found either at or beneath the land surface which are made up of discrete horizontal strata up to 0 10 m thick each showing a gradation in gypsum crystal size from less than 50 m at the top to more than 0 50 mm at the base 2 subsurface crusts of which there are two forms one made up of large lenticular crystals up to 0 50 m in diameter the desert rose crusts and the other a mesocrystalline form with gypsum crystals up to about 1 0 mm in diameter and 3 surface crusts which are subdivided into columnar powdery and cobble forms all of which are made up of predominantly alabastrine gypsum crystallites less than 50 m in diameter in southern tunisia and the central namib desert bedded crusts are found around ephemeral lakes and lagoons they are characterized by size graded beds gypsum contents of 50 80 by weight and comparatively high concentrations of sodium potassium magnesium and iron they are interpreted as shallow water evaporites which accumulate when saline pools evaporate to dryness desert rose crusts or croutes de nappe generally contain 50 70 by weight gypsum and have higher sodium concentrations than the second subsurface form texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals they are interpreted as hydromorphic accretions which precipitate in host sediments at near surface water tables through the evaporation of groundwater the second form of subsurface crust the mesocrystalline often occurs in close association with the various surface forms unlike the hydromorphic crusts they are not restricted to low lying terrain they are characterized by gypsum contents reaching 90 by weight and have a close chemical and textural similarity to columnar surface crusts this mesocrystalline form represents an illuvial accumulation the surface forms excluding the bedded crusts are exhumed examples at various stages of solutional degradation subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface results in displacive gypsum accumulation in the soil zone in southern tunisia the gypsum is derived from sand and dust deflated from evaporitic basins in the central namib salts dissolved in fog water are the most likely source where other salts are present differential leaching may form two tiered crusts calcrete gypsum or gypsum halite if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble gypsum crust genesis is restricted to arid environments and if their susceptibility to post depositional alteration is acknowledged they can provide valuable palaeoclimatic indicators

impingement of water jets on nonuniform streambed the effects of impingement of submerged water jets on a nonuniform stream bed were studied through laboratory experiments and theoretical analysis the variables are jet diameter jet velocity jet angle and the impinging height the effects on the nonuniform stream bed are scoured depth armored depth cleaned depth and width the data was correlated through the jet impingement diameter and a dimensionless impingement number

relationships between cyclicity and stromatolite form in the late proterozoic bitter springs formation australia stromatolite biostromes and bioherms in the lower two units of the late proterozoic loves creek member of the bitter springs formation represent shallowing upward and deepening upward sequences in the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence ooid and or peloid intraclast grainstones and small irregular bulbous and columnar stromatolites characterize the basal transgressive portion of cycles domal columnar and stratiform stromatolites comprise the bulk of the cycle these forms accreted in a gradually shallowing epeiric sea domal stromatolites predominate in the deeper parts of cycles here synoptic relief gradually increases upwards columnar and stratiform stromatolites predominate in the shallower parts of cycles where synoptic relief rapidly diminishes upwards in thin bedded dolo mudstones at the tops of cycles the co occurrence of desiccation cracks tepee structures scalloped dissolution surfaces gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure in contrast stromatolites in a unit at the base of the loves creek member accreted during a gradual rise in sealevel stratiform columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence but only the lower half of the shallowing upward cycle is represented synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit recognition of a deepening upward stromatolite sequence at the base of the loves creek member and a disconformity surface between this sequence and the underlying gillen member permits palaeoenvironmental re interpretation of the loves creek member as a single large scale sea level cycle

erosion of finite thickness sediment beds by single and multiple circular jets sediment management in reservoirs with the help of water jets has motivated this work erosion caused by single and multiple submerged circular turbulent wall jets on a noncohesive sediment bed of finite thickness lying on a fixed boundary was studied with the help of laboratory experiments different combinations of jet diameter jet separation and sediment thickness to jet diameter ratio were tested results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle froude number f0 given by the velocity at the nozzle and the effective diameter and submerged specific density of the sediment evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time t this time made dimensionless with a time scale tc involving the volume of sediment scoured and the rate of erosion was also related to the densimetric froude number a comparison with studi

stratigraphy and depositional environment of lower san andres formation in subsurface and equivalent outcrops chaves lincoln and roosevelt counties new mexico this paper establishes correlations from subsurface to outcrop and develops a depositional model for the lower san andres formation in new mexico five cyclic carbonate evaporite sequences numbered consecutively upward as units 1 2 3 4 and 5 were identified from measurement of approximately 1 750 ft 534 m of section in the hondo canyon area of lincoln county and examination of approximately 400 ft 122 m of core from six wells in the levelland slaughter trend of chaves and roosevelt counties an idealized sequence consists of open marine restricted subtidal intertidal and supratidal facies capped by anhydrite in the subsurface or an evaporite dissolution zone in outcrop lateral facies changes in outcrop indicate that the pedernal uplift provided a locus f r emergent conditions at low relative sea level stands through the deposition of unit 3 units 4 and 5 are incomplete highly variable sequences lateral facies changes are not present in outcrop indicating that the pedernal uplift exerted little influence on the deposition of these units preserved anhydrite cores capping p1 and p2 sequences equivalent to units 4 and 5 range up to 15 ft 5 m thick are relatively pure commonly lack underlying intertidal supratidal carbonates and contain some gypsum ghosts indicating that part of the anhydrite was deposited subaqueously we present a depositional model involving both subaqueous salina and subaerial sabkha deposition of the anhydrite the model accounts for the purity and thickness of the evaporite caps the rarity of subaerial exposure features variations in the nature of the dolomite anhydrite transition and the accumulation and preservation of thick subaqueous evaporites north of the study area

sedimentology of the cretaceous maha sarakham evaporites in the khorat plateau of northeastern thailand abstract evaporites of the cretaceous to early tertiary maha sarakham formation on the khorat plateau of southeast asia thailand and laos are composed of three depositional members that each include evaporitic successions each overlain by non marine clastic red beds and are present in both the khorat and the sakon nakhon sub basins these two basins are presently separated by the northwest trending phu phan anticline the thickness of the formation averages 250 m but is up to 1 1 km thick in some areas in both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation the stratigraphy lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition evaporites include thick successions of halite anhydrite and a considerable accumulation of potassic minerals sylvite and carnallite but contain some tachyhydrite and minor amounts of borates during the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline pan environment many halite beds however contain a curious sieve like fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow water gypsum the 34s isotopic values obtained from different types of anhydrite interbedded with halite range from 14 3 to 17 0 cdt suggesting a marine origin for this sulphate bromine concentration in the halite of the lower member begins around 70 ppm and systematically increases upward to 400 ppm below the potash rich zone also suggesting evaporation of largely marine waters in the middle member the initial concentration of bromine in halite is 200 ppm rising to 450 ppm in the upper part of this member the bromine concentration in the upper member exhibits uniform upward increase and ranges from 200 to 300 ppm the presence of tachyhydrite in association with the potassic salts was probably the result of 1 the large volumes of halite replacement of gypsum on a bed by bed basis releasing calcium back into the restricted waters of the basin and 2 early hydrothermal input of calcium chloride rich waters the borates associated with potash rich beds likely resulted from erosion and influx of water from surrounding granitic terrains however hydrothermal influx is also possible interbedded with the evaporites are non marine red beds that are also evaporative with displacive anhydrite nodules and beds and considerable amounts of displacive halite the 34s isotopic values of this anhydrite have non marine values ranging from 6 4 to 10 9 cdt these data indicate that the khorat and sakhon nakhon basins underwent periods of marine influx due to relative world sea level rise but were sporadically isolated from the world ocean

proterozoic coastal sabkha halite pans an example from the pranhita godavari valley south india abstract extensive occurrences of pseudomorphs and casts of halite in association with wave formed shallow water structures and evidence of emergence in quartzarenites at the base of the middle proterozoic pakhal group c 1276 20 ma in south india point to the development of sabkha environments in arid to semi arid elimatic conditions periodic inundation of the sabkha during storm and subsequent desiccation led to displacive growth of the halite and its dissolution within the sediment just below the surface the brine pools within the sabkha seem to have developed in fault controlled topographic depressions formed at the embryonic stage of an intra continental rift ocean or graben system

erosion by plane wall jets with minimum tailwater the results of an experimental study on the erosion of sand beds by plane water wall jets with minimum tailwater is presented a comparison is made with the erosion caused by the corresponding deeply submerged water wall jet

