meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

jurassic penetrative deformation and cenozoic uplift in the central high atlas morocco a tectonic model structural and orogenic inversions the central high atlas is characterized by a relatively mild deformation with a local development of a more or less penetrative cleavage in the axial part of the belt locally this cleavage and the plutonic bodies which outcrop in the core of most of the north east trending anticlines ridges with intrusions are overlain by a disconformable sequence that comprises red clastic deposits which are capped by cenomano turonian limestones the age of these red deposits are bracketted between bathonian to mid cretaceous therefore the spatial relationship of the cleavage with mafic intrusions in the axial part of the central high atlas suggests that it developed contemporaneously with the emplacement of magmas probably during mid jurassic times during this period the structural inversion occurred from a transtensional regime to a transpressive regime there are few indications that an important shortening persisted within the axial part of the central high atlas after the end of the jurassic along the northern and the southern borders of the belt however cretaceous and cenozoic strata were deposited their sedimentary facies bear witness of successive stages of the atlas evolution especially the rapidly coarsening up of the clastic deposits since the oligocene in these marginal basins indicates the onset of the rising up of the belt i e the orogenic inversion

lithologic controls on morphology of pressure dissolution surfaces stylolites and dissolution seams in paleozoic carbonate rocks from the mideastern united states abstract examination of pressure dissolution surfaces stylolites and dissolution seams in carbonate rocks from the mideastern u s demonstrates that the morphology of these surfaces varies with rock fabric one can quantify the morphology of these pressure dissolution surfaces using four parameters thickness t of the insoluble residue defining the surface amplitude of irregularity i of the surface between vertical offsets amplitude of vertical offsets o of the surface and frequency f of vertical offsets along the surface i and o increase with t reflecting progressive development of stylolites whereas f is proportional to 1 t apparently because the physical strength of a stylolite inhibits the formation of offsets mean values of f o and t are greater in grainstones and packstones than in mudstones and wackestones and the abundance of dissolution seams relative to that of stylolites decreases through the lithologic spectrum from mudstones to grainstones these relationships suggest that development of pressure dissolution surfaces to form large stylolites depends on the heterogeneity of the limestone fabric maximum values of t and o in grainstones correlate with cement abundance t and o are low in grain rich cement poor grainstones even though grains are favored as leading or penetrating elements of stylolite columns dissolution seams surfaces with o and f near zero are found only in rocks containing fine grained dolomite t and i of those seams decrease with increasing dolomite abundance in dolomitic limestones and t and o are lower in dolostones than in limestones taken together these relationships suggest that extreme abundance of pressure resistant components such as carbonate grains and dolomite inhibit development of pressure dissolution surfaces in contrast stylolites are most extensively developed i e i t o and f are greatest in carbonate rocks in which these pressure resistant elements are mixed with less resistant components such as cement and carbonate mud the latter rocks which include packstones well cemented grainstones and slightly dolomitic limestones may undergo preferential removal from the stratigraphic record by pressure dissolution

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

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

controls on stylolite development morphologic lithologic and temporal evidence from bedding parallel and transverse stylolites from the u s appalachians quantification of morphology of bedding parallel and transverse or tectonic stylolites from the southern appalachians reveals that regardless of tectonic setting or stylolite orientation lithologic heterogeneity controls stylolite development for example stylolites in more heterogeneous carbonate lithologies packstones and grainstones are more serrate than those in wackestones and mudstones similarly early forming stylolites are more serrate than late forming stylolites and computer simulations suggest that decreases in porosity or other lithologic heterogeneity cause the less serrate nature of late forming stylolites the linkage of a stylolite s morphology to its time of development should allow field estimation of timing of stylolitization and thus of timing of loading or compression although bedding parallel and transverse stylolites generally result from different kinds of compression morphological relations indicate that their development follows the same fundamental patterns bedding

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

middle jurassic shoaling of the central high atlas sea near rich morocco abstract the early and middle jurassic high atlas sea extended as a marine trough across what is now the central and eastern portions of the high atlas mountains southern morocco the sedimentary infilling of this sea is recorded by a thick sequence of limestones and marls punctuated by spectacular coral algal reef horizons a 1 200 m thick section of aalenian and bajocian sediments near rich morocco was studied in detail to document the upward shoaling basin to reef transition the lower 900 m of the section represents deeper water deposition it consists of monotonously interbedded dark hued marls and lime mudstones and wackestones that hear a pelagic fossil fauna and scattered turbidites of lime grainstone an overlying transitional sequence about 200 m thick records progressive shallowing as shown by 1 increased fossil diversity and abundance 2 an increase in algally micritized particles 3 a change from a pelagic to a benthic shelled fauna 4 an increase in limestone to marl ratio and 5 the appearance of small bioherms with scleractinian coral framework the sequence is capped by a 100 m thick horizon of very fossiliferous limestones and superbly preserved coral algal reefs that grew in shallow water several lines of evidence suggest that some of the reefs grew on submarine ridges that were uplifted by middle jurassic tectonic movements

