genesis of palimpsest cool water carbonate sediment on the continental margin of southern australia abstract the continental margin of southern australia is a site of extensive quaternary cool water carbonate sedimentation seafloor sediments are a mixture of holocene biofragments and late pleistocene relict and stranded particles relict sediments produced during marine isotope stages 3 and 4 are generally preserved as iron stained intraclasts such grains in the great australian bight are photozoan in character they include symbiont bearing benthic foraminifers and coralline algal particles and indicate that the wide shelves were bathed in shallow relatively warm oligotrophic waters and floored by marine grasses similar to many modern shallow inboard embayments in the region today in contrast east of the great australian bight relict sediments are more heterozoan in character with coralline algal and molluscan facies on the inner shelf and bryozoan sediments on the outer shelf stranded sediments marooned during the sea level rise associated with marine isotope stage 2 are gray to buff in color and composed of both biofragments and intraclasts these particles are mainly heterozoan in character throughout the region with subtropical photozoan marine grassbank facies localized to portions of the inner great australian bight stranded sediments are interpreted to have accumulated in an areally restricted photic zone and upon an overall narrow shelf covered in cool nutrient rich waters swell driven seafloor sediment disturbance to depths of 100 m along the southern australian margin in conjunction with reduced rates of sedimentation that characterize the cool water carbonate realm have resulted in mixing of sediment currently being produced with exhumed relict and stranded sediments generated during previous sea level stands the composition of these deposits and the nature of their pre burial alteration are a function of sea level history late pleistocene relict sediments originally accumulated in low energy inboard environments but were modified in the surf zone during subsequent sea level fluctuations this resulted in the formation of intraclasts abraded infilled and cemented biofragments of local origin the widespread distribution of intraclasts and of composite grain assemblages derived from multiple sedimentary environments that display highly variable degrees of seafloor alteration may be signatures of modern cool water carbonate assemblages worldwide while such palimpsest sediments may complicate paleoenvironmental interpretation their differentiation in the rock record can be utilized to discern subtleties in paleoceanography and sea level history

impact of high potential vorticity intrusions into the tropical upper troposphere in south atlantic on precipitation over northeast brazil 1 interannual variations of high potential vorticity intrusions at 10 s in the upper troposphere during ndjfm are shown to be negatively correlated with rainfall over northern northeast brazil neb that is higher intrusions associated with stronger equatorial westerlies may provoke droughts higher intrusions lead to the formation of an anomalous cyclonic vortex over neb which causes convergence in the upper troposphere and sinking motion at lower levels the number of intrusions in ndjfm has highest correlation significant at 99 confidence level with the rainfall of the principal rainy season in fmam

shallow burial dolomitization and dedolomitization of cenozoic cool water limestones southern australia geochemistry and origin abstract the gambier limestone a bryozoan rich cool water carbonate of late eocene to early miocene age in the otway basin of southeastern australia is about one third variably dolomitized limestone and dolostone individual dolomite crystals range from 10 to 500 m and are of three types 1 silt size crystals 4 64 m diameters having a brightly luminescent core 10 20 m of nearly stoichiometric dolomite with 1 mol mnco3 and sio2 but no fe surrounded by a thin dull or alternating dark and bright cl cortex that is nonstoichiometric with 44 46 mol mgco3 but sharp to diffuse zoning in mn and fe 2 sand size crystals which also have a brightly luminescent and stoichiometric core and dark and bright non stoichiometric thin inner cortex but are mostly a thick outer cortex at ca 50 m diameter that is nonluminescent and nonstoichiometric with some fe but no mn and 3 identical sand size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement these three crystal populations represent two distinct dolomites and their alteration product the smallest of the silt size crystals which are rarely preserved are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition the 18o values of dolomite rhombs 1 6 to 2 9 correlate positively with 13c values 0 7 to 3 0 with the lowest values from silt size or small sand size crystals and the highest values from large sand size crystals with leached unfilled cores low and variable 18o and 13c values in conjunction with high and variable mn and fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater higher values for the outer cortex in sand size crystals are typical of those expected for precipitation from normal or near normal seawater low sr mn and fe contents in the largest crystals in conjunction with fluid inclusions of seawater salinity in the outer cortex are also consistent with the bulk of the large crystals having precipitated from seawater using the sr isotope evolution curve for cenozoic seawater sr isotope ratios of the dolomites can be divided into four distinct groups which correspond to times of major transgressions this cenozoic example in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression regression offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained

deposition and textural evolution of cool water limestones outcrop analog for reservoir potential in cross bedded calcitic reservoirs the eocene miocene te kuiti group is a succession of cool water carbonate and siliciclastic rocks on the north island of new zealand the limestones are subtidal bioclastic and largely composed of bryozoans echinoderms and benthic foraminifers these rocks can be divided into a horizontally bedded facies assemblage and a cross bedded facies assemblage the horizontally bedded facies assemblage represents deposition on a wave storm dominated shelf and is composed of grainstones and packstones containing textures resulting from biologic processes the cross bedded facies assemblage represents deposition on a current dominated shelf and consists of grainstones with textures formed chiefly by physical processes the te kuiti group can be used as an outcrop analog for cross bedded calcite dominated hydrocarbon reservoirs porosity permeability and reservoir potential of the horizontally bedded facies assemblage decreased rapidly during late shallow burial owing to mechanical compaction and to the fine grain size close packing and ineffective early cementation of the rock coarser grain size open framework and earlier cementation within the cross bedded facies assemblage resulted in significant heterogeneity and good reservoir potential before moderate burial tabular lens shaped and sigmoidal reservoir units are proposed primary porosity and early developed traps are essential to reservoirs formed of cool water limestones and those carbonates from calcite seas the influence of depositional setting extends into the eventual bedding style texture physical properties and reservoir potential of the rock

nitrogen sources and recycling at subduction zones insights from the izu bonin mariana arc 1 we report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the izu bonin mariana ibm arc to investigate the sources flux and mass balance of nitrogen at a cool convergent margin the majority of samples have high n2 he 1217 17 300 and low co2 n2 exc 78 937 implying addition of nitrogen from the subducting slab this inference is supported by positive i e sediment like 15n values up to 5 5 in most samples the exception to these trends is agrigan in the mariana arc with low n2 he 200 high co2 n2 exc 1500 and negative 15n mixing calculations suggest an average of 34 of the nitrogen in our samples is derived from subducted sediment or 75 after correction for atmospheric contamination sediment derived n2 fluxes estimated by three different methods range from 0 25 108 to 1 11 108 mol yr 1 n2 representing 4 17 of the total nitrogen input flux or 11 51 of the sedimentary nitrogen input flux the altered oceanic crust is identified as an important contributor to the arc nitrogen budget and the 15n of the residual nitrogen subducted into the mantle is estimated at approximately 1 9 despite similarities in gas chemistry and 15n values our conclusions regarding nitrogen recycling for ibm are markedly different than those for the nicaraguan segment of the central american arc and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones the global nitrogen cycle is estimated to be in steady state suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume related rocks or secular variation in the isotopic composition of subducted sediments better constraints on nitrogen recycling at other arcs are required to test these conclusions

seismic architecture and seismic geomorphology of heterozoan carbonates eocene oligocene browse basin northwest shelf australia abstract seismic characterization of eocene oligocene heterozoan carbonate strata from the browse basin northwest shelf of australia defines marked progradation of nearly 10 km stratal terminations and stacking subdivide the succession into mappable seismic units stratal architecture and seismic geomorphology varies systematically through the succession individual surfaces discerned by toplap onlap and truncation outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18 sigmoidal clinoforms can include aggradation in excess of 200 m prograde more than 500 m and have slopes characterized by inclined wavy to discontinuous reflectors that represent ubiquitous gullies and channels in contrast the overlying tangential oblique clinoforms include downstepped shelf margins limited on shelf aggradation the associations of shelf aggradation shelf margin progradation and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation this distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded perhaps enhanced by downwelling comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture interpreted to be driven by distinct characteristics of heterozoan sedimentary systems

structure and origin of limestones sorby s presidential address of 1897 on the structure and origin of limestones was essentially an interim report of research in progress his petrographic approach to limestones stemming from three decades of research laid the foundations for a wide range of research lines some of which were not fully exploited for almost a century such as fluid inclusion studies in diagenesis the significance of many of his discoveries e g that some jurassic ooids and palaeozoic corals were originally calcitic had geochemical implications that have only recently achieved research prominence e g co 2 and the greenhouse earth in addition sorby s legacy was the example of his peerless approach to research applying ruthless empiricism to the problem at hand in the case of limestones this involved an application of meticulous descriptive petrography and innovative experimentation his approach to understand more fully the complex problems posed by carbonates remains unsurpassed involving a thorough integration of detailed observation imaginative thinking and judicious use of analytical techniques