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

upper triassic to lower cretaceous continental and coastal saline lake evaporites inthe atacama region of northern chile evaporites within upper triassic to lower cretaceous sequences in the atacama region of northern chile are interpreted as the deposits of continental and coastal saline lakes halite casts and finely laminated calcareous evaporites intercalated with alluvial and fluvial sediments are probably playa lake deposits these continental evaporites have been recognized in upper triassic alluvial sediments cifuncho formation in upper triassic lower jurassic braided river deposits basal unit of the pan de azucar formation and within a sequence of lower cretaceous aeolian alluvial and playa lake mudflat sediments quebrada monardes formation lower cretaceous evaporites between marine limestones lautaro formation and continental redbeds quebrada monardes formation were probably deposited in coastal saline lagoons produced during a regionally extensive marine regression these sequences and other similar successions in northern chile provide a record of almost continuous evaporite deposition and hence of arid to semi arid conditions since upper triassic times these conditions were primarily the result of a constant latitudinal position within the subtropical zone a contributary factor was the geographical position of the area initially on the west coast of gondwanaland and subsequently on the coast of south america with cold northward flowing ocean currents and offshore winds

effects of air entrainment on plunge pool scour plunge pool scour depths are normally assumed to be dependent on unit flow q and head drop h with a surprising variation among authors concerning the importance of h it is hypothesized by the writer that this process may be linked to plunge pool air entrainment tests are carried out using a hydraulic model on which q h and the air water ratio in the plunge pool could each be varied separately scour depths are found to depend only on q and with the apparent effects of h recorded in the past being possibly due to associated variations of with h equations are developed explaining the results in terms of forces on the particles of bed material these are shown to apply to a wider body of model test data and also to prototype data particular areas of relevant future research are also identified it is proposed that any future plunge pool scour studies should recognize air entrainment as a significant variable

microfossils and precambrian cambrian boundary stratigraphy at maldeota lesser himalaya an assemblage of problematical microfossils of precambrian cambrian boundary age is redescribed from the chert phosphorite member at the base of the lower tal formation of maldeota in the lesser himalaya of india this assemblage has previously been ascribed to various ages from precambrian to cretaceous but is held by us to contain maldeotaia bandalica protohertzina anabarica group trumpet shaped elements acicular elements a b conotheca sp ovalitheca cf multicostata allathecid sp a barbitositheca ansata hexangulaconularia cf formosa coleoloides aff typicalis hyolithellus aff insolitus h cf isiticus h vladimirovae spirellus shankari and olivooides multisulcatus these compare closely with assemblages found above the base of the first anabarites trisulcatus protohertzina anabarica zone in china and in the second pseudorthotheca costata zone of southern kazakhstan the stratigraphic setting of the krol tal succession is reviewed and several similarities are noted between the precambrian cambrian boundary successions of lesser himalaya in india and of yunnan and sichuan in southwest china indicating that correlation between them is possible at several levels

sedimentary facies depositional environments and palaeogeographic evolution of the neogene denizli basin sw anatolia turkey abstract the denizli basin southwestern anatolia turkey contains a record of environmental changes dating since the early miocene detailed facies analysis of the neogene formations in this half graben enables us to document successive depositional regimes and palaeogeographic settings sedimentation commenced in the early miocene with the deposition of alluvial fan and fluvial facies kizilburun formation at this stage alluvial fans sourced from elevated areas to the south prograded towards the basin centre the middle miocene time saw the establishment of marginal lacustrine and wetland environments followed by the development of a shallow lake sazak formation the uppermost part of this unit consists of evaporitic saline lake and saline mudflat facies that grade upward into brackish lacustrine deposits of late miocene pliocene age kolankaya formation the lake became shallower at the end of the pliocene time as is indicated by expansion shoreface foreshore facies in the early quaternary the denizli basin was transformed into a graben by the activation of ese trending normal faults alluvial fans were active at the basin margins whereas a meandering river system occupied the basin central part oxygen isotope data from carbonates in the successive formations show an alternation of wetter climatic periods when fresh water settings predominated and very arid periods when the basin hosted brackish to hypersaline lakes the neogene sedimentation was controlled by an active ese trending major normal fault along the basin s southern margin and by climatically induced lake level changes the deposition was more or less continuous from the early miocene to late pliocene time with local unconformities developed only in the uppermost part of the basin fill succession the unconformable base of the overlying quaternary deposits reflects the basin s transformation from a half graben into a graben system

microfacies depositional environment and diagenetic evolution controls on the reservoir quality of the permian upper dalan formation kish gas field zagros basin abstract the upper permian upper dalan formation contains one of the largest gas reservoirs in the world the formation consists of carbonates with some evaporite intercalations that developed on a gently sloping homoclinal carbonate ramp facing the late permian paleo tethys ocean this study focuses on the kish gas field zagros offshore basin situated between iran and qatar and is based on a 222 m thick continuous core based on the integration of core and wireline log data coupled with petrographic analyses of 580 thin sections three major depositional environments facies belts with 11 carbonate microfacies are identified these include 1 sabkha to tidal flat laminated to massive anhydrite dolomudstone with anhydrite nodules dolomudstone and intraclastic dolowackestone 2 lagoon and leeward shoals bioclastic wackestone dolowackestone to packstone and peloid dolopackstone and peloid bioclastic dolopackstone and 3 mobile windward sand shoal ooid peloid dolograinstone ooid dolograinstone ooid intraclast dolograinstone ooid bioclast dolograinstone packstone and coarse bioclast intraclast dolograinstone diagenetic evolution of the upper dalan formation is associated with evaporative marine shallow water normal marine meteoric and burial diagenetic environments common diagenetic effects include dolomite and calcite cementation mechanical and chemical compaction dissolution dolomitization and evaporative anhydrite mineralization reservoir quality is strongly affected by variations in the original rock fabrics and subsequent diagenetic alterations the most common pore types include interparticle moldic and connected vug fracture and cavernous the interparticle porosity permeability relationship for the studied facies suggests that the reservoir quality is not affected by different crystal sizes and most samples plot in the low porosity and low to high permeability field or display lucia class 1 or 2 petrophysical relationships the study shows that the pervasive pore filling anhydrite mineralization lead to a significant decrease in porosity and permeability poikilotopic anhydrite cement reduced matrix porosity but the pore size was less affected

aeolian landscapes in central australia gypsiferous and quartz dune environments from lake amadeus there are two different dune systems in central australia regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes these two systems especially gypsiferous dunes at lake amadeus the largest playa in central australia provide a sedimentary geomorphological and environmental history of the region during the late quaternary the gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below a mixture of gypsum sand and quartz sand no clay pellets have been found in the dune sequence in significant contrast to the gypsiferous clay dunes in other parts of australia three possible models of the environmental controls of gypsiferous dune formation are discussed the most plausible one suggests simultaneous gypsum precipitation and deflation sandsized gypsum was precipitated in a groundwater seepage zone around the playa margin during seasonally high water tables and these crystals were deflated onto land during dry intervals forming the marginal gypsiferous dunes these processes require conditions of high regional water table strong climatic seasonality and probably a windier and overall wetter climate at least two separate gypsiferous duneforming episodes can be recognized the age of formation of the younger one has been dated by thermoluminescence at 44 54 ka the gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals showing evidence of leaching dissolution and recrystallization it is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune after the construction of the younger gypsiferous dune there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa an equivalent aeolian sand layer was deposited within the playa soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation activation of the regional dunefields suggests a major period of dry climate which although not dated may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in australia

gypsum facies in the badenian middle miocene of southern poland middle miocene sulfate sediments south of the holy cross mountains southern poland comprise deep and shallow water as well as subaerial facies accompanied by carbonates and siliciclastics in the gypsum section 18 lithostratigraphic units have been distinguished the facies variety reflects distinct sedimentary conditions in the peripheral area of the evaporitic basin where the maximum water depth never exceeded some tens of meters the succession of facies is regressive and comprises six sedimentary cycles that reflect relative changes in sea level and in the physicochemical regime of the basin both of which were controlled by tectonic and climatic factors sea level fell five times during sulfate sedimentation the last sea level drop led to the almost total desiccation of the sea in the peripheral part of the basin

origin of polyhalite deposits in the zechstein upper permian zdrada platform northern poland polyhalite deposits in the zechstein upper permian of northern poland occur in the lower werra anhydrite in the zdrada sulphate platform the polyhalite appears to be a very early replacement of anhydrite the replacement was caused by the halite precipitating brines which contained potassium and magnesium ions the formation of polyhalite was preceded by the syndepositional anhydritization of the original gypsum deposit which has often preserved its primary textures this anhydritization on the platform and its slopes was a reaction of the precipitated gypsum in a hydrologically open evaporite basin with brines of salt basins adjacent to the sulphate platform these brines when nearly saturated with respect to halite and potassium and magnesium rich reacted with anhydrite to precipitate polyhalite along the slopes of the zdrada platform the oxygen and sulphur isotopic compositions of sulphate evaporites indicate that marine solutions were the only source of sulphate ions supplied to the zechstein basin and that anhydrite was transformed to polyhalite by reaction with marine brines more concentrated than those that precipitated precursor calcium sulphate minerals

origin and deformation of intra salt sulphate layers an example from the dutch zechstein late permian from salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale the relatively thick zechstein 3 anhydrite carbonate layer in the northern netherlands has been studied previously using 3 d seismic data observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub seismic scale imbedded in the zechstein 2 z2 salt core samples thin sections seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these z2 sulphate layers bromine analyses show that they mark a freshening event in the z2 salt which can be correlated over large distances in the northern netherlands their core calibrated log signature indicates that the z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite the mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows thin anhydrite polyhalite couplets and highs thicker anhydrite layers such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation salt internal geometries can further be used to distinguish between different deformation mechanisms in our study area the distribution of sulphate layers within the z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt related deformation

carbon isotope stratigraphy and the problem of a pre tommotian stage in siberia carbon isotopic oscillations are useful to elucidate the stratigraphy and biogeochemical events around the precambrian cambrian transition new isotopic data from the manykaj and emyaksin formations of the eastern anabar uplift siberia help to correlate the lower cambrian and neoproterozoic cambrian transitional beds across the siberian platform the similarity of trends and amplitudes of the carbon isotopic curves together with biostratigraphic and sequence stratigraphic markers from the anabar uplift provide a precise correlation with the southern part of the siberian platform diagenesis of argillaceous limestones of the emyaksin formation has appar ently not affected the primary isotopic variations the resulting curve is nearly identical in sections about 100 km apart in the tommotian atdabanian portion of the formation relatively frequent and pronounced isotopic oscillations in the lower beds of the emyaksin formation fit between features i and ii of the southern siberian isotopic reference scale but are undetected therein owing to the deposi tional hiatus at the base of the tommotian stage in its type section this confirms the transgressive onlap from the north suggested by previous studies and makes the appearance of the cambrian skele tal fossils on the siberian platform less abrupt the hiatus in the south appears to embrace at least two biostratigraphic zones as recognized in the north the case is strengthened for a pre tommotian cambrian stage in siberia the biostratigraphic framework for which has been elaborated earlier

the anatomy of a sulphate platform and adjacent basin system in the leba sub basin of the lower werra anhydrite zechstein upper permian northern poland the lower werra anhydrite zechstein upper permian deposits of the teba area originated in a deep basin setting in shallow to deep water conditions facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms this pattern of local platforms and adjacent basins developed during deposition in basinal areas the sequence is clearly transgressive whereas on platforms accumulation kept pace with subsidence after an initial transgression nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition massive anhydrite with pseudomorphs after upright growth gypsum crystals suggest rapid precipitation in a subaqueous environment and or fluctuating but generally high salinity conditions massive clastic sulphate originated due to periodic high energy events and resedimentation or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite massive textureless anhydrite is locally porous and passes upward into breccia indicating a strongly saline environment bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water bedded anhydrites contain portions which are graded intercalations of sulphate turbidites and upright growth gypsum suggest fluctuating water depths with comparatively deep water during turbidite deposition but shallower conditions during upright growth gypsum deposition the sequence observed in slope zones at platform basin margins detrital parautochthonous sulphate sand to graded beds to basinal laminites indicates that redistribution processes were important at the onset of the lower werra anhydrite deposition bathymetric relief existed between the central part of the basin and its margins where carbonate platforms remained subaerially exposed formation of local platforms and adjacent basins required a relatively high subsidence rate as pre existing relief cannot account for the total accumulated thickness of the lower werra anhydrite deposits one implication of this is that the main argument against the shallow water shallow basin evaporite basin model i e a very fast rate of subsidence may not be valid for the eba lower werra anhydrite basin

depositional environments of marine dominated bedded halite permian san andres formation texas the interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology facies studies supplemented by geochemical data define an evaporite shelf depositional environment for the halite of the san andres formation of the palo duro basin texas panhandle that is significantly different from modern and previously described ancient halite precipitating environments such as playas salinas sabkhas and barred basins a coastal palaeoenvironmental setting for the san andres halite is defined by its facies position between normal marine shelf carbonates and aeolian deposits the san andres brine pool extended over more than 10 000 km2 but was very shallow perennial brine pool conditions thick sequences of halite alternated with ephemeral brine pool conditions halite haloturbated mudstone halite mudstone interbeds and microkarst the depositional sequence in the perennial brine pool includes 1 flooding and minor dissolution of previously deposited halite 2 precipitation of a thin lamina of gypsum and 3 evaporation and precipitation of halite bottom growth forms of halite including chevrons and vertically elongated crystals are well preserved cumulates of foundered floating crystals and rafts were probably abundant but have been almost completely recrystallized mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine pool deposition and the establishment of subaerial conditions vertically elongated karst pits originate at the palaeosurfaces cross cut primary brine pool fabrics and are filled with geopetal sediment and coarse halite cement mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity the geochemistry of halite from both ephemeral and perennial brine pool environments reflects the frequent influx of marine brine indicating that communication with the marine environments to the south was good considering that restriction was sufficient to precipitate halite

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

lacustrine sedimentation in the diapir controlled miocene bicorb basin eastern spain abstract the miocene bicorb basin is a small elongated basin developed by normal faulting of a thick jurassic cretaceous carbonate succession and subsequent diapirism of upper triassic mudstones and evaporites the basin fill comprises a sequence over 650 m thick formed by two units the lower alluvial unit consists of a complex alternation of conglomerates sandstones mudstones and minor lacustrine limestones the upper unit comprises marginal alluvial and lacustrine deposits in which seven lacustrine facies associations have been distinguished the inner lacustrine deposits comprise mudstones and carbonates with minor evaporitic deposits at the northeast and southwest basin edges alluvial inputs led to the development of delta and fan delta environments where sandstone and conglomerate deposits dominate the latter stages of the lake evolution are represented by an expansive thick limestone sequence which close to the nw fault bounded margin pass laterally to breccias the hierarchical arrangement of the upper unit shows five different orders of sequences the first order sequence defines a major vertical trend of lake expansion whereas three second order sequences are linked to major flooding expansion episodes all these sequences are related to the tectonic evolution of the diapir metre scale third and fourth order sequences and fifth order sequences which consist of finely laminated rhythmite deposits probably record climatically forced processes the diapir graben system evolution and the climatic variations have exerted major controls on the sequential arrangement and evolution of the lacustrine system both factors have also strongly influenced the changes in the gastropod assemblages

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

a continuous high resolution record of late pleistocene climate variability from the estancia basin new mexico lake sediments that accumulated in the estancia basin central new mexico provide a detailed record of submillennial latest pleistocene climatic oscillations sedimentologic biologic and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k y b p as dated by 14c by means of accelerator mass spectrometry from shoreline and basin center deposits these dates determine the timing and duration of the episodes of wet and dry climate the lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca 20 to 15 k y b p a pronounced lowstand lasting 1 k y occurred between ca 15 and 14 k y b p followed by two more highstands of the lake between ca 14 and 12 5 k y b p desiccation of the perennial lake after 12 k y b p was followed by a final poorly dated highstand at ca 10 k y b p the record of lake level fluctuations from the estancia basin provides a high resolution record of changes in water budget in a basin the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination

petrography and genesis of quaternary coastal gypcrete in north kuwait arabian gulf abstract gypcrete crusts cover late quaternary playa deposits within the northern coastal flat of kuwait bay arabian gulf this study provides insight into the genesis of the gypcrete crust associated with the fluvial playa in an arid coastal area the playa sediments are mostly muddy sand rich in diagenetic gypsum the latter is concentrated at the top part of this playa where the gypcretic crust is developed field investigation megascopic microscopic and nanoscopic examination and xrd analysis revealed the occurrence of several types of gypcretes namely massive hard crust hard clustered gypsum roses gypseous rhyzocrete and powdery crust with the exception of the last type all gypcretes are formed largely of gypsum with subordinate amounts of bassanite two genetic types of bassanite are recognised these are replacive and precipitative replacive bassanite is the most common and is mainly represented by topotactic alteration of gypsum whereas the precipitative mode is mostly restricted to the powdery crumbled crust and root cavity fill the micromorphology of both types of bassanite is discussed it is suggested that the stability of bassanite is attributed to the elevated ambient temperatures in the presence of highly saline brine and organic acids