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

oblique extension in the jurassic trough of the central and eastern high atlas morocco a field structural analysis has been carried out in the high atlas mountains of morocco paleostress solutions provide information on the successive orientations of major stress patterns during mesozoic and cenozoic time an initial rift stage in the early jurassic is characterized by normal oblique faults bounding tilted blocks which are associated with tensional paleostress patterns whose minimum component trends west northwest east southeast faults parallel to this direction are interpreted as paleotransfer faults we assume that the divergent motion responsible for the opening of the rift system in the early jurassic was oriented west northwest east southeast subparallel to the paleotransfer faults and the trend of the minimum component a of the paleostress field the east west trending jurassic central eastem high atlas rift opened obliquely and not in a pure strike slip stress regime along east west striking faults as previously proposed a later stage of rifting middle jurassic is characterized by large normal faults and is supposed to accompany local movements probably due to gravity the uplift of the high atlas belt occurred mainly during the cenozoic period as a consequence of a north south to northwest southeast directed compression related to collision between europe and africa rcsumc une analyse structurale sur le terrain a kt6 effectuk dans les montagnes du haut atlas marocain les solutions du problbme des palkocontraintes renseignent sur les orientations successives des principaux modes de distribution des contraintes durant les pkriodes du mksozoique et du ctnozoique un stade initial de rift est apparu au jurassique prkcoce il est caractkrisk par des failles normales obliques dtveloppces en bordure de blocs basculcs elles sont associces au rkgirne des palhontraintes extensives avec une composante minimale orientce ouest nord ouest est sud est les failles paralltles i cette direction sont interprktks come des failles de palcotransfert nous supposons que le mouvement de divergence responsable de l ouverture du systkme de rift au jurassique prccoce ctait orient6 ouest nord ouest est sud est subparalltle aux failles de palkotransfert et a la direction de la composante minimale a3 du champ de pal6ocontraintes le rift jurassique de direction est ouest de la rcgion centrale orientale du haut atlas fut overt obliquement contrairement l hypothtse proposk antkrieurement d un regime coulissant de contrainte pure agissant le long des failles orientkes est ouest le stade tardif de distension jurassique moyen est caractcrisk par de grandes failles normales on assume qu il chit accompagnk de mouvements locaux probablement d origine gravitaire le soultvement de la ceinture du haut atlas a kt6 rkalisk principalement durant la pcriode du ccnozoique comme la cons6quence de la compression orientke nord sud 9 nord ouest sud est associce b la collision entre 1 europe et 1 afrique traduit par la rkdacti

hydrology of meteoric diagenesis residence time of meteoric ground water in island fresh water lenses with application to aragonite calcite stabilization rate in bermuda the average age of ground water that discharges at the shoreline from an island fresh water lens is equal to the volume of ground water in the lens divided by the total recharge to the island this island average residence time 0 is easily calculated by dupuit ghyben herzberg analysis dgh from estimates of the controlling variables recharge hydraulic conductivity and porosity it is estimated that 0 is usually on the order of 1 to a few tens of years in fresh water lenses of small width on the order of 100 to a few thousand meters strip islands where calcarenite is undergoing early diagenesis lateral variation in interstitial velocity and distribution of ground water age within the lens can be calculated from an approximate theory using dgh potentials and an assumption that discharge is uniformly distributed with depth results are within a few percent of those from rigorous flow net construction velocities range laterally through two orders of magnitude contours of ground water age are nearly horizontal over most of the lens at a depth of about 40 of the depth to the interface the ground water age is 0 2 and it equals 0 at about 60 of the depth to the interface flow net construction shows that velocities are greatest at the water table and decrease rapidly downward to the value given by dgh representative velocities halfway between the ground water flow divide and the shoreline are 10 to 100 m yr in these islands application of these calculation procedures to bermuda leads to a revised estimate of ground water age for water samples that have been used to estimate the rate of aragonite to calcite transformation in the fresh water lenses of that island this rate is an order of magnitude less than that in lenses in holocene oolitic cays of the bahamas the ratio of stabilization rate to amount of aragonite appears to be about the same in the two settings the value of the ratio implies a half life of 6 000 7 000 yr for aragonite to calcite transformation in these lenses

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

shear stress on a vertical strike slip fault in a layered medium due to tectonic loading san andreas fault system depth distribution of shear stress at a transform boundary induced by tectonic loading is analyzed the lithospheric plate is assumed to be made up of two elastic layers and is loaded by the far field shear stress the upper part of the plate boundary is locked the slipping region that underlies the locked part is modeled by a mode iii crack containing a breakdown zone owing to the existence of a breakdown zone the stress possesses a pronounced peak at the tip of the slipping region and the loading stress takes high values both in front of the slipping region and within the breakdown zone the stress in the upper part of earth s crust is little influenced by the form of the slipping region and is mainly affected by elastic properties of the medium the comparison of the present results with those based on observational data in particular the data pertaining to the 1979 imperial valley earthquake allow the formulation of two general suggestions related to the faulting process first the depth difference between the point of initiation and the point of deepest penetration of rupture associated with major earthquakes correlates with the size of the breakdown zone second the process of faulting in the top part of earth s crust is primarily controlled by medium rigidity which explains why faulting and seismicity are suppressed in sediments

strontium isotopes in early jurassic seawater abstract the analysis of well preserved well dated belemnites and oysters from the jurassic of great britain has resulted in a well constrained detailed seawater strontium isotope curve for the early jurassic the preservation of fossil low mg calcite was monitored using mn fe 13 c and 18 o iron was the most useful indicator with about 75 of the samples containing more than 150 ppm fe showing 87 sr 86 sr ratios elevated relative to adjacent points on the curve high mn concentrations less often correlated with elevated 87 sr 86 sr ratios however low mn concentrations 87 sr 86 sr 13 c and 18 o proved to be insensitive to diagenesis as it affects 87 sr 86 sr the principal features of the strontium isotope curve include a rise to about 0 70772 in the latest triassic and earliest jurassic hettangian from the hettangian the curve begins a roughly linear descent through the sinemurian and pliensbachian following a small levelling off and increase in the late pliensbachian the curve falls rapidly to its early jurassic minimum of 0 70706 it then gently increases through the toarcian until the falciferum zone where it shows an apparently abrupt increase to 0 70719 before continuing its slow increase to 0 70728 in the aalenian bajocian middle jurassic this reversal of the downwards trend established in the sinemurian and pliensbachian had not been previously identified the sinemurian and pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones 0 5 to 1 ma

comment on oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater by h qing and j veizer

quantitative interpretation of pulsed neutron capture logs part 2 inversion of measurements in thinly bedded formations we have developed an inversion method to reduce shoulder bed effects on pulsed neutron capture pnc logs for estimating layer by layer capture cross sections the method is based on a previously developed rapid approximation of pnc logs tests performed on synthetic examples that include a variety of lithology saturating fluid and bed thickness configurations confirm the efficiency reliability and stability of the inversion procedure inversion consistently improves the vertical resolution and definition of pnc logs across beds thinner than 45 cm our fast iterative algorithm inverts logs in seconds of cpu time and is therefore suitable for joint petrophysical interpretation with other open and cased hole logs

structural aspects of the middle and the high atlas morocco phenomena and causalities since the beginning of the mesozoic the structural development of the middle atlas and the central high atlas was controlled by a pre existent fault pattern which was reactivated repeatedly in various manners

a survey of multi stage diagenesis and dolomitization of jurassic limestones along a regional shelf to basin transect in the ziz valley central high atlas mountains morocco abstract petrographic and geochemical data from five localities in the ziz valley of morocco indicate that jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings burial diagenesis that was most pronounced in basinal settings and late diagenesis caused by compression and uplift of the high atlas mountains marine cements occur at all five localities from shelf to basin center although cement types vary from peloidal microcrystalline cements updip on the shelf to equant calcite in basinal settings presence of moldic grains and or mg poor fe poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region leaving only one locality unaffected by early meteoric processes 87 sr 86 sr ratios of 0 70810 0 70895 greater than 87 sr 86 sr of coeval limestones mg contents that decrease upward from 47 5 to 43 0 mol mgco 3 presence of dolomitized marine cements and dolomite cements that postdate marine cements but predate meteoric to burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis however poorer preservation of depositional fabrics lower 18 o values and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial whereas those updip did not compaction during diagenesis generated numerous concavo convex and sutured intergranular contacts at updip shelf downdip shelf and basinal localities where earlier meteoric cementation was not extensive compaction was insignificant in more extensively cemented mid shelf settings high sr 1200 3800 ppm and fe 1000 2300 ppm contents in brachiopod grains suggest that lmc components underwent some modification during burial in basinal settings in sr rich reducing waters fe contents of late intergranular cements increase from 2000 ppm at the basin s edge to as much as 6000 ppm in the basin s center bedding parallel stylolites occur at all localities the most negative 18 o values of sparry dolomites near the tizi n firest fault 6 2 vs pdb imply diagenetic temperatures of 65 85 c assuming water 18 o values of 0 0 2 0 vs smow those temperatures are much less than previous estimates of burial temperatures in the high atlas basin an isotopic gradient extrapolating to roughly 5 km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area comparison of morphologies of transverse stylolites which are found at all localities with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis uplift accompanying that compression allowed influx of low mg waters that along with other factors caused calcitization of dolomites the fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge opposite trends in earlier cements and reflecting uplift of the high atlas mountains and resultant changes in patterns of groundwater flow

structural chronology of the alleghanian orogeny in southeastern west virginia the trend change between central and southern appalachian structures is sharply defined in southeastern west virginia there n30 35 e trends coincident with major central appalachian folds such as the browns mountain anticline end abruptly at n60 e trending southern appalachian structures along the st clair fault analysis of local and regional folds cleavage bedding perpendicular stylolite seams and bedding and fault slickenlines reveals that layer parallel shortening directed n10 30 w occurred in nonfolded greenbrier group mississippian carbonates well into the present appalachian plateau area this structural event is early and is associated with the evolution of southern appalachian folds and faults south of the st clair fault central appalachian folds and mesoscopic structures were superimposed on this early layer parallel shortening fabric this structural chronology indicates that southern appalachian folds and faults predated the development of central appalachian structures in the region

mineralogical stabilization of high magnesium calcite geochemical evidence for intracrystal recrystallization within holocene porcellaneous foraminifera mineralogical stabilization of porcellaneous foraminifera is known to consist of chemical change without textural alteration at any scale however the nature of the alteration process has not been fully reconciled porcellaneous foraminifera from the freshwater diagenetic zone of two small islands in the schooner cays bahamas are in the midst of mg loss and delta 18 o change with no textural alteration at any scale these data indicate that the mineralogical stabilization process or recrystallization is a repetitive intracrystal incongruent dissolution precipitation reaction each recrystallization produces a calcite with a slightly lower mg content than its predecessor the rate of stabilization is dependent on time and hydrologic flux older phreatic zone material is the most altered and younger vadose zone material is the least altered numerical modeling of the chemical diagenesis indicates that the molar water rock ratio of a single recrystallization is mg between 0 0001 and 0 0003 these values are two orders of magnitude less than that generally assigned to calcite precipitation from a large fluid reservoir and they suggest that d mg is dependent on the type and scale of reaction though the nature of that dependency is unknown

a numerical model of the evolution of ocean sulfate and sedimentary sulfur during the last 800 million years abstract a reservoir model describing the time evolution of the sedimentary cycle of sulfur over the past 800 my has been developed as a first approximation the ocean sulfate concentration is assumed to be time independent with this assumption the model is integrated backward in time and a new initialization procedure is derived in order to calculate the present state of the system which must be compatible with both observational data and model equations the effects of a variation of the present state of the cycle on its past evolution are investigated it is found that when the present gypsum reservoir content is too low or when the weathering rate constants are too high no acceptable solution can be obtained for the evolution of the cycle since one reservoir is forced to depletion the sensitivity of the model to the mean isotopic composition of the sedimentary system and to the fractionation factor during pyrite formation is also studied moreover a model with time dependent ocean sulfate concentration was developed the existence of an acceptable solution appears to be linked to the steady state hypothesis for ocean sulfate since a model with no acceptable steady state solution may be integrated until t 800 my without any problem of reservoir depletion when the time dependent equations are used a tentative evolution of the ocean sulfate concentration is calculated it is shown that this concentration is negatively correlated to the 34s of seawater sulfate the carbon cycle is modelled in order to compare the calculated 13c of carbonate deposits to the observational data