microbially induced cementation of carbonate sands are micritic meniscus cements good indicators of vadose diagenesis characteristic fabrics such as micrite envelopes calcified filaments and micritic grain to grain bridges are observed in a modern subtidal firmground wood cay bahamas and in a variety of firm and hardgrounds of lower cretaceous and upper jurassic platform carbonates swiss and french jura mountains their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands meniscus type cements to distinguish them from vadose meniscus cements which clearly formed in subtidal environments are related to filament calcification trapping of percolating micrite and microbially induced carbonate formation such meniscus type cements are commonly micritic but meniscus shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed therefore an interpretation of vadose early diagenesis should not be based on meniscus cements alone similarly subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure

a quantitative analysis of recent arid coastal sedimentary facies from the arabian gulf coastline of abu dhabi united arab emirates abstract this study presents the results of the first quantitative analysis of surface sediments along multiple transects of the arabian gulf shoreline of abu dhabi united arab emirates the natural coastline of this low angle carbonate ramp depositional system is being anthropogenically modified at an unprecedented rate as this system is much employed as a recent analogue for many ancient depositional sequences it is imperative that sedimentological characteristics of natural facies occurrences are accurately recorded for employment in future applications we establish distinct relationships between the geomorphic setting and sediment characteristics siliciclastic grains admixed to this system are naturally sourced from the pliocene ghayathi formation and from aeolian sediment influx during dust storms this natural source of siliciclastic detritus is being increasingly superseded by siliciclastic material derived from anthropogenic activities evaporite minerals are similarly being contributed from anthropogenic sources the occurrence of mud to silt grade sediments is primarily controlled by the presence of a sediment binding microbial mat that prevents the removal of fine material during high energy events similarly the presence of a halite crust in the upper tidal flats and supratidal zone prevents aeolian deflation this study did not establish any clear relationship between coastal geomorphic setting and the bioclastic components present in the sediments this finding has major implications for the application of bioclasts in fine tuning palaeoenvironmental interpretation of ancient carbonate ramp coastal systems

marine diagenesis of lower ordovician carbonate sediments dumugol formation korea cementation in a calcite sea abstract the lower ordovician dumugol formation exhibits many features that indicate early lithification such as calcite nodules hardgrounds mud mounds and intraclasts detailed observations of these early lithified features reveal that rapid marine cementation was instrumental in their formation marine lithification took place in a low energy subtidal environmental that was influenced by intermittent storms marine cements include syntaxial overgrowth bladed calcite fibrous calcite and fine crystalline equant calcite cements syntaxial overgrowths precipitated on echinoderm grains and contributed to rapid marine lithification of echinoderm bearing sediments bladed fibrous and fine crystalline equant calcite cements precipitated in locally suitable sites but their occurrence is limited and thus played a minor role in marine lithification microcrystalline calcites also precipitated in lime mud rich fine grained sediments and participated in rapid marine lithification of the dumugol sediments the absence of aragonite allochems and cement and the predominance of calcite cement suggest that the dumugol sea was undersaturated with respect to aragonite but supersaturated with respect to calcite which is indicative of a calcite sea

botryoidal high magnesium calcite marine cements from the upper cretaceous of the mediterranean region abstract in both recent and ancient limestones marine cements which have a botryoidal morphology are generally thought to have precipitated as aragonite however upper cretaceous botryoidal cements from widespread mediterranean localities described here are interpreted as former high magnesium calcite petrographic features indicating an originally high magnesium calcite mineralogy include rhombic terminations to precursor crystallites and microdolomite inclusions minor element concentrations in the botryoids are depleted in strontium relative to calcitized skeletal aragonite a former calcitic mineralogy for cretaceous botryoids is in accordance with proposals that ocean chemistry at this time inhibited inorganic aragonite precipitation

carbonate cements in holocene beachrock example from bahiret et biban southeastern tunisia abstract holocene beachrock has been studied on the northern and western shores of bahiret el biban a shallow restricted lagoon in southeastern tunisia the beachrock consists of horizontal cemented layers which are 2 15 cm thick early cementation took place at the water table below beach ridges where geochemical hydrodynamic and possibly also microbiological conditions favored rapid precipitation of aragonite and or high mg calcite locally superimposed low mg calcite cements point to subsequent freshwater influence at one site six vertically stacked beachrock beds are found total rock 14c dating shows that the highest beachrock bed is older than the one of the same petrographic composition situated at the present groundwater level this implies a downward progression of cementation which probably monitored the sea level fall following a local highstand about 3000 years ago

chemistry of modern sediments in a hypersaline lagoon north of jeddah red sea abstract previous studies of modern peritidal sedimentary environments of the red sea such as hypersaline lagoons and sea marginal flats have concentrated on its northern part particularly in the gulf of aqaba however little is known about lagoon sediments in other localities along the red sea coastal stretches this paper deals with the chemical characteristics of the sediments of a hypersaline ras hatiba lagoon north of jeddah saudi arabia the chemistry of hypersaline lagoon sediments is considerably changed following the modifications to the water chemistry by evaporation and precipitation ras hatiba lagoon is a hypersaline elongated water body connected to the red sea by a narrow and shallow opening the total area of the lagoon is c 30 km 2 coarse bioclastic sands are dominant in the lagoon and mostly surround lithified calcareous grounds however fine silt and clay sediments are present in separate patches the sediments are rich in carbonates average 78 5 and organic carbon average 7 3 although they are negatively correlated calcium average 25 1 and magnesium average 10 8 show a similar distribution pattern in the lagoon sediments strontium average 5 2 is positively correlated with calcium sodium and potassium are relatively highly concentrated in the sediments average 118 ppm and 173 ppm respectively magnesium and strontium are of prime importance in the process of mineralization and diagenesis the sabkha formation surrounding the lagoon is of low carbonate and organic carbon content compared with the lagoon sediments whilst it is characterized by high magnesium sodium and potassium concentrations ras hatiba lagoon sediments and sabkha resemble those of the northern red sea in the gulfs of aqaba and suez and the arabian gulf in their major sedimentological and chemical characteristics

micritized skeletal grains in northern belize lagoon a major source of mg calcite mud abstract lime mud in northern belize lagoon is composed predominantly of silt size cryptocrystalline grains and clay size clumps both of which consist of anhedral equant crystals less than 0 5 m in size this mud is 70 90 mg calcite containing 9 10 mole mgco3 detailed petrographic morphologic and mineralogic analyses suggest that the cryptocrystalline grains are micritized skeletal grains and that the mud formed by abrasion and bioerosion of these altered grains micritization of skeletal grains occurred primarily by recrystallization of original skeletal carbonate laths prisms and blades of mg calcite aragonite and calcite to equant crystals of micritic mg calcite with additional alteration by micritic infilling of microborings and skeletal cavities because of d fferences between crystal morphologies and mineralogies of original skeletal components and their micritized counterparts mud produced by diminution of micritized grains is distinct from that produced by skeletal breakdown the abundance of cryptocrystalline grains in the bahamas and other shallow carbonate environments suggests that micritized grains are potentially an important source of lime mud that has not previously been identified

cementation patterns and genetic implications of holocene beachrocks from northeastern brazil abstract this paper describes the complex patterns of diagenesis in holocene beachrock ledges located along both the eastern and northern coasts of the state of rio grande do norte northeastern brazil these beachrocks show highly variable diagenetic processes fabrics and textures carbonate cements are exclusively composed by high mg calcite and occur in seven main morphologies 1 cryptocrystalline coatings 2 isopachous prismatic rims 3 equant spar 4 cryptocrystalline or micritic pore filling 5 pseudo peloidal aggregates 6 radial aggregates 7 isolated and randomly oriented clumps of scalenohedral crystals other diagenetic features include a carbonate siliciclastic mixed sediment b infiltrated micrite c authigenic smectitic clay minerals d organic films e chalcedony pore filling cement f vadose silty internal sediment the texture and composition of the cements are indicative of precipitation in the active marine phreatic zone the presence of vadose features and planktonic foraminifers within the infiltrated materials suggest that they were introduced in the backshore sands by muddy waters dislocated from the shelf during storms a general succession of diagenetic phases can be recognized comprising the precipitation of authigenic smectite micritic coatings radial aggregates isopachous rims of prismatic crystals or equant spar cryptocrystalline and pseudo peloidal pore filling cements and vadose infiltration of micritic carbonate siliciclastic mixed sediments or silty materials the absence of organic structures such as microbial bacteria or fungi filaments and bodies within the cements suggests that the mechanism behind beachrock cementation is essentially inorganic most probably the evaporation of entrapped seawater in response to the dry climatic conditions

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

a dolomitized shelfedge hardground in the northern gulf of mexico abstract an elongated calcareous hardground occurs on the shelfedge northeast of the mississippi trough high resolution seismic reflection profiles show that the hardground is in 90 110 m of water is 70 250 m wide at least 2 3 km long and of low relief 5 10 m the hardground is composed of dolostone and coquinoid limestone and exhibits a dead biofacies of nodular and encrusting calcium carbonate secreting organisms that are significantly different from those forms living in the adjacent unconsolidated sediments cementation of the hardground substrate by primary dolomite was initiated during early diagenesis close to the sea floor and progressed by concretionary growth under an interstitial environment partially open to marine molecular fluxes the bicarbonate necessary to form the dolomite came from an admixture of isotopically light thermogenic and sedimentary organic carbon sources and an isotopically heavier marine source exposed surfaces of the dolostone which have been heavily bioeroded are covered by an oxidized goethite rich reddish brown rind the coquina is primarily composed of whole and fragmented mollusk shells and was deposited in a higher energy environment than the precursor sediments of the dolostone the hardground was preserved during the holocene transgression because of its lithified nature and the top has been intermittently swept free of finer detritus by current action

composite micrite envelopes in the lagoon of abu dhabi and their application for the recognition of ancient firm to hardgrounds abstract micrite envelopes are common early diagenetic features in marine carbonates throughout the phanerozoic this paper documents a continuum of sub recent constructive and destructive composite micrite envelopes and discusses their application for the recognition of ancient marine firm to hardgrounds in the modern subtropical carbonate depositional environment of the abu dhabi coastal lagoon micrite envelopes are actively forming and are directly assigned to a range of processes and depositional environments i in the outer high energy lagoon bioclasts and ooids typically contain empty microendolithic borings ii in the unconsolidated sediments of the inner to middle lagoon destructive micrite envelopes yield fragments of their host grains that result from boring etching activity and are bound and covered by biofilms iii constructive envelopes are formed by high mg calcite micrite cements and coat destructive envelopes in the inner to middle lagoon or may nucleate on aragonite needle cements in the outer lagoon micrite cements that form constructive envelopes are typical early marine diagenetic features and are among the main fabrics of firm to hardgrounds calcitic skeletal grains such as benthic foraminifera and coralline red algae are less affected by microbioeroders compared to aragonitic ooids gastropods and bivalves high mg calcite constructive envelopes have a higher preservation potential relative to aragonite host particles and aragonite cements hence the presence and recognition of constructive micrite envelopes in the rock record may be instrumental to identifying ancient aragonitic as well as unconventional firm to hardgrounds that may otherwise be difficult to recognise

discontinuities in carbonate successions identification interpretation and classification of some italian examples abstract the growing use of unconformities as bounding surfaces for new types of stratigraphic units seems not to be matched by a corresponding effort on process oriented researches on unconformities themselves this paper aims at giving a contribution to classification and understanding of discontinuities in carbonate successions from the outcrop perspective based on some italian examples a number of integrated criteria geometry sedimentology diagenesis biostratigraphy is proposed to recognise stratigraphic breaks old terms are discussed and sometimes redefined while some new terms are introduced the term discontinuity surface ds is proposed as a general one to indicate any stratigraphic interface where an interruption of sedimentation can be proved a ds can be characterized as a firm ground or a rock ground depending on its coherence at the moment of the renewal of sedimentation polygenic and simple omission surfaces are separated within firm grounds if a recognizable gap is either present or not a third kind of firm ground revealed only by biostratigraphy has been called hidden discontinuity surface two categories of rock grounds are also distinguished hard grounds and inherited rock grounds if respectively generation of the ds and or deposition of the overlying sediments took place in the same environment of the underlying sediments or not a genetic interpretation for each type of discontinuity is proposed the largest variety of ds s occurs in pelagic sediments some discontinuities hard grounds and simple omission surfaces are attributed to increased bottom current activity during sea level falls others polygenic omission surfaces and hidden ds s are interpreted as submarine slide scars either with or without the overprint due to exposure on the seafloor the ds s separating different facies e g platform and pelagic ones are interpreted to be due to tectonics the relevance of the recognized ds s to sequence stratigraphy is briefly discussed

facies specific calcitic and bimineralic ooids from middle and upper cambrian platform carbonates western newfoundland canada abstract oolites in the port au port group consist of intercalated gray oolite deposited on the shallow subtidal mobile fringe of ooid sand shoals and brown oolite which accumulated on intertidal sand flats along the shoal crests gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe the present fabric of the ooids is the result of protracted burial diagenesis cortices of finely and coarsely preserved radial ooids excluding radial concentric ones and superficial radial ooids were mg calcite originally they changed to calcite still retaining fine fabric and subsequently to a variety of cortical fabrics during shallow burial and altered to blocky fabrics during deep burial concentric ooids and radial concentric ooids were bimineralic originally composed of aragonite now micrite and blocky calcite and mg calcite now radial fibrous calcite primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial resulting voids were filled with blocky calcite during deep burial correlation of original ooid mineralogy fabric and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in cambrian time this correlation is absent in modern deposits suggesting that more widespread chemical and organic controls now prevail

biofilm related calcification sediment trapping and constructive micrite envelopes a criterion for the recognition of ancient grass bed environments micrite envelopes are a common feature in carbonate sediments and are typically associated with the micrite filling of borings produced by microendolithic organisms these are referred to as destructive micrite envelopes and have long been recognized as reflecting an important early diagenetic process recent analysis of sediments collected from back reef environments at discovery bay north jamaica however has demonstrated envelope formation on the surfaces of carbonate grains clearly distinct from the micrite filling of microborings such constructive envelopes occur almost exclusively in sediments from grass bed environments and are always intimately associated with biofilms comprising abundant mucilage cyanobacteria bacteria and diatoms it is suggested that these envelopes represent a product of both biologically mediated micrite precipitation occurring within the biofilm mucilage and around the biofilm components i e cyanobacteria and diatoms and associated trapping of carbonate mud and fine grained sediment their recognition only within grass bed sediments may enable their use as a diagnostic feature of grass bed environments or vegetation stabilized substrates in the rock record

microboring versus recrystallization further insight into the micritization process sem observations of lightly etched thin sections of bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity a coccoid cyanobacterium tentatively identified as solentia sp bores tunnels which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain these tunnels are filled by radial fibrous aragonite which is precipitated as the microorganism advances extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes the rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring this type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization

grain susceptibility to the effects of microboring implications for the preservation of skeletal carbonates the boring activity of microendolithic organisms such as cyanobacteria chlorophytes rhodophytes and fungi represents a major destructive process affecting sediment preservation within reef environments this study demonstrates the presence of two distinct assemblages of microborers within sediments collected from the fringing reefs of north jamaica correlating to the upper 18 m depth photic zones the upper photic zone assemblage is dominated by cyanobacteria and chlorophytes whilst rhodophytes and fungi become more abundant with increasing water depth most significant from a grain preservation perspective is the variable nature of grain infestation observed between different reef sites and different carbonate grain types the highest degree of grain infestation occurs within shallow low energy back reef environments and the most susceptible grains at all sites are corals molluscs and foraminifera coralline algae halimeda and echinoid fragments are rarely heavily infested high rates of infestation at back reef sites result in rapid diminution of the most susceptible grains especially coral which are either underrepresented or contribute only to the finer sediment fractions in the subsurface fore reef grain assemblages undergo relatively little alteration microboring therefore has potential to bias the fossil record by removing the most susceptible skeletal grains the impact of microboring upon back reef grain assemblages must be considered when attempting to model depositional processes within both modern and ancient reef environments

ooids in purbeck limestones lowermost cretaceous of the swiss and french jura ooids occurring in the shallow water purbeckian carbonate sediments of the jura mountains can be grouped into six types gradations from one type to another and coexistence of the various types are common type 1 ooids are small and well rounded they display fine concentric micritic laminae in many cases their cortices are dissolved and replaced by void filling spar microsparitic neomorphic replacement occurs locally type 2 ooids are large and have irregular shapes they show fine micritic laminae and occasional layers of fine radial crystals they commonly evolve into oncoids ooids of type 3 display many fine radial cortical laminae and are patchily micritized they are medium in size and mostly well rounded this type of ooid may pass into large irregularly shaped coated grains type 4 ooids have 1 to 4 cortical laminae with a fine radial structure and patchy micritization they are medium in size and well rounded type 5 ooids have only one lamina with a coarse radial structure they are small and well rounded associated are spherical grains containing bundles of elongate crystals ooids of type 6 show superpositions of two or more different radial and or fine micritic laminae the cortical structure may also change laterally in the same lamina the preferential dissolution of type 1 ooid cortices to form oomoulds indicates a primary composition of unstable carbonate sedimentological features and comparison with modern ooid occurrences point to formation on high energy sandbars in normal marine waters type 2 ooids grew in marine lagoonal environments with quiet water and abundant cyanobacteria the radially structured ooid cortices of types 3 4 and 5 show no dissolution features this implies that they were originally composed of stable carbonates or that an unstable carbonate phase was transformed into a stable one at an early stage of diagenesis type 3 ooids occur together with marine faunas and indicate high water energy ooids of type 4 and type 5 originated probably from relatively quiet water of variable salinity coexistence of different ooid types and mixed forms of type 6 implies gradual or rapid changes in hydrodynamic geochemical and microbiological conditions which were a feature of the purbeckian depositional environments

palaeoclimate and the formation of sapropel s1 inferences from late quaternary lacustrine and marine sequences in the central mediterranean region abstract synchronous responses to climatic changes during the late pleistocene holocene transition are inferred from marine and lacustrine stratigraphic records in the central mediterranean region new stratigraphic data are presented from well dated sequences in the meso adriatic depression mad two crater lakes in the lazio region and in the tyrrhenian sea the sequences all span the last termination and the holocene but we focus here on the evidence in each record for the time period during which sapropel s1 formed in the mediterranean ca 9 0 to 6 8 cal kyr b p the new records provide evidence of palaeoenvironmental changes on land and sea that can be reconstructed at a high temporal resolution and which throw some light on the processes which led to the formation of s1 the collective evidence indicates that i organic rich sediments occurred in both the marine and the crater lake sites during the time of formation of the s1 sapropel ii there is evidence of increased stratification and anoxia in the sea water column during the period of s1 formation iii the s1 period in the study area is divisible into two sub phases s1a and s1b which reflect short term variations in oceanographic conditions stratification and anoxia iv changes in stratification in the marine column were contemporaneous with regional climate variations that are inferred from the terrestrial records we conclude that the key factor that initiated the formation of s1 was increased discharge of freshwater into the mediterranean following a change post 9 0 cal kyr b p to a warmer and wetter climate furthermore the period of s1 formation was interrupted by a short lived episode ca 500 years of comparatively cooler and drier conditions during the early middle holocene transition emht

submarine limestones in the nearshore environment off kuwait northern arabian gulf even with careful petrographic and mineralogic characterization of marine limestones intertidally and subtidally lithified rocks are often difficult to differentiate thus hindering an accurate delineation of the diagenetic environment limestones from water depths of 6 to 8 m off kuwait vary in petrographic character from oosparite and biosparite in which the cement is entirely aragonite to oomicrite and biomicrite in which at least some of the cement is microcrystalline magnesian calcite carbon 14 dates suggest that the oosparite may have lithified at depths shallower than at present possibly intertidally during a lower stand of sea level in contrast the biosparite oomicrite and biomicrite appear to be contemporaneous and to have lithified subtidally