the limitations of cement stratigraphy as revealed in some lower carboniferous oolites from south wales abstract the use of staining and cathodoluminescence to correlate carbonate cements is often coupled with the conception of diagenetic environments that are uniform through a large volume of rock at any instant in time thus it is often assumed that individual growth zones within carbonate cements are present throughout a unit and have a chronological significance detailed study of meteoric phreatic cements in the oolite group lower carboniferous south wales indicates that individual growth zones can only be traced with any certainty between adjacent pores and are often only identifiable in a few square millimetres of thin section additionally cement sequences with apparently identical cathodoluminescence appearance are often demonstrably of different ages the complex variation in cements through the oolite group partly reflects the extremely localised controls on groundwater chemistry in the shallow subsurface it is also due to the superimposition of a number of phases of subaerial exposure at successively higher levels during deposition of the group the chemistry of a calcite cement reflects the geochemical environment at the time of precipitation at that location in the diagenetic system it is likely that that environment has been frequently repeated both in space and time

formation of dolomite mottling in middle triassic ramp carbonates southern hungary abstract the middle triassic carbonates of the villany mountains were deposited on a homoclinal carbonate ramp many of the carbonates from the 700 m thick sequence show partial or complete dolomitization the present paper describes dolomites that occur in a limestone unit as irregular mottles and as pore and fracture filling cements replacement type scattered dolomite rhombs in the mottles having inclusion rich very dull luminescent cores and limpid non luminescent outer zones represent the initial phase of dolomitization the isotopic composition of these dolomites 13 c 2 30 vpdb 18 o 3 60 vpdb is similar to that of the calcitic micrite 13 c 2 6 vpdb 18 o 4 00 vpdb indicating that no external fluids were introduced during dolomite formation the elevated sr content of the micrites implies that sediment was originally aragonite or high mg calcite dolomitization took place in the burial realm from a marine pore fluid in a partly closed system later fracture related saddle dolomite reflects elevated formation temperatures and increasing burial five calcites were identified multiple generations of calcite filled fractures were formed during burial diagenesis generally having dull or no luminescence 13 c 1 80 vpdb 18 o 6 40 vpdb the latest phase calcites are related to karst formation having a very negative isotopic composition 13 c 5 0 to 7 2 vpdb and 18o 7 4 vpdb the karst related processes include dissolution calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite the timing of dolomitization is uncertain but the first phase took place in a partly closed system prior to stylolite formation late stage saddle dolomites were precipitated during maximum burial in the cretaceous the dissolution of dolomites and karst related calcite replacement was not earlier than late cretaceous

layered and laterally constrained 2d inversion of resistivity data in a sedimentary environment quasi layered models often can represent the actual geology more accurately than smooth minimum structure models we present a 2d inversion scheme with lateral constraints and sharp boundaries lci for continuous resistivity data all data and models are inverted as one system producing layered solutions with laterally smooth transitions the models are regularized through lateral constraints that tie interface depths or thicknesses and resistivities of adjacent layers a priori information used to resolve ambiguities and to add for example geological information can be added at any point of the profile and migrates through the lateral constraints to parameters at adjacent sites similarly information from areas with well resolved parameters migrates through the constraints to help resolve areas with poorly constrained parameters the estimated model is complemented by a full sensitivity analysis of the model parameters supporting quantitative evaluation of the inversion result a simple synthetic model proves the need for a quasilayered 2d inversion when compared with a traditional 2d minimum structure inversion a 2d minimum structure inversion produces models with spatially smooth resistivity transitions making identification of layer boundaries difficult a continuous vertical electrical sounding field example from sweden with a depression in the depth to bedrock supports the conclusions drawn from the synthetic example a till layer on top of the bedrock hidden in the traditional inversion result is identified using the 2d lci scheme furthermore the depth to the bedrock surface is easily identified for most of the profile with the 2d lci model which is not the case with the model from the traditional minimumstructure inversion

origin of dolomites in the cretaceous maha sarakham evaporites of the khorat plateau northeast thailand abstract the khorat plateau of northeast thailand and laos was an area of widespread deposition of evaporites and siliciclastics maha sarakham formation during the cretaceous and three types of dolomites are associated with this formation 1 limpid dolomite 2 coarse subhedral and euhedral dolomite and 3 saddle dolomite limpid dolomite is present as isolated crystals in recrystallized clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread basal anhydrite unit bau 1 1 m which defines the lowest member of the formation the 18 o pdb isotopic values and the 87 sr 86 sr ratios of the limpid dolomite are within the range of cretaceous seawater petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine fed aggrading halite salina dolomite formation was possibly influenced by bacterial metabolism the coarse crystalline and saddle dolomite types float within a matrix of coarse recrystallized anhydrite in the basal anhydrite the 18 o isotopic values of the coarse subhedral and euhedral dolomite types are more negative than the limpid dolomite and the 87 sr 86 sr ratios are more radiogenic than typical marine dolomite of the cretaceous saddle dolomite has even more negative 18 o isotopic values than the coarse euhedral dolomite the 87 sr 86 sr ratios of the saddle dolomite are also radiogenic the isotopic and petrographic data indicate that the dolomites in the basal anhydrite unit are late and authigenic but do not appear to replace a precursor carbonate rather they appear to have precipitated within the deep burial environment along a 1 2 m thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed the upwelling basinal waters presumably rose through the underlying siliciclastics of the khorat group and then moved laterally along the base of the maha sarakham salt the result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue the basal anhydrite unit

palaeomagnetism of the 1 89 ga boonadgin dykes of the yilgarn craton possible connection with india abstract a palaeomagnetic study was carried out on the newly identified 1888 9 ma boonadgin dyke swarm of the yilgarn craton in western australia the bonnadgin dykes yield a mean direction of magnetisation of d 143 i 13 k 37 and 95 8 based on samples from 10 diabase dykes with a corresponding palaeopole at 47 s 235 e a95 6 a positive baked contact test establishes the primary nature of the magnetisation the ca 1 89 ga palaeopole suggests that the yilgarn craton was near the equator at this time and the boonadgin dyke swarm can be interpreted to represent an arm of a radiating dyke swarm that shared the same plume centre with coeval mafic dykes in the dharwar and bastar cratons of southern india we therefore propose that the west australian craton wac consisting of the yilgarn and pilbara cratons and the south indian block sib consisting of the dharwar bastar and singhbhum cratons were connected ca 1 89 ga globally available high quality palaeopoles of similar age allow the west australian craton to be placed northwest of proto laurentia during the assembly of the supercontinent nuna

palaeogeography of the siberian platform during middle palaeozoic times 450 400 ma new palaeomagnetic evidence from the lena and nyuya rivers new reliable palaeomagnetic data from the siberian platform help in deciphering its palaeogeography during 450 400 ma geochronology of late devonian mafic sills provides time constraints for tectonic deformation along the southern margin of the siberian platform and thus a minimum age for the regional magnetic overprint from a late ordovician silurian sedimentary section of the nyuya syncline in the southern part of the siberian platform with the devonian sills hosted nearby in early palaeozoic sediments pre folding presumably primary magnetization was isolated from the sediments during stepwise thermal cleaning high unblocking temperatures imply that haematite is the main carrier of magnetization the sample mean direction for 37 ordovician samples from nine sites in stratigraphic coordinates is ds 168 5 is 5 ks 22 3 and 95 5 1 and for 77 silurian samples from six sites is ds 193 9 is 20 9 ks 16 and 95 4 2 another component recorded in both the silurian and ordovician samples is pre folding with a sample mean direction of ds 204 4 is 37 9 ks 27 2 and 95 2 7 for 104 samples from eight sites this component was probably formed during a regional remagnetization event which took place in post early silurian time putting this secondary component into a framework with available palaeozoic data and geochronology further constrains its age to be early devonian the isolated components yield new late ordovician early silurian and early middle devonian palaeomagnetic poles the revised middle palaeozoic segment of the apparent polar wander path apwp for the siberian platform provides new palaeogeographic constraints our data suggest that in late ordovician the platform was situated in equatorial latitudes and was rotated 180 with respect to its present position during middle late ordovician time the platform did not experience any notable latitudinal drift it started drifting to the north in the late ordovician and by the late silurian it had travelled 1500 km northwards and had rotated 30 counter clockwise ccw during late silurian time the platform continued northward drift and ccw rotation and by the early devonian it had drifted 1100 km northwards and rotated 10 ccw relative to its silurian position after that the rotation of the platform changed to clockwise cw and by the late devonian had drifted another 1500 km to the north and had rotated 60 cw we evaluated palaeomagnetically viable positions from 450 400 ma of the three largest laurasian cratons siberia baltica and laurentia based on the new data and previously published apwps contrary to several published reconstructions the siberian platform could not have been situated to the north of the caledonian suture in mid silurian time but was probably located either at the eastern or the western side of laurussia the new data are compatible with an early devonian position of siberia similar to the modern eurasian configuration they also support the post early or middle devonian relative rotation between the aldan and angara blocks of the siberian platform

estimation of porosity and specific yield by application of geoelectrical method a case study in western iran summary vertical electric soundings vess with a schlumberger electrode configuration were recorded in the mahidashat plain kermanshah province located in the western part of iran in order to estimate the hydraulic parameters of the aquifer system the geological formations surrounding the plain are mainly cretaceous limestones and plagioclases with regular layering of miocene age the northwestern part of plain is composed of red marls of eocene age in order to ascertain the subsurface geological framework the general distribution of resistivity response of the geological formations was obtained geoelectrical sections along a number of lines have been prepared probable lithological patterns from these sections have been identified in the present study an attempt to estimate porosity and specific yield from resistivity data has been made for alluvial aquifers in mahidashat plain the results indicated that the porosity varies from 0 18 to 0 66 and the average specific yield is 0 15 the estimated data are compared with field real data and their credibility was calculated by using the paired t test method

a hydrogeophysical model for relations between electrical and hydraulic properties of aquifers abstract factors influencing relations between electrical and hydraulic properties of aquifers and aquifer materials are discussed both direct and inverse relations are shown to be possible and to exist a general hydrogeophysical model is outlined for aquifer scale relations which are shown to depend primarily on two factors the character of the material level relationship and the mutual relation between the direction of groundwater flow aquifer layering and hydrogeophysical conditions in the aquifer

evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements1 spectral induced polarization as well as complex electrical measurements are used to estimate on a non invasive basis hydraulic permeability in aquifers basic laboratory measurements on a variety of shaly sands silts and clays showed that the main feature of their conductivity spectra in the frequency range from 10 3 to 103 hertz is a nearly constant phase angle thus a constant phase angle model of electrical conductivity is applied to interpret quantitatively surface and borehole spectral induced polarization measurements the model allows for the calculation of two independent electrical parameters from only one frequency scan and a simple separation of electrical volume and interface effects the proposed interpretation algorithm yields the true formation factor the cation exchange capacity and the surface area to porosity ratio which corresponds to the inverse hydraulic radius using a kozeny carman like equation the estimation of hydraulic permeability is possible

hydrogeophysical imaging of alluvial aquifers electrostratigraphic units in the quaternary po alluvial plain italy the integration of surface geological and geomorphological information with borehole point data and geophysical e g geoelectrical images of the subsurface yields spatially consistent representations of alluvial aquifers heterogeneity at different scales from depositional systems to basin fills such an approach requires a conceptual framework to match the stratigraphic units with their evidence from ground based dc resistivity methods to effectively fill the gaps between sparse borehole data and to obtain valid representations of sedimentary heterogeneities such an approach is applied to characterize two sites of the quaternary aquifers of the central po plain italy which represent 1 the middle upper pleistocene braided to meandering river depositional systems sitting on southalpine crust and 2 their down current counterparts where they are involved by the latest uplift and deformation due to the tectonic activity of the apennine frontal thrusts electrical resistivity was considered as a proxy of the litho textural properties of hydrofacies and their major hierarchical association at depth and was interpreted in accordance with the depth decreasing resolution of ground based resistivity methods thus it was possible to identify the geophysical signature of hydrostratigraphic units through electrostratigraphic units i e sedimentary volumes identified by resistivity contrasts that spatially preserve the vertical polarity hydrostratigraphy and electrostratigraphy were then joined together through a site specific relationship between electrical resistivity and hydraulic conductivity which takes into account the prevailing process of current conduction the litho textural properties of hydrofacies and the groundwater electrical conductivity at the scales of aquifer systems and complexes this approach permitted to establish the conceptual framework to match hydrostratigraphy electrostratigraphy average hydrodynamic properties and distribution of heterogeneities

reflux stratabound dolostone and hydrothermal volcanism associated dolostone a two stage dolomitization model jurassic lebanon the kesrouane formation which is characterized by pervasive dolomitization has a stratigraphic thickness that exceeds 1000 m it is part of a broad carbonate platform deposited in the levant region and represents 60 of the lebanese jurassic rocks two genetically distinct dolostones are recognized within this unit 1 fine to medium crystalline non planar grey dolostone and 2 coarse crystalline planar beige dolostone the former is stratabound and of early jurassic age 87 sr 86 sr 0ae707455 this dolostone locally exhibits pseudomorphs of evaporite nodules pointing towards seepage reflux dolomitization by hypersaline to marine related fluids exposures of the coarse crystalline dolostone are associated with regional pre cretaceous faults along which late jurassic volcanics also occur sedimentological and diagenetic considerations coupled with microthermometry support a hydrothermal origin for this dolostone with th values of primary inclusions between 50 and 80 c the related dolomitizing fluids are mesosaline 3ae5 12ae0 eq wt nacl and are believed to result from the mixing of evaporative brines and sea water dolomitization is thus believed to have occurred in two stages whereby fluids invaded the host rocks first by seepage reflux explaining the resulting early jurassic stratabound dolostone and later through fracture flow along the faults associated with the late jurassic volcanism explaining the coarse crystalline hydrothemal dolostone

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

modern and late holocene dolomite formation manito lake saskatchewan canada abstract major advances have occurred in our understanding of modern dolomite formation and penecontemporaneous dolomitization over the past several decades manito lake located in west central saskatchewan canada is a large 65 km 2 deep z max 22 m perennial saline 45 ppt tds lake in which modern and late holocene dolomite coexists with other endogenic and authigenic carbonate precipitates including aragonite monohydrocalcite calcite and mg calcite like many other lacustrine dolomites manito lake dolomite is microcrystalline less than 1 m to 5 m ca rich and poor to moderately ordered it occurs as relatively pure hardgrounds and as a component of nearshore microbialites it also forms isopachous cements in consolidated siliciclastic shoreline sediments manito lake dolomite is most likely forming by mainly biomediated precipitation at or near the sediment water interface i in pore spaces of coarse siliciclastic sediments i e beachrock ii as fine laminae associated with microbialites and iii as a major component of mudstone hardgrounds and pavements

modern and holocene carbonate sedimentology of two saline volcanic maar lakes southern australia the basin lakes are two adjacent maar lakes located in the centre of the western volcanic plains district of victoria australia both lakes are saline and alkaline west basin lake is meromictic whereas east basin is a warm monomictic lake the carbonate mineral suite of the modern offshore bottom sediments of these basins consists mainly of dolomite and calcite with smaller amounts of hydromagnesite and magnesite in west basin and monohydrocalcite in east basin the dolomite hydromagnesite magnesite and monohydrocalcite are endogenic in origin being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment water interface the calcite is biologically precipitated as ostracod valves in addition to the carbonates in the modern offshore deep water sediments the lakes also contain a girdle of nearshore carbonate hardgrounds both beachrock and microbialites algal boundstones are present these modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics morphologies and compositions in west basin the hardgrounds are composed mainly of dolomite hydromagnesite and magnesite whereas dolomite and monohydrocalcite dominate the east basin sediments aragonite high mg calcite kutnahorite siderite and protohydromagnesite also occur in these lithified carbonate units stratigraphic variations in the carbonate mineralogy of the holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the basins these changes in conjunction with fluctuations in organic remains and fossil content indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western victoria

diagenetically altered sabkha type pleistocene dolomite from the arabian gulf diagenetically altered pleistocene dolomite occurs in the shallow subsurface of the arabian gulf offshore of al jubayl saudi arabia this dolomite accumulated in relatively shallow marine to sabkha depositional environments in contrast with the thin extent of most other quaternary sabkha and sabkha related dolomite deposits these deposits comprise a thick 56 m accumulation additionally this pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous abu dhabi sabkha complex the pleistocene dolomite also has lower 13 and 18o values than the modern abu dhabi sabkha dolomite and higher values than those commonly reported for analogous dolomite from the ancient rock record the low 18o values in conjunction with the geological setting indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition thus the degree of ordering stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed holocene precursor

magnetostratigraphy of the moyero river section north western siberia constraints on geomagnetic reversal frequency during the early palaeozoic summary we present the magnetostratigraphy of the early palaeozoic moyero river section in north western siberia the section investigated which consists mainly of reddish limestones marls and greyish limestones extends from the upper cambrian to the lower silurian palaeomagnetic analyses reveal two magnetic components one is interpreted as a remagnetization acquired during siberian trap emplacement the other is believed to be early diagenetic consistent with the established lower palaeozoic apparent polar wander path for siberia reversed polarity dominates from the upper cambrian to the middle ordovician while normal polarity dominates from the upper ordovician to the lower silurian only a few reversals which are poorly defined are observed through the section in particular during the llandeilo these data are in agreement with previous magnetostratigraphic data from the baltic platform and siberia and indicate that magnetic reversal frequency was low during the ordovician because of the time delayed process of acquisition of the magnetization carried by haematite we cannot clearly ascertain if a reversed superchron occurred during the lower to middle ordovician however our results suggest that a 15 myr long reversed interval existed during the arenig

a volumetric approach for the resistivity response of freshwater shaly sandstones the dc electrical response of freshwater saturated shaly sandstone is analyzed on the basis of effective volume conductivity concepts for a concentrated mixture of solid grains in a continuous electrolyte the bulk conductivity of this model is physically controlled by 1 the effective porosity and the average electrical tortuosity of its free pore electrolyte and 2 the amount and concentration of bound water that coats charged solid surfaces mainly of clays and a corresponding averaged electrical surface tortuosity the latter is combined in an equivalent volume conductivity that is mainly due to the electrical double layers of charges developed at the clay electrolyte interfaces analytical expressions based on effective medium and general mixture theories are developed to describe both the whole rock conductivity and the specific conductivities of its constituent elements the derived equations for the bulk conductivity of the system describe with sufficient precision experimental core dat

the lower middle ordovician palaeofield of scandinavia southern sweden revisited abstract we present a palaeomagnetic pole for the arenig llanvirn swedish orthoceras limestones based on a new analysis of some earlier reported palaeomagnetic data this new pole 18 n and 046 e differs from the original estimate of 30 n and 46 e the revised pole probably represents the most reliable ordovician pole from baltica although some uncertainties remain concerning the relative importance of diagenetic depositional controls on remanence acquisition based on the new pole from the swedish orthoceras limestone and recent palaeomagnetic results from the northern scandinavian caledonides it is now evident that baltica lay within southerly latitudes of between 30 and 60 s in cambro ordovician times subsequent anticlockwise rotation relative to eastern avalonia southern britain may indicate dextral shear across the tornquist suture prior to mid late ordovician amalgamation of eastern avalonia and baltica

multiple step recrystallization within massive ancient dolomite units an example from the dinantian of belgium south of the caledonian brabant wales massif a more than 200 m thick tournaisian to lower visean replacive dolomite unit can be followed for several hundred kilometres from the boulonnais france to aachen germany field observations of features such as karst cavities occurring at the top of the lower visean dolomite which are filled by lower visean crinoidal limestone indicate that dolomitization and karstification took place during the early visean this early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric the minor element composition of the majority of the dolomites remains almost uniform throughout the entire studied area values for fe mn na and sr are normally in the range 700 4700 ppm 15 400 ppm 80 300 ppm and 50 200 ppm respectively the 13c values range 0 72 to 5 31 o mainly reflect the carbon isotopic composition of the precursor limestones the 18o values in contrast are highly variable ranging from 19 15 to 0 85 o this rather large range of 18o values is explained by multiple step re equilibration recrystallization during progressive burial and subsequent uplift of the dolomites these processes are also responsible for the high 87sr 86sr values of the dolomites which range from about 0 7088 to 0 7098 they are distinctly more radiogenic than lower visean marine carbonates 0 7076 0 7078 correlation however of 18o values or 87sr 86sr ratios with dolomite and or cathodoluminescenec cl textures has not been very successful this suggests that recrystallization may remain unrecognized if only petrographic techniques are used nevertheless certain cl textures can be related to specific interactions with the ambient recrystallizing fluids

dolomitization of the silurian niur formation tabas block east central iran fluid flow and dolomite evolution abstract the silurian niur carbonates at dahaneh kalut and ozbak kuh sections tabas block eastern central iran have been pervasively dolomitized four types of dolomite consisting of replacive dolomite rd1 rd3 and saddle dolomite cements cd were discriminated by texture cathodolouminscence petrography and geochemical analyses rd1 dolomite occurred as a very fine 10 25 m planar e to planar s dolomite that replaced marine limestones rd2 dolomite is a planar s to planar e medium crystalline dolomite 100 400 m rd3 dolomite is a very coarse 400 m 2 mm non planar crystalline dolomite with sweeping extinction and cd dolomite occurred as voids and fractures filling saddle dolomite 5 10 mm with strong sweeping extinction field and petrographic observations demonstrated that rd3 and cd occur only at the dahaneh kalut section and are related to mafic volcanic rocks and fractures at the base of the niur formation the 18 o 8 45 to 6 06 vpdb and 13 c 0 01 to 0 99 vpdb values suggest that rd1 dolomite was precipitated at the early stage of dolomitization from silurian seawater whereas rd2 dolomite formed from similar fluids at higher temperatures upon burial the 18 o 13 80 to 9 47 and 13 c 1 66 to 0 76 values and fluid inclusion data t h 122 to 175 c salinity 18 68 to 24 wt nacl eq revealed that rd3 and cd were precipitated from hot saline fluids with a magmatic origin the mn fe na and sr concentrations of rd3 and cd suggest that the responsible dolomitizing fluids were more enriched with respect to these elements and had a different origin relative to the rd1 and rd2 dolomitizing fluids these results revealed that the late ordovician to early silurian passive rifting phase generated a local thermal gradient that increased heat flow and thermal convection of diagenetic fluids that led to the precipitation of rd3 and cd as pervasive hydrothermal dolomite the network of fractures and faults that formed during the rifting phase provided open conduits for hydrothermal fluids these results emphasize the importance of both tectonic setting and thermal conditions in the generation and circulation of dolomitizing fluids as well as in massive dolomitization

emplacement ages of paleoproterozoic mafic dyke swarms in eastern dharwar craton india implications for paleoreconstructions and support for a 30 change in dyke trends from south to north abstract large igneous provinces lips and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents the dharwar craton of southern peninsular india represents a substantial portion of archean crust and has been considered to be a principal constituent of superia sclavia nuna columbia and rodinia supercontinents the craton is intruded by numerous regional scale mafic dyke swarms of which only a few have robustly constrained emplacement ages through this study the lip record of the dharwar craton has been improved by u pb geochronology of 18 dykes which together comprise seven generations of paleoproterozoic dyke swarms with emplacement ages within the 2 37 1 79 ga age interval from oldest to youngest the new ages integrated with u pb ages previously reported for the hampi swarm define the following eight swarms with their currently recommended names ne sw to ese wnw trending ca 2 37 ga bangalore karimnagar swarm n s to nne ssw trending ca 2 25 ga ippaguda dhiburahalli swarm n s to nnw sse trending ca 2 22 ga kandlamadugu swarm nw se to wnw ese trending ca 2 21 ga anantapur kunigal swarm nw se to wnw ese trending ca 2 18 ga mahbubnagar dandeli swarm n s nw se and ene wsw trending ca 2 08 ga devarabanda swarm e w trending 1 88 1 89 ga hampi swarm nw se ca 1 79 ga pebbair swarm comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features such as regional dharwar greenstone belts and the late archean ca 2 5 ga closepet granite batholith have led to the hypothesis that the northern dharwar block has rotated relative to the southern block by restoring a 30 counter clockwise rotation of the northern dharwar block relative to the southern block we show that pre 2 08 ga arcuate and fanning dyke swarms consistently become approximately linear two possible tectonic models for this apparent bending and concomitant dyke rotations are discussed regardless of which deformation mechanisms applies these findings reinforce previous suggestions that the radial patterns of the giant ca 2 37 ga bangalore karimnagar dyke swarm and probably also the ca 2 21 ga anantapur kunigal swarm may not be primary features

first paleomagnetic data for the new siberian islands implications for arctic paleogeography abstract in this paper we present new paleomagnetic and paleontological data from the ordovician and silurian carbonate rocks of kotelny island the anjou archipelago and from the ordovician turbidities of bennett island the de long archipelago it is assumed that both archipelagos belong to the nsi new siberian islands terrane a key tectonic element in the arctic region ages of the studied rocks have been established by paleontological data and lithological correlations our new data on conodonts combined with those from previous studies of ordovician and silurian fauna indicate a biogeographic similarity between the shelves of the siberian paleocontinent and the nsi in the early paleozoic three new paleomagnetic poles for the nsi 48 9 n 13 8 e a95 18 1 for 475 ma 45 5 n 31 9 e a95 11 0 for 465 ma and 33 7 n 55 7 e a95 11 0 for 435 ma fall between the south eastern part of central europe and the zagros mountains the similarity of paleomagnetic directions from kotelny and bennet islands confirms that both the anjou and de long archipelagos belong to the same terrane calculated paleolatitudes indicate that in ordovician silurian times this terrane has been located between 30 and 45 possibly in the northern hemisphere based on this observation we suggest a linkage between the nsi and the kolyma omolon superterrane comparison of apparent polar wander paths apwps of the nsi siberia and other cratons terranes suggests that the nsi drifted independently we demonstrate that the structural line between svyatoy nos peninsula and great lyakhovsky island is the continuation of the kolyma loop suture on the arctic shelf and expect that the continuation of the south anyui suture is to be found east of the nsi

paleomagnetism of the siberian traps new data and a new overall 250 ma pole for siberia the flood basalt province in siberia is one of the largest in the world but the number of reliable paleomagnetic data on these volcanics is still limited we studied lava flows and trap related intrusions from two areas in the north and west of the siberian platform a dual polarity characteristic component was isolated from most samples with the aid of stepwise thermal and alternating field demagnetization we then compiled all published paleomagnetic data on the siberian traps that have been obtained according to modern standards also included are presumably trap related overprint directions in paleozoic rocks although these overprints and trap results may locally differ the corresponding mean poles based on remagnetized sediments and volcanics show excellent overall agreement and justify pooling of both data types several ways of data grouping were attempted the trap mean pole proved to be rather insensitive to statistical treatment irrespective of the averaging procedure used the overall mean poles for the siberian traps nsp2 55 1 n 147 0 e n 8 k 123 a95 5 0 or nsp4 57 2 n 151 1 e n 8 k 192 a95 4 0 differ slightly but significantly from the coeval mean poles of baltica torsvik 2001 van der voo r and torsvik t h the quality of the european permo triassic paleopoles and its impact on pangea reconstructions in timescales of the paleomagnetic field j e t channell d v kent w lowrie and j g meert eds agu geophys monogr 2004 135 29 42 we consider possible causes for this difference and conclude that it could be explained either by persistent non dipole terms in the permo triassic geomagnetic field or widespread inclination shallowing in the european data

magnetic reversal frequency and apparent polar wander of the siberian platform in the earliest palaeozoic inferred from the khorbusuonka river section northeastern siberia summary marly limestones that crop out along the khorbusuonka river northeastern siberian platform define a well dated lower and middle cambrian sequence which was sampled for magnetostratigraphy in the upper part of the section from the uppermost lower cambrian toyonian stage to the middle cambrian amgan and mayan stages thermal demagnetization isolates two magnetic components carried either by magnetite haematite or both the first is parallel to the recent geomagnetic field at the site and is removed in the low to middle temperature range the second component is isolated at higher temperatures and has dual magnetic polarities this component yields a positive but partial since we consider only two sites fold test with other middle cambrian data previously obtained from the distant kulumbe river section and was probably acquired during sedimentation or at an early stage of diagenesis it records a very high magnetic reversal frequency of at least six to eight reversals per myr during the entire middle cambrian assuming a 10 myr duration for this epoch a much more complex palaeomagnetic behaviour is observed within the upper tommotian lower atdabanian lower cambrian part of the khorbusuonka section demagnetization diagrams show large overlaps between magnetic components which prevents the isolation of a clear magnetic component at high temperatures analysis of these overlaps is however consistent with the existence of an ill determined primary magnetic direction for the lower cambrian which is very different from that isolated in the middle cambrian although still in need of further confirmation the possibility that a large drift in siberian apparent polar wander occurred over a rather short time during the lower cambrian appears to be supported by our data

estimation of hydraulic parameters of shaly sandstone aquifers from geoelectrical measurements electrical and hydraulic conductivities of shaly sandstones are described using a capillary approach for a granular clay bearing material the clays are assumed to occur as shale shells uniformly coating the insulated sand grains the real and imaginary components of the complex electrical conductivity for this material model are written in terms of a unique solid matrix conductivity treated as a volumetric property above a critical salt concentration the conductive contribution of the shells is independent of the electrolyte salinity under this condition the equation for the bulk conductivity of the sandstone can be expressed in a simplified form however below the critical concentration the matrix conductivity is dependent on the conductivity of the water saturating the shaly component and can be expressed only by the complete equation the hydraulic conductivity for this model is expressed by a modified kozeny carman equation a new semi empirical equation relates the hydraulic conductivity for such rocks to their porosity formation resistivity factor and the electrical conductivity of its solid matrix these combined properties are described as the lithoporosity factor in this new formulation the petrophysical parameters involved are easily determined from the electrical geophysical measurements the performance of this equation is firmly tested with experimental laboratory data available in the literature its application is then extended to estimate the hydraulic parameters of a shaly sandstone aquifer in bahia brazil using either the borehole or the surface geoelectrical data examples are given to emphasize the combined use of electrical resistivity and induced polarization measurements in computing hydraulic properties

origin of lower ordovician dolomites in eastern laurentia controls on porosity and implications from geochemistry the catoche formation of the st george group in eastern laurentia western newfoundland consists of early ordovician arenigian shallow marine platform carbonates 160 m thick which were extensively dolomitized during the course of their diagenetic history the dolomites occur as both replacement and pore filling cements and are a major control on porosity distribution in the formation the origin and diagenetic history of the catoche dolomites at daniel s harbour western newfoundland were analysed in comparison to equivalent successions at port au choix pac and port au port peninsula pap to assay the reservoir potential of these dolomites in eastern laurentia petrographic examination identified at least three generations of dolomites in the catoche formation which are 1 an early replacement sub to eu hedral micritic dolomite 4 m 30 m d1 2 eu to sub hedral dolomite 70 m 1 mm often with cloudy cores and clear rims d2 and 3 subhedral to anhedral saddle dolomite cement 200 m 3 mm d3 the micritic dolomite d1 exhibits a dull cathodoluminescence cl under cathodoluminoscope whereas dolomite d2 exhibits consistent concentric cl zonation some subhedral crystals of d3 appear zoned both in plane polarized light and cathodoluminoscope otherwise d3 exhibits a dull cl stoichiometric dolomite occurs in all three generations with d2 as the dominant dolomite by abundance the low strontium 47 25 ppm content coupled with depleted 18o value of dolomitizing fluids 10 to 11 2 vsmow and near micritic grain size suggests an early precipitation of dolomite d1 at low temperatures of near surface conditions from solutions likely formed by mixing of early ordovician sea and meteoric waters in contrast microthermometric measurements of primary two phase fluid inclusions in dolomite d2 homogenization temperatures of 102 168 c with a salinity range of 19 8 25 eq wt nacl and dolomite d3 homogenization temperatures of 158 190 c with a salinity range of 20 2 22 2 eq wt nacl suggest that both dolomite generations were generated in mid to deep burial settings from high salinity low temperature 200 c hydrothermal fluids likely under suboxic conditions this is consistent with the low sr concentrations for d2 36 4 8 ppm and d3 38 7 9 ppm 18o values of dolomitizing fluids for d2 2 1 to 8 1 vsmow and d3 6 to 8 1 vsmow coupled with fe contents of d2 1684 1096 ppm and d3 1783 7 618 ppm as well as the respective mn d2 131 2 50 and d3 197 5 55 ppm concentrations ree and shale normalized reesn values of catoche carbonates indicate enrichment in rare earth element ree composition of the earliest calcite c1 relative to those of arenig seawater whereas the reesn profiles of the dolomite generations mimic that of calcite c1 the ce ce ce sn and la la la pr pr sn anomalies of the catoche dolomites are consistent with precipitation in equilibrium with source fluids in slightly oxic to suboxic conditions whereas eu eu eu cn anomalies suggest similar source fluids for d2 and d3 results of fluid inclusion gas analysis are consistent with petrographic features and geochemical compositions and support the exclusion of magmatic fluids during dolomitization visual estimates of porosity from thin sections indicate four porous 4 12 horizons vugs and intercrystalline pores are two types of porosity associated with the dolomites with the latter being the dominant type and associated mainly with dolomite d2

the relation between hydraulic transmissivity and transverse resistance in a complicated aquifer of glacial outwash deposits abstract the application of the electrical resistivity method to freshwater aquifers has shown a direct relationship between formation factor and aquifer permeability this is possible if the layer resistivity of the saturated zone can be obtained from an interpretation of the geoelectrical sounding curve without the ambiguity due to nonuniqueness this is usually the case for uncomplicated horizontal layer cases where the aquifer is sufficiently thick and has a resistivity which can easily be distinguished from the resistivities of the unsaturated zone and the acquitard in this study geoelectrical depth soundings after schlumberger were conducted over aquifers in glacial outwash deposits near a recessional moraine of rhode island in southern new england lateral as well as vertical changes in lithology in particular of the grainsize distribution present a complicated system of a multistory aquifer the depth soundings were interpreted in terms of layer resistivities thicknesses and transverse resistances in most cases the aquifer could be identified with an intermediate resistivity between that of the unsaturated zone and the fine grained aquitard results of the depth soundings were compared with test wells and pumping tests comparison of the ranges of hydraulic transmissivities with the transverse resistance shows a linear relationship between both values this suggests that even in complicated cases where test wells are of limited use the geoelectrical depth sounding method can be used to locate aquifers of high hydraulic transmissivity great care is necessary to measure reliably the water resistivity in the test well a further uncertainty is the depth of the filter screen relative to the bottom of the aquifer in light of the transitory nature between aquifer and aquitard the transverse resistance is a more unique integral electrical parameter than the layer resistivity this may explain why it shows a more reliable relationship with the hydraulic transmissivity in a complicated case than the relationship between formation factor and hydraulic conductivity which was found in simple cases

temporal correlation between dyke swarms and crustal extension in the middle palaeozoic vilyui rift basin siberian platform abstract this paper presents results from new 40ar 39ar isotope dating of nine mafic dykes from three large dyke swarms vilyui markha kontai dzherba and chara sinsk of the yakutsk vilyui large igneous province lip in addition to a reconstruction of the subsidence history of the middle palaeozoic vilyui paleorift basin eastern siberian platform all previously published 40ar 39ar and u pb dates are summarized statistical analysis of the dyke ages reveals repeated magmatic events in the study area two major pulses of mafic magmatism are identified one at the frasnian famennian boundary with a main peak at ca 374 1 ma and another in the latest devonian with a peak at ca 363 4 ma the time of maximum intensity of dyke intrusion coincides within uncertainty with rapid subsidence in the vilyui basin the minimum total volume of middle palaeozoic magmatism produced in the yakutsk vilyui lip is 100 215 k km3 which is much less than earlier estimates most of the mafic material within the yakutsk vilyui lip is related to the vilyui basin and associated dyke swarms backstripping analysis of sedimentation in depressions of the vilyui basin was carried out estimates were obtained for the spatial distribution of the stretching factor of the crust and mantle lithosphere averaging 1 17 and 1 44 respectively the amount of extension due to dyke intrusion is estimated to be 6 backstripping analysis of sedimentation in the vilyui basin was used to assess the effect of both intraplate far field forces and upwelling magma flows initiated by a mantle plume a numerical thermomechanical model was developed to investigate the relations between two possible mechanisms by which the vilyui rift was initiated intraplate extension passive rifting and the ascent of a mantle magmatic diapir active rifting a model considering both of these mechanisms shows the contribution of the far field extension forces and the effect of convective flows around the mantle plume assuming a temperature of 1500 c for a heat source operating for 10 myr the best fit model predicts the formation of the vilyui basin under extensional conditions with a depth integrated horizontal driving force of 2 0 1013 n m which corresponds to a lithostatic pressure of 0 73

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

induced polarization of shaly sands equations have been developed that relate induced polarization ip in shaly sands to measurable petrophysical parameters the induced polarization process has been modeled in terms of two mechanisms clay counterion displacement and membrane blockage the resulting equations can be used to determine shaliness brine conductivity and oil saturation from in phase and out of phase conductivities laboratory measurements have confirmed the ip dependence on these variables as well as on temperature

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

origin of island dolostones a case study from the cayman formation miocene cayman brac british west indies abstract cayman brac the easternmost of the cayman islands is 19 km long 1 5 to 3 km wide and rises 40 m above sea level at its eastern end geographically isolated by the deep oceanic waters of the caribbean sea this island with its thick succession of pervasively dolomitized tertiary rocks is a natural laboratory for assessing the origin of island dolostones dolostones in the miocene cayman formation 100 m thick with most crystals 55 mol caco3 geochemically they are characterised by 13c values of 1 6 to 3 5 18o values of 2 3 4 0 80 279 ppm sr 52 340 ppm fe and 9 82 ppm mn the inverse correlation between 18o and mol caco3 and positive correlation between sr and mol caco3 largely reflect kinetic effects on oxygen isotope fractionation and the sr partition behaviour between dolomite and water dolomite stoichiometry indicates formation from waters with a high mg ca ratio and 18o values corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation indicate formation from normal seawater like fluids under near surface conditions the 87sr 86sr ratios point to two phases of dolomitization phase i in the late miocene 6 8 ma caused partial dolomitization of the basal part of the cayman formation phase ii during the pliocene to early pleistocene 1 5 ma completed dolomitization of the cayman formation both phases of dolomitization were mediated by similar fluids although available information suggests that dolomitization was probably linked to sea level fluctuations the timing of dolomitization relative to the transgressive regressive cycle remains open to debate

powder x ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones abstract the ca molar ca ca mg 100 in sedimentary dolomite which ranges from 48 ca to 62 5 ca can be determined by atomic absorption spectrometry electron microprobe analysis or powder x ray diffraction analysis the latter procedure which relies on the fact that the position of the d104 reflection depends linearly on the ca content offers an easy and relatively inexpensive method of determining ca in dolomite to date such analyses have assumed that the dolostone is homogeneous with respect to the ca content of its constituent dolomite crystals herein the xrd technique is extended to compositionally heterogeneous dolostones a considerable amount of important information can be derived from x ray diffractograms of dolostones if they are analyzed with peak fitting techniques pf xrd specifically this technique allows 1 determination of the ca with an accuracy of 0 5 ca 2 identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their ca content and 3 derivation of the proportion of each population of dolomite crystals with an accuracy of 10 the pf xrd technique is a powerful analytical tool that provides valuable information on the major element composition of low fe dolomite compared to other techniques the pf xrd technique is accurate rapid relatively inexpensive and easy to use

sedimentary carbonates through phanerozoic time plate tectonic processes play a critical role in the origin and distribution of sedimentary carbonates through phanerozoic time the phanerozoic age distribution of sedimentary properties like calcite dolomite ratio inferred ooid and cement mineralogy and survival rate of continental carbonates is cyclic the cycles appear to be coupled to plate tectonic processes that give rise to global sea level change and changes in the properties of the ocean atmosphere system first order changes in sea level are driven by the accretion of mid ocean ridges high accretion rate high sea level low accretion rate low sea level although correlations between sea level and sedimentary carbonate properties are not strong high sea level over an extended period of time appears to be correlated with low calcite dolomite ratios lack of inferred aragonite ooids and cements and maxima in the survival rate of continental carbonates the opposite is true for extended periods of low mid ocean ridge accretion rates and global sea levels the lack of strong correlations may reflect an insufficient data base and the possibility of lags between sea level change and change in carbonate properties furthermore the survival rate of continental carbonates appears to be affected by differential cycling and therefore may not be directly related to accumulation rate it appears that the environmental conditions for early dolomitization and calcite ooid and cement formation are best met during extended times of high sea level when atmospheric co2 levels are high and the saturation state of seawater with respect to carbonate minerals relatively low during low sea levels early dolomitization is less favored and aragonite precipitates are more abundant because of low atmospheric co2 levels and enhanced seawater carbonate saturation states differential cycling has modified the phanerozoic sedimentary carbonate mass age distribution because of erosion of younger units within continental carbonate cycles it may be difficult to derive an unequivocal record of the partitioning of carbonate between the deep sea and shallow water realms of deposition during the phanerozoic this difficulty must be considered in further quantification of geochemical models describing the geologic history of atmospheric co2 and climate change

palaeomagnetism of lower cambrian sediments from the olenek river section northern siberia palaeopoles and the problem of magnetic polarity in the early cambrian summary new palaeomagnetic data from the lower and middle cambrian sedimentary rocks of northern siberia are presented during stepwise thermal demagnetization the stable characteristic remanence chrm directions have been isolated for three cambrian formations both polarities have been observed and mean chrm directions for normal polarity are kessyusa formation lower cambrian d 145 i 4o n 12 ug5 12 8 pole position cd 38 s a 165 e erkeket formation lower cambrian stratigraphically highly d 152 i 47 n 23 ug5 6 8 pole position cd 45 s a 159 e yunkyulyabit yuryakh formation middle cambrian d 166 i 33 n 38 ctg5 4 6 pole position 0 36 s l 140 e these poles are in good agreement with the apparent polar wander path based on the bulk of existing cambrian palaeomagnetic data from the siberian platform in cambrian times the siberian platform probably occupied southerly latitudes stretching from about 35 to o and was oriented reversely with respect to its present position siberia moved northwards during the cambrian by about 10 of latitude this movement was accompanied by anticlockwise rotation of about 30 the magnetostratigraphic results show the predominance of reversed polarity in the early cambrian and an approximately equal occurrence of both polarities in the part of the middle cambrian studied these results are in good agreement with the palaeomagnetic polarity timescale for the cambrian of the siberian platform constructed previously by khramov et al 1987

a new siberian record of the 1 0 gyr old maya superchron abstract we report new magnetostratigraphic data from the late mesoproterozoic linok formation consisting of clayed carbonates together with paleomagnetic data from the neoproterozoic carbonate sukhaya tunguska burovaya and turukhansk formations located in the turukhansk region northwestern siberian platform we also re analyzed demagnetization data previously obtained from the derevnya formation paleomagnetic analyses revealed up to four magnetization components first a steep inclination component likely of recent origin is isolated in all sections in the low temperature range second a magnetization is recovered in most samples between 250 c and 350 400 c it shows a single magnetic polarity and is interpreted as a remagnetization acquired during the kiaman permo carboniferous reverse superchron third a magnetization component is isolated between 400 c and 500 c in samples from the burovaya and turukhansk formations this component is interpreted as a remagnetization that occurred before the end of the neoproterozoic a fourth high temperature ht magnetization component is observed up to 575 c in samples from the linok and burovaya formations and up to 680 c for those of the derevnya and turukhansk formations a series of magnetic polarity reversals is observed in the lower and middle parts of the linok formation whereas a single polarity interval is found in its upper part the syn depositional origin of this magnetization is attested by positive reversal conglomerate and fold tests the ht magnetization component isolated in the derevnya and burovaya formations shows a single magnetic polarity its primary origin is supported by the fact that the derevnya and burovaya ht mean directions are positioned in the continuity of the directional evolution observed through the linok formation it is further constrained by very consistent late mesoproterozoic to early neoproterozoic apparent polar wander paths obtained from different regions of the siberian platform a single magnetic polarity reversal is observed in a 40 m thick section of the turukhansk formation the ht directions found in the upper part of the linok formations over a stratigraphic thickness of 200 m and in the derevnya and burovaya formations strengthen the existence of the normal polarity maya superchron around 1 0 ga at the transition between the mesoproterozoic and the neoproterozoic

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

origin of dolomite in the late cretaceous paleocene limestone turbidites eastern pontides turkey abstract the late cretaceous paleocene tonya formation of the hacimehmet region in trabzon eastern pontides turkey is composed of limestone turbidites these limestone turbidites were deposited at water depths of around 500 to 600 m and remained buried until uplifted in late paleocene time the lower part of this formation which is made up of clast supported calcirudites and pebbly calcarenites is partially to completely dolomitized two morphological types of dolomite are identified 1 fine to medium crystalline dolomite rd1 2 coarse to very coarse crystalline dolomite rd2 each one of the dolomite types is non stoichiometric ca rich ca 57 61 mg 43 39 and has similar geochemical features and uniform dull red brown luminescence these dolomites have 18 o pdb values from 2 6 to 3 3 average 3 0 13 c pdb values from 2 3 to 3 1 average 2 7 and 87 sr 86 sr ratios from 0 707719 to 0 707772 fluid inclusion data of rd2 dolomites yield homogenization temperatures t h of 48 58 c and salinities of 20 4 to 21 7 wt nacl equivalent the sr concentration in dolomites varying from below a detection limit of 20 to 360 ppm is not homogeneous within each dolomite crystal the na concentrations of dolomites are similar varying from 600 to 760 ppm average 680 ppm and are higher than those for dolomites formed in normal marine environments the fe and mn concentrations range from 2500 to 7000 ppm average 3800 ppm and from 50 to 850 ppm average 330 ppm respectively considering the petrography paragenesis trace element and stable isotope data the dolomites in the tonya formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths dolomitization in the tonya formation was most probably driven by seawater that was slightly modified through rock water interaction and in part at elevated temperatures

coastal aquifer assessment based on geological and geophysical survey northwestern crete greece groundwater preservation comprises a major problem in water policy the comprehension of the groundwater hydraulic systems can provide the means to approach this problem generally drilling is expensive and time consuming on the other hand new techniques have been applied during the last few decades that provide useful information on the depth and quality of aquifers among them transient electromagnetic method tem is an appealing method that provides fast results with minimum field crew and solves several hydrogeological problems many portable systems for single site measurements are commercially available the tem fast 48hpc was used for acquiring 106 soundings in the northwestern crete in greece for defining the hydrogeological characteristics of the study area since there were no available data from boreholes detailed geological hydrolithological and tectonic survey was applied prior to the geophysical measurements all the data were integrated to produce a secure and reliable hydrogeological model for the study area prior to any future hydrowell specifically geometrical and hydraulic data of the study area groundwater were acquired two unconnected aquifers were detected and their possible contamination due to saltwater intrusion was analyzed and eliminated moreover a location for borehole construction and groundwater pumping based on the potential of the aquifer system was proposed finally the contribution of tem and electrical resistivity tomography geophysical methods in studying complex coastal aquifers is shown by this work

genesis of fabric destructive dolostones a case study of the brac formation oligocene cayman brac british west indies abstract the origin of fabric destructive and fabric retentive textures in dolostones of all ages remains open to debate evolution of the fabric destructive textures which effectively destroys all traces of the original limestones remains particularly controversial on cayman brac a small island located in the caribbean sea the brac formation lower oligocene is formed of limestones and fabric destructive dolostones the top of this formation is defined by the brac unconformity which developed as a karst surface during the late oligocene and early miocene the overlying cayman formation lower to middle miocene is pervasively dolomitized and characterized by fabric retentive textures that have preserved the depositional textures of the original limestones the stratigraphic juxtaposition of these two formations provides an ideal opportunity to compare the factors that controlled the development of fabric destructive and fabric retentive fabrics in dolostones the dolostones in the brac formation are formed of dolomite that replaced the limestone and dolomite cements typically zoned that were precipitated in pores throughout the dolostones interpretations based on the composition of the dolomite 13 c values 18 o values and sr fe and mn contents indicate that dolomitization of both the brac formation and the cayman formation was probably controlled by similar slightly modified seawater with the distribution of the dolomite in the limestones and the evolution of dolostone textures being controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids the large sucrosic dolomite crystals which are commonly zoned probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation

spatial variations in the stoichiometry and geochemistry of miocene dolomite from grand cayman implications for the origin of island dolostone abstract the cayman formation miocene 140 m thick on grand cayman is incompletely dolomitized with the most of the dolomite restricted to the peripheral part of the island these calcium rich dolomites with 50 60 mol caco 3 ca are divided into low calcium dolomite lcd ca 55 despite the small size of the island 6 8 km wide the percentages of lcd and hcd the ca and the geochemical properties of the dolomites show geographic variations relative to the surrounding shelf edge accordingly the cayman formation on the east end of the island is divided into the peripheral dolostone shelf edge 1 5 km inland transitional dolostone 1 5 2 7 km inland and the interior limestone and dolostone 2 7 km to island center from the peripheral dolostone to the interior limestone and dolostone there is an increase in the percentage of hcd and ca in the dolomite and decreases in the 18 o and 13 c values of the dolomite interpretations based on the oxygen and carbon isotopic compositions indicate that seawater is the source of mg for dolomitization the concentric pattern of dolomitization on the island reflects the fact that seawater flowed into the island from all directions during dolomitization the lateral inland variations in the dolomite stoichiometric and isotopic properties reflect the gradual modification of seawater by water rock interaction along the flow path from the shelf edge to the island center 87 sr 86 sr ratios indicate that two phases of dolomitization late miocene early pliocene and late pliocene early pleistocene were responsible for dolomitization of the cayman formation it is very likely that during both phases the carbonate platform was subaerially exposed and that the pump for circulating the seawater through the island was related to the seawater freshwater mixing zone the dolomitization model developed from grand cayman may be applicable to many other island dolostones affected by long term sea level changes given that the diagenetic potential of dolostone is directly linked to its stoichiometry the distribution of the lcd hcd will influence the petrographic properties geochemical signatures and reservoir potential of the dolostones

paleomagnetism of the ulkan massif se siberian platform and the apparent polar wander path for siberia in late paleoproterozoic early mesoproterozoic times abstract paleomagnetic studies on paleoproterozoic rocks of the ulkan massif from the south eastern siberian platform yielded two ancient paleomagnetic directions and made it possible to calculate poles corresponding to them for two time intervals in the late paleoproterozoic 1732 ma for volcanogenic sedimentary rocks of the elgetey formation plat 7 1 7 1 plong 183 5 3 5 dp 10 5 dm 16 4 positive fold reversals and conglomerates tests and b less than 1720 ma for granites of the ulkan complex plat 42 1 42 1 plong 249 4 69 4 dp 3 4 dm 5 6 the analysis of the existing paleomagnetic data for siberia in late paleoproterozoic early mesoproterozoic times allowed a total of 12 new poles to be defined with five of them considered key poles table 3 based on which the apwp for siberia was generated on the basis of the data presented here one can conclude that in the context of the paleoproterozoic supercontinent model preference is given to the position of paleoproterozoic north poles of siberia in the indian ocean and australia rather than in the atlantic ocean

dolomitization of the oligocene miocene bluff formation on grand cayman british west indies the oligocene miocene bluff formation on grand cayman is formed of hard white finely crystalline dolostone in which the precursor textures and fossil microstructures are commonly preserved the d

origin of the breno and esino dolomites in the western southern alps italy implications for a volcanic influence the esino limestone of the western southern alps represents a differentiated ladinian lower carnian carbonate platform comprised of margin slope and peritidal inner platform facies up to 1000 m thick a major regional subaerial exposure event lead to coverage by another peritidal lower carnian carbonate platform breno formation multiphase dolomitization affected the carbonate sediments petrographic examinations identified at least three main generations of dolomites d1 d2 and d3 that occur as both replacement and fracture filling cements these phases have crystal size ranges of 3 35 m dolomicrite d1 40 600 m eu to subhedral crystals d2 and 200 m to 5 mm cavity and fracture filling anhedral to subhedral saddle dolomite d3 respectively the fabric retentive near micritic grain size coupled with low mean sr concentration 76 37 ppm and estimated 18o of the parent dolomitizing fluids of d1 suggest formation in shallow burial setting at temperature 45 50 c with possible contributions from volcanic related fluids basinal fluids circulated in volcaniclastics or related to volcanic activity which is consistent with its abnormally high fe 4438 4393 ppm and mn 1219 1418 ppm contents the larger crystal sizes homogenization temperatures d2 108 9 c d3 111 14 c of primary two phase fluid inclusions and calculated salinity estimates d2 23 2 eq wt nacl d3 20 4 eq wt nacl of d2 and d3 suggest that they formed at later stages under mid to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity which is supported by higher estimated 18o values of their parent dolomitizing fluids this is also consistent with their high fe 4462 4888 ppm and 1091 1183 ppm respectively and mn 556 289 ppm and 1091 1183 ppm contents and low sr concentrations 53 31 ppm and 57 24 ppm respectively the similarity in shale normalized sn ree patterns and ce ce ce sn and la pr pr sn anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor positive eu anomalies coupled with fluid inclusion gas ratios n2 ar co2 ch4 ar he high f concentration high f cl and high cl br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks which is consistent with the co occurrence of volcaniclastic lenses in the investigated sequence

estimation of aquifer transmissivity from dar zarrouk parameters in porous media the aim of this paper is to establish an analytical relationship between the aquifer parameters for porous media and the so called dar zarrouk parameters so that the former can best be estimated from the surface resistivity measurements taking the two fundamental laws viz darcy s law of fluid flow and ohm s law of current flow analytical relationships between transmissivity and transverse resistance as well as transmissivity and longitudinal conductance have been established it was found that the relationship between transverse resistance and transmissivity is more meaningful as the product of hydraulic conductivity and electrical conductivity of an aquifer may be assumed to be constant for an area with a particular geological setting and range of water quality this is verified by analysing the data given by kelly

synsedimentary dolospar cementation a possible devonian example in the camsell formation northwest territories canada1 a unique association of beds containing fenestral pore filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the lower devonian camsell formation many of these sandstones consist almost entirely of dolospar sand sedimentological petrographic cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during camsell deposition from fenestral dolostone beds that directly underlie these sandstones erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization or dolomite cementation of the pore filling carbonate cements within fenestrae of the fenestral fabrics occurred before erosion and deposition of dolospar sand the fine details of the cathodoluminescent zonations suggest that the pore filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures this unique occurrence may document synsedimentary devonian cementation by medium to coarsely crystalline dolomite

effectiveness of geophysical surveys for water wells relocation in overexploited aquifers the example of maggiore and traversola valleys northwestern italy aquifer overexploitation is a common problem in drinking water management this will become more and more important given the general reduction of water resources to overcome overexploitation one of the most adopted solutions is the relocation of extraction wells to establish the positions of new wells a precise knowledge of the hydrogeological setting is required specific surveys are therefore necessary to obtain information over wide investigation zones geophysical methods particularly electromagnetic and electrical can be useful with this aim in the present paper a case history on the combined use of electric resistivity tomography and time domain electromagnetic soundings is reported surveys have been performed within the maggiore and traversola valleys to investigate the uppermost part of the quaternary deposits hosting the near surface aquifer the electromagnetic data have been inverted with a spatially constrained approach by assuming a quasi 1d model of the subsurface geophysical surveys allowed for depicting the depth and lateral continuity of the supposed aquifer level in the surveyed area up to a depth of about 200 m and proposing potential positions for well relocation

a geophysical and geochemical approach for seawater intrusion assessment in the acquedolci coastal aquifer northern sicily vertical electrical sounding ves surveys and chemical analyses of groundwater have been executed in the coastal plain of acquedolci northern sicily with the aim to circumscribe seawater intrusion phenomena this urbanized area is representative of a more general problem interesting most of mediterranean littoral areas where intensive overdraft favors a heavy seawater intrusion through the coastline aquifer resistivity seems decisively to be conditioned by the chlorine and magnesium content in the main aquifer of the region schlumberger ves together with piezometric and chemical physical information of groundwater allowed us to perform hydrogeological and geophysical elaborations to propose the occurrence of a relatively narrow belt marked by fresh salt water mixing in the considered plain pollution risk studies have been already realized by authors with the proposal to identify by parametric and synthetic zoning of significant hydrogeological elements the most vulnerable sectors in detail an intrinsic vulnerability mapping has been already performed applying the well known sintacs system this paper intends to give in this sector of sicily an example of integration of different methodologies showing the role of geophysics to describe the degradation of aquifers on the whole as well as to assess their pollution risk better

estimation of aquifer hydraulic parameters from surficial geophysical methods a case study of keritis basin in chania crete greece summary knowledge of aquifer parameters is essential for the management of groundwater resources conventionally these parameters are estimated through pumping tests carried out on water wells few boreholes may be available and carrying out pumping tests at a number of sites may be costly and time consuming the application of geophysical methods in combination with pumping tests provides a cost effective and efficient alternative to estimate aquifer parameters a geophysical method is used to obtain aquifer characteristics that are estimated through the pumping tests a correlation is established between these parameters at other sites where pumping has not been carried out in this way the entire investigation area could be covered to characterize an aquifer system this study has been carried out in the keritis basin in chania crete greece where the aquifer characteristics are required for the management of groundwater in the region

paleomagnetism of ca 2 13 2 11 ga indin and ca 1 885 ga ghost dyke swarms of the slave craton implications for the slave craton apw path and relative drift of slave superior and siberian cratons in the paleoproterozoic of 35 archean cratons that have been identified around the globe only one the superior craton of the canadian shield has a reasonably well defined apparent polar wander apw path for much of paleoproterozoic time based on key i e well defined and precisely dated paleopoles as a result it has been difficult to compare the drift of these cratons or reliably test continental reconstructions of the archean cratons in the paleoproterozoic based on paleomagnetism in this study we report key paleopoles for the 2 13 2 11 ga northwest to north northwest trending indin dyke swarm 36 n 76 w a 95 7 and the 1 885 ga northeast to north northeast trending ghost swarm 2 n 106 w a 95 6 of the yellowknife region of the slave craton u pb baddeleyite ages have been determined at paleomagnetic sampling sites and baked contact tests establish that the remanences are primary combined with paleopoles from other precisely dated dyke swarms of the slave craton these data define a rudimentary apw path between ca 2 23 and 1 885 ga and permit a comparison of the drift of the slave and superior cratons over this interval both the indin and ghost poles are precisely matched in age with key poles on the superior apw path the slave and superior paths are not superimposed demonstrating that the two cratons were not in their present relative locations they have different overall shapes indicating relative drift during at least a portion of the period however the earlier 2 23 2 21 ga portions of the tracks appear broadly similar and could permit the two cratons to be on a single tectonic plate at that time although separated by a great distance a comparison of the ghost pole and a coeval key pole for siberia permits a 1 88 ga reconstruction with southern siberia facing the northern slave laurentia margin broadly similar to reconstructions that have been proposed at least as late as 1 38 ga although the distance between the cratons is still poorly constrained

a grain conductivity approach to shaly sandstones the effects of clay conduction on the electrical behavior of shaly sandstones at low frequencies are analyzed by considering self similar mixtures of conductive grains as representative of these porous rocks an essential step in this model is the representation of the surface conductivities of the clay platelets by an equivalent volume conductivity based on their pattern of occurrence the clays are treated either as a continuous coating over the sand grains or as individual detrital grains mixed with a nonconductive mineral matrix in the first case the solids are modeled as layered conductive particles taken as spheres to a first approximation in the second case we generalize from the results of a three component self similar mixture of spherical particles for linear approximations the models are described by expressions quite similar to the frequently used waxman smits and dual water equations but dependent upon the clay type concentration and its distribution in the rock the models are written in terms of geometric parameters readily available from well log measurements the derived analytical expressions are successfully applied to describe a set of available experimental core and geophysical log data the results clearly demonstrate the applicability of the equations to well log interpretation as a practical procedure for computing formation factors and shaliness of argillaceous sandstones

systematic decrease of high δ13c values with burial in late archaean 2 8 ga diagenetic dolomite evidence for methanogenesis from the crixas greenstone belt brazil abstract remnants of fe poor 2700 4400 ppm dolomite crystals crystal aggregates and microconcretions are cryptically distributed within stratiform bodies tens of metres across of metamorphic fe rich 3 6 wt fe dolomite that occur in graphitic schists of the 2 8 ga ribeira o das antas formation in the crixas greenstone belt brazil fe poor dolomite is a diagenetic carbonate formed during burial of organic bearing muddy sediments using cathodoluminescence microscopy a consistent sequence of dolomite growth stages is well defined within individual crystals crystal aggregates and microconcretions from comparison with similar textures in phanerozoic carbonates we suggest that the early dolomite formed during progressive burial aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction the 13cdolomite values of crystals and concretion cores are very high 12 14 pdb and a negative shift of about 7 occurs over millimetres in subsequent dolomite generations early diagenetic fe poor dolomite and late stage fe rich dolomite show overlapping ranges of initial 87sr86sr ratios between 0 7080 and 0 7164 and 18o values 14 to 16 pdb these similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids yet the well preserved growth zonation of the fe poor dolomite suggests that such exchange did not follow a process of recrystallization the decrease in the 13c of dolomite is correlated with crystal and concretion growth during burial this behaviour is interpreted to indicate a change in the 13c of pore water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis the crixas dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early precambrian the pattern of 13c values found for the crixas dolomite is also documented in other early to middle precambrian dolomites and limestones previous interpretations have considered positive 13c values to be the result of primary oceanographic controls whereas variability in 13c is commonly linked to metamorphic alteration we suggest that the role played by organic diagenesis in controlling the 13c values of diagenetic carbonates has been underestimated

ordovician magnetostratigraphy a correlation of global data palaeomagnetic studies where primary magnetizations are established from well dated rocks have been compiled to construct a magnetostratigraphic time scale for the ordovician period an excellent correlation of magnetozones is observed between independent studies of the baltic and south siberian platform sequences supplemental datasets from other continents concur with the baltic and siberian polarity data early ordovician times were mainly characterized by a reverse field rapid reversals then occurred during llanvirn and llandeilo times late ordovician times were dominated by a normal polarity field some gaps remain in the polarity record notably for late caradoc early ashgill and mid tremadoc times

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

diagenesis of fibrous magnesian calcite marine cement implications for the interpretation of δ18o and δ13c values of ancient equivalents fibrous high mg calcite hmc marine cement from the pliocene hope gate formation jamaica occurs as 1 2 mm thick isopachous coatings on skeletal grains in local occurrences of halimeda grainstone and is commonly syntaxially overgrown by clear prismatic low mg calcite lmc interpreted as a mixing zone cement the presence of inclusions at discrete levels in the fibrous marine cement produces internal growth banding wherein clear inclusion free zones alternate with cloudy inclusion rich zones clear zones have uniform mg contents 13 7 0 2 mol mgco3 and stable isotopic compositions 18o 0 8 0 1 13c 3 2 0 1 consistent with a marine origin in contrast inclusion rich zones contain fine scale intergrowths of lmc dolomite and hmc and have carbon and oxygen isotopic compositions that define a linear mixing trend between compositions of inclusion free hmc and prismatic lmc cement overgrowths petrographic observations indicate that the mixing trend defined by isotopic data from inclusion rich zones is related to the addition of large volumes of secondary lmc as cement in primary pore space as intergrowths among fibers of primary hmc marine cement and or as replacements of primary hmc crystallites dolomite is present in inclusion rich zones in volumes generally compatible with amounts predicted to result from the closed system stabilization of hmc to lmc and dolomite in addition isotopic and geochemical data from some areas of inclusion rich zones are consistent with mass balance constraints assuming a closed system which require that bulk isotopic and geochemical compositions are retained these relations suggest that small remnants of hmc in inclusion rich zones of hope gate marine cement have altered to lmc and dolomite while isolated from system pore fluids such that primary marine isotopic signatures were imparted on diagenetic products similar petrographic geochemical and isotopic relations have been noted in ancient occurrences suggesting that 18o and 13c values from some former marine cements should be useful for the determination of isotopic compositions of original marine cements and ultimately for the determination of ocean chemistry in the geologic past

dolomitization of the lower ordovician watts bight formation of the st george group western newfoundland evidence of hydrothermal fluid alterationgeological survey of canada contribution 20080544 the watts bight formation in western newfoundland consists of a lower ordovician succession of shallow water carbonates and has been extensively dolomitized these dolomites occur as both replaceme

paleomagnetism of mesoproterozoic margins of the anabar shield a hypothesized billion year partnership of siberia and northern laurentia abstract siberia and laurentia have been suggested as near neighbors in proterozoic supercontinents nuna and rodinia but paleomagnetic evidence has been sparse and ambiguous here we present four new paleomagnetic poles from undeformed paleo mesoproterozoic lower riphean sedimentary rocks and mafic intrusions of the northwestern anabar uplift in northern siberia combining these results with other proterozoic data from siberia and laurentia we propose a tight juxtaposition of those two blocks euler parameters 77 098 137 for anabar to north america spanning the interval 1 7 0 7 ga constituting a long lived connection that outlasted both the nuna and rodinia supercontinental assemblages

palaeomagnetism of the lower ordovician orthoceras limestone st petersburg and a revised drift history for baltica in the early palaeozoic summary palaeomagnetic investigation of lower ordovician limestone in the vicinity of st petersburg yields a pole position at latitude 34 7 n longitude 59 1 e dp dm 5 7 6 4 a probable primary remanence origin is supported by the presence of a field reversal the limestone carries one other remanent magnetization component associated with a mesozoic remagnetization event an apparent polar wander path is compiled for baltica including the new result ranging in age from vendian to cretaceous ages of the published lower to midpalaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of tucker mckerrow 1995 the new arenig result is the oldest of a series of ordovician and silurian palaeomagnetic pole positions from limestones in the baltic region there are no data to constrain apparent polar wander for the tremadoc cambrian and latest vendian if the fen complex results previously taken to be vendian in age c 565 ma are reinterpreted as permian remagnetizations an early ordovician cambrian vendian cusp in the polar wander path for baltica is eliminated the apparent polar wander curve might then traverse directly from poles for vendian dykes on the kola peninsula c 580 ma towards our new arenig pole c 480 ma the consequence of this change in terms of the motion of baltica in cambrian times is to reduce significantly a rotational component of movement the new arenig pole extends knowledge of ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of baltica in some published palaeogeographies exclusion of the fen complex result places baltica in mid to high southerly latitudes at the dawn of the palaeozoic consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions

field geometry petrography and geochemistry of a dolomitization front late jurassic central lebanon this contribution describes the field geometry petrography and geochemistry of a well exposed dolomitization front in upper jurassic carbonates and attempts to highlight the sedimentological structural and relative sea level controls on multiphase dolomitization and related diagenetic events the data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body whose dimensions are well defined in the field local microbial intraclastic dolomites of late tithonian age accumulated in a hypersaline lagoon during relative sea level fall these pre date beige hydrothermal dolostones 51 to 55 mol caco3 18o 9 3 to 4 0 v pdb 13c 1 5 to 2 1 v pdb 87sr 86sr 0 70742 matrix porosity 6 klinkenberg permeability 0 5 md whose dolomitizing fluid circulated along faults and invaded the nearby facies first the burrows were dolomitized then the bulk rocks resulting in the investigated tongue shaped dolomite body upon late jurassic early cretaceous uplift near surface water percolated through and altered the underlying beige dolostones this event was followed by a ferroan dolomite cement phase which occurred during further burial this contribution featuring a well defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization

aquifer transmissivity of porous media from resistivity data to optimize the information cost ratio and avoid the indiscriminate and excessive use of drilling and pump testing to calculate aquifer transmissivity an analytical relationship between modified transverse resistance and aquifer transmissivity has been developed for estimating transmissivity from resistivity sounding data the relation takes into consideration the variation in the quality of groundwater the relation has been tested successfully for the glacial aquifers of rhode island u s a and alluvial aquifers of three different areas of uttar pradesh india the practical applicability of the relation lies in the fact that if hydraulic conductivity is known for any reference point of a porous homogeneous aquifer one can get fairly good idea of the transmissivity of the aquifer at other locations within a basin from surface geo electrical measurements

application of geophysical methods in describing spatial variability of saturated hydraulic conductivity in the zone of aeration abstract an attempt is made to apply surface geophysical methods for the determination of spatial variability of saturated hydraulic conductivities methods of vertical electrical sounding and dipole electromagnetic profiling were tested in an area of 100 100 m measurements were taken on a square grid 20 by 20 m the soil profile formed by coarse acid brown soil of relatively high permeability underlain by a weathered gneiss substratum is represented by four geoelectrical layers the saturated hydraulic conductivities of both individual soil layers and the integral soil medium were compared with the geophysical parameters they were estimated by a laboratory constant head permeameter and in the field by ponding infiltration measurements the hydraulic conductivities of the individual soil layers were statistically derived from the resistivities of the layers while the estimation of the conductivities of the integral soil medium generally needed some multivariate treatment of the geoelectrical data it was concluded that the geoelectrical methods may be effectively applied to determine the spatial variation pattern of both individual and integral saturated hydraulic conductivities in the field to estimate their relations to the deeper structural and tectonic phenomena and to differentiate soil layers according to their properties

dolomitization of the lower ordovician catoche formation implications for hydrocarbon exploration in western newfoundland the lower ordovician st george group in western newfoundland consists of a sequence of subtidal and peritidal carbonates which are extensively dolomitized the current study investigates the diagenetic evolution of the catoche formation from the port aux choix and port au port peninsulas in order to study the controls on reservoir quality in western newfoundland the catoche formation dolomites are classified into three main generations early and pervasive replacement dolomite d1 indicates that dolomitization began during early stages of diagenesis stable isotope and trace element data indicate significant variations between d1 dolomite on the port aux choix and port au port peninsulas the depleted 18o signature of d1 dolomite fluids 8 7 1 3 vpbd on the port aux choix peninsula is consistent with partial dolomitization associated with mixing of seawater and meteoric waters on the flanks of structural highs in contrast 18o values 6 1 0 7 vpbd and trace element data from the port au port peninsula indicate that pervasive d1 was associated with mixing of possibly post evaporitic brines with meteoric waters later stage replacement dolomites d2 are associated with enhancement in porosity through the development of intercrystalline pores while latest stage saddle dolomite d3 significantly occluded the pores in some horizons d2 dolomite formed due to the influx of warm 100 c saline 15 eq wt nacl fluids intercrystalline porosity in d2 formed due to the dolomitization of precursor calcite due to the lower molar volume of dolomite compared to calcite therefore porosity development is lower on the port au port peninsula with no significant volume change during the recrystallization of the pervasive early d1 dolomicrite similarly extensive porous horizons on the port aux choix peninsula are related to the limited extent of d1 dolomitization this suggests that the quality of a potential dolomite reservoir is strongly controlled by tectonic and diagenetic history of host carbonates

global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

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

dolostones from grand cayman british west indies abstract pervasively dolomitized tertiary strata on grand cayman are dominated by heterogeneous dolostones that are composed of both low ca calcium dolomite lcd 55 mol caco3 homogeneous dolostones formed of either lcd or hcd are rare some dolostones contain dolomite crystals that are characterized by oscillatory zones 1 10 m thick of lcd and hcd whereas others have rims of lcd surrounding cores of hcd in some dolostones bioclasts formed of lcd are embedded in a matrix of hcd an 1 18o difference between the lcd and hcd can be attributed to the natural covariance that exists between the 18o and mol caco3 in dolomite there are no significant differences in the 87sr 86sr ratios of the lcd and hcd there is no consistent vertical or lateral pattern to the distribution of lcd and hcd in the cayman dolostones even in closely spaced sampling sites such variability is more consistent with growth of individual dolomite crystals being controlled by processes that operated around the surfaces of individual crystals rather than by large scale changes in the dolomitizing fluids this interpretation is consistent with the development of oscillatory zoning which is attributed to precipitation of different phases or minerals from the same parent fluid because of competition between interface controlled and diffusion controlled growth kinetics dolomitization of the tertiary strata on grand cayman was probably mediated by seawater or modified seawater nevertheless small scale intrinsic factors controlled the incorporation of ca in the dolomite lattice and accordingly the geochemical parameters that are intimately linked to the mol caco3 these small scale intrinsic controls must be factored into any model to explain the formation of tertiary island dolostones

toward constraining the long term reversing behavior of the geodynamo a new maya superchron 1 billion years ago from the magnetostratigraphy of the kartochka formation southwestern siberia abstract we present new magnetostratigraphic data obtained from the late mesoproterozoic 1 ga kartochka formation in the east angara terrane of the yenisey ridge region southwestern siberian platform a 200 m thick section encompassing the carbonate kartochka formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites paleomagnetic analyses revealed the existence of two magnetization components a low unblocking temperature component which likely has a viscous origin was first isolated below 200 c a characteristic component carried either by magnetite or by a mixture of magnetite and hematite was then isolated in the medium to high temperature range up to 565 585 c or 680 c the hematite bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening a positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition this primary origin was further verified by comparison between the paleomagnetic poles derived for the kartochka formation and other late mesoproterozoic siberian sections all data from the kartochka formation contain a single magnetic polarity assumed to be normal this long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the siberian uchur maya region the new data fortify evidence of a normal polarity superchron spanning the 1000 ma old mesoproterozoic neoproterozoic boundary which we propose to call the maya superchron these data confirm the occurrence of sharp transitions between a frequently reversing regime and a non reversing regime of the geodynamo this behavior also observed in other regions may represent a consistent property of the proterozoic geomagnetic field together with changes in the amount of time spent in the superchron regime they may testify to a different field behavior during the late precambrian than during the phanerozoic this difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core mantle boundary during the precambrian possibly as a consequence of the inner core not yet being nucleated at this time

laterally constrained inversion of ground roll from seismic reflection records seismic reflection data contain surface waves that can be processed and interpreted to supply shear wave velocity models along seismic reflection lines the coverage of seismic reflection data allows the use of automated multifold processing to extract high quality dispersion curves and experimental uncertainties in a moving spatial window the dispersion curves are then inverted using a deterministic laterally constrained inversion to obtain a pseudo 2d model of the shear wave velocity a monte carlo global search inversion algorithm optimizes the parameterization when the strategy is used with synthetic and field data consistent final models with smooth lateral variations are successfully retrieved this method constitutes an improvement over the individual inversion of single dispersion curves

burial dolomitisation in a non tropical carbonate petroleum reservoir the oligocene tikorangi formation taranaki basin new zealand abstract the oligocene tikorangi formation is a subsurface non tropical limestone rich fracture producing oil reservoir in taranaki basin new zealand the formation is distinct from uplifted and exposed age equivalent new zealand limestones due to partial dolomitisation by modest quantities of silt sized scattered inclusion rich euhedral dolomite rhombs occasionally displaying idiotopic fabrics rhombs commonly have dull luminescent fe rich cores and oscillatory bright and dull concentric outers the dolomites are non stoichiometric high calcian and highly ferroan varieties dolomite 18 o and 13 c values are moderately depleted while associated precursor calcite phases are less depleted petrographic geochemical and geohistory evidence suggest that the dolomite formed during and following the later stages of pressure dissolution cementation in a relatively closed moderately deep burial environment ca 0 85 1 9 km burial depth ca 33 65 c in the early miocene ca 24 18 ma from warm saline enriched pore fluids by mainly mimic fabric selective replacement of interparticle and rarely intraparticle micrite matrix dolomitisation did not affect skeletal grains or calcite cements or develop any secondary porosity mg supply was restricted and was likely sourced internally from micritic inter and intraparticle precursor material and from the pressure dissolution of calcitic skeletons importantly dolomitisation occurred prior to compression related brittle fracturing of the carbonates responsible for creating extensive hydrocarbon bearing fracture networks because of the greater susceptibility of the dolomitised limestones to brittle fracturing their location and distribution may have important implications for hydrocarbon prospectivity and production within the tikorangi formation

early dolomitization in the lower cretaceous shallow water carbonates of southern apennines italy clues about palaeoclimatic fluctuations in western tethys abstract a multidisciplinary study of the dolomitized bodies present in the lower cretaceous platform carbonates of mt faito southern apennines italy was carried out in order to explore the connection between early dolomite formation and fluctuating climate conditions the berriasian aptian investigated succession is 466 m thick and mainly consists of shallow water lagoonal limestones with frequent dolomite caps the dolomitization intensity varies along the succession and reaches its peak in the upper hauterivian lower barremian interval where it is present a completely dolomitized interval about 100 m thick field relations petrography mineralogy and geochemistry of the analyzed dolomite bodies allowed identifying two populations of early dolomites a fine medium crystalline fmdol and a coarse crystalline dolomite cdol both interpreted as the product of mesohaline water reflux according to our interpretation fmdol precipitated from concentrated brines in the very early stage of the reflux process producing typical sedimentary features as dolomite caps in the successive step of the process the basin ward latent reflux precipitated cdol from less concentrated brines a peculiar feature of the studied succession is the great consistency between stratigraphic distribution of dolomite bodies and their geochemical signature the completely dolomitized hauterivian barremian interval in fact is characterized by geochemical values suggesting an origin from distinctly saltier brines considering that the observed near surface dolomitization process is controlled by physical and chemical parameters reflecting the paleoenvironmental and paleoclimatic conditions during dolomite formation we propose that the stratigraphically controlled dolomitization intensity reflects periodic fluctuations in the salinity of dolomitizing fluid in turn controlled by long term climate oscillations the present work highlights that the stratigraphic distribution of early diagenetic dolomite may be used as proxy to define the climatic fluctuations that have influenced the sedimentary dynamics in the early cretaceous moreover considering that a comparable early dolomite distribution is present also in the dinaric platform we suggest that a regional scale climate control acted on early dolomite formation and distribution refining the knowledge of such a key control may have a significative impact on hydrocarbon reservoir characterization and exploration in the periadriatic area

results of lithospheric studies from long range profiles in siberia several long range seismic profiles obtained during the last ten years in siberia show the complicated lithospheric structure of the siberian platforms the three component observations conducted at distances up to 3000 km made it possible to obtain information on p and s velocities in the crust on p velocity and q factor for the upper mantle and on the seismic boundaries responsible for reflected refracted and converted waves down to a depth of 400 700 km the crustal models are typical of old platforms of eurasia the average thickness of 40 km three layers with p velocities 6 2 6 5 7 0 km s and thicknesses of 10 15 km are distinguished the depth to the m discontinuity varies from 45 50 km beneath the old tunguss depression to 35 40 km beneath the younger vilyui basin the most complicated moho structure is observed in the boundary between the west siberian and the siberian platforms a strong inhomogeneity of p velocity models was revealed for the upper mantle the horizontal inhomogeneities are more larger in the uppermost mantle to depths of 80 100 km where p velocities vary from 8 0 8 2 km s beneath the young west siberian plate to 8 4 8 6 km s beneath some blocks of the siberian craton the fine vertical inhomogeneity was studied with reflections correlated after computer processing of seismograms they outlined several low velocity layers 20 50 km thick the layers were characterized by low q as well intensive waves were recorded from the transition zone between the upper and lower mantle the top of the zone is nearly horizontal in the area its depth is 400 25 km the bottom of the zone lies at about 700 km

salt water intrusion mapping by geoelectrical imaging surveys the geoelectrical imaging method is a tool for mapping the intrusion boundary between fresh water and saline water due to its inherent capability to delineate the lateral changes in pore water salinity a new field survey technique that can be used for environmental and geotechnical investigations has been developed this study evaluates the multiscale survey technique as a tool employed in electrical imaging to detect the salt water intrusion boundary in yan state of kedah northwest malaysia the technique was incorporated into these surveys and it has proved to be a robust method for accurately mapping the fresh water saline water boundary the resulting resistivity sections from these surveys were consistent with both the available geological and borehole information from the area and the previous resistivity surveys conducted by the geological survey of malaysia at those sites

new middle cambrian and middle ordovician palaeomagnetic data from siberia llandelian magnetostratigraphy and relative rotation between the aldan and anabar angara blocks abstract whether siberia was a coherent tectonic unit throughout the phanerozoic or whether significant rotations occurred in early palaeozoic times between the anabar angara and aldan blocks is still a matter of debate addressing this problem a detailed palaeomagnetic study of sections of mid cambrian and llandeillian age has been carried out along the river lena river maya river kulumbe and river stolbovaya with a total of 403 collected hand samples a section of llandeilian age was also investigated on the russian platform near st petersburg the studied rocks are carbonates or fine grained terrigenous sediments of reddish and greyish colors stepwise thermal demagnetization experiments reveal a rather simple directional behavior in most samples after removal of a secondary magnetizations at low to intermediate temperatures a high temperature component is isolated which is carried by magnetite and or hematite positive reversal tests and fold tests clearly demonstrate the primary character of the latter component while only reversed polarity directions are obtained in the lower two thirds of the llandeilo six magnetozones of normal polarity are found in the upper llandeilo which mark the end of the moyero reverse superchron together with previously published paleomagnetic data our new results show rather systematic difference in declinations of 20 between the aldan and anabar angara blocks this argues in favor of a relative rotation between these two blocks with an euler pole of rotation located at latitude 62 n and longitude 117 our interpretation is further supported by existing results from deep sounding seismic experiments yielding a very similar pole of rotation

upper cambrian to middle ordovician magnetostratigraphy from the kulumbe river section northwestern siberia abstract we present the magnetostratigraphy of an upper cambrian to middle ordovician sedimentary section sampled along the kulumbe river in northwestern siberia thermal demagnetization yields the determination of two magnetic components a component of steep inclination is interpreted as a remagnetization acquired during the emplacement of the siberian traps at the permian triassic boundary another component with a moderate inclination essentially carried by hematite and sometimes by magnetite likely represents a syndepositional or nearly syndepositional magnetization of the studied deposits a sequence of 17 magnetic intervals is observed between the upper part of the upper cambrian and the tremadoc three of them being poorly defined by single samples we also find that the llanvirn magnetostratigraphic sequence contains only one reversed polarity interval together with magnetostratigraphic results previously obtained from the moyero river section from siberia our data suggest the occurrence of a 25 30 myr long period during the lower and the middle ordovician characterized by a very low magnetic reversal frequency possibly containing a superchron and dominated by reversed polarity this geomagnetic feature would be compatible with a time constant of about 150 myr in the reversal process through the whole phanerozoic

ordovician magnetostratigraphy llanvirn caradoc limestones of the baltic platform summary palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the baltoscandian ordovician carbonates after removing a ubiquitous permocarboniferous 287 f 14 ma remagnetization between 200 and 500 c a llanvirncaradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550 580 c three reversed se down and three normal nw up antipodal polarity intervals have been recognized a primary early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons primary magnetizations are resident in detrital biogenic or early diagenetically formed single and pseudo single domain magnetite phases and subordinate early diagenetic pigmentary haematite the recognition of a primary remanence within these well dated ordovician carbonates has the following important tectonic and magnetostratigraphic consequences 1 accurate time calibration of the baltic apw path implies that rapid counterclockwise rotation took place in late tremadoc and llandeilo times the arenigllanvirn epochs are characterized by a still stand baltica occupied intermediate to high southerly latitudes during the early ordovician tremadoc llanvirn systematic northward drift is recognized in post llanvirn times 2 a time calibrated ordovician reversal stratigraphy is proposed presently available data suggest the geomagnetic field was predominantly reversely polarized during tremadoc and arenig times two normal polarity zones of short duration are identified within mid llanvirn and mid llandeilo strata discontinuities within the succession may mask other short period events late llandeilo to mid caradoc times were then characterized by a normal polarity field

energetics of calcium rich dolomite abstract the enthalpy of formation of sedimentary ca rich dolomite has been determined by oxide melt calorimetry the results show that ca rich dolomite is energetically different from well ordered stoichiometric dolomite the enthalpy of formation from calcite and magnesite becomes strongly more endothermic with increasing excess ca content this supports the idea that the substitution of ca in the mg layer of dolomite is energetically unfavorable ca rich dolomite is unstable relative to well ordered stoichiometric dolomite and calcite the enthalpy behavior for excess ca substitution in dolomite is different from that of mg substitution in calcite the enthalpy change is much larger in magnitude in dolomite and is more strongly dependent on composition differences in cation order as well as the presence of a modulated structure and low symmetry domains in ca rich dolomite cannot be discerned from the enthalpy data the findings confirm that the growth and occurrence of ca rich dolomite in sedimentary environments must be attributed to kinetic factors rather than to equilibrium

diagenesis in mesozoic carbonate rocks in the north pyrenees france from mineralogy and fluid inclusion analysis example of rousse reservoir and caprock abstract fluid rock interaction is of key interest for deciphering the geodynamic history of complex geological environments in this paper we describe the mineralogy and petrography of the rousse reservoir the upper jurassic dolomie de mano formation as well as the reconstruction of its thermal and pressure history we also investigate the base of the reservoir caprock the campanian breccia a satellite of the giant gas field of lacq in the south west of france rousse was a gas producer until it moves to a co2 storage site in 2010 rousse is located within a deep isolated faulted carbonate jurassic horst overlain by a 4500 m thick overburden that is composed of a series of turbiditic flysch deposits of upper cretaceous cenomanian to tertiary eocene age cores were sampled from two wells rousse 1 and 2 and were analyzed by icp ms xrd sem epma tem cathodoluminescence and fluid inclusion techniques pressure temperature p t conditions were derived by modeling data from fluid inclusions trapped in the fracture minerals of the dolomie de mano formation and the campanian breccia in the dolomie de mano diagenesis was dominated by three dolomite episodes followed by a calcite stage in fractures the second and third dolomitizations were associated with fracturing episodes these dolomites precipitated from low saline

diagenesis and crystal caskets echinoderm mg calcite transformation dry canyon new mexico u s a abstract the stereom of living echinus esculentus skeletons is composed of mg calcite interambulacral plates 6 mole mgco3 spines 4 mole mgco3 and is compared with the stereom of pennsylvanian echinoderms from the holder formation dry canyon some dry canyon echinoderms had their pore system 55 volume filled by magnesian ferroan calcite cement before the stereom transformed type 1 into small 20 m diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns the margins of some type 2 ossicles set in a clay matrix are fe rich and contain barite crystals the fe2 and ba2 were transferred from the surrounding clay hundreds of microns into these ossicles type 1 transformation was catalyzed by internal water from the stereom the presence of a ferroan calcite crystal casket confined alteration to the skeletal mg calcite type 2 transformation involved both stereom and pore filling mg calcite cement the unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased the transformation of dry canyon echinoderms was probably triggered or accelerated by rising temperature caused by burial the bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original major element chemistry of echinoderm skeletons minor amounts of mg calcite cement that occur in dry canon type 1 ossicles indicate the chemistry of porewater close to the sediment water interface fossil echinoderms are preserved in many ways and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions the stereom is destroyed in most fossil echinoderms and even in those where the stereom shape is preserved its internal structure is changed echinoderm mg calcite does not transform with perfect textural preservation

the use of geophysical prospecting for imaging active faults in the roer graben belgium as part of a paleoseismological investigation along the bree fault scarp western border of the roer graben various geophysical methods electrical profiling electromagnetic em profiling refraction seismic tests electrical tomography ground penetrating radar gpr and high resolution reflection seismic profiles were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters these geophysical investigations in parallel with geomorphological and geological analyses helped in the decision to locate trench excavations exposing the fault surfaces the results could then be checked with the observations in four trenches excavated across the scarp geophysical methods pointed out anomalies at all sites of the fault position the contrast of physical properties electrical resistivity and permittivity seismic velocity observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change