diagenetic evolution of tortonian temperate carbonates close to evaporites in the granada basin se spain abstract the granada basin se spain is a small basin located in the central part of the betic cordillera structured as such in the late tortonian and initially connected to the atlantic ocean and to the mediterranean sea during the late tortonian normal marine conditions prevailed leading to the deposition of skeletal carbonate sediments on platforms around structural highs the marine connections were later interrupted first to the atlantic ocean and then to the mediterranean sea and a thick evaporite sequence marking the transition from marine to continental conditions was deposited during the latest tortonian in this work the diagenetic evolution of the tortonian temperate carbonates ttc underlying and close to the evaporite bodies is revealed and discussed the diagenetic study includes petrographic analyses conventional petrography cathodoluminescence and fluorescence geochemical analyses major minor and trace elements and 13c and 18o stable isotopes and microthermometry of fluid inclusions in the ttc marine diagenetic processes such as micritization and fibrous calcite cement precipitation and mechanical compaction took place during or just after deposition eogenesis an initial burial event mesogenesis 1 is characterized by 1 stabilization of the temperate water carbonates by freshwater and 2 porosity occlusion via precipitation of low mg bladed and syntaxial mosaic calcite cements the ttc were then subaerially exposed or got close to the surface during evaporite deposition and underwent pedogenesis mg smectite infiltration and pyrite formation telogenesis 1 subsequent brine related diagenetic alterations such as dolomitization and silica halite and sylvite replacements of carbonate grains occurred during a second burial episode mesogenesis 2 concomitant with the messinian lacustrine deposition this being followed by chemical compaction stylolite formation finally the area was uplifted and the ttc exhumed microstalactitic dripstone and fibre whisker calcite cement precipitation and extensive dissolution relate to this pliocene quaternary late event telogenesis 2 in the study case diagenetic history is closely linked to basin evolution as diagenetic pathways of carbonate rocks were related to major geodynamic events including basin restriction leading to evaporite deposition and several episodes of subsidence and uplift up to now only very few diagenetic studies have attempted to demonstrate this correlation between diagenetic history and basin evolution

early diagenesis of carbonates on a cool water carbonate shelf southern australia abstract the southern australian continental margin is an immense cool water carbonate factory the high energy open shelf is blanketed by heterozoan carbonates of both holocene and late pleistocene age three distinct grain types have been identified and radiocarbon dated these include 1 relict grains highly abraded iron stained intraclasts formed during the intermediate sea level stands of marine isotope stages 3 and 4 2 stranded grains gray and buff colored abraded biofragments and intraclasts marooned during the sea level rise associated with marine isotope stage 2 and 3 holocene grains unaltered biofragments formed during marine isotope stage 1 optical mineralogical and chemical characterization of grains of each age has revealed pathways of early diagenesis in this realm both calcitic and aragonite biogenic grains undergo dissolution while dissolution of calcitic components is minor and incomplete relict sediments are predominantly mg calcite aragonitic skeletons undergo significant dissolution within 30 000 years in contrast to early dissolution of aragonite micritic calcite cement with 12 mol mgco3 is a significant diagenetic phase in these sediments infilling many stranded and most relict skeletons similar cement precipitates between grains forming small numbers of cemented grain aggregates in most samples microprobe and icp ms analysis demonstrates fe oxides mn oxides and associated clay minerals discolor stranded and relict grains by coating grain surfaces and infesting micropores such coatings begin to form on holocene grains during or soon after formation the oxides scavenge trace elements including v ti cr and ree altering the elemental compositions of biogenic carbonate

cool water carbonate sedimentology and eustasy pleistocene upper slope environments great australian bight site 1127 odp leg 182 the southern australian shelf is the largest area of the globe composed of cool water carbonates and provides the basis for the development of actualistic models of such sediments ocean drilling program leg 182 drilling of the sedimentary succession provides essential information to also decipher the dynamics of this depositional system sediment samples and geophysical logs from a core drilled on the eucla plateau off the coast of southern australia site 1127 have been used to study sedimentary cyclicity in an 800 m thick cool water carbonate pleistocene wedge this periplatform succession is dominated by bioclastic packstone and wackestone with minor intercalations of bioclastic grainstone fluctuations of stable oxygen isotope values in bulk carbonate samples parallel global climatic and related sea level fluctuations as predicted in the specmap curve the sea level controlled sedimentary cycles of the slope succession are several tens of meters thick lowstand deposits are relatively fine grained and include abundant sponge spicules whereas highstand deposits are coarser grained and contain abundant high mg calcite bioclasts aragonite content makes up to 36 of the cool water carbonates aragonite originates from tunicate spicules primary sea level controlled carbonate mineralogical and textural changes are likely to promote facies dependent differential diagenesis during later burial stages of such successions

reefal coralline algal build ups within the arctic circle morphology and sedimentary dynamics under extreme environmental seasonality abstract carbonate frameworks secreted by phototrophic organisms within the arctic circle are not well documented underwater surveys of the inner shelf off troms northern norway 70 n reveal extended fixed algal build ups which are fringed by rhodolith belts affected by storms reefd growth by coralline algae under temperature and light regimes of extreme seasonality is made possible because of a decoupling of carbon fixation during summer and utilization of stored carbon during the period of winter darkness although the annual growth of the framework constructing algae is comparatively low the annual carbonate production rate is similar to subtropical tropical counterparts because of a remarkably high standing stock early diagenetic alteration is restricted to intraparticle cementation processes which start in vivo bioerosional destruction is the dominant control on the preservation of high latitude build ups preservation of holocene autochthonous coralline algal biostromes is enhanced by rapid burial during storm events redeposition during storms is the most important process in forming a distinct sedimentary facies zonation

sedimentation rates and growth potential of tropical cool water and mud mound carbonate systems carbonate fixation in the ocean proceeds in three basic modes abiotically biotically induced with organic trigger and biotically controlled where organisms determine timing location and composition of the product the three modes combine in a variety of ways to produce carbonate sediment when viewed on the scale of formations and global facies belts three benthic carbonate production systems or factories emerge 1 the tropical shallow water system dominated by biotically controlled mainly photoautotrophic and abiotic precipitates 2 the cool water system dominated by biotically controlled mainly heterotrophic precipitates and 3 the mud mound system dominated by abiotic and biotically induced mainly microbial precipitates the sedimentation rates of all three factories decrease as the time span of observation increases this scaling trend is not just an artifact of ratio correlation it is almost certainly related to the episodic pulsating nature of sedimentation the growth potential of the three systems can be estimated by drawing the envelope of the maximum observed rates of aggradation the tropical system shows the highest rates 10

heterozoan carbonates in subtropical to tropical settings in the present and past water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments however parameters other than temperature also strongly influence ecosystems and consequently the carbonate grain associations in the resulting carbonate rock among those factors are biological evolution water energy substrate water chemistry light penetration trophic conditions co2 concentrations and mg ca ratios in the seawater increased nutrient levels in warm water settings for example lead to heterotrophic dominated associations that are characteristic of temperate to cool water carbonates failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past modern analogues of low latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm water carbonates careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate

petrogenesis of cenozoic temperate water calcarenites south australia a model for meteoric shallow burial diagenesis of shallow water calcite sediments abstract the gambler limestone is one of several extensive shallow water shelf carbonates of eocene to miocene age exposed along the southern margin of australia it is a muddy to grainy bryozoan calcarenite with accessory benthonic foraminifers and echinoids the sediments originally composed almost entirely of calcite or mg calcite have been in vadose and phreatic environments for over 10 ma yet are virtually unlithified the only cements of any consequence are epitaxial on echinoids numerous karst features dolines caves speleothems and surface karren attest to prolonged residence in the meteoric zone the gambler limestone is presently one of the best fresh water aquuifers in australia most flow occurs through intergranular pores in sediments with over 30 porosity cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m the overlying naracoorte limestone miocene a calcarenite of warmer water aspect contains numerous aragonite molds and is well cemented we propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite rich tropical sediments it is probable that many similar early and middle paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure

petrofacies analysis of carbonate rocks example from lower paleozoic hunton group of oklahoma and texas a petrophysical classification of hunton group dolomitized carbonates lower paleozoic in the anadarko basin of oklahoma and texas is based on porosity mercury recovery efficiency median pore throat size normalized pore throat size the shapes of capillary pressure curves and lithology five petrophysical facies petrofacies include i porous and permeable dolostones ii skeletal grainstones and packstones iii low porosity low permeability dolomitized oomoldic ooid grainstones iv cemented ooid grainstones cherts and cherty carbonates and v lime mudstones and tight fine grained dolostones petrofacies i porous dolostone is the most variable this variability reflects the inherent variability of dolostones and the probability that dolomitization of t e hunton group occurred in more than one episode four subpetrofacies include ia crystalline and sucrosic dolostones ib vuggy dolostones iva cemented ooid grainstones and ivb cherts and cherty mudstones and wackestones the petrofacies form mappable subsurface units that can be used to predict the distribution of petroleum reservoirs e g petrofacies i and ii and of potential sealing units petrofacies iii iv and v and to identify potential stratigraphic traps petrofacies and depositional lithofacies are discordant indicating that diagenetic processes did not respect lithofacies boundaries the best reservoir rocks sucrosic fossiliferous dolostones of subpetrofacies ia and crinoidal grainstones of petrofacies ii have both high porosities mean 5 2 6 9 and high mercury recovery efficiencies mean 30 7 36 1 these rocks are characterized by moderately sized throats median 2 6 8 8 m mesokurtic to platykurtic pore throat size distributions and intermediate or gently sloping cumulative mercury intrusion capillary pressure curves higher porosity rocks with low mercury recovery efficiencies mean porosity 11 8 re 25 for example vuggy and moldic dolostones of subpetrofacies ib would not perform well during primary oil recovery but would be good candidates for enhanced oil recovery for reservoir rocks that are oil wet capillary pressure dat must be interpreted differently than is done in this paper most petrofacies can be distinguished by the shapes of capillary pressure curves alone however bivariate plots of petrophysical parameters also allow discrimination between petrofacies the negative log of median pore throat size plotted vs either porosity or mercury recovery efficiency provides the most readily interpretable graphic method of assigning samples to petrofacies such plots are particularly useful in highlighting samples that are lithologically similar to but petrophysically different from samples typical of a particular petrofacies

environmental factors influencing skeletal grain sediment associations a critical review of miocene examples from the western mediterranean the presence of foramol rhodalgal and bryomol skeletal grain associations in ancient shallow marine limestones is commonly interpreted as evidence for non tropical palaeoclimate despite temperature being only one of several factors influencing skeletal grain associations such interpretations neglect the multitude of factors other than temperature that influence carbonate producing biota these include nutrients water energy water transparency depth of the sea floor salinity oxygen ca2 and co2 concentrations mg ca ratio alkalinity substrate requirements competitive displacement as well as biological and evolutionary trends this uniformitarian approach also disregards the probability that conditions of present day biological systems may not be representative of past conditions of analogous systems here the importance of considering these other factors is illustrated through two examples of carbonate platforms in the western mediterranean these platforms are dominated by foramol rhodalgal and bryomol associations of miocene age in spite of having formed in tropical conditions the platforms discussed are 1 the lower tortonian ramp on menorca balearic islands and 2 the lower middle miocene ramps of the central apennines italy evidence for tropical conditions in the mediterranean during the period of growth of these platforms is provided by species of red algae and larger foraminifera by data from coeval continental basins and by global oxygen isotope data

models of porosity formation and their impact on reservoir description lisburne field prudhoe bay alaska the lisburne field at prudhoe bay alaska produces from shelfal carbonates in the pennsylvanian wahoo formation four major factors control reservoir behavior 1 depositional stratification 2 a fractured permeable subunconformity alteration zone saz 3 multiple episodes of porosity formation and 4 faulting this paper is the first written description of facies cyclicity and diagenetic processes as they apply to porosity formation and reservoir modeling in the lisburne field successive depositional cycles in the wahoo formation pass from ooid skeletal grainstones deposited in shoal complexes to oncolitic packstones and skeletal peloidal wackestones formed in restricted lagoonal environments geochemical data and crosscutting relationships between porosity and unconformities pressure solution features fractures and faults provide evidence of three distinct episodes of porosity formation earliest porosity is probably due to periodic localized exposure during the pennsylvanian a second stage of porosity is associated with shallow burial dolomitization that probably began during permian triassic subaerial exposure reservoir quality in dolomites varies with the degree of neomorphic recrystallization the third stage began in the cretaceous and lasted in o the tertiary and is associated with final burial and hydrocarbon maturation this burial dissolution event also opened existing fault systems creating a complex reservoir most lisburne field effective porosity is of late burial origin due to either dolomitization or dissolution faults and the saz act as giant collectors of oil from low permeability matrix and they have reduced the number of wells needed for field development faults also complicate waterflood implementation and maintenance of uniform reservoir pressure field studies such as these demonstrate the importance understanding the diagenetic history of a reservoir can have for field management and development planning

dissolution behaviour of bryozoan sediments taphonomic implications for nontropical shelf carbonates abstract diagenesis of carbonate sediments on temperate marine shelves is commonly destructive in contrast to a dominantly constructive regime in tropical waters in some cases dissolution can be important in temperate shelf waters particularly affecting the metastable carbonate minerals if dissolution does occur it can be an important taphonomic process reducing skeletal remains available for fossilization and biasing fossil assemblages the dissolution behaviour of bryozoans has been investigated in an acid bath laboratory experiment using nontropical carbonate sediments from the new zealand shelf bryozoans of various shapes growth forms and compositions low to high mg calcite and aragonite were extracted and their weight loss over time was measured results show that variations in stability of carbonate mineralogies are insufficient to predict relative dissolution rates if shelf dissolution occurs even if only periodically one might anticipate that the less stable aragonite and high mg calcite skeletons would dissolve first in fact this experiments shows that small delicate skeletons with non compact morphologies dissolve most rapidly apparently with little regard for mineralogy fossil bryozoan assemblages from temperate marine shelf environments may therefore not only reflect the preferential loss of certain aragonitic species but also the loss of small delicate species new zealand s nontropical tertiary bryomol limestones often lack aragonitic skeletons they may also be relatively depleted in small delicately branched bryozoans in contrast tropical shelf carbonate sediments can retain more delicate skeletons through early cementation and lack of dissolution a difference in character between tropical and temperate carbonate shelf deposits may therefore be in the proportion of small delicate bryozoan colonies

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

diagenesis and compositional partitioning of quaternary cool water carbonate aeolianites southeastern australia abstract middle to late pleistocene stranded carbonate sea beaches adjacent aeolian dunes and pleistocene aeolianites together with holocene marginal marine calcareous sediments across southeastern australia contain the longest record of such deposition in the modern world stretching back to at least 900 ky mis 23 the grains are were derived from a suite of cool water heterozoan carbonate sediments produced on the adjacent offshore shelf most modern shelf carbonate sediments in the region today are aragonite rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record beach dune sediments are derived mainly from the shallow 30 meters deep adjacent seafloor they are mollusk rich but with most bryozoan particles having been destroyed in the surf zone the mollusks are dominantly aragonite these nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite poor open shelf sediments aragonite skeletal fragments in the dunes progressively dissolve with time under a semi arid climate and produce diagenetic low magnesium calcite meteoric cements a well known process common in tropical aragonitic pleistocene aeolianites the loss of metastable minerals with time is matched by a progressive and well understood change in stable isotopes such alteration in this environment however seems to take longer than in tropical carbonates with lithification herein not beginning until 200 ka and complete cementation being achieved in 450 ky thus neither aeolianites nor offshore marine carbonates record the composition of the original sediment aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks this partitioning revealed for the first time answers the conundrum as to why some cool water neritic shelf and coastal sediments in the rock record are compositionally different

the relative roles of compaction and early cementation in the destruction of permeability in carbonate grainstones a case study from the paleogene of west central florida u s a abstract how compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented in this study the destruction of permeability by compaction relative to early cementation is examined in forty five shallow buried 30 440 m grainstones with a range in permeability of 12 to 5180 md compaction and cementation indices calculated from thin section point count data indicate a group of grainstones dominated by cementation and another by compaction log linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes the nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability as cement abundance increases in the cement dominated samples a continuous distribution of cement develops along grain surfaces which is followed by the development of partial and then complete pore filling mosaics permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats permeability loss results from porosity reduction progressive constriction of pore throats and increased tortuosity of flow paths once permeability is reduced to tens of millidarcies effective pore radii are all less than 1 m and many are less than 0 1 m but there is no evidence for complete blocking of all pore throats in this suite of samples in absolute terms the pore lining cements reduce permeability by thousands of millidarcies whereas the pore filling cements reduce permeability only by hundreds of millidarcies mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction dominated samples linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases however there is no concurrent change in the amount of pore space accessed by large or small throats indicating that mechanical compaction does not uniformly constrict or seal pore throats instead the data suggest that mechanical compaction reduces permeability by reducing porosity lengthening pore throats and increasing tortuosity at about 500 md the compaction dominated trend steepens on the log linear plot of permeability versus porosity a change that corresponds to the development of grain to grain pressure solution unlike mechanical compaction grain to grain pressure solution constricts and eliminates pore throats and isolates pore space resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats in contrast to cement dominated samples the most effective pore radii are still greater than 1 m at tens of millidarcies of permeability such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample