nature of the continent ocean transition on the non volcanic rifted margin of the central great australian bight a region of 50 120km width defines the continent ocean transition cot in the central great australian bight it is characterized by a thin apron of post break up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the cot and basement ridge modelled gravity and magnetic data and features interpreted from seismic data are consistent with aspects of extensional and break up models proposed for the west iberia margin many of the features and relationships observed beneath the outer margin of the central great australian bight can be explained by extension within a lithosphere scale pure shear environment involving four layers brittle upper crust and upper mantle and ductile lower crust and lower lithospheric mantle the cot is interpreted to be underlain by extended continental lithosphere thus the continentocean boundary is unequivocally defined between oceanic crust and the cot and appears to be associated with sea floor spreading magnetic anomaly 33 indicating that break up and sea floor spreading did not commence until c 83 ma early carnpanian time later than the currently accepted 95ma age the major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting the magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly but one that does not relate to break up and sea floor spreading

seismic stratigraphy and sediment distribution on the wilkes land and terre adelie margins east antarctica abstract seismic reflection data show the existence of two major sedimentary basins along the continental margin of wilkes land and terre adelie east antarctica that contain more than 5 s twt 9 km of sediments four seismic megasequences are identified ms4 to ms1 that are bounded by basement unconformities of interpreted turonian maastrichtian and early middle eocene age and the seafloor the 4 5 km thick rift and pre rift sediments are concentrated in a margin parallel basin sabrina basin on the basis of seismic correlation with the australian margin this basin is interpreted to be of late jurassic to mid cretaceous age the post rift sediments are generally thick along the margin and in the adjacent deep ocean basin but are particularly thick in a major depocentre off west wilkes land named here the budd coast basin bcb the bcb contains a maximum observed thickness of 5 s twt 9 km of post rift sediments and its location suggests that the sediments were largely derived from a sub glacial basin currently occupied by the totten glacier within the post rift sediments a major regional unconformity is identified that is interpreted to be of early middle eocene age 45 ma while some previous interpretations have suggested that this unconformity marks the onset of the glaciation of east antarctica the unconformity is interpreted here to be the product of the major global plate reorganisation in the middle eocene and the coincident sharp increase in the sea floor spreading rate between australia and antarctica a seismic unconformity that can be correlated with the onset of glaciation is not identified and the position of glacial onset within the sedimentary section can only be estimated through the extrapolation of sedimentation rates from the underlying section and from deep sea drilling project site 269 the post middle eocene section is much thicker than the underlying post rift section and this is interpreted to be largely due to the very high sediment fluxes associated with the period of polythermal glaciation from 34 9 ma sediment isochron and isopach maps derived from a regional seismic grid show that the depocentre of the bcb trends approximately north south beneath the continental slope but is deflected to the east beneath the deep ocean basin this distribution indicates that deep marine processes were important in lateral along margin sediment transport a west to east direction of movement is inferred by the sediment distribution this direction is opposite to the present day marginal currents along the continental slope and rise

pyrite from concretion pipes in mesozoic shales mineralogical and chemical evidence of hydrothermal origin abstract the origin of mineralized concretion pipes pyrite ba sr sulphates ankerite and other carbonates found in the jurassic black shales of southeastern france the terres noires formation is ascertained in terms of the characteristics of pyrite the morphology 1 is euhedral the habit 2 is cubic or pentagonododecahedral the size of the pipes 3 ranges from a few cubic micrometres to a few cubic centimetres the chemical composition 4 includes 130 700 ppm co 0 7490 ppm ni 100 2490 ppm as up to 9070 ppm cu and up to 300 ppm zn and se sulphur isotopes 34 s cdt 5 7 1 compared to 15 1 17 for jurassic waters and the concentric spatial arrangement of pyrite crystals 5 involves a shale impregnation and is consistent with a synsedimentary exhalation process a hypothetical model of concretion mineralization development is proposed which involves a change from an arseniferous chemistry to a nickeliferous chemistry the concretion pipes could be good indicators of fluid circulations and thermal anomalies in palaeosedimentary environments such as passive continental margins

why are marine magnetic anomalies suppressed over sedimented spreading centers the absence of lineated marine magnetic anomalies over sedimented recent spreading centers has been observed in several areas we propose that magnetic anomalies can be suppressed as a result of pervasive hydrothermal reactions underneath thick blankets of sediment the relatively impermeable sediment cover produces comparatively closed hydrothermal systems which increase the residence time of the hot fluids in basaltic layer 2 causing more thorough leaching of the remanence carrying iron titanium oxides and diminishing the marine magnetic anomalies this contrasts with lesser alteration at sparsely or unsedimented zones of extension with more open circulation conversely spreading ridges and oceanic crust characterized by absent or subdued lineated anomalies may signal sites of extensive hydrothermal mineralization this process might also provide an explanation for some magnetic quiet zones such as in the gulf of california the gulf of aden and the northern red sea

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

geochemical investigation of oils and source rocks from qianjiang depression of jianghan basin a terrigenous saline basin china abstract six oils and seven suspected source rocks from the jianghan saline basin in the central eastern part of china have been examined using a variety of geochemical techniques the results from this study show that oils from the third and fourth sections of the qianjiang formation eocene are derived from similar source materials but have a different source from the oils of xingouzui formation eocene in the same basin the oils of the xingouzui formation eocene are more mature and probably have a greater input of terrigenous source material and in all probability have migrated greater distances than the qianjiang oils oils from these sections are derived from an environment characterized as being extremely anoxic strongly reducing and highly saline the suspected source rocks examined in the study were from the qianjiang formation and were generally mature except for the shallowest sample the biomarker distributions in the suspected source rocks examined from the qianjiang formation were very different from those of the oils present in the same sections it is proposed that oils from the qianjiang formation have sources in addition to or exclusive of the suspected source rocks examined in this study

structure and significance of c30 4 methyl steranes in lacustrine shales and oils abstract the structure of the abundant c 30 4 methyl steranes of ancient freshwater lacustrine sediments and derived oils is of interest as their precursors the 4 methyl and in particular 4 23 24 trimethyl sterols dinosterols are reported to be specific to dinoflagellates this study employs a novel liquid crystal gc phase combined with gc ms and gc ms ms to separate and characterise mixtures of c 30 4 methyl sterane standards not resolvable on non polar phases this technique is then applied to identify the c 30 4 methyl steranes in an ancient freshwater sediment a freshwater lacustrine and a marine derived oil a comparison is also made with the 4 methyl steranes in marine sediments containing varying amounts of dinoflagellate cysts the c 30 4 methyl steranes of the freshwater lacustrine samples have been shown to possess a 4 methyl 24 ethylcholestane structure while both 4 methyl 24 ethylcholestanes and 4 23 24 trimethyl 5 h cholestanes dinosteranes were found in the marine dinoflagellate rich sediments while still indicative of dinoflagellates 4 methyl 24 ethyl sterols are generally not the major dinoflagellate sterol and alternative explanations for the abundance of 4 methyl 24 ethylcholestanes in ancient lacustrine sediments should be considered possibly ancient freshwater dinoflagellates synthesised principally the 4 methyl 24 ethyl form alternatively a source from an unrecognized algal class is possible while a source from bacteria which are known to synthesise certain 4 methylsterols is discussed

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

complex patterns of steroidal biomarkers in tertiary lacustrine sediments of the biyang basin china gas chromatography gc gas chromatography mass spectrometry gc ms and gas chromatography mass spectrometry mass spectrometry gc ms ms with co injected synthetic standards were used to analyse the biomarker patterns of some tertiary lacustrine clayey dolomites from the biyang basin china the lithology low pr ph ratio and high gammacerane content of these sediments indicated that high salinity prevailed during their deposition the distributions of steroidal hydrocarbons were particularly unusual and several pseudohomologous series including regular steranes c27 c29 4 methyl steranes c28 c30 including dinosteranes 3b ethyl steranes c29 c31 lanostanes c30 c32 and a variety of other 3b alkylated steranes were identified 3b n propylcholestane 3b n propylstigmastane and 4 4 dimethylcholestane were identified using authentic standards and this is the first time these compounds have been unambiguously characterised in sediments crown copyright 2001 published by elsevier science ltd all rights reserved

evidence for unusually thin oceanic crust and strong mantle beneath the amazon fan we used seismic and gravity data to determine the structure of the crust and mantle beneath the amazon fan seismic data suggest that the crust is of oceanic type and is unusually thin 4 km compared to elsewhere in the atlantic we attribute the thin crust to ultraslow seafloor spreading following the breakup of south america and africa during the early cretaceous gravity data suggest that the fan was emplaced on lithosphere that in creased its elastic thickness te and hence strength following rift ing the increase from 10 km to 40 km is greater however than would be expected if te were determined by a single controlling isotherm based on a cooling plate model hence we conclude that the amazon fan has been emplaced on and is supported by un usually thin oceanic crust and strong mantle

subsidence history crustal structure and thermal evolution of the valencia trough a young extensional basin in the western mediterranean the valencia trough is an extensional basin that began rifting during the late oligoceneearly miocene yet it is located within a region of compression between the european and african plates seismic reflection seismic refraction well gravity and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough although the trough appears symmetric on bathymetric maps seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a transect of the trough between the catalan coastal ranges and mallorca there is good agreement between the predicted depth to the mono based on a stretching model and the seismically constrained depth beneath the catalan margin but poor agreement beneath mallorca the seismic moho beneath mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust fold loading geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle the catalan margin is characterized by a thermal and mechanical boundary which marks the transition between cold unstretched lithosphere and hot thin lithosphere while mallorca is located above a broad region of lithospheric thinning the subsidence history and amount of crustal thinning in the catalan margin resemble that of typical atlantic type nonvolcanic margin however a narrow transition zone 60 km the lack of any evidence for a conjugate margin and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings

early diagenesis and its relationship to depositional environment and relative sea level fluctuations upper cretaceous marshybank formation alberta and british columbia early diagenesis of the upper cretaceous late coniacian to early santonian marshybank formation was controlled by depositional environment composition of depositional water fe and organic content of the sediment sedimentation rate proximity to the shoreline and influx of meteoric water related to relative sea level fall five depositional environments each characterized by a distinct early diagenetic mineral assemblage have been recognized offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions composed of early calcite and siderite intense sulphate reduction promoted by the dysaerobic depositional water was the primary influence on early diagenesis offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions composed of two generations of siderite s1 and s2 in this environment methanogenesis rather than sulphate reduction was more important early diagenesis of the inner shelf sands was generally limited however in sands deposited proximal to the shoreline mixing of marine and meteoric waters promoted crystallization of fe rich chlorite and siderite the shoreface was characterized by dissolution of detrital minerals in the upper portion and precipitation of kaolinite or illite smectite in the lower portion in the coastal plain environment brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite ankerite rather than siderite is the typical early diagenetic carbonate the 18o values of the earliest cements i e early calcite siderite s1 inner shelf siderite indicate crystallization from a low 18o marine derived porewater assuming crystallization at 25 c a 18o value of about 7 smow can be estimated for the seaway during marshybank formation time similar calculations for the overlying dowling member puskwaskau formation suggest that the 18o value of the seaway increased to about 4 smow consistent with its transgressive nature very low 18o values are exhibited by siderite s2 these results indicate crystallization during intermediate diagenesis 60 c from meteoric water 15 smow that entered the marshybank formation during sea level lowstand

a marine geophysical study of the wilkes land rifted continental margin antarctica summary the wilkes land margin of east antarctica conjugate to the southern australian margin is a non volcanic rifted margin that formed during the late cretaceous during 2000 01 and 2001 02 geoscience australia acquired 10 000 line km of seismic reflection and refraction magnetic anomaly and gravity anomaly data over the margin we have used the seismic data to estimate the sediment thickness along the margin the data reveal a deep 11 km rift basin that contains over 9 km of sediments seaward of the totten glacier western wilkes land the limits of oceanic and continental crust that underlies the thick post rift and the significantly thinner pre and syn rift sediments are equivocal seismic reflection data suggest a 30 100 km wide continent ocean transition zone cotz along the margin the cotz extends over 400 km seaward of the shelf break off the eastern wilkes land terre ad elie sector

sulphur speciation in anoxic hypersaline sediments from the eastern mediterranean sea abstract the anoxic hypersaline tyro and bannock basins are among the most sulphidic bodies of water in the marine environment h2s 2 mm we report the distribution of elemental sulphur acid volatile sulphur avs organic polysulphides humic sulphur 0 5 m naoh extractable and pyritic sulphur in the sediments from these basins pyritic sulphur appears to be the main phase of inorganic reduced sulphur 50 80 of the total sulphur pool and is at the same level about 250 moles per gram dry weight in cores recovered from the two basins remarkably humic sulphur has been found to account for 17 28 of the total sulphur pool in the tyro basin and for 10 43 in the bannock basin sulphur isotope data show negative 34s values for both pyritic sulphur 34s 19 c to 39 and humic sulphur 34s 15 to 30 this indicates that both pyritic and humic sulphur originate from microbially produced hzs neither basin shows a significant correlation between pyritic sulphur and organic carbon suggesting syngenetic pyrite formation additionally the degree of pyritization in these sediments dop is 0 62 indicates that pyrite formation is limited by the reactivity of fe humic sulphur correlates with the pyritic sulphur distribution and seems to be related with gelatinous pellicles bacterial mats at the interface between oxic seawater and brine both pyritic and humic sulphur can be formed in the water column at the interface of oxic seawater and brine by a syngenetic pathway about 5 of the pyrite is formed in the sediments by diagenetic processes the presence of a bacterial mat and high hzs content in the brines favour such conditions however most of the humic sulphur inventory can be supported by diagenetic processes although these basins differ in their major element chemistry their reduced sulphur species chemistry appears to be similar

