supratidal evaporitic dolomite at ojo de liebre lagoon mineralogical and isotopic arguments for primary crystallization abstract this pair deals with a new site of dolomite formation in supratidal evaporite flats at the southernmost edge of the ojo de liebre evaporitic complex baja california mexico the sedimentary context is mostly siliciclastic except for calcareous outcrops around the flats both chemical and isotopic data on the interstitial solutions demonstrate a mixing between mg2 rich marine derived brines and calcium bicarbonate bearing continental waters which are subjected to concentration by capillary evaporation the lack of any in situ solid caco3 in the sedimentary matrix argues for direct crystallization of dolomite from aqueous solutions this is documented by the oxygen isotope compositions of the dolomite which are consistent with deposition under equilibrium conditions of ambient temperature and 18o of water of the interstitial solutions furthermore the low 13c values of the dolomites and the absence of living plants or organisms in the basin indicate that a fraction of the total dissolved inorganic carbon reservoir is biogenic and is mostly provided by continental groundwaters therefore crystallization of dolomite occurs within the sands infiltrated by solutions whose state of saturation relative to dolomite is mainly governed by the chemistry of continental groundwaters mixing with the inflowing marine brines

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

did late palaeoproterozoic assembly of proto australia involve collision between the pilbara yilgarn and gawler cratons geochronological evidence from the mount barren group in the albany fraser orogen of western australia abstract the mount barren group which crops out on the southern margin of the yilgarn craton has been considered an allochthonous synorogenic sequence to a mesoproterozoic 1300 ma collision between southwestern australia southeastern australia and eastern antarctica during assembly of east gondwana reassessment of its stratigraphic geometry confirms the historical perspective that nonconformable and faulted contacts are preserved on autochthonous archaean orthogneiss of the yilgarn craton emphasising that it is not an allochthonous sequence shrimp u pb dating of early diagenetic xenotime establishes a late palaeoproterozoic 1696 7 ma depositional age for the mount barren group thereby negating a mesoproterozoic synorogenic origin the mount barren group now correlates in time with strike slip sequences in the capricorn orogen approximately 1000 km to the north and which were a post collisional response to 1750 ma sinistral transform collision between the pilbara and yilgarn cratons shrimp u pb detrital zircon geochronology of the mount barren group therefore has an important role in establishing if pilbara yilgarn transform collision were either independent of or a component in the late palaeoproterozoic assembly of a proto australian continent detrital zircon age data from four samples of different sandstone beds in the mount barren group define complex age spectra composed of seven significant provenance age subpopulations comparison to age spectra from provinces with a possible geotectonic link to the yilgarn craton indicates that either the pilbara craton or the gawler craton was the source terrain a barrier to a dispersal system link to the pilbara craton points to the gawler craton as the source a provenance link that was also made for the now redundant 1300 ma scenario the 1696 7 ma depositional age and zircon provenance analysis support a contentious hypothesis involving coeval collisions between the pilbara yilgarn and yilgarn gawler cratons during assembly of a proto australian continent some 500 400 million years earlier than envisaged by other reconstructions

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

a structural and metamorphic traverse across the albany mobile belt western australia the albany mobile belt is divided into three domains the southern central and northern domains based on their aeromagnetic response and their distinctive structural and metamorphic characteristics amphibolite facies rocks of the northern domain are reworked archaean yilgarn block gneisses and dolerite dykes with a progressive southwards increase in the intensity of deformation and metamorphism the northern domain central domain contact is marked by an oblique thrust zone with a component of dextral transcurrent movement developed during the second deformation event d2 the central domain consists of two pyroxene granulites together with quartz magnetite gneiss which has undergone penetrative ductile non coaxial deformation with a dominant dextral transcurrent component during the first deformation event d1 and has subsequently been folded around kilometric and metric scale folds overturned to the nnw during d2 the central domain grades transitionally southwards into the southern domain where retrograde amphibolite facies rocks dominate the albany fraser province is a proterozoic mobile belt formed during two prolonged tectonothermal events both of which consist of a combination of dextral transcurrence and northwards thrusting

the composite magnetic anomaly map of the east antarctic abstract airborne and marine magnetic observations in east antarctica and adjacent seas of the indian ocean were compiled for a magnetic anomaly map of the antarctic for east antarctica over 260 000 line km of russian reconnaissance magnetic data were used that had been collected since 1955 mainly at line spacings of about 5 20 and 50 km for the offshore areas magnetic data from american australian german japanese and russian marine expeditions were incorporated digitally recorded data and data digitized from published and unpublished maps and profiles were included in the compilation local grids of these data were developed and merged into a regional grid at an interval of 5 km the prime product of this compilation is a shaded relief map that shows the most complete and coherent perspective to date of the region s magnetic character in combination with other types of data the compilation provides new insight on the tectonic features and history of this largely inaccessible region of the world it maps out approximately 4300 km of the antarctic continental margin magnetic anomaly acmma related to gondwana breakup new cratons and mobile belts and large submarine igneous provinces

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

the mesoproterozoic rayner province in the lambert glacier area its age origin isotopic structure and implications for australia antarctica correlations within the rayner province in the lambert glacier area two proterozoic lithotectonic zones are distinguished which differ in age and lithology in the fisher zone a significant juvenile component 1nd t 1 4 2 to 4 is represented by mafic intermediate and felsic plutonic and volcanic rocks the tectonomagmatic processes were concentrated at c 1400 1200 ma in the beaver zone orthogneisses record multiple emplacement metamorphism and ductile deformation at c 1150 930 ma new u pb shrimp ii zircon ages c 1140 ma and 1095 ma protolith emplacement c 950 850 and c 540 ma metamorphism chemical and sm nd isotopic data indicate that both zones have their continuations in the eastern amery ice shelf coast the rayner province has some geological features in common with the albany fraser orogen in western australia suggesting that these regions evolved in similar tectonic environments at c 1400 1300 ma followed by steady closure from east to west of the ocean that separated the mawson continent from western australia supplementary material chemical compositions of rocks a summary of trace element ratios a summary of published u pb dates field photo views new u pb analyses by sims shrimp ii and new sm nd data are available at www geolsoc org uk sup18621 the crystalline basement of east antarctica comprises a few relatively small archaean cratons enclosed in a vast proterozoic circum east antarctic mobile belt ceamb yoshida kizaki 1983 ivanov kamenev 1990 tingey 1991 yoshida 1992 black et al 1992a development of the mobile belt occurred largely between c 1150 ma and c 950 ma fitzsimons 2000a 2003 and references therein the ceamb was correlated with the grenville province of north america and thought to have been conjugated within the rodinia supercontinent hoffman 1991 dalziel 1991 condie 2001 in some areas the ceamb was affected by major tectono thermal reworking at c 550 500 ma and this suggested a model of continental collision resulting in the amalgamation of gondwana at that time shiraishi et al 1992 fitzsimons 2000a boger et al 2001 typical rock assemblages of the ceamb were first described in western enderby land by kamenev et al 1965 and kamenev 1972 these authors distinguished the rayner metamorphic complex considering it to be a proto geosynclinal formation other investigators considered the rayner complex to be strongly and probably repeatedly reworked earliest precambrian crust with the rayner structural episode c 1000 950 ma being the most prominent event sandiford wilson 1984 sheraton et al 1987 u pb zircon age data obtained from various parts of the ceamb showed considerable spread e g fitzsimons 2000b and three separate tectonic provinces have now been from harley s l fitzsimons i c w zhao y eds 2013 antarctica and supercontinent evolution geological society london special publications 383 35 57 first published online may 20 2013 http dx doi org 10 1144 sp383 1 the geological society of london 2013 publishing disclaimer www geolsoc org uk pub ethics by guest on december 12 2013 http sp lyellcollection org downloaded from

determination of paleoheat flux from vitrinite reflectance data a method is presented for determining paleoheat flux from the reflectance of vitrinite collected from wells the maturation of vitrinite is modeled using a time temperature integral that assumes pseudo first order kinetics in testing the numerical algorithm it was necessary to introduce a critical temperature of 295 k below which vitrinite does not mature this result is consistent with the latest findings of independent researchers the degree of consistency obtained from a number of wells is not perfect much of the inconsistency can be attributed to large uncertainties in the vitrinite reflectance readings to test the method heat flow history was determined for the north sea basin and the national petroleum reserve of alaska quantitative results were obtained detailing the increase and or decrease of heat flux with time and were consistent with the qualitative behavior inferred for paleoheat flow from current geologic models

