isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis abstract diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute hf the weakened interface breaks when grains are agitated ultrasonically subsequent particle size separation is an effective means of concentrating the diagenetic component

submillimeter scale zonation of δ18o in quartz and feldspar isle of skye scotland abstract laser probe analysis of oxygen isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains two samples collected within 10 m of each other in the maol na gainmhich epigranite mge isle of skye scotland are similar petrographically yet display very different 18o zonation patterns sk1 has isotopically zoned quartz 18 o 1 8 8 2 with intragranular gradients of 18o as great as 5 mm and individually homogeneous grains of k feldspar 0 2 which also display a large range of values 18 o 1 7 to 7 3 oxygen isotope gradients of more than 11 mm were measured in this sample in contrast sk4 has homogeneous quartz 18o 5 7 0 3 n 14 and a zoned k feldspar phenocryst 18 o 3 3 to 6 9 isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains the isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the mge pluton although the average values of 18o obtained by laser probe are identical to the average bulk values values in k feldspar suggest exchange at 400 c with infiltrating meteoric water 18 o 10 the marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous these microanalytical data for quartz and feldspar provide the first oxygen isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures

diagenesis of a mixed siliciclastic evaporitic sequence of the middle muschelkalk middle triassic the catalan coastal range ne spain the middle muschelkalk middle triassic of the catalan coastal range north east spain comprises sandstone mudstone anhydrite and minor carbonate layers interbedded sandstones and mudstones which are dominant in the north eastern parts of the basin are terminal alluvial fan deposits south westward in the basin the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha mudflat environments the diagenetic and pore water evolution patterns of the middle muschelkalk suggest a strong facies control during eodiagenesis formation of microdolomite anhydrite baryte magnesite k feldspar and mixed layer chlorite smectite was favoured within and adjacent to the sabkha mudflat facies whereas calcite haematite mixed layer illite smectite and quartz formed mainly in the alluvial facies low 18osmow values for microdolomite 23 7 to 28 4 and k feldspar overgrowths 17 3 to 17 7 suggest either low temperature isotopic disequilibrium or precipitation from low 18o porewaters low 18o waters might have developed at least in part during low temperature alteration of volcanic rock fragments during mesodiagenesis precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones whereas recrystallization of microdolomite was dominant in the sabkha mudflat facies the isotopic compositions of these mesogenetic phases reflect increasing temperature during burial upon uplift and erosion telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite the isotopic composition of the calcite 18osmow 21 5 to 25 6 o 13c 7 7 to 5 6 o and presence of haematite indicate infiltration of meteoric waters

thermo chronological approach to reservoir diagenesis in the offshore angola basin a fluid inclusion 40 ar 39 ar and k ar investigation 1 microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in neocomian sandstones and albian dolomitic reservoirs from the offshore angola basin have allowed determination of the age and the physical chemical conditions for fluid migration in a late diagenetic stage the 40 ar 39 ar laser probe dating of adularia overgrowths from the albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early miocene 25 6 or 3 2 ma the k ar age of authigenic illites from the neocomian sandstones is slightly younger 19 or 1 ma and the crystallization of regular illite smectite mixed layers is related to a previous eocene to paleocene diagenetic stage 51 58 ma there appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the miocene the fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids assuming that pressures during the miocene were close to the present reservoir pressures and by intersecting the isochores of aqueous and oil inclusions assuming they were trapped at strictly similar pressure and temperature conditions the fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content aliv of authigenic chlorites 168 206 degrees c in the albian reservoirs at depths between 2350 and 3580 m and 175 237 degrees c in the neocomian sandstones at depths between 2770 and 4470 m in both reservoir formations miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient

separation kar dating and 18o16o ratio measurements of diagenetic k feldspar overgrowths an example from the lower cretaceous arkoses of the angola margin abstract k feldspar overgrowths adularia on detrital k feldspar were removed from the lower cretaceous 130 115 ma arkoses of angola and concentrated for oxygen isotopic analysis and k ar dating the concentration technique consisted of etching with dilute h2sif6 followed by ultrasonic agitation and particle size separation the diagenetic k feldspar content of each of fourteen subfractions as estimated by cathodoluminescence was between 45 and 92 percent the rest being mostly detrital microcline and in some cases small amounts of quartz and or albite extrapolation of 40 ar 40 k and 18 o 16 o ratios indicated a k ar age of the adularia of 98 16 ma and a 18o of 20 2 0 8 per mil smow the age is consistent with petrographic observations which suggest that precipitation of adularia occurred relatively early in the diagenetic history the k ar age together with the 18o value burial history and fluid inclusion data indicate that adularia formed at relatively shallow depth

tomographic vp and vs structure of the california central coast ranges in the vicinity of safod from controlled source seismic data summary a seismic reflection refraction survey across the san andreas fault near parkfield california has refined our knowledge of the upper crustal structure of the central california coast ranges at the san andreas fault observatory at depth safod the survey consisted of a 46 km long line of seismographs 25 50 m spacing and 63 explosions 25 200 kg nominal spacing of 500 m with some gaps the traveltimes of refracted p and s waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce p and s wave velocity models vp vs along the profile extending to as much as 5 km depth the vp and vs models show a prominent lateral drop in velocities a few hundred metres northeast of safod between the drill hole and the san andreas fault the vp model shows particularly well a southwest dipping velocity inversion beneath safod the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below in addition to vp and vs models a vp vs model was derived a vp vs ratio lower than 1 73 is seen only at depth in a narrow zone beginning at the target earthquakes for safod and extending downward and northeastward into the north america plate clusters in the parameter space spanned by vp vs ratios and vp can be identified by two different methods one more intuitive analytical method and one more abstract method based on neural network techniques these clusters are correlated to different rock types based on laboratory and in situ data these clusters are remapped back into x z plane along the profile prominent features mapped this way include salinian granitic rocks beneath and west of safod and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the buzzard canyon fault bcf system these sedimentary rocks extend from this fault to the san andreas fault system unfortunately our cluster analysis shows no significant discontinuity at the san andreas fault owing presumably to the fact that the san andreas fault is located within sedimentary rocks having similar elastic properties this paper is an attempt to downward continue a geological map by geophysical means based on elastic properties of rock samples from the region

upper mantle structure at walvis ridge from pn tomography abstract passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up within the lispwal lithospheric structure of the namibian continental passive margin at the intersection with the walvis ridge from amphibious seismic investigations project combined on and offshore seismic experiments were designed to characterize the southern african passive margin at the walvis ridge in northern namibia in addition to extensive analysis of the crustal structures we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the walvis ridge with the namibian coast upper mantle pn travel time tomography from controlled source amphibious seismic data was used to investigate the sub moho upper mantle seismic velocity we succeeded in imaging upper mantle structures potentially associated with continental break up and or the tristan da cunha hotspot track we found mostly coast parallel sub moho velocity anomalies interpreted as structures which were created during gondwana break up

an integrated thermal and isotopic study of the diagenesis of the brent group alwyn south u k north sea abstract the petrography fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore fluid history of the middle jurassic brent group reservoir sandstones in the alwyn south area of the u k north sea the study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels the cement assemblage of kaolinite quartz and illite has resulted in severe deterioration of otherwise good reservoir quality early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation temperature estimates for kaolinite precipitation of 80 c and 18o of 15 3 suggest co existing fluids of 18o 3 quartz cementation overlapped both kaolinite and illite formation fluid inclusion data indicate that quartz cementation took place at temperatures of 109 7 c pore fluid salinities were 4 wt nacl with a h2o o isotopic composition of 1 o 0 5 smow the fluids which precipitated coexisting illite were compositionally homogeneous with equilibrium 18o water compositions of 0 5 smow illite sd values range from 33 to 50 smow these fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform marine signature this is consistent with the predominantly marine to paralic depositional context of the brent group in alywn south illite precipitation was followed by hydrocarbon emplacement between the middle eocene and lower oligocene

optically continuous silcrete quartz cements of the st peter sandstone high precision oxygen isotope analysis by ion microprobe a detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried 1 km quartz arenites of the st peter sandstone in sw wisconsin constrains temperature and fluid sources during diagenesis quartz overgrowths are syntaxial optically continuous and show complex luminescent zonation by cathodoluminescence detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination resulting

hump shaped 40ar 39ar age spectra in k feldspar and evidence for cretaceous authigenesis in the fountain formation near eldorado springs colorado the fountain formation near eldorado springs co usa shows evidence of alteration by hydrothermal fluids that precipitated authigenic potassium feldspar adularia as rims on detrital feldspars and as interstitial cement deposits detailed 40ar 39ar step heating experiments of samples from the fountain formation reveal age spectra having a characteristic hump at laboratory temperatures between 900 and 1080 c the humps appear to be related to the presence of adularia laser analyses of small grains of adularia indicate that the hump can be physically dissected unlike age gradients found in igneous feldspars such behavior is consistent with the mixing of gas from two or more generations of k feldspar each having unique diffusion properties the feldspars studied here indicate that two pulses of 150 c hydrothermal fluids migrated through the fountain formation at 135 and 94 ma prior to the main phase of laramide tectonic activity in the region the limited occurrence of authigenic cements suggests that reactivation of the underlying precambrian idaho springs ralston creek shear zone was significant enough to heat and mobilize large quantities of meteoric fluids in addition these data also suggest that cretaceous movements along the transcontinental arch correlated with stratigraphic events began approximately 40 million years earlier than previously thought

seismic imaging of the waltham canyon fault california comparison of ray theoretical and fresnel volume prestack depth migration abstract near vertical faults can be imaged using reflected refractions identified in controlled source seismic data often theses phases are observed on a few neighboring shot or receiver gathers resulting in a low fold data set imaging can be carried out with kirchhoff prestack depth migration in which migration noise is suppressed by constructive stacking of large amounts of multifold data fresnel volume migration can be used for low fold data without severe migration noise as the smearing along isochrones is limited to the first fresnel zone around the reflection point we developed a modified fresnel volume migration technique to enhance imaging of steep faults and to suppress noise and undesired coherent phases the modifications include target oriented filters to separate reflected refractions from steep dipping faults and reflections with hyperbolic moveout undesired phases like multiple reflections mode conversions direct p and s waves and surface waves are suppressed by these filters as an alternative approach we developed a new prestack line drawing migration method which can be considered as a proxy to an infinite frequency approximation of the fresnel volume migration the line drawing migration is not considering waveform information but requires significantly shorter computational time target oriented filters were extended by dip filters in the line drawing migration method the migration methods were tested with synthetic data and applied to real data from the waltham canyon fault california the two techniques are applied best in combination to design filters and to generate complementary images of steep faults

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

no moho uplift below the baikal rift zone evidence from a seismic refraction profile across southern lake baikal 1 the late cenozoic baikal rift zone brz in southern siberia is composed of several individual topographic depressions and half grabens with the deep lake baikal at its center we have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the baikal explosion seismic transects best project across the rift zone in the southern part of lake baikal the seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2 d ray tracing of first arrivals and reflections the velocity model shows a gently deepening moho from the siberian platform 41 km depth into the sayan baikal fold belt 46 km depth we can exclude the presence of any moho uplift around the 10 km deep sedimentary graben structure of southern lake baikal the lower crust includes a distinct 50 80 km wide high velocity anomaly 7 4 7 6 0 2 km s slightly offset to the northeast from the rift axis we interpret this feature as resulting from mafic intrusions their presence may explain the flat moho by compensation of lower crustal thinning by intrusion of mafic melts the pn wave velocities 8 15 8 2 km s are normal for the area and do not show any sign of decompression melting in the sub moho mantle such that possible lithosphere thinning has not reached the base of the crust on the basis of the results of the best project we suggest that the brz is formed by passive rifting in the rheologically weak suture between the siberian platform and the amurian plate

oxygen isotope analyses of fine silica grains using laser extraction technique comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation the laser fluorination technique reported here for analyzing the oxygen isotope composition d 18 o of fine quartz size fractions 50 20 20 10 10 5 5 2 2 1 and 1 lm has been validated by comparison with the ion microprobe technique it yields accurate d 18 o data with an external precision better than 0 15 this is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis both techniques were used to investigate d 18 o composition of a cretaceous quartzite and silcrete sequence from the southeast of france quartzite cements average 31 04 1 93 they formed from mid cretaceous seawater higher in the series silcretes cements average 26 66 1 36 they formed from upper or post upper cretaceous soil water and groundwater oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another including from an upper pedogenic silcrete to a lower groundwater silcrete occurred in a closed or weakly evaporating hydrological system

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

crustal structure of northwest namibia evidence for plume rift continent interaction the causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute seismic tomography and other studies suggest that hot mantle material rising from the core mantle boundary cmb might play a significant role in the formation of such hotspot trails an important area to verify this concept is the south atlantic region with hotspot trails that spatially coincide with one of the largest low velocity regions at the cmb the african large low shear wave velocity province the walvis ridge started to form during the separation of the south american and african continents at ca 130 ma as a consequence of gondwana breakup here we present the first deep seismic sounding images of the crustal structure from the landfall area of the walvis ridge at the namibian coast to constrain processes of plume lithosphere interaction and the formation of continental flood basalts parana and etendeka continental flood basalts and associated intrusive rocks our study identified a narrow region 100 km of high seismic velocity anomalies in the middle and lower crust which we interpret as a massive mafic intrusion into the northern namibian continental crust seismic crustal reflection imaging shows a flat moho as well as reflectors connecting the high velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the etendeka continental flood basalt we suggest that the observed massive but localized mafic intrusion into the lower crust results from similar sized variations in the lithosphere i e lithosphere thickness or preexisting structures

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

oxygen isotope studies of clastic diagenesis in the lower cretaceous viking formation alberta implications for the role of meteoric water

linking rift propagation barriers to excess magmatism at volcanic rifted margins break up related extrusive magmatism imaged in reflection seismic data as seaward dipping reflectors sdrs extends symmetrically along the volcanic margins of the atlantic ocean recent research found distinct along margin variations in the distribution of sdrs and abundance of volcanic material was found to be spatially linked to transfer fault systems these segmented the propagating rift that later developed into the ocean and are interpreted as rift propagation barriers based on these observations we develop a numerical model which shows that rift parallel mantle flow and locally enhanced rates of volcanism are the result of delays in rift propagation and segmented opening our model suggests that segmentation is one of the major factors in the distribution and localization of riftrelated extrusive magmatism we conclude that in addition to mantle temperature and inherited crustal structures e g weaknesses from previous rift episodes rift propagation delay plays an important role in the distribution of extrusive volcanism at volcanic passive margins by controlling the mantle flow beneath the rift axis

subsidence rates in the po plain northern italy the relative impact of natural and anthropogenic causation abstract the central eastern po plain in northern italy is a rapidly subsiding sedimentary basin that hosts about 30 of the italian population and 40 of italy s total productive activities subsidence rates range from 0 to 70 mm year the maximum occurring in synclinal areas at the po delta and near bologna the minimum located at the top of buried probable tectonically active anticlines mirandola ferrara we show that modern subsidence is at least an order of magnitude higher than due solely to long term natural processes this implies that most subsidence in the po plain has been induced by human activities as previously noted we suggest that the main factor controlling modern subsidence is water withdrawal which was particularly intense during the second half of the 20th century coinciding with accelerating economic growth we also evaluate the impact of rapid subsidence on floods in low areas by comparing subsidence velocity maps with flood maps this shows that there is a clear cut correlation between flood frequency and rapid subsidence in contrast few floods occurred in low subsidence areas which generally correspond with the top of buried anticlines we finally note that the anthropic caused increase in subsidence has now greatly increased the potential for additional floodings

pore water evolution in oilfield sandstones constraints from oxygen isotope microanalyses of quartz cement abstract oxygen isotope microanalyses of authigenic quartz in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the brae formation in the miller and kingfisher fields north sea oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a cameca ims 4f ion microprobe in conjunction with quartz cement paragenesis in the reservoirs constrained from textural and cathodoluminescence cl microscopy studies pore water evolution was reconstructed from the time of deposition of the sandstones in the upper jurassic until the present cl photomicrographs of quartz overgrowths in the brae formation sandstones show three cement zones a b and c which can be related to different oxygen isotope compositions 1 the earliest and thinnest zone a homogeneous cl pattern with probable 18 o values between 23 and 26 direct measurements were not possible precipitated in the sandstones at temperatures 18 o values between 15 and 18 precipitated in the sandstones most likely between 70 and 90 c 3 the third zone c homogeneous cl pattern and directly measured 18 o values between 16 and 22 precipitated in the sandstones most likely at temperatures 90 c calculated oxygen isotope compositions of pore waters show that zone a quartz cements and enclosing concretionary calcite precipitated from a meteoric type fluid 7 during shallow burial 18 o 7 to a more 18 o enriched fluid 18 o 4 as burial continued to 3 0 km data from zone c quartz cements are consistent with further fluid evolution from 18 o 4 to basinal type fluids with 18 o similar to the present day formation water oxygen isotope composition 0 6 at 4 0 km burial a similar pore water evolution can be derived for all three reservoirs studied indicating that hydrogeologic evolution was similar across sandstones of the whole brae formation the quartz cement zones observed in the brae formation sandstones and the pore water history derived for the area studied is analogous to published petrographic and pore water evolution data from the nearby brent group reservoirs and from reservoirs located in the haltenbanken area on the atlantic margin offshore norway considering quartz cement is a major porosity occluding phase in many reservoir sandstones and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

seismic structure of the lithosphere beneath nw namibia impact of the tristan da cunha mantle plume northwestern namibia at the landfall of the walvis ridge was affected by the tristan da cunha mantle plume during continental rupture between africa and south america as evidenced by the presence of the etendeka continental flood basalts here we use data from a passive source seismological network to investigate the upper mantle structure and to elucidate the cretaceous mantle plume lithosphere interaction receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km we interpret this interface as the relic of the lithosphereasthenosphere boundary lab formed during the mesozoic by interaction of the tristan da cunha plume head with the pre existing lithosphere the velocity contrast might be explained by stagnated and frozen melts beneath an intensively depleted and dehydrated peridotitic mantle the present day lab is poorly visible with converted waves indicating a gradual impedance contrast beneath much of the study area converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1 5 s earlier than in the landward plume unaffected continental interior suggesting high velocities in the upper mantle caused by a thick lithosphere this indicates that after lithospheric thinning during continental breakup the lithosphere has increased in thickness during the last 132 myr thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here we propose that the remnant plume material which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration significantly contributed to the thickening of the mantle lithosphere

minerological and o isotope studies of diagenesis and porewater evolution in continental sandstones cretaceous belly river group alberta canada abstract the mineralogy and o isotope geochemistry of diagenetic phases from continental sandstones of the upper cretaceous belly river group west central and south western alberta have been determined diagenesis of the continental sandstones is dominated by crystallization of clay minerals including fe rich chlorite 18 o 3 3 to 9 3 i s c s and illite 18 o 9 5 to 10 7 kaolinite 18 o 5 9 to 12 7 s i and smectite 18 o 11 0 to 14 0 other diagenetic phases include quartz overgrowths 18 o 12 0 to 21 0 and calcite 18 o 8 5 to 12 9 13 c x02212 14 3 to 2 0 chlorite generally crystallized first in the paragenetic sequence followed by quartz i s calcite and kaolinite and then s i and smectite interpretation of these data suggests that early diagenesis occurred in the presence of low 18 o meteoric water whose 18 o value rose moderately during subsequent burial highest burial temperatures 110 c were probably achieved in the south western portion of the study area and are reflected by the appearance of r1 ordered i s in this locality post laramide influx of meteoric water subsequently resulted in a substantially lower porewater 18 o value values of 18 o for diagenetic minerals from the continental sandstones and immediately underlying deltaic and shoreline basal sandstones are systematically lower by 1 to 6 than equivalent phases from the lower cretaceous marine sandstones of the viking formation relative differences in the oxygen isotopic compositions of connate waters freshwater vs sea water have been preserved surviving 18 o enrichment during burial and 18 o depletion during post laramide influx of meteoric water while substantial meteoric dilution of the porewater has occurred flushing of evolved connate waters has not been complete

oxygen isotope microanalysis by secondary ion mass spectrometry suggests continuous 300 million year history of calcite cementation and dolomitization in the devonian bakken formation calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems unravelling detailed histories and timescales of cementation and replacement is however often compromised by the limited spatial resolution of many analytical techniques in some cases multiple grains are co analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish in this study we have used 10 micrometer resolution in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual 10 50 micrometer size crystals of dolomite and calcite from six samples in a single core of upper devonian middle bakken member siltstones and sandstones the major tight oil formation of the williston basin usa this amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred petrographic data show that microcrystalline calcite cement is an early phase and combined with mineralogical data suggest that much of the dolomite replaces calcite over spatial scales of less than a centimeter analyses of individual calcite crystals have a range of 5 for 18o in the group of crystals and for the group of individual dolomite crystals 10 these sub centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low resolution studies may have inadvertently analyzed mixed phases there is no relationship between dolomite texture and isotopic composition at this spatial scale microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen but cannot be resolved with a 10 micrometer spot size assuming since it is an early cement that calcite precipitated from seawater 18o 1 5 it formed at ca 15 40 c mainly at the lower temperatures present day formation waters in devonian rocks in this region have oxygen isotope compositions of 7 8 vsmow using these values as a likely dolomitizing fluid we suggest that dolomitization occurred continuously between 40 and 140 c over 150 200 million years most likely in a fluid with a high mg ca ratio resulting from gypsum formation in local evaporites we suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of mg in a very sluggish flow regime dolomitization is incomplete because of a limited supply of mg

methodological improvements for investigating silcrete formation petrography ft ir and oxygen isotope ratio of silcrete quartz cement lake eyre basin australia the oxygen isotopic composition 18o of the cement of a silcrete sample from the cordillo silcrete late eocene early oligocene of lake eyre basin australia was investigated the massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater in order to avoid errors inherent in estimates of the cement 18o value from isotopic analyses of bulk material and mass balance considerations methodological improvements are described the silcrete cement is physically separated from the detrital phases its purity is checked using cathodoluminescence cl spectroscopy optical microscopy x ray diffraction xrd analyses and fourier transform infrared ft ir spectroscopy show the cement is made of three phases i overgrowth ii crypto crystalline quartz and iii micro crystalline quartz thus its 18o value must be interpreted as that of a mixture although the mixture consists mainly of crypto crystalline quartz as the amount of si oh exchangeable oxygen in the cement would bias the results by an amount less than the analytical error 0 09 it is unnecessary to go through an equilibration procedure prior to isotopic analysis the 18o value of the cement sample is between 24 8 and 25 8 relative to v smow assuming an eocene oligocene temperature of 15 20 c in agreement with published regional temperature estimates the inferred 18o value of the quartz forming solution was between 6 9 and 12 2 this is lower than the minimum 18o value of 6 70 estimated for the modern groundwater near the sample site a plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one from shallow groundwater frequently recharged in a setting of slow tectonic uplift

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

diagenesis and isotopic evolution of porewaters in the alberta deep basin the falher member and cadomin formation abstract diagenesis and porewater evolution in the alberta deep basin have been studied by combining results from petrologic stable isotope and fluid inclusion analyses of diagenetic minerals four stages of diagenesis and porewater evolution corresponding to burial and subsequent uplift and erosion have been identified in the falher member of the spirit river formation and the cadomin formation stage 1 deposition and burial is marked by early precipitation of hematite siderite or chlorite and the dissolution of unstable detrital grains followed by later albitization precipitation of illite and local calcite cement and pressure solution of chert grains during this stage 18o values of porewaters in the falher member increased from 10 3 to 3 infiltration of saline fluids from pre cretaceous units may have produced porewaters in the cadomin formation with 18o values as high as 4 to 7 stage 2 maximum burial and relief is dominated by precipitation of quartz druse in conglomerates and horizontal fractures porefluids during stage 2 had 18o values of 3 at 190 c 2 3 wt dissolved solids in the falher member and at 150 c in the cadomin formation during stage 3 uplift and erosion precipitation of dickite occurred in both the falher member and the cadomin formation from evolved meteoric water as temperatures and 18o values of the porewater decreased in addition precipitation of ankerite and calcite was initiated in the falher member as the influx of evolved meteoric water continued finally in stage 4 maximum generation of methane from interbedded coals methane saturation of the porespaces marked the end of diagenesis in the down dip gas saturated part of the deep basin

3 d magnetotelluric image of offshore magmatism at the walvis ridge and rift basin highlights we report on marine 3d magnetotelluric study on walvis ridge derived 3d electrical resistivity model shows a large scale resistive zone which we link to crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting smaller scale resistive region is attributed to magma ascent during rifting rift basin is identified by low resistivity region the namibian continental margin marks the starting point of the tristan da cunha hotspot trail the walvis ridge this section of the volcanic southwestern african margin is therefore ideal to study the interaction of hotspot volcanism and rifting which occurred in the late jurassic early cretaceous offshore magnetotelluric data image electromagnetically the landfall of walvis ridge two large scale high resistivity anomalies in the 3 d resistivity model indicate old magmatic intrusions related to hot spot volcanism and rifting the large scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2 d offshore seismic profiles one of the high resistivity anomalies above 500 m has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust one of the arms stretches underneath walvis ridge the shape is suggestive of crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting a second smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting we attribute a low resistivity anomaly east of the continent ocean boundary and south of walvis ridge to the presence of a rift basin that formed prior to the rifting

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

uppermost mantle and crustal structure at tristan da cunha derived from ambient seismic noise abstract according to classical plume theory the tristan da cunha hotspot located 400 km east off the mid atlantic ridge is thought to have played a major role in the rifting while creating an aseismic walvis ridge during and after the breakup of the south atlantic margins volcanic activity on the tristan da cunha island shows that the hotspot might still be there influencing the upper mantle and crustal structure in this study we present ambient noise data from 24 broadband obs around tristan da cunha and a seismic station on nightingale island which provide first constraints on the crustal and uppermost mantle structure around the island by combining ambient noise techniques dispersion curve analysis of rayleigh waves 2d tomographic inversion of travel times and 3d depth inversion of dispersion data we derived a 3d v s velocity model around the archipelago of tristan da cunha the model shows an isolated vertically sharp bounded thickened and modified crust beneath the islands surrounded by thin oceanic crust

illitization and paleothermal regimes in the middle ordovician st peter sandstone central michigan basin k ar oxygen isotope and fluid inclusion data hydrocarbon reservoirs occur in the middle ordovician st peter sandstone in the central michigan basin at depths of 1 5 3 5 km and are diagenetically altered latest diagenetic cements include saddle dolomite pervasive microcrystalline illite and chlorite and quartz a k ar and 18o 16o study of the fine grained authigenic illite in 25 samples from 16 wells covering a large area within the basin yields k ar ages ranging from 367 to 322 ma and dgr18o values between 12 7 and 16 9 pmil smow the dgr18o values of diagenetic quartz overgrowths range from 15 2 to 18 9 pmil fluid inclusion temperatures in the quartz cement range from 70 to 170 c reflecting multiple generations of diagenetic quartz and or precipitation over m st of the diagenetic history reequilibrated fluid inclusions in the saddle dolomite cement yield temperatures ranging from 90 to 150 c a regionally significant episode of illitization occurred during the late devonian mississippian temperatures of illite formation are indirectly estimated to be in the range of 125 170 c and most paleodepths of illitization are between 2 8 and 3 2 km these results imply that 1 illite formed from 18o rich fluids and 2 elevated geothermal gradients i e greater than 34 c km existed in the michigan basin in the late paleozoic the k ar ages and the dgr18o values are not correlated to present depths of the samples or paleodepths of illitization illites with young ages and low dgr18o values tend to be geographically distributed along the north south branch of the buried precambrian rift the dgr18o values of the diagenetic quartz follow a similar trend the spread of illite k ar ages and dgr18o values and their geographic distribution are best explained as reflecting abnormally high thermal regimes in the part of the basin located above the presumably highly fractured basement along the rift

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

influence of early diagenesis on reservoir quality of rotliegende sandstones northern netherlands reservoir quality of rotliegende sandstones lower permian in the northern netherlands was significantly affected by early diagenetic processes that are related to sedimentary facies in marginal desert playa lake environments in the past burial related products and processes have received more attention than early diagenetic processes neglecting the effects that early diagenesis can have on reservoir quality and well productivity of rotliegende sandstones shallow groundwater related diagenesis is a major factor affecting rotliegende reservoir sandstones the precipitation of the main porosity occluding cements dolomite anhydrite and quartz occurred early in the diagenetic history oxygen isotopes of dolomite d18o 18 4 to 23 4 smow suggest that meteoric water has played a major role throughout the diagenetic history and that precipitation of these cements occurred at low temperatures 80 c the strontium isotopes of dolomite cements 87sr 86sr 0 70935 0 71387 and the sulfur isotopes of anhydrite cements d34s 6 7 to 9 7 cdt rule out permian or younger seawater as a source for these cements and are consistent with formation from meteoric water within a continental basin an early diagenetic origin of the cements is also suggested by the fact that the majority of sandstone samples have lost more porosity due to cementation than through compaction most samples with high 20 amounts of porosity loss via cementation were affected by early dolomite and anhydrite the relative abundance of dolomite and anhydrite cements can be correlated with depositional environments wet depositional environments e g interdune and fluvial sandstones with water tables close to the surface show the highest amount of dolomite and anhydrite cements whereas dry environments with relatively deep water tables e g dune sandstones are characterized by low amounts of cements the precipitation of early diagenetic cements has strongly influenced the present day porosity patterns with depositional environments interpreted as drier showing less cements and significantly altering the porosity patterns formed during early diagenesis knowledge of the influence of sedimentology and paleogeography on the diagenetic patterns is a key element for improved understanding and prediction of reservoir quality in the rotliegende sandstones of the northern netherlands

south atlantic opening a plume induced breakup upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup the formation of the walvis ridge during the opening of the south atlantic is related to a putative plume induced breakup we investigated the crustal structure of the walvis ridge southeast atlantic ocean at its intersection with the continental margin and searched for anomalies related to the possible plume head the overall structure we identify suggests that no broad plume head existed during opening of the south atlantic and anomalous mantle melting occurred only locally we therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting leaving a track of kimberlites in the african craton

ion microprobe analyses of δ18o in early quartz cements from 1 9ga granular iron formations gifs a pilot study abstract the low 18 o values of precambrian cherts have been widely used to infer that temperatures were higher and or seawater 18 o was lower compared to today s oceans however the precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water suggesting diagenetic alteration is an important cause of the widespread low 18 o values many pores between sand size clasts in granular iron formations gifs are filled with coarsely crystalline quartz cements that are texturally primary in unmetamorphosed gif samples from the 1 9 ga gunflint and sokoman iron formations synsedimentary clasts of gif and high minus cement porosities the volume of pore space occupied by recognizable cement inversely proportional to the degree of compaction indicate such cements precipitated directly from pore water near the depositional interface growth bands identified in quartz cement crystals using cathodoluminescence cl imaging further confirm their void filling and unrecrystallized nature in order to test the effect of neomorphism on the 18 o of silica in these rocks unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe values of 18 o range from 23 5 to 26 4 vsmow in cements and 21 3 26 8 in clasts in samples from the gunflint iron formation samples from the sokomon iron formation record deeper more evolved pore waters and in some cases contain lower 18 o cements in individual pores of gunflint samples cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed overall the 18 o of the cements are similar to the most elevated 18 o reported from cherts of similar age but span a substantially smaller range this suggests most of the lower values reported previously reflect late diagenetic conditions at greater burial depths the shallow possibly restricted seas above the gunflint and sokoman gifs appear to have been warmer and may have been lower in 18 o than present day seawater but they may not have been representative of global seawater

ion microprobe analysis of 18o 16o in authigenic and detrital quartz in the st peter sandstone michigan basin and wisconsin arch usa contrasting diagenetic histories abstract the oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures compositions and origins of pore occluding fluids during diagenesis but quartz overgrowths are too fine grained to be amenable to conventional isotopic analysis we have used a cameca ims 4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the ordovician st peter sandstone from the michigan basin and wisconsin arch midwestern usa ion microprobe isotopic analyses have been successfully accomplished with an internal precision of 1 1 and a spatial resolution of 20 30 m at low mass resolution using a high voltage offset technique repeated analyses of the quartz standard demonstrate a reproducibility of close to 1 1 sd in good agreement with that expected from counting statistics the four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated sieved and analysed as a function of sieve size by conventional bulk mineral techniques conventional and ion microprobe analyses are mutually consistent supporting the accuracy of the ion microprobe analyses within sample isotopic variations of up to 13 and micro scale isotopic variations of at least 4 over a distance of 100 m have been measured within quartz overgrowths in a sandstone from the wisconsin arch overgrowths are uniformly higher in 18o than detrital grains and gradients of up to 25 exist across a few microns 18o enriched quartz overgrowths in sandstones from the wisconsin arch show complex cl zonation and reflect one of two possible processes 1 low temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids 2 higher temperature quartz precipitation during episodic gravity driven upwelling of warm basinal fluids of comparable isotopic composition to michigan basin fluids from the illinois basin related to evolution of mississippi valley type pb zn ore forming fluids quartz overgrowths in michigan basin sandstones which derived their silica locally by pressure solution are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation detrital quartz shows significant intea grain and intergrain isotopic variation 5 13 but no significant inter sample variation consistent with a uniform source of granitoid and metamorphic quartz in space and time

oxygen isotope microanalyses of diagenetic quartz possible low temperature occlusion of pores abstract secondary ion mass spectrometry sims was used to obtain 18 o measurements on 20 m spots in authigenic and detrital quartz from mississippian sandstones of the western canadian sedimentary basin detrital quartz shows a large range in 18 o from 5 to 25 which overlaps the analyses of authigenic cements 20 34 combining these data with the burial history and precipitation sequence of minerals indicates that meteoric water may have precipitated quartz at temperatures between 15 and 35 c over a period of 180 ma 300 120ma this nearly isothermal precipitation may have occluded pores in samples from the central portion of the basin but quartz precipitation continued up 75 c at the basin margin conventional bulk analyses of these samples gave 18 o values of 13 4 for detrital quartz and 19 8 for authigenic quartz models based on bulk isotopic analyses can bias the interpretation of the timing of diagenetic quartz precipitation toward much higher temperatures the sims measurements in diagenetic silicates can identify discrete episodes of precipitation allowing a more accurate interpretation for the timing of diagenetic events

off rift volcanism in rift zones determined by crustal unloading when continents are stretched over a long period of time deep elongated rift valleys form at earth s surface and zones of ponded magma centred beneath the rift form at the crust mantle boundary1 2 ascending magma sometimes erupts within the rift valley3 4 or counterintuitively at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth5 6 7 8 the controls on the distribution of this off rift volcanism are unclear here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley whereas others are confined to the valley we find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure caused by crustal thinning and faulting along the rift borders when gravitational unloading dominates over tectonic stretching forces dykes ascending from the ponded magma are steered towards the rift sides eventually causing off rift eruptions our model also predicts the formation of stacked magma sills in the lower crust above the magma ponding zone as well as the along rift propagation of shallow dykes during rifting events consistent with observations of magmatism and volcanism in rift zones globally we conclude that rift topography induced stress changes provide a fundamental control on the transfer of magma from depth to the surface

imported quartz cement in aeolian sandstone grew from water of uniform composition but has complex zonation the origins and volumes of waters which mass transport silica in sedimentary basins remain obscure many previous analytical studies have illustrated cases where quartz cements in sandstones originate from complex and variable fluids by contrast we show by using a combination of separation and analytical techniques that in lower permian sandstones of the southern north sea the cementing fluid was isotopically uniform during growth of quartz cement with a 1b o of 19 6 1 0 v smow in this relatively uniform fluid quartz cements grew and developed complex cathodoluminescence cl zonations petrographic data show that 8 10 quartz cement locally 30 was imported into this 380 km 2 180 m thick aeolian sandstone and cement distribution controlled by depositional permeabilities we infer a large scale high volume flux of evolved meteoric fluid during 2 km deep burial and show that complex cl zonation may arise from relatively subtle changes in water composition

microspectrofluorimetric and microthermometric evidence for variability in hydrocarbon fluid inclusions in quartz overgrowths implications for inclusion trapping in the alwyn north field north sea abstract hydrocarbon inclusions trapped in quartz cement from alwyn north field sandstones of well 3 9a 6 north sea have been analyzed using microthermometric and microspectrofluorimetric methods they are two phased with consistent gas oil ratios and mainly occur along the detrital grain overgrowth boundary their homogenization temperatures vary from 60 to 129 c with a great variability within a single fluid inclusion assemblage up to 40 c the fluorescence colour of the hydrocarbon inclusions has been characterized using a microspectrofluorimeter which allows the analysis of very small inclusions 1 7 m with good reproducibility hydrocarbon inclusions display large variations in their fluorescence colour converted in chromaticity coordinates even within a single fluid inclusion assemblage moreover they are concentrically distributed within the detrital grain overgrowth interface according to the relationship between api gravity and fluorescence colour established with reference to crude oils from alwyn north field this observation is interpreted as representing compositional variations such compositional variability in a single fluid inclusion assemblage could result from either the progressive entrapment of oils in which api gravity evolves through time and or the fractionation of hydrocarbons at the time of trapping

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

timing and conditions of permian rotliegende sandstone diagenesis southern north sea k ar and oxygen isotopic data illitic clay ranging from pure illite to highly illitic illite smectite i s is the most abundant diagenetic phase in the eolian and sabkha facies of the rotliegende formation of the southern north sea and northeastern netherlands most k ar ages of diagenetic illite and i s in the eolian sandstones are between 100 and 175 ma illite formation is related to two major phases of tectonic activity the jurassic kimmerian orogenic movements and late cretaceous early tertiary inversion movements the burial depth of a sample at the time of diagenetic illite formation can be derived from the k ar age and the appropriate burial history curve illite formed in different localities at different times and at different depths generally diagenetic illite is more abundant in samples more deeply buried at the time of illite formation the correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties specifically we would expect reservoir permeability of the rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the late jurassic through the late cretaceous calculated dgr18o values of most illite forming fluids lie between 0 and 4 pmil smow indicative of meteoric or marine waters modified by interaction with rock however 18o depleted fluids as low as 5 pmil indicative of a major meteoric component are inferred to have been involved in the formation of illite in the groningen gas field at a time when nearby sections of the rotliegende were eroded and exposed at the surface

the wide angle seismic image of a complex rifted margin offshore north namibia implications for the tectonics of continental breakup highlights up to 33 km thick dominantly gabbroic crust beneath walvis ridge massive gabbro addition with subjacent cumulates to the cob south of walvis ridge slow upper mantle at 35 km depth beneath etendeka plateau 4 6 km thick oceanic crust in the angola basin north of florianopolis transform velocity models suggest a dominant tectonic control on the location of magmatism abstract voluminous magmatism during the south atlantic opening has been considered as a classical example for plume related continental breakup we present a study of the crustal structure around walvis ridge near the intersection with the african margin two wide angle seismic profiles were acquired one is oriented nnw sse following the continent ocean transition and crossing walvis ridge a second amphibious profile runs nw se from the angola basin into continental namibia at the continent ocean boundary cob the mafic crust beneath walvis ridge is up to 33 km thick with a pronounced high velocity lower crustal body towards the south there is a smooth transition to 20 25 km thick crust underlying the cob in the walvis basin with a similar velocity structure indicating a gabbroic lower crust with associated cumulates at the base the northern boundary of walvis ridge towards the angola basin shows a sudden change to oceanic crust only 4 6 km thick coincident with the projection of the florianopolis fracture zone one of the most prominent tectonic features of the south atlantic ocean basin in the amphibious profile the cob is defined by a sharp transition from oceanic to rifted continental crust with a magmatic overprint landward of the intersection of walvis ridge with the namibian margin the continental crust beneath the congo craton is 40 km thick shoaling to 35 km further se the velocity models show that massive high velocity gabbroic intrusives are restricted to a narrow zone directly underneath walvis ridge and the cob in the south this distribution of rift related magmatism is not easily reconciled with models of continental breakup following the establishment of a large axially symmetric plume in the earth s mantle rift related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism dividing the margin in a magma dominated southern and an essentially amagmatic northern segment

the onset of walvis ridge plume influence at the continental margin highlights crustal structure of walvis ridge reveals high seismic velocities in the lower crust intruding the african continent this modified crust is localized to approx 100 100 km within the continent no indication for a large plume head observed the opening of the south atlantic is a classical example for a plume related continental breakup flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at tristan da cunha to determine the effect of the proposed plume head on the continental crust we acquired wide angle seismic data at the junction of the walvis ridge with the african continent and modelled the p wave velocity structure in a forward approach the profile extends 430 km along the ridge and continues onshore to a length of 720 km crustal velocities beneath the walvis ridge vary between 5 5 km s and 7 0 km s a typical range for oceanic crust the crustal thickness of 22 km however is approximately three times larger than of normal oceanic crust the continent ocean transition is characterized by 30 km thick crust with strong lateral velocity variations in the upper crust and a high velocity lower crust hvlc where velocities reach up to 7 5 km s the hvlc is 100 to 130 km wider at the walvis ridge than it is farther south and impinges onto the continental crust of the kaoko fold belt such high seismic velocities indicate mg rich igneous material intruded into the continental crust during the initial rifting stage however the remaining continental crust seems unaffected by intrusions and the root of the 40 km thick crust of the kaoko belt is not thermally abraded we conclude that the plume head did not modify the continental crust on a large scale but caused rather local effects thus it seems unlikely that a plume drove or initiated the breakup process we further propose that the plume already existed underneath the continent prior to the breakup and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts