effect of pore structure on electrical resistivity in carbonates the electrical resistivity log has proven to be a powerful tool for lithology discrimination correlation porosity evaluation hydrocarbon indication and calculation of water saturation carbonate rocks develop a variety of pore types that can span several orders of magnitude in size and complexity a link between the electrical resistivity and the carbonate pore structure has been inferred although no detailed understanding of this relationship exists seventy one plugs from outcrops and boreholes of carbonates from five different areas and ages were measured for electrical resistivity properties and quantitatively analyzed for pore structure using digital image analysis from thin sections the analysis shows that in addition to porosity the combined effect of microporosity pore network complexity pore size of the macropores and absolute number of pores are all influential for the flow of electric charge samples with small pores and an intricate pore network have a low cementation factor whereas samples with large pores and a simple pore network have high values for cementation factor samples with separate vug porosity have the highest cementation factor the results reveal that 1 in carbonate rocks both pore structure and the absolute number of pores and pore connections seem more important in controlling the electrical resistivity instead of the size of the pore throats as suggested by previous modeling studies 2 samples with high resistivity can have high permeability large simple pores facilitate flow of fluid but fewer numbers of pores limit the flow of electric charge and 3 pore structure characteristics can be estimated from electrical resistivity data and used to improve permeability estimates and refine calculations of water saturation

pan african tectonics in northwestern cameroon implication for the history of western gondwana abstract three main tectonic events related to pan african collision and post collision evolution have been identified in cameroon i crustal thickening ii left lateral wrench movements and iii right lateral wrench movements successively correlation of these events throughout the pan african belt and an integrated synthesis that includes the domains of central africa trans sahara and ne brazil enable us to define an indent type structure in northwestern cameroon this one suggests the existence of a rigid prong in this area which collided with the sao francisco congo craton sfcc active margin at ca 640 ma prior to subsequent widespread remobilization and granitization that partly affected both landmasses between 640 and 580 ma major indent related shear zones were further reactivated during regional clockwise rotation of the newly stabilized domain of northwestern cameroon suggesting that collision with the west african craton wac was still active after 580 ma geodynamic implications of this model are discussed in the context of the pan african global evolution in western gondwana

development of microporosity in a micritic limestone reservoir lower cretaceous middle east abstract abundant intercrystalline microporosity is developed within the matrix of micritic limestones over several hundreds of meters of section in the thamama group lower cretaceous at sajaa field eastern u a e sediments in the upper shuaiba formation uppermost tamama group include muddy rudist algal coralgal and orbitolina dominated lithofacies matrix is composed of euhedral to subhedral micro rhombic low mg calcite crystals with average diameters of about 5 m microporosity development was not related to leaching chalkification of lithified micrite during post depositional emergence of the shuaiba formation features diagnostic of emergence are lacking in core below the upper contact of the unit matrix microporosity is believed to have been an early developed property of micritic limestones trace element compositions of matrix reflect a diagenetic process that involved conversion of a calcitic lime mud into micro rhombic micrite and microspar using a closed system geochemical model the observed matrix oxygen isotope compositions 18 o of 4 to 5 pdb are compatible with dissolution of aptian lime mud 18 o of 2 pdb and reprecipitation of calcite in fluids of marine composition under conditions corresponding to burial depths of 550 800 m porosity of 30 50 at 40 50 c closed system conditions are consistent with textural constraints for limited or arrested cementation in the matrix chemical compaction was responsible for partial occlusion of matrix microporosity as microporosity is decreased in zones of intense stylolitization slight corrosion of matrix crystals and leaching of peloids and orbitolina postdated cementation and chemical compaction during deep burial

prominence of ichnologically influenced macroporosity in the karst biscayne aquifer stratiform super k zones a combination of cyclostratigraphic ichnologic and borehole geophysical analyses of continuous core holes tracer test analyses and lattice boltzmann fl ow simulations was used to quantify biogenic macroporosity and permeability of the biscayne aquifer southeastern florida biogenic macroporosity largely manifests as 1 ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems ophiomorpha and 2 biomoldic macroporosity originating from dissolution of fossil hard parts principally mollusk shells ophiomorpha dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the biscayne aquifer in a 345 km 2 study area stratiform tabular shaped units of thalassinidean associated macroporosity are commonly confi ned to the lower part of upward shallowing highfrequency cycles throughout aggradational cycles and in one case they stack vertically within the lower part of a high frequency cycle set broad continuity of many of the macroporous units concentrates groundwater fl ow in extremely permeable passageways thus making the aquifer vulnerable to long distance transport of contaminants ichnogenic macroporosity represents an alternative pathway for concentrated groundwater fl ow that differs considerably from standard karst fl ow system paradigms which describe groundwater movement through fractures and cavernous dissolution features permeabilities were calculated using lattice boltzmann methods lbms applied to computer renderings assembled from x ray computed tomography scans of various biogenic macroporous limestone samples the highest simulated lbm permeabilities were about fi ve orders of magnitude greater than standard laboratory measurements using air permeability methods which are limited in their application to extremely permeable macroporous rock samples based on their close conformance to analytical solutions for pipe fl ow lbms offer a new means of obtaining accurate permeability values for such materials we suggest that the stratiform ichnogenic groundwater fl ow zones have permeabilities even more extreme 2 5 orders of magnitude higher than the jurassic super k zones of the giant ghawar oil fi eld the fl ow zones of the pleistocene biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of origin and evolution in other carbonate aquifers as well as petroleum reservoirs

cenomanian early turonian carbonate platform of northern oman stratigraphy and palaeo environments abstract the jebel akhdar and adam foothills outcrops in northern oman arabian peninsula offer the opportunity to study in detail the lateral and vertical changes of a cenomanian early turonian carbonate platform formed by the natih formation wasia group eleven sections with less than 5 km offset were logged to express metre scale lithological and biostratigraphical vertical changes the first rudist zonation is established complementing classical benthonic foraminifer and ammonite biozones typical cenomanian rudist association caprinids and radiolitids occur in the natih e c d a lower part whereas lower turonian hippuritid rudists are represented in the upper part of the natih a a sedimentological and palaeoecological approach leads to identification and interpretation the facies and discontinuities of the natih carbonate platform and to a reconstruction of its spatial organization application of the concepts of sequence stratigraphy allows interpretation of the units of the natih in terms of systems tracts an attempt is made to correlate the sequence stratigraphical model to eustatic sea level changes and regional tectonic events three palaeoenvironmental models taking into account bio and lithofacies are proposed

early diagenetic microporosity in the cotton valley limestone of east texas abstract the upper jurassic cotton valley limestone was deposited on a mature ramp where monotonous regional slopes were punctuated by salt generated structures and basement topography the strandline and the crests of paleobathymetric highs were blanketed by oolitic and palletoidal grainstones the ratio of grainstones to mudstones increases in the upper cotton valley reflecting a regional shallowing phase thinner shoaling upward sequences are present but they do not correlate easily especially from basement highs to salt domes probably because salt movement occurred during deposition of cotton valley rocks the complex diagenetic history of the cotton valley inferred from cross cutting features observed in thin section and from trace element and stable isotope content includes the origin of chalky microporosity especially in ooids this intraparticle intercrystalline porosity occurs in a fabric of equant subhedral to euhedral low mg calcite micro rhombs which appear to have developed at the expense of an acicular precursor such microporous ooids are present mainly on the crests of paleobathymetric highs nearby offstructure ooids contain a mixture of micritic and well preserved ooids however these micritic ooids are different from the micro rhombic microporous ones on the highs the well preserved low mg calcite ooids from offstructure positions exhibit relict acicular microstructures in some of their lamellae the chalky microporosity is crosscut by virtually every other diagenetic feature in the cotton valley limestone the 13c values from individual microporous ooids range from 1 65 to 2 76 pdb which is not in the range of values associated with precipitation in a hydrocarbon rich environment the formation of microporosity was followed by the formation of embayed grain contacts pore filling cementation grain compaction stylolite formation replacements by quartz and rhombic dolomite fracturing fracture filling cementation saddle dolomite late leaching and formation of authigenic sulfides the microporosity is interpreted to have been formed in the near surface diagenetic environment early in the burial history of the cotton valley as there is no evidence of vadose diagenesis the ooids on the crests of calcarenite shoals must have been placed is disequilibrium with their surroundings by a change in water chemistry probably as a consequence of regression and an influx of fresh water the introduction of hydrocarbons appears to be contemporaneous with such late diagenetic features as saddle dolomite and authigenic sulfides however the extent to which those fluids affected the micro rhombic calcite crystals appears to be negligible

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

geodynamic evolution of the pan african belt in central africa with special reference to cameroon the pan african belt in central africa has benefited from the many petrographic structural and geochronological studies in the recent years that have improved our understanding of the belt however those studies have also produced various and often divergent evolutionary models for the belt some of which do not even involve well defined cratons following a review of the available data in cameroon we propose a model of continent continent collision that involved the congo craton and the north central cameroon active margin showing archean to paleoproterozoic inheritances this model is based among others on i the prominent role of the congo craton as demonstrated by the regional extension of external nappes on its northern edge and the concomitant exhumation of the 620 ma granulitic rocks believed to have formed at the root of the collision zone and ii the late development of a strike slip fault system in central cameroon as the result of horizontal movement following the multistage collision

palaeogeographical reconstructions of the pan african brasiliano orogen closure of an oceanic domain or intracontinental convergence between major blocks abstract a reconstruction of the pan african brasiliano orogen has become possible thanks to new data from the southern pan african belt and by using published data geological mapping which included structural thermobarometric and geochemical studies in togo and neighbouring countries have enabled to define the existence of a preorogenic continental rift whose closure followed soon after its opening continental crust convergence induced an oblique collision that caused nappe stacking this involved external nappes thrust over the west african craton intermediate nappes including metabasic rocks with eclogitic assemblages and internal nappes composed of highgrade anatectic gneiss and intrusive granite and charnockite comparison with data from other regions hoggar adrar des iforas northeastern brazil nigeria central africa has led to the recognition of a pre orogenic continental rift that bordered the west africa sao francisco congo cratons in the southern part of the chain farther north in the hoggar and adrar des iforas this continental rift opened on a wider oceanic domain which involved a passive margin along the west african craton and an active margin to the east after these rifting and subduction stages continental collision followed that was dominated by strike slip movement oblique to the plate boundaries the general organization of the pan african brasiliano belt can be considered as the result of a confrontation between three major continental domains these were the west african and sao francisco congo cratons and a reworked shield composed in its inner part of polycyclic basement rocks and bounded by a fringe of monocyclic metasedimentary thrust belts that faced the cratons

akimotoite mg fe sio3 a new silicate mineral of the ilmenite group in the tenham chondrite akimotoite mg fe sio 3 a new silicate mineral of the ilmenite group was found in the shock metamorphosed tenham chondrite it occurs as aggregates adjacent to clinoenstatite in fragments within shock induced melt veins chemical analyses show the simplified formula to be mg sub 0 79 fe sub 0 21 sio 3 the same as for clinoenstatite selected area electron diffraction saed patterns correspond to the synthetic mg fe sio 3 ilmenite phase with space group r3 lattice parameters derived from saed patterns are a 0 478 5 nm c 1 36 1 nm and v 0 269 8 nm 3 in the hexagonal setting the calculated density is 4 0 1 g cm 3 akimotoite in this occurrence is thought to have been transformed from original orthoenstatite in a solid state reaction produced by a shock event peak pressure and temperature generated by shock events in tenham are estimated to be 22 gpa max max 2000 degrees c assuming that equilibrium crystallization of aluminous majorite occurred in the melt veins this new mineral was named after syun iti akimoto

improving geochronological framework of the pan african orogeny in cameroon new sims zircon and monazite u pb age constraints abstract high precision geochronological framework is crucial to understanding of the evolution of orogenesis as part of the pan african north equatorial fold belt the neoproterozoic fold belt of cameroon nfbc involved polyphase deformation d1 to d4 phases metamorphism up to granulite facies and widespread crustal anatexis and magmatism while a general geochronological framework has been established controversy still exists on the temporal evolution of the metamorphic and magmatic events associated with polyphase deformation in particular the accurate age and geodynamic manifestations underlining the end of the orogeny are yet to be well constrained we conduct in this work integrated analyses of sims u pb zircon and monazite dating and in situ zircon hf o isotopes on the orthogneiss leucosome and granite samples from wum and yaounde of southwestern cameroon in order to provide further constraints on temporal evolution of the nfbc our high precision sims u pb age data demonstrate that the magmatic protoliths of orthogneisses from wum and yaounde were emplaced synchronously at ca 620 ma they were generated by partial melting of the thickened paleoproterozoic crustal basement rocks with zircon hf tdmc model age of ca 1 9 2 0 ga metamorphic zircons and two generations of monazites from an in situ leucosome within the granulitic paragneiss of the yaounde group yield indistinguishable ages of ca 610 ma these new results in combination with previous isotopic data suggest that the yaounde group supracrustal rocks with zircon hf tdmc model age of ca 1 7 ga experienced a very rapid evolution from sedimentation at

discrimination of effective from ineffective porosity in heterogeneous cretaceous carbonates al ghubar field oman the natih e heavy oil reservoir 21 api at al ghubar field oman has produced less than 5 of the calculated oil in place porosity logs used to calculate reserves show high porosity throughout the reservoir but further analysis of the only continuous core taken from the field indicates that much of the porosity is ineffective there are four heavily oil stained high permeability skeletal pelletal grainstone units with interparticle porosity in the core that probably contributed most of the production the four permeable grainstone units occur at the top of small scale accommodation cycles that have wackestone and packstone bases these grainstones make up about 20 of the total thickness of the porous natih e reservoir the other 80 is composed of packstone and wackestone with ineffective microporosity interparticle porosity in burrows and isolated moldic and intraskeletal porosity the small scale reservoir bearing cycles can be correlated across the field using the separation between the medium and deep induction curves as a guide resistivity logs are the most reliable tool to distinguish effective from ineffective porosity most high permeability grainstone units have deep induction values more than 100 ohm m and separation of more than 10 ohm m between the medium and deep induction curves the ineffective intervals with microporosity burrow porosity and moldic porosity have lower resistivity and little separation between the medium and deep induction curves

u pb dating of precambrian rocks from northern cameroon orogenic evolution and chronology of the pan african belt of central africa abstract northern cameroon has been classically divided into an old basement complex a middle proterozoic cover series intermediaires and granites emplaced during the pan african orogeny new u pb geochronological data on different rock units from this region are presented a rhyolite from the poli schists series intermediaires gives an age close to 800 ma migmatitic gneisses of the basement complex seem to derive from different detrital or magmatic sources the metadiorites and foliated granites of the basement complex give ages around 630 ma the 800 ma age is interpreted as dating volcanic activity probably related to the initial stage of the pan african evolution ages around 630 ma date a syn tectonic plutonism dominated by a regional scale basic to intermediate plutonic suite bip zircons of the bip suite are almost concordant and show no evidence of inheritance the suite shows calc alkaline affinity and relatively low 87 sr 86 sr initial ratios thus the bip suite represents material newly added to the crust during the panafrican orogeny these data are at variance with previous interpretations of this part of the mobile belt as old precambrian crust reactivated during the pan african orogeny the previous distinction between the series intermediaires and a regional basement complex must be reappraised similar ages being found in both units a chronological succession of the pan african evolution is established and tentatively compared with that of the trans saharan belt in nigeria and hoggar

morphometry of micrite particles in cretaceous microporous limestones of the middle east influence on reservoir properties abstract microporosity may account for a significant part of the total porosity of cretaceous limestone reservoirs of the middle east in these microporous facies porosity is moderate to excellent up to 35 while permeability is poor to moderate up to 190 md micritic limestones also may form dense layers with very low porosity and permeability values micritic samples were collected from three fields of the habshan and mishrif formations to examine the spatial relationship with their porosity permeability and pore throat radius distributions two key parameters of the micritic particles are studied using scanning electron microscopy their morphology shape and inter crystal contacts and their crystallometry results reveal that micrite matrixes can be subdivided into three petrophysical classes class c strictly microporous limestones with coarse punctic to partially coalescent micrites is made up of coarse 2 m polyhedral to rounded micritic crystals it has good to excellent porosity 8 28 poor to moderate permeability 0 2 190 md and a mean pore threshold radius of more than 0 5 m the class c is usually observed in rudist shoal facies where relatively high hydrodynamic energy disfavoured deposition of the finer micritic crystals it also developed within meteoric leaching intervals below exposure surfaces class f strictly microporous limestones with fine punctic to partially coalescent micrites is composed of fine

decomposition of k amphibole at high pressures and implications for subduction zone volcanism abstract the stability of k amphibole has been studied at pressures 12 6 16 5 gpa and at temperatures 940 1450 c k amphibole decomposes into an assemblage of clinoenstatite diopside an unknown phase x stishovite fluid at pressures above 16 gpa and temperatures below 1200 c the phase boundary has a negative clapeyron slope and the high temperature assemblage is clinoenstatite diopside x wadeite type k 2 si 4 o 9 fluid at pressures 14 16 gpa the x phase has a cation ratio of approximately k mg si 1 2 2 and contains 1 7 0 1 wt h 2 o as determined by sims measurements leading to a formula of k 4 mg 8 si 8 o 25 oh 2 the present results suggest that the decomposition of k amphibole in the dragged hydrous peridotite layer at the base of the mantle wedge may produce certain amounts of h 2 o rich fluid at 14 16 gpa 450 km depth while some of h 2 o is trapped in the new hydrous phase x and is further carried into deeper regions of the mantle the aqueous fluid released by the decomposition of k amphibole should react with phase to form hydrous phase in the mantle transition region thus the dehydration of k amphibole would not cause any volcanic activities in the back arc regions in contrast to the dehydrations of amphibole chlorite and phlogopite which are presumably responsible for the first and the second volcanic chains however some of the volcanic activities such as in muriah indonesia may be related to the dehydration of k amphibole in unusually hot regions above the subducting slab

characterization and comparison of structural and compositional features of planetary quadrilateral pyroxenes by raman spectroscopy this study reports the use of raman spectral features to characterize the structural and compositional characteristics of different types of pyroxene from rocks as might be carried out using a portable field spectrometer or by planetary on surface exploration samples studied include lunar rocks martian meteorites and terrestrial rocks the major structural types of quadrilateral pyroxene can be identified using their raman spectral pattern and peak positions values of mg mg fe ca of pyroxene in the mg fe ca quadrilateral can be determined within an accuracy of 0 1 the precision for ca mg fe ca values derived from raman data is about the same except that corrections must be made for very low ca and very high ca samples pyroxenes from basalts can be distinguished from those in plutonic equivalents from the distribution of their mg mg mg fe and wo values and this can be readily done using point counting raman measurements on unprepared rock samples the correlation of raman peak positions and spectral pattern provides criteria to distinguish pyroxenes with high proportions of non quadrilateral components from mg fe ca quadrilateral pyroxenes

planar deformation features in quartz from impact produced polymict breccia of the xiuyan crater china the 1 8 km diameter xiuyan crater is an impact structure in northeastern china exposed in a proterozoic metamorphic rock complex the major rocks of the crater are composed of granulite hornblendite gneiss tremolite marble and marble the bottom at the center of the crater covers about 100 m thick lacustrine sediments underlain by 188 m thick crater fill breccia a layer of polymict breccia composed of clasts of granulite gneiss hornblendite and fragments of glass as well as clastic matrix occurs near the base in the depth interval from 260 to 295 m an investigation in quartz from the polymict breccia in the crater fill units reveals abundant planar deformation features pdfs quartz with multiple sets of pdfs is found in clasts of granulite that consist of mainly quartz and feldspar and in fine grained matrix of the impact produced polymict breccia a universal stage was used to measure the orientation of pdfs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290 m forty four percent of the quartz grains contain three sets of pdfs and another 40 contain two sets of pdfs the most abundant pdfs are rhombohedron forms of pf10 12g xf10 13g a ndr zf10 11g with frequency of 33 5 22 3 and 9 6 respectively a predominant pdf form of f10 12g in quartz suggests a shock pressure 20 gpa the occurrence of pdfs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater which thus appears to be the first confirmed impact crater in china

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

sedimentological and marine eogenetic control on porosity distribution in upper cretaceous carbonate turbidites central albania results of a detailed petrographic and stable isotope study illustrate that sedimentological differences and eogenetic dissolution precipitation processes controlled porosity distribution within carbonate turbidites of the ionian basin central albania based on lithology characteristics and porosity distribution observed in outcrop individual turbidite beds can be subdivided into four distinct intervals i e from base to top a a non porous wackestone floatstone or packstone followed by b porous packstone grainstone that grades into c wackestone and d non porous mudstone with pelagic foraminifera wackestone interval c is characterized by an alternation of porous and non porous laminae changes in turbidity current flow regime controlled the initial presence of matrix micrite giving rise to both matrix and grain supported lithologies within turbidite sequences these are non porous and porous respectively four eogenetic diagenetic processes dissolution cementation neomorphism and compaction acted shortly after deposition and modified primary porosity characteristics and distribution alteration by meteoric water is excluded based on the continuous burial until the oligocene of the studied deep marine carbonates moreover the stable isotope data with values between 2ae1 and 0ae7 for d 18 ov pdb and between 1 and 3 for d 13 cv pdb favour alteration by marine derived pore waters compaction and aggrading neomorphism occurred dominantly in intervals characterized by higher matrix micrite content i e the floatstone base and the wackestone mudstone upper turbidite part framework stabilizing cementation occurred dominantly in the packstone grainstone middle part of the turbidite beds in the latter porous lithologies matrix micrite was not compacted because of the grain fabric and the framework stabilizing cements here neomorphism of micrite into microrhombic euhedral calcite occurred and microporosity was preserved

electrical and fluid flow properties of carbonate microporosity types from multiscale digital image analysis and mercury injection electrical and fluid flow properties of porous media are directly related to the morphology of pores and the connectivity of the pore network both are closely linked to the amount and type of intrinsic microporosity in carbonate rocks which is not resolved by conventional techniques broad ion beam bib milling produces high quality true two dimensional cross sections for scanning electron microscopy sem and enables accurate quantification of carbonate microporosity for the first time the combination of bib sem mosaics with optical micrographs yields a multiscale digital image analysis msdia spanning six orders of magnitude in this paper the pore structures of 12 different carbonate rock samples from various rock types are quantified using msdia mercury injection capillary pressure measurements are used to assess pore throat properties the quantified pore structure parameters are correlated with plug measurements of electrical resistivity and permeability results indicate that petrophysical properties are closely linked to the type of microporosity which is distinctive for a certain rock type rock types with crystalline microporosity such as mudstone and dolomite generally show good connectivity in which the size of the pore network determines if the rock favors either hydraulic or electric flow rock types with intercement or micromoldic microporosity such as bindstone and travertine show variations in connectivity due to layering and moldic micropores of biological origin furthermore pore size distributions psd follow a power law in all samples despite their depositional and diagenetic differences the slope of the psd correlates with the electric properties in which samples with a steeper slope show lower cementation factors the linearity of the power law distribution enables predictions of pore populations outside the investigated length scales

u pb zircon and microfabric data of meta granitoids of western cameroon constraints on the timing of pluton emplacement and deformation in the pan african belt of central africa abstract striking characteristics of the western neoproterozoic belt of cameroon nfbc are the large volume of granitoids and crustal scale shear zones new structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the atlantic ocean three different complexes have been identified in the study area the migmatitic complex of foumbot mcf the metagranitoid complex of bangwa bc and the batie pluton bp the mcf was intruded by the bc while the bp cuts through the bc u pb zircon dating of metaleucogranite and metagranodiorite of the bc yielded concordant to subconcordant ages of 638 2 ma and 637 5 ma respectively a concordant u pb zircon age of 602 1 4 ma has been obtained from porphyrogranite of the bp these ages are interpreted as emplacement ages continuous deformation from magmatic to solid state flow along the bp margins and the sub parallelism of the steep solid state foliation in the bp margins with the foliation in the surrounding bc and mfc suggest synkinematic emplacement of the bp along crustal scale nne to ene trending strike slip shear zones subhorizontal foliations in migmatitic gneiss xenoliths found in the bc suggest that the major transcurrent motion was preceded by thrusting the new data confirm previous assumptions that the western nfbc is equivalent to parts of the borborema province of brazil there are geochronological correlations between the studied meta granitoids and brasiliano pre to syn transcurrent granitoids of the borborema province

middle cretaceous carbonate reservoirs fahud field and northwestern oman reservoir facies in fahud field and throughout northwestern oman are in shallow shelf carbonates of the middle cretaceous mishrif and mauddud formations interparticle porosity formed in the mishrif as sand aprons of lithoclast and skeletal grainstones surrounding fault block islands and less commonly in the mauddud as biostromes of rudist packstones moldic porosity after fine rudist debris is more common than interparticle porosity and occurs in thicker stratigraphic units interpreted to have formed locally in meteoric water lenses associated with islands and regionally during subaerial exposure associated with sea level lows

origin of micro rhombic calcite matrix within cretaceous reservoir rock west stuart city trend texas abstract cores from four wells from the west stuart city trend in la salle and webb counties texas were analyzed to define depositional and diagenetic facies and to determine factors controlling porosity distribution in all 1187 ft 362 m of core and 220 thin sections were examined in detail and supplemented by sem analyses of fractured surfaces and plastic casts of pore systems a comparison of lithofacies and ecologic facies to permeability and porosity values compiled from core data revealed that zones with permeabilities greater than 0 1 md and porosities of at least 6 were associated with rudistid grainstones cemented by isopachous submarine cement and packstones with a finely crystalline rhombic calcite matrix isotopic and trace element analyses of both isopachous submarine cements and micro rhombic matrix suggest a common origin the precursor to the micro rhombic calcite is believed to have been peloidal high magnesian calcite this internal marine sediment may be analogous to the peloidal fabrics that have been reported from holocene and pleistocene carbonates diagenetic equilibration of both submarine cements and peloidal infill is believed to have occurred during burial either in marine pore waters at elevated temperatures or in restricted flow phreatic freshwaters

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

shock metamorphic features in amphiboles from the xiuyan crater of china amphibole bearing gneiss fragments are common in the impact breccias of the xiuyan crater china three kinds of amphibole bearing gneiss fragments with different shock metamorphic levels have been identified shock metamorphic features of amphiboles in these gneisses were investigated in situ by optical microscope electron microprobe raman spectroscopy and x ray diffraction amphiboles in the weakly shocked gneiss shock pressure less than 10 gpa basically remain intact amphiboles in the moderately shocked gneiss shock pressure range between 35 and 45 gpa show strong deformation reduced optical interference color and partial loss of oh in the strongly shocked gneiss shock pressure above 50 gpa amphiboles are completely melted and dendritic pyroxenes crystallize from the melt the formation of dendritic pyroxenes shows nearly complete loss of water in the amphibole melt at shock induced high temperature above 1 500 c the occurrence of both diopside and pigeonite dendrites crystallized in the same amphibole melt shows inhomogenous melt composition and rapid cooling of the melt

high resolution seismic stratigraphy of the shu aiba and natih formations in the sultanate of oman implications for cretaceous epeiric carbonate platform systems

impact induced devolatilization and hydrogen isotopic fractionation of serpentine implications for planetary accretion the degree of impact induced devolatilization of nonporous serpentine porous serpentine and deuterium enriched serpentine was investigated using two independent experimental methods the gas recovery method and the solid recovery method yielding consistent results the gas recovery method enables determination of the chemical and hydrogen isotopic composition of the recovered gases experiments on deuterium enriched serpentine unambiguously identify the samples as the source of the recovered gases as opposed to other possible contaminants for shock pressures near incipient devolatilization p initial 5 0 gpa the hydrogen isotopic composition of the evolved gas is similar to that of the starting material for higher shock pressures the bulk evolved gas is significantly lower in deuterium than the starting material there is also significant reduction of h 2o to h 2 in gases recovered at higher shock pressures probably caused by reaction of evolved h 2o with the metal gas recovery fixture the hydrogen isotopic fractionation between the evolved gas and the residual solid indicates nonequilibrium kinetic control of gas solid isotopic ratios in contrast gaseous h 2o h 2 isotopic fractionation suggests high temperature 800 1300 k isotopic equilibrium between the gaseous species indicating initiation of devolatilization at sites of greater than average energy deposition i e shear bands impact induced hydrogen isotopic fractionation of hydrous silicates during accretion can affect the distribution of hydrogen isotopes of planetary bodies during accretion leaving the interiors enriched in deuterium the significance of this process for planetary development depends on the models used for extrapolation of the observed isotopic fractionation to devolatilizations greater than those investigated experimentally and assumptions about timing and rates of protoatmosphere loss frequency of multiple impacts and rates of gas solid or gas melt isotopic re equilibration a simple model indicates that substantial planetary interior enrichments of d h relative to that of the incident material can result from impact induced hydrogen fractionation during accretion

shock metamorphism of enstatite chondrites abstract we have extended the shock metamorphism classification scheme of stoffler et al 1991 to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites orthopyroxene exhibits the following sequence of progressive shock effects shock stage s1 unshocked sharp optical extinction s2 very weakly shocked undulose extinction s3 weakly shocked development of clinoenstatite lamellae parallel to 100 s4 moderately shocked weak mosaicism s5 strongly shocked strong mosaicism as in the stoffler et al 1991 scheme stage s5 is characterized principally by the solid state transformation of crystalline plagioclase into maskelynite most el3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles el5 and el6 chondrites are all shock stage s2 because opaque veins are rare in s2 ordinary chondrites it is plausible that the centimeter size kamacite veins in atlanta and blithfield and the 1 6 cm long oldhamite rich vein in jajh deh kot lalu all el6 formed when their hosts were shocked to s3 s5 levels because removal of shock stage s3 s5 features including repair of shock damaged orthopyroxene requires levels of metamorphism comparable to those experienced by petrologic type 5 to 6 chondrites we infer that proto el6 material was shocked to s3 s5 levels prior to peak metamorphism and shocked again to stage s2 after metamorphism overall enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites

cathodoluminescence studies and u pb dating of zircons in pre mesozoic gneisses of the tauern window implications for the penninic basement evolution the pre mesozoic basement in the eastern and central part of the tauern window eastern alps consists mainly of anatectic plagioclase gneisses and amphibolites intruded and migmatized by carboniferous granitoids the gneisses are geochemically similar to calcalkaline dacites of modern primitive island arcs cathodoluminescence investigations and u pb data on zircons indicate an uppermost proterozoic or cambrian extrusion age of the dacitic protoliths and a carboniferous syngranitoid anatexis around 320 ma high k calcalkaline meta rhyolites in the north central tauern window form a separate part of the basement not affected by carboniferous anatexis and plutonism their protoliths extruded in the lower carboniferous in the permian high k granitoids intruded the anatectic basement

monazite and xenotime u th pb geochronology by ion microprobe dating highly fractionated granites at xihuashan tungsten mine se china zircon monazite and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high u th and low common pb contents however zircons that have crystallized in highly fractionated granites often have such high u contents that radiation damage can lead to scattered u pb ages when measured with secondary ion mass spectrometry sims in this study monazite and xenotime were separated from a number of highly fractionated granites at the xihuashan tungsten mine southeast china for alternative dating methods by sims for monazite analysis obvious excess 204 pb signal mainly from interference of 232 th 144 nd 16 o 2 was observed in high th 2 wt monazite which hinders 204 pb based common pb correc tions a 207 pb based common pb correction method was used instead by employing power law relationships between pb u versus uo2 u p b th versus tho2 th and suitable exponentials monazites with tho2 contents in the range of 3 19 do not exhibit this matrix effect inde pendent sims u pb ages and th pb ages of three phases of xihuashan granite samples were consistent with each other and yielded dates of 158 7 0 7 158 0 0 7 and 156 9 0 7 ma respectively xenotime does show marked matrix effects due to variations of u th and y or total rare earth element ree referred as rree hereafter contents suitable correction factors require end member standards with extremely high or low u th and y or rree con tents no excess 204 pb was observed indicating that the 204 pb based common pb correction method is feasible independent 207 pb 206 pb ages can be obtained although multi collector mode is necessary to improve precision the main difficulties with dating xenotime are when high th u mineral inclusions are ablated we can identify when this occurs however by comparing the measured uo2 u and tho2 th with those in xenotime standards three xeno time samples from the first phase of xihuashan granite yielded a weighted mean 207 pb 206 pb date of 159 5 4 4 ma mswd 1 0 and a 206 pb 238 u date of 159 4 0 9 ma mswd 1 6 which are consistent with mona zite u pb and th pb ages from the same granites this study demonstrates that monazite and xenotime are better sims chronometers for highly fractionated granites than zircon which can yield doubtful ages due to high u contents

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

the pan african quartz syenite of guider north cameroon magnetic fabric and u pb dating of a late orogenic emplacement the small guider pluton 70 km2 in between the cities of maroua and garoua north cameroon is one of the quartz syenite bodies that intrude pan african orthogneisses and that are aligned along the western side of the nne directed poli lere volcano sedimentary corridor the guider syenite is here studied for its internal structures using field and anisotropy of magnetic susceptibility measurements and for its emplacement age of 593 4 ma u pb zircon 20 ma younger than the beginning of gneissification of the country rocks mainly made of deformed granodiorites and diorites in these orthogneisses constantly nne trending vertical foliations variously plunging stretching lineations and fold axes as well as kinematic markers attest to a transpressive and possibly dextral regime the guider syenite rich in k feldspar and magnetite displays typical magmatic microstructures with incipent solid state features such as chess board subgrains in quartz its magnetic fabric displays a well defined feeding zone in its centre and western side attesting to its unrooted nature however n to ne trending lineations at its northern and southern ends parallel to the overall linear structure of the country rocks point to its late orogenic emplacement our results reveal that the ne trending and dextral transpressive shear zone that acted in the guider area ended after the emplacement of the syenite body they specify the age of collage of the nw cameroon domain with the mayo kebbi domain they also show that the tectonic evolution of guider area is closer to the e nigerian domain to the west than to its eastern neighbouring mayo kebbi and adamawa yade domains the connection of our sector with e brazil is briefly discussed

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

pore structure and fractal analysis of lower carboniferous carbonate reservoirs in the marsel area chu sarysu basin abstract evaluation of carbonate heterogeneity closely related to pore geometry and connectivity is important in oil and gas field production and reservoir prediction to profoundly understand the heterogeneity of lower carboniferous carbonate reservoirs in the marsel area including visean c1v and serpukhovian c1sr the pore structure and fractal characteristics are investigated using routine petrophysical measurements x ray diffraction xrd cast thin section analysis scanning electron microscopy sem and mercury injection capillary pressure micp tests the origin of potentially prolific reservoir the relationships between fractal dimension and reservoir physical properties mineral composition and pore structure are also discussed xrd results reveal that the mineral compositions are highly heterogeneous in c1v and c1sr routine petrophysical experiments indicate that c1v and c1sr are generally tight however some high porosity permeability zones exist in c1sr which is described as a potentially prolific reservoir the cast thin section and sem analyses identify three main pore types in the potentially prolific reservoir interparticle dissolution pores intercrystalline dissolution pores and moldic pores the micp data reflect that the pore throat radii in the prolific reservoir range from 0 29 to 4 6 m whereas those in tight section are typically

assessment of shock effects on amphibole water contents and hydrogen isotope compositions 1 amphibolite experiments abstract kaersutitic amphiboles found within a subset of the martian meteorites have low water contents and variably heavy hydrogen isotope compositions in order to assess if impact shock induced devolatilization and hydrogen isotope fractionation were determining factors in these water and isotopic characteristics of the martian kaersutites we conducted impact shock experiments on samples of gore mountain amphibolite in the ames vertical gun range avgr a parallel shock experiment conducted on an anorthosite sample indicated that contamination of shocked samples by the avgr hydrogen propellant was unlikely petrographic study of the experimental amphibolite shock products indicates that only 10 of the shock products experienced levels of damage equivalent to those found in the most highly shocked kaersutite bearing martian meteorites 30 35 gpa ion microprobe studies of highly shocked hornblende from the amphibolite exhibited elevated water contents h 2 o 0 1 wt and enriched hydrogen isotope compositions d 10 relative to unshocked hornblende water and hydrogen isotope analyses of tens of milligrams of unshocked moderately shocked and highly shocked hornblende samples by vacuum extraction uranium reduction and isotope ratio mass spectrometry irms respectively are largely consistent with analyses of single grains from the ion microprobe the mechanisms thought to have produced the excess water in most of the shocked hornblendes are shock induced reduction of hornblende fe and or irreversible adsorption of hydrogen addition of the isotopically enriched martian atmosphere to the martian meteorite kaersutites via these mechanisms could explain their enriched and variable isotopic compositions alternatively regrouping the water extraction and irms analyses on the basis of isotopic composition reveals a small but consistent degree of impact induced devolatilization 0 1 wt h 2 o and h isotope enrichment d 10 extrapolating the shock signature of the regrouped data to grains that experienced martian meteorite like shock pressures suggests that shock induced water losses and hydrogen isotope enrichments could approach 1 wt h 2 o and d 100 respectively if these values are valid then impact shock effects could explain a substantial fraction of the low water contents and variable hydrogen isotope compositions of the martian meteorite kaersutites

stratigraphic well correlations for 3 d static modeling of carbonate reservoirs the principles and purposes of stratigraphic well correlation in carbonate sedimentary systems are defined and discussed within the context of static reservoir modeling the challenge of well correlations is to relate the heterogeneities measured at core and well scales to the spatial heterogeneities at reservoir and flow unit scales the introduction of a priori knowledge in the process of stratigraphic well correlation is critical to support the stratigraphic rules and to establish a coherent geological and petrophysical concept the links between well correlation and geostatistics are discussed with regard to the stationarity hypothesis and property trend analysis we stress that wells are incomplete and biased samples of the geological reality which is not dependent unlike the dynamic reservoir behavior on the well numbers location and spacing stratigraphic rules are applied as a function of the well spacing relative to the geological reality a simple trigonometric method combining angle of base profile paleobathymetry and well spacing is introduced to check the validity of the well correlation in carbonate ramp like systems two models based respectively on outcrop and subsurface with seismic data are discussed in detail to show the combined influence of the data set sedimentary systems and diagenetic transformations on stratigraphic well correlations

petrology of high pressure granulite facies metapelites and metabasites from tchollire and banyo regions geodynamic implication for the central african fold belt cafb of north central cameroon the tchollire and banyo high pressure granulites occur mainly as highly strained small lenses bands or elongated bodies interbedded with gneisses and migmatites in the central african fold belt cafb in north central cameroon they were previously attributed to palaeoproterozoic but are now shown to be pan african these granulites are made up of two occurrences of metapelites with garnet kyanite sillimanite cordierite biotite quartz plagioclase and metabasites containing garnet clinopyroxene orthopyroxene hornblende quartz plagioclase eight samples were analysed in detail by electron microprobe for mineral chemistry parageneses in both metapelites and metabasites highlight three main stages witnessed by prograde peak and retrograde mineral assemblages the prograde stage is preserved as well defined inclusion trails of kyanite biotite plagioclase quartz and rutile within porphyroblasts of garnet in metapelites or with more or less clinopyroxene hornblende plagioclase quartz ilmenite rutile and apatite within porphyroblasts of garnet in metabasites the peak stage displaying heterogranular granoblastic texture is characterised by porphyroblastic garnet kyanite k feldspar biotite in metapelites and garnet clinopyroxene plagioclase quartz in metabasites this was followed by decompression and cooling during the retrograde stage marked by aggregate of sillimanite prisms presumably after kyanite and cordierite corona around garnet in metapelites and by symplectites or vermicular structures of orthopyroxene plagioclase and orthopyroxene corona around garnet in metabasites geothermobarometric study shows that granulite facies metapelites and metabasites from both tchollire and banyo regions recrystallised under peak pressure temperature conditions of 13 14 kbar and 800 900 c they experienced similar clockwise p t path with nearly isothermal decompression our results provide evidence for a substantial crustal thickening during the pan african continent continent collision but show that there is no significant isothermal decompression as commonly observed in recent collisional orogens the reconstructed p t paths are in some ways reminiscent of the ones reported in precambrian mixed hot orogens

tectonic evolution of the yaounde segment of the neoproterozoic central african orogenic belt in southern cameroon the deformation history of the neoproterozoic central african orogenic belt in southern cameroon is well recorded in the low to high grade rocks outcropping in the area around yaounde the fabrics in these rocks are con sistent with two main ductile deformation events d1 and d2 d 1 predated emplacement of calc alkaline dioritic bodies and caused the formation of nappes that resulted in high pressure granulite metamorphism of soft sediments a strong overprinting of these nappes during d2 symmetric extension probably associated with large scale foliation boudinage and or gneissic doming and intense magmatic underplating gave rise to regional flat lying fabrics the latter were further buckled by d3 and d4 folding phases defining a vertical constriction occurring with a major east west to nw se shortening direction the corresponding f3 and f4 folds trend north south to ne sw and east west to nw se respectively and represent the main regional strain patterns based on the east west to nw se maximum shortening orientation indicated by f3 folds it is proposed that the nappe stacking phase d 1 occurred in the same direction the deformation history in the area can thus be described as corresponding principally to alternating east west to nw se contractions and north south to ne sw orogenic parallel extensions at the regional scale this could be due to the transaharan east west collisional system

coesite and quartz characteristic of crystallization from shock produced silica melt in the xiuyan crater article i nfo abundant coesite and quartz coexist with the shock produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the xiuyan crater quartz occurs as idiomorphic needle like and spherulitic crystals and polycrystalline aggregates coesite occurs as granular or idiomorphic dendritic and needle like crystals and polycrystalline aggregates of stringers the occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt during decompression coesite firstly crystallizes from shock produced dense silica melt at high pressure as pressure decreases to the stability field of quartz quartz crystallizes from the melt whereas coesite remains metastable state a growth rate of 10 3 m s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10 ms in the coesite stability field for the crater higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially the occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater 2010 elsevier b v all rights reserved

stratigraphic organization of carbonate ramps and organic rich intrashelf basins natih formation middle cretaceous of northern oman the middle cretaceous carbonate deposits in the middle east are among the most productive oil bearing stratigraphic intervals in the world containing numerous giant fields in for instance the united arab emirates mauddud and mishrif formations iran sarvak formation and oman natih formation one of the main reasons for this concentration of hydrocarbons is a geological factor the coexistence of both reservoir facies and source rocks in the same depositional sequences due to the repeated creation of organic rich intrashelf basins this is demonstrated in a high resolution sequence stratigraphic study of the natih formation in oman which shows distinct and predictive patterns in the distribution and geometries of reservoir source rock and seal facies the sequence stratigraphic model presented here may serve as a reference for time equivalent deposits in the middle east the sedimentological analysis showed that the natih formation was formed by the alternation of two types of depositional systems 1 a flat bedded mixed carbonate clay ramp dominated by benthic foraminifera and 2 a carbonate dominated ramp bordering an intrashelf basin with abundant rudists in the mid ramp environment and organic rich basinal facies three fully developed third order sequences are distinguished showing a similar evolution of the depositional system with a mixed carbonate clay ramp system at the base followed by a carbonate dominated ramp system in the upper part variations occur on this pattern however depending on the relative influence of eustasy environmental factors and tectonism the late albiannearly cenomanian sequence i shows an evolution from a mixed flat ramp to a carbonate dominated ramp and organic rich intrashelf begin page 22 basin and sedimentation is predominantly controlled by eustatic sea level in the middle cenomanian sequence ii the evolution from a mixed ramp to a carbonate ramp is also observed but no intrashelf basin topography was developed in the studied area this may be due to the high influx of clay that influenced the environment in this sequence inhibiting the carbonate production probably in combination with the lack of sufficient creation of accommodation space the late cenomaniannearly turonian transgressive part of sequence iii shows a similar evolution to that observed in sequence i with the development of an organic rich intrashelf basin during highstand however a tectonically controlled sedimentation pattern is observed with the development of forced regressive wedges due to the flexural bulge of the foreland basin intrashelf basin formation occurred twice in the transgressive part of the third order depositional sequences of the natih formation our study shows that this is mainly the result of differential sedimentation rates that is the dynamics of the carbonate sedimentary system itself in response to rapid rises in relative sea level probably of eustatic origin tectonism was only a minor factor in the creation of the basin topography possibly through the creation of small initial relief the accumulation of the organic matter is not only a result of the creation of a sufficiently deep water column to guarantee dysaerobic conditions for its preservation the late albian and late cenomaniannearly turonian were also periods of generally favorable conditions worldwide for high organic matter productivity the time lines and stratigraphic architecture of the third order sequences presented here have an application potential at the scale of the arabian plate the general sedimentation pattern is predicted by our model but modifications due to different local conditions are likely to occur

natural occurrence of reidite in the xiuyan crater of china the high pressure minerals of reidite and coesite have been identified in the moderately shock metamorphosed gneiss shock stage ii 3545gpa and the strongly shock metamorphosed gneiss shock stage iii 4555gpa respectively from the polymict breccias of the xiuyan crater a simple impact structure 1 8km in diameter in china reidite in the shock stage ii gneiss displays lamellar textures developed in parental grains of zircon the phase transformation of zircon to reidite likely corresponds to a martensitic mechanism no coesite is found in the reidite bearing gneiss the shock stage iii gneiss contains abundant coesite but no reidite is identified in the rock coesite occurs as acicular dendritic and spherulitic crystals characteristic of crystallization from shock produced silica melt zircon in the rock is mostly recrystallized the postshock temperature in the shock stage iii gneiss is too high for the preservation of reidite whereas reidite survives in the shock stage ii gneiss because of relatively low postshock temperature reidite does not occur together with coesite because of difference in shock induced temperature between the shock stage ii gneiss and the shock stage iii gneiss

micro rhombic calcite and microporosity in limestones a geochemical study of the lower cretaceous thamama group u a e abstract euhedral micro rhombic calcite forms a microporosity network in limestones of the lower cretaceous thamama group u a e isotopic and trace element data indicate that micro rhomb formation was a two step process associated with diagenesis in meteoric waters the micro rhombic calcite and the micropores are both products of the second event sr fe mn and mg contents of the micro rhombs are all relatively low ranging from 108 to 310 ppm 34 to 308 ppm 3 to 32 ppm and 0 42 to 0 68 mole respectively the low sr and mg contents indicate extensive flushing of these constituents from the rock the low fe and mn contents are probably inherited from the original sediments and indicate the lack of any fe or mn in the pore fluids during micro rhomb formation the 13c of the micro rhombs 3 1 to 4 5 pdm are also primary unaltered by diagenesis the 13o of the micro rhombs 4 5 to 6 0 pdb reflect alteration by mateoric waters a large number of pore volumes and an open water buffered system would be necessary to flush sr and mg yet decreasing 18o with increasing porosity indicates that 18o values in non porous intervals are partially rock buffered two diagenetic events are thus implicated the first flushing sr and mg and lowering 18o in all rocks and the second event actually forming the micro rhombs and further altering 18o as a function of porosity the first diagenetic event was most likely mineralogical stabilization in an unconfined meteoric aquifer immediately after shuaiba deposition if mineralogical stabilization was completed in the first event then calcite to calcite recrystallization is implicated for the second event the second event may have occurred in the unconfined meteoric aquifer or it may have occurred in a confined meteoric aquifer during the early late cretaceous

relation between stratigraphic architecture and multi scale heterogeneities in carbonate platforms the barremian lower aptian of the monts de vaucluse se france abstract understanding the relationship between multi scale sedimentary heterogeneities and the stratigraphic architectures of carbonate platforms improves the interpretation of paleoenvironmental evolution in time and space such an approach improves the prediction of the spatial distribution of reservoir properties within carbonate stratigraphic units the reservoir scale stratigraphic anatomy and the growth history of a barremian aptian carbonate platform monts de vaucluse se france was based on 1 high resolution measured sections on outcrops 2 detailed microfacies analyses 3 paleoenvironmental interpretations 4 biostratigraphy using rudist bivalves foraminifera calcareous algae and locally echinoderm and ammonites and 5 sequence stratigraphic interpretations with a special focus on the identification of discontinuity surfaces with a significant stratigraphic correlation potential and critical environmental meaning the platform succession consists of subtidal mainly infralittoral with rare inter to supratidal deposits thirty types of elementary facies grouped in 12 facies associations have been defined the vertical and lateral changes in paleoenvironments together with the development of remarkable stratigraphic surfaces such as subaerial exposures marine erosional surfaces and drowning surfaces allow the definition of five stratigraphic sequences ranging from the late barremian to the earliest aptian these depositional sequences can be grouped into two distinct carbonate sedimentary systems characterized by specific sedimentary profiles and skeletal and non skeletal components 1 a carbonate platform with stronger hydrodynamic gradients rimmed by a narrow belt of high energy deposits with corals and rudistid rudstone facies and 2 a carbonate ramp dominated by orbitolinids fine grained facies including an elongated high energy oolitic shoal the two dimension architecture of the depositional sequences dimensioned geometries thickness and facies variations and the morphology of the shelf reflect a strong interaction between tectonic differential subsidence sediment production and global environmental conditions at the km scale the gorges de la nesque stratigraphic motif could represent a nested stratigraphic structure within the large scale model which could be implemented for up scaling petrophysical properties from the well to a full field model the association of coral facies to the rudist dominated system in the provence platform is a very clear proxy of the polarity of the carbonate platform and is critical for subsurface facies property modeling based on well data finally at the global scale the stratigraphic architecture and the step wise drowning events of the provence platform confirm and document the precursor events associated with environmental perturbations that affected the carbonate factory prior the oceanic anoxic event 1a oae1a

oh in zoned amphiboles of eclogite from the western tianshan nw china chemically zoned amphibole porphyroblast grains in an eclogite sample ws24 7 from the western tianshan nw china have been analyzed by electron microprobe emp micro fourier transform infrared micro ftir and micro raman spectroscopy in the oh stretching region the emp data reveal zoned amphibole compositions clustering around two predominant compositions a glaucophane end member bna2cm2 3 m3 2tsi8 oh 2 in the cores whereas the mantle to rim of the samples has an intermediate amphibole composition a0 5bca1 5na0 5cm2 4 5 m0 53 tsi7 5al0 5 oh 2 a na and or k m2 mg and fe2 m3 fe3 and or al between winchite and ferro winchite and katophorite and mg katophorite furthermore we observed complicated ftir and raman spectra with oh stretching absorption bands varying systematically from core to rim the ftir raman spectra of the core amphibole show three lower frequency components at 3 633 3 649 3 651 and 3 660 3 663 cm 1 which can be attributed to a local o 3 h dipole surrounded by m 1 m 3 mg3 m 1 m 3 mg2fe2 and m 1 m 3 fe2 3 respectively an empty a site and tsi8 environments on the other hand bands at higher frequencies 3 672 3 673 3 691 3 697 and 3 708 cm 1 are observable in the rims of the amphiboles and they indicate the presence of an occupied a site the ftir and raman data from the oh stretching region allow us to calculate the site occupancy of the a m 1 m 3 t sites with confidence when combined with epm data by contrast m 2 and m 4 site occupancies are more difficult to evaluate we use these samples to highlight on the opportunities and limitations of ftir oh stretching spectroscopy applied to natural high pressure amphibole phases the much more detailed cation site occupancy of the zoned amphibole from the western tianshan have been obtained by comparing data from micro chemical and ftir and or raman in the oh stretching data we find the following characteristic substitutions si t site mg fe m 1 m 3 site al t site al m 1 m 3 site tschermakite ca m4 site a site na m4 site na k a site richterite and ca m4 site mg fe m 1 m 3 site na m4 site al m 1 m 3 site glaucophane from the configurations observed during metamorphism

metasedimentary melting in the formation of charnockite petrological and zircon u pb hf o isotope evidence from the darongshan s type granitic complex in southern china abstract charnockites are opx bearing igneous rocks commonly found in high grade metamorphic terranes despite being volumetrically minor they show a wide range in both bulk geochemistry and intensive parameters they form a characteristic component of the amcg anorthosite mangerite charnockite granite suite but their association with typical s type granites is less well known the darongshan s type granitic complex dsgc in guangxi province southern china contains granites varying in mafic silicate mineral assemblages from bt crd darongshan suite to opx grt bt crd jiuzhou suite and opx crd bt taima suite corresponding to a geochemical transition from magnesian calc alkalic to ferroan calc alkalic however its genesis even the accurate age of intrusion remains highly contentious despite intensive research in order to understand the genesis of charnockite and its genetic relationship with s type granite here we first determined zircon u pb ages of each suite using a sims on the basis of a detailed petrological study zircon u pb ages show that all suites of the complex were emplaced contemporaneously at ca 249 ma monazite apparent u pb ages are indistinguishable from zircon u pb ages within analytical error further in situ zircon hf o isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks 18ozircon ca 11 ehf t zircon ca 10 log fmq 0 mn in apatite oxybarometer with rare material input from the mantle the variation in 18o 7 8 12 9 is more likely a result of hybridization whereas that in ehf t 31 9 to 1 8 is a result of both hybridization and disequilibrium melting the variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite dehydration melting nevertheless heat input from mantle through basaltic intrusion underplating is considered to play a major role in high temperature 850 c melting at mid crustal levels i e the cordierite stable field for generation of the granitic complex we interpret that the granites were intruded in a back arc setting and basaltic magmatism was directly associated with slab roll back and tearing during the latest permian and early triassic times

impact induced dehydration of serpentine and the evolution of planetary atmospheres shock recovery experiments in the 25 to 45 gpa range on antigorite serpentine determine the amount of shock induced loss of structural water as a function of shock pressure infrared absorption spectra of shock recovered samples demonstrate systematic changes in the amount of structural water and molecular surface adsorbed water these yield qualitative estimates of shock induced water loss and demonstrate that a portion of the shock released structural water is readsorbed on interfacial grain surfaces determination of the post shock water content of the shocked samples relates shock induced water loss and shock pressure based on the present results and theoretical predictions we conclude that shock pressures of from 20 to 60 gpa induce incipient to complete water loss respectively this result agrees closely with theoretical estimates for total dehydration the dehydration interval and the activation energies for dehydration in shocked samples decrease systematically with increasing shock pressure as experienced by the sample we believe the present experiments are applicable to describing dehydration processes of serpentine like minerals in the accretional environment of the terrestrial planets we conclude that complete loss of structural water in serpentine could have occurred from accretional impacts of 3 km sec when earth and venus have grown to about 50 of their final size accreting planetesimals impacting mars never reached velocities sufficient for complete dehydration of serpentine our results support a model in which an impact generated atmosphere hydrosphere forms while the earth is accreting

quantifying flow in variably wet microporous carbonates using object based geological modeling and both lattice boltzmann and pore network fluid flow simulations abstract micropores can constitute up to 100 of the total porosity within carbonate hosted hydrocarbon reservoirs usually existing within micritic fabrics here three dimensional computational representations of end point micritic fabrics are produced using a flexible object based algorithm to further our understanding of the contribution that micropores make to flow by methodically altering model parameters we explore the state space of microporous carbonates quantifying single and multiphase flow using lattice boltzmann and network models in purely micritic fabrics average pore radius ranging from 0 26 to 0 44 m was found to have a positive correlation with single phase permeability 1 7 to 2 7 md respectively similarly increasing average pore size resulted in decreasing residual oil saturation under both water wet and 50 fractionally oil wet states permeability was found to increase by an order of magnitude from 0 6 to 7 5 md within fabrics of varying total matrix porosity from 18 to 35 because of increasing pore size 0 37 to 0 56 m respectively but minimal effect on multiphase flow was observed increased pore size due to micrite rounding notably increases permeability in comparison with original rhombic fabrics with the same porosity but multiphase flow properties are unaffected finally when moldic mesopores are added to a micritic matrix they impact flow when directly connected otherwise micropores control single phase permeability magnitude importantly recovery is dependent on both wetting scenario and pore network homogeneity under water wet imbibition increasing proportions of microporosity yield lower residual oil saturations together these results quantify the importance of micropores in contributing to or controlling overall flow and sweep characteristics in such fabrics

quantification of pore structure and its effect on sonic velocity and permeability in carbonates carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude traditional pore type classifications describe these pore structures but are inadequate for correlations to the rock s physical properties we introduce a digital image analysis dia method that produces quantitative pore space parameters which can be linked to physical properties in carbonates in particular sonic velocity and permeability the dia parameters derived from thin sections capture two dimensional pore size domsize roundness aspect ratio ar and pore network complexity poa comparing these dia parameters to porosity permeability and p wave velocity shows that in addition to porosity the combined effect of microporosity the pore network complexity and pore size of the macropores is most influential for the acoustic behavior combining these parameters with porosity improves the coefficient of determination r2 velocity estimates from 0 542 to 0 840 the analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small complicated pores estimates of permeability from porosity alone are very ineffective r2 0 143 but can be improved when pore geometry information poa r2 0 415 and domsize r2 0 383 are incorporated furthermore results from the correlation of dia parameters to acoustic data reveal that 1 intergrain and or intercrystalline and separate vug porosity cannot always be separated using sonic logs 2 p wave velocity is not solely controlled by the percentage of spherical porosity and 3 quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates

amphiboles in the igneous environment the time is ripe for a fresh inventory of occurrences of amphibole group minerals in igneous rocks here i build on the available resources for example on the review of wones and gilbert 1982 in volume 9b of the rimg series they entitled their chapter amphiboles in the igneous environment i adopt the same title but intend to make the scope of my review somewhat broader by including non igneous rocks formed in the igneous environments much has been written about occurrences of amphibole group minerals over the last quarter of a century much of it ably summarized by deer et al 1997 and it is quite impossible to do more here than to summarize representative findings concerning the textural development and broad compositional features of minerals of the amphibole group that are important in igneous contexts amphibole group minerals are widespread in igneous rocks produced in zones undergoing extension as well as in rocks associated with subduction systems each setting may be associated with specific late stage metasomatic phenomena in which amphibole group minerals play a key role metasomatic overprints involving an amphibole also are widespread in the mantle and act as an essential step in the fertilization of sterile mantle if it is to produce alkali basaltic magmas by partial melting finally selected findings are summarized here on amphibole group minerals in igneous environments in other parts of the solar system amphibole group minerals have generally been underutilized in petrological studies reasons are not hard to find the full characterization of an amphibole presents serious challenges so much so that most investigators have been constrained to examine only those major elements that can be assessed in routine electron microprobe analyses as hawthorne and oberti 2006 pointed out in their essay on the future of schemes of classification it has become abundantly clear that progress is within

deep burial microporosity in upper jurassic haynesville oolitic grainstones east texas abstract secondary micromoldic porosity generated during deep burial diagenesis occurs pervasively in upper jurassic haynesville oolitic grainstones in east texas and constitutes the major pore type in these gas reservoirs petrographic and geochemical relationships establish that development of this microporosity postdates emplacement of bitumen and most pressure solution fabrics in the reservoir grainstones microporosity development is strictly controlled by depositional texture and is restricted to either active shoal complex grainstones or thicker grainstones shed downramp by storm processes haynesville diagenetic and porosity relationships are consistent along the entire length of the east flank of the east texas basin a distance greater than 100 km identical relationships have also been observed along the west flank of this basin haynesville micromoldic porosity development is confined principally to ooids but also occurs in normally stable calcitic skeletal grains like oysters resultant micropores are a few microns across or less complete dissolution of ooids to form oomoldic macroporosity is not observed in haynesville limestones nearly all primary porosity in the haynesville is now occluded by carbonate cement confirmation of a late deep burial origin for haynesville secondary microporosity is based on physical relationships observed in numerous cores regional petrography and geochemical data collectively these observations demonstrate that haynesville sediments were never locally or regionally exposed to freshwater but have undergone progressive burial diagenesis punctuated by a major late dissolution event which created the microporosity key observations which support haynesville deep burial microporosity development include 1 petrographic relationships which demonstrate microporosity developed after emplacement of bitumen 2 lack of subaerial exposure features in core both atop shoal complexes and at the contact between the haynesville and bossier sequences 3 absence of freshwater porosity fabrics 4 lack of precompactional freshwater cements 6 pervasive pressure solution in the microporous grainstones including extensive grain interpenetration 6 preservation of abundant microporosity directly adjacent to pressure solution seams in porous grainstones 7 development of identical microporosities in mineralogically stable calcitic grains which rarely leach in freshwater 8 microporosity development which postdates formation and cementation of fractures in the reservoir facies and 9 calcite cements occluding primary porosity whose geochemical attributes are inconsistent with precipitation from freshwater but are consistent with a burial origin this regional core based case study reveals that the contact between the haynesville and overlying bossier shale is a marine drowning unconformity physical petrographic and geochemical evidence for regional subaerial exposure at the end of haynesville deposition due to a eustatic fall in sea level is not present questioning the validity of interpreting local or global sea level history from seismic and well log data in the absence of critical core control

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

the velocity deviation log a tool to predict pore type and permeability trends in carbonate drill holes from sonic and porosity or density logs the velocity deviation log which is calculated by combining the sonic log with the neutron porosity or density log provides a tool to obtain downhole information on the predominant pore type in carbonates the log can be used to trace the downhole distribution of diagenetic processes and to estimate trends in permeability laboratory measurements on over 300 discrete carbonate samples reveal that sonic velocity is a function not only of total porosity but also of the predominant pore type in general there is an inverse porosity velocity correlation but significant deviations occur from this relationship for certain pore types frame forming pore types such as moldic or intrafossil porosity result in significantly higher velocity values at equal total porosities than do pore types that are not embedded in a rigid rock frame such as interparticle porosity or microporosity the results of the laboratory measurements can be applied to expand interpretations of standard wireline log data as shown in this study on two drill holes through neogene carbonates from the great bahama bank the velocity deviation log is calculated by first converting porosity log data to a synthetic velocity log using a time average equation the difference between the real sonic log and the synthetic sonic log can then be plotted as a velocity deviation log because deviations are the result of the variability of velocity at a certain porosity the deviation log reflects the different rock physical signatures of the different pore types positive velocity deviations mark zones where velocity is higher than expected from the porosity values such as zones where frame forming pore types dominate zero deviations show intervals where the rock lacks a rigid frame such as in carbonates with high interparticle porosity or microporosity negative deviations mark zones in which sonic log velocities are unusually low caused for instance by a cavernous bore hole wall fracturing or possibly by a high content of free gas by tracing the velocity deviations continuously downhole one can identify diagenetic zones that are characterized by these different pore types in addition this method can be used to observe permeability trends because pore types influence the permeability of the rock

origin and evolution of the late precambrian high grade yaounde gneisses cameroon abstract the yaounde series is composed of low to high grade garnet bearing schists and gneisses belonging to the pan african north equatorial fold belt the high grade rocks consist of kyanite garnet gneisses and garnet plagioclase gneisses containing layers of calcsilicate rocks marbles quartzites and magnetite rich orthopyroxenites the chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite rocks dolomitic marls evaporitic beds quartzites and iron rich sediments some volcanogenic greywackes may have been of alkaline affinity this sequence was probably deposited in shallow water near shore semi barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the congo craton the yaounde gneisses were deformed during two main tectonic phases the first phase d1 is marked by a layering s0 s1 resulting from tectonic transposition the second one d2 corresponds to tangential tectonics with isoclinal folds and flat lying shear zones associated with a s2 schistosity and a l2 lineation a third phase late d2 is marked by the development of low temperature mylonitic shears scarce wrench faults indicate later e w compression the d1 d2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high pressure granulite facies conditions t 750 800 c p 10 12 kbar these peak metamorphic conditions triggered a widespread migmatization dated at 565 22 ma and coeval with the d2 event the gradual decrease of the metamorphic conditions toward the south and the existence of a syn d2 retrograde inverse metamorphic gradient suggest that the overall structure of the yaounde series is a large tectonic nappe thrust onto the congo craton a geotectonic scheme for the north equatorial fold belt is discussed