heavy mineral sands in the eucla basin southern australia deposition and province scale prospectivity the marine eucla basin in southern australia is emerging as a major new heavy minerals province in australia beach placers are associated with a series of partially buried cenozoic coastal barrier sands formed along an arcuate 2 000 km long basin margin the trace of which is up to 320 km inland of the present coastline the presence of high grade deposits with dominant zircon over ilmenite and lesser amounts of rutile and leucoxene was established with the discovery of the jacinth and ambrosia heavy mineral deposits in late 2004 an additional 10 heavy mineral prospects were subsequently identified and are at various stages of evaluation the eucla basin and its adjacent paleovalley system have a large areal extent that contains a complicated succession of marine and nonmarine strata spanning a wide range of depositional environments four distinct constructional phases for the development of shorelines can be recognized and correlated with major third order sea level events established by others from the marine depositional record as occurring during the middle eocene 42 5 ma late middle eocene 39 36 ma late eocene 36 34 ma and miocene pliocene 15 2 6 ma prevailing westerly winds built extensive dune systems by longshore drift sediment movement was from west to east detrital zircon rocks from the ooldea and barton barriers show a distribution of zircon age that is consistent with the proterozoic musgrave province to the north of the basin as the dominant primary source area of the heavy minerals with a contribution from the albany fraser orogen to the west the likelihood is that these heavy minerals have been recycled via sedimentary basins that flank the musgrave province and include the neoproterozoic to cambrian officer basin and permian to mesozoic deposits of the bight basin our current depositional model is summarized as follows 1 initial rapid transgression and deposition of a shallow marine sand sheet subsequently overlain by shallow marine limestone during middle eocene 2 a major eocene transgression and deposition of a shelf barrier and lagoonal shoreface marine complex during the late middle eocene 3 further transgression and highstand deposition during the late eocene 4 renewed transgression of barrier lagoonal and possibly flooding deltaic sand blanket in the southeastern coastal plain with neotectonic uplift tilting in the western eucla margin during mio pliocene time each stage of reworking increased the potential for heavy mineral concentration in placer deposits

the role of the calcium carbonate calcium phosphate switch in the mineralization of soft bodied fossils authigenic minerals play an important role in the preservation of most soft bodied fossils the greatest detail is preserved in apatite calcium phosphate but its precipitation is usually inhibited by the high concentrations of hco 3 in aqueous settings nonetheless investigations of soft bodied biotas have revealed very early authigenic calcite crystal bundles in close association with phosphatized soft tissues this demonstrates that the geochemical controls on soft tissue mineralization are dynamic and act on a very local scale direct comparisons with experimental results permit the conditions of fossilization to be inferred

stress state in the cuu long and nam con son basins offshore vietnam abstract the results presented in this paper are the first published estimates of the complete stress tensor in the cuu long and nam con son basins offshore vietnam we analysed in situ stress and pore pressure fields in the sedimentary formations using data from petroleum exploration and production wells to evaluate the stress state in these basins the data were obtained from the seafloor to 4300 m burial depth and include both hydrostatic and overpressured sections minimum horizontal stresses were obtained from extended leak off tests and mini fracture tests maximum horizontal stresses were estimated from drilling induced fracture parameters and borehole breakout widths in twelve wells using rock properties measured in the laboratory or estimated empirically from wireline logs seven data points are located in sediments and seventeen data points in igneous basement rocks at depths greater than 3000 m the estimated magnitudes of h are 70 110 of the v magnitudes considering the errors in the stress magnitude estimates their relative magnitudes suggest that a borderline normal strike slip stress regime presently exists in normally pressured sequences of the nam con son and cuu long basins of the twenty four data points twenty have effective stress ratios at a critical stress state for faulting on the assumption that there are faults present that are optimally oriented for failure with friction coefficients of 0 5 the results suggest that the stress state in these basins is generally critical

the stratigraphic relationship between the shuram carbon isotope excursion the oxygenation of neoproterozoic oceans and the first appearance of the ediacara biota and bilaterian trace fossils in northwestern canada abstract a mechanistic understanding of relationships between global glaciation a putative second rise in atmospheric oxygen the shuram carbon isotope excursion and the appearance of ediacaran type fossil impressions and bioturbation is dependent on the construction of accurate geological records through regional stratigraphic correlations here we integrate chemo litho and sequence stratigraphy of fossiliferous ediacaran strata in northwestern canada these data demonstrate that the fad of ediacara type fossil impressions in northwestern canada occur within a lowstand systems tract and above a major sequence boundary in the informally named june beds not in the early ediacaran sheepbed formation from which they were previously reported this distinction is substantiated by 13 c carb chemostratigraphy of the sheepbed carbonate which overlies the sheepbed formation and the gametrail formation which overlies the june beds the sheepbed carbonate hosts heavy 13 c carb values whereas the gametrail formation contains a large 13 c carb excursion which we correlate with the globally recognized shuram excursion stratigraphically above the gametrail excursion the first bilaterian burrows are present in the basal blueflower formation together these data allow us to construct an age model for ediacaran strata in northwestern canada and conclude that a purported shift in fe speciation in the sheepbed formation significantly predates the shift recorded above the ca 582 ma gaskiers glaciation in newfoundland and the first appearance of ediacaran biota the gametrail excursion shares many characteristics with shuram negative 13 c carb excursion 1 13 c carb and 18 o carb covary 2 13 c carb and 13 c org do not covary 3 the excursion is developed during a transgressive systems tract and recovers in an highstand systems tract and 4 values in some sections are well below mantle 13 c input values but are variable between sections we relate regional lateral variability in the magnitude and character of this excursion to condensation and diachronous deposition during the transgression and local authigenic carbonate production in light of these observations we explore a variety of models for the genesis of the shuram excursion and suggest that the location and amount of authigenic carbonate production played a role in the excursion

aragonite crystal fans in neoproterozoic cap carbonates a case study from brazil and implications for the post snowball earth coastal environment abstract neoproterozoic cap carbonates are characterized by anomalous sedimentary features including sea floor precipitates represented by aragonite pseudomorph cement crusts and crystal fans these features are found in restricted periods of geological history and their generation in the context of the neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation new sedimentary and geochemical results on well preserved neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the sete lagoas formation bambui group central brazil are reported the micrite settling facies shows the highest abundance of crystal fans arranged in centimeter scale layers interfingered with the micrite matrix this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters a numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pco2 in these conditions the mg ca ratio of the neoproterozoic seawater is calculated at ca 1 2 alkalinity oversaturation is monitored by a variable evaporation degree aragonite formation is kinetically favored and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pco2 hence alkalinity is higher these modeling conditions do not preclude the contribution of incomplete organic matter degradation to alkalinity as suggested by negative carbon isotope signatures in addition the large number of aragonite micrite pairs suggests a seasonal or paleoclimatic forcing other cases of abiotic aragonite precipitation through time are also briefly examined by comparison

well planning for swf and overpressures at the kestrel well editor s note this article is excerpted from a presentation at the 2001 otc drilling in deepwater has forced a more complete understanding of and appreciation for pore pressures and fracture gradients shallow water flow swf sands perhaps exemplify the most extreme example of how narrow the window between pore pressures and fracture gradients can be as stated by bp s mark alberty at the 2000 otc paper otc11971 swf sands are overpressured unconsolidated sands encountered before pressure control is in place on the wellhead assembly they may flow into the well if their pore pressures are not balanced by the mud weights however if the mud weights are too highly overbalanced the sands may fracture resulting in lost returns and loss of well control the margins for wellbore pressures may be less than 100 psi for shallow depths below mudline in deepwater wells the kestrel well was drilled in a water depth of 2795 ft in mississippi canyon block 632 an area of well documented swf data from nearby wells were incorporated in designing the casing and mud weight program essential to the design were predictions of pore pressures and lithology efficient well design required a pore pressure prediction for the swf section and the deeper overpressured sections accurate predictions required input from a pore pressure analyst geophysicists seismic interpreters operations geologists and drilling engineers the kestrel well was designed with information from a number of wells but a great deal of the most important information came from the mississippi canyon 718 1 well approximately 7 miles wsw of the kestrel well planning for swf in this well was very well presented at the otc in 1996 identification of flowing water sand drilling hazards in the deepwater gulf of mexico by byrd et al figure 1 is a seismic line extracted from a 3d seismic data

microfossils associated with silicified stratifera undata komar 1966 from the late proterozoic bambui group south central brazil abstract small partly silicified laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower bambui group late proterozoic near unai minas gerais south central brazil are here described and classified as stratifera undata komar 1966 rare moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms there being no convincing evidence of filamentous forms if in fact coccoidal micro organisms were the overwhelmingly dominant mat formers this represents an unusual situation among silicified microflorules from morphologically distinct precambrian stromatolites yet is quite similar to that observed in another form of stratifera s biwabikensis from the gunflint iron formation which like the unai stromatolites also formed under permanently submerged conditions s undata is now known in brazil from similar upper proterozoic settings in two very widely separated localities which suggests its potential use in regional stratigraphic correlation as a result of this study we recommend that greater attention be paid to the simpler stromatolite morphologies especially when micro fossiliferous since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same more complex forms inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper precambrian rocks

subsurface fractures in situ stress and permeability variations in the walloon coal measures eastern surat basin queensland australia abstract in situ stress pattern and subsurface fractures control the production of coal seam gas csg reservoirs and hence comprehensive analyses are required for better development of these reservoirs the surat basin is a prolific csg province with major economic impacts in eastern australia recent studies have reported significant stress variations in this basin due to local geological structures however a systematic analysis of localised stress perturbations within the basin and their impacts on reservoir permeability have not been presented thus far in the published literature in this paper we combine different sets of data including conventional log data wellbore image logs 2d seismic lines and interpreted permeability from well test data in order to characterise the role of geological structures in the subsurface fluid flow of the walloon coal measures in eastern surat basin analysis of 7 3 km of borehole image log across 33 vertical wells revealed a regional ene wsw orientation of the maximum horizontal stress shmax in the study area however there are massive variability in the shmax orientation spatially and with depth analysis of 2d seismic lines highlight the role of geological structures in stress perturbation and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub parallel to the acute angle of local shmax orientation interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in situ stress rotation is observed this indicates the influence of local shmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the surat basin