benchmarks and sediment source s of the 1755 lisbon tsunami deposit at boca do rio estuary article standardizing the signature of tsunami deposits has been identified as a major limitation for the identifica tion of paleo tsunami deposits this limitation mostly arises from the strongly source dependent nature of these deposits which in turn determines their composition and depositional architecture and from the effect of the local morphology of the corresponding depositional environment here we provide new high resolution mineralogical geochemical and micro macrofauna data of the 1755 tsunami layer of boca do rio estuary algarve portugal with the aim of unraveling the signatures of estuarine tsunami deposits and linking them to possible sediment sources we also apply for the first time diffuse reflectance spectropho tometry drs analysis our results show that the 1755 tsunami deposit of the boca do rio estuary is featured by an enhancement in sr and ca which are linked to the input of biogenic and detrital carbonates shell frag ments and limestone clasts from the beach foreshore and a strong depletion in most terrestrial and marine sensitive indicators the latter is interpreted as resulting from the reworking of the estuarine clays and subsequent dilution within a huge volume of sand eroded from the coastal barrier it confirms that in the case of the boca do rio estuary the sediment source is essentially proximal and coastal textural and min eralogical features between the base and the top of the tsunami layer suggest the imprint of run up and back wash currents derived from a unique wave micro and macrofauna analysis and drs data of the siliciclastic fraction show slight but significant environmental changes occurring just after the tsunami which could be provoked by an eventual closure of the estuary mouth

paleoreconstruction by biological markers abstract during diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons the following four basic events needed for paleoreconstruction can be monitored by biological markets 1 sourcing 2 maturation 3 migration and 4 biodegradation actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated cretaceous and phosphoria sourced oils in the wyoming thrust belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry identification of recently discovered biological markers head to head isoprenoids allows subtle source differentiation between oils from sumatra the degree of crude oil maturation in basins from california alaska russia wyoming and louisiana can be assessed by specific novel biomarker ratios 20s 20r sterane epimers field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock an example of immature sumatra rock is given extensive migration is documented by biomarkers in several oils the effect of biodegradation on biomarker steranes and terpanes is exemlified in a california basin all biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo reconstruction

iron oxyhydroxide and sulphide mineralization in hydrocarbon seep related carbonate submarine chimneys gulf of cadiz sw iberian peninsula this paper aims to investigate the iron mineralization hosted in the submarine hydrocarbon seep related carbonate chimneys from the gulf of cadiz continental slope sw iberia chimneys are made of a general fine groundmass of major fe rich dolomite ankerite and calcite small grains of quartz and phyllosilicates and some foraminifer tests and ostracod shells primary porosity is frequently generated inside foraminifer chambers and ostracod shells and is filled with closely packed microcrystals of iron oxyhydroxide and sulphide minerals forming framboids some extremely peculiar multiframboidal textures are also detected corresponding to microcrystal groups spherical framboids and euhedral crystals without framboidal texture the mineralogy observed is mainly goethite pyrite pseudomorphs traces of tiny grains of pyrite were also observed cubic octahedral and pyritohedral are the habits observed in both minerals chemical analyses of framboids and euhedral crystals display high often erratic amounts of as co ni and mo in oxyhydroxides and mo pb v and co in sulphides versus almost always negligible concentrations of cu and zn a textural geochemical and mineralogical evolution is proposed to explain the coexistence of different morphologies in the multiframboidal texture a growth and aggregation of microcrystals as typical framboidal type mineralogical associations b development of euhedral habits c coalescence and homogenization of the microcrystal into large size euhedral to anhedral crystals and d formation of euhedral crystals or polycrystalline masses with complete loss of framboidal texture along this process an increase of the concentration of fe s mn and ti linked to a decrease in mg si al as p ca and v was observed the study of the iron mineralization in the gulf of cadiz is a first and can give clues to understand the complex geobiological interactions in this and other similar extreme hydrocarbon bearing submarine ecosystems

pyritization a palaeoenvironmental and redox proxy reevaluated abstract pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments two sites 40 m apart with very different physicochemical properties were sampled seasonally the sites differed from each other in 1 the amount of tidal flushing 2 vegetation 3 redox condition below the sediment water interface 4 carbon content and 5 solid phase iron oxyhydroxide and iron sulfide content of the sediment the calculated degree of pyritization dop varies between 0 44 and 1 0 depending on the site and the season different dop values were obtained for the same samples when different techniques were applied for extraction of reactive iron framboidal pyrite was dominant however other textural forms of pyrite were also observed the size distribution of the individual pyrite crystals and the framboids was fairly narrow with the majority of them having diameters

biodegradation of tar sand bitumens from the ardmore and anadarko basins carter county oklahoma abstract a series of tar sand bitumens collected from a single shallow well in the ardmore basin carter county oklahoma has been characterized by various geochemical techniques particular emphasis has been placed on determination of variations in biomarker distributions both hydrocarbons and porphyrins as a function of biodegradation the tar sand bitumens have been severely biodegraded and most of the n alkanes low molecular weight cycloalkanes isoprenoid alkanes c27 c29 steranes light aromatics and sulfur compounds have been removed in addition the hopane distributions have been altered to differing degress with the pseudohomolugoues above c30decreasing in concentration prior to the c20 21 the triaromatic steroid hydrocarbons were also altered with preferential removal of 20r epimers c20 21 and c27 28 diasterane and c30 sterane distributions appear to be relatively unaffected by biodegradation the high resistance of tricyclic terpanes the c24 tetracyclic terpane and monoaromatic steroid hydrocarbons to biodegradation indicate that the distributions of these compounds are well suited to serve as correlation parameters in heavily altered samples it was also observed in this study that the main ring structure of the porphyrin molecule was not affected by these very high levels of biodegradation previously hypothesized changes in porphyrin structure due to biodegradation have included n alkyl side chain cleavage and possible conversion from dpep to etio type porphyrins an investigation has also been made with an active seep collected from sulphur in the anadarko basin oklahoma and the results show that the relative rate of biodegradation of various classes of biomarkers differs between these two locations although it was proposed that the original oil responsible for these two seeps is derived from the same source rock formation

factors controlling the style of continental rifting insights from numerical modelling passive continental margins similarly formed by lithospheric stretching and rifting can differ significantly in terms of width ranging from 50 to 400 km or subsidence history our interest lies in determining what properties of the lithosphere and what stress and strain boundary conditions control this regional variability and the siting of rupture continental breakup occurs probably as the result of a stretching instability which develops in a zone of material or geometrical heterogeneity to analyze the factors controlling these instabilities we use a numerical thermo mechanical model of lithosphere extension based on a finite element program the particular program was chosen because it combines constitutive models appropriate to our knowledge of lithosphere rheology elasticity plasticity and nonlinear temperature dependent creep with a kinematic formulation for large inelastic strain we choose to weaken the plate by locally increasing the crustal thickness by a very small amount and examine how this defect or perturbation acts to localize the deformation we compare the geometry and timing of necking for relatively wet versus dry rheology cold versus warm geotherm and fast versus slow strain rate we examine as well how the width and amplitude of the perturbation itself affects the deformation history since a velocity boundary condition is applied the required extensional forces are evaluated and compared to estimates of plate driving forces conditions that favor deformation by plasticity rather than creep cold geotherms dry rheologies lead to a more localized extension and rapid rupture but the forces involved 1013 n m are larger than typical plate driving forces increasing the amplitude of the defect i e crustal thickness contrasts accelerates necking as well cooling of the thinned area becomes significant at low extension rates depending on the initial rheology cooling simply delays the moment when deformation becomes unstable or conversely inhibits necking

pyrite framboids and their development a new conceptual mechanism it has been suggested that the two morphologies of sedimentary pyrite framboids and euhedra may reflect two distinct pathways of pyrite formation framboids form indirectly via iron monosulphides whereas euhedra form from direct precipitation from solution a third pathway which is bridging these two forms is proposed here namely the continuous growth from a monosulphide globule through framboids to a euhedral single crystal it is also suggested that framboids probably occur over a range of three orders of magnitude from the least complex microframboids through framboids to polyframboids

a transect across australia s southern margin in the otway basin region crustal architecture and the nature of rifting from wide angle seismic profiling abstract the otway basin in southeastern australia formed on a triangular shaped area of extended continental lithosphere during two extensional episodes in cretaceous to miocene times which ultimately led to the separation of australia and antarctica the velocity structure and crustal architecture of the otway continental margin has been interpreted from offshore onshore wide angle seismic profiling data along a transect extending from near the northern otway basin margin with palaeozoic outcrop to the deep ocean basin under the southern ocean along this transect the otway continental margin ocm transect the onshore half graben geometry of early cretaceous deposition gives way to a 5 km thick basin sequence p wave velocity 2 2 4 6 km s extending down the continental slope offshore to at least 60 km from the shoreline at 120 km from the nearest shore sonobuoy data indicate a 4 5 km sedimentary sequence overlying 7 km of crustal basement rocks above the moho at 15 km depth water depth 4220 m conspicuous strong moho reflections are evident under the continental slope at about 10 2 s twt basement is interpreted to be attenuated faulted palaeozoic rocks of the delamerian and lachlan orogens intruded with jurassic volcanics that thin from 16 km onshore to about 3 5 km at 120 km from the nearest shore these rocks comprise a 3 km section that has a velocity of 5 5 5 7 km s overlying deeper basement with a velocity of 6 15 6 35 km s over the same distance the moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore and then to about 12 km in the deep ocean at the limits of the profile water depth 5200 m the continent ocean boundary cob is interpreted to be at a prominent topographic inflection point at the bottom of the continental slope in 4800 m of water p wave velocities in the lower crust are 6 4 6 8 km s above a transition to the moho with an upper mantle velocity of 8 05 km s there is no evidence of massive high velocity 7 km s intrusives underplate material in the lower crust nor any syn rift or early post rift subaerial volcanics indicating that the otway continental margin can be considered a non volcanic margin similar in many respects to some parts of the atlantic ocean margins e g the nova scotia newfoundland margin off canada and the galicia bank off the iberian peninsula

a geophysical transect of the adelie margin east antarctica abstract in 1982 a geophysical survey of the antarctic margin including multichannel seismic gravity magnetic and bathymetric surveying was carried out off adelie coast wilkes land and in the eastern ross sea of the 5000 km of lines recorded 3000 km were in the adelie coast area lines atc 101 102 approximately following meridian 138 e show the first complete transect of the adelie coast margin from the southeast indian abyssal plain at dsdp site 269 to the continental shelf these lines reveal a thick sedimentary series divided into three main acoustic units by two major unconformities considered to be upper eocene 42 ma and late oligocene 25 ma oceanic or continental basement can be traced under the whole area and the ocean continent boundary clearly lies beneath the lower continental slope a deep high amplitude low frequency horizon extending under the oceanic basement for over 300 km of line is considered to be the moho our interpretation supports a recent revision be cande and mutter proposing an early upper cretaceous opening between australia and east antarctica

dinoflagellate origin for sedimentary 4α methylsteroids and 5α h stanols biological marker compounds provide useful tools for evaluating the depositional environments of recent and ancient sediments and petroleums1 largely through established relationships between the organic matter of sediments and source organisms1 2 such relationships are best tested and explored by investigating the lipid components of natural populations of autochthonous biota that can be clearly recognized as contributors to underlying bottom sediments in this context we have studied the lipids of the freshwater dinoflagellate peridinlum lomnickii woloszynska order peridiniales class dinophyceae collected from the waters of priest pot3 a eutrophic lake in the english lake district its distributions of both 4 methylsterols and 4 methylstanones closely resemble those of the underlying bottom sediments demonstrating that dinoflagellates are important contributors of these sedimentary compounds the 4 methylsteroidal hydrocarbons found in ancient sediments and petroleums4 6 are presumably diagenetic products of such 4 methylsteroids and therefore reflect dino flagellate inputs to the original depositional environments furthermore the prominence of 5 h cholestan 3 ol in p lomnickii suggests that dinoflagellates may be the long sought direct biological source of sedimentary 5 h stanols

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

sulphur isotope geochemistry of pyrite from the upper cretaceous marshybank formation western interior basin abstract variations in the texture and sulphur isotopic composition of pyrite in the upper cretaceous marshybank formation are linked to depositional environment abundant framboidal and lesser euhedral pyrite precipitated in offshore marine rocks moderate quantities of pyrite also crystallized in brackish coastal plain rocks however in contrast to marine pyrite coastal plain pyrite is dominantly euhedral in texture reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration in the marine rocks pyrite 34s values range from 35 7 to 27 4 avg 4 8 canyon diablo troilite cdt pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition 49 8 to 10 6 cdt however isotopic values are often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments pyrite in coastal plain rocks is characterized by relatively high 34s values 4 2 to 35 5 avg 13 2 cdt while the overlying sideritized conglomerates have the lowest 34s values reported for cretaceous rocks from the western interior basin of north america 49 8 to 41 7 cdt very low 34s values which are only observed in the marine rocks are indicative of microbial sulphate reduction and pyrite formation in a sulphate replete i e open system higher 34s values up to 18 which were obtained for both the marine and coastal plain rocks are indicative of progressive pyrite crystallization in a sulphate limited i e closed system such conditions are expected in marine sediments as burial occurs and in brackish i e low sulphate sediments pyrite with very high 34s values 18 is common in the coastal plain rocks these high values are the result of influx of 34s enriched residual sulphide derived from overlying marine units a minor amount of 34s enriched pyrite is also present within and proximal to marine sedimentary unit h which was deposited at the same time as the coastal plain rocks we hypothesize that porewater containing 34s enriched residual sulphide flowed from the coastal plain and along unit h into the marine sediments thus producing anomalously high 34s pyrite

zechstein main dolomite oil characteristics in the southern permian basin i polish and german sectors abstract geochemical analyses were used to classify 39 zechstein late permian lopingian main dolomite ca2 crude oil samples from fields in the eastern and southern sector of the southern permian basin spb of europe and to provide new insights into the origin of the oil geochemical data indicate that ca2 oils were generated in the early to late oil window and are mostly non waxy oils various biomarker and stable carbon isotopic ratios were used to identify source and depositional settings for source rocks of ca2 oils arranged within 10 distinct oil groups specifically the geochemical analyses and oil oil correlations revealed a set of characteristic biomarkers including an even over odd predominance eop for the c20 30 n alkanes c40 carotenoid occurrence isorenieratane chlorobactane isorenieratane bisnorhopane hopane bnh h ratios 0 1 high abundances of c35 homohopanes and elevated concentrations of c32 and c34 homohopanes a predominance of c29 homologues among 4 desmethyl steranes in the majority of oil samples and a high abundance of diasteranes stable carbon isotopes and biomarkers provided ample evidence that ca2 oils were generated from predominantly algal rich marly carbonate evaporite source rocks located in the lower slope shallow basin and lagoonal facies of the ca2 basin all deposited under suboxic anoxic euxinic conditions in the case of all higher maturity oils the source rocks could not be reliably identified but high 2 c24tet c23 values suggest a carbonate evaporite depositional setting

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

genesis of large siliceous stromatolites at frying pan lake waimangu geothermal field north island new zealand lilypad stromatolites up to 3 m long and 1ae5 m wide were found to be actively growing in the shallow marginal waters of frying pan lake and its outflow channel these stromatolites composed of phormidium 90 fischerella and a variety of other microbes develop through a series of distinct growth stages dark green microbial mats cover the floor of the outflow channel and give rise to columns of various sizes and shapes in the shallower marginal waters once the columns reach the water level the mats spread laterally to form a lilypad stromatolite the lilypads are characterized by a raised dark green rim 4 5 mm high that encircles a flat interior covered with a distinctive orange red mat the microbes forming the columns and lilypad plate are being actively silicified the stromatolites are formed of i flat lying phormidium filaments p laminae ii upright filaments of phormidium that are commonly associated with fischerella u laminae and iii mucus diatoms and pyrite framboids m laminae p laminae dominate most of the columns with tripartite cycles of p u to m laminae being found mostly in the upper parts of the stromatolites the transition from the p to u laminae is marked by a change in the growth pattern of the phormidium and branching of fischerella which was probably triggered by a change in environmental conditions in the frying pan lake outflow channel this change may be related to fluctuations in water level and flow rates that are caused by periods of heavy rain seasonal changes long term variations in rainfall and or the unique 40 day hydrological cycle that exists between frying pan lake and inferno crater which is a nearby hydrothermal crater lake

an organic geochemical investigation of crude oils from shanganning jianghan chaidamu and zhungeer basins people s republic of china abstract eighteen oils from four different oil producing basins in china have been characterized using a variety of organic geochemical techniques the source materials for these oils were deposited under a variety of environmental conditions and this study investigates the use of geochemical parameters in differentiating the resultant oils the parameters discussed are based on relative amounts and distributions of both individual and groups of biomarkers including organosulphur compounds and the carbon isotopic composition of the aliphatic and aromatic fractions examination of the oils showed that those from the zhungeer basin thought to be derived from source rocks formed in a brackish lacustrine environment possessed high concentrations of tricyclic terpanes relative to pentacyclic hopanes and a predominance of 24 methyl and 24 ethylcholestanes c28 and c29 respectively over cholestanes c27 which were virtually absent oils believed to be derived from source rocks deposited in the saline depositional environment of the jianghan basin were characterized by a large amount of gammacerane relative to 17 21 30 homohopanes c31 22r and 22s in general those oils considered to be derived from source rocks deposited in fresh water lacustrine environments shanganning and chaidamu basins had small amounts of tricyclic terpanes relative to pentacyclic hopanes the presence of an unknown c30 pentacyclic terpane small amounts of extended hopanes c31 c35 relative to 17 21 hopane and relatively large amounts of 24 ethylcholestanes 20r and 20s relative to total stearanes it appears therefore that the distribution of biomarkers may be used to differentiate oils derived from different lacustrine environments

sulfur rich petroleum derived from lacustrine carbonate source rocks in bohai bay basin east china abstract crude oils produced from the zhanhua depression bohai bay basin are high in sulfur and differ from most lacustrine oils in this basin and elsewhere physical properties biomarker ratios and aromatic hydrocarbon assemblages reveal that they are derived from carbonate source rock horizons representative source rock samples with abundant organic matter toc 1 and s 2 5 mg hc g rock were selected for further correlation and their high bitumen contents indicated their indigenousness to saline or carbonate depositional environments the results of oil source rock correlation suggest that these sulfur rich heavy oils originate from es 3 4 carbonates and marls limited to the bonan sag maturity assessments with burial history showed that they were expelled at the early oil generation window high levels of 20s 5 14 17 and 5 14 17 steranes together with low values for the ts ts tm ratio and two methylated aromatic ratios mdr and mpi 1 were observed in these oils sulfur associated with evaporite and carbonate environments is a significant factor not only in controlling the occurrence of biomarker isomers but also for petroleum formation at abnormally low thermal stress levels

high abundance of carotanes in the brackish saline lacustrine sediments a possible cyanobacteria source abstract the organic rich oil shales of low maturity and tight oils from the middle permian lucaogou formation were formed in a brackish saline lacustrine environment in the jimusaer sag junggar basin they are characterized by remarkably abundant carotane and carotane the biological sources and favourable sedimentary environments of carotanes formation remain obscure organic geochemical analyses of 17 oil shales and 20 tight oils from the jimusaer sag reveal that reducing evaporitic and carbonate rich shallow water environment is favourable for carotane accumulation geochemical studies show that carotane abundance is controlled by both source of organic matter and sedimentary environment rather than thermal maturity in the study area even though both higher plants and microalgae are reported to be precursors of carotanes cyanobacteria may be the main biological sources of carotanes in the lucaogou formation based on cyanobacteria derived biomarkers such as the c39 2 6 dimethyl benzene substituted isoprenoid 2 methylhopanes 7 and 8 monomethylhepadecanes the airborne volcanic ash containing ample elements e g mg and fe caused by the intermittent volcanic eruptions during the middle permian most likely contributed to the flourish of cyanobacteria in the study area

novel cyclised and aromatised diagenetic products of β carotene in the green river shale abstract a suite of novel diagenetic products of carotene has been identified in an extract of a sample from the green river shale identification is mainly based on comparison of mass spectra with those of diagenetic products of another carotenoid isorenieratene these compounds were formed by i cyclisation and aromatisation of the polyene isoprenoid chain ii explusion of toluene and m xylene from the polyene isoprenoid chain and iii aromatisation of the 1 1 5 trimethylcyclohex 5 enyl moieties resulting in 1 2 dimethylbenzene end groups reduction of the double bonds of carotene is the most important diagenetic process however as judged from the high relative amount of carotane 63 of quantified diagenetic products of carotene these results indicate that the fossilisation potential of carotenoids is greater than previously thought

australian antarctic depression from the mid ocean ridge to adjacent continents a morphotectonic depression some 15 106 km2 in area and centred on the australia antarctic discordance1 extends across the southern half of australia through the south east indian ocean to wilkes land in antarctica fig 1a b the depression is co extensive with a negative satellite free air gravity anomaly2 which suggests that the region is underlain by downward convecting asthenosphere here i outline the depression and explore its age in a pre breakup 55 myr reconstruction fig 1c the eastern highlands of australia and the transantarctic mountains are colinear because the eastern highlands were notably high during the late cretaceous and shared with the transantarctic mountains a history of volcanism in the jurassic both highlands probably constituted the eastern flank of the past depression in the centre of which thick 8 km late cretaceous sediment in the ceduna saddle3 and a possible mirror image in offshore antarctica mark its depocentre a broad area of late carboniferous marine sediments in southern australia4 may represent the earliest sign of the depression

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

warm humid paleoclimate control of salinized lacustrine organic rich shale deposition in the oligocene hetaoyuan formation of the biyang depression east china abstract lacustrine organic rich shales are widely developed within the oligocene biyang depression in the nanxiang basin in east china and across southeast asia these shales have been proven as hydrocarbon source rocks of crude oil as well as sources and reservoirs of shale oil understanding the deposition of these shales is essential to reconstruct the paleoenvironment and paleoclimate during the oligocene in east china as well as target the potential shale oil resources based on x ray diffraction organic and inorganic petrologic organic and inorganic geochemical and stable isotopic analyses paleoenvironment paleoclimate and hydrocarbon potential of the lacustrine organic rich shales in the oligocene hetaoyuan formation were investigated the mineral composition of hetaoyuan formation organic rich shales is dominated by clay minerals followed by quartz plagioclase calcite dolomite pyrite and gypsum six lithotypes are identified including siltstone silty shale clay shale calcareous shale dolomite shale and massive mudstone the organic matter om of the organic rich shales is dominated by type i kerogen lamalginite maturity parameters tmax biomarker ratios and calculated vitrinite reflectance suggest that the om enters the oil window maturity high toc and generative potential together with high contents of brittle minerals including quartz plagioclase and carbonates reveal the oligocene hetaoyuan formation hold good to very good potential of shale oil exploitation six organic rich shale layers have been identified in the hetaoyuan formation shale layer 1 to 6 from top to bottom among them shale layers 5 and 3 hold the best shale oil potential and therefore be investigated in the present paper shale layer 5 was deposited in a semi deep lacustrine environment under the moderately warm humid paleoclimate further dividing into lower middle and upper units relatively low toc contents av 2 6 wt in the lower unit are probably caused by high sedimentation rate high bioproductivity under favorable preservation conditions led to relatively high toc contents av 3 6 wt in the middle unit less reducing conditions together with moderate low bioproductivity resulted in relatively low toc contents av 2 6 wt in the upper unit warm humid paleoclimate prevailed during deposition of shale layer 3 deepened water depth and formed a deep lacustrine environment the stratified water column led to stable anoxic condition and photic zone euxinia in the saline deep lacustrine environment which enabled the accumulation of high amounts of om av 4 9 wt toc under excellent preservation av hi 710 mg hc g toc the results of core scanning analysis indicate that formation of the laminated shales probably controlled by seasonal fluctuations of paleoclimate

tetrahymanol its widespread occurrence and geochemical significance abstract the occurrence of tetrahymanol gammaceran 3 ol in sediments from various marine depositional environments such as the santa monica basin and palos verdes shelf southern california bight the santa barbara basin off the northern california borderland the atlantic shelf slope and rise and the antarctic region as well as in bacterial algal mats from the santa barbara basin and baja california together with its detection in sediments from the peru upwelling region odp leg 112 and baffin bay odp leg 105 suggests that tetrahymanol occurs ubiquitously in marine samples tetrahymanol is the only known likely biological precursor of gammacerane which is found in many petroleums and shales the common occurrence of tetrahymanol in marine environments implies that primitive organisms similar to tetrahymena or organisms other than tetrahymena other protozoa bacteria are also likely to contain this compound its isomer diplopterol has also been detected in several sediment sections

porphyrin distributions in crude oils from the jianghan and biyang basins china abstract a number of oils tar sands and extracts of suspected source rocks from the jianghan and biyang basins of china have been studied to investigate their porphyrin content the results obtained suggest that these distributions are controlled to a large extent by the nature of the depositional environment and the maturity levels of the sediments it is proposed that the predominance of ni porphyrins in the sediments of these basins is caused mainly by the high ni content of the groundwater the relatively high content of a c32 porphyrin isomer with a five membered ring with a methyl substituent is indicative of environments with very high salinity the porphyrin distributions have been affected through thermal maturation with the major changes being dealkylation alkylation within etio porphyrins the instability of ni dpep porphyrins is suggested as a major reason for the rarity of dpep porphyrins in chinese oils the ratios of dpep etio or dpep etio dpep c 31 etio a c 28 etio b and etio a c 31 c 30 are useful as thermal maturity parameters and the results also indicate that the percentage of dpep porphyrins decreases through migration

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

sensitivity of biomarker properties to depositional environment and or source input in the lower toarcian of sw germany abstract a sequence of lower toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil to source correlation lower liassic marls are representative of a well aerated shallow water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions various properties have been found to vary considerably within these two units amongst the most important are carbon isotopic composition of the kerogen the pristane phytane ratios nickel as opposed to vanadyl porphyrins and the c27 dia regular steranes although maturation within the profile does not change some of the maturation dependent biomarker properties such as the monoaromatic steroid side chain cracking and the tm ts ratio exhibit large changes which can be assigned to diagenetic processes another maturation dependent property the 20s 20r epimerization of c29 steranes exhibits smaller changes which could also be due to early diagenetic processes the study suggests that reducing and oxidizing conditions i e eh and ph in the sediment exert an influence on several biomarker precursor product pathways in maturation studies initial variations due to depositional conditions have therefore to be taken into account

variations of calcium carbonate organic carbon and iron sulfides in anoxic sediment from the orca basin gulf of mexico abstract sediment from the anoxic orca basin is highly laminated and enriched in iron sulfides and organic carbon unlike sediment from the surrounding northwest gulf of mexico continental slope black laminae are enriched in calcium carbonate 19 vs 15 organic carbon 1 8 vs 0 7 and iron sulfides 0 7 vs 0 4 compared to light gray laminae however more of the fes is converted to fes2 in the light gray laminae dark gray laminae of intermediate composition are also common the laminae probably reflect annual and longer term variations in organic matter supply to the basin and variations in mixing at the brine seawater interface

characteristics of magnetic carriers responsible for late paleozoic remagnetization in carbonate strata of the mid continent u s a magnetic analysis of carbonate strata of the mid continent region of the united states indicates that the late paleozoic remagnetization already recognized in the appalachians also affects almost the entire mid continent magnetic intensity is regionally variable with more intense magnetizations occurring on intracratonic arches magnetization is carried predominately by magnetite which often occurs in tiny hollow spheroids composed of well formed octahedral microcrysts spheroid chemistry and morphology suggests that the magnetite is authigenic composed of iron extracted from pre existing pyrite or clay the timing of the remagnetization the character of the magnetite and the regional variation in magnetic intensity support the idea that the remagnetization is a diagenetic consequence of tectonically driven brine migration

the trace metal geochemistry of pore water brines from two hypersaline lakes abstract pore water brines from modern hypersaline lake sediments have been analyzed to determine the nature of brine trace metal interaction in these environments brine samples were collected from the surface waters and sedimentary pore waters of two hypersaline lakes solar lake eastern sinai and goto meer bonaire netherlands antilles metal concentrations in the oxic brines appear to be related to the strength of their respective chloride complexes with pb and zn more enriched than cu pore water cu zn and fe concentrations increased across the sediment water interface in both lakes all three metal profiles follow a similar pattern with depth in the pore water higher concentrations were observed in the goto meer brines strontium isotopes were utilized to qualify better the mixing of the brines in solar lake although these metals were observed at higher concentrations than sea water in these sulfidic brines their concentrations are not close to what is needed for the formation of ore deposits

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

a geochemical investigation of crude oils and source rocks from biyang basin china abstract china has a number of petroliferous lacustrine sedimentary basins of varying salinity and age mainly eocene a geochemical investigation has been undertaken on several oils and source rocks from the eocene lacustrine biyang basin the distributions of n alkanes isoprenoids steranes and terpanes have been studied and used to characterize the sedimentary environment of deposition maturity biodegradation and undertake possible correlations the ratios of c 30 hopane gammacerane 4 methyl steranes regular steranes steranes hopanes c 21 tricyclic c 30 hopane are proposed to be indicative of the depositional environment whereas s carotane appears to be a source related indicator the geochemical data obtained in this study suggest that the major source rocks in the biyang basin were deposited in a saline hypersaline depositional environment

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

genesis of the lower triassic bonebeds from gogolin s poland the impact of microbial mats on trapping of vertebrate remains abstract three bone bearing horizons consisting of seven bone bearing beds with fish and reptile remains were recently discovered in the uppermost rot buntsandstein lower triassic peritidal and shallow marine carbonates in the vicinity of gogolin s poland the aim of this study is to recognize the genesis and depositional environments of the bonebeds detailed fieldwork microfacies analysis and sem eds analysis reveal that the vertebrate remains occur to a great extent alongside evidence for former microbial activity the reptile remains represent at least dactylosaurus and nothosaurus sauropterygia genera while the fish remains belong to the chondrichthyes and actinopterygii the vertebrate remains were deposited in a transitional zone between the costal sabkha and the epicontinental sea the first bone bearing horizon originated in the most landward setting in the uppermost intertidal zone likely at the margins of a pond or coastal lagoon the second bone bearing horizon was formed in the upper intertidal zone the vertebrate remains of the third bone bearing horizon were accumulated in two different settings in the lower intertidal zone in a small palaeopond and in the subtidal zone in a protected back shoal lagoon three bone accumulations occur within planar stromatolites while the other bone bearing beds reveal more indirect evidence for former microbial activity in our opinion the presence of the sticky microbial mats in the intertidal settings was the crucial factor that allowed the establishment of and controlled these accumulations of vertebrate remains the soft gluey substrates trapped vertebrate remains that were transported across them once trapped the vertebrate bioclasts were efficiently immobilized and thus protected against damage due to sedimentological agents the lack of any orderly arrangement of the vertebrate bioclasts suggests that they may have been delivered to the individual mat surfaces by diverse media from various directions and also at different times

pyrite and oxidized iron mineral phases formed from pyrite oxidation in salt marsh and estuarine sediments abstract we used scanning electron microscopy and energy dispersive x ray analysis to examine sediments from vegetated portions of three salt marshes the great sippewissett marsh cape cod ma sapelo island georgia and the hackensack meadowlands n j and from the sediments of an estuary newark bay n j pyrite particles were abundant in sediments from all sites both fine grained pyrite crystals and framboids were found single fine grained crystals diameter 0 2 to 2 0 micrometers predominated in all samples strong evidence for rapid formation of pyrite we also found both microcrystalline and framboidal iron oxyhydroxide phases in many of the sediment samples this is evidence of pyrite oxidation within the sediments and suggests that iron is conserved in salt marshes even as pyrite is oxidized the thermodynamic stability of iron phases in marsh sediments and recent pyrite oxidation studies in coal suggest goethite as the crystalline iron oxyhydroxide phase present in addition we sometimes found a red amorphous coating on grass roots from the great sippewissett and sapelo island marshes this coating is likely a form of hydrated iron iii oxide

characteristics of selected petroleum source rocks xianjiang uygur autonomous region northwest china 1 the sedimentary basins of xinjiang uygur autonomous region china are moderately to poorly explored for petroleum volumetric adequacy of petroleum source rocks is a critical exploration risk in these basins particularly because source rock data are limited this study provides new source rock data and speculatively assesses the source rock potential of xinjiang basins the junggar zhungaer basin the best explored of the xinjiang basins and containing a giant oil field is underlain in many areas by an upper permian lacustrine oil shale sequence remarkable for its organic richness and oil source quality depending on its position in the basin the permian section ranges from immature to overmature and is inferred to be the principal source of oil in the basin upper triassic middle jurassic coal measures including lacustrine rocks constitute a secondary source rock sequence in the basin the smaller intermontane turpan tulufan basin contains a very similar upper triassic middle jurassic sequence which where sufficiently buried probably comprises the only significant oil source sequence in the basin the vast tarim talimu basin offers the greatest variety of potential source rocks of all xinjiang basins but remains the least well documented from limited but geologically planned and focused sampling cambrian carboniferous and permian strata are not considered major oil contributors in the dominantly shallow marine paleozoic section of the northern tarim basin only ordovician black shales appear to have significant potential the upper triassic middle jurassic sequence of the northern tarim basin is similar to that of the junggar and turpan basins a section rich in coal and lacustrine shale that constitutes another potentially significant oil source due to the size stratigraphic packaging and structural relief of the northern tarim basin paleozoic and mesozoic potential il source beds range from immature to overmature

the effect of nutrient rich effluents from shrimp farming on mangrove soil carbon storage and geochemistry under semi arid climate conditions in northern brazil a semi arid mangrove estuary system in the northeast brazilian coast ceara state was selected for this study to i evaluate the impact of shrimp farm nutrient rich wastewater effluents on the soil geochemistry and organic carbon oc storage and ii estimate the total amount of oc stored in mangrove soils 0 40 cm wastewater affected mangrove forests were referred to as wam and undisturbed areas as non wam redox conditions and oc content were statistically correlated p 400 mv in non wam soils and significantly higher from 66 to 411 mv in the dry season p 0 01 oc contents 0 40 cm soil depth were significantly higher p 0 01 in the wet season than the dry season and higher in non wam soils than in wam soils values of 8 1 and 6 7 kg m 2 in the wet and dry seasons respectively for non wam and values of 3 8 and 2 9 kg m 2 in the wet and dry seasons respectively for wam soils p 0 01 iron partitioning was significantly dependent p 0 05 on type of land use with a smaller degree of pyritization and lower fe pyrite presence in wam soils compared to non wam soils basal respiration of soil sediments was significantly influenced p 0 01 by type of land use with highest co2 flux rates measured in the wam soils mean values of 0 20 mg co2 h 1 g 1 c vs 0 04 mg co2 h 1 g 1 c the oc storage reduction in wam soils was potentially caused i by an increase in microbial activity induced by loading of nutrient rich effluents and ii by an increase of strong electron acceptors e g no3 that promote a decrease in pyrite concentration and hence a reduction in soil oc burial the current estimated oc stored in mangrove soils 0 40 cm in the state of ceara is approximately 1 million t

fe rich microspheres pseudomorphs after pyrite framboids in holocene fluvial deposits from ne spain relationship with environmental conditions and bacterial activity abstract three iron oxides rich microsphere types type i to iii were detected in an holocene 17 m thick stratigraphic succession located in the iberian range ne spain lithofacies features indicate that the studied materials were generated in an alluvial dominated setting with a channeled area fringed by floodplain zones during high water levels and high energy floods gravels and sands deposited in the active area and in lateral overbank areas in these lateral areas mud settling took place when flood decreased and then anoxic conditions could be reached due to microbial oxidation of organic matter and the low permeability of the marly sediment x ray diffraction xrd analysis of 32 samples and microtextural observation of 10 samples by field emission scanning electron microscopy fesem revealed the occurrence of fe oxi hydroxides microspheres showing different surficial structure these microspheres are pseudomorphs after pyrite framboids although the formation of some primary fe oxy hydroxides aggregates cannot be rejected pyrite framboids genesis in sediments underlying oxic dysoxic water column would have been favored by anoxic conditions reached in lateral overbank areas after main flooding involving the activity of fe reducing bacteria and sulfate reducing bacteria srb given the high so4 availability provided by the highly mineralized groundwater from the upstream banos de arino spring subsequent change to oxic conditions during the exposition of the floodplain enhanced the transformation of pyrite into fe oxy hydroxides as well as in microenvironments around cracks and roots pyrite oxidation likely took place with the implication of neutrophilic iron oxidizing bacteria and archaea living in microaerophilic conditions as well as cyanobacteria given the close association of fe oxides framboids with microbial features in addition pyrite and or fe oxi hydroxides framboids from marl levels could have been incorporated to sand sediments during further overbank flooding episodes under these new oxic conditions pyrite oxidation would have been favored and even the formation of primary fe oxy hydroxides

biological marker compounds as indicators of the depositions history of the maoming oil shale abstract the eocene maoming oil shale from guangdong province occurs as a laterally uniform stratigraphic section typically 20 25 m thick from which the aliphatic hydrocarbon constituents of six representative samples were investigated using gc and c gc ms the sediments evaluated included the basal lignite a vitrinite lens from the overlying claystone and four intervals from the massive oil shale bed as expected the lignite and vitrinite differ markedly from the oil shales the lignite is dominated by bacterial hopanoids and components of higher plant origin including c 29 steroids and triterpenoids such as oleanenes visually the oil shale samples show corroded and degraded phytoclasts spores wispy particles of fluorescent organic material attributable to dinoflagellates and especially in the uppermost sample colonial algal bodies the distributions of biological markers in the oil shales show many features in common notably a dominance of dinoflagellate derived 4 methylsteroids and a significant proportion of higher plant derived n alkanes with marked odd over even carbon number predominance overall they exhibit several features that resemble characteristics of the messel shale the hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales the aliphatic hydrocarbons of the uppermost shale are dominated by a number of c 31 and c 33 botryococcane homologues and other unusual branched alkanes possibly derived from green algae all of the samples are immature overall molecular and microscopic examination of the stratigraphic succession of the maoming oil shale suggests a shallow lacustrine environment within which peats were deposited this lake subsequently deepened to support abundant algal populations especially dinoflagellates culminating in a dominance of botryococcoid algae

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

authigenic carbonates from the darwin mud volcano gulf of cadiz a record of palaeo seepage of hydrocarbon bearing fluids hydrocarbon rich fluids expelled at mud volcanoes mvs may contribute significantly to the carbon budget of the oceans but little is known about the long term variation in fluid fluxes at mvs the darwin mv is one of more than 40 mvs located in the gulf of cadiz but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity in order to test this idea we have conducted petrographic chemical and isotopic analyses of the carbonate crust in addition a 1 d transport reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present the carbonate crusts mainly comprise of aragonite with a chaotic fabric exhibiting different generations of cementation and brecciation the crusts consist of bioclasts and lithoclasts peloids intraclasts and extraclasts immersed in a micrite matrix and in a variety of cement types microsparite botryoidal isopachous acicular radial and splayed fibrous the carbonates are moderately depleted in 13c 13c 8 1 to 27 9 as are the pore fluids 13c 19 1 to 28 7 which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons like the gases that seep from the mv today the carbonate 18o values are as high as 5 1 and it is most likely that the crusts formed from 18o rich fluids derived from dehydration of clay minerals at depth pore fluid modelling results indicate that the darwin mv is currently in a nearly dormant phase seepage velocities are 0 09 cm yr 1 thus the thick carbonate crust must have formed during past episodes of high fluid flow alternating with phases of mud extrusion and uplift highlights results of pore fluid modelling indicate low seepage activity at localised sites pore fluids are supersaturated with respect to hydrocarbons of thermogenic origin aom supports vent fauna and results in the formation of authigenic carbonates the carbonate crust has a brecciated appearance and mainly consists of aragonite the crust formation seems to be regulated by changes in fluid and mudflow activity

the origin and fate of 4 methyl steroid hydrocarbons i diagenesis of 4 methyl sterenes abstract treatment of 4 methylcholest 4 ene under mild acid conditions at low temperatures gives chemical evidence for certain features seen in the distributions of sedimentary 4 methyl steroid hydrocarbons and further indicates that many low temperature diagenetic reactions of steroids are explicable in terms of acid catalysed rearrangements specifically the results provide i indirect evidence that the 4 ene skeleton is a key intermediate in the dehydration of 4 methyl stanols in sediments ii an explanation for the distribution of 4 methyl sterenes and a nor sterenes in the lacustrine messel shale eocene iii an explanation for the presence of 4 methyl steranes in relatively immature sedimentary rocks despite the precursor stanols having the 4 methyl configuration with increasing maturity in the paris basin shales lower toarcian the less stable 4 methyl steranes decrease gradually in abundance relative to their 4 methyl counterparts at a rate fairly similar to the change in pristane stereochemistry

depositional environment and factors controlling β carotane accumulation a case study from the jimsar sag junggar basin northwestern china abstract a combined investigation of bulk geochemistry and molecular geochemistry was carried out on 31 source rock samples from the lower permian lucaogou formation in the jimsar sag of the junggar basin northwestern china geochemical characterization reveals the depositional environments and other factors favorable for the generation and accumulation of carotane an important hydrocarbon precursor many core samples have a total organic carbon content 1 and rock eval s2 values 2 mg hc g rock suggesting relatively high hydrocarbon generation potential biomarkers in the lucaogou source rock extracts are characterized by short to long chain n alkanes a wide range of pr ph ratios 0 74 1 42 and carotane index values carotane cmax 0 16 1 92 high concentrations of c29 sterane relatively low tricyclic terpane content and moderate gammacerane index values 0 14 0 29 this is consistent with a weakly oxidizing to weakly reducing depositional environment with fresh to brackish water conditions rapid input of terrigenous organic matter and a relatively small contribution of aquatic algal bacterial organic matter cross plots of biomarker parameters and the carotane index show that carotane in the lucaogou source rocks is mainly derived from terrigenous organic matter and is found predominantly in sediments deposited in weakly reducing brackish environments it does not appear to have been affected by thermal maturation