the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

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

one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

rock composition dolomite stoichiometry and rock water reactions in dolomitic carbonate rocks dolomitic carbonate rock units from a variety of phanerozoic localities and depositional environments throughout north america were evaluated with respect to their dolomite content and dolomite composition by x ray diffraction analysis dolomite compositions range from 48 to 57 mole caco 3 with well defined modes at 51 nearly stoichiometric dolomite and 55 distinctly calcian dolomite caco 3 values of percentage of dolomite also exhibit a pronounced albeit less well defined bimodal distribution with modes at 97 dolostones and 20 dolomitic limestones dolomite similar distributions characterize most other published data on phanerozoic dolomitic carbonates these data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite 1 dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution calcite and dolomite precipitation reactions and 2 dolostones originate in diagenetically open systems during th

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

the influence of organic matter in organogenic dolomitization we hypothesize that the decomposition of organic debris derived from plants planktonic animals and algae by sulfate reducing bacteria and specifically by the enzymatic degradation of protein is vital to organogenic dolomitization the role of this degradation is to increase both alkalinity and ph of the pore waters providing the necessary solution and surface chemistries for dolomitization to occur ammonia derived by the enzymatic breakdown of proteins is the only simple base available to supply the ph conditions necessary for dolomitization protein is abundant in organic matter derived from a marine source because of the large enthalpies of hydration of mg super 2 and ca super 2 ions we propose that dolomitization proceeds with the transfer of neutral ion pairs caco 3 and mgco 3 to and from the dolomite surface within a replacement zone for this reason dolomitization will occur only when the concentration of the co 3 super 2 ion is high and in large excess to the hco 3 super ion in solution and at the carbonate crystal surface these conditions will be met when the ph and or ionic strength is high and the carbonate alkalinity is also high

holocene carbonate sedimentation on the west eucla shelf great australian bight a shaved shelf the southern continental margin of australia is a cool water carbonate sedimentary province located in a high energy swell dominated oceanographic setting a vibrocore transect of 14c dated sediments across the centre of the eucla shelf is the first record of holocene shelf deposition in the great australian bight much of the seafloor shallower than 70 m water depth the base of wave abrasion is bare cenozoic limestone in some places encrusted by late pleistocene coral rich limestone that is cemented by high magnesium calcite 12 mole mgco3 the areally extensive 100 km wide hard bored substrate supports an epibiota of coralline algae minor bryozoans and soft algae or is covered by patches of holocene sediment up to 1 5 m thick generally a basal bivalve lag 3 ka overlain by quartzose bioclastic palimpsest sand this pattern of active carbonate production but little accretion on the wave swept mid to inner shelf is similar to that on other parts of the southern australian continental margin the term shaved shelf is proposed for this style of carbonate platform formed by alternating periods of sediment accretion cementation and erosion the palimpsest sand is typically rich in bivalves coralline algae and locally detrital dolomite outer shelf holocene sediment below the base of wave abrasion but inboard of the shelf edge is a metre thick unit of fine microbioclastic muddy sand with minor delicate bryozoans overlying a 9 13 ka rhodolith gravel some of this outer shelf sediment appears to have been resedimented the shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production the holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers cf marginopora and depleted in bryozoans as compared to coeval deposits on the lacepede and otway shelves off southeastern australia these differences are interpreted to reflect warmer waters of the leeuwin current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

late quaternary climates of the australian arid zone a review abstract aridity in australia has generally been associated with glacial intervals of the last few glacial cycles the causes and nature of aridity varied from north to south over the continent in northern and central australia reduced monsoon rain in global cold stages caused lakes and rivers to dry vegetation to become more sparse sand dunes to become active and dust advection to increase while the monsoon is correlated with warmer global temperatures and high sea levels overall the exact timing of the greatest development of the monsoon in stage 5 remains uncertain in southeastern australia the proxy records give a less clear picture while vegetation seems to have been more sparse in cold stages and sand dune activity and dust flux consequently greater records of runoff in rivers and lakes have mixed or opposite behaviour to northern or central australia the nature of aridity in southeastern australia is not analogous to conditions anywhere on the continent today greatly reduced temperatures over the central and southern interior greater influence of snow and periglaciation on runoff high groundwater tables lower rainfall and reduced atmospheric carbon dioxide levels may all have contributed to and interacted to produce the reconstructed picture of a cold sparsely vegetated arid landscape with large rivers and perennial lakes the largest changes in circulation patterns over the glacial cycle probably occurred in the location and or intensity of summer tropical convergence in northern australia over southern australia changes to the temperature and humidity of the westerly circulation have been more significant than the small fluctuations in latitude of the sub tropical high pressure ridge

andesites and dacites from daisen volcano japan partial to total remelting of an andesite magma body voluminous andesite and dacite lavas of daisen volcano sw japan contain features suggesting the reverse of normal fractionation anti fractionation in the sense that magma genesis progressed from dacite to andesite accompanied by rises in temperature a positive correlation exists between phenocryst content 0 40 vol and wt sio2 61 67 phenocryst rich dacites contain hornblende and plagioclase that are generally unaltered clear and euhedral however phenocryst poor rocks contain sieve textured plagioclase resorbed plagioclase and opacite in which hornblendes are pseudomorphed some daisen rocks contain two coexisting pyroxenes many orthopyroxene phenocrysts from two pyroxene lavas have high ca overgrowth rims up to 50 mm a feature consistent with crystallization from a higher temperature magma than the core rim compositions are similar from phenocryst to phenocryst in individual samples temperatures of 800 900 c are obtained from the cores whereas temperatures of 1000 1100 c are indicated for the rims lavas ranging from aphyric andesite 61 wt sio2 to phenocryst rich dacite 67 wt sio2 have similar sr sr 0 7045 0 7052 and nd nd 0 5127 0 5128 isotopic variability within daisen volcano is likely to be mantle derived reflecting isotopic variability within the magma source region associated with a single mantle diapir the daisen andesites and dacites have the same trace element signatures as the associated basalts and were probably derived from primary magmas at the same general depth 60 km our interpretation is that mantle derived hydrous magnesian andesite generated in the same mantle diapir as coexisting basalt magma may be parental to the hydrous calc alkaline magmas in daisen volcano we suggest a two stage process involving mid crustal solidification of bodies of this calcalkaline magma followed by varying degrees of partial melting from body to body to produce the magmatic trends and phenocryst zoning patterns observed the heat required for this melting according to our model was supplied by the intermittent rise of subjacent basaltic magma

strontium isotopic compositions of cambrian upper miaolingian furongian series dolomites from south eastern sichuan basin china significance of sources of dolomitizing fluids and timing of dolomitization abstract tracing the nature of dolomitizing fluids and constraining the dolomitization time are of great significance for understanding dolomite genesis and reservoir formation especially for ancient dolomites strontium isotopes can be used as very useful tools to study this aspect thick dolomite bodies developed in the loushanguan group of the upper miaolingian furongian series cambrian in the south eastern margin of the sichuan basin china these dolomites are probably good hydrocarbon reservoirs little research has been carried out on the sr isotopes in these ancient dolomites three different textures of dolomites have been identified in the study area dolomicrite fabric retentive oolitic dolomite and fabric obliterative dolomite the dolomicrite displays a higher sr isotopic value avg 0 71052 compared with the contemporaneous seawater as well as a high sr and low mn content these chemical characteristics indicate that the dolomitizing fluids of the dolomicrite were mainly derived from modified seawater mixed with a little continental input the 87 sr 86 sr ratios avg 0 70936 of the fabric retentive oolitic dolomites are close to those of normal seawater indicating that the dolomitizing fluids were derived from the contemporaneous seawater and that they possibly formed soon after sedimentation as a result of an early replacement during shallow burial the low sr isotopic values avg 0 70854 of the fabric obliterative dolomites indicate that these dolomitizing fluids are related to late cambrian seawater and that the dolomites are the result of recrystallization of the fabric retentive dolomites in a relatively closed diagenetic system during burial with combined characteristics of relatively high sr and low mn contents the recrystallization began approximately 490 ma and was completed not earlier than 485 ma this study is not only beneficial to understand the characteristics and multistage process of the dolomitization of ancient cambrian dolomites but provides a reference for sourcing dolomitizing fluids and constraining the dolomitization time using sr isotopes

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

apoyo caldera nicaragua a major quaternary silicic eruptive center apoyo caldera near granada nicaragua was formed by two phases of collapse following explosive eruptions of dacite pumice about 23 000 yr b p the caldera sits atop an older volcanic center consisting of lava flows domes and ignimbrite ash flow tuff the earliest lavas erupted were compositionally homogeneous basalt flows which were later intruded by small andesite and dacite flows along a well defined set of n s trending regional faults collapse of the roof of the magma chamber occurred along near vertical ring faults during two widely separated eruptions field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast followed by eruption column collapse which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse a period of quiescence was marked by the eruption of scoria bearing tuff from the nearby masaya caldera and the development of a soil horizon violent plinian eruptions then resumed from a vent located within the caldera a second phase of caldera collapse followed accompanied by the effusion of late stage andesitic lavas indicating the presence of an underlying zoned magma chamber detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume mapping of the apoyo airfall and ignimbrite deposits gives a volume of 17 2 km3 within the 1 mm isopach crystal concentration studies show that the true erupted volume was 30 5 km3 10 7 km3 dense rock equivalent approximately the volume necessary to fill the caldera a vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions this vent area is controlled by n s trending precaldera faults related to left lateral motion along the adjacent volcanic segment break fractional crystallization of calc alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at apoyo prior to caldera collapse highly atypical tholeiitic magmas resembling low k high ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber the injection of tholeiitic magmas may have contributed to the paroxysmal caldera forming eruptions

a review of the nature and origin of limestone microporosity abstract limestone microporosity is ubiquitous and extensively developed in most phanerozoic limestones from an economic perspective microporosity is important because it contributes substantially to the carbonate pore system which can host significant volumes of water and hydrocarbons therefore determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations reservoir characterization and fluid flow simulations from an academic standpoint microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments a fundamental process in carbonate diagenesis many types of micropores contribute to what has been referred to as microporosity but the vast majority is hosted among low magnesium calcite lmc microcrystals that are present in limestone matrix and allochems geochemical textural and mineralogical data from natural settings and laboratory experiments indicate that lmc microcrystals are diagenetic in origin more specifically these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high magnesium calcite hmc minerals and precipitation of lmc microcrystal cements the stabilization process is inferred to take place in the meteoric marine and burial diagenetic realms although it has not been directly observed carbon and oxygen isotopes as well as trace element data suggest that lmc microcrystals form during burial diagenesis in marine like fluids evidence suggests that porosity is not generated during this dissolution precipitation process but rather inherited from the precursor sediments the final arrangement of the micropores in a limestone however depends on the precise diagenetic pathway lmc microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size the three main textural classes granular framework fitted mosaic and clustered have been recognized across a wide range of ages depositional settings burial depths and precursor types and are characterized by distinct petrophysical properties such as porosity permeability and pore throat size observations from modern sediments also support the hypothesis that lmc microcrystals develop from aragonite and hmc dominated lime mud the origin of lime mud has been extensively studied but still highly debated of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the phanerozoic microporous limestones comprised of lmc microcrystals are equally abundant during times of aragonite seas and calcite seas which suggests that no special mineral precursor is required microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents unlike shallow marine limestones deep marine sediments start out as mostly lmc therefore mineralogical stabilization is not a significant process in chalk diagenesis

marine cements in mid tertiary cool water shelf limestones of new zealand and southern australia large areas of southern australia and new zealand are covered by mid tertiary limestones formed in cool water shelf environments the generally destructive character of sea floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates yet these limestones include occasional units with marine cements 1 within rare in situ biomounds 2 within some stacked cross bedded sand bodies 3 at the top of metre scale subtidal carbonate cycles and 4 most commonly associated with certain unconformities the marine cements are dominated by isopachous rinds of fibrous to bladed spar interstitial homogeneous micrite and interstitial micropeloidal micrite often precipitated sequentially in that order internal sedimentation of microbioclastic micrite may occur at any stage the paradox of marine cemented limestone units in an overall destructive cool water diagenetic regime may be explained by the precipitation of cement as intermediate mg calcite from marine waters undersaturated with respect to aragonite in some of the marine cemented limestones aragonite biomoulds may include marine cement sediment internally suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences we suggest that marine cementation occurred preferentially but not exclusively during periods of relatively lowered sea level probably glacio eustatically driven in the mid tertiary at times of reduced sea level there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites which now experienced increased swell wave and or tidal energy levels fostering sediment abrasion and reworking reduced sedimentation rates and freer exchange of sediment pore waters energy levels were probably also enhanced by increased upwelling of cold deep waters onto the southern ocean margins of the australasian carbonate platforms where water mass mixing warming and loss of co2 locally maintained critical levels of carbonate saturation for sea floor cement precipitation and promoted the phosphate glauconite mineralization associated with some of the marine cemented limestone units

geochronology and stratigraphy of the ignimbrites from the 21 30 s to 23 30 s portion of the central andes of northern chile the central volcanic zone cvz of the andes which extends from 14 to 28 s is one of the largest provinces of late tertiary to recent ignimbrite volcanism in the world in the 21 30 s to 23 30 s portion of the cvz in northern chile ignimbrite volcanism was initiated at the beginning of the late miocene 10 4 ma ago and continued until the recent the stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions 100 km3 the artola ignimbrite member 9 4 ma the sifon ignimbrite member 8 3 ma the pelon ignimbrite member 5 5 ma the puripicar ignimbrite member 4 2 ma and the atana ignimbrite member 4 1 ma at least three of these represent only portions of larger ignimbrite sheets erupted from centres of sw bolivia two of the ignimbrites the sifon and atana ignimbrites must represent erupted volumes in excess of 1000 km3 each the major units which represent 90 of the total volume of ignimbrite in this area are intercalated with smaller 50 km3 less extensive volcanic units some of which were erupted from source structures within the study area the upper tertiary to recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons the ignimbrite formations can be divided into two groups on a geographic basis those in the northern and central portions of the study area are known as the san bartolo group while those in the southern portion belong to the silapeti group these two groups appear to be contemporaneous although the silapeti group may represent a period of ignimbrite volcanism which was initiated later about 6 ma ago the ignimbrite formation of sw bolivia and the punense arancaniano formation of nw argentina are equivalent to the san bartolo and silapeti groups based on correlation of the stratigraphy into adjacent areas in sw bolivia and nw argentina the beginning of the late miocene 10 4 ma marks the onset of the most intense phase ignimbrite volcanism in the cvz this phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the quechua phase of deformation and subduction approximately 10 000 km3 of ignimbrite erupted over a period of 9 ma in the 21 30 23 30 s portion and led to the development of a major ignimbrite province

dolomitization and hydrotectonic model of burial dolomitization of the furongian lower ordovician carbonates in the tazhong uplift central tarim basin nw china implications from petrography and geochemistry abstract the nature of the dolomitizing fluids and genesis of the furongian lower ordovician dolomites and dolomite cements are interpreted on the basis of petrography geochemistry trace elements rees o c sr isotopes and fluid inclusion microthermometry these carbonate strata were deposited on a shallow marine platform in the tazhong uplift in central tarim basin and were extensively dolomitized based on field investigations and petrographic examination two types of fabric retentive dolomitic textures dolomicrites dm and microbial dolomudstone dms and three types of fabric obliterated dolomitic textures very fine to fine crystalline planar e s dolomite rd1 fine to medium crystalline planar e s dolomite rd2 and medium to coarse crystalline nonplanar a planar e s dolomite rd3 are identified two types of dolomite cements medium to coarse crystalline dolomite cement dc1 and coarse to very coarse crystalline saddle dolomite cement dc2 are identified the ree patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard the dc2 dolomite cement and later stage calcite cement lscal display positive europium anomalies eu and positive cerium anomalies ce whereas the dm dms rd1 rd2 rd3 and early stage calcite cement escal samples display positive ce light ree lree enrichment and negative eu the dm and dms were formed through seawater flux and mesosaline brine derived from evaporative seawater during early burial seepage reflux dolomitization and microbial dolomitization respectively the preferential occurrence of rd1 along low amplitude stylolites suggest that their formation may have linked to pressure dissolution by which mg ions were probably released for replacement dolomitization during late early burial to early shallow burial dolomitization moreover the tightly packed and curved crystal faces of rd2 and rd3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the rd1 and or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow burial dolomitization the average range of the homogenization temperatures th of cd2 is 132 4 c 198 7 c and mostly higher than the estimated ambient temperature 95 c 150 c for the furongian lower ordovician strata implying that the growth of dc2 in the study area was the result of hydrothermal dolomitization the comparable isotopic 18o 13c and 87sr 86sr values between rd2 rd3 dc1 dc2 dolomites and coeval neoproterozoic to early phanerozoic seawater values in addition to the relatively high th and salinity levels in these dolomites indicates that their dolomitizing fluid were derived from radiogenic 87sr enrich coeval seawater during late shallow burial dolomitization and hydrothermal dolomitization the hydrotectonic driving model of burial dolomitization of the furongian lower ordovician carbonates in the tazhong uplift resulted from compressional tectonics that affected the tarim basin during cambrian to middle ordovician and late ordovician period this model is characterized by fluid flow in response to significant thermal heating pressure loading and enhanced fluid expulsion by squeezing and the resultant higher temperature mg rich connate waters and hydrothermal basinal brines are conveyed by convective advective flows along permeable horizons fractures and overthrust fault planes resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites forming irregular networks of dolomites and calcite cements in the host carbonate rocks

characteristics of deep marine dolomite abstract deep marine sediment has not been subjected to the influences of meteoric waters deep burial or tectonic stress that have profoundly altered most rocks available to us in continental exposures hence it preserves the record of formation and early development of its dolomite evaluation of the dsdp x ray and smear slide data files indicates that dolomite is a common component in deep marine sediments it averages 1 and is found in all sampled ocean basins throughout post jurassic time dolomite abundance is not related to the abundance of any other detrital authigenic or alteration mineral except aragonite petrographic examination of 48 samples suggests that deep marine dolomite crystal size and appearance are similar to the size and appearance in modern supratidal deposits do omite does not increase in abundance or in crystal size downhole nor does crystal size increase with increasing age the dolomite is commonly nonstoichiometric avg 56 caco3 and poorly ordered globally it does not change in stoichiometry or order with depth or age dolomite in deep marine sediments does not originate from unusual or regionally important conditions it entered the sediments early most probably as a chemical precipitate from pore waters an uncertain amount estimated to be 10 is detrital

partially disordered dolomite microstructural characterization of abu dhabi sabkha carbonates transmission and analytical electron microscopy observations document at least three carbonate phases in sediments of the abu dhabi sabkha aragonite ordered dolomite often showing modulated microstructure and a mostly disordered calcium magnesium carbonate with submicrometer sized ordered domains it is suggested that ordered and partially ordered dolomite formed mainly by direct precipitation from pore fluids with changing temperature and chemistry in response to environmental parameters

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

hydrogen sulfide hydrates and saline fluids in the continental margin of south australia during the drilling of the southern australian continental margin leg 182 of the ocean drilling program fluids with unusually high salinities to 106 were encountered in miocene to pleistocene sediments at three sites 1127 1129 and 1131 high contents of h2s to 15 ch4 50 and co2 70 were also encountered these levels of h2s are the highest yet reported during the history of either the deep sea drilling project or the ocean drilling program the high concentrations of h2s and ch4 are associated with anomalous na cl ratios in the pore waters although hydrates were not recovered and despite the shallow water depth of these sites 200 400 m and relative warm bottom water temperatures 11 14 c we believe that these sites possess disseminated h2s dominated hydrates this contention is supported by calculations using the measured gas concentrations and temperatures of the cores and depths of recovery high concentrations of h2s necessary for the formation of hydrates under these conditions were provided by the abundant so42 caused by the high salinities of the pore fluids and the high concentrations of organic material one hypothesis for the origin of these fluids is that they were formed on the adjacent continental shelf during previous lowstands of sea level and were forced into the sediments under the influence of hydrostatic head

paleogene geochronology an integrated approach geochronology is the conceptual division of continuous time as measured geochronometry by the progression in an ordinal series of events this is best achieved by an approach which integrates four independent data sets magnetostratigraphy seafloor spreading magnetic lineation patterns biostratigraphy and isotopic ages this integrated approach results in an ordinal framework which can measure time with greater resolution though perhaps less accuracy than a radiometric approach alone a comparative analysis of two recently proposed paleogene geochronologic time scales is presented

composition and genesis of temperate shallow marine carbonate muds spencer gulf south australia abstract the origin of carbonate muds grain sizes 63 m in shallow nontropical marine environments is poorly documented coarse grained carbonates often characterize coastal temperate seafloors but many such deposits also have abundant carbonate muds spencer gulf a large shallow embayment along the southern australian margin 60 m water depth contains mixtures of carbonate gravels sands and muds with muds constituting up to 84 volumetrically benthic environments include dense seagrass meadows sand barrens and rhodolith pavements where bivalves benthic foraminifera coralline algae and bryozoans dominate heterozoan carbonate assemblages mud grain morphologies were investigated using scanning electron microscopy leading to the conclusion that muds are predominantly composed of skeletal fragments bivalves benthic foraminifera ascidians echinoderms and coralline algae interpreted to have formed due to breakdown of rigid material via maceration x ray diffraction analysis confirms that these muds contain combinations of intermediate mg calcite low mg calcite and aragonite with concentrations varying among locations according to skeletal grain types spencer gulf muds differ in mineralogy and composition from those of classic models of tropical and deep water carbonate mud deposition unlike shallow water tropical carbonate muds these temperate muds are not dominated by aragonite and there is no evidence of carbonate precipitation from seawater pelagic organisms contribute only trivial amounts to these deposits in contrast to deep water muds

genetic sequence stratigraphy of cool water slope carbonates pleistocene eucla shelf southern australia the south australian eucla shelf belongs to the world s largest cool water carbonate sedimentary system during the pleistocene it exported large amounts of sediment to the shelf edge and upper slope resulting in an expanded sedimentary wedge wedge internal clinoforming seismic reflectors suggest a stacking of the deposits into genetic sequences high resolution stable oxygen and carbon isotope point counting grain size and carbonate mineralogical xrd analyses were carried out to characterize these genetic sequences along a dip parallel transect of three odp leg 182 drill holes located between the shelf edge and upper slope oxygen and carbon isotope fluctuations show that the genetic sequences formed as a response to sea level fluctuations within the genetic sequences facies differentiation and sediment volume partitioning occur along the transect lowstand deposits are fine grained and contain more sponge spicules and micrite highstand deposits are coarse grained with tunicate spicules brown bioclasts as well as bryozoan and corallinacean debris boundaries separating highstand and lowstand deposits are triggered by sea level fall and are expressed as abrupt grain size changes or as turning points in grain size trends analyzed components vary in abundance along the transect genetic sequences show dip parallel variations in thickness combined with changing relative proportions of lowstand versus highstand deposits

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

the palaeogene time scale radiometric dating versus magnetostratigraphic approach numerical geological time scales rest firmly on the analysis of radiometric data since the discovery of oceanic magnetic stripes and their signification alternative time scales have been constructed whose numerical predictions are based on the hypothesis of a constant rate of sea floor spreading the most recent such scale uses only five tie points plus present day to subdivide a span of over 80 million years the present paper demonstrates that the rate of ocean floor spreading has varied markedly and without obvious pattern both in time and in space thus magnetostratigraphic scales based on large interpolations are fundamentally flawed an alternative scale based on a consideration of all radiometric data and using magnetostratigraphic data only for small interpolations is commended

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

effects of mineral composition and heterogeneity on the reservoir quality evolution with co2 intrusion during the diagenesis of rocks in a sedimentary basin an intrusion of co2 could cause dissolution or and precipitation of the surrounding rocks as a result the reservoir quality may be altered the mineral composition and heterogeneity have pronounced impacts on the geochemical reaction and reservoir quality evolution a numerical simulation method is employed to investigate the influences of primary mineral on the diagenesis based on the measured data from the songliao basin a total of 26 two dimensional models with different mineral composition are set up to mimic the regionally heterogeneous distribution of mineral composition the monte carlo method is employed a co2 gas reservoir of magma intrusion origin located in the songliao basin northeastern china is selected for the present study the reservoir is an ideal site for investigating the impact of mineral heterogeneity on diagenesis after the co2 intrusion in this reservoir with the presence of high pressure co2 the mineral heterogeneity causes significant dissolution and precipitation of minerals which decreases the reservoir porosity and degrades the reservoir quality the geochemical reactions caused by different mineral composition vary widely the mineral heterogeneity causes similar distributions of the geochemical reactions and reservoir quality evolution dominant secondary minerals are dawsonite in the early diagenesis and ankerite in the late stage the diagenetic sequences modeled by our numerical simulations reproduce the petrographic and geochemical data well

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

carbonate sedimentation and diagenesis associated with mg ca chloride brines the permian san andres formation in the texas panhandle abstract the san andres evaporitic sequence in the palo duro basin includes several thick carbonate units in its lower part and many thin units in its upper part evaporites are concentrated in the northern part of the northern shelf of the midland basin and carbonates predominate to the south within the palo duro basin the carbonates in the lower part of the sequence differ from those in the upper part in containing skeletal lithofacies and in having lower mn fe and terrestrial organic content the br content in halite in the lower part of the sequence is consistently high whereas halite in the upper part is commonly br depleted the sr content in dolomite calcite and anhydrite and 18o values are about the same in the en ire san andres sequence in the palo duro basin depleted 13c values in dolomite and low pristane phytane ratios occur toward the south na cl and k cl ratios attributed to liquid inclusions in almost all carbonates are characteristic of marine brines evaporated beyond halite saturation na in the dolomite lattice is generally low and increases from north to south at the same stratigraphic levels the lower part of the formation was deposited in a broad shelf basin or lagoon sufficiently deep to maintain long periods of steady state circulation during which little halite dissolution and no potash magnesia mineral precipitation occurred the upper part of the formation was deposited in smaller water bodies sensitive to fluctuations in inflow of marine water as well as meteoric water diagenesis of the carbonates occurred in contact with saline mg ca chloride brines evolved through anhydrite and halite precipitation skeletal mold formation anhydrite cementation dolomitization and high sr calcite cementation associated with celestite are all cogenetic products of such a brine rock interaction 18o of dolomite calcite and chert indicates apparent equilibrium relationships with the same solution temperatures have never exceeded 40 to 45 c and imply the 18o of such a solution to be about 3 the somewhat depleted 18 up o composition of the proposed halite saturated brine resulted from the reversal in the dependence of 18o on increased evaporation in highly saline brines the proposed diagenetic model implies that following early diagenetic processes the rock sequence remained as a closed but compacting system unchanged by later fluid introduction low sr values in dolomite are indicative of recrystallization and not fresh water diagenesis

dynamics of a high energy dissipative surf zone vright l d guza r t and short a d 1982 dynamics of a high energy dissipative surf zone mar geol 45 41 62 pressure and horizontal current u v time series were measured at different positions across the inner 150 m of a wide 500 m surf zone of a microtidal high wave energy beach incident waves had average heights of 3 4 m with maxima of 5 m and periods of 12 to 15 sec bores of broken waves diminished in height at a nearly constant rate as they progressed across the surf zone the ratio of bore height h to local water depth h was everywhere less than 1 for even the highest bores and was on the order of 0 40 for the significant bores at incident wave frequencies rip circulation was weak or absent but a moderate longshore current was present shore normal flows were vertically segregated with strong net onshore flows prevailing just below the surface accompanied by weaker net seaward flows near the bed spectra of water surface oscillations as determined from pressure u and v reveal that most of the energy in the inner surf zone was at infragravity frequencies periods greater than 30 sec shoreward decay of wave energy at incident wave frequencies was accompanied by shoreward growth of infragravity energy near the beach the infragravity oscillations had heights on the order of 1 m cross spectra show that the infragravity oscillations were standing in the shore normal direction from the relative magnitudes of infragravity versus incident wave currents it is inferred that the surf beat may be an order of magnitude more important than incident waves to the transport of sediment in the inner surf zone

the carbonate system in hypersaline solutions alkalinity and caco3 solubility of evaporated seawater changes in total alkalinity and in ph during the evaporation of seawater in the laboratory were studied for an evaporation range extending between 1 1 and 26 as defined by the concentration ratio of mg in the evaporated sample relative to mean ocean water the evaporation experiments were performed with and without suspended aragonite the results of the two experimental sets define an alkalinity field for evaporated marine derived brines data from naturally formed marine brines fall within the experimental field deviations therefrom can be explained by changes in alkalinity unrelated to caco precipitation dissolution reactions

the role of diffusion during carbonate diagenesis abstract the dissolution reprecipitation process which characterizes the early mineralogical stabilization of carbonate sediments takes place in a somewhat open chemical system studies of the trace element chemistries and isotopic compositions of carbonate grains before and after stabilization have documented the exchange of ions between the diagenetic site and the aquifer solution if the pores at reaction zone and in the immediate neighborhood of the diagenetic site are of insufficient diameter to allow effective solution flow the exchange of ions between the diagenetic site and the aquifer solution the large volume of water which flows through the macropores of the carbonate sediment takes place by aqueous diffusion this paper examines diffusion applied specifically to the conditions of carbonate diagenesis the diffusion equation modified to these circumstances serves as a guide to understanding the diffusion process at a given temperature three variables determine the efficacy of diffusion the diffusion coefficient the pore path and geometry and the concentration gradient the diffusion coefficient may be considered a measure of the inherent velocity at which a given ion travels under standard conditions the double positive cations common in carbonates ca2 sr2 ba2 zn2 mg2 etc and the carbonate anion all have similar diffusion coefficients thus this term of the equation exerts little influence during normal carbo ate diagenesis the pore path and geometry refer to the distance from aquifer to reaction zone and the diameter shape and directness of the pore passage this term is difficult to quantify in diagenetic systems but is constant for different ions at a single diagenetic site the longer and the more complex the path the less effective diffusion is there is a general facies dependence to this term grainstones have short paths from reaction zone to aquifer due to their permeability whereas mudstones and wackestones are characterized by long and torturous paths through the mud grains to the aquifer solution exchange of ions in the fast situation is more effective than in the second the concentration gradient the difference between the concentration of an ion in th reaction zone and in the proximal aquifer solution is the most variable parameter in diffusion the sign of the gradient which may be positive or negative indicates the direction of diffusion transport of the ions toward the region of lower concentration one cation may diffuse from the reaction zone while simultaneously a second cation diffuses toward the reaction zone this two way traffic has been documented in diagenetic calcite corals sr2 diffuses away from and mg2 diffuses toward the diagenetic site additionally one cation may diffuse effectively whereas a second diffuses ineffectively the latter species being characterized by a weaker concentration gradient thus a given diagenetic site may be chemically open for one cation and simultaneously somewhat closed for another application of diffusion principles to the aragonite calcite transformation during early diagenesis indicates the compositions of solutions and diagenetic calcites undergo temporal changes early on with few active diagenetic sites sr2 diffuses effectively from the reaction zone to the aquifer since there is a steep concentration gradient diagenetic calcites are relatively low in sr2 because of its effective removal from the diagenetic site as the number of active diagenetic sites increases so much sr2 may be delivered to the aquifer that sr2 builds up in the aquifer solution and the concentration gradient shallows diffusion from individual sites becomes less effective sr2 concentrates in the reaction zone and diagene ic calcites enriched in sr2 are produced as more and more aragonite is consumed a final phase is reached in which less total sr2 is delivered to the aquifer and the system returns to the conditions characteristic of the early phase

palaeocene oligocene tectonics of nw europe s u m m a r y during the palaeogene in nw europe a gallic subplate was very nearly created this plate was almost entirely bordered by plate boundaries although not of equivalent ages to the n the n atlantic began to open about 56 ma ago the alpine convergent boundary extended southwestwards to the pyrenean subduction zone on the ne side a n british pataeocene early eocene fracture dyke boundary appears to pass southeastwards across the north sea into a lutetian to burdigalian rift system there is an age disparity between the fracturing and nw dyke intrusion in britain and the fracturing and formation of the rhine graben the last dykes in britain ypresian were intruded not long before the important subsidence of the rhine graben earliest lutetian but their causes were different the british dyke fractures propagated southeastwards via the cleveland dyke towards the north sea whereas the rhine graben propagated northwards towards the zuider zee depression however the structures from both sides converged in the sole pit basin in the north sea with the result that a continuous though not coevally originated fracture system developed from nw scotland to the alps during the early tertiary there was a wide range of tectonic activity throughout nw europe fig 1 in nw britain lavas were extruded and plutonic complexes and extensive dyke swarms were intruded in the north sea tufts were deposited as a result of volcanism along the rockall faeroe rift reactivation of faults con trolled magmatism localization of gas fields devolatilization of coal and inversion of basins caused uplift deformation and erosion along the rhine graben doming subsidence and rift propagation took place in association with alka line volcanism and plutonism coevally with the meso alpine compressive deformations that oc curred in the alps faulting controlled sedimen tation patterns during inversion of basins in southern england the english channel northern france and holland old rift faults were reacti vated in the western approaches basin thrust faults formed in the southern bay of biscay contemporaneously with thrusting in the pyre nees and variscan massifs were tilted and uplifted along steep reverse faults the main tectonic activity lasted from about 60 ma to about 30 ma but it was diachronous generally earlier in the nw and later in the se the igneous activity in northern britain was related to the opening of the n atlantic in the late palaeocene but the formation of the rhine graben and the inversion of sedimentary basins in the north sea and near the english channel were caused by major orogenic events in the alpine region in the late eocene to mid oligocene frisch 1981 tectonic phenomena in the north sea provide the link between those in northern britain and those in the rhine graben the aim of this paper is to demonstrate that these movements from the atlantic and alpine sides almost fragmented the lithospheric plates of nw europe during the creation of a gallic sub block an earlier northern boundary from nw scotland to northern germany being transected by a later eastern boundary along the rhine graben

sedimentology mineralogy and isotopic analysis of pellet lake coorong region south australia gravity cores of holocene sediments from a shallow ephemeral lake in the coorong region pellet lake southeastern coastal australia show a mineral assemblage and sequence particular to its hydrology the mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core i e organic rich aragonite to magnesite hydromagnesite aragonite followed by a relative decrease in salinity i e magnesite aragonite hydrornagnesite to aragonite hydromagnesite in the upper portions of the core this sequence is capped by 0 4 m of micritic dolomite and minor amounts of hydromagnesite with the relative abundance of dolomite increasing upwards three stratigraphically and spatially distinct dolomite units upper lower and margin are recognized using stable carbon and oxygen isotope data unit cell calculations and mgc03 mole per cent data of the dolomite detailed x ray diffraction xrd analyses of samples with more than 80 dolomite shows that the dolomite is ordered average unit cell parameters calculated from the xrd patterns indicate that the upper dolomite unit has crystal lattices expanded in the c direction c 16 09 a relative to ideal dolomite c 16 02 a and contracted in the a direction a 4 796 a relative to ideal dolomite a 4 812 a the mol fraction of mgc03 in the upper dolomite shows up to 4 0 f 2 0 mole per cent excess mg in the dolomite crystal lattice calculated from xrd this unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated mg ca ratios transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution the modulated microstructure found in calcium rich dolomite is completely lacking dolomite ordering reflections are present in electron diffraction patterns but are weak stable oxygen and carbon isotope values of the upper dolomite are tightly grouped ave 6l80 x 7 55 6i3c 4 10 yet show three upward lightening oxygen cycles the oxygen cycles correlate with three upward decreases in the calculated mg content of the dolomite zone these cycles may indicate the increased importance of rain water dilution of the brine at times when the water in the lake was at its shallowest levels analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit the lower dolomite is close to stoichiometric has less evaporitic stable isotope values than the upper dolomite and has only a slightly expanded c axis the margin dolomite is ca rich has a more homogeneous microstructure and has expanded a and c axes the abundance of relatively soluble mg bearing phases such as hydromagnesite and magnesite may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment this process may leave a finely laminated dolomicrite deposit which retains little if any evidence of evaporite minerals

synsedimentary cemented calcarenite layers in oligo miocene cool water shelf limestones eucla platform southern australia abstract the abrakurrie limestone is a bryozoan rich cool water open shelf unit of oligocene middle miocene age beneath the nullarbor plain the sediments have been in the meteoric diagenetic environment never buried more than a few hundred meters since middle miocene time the unit is composed of meter scale subtidal cycles each of which is capped by an erosion surface the upper several centimeters of most cycles are well lithified forming distinctive hard layers in otherwise friable carbonates intergranular cements in these layers are now isopachous rinds of low magnesium calcite crystals are inclusion rich generally fibrous and have a speckled cathodoluminescence signature cements in intergranular pores at the tops of cycles are overlain by interstitial marine sediment the cem nts are interpreted to have been marine precipitates in the form of magnesium calcite originally containing from 8 12 mole mgco3 and having a 18o value of 1 0 0 5 versus pdb stable carbon and oxygen isotope values in these interpreted marine hardgrounds are distinctly more positive than values from underlying soft sediments reflecting alteration under lower water rock ratio conditions due to reduced permeabilities covariance in stable isotope values from different localities suggests diagenesis in groundwaters of mixed composition fluid inclusions indicate neomorphism in fluids with salinities equal to or slightly greater than seawater alteration is envisaged to have taken place in a paleogroundwater system of variably mixed saline continental waters and seawater occurrence of similar to identical cemented layers and cements in coeval cool water new zealand shelf carbonates suggests that such cementation was a sporadic but widespread phenomenon on mid cenozoic platforms in this area by inference cool water carbonates of all ages may contain units lithified on the seafloor by cements containing low to intermediate amounts of magnesium cool water is not an inhibitor to synsedimentary cementation

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

sealevel changes control diagenetic dolomite formation in hemipelagic sediments of the peru margin abstract the first experimentally determined temperature dependent oxygen 18 fractionation factor between dolomite and water at low temperatures vasconcelos c mckenzie j a warthmann r bernasconi s 2005 calibration of the 18o paleo thermometer with dolomite formed in microbial cultures and natural environments geology 33 317 320 allows now the precise calculation of temperatures during early diagenetic dolomite precipitation we use 18o values of early diagenetic dolomite beds sampled during odp legs 112 and 201 on the peru continental margin sites 1227 1228 and 1229 meister p mckenzie j a vasconcelos c bernasconi s frank m gutjahr m schrag d p 2007 dolomite formation in the dynamic deep biosphere results from the peru margin opd leg 201 sedimentology 54 1007 1032 to calculate paleo porewater temperatures at the time of dolomite precipitation we assumed unaltered seawater 18o values in the porewater which is supported by 18o values of the modern porewater presented in this study the dolomite layers in the pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 c lower than today since sites 1228 and 1229 are located at 150 and 250 m below sealevel respectively their paleo porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands thus pleistocene dolomite layers in the peru continental margin probably formed during glacial times this finding is consistent with a model for dolomite precipitation in the peru margin recently discussed by meister et al meister p mckenzie j a vasconcelos c bernasconi s frank m gutjahr m schrag d p 2007 dolomite formation in the dynamic deep biosphere results from the peru margin opd leg 201 sedimentology 54 1007 1032 where dolomite forms episodically at the sulphate methane interface it was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column but dolomite layers may only form when the sulphate methane interface stays at a fixed depth for a sufficient amount of time we hypothesize that the sulphate methane interface persists within toc rich interglacial sediments while this zone is buried by toc poor sedimentation during glacial times thus the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations a similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a milankovitch type pattern observed in the geological record such as in the miocene monterey formation

a comparison of nanometer scale growth and dissolution features on natural and synthetic dolomite crystals implications for the origin of dolomite abstract this paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy synthetic dolomite was formed by replacement of calcite at 218 c in mg ca cl solutions the first products to form during dolomite synthesis experiments as detected with x ray diffraction xrd are invariably poorly ordered nonideal dolomite as the reaction proceeds the degree of cation order increases yet the products remain relatively disordered upon examination the surfaces of nonideal dolomite are covered with mounds 10 200 nm wide and 1 20 nm high following depletion of the calcite reactant dolomite becomes relatively well ordered and stoichiometric ideal the surfaces of ideal dolomite are covered with broad flat layers separated by steps that measure tenths to tens of nanometers high comparison of natural and synthetic dolomite surfaces after etching in 0 5 h2so4 for 10 20 seconds indicates that high temperature synthetic and low temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite in contrast synthetic and low temperature ideal dolomite surfaces when etched are characterized by flat layers with deep euhedral etch pits these results suggest that despite the wide range in formation conditions natural and synthetic dolomites form by the same growth mechanisms furthermore the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth

magnetostratigraphic correlation of palaeogene sediments in the hampshire and london basins southern uk magnetostratigraphic studies of the late palaeocene and early to middle eocene formations in the hampshire and london basins indicate that the geomagnetic field maintained a predominantly reverse polarity throughout the period of deposition of these sediments eight discrete normal polarity magnetozones have been identified within this dominantly reverse polarity interval three of these lie within the bracklesham group three within the london clay formation one in the oldhaven 9formation9 and one within the lowermost part of the thanet formation assuming that the magnetozone boundaries represent absolute time planes except where they correspond with stratigraphic hiatuses they may be used to provide a time framework for chrono stratigraphic correlation between sections and for investigating the dynamics of early palaeogene sedimentation patterns in this region the results indicate that the majority of the marine transgressive and regressive events which form the basis of king9s 1981 subdivisions of the london clay occurred very rapidly and may be regarded as being effectively 9instantaneous9 in marked contrast the boundaries between the sedimentary cycles of the bracklesham group represented by the wittering earnley selsey and huntingbridge divisions are significantly diachronous in relation to the absolute time planes provided by the magnetozone boundaries it is clear that the bracklesham group transgressive events occurred systematically earlier in the east than in the west as might be expected for westward directed transgressions certain biostratigraphic zonal boundaries within the bracklesham group appear also to have been influenced by the changing environmental conditions which accompanied these transgressive regressive cycles

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

multiple dolomitization episodes in deep water limestones of the triassic lagonegro basin southern italy from early reflux to tectonically driven fluid flow abstract massive dolomitization does not commonly occur in deep water carbonates however this regional study on outcrops and subsurface samples of triassic lagonegro basinal carbonates from the southern apennines fold and thrust belt italy shows that the evolution of the lagonegro basin was punctuated by two events of massive dolomitization these processes resulted in two distinct dolomite types displaying different petrographic and geochemical features a medium crystalline and fabric preservative dolomite in the northern area molise campagna and san fele mount pierno contrasts with a medium to coarse crystalline and fabric destructive dolomite in the southern area from pignola to the high val d agri geochemical data increase in 18o with respect to typical late triassic seawater values high sr and low mn and fe content sr isotopes consistent with late triassic seawater indicate that the dolomite in the northern area is the result of an early replacement caused by a marine and possibly oxidizing fluid during shallow burial dolomitizing fluids likely were expelled through reflux from the norian apenninic carbonate platform towards the proximal lagonegro basin this reflux was promoted by the norian peculiar paleoceanographic and climatic conditions in the tethyan region and affected only the proximal basinal facies dolomites in the southern area display a decrease in 18o compared to late triassic seawater values and show lower sr higher mn and fe contents and higher sr isotope ratios compared to the early dolomites of the northern area fluid inclusion microthermometry reveals homogenization temperatures between 70 and 120 c and salinities of 1 9 to 6 4 wt nacl eq for saddle dolomites these geochemical signatures indicate that the southern area was affected by late diagenetic burial dolomitization interpreted to be the result of large scale tectonically driven fluid flow during the second compressional phase characterizing the southern apennines fold and thrust belt emplacement the dolomite distribution across the lagonegro basin northern and southern is possibly controlled by the paleogeographic distribution of the facies proximal vs distal as well as by their proximity to thrust faults the lagonegro case study demonstrates that basinal successions may undergo multiple dolomitization episodes in a manner similar to carbonate platform successions from early shallow to deep burial diagenesis one of the relevant implications is that in areas of restricted oceanographic circulation during periods of aridity reflux dolomitization may extend beyond the platform margin into deep water carbonates having the potential to alter their reservoir quality

nannoplankton zonation and the palaeocene eocene boundary beds of nw europe an indirect correlation by means of volcanic ash layers correlation of early palaeogene ash layers in dsdp hole 550 leg 80 goban spur with those of the north sea basin indicates that the base of nannoplankton zone np10 approximates the base of the sele formation of the north sea area the bases of the danish ash series the german ash bearing untereozan 1 and the paris basin sparnacian and the top of the woolwich reading bottom bed of the london basin if as is now common practice the palaeocene eocene boundary is placed at the np9 np10 boundary then the units listed above would constitute the basal units of the eocene in their respective countries the cessation of large scale pyroclastic activity took place at about the middle of the zone np10 interval

book reviewxvit 263 p 25 us 49 50 hardbound 11 95 us 19 95 paperback a geologic time scale w b harland et al cambridge university press washington dc usa 1990 isbn 0 521 38361 7

co generation of hydrogen sulfide and methane in marine carbonate sediments sulfate reduction and methanogenesis are considered to be mutually exclusive microbial reactions in marine sediments typically methane does not appear in significant concentrations in sediment pore waters until almost all dissolved sulfate has been reduced to sulfide an exception to this commonly accepted pattern occurs in an approximately 500 meter thick sequence of quaternary carbonates on the continental margin of the great australian bight an unusual combination of geochemical and sedimentological conditions leads to extensive simultaneous sulfate reduction and methane production throughout the 500 m interval a probable explanation for the co production of these reduced gases in this deeper biosphere is the presence of noncompetitive substrates for the two types of microbiota

degree of supersaturation and precipitation of organogenic dolomite this paper integrates the degree of carbonate saturation into a general model for the origin of dolomite in organic rich marine sediments degradation of organic matter by sulfate reducing bacteria promotes early dolomite precipitation by simultaneously increasing the carbonate alkalinity and lowering the sulfate ion concentration to near zero the interstitial water in organic rich sediments of several deep sea drilling project sites where dolomite is believed to be precipitating is up to 10 sup 3 times supersaturated with respect to near ideal dolomite and up to 10 times supersaturated with respect to calcite the highly supersaturated interstitial waters appear to overcome low temperature kinetic barriers to precipitate a nonideal fine grained calcian dolomite the degree of supersaturation with respect to dolomite decreases and the interstitial water approaches carbonate equilibrium with increasing burial depth at several of these sites removal of sulfate ion is important because sulfate ion may inhibit dolomite precipitation a calcareous sediment is not required but the presence of calcite or aragonite appears to greatly enhance dolomitization by providing an additional source of ca and carbonate ions

origin of silicic magmas along the central american volcanic front genetic relationship to mafic melts abstract silicic pyroclastic flows and related deposits are abundant along the central american volcanic front these silicic magmas erupted through both the non continental chorotega block to the southeast and the paleozoic continental chortis block to the northwest the along arc variations of the silicic deposits with respect to diagnostic trace element ratios ba la u th ce pb oxygen isotopes nd and sr isotope ratios mimic the along arc variation in the basaltic and andesitic lavas this variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas carr m j feigenson m d bennett e a 1990 incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the central american arc contributions to mineralogy and petrology 105 369 380 patino l c carr m j and feigenson m d 2000 local and regional variations in central american arc lavas controlled by variations in subducted sediment input contributions to mineralogy and petrology 138 3 265 283 with respect to along arc trends in basaltic lavas the largest contribution of slab fluids is in nicaragua and the smallest input from the slab is in central costa rica similar trends are observed in the silicic pyroclastic deposits data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high k 2 o na 2 o silicic magmas by fractional crystallization or partial melting of low k 2 o na 2 o sources however fractional crystallization or partial melting of medium to high k basalts can produce these silicic magmas we interpret that the high silica magmas associated central america volcanic front are partial melts of penecontemporaneous mantle derived evolved magmas that have ponded and crystallized in the mid crust or are melts extracted from these nearly completely crystallized magmas

late quaternary fluctuations of palaeoproductivity in the murray canyons area south australian continental margin abstract two sediment cores from the murray canyons area south of kangaroo island south australia were investigated for proxy records to reconstruct past productivity of the surface waters in the area over the last 175 ka the proxies used included concentrations of aragonite low and high mg calcite total carbonate total organic carbon sulfur and 13c of globigerina bulloides cyclic increases in palaeoproductivity were observed to be in tune with insolation minima at 30 s the atmospheric conditions during insolation minima were comparable to present winter patterns when strong westerly winds dominate over the area and bring dust from the central desert areas during the last insolation minimum last glacial maximum lgm the murray canyons laid directly under the eastern australian dust plume dust could have fertilized surface waters and initiated the observed productivity increases on a precessional time scale the core from the more westerly canyons is richer in organic matter this could be attributed to the influx of organic matter from shallower water which is carried by an overflow of hypersaline water from the spencer gulf to the deep ocean using the western canyons as conduits there is no evidence that the palaeo murray river which debouches close to the core sites during sealevel lowstands was a major source of nutrients for surface waters although total carbonate concentrations remained high planktonic foraminifers were fewer in numbers during these periods possibly due to reduced light penetration caused by suspended river material a deglacial minimum in 13c of globigerina bulloides observed in other cores from the southern hemisphere and attributed to a major hydrographic change south of the polar front is also visible in our two cores thus attesting to the global significance of the event

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

dolomitizing seas in evaporitic basins a model for pervasive dolomitization of upper miocene reefal carbonates in the western mediterranean petrographic stratigraphic and stable isotopic data from a dolomitized upper miocene reef complex on mallorca spain coupled with the presence of extensive messinian subtidal evaporite deposits throughout the western mediterranean suggests that messinian evaporitic brines were responsible for dolomitization thermodynamic reaction path modeling was undertaken to more rigorously and quantitatively demonstrate the dolomitizing potential of this evaporitic system these calculations provide maximum amounts of dissolution or precipitation moles l to maintain a specified fluid rock equilibrium significant amounts of dolomite can be created when brines ar concentrated by evaporation prior to reaction with calcite heavy stable isotopes of mallorcan dolomites are consistent with an evaporitic origin however lighter stable isotopes do not preclude this model calculations show that mixing evolved brines with unaltered seawater or continental waters produces a suite of fluids with much greater dolomitization potential than seawater and oxygen isotope signatures approaching those of seawater this evaporitic basin model serves as one hypothesis for the many dolomitized upper miocene reef complexes around the western mediterranean and may be applicable to many older phanerozoic carbonates associated with basinal evaporites

composition of carbonate overgrowths produced on iceland spar calcite crystals buried in bahamian carbonate rich sediments abstract high magnesium calcite overgrowths were precipitated on iceland spar calcite crystals buried in bahamian sediments their composition was within the range of carbonate cements in this area the precipitation rate of the overgrowths was slower and the magnesium content higher than predicted from laboratory experiments possibly due to adsorption of organic compounds

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

silicic ignimbrites within the costa rican volcanic front evidence for the formation of continental crust abstract the origin of silicic magmas greater than 65 silica in areas without old evolved continental crust has long been an enigma in oceanic arcs silicic volcanic deposits are not common which has generally been attributed to subduction related magmas rising without stalling or fractionating through a relatively thin mafic oceanic crust the volcanic arc in costa rica is built on a thick 40 km oceanic plateau and extensive silicic magmatism dominated by silicic ignimbrites has occurred in the region in the absence of old evolved continental crust the silicic deposits display the common large ion lithophile element lile enrichment and high field strength element hfse depletion observed in magmas generated by subduction processes which are not characteristic of melts generated from oceanic plateaus key trace element ratios in the ignimbrites vary with position along the volcanic front and this variation is similar to that of the volcanic front lavas ignimbrites contain unique cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit we propose that the caribbean oceanic plateau in costa rica is being converted to continental crust in large part by the emplacement of silicic magmas in this area the origin of these silicic magmas is due to reprocessing juvenile mantle derived subduction related magmas that have ponded in the crust

salinity reflux and dolomitization of southern australian slope sediments the importance of low carbonate saturation levels anomalously saline waters in ocean drilling program holes 1127 1129 1130 1131 and 1132 which penetrate southern australian slope sediments and isotopic analyses of large benthic foraminifera from southern australian continental shelf sediments indicate that pleistocene holocene meso haline salinity reflux is occurring along the southern australian margin ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50 salinity a well flushed zone at the top of all holes contains pore waters with normal marine trace element contents alkalinities and ph values dolomite precipitation occurs directly below the well flushed zone in two phases phase 1 is a nucleation stage associated with waters of relatively low ph ca 7 caused by oxidation of h2s diffusing upward from below this dolomite precipitates in sediments 80 m below the sea floor phase 2 dolomite grows on dolomite cores precipitated during phase 1 phase 2 dolomite has d 13 c values similar to those of the surrounding bulk carbonate and high sr values relative to phase 1 dolomite consistent with having formed in waters affected by aragonite and calcite dissolution the nucleation stage in this model phase 1 challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water

fine grained sediment lofting from meltwater generated turbidity currents during heinrich events turbidity currents generated from sediment carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater from experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal this ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows evidence for large scale lofting of suspended sediment from the top of giant sand and gravel carrying turbidity currents in the labrador sea comes from two independent lines of observations 1 the first is a distinct sedimentary facies consisting of stacked centimeter thick graded mud layers that contain grains of ice rafted debris ird supported by the mud deposition of these unusual layers requires a gradedlayer forming process that is slow enough to allow the incorporation of ird this is not possible with normal mud carrying turbidity currents 2 the second observation is the presence of a huge abyssal sand and gravel plain in the central labrador sea that received its sediment from bed load rich meltwater discharges from the hudson strait outlet of the pleistocene laurentide ice sheet these discharges turned into turbidity currents that released rising columns of freshwater that carried fine grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater deposition from these slow turbid interflows would allow the incorporation of ird in the accumulating graded mud deposits the ird spiked graded mud facies is restricted to heinrich layers within 300 km radius of the hudson strait ice stream terminus tying the sand carrying turbidity currents via fine grained sediment lofting to heinrich events estimated total discharge volumes of individual currents are on the order of 10 3 km 3 supporting the notion that heinrich ice rafting events were times of maximum meltwater generation

synsedimentary detrital dolomite in quaternary cool water carbonate sediments lacepede shelf south australia carbonate sediments on the lacepede shelf contain up to 25 colorless to orange to dark red dolomite particles as either single rhombs or rhomb clusters this dolomite is present in variable amounts across the shelf in quartzose bivalve bryozoan sediments which are a mixture of modern and relict late pleistocene particles rhombs range from sharply edged to abraded to completely rounded abraded aggregates are loosely cemented by high mg calcite and in places have pristine unabraded rhombs attached the dolomite is ca rich 44 mol mgco 3 fe poor and intricately zoned as seen under cathodoluminescence sr isotopes are compatible with precipitation from seawater of modern composition crystals from one locality have a 14 c age of 28 ka stable isotopes of carbon and oxygen are similar to those of dolomite precipitated from seawater the size composition and geochemistry of the crystals are incompatible with their originating in nearby holocene coorong and older tertiary dolomites the dolomite is interpreted as having formed initially during late pleistocene time a period of fluctuating sea level on the lacepede shelf in mildly reducing sediments just below the sediment water interface the crystals have since been reworked somewhat and dolomite may be forming on these nuclei today such scattered dolomite could be interpreted as postdepositional in ancient limestones when these sediments are buried the rhombs might act as nuclei for further precipitation of dolomite from subsurface fluids

skeletal aragonite dissolution from hypersaline seawater a hypothesis abstract hypersaline seawater has often been invoked as a mechanism to explain the pervasive dolomitization of ancient platform carbonates but its potential in causing skeletal aragonite dissolution of these carbonates has rarely been investigated previous experimental and theoretical studies have demonstrated that hypersaline seawater is undersaturated with respect to aragonite when evaporation reaches a certain degree it is contended here that similar undersaturation could also have occurred in ancient evaporitic seas geological evidence from the miocene carbonates of se spain gulf of suez red sea and iraq suggests that this may have been the case despite differences in their geological settings these carbonates have common diagenetic features including 1 widespread dissolution of skeletal aragonite with little or no calcite cementation and 2 pervasive dolomitization dolomite occurs as both a replacement of mg calcite and mouldic pore filling cement the association of the dolomites with evaporites their relatively heavy oxygen isotopic values and widespread distribution suggest a hypersaline seawater origin of these dolomites petrographic data indicate that skeletal aragonite was dissolved during dolomitization because the aragonite fossil moulds contain dolomite cements but no pre dolomitization calcite cements implying that the dolomitizing fluids hypersaline seawater were probably undersaturated with respect to aragonite the dissolved calcium may have been subsequently incorporated into the dolomite this may help to explain the lack of calcite cementation despite the extensive skeletal aragonite dissolution

palaeoecological evidence for early aragonite dissolution in ancient calcite seas it has been proposed that shallow carbonate seas of upper ordovician and jurassic ages precipitated calcite as the primary calcium carbonate phase1 3 unlike such settings today it is expected that such seas were undersaturated with respect to the more soluble aragonite so that aragonitic invertebrate skeletons would have tended to dissolve here we describe examples in which this seems to have been the case our evidence consists of submarine cemented horizons in which aragonite fossils have been dissolved out to leave open moulds the walls of the moulds are encrusted by contemporaneous organisms confirming the early timing of the aragonite dissolution

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

eocene magnetic stratigraphy at gubbio italy and its implications for paleogene geochronology the sequence of polarity zones established by alvarez and others for the cretaceous and paleocene in the scaglia formation in the bottaccione gorge at gubbio has now been extended through the uppermost paleocene into the upper eocene globigerinatheka semiinvoluta zone anomaly 18 the relationships between the standard paleogene foraminiferal zones and the sea floor anomalies are defined with the exception of the mid eocene zones of morozovella lehneri orbulinoides beckmanni and the base of the truncorota loides rohri zone which are obscured by inadequate preservation and a fault gap the base of the eocene lies above anomaly 25 the lower to middle eocene contact base lutetian lies in the upper part of anomaly 22 the base of the bartonian lies below the base of anomaly 18 and the base of the priabonian upper eocene lies within anomaly 18 radiometric ages appear to diverge systematically from a geochronology based on steady sea floor spreading rates suggesting that spreading may have been as much as a factor of 3 slower in the paleocene and the late eocene than in the early and middle eocene

magnetostratigraphy of the jurassic cretaceous boundary there is no internationally recognized standard for the jurassic cretaceous boundary a precise biostratigraphical correlation between different paleobiogeographic provinces is impossible due to barriers blocking faunal exchange this problem may be overcome by using magnetic polarity events which are globally isochronous the magnetic polarity time scale of the latest jurassic and earliest cretaceous enables determination of the relative timing and age span of several possible definitions of this system boundary among different biostratigraphies within the tethys atlantic faunal realm therefore the polarity scale can be used as a regional standard for this interval of time the base of reversed polarity chron m18 falls in the middle of the interval covered by various recent definitions of the tethyan jurassic cretaceous boundary if polarity chron m18 also proves to be recognizable in the boreal realm then its basal magnetic reversal could serve as a precise globally correlatable event for defining the jurassic cretaceous boundary

correlating the eocene oligocene mammalian biochronological scale from sw europe with the marine magnetic anomaly sequence magnetostratigraphic studies have been undertaken on sedimentary continental formations from western france and spain which have yielded palaeogene mammals the results obtained provide new constraints on the chronological distribution of the mammalian reference levels a magnetozone sequence associated with the reference levels is defined the correlation of this sequence with the geomagnetic polarity time scale is proposed this correlation links the late mid eocene to oligocene mammalian biochronological scale from western europe to other stratigraphic scales a numerical calibration of the palaeogene polarity sequence is made taking into account recent radiometric dating the equations to convert the ages proposed by several authors are given

evaluating reflux dolomitization using a novel high resolution record of dolomite stoichiometry a case study from the cretaceous of central texas usa

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

surficial sediments of the great australian bight facies dynamics and oceanography on a vast cool water carbonate shelf abstract the great australian bight gab the largest sector of the southern australia continental margin is a site of cool water carbonate sedimentation throughout ranging from locally warm temperate inboard to cool temperate outboard surficial sediments are a mixture of calcareous pleistocene skeletal and lithic intraclasts relict grains and holocene biofragments with minor amounts of quartz inboard the inner shelf is an area of abundant macrophytes and seagrasses active carbonate sediment production and accumulation and little relict sediment the huge middle portion is a shaved shelf with active sediment winnowing and mostly relict sediment the outer shelf and upper slope is a variably productive sediment factory characterized by prolific calcareous epibenthic growth on hard substrate subaqueous islands shedding particles into surrounding sands and muds patterns of holocene sedimentation are linked to modern oceanographic parameters in this high energy setting characterized by overall downwelling prolific rhodoliths occur on the nw inner shelf where shallow summer waters are the warmest in the gab these warm saline nutrient depleted waters then drift eastward across the shelf suppressing heterozoan carbonate production on the central and eastern mid shelf this arrested production in the eastern gab is countered locally by summer coastal upwelling along western eyre peninsula with bryozoan rich sediment extending well inboard onto the mid shelf the outer shelf and upper slope is an area of prolific bryozoan growth likely linked to upwelling except in the central gab a region of year round downwelling where the area is one of off shelf fine sediment transport and carbonate mud deposition these patterns in the central gab at least are present in the underlying holocene and pleistocene suggesting that the general modern oceanographic dynamics and resultant carbonate sedimentation have persisted throughout the quaternary

multi method radiometric dating of volcano sedimentary layers from northern italy age and duration of the priabonian stage abstract a comparison between conventional k ar biotite ages and fission track zircon and apatite and u pb zircon ages obtained from stratigraphically well constrained priabonian late eocene volcano sedimentary deposits of northern italy is presented two sections at priabona one level and possagno two levels were dated the application of fission track dating appears fruitful for obtaining reasonably precise 4 to 5 2 errors ages useful for time scale calibration the concordancy of apatite and zircon fission track ages and the reproducibility of results provide the time of volcanic eruption and deposition the u pb analysis of the zircons has not been unsuccessful but discordancy does not permit accurate dating significant dates obtained from possagno are k ar method 35 0 0 5 ma duplicate analysis on k rich biotite from the same level fission track dating method 35 8 1 4 ma weighted mean age on 2 apatite and 3 zircon separates from the same level u pb method 36 7 1 0 ma maximum age of discordant zircons from the same level the comparison between the present results and recent multi method and multi laboratory results obtained from time equivalent priabonian late eocene biotite rich layers from the apennines shows perfect agreement and supports the location of a priabonian stage between about 37 5 ma and about 33 7 0 5 ma the alternative ages preferred by the decade of north american geology convention should be abandoned and a large portion of this scale revised accordingly

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

isotopic composition of a large photosymbiotic foraminifer evidence for hypersaline environments across the great australian bight during the late pleistocene abstract analysis of carbon and oxygen isotopic compositions of large benthic foraminifera tests marginopora vertebralis that lived in the great australian bight during the late pleistocene reveal that the tests are enriched by 1 to 3 in both 18o and 13c relative to modern specimens from the same region the intolerance of m vertebralis for cool waters negates lower ocean water temperature as an explanation for such high 18o values the oxygen isotopic compositions are thus interpreted to reflect tests secreted in hypersaline waters of up to 56 ppt salinity concentrated from seawater by evaporation m vertebralis thrives today in waters of similar salinity at shark bay western australia the pleistocene sedimentary assemblage supports an interpretation that environments broadly similar to those in outer modern day shark bay were wide spread across the great australian bight during portions of marine isotope stages 2 3 and 4 the high 13c values of the pleistocene m vertebralis are interpreted to reflect enhanced photosynthetic activity that depletes dissolved carbonate in 12c in such shallow saline settings these hypersaline environments formed during periods of lower sea level when shallow waters