corrigendum strange bedfellows glacial diamictite and cap carbonate from the marinoan 635 ma glaciation in namibia tectonic fold tests conducted in namibia demonstrate that the inclination with respect to bedding of geoplumb palaeovertical tubular structures in the marinoan 635 ma syndeglacial cap dolostone is mainly the result of tectonic strain therefore tubestone inclination data cannot be used to estimate the gradient of the sea floor on the foreslope of the otavi carbonate platform during the marinoan glaciation a gradient steeper than 0 1 slope angle ca 5 7 implying a glacial base level fall 0 5 km is nevertheless supported by boulder size intraclast debrite in the falling stand wedge directly beneath the glacigenic sequence cryogenian oceans lacked skeletal carbonate production raising the carbonate saturation state and persistent deep water anoxia excluded acid producing aerobic respiration facilitating early diagenetic carbonate precipitation lithification and steep submarine slopes

origin of overpressure and pore pressure prediction in the baram province brunei accurate pore pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited tertiary baram delta province where all economic fields exhibit overpressures commonly of high magnitude and with narrow transition zones a pore pressure database was compiled using wireline formation interval tests drillstem tests and mud weights from 157 wells in 61 fields throughout brunei overpressures are observed in 54 fields both in the inner shelf deltaic sequences and in the underlying prodelta shales porosity vs vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner shelf deltaic sequences however the geology of brunei precludes overpressures in the inner shelf deltaics being generated by any conventional fluid expansion mechanism e g kerogen to gas maturation and we propose that these overpressures have been vertically transferred into reservoir units via faults from the prodelta shales sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer and hence different pore pressure prediction strategies need to be applied in the prodelta shales and inner shelf deltaic sequences sonic and density log data detect overpressures generated by disequilibrium compaction and pore pressures are accurately predicted using an eaton exponent of 3 0 sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly and pore pressures are reasonably predicted using an eaton exponent of 6 5

carbonate deposition climate stability and neoproterozoic ice ages the evolutionary success of planktic calcifiers during the phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate ion concentration the saturation dependent preservation of carbonate in sea floor sediments before this buffering was primarily accomplished by adjustment of shallow water carbonate deposition to balance oceanic inputs from weathering on land neoproterozoic ice ages of near global extent and multimillion year duration and the formation of distinctive sedimentary cap carbonates can thus be understood in terms of the greater sensitivity of the precambrian carbon cycle to the loss of shallow water environments and co 2 climate feedback on ice sheet growth

petrographic and sims pyrite sulfur isotope analyses of ediacaran chert nodules implications for microbial processes in pyrite rim formation silicification and exceptional fossil preservation abstract the lower ediacaran doushantuo formation in the yangtze gorges area contains exceptionally preserved microfossils including the earliest known animal resting eggs and embryos these fossils are preserved in cm sized chert nodules which typically have a microbial mat fragment in the center a silica cortex a pyrite rim and an outer rim of blocky calcite petrographic analysis indicates that the formation of the blocky calcite rim postdates that of the pyrite rim and silica cortex the pyrite rim grew centripetally during early diagenesis representing a reaction front that was determined by the dynamics between ambient fe2 and h2s the latter of which was derived from bacterial sulfate reduction bsr of mat fragment in nodule center the silica cortex was formed pervasively through replacement of carbonate sediments prior to compaction secondary ion mass spectroscopy sims sulfur isotopes of individual pyrite crystals 34spy sims in pyrite rims and matrices show highly positive values 15 2 39 8 the pyrite rims do not show an isotopic gradient between small crystals formed in outer rim during early diagenesis and large crystals formed in inner rim during subsequent overgrowth although rim pyrite in the same chert nodule has consistent 34spy sims values there are significant spatial and stratigraphic variations in 34spy sims values of both matrix and rim pyrite overall isotopic fractionation between pyrite and carbonate associated sulfate cas is small

systematic assessment of fault stability in the northern niger delta basin nigeria implication for hydrocarbon prospects and increased seismicities abstract accurate information on fault networks the full stress tensor and pore fluid pressures are required for quantifying the stability of structure bound hydrocarbon prospects carbon dioxide sequestration and drilling prolific and safe wells particularly fluid injections wells such information also provides essential data for a proper understanding of superinduced seismicities associated with areas of intensive hydrocarbon exploration and solid minerals mining activities pressure and stress data constrained from wells and seismic data in the northern niger delta basin nndb nigeria have been analysed in the framework of fault stability indices by varying the maximum horizontal stress direction from 0 to 90 evaluated at depths of 2 km 3 5 km and 4 km we have used fault dips and azimuths interpreted from high resolution 3d seismic data to calculate the predisposition of faults to failures in three faulting regimes normal pseudo strike slip and pseudo thrust the weighty decrease in the fault stability at 3 5 km depth from 1 2 mpa to 0 55 mpa demonstrates a reduction of the fault strength by high magnitude overpressures pore fluid pressures 50 mpa have tendencies to increase the risk of faults to failure in the study area statistical analysis of stability indices si indicates faults dipping 50 60 80 90 and azimuths ranging 100 110 are most favourably oriented for failure to take place and thus likely to favour migrations of fluids given appropriate pressure and stress conditions in the dominant normal faulting regime of the nndb a few of the locally assessed stability of faults show varying results across faulting regimes however the near similarities of some model based results in the faulting regimes explain the stability of subsurface structures are greatly influenced by the maximum horizontal stress shmax direction and magnitude of pore fluid pressures

neoproterozoic peritidal phosphorite from the sete lagoas formation brazil and the precambrian phosphorus cycle the neoproterozoic sete lagoas formation ca 610 ma of the sao francisco basin brazil is a succession of siltstone limestone and phosphorite phosphorite forms part of a previously unrecognized 150 to 200 m thick unconformity bounded depositional sequence lithofacies stacking patterns indicate that deposition was punctuated by higher order fluctuations in base level that produced aggradational parasequences these shallowing upward cycles record the progradation of phosphate rich intertidal flats over shallow subtidal deposits as accommodation filled the presence of mudcracks authigenic chert nodules lack of coarse terrigenous clastics and the abundance of silt with fine abraded quartz grains suggests accumulation along an arid coastline with significant aeolian input delivery of phosphorus adsorbed on aeolian fe oxyhydr oxide and clay is interpreted as having stimulated phosphogenesis in peritidal environments lithofacies associations indicate that windblown phosphorus promoted the establishment of cyanobacterial communities along the coast which produced photosynthetic oxygen and the suboxic conditions necessary for the precipitation of authigenic carbonate fluorapatite as in other precambrian phosphatic systems nearshore oxygen oases were a prerequisite for phosphorite accumulation because redox sensitive phosphogenic processes were pushed into the sediment to concentrate phosphorus in more distal anoxic environments phosphorite could not form because these biotic and abiotic processes were suspended in the water column which cycled phosphorus in sea water rather than at the sediment water interface such shallow water phosphorite is unlike larger younger neoproterozoic phanerozoic phosphatic deposits inferred to have formed in deeper upwelling related environments the increasing size of phosphatic deposits through the latest precambrian is interpreted as reflecting the progressive ventilation of the oceans during the neoproterozoic oxygenation event and resultant expansion of phosphogenic environments into distal settings the widespread cycling of bioavailable phosphorus at the sea floor not only produced the first true phosphorite giants but may have also been an important precondition for the evolution of multicellular animals

the present day state of tectonic stress in the darling basin australia implications for exploration and production knowledge of the full present day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs the darling basin of new south wales australia is an old sedimentary basin late cambrian silurian to early carboniferous in which there was limited information about the present day stress field prior to this study in this paper we evaluate the contemporary stress field of the darling basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability fracture fault generation and reactivation our interpretations of borehole failures in borehole image logs reveal a prevailing east west orientation of the maximum horizontal stress throughout the darling basin the estimates of the magnitudes for the vertical minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike slip faulting stress regime at 600 700 m depths where the magnitude of vertical stress and minimum horizontal stress are close to each other however the presence of borehole breakouts and drilling induced tensile fractures that we observe in the image logs at greater depths 900 2100 m indicate a transition into a strike slip tectonic stress regime below a depth range of approximately 700 900 m these findings are in agreement with overcoring stress measurements east and west of the investigated wells furthermore there are several neogene to recent geological structures in the study area that indicate thrust faulting with an east west oriented maximum horizontal stress orientation around this old sedimentary basin the consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data however the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike slip stress regime sh sh similar to sv indicated by petroleum data and highlights a potential pitfall of using neotectonic structures in geomechanical models in particular careful attention and verification should be made when using neotectonic structures for input calibration or confirmation of geomechanical models especially in intraplate tectonic settings such as australia c 2016 elsevier ltd all rights reserved

vertically transferred overpressures in brunei evidence for a new mechanism for the formation of high magnitude overpressure overpressures in sedimentary basins are commonly assumed to be the result of two distinct and separate mechanisms disequilibrium compaction and fluid expansion however the potential for overpressures to be redistributed or transferred to other pressure compartments over time has been seldom considered and rarely demonstrated pore pressure data and velocity effective stress plots from 61 fields across the baram delta province of brunei northwest borneo reveal that two different types of overpressure occur in stratigraphically defined sections the basal pro delta shales contain overpressures generated by disequilibrium compaction whereas the overlying sand shale deltaic sequence contains overpressures that appear to be generated by fluid expansion however the geology of the deltaic sequences and high magnitude of the pore pressures precludes the overpressures in the deltaic sequences being generated by any conventional fluid expansion mechanism such as kerogen to gas maturation or clay diagenesis the fluid expansion overpressures are located in fields that were inverted during the pliocene an event that resulted in large scale fluid migration from the pro delta shales into the deltaic sequences including charging of the numerous oil fields in the inner shelf hence we propose that the overpressures in brunei provide the first evidence for a new overpressuring mechanism whereby overpressured fluids have been vertically transferred from the pro delta shales into the deltaic sequences during basin inversion furthermore vertical transfer may be a mechanism for explaining overpressures observed in basins that have been recently uplifted or inverted

relationship between structural style overpressures and modern stress baram delta province northwest borneo christopher k morley mark tingay and richard hillis australian school of petroleum university of adelaide adelaide australia rosalind king

identification and calculation of transfer overpressure in the northern qaidam basin northwest china the origin of the overpressure in the northern qaidam basin has not been clearly understood which has caused some difficulties in hydrocarbon exploration using a compaction study we applied a modified acoustic velocity and effective stress diagram to identify the overpressure transfer in the study area this phenomenon has not been discussed in previous studies for the present study we approximately calculated the magnitude of the transfer overpressure and analyzed the cause of the overpressured aquifer at the crest of the anticline in the study area our study indicates that the effect of overpressure transfer is very distinct and the largest contribution to the total overpressure is 57 the main media of overpressure transfer include vertical faults and lateral conducting layers the vertical faults can connect deep overpressured strata and the lateral conducting layers can connect overpressured strata at the top and wing of the anticline during anticline formation the crest fractures and then the overpressured water in the anticline wing flows into the fractured crest and forms the overpressure compartment that prevents the charging of deeper natural gas

evidence for overpressure generation by kerogen to gas maturation in the northern malay basin gas generation is a commonly hypothesized mechanism for the development of high magnitude overpressure however overpressures developed by gas generation have been rarely measured in situ with the main evidence for such overpressures coming from source rock microfractures the physical necessity of overpressures for primary migration laboratory experiments and numerical modeling indeed previous in situ observations suggest that gas generation only creates highly localized overpressures within rich source rocks pore fluid pressure data and sonic velocity vertical effective stress plots from 30 wells reveal that overpressures in the northern malay basin are primarily generated by fluid expansion and are located basinwide within the miocene 2a 2b and 2c source rock formations the overpressures are predominantly associated with gas sampled in more than 83 of overpressure measurements and have a sonic density response consistent with gas generation the association of fluid expansion overpressures with gas combined with the sonic density response to overpressure and a regional geology that precludes other overpressuring mechanisms provides convincing in situ evidence for basinwide gas generation overpressuring overpressure magnitude analysis suggests that gas generation accounts for approximately one half to two thirds of the measured excess pore pressure in the region with the remainder being generated by coincident disequilibrium compaction thus the data herein suggest that gas generation if acting in isolation is producing a maximum pressure gradient of 15 3 mpa km 0 676 psi ft and not lithostatic magnitudes as commonly hypothesized the gas generation overpressures in this article are not associated with a significant porosity anomaly and represent a major drilling hazard with traditional pore pressure prediction techniques underestimating pressure gradients by 2 3 1 5 mpa km 0 1 0 07 psi ft

in situ stress analysis in the yinggehai basin northwestern south china sea implication for the pore pressure stress coupling process abstract the yinggehai basin is located in the triangle between the sunda block south china block and the south china sea and is an interesting target for in situ stress investigation not only because of its specific tectonic position but also due to its abnormally high pore pressures magnitudes and orientations of in situ stress were directly constrained by measurements operated in the vertical boreholes that were drilled in the yinggehai basin gradients and magnitudes of the least horizontal stress remarkably increase with overpressure enhancement indicating that the least principal stress and pore pressure are in fact coupled the ratios of pore pressure stress coupling h pp are similar in the north and south central yinggehai basin despite the burial depths of overpressure top are different however a deeply buried strike slip faulting regime under wnw ese oriented horizontal compression was identified in the central yinggehai basin where the h pp ratios have intermediate values ranging from that for an idealized normal faulting stress regime to that for the southeastern south china a strong strike slip or even reverse faulting stress regime additionally the wnw ese orientations of maximum horizontal stress determined from drilling induced tensile fractures in the yinggehai basin are inappropriate for the present maximum horizontal stress derived from earthquake focal mechanism solutions in the northern segment of the red river faults suggesting that the dextral movement of red river faults has slightly influenced the present stress state in the yinggehai basin the present stress state of the yinggehai basin was primarily generated by the ese absolute horizontal motion of the sunda and south china blocks

seafloor precipitated carbonate fans in the neoproterozoic rainstorm member johnnie formation death valley region usa cm sized carbonate seafloor fans occur in the neoproterozoic rainstorm member of the johnnie formation death valley usa the fans formed in a mixed carbonate clastic succession near storm wave base at the base of parasequences on a storm dominated ramp petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis although not directly dated the rainstorm member preserves a large magnitude negative carbon isotopic anomaly down to 11 pdb tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 ma and 550 ma thus the age distribution of seafloor aragonite fans in neoproterozoic strata appears more widespread than previously thought occurring in strata significantly younger than the last widespread neoproterozoic glaciation rainstorm member carbonate fans and oolitic units representing time correlative shallower environments record similar carbon isotope ratios during the negative carbon isotopic anomaly the carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well mixed rather than stratified during the late neoproterozoic isotopic anomaly in addition the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon on a m to cm scales suggests that the local source of alkalinity required for fan growth was derived from a well mixed reservoir likely seawater rather than local diagenetic fluids increased alkalinity and the presence of inhibitors to carbonate nucleation perhaps fe2 under anoxic conditions likely fostered precipitation of aragonite crystal fans on the seafloor

estimating pore fluid pressure stress coupling abstract fracture pressure fp prediction is a vital component in well planning for safe drilling in high pressure areas in the estimation of seal capacity and seal breach risk analysis for fluid injection and in fracture stimulation for oil gas recovery from tight reservoirs and mature source rocks coupling of pore fluid pressure pp and fp at basin scale has been recognised for several decades with coupling ratio values mostly between 0 46 and 0 87 i e for every 10 mpa of pp increase at constant depth fp increases by 4 6 8 7 mpa these values are similar to estimates for reduction of fp with reservoir depletion of an oil gas field during production poroelasticity has been suggested as the principal reason for coupling most previous coupling values were generated by examination of pressure gradients a new method to estimate pore fluid pressure stress coupling is proposed which plots the measured value of fp from a leak off test lot minus the expected fp value on the trend for normal pressures termed the fracture pressure residual fpr against overpressure op which is the magnitude of pp above normal at the same depth the coupling value is now the slope of fpr against op fpr op coupling values from 11 global basin datasets range from 0 24 to 0 43 approximately half the previous quoted basin scale coupling values a pressure depth model for rapid deposition on a continental margin with op exclusively generated by disequilibrium compaction and no change in effective stress in the overpressured section represents a base case for compaction op without poroelasticity this model helps to reveal 1 that coupling values vary with the ratio of fp to lithostatic pressure sv 2 since the 11 case study coupling values exceed the base case one or more other minor coupling mechanisms are involved potentially including poroelasticity 3 values may be independent of tectonic environment 4 one case study involving carbonate chalk is within the same range as those from siliciclastic sediments and 5 the high coupling pore pressure stress coupling values in the literature result from using fluid and fracture gradients including the shallow data where pp is normal which distort the analysis

present day stress of the central persian gulf implications for drilling and well performance abstract the present day state of stress in the persian gulf is poorly understood but has significant impacts on well drilling and performance the upper permian to lower triassic formation of kangan dalan persian gulf exhibits a complex structural context in the neighborhood of the oman mountains and the zagros orogenies this formation is divided into four reservoir layers k1 to k4 where three main lithologies limestone dolomite and anhydrite are alternating we conduct an analysis of the present day stress and natural fractures at the wellbore using full bore fmi logs leak off test and density logs for this purpose borehole breakout and tensile fracture data are used to determine orientation of s h furthermore density log leak off test and kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of s v s h and s h respectively vertical stress s v gradient at 3100 m depth approximates 20 mpa km 2 9 psi m indicating a bulk density of 2 04 g cm 3 a total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262 m are observed in two wells indicating a mean maximum horizontal stress s h orientation of n53 18 45 for drilling induced tensile fracture ditf data and n50 10 79 for breakout data the mean orientation of s h rotates counterclockwise with depth from k2 n70 4 2 to k4 n40 5 1 reservoirs noticed correlation between these data and stress orientations from earthquake focal mechanism solution first of all indicates that the stresses are linked to the resistance forces generated by the arabia eurasia collision at the zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones in the kangan dalan formation the nw se main open fracture direction is found as a common regional direction which is sub perpendicular to the present day maximum horizontal stress minimum horizontal stress s h gradient in reservoir sections is estimated to be equal to 17 mpa km 2 5 psi m the concluded strike slip stress regime s h s v s h in the study area is consistent with the compressive regime in the zagros thrust fold belt the present day stress in the kangan dalan formation has implications for wellbore stability lost circulation and well inflow performance relationship ipr wells are more unstable if deviated toward the s v direction whereas well productivity and mud loss increase in wells deviated toward s h which conveys the idea of a strike slip faulting effect tends to keep the natural fractures open in that direction

3d spatial variation in vertical stress in on and offshore netherlands integration of density log measurements and basin modeling results abstract the principal vertical stress at a certain depth is equivalent to the weight of the overburden rocks and pore fluids at that depth vertical stress trends will be similar across areas with a simple layer cake lithostratigraphic sequence and similar burial history these conditions are not fulfilled for the lithostratigraphy and burial history of the onshore and offshore netherlands burial histories vary over short distances and resulted in highs platforms and inverted basins with different lithostratigraphic sequences and porosity distributions density log measurements and basin modeling results were used to assess the spatial variation of vertical stress in the dutch subsurface this paper presents the approach and assessed spatial variation in vertical stress the 3d basin modeling proved to be a good tool for correcting and extending density log derived vertical stress depth relations results show that the vertical stress varies spatially and that the vertical stress gradient is less than the standard gradient in large part of the area the largest local deviations from the standard gradient occur in areas with a thick package of low density cenozoic sediments and large low density salt structures i e in the northern offshore relatively old and more dense stratigraphic units occur at relatively shallow depth on the highs and in strongly inverted parts of the mesozoic basins in these areas the vertical stress reaches and follows the standard vertical stress gradient at shallower depths than in other areas

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

transverse drilling induced tensile fractures in the west tuna area gippsland basin australia implications for the in situ stress regime abstract drilling induced tensile fractures ditfs have been interpreted on image logs from vertical wells in the gippsland basin offshore southeastern australia interpreted axial vertical ditfs have previously been well described worldwide we also interpret transverse horizontal ditfs which are horizontal fractures that are electrically conductive non planar bimodal and constrained to the tensile region of the wellbore elasticity theory predicts formation of both transverse and axial drilling induced tensile fractures ditfs in vertical wells depending on the magnitude of the principal in situ stresses pore pressure and mudweight drilling induced tensile fractures initiate in very specific stress environments axial ditfs can closely constrain a lower bound to the maximum horizontal stress s h max magnitude where the minimum horizontal s h min stress is known if transverse ditfs are observed they can constrain a lower bound to maximum and minimum horizontal stress magnitudes the observation of transverse ditfs on image logs can constrain the stress field to one on the border of strike slip and reverse faulting s h max s h min s v without requiring knowledge of the s h min or s h max magnitude the observation of transverse ditfs in the west tuna area combined with wireline log data leak off tests and pore pressure data are used to constrain the in situ stress tensor the interpreted in situ stress tensor lies on the border of a strike slip and reverse faulting regime s h max 40 5 mpa km s h min s v 21 mpa km interpreted data from leak off tests in the west tuna area confirm that s h min s v

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

origin of abnormal pressure in the upper paleozoic shale of the ordos basin china abstract overpressure in the upper paleozoic shale of the ordos basin upsob in china occurred mainly in the early cretaceous and is modeled using the petromod simulation method in this paper the origin of paleo overpressure was analyzed based on the multiple logging combination method bowers method and the velocity density cross plot method using logging data and shale porosity the causes of the underpressure and normal pressure and the history of their evolution were investigated based on a large amount of drilling and pressure data for the basin the results indicate that paleo overpressure in the upsob produced in the early cretaceous was not the cause of disequilibrium compaction but this occurred owing to increased hydrocarbon generation since the late cretaceous during the overall uplift of the ordos basin the decrease in temperature has led to a reduction in pore pressure of 23 93 34 35 mpa accounting for 56 69 69 47 of total pressure loss a combination of the expansion and diffusion of natural gas has led to a decrease in pore pressure of 9 52 22 60 mpa accounting for 27 63 41 83 of total pressure loss the effects of pore rebound on pressure loss were weak therefore temperature decrease alone could not have led to the current underpressure and normal pressure and the expulsion of gas owing to tectonic uplifting might have been the most important factor the upsob likely had a relatively high gas expulsion efficiency especially during late stage tectonic uplifting which may be a major risk for shale gas exploration

plate boundary forces are not enough second and third order stress patterns highlighted in the world stress map database 1 the world stress map project compiles a global database of contemporary tectonic stress information of the earth s crust early releases of the world stress map project demonstrated the existence of first order plate scale stress fields controlled by plate boundary forces and second order regional stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound the 2005 release of the world stress map project database provides for some areas high data density that enables us to investigate third order local stress field variations and the forces controlling them such as active faults local inclusions detachment horizons and density contrasts these forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic we present and discuss examples for venezuela australia romania brunei western europe and southern italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns

quantifying cretaceous cenozoic exhumation in the otway basin southeastern australia using sonic transit time data implications for conventional and unconventional hydrocarbon prospectivity in prospective basins affected by exhumation uncertainty commonly exists regarding the maximum burial depths of source reservoir and seal horizons one such basin is the otway basin an important gas province in southeastern australia which has witnessed several exhumation events here we present estimates of net exhumation magnitudes for 110 onshore and offshore petroleum wells based on the sonic transit time analyses of lower cretaceous fluvial shales our results show significant post albian net exhumation in the eastern onshore otway basin 1500 m 4920 ft and a generally minor net exhumation 200 m 655 ft elsewhere in the otway basin consistent with estimates based on thermal history data the distribution of net exhumation magnitudes in relation to mid cretaceous and neogene compressional structures indicates that exhumation was dominantly controlled by short wavelength basin inversion driven by plate boundary forces deeper burial coupled with high geothermal gradients in the onshore eastern otway basin and along the northern basin margin during the early cretaceous have rendered lower cretaceous source rocks mostly overmature with any remaining hydrocarbons from the initial charge likely to be trapped in tightly compacted reservoirs and or secondary fracture related porosity however the embrittlement of these reservoirs during their deeper burial may present opportunities for the development of low permeability plays through hydraulic fracturing where smectite clay minerals are illitized source rocks at near maximum burial at present day are at temperatures suitable for gas generation with key controls on prospectivity in these areas including the sealing potential of faulted traps and the relationship between charge and trap development

seafloor precipitates and c isotope stratigraphy from the neoproterozoic scout mountain member of the pocatello formation southeast idaho implications for neoproterozoic earth system behavior abstract macroscopic seafloor precipitated crystal fans are rare in post paleoproterozoic strata but they are more common in the unusual carbonates that cap certain neoproterozoic glacial strata the uppermost carbonate and marble unit of the scout mountain member of the neoproterozoic pocatello formation younger than 667 5 ma contains seafloor fans records negative 13 c values and lies within a transgressive sequence all features consistent with such post glacial carbonate units it does not however rest directly upon a known glaciogenic deposit or sequence boundary instead it occurs within a cyclic upward fining succession that overlies an erosional contact on the pink dolostone post glacial carbonate atop the late sturtian scout mountain member diamictites it is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the pocatello formation alternatively it may imply that means independent of glaciation such as overturn of an anoxic ocean foster the deposition of cap like carbonates independent of post glacial processes thus neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low latitude glaciation additionally the radiometrically calibrated 13 c record of the pocatello formation demonstrates that the interval from 700 to 580 ma has more variation than previous compilations would suggest

late proterozoic aragonitic cement crusts bambui group minas gerais brazil the late proterozoic pedro leopoldo facies bambui group in the vicinity of belo horizonte brazil comprises alternating laminated microsparitic limestones 10 35 mm thick beds and fibrous limestones 10 55 mm thick the latter are composed of a mosaic of sparry calcite crystals these irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared off growth zones ghosts of an original fibrous mineral hexagonal in cross section are visible the petrographic characteristics very high strontium content and low magnesium content of the fibrous beds as well as microspar beds strongly argue for an original aragonitic mineralogy the rays are interpreted as having formed by precipitation at the sediment water interface whereas the micrite was precipitated from the water column prior to deposition on the sea floor the lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions

paleoceanographic constraints on precambrian phosphorite accumulation baraga group michigan usa abstract the palaeoproterozoic baraga group ca 1850 1 ma of northern michigan is a 1200 m thick sedimentary succession of marine clastic iron formation chert and phosphatic sedimentary rocks that accumulated at the peak of the world s first major phosphogenic episode lithofacies stacking patterns are interpreted to record the flooding of the nuna continental margin during two sea level cycles the base of the first sequence is marked by a transgressive lag on archean basement that is transitional into a highstand accumulation of sandstones deposited in peritidal environments the bottom of the second sequence is characterized by a chert and carbonate unit with numerous subaerial exposure surfaces deposited in intertidal to supratidal environments this chert grades upward into subtidal deposits composed of interbedded organic rich mudstone and sandstone the highstand and falling stage systems tracts are composed of progradational deltaic deposits sequences were framed using the newly discovered sudbury impact ejecta horizon as a datum its emplacement approximately coincides with unrelated environmental changes that increased the delivery of river borne sediment to the nuna margin iron bearing minerals ankerite siderite and pyrite show that ferrous iron was present in peritidal to deep marine environments ankerite dominates nearshore settings where pyrite is absent pyrite occurs in deeper progradational deltaic deposits where sulfide was produced by bacterial sulfate reduction phosphorite is restricted to shallow water sediments of the first sequence and the lowstand of the second sequence precipitation is interpreted to be the result of fe redox pumping just below the sediment water interface where photosynthetically produced nearshore oxygen oases impinged on the seafloor such shallow water phosphorite accumulation is in stark contrast to many phanerozoic depositional systems in which phosphogenesis occurs across the shelf this difference likely reflects the dissimilarity in the oxygenation state of the seafloor in the precambrian fe redox pumping and thus phosphogenesis was restricted to shallow water settings with a suboxic seafloor in the phanerozoic phosphorite forms in the full spectrum of shelf environments because the entire seafloor is generally well oxygenated the concentration of bioavailable p in neritic environments during the proterozoic may have played a major role in the development of benthic microbial ecosystems and evolving eukaryotes

variation of vertical stress in the carnarvon basin nw shelf australia abstract variations in vertical stress magnitudes can be attributed to lateral facies variations diagenesis localized uplift and disequilibrium compaction overpressure vertical stress magnitudes have been calculated using density and checkshot data from 47 wells in the carnarvon basin on the nw shelf of australia the vertical stress ranges from 20 2 mpa km 1 to 23 8 mpa km 1 below seabed demonstrating a significant variation across the basin the largest vertical stress magnitudes were calculated for the dampier sub basin the eastern barrow sub basin and on the peedamullah shelf the smallest vertical stress magnitudes were calculated in the kangaroo syncline the western barrow sub basin and offshore across the exmouth plateau high rock densities thus high vertical stress magnitudes can be due to uplifted rock that is over compacted relative to its current depth in the carnarvon basin areas of elevated vertical stress magnitudes correspond to areas of identified uplift uplift at the bambra anticline has previously been estimated to be approximately 900 m calculations within the bambra 4 well demonstrate that a 900 m increase in burial depth of a regionally consistent lithostratigraphic unit the muderong shale equates to a 0 22 g cm 3 increase in rock density thus the difference between uplifted over compacted rock and normally compacted rock no uplift is equivalent to a difference in the vertical stress magnitude of 2 1 mpa km 1 at the bambra anticline this value totalling the difference between over compacted uplifted and normally compacted sedimentary rock 2 1 mpa km 1 accounts for some part of the 3 6 mpa km 1 variation in vertical stress magnitude observed across the carnarvon basin therefore recent tectonic uplift has had a significant impact on the present day magnitude of the vertical stress and is interpreted to be responsible for the majority of observed variation of vertical stress in the carnarvon basin

geological and exploration models of beach placer deposits integrated from case studies of southern australia abstract the processes leading to the formation of beach placer deposits generally begin inland and terminate at the coast including source rocks being weathered eroded and then transported by streams and rivers to the coast where the sediments are deposited in a variety of coastal environments the coastal sediments are reworked by the action of waves tides longshore currents and wind which are effective processes for sorting the mineral grains based on differences in their size and density resulting in laminated or lens shaped packages of sediments up to tens of meters thick that are rich in heavy minerals detailed studies of sedimentary basins as well as peripheral paleo valleys that drained sediment source areas are important tools in the exploration for heavy mineral resources knowledge of the paleo basin associated valley architecture and the source of heavy minerals concentrated in the shorelines and valleys are useful guides to the potential for and location of economic deposits evidence from sedimentology can be combined with that from other geological and geophysical characteristics to arrive at a general reconstruction of basin and paleovalley architecture and depositional environments complex paleogeography of the shorelines can influence or determine the sites of heavy mineral concentration heavy mineral sands tend to concentrate in certain shoreline settings during storm activity repeated storm erosion and reworking over centuries e g the southeastern coast of australia or millennia e g the eucla and murray basins of australia can progressively enrich heavy mineral sand deposits preservation of these deposits over a geological timeframe of millions of years can ensue through subsidence of coastal sediments and during sea level change that results in either shorelines migrating inland marine transgression or seaward marine regression potentially burying or stranding earlier deposits or reworking them to form younger deposits refinements in remote sensing and geophysical techniques data processing sedimentology and computer aided interpretations provide effective economic and efficient methods for modeling coastal reconstructions and for exploring provinces and terrains prospective for heavy mineral sand deposits landscape topography analysis combined with geophysical methods that can resolve physical property contrasts between the shoreline sediments and underlying sequences are increasingly used in mineral exploration to locate and to reconstruct paleoshorelines and paleovalleys australia has modern and ancient beach placer deposits both of which show many similar geologic features the formation of these heavy mineral deposits provides one of the best examples of applying knowledge of modern systems e g the west and east coasts of australia as an analogue to interpret and understand the geology and form of ancient deposits e g the eucla and murray basins of southern australia this study provides descriptive and exploration models of australia s heavy mineral sand deposits formed in coastal environments which can be applied to similar settings worldwide

recent discoveries of proterozoic microfossils in south central brazil abstract twenty two precambrian microfossiliferous localities seven newly reported here are known from the sao francisco block and associated brasilia fold belt of south central brazil reported from outcrop and drill core samples of mesoand neoproterozoic cherts carbonates and siliciclastics of the 1170 to 950 ma old paranoa group and the 950 to 700 ma old sao francisco supergroup composed of the jequitai formation and the overlying bambui group representatives of nearly 30 genera of proterozoic microfossils chiefly cyanobacteria and subsidiary acritarchs have been reported from these strata preserved mostly within carbonaceous black chert in flat laminated and high relief stromatolites and represent a wide spectrum of environments evidently ranging from nearshore lagoonal to supratidal intertidal subtidal and offshore marine settings contrary to previously held views the microfossils are generally more abundant and better preserved in the older unit paranoa group than in the younger one sao francisco supergroup nevertheless among the newly reported assemblages one from the lower part of the bambui group stands out as probably the best preserved and most diverse microbiota known from any precambrian unit of south america although paleobiologic studies of the brazilian proterozoic are still in a formative stage recent discoveries are highly promising such as the detection in the work presented here of possible differences 1 between the microfossil sizes in stromatolites of the paranoa and bambui groups 2 in the abundance and size of acritarchs with respect to lithology and facies and 3 in the microfossil assemblages preserved in morphologically distinct stromatolites

phosphogenesis aragonite fan formation and seafloor environments following the marinoan glaciation abstract carbonates in the sete lagoas formation sao francisco craton brazil preserve a record of chemical biological and oceanographic changes that occurred during the ediacaran period the base of this formation constitutes a post glacial cap carbonate which contains seafloor precipitates carbonate and barite crystal fans as well as various authigenic and diagenetic minerals apatite pyrite and barite here we present petrographic and geochemical data on this unit and discuss the significance of its mineral association for marine environments following the marinoan snowball earth glaciation for the first time we report well developed apatitic cements in a neoproterozoic cap carbonate isopachous and intergranular void filling cements encrust and surround seafloor precipitated fan crystals that precipitated as aragonite we propose a model for the origin of this mineral association which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions according to this model the boundary between oxic and anoxic conditions was located at or just below the sediment water interface burial of iron oxyhydr oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction iron reduction released fe2 which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor concurrently iron pumping shuttled phosphate from the water column to the sediment and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways created conditions conducive to phosphate mineralization this model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation in addition to high alkalinity and that fe2 serves as this inhibitor overall our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the ediacaran illuminates the paleoenvironments of cap carbonates with seafloor precipitates and contributes to understanding of phosphogenesis following low latitude glaciations

terminal proterozoic cyanobacterial blooms and phosphogenesis documented by the doushantuo granular phosphorites ii microbial diversity and c isotopes an unprecedented period of phosphogenesis along with massive deposition of black shales major perturbations in the global carbon cycle and the rise of atmospheric oxygen occurred in the terminal proterozoic in the aftermath of the marinoan glaciation although causal links between these processes have been postulated evidence remains challenging correlated in situ micro analyses of granular phosphorites from the ediacaran doushantuo formation in yichang south china suggested that cyanobacteria and associated extracellular polymeric substances eps might have promoted aggregated granule growth and subsequent phosphatization she et al 2013 here we present new paleontological data for the doushantuo phosphorites from yichang which combined with raman microspectroscopy and carbon isotope data further document links between the biology of cyanobacteria and phosphogenesis mapping of microfossils in thin section shows that most phosphatic granules contain microfossils that are dominated by colonies of myxococcoides along with several filamentous genera generally considered to represent cyanobacterial sheaths in addition the phosphorites and associated rocks have 13corg values in the range of 26 0 to 29 7 vpdb consistent with photoautotrophic carbon fixation with the rubisco enzyme close association of phosphorites with the marinoan tillites in stratigraphic level supports a genetic link between deglaciation and phosphogenesis at least for the doushantuo occurrence our new data suggest that major cyanobacterial blooms probably took place in the terminal proterozoic which might have resulted in rapid scavenging of bioavailable phosphorus and massive accumulations of organic matter om within a redox stratified intra shelf basin the om bound phosphorus could have liberated by microbial sulfate reduction and other anaerobic metabolisms and subsequently concentrated by fe redox pumping below the chemocline upwelling of the bottom waters or upward fluctuation of the chemocline might have brought p enriched waters to the photic zone where it was again scavenged by cyanobacteria through their eps to be subsequently precipitated as francolite the feedbacks between enhanced continental weathering cyanobacterial blooms carbon burial and accelerated phosphorus cycle thus controlled the marine biogeochemical changes which led to further oxygenation of the atmosphere and oceans ultimately paving the way for the rise of metazoans