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

in situ u pb geochronology and geochemistry of a 1 13 ga mafic dyke suite at bunger hills east antarctica the end of the albany fraser orogeny abstract antarctica contains continental fragments of australian indian and african affinities and is one of the key elements in the reconstruction of nuna rodinia and gondwana the bunger hills region in east antarctica is widely interpreted as a remnant of the mesoproterozoic albany fraser orogen which formed during collision between the west australian and mawson cratons and is linked with the assembly of rodinia previous studies have suggested that several generations of mafic dyke suites are present at bunger hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints new in situ shrimp u pb zircon and baddeleyite dates of respectively 1134 9 ma and 1131 16 ma confirm an earlier rb sr whole rock age estimate of ca 1140 ma for emplacement of a major mafic dyke suite in the area existing and new geochemical data suggest that the source of the dyke involved an emorb like source reservoir that was contaminated by a lower crust like component the new age constraint indicates that the dykes post date the last known phase of plutonism at bunger hills by ca 20 million years and were emplaced at the end of stage 2 of the albany fraser orogeny in current models post orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at bunger hills a major episode of mafic dyke emplacement following uplift cooling and plutonic activity with increasing mantle input suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse if the undated olivine gabbro dykes with similar trend geochemistry and petrology at windmill islands are coeval with the ca 1134 ma dyke at bunger hills this would suggest the presence of a major dyke swarm at least 400 km in extent in such case the dykes could have been emplaced laterally from a much more distant mantle source possibly a plume and interacted with the locally heterogeneous and variably metasomatised lithosphere

regional correlation of mesoproterozoic structures and deformational events in the albany fraser orogen western australia geologic mapping and meso and microstructural analysis of a previously unstudied area in the western part of the mesoproterozoic albany fraser orogen western australia leads to a correlation of deformational events along the exposed length of the orogen the earliest deformational event d1 accompanied granulite facies metamorphism and is characterized by a locally preserved subhorizontal foliation and associated recumbent folds this event is interpreted to record northwest southeast contraction the second event d2 produced a subvertical east northeast striking foliation and variably plunging upright folds northwest vergent folds toward 333 and sparse kinematic indicators suggest southeast up tectonic transport with a dextral component of movement a third deformational event d3 is manifested by a conjugate set of mylonitic to cataclastic shear zones with a dominant dextral set oriented at 105 and a subordinate sinistral set oriented at 010 the bisector of the average orientations for the shear zones is 325 consistent with northwest southeast shortening like the d2 structures the d3 structures may represent deformation in a dextral transpressive setting d4 structures include two subvertical joint sets one oriented at 355 and the other at 105 all structures are similar in geometry and kinematics to structures near albany which have been constrained to 1190 1170 ma precambrian res 59 1992 95 as well as structures east of esperance that are associated with a major tectonothermal event at 1345 1260 ma precambrian res 102 2000 155 additional geochronological data particularly from the western albany fraser orogen are needed to resolve this apparent discrepancy there is a near one to one correlation in lithology structural style metamorphic grade kinematics and possibly timing of deformational events between the albany fraser orogen and the bunger hills of east antarctica supporting the previously suggested ties between these two areas marked dissimilarities however in model tdm ages u pb zircon dates kinematics and timing of deformation and timing of metamorphism between the albany fraser orogen and the oaxacan complex of southern mexico raise serious doubts regarding the viability of rodinia reconstructions that juxtapose those two mesoproterozoic terranes

thermal history of michigan basin the elevated organic maturity observed in shallowly buried units from the michigan basin implies that higher temperatures and thicker overburdens once existed in the basin evidence from sediment accumulation rates regional dips and maturity of pennsylvanian age coals suggests that up to 1 000 m 3 280 ft of sediment were removed by erosion prior to the late jurassic when the basin became stable geothermal gradients during paleozoic basin subsidence probably ranged from 35 to 45 c km 19 to 25 f 1 000 ft in contrast to the average present value of 25 c km 14 f 1 000 ft depths to the top of the oil window ranged from 1 900 to 2 300 m 6 230 to 7 540 ft during the paleozoic post pennsylvanian erosional uplift and further thermal m turation of the basin have combined to raise the top of the oil window to its present level of 500 m 1 640 ft

petrogenesis of plutonic rocks in a proterozoic granulite facies terrane the bunger hills east antarctica abstract several compositionally varied suites of plutonic rocks gabbro quartz monzogabbro granite quartz monzodiorite in the bunger hills were emplaced at depths of 20 km over a relatively short time interval 1170 1150 ma during the waning stages of granulite facies metamorphism the peak of which occurred 1190 ma ago alkaline syenite to granite rocks were emplaced west of the denman glacier at david island 515 ma ago most of these rocks crystallised from magmas derived by melting of heterogeneous mantle comprising at least two components nb poor long term enriched lithosphere and a nb rich oib ocean island basalt type within plate component metasomatic enrichment of the source of the plutonic rocks as well as that of 500 ma alkaline dykes may have been contemporaneous with continental crust formation in the area during the late archaean or early proterozoic much of the chemical variation of the different suites can be explained by fractional crystallisation but other processes particularly variations in the extent and or p t conditions of melting were also important crustal contamination does not appear to have been a major factor in the petrogenesis of the bunger hills magmas but cannot be entirely discounted particularly for some of the granitic rocks the more evolved david island syenitic intrusives may have been derived by melting of lower crustal rocks

in situ shrimp u pb dating of monazite integrated with petrology and textures does bulk composition control whether monazite forms in low ca pelitic rocks during amphibolite facies metamorphism abstract bulk composition and specific reaction history among common silicate minerals have been proposed as controls on monazite growth in metapelitic rocks during amphibolite facies metamorphism it has also been implied that monazite that formed during greenschist facies metamorphism may be preserved unchanged under upper amphibolite facies conditions if correct this would make the interpretation of monazite ages in polymetamorphic rocks exceedingly difficult because isotopic dates could vary significantly in rocks that have experienced identical metamorphic conditions but differ only slightly in whole rock composition low ca pelitic schists from the mount barren group in southwestern australia display a range of whole rock compositions in afm space and different peak mineral assemblages resulting from amphibolite facies metamorphism 8 kb 650 c in this study we test whether bulk composition controls the formation of monazite through geochronology and textural evidence linking monazite growth with deformation and peak metamorphism x ray element mapping of monazite from the metapelitic rocks reveals concentric zoning in many grains with compositionally distinct cores and rims in situ shrimp u pb geochronology of monazite yields two 207 pb 206 pb age populations the cores and texturally early monazite give an age of 1209 10 ma interpreted to record prograde metamorphism whereas the rims and late monazite define a single population of 1186 6 ma which is considered the likely age of peak thermal metamorphism the growth of monazite was widespread in low ca pelitic schists representing a broad range of compositions in afm space indicating that variations in bulk composition in afm space did not control the formation of monazite during amphibolite facies metamorphism in the mount barren group

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

post orogenic 1500 ma thermal history of the palaeo mesoproterozoic mt isa province ne australia abstract we present hornblende white mica biotite and alkali feldspar 40 ar 39 ar data from paleo mesoproterozoic rocks of the mt isa inlier australia which reveal a previously unrecognised post orogenic non linear cooling history of part of the northern australian craton plateau and total fusion 40 ar 39 ar ages range between 1500 and 767 ma and record increases in regional cooling rates of up to 4 c ma during 1440 1390 and 1260 1000 ma forward modelling of the alkali feldspar 40 ar 39 ar arrhenius parameters reveals subsequent increases in cooling rates during 600 400 ma the cooling episodes were driven by both erosional exhumation at average rates of 0 25 km ma and thermal relaxation following crustal heating and magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing in the arunta block and gawler craton late mesoproterozoic cooling could have either been driven by increased rates of exhumation or a result of thermal relaxation following a heat pulse that was synchronous with dyke emplacement in the arunta musgrave and mt isa province as well as grenville aged orogenesis in the albany fraser belt latest neoproterozoic cambrian cooling and exhumation was probably driven by the convergence of part of the east antarctic shield with the musgrave block and western australia petermann ranges orogeny as well as collisional tectonics that produced the delamerian ross orogen major changes in the stress field and geothermal gradients of the australian plate that are synchronous with the assembly and break up of parts of rodinia and gondwana resulted in shearing and repeated brittle reactivation of the mt isa inlier probably via the displacement of long lived basement faults within the northern australian craton

recent advances in petroleum geochemistry applied to hydrocarbon exploration this paper presents a critical review of recent advances in the study of generation maturation and migration of petroleum the different source potentials for generating hydrocarbons depend on the type of kerogen in addition to the classic types i ii and iii a residual oxidized organic matter is also observed which has no potential for hydrocarbon generation coal is widely considered as a potential source for gas but some coals may generate sizeable amounts of oil as well the development of routine analytical tools particularly pyrolysis makes possible the preparation of geochemical logs that are the basis for screening samples and for interpreting other analyses among optical techniques the major subjects of discussion concern the nature of amorphous organ c matter which may encompass different chemical types of kerogen and the applicability of vitrinite reflectance techniques to type i and ii kerogens following important advances in identification of biological markers in sediments and crude oils these markers are used for oil source rock correlation and proposed for reconstruction of depositional environment and subsequent thermal evolution migration of hydrocarbons is now better understood and the importance of a hydrocarbon phase migration is widely recognized overpressuring of pore fluids is mainly responsible for expulsion of petroleum geologic models of basin evolution subsidence compaction thermal history and geochemical models hydrocarbon generation and migration become progressively available their integration int a comprehensive model should be one of the major developments in future petroleum exploration

reservoir assessment of middle jurassic sandstone dominated formations in the egersund basin and ling depression eastern central north sea abstract reservoir quality assessment was conducted from petrophysical analysis and rock physics diagnostics on 15 wells penetrating middle jurassic sandstone reservoir formations in different regions of the eastern central north sea seismic interpretation on available 3d and 2d seismic reflection data was utilized to map thickness variations and to draw broad correlations to structural features such as salt structures and faults in the central egersund basin the sandnes formation shows good reservoir properties gross thickness 107 147 m n g 33 53 while the bryne formation exhibits poorer reservoir quality n g

regional epigenetic dolomitization in the bonneterre dolomite cambrian southeastern missouri a dolomite bed averaging 6 m in thickness forms the base of a limestone and shale facies of the bonneterre dolomite cambrian southeastern missouri the dolomite immediately overlies the lamotte sandstone and covers an area of more than 16 600 km 2 it is proposed that this basal dolomite formed as a result of interaction with warm basin derived water circulating through the underlying lamotte sandstone and that the dolomitization event coincided with the emplacement of mississippi valley type sulfide ores in the nearby viburnum trend this hypothesis is supported by 1 petrographic evidence that suggests an origin at temperatures 50 c 2 cathodoluminescent microstratigraphy of dolomite cements which is correlative with that in gangue dolomite cements in the nearby orebodies and 3 stable oxygen and carbon isotope values that are consistent with a basinal origin of the dolomitizing water the basal dolomite bed may represent one of the most extensive epigenetic dolomites yet documented this implies that a basinal water can alter rocks over a wide area at a considerable distance from its source

gondwanaland significance of lower palaeozoic deformation in central india and sw western australia the stirling range formation of western australia and the vindhyan supergroup of central india have undergone two identical lower palaeozoic deformation events folding related to dextral reactivation of basement structures suggests a possible continuation of the indian narmada son brahmaputra lineament into sw western australia subsequent open folding during ne sw crustal shortening in both terranes is possibly related to gondwanaland assembly whilst the shillong group in ne india appears similar to the stirling range formation and vindhyan supergroup further geochronology is required to substantiate a common tectonic history

stable isotope study of karstic related dolomitization jurassic rocks from the coastal plain israel the middle upper jurassic section in israel contains scattered areas of dolomitization within predominantly limestone formations one of these dolomitization sites in the subsurface in the ashdod gan yavne area is associated with karst features as well as mississippi valley type mineralization saddle dolomite fluorite quartz bituminous clay calcite and hydrocarbons four distinct carbonate phases limestone coarsely crystalline brown host rock dolomite white sparry saddle dolomite and white vein calcite were analyzed for stable isotopes of carbon and oxygen these phases are believed to represent successive stages of diagenesis epigenesis of the ashdod gan yavne deposits water taken from fluid inclusions from saddle dolomite and calcite pairs was analyzed for stable isotopes of hydrogen as well in all but a few samples the carbon isotope ratio 1 or 1 per thousand was identical for all the carbonate phases studied the oxygen isotope ratio for the limestones in all but a few cases was much heavier 4 5 to 7 0 per thousand than that of the host rock dolomites 5 7 to 11 3 per thousand and is interpreted as reflecting dolomitization following limestone deposition the oxygen isotopes in the host rock dolomites show geographic as well as stratigraphic variations the host rock dolomites in the zohar formation in the ashdod area have characteristically less negative delta 18 o values 6 7 to 9 0 per thousand than the host rock dolomites in the shderot formations in the ashdod area and the dolomites from both the zohar and shderot formations in the ashqelon area delta 18 o 9 2 per thousand the oxygen isotope results can be interpreted as evidence for either two different events of dolomitization one regional and the other restricted to the ashdod area or a single event of dolomitization affecting two separate aquifers which differ in their oxygen isotope composition saddle dolomites which were earlier thought to be a separate phase of epigenesis representing the earliest stage of mineralization postdating dolomitization showed isotopic values identical to the host rock dolomites and showed the same formational and geographic variations as the host rock dolomites isotopic signatures of the vein calcites believed to be a much later stage of mineralization showed delta 18 o values of 5 6 to 10 8 per thousand similar to the trend expected for dolomite calcite pairs in the zohar formation and the opposite trend in the shderot formation a large range of delta d values 38 to 58 per thousand was measured from fluid inclusions from saddle dolomites and vein calcites this range is significantly different from the hydrogen isotope composition presently found in formation waters as well as local meteoric water

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

caprock characterization of upper jurassic organic rich shales using acoustic properties norwegian continental shelf abstract our analysis of a comprehensive well log database and complementary mineralogical and geochemical information indicates that the risk for upper jurassic shales on the norwegian continental shelf ncs to permit severe leakage of hydrocarbons from the reservoir is generally low even in the case of substantial uplift the content of brittle minerals organic content and compaction are dominant factors that explain the observed discrepancies in acoustic properties of organic rich caprock shales in particular variations in silt clay content in clay dominated shales are found to primarily influence sonic velocity and to correlate closely with gamma ray where the uranium contribution is limited grey shales changes in organic content exhibit a stronger density component and are seen to counteract or mask the compaction effect on velocity and density in kimmeridgian black shales the hekkingen draupne and tau formations are distinctly different from the underlying grey shale formations in acoustic properties despite that the latter group also contains significant amounts of organic matter based on the low permeability and high capillary sealing capacity of clay dominated shales we conclude that even for a silty seal migration through the caprock matrix is highly unlikely furthermore tectonic fracturing is an ineffective leakage mechanism when the seal is poorly consolidated cemented prior to uplift brittleness related to both mineralogical composition and consolidation is consequently a crucial parameter for predicting seal integrity in exhumed basins our rock physics framework and interpretations relate this rather qualitative parameter to acoustic properties and thus to seismic data

multiple episodes of dolomitization and dolomite recrystallization during shallow burial in upper jurassic shelf carbonates eastern swabian alb southern germany abstract the upper jurassic of the eastern swabian alb is composed of oolitic platform sands with associated microbe siliceous sponge mounds at the platform margins they are surrounded by argillaceous or calcareous mudstones and marl limestone alternations deposited in adjacent marl basins partial to complete dolomitization is predominantly confined to the mound facies six types of dolomite as well as one type of ankerite document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization the earliest massive matrix dolomitization is ca rich has slightly depleted oxygen isotope values relative to late jurassic seawater and carbon isotopic values in equilibrium with late jurassic seawater this initial massive matrix dolomitization occurred during latest jurassic to earliest cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 c hydrologic conditions and mass balance calculations indicate that burial compaction provided sufficient fluids for dolomitization mg is derived from negligibly modified seawater that was expelled from the adjacent off reef strata into the mound facies position of the mounds along the platform margins controlled the distribution of the shallow burial dolomite covariant trends between textural modification increasing stoichiometry partial changes in trace element content mn fe sr and depletion in stable isotopes as well as distinctive cl pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures strong sr enrichment of the second phase of recrystallized dolomite is ascribed to sr rich meteoric waters descending from overlying aragonite bearing reef limestones or evaporite bearing peritidal carbonates late stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial ankerite associated with sulphide and sulphate minerals and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during late cretaceous to tertiary alpine orogenesis

geothermics of the williston basin in canada in relation to hydrodynamics and hydrocarbon occurrences over 8 400 bottom hole temperature bht values from the canadian part of the williston basin were analyzed and a temperature high was discovered in the weyburn area of southeastern saskatchewan geothermal gradients thermal conductivities and heat flow have been investigated for most of the mesozoic cenozoic clastic unit as well as the upper paleozoic carbonate evaporite unit regional heat flow variations with depth occur which are closely related to the hydrodynamics governed by the topography and geology the blanketing effect of low conductivity shaly formations may cause a temperature anomaly in the south where the thickest phanerozoic cover exists however the weyburn high can be explained only partially in this way hydrodynamics has also contributed to formation of the temperature anomaly there the process of forming the anomaly by the blanketing effect and hydrodynamics also contributed to oil deposition there is a correlation between mississippian oil occurrences in the southeastern part o

relationships between thermal maturity indices calculated using arrhenius equation and lopatin method implications for petroleum exploration thermal maturity can be calculated with time temperature indices tti based on the arrhenius equation using kinetics applicable to a range of types ii and iii kerogens these ttis are compared with tti calculations based on the lopatin method and are related theoretically and empirically via vitrinite reflectance to the petroleum generation window the ttis for both methods are expressed mathematically as integrals of temperature combined with variable linear heating rates for selected temperature intervals heating rates control the thermal maturation trends of buried sediments relative to arrhenius ttis lopatin ttis tend to underestimate thermal maturity at high heating rates and overestimate it as low heating rates complex burial histories applicable to a range of tectonic environments illustrate the different exploration decisions that might be made on the basis of independent results of these two thermal maturation models

crustal residence history and garnet sm nd ages of high grade metamorphic rocks from the windmill islands area east antarctica nd whole rock data from the windmill islands area yield early proterozoic to middle archaean nd model ages these crustal residence times are consistent with regional correlations with other parts of antarctica bunger hills denman glacier area and the albany fraser orogen of south western australia during the mid proterozoic and thus support reconstructions with a continuous mid proterozoic orogen in these areas the new nd isotope data provide strong evidence that no age boundary exists between the higher and lower grade parts of the windmill islands area and that the metamorphic complex represents a single terrane with a common crustal history the data support the notion of a time link between the occurrence of intrusive charnockites c type magmas and high grade metamorphism the magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components i e older sediments ca 3 2 2 6 ga and a younger mafic component ca 1 9 ga two garnet sm nd isochrons yield ages of 1156 17 ma and 1137 2 5 ma and are identical to shrimp u pb results on monazite from these samples a garnet sm nd age of 1123 13 ma for the ford granite is significantly younger than the shrimp u pb zircon age for this sample the difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism

reworking of archaean and early proterozoic components during a progressive middle proterozoic tectonothermal event in the albany mobile belt western australia abstract isotopic data derived mainly from u pb zircon ion microprobe analyses are presented for each of the three tectonic domains of the albany mobile belt western australia they show that not only the northern domain but also parts of the central domain originally crystallised about 3 100 ma ago they represent reworked archaean rocks similar to those of the adjoining yilgarn block on the basis of sm nd model ages a somewhat younger less precisely defined crustal formation event of probable early proterozoic 2100 ma age can be inferred for the southern domain and the remainder of the central domain the younger component of the albany belt is older than some previous estimates being of similar initial age to most of the early proterozoic orogenic provinces of northern and central australia interpretation of the sm nd data is not strictly based on conventional model ages for it appears that significant fractionation of the rare earth elements occurred during middle proterozoic crustal melting pegmatites and granites dated by u pb on zircon provide a temporal framework for the middle proterozoic evolution of the albany mobile belt progressive deformation incorporating foliation development up to four superimposed fold generations late tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span about 1190 ma ago dextral transpression and thrusting at 1190 ma postdates granulite facies metamorphism and major deformation events in the fraser mobile belt the widespread 1190 ma rocks are of comparable age to intrusives rocks emplaced during a similar high grade metamorphic event in the bunger hills of antarctica a region that is commonly juxtaposed with the albany mobile belt in gondwana reconstructions

a model of hydrocarbon generation from type i kerogen application to uinta basin utah we have developed a computer model that can predict when and how much oil and gas are generated from a source rock during its burial and later uplift kinetic parameters for the oil and gas generation reactions are obtained from high pressure pyrolysis experiments carried out over a wide range of heating rates and temperatures in our kinetic model which applies only to green river shale we use a single activation energy of 52 4 kcal mole and different pre exponential factors for different products of primary pyrolysis which allows us to extrapolate laboratory derived kinetics to geologic heating rates this model is in contrast to the wide distributions of activation energies or artificially low apparent activation energies used in some models of petroleum formation hen extrapolated to geologic heating rates on the order of 10 c m y our kinetics show that the temperature of the maximum rate of oil generation tp changes by about 15 c when the heating rate is changed by an order of magnitude changes in pressure have relatively minor effects on the kinetics of oil generation but are important for gas generation reactions we used geophysical data from oil fields in the uinta basin of utah to develop a thermal history model of green river formation source rocks this time temperature history was used to predict the maturation level of the kerogen at a given depth and to predict changes in the compositional characteristics of the oil the shape of calculated oil generation rate curves as a function of depth in the basin mimics the shape of the overpressure curves this similarity suggests that oil gas generation may be an important cause of overpressuring maturation levels and compositional characteristics of the oil predicted by our model agree very well with characteristics of the oil recovered from the basin

mechanisms for folding of high grade rocks in extensional tectonic settings abstract this review of structures developed in extensional high grade terrains combined with results of centrifuge analogue modelling illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust thickened orogen several extensional fold mechanisms such as folding within detachment shear zones are similar to those in contractional settings the metamorphic p t t path and not fold style or mode of formation is therefore required to determine the tectonic setting in which some folds developed other mechanisms such as rollover above and folding between listric normal shear zones and folding due to isostatic adjustments during crustal thinning are unique to extensional tectonic settings several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation it is shown that isoclinal recumbent folds refolded by open upright folds may develop during regional extension in the deep crust folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment or from enhanced back rotation of layers between normal shear zones during back rotation folding layers rotated into the shortening field undergo further buckle folding and all may rotate towards orthogonality to the maximum shortening direction this mechanism explains the presence of many transposed folds folds with axial planar pegmatites and folds with opposite vergence in extensional terrains examples of folds in high grade rocks interpreted as forming during regional extension included in this paper are from the grenville province of canada norwegian caledonides albany mobile belt and leeuwin complex of western australia ruby mountains in the basin and range province of nevada the at sund area of greenland the napier complex of enderby land in east antarctica and the kigluaik mountains in western alaska

timing and chemistry of fluid flow events in the lawn hill platform northern australia mudrocks and carbonates of the isa superbasin in the lawn hill platform in northern australia host major base metal sulfide mineralization including the giant strata bound century zn pb deposit mineral paragenesis stable isotope and k ar dating studies demonstrate that long lived structures such as the termite range fault acted as hot fluid conduits several times during the paleoproterozoic and mesoproterozoic in response to major tectonic events illite and chlorite crystallinity studies suggest the southern part of the platform has experienced higher temperatures up to 300 degrees c than similar stratigraphic horizons in the north the irregular downhole variation of illite crystallinity values provides further information oil the thermal regime in the basin and shows that clay formation was controlled not only by temperature increase with depth but also by high water rock ratios along relatively permeable zones k ar dating of illite in combination with other data may indicate three major thermal events in the central and northern lawn hill platform lit 1500 1440 to 1400 and 1250 to 1150 ma this study did not detect the earlier century base metal mineralizing event at 1575 ma 1500 ma ages are recorded only in the south and correspond to the age of the late isan orogeny and deposition of the lower roper superbasin they may reflect exhumation of a provenance region the 1440 to 1300 ma ages are related to fault reactivation and a thermal pulse at similar to 1440 to 1400 ma possibly accompanied by fluid flow with subsequent enhanced cooling possibly due to thermal relaxation or further crustal exhumation the youngest thermal and or fluid flow event at 1250 to 1150 ma is recorded mainly to the cast of the tern lite range fault and may be related to the assembly of the rodinian supercontinent fluids in equilibrium with illite that formed over a range of temperatures at different times in different parts of the platform have relatively uniform oxygen isotope compositions and more variable hydrogen isotope compositions delta o 18 3 5 9 7 parts per thousand v smow delta d 94 to 36 parts per thousand v smow the extent of the 180 enrichment and the variably depleted hydrogen isotope compositions suggest the illite interacted with deep basin hypersaline brines that were composed of evaporated seawater and or highly evolved meteoric water siderite is the most abundant iron rich gangue phase in the century zn pb deposit which is surrounded by all extensive ferroan carbonate alteration halo modeling suggests that the ore siderite formed at temperatures of 120 degrees to 150 degrees c whereas siderite and ankerite in the alteration halo formed at temperatures of 150 degrees to 180 degrees c the calculated isotopic compositions of the fluids are consistent with o 18 rich basinal brines and mixed inorganic and organic carbon sources 6180 3 10 parts per thousand v smow delta c 13 7 to 3 parts per thousand v pdb in the northeast lawn hill platform carbonate rich rocks preserve marine to early diagenetic carbon and oxygen isotope compositions whereas ferroan carbonate cements in siltstones and shales in the desert creek borehole are o 18 and c 13 depleted relative to the sedimentary carbonates the good agreement between temperature estimates from illite crystallinity and organic reflectance 160 degrees 270 degrees c and inverse correlation with carbonate delta o 18 values indicates that organic maturation and carbonate precipitation in the northeast lawn hill platform resulted from interaction with the 1250 to 1150 ma fluids the calculated isotopic compositions of the fluid are consistent with evolved basinal brine delta o 18 5 1 9 4 parts per thousand v smow delta c 13 13 2 to 3 7 parts per thousand v pdb that contained a variable organic carbon component from the oxidation and or hydrolysis of organic matter in the host sequence the occurrence of extensive o 18 and c 13 depleted ankerite and siderite alteration in desert creek is related to the high temperature of the 1250 to 1150 ma fluid flow event in the northeast lawn hill platform in contrast to the lower temperature fluids associated with the earlier century zn pb deposit in the central lawn hill platform

origin of dolomitization on the barbwire terrace canning basin western australia a thick areally extensive subsurface sequence of upper devonian carbonates occurs on the barbwire terrace in the canning basin of western australia it is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized slightly deeper water mark have remained as limestones the major regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite up to 600 m with no primary calcite a small volume of evaporitic supratidal dolomite is present at one location this dolomite is derived from highly saline fluids developed in an arid supratidal environment replacement dolomite of the regional dolomite type has a xenotopic form with undulose extinction and irregular crystal boundaries in addition saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite this suggests the regional dolomite type was precipitated at slightly elevated temperatures dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution geochemically the synsedimentary supratidal and regional dolomite types are quite distinctive supratidal dolomites have 6 o values which are significantly higher 6180 2 to i pdb than the regional dolomite type 6l80 9 to 2 pdb assuming the lowest 6 o values for the sabkha dolomite represent replacement in marine waters the oxygen isotopic composition for upper devonian canning basin marine dolomite would be around 6l80 2 pdb the petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin however sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the barbwire terrace therefore it is suggested that dolomitization may have taken place in a near surface environment with a major recrystallization event superimposed during burial diagenesis

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

origin of dolomite in the eocene castle hayne limestone north carolina abstract a locally dolomitized 11 0 m section of the castle hayne limestone is exposed in the martin marietta aggregates quarry in new hanover county a 1 0 m zone 6 4 m below the overlying unconformity consists entirely of sucrose textured dolomite with intercrystalline and moldic porosity with vague outlines of the original allothems the dolomite is nonstoichiometric 48 mole mgco3 nonferroan and occurs as very fine to fine anhedral to subhedral crystals commonly with cloudy centers the percentage of dolomite decreases fairly uniformly above and below this zone and 3 6 m below the upper unconformity the unit is undolomitized above and below the zone of maximum dolomitization the dolomite selectively replaces the micrite matrix and calcite allochems are unaffected s dolomitization increases towards a maximum calcite allochems are replaced the lack of sabkha type sedimentary structures evaporite minerals and the presence of a stenohaline marine shelf fauna suggest hyposaline dolomitization consistent with the mixing zone model analyses of the na sr and cl content in acid soluble material from a core drilled at this locality reveal that the sr content systematically increases from 302 ppm at the top of the core to 524 ppm at the base the sr data suggest that the calcite and dolomite phases have been depleted in sr perhaps by hyposaline fluids the strong positive correlation between sr content and depth suggests a memory of a stratified mixing zone fluid sodium ranges from a low of 252 ppm in calcite to a high of 1 500 ppm in dolomite and a strong positive correlation exists between percentage of dolomite and na concentration the molar ratio of na cl ranges from greater than 2 1 to 11 1 thus the high na values cannot be attributed to high salinity fluid inclusions in the dolomite although the high na values associated with the dolomite suggest saline hypersaline dolomitization the acid soluble na was derived from a heulandite group zeolite which is present in the dolomites the interlocking nature of the dolomite and zeolite crystals the euhedral morphology of the zeolite and the strong correlation between percentage of dolomite to na concentration suggest that both mineral phases are authigenic and formed penecontemporaneously unless the na distribution can be documented these data suggest that whole rock na concentrations in ancient dolomites may not be an accurate indicator for hyposaline versus hypersaline dolomitization the laterally equivalent undolomitized biomicrudite facies of the castle hayne limestone has essentially no matrix porosity and little secondary moldic porosity thus dolomitization has greatly enhanced the porosity and permeability of the biomicrudite facies in this area mixing zone dolomitization caused by a lowering of eustatic sea level in conjunction with favorable hydrologic and geologic conditions can have a profound effect on reservoir properties and distribution

modelling of hydrocarbon generation patterns in the egersund sub basin north sea abstract hydrocarbon generation in the egersund sub basin has been modelled using empirical lopatin type and kinetic models and two prototypes of source rocks from wells 9 4 4 and 18 11 1 the amount of oil generated changes with initial heat flow type of generation model activation energies and source rock richness but does not follow a straightforward pattern for example at lower thermal histories the amount of oil generated up to the present changes exponentially with initial heat flow variation due to change of activation energy distribution is linear to exponential depending on heating rate and variation due to source rock potential is linear the empirical model for oil generation is less sensitive to temperature and more sensitive to time than the kinetic models and appears to overestimate the amount of oil generated at low heating rates but underestimates it at high heating rates hydrocarbon yields predicted by the empirical model are usually 4 5 times higher than those predicted by the kinetic models for the drainage area of well 9 2 1 variation of initial heat flow between 1 15 and 1 25 hfu heat flow units results in a variation of output yield by a factor three if a minimum oil saturation of the pore volume of 10 is required for long distance migration the empirical model should have resulted in accumulations or oil shows also in the marginal drainage areas although not unambiguous the present hydrocarbon distribution can be explained best by generation from the kinetic model at thermal histories represented by initial heat flow between 1 20 and 1 30 hfu

seismic geohistory analysis a case history from the canning basin western australia seismic geohistory analysis is a method of reconstructing an interpreted seismic section assuming generalized porosity traveltime and depth traveltime relations the method can indicate timing of fault movements juxtaposition of units across faults in the past and depth or traveltime to specific reflection horizons throughout geologic time the method applied to the lennard shelf of the canning basin shows that the major movements of the pinnacle fault system occurred during the ordovician and middle carboniferous the movements reflect subsidence of the fitzroy trough which is basinward of the lennard shelf the analysis also shows a prograding sediment wedge on the margin of the lennard shelf to have had temperatures in the range 110 116 c since permian times being within the thermal oil window for the past 280 ma the sediment wedge may be a good hydrocarbon prospect

early permian sediment provenance and paleogeographic reconstructions in southeastern gondwana using detrital zircon geochronology northern perth basin western australia abstract the perth basin western australia forms part of a north trending belt of intracratonic rift basins that initiated in southeastern gondwana in the late paleozoic existing provenance studies have highlighted the dominance of proterozoic zircons derived from southerly terranes such as the albany fraser orogen leeuwin complex and more distant provinces in east antarctica with limited input from the adjacent archean western australian craton new u pb geochronology data were acquired using a sensitive high resolution ion microprobe shrimp ii on detrital zircons from lower permian formations exposed in the northeastern perth basin in contrast to previous studies archean zircons showing fine oscillatory zoning dominate the age spectra with up to 90 grains ranging between 3 0 and 2 6 ga indicating that granitic parent rocks of the adjacent craton were a major sediment source during the early permian a minor paleoproterozoic age population from the gascoyne complex of the capricorn orogen was likely transported southward and dispersed in the basin by a combination of longshore drift and tidal currents meso and neoproterozoic detritus became prominent only from the middle artinskian and may relate to the establishment of large northward flowing river systems draining the albany fraser province the prydz leeuwin belt and or the wilkes land in east antarctica this study shows that integrating depositional processes and stratigraphic evolution of sedimentary formations with provenance analysis can enhance reconstruction of dispersal pathways to and within basins

calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

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

40ar 39ar thermochronology of the southern gawler craton australia implications for mesoproterozoic and neoproterozoic tectonics of east gondwana and rodinia 40ar 39ar thermochronology of the southern gawler craton australia reveals a protracted history of slow cooling with local crustal heating rapid cooling and fault reactivation between 1700 and 700 ma hornblendes from most of the southern part of the craton including the kalinjala mylonite zone yield ages 1580 1610 ma with younger more discordant samples showing evidence of retrogression these results indicate that the region cooled below 500 c 100 m y after the kimban orogeny and coincident with the gawler range hiltaba suite magmatism to the north perhaps associated with postorogenic extension biotite and k feldspar data indicate that regional cooling after this time was more gradual with a few exceptions locally in the southeastern eyre peninsula rapid cooling at 1425 ma is indicated by nearly concordant hornblende and biotite ages and may record a phase of regional extension other exceptions include areas of lower grade archean rocks that cooled to 350 c by 1700 1640 ma greenschist facies reactivation of shear zones occurred at 1100 ma during the musgravian orogeny on the basis of biotite single grain data and retrogressed amphiboles k feldspar age spectra record cooling at 700 800 ma following regional intrusion of the gairdner dike swarm and or denudation during rifting along the pacific margin of east gondwana and breakup of rodinia tectonothermal events revealed by these results previous geochronological and geological studies and available paleomagnetic data are comparable to other cratons and mobile belts in australia east gondwana and laurentia providing evidence that southeastern australia and north america were adjacent during much of proterozoic time

paleoproterozoic ca 1 9 ga megascopic life on land in western australia abstract controversial hairpin shaped trace fossils myxomitodes stirlingensis and discoid fossils cyclomedusa davidi are here reinterpreted in a reassessment of sedimentology and paleosols of the 1 9 0 1 ga stirling range formation of western australia paleosols in the stirling range formation were recognized from complex cracking patterns peds and cutans chemical and mineralogical zonation soil horizons and poikiloblastic crystals desert roses redox profiles of the paleosols are evidence of well drained profiles of a coastal plain under a low oxygen 433 116 ppmv and carbon dioxide 564 64 ppmv atmosphere gypsum pseudomorphs in paleosols that are deeply weathered chemically represent a persistence of acid sulfate weathering better known in archean paleosols before the great oxidation event at 2 45 ga chemical composition of the paleosols is evidence of a humid 1478 182 mm mean annual precipitation temperate 11 3 0 5 c mean annual temperature paleoclimate their paleoenvironmental setting was a coastal plain on a passive tectonic margin megafossils were found on paleosols of western red sandstones of the stirling range formation not in gray shales and sandstones of marine facies to the east trace fossils myxomitodes stirlingensis and discoid fossils are both found at the surface of well drained paleosols and so were not metazoan trails nor sea jellies myxomitodes was more likely a grex slug trail of soil slime molds and the cyclomedusa discoids were probably microbial colonies pervasive microfilamentous structures in the paleosol surface are additional evidence of paleoproterozoic microbial earth ecosystems

evolution of oil generative window and oil and gas occurrence in tertiary niger delta basin assuming a simple model of delta development involving progradation and uniform burial at 500 m m y 1 640 ft m y to present depths oil genesis nomographs derived from the time temperature index tti method were constructed for geothermal gradients ranging from 2 2 to 5 1 c 100 m 1 2 2 8 f 100 ft of the niger delta and used in mapping the positions depth temperature of the top of the oil generative window ogw at various times between 40 m y b p and the present during the active subsidence phase oil generation within any megasedimentary unit was initiated at a temperature of 140 146 c 284 294 8 f and depth of 3 000 5 200 m 9 843 17 060 ft within 7 11 m y after deposition of the potential source rocks after cessation of subsidence vertical upward movement of the ogw by 800 1 600 m 2 625 5 249 ft was accompanied by a temperature lowering of 23 54 c 41 97 f producing correspondingly heavier crudes in a central belt of the delta hydrocarbon generation and expulsion from the lower part of the agbada formation predate the cessation of subsidence and structural deformation whereas in other areas it postdates the cessation of subsidence and structural deformation in this central belt the agbada is the major oil source with the akata serving as a gas source in the other areas both the agbada and akata constitute oil sources which implies that the thermal conditions rather than the kerogen type influence the oil gas mix in the niger delta basin

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

quantitative evaluation of lower cretaceous mannville group as source rock for alberta s oil sands the western canada sedimentary basin hosts about 12 billion bbl of conventional oil in devonian to cretaceous reservoirs lower cretaceous oil sands contain an additional 1 3 trillion bbl in place the oil sands represent the biodegraded remnants of supergiant conventional oil deposits the source for which has been thought to be mature rocks of the equivalent age mannville group this work shows however that the known mannville rocks alone are incapable of generating the required volume of hydrocarbons the volume of hydrocarbons generated in the mannville beneath central alberta was calculated by combining measured geochemical and geologic data with lopatin s method for thermal maturation original hydrocarbon generative capacity of the mannville rocks was calculated from geochemical analyses of immature samples using average values for total organic carbon toc 1 3 and rock eval hydrogen index 100 mg hc g toc maximum hydrocarbon generation per unit volume of source rock was calculated the maturation model was then employed to estimate the extent to which maximum yield has actually been achieved total volume of source rock in the basin was obtained from isopach maps of mannville shale multiplication of actual oil generation per unit volume by source rock volume gave a generated volume of 450 billion bbl these calculated values are exceedingly optimistic however because they ignore inefficiencies in expulsion and migration inclusion of oil generated in mannville equivalent source rock in the foothills belt would less than double this quantity assuming that source potential there is similar to that of the central alberta rocks it is clear that the mannville group of the western canada sedimentary basin cannot be the major source of alberta s oil sand hydrocarbons either the hydrocarbons were generated in as yet undocumented very organic rich mannville equivalent rocks in the foothills or they are derived from multiple sources throughout the basin

petrophysical implications of source rock microfracturing abstract hydrocarbon generation induced microfracturing in source rocks is important affecting both primary migration of hydrocarbons and unconventional shale plays in this study petrographic observations rock eval and total organic carbon content toc data and log derived physical properties were analyzed in order to evaluate eventual petrophysical implications of microfracturing induced by hydrocarbon generation petrographically evidences of microfracturing both normal and parallel to bedding have been found horizontal microfractures typically cross cut each other at an angle between 30 and 90 horizontal microfractures are readily imaged compared to vertical microfractures vertical microfractures were clearly observed only in rare cases in samples with a high degree of microfracturing in the case of moderate microfracturing the vertical component of microfractures may be too small to be detectable using conventional microscopy techniques longer vertical components of the microfractures may be facilitated by higher toc and particularly dense laminated organic matter fabric as well as higher thermal maturity levels clay minerals particularly smectites are also playing a role in reducing permeability and thereby increasing the local pressure generated by hydrocarbon generation eventually resulting in microfracturing log derived petrophysical properties particularly p wave velocity vp and formation resistivity can give information on microfracturing potential even though the techniques can t resolve the microfractures directly vp has been postulated to be sensitive to organic matter quantity and fabric whereas formation resistivity is largely governed by fluid content and clay minerals a reverse trend of increasing resistivity with decreasing vp was found and is linked to higher hydrocarbon saturation and toc and or dense laminated fabric of organic matter favoring microfractures the implications of the log derived petrophysical results are in agreement with the direct petrographic observations formation resistivities in the source rocks however are lower than those observed in non clay dominated hydrocarbon saturated intervals this indicate that low permeable organic rich source rocks with a high clay content particularly smectites may facilitate formation of the hydrocarbon filled vertical microfractures even at relatively low thermal maturities

insights into the dolomitization process and porosity modification in sucrosic dolostones avon park formation middle eocene east central florida u s a abstract the avon park formation middle eocene in central florida u s a contains shallow water carbonates that have been replaced by dolomite to varying degrees ranging from partially replaced limestones to highly porous sucrosic dolostones to less commonly low porosity dense dolostones the relationships between dolomitization and porosity and permeability were studied focusing on three 305 m long cores taken in the city of daytona beach stable isotope data from pure dolostones mean 18o 3 91 v pdb indicate dolomite precipitation in eocene penesaline pore waters which would be expected to have been at or above saturation with respect to calcite nuclear magnetic log derived porosity and permeability data indicate that dolomitization did not materially change total porosity values at the bed and formation scale but did result in a general increase in pore size and an associated substantial increase in permeability compared to limestone precursors dolomitization differentially affects the porosity and permeability of carbonate strata on the scale of individual crystals beds and formations at the crystal scale dolomitization occurs in a volume for volume manner in which the space occupied by the former porous calcium carbonate is replaced by a solid dolomite crystal with an associated reduction in porosity dolomite crystal precipitation was principally responsible for calcite dissolution both at the actual site of dolomite crystal growth and in the adjoining rock mass carbonate is passively scavenged from the formation which results in no significant porosity change at the formation scale moldic pores after allochems formed mainly in beds that experienced high degrees of dolomitization which demonstrates the intimate association of the dolomitization process with carbonate dissolution the model of force of crystallization controlled replacement provides a plausible explanation for key observations concerning the dolomitization process in the avon park formation and elsewhere 1 volume for volume replacement at a crystal scale 2 coupled growth of dolomite crystals and dissolution of host calcium carbonate matrix and 3 automorphic replacement by euhedral dolomite crystals the force of crystallization model also does not require an influx of externally derived water that is undersaturated with respect to calcite to dissolve calcite a fact that could simplify diagenetic models of porosity generation in dolostones the later addition of external carbonate can result in a substantial reduction in porosity by the precipitation of dolomite cement which could convert a high porosity sucrosic dolostone into a dense paleozoic type dolostone

on the accuracy of modelling hydrocarbon generation and migration the egersund basin oil find norway abstract quantitative estimates of kerogen transformation and hydrocarbon charge to the structure of block 9 2 norwegian north sea were obtained by integrated basin modelling a probability distribution was assigned to each input parameter and the sensitivity to these parameters was investigated by subsequently varying each parameter within the assigned ranges the inaccuracy in the modelling results were mainly due to imprecise knowledge of hydrocarbon generation kinetics depth conversion and present day virgin rock temperature kerogen transformation modelling and source rock richness thickness areal extent volumetric charge calculations monte carlo latin hypercube sampling based on the probability distribution of each parameter was performed giving a 40 probability for a dry structure a mean value of 10 7 106sm3 recoverable oil and a considerable high side potential 4 probability for more than 50 106sm3

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

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

palaeomagnetic dating and tectonic significance of dolerite intrusions in the albany mobile belt western australia abstract dolerite dykes and sills in the mesoproterozoic albany mobile belt and neoproterozoic stirling range formation metasediments in southwestern australia provide evidence for extensional events between australia east antarctica and greater india paleomagnetic analysis shows that primary remanence was carried by single domain magnetite for samples from seven localities palaeomagnetic poles for the majority of sites fall close to the earliest cambrian part of the australian apparent polar wander path apwp the interpreted earliest cambrian dykes intrude both extensional dominant and shear fractures indicating nne ssw extension between the western australian and east antarctic shields and a horizontal ese wnw maximum compressive stress dyke emplacement appears to be contemporaneous with mafic volcanism in central and northern australia and igneous intrusion in once contiguous parts of the east antarctic shield these features are interpreted as being related to gondwanaland assembly and may represent a response to far field stresses resulting from the collision between east and west gondwanaland a second suite of nw striking mid carboniferous dykes is related to ne sw extension and rifting between greater india and the western australian shield during early stages in the formation of the perth basin in a sinistral transtensional regime along the southern western australian margin a palaeopole from the primary remanence in a nnw striking dolerite dyke falls on the poorly defined triassic to early cretaceous part of the australian apwp indicating a possible third phase of dyke emplacement regional tectonic constraints for rifting in the perth basin suggest that this youngest dyke most likely intruded during the triassic

geology and hydrocarbon accumulations columbus basin offshore trinidad the columbus basin situated on the eastern shelf of trinidad lies at the eastern extremity of a belt of severe deformation along the northern boundary of south america that has been affected by compressional and wrench tectonics in pliocene pleistocene time two major structural trends are present in the columbus basin a series of east northeast trending anticlines and north northwest oriented normal faults the basin was filled during late miocene to holocene time with sediments deposited by an ancestral orinoco river draining a hinterland to the southwest the pliocene pleistocene section which contains the hydrocarbon accumulations in the columbus basin was deposited in three coarsening upward sedimentary sequences followed by a late pleistocene transgressive sequ nce traps for hydrocarbon accumulation were formed by an easterly trending pliocene pleistocene wrench system with associated east northeast oriented anticlines combined with north northwest oriented normal faults oil was sourced in the late miocene lower cruse formation whereas gas was derived both from pliocene pleistocene pro delta shales and as a late high temperature phase of lower cruse hydrocarbon generation the north northwest faults formed migration conduits from the oil source rock to pliocene pleistocene reservoirs the temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas the size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal particularly in the western half of th basin

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

hydrocarbon generation and migration from jurassic source rocks in the e shetland basin and viking graben of the northern north sea in the e shetland basin oil generation began 65 ma ago peak oil generation maturity occurs today at 3250 m 0 7 r 0 and was first reached 40 50 ma ago the oil generation threshold is at 2500 m highest oil saturations in the kimmeridge clay occur at 0 8 r 0 oil expulsion efficiencies are 20 30 oil phase migration has probably occurred through oil wet kerogen laminae and through interconnected large pores aided by low oil water interfacial tensions oil migrated along strong lateral fluid pressure gradients from overpressured source rocks in half grabens to jurassic reservoirs in tilted fault blocks in the viking graben the kimmeridge clay is at oil floor maturity below 4500 m oil and peak oil generation began 7c 80 and 55 65 ma ago respectively 40 ma ago the kimmeridge clay passed through peak generation and gas generation by cracking of oil had begun peak dry gas generation from brent coals occurs today below 5000 m and began 40 ma ago the frigg field gas probably generated from late jurassic source rocks migrated through microfractures in overpressured mudstones below 3500 m above 3500 m methane probably migrated in aqueous solution and was exsolved in the early tertiary aquifer

constraints of fluid inclusions and c o isotopic compositions on the origin of the dolomites in the xisha islands south china sea abstract the dolomite problem is a key issue in research on carbonate diagenesis and it is still a controversial scientific problem the origin of the neogene dolostones in the xisha islands has remained disputed until now however the geochemical characteristics of dolomite hosted fluid inclusions can potentially provide a relatively direct record of the origin and evolution of their dolomitizing fluid in this article based on the samples selected from the well xike 1 also known as zk 1 and xk 1 reef core of the xisha islands the hydrogen isotopic compositions and gas and liquid compositions of the fluid inclusions hosted in calcite and dolomite were analyzed meanwhile the isotopic compositions of carbon and oxygen in the calcite and dolomite minerals were also tested the origin of the dolomitizing fluid was traced in detail which provided important evidence for establishing the dolomitization model of the primary dolomites in the study area the main gas compositions of the dolomite hosted fluid inclusions are co2 h2o n2 and o2 which are similar to those of the atmosphere indicating that the dolomites in this area experienced a partly oxygenated diagenetic environment from high to low the cation concentrations of the liquid component in the dolomite hosted fluid inclusions are mg2 ca2 na and k whereas their main anion compositions are so42 cl and f which are characterized by high so42 concentrations and low cl br ratios the 13c values pdb of the dolomites range from 1 9 to 3 0 and their 18o values pdb range from 2 1 to 4 7 the calculated z values which can be used to trace the salinity of diagenetic fluid based on the 13c and 18o values of the dolomites range from 133 0 to 135 8 which are higher than that of modern seawater approximately 120 these data indicate that the dolomitizing fluid of the primary dolomites in the xisha islands is the high salinity concentrated seawater derived from a surface environment in addition the model of seepage reflux dolomitization can better interpret the dolomitizing process of the primary dolomites in the xisha islands

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

burial diagenesis in the upper devonian reef complexes of the geikie gorge region canning basin western australia the devonian carbonates of the geikie gorge region canning basin have undergone a long and complex diagenetic history that began in devonian seawater with extensive marine cementation of platform margin lithologies devonian lower carboniferous burial diagenesis was the most important porosity occluding episode because almost all primary porosity was destroyed by equant calcite cements nonluminescent to brightly luminescent to dully luminescent during this interval dolomitization and consequent secondary porosity development also occurred during early burial diagenesis the distribution and geochemistry of the major calcite cements and dolomite types are consistent with these phases having been precipitated from connate marine or basinal brines karstification and mi or calcite cementation took place during late carboniferous subaerial exposure minor calcite cementation occurred during permian cenozoic burial predominantly in secondary porosity within pervasively dolomitized lithologies karstification dedolomitization and calcite recrystallization took place in association with cenozoic meteoric diagenesis secondary moldic and intercrystalline porosity within the completely dolomitized lithologies were the longest lived porosity types in the carbonates some secondary porosity escaped both devonian carboniferous and permian cenozoic burial cementation probably due to a lack of nucleation sites for calcite cements within completely dolomitized lithologies

origin and modification of cambrian dolomites red heart dolomite and arthur creek formation georgina basin central australia the early to middle cambrian red heart dolomite and lower arthur creek formation of the southern portion of the georgina basin australia is an entirely dolomitized succession of shallow water evaporitic mudflat and deeper water subtidal lithologies three types of dolomite have been identified and are interpreted as 1 syndepositional dolomite 2 regional replacement dolomite and 3 void filling dolomite cement syndepositional dolomite derived from saline pore fluids developed in a sabkha environment is a minor dolomite type with very fine crystal mosaics and has a mottled non zoned cathodoluminescence the widespread regional replacement dolomite ranges from fine to medium crystalline forming mainly planar s and non planar a crystal mosaics and displays blotchy mottled non zoned cathodoluminescence void filling dolomite commonly forms planar s to planar e medium to very coarse crystal mosaics rare non planar c very coarsely crystalline saddle dolomite also exists void filling dolomite has a successively zoned cathodoluminescence pattern from non to brightly to dully luminescent geochemically the syndepositional dolomite has 18o pdb values ranging between 5 3 and 8 6 o regional replacement dolomites exhibit a wide range of 18o values from 3 3 to 10 9 o whereas void filling dolomite has 18o values ranging from 10 8 to 14 3 o all three dolomite types have similar 13c pdb values in the range between 1 7 and 1 7 o three initial dolomitization episodes are interpreted 1 a sabkha stage forming the syndepositional dolomite and dolomitizing the evaporitic mudflat lithologies 2 a brine reflux stage replacing the subtidal lithologies and 3 a burial stage forming the void filling dolomite type final dolomite stabilization occurred during burial at elevated temperatures in the presence of basinal fluids resulting in progressive recrystallization and stabilization of the earlier formed syndepositional and replacement dolomites both textural and geochemical evolution should be taken into account when studying the origin of dolomites based on their present geochemical composition sulphates are represented by very fine crystalline syndepositional anhydrite in association with the syndepositional dolomite and coarse to very coarse anhydrite cement evaportic mudflat sabkha and burial environments are inferred for the origin of the former and the latter anhydrite types respectively evaporite dissolution breccias indicative of the former presence of evaporites are common throughout the succession

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

epigenetic dolomitization of the přaidoli formation upper silurian the barrandian basin czech republic implications for burial history of lower paleozoic strata stratabound epigenetic dolomite occurs in carbonate facies of the barrandian basin silurian and devonian czech republic the most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated p idoli and lochkov formations deposited on a carbonate slope medium crystalline 100 400 m inclusion rich xenotopic matrix dolomite 18o 4 64 to 3 40 pdb 13c 1 05 to 1 85 pdb which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type coarse crystalline saddle dolomite 18o 8 04 to 5 14 pdb 18c 0 49 to 1 49 pdb which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event in some vugs saddle dolomite coprecipitated with petroleum inclusion rich authigenic quartz crystals and minor sulfides which in turn were post dated by semisolid asphaltic bitumen the interpretation of the dolomitization remains equivocal massive xenotopic dolomite although generally characteristic of a deeper burial setting may have been formed by a recrystallization of an earlier possibly shallow burial dolomite deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the p idoli sequence appears as a viable explanation for this dolomitization overprint saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone the presence of saddle dolomite the fluid inclusion composition of associated quartz crystals and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160 c although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures burial heating beneath approximately 3 km thick sedimentary overburden of presumably post givetian strata no longer preserved in the basin appears to be the most likely interpretation this interpretaion may imply that the magnitude of post variscan erosion in the barrandian area was substantially greater than previously thought

two hundred million years of karst history dachstein limestone hungary platform carbonates of the upper triassic dachstein limestone in naszaly hill have been karstified extensively over the past 200 million years they provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequences seven karstic phases are recognized in the area each of which include polyphase karstic events the first karst phase was associated with the lofer cycles meteoric waters caused dissolution enlarged fractures and cavities were filled by marine and or vadose silts and cement the second karst phase was caused by local tectonic movements bedding plane controlled phreatic caves were formed and filled by silts the third karst phase lasted from the end of the triassic to the eocene this was a regional multiphase karstic event related to younger composite unconformities bauxitic fill is the most characteristic product of this phase the karst terrain reached its mature or senile stage with very little porosity narrow veins and floating rafts of white calcite marks karst phase 4 which resulted from hydrothermal activity associated with palaeogene magmatism the early rupelian phase of alpine uplift caused large scale rejuvenation of the former karst terrain karst phase 5 subsequently naszaly hill was buried as an area of juvenile karst with significant porosity a second period of hydrothermal activity in the area karst phase 6 was induced by miocene volcanism which resulted in wide pale green calcite veins finally karst phase 7 was of tectonic origin following the most recent miocene uplift of the naszaly hill the carbonates have again become the site of vadose karst development

meteoric modification of early dolomite and late dolomitization by basinal fluids upper arbuckle group slick hills southwestern oklahoma 1 thick massive dolomite units from the upper arbuckle lower ordovician peritidal carbonate sequence slick hills southwestern oklahoma provide insight into two important problems 1 modification of early dolomite and 2 late dolomitization early dolomite volumetrically dominant is nonluminescent to dully luminescent 87 sr 86 sr ratios of most early dolomite samples are similar to both early ordovician seawater and associated limestones consistent with an early origin from coeval seawater all early dolomite samples are characterized by low delta 18 o values 5 1 to 10 0 o oo pdb indicating postdepositional modification meteoric water buffered by younger carbonates with lower 87 sr 86 sr ratios was probably responsible for modification because lower sr concentrations delta 18 o values and 87 sr 86 sr ratios characterize the more intensively modified early dolomite samples meteoric modification probably occurred during emergence of the carbonate platform in the ordovician late dolomite volumetrically minor is brightly luminescent and occurs mainly in transition zones between early dolomite and associated limestone late dolomite has high 87 sr 86 sr ratios delta 18 o values 5 9 to 2 5 o oo pdb and fe and mn concentrations relative to early dolomite and associated limestone 87 sr 18 o fe and mn enriched fluids derived from the anadarko basin during the late paleozoic are proposed to have formed the late dolomite the fluids migrated through preexisting early dolomite aquifers and dolomitized limestones near the early dolomite limestone contacts which existed at that time although early dolomite remained more porous than limestone during early burial burial alteration by solution compaction and cementation by late dolomite have reduced porosity considerably better reservoir quality is expected to be associated with late dolomite in the arbuckle group

bulk chemical control on metamorphic monazite growth in pelitic schists and implications for u pb age data several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock forming minerals but little attention has been paid to the control that bulk composition might have on these reactions in this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the mount barren group of the albany fraser orogen western australia reflect differing bulk compositions a new sensitive high mass resolution ion microprobe shrimp u pb monazite age of 1027 8 ma for pelitic schist from the mount barren group contrasts markedly with previously published shrimp u pb monazite and xenotime ages of c 1200 ma for the same area all dated samples experienced identical metamorphic conditions but preserve different mineral assemblages due to variable bulk composition monazite grains dated at c 1200 ma are from relatively magnesian rocks dominated by biotite kyanite and or staurolite whilst c 1027 ma grains are from a ferroan rock dominated by garnet and staurolite the latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite but this reaction was not intersected by more magnesian compositions which are instead dominated by monazite that grew during an earlier greenschist facies metamorphic event these results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes samples too magnesian to ever grow garnet or too iron rich to undergo garnet breakdown are likely to yield older monazite and the age difference can be significant in terranes with a polymetamorphic history

folding in high grade rocks due to back rotation between shear zones abstract folds with a sense of asymmetry opposite to the bulk shear sense may form in high grade rocks due to back rotation between ductile shear zones layers back rotated into the shortening field can undergo additional buckle folding with progressive deformation shear zones and domains of back rotated foliation between shears rotate towards the x y flattening plane of the finite strain ellipsoid in high grade gneiss the migration of melt into shear zones may facilitate displacement along them and accentuate back rotation of material between shear zones folding during back rotation occurs when the spacing between shear zones does not increase sufficiently to accommodate the length of back rotated layers or between non parallel shear zones folds formed by back rotation between shear zones commonly have pegmatites sub parallel to their axial planes thickened overturned limbs and occur in localised packages associated shear zones have the same sense of shear on both attenuated limbs of folds formed by back rotation examples are given for folds between transcurrent and normal ductile shear zones in the albany mobile belt and leeuwin complex western australia and the dharwar craton india

the role of subgrain boundaries in partial melting abstract evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the lost creek gneiss of the llano uplift central texas the wet mountains of central colorado and the albany fraser orogen southwestern australia domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph subgrain boundaries consist of arrays of dislocations within a crystal lattice and unlike fractures would not act as conduits for melt migration instead the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there preferential melting in high strain locations may be attributed to strain energy which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds and or the presence of small quantities of water associated with dislocations which may enhance diffusion rates or locally lower the temperature needed for partial melting

structure of the western albany mobile belt southwestern australia evidence for overprinting by neoproterozoic shear zones of the darling mobile belt abstract the albany mobile belt is dominated by ductile fabrics and regional structures formed during progressive dextral transpression at 1190 ma d1 resulting from nw se to nnw sse shortening and represents the oblique convergence between the west australian and east antarctic shields deformation was accompanied by metamorphism up to granulite facies and granitoid intrusion neoproterozoic extension and progressive deformation within a sinistral transcurrent to transtensional shear zone in the leeuwin complex of the southern darling mobile belt may be contemporaneous with transpressional shearing along the donnybrook nannup shear zone on the western yilgarn craton margin during this neoproterozoic event e w mesoproterozoic trends in the western albany mobile belt have been rotated into a n s orientation ductile reactivation and overprinting of mesoproterozoic structures is accompanied by retrogression of granulite facies assemblages to amphibolite facies migmatisation and pegmatite and granitoid intrusion in demonstrating that the n s orientation of the western albany mobile belt results from a younger event a continuation of this mobile belt is expected in northeast india whilst a continuation of the darling mobile belt may exist in the mirnyy area of east antarctica folding and the formation of conjugate brittle ductile shear zones during ne sw shortening and early cambrian conjugate faulting and jointing resulting from ese wnw shortening is common to both the western albany mobile belt and leeuwin complex

timing of plutonism in the proterozoic albany mobile belt southwestern australia abstract zircon u pb ages for two adamellites from the albany mobile belt in the albany fraser orogen of southwestern australia fall within the range 1174 12 ma and are interpreted as dating post tectonic granite emplacement in the belt slightly younger k ar ages on hornblendes from granulite and amphibolite facies metamorphic rocks from the southern part of the belt are interpreted as cooling ages recording uplift associated with the closing stages of orogeny and granite emplacement a u pb age of 1289 10 ma for zircons from an enderbitic gneiss adjacent to the 1177 12 ma old albany adamellite is interpreted as dating the emplacement of the granodioritic parent of the gneiss probably during granulite facies metamorphism this together with a previously reported rb sr age of 1300 ma for the granulite facies metamorphism in the northeast section of the albany fraser orogen suggests that granulite facies conditions were widespread in the orogen at 1300 ma previous sm nd results suggest that the granitoids were derived from 2000 ma old crust so far no older zircon xenocrysts which could support this model have been found in the granitoids

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago