venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

microfossils or pseudofossils cu co sulphide microfeatures in the neoproterozoic katangan supergroup democratic republic of congo abstract in this paper we investigate a group of microfeatures discovered in the mineralised neoproterozoic mines subgroup of the katangan copperbelt democratic republic of congo drc at the mashitu south cu co deposit the features are bacillus shaped composed of cu co sulphides and occur within syn diagenetic apatite veins in a low grade metasedimentary shale they exhibit a range of evidence for biogenicity including indicators of cellular complexity and were therefore investigated as candidate microfossils of chasmoendolithic prokaryotes however our study found that the features are likely to be pseudofossils formed from abiogenic rutile crystals which have been replaced by sulphides in such a way as to mimic biogenicity indicators this method of sulphide replacement adds to the list of possible false positives in the search for early life and emphasises the importance for micropalaeontological studies to consider such features with a sceptical holistic approach and rigorous scientific method

fossil sulphate reducing bacteria in the bleiberg lead zinc deposit austria polished ore samples from the world class bleiberg lead zinc deposit in austria were studied by field emission scanning electron microscopy fesem at magnifications in the range of 105 106 the zinc ore shows nano sized sphalerite filaments and spherules which are morphologically similar to recent biofilms of sulphate reducing bacteria the activity of sulphate reducing bacteria is suggested by a large 34s difference of 40 between coeval seawater sulphate 16 and sulphide sulphur 25 in the bleiberg ores and by variable sulphur valences peloids of sphalerite zn bearing calcite and pyrite have features typical of bacterial colonies combined with geological and mineralogical evidence a significant role of bacteria during ore deposition at bleiberg is likely

dating of u rich heterogenite new insights into u deposit genesis and u cycling in the katanga copperbelt abstract the katanga copperbelt region of the democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves heterogenite coooh is the most abundant co bearing secondary mineral in the region its occurrence is the result of oxidation of cu co sulfides and associated co reprecipitation in the uppermost part of the deposits during the pliocene in addition to sediment hosted copper and cobalt ore deposits the katanga copperbelt also hosts numerous uraniferous mineral occurrences and deposits which can be associated with heterogenite within these deposits heterogenite can have high concentrations of u up to 3 5 and pb up to 4 in situ sims u pb ages were obtained for heterogenite samples from the u deposits of shinkolobwe kalongwe and kambove these analyses yield distinct neoproterozoic ages at 876 ma 823 ma and in the 720 to 670 ma age range as the geological context prevailing at those times was not favorable for heterogenite formation these ages most probably record geological events that are not the formation of the mineral itself for instance the heterogenites could have inherited the u pb signature of a u rich mineral most likely uraninite formed and or yet reworked at 876 ma 823 ma and in the 720 to 670 ma time interval and spatially associated with primary co sulfides in this hypothesis the ages obtained in this paper are significant for understanding the cycling and re deposition of u at given moments in the regional geological history in such context the 876 ma and the 823 ma age are consistent with syn early diagenetic concentration of uranium in sediments of the katanga basin the 720 to 670 ma ages are interpreted as a phase of u remobilization related to hydrothermal fluid circulation induced by late nguba proto oceanic rifting or by early stages of congo kalahari craton convergence and associated hydrothermal circulation

bacterially mediated authigenesis of clays in phosphate stromatolites authigenic clays in close textural relation to carbonate fluorapatite within finely laminated phosphate stromatolites of upper jurassic age have been studied using scanning electron microscopy sem transmission electron microscopy tem and analytical electron microscopy aem stromatolite laminae consist of hexagonal prisms of francolite sizes ranging between 0 1 and 1 m that are surrounded by poorly crystalline smectite and amorphous fe si al oxyhydroxides microanalyses show that smectite is fe rich with highly variable composition particularly regarding fe and si contents smectite has significant beidellitic montmorillonitic and non tronitic substitutions although the lack of fringe contrast in some areas adjacent to the smectite packets with 1 0 1 3 nm spacing is due to differences in orientation of layers textural and analytical data clearly indicate the presence of fe si al amorphous phases intimately intergrown with smectite the occurrence of poorly crystalline smectite and associated amorphous phases within microbially precipitated stromatolite laminae both as envelopes around and as pore fillings between extremely small calcium phosphate crystals demonstrates authigenic smectite growth from a precursor fe si al amorphous material this material is formed in close association with a phosphate rich precursor the textural and structural relations the preservation of chemical precursors of glauconite such as nontronitic montmorillonite and the presence of fe si al amorphous mineral phases imply crystallization of the observed crystalline phases from synsedimentary bacterially precipitated amorphous precursors during early diagenesis in postoxic environments carbonate fluorapatite was the first phase to crystallize from the primary gel smectite and associated amorphous fe si al oxyhydroxides were the residual material of the crystallization process the slow rate of transformation at low temperatures from fe si al rich gels to smectite explains the textural relations between the poorly crystalline phases and the phosphate crystals as well as the preservation of amorphous substances in relation to clays authigenic smectite represents the first step in glauconitization

pseudomicrofossils and altered microfossils from a middle proterozoic shale belt supergroup montana abstract pseudomicrofossils and authentic microfossils occur in the approximately 1 3 ga old appekunny argillite lower belt supergroup glacier national park montana these microstructures occur in finely laminated dark gray and black mudstones that were deposited in an offshore environment and were subsequently subjected to moderate burial metamorphism the pseudomicrofossils consist of 0 1 0 3 m thick organic envelopes that encapsulate 1 20 m diameter authigenic crystals these crystals petrographically identified as apatite ca 5 po 4 3 oh f cl occur isolated in clusters and in filiform aggregates up to 200 m long the organic envelopes apparently originated through the accretion around these crystals of organic matter which was finely disseminated throughout the sedimentary matrix the authentic microfossils are poorly preserved sphaeromorphs which unlike the pseudomicrofossils are highly compressed parallel to lamination these microfossils are several tenths of a micrometer to ca 1 m thick and range from 22 82 m long and 17 71 m wide they commonly exhibit tears and folds that evidently were produced during compaction of the enclosing argillaceous matrix due to poor preservation these specimens cannot be assigned to known proterozoic sphaeromorph genera inasmuch as the pseudofossils could be misinterpreted as microfossils they well illustrate the need to exercise caution when interpreting organic microstructures preserved in precambrian strata not being compressed parallel to lamination the pseudomicrofossils are readily distinguished from the authentic microfossils which are strongly compressed parallel to lamination the authentic microfossils demonstrate that organic walled microfossils can be preserved in strata subjected to moderate burial metamorphism and they provide information useful for recognizing and interpreting microfossils in similarly metamorphosed argillaceous strata

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

role of the bacterium thiobacillus in the formation of silicates in acidic mine tailings acidophilic chemolithotrophic bacteria such as thiobacillus are commonly found in sulfidic mine tailings they oxidize fe ii and reduced sulfur species from sulfide minerals left in tailings and produce acidic leachates loaded with heavy metals our study of the kam kotia mine tailings cu zn ore an abandoned mine located near timmins ontario canada showed that thiobacillus played an important role in the in situ formation of silicates transmission electron microscopic observations of ultrathin sections of tailings revealed the presence of bacteria with fine grained silica sio2 associated with their cell walls under laboratory conditions mimicking the chemistry of the pore waters of the tailings i e identical ph fe si and so4 thiobacillus promoted the rapid formation 48 h of amorphous silica at low ph values ph 2 2 2 3 while an abiotic system with the same chemical conditions did not show silica precipitation however in the presence of fe oxides silica formation was not observed because dissolved si was preferentially sorbed to the surface of the oxides in addition when the cells were grown in the presence of micas biotite and muscovite under acidic conditions mixed mg al si fe silicate precipitates were formed on the surface of thiobacillus these results indicate that si cycling and solubility in mine tailings could be strongly influenced by bacteria especially in sulfide depleted tailings like the kam kotia ones where the formation of fe oxide is negligible

a diverse and exquisitely preserved organic walled microfossil assemblage from the meso neoproterozoic mbuji mayi supergroup democratic republic of congo and implications for proterozoic biostratigraphy abstract a well preserved and diversified microfossil assemblage is reported from the meso neoproterozoic mbuji mayi supergroup in the kasai oriental province central part of democratic republic of congo a total of 49 taxa belonging to 27 genera were identified including 11 species of unambiguous eukaryotes 10 species of possible eukaryotes or prokaryotes and 28 species of probable bacteria this assemblage is more diverse than previously reported but includes taxa reported in coeval worldwide assemblages it is characterized by abundant sphaeromorphs filamentous colonial aggregates and filamentous forms as well as a relatively low diversity of acanthomorphs including the late mesoproterozoic and early neoproterozoic index fossil trachyhystrichosphaera aimika reported for the first time in central africa this species co occurs with other taxa also reported for the first time in africa trachyhystrichosphaera botula jacutianema solubila cf tappania sp valeria elongata and numerous other taxa correlation with other geochronologically constrained successions that contain trachyhystrichosphaera confirms t aimika as promising index fossil to define the late mesoproterozoic early neoproterozoic interval the available biostratigraphic data enable to suggest a minimum tonian age for the mbuji mayi supergroup this age is consistent with the published and new geochronological data comparison with worldwide proterozoic assemblages permits to define microfossil assemblages useful for biostratigraphy this study significantly improves our understanding of the diversity of the late mesoproterozoic early neoproterozoic biosphere and in particular the diversification of early eukaryotes preserved in the democratic republic of congo rock record and more broadly in africa where micropaleontological investigations are sparse

diverse microstructures from archaean chert from the mount goldsworthy mount grant area pilbara craton western australia microfossils dubiofossils or pseudofossils abstract a diverse assemblage of indigenous carbonaceous microstructures classified here as highly probable microfossils to pseudomicrofossils is present in the ca 2 97 ga farrel quartzite gorge creek group at mount grant and mount goldsworthy pilbara craton western australia the microstructures are an integral part of the primary sedimentary fabrics preserved in black chert beds the interbedding of chert with layers of large silicified crystal pseudomorphs and fine to coarse grained volcaniclastic clastic beds indicate deposition in a partially evaporitic basin with terrigenous clastic and volcaniclastic input similar associations of microstructures are present at the same stratigraphic level in outcrops more than 2 km apart four major microstructural types are present thread like film like spheroidal and lenticular to spindle like each of which can be further subdivided into several sub types most of the microstructures were deposited as part of an assemblage of chemical and clastic sediments although there are some thread like microstructures present for which a synsedimentary origin cannot be confirmed many specimens appear to have originally had flexible but breakable walls and some occur in colony like aggregations size distributions for the four major types are generally narrow a feature typical of biogenic structures the microstructures are composed of disordered carbon as revealed by raman spectroscopy and the bulk isotopic composition of the carbon is 13 c the combined morphological and geological evidence suggests that the film like structures small spheres associated with films large spheroids and spindle like structures are probable to highly probable fossil remains of microorganisms the morphological variety among the microstructures suggests that a diverse microbial ecosystem flourished in the pilbara region during the archaean

low temperature sulphate reduction biological versus abiological sulphate is considered to have been a major source of sulphide in strata bound and stratiform base metal sulphide deposits many of these deposits however appear to have been formed at moderate temperatures 200 c which poses the question by what mechanism s was sulphate reduced to sulphide two modes of reduction have been established experimentally 1 catalysis by sulphate reducing bacteria which at present is only known to occur below ca 100 c and 2 abiological reduction by ferrous iron or organic matter which has only been clearly shown above ca 250 c several attempts have been made to demonstrate abiological reduction below 200 c and some new data are presented here although the results do not exclude the possibility that such a reaction may be geochemically significant there has been no unequivocal demonstration of nett sulphide formation from sulphate at these temperatures recent studies of the microbiology of hydrothermal regions have opened up the prospect of bacterial sulp

nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the 1 9 ga gunflint chert the 1 88 ga gunflint biota is one of the most famous precambrian microfossil lagerstatten and provides a key record of the biosphere at a time of changing oceanic redox structure and chemistry here we report on pyritized replicas of the iconic autotrophic gunflintia huroniospora microfossil assemblage from the schreiber locality canada that help capture a view through multiple trophic levels in a paleoproterozoic ecosystem nanoscale analysis of pyritic gunflintia sheaths and huroniospora cysts reveals differing relic carbon and nitrogen distributions caused by contrasting spectra of decay and pyritization between taxa reflecting in part their primary organic compositions in situ sulfur isotope measurements from individual microfossils 34sv cdt 6 7 to 21 5 show that pyritization was mediated by sulfate reducing microbes within sediment pore waters whose sulfate ion concentrations rapidly became depleted owing to occlusion of pore space by coeval silicification three dimensional nanotomography reveals additional pyritized biomaterial including hollow cellular epibionts and extracellular polymeric substances showing a preference for attachment to gunflintia over huroniospora and interpreted as components of a saprophytic heterotrophic decomposing community this work also extends the record of remarkable biological preservation in pyrite back to the paleoproterozoic and provides criteria to assess the authenticity of even older pyritized microstructures that may represent some of the earliest evidence for life on our planet

fossilization potential of iron bearing minerals in acidic environments of rio tinto spain implications for mars exploration abstract acidic waters of the rio tinto southwestern spain evaporate seasonally precipitating a variety of iron sulfide and oxide minerals schwertmannite and nanophase goethite form thin laminae on biological and detrital grain surfaces replicating among other things the morphologies of insect cuticle plant tissues fungi algae and bacteria intergrain cements also incorporate bacterial cells and filaments other sulfate minerals precipitated in rio tinto environments are transient and contribute little to short term preservation because the rio tinto has been cutting its current valley for several million years terrace deposits provide a window on longer term fossil preservation early and later diagenesis are recorded in terrace deposits formed about one thousand and two million years ago respectively the sedimentary structures and mineralogies of these deposits suggest that they formed under physical and chemical conditions comparable to those of modern rio tinto sediments the terrace deposits show quantitative loss of sulfate minerals increasing crystallinity of goethite and in the older terrace replacement of goethite by hematite fossils formed originally by schwertmannite and nanophase goethite replication persist through diagenesis preserving a long term record of local biological diversity fossil preservation by iron oxides in the acidic environment of rio tinto suggests that if life was present when sedimentary rocks formed at meridiani planum mars precipitated minerals could record their presence

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

critical testing of earth s oldest putative fossil assemblage from the 3 5ga apex chert chinaman creek western australia abstract structures resembling cyanobacterial microfossils from the ca 3465 ma old apex chert of the warrawoona group in western australia have until recently been accepted as providing the oldest morphological evidence for life on earth and have been taken to support an early beginning for oxygen releasing photosynthesis eleven species of filamentous prokaryote principally distinguished by shape and geometry have been put forward as meeting the criteria required of authentic archaean microfossils they were contrasted with other microfossils that were dismissed as either unreliable or irreproducible the aim of this paper is to provide a detailed account of research recently reported by us on the type and recollected material involving optical and electron microscopy digital image analysis and other techniques all previously figured holotype materials are illustrated here and the context for all the published materials is re evaluated the apex chert microfossils occur near the top of a 1 5 km long chert dyke complex associated with major synsedimentary growth faults highly localised glassy felsic tuffs erupted explosively from this and other fissures during the early stages of volcanism and were followed by the deposition of essentially hydrothermal black and white baso4 rich cherts that infiltrated the feeder dykes underplating and dilating adjacent stratiform cherts before the start of the next volcanic cycle the apex chert microfossils occur within multiple generations of these metalliferous hydrothermal vein cherts some 100 m down the dyke system comparable structures occur in associated volcanic vent glass and in hydrothermal cherts at least 1 km deep we find no supporting evidence for a primary biological origin we reinterpret the purported microfossil like structures as pseudofossils that formed from the reorganization of carbonaceous matter mainly during recrystallization from amorphous to spherulitic silica

tomographic reconstruction of near borehole slowness using refracted borehole sonic arrivals two dimensional 2 d reconstructions of the near borehole slowness field are computed using arrival times of refracted borehole sonic arrivals first arrival traveltimes derived from both computer simulations and field data from full waveform sonic tools were inverted for the near borehole formation slowness both axially along the borehole and radially away from the borehole the inversion is nonlinear the solution is obtained by means of a series of linear inversions followed by provisional ray tracings each iteration involves the application of a tomographic reconstruction algorithm similar to those used in seismic crosswell tomography or medical imaging applications the technique was demonstrated using ray theoretic examples to simulate radial variations in slowness in addition full waveforms were generated using two and a half dimensional 2 5 d fdm computer models the finite difference method fdm computer models were used to test the validity of the ray theoretic approximation used in the

sequence and carbon isotopic stratigraphy of the neoproterozoic roan group strata of the zambian copperbelt abstract the neoproterozoic roan group in northern zambia is host to numerous world class stratiform sediment hosted cu ores that occur around the transition from basal continental clastics to an overlying shallow marine succession the latter can be considered in terms of six sedimentary facies deposited within a mixed clastic and carbonate barred basin margin environment sequence stratigraphic analysis based on palaeo bathymetric cycles defined by vertical facies changes in sections constructed from detailed logging of diamond drill cores allows the definition of seven sedimentary sequences accommodation for the lowermost two sequences which host the bulk of the copper ores was generated by half graben development during active extension the subsequent two sequences record denudation of tectonically generated relief and evolution to a laterally extensive low relief basin margin carbonate platform during a period of tectonic quiescence the uppermost sequences that coincide with the mwashia subgroup record the resumption of tectonically generated accommodation that may reflect the onset of the breakup of rodinia carbon isotopic profiles through the roan succession in zambia show two secular 13c excursions of more than 10 to values of

morphotectonics of the central muertos thrust belt and muertos trough northeastern caribbean multibeam bathymetry data acquired during the 2005 spanish r v hesperides cruise and reprocessed multichannel seismic profiles provide the basis for the analysis of the morphology and deformation in the central muertos trough and muertos thrust belt the muertos trough is an elongated basin developed where the venezuelan basin crust is thrusted under the muertos fold and thrust belt structural variations along the muertos trough are suggested to be a consequence of the overburden of the asymmetrical thrust belt and by the variable nature of the venezuelan basin crust along the margin the insular slope can be divided into three east west trending slope provinces with high lateral variability which correspond to different accretion stages 1 the lower slope is composed of an active sequence of imbricate thrust slices and closed fold axes which form short and narrow accretionary ridges and elongated slope basins 2 the middle slope shows a less active imbricate structure resulting in lower superficial deformation and bigger slope basins 3 the upper slope comprises the talus region and extended terraces burying an island arc basement and an inactive imbricate structure the talus region is characterized by a dense drainage network that transports turbidite flows from the islands and their surrounding carbonate platform areas to the slope basins and sometimes to the trough in the survey area the accommodation of the ongoing east west differential motion between the hispaniola and the puerto rico virgin islands blocks takes place by means of diffuse deformation the asymmetrical development of the thrust belt is not related to the geological conditions in the foreland but rather may be caused by variations in the geometry and movement of the backstop the map view curves of the thrust belt and the symmetry of the recesses suggest a main north south convergence along the muertos margin the western end of the investigator fault zone comprises a broad band of active normal faults which result in high instability of the upper insular slope

neoproterozoic allochthonous salt tectonics during the lufilian orogeny in the katangan copperbelt central africa the neoproterozoic katangan rocks in the copperbelt of shaba democratic republic of the congo and zambia contain the world s largest concentration of stratiform copper cobalt ores we propose that during the lufilian orogeny the katangan basin was radically transformed first by extrusion of allochthonous evaporites then by orogenic shortening salt tectonics plausibly explains spectacular breccias underlying 25 000 km2 and containing gigaclasts up to 10 km long the brecciated shaban roan supergroup the lowermost katangan unit forms regional detachments and diapirs the former existence of roan evaporites is indicated by sabkha facies crystals and pseudomorphs of gypsum and anhydrite stratigraphic gaps underlain by collapse breccias chloride inclusions in ores hydrothermally propylitized ore hosts and saline springs salt tectonics began during deposition of the roan supergroup between 1050 and 950 ma in mid roan time small walls and extrusions of evaporite gigabreccia began to be emplaced in early kundelungu time 940 850 ma evaporitic diapirs enlarged lufilian deformation began between 850 and 650 ma by laterally squeezing diapirs to form salt welds then a large sheet of commingled roan evaporite and carbonate dominated sediments and ores was extruded northward by 65 km extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits continued shortening then emplaced large thrust sheets lubricated by the preexisting salt sediment extrusive sheet restoration suggests that the lufilian foreland in this area shortened from 193 km to 85 km the depositional northern edge of the evaporite basin controlled the shape of the outer lufilian arc we infer three original salt provinces the large scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other precambrian basins

morphostructure at the junction between the beata ridge and the greater antilles island arc offshore hispaniola southern slope abstract oblique convergence between the caribbean plate s interior and the inactive greater antilles island arc has resulted in the collision and impingement of the thickened crust of the beata ridge into southern hispaniola island deformation resulting from this convergence changes from a low angle southward verging thrust south of eastern hispaniola to collision and uplift in south central hispaniola and to left lateral transpression along the southern peninsula of haiti in western hispaniola using new swath bathymetry and a dense seismic reflection grid we mapped the morphological structural and sedimentological elements of offshore southern hispaniola we have identified four morphotectonic provinces the dominican sub basin the muertos margin the beata ridge and the haiti sub basin the lower slope of the muertos margin is occupied by the active muertos thrust belt which includes several active out of sequence thrust faults that were they to rupture along their entire length could generate large magnitude earthquakes the interaction of the thrust belt with the beata ridge yields a huge recess and the imbricate system disappears the upper slope of the muertos margin shows thick slope deposits where the extensional tectonics and slumping processes predominate the northern beata ridge consists of an asymmetrically uplifted and faulted block of oceanic crust our results suggest that the shallower structure and morphology of the northern beata ridge can be mainly explained by a mechanism of extensional unloading from the upper cretaceous onward that is still active residually along the summit of the ridge the tectonic models for the northern beata ridge involving active reverse strike slip faults and transpression caused by the oblique convergence between the beata ridge and the island arc are not supported by the structural interpretation the eastern bahoruco slope an old normal fault that acts as a passive tear fault accommodating the sharp along strike transition from low angle thrusting to collision and uplifting

venezuela basin of the caribbean sea stratigraphy and sediment distribution abstract the venezuela basin of the caribbean sea averages approximately 4800 m deep and is underlain by oceanic type crust it is bounded on the east and west by pre cenozoic ridges and on the north and south by late cenozoic deformed belts the age of the crust underlying the venezuela basin has not been firmly established but based upon drilling it is at least as old as late cretaceous the age lithology and origin of rock which underlies seismic reflecting horizon b the deepest horizon encountered by drilling is uncertain rocks which coincide with horizon b are tholeiitic basalt sills intruding pelagic carbonate sedimentary rock of late cretaceous age sedimentary strata above horizon b in the central venezuela basin are well known from deep sea drilling and by seismic delineation of their thickness the dominant sediment is pelagic carbonate seismic horizon a marks the onset of lithification pelagic chalk and marl oozes overlie limestones and marl sediments in the a b interval average approximately 500 m in thickness but vary from 0 to over 1000 m in thickness the thickest sections coincide with the venezuelan abyssal plain and with the base of the beata ridge in the northwest corner of the basin in the south central portion of the basin most of the a b section is absent the post a sediments average approximately 400 m in thickness with variations between 100 and 2600 m the principal thickening in this section is in the southeastern corner of the basin bounded by the curacao and aves ridges abyssal plain turbidites appear to be limited to the post a section

prediction of fracture density using genetic algorithm support vector machine based on acoustic logging data the existing methods of well logging interpretation cannot be applied accurately to the exploration and evaluation of carbonate reservoirs because of the fracture development based on the fracture density obtained by core analysis in a carbonate reservoir located in the ordos basin the west of china three types of fracture density low medium and high fracture density of the target formation were identified we investigated the effect of fractures on acoustic logging signals in time and frequency domain by hilbert huang transform hht and extracted 11 features in time domain and 9 features in frequency domain then we reduced the features in time and frequency domain to three principal components by principal component analysis pca finally a new prediction model of ga svm method based on hilbert huang transform of acoustic logging data was reported to predict the fracture density the results show that fracture density has a greater effect on the attenuation of imf2 and imf3 components for thre

shallower structure and geomorphology of the southern puerto rico offshore margin oblique convergence between the north american and caribbean plates along the eastern greater antilles island arc has yielded the asymmetric muertos thrust belt in the backarc region offshore south of puerto rico this thrust belt disappears and is replaced by a succession of neesw and eew trending deep basins and steep ridges that characterize the western anegada passage resulting in a complex deformation pattern using new high resolution multibeam bathymetry and seismic reflection profiles we studied the geomorphology and shallower structure of the southern puerto rico offshore margin we have identified four morphotectonic provinces the puerto rican sub basin and muertos trough the muertos margin the insular shelf and the western anegada passage the muertos margin province shows two distinct slope sub provinces the active muertos thrust belt e which includes lower and upper thrust belts with distinct deformational styles and lateral continuity e and the shelf slope highly incised by a dense canyon network this network is disrupted by the investigator fault zone consisting of a 130 km long eew trending band of active extensional deformation the investigator fault zone shows differential surface expression caused by along strike changes in the magnitude and distribution of the deformation though this deformation is driven by a nes oriented extension in the western anegada passage province the whiting basin and whiting and grappler ridges are formed by large dip slip normal faults driven by a nwese oriented extensional regime the western st croix rise shows a complex structure where the neesw trending nw dipping normal faults observed at the summit of the rise predate the eew bounding faults that could accommodate the extensional deformation at the present this study provides detailed observations on the active tectonic and sedimentary processes to help future studies assessing the natural resources and the seismic and tsunamigenic hazard in the puerto rico region

anhydrite pseudomorphs and the origin of stratiform cu co ores in the katangan copperbelt democratic republic of congo the stratiform cu co ore mineralisation in the katangan copperbelt consists of dispersed sulphides and sulphides in nodules and lenses which are often pseudomorphs after evaporites two types of pseudomorphs can be distinguished in the nodules and lenses in type 1 examples dolomite precipitated first and was subsequently replaced by cu co sulphides and authigenic quartz whereas in type 2 examples authigenic quartz and cu co sulphides precipitated prior to dolomite and are coarse grained the sulphur isotopic composition of the copper cobalt sulphides in the type 1 pseudomorphs is between 10 3 and 3 1 relative to the vienna canyon diablo troilite indicating that the sulphide component was derived from bacterial sulphate reduction bsr the generation of text hco 3 during this process caused the precipitation and replacement of anhydrite by dolomite a second product of bsr is the generation of h2s resulting in the precipitation of cu co sulphides from the mineralising fluids initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core precipitation of authigenic quartz was most likely induced by a ph decrease during sulphide precipitation fluid inclusion data from quartz indicate the presence of a high salinity 8 18 eq wt nacl fluid possibly derived from evaporated seawater which migrated through the deep subsurface 87sr 86sr ratios of dolomite in type 1 nodules range between 0 71012 and 0 73576 significantly more radiogenic than the strontium isotopic composition of neoproterozoic marine carbonates 87sr 86sr 0 7056 0 7087 this suggests intense interaction with siliciclastic sedimentary rocks and or the granitic basement the low carbon isotopic composition of the dolomite in the pseudomorphs 7 02 and 9 93 relative to the vienna pee dee belemnite v pdb compared to the host rock dolomite 4 90 and 1 31 v pdb resulted from the oxidation of organic matter during bsr

rare earth elements in deep water articulated brachiopods an evaluation of seawater mass abstract modern articulated brachiopods of the orders rhynchonellids and terebratulids were obtained from below the neritic zone of the caribbean sea north atlantic south pacific and southern oceans to evaluate the proxy potential of their rare earth element ree contents shells of the two orders display similar ree distribution coefficients that vary consistently within the series in response to changes in ionic radii relative to that of the ca 2 cation which in turn excludes the possibility of a biological control on their ree incorporation the calculated log k d values for the brachiopods are similar to those documented in other investigations and as a consequence shell calcite is enriched in ree concentrations relative to ambient water masses by an average of 0 81 10 5 brachiopods from the four areas yielded lree sn and mree sn characteristics of their ambient water mass exhibiting gradual enrichments with increasing atomic number as well as negative ce sn excursions their hree sn values on the other hand define a trend that varies with water depth overall the ce ce profile of open water deep sea brachiopods is coupled with that of the ambient seawater our study confirms the robustness of rees in deep water 500 m articulated brachiopods and supports their use as a proxy of paleo oceanography specifically water mass and redox

metal fixation by bacterial cell walls all biomass contains a significant quantity of metallic constituents and mineralization in living and dead biodebris may contribute to element transport from the hydrosphere into sediments the anionic cell walls of bacteria are remarkable in their ability to fix metals and provide sites for nucleation and growth of minerals results presented show the types of cell wall polymers that are responsible for metal binding in walls of gram positive and gram negative bacteria

pressure solution and hydrothermal recrystallization of carbonate sediments an experimental study reply abstract the extent of calcite recrystallization was determined in pressure solution and hydrothermal experiments which were conducted on deep sea carbonates of low mg calcite iceland spar and reagent grade calcite powder in the pressure solution experiments wet sediments were subjected to confining pressures of 500 1500 bars and pore pressures of 150 500 bars at temperatures between 22 and 180 c for 21 240 h the hydrothermal experiments were performed in sealed teflon coated bombs at 200 c zero effective stress in sulfate free sea water for two weeks the hydrothermal system was solution dominated the extent of calcite recrystallization was determined by measuring the oxygen isotopic compositions of the pore fluids and solids before and after each experiment scanning electron microscope observations porosity and specific surface area measurements were performed in fine grained carbonate samples subjected to high effective stresses the mechanism for recrystallization apparently involves both relief of strain energy at grain to grain contacts and decrease in surface free energy by decreasing the surface area while in coarse grained carbonates relief of strain energy appears to be the most important control of recrystallization in the hydrothermal experiments however decrease in surface free energy is the only driving force for recrystallization effective stress increased the rate of calcite recrystallization in both pressure solution and hydrothermal experiments clay minerals retarded the reaction the effects of diatomite and basaltic glass on the extent of calcite recrystallization was investigated only in the hydrothermal experiments both admixed non carbonate materials retarded the reaction diatomite being the most effective inhibitor surface chemical reactions seem to be responsible for the observed inhibitions of calcite recrystallization increases in the extent of calcite recrystallization with increasing ionic strength were observed in hydrothermal experiments in nacl solutions of five different ionic strengths

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

the nature of fossil bacteria a guide to the search for extraterrestrial life in an attempt to establish reliable criteria for the identification of potential fossil life in extraterrestrial materials the fossilizable characteristics of bacteria namely size shape cell wall texture association and colony formation are described and an overview is given of the ways in which fossil bacteria are preserved as compressions in fine grained sediments preservation in amber permineralized by silica replacement by minerals such as silica pyrite fe mn oxides calcite phosphate and siderite or as molds in minerals the problem of confounding minerally replaced bacteria with non biological structures having a bacterial morphology is addressed examples of fossilized bacteria from the early archaean through to the recent are used to illustrate the various modes of preservation and the morphology of fossil bacteria

bacteria as nucleation sites for authigenic minerals in a metal contaminated lake sediment abstract sediment samples contaminated with metals arising from mine tailings drainage were obtained from lower moose lake in the onaping region near sudbury ontario canada the samples were examined by electron microscopy selected area electron diffraction and energy dispersive x ray spectroscopy individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic fe al silicate the principal metal sulfide species associated with the bacteria were amorphous mackinawite fes 1 x and microcrystalline millerite nis trace amounts of cu and zu were also detected in some of the sulfide precipitates at least two structural forms of the fe al silicate were present and energy dispersive x ray spectroscopy point analyses revealed corresponding differences in chemical composition poorly ordered limonitic clay type phases had a granular morphology and contained less fe than well developed crystalline material which generated hexagonal diffraction patterns with reflections d 4 60 and 2 55 characteristic of an interstratified chamositic clay

imaging of near borehole structure using full waveform sonic data the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near borehole structural features these arrivals are used to form an image of the near borehole structural features in a manner similar to seismic migration possible uses of this technique include horizontal well logging structural dip and contour determination fault salt dome pinnacle reef and fracture zone imaging and eor steam flood monitoring since both the source and the receivers pass through structures that cross the borehole the downdip structure and the updip structure can be imaged separately the technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis separate matrices are formed for the updip and for the downdip raypaths the basic technique is illustrated with synthetic data generated to approximate the case of a sonic tool logging

insights into cyanobacterial fossilization in ediacaran siliciclastic environments ediacaran sedimentary successions are noted for the preservation of microbes and microbial textures on the surfaces of sandstones and siltstones although microorganisms have been preserved in coarse grained siliciclastic sand throughout geologic history the exceptional preservation of microbes in ediacaran sediments suggests the potential for a unique taphonomic window here we identify conditions conducive to the fossilization of filamentous cyanobacteria growing in the presence of siliciclastic sand and demonstrate that the sheaths of filamentous cyanobacteria can become coated by clay minerals within days under oxic conditions smooth extensive mineral coatings develop in the presence of 5 6 to 55 6 mg l of suspended clay and 0 1 mm or greater concentrations of dissolved silica thus elevated concentrations of seawater silica and the delivery of suspended clays promote microbial preservation on sandy and silty surfaces these factors likely facilitated microbial fossilization in coarse grained siliciclastic sand throughout the ediacaran period and may have also contributed to microbial fossilization in siliciclastic deposits at other times throughout earth s history

late cretaceous miocene diachronous onset of back thrusting along the south caribbean deformed belt and its importance for understanding processes of arc collision and crustal growth 1 the venezuelan basin is obliquely subducted to the east southeast beneath the continental south american plate at the east west trending south caribbean deformed belt scdb a 50 100 km wide wedge of accreted sedimentary rocks derived both from offscraping of the 0 5 to 1 km thick sedimentary cover of the subducting venezuelan basin and from incorporation of clastic sediments derived from the continental margin of south america in this paper we describe the structure and sequence stratigraphy of five late cretaceous to recent sedimentary sequences overlying a 230 000 km2 area of the southern venezuelan basin and beata ridge the data set includes 5900 km of 2d seismic reflection data that are tied to lithologic and age data taken from dsdp sites 146 149 and 150 in the central part of the venezuelan basin we use observed patterns of wedging in mapped and dated sedimentary sequences adjacent to the scdb to determine the age of the diachronous west to east initiation of thrusting and subduction at the scdb the onset of subduction in the scdb corresponds with the cessation of thrusting from west to east along the zone of collision between the intraoceanic caribbean arc and the south american continent this correlation in the age of deformation indicates that collision between the caribbean arc and the south american continent led to subduction polarity reversal a process that is commonly observed at various stages of tectonic evolution in other areas of active arc continent collision including japan and sunda

multipole borehole acoustic waveforms synthetic logs with beds and borehole washouts we give a two dimensional 2 d velocity stress finite difference vs fd formulation for the multipole borehole acoustic logging problem irregular boreholes horizontal bedding and axially varying alteration are encompassed by the model excellent agreement is obtained with transform techniques for constant radius boreholes and homogeneous formations the method is used to generate waveforms for many positions of transmitter and receiver array relative to beds and washouts as in logging first motion and semblance techniques are applied to the waveforms to extract formation slownesses resulting in synthetic multipole logs which demonstrate the response of ideal multipole logging tools and signal processing to several environmental effects for monopole compressional first motion logs residual slowness errors remain at borehole washouts after borehole compensation these errors increase as the effective measure point in the waveforms is moved back in time from true first motion admitting greater in

exceptionally preserved fossil assemblages through geologic time and space abstract geologic deposits containing fossils with remains of non biomineralized tissues i e konservat lagerstatten provide key insights into ancient organisms and ecosystems such deposits are not evenly distributed through geologic time or space suggesting that global phenomena play a key role in exceptional fossil preservation nonetheless establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages to this end we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life 610 to 3 ma our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters each signifying an ancient geographic region up to hundreds of kilometers in scale which repeatedly developed conditions conducive to soft tissue preservation using a novel hierarchical clustering approach we show that these clusters decrease in number and shift from open marine to transitional and non marine settings across the cambrian ordovician interval conditions conducive to exceptional preservation declined worldwide during the early paleozoic in response to transformations of near surface environments that promoted degradation of tissues and curbed authigenic mineralization potential we propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation which affected virtually all taphonomic pathways in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation after these transitions exceptional preservation rarely occurred in open marine settings excepting times of widespread oceanic anoxia when low oxygen levels set the stage with these patterns non marine cluster count is correlated with non marine rock quantity and generally decreases with age this result suggests that geologic processes which progressively destroy terrestrial rocks over time limit sampling of non marine deposits on a global scale future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record

precambrian microbe like pseudofossils a promising solution to the problem of various problems that have hindered progress in documenting the precambrian history of life the difficulty in distinguishing between bona fide microbial fossils and nonbiological microscopic pseudofossils has been among the most serious though errors in the interpretation of putative precambrian fossil microbes have diminished greatly over recent years mistakes continue to be made we suggest that such errors can be avoided by the use of a multifaceted strategy based on a specified series of biologically definitive characteristics that document the presence of interrelated biological morphology and biologically derived chemistry to illustrate this promising approach we use optical microscopy confocal laser scanning microscopy and raman spectroscopy together to distinguish between authentic microbial fossils and microscopic look alikes both coccoidal and filamentous rock embedded in five proterozoic and two archean geological units bona fide fossils of the 800 ma bitter springs formation of central australia and 3050 ma farrel quartzite of northwestern australia and objects we regard to be pseudofossils from the 770 ma chanda limestone of southern india 800 ma myrtle springs formation of south australia 1020 ma lakhanda formation of southeastern siberia 1700 ma vempalle formation of central india and 2629 ma marra mamba iron formation of northwestern australia the results demonstrate that no single criterion by itself is sufficient to establish the biological origin of such objects instead as shown here this problem appears solvable by the use of an interdisciplinary approach that combines the data and techniques of geology biology and chemistry