association of redeposited salt breccias and potash evaporites in the lower miocene of stebnyk carpathian foredeep west ukraine abstract potash evaporites of the lower miocene vorotyshcha suite are sandwiched in salt breccias in the ukrainian part of the carpathian foreland basin these breccias have traditionally been interpreted as tectonically deformed units related to the overthrust nappes of the carpathians most clasts consist of intrabasinal sandstone marlstone halite and anhydrite but exotic clasts consisting of rocks that are characteristic of the carpathian flysch rocks are also present breccia beds are intercalated with salt layers containing chevron crystals and cubic hopper crystals and some layers are composed of detrital salt these salt layers rarely display graded bedding or cross lamination and continue over great distances there are also slump structures associated with the bedded salts the salinity in the vorotyshcha basin was extremely high but intermittent inflows caused brine dilution so that it could maintain halite precipitation for a long time during long periods of evaporative concentration and reduced inflow brines in the basin could reach the sylvite saturation level so that potash chlorides and sulfates started to precipitate the potash precipitated subaqueously on the basis of evidence from the study of fluid inclusions the depth of the brine is interpreted to have been 20 30 m in agreement with the supposed brine stratification in a basin subjected to influxes of sediment laden waters the breccias are debris flow deposits they are commonly tectonically deformed but analysis of the relation of breccias to evaporites suggests that both were formed in water ten to a few tens of meters deep it therefore seems that halite breccias in other foreland basins could have had a sedimentary origin

lower proterozoic red beds evaporites and secondary sedimentary uranium deposits from the east arm great slave lake canada the east arm graben contains a thick sequence of well preserved unmetamorphosed sedimentary rocks three units have been recognized the wilson island group c 2400 2300 ma the great slave supergroup c 2300 1790 ma and the et then group c 1700 1600 ma the wilson island group contains red beds and overlies an apparently normal regolith formed on archaean granites the great slave supergroup contains a normal regolith numerous red beds glauconitic horizions biogenic hydrocarbons and sedimentary uranium deposits there is also abundant evidence for the former presence of evaporites in that part of the sequence that was deposited within 30 of the palaeoequator the uranium deposits originated during low temperature diagenesis by precipitation from oxidizing waters in areas of early accumulated hydrocarbons these observations suggest that the lower proterozoic climate atmosphere and hydrosphere were similar to modern ones and that uranium prospectors in such old terrains should look for chemical deposits as well as the classic placer types

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

origin of the infracambrian salt basins of the middle east abstract during the infra and lower cambrain the arabian gulf and zagros mountains hormuz formation south oman ara formation kerman in central iran ravar formation and the salt range province in pakistan salt range formation were all sites of salt precipitation in subsiding rift basins along the middle eastern edge of gondwanaland also from 600 to 540 ma a rift system extended westwards from the sinai peninsula and north egypt across ne africa while a second rift system about 600 to perhaps 510 ma extended northwards from the sinai peninsula across the jordan valley and dead sea into se turkey the development of these rifts was contemporaneous with the left lateral najd strike slip fault system which dislocated the arabian peninsula by 300 km the najd system may be interpreted as a transform fault system which accommodated rifting between ne egypt the jordan valley north pakistan kerman and south oman the najd system is parallel to the zagros line and the latter is also the eastern depositional limit of the infracambrian hormuz formation the infracambrian zagros line may be interpreted as a right lateral strike slip fault system which caused infracambrian rifting in the arabian gulf and zagros mountains palaeontological and radiometric age constraints together with stratigraphic considerations suggest that the syn rift evaporites were first precipitated during a transgression which commenced in the latest precambrian 600 ma and again during a lower cambrian regression the regression terminated a period dominated by marine carbonates and evaporites and initiated the deposition of progradational alluvial to marginal marine sediments finally in the late lower cambrian and middle cambrian transgressive marine deposits swept across the middle east

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

evaporitic subtidal stromatolites produced by in situ precipitation textures facies associations and temporal significance abstract the transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters to isopachous and continuously laminated on a scale of a centimeter to a few meters the isopachous lamination texture may be composed of either 1 micritic or radial fibrous calcite or 2 dolomite isopachous stromatolitic laminae are remarkably uniform varying little in thickness over a given lateral distance in contrast to stromatolites formed of peloidal laminae which show marked variation in thickness over an equivalent lateral distance these isopachous textures are uncommon on most open marine carbonate platforms and apparently developed in transitional carbonate to evaporite settings because of increasing temperature salinity and anoxia related to water stratification which would have created ecologic restriction and an opportunity for stromatolite growth stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea floor encrusting calcite and possibly dolomite whereas the stromatolites composed of peloidal discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats while the presence of microbes in almost all near surface environments nullifies use of the term abiotic to describe most precipitated minerals we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats these transitional stromatolite facies are best developed atop proterozoic and paleozoic carbonate platforms that underlie major evaporite successions however inspection of jurassic and younger evaporite basins such as the messinian of the mediterranean region shows that stromatolites with thin isopachous lamination and radial fibrous textures though present are rare instead these facies may have been replaced by stromatolites with peloidal clastic textures and by low diversity diatomaceous and coccolith mudstones accumulation of the mudstones would have imposed two important effects 1 production of coccoliths would have helped extract calcium carbonate from seawater thus lowering the growth potential for precipitation of sea floor encrusting stromatolites 2 settling of both coccoliths and diatoms would have created a sediment flux to the sea floor which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals

evolution of an early proterozoic foreland basin carbonate platform lower pethei group great slave lake north west canada the taltheilei utsingi mclean and blanchet formations form a 175 390 m thick carbonate platform to basin succession in the lower part of the paleoproterozoic pethei group preserved in the eastern arm of great slave lake carbonates accumulated along the south east margin of the slave craton within a foredeep formed during the collision of the slave and churchill cratons the rocks include eight predominantly microbial carbonate facies that comprise five facies associations representing 1 shallow water rimmed shelf 2 shallow water open shelf 3 shallow water ramp 4 upper slope and deep ramp and 5 lower slope and basin plain environments microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud these wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower pethei sea floor from ramp to open shelf to shallow rimmed shelf with associated slope and basin plain deposition repeated relative sea level changes influenced platform growth this resulted in five shallowing upward packages each separated by an incipient drowning event of varying magnitude antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval this repeated pattern is attributed to interaction between a the inherent tendency of microbial carbonates to aggrade vertically b changing sedimentation rates and c readjustments of relative base level the lower pethei succession is one of few paleoproterozoic examples of carbonate platform growth within a foreland basin it has 1 a low gradient profile 2 extensive slope and basin plain carbonate production and sedimentation 3 no ooids 4 minor terrigenous clastic sediments and 4 a mobile submergent shelf rim lacking substantial carbonate sand shoals

performance of embankments on liquefiable soils improved with dense granular columns observations from case histories and centrifuge experiments abstractdense granular columns dgcs are generally known to mitigate the liquefaction hazard through a combination of 1 installation induced ground densification 2 enhanced drainage and 3

peritidal carbonate platform growth and cyclicity in an early proterozoic foreland basin upper pethei group northwest canada abstract the upper pethei group is a 170 235 m thick largely limestone succession located in the east arm of great slave lake northwest canada strata accumulated in a paleoproterozoic foreland basin during the late stages of marine deposition rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages 1 marginal peritidal complex 2 back barrier lagoon 3 shelf margin 4 subtidal shelf and 5 slope and basin carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae water column precipitation of carbonate mud and generation and transport of nonskeletal grains stromaclasts ooids oncoids peloids and intraclasts the upper pethei platform is interpreted to have evolved through three basic platform geometries peritidal rimmed shelf subtidal rimmed shelf and subtidal open shelf in response to changes in the rate of creation of accommodation space the peritidal complex and rim behaved in a manner consistent with models of phanerozoic reefal rims and alternatively kept up gave up or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation three scales of cycle are recognized 1 overall formation scale upward deepening of the platform 2 laterally traceable decameter scale shallowing upward cycles separated by marine flooding surfaces and 3 laterally discontinuous meter scale shal owing upward peritidal cycles the first two cycle types are interpreted to have been allogenic controlled by changes in eustasy and subsidence rates meter scale cycles are interpreted as mainly autogenic having formed in an aggradational tidal island model the upper petrel platform is similar to other paleoproterozoic foreland basin carbonate platforms important differences however include 1 laterally discontinuous peritidal cycles 2 lack of widespread sand shoals inboard of the shelf rim 3 a thick predominantly carbonate slope and basin succession 4 moderate to low platformal micrite content 5 extensive lagoonal ooid sands and 6 low contents of terrigenous clastics

herringbone calcite petrography and environmental significance abstract herringbone calcite is a previously undescribed carbonate cement and sea floor precipitate that is common in archean carbonates but rare in proterozoic and phanerozoic rocks it is abundant in the 2520 ma campbellrand malmani platform south africa where field relationships such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones demonstrate that it precipitated from ambient marine water this interpretation is supported by depositional relationships in the 2 6 ga huntsman limestone of the bulawayo greenstone belt zimbabwe the 2 6 ga carawine dolomite australia the 1 90 ga rocknest formation and the 1 8 1 2 ga dismal lakes group canada the ordovician porterfi ld carbonate buildup virginia and various silurian carbonate buildups in the midcontinent united states each of these occurrences is associated with anaerobic depositional environments or organic rich sediments herringbone calcite consists of alternating light and dark crenulated bands each light dark pair is 0 5 1 0 mm thick microscopically each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement the bases of the crystals are optically unoriented but upwards in each crystal the optical c axis rotates until it is perpendicular to crystal elongation the tops of the elongate crystal are thus optically aligned and length slow the light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals whereas the dark bands correspond to the optically unoriented lower parts of the elongate crystals microspar crystals are also present in some dark ba ds a mg calcite precursor for herringbone calcite now preserved as low mg calcite or dolomite is supported by the presence of micro dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low mg calcite precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface since herringbone calcite is associated with anaerobic depositional environments the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water thus the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time

synsedimentary cements as paleoproterozoic platform building blocks pethei group northwestern canada peritidal and subtidal carbonates of the pethei group are a complex arrangement of fibrous and micritic synsedimentary precipitates carbonate grains and detrital micrite synsedimentary precipitates present largely as stromatolitic laminae constitute the bulk of the pethei platform providing about 75 of the total carbonate they occur as stromatolitic precipitates precipitates closely associated with stromatolitic laminae which constitute about 60 of the rock and encrusting precipitates precipitates outside stromatolites and between grains i e cement which constitute about 15 of the rock the most abundant precipitates are fibrous in the form of stromatolitic microdigitate stalks tufa and masses of radial fans as well as encrusting pore filling fans isopachous rinds and laminar crusts micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds the ratio of fibrous to micritic precipitates is 2 1 stromatolitic precipitates are arranged in three distinct microfabrics 1 ubiquitous spar micrite couplets of fibrous and clotted micrite laminae 2 deep water vertical fibrous masses that precipitated over micritic precipitate cores and 3 supratidal microdigitate crusts of branching fibrous stalks micritic cements are interpreted to be of biogenic origin having precipitated somehow in association with microorganisms fibrous precipitates are interpreted to be largely abiotic since stromatolitic and encrusting precipitates are petrographicaily similar they probably share similar origins there is little evidence of sediment trapping and binding as a mechanism for stromatolite formation the abundance of synsedimentary precipitates varies with platform geometry subtidal ramps subtidal rimmed shelves subtidal open shelves peritidal rimmed shelves while the basic building blocks of the pethei platform synsedimentary precipitates carbonate grains and detrital micrite are the same as those in phanerozoic platforms their distribution and relative proportions are different these differences reflect a proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation

the origin of evaporitive limestones an example from the messinian of sicily italy a good example of an extensive evaporative carbonate deposit which has altered little since formation is found in the messinian upper miocene evaporites of sicily in this study these messinian carbonates the calcare di base are examined through a combination of field observations together with petrologic mineralogic and stable isotope studies these evaporitic carbonates are found in massive nearly featureless beds both calcite and aragonite commonly composed of automorphic breccias and or weakly layered micrite additionally fine scale features are observed within the carbonates such as small algal stromatolites and algally controlled laminae petrologic study of thin sections reveals ubiquitous algal filaments and grumeleuse to pelletal micrites as the primary components this suggests by analogy to modern hypersaline environments that the original calcium carbonate precipitated as algally controlled aragonite in a hypersaline environment displacive and interbedded halite still present within the deposit in the subsurface in some areas has been leached in others leaving residual breccias cubic voids and halite pseudomorphs the carbonate has been partially calcitized during one or more phases of early diagenesis the diagenetic systems which affected these sediments were not wholly open the halite was removed in some areas early in the sediment s history but after partial lithification probably synsedimentary as indicated by breccias containing well preserved halite voids these breccias lie within a nonhalite bearing micritic matrix later alteration has removed more of the halite leaving cubic void spaces porous layers and halite pseudomorphs filled with calcite celestite and strontianite the presence in some areas of considerable amounts of unaltered aragonite and in other regions of secondary strontium minerals indicate that the systems which affected these sediments were not open ones based on the oxygen isotope values obtained from the unaltered aragonites it appears that the large volumes of calcium carbonate in this deposit were derived from a combination of continual influx of seawater together with a probable contribution from continental sources coming into a restricted hypersaline environment at the margins of the basin the isotopic composition of the altered carbonates suggests that meteoric waters participated in the diagenesis while secondary bacterial processes oxidation sulfate reduction and methanogenesis contributed carbonate ions depleted in carbon 12 to the final product

late miocene events in the mediterranean as recorded by carbonate evaporice relations the messinian stage in the mediterranean region is known mainly because of a major evaporitic event whose causes and global implications are still under discussion correlations between basinal evaporite bearing series and marginal carbonate platforms allow for improvement of the classical depostional models especially concerning the eustatic control of the environmental changes in the mediterranean these changes were controlled by the interplay of aperiodic and periodic factors 1 tectonics related to the relative motions of the african and eurasian plates and resultant linear increase in the basin restriction 2 sea level fluctuations that alternately aggravated the restriction or allowed marine replenishment and 3 climatic fluctuations that controlled the level of the mediterranean during stages of isolation the messinian sedimentary succession reflects a typical sequence of progressive isolation marine marls yields laminites yields transitional deposits yields evaporites yields lacustrine deposits the first stage of the so called salinity crisis was characterized by a mediterranean lowstand that resulted in erosion of former platforms and deposition of the lower evaporites during the next stage sea level rose tb3 3 cycle high enough to temporarily replenish the basin cyclic upper evaporites with development of stromatolite rich deposits finally the uppermost messinian freshwater to brackish lacustrine deposits reflect both themore cessation of sea water input maximum tectonic isolation low sea level and freshwater input related to increasing runoff less

the coral reef of nijar messinian uppermost miocene almeria province s e spain abstract the nijar reef is a thoroughly exposed and representative example of the messinian coral reefs outcropping along the periphery of the western mediterranean facies patterns in the nijar reef complex are presented in this paper as a generalized guide or model for the study of other messinian reefs messinian paleogeography of this area was characterized by an archipelago whose islands are present day sierras at the beginning of the messinian and before development of the reefs marine terrigenous sediments marginal terrigenous complex were deposited on an irregular erosion surface on the metamorphic basement the reef complex developed as a belt fringing the positive areas islands when terrigenous deposition decreased the reef complex consists of 1 reef core constructed by a framework of vertical branching colonies of porites coated by laminated micrite probably submarine cements which form pinnacles as well as thickets and 2 fore reef made of sloping beds of broken skeletons and debris the fore reef is subdivided into i reef talus slope ii proximal slope and iii distal slope the reef talus forms massive strata with dips of 20 30 and abundant pebble to boulder size slumped blocks of reef core and coral breccia the proximal slope shows dips of 10 20 of well bedded thick strata with internal parallel layering of granule to sand size skeletal calcarenites the distal slope is made of predominant bioturbated skeletal calcisiltites and fine grained calcarenites with ngles lower than 10 the distal slope grades into the basinal pelagic marls of the nijar plains the original reef morphology is well preserved in present day topography with relicts of pinnacles reef valleys and slopes messinian volcanic highlands located to seaward complicated the local paleogeography with atoll like features reef growth was concomitant with a gradual fall of sea level and occurred before the messinian gypsum was deposited on top of the basinal marls in what is now the plains area the reef complex is partially truncated by a major erosion surface and it is overlain by oolitic shoals and stromatolites of the so called terminal carbonate complex the terminal complex is interpreted as an upslope equivalent in age to the messinian upper evaporite and it implies a new cycle of sea level rise

proterozoic ministromatolites with radial fibrous fabric small digitate stromatolites with diameters in the range of 0 2 20 mm ministromatolites are common in early and middle proterozoic carbonate sequences and extend stratigraphically from the archaean to the holocene an occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial fibrous fabric and microscopically crinkled microcrenate lamination is described from the 1 9 gyr old belcher supergroup mcleary formation in southeastern hudson bay canada the structures which can be considered to be a variety of tufa are unusually well preserved because of early diagenetic silicification columnar types are referable to pseudogymnosolen asperia and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern canada where the fabric is normally preserved by a secondary mosaic of dolomite the textural evidence of angulate cross sections and rectilinear divergent patterns indicates that the radial fibrous fabric represents primary or very early diagenetic precipitation and that pseudogymnosolenids with mosaic dolomite originally also had radial fibrous structure the precipitation may have been within or on microbial mats

mixed carbonate siliciclastic sedimentation on a miocene fault block gulf of suez egypt abstract the miocene sediments at abu shaar red sea egypt include both carbonates and detrital silicates whose distribution is closely related to a pre miocene basement fault block as such they demonstrate the relationships between local distensional tectonics and sedimentation because of the vertical bathymetric relief of the basement block the carbonates include both a spectacular talus and massive platform facies in spite of the presence of scattered corals sedimentation is not truly reefal as deposits consist essentially of muddy bioclastic and silicate sands the most typical ecological elements are numerous spectacular stromatolites rigidity of the sedimentary mass was mainly induced by early marine cementation carbonate siliciclastic transitions on and around the abu shaar platform were determined mainly by basement morphology and related pre miocene tectonics

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

early triassic stromatolites as post mass extinction disaster forms aftermaths of mass extinctions have been thought to be characterized by relaxation of ecological constraints accompanied by increased prominence of opportunistic generalists such taxa termed disaster forms have been shown to increase dramatically in range and abundance after several mass extinction events the cambrian ordovician stromatolite decline in normal marine level bottom environments has been explained as a direct or indirect consequence of increases in ecological constraints such as greater levels of predation and or bioturbation of microbial communities caused by early paleozoic benthic invertebrate evolution and diversification thus one would predict that in post ordovician strata stromatolites might appear in normal marine level bottom environments as disaster forms in the aftermaths of mass extinction particularly devastating to the benthic biota such as during early triassic time mounded stromatolites are present in two beds up to 1 5 m thick of the lower triassic spathian virgin limestone member moenkopi formation in the southwestern spring mountains of nevada stromatolites from level bottom normal marine subtidal environments have also been described from other lower triassic strata in north america europe and asia these stromatolites unusual in level bottom normal marine settings may have developed locally during the long aftermath 4 5 m y of the permian triassic mass extinction because of partial relaxation of the ecological constraintsmore that typically restricted them from unstressed subtidal normal marine level bottom environments less

facies and geologic history of an exposed miocene rift margin carbonate platform gulf of suez egypt abu shaar is a small exhumed carbonate platform of lower to middle miocene age on a horst of precambrian crystalline rock along the western side of the gulf of suez the reef rimmed complex is entirely dolomite but with exquisitely preserved primary and diagenetic microfabrics similar platforms some of which contain hydrocarbons are buried beneath evaporites in the gulf the platform developed in three stages defined as local members of the rudeis formation the first stage records deposition during progressive marine onlap as aprons of mixed terrigenous carbonate sediments evolved into a wide reef rimmed platform synsedimentary platform margin collapse subsequently removed the most seaward portions of the reefs along the eastern gulf facing margin these truncation surfaces were then covered by a second phase of reef growth and deposition of fore reef carbonates the third and final stage deposited following a sea level drop and subaerial exposure is a series of well bedded peritidal carbonates that become progressively more evaporitic upward fore reef facies are again truncated by synsedimentary slope failure former evaporites are now recorded as collapse breccias contorted bedding black chert nodules and evaporite molds porosity is mainly secondary as vugs and molds and is highest in platform interior facies these more porous sediments contained more aragonitic components originally and were less affected by submarine cementation in contrast the higher degree of synsedimentary lithification of platform margin reef and fore reef sediments is comparable with what is commonly observed in modern reef dominated platform margins

messinian stromatolite thrombolite associations santa pola se spain an analogue for the palaeozoic stromatolite thrombolite associations are the dominant facies forming large portions of the santa pola carbonate platform se spain during deposition of the terminal carbonate complex tcc the tcc the last period of marine sedimentation in the western mediterranean associated with the messinian salinity crisis comprises a ne sw trending thrombolite reef with occasionally interlayered stromatolite horizons and a predominantply stromatolite and oolite facies in the back reef area the stromatolites are mainly dome shaped but fine columnar or wavy undulose forms can occur the stromatolites form huge bioherms extending tens to hundreds of metres they are finely laminated with alternating layers of dolomicrite and dolomicrospar the dolomicrite layers appear to be a primary dolomite precipitate whereas the dolomite crystals in the dolomicrospar layers apparently formed around a meta stable nuclei which was subsequently dissolved or degraded the low content of sand sized particles in the stromatolitic layers indicates formation under low energy conditions possibly on a tidal flat as reported from other areas in the western mediterranean deposition of the tcc at santa pola was apparently cyclic whereby stromatolites generally terminate each depositional cycle subtidal conophyton stromatolites possibly the only known occurrence younger than palaeozoic are however found on the reef slope at the base of the first tcc depositional cycle the dolomitic nature of the unadulterated stromatolitic laminations and the association of stromatolites and thrombolites as platform builders were a common feature in the early palaeozoic but are unusual in post ordovician carbonate facies we propose that the conditions during tcc deposition were very restricted possibly reflecting an environment similar to that of the early palaeozoic

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

petrogenesis of 1 9 ga limestones and dolostones and their record of paleoproterozoic environments abstract fibrous calcite cements from limestones of the paleoproterozoic pethei group in northwest canada have relatively constant 13 c values near 1 that indicate the c exogenic cycle of ca 1 9 ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the phanerozoic 87 sr 86 sr ratios of these cements are near 0 7051 and support a progressively increasing influx of radiogenic sr from continental weathering into the paleoproterozoic oceans 18 o values of the cements vary from 14 to 8 5 although the original values of the cements were probably higher because the cements have been subseqently altered as evidenced by their variable trace element contents proposed higher temperatures or substantially lower 18 o values of ancient seawater based on such values may be in error fabric retentive dolomites generally regarded as syndepositional also have variable 18 o values the highest of which are ca 30 greater than the highest values of the fibrous calcite cements the dolomites have more radiogenic sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones these fluids may have partially altered the 18 o values and chemical compositions of the fibrous calcite cements late stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar 18 o values and radiogenic sr isotopic ratios as the fabric retentive dolomites if these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions

impact melt rock clasts in the hvittis enstatite chondrite breccia implications for a genetic relationship between el chondrites and aubrites the hvittis enstatite chondrite breccia contains 5 2 vol dark clasts which appear to be impact melt rocks the largest clast clast a is 2 5 cm in diameter it has an igneous texture no chondrules and is depleted in metallic fe ni and sulfide its fine grain size low ni kamacite and schreibersite and rare martensite resulted from rapid cooling the abundance of melt rock clasts in hvittis suggests that the breccia resided at its parent body surface however the apparent absence of el chondrites with solar wind implanted rare gases suggests that the el parent body was too small to develop a substantial regolith the bulk composition of clast a is very similar to that of average aubrites except for a sevenfold enrichment in s cr and mn these elements are concentrated in the sulfides it thus is possible that the clast was derived from one of the sulfide rich regions of the hvittis matrix if so then aubrites may be genetically related to el chondrites

late precambrian cambrian sediments of huqf group sultanate of oman the huqf group is the oldest known sedimentary sequence overlying crystalline basement in the sultanate of oman it crops out on a broad regional high the huqf axis which forms a dominating structural element on the southeastern edge of the arabian peninsula subsurface and outcrop evidence within and outside of oman suggests that the sediments of the huqf group lie within the age span of late precambrian to early middle cambrian the huqf group is subdivided into five formations corresponding to an alternation of clastics abu mahara and shuram formations and carbonates khufai and buah formations deposited in essentially shallow marine to supratidal or fluviatile conditions and terminated by an evaporitic sequence ara formation evaporites are absent on the huqf axis but they are thickly developed to the west over a large part of southern and central oman where they acted as the major structure former of most of oman s oil fields and even locally pierced up to the surface in the subsurface the huqf group has been reached by the drill mainly in southern oman where its lithostratigraphy resembles that defined in the type area in central oman the oldest sediments exposed in the core of the oman mountains may be correlative with the huqf group regional correlations suggest that the predominantly carbonate evaporitic facies of the huqf group was widely distributed in late precambrian early cambrian time the huqf basin is tentatively considered part of a belt of evaporitic basins and intervening carbonate platforms which stretched across the pangea landmass from the indian subcontinent salt range of pakistan through south yemen oman and saudi arabia into the gulf states and iran hormuz series and carbonate platform north of the zagros

a summary of the geology and oil habitat of the eastern flank hydrocarbon province of south oman abstract the eastern flank hydrocarbon province borders the southeastern edge of the south oman salt basin in the southern part of oman an intensive exploration programme by pdo over the past seven years has led to the discovery of almost 2 10 9 m 3 of oil with current production of some 15000 m 3 d from six fields in stark contrast to other oil habitats of the arabian peninsula the main play involves rock units and geological events ranging in age from late precambrian to quaternary times while trap development and charging are closely related to syn tectonic salt dissolution the principal reservoirs and seals are poorly consolidated palaeozoic clastics which drape due to salt dissolution over residual cores principally composed of either lower palaeozoic sandstones haima group or late proterozoic carbonates huqf group or some combination of these oils have been geochemically correlated with algal source rocks of the huqf eastern flank structures are considered to have been charged from late mesozoic times onwards in conjunction with progressive subsurface salt removal and the onset of maturation of local source rocks the effectiveness of charging along the retreating salt edge is indicated by the small percentage of dry wells major oil zones are found in both anticlinal and truncation traps which are commonly developed around sandstone haima group cores oils show a wide range of characteristics but are typically heavy early expulsion crudes high sulphur with little associated gas bacterial transformation is not uncommon producing reservoirs mainly occur at relatively shallow depths 600 1200 m primary recovery factors of around 7 reflect the high density and viscosity of the eastern flank crudes and have initiated interest in enhanced oil recovery eor techniques two such pilot projects involving steam and polymer flooding are due to commence in 1984 and if successful will substantially increase the reserve base of these fields

radiaxial calcite alteration product of and petrographic proxy for magnesian calcite marine cement radiaxial calcite is a common and distinctive pore fill in ancient limestones but the origin of this fabric is highly controversial we present data from the late cretaceous equatorial pacific that suggest that radiaxial calcite forms through alteration of early magnesian calcite 7 mol mgco 3 marine cement these data suggest that radiaxial calcites are not straightforward indicators of pore fluid chemistry on the other hand the presence of radiaxial calcite in limestones may provide a distinctive petrographic proxy for earlier magnesian calcite cement and thus a generally warm caco 3 saturated marine environment

evidence for primary aragonite precipitation lower proterozoic 1 9 ga rocknest dolomite wopmay orogen northwest canada abundant precipitation of aragonite is suggested by well preserved fabrics in dolostones of the lower proterozoic 1 9 ga rocknest formation fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans and cement crusts in cryptalgal tufas aragonite was common on the 1 9 ga rocknest shelf as it is on modern tropical shelves a search for similar marine cement fabrics in other precambrian carbonates should help resolve whether aragonite was an important precipitate from precambrian tropical seas implying an ocean carbonate chemistry similar to today or if aragonite precipitation was restricted to rare times in the past calcite and even dolomite being the main carbonates precipitated as recently suggested by some authors

tectonic control on carbonate platform facies distribution and sequence development miocene gulf of suez abstract mid miocene deposition in the southwestern gulf of suez was governed by an extensional fault block topography shallow marine carbonates and clastics were deposited around the exposed block crests and in marginal half grabens of the rift while central troughs were filled by slope aprons and turbiditic and pelagic mudstones fan deltas developed at points along the fault block escarpments syn rift platform carbonates from one block crest esh el mellaha are described here the platform evolved in three stages generating three sedimentary sequences sequences 1 and 2 were separated by boundary fault reactivation sequences 2 and 3 by sea level drawdown exposure and karstification the sediments comprise six lithofacies associations la 1 6 reef la 1 fore reef la 2 and back reef embayment la 3 sediments form a small reef rimmed platform in sequence 1 at the southern end of the block fringing reefs and reef lagoon systems also developed along the footwall escarpment and on fan deltas which bridged the block bounding fault fringing reefs in sequence 2 generated a carbonate talus veneer la 4 along the whole 80 km length of the esh el mellaha fault scarp carbonate slope and reefal sediments preserved on the footwall exhibit abundant evidence of syndepositional fault movements with truncation of escarpment fringing sequences and repeated generation of reefal wedges minor drowning at the sequence 1 2 boundary caused deepening of lagoons progradation of leeward reefal margins with lagoonal patch reef development and back stepping and vertical accretion of the reefal rim platform development was followed by algal laminated dolomitic sediments la 5 in the area of the old platform interior sequence 3 much of the esh el mellaha ridge remained exposed during platform growth and shed coarse alluvial clastic sediments la 6 into the peripheral marine environments the relationships demonstrated between miocene sedimentation and structural setting have parallels in the modern environments of the zeit bay area