the proterozoic phanerozoic transition and the precambrian cambrian boundary abstract a brief summary of the work from 1972 to 1981 of the international geological correlation programme s project no 29 working group on the precambrian cambrian boundary is given and the position reached in the opinion of the author in the first quarter of 1981 is outlined two figures give both the coverage of the project s research in global terms and tentative conclusions regarding the position of the precambrian cambrian boundary in selected critical sections plenary decisions of the working group at conferences in paris 1974 and cambridge 1978 are outlined in conclusion the stratigraphic value of archaeocyathid zonation small tommotian shelly fossils trace fossils and trilobites are discussed and the main problems thought to remain in the next few years are noted 1 1 location and accessibility of the global boundary section and point possibly in asia 2 2 intercontinental correlation 3 3 palaeobiological research

effect of upward seepage on scour and flow downstream of an apron due to submerged jets the upward seepage through the bed sediment downstream of an apron of a sluice gate structure is a common occurrence due to afflux of the flow level between the upstream and downstream reaches of a sluice gate the result of an experimental investigation on the characteristics of the scour hole and the flow field downstream of an apron due to submerged jets under the influence of upward seepage through the bed sediment is presented experiments were run for the conditions of submerged jets having submergence factors from 0 99 to 1 72 and jet froude numbers from 3 15 to 4 87 over beds of sediments median sizes 0 8 1 86 and 3 mm downstream of an apron under upward seepage velocities the characteristic lengths of the scour hole determined from the scour profiles are the maximum equilibrium scour depth the horizontal distance of the location of maximum scour depth from the edge of the apron the horizontal extent of the scour hole from the edge of the apron the dune height and the horizontal dist

cohesion influences on erosion and bed load transport 1 studies on erosion and transport rates of cohesive sediment mixtures containing gravel are not reported so far in the literature to the best of our knowledge results from laboratory experiments on the process of erosion of cohesive sediment mixtures by open channel flow are presented the sediment used for experimentation consisted of fine gravel mixed with clay in proportions varying from 10 to 50 and fine gravel fine sand in equal proportion by weight mixed with clay proportions again varying from 10 to 50 by weight experimental observations have revealed that the process of detachment of cohesive sediment mixtures containing gravel is significantly different from that of cohesionless sediments also it is different from the detachment process of cohesive sediment mixtures not containing coarser size material experimental data on transport rates of the bed load of the cohesive sediment mixtures are presented on the basis of dimensional considerations a computational procedure is proposed to determine the bed load transport rates generated from erosion of the different size fractions of sediment mixtures clay percentage and unconfined compressive strength of the sediment bed were the main parameters controlling bed load transport rate of such sediments in the absence of cohesion the proposed method reduces to the patel and ranga raju 1996 method for bed load transport rate computation of nonuniform cohesionless sediments

local scouring due to turbulent water jets downstream of a trapezoidal drop laboratory experiments and stability analysis 1 three series of laboratory experiments were run varying the flume water discharge and the downstream water depth to investigate local scouring downstream of a trapezoidal drop followed by a rigid apron three different kinds of jets developing downstream of the drop were observed a submerged jet a surface wave jet and an oscillating jet the oscillating jet is defined as a jet which changes intermittently between a surface wave jet and a submerged jet a different development of the scouring process due to the different kinds of jet was observed a submerged jet produced a local scouring with a downstream dune while a surface wave jet produced a local scouring with both an upstream and a downstream dune the scouring process due to the oscillating jet was the result of the interaction of the scouring processes due to a submerged jet and a surface wave jet a stability analysis of the surface wave jet was performed with both a flat and a scoured bed by means of the modified de saint venant equations that is with correction terms accounting for the curvature of the streamlines the stability of the surface wave jet is affected by the presence of the scoured bed which weakens its stability properties

distribution patterns of gypsum and kalistrontite in a dry lake basin of the southwestern kalahari omongwa pan namibia the deposits of the omongwa pan southwestern kalahari namibia are partly gypsiferous and locally contain small amounts of kalistrontite the distribution patterns of gypsum caso4 2h2o and kalistrontite k2sr so4 2 are mainly determined by groundwater depth and by the lithological composition of the deposits the latter determines the hydraulic conductivity for saturated and unsaturated flow and therefore controls the depth of the interval within the deposits where evaporation and mineral precipitation can take place other factors that affect the distribution of the evaporite minerals are the patterns of groundwater flow within the basin and the occurrence of a redistribution of salts during short flooding stages gypsum occurs as crystals of four distinct morphological types which each developed in different conditions prismatic crystals formed both as subaqueous precipitates and as crystals that developed within the sediment matrix of a brine saturated surface layer sub euhedral tabular crystals formed within brine filled macropores hemi bipyramidal crystals formed in a nearly permanently brine saturated part of a subsurface horizon phreatic zone as the product of recrystallization of an older gypsum occurrence and lenticular crystals formed by intrasediment growth in the vadose zone the development of these morphological types of gypsum crystals is marked by differences in the differential inhibition of growth of the various crystal forms the further growth of lenticular crystals which are most strongly affected by differential growth inhibition is still characterized by a difference in degree of inhibition between the 111 and 103 forms expressed as a change in orientation of the plane of flattening copyright 1999 john wiley sons ltd

erosion by planar turbulent wall jets new scaling laws are presented for the spatial variation of the mean velocity and lateral extent of a two dimensional turbulent wall jet flowing over a fixed rough boundary these scalings are analogous to those derived by wygnanski et al 1992 for the flow of a wall jet over a smooth boundary they reveal that the characteristics of the jet depend weakly upon the roughness length associated with the boundary as confirmed by experimental studies rajaratnam 1967 these laws are used in the development of an analytical framework to model the progressive erosion of an initially flat bed of grains by a turbulent jet the grains are eroded if the shear stress exerted on the grains at the surface of the bed exceeds a critical value which is a function of the physical characteristics of the grains after the wall jet has been flowing for a sufficiently long period the boundary attains a steady state in which the mobilizing forces associated with the jet are insufficient to further erode the boundary the steady state profile is calculated separately by applying critical conditions along the bed surface for the incipient motion of particles these conditions invoke a relationship between the mobilizing force exerted by the jet the weight of the particles and the local gradient of the bed use of the new scaling laws for the downstream variation of the boundary shear stress then permits the calculation of the shape of the steady state scour pit the predicted profiles are in good agreement with the experimental studies on the erosive action of submerged water and air jets on beds of sand and polystyrene particles rajaratnam 1981 the shape of the eroded boundary at intermediate times before the steady state is attained is elucidated by the application of a sediment volume conservation equation this relationship balances the rate of change of the bed elevation with the divergence of the flux of particles in motion the flux of particles in motion is given by a semi empirical function of the amount by which the boundary shear stress exceeds that required for incipient motion hence the conservation equation may be integrated to reveal the transient profiles of the eroded bed there is good agreement between these calculated profiles and experimental observations rajaratnam 1981

lithofacies diagenetic spectra and sedimentary cycles of messinian late miocene evaporites in se spain abstract messinian evaporites have been deposited within a number of connected basins that formed the so called betic strait in se spain during the late miocene pre syn and postevaporitic sedimentary successions may well be compared with other strata of the same age surrounding the mediterranean sea there is no evidence for a cataract like connection of these basins with a strong unidirectional flow regime before and during the formation of messinian evaporites as previously suggested the most frequently occurring gypsum lithofacies presumably related to shallow water depositional environments are crystalline selenitic gypsum grass like gypsum laminated gypsum and cross bedded gypsarenite only few real sabkha type chicken wire alabastrine gypsum have been identified evenly laminated gypsum together with graded gypsarenites turbidites gypsrudites debrites and slumps is restricted to basins or parts of basins with considerable paleoslopes evidence for diagenetic overprinting is abundant in all outcrop areas studied this includes the formation of giant gypsum twin crystals super cones and alabastrine nodular gypsum the formation of messinian evaporites in se spain is thought to be controlled by a superimposed third order sea level lowstand smaller sedimentary units fall well into the range of fourth to sixth order cyclic and rhythmic successions they are clearly related to basin dynamics or to autogenetic processes of the local facies zone facies dynamics none of the smaller scale cycles fourth and higher order as observed in outcrops are related to high frequency eustasy

pre tommotian age of the lower pestrotsvet formation in the selinde section on the siberian platform carbon isotopic evidence carbon isotopic data from the selinde section in the southeastern part of the siberian platform area are correlated with the reference isotopic profile from the lower cambrian stratotype sections of the lena aldan region but also show additional 13 c excursions unrecognized there the chemostratigraphic correlation suggests that the geological and fossil record of the lower pestrotsvet formation in the selinde section has a deeper history than the stratotype region this conclusion is important for both constraining the age of the earliest cambrian marine transgression on the siberian platform and providing a clearer understanding of the pace and order of early cambrian geochemical and biological events

traces and dubiotraces examples from the so called late proterozoic siliciclastic rocks of the vindhyan supergroup around maihar india signatures of biogenic activity registered in the form of burrows and bioturbation of sediments are recorded for the first time from the precambrian siliciclastic tidal flat deposits of the bhander group the uppermost sub division of the vindhyan supergroup exposed near maihar central india the burrows vary from large diameter near vertical stubby forms to microscopic thread like features cutting across the physical sedimentary structures associated with the burrows the sediments also register long and curved scratch markings bean seed or lozenge shaped projections micro mud volcano like protuberances on the positive of sandstone sole thin section study of the large diameter burrows discloses two different patterns in the nature of burrow fill 1 staggered concave upward internal laminae showing v in v or broad based u in u structures resembling monocraterion and 2 an ill defined arrangement of the upwarped laminae of the burrow fill the stubby thumb like burrow being bordered by clay lining on the burrow wall dissection of the bean shaped forms reveals that these represent the lower part of diplocraterion burrows however the scratch makings and other features are grouped as dubiotraces as generation of similar structures by physical processes can not be ignored intense bioturbation of the sediments in which the burrows are located might suggest that some of the trace makers were definitely deposit feeders due to the lack of proper radiometric ages data the lower part of the bhander group of rocks of central india has been assigned a middle to late riphean age on the basis of stromatolite assemblages however the trace fossils described in this paper might sharpen our earlier concept of vindhyan sedimentation by fixing these burrowed rocks very close to the precambrian cambrian boundary

carbonate microbialites and hardgrounds from manito lake an alkaline hypersaline lake in the northern great plains of canada abstract manito lake is a large perennial na so 4 dominated saline to hypersaline lake located in the northern great plains of western canada significant water level decrease over the past several decades has led to reduction in volume and surface area as well as an increase in salinity the salinity has increased from 10 ppt to about 50 ppt tds this decrease in water level has exposed large areas of nearshore microbialites these organogenic structures range in size from several cm to over a meter and often form large bioherms several meters high they have various external morphologies vary in mineralogical composition and show a variety of internal fabrics from finely laminated to massive in addition to microbiolities and bioherms the littoral zone of manito lake contains a variety of carbonate hardgrounds pavements and cemented clastic sediments dolomite and aragonite are the most common minerals found in these shoreline structures however calcite after ikaite monohydrocalcite magnesian calcite and hydromagnesite are also present the dolomite is nonstoichiometric and calcium rich the magnesian calcite has about 17 mol mgco 3 ams radiocarbon dating of paired organic matter and endogenic carbonate material confirms little or no reservoir affect although there is abundant evidence for modern carbonate mineral precipitation and microbialite formation most of the larger microbialites formed between about 2300 and 1000 cal bp whereas the hardgrounds cements and laminated crusts formed about 1000 500 cal bp

geochemistry of acid brines lake tyrrell victoria australia abstract pore water groundwater and sediments were collected from lake tyrrell to study the evolution of brines in an acidic environment lake tyrrell is a 150 km2 inland salt playa acid saline groundwater enters the playa through springs along the lake margin and is evapo concentrated past halite precipitation the brines were found to contain sea salts but some solutes do not follow the seawater evaporation curve processes that occur in neutral alkaline systems e g gypsum formation occur here but precipitation of alunite kal3 so4 2 oh 6 jarosite kfe3 so4 2 oh 6 and fe oxides also affect solute behavior processes that occur in the recharge area and along the groundwater flow path k uptake calcrete and gypcrete formation affect the brine s chemical signature regardless of the chemical variability found among the samples solutes have evolved from a common source sea salt acidification which appears to occur early in the evolution of the groundwater is related to pyrite oxidation and may be enhanced by precipitation of fe oxides alunite and jarosite and evaporation acidity is maintained in the system because of the lack of buffering capacity of aquifer minerals we suggest that this acid brine from australia is another end member in the eugster jones hardie model for the evolution of water by evaporation in closed basins

epitaxial salt reefs and mushrooms in the southern dead sea salt usually accumulates in shallow ephemeral brines as simple flat beds however in permanent brines deepening in solar evaporation ponds in the southern dead sea salt accumulation is complicated by the growth of isolated vegetable like salt structures

fluvial evaporitic and shallow marine facies architecture depositional evolution and cyclicity in the sivas basin lower to middle miocene central turkey abstract lower to middle miocene rocks in the karayun region of the sivas basin are represented by the agilkaya and egribucak formations forming a 4 km thick continuous section each formation is subdivided into three members representing similar lithofacies the lower members are made up of fluvial sheet sandstone and red mudstone that migrated laterally on a flood basin within a semi arid fan system the middle members are composed of bedded to massive gypsum and red green mudstone of a coastal and or continental sabkha environment they are intercalated with lagoonal dolomitic limestone and bituminous shale in the agilkaya formation and fluvial red pink sandstone red mudstone in the egribucak formation the upper members are made up of shallow marine fossiliferous mudstone and sandy limestone rapid vertical and horizontal facies changes in both formations reflect the locally subsiding nature of this molassic basin the overall section can be also subdivided into three orders of fining upward cycles type i cycles correspond to the agilkaya and egribucak formations each around 2 km thick and indicate two long term transgressive phases controlled by tectono eustatic factors type ii cycles 140 m thick on average are formed by alternations of fluvial sheet sandstone and red mudstone capped by up to 50 m thick red flood basin mudstone a 400 ka period milankovitch cyclicity related to astronomical climatic fluctuations is suggested as a governing mechanism although subsidence cannot be excluded the existence of laterally persistent fluvial sheet sandstone overlain by red mudstone that forms fining upward type iii cycles 10 m thick on average is attributed to changes in subsidence rate rhythmic alternations between gypsum beds and red green mudstone and or dolomitic limestone and bituminous shale observed in the middle members might be related to minor periodical climatic changes rather than to tectonics

marine to non marine sedimentation in the upper miocene evaporites of the eastern betics se spain sedimentological and geochemical evidence abstract several neogene basins in the eastern betics contain a record of thick upper miocene evaporites the inner basins those located distal to the mediterranean coast include evaporite units ranging from the late tortonian to the early messinian whereas the marginal basins those located in the vicinity of the mediterranean coast include evaporite units corresponding to the late messinian that are linked to the salinity crisis the sedimentological and geochemical comparison of the evaporites in the inner and marginal basins enable us to show their differences in the modes of deposition under local structural controls and differentiated palaeogeography in the inner basins of the murcia alicante region a number of evaporite units have been differentiated whose gypsum lithofacies have 1 variable strontium contents with mean values primary gypsum between 1121 and 2380 ppm 2 isotopic compositions of sulphur 34 s ranging from those characteristic of the tertiary marine water 20 3 to 25 5 to those characteristic of the triassic sulphates 8 0 to 16 0 the latter suggesting recycling of the sulphate anion in meteoric waters intermediate values 17 4 to 19 6 are attributed to mixed mother brines and 3 strontium ratios 87 sr 86 sr comprised between 0 70804 and 0 70888 these values range from those of the tertiary marine water to those exceeding the triassic sulphates suggesting additional continental contributions from country rocks and hydrothermal solutions the comparison between the sulphur and strontium isotopic compositions of the sulphates together with the study of the biota present in the marls associated with the gypsum allows us to distinguish between marine continental and mixed units in the marginal basins of the eastern betics where single evaporite units characterized by selenitic gypsum prevail 1 the strontium contents are more homogeneous with mean values primary gypsum between 493 and 625 ppm 2 the isotopic compositions of oxygen 18 o and sulphur 34 s are comprised between 12 2 and 14 8 and between 22 0 and 23 5 respectively and 3 the strontium ratios 87 sr 86 sr vary from 0 70890 to 0 70895 all these isotopic values reveal a marine origin and the lack of contribution of significant continental water during the late messinian evaporite deposition in the betic strait these characteristics are similar to those of the lower evaporite

sulfate minerals in evaporite deposits evaporite deposits preserve a wealth of information on earth s past surface conditions the deposits are sensitive indicators of depositional environment and climate recorded in a wide variety of mechanically and chemically produced sedimentary structures and fabrics unique to evaporite deposits is the record they carry of the hydrochemistry of surface waters including seawater sulfate minerals are important components in understanding the hydrochemistry of ancient surface waters sedimentological aspects of evaporite deposits were compiled by melvin 1991 in the compilation sedimentology specific to ca bearing sulfates was described by warren 1991 lowenstein and hardie 1985 handford 1991 and smoot and lowenstein 1991 discussed the sedimentology of evaporites in general these general principles can be applied specifically to the sulfate minerals in evaporite deposits sedimentological aspects of sulfate minerals in evaporite deposits are not discussed here for this information the reader is referred to the above references hardie et al 1985 and spencer and lowenstein 1988 discussed the petrography of evaporites and the interpretation of fabrics of evaporite minerals examples of petrographic aspects of sulfate minerals in evaporites are presented here along with references specific to the sulfate minerals for a more complete understanding of these aspects the reader is referred to the above references the major focus of this chapter is on the chemical aspects of the common sulfate minerals that occur in evaporite deposits the discussion begins with an explanation of the concept of chemical divides and the determination of evaporation paths and the evolution of brines in accordance with mineral solubility and the composition of dilute inflow waters determination of the precipitation pathways of the more soluble salts is more complex examples of sulfate mineral assemblages in evaporation systems calculated via thermochemical modelling are given for both marine and non marine systems the mineralogy of evaporite deposits is

the kuranakh epithermal gold deposit aldan shield east russia abstract the kuranakh deposit one of the largest lode gold deposits in russia is located within the central aldan ore district on the southern flank of the siberian craton the host rocks are flat lying jurassic arkose and lower cambrian limestone and dolomite overlying a precambrian metamorphic basement the hydrothermal mineralizing event is associated with mesozoic igneous activity in the mine area this igneous activity is manifested by three swarms of dikes with a few small plugs and sills of bostonite microgabbro and minette gold mineralization is spatially related to the dikes which may be both pre ore and post ore in age the kuranakh deposit is interpreted as a low sulfidation epithermal gold deposit with quartz adularia alteration several sub horizontal blanket or ribbon like orebodies up to 50 m thick occur mainly along the karstified contact between cambrian calcareous footwall rocks and overlaying jurassic clastic rocks within a narrow but very long zone of about 25 km originally gold mineralization was associated with pyrite arsenopyrite sphalerite and galena however total sulfides constituted only a few percent of the total rock mass the deposit has been thoroughly oxidized and only traces of arsenopyrite and pyrite are rarely found gold occurs primarily as mineral grains less than 5 m in size usually contained within friable grains of porous goethite studies of fluid inclusions show a range of homogenization temperatures from 80 c to 220 c but generally ranging from 110 c to 160 c the center of the heat source may have been located in the southern end of the deposit at its transition to the gold and uranium mineralization hosted in the precambrian basement of the siberian craton

local scour downstream of positive step stilling basins this note focuses on the temporal and spatial evolution of local scour below low head spillways steady flow experiments were carried out in a 1 m wide and 20 m long rectangular straight channel the jet was generated by an ogee crest spillway followed by a positive step stilling basin nearly uniform sandy beds were generally tested but additional tests were also performed with a special bed of lead spheres to circumvent the combination of local and general scour phenomena tailwater depths were set such that tailwater flow intensities were below the threshold of sediment motion as a consequence for each run a submerged hydraulic jump formed tests were of long durations of order of days mainly to achieve conditions of quasi equilibrium based on the data collected literature approaches are discussed then empirical models are proposed to estimate 1 the maximum scour depth at the quasi equilibrium stage and its horizontal distance from edge of stilling basin 2 the time variation of scour depth and 3 the axial scour profiles the proposed equations agree well with experimental data findings also highlight that affinity rather than similarity may be the typical property of low angle eroding jets

lithofacies and water body record of messinian evaporites in nijar basin se spain abstract the depositional lithofacies and fabrics of messinian evaporites have been studied to evaluate the palaeodepth of water body in nijar basin se spain the nijar messinian strata consist of three formations abad marls yesares evaporites and feos rocks gypsum deposits dominate the yesares formation and the basal feos formation six gypsum lithofacies have been identified including coarse twinned selenite grass like selenite hemiradial to radial selenite turbidite selenite in the yesares formation and gypsum conglomerate laminated gypsum and hemiradial to radial selenite in the feos formation the size of yesares selenites decreases upward while carbonate content and trace element abundance increase in each selenite bed and the whole formation suggesting a shallowing up sequence these gypsum lithofacies were deposited in subaqueous environments commonly at 10 30 m and occasionally up to 100 m or deeper for the turbiditic selenites based on the interpretations of gypsum fabrics marl intervals basin configuration and geochemistry the sea level of nijar basin experienced two major drops with a magnitude of 40 m before and after the deposition of yesares evaporites however the basin was never desiccated during the evaporite deposition and the change of sea level for the alternative deposition of gypsum beds and marl intervals was commonly around 50 80 m probably resulting from the glacio eustacy fluctuations feos gypsum reflects another evaporative event and is correlated to the upper evaporite while the yesares selenite is correlated to the lower evaporite based on the interpretations of lithofacies geochemistry and sealevel change as seawater in nijar basin was continuously supplied by the central mediterranean basins and remained in subaqueous conditions during the messinian salinity crisis therefore the desiccation theory needs to be reexamined

mineralogy of salt efflorescences in paddy field soils of kollo southern niger abstract this study examines the mineralogy of salt crusts developed on the irrigated paddy soils in the soudano sahelian region of niger it involved field observations supplemented by laboratory determinations powder xrd sem edax ir spectral analyses the salt crusts were found in the concave zones near to the dike along the niger river they consisted of white mamillated konyaite crystals supported by a continuous hexahydrite crust upslope konyaite disappeared progressively and the discontinuous crust was constituted by hexahydrite only at a few centimeters below the soil surface epsomite crystallized in the cracks and voids in the subsoil gypsum and calcite were deposited the salt minerals were distributed vertically and laterally in the soil horizons according to the chemical composition of shallow groundwater in particular the namg ion ratio the water table depth and the extent of surface runoff human activity particularly superfluous irrigation constitutes the most pertinent explanation of the salinization processes at kollo

pleistocene aeolianites at cape spencer south australia record of a vanished inner neritic cool water carbonate factory aeolianites are integral components of many modern and ancient carbonate depositional systems southern australia contains some of the most impressive and extensive late cenozoic aeolianites in the modern world pleistocene aeolianites on yorke peninsula are sculpted into imposing seacliffs up to 60 m high and comprise two distinct imposing complexes of the late pleistocene bridgewater formation the lower aeolianite complex which forms the bulk of the cliffs is a series of stacked palaeodunes and intervening palaeosols the diagenetic low mg calcite sediment particles are mostly bivalves echinoids bryozoans and small benthic foraminifera this association is similar to sediments forming offshore today on the adjacent shelf in a warm temperate ocean by contrast the upper aeolianite complex is a series of mineralogically metastable biofragmental carbonates in a succession of stacked lenticular palaeodunes with impressive interbedded calcretes and palaeosols bivalves geniculate coralline algae and benthic foraminifera together with sparse peloids and ooids dominate sediment grains fragments of large benthic foraminifera including marginopora vertebralis a photosymbiont bearing protist are particularly conspicuous palaeocean temperatures are interpreted as having been sub tropical somewhat warmer than offshore carbonate factories in the region today the older aeolianite complex is tentatively correlated with marine isotope stage 11 whereas the upper complex is equivalent to marine isotope stage 5e marine isotope stage 5e deposits exposed elsewhere in southern australia glanville formation are distinctive with a subtropical biota including marginopora vertebralis thus in this example palaeodune sediment faithfully records the nature of the adjacent inner neritic carbonate factory by inference aeolianites are potential repositories of information about the nature of long vanished marine systems that have been removed due to erosion tectonic obliteration or are inaccessible in the subsurface such information includes not only the nature of marine environments themselves but also palaeoceanography

prokaryotic and eukaryotic microfossils from a proterozoic phanerozoic transition in china cherts of the doushantuo formation in the yangtze gorges south china contain a superbly preserved heterogeneous assemblage of bacteria cyanobacteria planktonic algae submillimetre sized burrows and problematica the microbiota has a bearing on defining the precambrian cambrian boundary baltisphaeridium and associated spinose microfossils in the chert suggest an early cambrian age whereas other acritarchs vendotaenids and pretrilobite small shelly fossils from the conformably overlying the dengying formation indicate a late proterozoic age for the doushantuo this mixed benthic planktonic assemblage preserved in silicified laminated microbial mat like fabrics provides a unique sample of diverse microbiotic elements existing at the dawn of the phanerozoic and records the earliest evidence of meiofauna

lake greenly eyre peninsula south australia sedimentology palaeoclimatic and palaeohydrologic cycles abstract lake greenly is a continental playa located in a mid latitude region 34 30 s 33 m above mean sea level on eyre peninsula south australia the uppermost 3 m of central lake greenly sediments contain two complete cycles and one half cycle of carbonate evaporite deposition the sequence is represented by alternating units of protodolomite and fine grained gypsarenite capped by 30 cm thick surficial low magnesian calcite mud which grades laterally into skeletal peloidal sand and beachrock near the lake margin the favoured depositional model suggests that calcite was deposited at the sediment water interface during freshening episodes marked by higher lake levels and lower salinities at the same time individual flooding events deposited skeletal peloidal sand as thin interbeds which thicken and coarsen laterally towards the lake margin the dolomite was probably deposited during periods of low levels and high brine concentrations caused by increased aridity and evaporation and represents a dolomitization front during this stage water was drawn up by evaporative pumping with subsequent precipitation of gypsum and halite at the sediment air interface although gypsum precipitation did not control dolomitization it probably assisted the process by raising the mg ca ratio and reducing the number of competing ions until formation of protodolomite by early diagenetic replacement of calcite took place

trace fossils from the precambrian cambrian boundary candidate at meishucun jinning yunnan china the precambrian cambrian boundary candidate section at meishucun china has yielded trace fossils which are abundant at some horizons the earliest occur in unit 3 of the zhongyicun member approximately 8 m above the lower selected stratotype reference point for the boundary and include arenicolites sp asteriacites sp neonereites biserialis n uniserialis and sellaulichnus meishacunensis the next trace fossil bearing horizon is in unit 6 of the zhongyicun member where cochlichnus sp monomorphichnus sp neonereites biserialis and n uniserialis occur immediately above in unit 7 are cruziana sp didymaulichnus miettensis monomorphichnus sp and rusophycus sp in the badaowan member at the top of the section there are didymaulichnus sp and taphrhelminthopsis circularis in unit 9 arenicolites sp diplocraterion sp gordia molassica skolithos sp and t circularis in unit 11 and gordia meandria plagiogmus sp skolithos sp and t circularis in unit 12 comparison of this trace fossil distribution with that in key precambrian cambrian boundary sections in other countries indicates that the ranges of a few trace fossils cross the boundary e g didymaulichnus neonereites planolites but most appear only in the cambrian different ichnogenera seem to appear at various levels above the boundary arenicolites and asteriacites are among the first while taphrhelminthopsis circularis is only encountered higher in all sequences some have only been recorded at much higher levels and relatively close to the first appearance of trilobites e g cruziana diplocraterion rusophycus this suggests that the first appearance of specific trace fossils or groups of trace fossils may be valuable for locating the boundary in some sections and for correlating late precambrian and early cambrian strata

erosion by submerged circular jets an experimental study on the erosion of sand beds by impinging submerged circular water jets shows similarities between air sand system and water sand system it is shown that there are significant differences between the two systems regarding the depth and radial extent of the scour hole

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

local scour due to submerged horizontal jet the phenomena of local scour and sediment transport due to a horizontal jet issuing from a sluice and flowing over a rigid apron then on to an erodible bed have been investigated based on experimental data the scour characteristics have been correlated through the development of empirical expressions for the time to reach equilibrium stage the locations of maximum scour depth and peak of dune and the variation of maximum scour depth with time similarity criteria for the predication of development of the scour hole and sediment transport have also been developed and apart from the flow parameters and sediment properties the jet diffusion characteristics play an important role in these processes the empirical relationships developed in this study highlight the necessity of an individual assessment when developing the similarity criteria for a particular flow situation

hydraulics of submerged jet subject to change in cohesive bed geometry the results of an experimental investigation of the time variation of scour hole and the flow characteristics of the quasi equilibrium state of scour of a cohesive bed downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented experiments were carried out with natural cohesive sediment for various sluice openings jet velocities and lengths of apron attempts are made to explain the similarity existing either in the process of scour or in the scour profiles that the scour holes follow downstream of an apron the scour profiles at different times follow a particular geometrical similarity and can be expressed by a polynomial using relevant parameters the characteristic parameters affecting the time variation of scour depth are identified based on the physical reasoning and dimensional analysis an equation for time variation of maximum scour depth is obtained empirically the diffusion characteristics of the submerged jet growth of boundary layer thickness velocity distribution within the boundary layer and shear stress at the quasi equilibrium state of scour are also investigated the expression of shear stress is obtained from the solution of the von karman momentum integral equation

cyclonic brine flow pattern recorded by oriented gypsum crystals in the badenian evaporite basin of the northern carpathian foredeep abstract in the middle miocene badenian within sulfate evaporites formed in the northern carpathian foredeep basin of poland ukraine and the czech republic the apices of the primary bottom grown gypsum crystals selenites are similarly aligned over broad areas and have a common azimuth this crystal orientation is interpreted as the product of a consistent direction of a brine inflow during gypsum crystallization and such a unique commonality is used as a tool for paleocurrent analysis in order to recognize both the local and the regional brine paleocurrents in the basin earlier sedimentologic and stratigraphic studies proved that these beds of oriented selenites were deposited at the same or nearly the same time in a brine not more than a few meters deep it is interpreted that the oriented selenites recorded only the mean flow within the basin mainly from the periods of mixing in the dry season lowstands when the whole brine column down to the bottom was ca sulfate oversaturated the measured paleocurrent vectors showed that the brines flowed along the marginal zone of the carpathian foredeep basin predominantly in a counterclockwise direction this longshore flow is interpreted as a typical cyclonic circulation analogous to those present in recent lakes and semi closed basins in the northern hemisphere where counterclockwise mean water drift is directly or indirectly forced by earth rotation the coriolis effect the detected brine flow directions do not reflect downslope gravity flow of dense brines into the central basinal depths or subbasins that were filling with the halite facies

scour downstream of an apron due to submerged horizontal jets the results of an experimental investigation on scour of noncohesive sediment beds uniform and nonuniform sediments downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented attempts are made to explain the similarity existing in the scour process and profiles including dune in the downstream of the scour hole the scour profiles at different times follow a particular geometrical similarity and can be expressed by the combination of two polynomials using experimental scour depth at different times the time variation of scour depth is scaled by an exponential law where time scale increases linearly with densimetric froude number the equilibrium scour depth related to the sediment size relative to the sluice opening decreases with increase in sediment size and sluice opening on the other hand the equilibrium scour depth increases with increase in densimetric froude number the variation of equilibrium scour depth with tailwater depth indicates a critical tailwater depth corresponding to a minimum equilibrium scour depth the effect of sediment gradation on scour depth is pronounced for nonuniform sediments which reduce scour depth significantly due to formation of an armor layer and therefore prompted study of the reduction of scour depth by a launching apron placed downstream of the rigid apron the results show that the average reduction of scour depth by placing a launching apron was 39 having a maximum of 57 3 and a minimum of 16 2 the characteristic parameters affecting maximum equilibrium scour depth are identified based on the physical reasoning and dimensional analysis equation of maximum equilibrium scour depth obtained empirically agrees well with the experimental data

precambrian cambrian boundary candidate aldan river yakutia u s s r the current situation in 1982 regarding the status and validity of the ulakhan sulugur section on the aldan river as a candidate for the global stratotype section and point for the precambrian cambrian pc c boundary is reviewed with details of new work since 1973 and consequent revisions of views on controversial topics on a regional continental and intercontinental scale of stratigraphic correlations with a short note regarding accessibility

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

influence of cohesion on the incipient motion condition of sediment mixtures 1 previous investigations on initiation of motion and erosion of cohesive sediment mixtures have not included gravel within the mixtures experimental results on the incipient motion characteristics of cohesive sediment mixtures containing gravel are presented in this paper the sediment used consisted of fine gravel mixed with clay in proportions varying from 10 to 50 by weight and fine gravel with fine sand mixed in equal proportion with clay proportions again varying from 10 to 50 by weight the stages of the incipient condition of motion were visually identified three modes of initiation of motion of cohesive sediment mixtures were noticed namely pothole erosion line erosion and mass erosion the modes of initiation of motion changed mainly with clay percentages in the mixture its antecedent moisture characteristics and the applied shear stress analysis of the experimental data from the present study and the literature revealed that depending upon the flow and sediment characteristics the critical shear stress of the incipient motion condition in cohesive sediment mixtures can be up to 50 times larger than the critical shear stress of cohesionless sediments having similar arithmetic mean size as the cohesive sediments on the basis of dimensional considerations a relationship is proposed for the determination of critical shear stress of the incipient motion condition in cohesive sediment mixtures also containing gravel the variables namely clay percentage void ratio and unconfined compressive strength of the sediment bed are noticed to be the main parameters controlling the incipient motion condition of the cohesive sediments in the absence of cohesion the proposed relationship reduces to the critical shear stress for the incipient motion condition of the cohesionless sediment mixture having the same arithmetic mean size the condition for the threshold of erosion of cohesive sediment mixtures by the flow is thus parameterized in the present study

diagenetic history and hydrocarbon potential of upper permian carbonate buildups wegener halvo area jameson land basin east greenland 1 the upper permian of jameson land includes two carbonate sequences the karstryggen and wegener halvo formations the karstryggen formation contains hypersaline carbonates and localized evaporites that were heavily weathered and dissected prior to deposition of the overlying strata the overlying wegener halvo formation represents an abrupt and extensive marine inundation over the underlying karstified karstryggen surface bryozoan brachiopod algal cement buildups of the wegener halvo formation are localized on karstic highs and show up to 150 m of depositional relief in situ mound core deposits are flanked by allodapic limestones which pass laterally into intermound calcareous shales the diagenetic histories of the core and flank facies are very different core facies porosity was intially obliterated by marine cements but repeated meteoric exposure altered unstable core facies constituents this alteration produced extensive secondary porosity through grain and cement leaching with local collapse brecciation flank strata however underwent little sea floor diagenesis and low permeability and mineralogically stable grain composition protected these strata from meteoric alteration early cementation and stabilization of core strata led to minimal burial diagenetic porosity loss uncemented flank beds however underwent profound mechanical and chemical compaction during this stage thus at the time of hydrocarbon generation distal flank beds had less than 2 porosity coarse upper flank beds had 5 10 remnant primary porosity and core facies deposits had 8 12 secondary pore space hydrocarbons generated from surrounding marine shales charged many of the bioherms with oil subsequent fracturing and hydrothermal fluid flow however flushed hydrocarbons and filled pores with ferroan calcite barite fluorite galena and baroque dolomite this heating and flushing is thought to have been especially intense in the wegener halvo region thus more basinal areas may still have reservoirs containing significant oil in equivalent upper permian limestones if as is likely the sea level changes affecting the greenland permian were eustatic then this study may provide significant clues to porosity development throughout the largely unexplored northern zechstein basin and the arctic basin of the barent sea this study also provides some important connections to the probably time equivalent guadalupian carbonate reservoir rocks of west texas new mexico and wyoming

unusual twinning features in large primary gypsum crystals formed in salt lake conditions middle miocene madrid basin spain palaeoenvironmental implications reply abstract a gypsum formation in the middle miocene saline lake deposits of the madrid basin is composed of large twinned crystals displaying an unusual habit characterised by the twin re entrant angles systematically opening downward such a habit is an exception to the commonly accepted mottura s rule for the growth of large twinned crystals which have been observed in marine derived evaporitic settings the crystals twinned along 100 are composed of elongated sub crystals formed by the intersection of the pinacoid 010 and the hemipyramid 111 field and petrographic evidence indicate a primary origin by subaqueous growth on the bottom ruling out a secondary origin brine composition dominated by ca na so 4 cl can be one of the several factors potentially controlling the selective growth of this habit further observations could indicate that such inverted twinned crystals may be considered diagnostic of gypsum growth in continental settings as such a pattern has not been recognised in marine evaporite formations

gypsum ground a new occurrence of gypsum sediment in playas of central australia abstract there are many playas dry salt lakes in arid central australia regional rainfall about 250 mm y and pan evaporation around 3000 mm y highly soluble salts such as halite only appear as a thin several centimetres thick white ephemeral efflorescent crust on the dry surface gypsum is the major evaporite precipitating both at present and preserved in sediment sequences one type of gypsum deposit forms a distinctive surface feature which is here termed gypsum ground it consists of a thick up to 80 cm gypsum zone which rises from the surrounding smooth white playa surface and is overlain by a heaved brown crust the gypsum zone with an average gypsum content above 60 consists of pure gypsum sublayers and interlayered clastic bands of sandy clay the gypsum crystals are highly corroded especially in the direction parallel to the c axis and on the upper sides where illuviated clay has accumulated in corrosion hollows overgrowth parallel to the a and b axes is very common forming highly discoidal habits these secondary changes corrosion and overgrowth are well developed in the vadose zone and absent from crystals below the long term watertable depth around 40 cm these crystal characteristics indicate a rainwater leaching process at lake amadeus one of the largest playas 800 km2 of central australia such gypsum ground occupies 16 of the total area the gypsum ground is interpreted as an alteration of a pre existing gypsum deposit which probably extended across the whole playa before breaking down leaving a playa marginal terrace and several terrace islands within the gypsum ground this pre existing gypsum deposit preserved in the residual islands consists of pure pale sand sized lenticular crystals it is believed to have been deposited during an episode of high regional watertable causing active groundwater seepage and more frequent surface brine in the playa a later fall in watertable probably resulting from climatic change caused the degradation of the gypsum deposit by dissolution and leaching processes the common distribution of the gypsum ground and marginal terraces in the playas of central australia demonstrates the extent of this hydrologic and climatic history

skeletal microfauna of meishucunian and qiongzhusian precambrian cambrian boundary age from the ganga valley lesser himalaya india the earliest skeletal microfauna of precambrian cambrian age recovered from the lower tal sequence chert phosphorite to calcareous members of the tal formation exposed in the ganga valley lesser himalaya uttar pradesh india has been grouped into three assemblages in ascending order these are assemblage i containing anabarites trisulcatus missarzhevsky tiksitheca korobovi miss circotheca sp turcutheca sp spirellus columnorus jiang and olivooides alveus qian assemblage ii yielding allonia erromenosa jiang a sp cf a erromenosa jiang dimidia simpleca jiang d sp cf d simpleca jiang and hyolithellus sp and assemblage iii comprising pelagiella lorenzi kobayashi auriculatespira madianensis zhou xiao and a andunca he pei the assemblages i and ii are correlatable to the meishucunian zone i and zone iii respectively and the assemblage iii to the qiongzhusian stage of china thus the lower tal sequence is assigned to precambrian cambrian age

foraminiferal record of marine transgression during deposition of the middle miocene badenian evaporites in central paratethys borkow section polish carpathian foredeep benthic and planktonic foraminifera from a marly clay intercalation sandwiched between mid badenian middle miocene gypsum deposited in an environment of an evaporitic shoal 1 m deep at borkow southern poland indicate a major marine flooding event in the previously isolated carpathian foredeep basin central paratethys after this very short term environmental change benthic foraminifers started to colonize a new niche which was previously defaunated and the pattern of benthic foraminiferal colonization is similar to that related to the reflooding which terminated the badenian evaporite deposition the benthic foraminifer assemblages are composed of pioneer opportunistic r selected species dominated by elphidiids the connection of the carpathian foredeep basin with the marine reservoir was short lived the marly clay intercalations in evaporite sequences originating in bared basins can thus register major environmental changes

carbon isotope stratigraphy of the precambrian cambrian sukharikha river section northwestern siberian platform a high resolution carbon isotope profile through the uppermost neoproterozoic lower cambrian part of the sukharikha section at the northwestern margin of the siberian platform shows prominent secul

climatic and geomorphic controls on rift sedimentation oligo miocene syn rift facies in the gulf of aden yemen existing models for sedimentation in extensional basins provide a basis for the analysis of their facies distribution and stratigraphy but do not include all the variables which control rift basin sedimentation tectonic controls on basin geometry and drainage systems are well established as are the effects of relative base level change but climatic controls have been considered only for continental basins and the ways in which the relief and geology of the hinterland influence sedimentation have received little attention a study of oligo miocene sediments in the gulf of aden illustrates the need to take into account the geology and geomorphology of the rift flanks when considering the supply of detritus to the rift basin it is also shown that climate is an important factor in controlling the facies distributions in marginal marine environments in rift settings rifting in the gulf of aden commenced in the oligocene and continued until a passive margin developed in the late miocene syn rift deposits exposed in a coastal strip east of mukalla in eastern yemen can be divided into two sequences separated by an angular unconformity sequence 1 occurs unconformably on pre rift limestones and includes localised alluvial fan deposits algal limestones and gypsiferous sediments formed in evaporitic mudflats sequence 2 is more widespread and up to 1500 m thick it comprises sandy to pebbly braided river deposits lagoonal marls localised patch reef limestones and extensive coastal sabkha deposits towards the end of sequence 2 lateral facies variations were controlled by individual fault blocks which compartmentalised sedimentation the predominance of fine grained coastal plain and shallow marine facies suggests that there was little coarse clastic supply from the rift flank this was partly a consequence of limited uplift of the rift shoulder and also the hinterland bedrock which being largely palaeogene carbonate rocks supplied a low proportion of coarse bedload to the basin a semi arid to arid palaeoclimate resulted in widespread syn rift coastal evaporate deposits this case study from the gulf of aden shows that a comprehensive framework for the analysis of sedimentation in extensional basins should include a consideration of the palaeoclimate hinterland bedrock and rift flank relief as well as tectonic and base level controls