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

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

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

aftermath of a snowball earth 1 using a simple 3 box model of the ocean atmosphere system we simulate the cycling of carbon and strontium in the aftermath of a global glaciation model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pco2 conditions as well as ocean mixing air sea gas exchange and biological productivity the 13c of the first carbonate precipitated after the glaciation depends on the pco2 temperature the saturation state of the surface ocean and kinetic effects associated with mineral precipitation with no biological productivity the model produces 13c values between 1 and 3 consistent with observations this is in direct contradiction with arguments by kennedy et al 2001a who suggest that the 13c value of dissolved carbon in a snowball ocean and directly afterward must be 5 kennedy et al assume the carbon isotope cycle is in steady state which does not apply to a global glaciation and also neglect any effect of high pco2 on the carbonate chemistry of seawater a major difference between our findings and the qualitative predictions of hoffman et al 1998 is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves as a result the 2 drop in the 13c observed in the cap dolostone is unlikely to be the product of rayleigh distillation of atmospheric co2 via silicate weathering instead we interpret the 2 drop in the 13c values as indicative of an increase in sea surface temperature which lowers the fractionation between co2 and carbonate kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important rayleigh distillation of atmospheric co2 via silicate weathering is a viable explanation for the continued drop in the 13c values in the limestone sequence above the cap dolostone with biological productivity and carbonate weathering driving a slow increase in 13c values once pco2 levels decline our study also simulates the cycling of strontium in seawater in contrast to the finding of jacobsen and kaufman 1999 and kennedy et al 2001a model simulations show a drop in 87sr 86sr of less than 0 001 during 5 million years of global glaciation and an increase of less than 0 001 over the entire episode of silicate weathering our calculations emphasize the importance of considering the changes in seawater chemistry due to high pco2 in evaluating the snowball earth hypothesis

estimating hinterland exhumation from late orogenic basin volume nw borneo a new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late tertiary basin and topographic development in nw borneo sediment volumes for the baram deltaic province were estimated for four time periods latest early middle and late miocene and pliocene recent using 2d and 3d seismic horizons wells and outcrops volume restoration onto the palaeo sediment source area determined exhumation amount at the drainage divide c 5 km from 17 ma to recent and provided a reasonable match with other denudation estimation methods cooling ages and an older regional sediment restoration study using 1979 vintage sediment isopach maps restoration took into account 1 changing sediment densities during the erosion deposition cycle 2 changing area of the sediment source province with time e g changing shoreline location as a result of eustacy uplift and delta progradation tectonic shortening 3 uplift and partial erosion of baram deltaic province sediment denudation at the drainage divide for the middle miocene late miocene and pliocene recent has proceeded at a similar rate for each period initial uplift of central borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under nw borneo in the early miocene however the absence of decay in the erosion rates with time from the middle miocene to recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere slab detachment is the favoured explanation

phosphorus sources for phosphatic cambrian carbonates the fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating but time bounded window into ediacaran cambrian animal evolution for reasons that remain unknown phosphatic fossil preservation declined significantly through cambrian series 2 here we investigate the phosphorus p sources for phosphatic cambrian carbonates presenting sedimentological petrographic and geochemical data from the cambrian series 2 3 thorntonia limestone australia some of the youngest cambrian strata to display exceptional phosphatic preservation of small shelly fossils we find that within thorntonia sediments phosphate was remobilized by organic decay and bacterial iron reduction with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils we discuss the merits of bioclastic derived organic matter bound or iron bound p as potential sources to these strata petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the p for fossil preservation in contrast high organic carbon contents imply significant organic fluxes of p to thorntonia sediments sedimentology and iron speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic ferruginous conditions in subsurface water masses suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant p source to thorntonia sediments simple stoichiometric models suggest that by themselves neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment thus we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the thorntonia limestone

reprint of comparison of modern fluid distribution pressure and flow in sediments associated with anticlines growing in deepwater brunei and continental environments iran abstract differences in fluids origin creation of overpressure and migration are compared for end member neogene fold and thrust environments the deepwater region offshore brunei shale detachment and the onshore arid central basin of iran salt detachment variations in overpressure mechanism arise from a the availability of water trapped in pore space during early burial deepwater marine environment vs arid continental environment and b the depth temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development chemical reactions associated with smectite rich mud rocks in iran occur shallow 1900 m smectite to illite transformation causing load transfer related moderate overpressures whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore brunei the basal detachment in deepwater brunei generally lies below temperatures of about 150 c where chemical processes and metagenesis are inferred to drive overpressure development overall the deepwater brunei system is very water rich and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines the result is a wide variety of fluid migration pathways and structures from deep to shallow levels particularly mud dykes sills laccoliths volcanoes and pipes fluid escape pipes crestal normal faults thrust faults and widespread inflationary type overpressure in the central basin the near surface environment is water limited mechanical and chemical compaction led to moderate overpressure development above the upper red formation evaporites only below thick early miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms fluid leakage episodes across the evaporites have either been very few or absent in most areas locations where leakage can episodically occur e g detaching thrusts deep normal faults salt welds are sparse however in both iran and brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds

present day stress orientation in brunei a snapshot of prograding tectonics in a tertiary delta the baram delta province of nw borneo is unusual when compared with most other tertiary deltas as it has built up upon an active margin hence structures observed in the baram delta province are the result of both margin parallel gravity driven deltaic tectonics and approximately margin normal transpressive tectonics associated with the active margin image and dipmeter logs have been examined for breakouts and drilling induced tensile fractures ditfs in 47 wells throughout brunei breakouts and ditfs observed in 19 wells suggest that the maximum horizontal stress is oriented margin normal nw se in the proximal parts of the basin and margin parallel ne sw in the outer shelf region the margin parallel outer shelf stress field is interpreted as a local deltaic stress field caused by the shape of the clastic wedge the margin normal maximum horizontal stress in the inner shelf is interpreted to reflect basement stresses associated with the active margin however the maximum horizontal stress in the inner shelf is approximately perpendicular to the strike of miocene pliocene normal growth faults suggesting that maximum horizontal stress in the inner shelf has rotated from margin parallel deltaic to margin normal basement associated over time hence approximately the same stress rotation has occurred over time in the inner shelf as is currently observed spatially from the outer to inner shelf the spatial and temporal stress rotations in brunei are thus interpreted to be the result of deltaic and basement associated tectonic regimes that are prograding basin wards the proximity of the active margin has resulted in progressive uplift and inversion of the hinterland that has forced the delta system to prograde rapidly the zone of active deltaic growth faulting and margin parallel maximum horizontal stress has shifted basin wards prograded as the delta system has rapidly prograded across the shelf after uplift and delta progradation the old growth faults of the inner shelf ceased being active and have then been successively reactivated by a similarly prograding margin normal inversion front

time average equation revisited expressions that relate velocity to porosity and to pore fluid compressibility are among the most important deliverables of rock physics such relations are used often as additional controls for inferring porosity from well logs as well as in situ indicators of pore fluid type the oldest and most popular is the wyllie et al 1956 equation p s f where p is the measured traveltime of a p wave s is the traveltime expected in the solid phase material and f is the traveltime expected in the pore fluid it follows from equation 1 that 1vp 1 vps vpf where is porosity vp is the measured p wave velocity and vps and vpf are the p wave velocities in the solid and in the pore fluid phases respectively

sequence stratigraphic framework of proterozoic successions in eastern brazil abstract first order regional unconformities separate proterozoic supracrustal rocks of the gas bearing sao francisco craton and surrounding thrust and fold belts eastern brazil into four genetic stratigraphic units which constitute 1st order basin fill sequences espinhaco canastra paranoa vazante macaubas and bambui sequences the espinhaco 1st order sequence ca 1 73 1 50 ga records a stage of aborted lithospheric stretching of the sao francisco congo continental mass the espinhaco basin displays a steer s head geometry where four 2nd order sequences are recognized prerift rift transitional and flexural third order rift propagation unconformities allowed the definition of three 3rd order synrift sequences within the rift 2nd order sequence the 2nd order flexural sequence of the espinhaco basin can be subdivided into three 3rd order transgressive regressive sequences the canastra paranoa vazante and macaubas 1st order sequences ca 1200 630 ma comprise rift to drift successions deposited during the breakup of the supercontinent rodinia and the opening of the brazilide and adamastor oceans respectively to the west and east of the sao francisco paleocontinent according to seismic data the passive margin canastra paranoa vazante 1st order sequence can be divided into two 2nd order sequences rift and passive margin the passive margin 2nd order sequence can be further subdivided into three 3rd order sequences the outcropping canastra paranoa vazante sequence consists mainly of shallow marine deposits which contain black shales marls with intervals showing high total organic carbon toc contents comprising one of the highest potential hydrocarbon source rocks of the sao francisco basin the bambui foreland 1st order sequence ca 750 550 ma formed as a consequence of thrust loading related to shortening in the brasilia fold belt on the western flank of the sao francisco craton during the closing of the brazilide ocean and the assembly of the west gondwana supercontinent the bambui sequence shows two distinct successions which reflect the main basin paleogeographic settings foredeep and flexural ramp three transgressive progradational 2nd order sequences can be recognized in the ramp succession of the bambui basin

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

terrestrial or waterlain glacial diamictites three case studies from the late precambrian and late ordovician glacial drifts in west africa abstract three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the west african cratonic platform are described they belong to the late precambrian and upper ordovician glacial periods and the paleogeography is summarized the late precambrian diamictites of the jbeliat group in adrar of mauritania displays characteristics of tillite deposited as lodgement or melt out till detail studies on structure clast shape nature and fabric texture in thin section clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification compared to this first example the two other diamictites from the late precambrian bakoye group in western mali and from the upper ordovician tichit group in southeastern mauritania have obviously been deposited in an aquatic environment waterlain glacial diamictites by the action of ice rafting bottom current and flow processes these examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships however detailed sedimentological analyses remain essential for precise facies identification especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes

influence of basement structures on in situ stresses over the surat basin southeast queensland the jurassic to cretaceous sedimentary rocks of the surat basin in southeast queensland host a significant volume of coal seam gas resources consequently knowledge of the in situ stress is important for coal permeability enhancement and wellbore stability using wireline log data and direct stress measurements we have calculated stress orientations from 36 wells and stress magnitudes from 7 wells across the surat basin our results reveal a relationship between high tectonic stress and proximity to structures within the underlying basement rocks the influence of tectonic stresses is diminished with depth in areas with thicker sedimentary cover that are relatively far from the basement structures we suggest that this relationship is due to the redistribution of in situ stresses around areas where basement is shallower and where basement structures such as the leichhardt burunga fault system are present this behavior is explained by a lower rigidity in the thickest basin cover which reduces the ability to maintain higher tectonic stress over the entire surat basin a significant amount of variability in in situ stress orientation is observed the authors attribute this stress variability to complex plate boundary interactions on the northern and eastern margins of the indo australian plate

paleostress pattern and salt tectonics within a developing foreland basin north western subhercynian basin northern germany analysing the paleostress field in sedimentary basins is important for understanding tectonic processes and the planning of drilling campaigns the subhercynian basin of northern germany is a perfect natural laboratory to study the paleostress field in a developing foreland basin the simple layer cake geometry of the basin fill is dominated by several piercing and non piercing salt structures we derived the paleostress field from the orientation of fracture sets faults slickensides and stylolites on a regional scale the basin fill is characterized by a horizontal compressional paleostress vector that is mainly nne ssw oriented which reflects the late cretaceous inversion phase in central europe we show that the local paleostress field is distinctly perturbated due to the salt structures along the edge of the salt pillows the maximum horizontal paleostress vector is deflected by up to 90 from the regional trend in the case of the elm salt pillow it forms a radial pattern restoration of balanced cross sections demonstrates at least 9 of the shortening of the north western part of the subhercynian basin was achieved by folding the salt structures in the north western subhercynian basin are the result of varying stress conditions initial extension in the triassic caused first salt movements that prevailed during the jurassic and early cretaceous most important is the late cretaceous contractional phase that shortened the diapirs and led to the formation of the salt pillows between diapirs due to detachment folding we derive four main controlling factors for such salt dominated contractional basins 1 the wedge shape basin fill is the product of the dynamic load at the southern margin of the basin 2 a basal salt layer fed the diapirs and acted as a detachment horizon during the later shortening 3 detachment folding was the dominating deformation mechanism during contraction and 4 the pre existing diapirs controlled the position of the detachment folds

late cenozoic reverse faulting in the fall zone southeastern virginia abstract a set of en echelon reverse faults cut paleozoic metamorphosed igneous rocks of the piedmont and overlying late cenozoic sediments at the old hickory heavy mineral deposit in the fall zone of southeastern virginia diorite of the eastern slate belt was faulted over nearshore to shore face deposits of the pliocene yorktown formation these nw se striking faults experienced oblique dip slip movement with a maximum displacement of up to 6 m on individual faults faults tip out along strike and are overlain by distinct cobble beds suggesting that sediment deposition and faulting were contemporaneous deformation at old hickory may have been formed by reactivation of existing paleozoic structures under a regionally extensive compressional stress field parallel to the modern one

phosphogenesis around the proterozoic phanerozoic transition there was a very significant and widespread phosphogenic event around the proterozoic phanerozoic transition this is reflected in the occurrence of many major and minor latest proterozoic and cambrian phosphate deposits phosphorites in asia australia africa and south america as well as minor occurrences in many other parts of the world the major peak in phosphogenesis was in the tommotian with lesser events in the ediacaran and the lenian bracketed by other minor though significant events recent evidence on the timing of proterozoic phosphogenesis does not now support the suggestion of a phosphogenesis glaciation link these peaks in phosphogenesis are taken as evidence that there was a marked increase in the phosphate content of the shallow seas at about this time possibly as a result of the circulation of deep phosphorus rich ocean waters into the photic zone not only was this responsible for the formation of phosphorites but it also had a profound effect on the biota and may have been a driving mechanism for the cambrian faunal explosion and the initiation of biomineralization

anatomy and origin of a cretaceous phosphorite greensand giant egypt late cretaceous epicontinental phosphorites porcelanites cherts dark coloured shales glauconitic sandstones and bioclastic and fine grained carbonate rocks in egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics two main depositional realms are interpreted and correlated through sequence stratigraphic analysis 1 a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon rich shales biosiliceous sediments and thick phosphorites and 2 a relatively high energy depositional regime accompanying sea level fall during which deltas advanced glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis thereby promoting solution collapse phenomena in associated cherts lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments in eastern egypt the phosphorites form winnowed lag layers some of which may have been redeposited down slope in structural lows in the west these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning however direct desorption of phosphorus to pore waters from detrital iron oxyhydroxide phases also may have been important in the cretaceous setting the iron reduced in this process being available for incorporation in glauconites the main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters this study suggests a model for the common coexistence of glauconites and phosphorites in the geological record although upwelling is often advocated as the origin of nearly all giant phosphorite deposits we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and or reduction in bottom waters or pore waters

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

the puzzle assembled ediacaran guide fossil cloudina reveals an old proto gondwana seaway abstractduring the ediacaran the clymene ocean separated the lau rentia amazonia and rio apa cratons from several landmasses to the west forming the proto gondwana supercontinent however no clear evidence about the existence of ediacaran epeiric seas over those landmasses has been found here we report and discuss the discovery of the ediacaran guide fossil cloudina sp associated with other metazoan body and trace fossils in the bambui group central eastern brazil the ediacaran age of the bambui group and the paleogeographic position of cloudina bearing successions in brazil antarctica namibia and argentina suggest a scenario of ocean con nectivity among coeval intracratonic basins of south america africa and antarctica at the end of neoproterozoic time the new nding epitomizes one of the most important paleontological discoveries ever made in south america helping to solve an old paleogeographic puz zle of the gondwana supercontinent introduction the bambui group crops out in central eastern brazil and is one of the most studied neoproterozoic sedimentary successions in south america it comprises an extensive at lying cratonic cover in erosional contact with paleoproterozoic and archean basement rocks of the sao francisco craton in its eastern and westernmost occurrences the bam bui group is deformed and metamorphosed by the brasilia and aracuai mobile belts respectively fig 1a the regional tectonostratigraphy has been interpreted as re ecting deposition in a foreland basin related to gondwanan collisions involving the sao francisco craton pimentel et al 2011 the bambui group bg overlies rocks of the macaubas group and jequitai formation and the carrancas conglomerate units supposedly de posited during sturtian glaciation the unit is 700 1000 m thick misi et al 2007 and comprises limestones and dolomites of the sete lagoas for mation at the base that grade upward to shales and siltstones of the serra de santa helena formation followed by marls siltstones limestones and sandstones of the lagoa do jacare serra da saudade and tres marias formations fig 1b the precise age of the bg is not well established precluding accurate correlation with other supposedly coeval successions such as the corumba group brazil itapucumi paraguay arroyo del soldado uruguay si erras bayas argentina and nama group namibia the presence of

variation in vertical stress in the baram basin brunei tectonic and geomechanical implications the vertical or lithostatic stress is an important factor in tectonic and geomechanical studies and is commonly used in the prediction of pore pressures and fracture gradients however the vertical stress is not always calculated in situ and the approximation of 1 0 psi ft 22 63 mpa km is often used for the vertical stress gradient vertical stress has been determined in 24 fields in the baram basin brunei using density log and checkshot velocity survey data the baram basin shows a variation in vertical stress gradient between 18 3 and 24 3 mpa km at 1500 m depth below the surface this variation has a significant effect on in situ stress related issues in field development such as wellbore stability and fracture stimulation the variation is caused by a bulk rock density change of 2 48 2 07 g cm 3 from the hinterland of the delta to its front differential uplift and erosion of the delta hinterland and undercompaction associated with overpressure are the interpreted causes of the density and hence vertical stress variation q 2004 elsevier ltd all rights reserved

balancing deformation in nw borneo quantifying plate scale vs gravitational tectonics in a delta and deepwater fold thrust belt system abstract recent gps measurements demonstrate that nw borneo undergoes 4 6 mm of plate scale shortening a year which is not accommodated by plate scale structures the only geological structure in nw borneo described to accommodate on going shortening is the baram delta system located on the outer shelf to basin floor delta toe fold thrust belts are commonly thought to be caused by margin normal compressional stresses generated by margin parallel upslope gravitational extension the baram delta system is divided into three neotectonic provinces 1 an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province 2 an outer shelf region of present day deltaic extension and 3 a compressional delta toe however it is uncertain whether compression in the delta toe area is purely driven by gravity i e shortening is less than upslope extension or if there is a component of regional shortening involved which can only be demonstrated if shortening is greater than upslope extension in order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the baram delta system we have used a geomechanical code based on the finite element method and conservation of mass and momentum dynel 2d this enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top with the system exhibiting 1 8 shortening overall or 2 0 km this additional shortening is therefore attributed to plate scale shortening across nw borneo produced by far field compression using a convergence rate of 4 mm y 1 demonstrated by gps measurements the delta toe has accommodated far field compression only in the last 0 5 my

heavy mineral deposits in the upper coastal plain of north carolina and virginia approximately 22 7 million metric tons 25 million short tons of heavy minerals at an average grade of 6 wt percent in 377 8 million metric tons 416 million short tons of sand have been delineated in 19 deposits in the upper coastal plain of north carolina and virginia average concentrations of the principal minerals are ilmenite 60 percent rutile 2 5 percent zircon 12 5 percent staurolite 8 5 percent tourmaline 0 7 percent kyanite 3 0 percent sillimanite 1 3 percent and others principally limonite 11 5 percent these deposits formed during a worldwide pliocene transgressive regressive event that occurred between 3 5 and 3 0 ma the deposits formed as beach or dune sands during the regressive phase of the event over an elevation range of 96 m 315 ft to 53 m 175 ft other major deposits that are interpreted to have formed during the pliocene transgressive regressive event are as follows trail ridge florida enneaba western australia lakehurst new jersey and deposits of the western murray basin victoria australia combined reserves of these pliocene deposits exceed 317 5 million metric tons 350 million short tons of heavy minerals the potential for new discovery of pliocene deposits elsewhere in the world is judged to be favorable

shipboard magnetic field noise reveals shallow heavy mineral sediment concentrations in chesapeake bay abstract shipboard magnetic field data collected over chesapeake bay exhibit low amplitude short wavelength anomalies that most likely indicate shallow concentrations of heavy mineral sediments piston core layers and black sand beach samples exhibit enhanced magnetic susceptibilities and carry remanent magnetization with mineralogical analyses indicating ilmenite and trace magnetite and or maghemite and hematite the anomalies are subtle and would be filtered as noise using traditional approaches but can instead be highlighted using spectral methods thus providing nearly continuous coverage along survey tracks the distribution of the anomalies provides constraints on relevant sorting mechanisms comparisons to sonar data and previous grab samples show that two of three areas surveyed exhibit short wavelength anomalies that are clustered over sand covered areas suggesting initial sorting through settling mechanisms this is supported by a correlation between core magnetic susceptibility and grain size near the choptank river where sediment resuspension is wave dominated anomalies show a sharp decrease with seafloor depth that cannot be explained by signal attenuation alone in pocomoke sound where both tidal currents and wave action impact sediment resuspension anomalies show a more gradual decrease with depth near the mouth of the bay where there is a higher influx of sediments from the continental shelf short wavelength anomalies are isolated and do not appear to represent heavy mineral sand concentrations these combined observations suggest the importance of further sorting by erosional processes in certain parts of the bay additionally comparisons of these data to cores sampling pre holocene sediments suggest that the sorting of heavy minerals in higher energy shallow water environments provides a mechanism for correlations between core magnetic susceptibility and sea level changes

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

stress magnitudes across uk regions new analysis and legacy data across potentially prospective unconventional resource areas abstract stress magnitude data across the uk is limited spatially and stratigraphically with information available for only 21 sites in the latest release of the world stress map this information is largely derived from geothermal resource exploration and radioactive waste storage site assessment active exploration of unconventional resources in the uk has highlighted a lack of information to adequately characterise the stress field in particular in regions underlain by potentially prospective shale formations understanding the in situ stress conditions is critical to the planning of sub surface operations and the potential extraction of unconventional resources legacy stress magnitude data from 75 sites is combined with new analysis of wireline data to re characterise the stress field across two regions which are underlain by the bowland shale formation which has resource potential for unconventional hydrocarbons these regions are east yorkshire and north nottinghamshire and cheshire and lancashire vertical stress gradients vary between 23 and 26 mpakm 1 for the regions studied pore pressure is similar for both regions and is hydrostatic with a gradient of 10 19 mpakm 1 lower bounds for the minimum horizontal stress have been estimated from the available data and show that the magnitude of the minimum horizontal stress is 2 6 mpakm 1 higher to the east of the pennines the compiled legacy data show that the maximum horizontal stress is consistently greater than the vertical stress which in turn is greater than the minimum horizontal stress indicating that at depth within the two regions the faulting regime is predominantly strike slip

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

incompatible stress regimes from geological and geomechanical datasets can they be reconciled an example from the carnarvon basin western australia abstract knowledge of the in situ stress field is essential in petroleum basins as it exerts a prime control over seal integrity fracture stimulation wellbore stability and fluid flow previous geomechanical analysis of australia s carnarvon basin the continent s premier hydrocarbon region highlights a normal to strike slip fault stress regime however neotectonic evidence and contemporary seismicity suggest the possibility of a strike slip to reverse fault stress regime we attempt to reconcile these conflicting datasets through new analysis of the in situ stresses principal stress magnitudes and orientations were defined with data from 76 previously unanalysed petroleum wells wellbore image logs yield a maximum horizontal stress orientation of 113 n consistent with modelling of the stress field throughout the indo australian plate vertical stress magnitudes are estimated to range from 20 mpa km 1 to 22 4 mpa km 1 leak off tests lot from 42 wells are analysed using two methodologies one assuming tensile failure as traditionally accepted the other assuming shear failure as outlined by couzens schultz and chan 2010 and result in two different sets of horizontal stress magnitude estimates traditional interpretation of lots results in a minimum horizontal stress gradient of 16 8 mpa km 1 and a maximum horizontal stress gradient of 21 8 mpa km 1 the new method for lot interpretation results in a minimum horizontal stress gradient of 18 1 mpa km 1 and a maximum horizontal stress gradient of 25 4 mpa km 1 inclined natural fractures observed at lot depths supports application of the new method of lot interpretation traditional interpretation of stress magnitudes implies a normal to strike slip fault stress regime while the new method implies a strike slip fault stress regime this latter interpretation is favoured by the authors as it allows for a reconciliation of the geological and geomechanical datasets the assessment undertaken herein allows for reinterpretation of stress magnitudes in seismically active basins hosting differing geomechanical and neotectonic regimes potentially altering existing understanding of seal integrity fracture stimulation wellbore stability and fluid flow

simultaneous remagnetization and u pb isotope resetting in neoproterozoic carbonates of the sao francisco craton brazil abstract the southern part of the neoproterozoic sao francisco basin in minas gerais state brazil can be divided into three structural domains a the central part of the basin where the rocks are undeformed b the western domain where the rocks have been deformed by the 600 550 ma brasilia fold belt and c the eastern domain where the rocks have been affected by the 600 550 ma aracuai fold belt u pb and pb isotopic data rock magnetism data and paleomagnetism data from the carbonates from different domains support a close connection between a pervasive remagnetization and a large scale fluid percolation event that strongly affected the isotopic system of these rocks at 530 500 ma during the last stage of the brasiliano pan african orogeny a pb pb isochron age of 686 69 ma has been determined from undeformed carbonates in the center of the basin and it is interpreted as the minimum depositional age however most of the pb pb and the u pb ages obtained from deformed as well as other undeformed carbonates fall in an interval of 550 500 ma carbonates containing radiogenic crustal pb with an isotopic signature of the archean paleoproterozoic basement were found in the central portion of the basin which was not affected by deformation suggesting that this pb was incorporated into the carbonates through fluids which promoted the resetting of the isotope system and severe remagnetization in the carbonates the post depositional character of the characteristic magnetizations is strengthened by the following a the disclosed rock magnetic properties such as wasp waisted hysteresis loops anomalously high hysteresis ratios and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates b thermomagnetic analysis and scanning electron microscopy suggest authigenic magnetite as the main magnetic carrier c moderate to high paleolatitudes inferred from paleomagnetic data for the study area would require a different climate pattern during the sedimentation of the wide carbonate platforms d magnetization directions with a single polarity were found along the whole sedimentary sequence e mean magnetization components identified in the carbonates show lower dispersion than would be expected if the secular variation of the geomagnetic field was fully averaged out f paleomagnetic poles from carbonate sequences and adjacent brasiliano metamorphic rocks are similar and coincide with high quality gondwanan paleomagnetic poles for the 530 500 ma interval the similarity between paleomagnetic and isotopic results from the bambui and the salitre carbonates ca 1000 km to the northeast implies a large scale fluid percolation event that simultaneously affected the whole basin the paths of these fluids may have been along old basement faults reactivated during the last stage of the brasiliano pan african orogeny

lower cambrian apatitic stromatolites and phospharenites related to the glacio eustatic cratonic rebound sahara algeria abstract at the end of the early cambrian glacial event occurring on the west african craton the tillite surface was marked especially in the northeastern part of the taoudenni basin by noticeable topographic relief after deglaciation the eustatic rise in sea level led to deposition over all the west african craton of a thin but widespread carbonate unit containing lower cambrian shelly fossils at the ice cap periphery isostatic rebound of the craton resulted in uplift of the topographic highs and weathering of the carbonate during fluctuations of relative sea level before the return to marine transgressive conditions a succession of events took place first in the submerged areas stromatolitic phosphorite in flat domal to columnar buildups accreted followed by growth of dolomitic ministromatolites laterally phospharenitic beds oncolites and phosphatic grains accumulated under water in the depressions the stromatolites and oncolites are made of carbonate fluorapatite characteristic of coastal environments glauconite is closely related to apatite and suggests conditions of a sea level rise in respect to phosphatization precipitation of dolomite calcite and evaporite postdates phosphatization and glauconitization indicating a regression and a chemical concentration due to a warm and perhaps arid climate apatitic stromatolites and oncolites resulted from in situ precipitation within bacterial mats this phosphogenesis corresponds to the major early paleozoic phosphogenetic event

present day stress orientations and tectonic provinces of the nw borneo collisional margin received 23 september 2009 revised 17 may 2010 accepted 9 june 2010 published 16 october 2010 1 borehole failure observed on image and dipmeter logs from 55 petroleum wells across the nw borneo collisional margin were used to determine maximum horizontal stress sh orientations combined with seismic and outcrop data they define seven tectonic provinces the baram delta deepwater fold thrust belt exhibits three tectonic provinces its inner shelf inverted province sh is nw se margin normal its outer shelf extension province sh is ne sw margin parallel and its slope to basin floor compression province sh is nw se margin normal in the inverted province sh reflects inversion of deltaic normal faults the sh orientations in the extension and compression provinces reflect deltaic gravitational tectonics the shale and minibasin provinces have been recognized in offshore sabah in the shale province sh is n010 e which aligns around the boundary of a massif of mobile shale currently no data are available to determine sh in the minibasin province in the balingian province sh is ese wnw reflecting ese absolute sunda plate motions due to the absence of a thick detachment seen elsewhere in nw borneo the central luconia province demonstrates poorly constrained and variable sh orientations these seven provinces result from the heterogeneous structural and stratigraphic development of the nw borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages

direct dating of the sete lagoas cap carbonate bambui group brazil and implications for the neoproterozoic glacial events the end of the neoproterozoic is punctuated by glacial deposition but the chronology of these deposits is hindered presently by the paucity of geochronological data here we present new radiometric dating for the basal sete lagoas cap carbonate deposits that overlie glacial units in the sao francisco craton six samples from aragonite pseudomorph crystal rich facies showing pristine textures and constant 87sr 86sr ratios around 0 7075 yielded a pb pb isochron age of 740 22 ma which is interpreted as the depositional age for these remarkably preserved rocks this age can be used to infer a low to moderate palaeolatitude of 20 30 for carbonate and glacial deposition in addition as it overlaps the ages obtained for the oldest neoproterozoic glacial successions our result reinforces the idea of a long standing sturtian interval suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 ma